LINEAR AND NONLINEAR OPTICAL PROPERTIES

LINEAR AND NONLINEAR OPTICAL PROPERTIES OF POLYMER-ZnO-CuO NANOCOMPOSITES

HAIDER MOHAMMED SHANSHOOL

THESIS SUBMITTED IN FULFILMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

FACULTY OF SCIENCE AND TECHNOLOGY UNIVERSITI KEBANGSAAN MALAYSIA BANGI 2016

ii

DECLARATION

I hereby declare that the work in this thesis is my own except for quotations and summaries which have been duly acknowledged.

January 2016

HAIDER MOHAMMED SHANSHOOL P59650

iii

ACKNOWLEDGMENTS First and foremost, I must say “Alhamdulillah” and I must thank the Almighty God for giving me the strength, patience, courage, and determination in completing this work. I would like to express my deepest appreciation to my supervisor, Professor Emeritus Dato’ Dr Muhamad Yahaya, for his incredible guidance, continuous support, and unfailing encouragement. He always had the time for me. His high standard of diplomatic power and professionalism has set a great model for me to adhere to. . Furthermore, many special thanks to my co. supervisor Professor, Dr Wan Mahmood Mat Yunus, for he readily provided his technical expertise throughout the period of my study. He spent a lot of time teaching me how to be a good researcher and how to write quality academic papers. On top of that, I am extremely thankful to Dr Tan Sintee for her great discussion and helpful suggestions, particularly in starting my lab work, as well as her contribution in FESEM and EDX characterizations and conceptual advice.Along the line, special gratitude is also conveyed to my lab mate, Ibtisam Yahya for her continuous assistance and discussion. All her help in lab work is gratefully acknowledged. Thanks also to all staff members of the Department of Applied Physics and Centre for Research and Instrumentation (CRIM) at UKM, as well as the Department of Physics at UPM for their cooperation to provide the requirements of my work. My gratitude also goes to my lab mates at UKM and UPM; Hind Fadhel, Titian, Chun Hui, Hock Beng, Huwaida, Marjoni, Abdelelah Ali, and Lina for their understanding and help. Not forgotten, my office in Iraq, the Laser Research Centre in the Ministry of Science and Technology, is gratefully acknowledged for giving me the opportunity to pursue my studies. Besides, I would like to extend my gratitude to all my colleagues, mainly Dr Nassir Mahdi and Dr Hassan Ali for their never-ending support and well wishes. Moreover, my appreciation to my colleagues at my office in Iraq who studied at UPM; Khalil Ibrahim and Mohammed Jabar, for their assistance in laser lab (UPM). I am also heavily indebted to my close friends; Zahid Hassan and Laith Smaism for their kindness and moral support offered during my study. I would like to dedicate this work in the memory of my father and mother who encouraged me to complete my higher studies. Not forgetting, I also would like to acknowledge my brothers in Iraq, Ali, Thamir, and Tholfikaar, by saying ‘thank you’ for their prayers and encouragements. Last but not least, to my dearest wife, my three sons and daughter, thank for your patience, continuous support, understanding and the wonderful environment you all provided throughout the completion of this work.

iv

ABSTRACT

Polymer/ZnO nanocomposites have received a great interest in linear and nonlinear optical properties because of their potential application such as optical switching and optical limiting devices. This thesis reported the study concerning the linear and nonlinear optical properties of ZnO nanoparticles which embedded in the polymer matrix. A flexible foil-like polymer/ZnO nanocomposite was prepared via casting method. In the typical procedure, various concentrations of ZnO nanoparticle (0, 1, 3, 5, 10, and 15wt %) that acted as a filler were incorporated into different types of polymer matrix which were polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), polystyrene (PS) and polyvinyl alcohol (PVA) by mixing approach. Moreover, linear and nonlinear optical modification on as-prepared polymer/ZnO nanocomposites, as a foil, composed of 10wt% was also studied through the addition of (1wt %) CuO nanoparticles. Then, PMMA/ZnO and PVDF/ZnO nanocomposites were chosen to prepare as a thin film on quartz by spin coating and casting respectively. It was found the as-prepared samples shown an intense linear absorption spectra at the range wavelength of 370-377nm indicating the existence of ZnO nanoparticles arise from the exciton transition. The linear transmittance spectra was found inversely proportional to the ZnO nanoparticles concentration while reverse phenomenon was observed in reflectance spectra in the UV region. In addition, it was noticed that the optical band gap of the samples were red shifted, that was proved from photoluminescence (PL) analysis. Furthermore, the purity and composition of the nanocomposites were confirmed via FTIR, EDX analysis and EDS mapping. Surface morphology of samples was tested by FESEM that were shown the dispersion of ZnO nanoparticles successfully. The optical absorptive and refractive nonlinearities parameters such as absorption coefficient and refractive index of the as-prepared sample were analysed using an open and closed aperture single beam Z-scan technique via Q-switched Nd-YAG pulse laser at 532nm. The nonlinear refractive index was in the order of 10-12 cm2/W with a negative sign whereas the nonlinear absorption coefficient was in the order of 10-7cm/W for samples prepared as foil. The real part, imaginary part and the absolute value of the third order nonlinear optical susceptibility χ (3) were calculated. The χ(3) was in the order of 10-6 esu for foil samples which indicated the effect of the percentage of ZnO nanoparticles and the types of polymer matrix on those parameters. The effect of adding CuO nanoparticles to nanocomposites as foil was observed, and enhanced their linear absorption and nonlinear optical properties. Consequently, a good optical limiting was obtained. In order to evaluate the suitability of the samples as optical limiting, the optical limiting threshold of the samples was measured by transmittance technique. The results showed that the prepared nanocomposites can be considered as an excellent candidate for optical limiting devices, which clearly affected by the concentration of ZnO nanoparticles and polymer matrix. Nanocomposites PMMA/ZnO/CuO and PS/ZnO/CuO showed the low optical limiting threshold, which were equal to 60Mw/cm2 and 50Mw/cm2 respectively.

v

ABSTRAK

Nanokomposit Polimer/ZnO telah menarik perhatian yang semakin tinggi terutamanya dalam sifat optik linear dan tak-linear kerana bahan ini berpotensi dijadikan peranti suis optik dan pengehad optik. Tesis ini melaporkan kajian mengenai zarah ZnO bersaiz nano yang dimasukkan dalam matrik polimer. Nanokomposit polymer/ZnO yang luwes (fleksibel) berbentuk keranjang telah disediakan melalui kaedah tuangan. Secara lazimnya, kepekatan nanozarah ZnO yang berbeza (0, 1, 3, 5, 10 dan 15 wt%) bertindak sebagai pengisi telah dimasukkan ke dalam matrik polimer yang berlainan, iaitu polimetil metakrilat (PMMA), polivinilidena fluorida (PVDF), polistiren (PS) dan polivinil alkohol (PVA) dengan kaedah campuran. Selain itu, pengubahsuaian sifat optik linear dan tak-linear dengan pernambahan nanozarah CuO (1wt%) terhadap sampel polimer/ZnO juga telah di kaji. Kemudian, nanokomposit PMMA/ZnO dan PVDF/ZnO telah disediakan dalam bentuk filem nipis pada permukaan kuarza dengan kaedah saduran berputar dan tuangan. Didapati, sampel yang disediakan telah menunjukkan spektrum penyerapan linear yang tinggi pada panjang gelombang 370-377 nm yang menerangkan keujudan nanozarah ZnO yang timbul dari transisi eksiton. Spektrum transmisi linear didapati berkadar songsang dengan kepekatan nanozarah ZnO dan sebaliknya untuk spektrum kepantulan. Di samping itu didapati juga, jurang jalur optik sampel menunjukkan anjakan merah dari analisis fotopendarcahaya (PL). Ketulenan dan komposisi nanokomposit telah disahkan melalui analisis FTIR dan EDX, serta pemetaan EDS. Mofologi permukaan sampel yang di uji melalui FESEM telah menunjukkan taburan nanozarah ZnO dengan baik. Parameter bagi penyerapan ketaklinearan optik seperti pekali penyerapan dan indeks biasan sampel telah dianalisis melalui teknik imbasan-Z, alur tunggal apertur buka, dan tertutup dengan menggunakan Laser denyutan Nd-YAG bersuis Q pada 532 nm. Indeks biasan taklinear adalah dalam tertib 10-12 cm2/W dengan tanda negatif, manakala pekali penyerapan tak-linear adalah dalam tertib 10-7 cm/W bagi sampel berbentuk keranjang. Bahagian nyata, bahagian khayalan serta nilai mutlak untuk kerentanan optik tak-linear tertib ketiga χ(3) telah dikomputasi. Ketiga-tiga parameter ini dipengaruh oleh peratusan nanozarah ZnO dan jenis matrik polimer yang digunakan. Nilai χ(3) adalah dalam tertib 10-6 esu bagi sampel bentuk keranjang. Penyerapan linear dan sifat optik tak-linear telah ditingkatkan dengan penambahan nanozarah CuO. Penghad ambang optik sampel telah diukur dengan teknik transmisi bagi menilai kesesuian sampel sebagai penghad optik. Hasil kajian telah menunjukkan sampel nanokomposit ini merupakan calon yang baik untuk peranti pengehad optik yang dipengaruhi oleh kepekatan nanozarah ZnO dan matrik polimer. Ambang pengehad optik yang rendah adalah daripada sampel PMMA/ZnO/CuO dan PS/ZnO/CuO yang masing-masing bernilai 60 Mw/cm2 dan 50 Mw/cm2.

vi

CONTENTS

Page DECLARATION

ii

ACKNOWLEDGEMENTS

iii

ABSTRACT

iv

ABSTRAK

v

CONTENTS

vi

LIST OF FIGURES

x

LIST OF TABLES

xv

LIST OF ABBREVIATION AND SYMBOLS

xvi

CHAPTER I

INTRODUCTION

1.1

Introduction

1

1.2

Zinc Oxide

2

1.3

Copper oxide

4

1.4

Polymer matrix

5

1.4.1 1.4.2 1.4.3 1.4.4

5 5 6 7

Poly (methyl methacrylate) (PMMA) Poly (vinylidene fluoride) (PVDF) Polyvinyl alcohol (PVA) Polystyrene (PS)

1.5

Nonlinear Optics

7

1.6

Optical limiting

8

1.7

Techniques of determining nonlinear optical properties

9

1.8

Problem statement

9

1.9

Importance of the work

10

1.10

Objective

11

1.11

Scope

12

1.12

Thesis organization

12

vii

CHAPTER II

LITERATURE REVIEW

2.1

Introduction

14

2.2

Brief history of the ZnO nanoparticles and its nanocomposites

14

2.3

Literature Review

14

2.4

ZnO nanoparticles as pure, colloid and capped

15

2.4.1 2.4.2

15 17

2.5

2.6

2.7

The Studies of Nonlinear Optical Properties The Studies of Linear Optical Properties

Doping of ZnO nanoparticles

20

2.5.1 2.5.2

21 23


Polymer-ZnO nanocomposites

26

2.6.1 2.6.2

26 29


Z-scan technique

31

2.7.1

31

Literatures in Relation to the Z-scan

CHAPTER III

METHODOLOGY

3.1

Introduction

39

3.2

Preparation method

41

3.2.1 3.2.2

Casting method Spin coating

41 42

3.3

Agglomeration and aggregation

42

3.4

Materials

43

3.4.1 3.4.2 3.4.3 3.4.4

43 44 44 44

3.5

Polymers Zinc oxide (ZnO) Copper oxide (CuO) Solvents

Preparation of polymer-ZnO nanocomposites

45

3.5.1 3.5.2 3.5.3 3.5.4

46 48 50 51

PMMA/ZnO nanocomposites PVDF/ZnO nanocomposites PS/ZnO nanocomposites PVA/ZnO nanocomposites

3.6

Preparation of polymer/ZnO/CuO nanocomposites

51

3.7

Characterization methods

53

3.7.1 3.7.2 3.7.3 3.7.4

53 53 53 53

Field Emission Scanning Electron microscope Energy dispersive X-ray (EDX) EDS mapping Photoluminescence (PL)

viii

3.7.5 3.7.6 3.7.7

Fourier Transfer Infrared (FTIR) Ultraviolet- visible Spectroscopy Z-scan technique

54 54 54

CHAPTER IV

RESULTS AND DISCUSSION

4.1

Introduction

56

4.2

FTIR Analysis

56

4.2.1 4.2.2 4.2.3 4.2.4

56 58 59 60

4.3

4.4

PMMA/ZnO PVDF/ZnO PVA/ZnO PS/ZnO

Field Emission Scanning Electron microscope

61

4.3.1 4.3.2 4.3.3 4.3.4 4.3.5

PMMA/ZnO nanocomposites PVDF/ZnO nanocomposites PVA/ZnO nanocomposites PS/ZnO nanocomposites Polymer/ZnO/CuO nanocomposites

61 62 64 64 65

Energy Dispersive X-ray(EDX) and EDS Mapping

67

4.4.1 4.4.2 4.4.3 4.4.4

PMMA/ZnO nanocomposites PVDF/ZnO nanocomposites PVA/ZnO nanocomposites PS/ZnO nanocomposites

67 67 69 70

4.4.5

Polymer/ZnO/CuO nanocomposites

71

4.5

Photoluminescence (PL)

73

4.6

Transmittance

74

4.6.1 4.6.2

74 77

4.7

4.8

4.9

4.10

Polymer/ZnO nanocomposites Polymer/ZnO /CuO nanocomposites

Absorption

78

4.7.1 4.7.2

78 81

Polymer/ZnO nanocomposites Polymer/ZnO/CuO nanocomposites

Reflectance

82

4.8.1 4.8.2

82 84


Linear absorption coefficient (α)

85

4.9.1 4.9.2

85 86


Energy gap (Eg)

88

4.10.1 Polymer/ZnO nanocomposites 4.10.2 Polymer/ZnO/CuO nanocomposites

88 92

ix

4.11

4.12

Extinction coefficient (K)

94


94 95

Refractive index (n)

96


96 99

4.13

Nonlinear Optical properties

100

4.14

Nonlinear refractive index (Closed aperture)

101


101 109

Nonlinear absorption coefficient (Open aperture)

111


111 119

4.16

Third-order nonlinear optical susceptibility (3)

121

4.17

Optical limiting

122

4.15

4.17.1 Optical limiter

123

4.17.2 Measurements of optical limiting threshold

123


124 127

CHAPTER V

CONCLUSIONS

5.1

Conclusions

129

5.2

Main Contribution

132

5.3

Recommendation for the Future Research

133

REFERENCES

134

APPENDIX A

List of Publications

153

B

Curriculum Vitae

156

x

LIST OF FIGURES

Figure No.

Page

1.1

An ideal optical limiter

8

3.1

Preparation of polymer/ZnO nanocomposites

40

3.2

Preparation of polymer/ZnO/CuO nanocomposites

40

3.3

Characterization of the as-prepared samples

41

3.4

Spin coating method

42

3.5

Preparation method of pure polymer and polymer-ZnO nanocomposites as foil

46

3.6

Preparation method of pure PMMA and PMMA-ZnO nanocomposites as thin film

48

3.7

Preparation method of pure PVDF and PVDF/ZnO nanocomposites as film on quartz

50

3.8

Preparation method of pure polymer and polymer/ZnO/CuO nanocomposites as foil

52

3.9

Z-scan Experiment Setup

55

4.1

FTIR of PMMA/ZnO nanocomposites

57

4.2

FTIR of PVDF/ZnO nanocomposite

58

4.3

FTIR of PVA/ZnO nanocomposites

59

4.4

FTIR of PS/ZnO nanocomposites

60

4.5

Top view FESEM images of (a) thin film based PMMA (b) foil like PMMA (c) thin film based PMMA/ZnO (d) foil like PMMA/ZnO and cross section view of (e) thin film based PMMA and (f) ) thin film based PMMA/ZnO

62

4.6

Top view FESEM images of (a) film based PVDF (b) foil like PVDF (c) film based PVDF/ZnO (d) foil like PVDF/ZnO and cross section view of (e) film based PVDF (f) film based PVDF/ZnO

63

4.7

Top view FESEM images magnification 10 k of (a) PVA (b) PVA/ZnO, and magnification 30 k of (c) PVA (d) PVA/ZnO

64

4.8

Top view FESEM images magnification 10 k of (a) PS (b) PS/ZnO, and magnification 30 k of (c) PS (d) S/ZnO

65

xi

4.9

Top view FESEM images of nanocomposites (a) and (b) PMMA/ZnO/CuO (c) and (d) PVDF/ZnO/CuO (e) and (f) PVA/ZnO/CuO (g) and(h) PS/ZnO/CuO

66

4.10

(a) EDX of thin film based PMMA; (b) EDX of foil like PMMA (c) EDS mapping of thin film based PMMA; (d) EDS mapping of foil like PMMA; (e) EDX of thin film based PMMA/ZnO; (f) EDX of foil like PMMA/ZnO, (g) EDS mapping of thin film based PMMA/ZnO; (h)EDS mapping of foil like PMMA/ZnO

68

4.11

(a) EDX of film based PVDF; (b) EDX foil like PVDF, (c)EDS mapping of film based PVDF, (d) EDS mapping of foil like PVDF;(e) EDX of film based PVDF/ZnO; (f) EDX of foil like PVDF/ZnO; (g)EDS mapping of film based PVDF/ZnO; (h) EDS mapping of foil like PVDF/ZnO

69

4.12

(a) EDX of PVA;(b)EDX of PVA/ZnO, (c) EDS mapping of PVA (d) EDS mapping of PVA/ZnO

4.13

(a)EDX of PS (b) EDX of PS/ZnO (c)EDS mapping of PS (d) EDS mapping of PS/ZnO

71

4.14

(a) EDX of PMMA/ZnO/CuO; (b) EDX of PVDF/ZnO/CuO (c) EDS mapping of PMMA/ZnO/CuO (d) EDS mapping of PVDF/ZnO/CuO; (e) EDX of PVA/ZnO/CuO; (f) EDX of PS/ZnO/CuO; (g) EDS mapping of PVA/ZnO/CuO (h)EDS mapping of PS/ZnO/CuO

72

4.15

PL of polymer/ZnO nanocomposites (a) PMMA/ZnO (b) PVDF/ZnO (c) PVA/ZnO (d) PS/ZnO

74

4.16

Transmittance spectra of nanocomposites (a) thin film based PMMA/ZnO (b) foil like PMMA/ZnO (c) film on quartz based PVDF/ZnO (d) foil like PVDF/ZnO (e) foil like PVA/ZnO (f) foil like PS/ZnO

77

4.17

Transmittance spectra of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

78

4.18

Absorption spectra of nanocomposites (a) thin film based PMMA/ZnO (b) foil like PMMA/ZnO (c) film on quartz based PVDF/ZnO (d) foil like PVDF/ZnO (e) foil like PVA/ZnO (f) foil like PS/ZnO

80

4.19

Absorption spectra of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

81

4.20

Reflectance spectra of nanocomposites (a) thin film PMMA/ZnO (b) foil like PMMA/ZnO (c) film on quartz based PVDF/ZnO (d) foil like PVDF/ZnO (e) foil like PVA/ZnO (f) foil like PS/ZnO

83

70

xii

4.21

Reflectance spectra of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

84

4.22

Linear absorption coefficient spectra of nanocomposites (a) thin film based PMMA/ZnO (b)foil like PMMA/ZnO (c) film on quartz based PVDF/ZnO (d)foil like PVDF/ZnO (e) foil like PVA/ZnO (f) foil like PS/ZnO

87

4.23

linear absorption coefficient spectra of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

88

4.24

Energy gap of nanocomposites (a)Thin film based PMMA/ZnO (b) Foil like PMMA/ZnO (c) Film on quartz based PVDF/ZnO (d) Foil like PVDF/ZnO (e) Foil like PVA/ZnO (f) Foil like PS/ZnO

91

4.25

Energy gap of polymer-ZnO nanocomposites versus ZnO nanoparticles %

92

4.26

Energy gap of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnOCuO (d) PS/ZnO

93

4.27

Extinction coefficient of nanocomposites (a) Thin film based PMMA/ZnO (b) Foil like PMMA/ZnO (c) Film on quartz based PVDF/ZnO (d) Foil like PVDF/ZnO (e) Foil like PVA/ZnO (f) Foil like PS/ZnO

95

4.28

Extinction coefficient of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

96

4.29

Refractive index of nanocomposites (a) thin film based quartz based PVDF/ZnO (d) foil like PVDF/ZnO (e) foil like PVA/ZnO (f)foil like PS/ZnO

98

4.30

Refractive index of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

99

4.31

The normalized transmittance as a function of sample position in the closed aperture Z-scan for nanocomposites thin film based PMMA/ZnO (a)ZnO(1 wt%) and (b) ZnO(15 wt%). Foil like PMMA/ZnO (c) ZnO(1 wt%)and (d) ZnO(15 wt%)

105

4.32

Nonlinear refractive index (n2) as a function of ZnO wt% for nanocomposites (a)Thin film based PMMA/ZnO ( b) Foil like PMMA/ZnO

106

4.33

The normalized transmittance as a function of sample position in the closed aperture Z-scan for nanocomposites Film on quartz based PVDF/ZnO (a)ZnO(1 wt%) and (b) ZnO(10 wt%). Foil like PVDF/ZnO (c) ZnO(1 wt%)and (d) ZnO(10 wt%)

106

xiii

4.34

Nonlinear refractive index (n2) as a function of ZnO wt% for nanocomposites (a)as film on quartz based PVDF/ZnO (b) Foil like PVDF/ZnO

107

4.35

The normalized transmittance as a function of sample position in the closed aperture Z-scan for foil like PVA/ZnO nanocomposites (a)ZnO(1 wt%) and (b) ZnO(15 wt%)

107

4.36

Nonlinear refractive index (n2) as a function of ZnO wt% for foil like PVA/ZnO nanocomposites

108

4.37

The normalized transmittance as a function of sample position in the closed aperture Z-scan for foil like PS/ZnO nanocomposites (a)ZnO(1 wt%) and (b) ZnO(15 wt%)

108

4.38

Nonlinear refractive index (n2) as a function of ZnO wt% for foil like PS/ZnO nanocomposites

109

4.39

The normalized transmittance as a function of sample position in the closed aperture Z-scan for nanocomposites foil like (a) PMMA/ZnO (b) PMMA/ZnO/CuO (c) PVDF/ZnO (d) PVDF/ZnO/CuO (e) PVA/ZnO (f) PVA/ZnO/CuO (g)PS/ZnO (h) PS/ZnO/CuO

110

4.40

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites (a) Thin film based PMMA/ZnO (b) Foil like PMMA/ZnO

113

4.41

Nonlinear absorption coefficient (β) as a function of ZnO wt% for nanocomposites (a) Thin film based PMMA/ZnO (b) Foil like PMMA/ZnO

113

4.42

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites as (a) Film on quartz based PVDF/ZnO (b) Foil like PVDF/ZnO

114

4.43

Nonlinear absorption coefficient (β) as a function of ZnO wt% for nanocomposites (a) Film on quartz based PVDF/ZnO (b) Foil like PVDF/ZnO

114

4.44

The normalized transmittance as a function of sample position in the open aperture Z-scan for foil like PVA/ZnO nanocomposites

115

4.45

Nonlinear absorption coefficient (β) as a function of ZnO wt% for foil like PVA/ZnO nanocomposites

115

4.46

The normalized transmittance as a function of sample position in the open aperture Z-scan for foil like PS/ZnO nanocomposites

116

4.47

Nonlinear absorption coefficient (β) as a function of ZnO wt% for foil like PS/ZnO nanocomposites

116

xiv

4.48

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites foil (a)PMMA/ZnO and PMMA/ZnO/CuO (b) PVDF/ZnO and PVDF/ZnO/CuO (c) PVA/ZnO and PVA/ZnO/CuO (d)PS/ZnO and PS/ZnO/CuO

119

4.49

Setup of transmittance technique

124

4.50

Optical limiting threshold of PMMA/ZnO nanocomposites

125

4.51

Optical limiting threshold of polymer/ZnO nanocomposites with different polymer matrix

126

4.52

Optical limiting threshold of nanocomposites (a) PMMA/ZnO and PMMA/ZnO/CuO (b) PVDF/ZnO and PVDF/ZnO/CuO (c) PVA/ZnO and PVA/ZnO/CuO (d) PS/ZnO and PS/ZnO/CuO

128

xv

LIST OF TABLES

Table No.

Page

2.1

Summary of the values of β, n2, χ(3) in literatures review

38

4.1

Energy gap of polymer/ZnO and polymer/ZnO/CuO

93

4.2

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PMMA/ZnO nanocomposites as thin film for different concentrations of ZnO nanoparticles

117

4.3

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PMMA/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

117

4.4

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PVDF/ZnO nanocomposites as film for different concentrations of ZnO nanoparticles

117

4.5

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PVDF/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

118

4.6

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PVA/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

118

4.7

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PS/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

118

4.8

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3) | for PMMA/ZnO (10 wt %) and PMMA/ZnO (10 wt %) /CuO (1 wt %) nanocomposites as foil

120

4.9

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PVDF/ZnO (8 wt %) and PVDF/ZnO (8 wt %) /CuO (1 wt %) nanocomposites as foil

120

4.10

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PVA/ZnO (10 wt %) and PVA/ZnO (10 wt %) /CuO (1 wt %) nanocomposites as foil

120

4.11

Values of Leff , β,Φ°,n2, Re χ(3) , Im χ(3) and |χ(3)| for PS/ZnO (10 wt %) and PS/ZnO (10 wt %) /CuO (1 wt %) nanocomposites as foil

121

xvi

LIST OF ABBRIVIATIONS AND SYMBOLS

µ

Density

C

Carbon

Cu

Copper

CuO

Copper Oxide

d

Thickness

DMF

Dimethylfomamide

EDX

Energy dispersive x-ray

Eg

Energy gap

F

Fluorine

FESEM

Field emission scanning electron microscope

FTIR

Fourier transfer infrared

h

Planck’s constant

Im χ(3)

Imaginary part of third order susceptibility

K

Extinction coefficient

Leff

Effective length

Mw

Molecular weight

n

Refractive index

n2

Nonlinear refractive index

O

Oxygen

ɸ◦

Nonlinear Phase shift

PL

Photoluminescence

PMMA

Poly (methyl methacrylate)

PS

Polystyrene

PVA

Polyvinyl alcohol

PVDF

Poly (vinylidene fluoride)

R

Reflectance

ra

Radius of aperture

Re χ(3)

Real part of third order susceptibility

S

Linear transmittance of aperture

Si

Silicon

xvii

T

Transmittance

Tp

Normalized peak transmittance

Tv

Normalized valley transmittance

UV

Ultraviolet

UV-Vis

Ultraviolet- visible spectroscopy

wa

The beam waist on the aperture

wo

The radius of the laser beam at the focus

wt%

Weight percentage

Zn

Zinc

ZnO

Zinc Oxide

Zo

Rayleigh length

α

Linear absorption coefficient

β

Nonlinear absorption coefficient

ν

Frequency

χ(3)

Third order susceptibility

𝜆

Wavelength

1

CHAPTER I

INTRODUCTION

1.1

INTRODUCTION

Recently, significant scientific and technological interest have focused on polymer inorganic nanocomposites. Those nanocomposites are a kind of composite materials comprising of relatively small inorganic nanoparticles content at nanometer-sized, which are

dispersed in a polymer matrix uniformly.The addition of inorganic

nanoparticles into a polymer matrix will change both properties of inorganic nanoparticles and polymer, hence it will enhanced and advanced new functions can be generated to the nanocomposite(Li et al. 2010).The combination of organic polymer and inorganic nanoparticles will offer materials with improved mechanical, electrical, optical, magnetic, thermal and many other specific properties. So their effect on composite properties is either enhanced or the same impact is achieved at lower concentrations of filler (Anžlovar et al. 2011a).By successfully joining materials of different characteristics to prepare nanocomposites a single material with novel properties can be obtained which can be used to provide high-performance novel materials and can applied in many scientific fields. However polymers have flexible lightweight and they are produced at a low cost, so they are widely used in the optoelectronics industry and they play important roles in various applications. The optical properties of nanocomposites have received much attention because they are different from individual polymers. In addition, they offer unique properties which greatly differ from that of conventional materials (Indolia and Gaur 2013) . The optical properties of nanocomposites are certainly influenced by many factors such as the size of filler, size distribution, degree of dispersion, and filler contents (Rajagopalan and Khanna 2013). The important factor that affects many optical

2

properties of nanocomposites is the size of the inorganic nanoparticles as nano-fillers (Li et al. 2010). According to the latest study, nanoparticles of size (50- 100 nm) lead to reduction of scattering light because they are significantly smaller than the wavelength of light. So nanocomposites containing nanoparticles act as optically homogeneous materials that modify optical properties, hence attract much interest. For example, the refractive index of polymers is usually 1.3-1.5,but when nanoparticles are added, the refractive index can be increased up to 2.5 (Chen et al. 2003).

1.2

ZINC OXIDE

Zinc oxide (ZnO) is one of the groups of II–VI semiconductors. It is the most attractive semiconductor due to the unique combination of electrical and optical properties, and important features like biocompatibility, long-term environmental stability, non-toxicity and low cost (Jeeju et al. 2012).

ZnO nanoparticles are considered nano-sized at dimensions between 1 and 100 nm. When it becomes nano-sized, the surface area to volume ratio (sa/vol) becomes very large. The sa/vol of most materials increases gradually as their particles become smaller, which then increase the adsorption of the surrounding atoms and changes their properties and behaviour. Materials that are reduced to nano-scale can suddenly show different properties and behaviour, compared to what they exhibit at macroscale, thus enables unique applications (Hristozov and Malsch 2009) .

The following should be noticed in order to determine the importance of ZnO nanoparticles: 

ZnO nanoparticles are produced with different particle sizes. They are nano powder, 100% crystalline, and non-porous. However, they incline to aggregate because of their high surface energy (Liu et al. 2006) .



Existing technologies are also being revolutionized by ZnO nanoparticles. The future where ZnO devices become part of our daily lives is already approaching (Irimpan 2008).

3



The European Commission has listed ZnO nanopowder as one of the major nanomaterials expected to be intensely commercialized in 2006-2014 (Mahmud 2008).



However, there is less understanding and knowledge about its nonlinear optical properties compared to its linear optical properties (Shim et al. 2010; Kumari et al. 2012).

ZnO, as a p-type semiconductor oxide, has a wide band gap (3.37 eV) which makes it an efficient UV absorber. The absorption edge of ZnO is about 368 nm. Due to its large exciton binding energy (60meV) which is much larger than the room temperature thermal energy (26meV), ZnO has become a promising material for optoelectronic devices of short wavelength, especially UV laser diode and lightemitting diodes.Thus, it refers stability(Hassan and Hashim 2013) of the electron-hole pairs as constant even at room temperature. Besides that, ZnO can be synthesized in a wide range of particle sizes and shapes at low cost preparation method( Anžlovar et al.2008 ; Seow et al. 2009 ; Saleem et al. 2012 ; No et al. 2013 ; Nagarani and Vasu 2013 ; Son et al. 2009).

ZnO has been added as fillers in many polymers, such as PMMA, PS, PVDF, PVA,PVC and PC (Indolia and Gaur 2013 ; Chen et al. 2003 ; Jeeju et al. 2012 ; Sangawar and Golchha 2013 ; El-kader et al. 2013 ; Abdul Nabi et al. 2014 ; Al Jaafari and Ayesh 2011).

Recently, many studies have described the preparation of nanocomposite that contains polymer and inorganic nanoparticles of ZnO(Chen et al. 2003 ; Sharma et al. 2011 ; Kulyk et al. 2011; Wong et al. 2012). Firstly, the nano- fillers of ZnO are synthesized or purchased (as ready nanoparticles), then it is added to the dissolved polymer directly or dispersed in the monomer followed by polymerization (Li et al. 2010).

The most important thing that should be noticed is when preparing nanocomposite that includes nanoparticles of ZnO, the poor bonding between the

4

polymer and nanoparticles may present defects and voids that have a harmful effect on the optical and mechanical properties.

The study of optical characteristics of ZnO nanoparticles and its nanocomposites, either linear optical like transmittance, absorbance, and reflectance, or nonlinear optical such as nonlinear refractive index and nonlinear absorption coefficient, should receive more interest to determine (Nagaraja et al. 2013) the suitable materials for nonlinear applications .

Nowadays: 

ZnO is used as a semiconductor material in many applications such as chemical and gas sensors

due to its unique electrical and optical properties

(Wasan et al.2012). 

ZnO is a promising semiconductor for optical devices due to its nonlinear optical properties and ZnO films can be applied in various areas. However, its properties are strongly dependent on the preparation method and doping species (Wang et al. 2003).

1.3

COPPER OXIDE (CuO)

Copper oxide (CuO) can be defined as a p-type semiconductor oxide with a narrow band gap of about 1.2-1.4 eV at room temperature. CuO has a monoclinic crystal structure. The copper atom is surrounded by four oxygen atoms in an approximately square planar design (Al-Gaashani 2012). It has been attracting attention due to its multipurpose applications in optoelectronic devices which are effective at ultraviolet and blue regions. Its excitation binding energy is very large (60 meV) at room temperature.It has two absorption peaks at 391nm and 485nm(Jundale et al. 2013). CuO nanoparticles showed different peak absorption at 370-390nm depending on the particle size; i.e., they exhibit blue shift at the absorption peaks with decreasing of nanoparticle size (Dagher et al. 2014).

5

The literature revealed that doping another nanoparticles material modified the ZnO nanoparticles (Li et al. 2010).

Ando et al, Chen et al, and Shahmiri et al. stated that CuO nanoparticles have been showing high optical nonlinearity, χ

(3)

whose value is in order of 10-8 (e s u)

(Ando et al. 1995; Chen et al. 2009; Shahmiri et al. 2013).

Chen et al,and Anand et al. mentioned that CuO nanoparticles film was found to be a good optical limiter under high repetition rate laser with ultrafast pulse duration (Chen et al. 2013; Anand et al. 2014).

1.4

POLYMER MATRIX

In general, the optical, mechanical, and electrical properties of polymeric matrices will be modified with the use of nanomaterial as filler (Jeeju 2012).The polymers which use as polymer matrix are:

1.4.1 Poly (methyl methacrylate) (PMMA)

PMMA has received unlimited attention and significant attributes due to its unique properties such as rigid, hard, high transparency in the visible region, low optical absorption, thermal capability, electrical performance, excellent

mechanical

properties, low cost, simple synthesis, and low refractive index. PMMA is preferred as matrix due to its good resistance to outdoor weather and hydrolysis.Since it is thermoplastic, it can be designed to any shape desired (Anžlovar et al. 2011a). Besides,it can be used in nonlinear optics as nanocomposites. However, PMMA has low nonlinear refractive index, n2, so it cannot be used alone as an NLO medium (Dorranian et al. 2012) .

1.4.2 Poly (vinylidene fluoride) (PVDF)

PVDF has been the centre of scientific interest. It is a semi crystalline polymer which has a special feature that is its uncommon polymorphism; and this distinguishes it

6

from other polymers. It has five crystalline phases named α, β, γ, δ and ε (Ahmed et al. 2013). During the crystallization, the α phase is formed, which represents the most stable polymorph of PVDF (Yousefi 2011b) and it can be obtained easily from the melt. Whereas,the β phase can be produced by mechanical deformation and it receives the greatest attention technologically due to its greater piezoelectric properties(Ahmed et al. 2013). During the preparation of a PVDF sample, usually more than one phase are produced depending on the different parameters. These parameters such as temperature, type of solvent, ambient conditions, solution concentration and crystallization rate; depend on the rate of solvent evaporation. Although fine control is still lacking, researchers still focus on evaporation rate. The formation of β phase occurs with low solvent evaporation rates, while high evaporation rates mostly produce α phase (Chinaglia et al. 2010).In recent years, many studies have focused on crystalline structure and crystalline phases that are produced during the preparation of PVDF films. Most of the studies focused on the formation of β phase (Ahmed et al. 2013)and sometimes γ phase because they are very important to be used as piezoelectric material due to its high piezoelectric activity. In addition, they usually use DMF, HMPA, DMSO, and acetone and mixed DMF - acetone as the solvent. Despite few studies have been looking at the optical properties of PVDF, there is a clear lack of details of those studies. Besides the study of nonlinear optical properties of pure PVDF and its nanocomposites is done for the first time in this work.

1.4.3 Polyvinyl Alcohol (PVA)

PVA has attractive properties such as nontoxic, water soluble polymer, strong film forming ability, good charge storage capacity, high dielectric strength, and its optical and electrical properties depend on the types of doping filler. In addition, it could function with high performance as a polymer matrix or a template that contains nanostructured materials. Also, PVA has a carbon chain backbone with hydroxyl groups, which can act as the source of hydrogen bonding to enhance the formation of polymer complex (El.Sayed et al. 2014;Hamdalla et al. 2015).

7

1.4.4 Polystyrene (PS)

PS is an amorphous, optically clear thermoplastic material, flexible as thin films, and it can be naturally transparent but it can be coloured with colorants. Structurally, it has long hydrocarbon chain with a phenyl group attached to each carbon atom.Polystyrene is used as a host matrix due to its ideal properties for investigating the optical properties of its nanocomposite (Jeeju 2012).

1.5

NONLINEAR OPTICS

It is the phenomena that occur as a consequence of the modification of optical properties of a material system in the presence of intense light (Sutherland 2003; Boyed 2007).

Through the incidence of a high intensity radiation on the material, depending on its absorptive property, the transmittance through materials may increase or decrease. Some of the materials show saturable absorption phenomena when the transmittance increases with an increase in incident intensity, but other materials which exhibit a decrease in transmittance with an increase in incident intensity are indicate as reverse saturable absorption (Haripadmam et al. 2012). That materials have optical limiting properties.

During the past few years,a study on the third order nonlinear susceptibility of ZnO nanoparticles showed it was approximately 500 times greater than that of ZnO in bulk. So, ZnO nanoparticles and its nanocomposites received more attention (Haripadmam et al. 2012).

During recent years, nonlinear optical properties have been getting a great interest by both experimentalists and theoreticians due to the wide range of applications in computing and optical signal processing. The third-order optical nonlinearities and optical limiting materials have been investigated (Dorranian et al. 2012) extensively for their applications in high-speed optical switches, optical phase conjugation and optical limiting devices.

8

1.6

OPTICAL LIMITING

Optical limiting is a nonlinear optical process where the transmittance of a material decreases as the incident light intensity increase. Optical limiter is one of the most important types of devices used to control the amplitude of high light intensity.

Figure 1.1

An ideal optical limiter

In the visible spectral region, the maximum exposure of laser light that is allowed for human eyes is around 2.5 mW/cm2 (Mathews et al. 2007).

For an ideal optical limiting material, Figure 1.1, high transmission is observed at normal light, whereas low transmission is detected at intense light. So, it is used as a protection for human eyes and optical detectors. The self-defocusing in conjunction with the nonlinear absorption process in semiconductors will enhance the optical limiting performance (Aleali and Mansour 2010).The satisfaction of strong nonlinear refraction enables the ability of optical switching, while the presence of strong nonlinear absorption produces good optical limiting (Kumar et al. 2008).

9

1.7

TECHNIQUES PROPERTIES

OF

DETERMINING

NONLINEAR

OPTICAL

There are different measurement techniques (Irimpan 2008) to determine nonlinear optical properties characterization of materials such as: degenerate four waves mixing, three waves mixing, optical Kerr effect, ellipse rotation, interferometric methods, beam self-bending, third harmonic generation, two photon fluorescence,photo thermal, photo acoustic techniques, beam distortion measurements and Z-scan technique.

Originally(Gomes et al. 2007; Liu and Tomita 2012), a single beam Z-scan technique is considered as the best technique to characterize the nonlinear optical properties of numerous nonlinear optical materials.

1.8

PROBLEM STATEMENT

ZnO has attractive properties, and has been cited as the material of 21st century by Materials Today Journal (Wang 2004). However, there are less information and knowledge about the nonlinear optical properties of ZnO and its nanocomposites compared to its linear optical properties and other properties (Shim et al. 2010; Kumari et al. 2012).In the present work, nanocomposites of polymer-ZnO as foils have been successfully prepared by using the casting method. Technological and wellknown scientists have been focusing on the study of polymer-ZnO nanocomposite prepared as a thin film deposition on the substrate.

The study of nonlinear optical properties should be focused due to the following reasons: 

For many optoelectronic applications such as optical limiting and switching, the knowledge of nonlinear optical properties of materials is necessary.



For nonlinear optical regime, there are a number of interesting phenomena such as second harmonic generation, third harmonic generation,and optical limiting.These properties are absent in the linear regime (Shim et al. 2010; Krishnan 2005).

10



On the other hand, despite the pioneer experiments done, it was only since the 1990’s that the study of nonlinear effects in nanostructured materials became a very active research domain.



In spite of the many studies that had been conducted, there are a need to enhance the information of all photonics technologies, and contribute to the knowledge pertaining to the suitability of using nonlinear optical materials as optical limiting, especially the common usage of optical detectors, and sensors for different scientific purposes (Jeeju et al. 2012). The optical limiting devices are needed to protect the photosensitive components from high intensity laser radiation.

Our work is considered as a contribution to fill the lack of nonlinear optical studies on polymer-ZnO nanocomposites. The studies on optical nonlinearity are important for the development of new materials for the applications in ultrafast optical devices. To the best of our knowledge, no report has been published describing the preparation of polymer/ZnO or polymer/ZnO/CuO nanocomposites as foils for that purpose. 1.9

IMPORTANCE OF THE WORK

The importance of studying linear and nonlinear optical properties is summarized as follows: 

The development of a new concept of the possibility of preparing a new nanocomposite with doping nanoparticles of materials.



Identify the Z- scan technique and how it is used to study the nonlinear optical properties of materials.



Opportunity to determine and choose the nanoparticles of materials and nanocomposites in which the linear and nonlinear optical properties are known so it can be used in optoelectronics applications.

11

1.10

OBJECTIVES

The main objective of this study is to evaluate and enhance the nonlinear optical properties of polymer-ZnO nanocomposites. In order to achieve the main objective, several sub-objectives need to be accomplished:

1.

To prepare polymer-ZnO nanocomposites

A.

As foils by using the casting method; ZnO nanoparticles are used as fillers and four different polymers are used as polymer matrix:

B.

i.

PMMA/ZnO nanocomposites.

ii.

PVDF/ZnO nanocomposites.

iii.

PVA/ZnO nanocomposites.

iv.

PS/ZnO nanocomposites.

As film on quartz, two polymers are chosen to compare the effect of preparation method:

2.

i.

PMMA/ZnO nanocomposites prepared by applying spin coating.

ii.

PVDF/ZnO nanocomposites prepared by applying casting method.

To enhance the linear and nonlinear optical properties of nanocomposites, CuO nanoparticles was added to the nanocomposites that have been prepared as foils with 10% ZnO nanoparticles.

3.

To study the linear optical properties of nanocomposites and describe their optical

4.

characteristics.

To evaluate the nonlinear optical properties; nonlinear absorption coefficient and nonlinear refractive index by using Z-scan technique (single beam method).Then, they are used to calculate the third order nonlinearity and the transmittance technique is used to explore the optical limiting property.

12

1.11

SCOPE

This research aims to prepare nanocomposites as foils which contain one of the individual polymers (PMMA, PVDF, PS and PVA) as the polymer matrix and ZnO nanoparticles as its fillers.To study the effect of preparation method, PMMA/ZnO and PVDF/ZnO nanocomposites are prepared by using other methods.The spin coating method is used to prepare thin film on quartz of PMMA/ZnO nanocomposites. Then, casting method is used to prepare the film on quartz of PVDF/ZnO nanocomposites.

Besides ZnO nanoparticles, CuO nanoparticles are also added to each nanocomposite as foils with particular percentage of ZnO nanoparticles to prepare polymer/ZnO/CuO nanocomposites as foils.

The UV-Visible(UV-Vis) spectroscopy is used to show the transmittance, absorbance, and reflectance spectra.The energy gap (Eg), linear absorption coefficient (α),extinction coefficient (K),and refractive index (n) are calculated.The FTIR of the samples is then analyzed.FESEM images of the samples and EDX and EDS mapping are checked. The photoluminescence (PL) spectra are recorded.

By using the Z-scan technique (single beam method), the nonlinear absorption coefficient and nonlinear refractive index are evaluated, and then they are used to calculate the third order nonlinearity of the samples. Then, by using transmittance technique, the optical limiting property of the samples is explored.

1.12

THESIS ORGANIZATION

This thesis consists of five chapters. Chapter I gives a brief introduction of ZnO nanoparticles and its nanocomposites, CuO nanoparticles, nonlinear optics, and optical limiting. The problem statement identifies the research importance and, the direction of the research, then the objectives and its scope are elaborated.

13

Chapter II is a literature review of the subject of interest. Chapter III describes the methodology.Chapter IV shows the collection data and discusses the measurement results. Then describes the application of the obtained results to get a good optical limiter.

Chapter V concludes the thesis, which reviews the major contributions and limitation of the linear and nonlinear optical properties of the nanocomposites as well as putting forward suggestions for future work.

14

CHAPTER II

LITERATURE REVIEW

2.1

INTRODUCTION

The background history of the study is presented.It concludes the linear and nonlinear optical properties of ZnO nanoparticles; firstly, as pure, colloid, and capped; secondly, the ZnO nanoparticles are doped with another nanoparticles material; and thirdly, polymer–ZnO nanocomposite.Besides, the history of discovery and development of Zscan technique is shown.

2.2

BRIEF HISTORY OF ZnO NANOPARTICLES AND ITS NANOCOMPOSITES

For more than 75 years, ZnO has been considered a familiar field in scientific study. It has been highlighted as the subject of thousands of studies. The first study of ZnO was in 1935 (Bunn 1935). The literature reveals that few studies have been conducted on nonlinear optical properties of ZnO nanoparticles and its nanocomposites and its comparison to other studies of its linear optical properties.The literature showed that it has been known as a candidate for optoelectronic devices.

2.3

LITERATURE REVIEW

The literature review represents two parts: part one represents the ZnO nanoparticles and its nanocomposites which are divided into three sections and every section is divided into two subsections which are nonlinear optical properties and linear optical properties.First section is ZnO nanoparticles as pure, colloid and capped. Secondly, doped ZnO nanoparticles.lastly, polymer-ZnO nanocomposites. Part two covers the

15

research works related to the discovery and development of the Z-scan technique to study the nonlinear optical properties.

2.4

ZnO NANOPARTICLES AS PURE, COLLOID AND CAPPED

Normally, ZnO has different favourable properties such as wide band gap, high luminescence room temperature, good transparency, high electron mobility, and ultra violet absorbance.Nevertheless, ZnO nanoparticles have been studied for the advantages connected with quantum

confinement including increased absorption

efficiency, reduced dark noise, potential for large area, and lower cost devices. Also, they have attracted a large research funding due to their high multipurpose and promising applications in ultraviolet applications including catalysis, sunscreen, paint, and polymer nanocomposite.

In addition, they are applied in optoelectronics such as light emitting diodes, field effect transistors, field emitters, solar cells, toners and sensors. Moreover, they are also used in biotechnology fields such as antibacterial and biosensors. In addition to the above mentioned applications, ZnO can also be used in varistor, ferrite, cement, ceramic flux, pigment, animal food, pollutant filter, dental filling, hydrogen fuel and nano-textile.

2.4.1

The Studies of Nonlinear Optical Properties

Rong-yao et al.(1998) have added ZnO nanoparticles into DBS- xylene.The nonlinear optical properties of the samples were studied using the Z- scan technique.The results showed that the samples revealed fast and large optical nonlinear response below the energy gap; hence they may have the potential to be applied as an optical switch.

ZnO nanoparticles capped with polyvinylpyrrolidone (PVP)have been synthesized in a previous study(Guo et al. 2000).The capped ZnO nanoparticles did not show any observable changes after more than half a year,a contrast to non-capped ZnO nanoparticles. The optical properties were studied, but the degenerate four-wavemixing (DFWM) technique was used to measure χ (3).

16

The effects of particle shape and host on optical nonlinearities of ZnO nanorods and ZnO nanoparticles dispersed in water and ethanol were studied by (Chang and Song 2007). The Z- scan technique with a laser of wavelength 532nm, pulse duration of 8 ns, and pulse energy about 100μJ was used. The samples were placed in a cell with a thickness of 2mm. The third-order optical nonlinearities demonstrated that the ZnO nanorods were bigger than that of ZnO nanoparticles. In addition, water suspension for ZnO nanorods was bigger than that of ethanol suspension.

In a related study, the comparison of the third order nonlinear optical properties of ZnO nanorods and ZnO nanoparticles, which were dispersed in water , ethanol and N,N- dimethyl formaide forming suspensions were done (Chang et al. 2008).The measurements of the third order nonlinear optical properties were completed by using the Z-scan technique.The results showed that the third order of ZnO nanoparticles was smaller than that of ZnO nanorods. They concluded that the shape of nanomaterials has a strong effect on the third nonlinear optical performance.

Ara et al. (2008) prepared ZnO nanoparticles colloids and the Optical properties of the samples were studied.The peak absorption of the sample was 373.5 nm, while the band gap was about 3.31 eV. Also,the Z- scan technique with continuous wave He - Ne laser (wavelength of 632.8 nm and power of 50mW) was used to measure the nonlinear refractive index and nonlinear absorption coefficient.The results indicated the presence of self - defocusing and two photon absorption.

ZnO as a colloidal solution has been synthesized in a previous study(Sreeja et al.2008). A 1 mm cuvette was used to study the nonlinear optical properties. In addition, other samples were prepared by dispersing ZnO nanoparticles in poly vinyl alcohol (PVA) and emptied on a glass substrate. The nonlinear optical properties of the samples were studied by using the Z- scan technique (laser of wavelength 532nm and ns pulses). A High value of two photon absorption coefficient was observed in which it resulted in ZnO as an ideal optical limiter in novel applications.

17

The three-photon absorption was observed (Suhail et al. 2011), when the nonlinear absorption coefficient for ZnO nanostructure thin film was measured using a fully computerized Z-scan system (laser of wavelength 800 nm and pulse duration of 60 fs ).The value of the nonlinear absorption coefficient was found to be ten times of magnitudes higher than the bulk value. Moreover the optical properties were investigated for a sample with a thickness about 900nm which was prepared by applying chemical spray pyrolysis technique.

Raied et al. (2012) prepared ZnO nanostructure film with a thickness around 2 μm by applying the chemical spray pyrolysis technique.The optical properties of the sample such as absorption, reflection, transmission, Raman and PL spectra were studied.The three photon absorption coefficient was measured by using the same Zscan technique which was used in the previous research. The value was approximately five times higher than the bulk value.

The effect of surface roughness on nonlinear optical properties of annealed ZnO thin films has been studied by (Kumari et al. 2012). They found that the nonlinear optical properties were dependent on surface roughness

Nagaraja et al. (2013) prepared ZnO thin films on a quartz substrate.The effect of annealing on structural and third order nonlinearity was then evaluated. The samples showed optical limiting property.

2.4.2

The Studies of Linear Optical Properties

Different sizes of ZnO nanoparticles were prepared by (Zhang et al. 2004) using the sol-gel and precipitation method. The characteristics of the samples were studied.

Meanwhile, Damonte et al. (2004) produced ZnO nanoparticles by using mechanical milling. The structural and optical characterizations were studied using xray diffraction spectra, scanning electron microscopy, positron annihilation spectroscopy, and PL spectroscopy.

18

ZnO nanoparticles were obtained by using wet chemistry method in a previous study (Lojkowski et al. 2005).Then, their structural and optical properties were evaluated.

The synthesis of various sizes (nano and micro) of ZnO has been done using different techniques by (Gupta et al. 2006).Moreover,the optical properties of the samples were investigated, thus the micro and nano crystals were explored. They can be applied in bio-sensors and also can be an active medium for laser oscillations.

ZnO nanoparticles were prepared using a simple polyol method by (Lee et al. 2008). The shape and size of the samples were controlled.The amount of water and the process of its addition played a significant role in the formation of the properties of the synthesized particles.

Soosen et al. (2009) have synthesized ZnO nanoparticles through chemical method. Polyvinyl-pyrrolidone (PVP) was used as the capping agent. Moreover, by using TEM, XRD and UV-Vis spectroscopy, the optical properties and structure of the samples were studied.

Ooi et al. (2010) prepared ZnO nanoparticles thin film and then the structural and optical properties of the samples were studied.The strong single emission peak was shown at a wavelength of 370nm in the UV region.

The optical properties of ZnO nanoparticles capped with polymers have been investigated by Tachikawa et al. (2011). Polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) were used as the capping agents. The sol- gel method was used to synthesize the ZnO nanoparticles. Fluorescence and absorption spectra were measured. The optical properties drastically changed when the timing of the addition of the polymer to the ZnO nanoparticle solution was varied.

Qin et al. (2011) prepared ZnO nanoparticles coated with polyvinylalcohol (PVA).The layer of the PVA was used to passivate the ZnO nanoparticles. During the study of the optical characteristics of the samples, the enhancement of UV emission

19

and an increase in photocurrent sensitivity were observed in the PVA coated ZnO nanoparticles. As a conclusion, the samples could be applied to prepare low cost, sensitive, and visible blind UV photo detectors.

Ghaffarian et al. (2011) synthesized ZnO nanoparticles by using spray pyrolysis method.The structure and morphology of the samples were studied using XRay diffraction and transmission electron microscopy. Correspondingly, ZnO nanoparticles were prepared by (Askarinejad et al. 2011) using a novel sonochemical route and then the optical properties of the samples were investigated.

Conde et al. (2011) prepared ZnO nanoparticles by using the chemical route.Their optical properties were investigated and the results showed a good absorption at a wavelength of 380 nm in the UV region.

Sivakumar et al. 2012) prepared ZnO nanoparticles thin films by using the sol gel dip coating method with various pH values. Optical characteristics were evaluated which showed the band gap is dependent on PH values.

In a related study, ZnO nanoparticles in the presence of folic acid were synthesized by using the sol gel technique(Dutta and Ganguly 2012).The measurements of the X-ray diffraction and UV-Vis Spectroscopy were conducted. In addition, the optical properties of the samples were studied using transmission electron microscopic, Fourier transmission infrared, and room temperature photoluminescence.

Chang and Chen 2012) fabricated ZnO nanoparticles mixed with poly acrylic acid, polyvinyl alcohol and octanol that formed a thin film on Al2O3 substrate to be used as an ethanol gas sensor.

Thin film ZnO nanoparticles with various thicknesses of layer were deposited using sol-gel spin coating method (Shariffudin et al. 2012). Structural, electrical and optical properties of the samples were investigated. The optical band gap increased as

20

the thickness of the thin film increased. Also, the optical transmittance spectra of all samples were more than 88% in the visible and NIR regions.

Through the novel chemical route, ZnO nanoparticles powder were synthesized (Khan et al. 2012) .The optical properties of the powder samples were characterized and studied by using X-ray diffraction(XRD),scanning electron microscopy (SEM),UV-Vis spectroscopy and PL spectroscopy. A blue shift in the absorption spectrum and PL spectroscopy was observed.

Aznan and Johan (2012) produced ZnO nanoparticles by mechanical milling.The XRD, UV-Vis and PL were studied. The results showed that the increase in absorbance depends on the milling time.

Hassan and Hashim (2013) prepared ZnO nanoparticles thin films by the oxidation of high purity zinc powder.The morphology and structure of the samples was characterized, thereby the photoluminescence (PL) spectra of the samples were measured.

In previous study, ZnO nanoparticles were prepared with PVA as the chelating agent (Mallika et al. 2015). The UV-Vis spectroscopy, PL, X-ray, FTIR and SEM were used to study the annealing effect on the structural and optical properties of the samples. The absorption spectra showed the absorption was red-shifted, while the calculation of energy band gap showed a blue-shifted as the annealing temperatures increased.

2.5

DOPING OF ZnO NANOPARTICLES

ZnO is known as a highly efficient UV light emitter, and a candidate for optoelectronic devices. To extend the application of optoelectronic devices in the future, the following features should be fulfilled: nanomaterials, high performance, small sizes, high speed of operation, and doped with nanoparticles of other materials. Recently, the doping of ZnO with nanoparticles of other materials has attracted the attention of many researchers. It has been observed that by controlling the dopants and

21

its concentrations, the physical properties of ZnO nanoparticles such as optical, electrical, and magnetic can be modified and enhanced. That modification and enhancement were obtained due to the change in the electronic structure and the band gap. In this field, the studies are very important to create applications that can be applied in optical devices.

2.5.1


The nonlinear absorption of Cu doped with ZnO has been studied by Ryasnyansky et al. (2005). The values of nonlinear absorption coefficient were measured using the Zscan technique with a laser of wavelength 532 nm with nanosecond and picosecond pulses.

In another study, Ryasnyanskiy et al. (2007) prepared Au doped with ZnO as thin film. The Z-scan technique at laser of wavelength 532 nm and pulse duration of 7 ns was used to investigate the nonlinear refraction coefficient and the real part of the third-order nonlinear susceptibility of the samples.

ZnO thin films on glass have been synthesized by Ozga et al. (2008) using chemical vapour deposition. The Au nanoparticles were attached to the surfaces of ZnO by applying a seed-mediated growth technique. Nd: YAG laser (1.06 µm) with a pulse duration of 30 ps was used as a source in the Z-scan technique which was then utilized to study the SHG of the samples and nonlinear optical absorption that showed two-photon absorption.

By using nanosphere lithography, the Au/ZnO nanoparticle have fabricated arrays on fused quartz substrate (Ning et al. 2008).The absorption peak was observed at 570 nm . The Z-scan method with a laser of wavelength 532 nm and pulse duration of 10 ns was used to measure the third-order nonlinear optical susceptibility (real and imaginary parts). As a result, the Au/ZnO nanoparticles arrays have a great potential application in the future of optical devices.

22

Al with concentrations of 0, 3, 5 and 7 % were doped with ZnO (Bahedi et al. 2009) . Spray pyrolysis method was used to deposit the thin films on glass substrate. The third order nonlinear optical susceptibility was studied and it increased with the incorporation of aluminium.

Karthikeyan et al. (2009 a) prepared cobalt doped with ZnO nanoparticles by using simple wet chemical method.The linear optical properties were studied such as absorption,PL , morphology, and local structure of the particles. The open aperture Zscan technique (laser of wavelength 532 nm and pulse duration of 5 ns) was used to study the nonlinear optical properties. The optical limiting property was enhanced by the high concentration of cobalt due to the three - photon absorption.

The preparation of Bi- doped with ZnO nanoparticles was done by Karthikeyan et al. (2009 b) using wet chemical method at room temperature.The absorption, size and shape of the particles, and PL of the samples were studied.The nonlinear optical properties were evaluated using the open aperture Z-scan with a laser of wavelength 532nm and pulse duration 5ns. The results showed that the nonlinear absorption occurred due to the three-photon absorption process.

The optical absorption and PL of Na doped with ZnO have been studied by (Karthikeyan et al. 2009 c). The nonlinear optical properties of the samples were studied using the Z-scan with pulse laser of wavelength 532 nm and pulse duration 5 ns. The results showed the ability of the samples to be used as an optical limiter.

Irimpan et al. (2010) prepared ZnO-Ag, ZnO-Cu, ZnO-CdS, ZnO-TiO2, ZnOSiO2 and ZnO-TiO2-SiO2 nanocomposites by using colloidal chemical synthesis.A cuvette (thickness of 1 mm) contained collodial ZnO nanocomposites was used as the sample. Optical absorption measurements of the samples were determined.The results showed a very strong UV emissions for ZnO-Ag, ZnO-Cu and ZnO-SiO2 nanocomposites, while visible emission for ZnO-Cu nanocomposite and its magnitude was more than ten times stronger than that of pure Cu. In addition, the emission peaks of ZnO-CdS and ZnO-TiO2 samples changed based on the changes in the band gap. Nonlinear optical absorption was studied using the Z- scan technique with a laser of

23

wavelength 532nm and pulse duration of 7ns.Two photon absorption was observed. As a result these samples are available to be used as optical limiters.

Torres-Torres et al. (2012) prepared Ag doped with ZnO thin films by applying ultrasonic spray pyrolysis process.The magnitude of the third order nonlinearities was evaluated.

In another study, Tamgadge et al. (2014) prepared thin films of PVA/ZnO nanocomposites and PVA/ZnO doped with strontium.The linear optical properties were studied using UV–Vis spectroscopy and FTIR. The nonlinear absorption coefficient and nonlinear refractive index of the samples were evaluated by using the Z-scan technique.

2.5.2


ZnO-MgF2 thin films were prepared by Fujihara et al.(2001) using the sol-gel method. The concentration of ZnO used was varied (30, 40 and 50 mol. %). The results showed the transmittance of the films that were heated at 400 ОC was lower than the transmittance of the films that were heated at 300 ОC. A green PL emission from the films was observed at room temperature which depended on heat-treatment and time.

Aluminum (of verified concentrations) doped with ZnO nanoparticles were synthesized by Suliman and Tang (2007).The absorption and transmittance of the ZnO were studied.The samples showed high absorption in the visible region.

Mg and In doped with ZnO nanoparticles were prepared by Li et al. (2009) using flame spray synthesis method.The structure of the samples was studied and the results showed that the band gap of the Mg doping increased, while that of In doping decreased. Furthermore, by using PL spectra, the strong UV emission and weak visible emission were explored.

ZnO thin films covered by TiO2 nanoparticles were synthesized by applying electron beam evaporation (Xu et al. 2009).The high increment of UV emission in the

24

ZnO thin films covered by TiO2 nanoparticles was investigated but the green emission was suppressed.

ZnO thin films contained different concentrations of Ag-nanoparticles were prepared using sol-gel method (Hong et al. 2009).The optical and electrical properties of the samples were determined. Also, the incorporation of Ag nanoparticles into Al doped with ZnO thin films was examined.

Different concentrations of vanadium doped with ZnO nanoparticles were synthesized by (Tahir et al. 2009).The diffuse reflectance spectroscopy of the samples was evaluated.A linear increase in the band gap energy along with the increase of Vdoping was observed.

A thick film of Sb-doped ZnO nanoparticles was prepared using sol-gel technique by Kashyout et al. (2010). A decrease in the transmittance values and an increase in the quantity of Sb were observed. The shift in the transmittance edge to additional UV values was also observed. Moreover, the value of the band gap increased as the Sb doped increased.

Undoped copper and copper doped with ZnO nanoparticles were synthesized by Chauhan et al. (2010 a).The value of the band gap of the prepared samples was 3.15 for undoped copper,but its value decreased to 2.92 for doped copper.The measurement of absorption showed a red shift in the absorption band edge of the copper doping samples. By using same method, undoped silver and silver doped with ZnO nanoparticles were synthesized using co-precipitation method (Chauhan et al. 2010 b).The optical properties of the samples were investigated. The absorption of Ag doping samples specified a red shift in the absorption band edge.

Verified concentrations of calcium doped with ZnO thin films were deposited on glass substrates using RF magnetron sputtering (Ayadi et al. 2010).The results of optical properties showed high transparency in the visible and near IR spectra.

25

Sridevi and Rajendran (2010) synthesized lanthanum doped with ZnO nanoparticles by using hydrothermal method.The La doped with ZnO samples indicated higher luminescence activity compared to the sample of pure ZnO in which it increased as the La concentration of the samples increased.The absorption spectrum edge of the La doped ZnO clearly showed a blue shift.

Al doped ZnO nanoparticles were prepared by Tarasov and Raccurt (2011) using wet chemical deposition method.The optical properties showed 90% transparency in the visible spectrum.

Different percentages of Mn doped ZnO were synthesized by applying precipitation method (Abdollahi et al. 2011).Characterization was carried out and photodegradation under visible- light irradiation was studied. Besides, the band gap was also measured. Mg (with different concentrations) doped with ZnO nanoparticles were synthesized by Nursyahadah et al. (2011) by using mechanochemical processing.The UV-Vis spectrometer was used to investigate the optical properties of the samples.The results showed that the value of the band gap of the samples increased as the Mg doping increased.

Ag doped ZnO nanoparticles were prepared by Van Nghia et al.(2012).The absorption peak was observed at 460 nm in the visible region.Therefore,the modification decreased the band gap energy of ZnO.

By utilizing the simple sol-gel technique Pandiyarajan and Karthikeyan (2012) synthesised Cr doped ZnO nanoparticles.The structural and optical properties of the samples were studied. The absorption spectral indicated a blue shift due to the Cr doping.

Kant and Kumar (2012) prepared Ni doped ZnO using the sol -gel method.The structural and optical properties of the samples were studied.The UV - Vis spectra presented a deviation in the absorption at the blue region as the concentration of Ni increased.

26

Zn particles capped with ZnO nanoparticles were prepared by Morozov et al. (2015). The structural and optical properties of the samples were studied using TEM, X-ray diffraction,

UV-Vis

spectroscopy,

FT-IR

spectroscopy,

and

Raman

spectroscopy.

Undoped ZnO nanoparticles and doped with Al were synthesized by Vadivel et al. 2015). XRD, FTIR, SEM, UV-Vis spectroscopy, and PL were used to study the structural and optical properties of the samples.The doping of Al shifted the edge of absorption band slightly to a higher wavelength region. The value of the band gap and the PL spectra revealed a decrement after doping.

2.6

POLYMER-ZnO NANOCOMPOSITES

In the last two decades, technological and significant scientific studies have been focusing on polymer/inorganic nanocomposites,which are used to provide highperformance novel materials that can be applied in many industrial fields.However, polymers have lightweight flexibility and they are produced at low cost. Hence, they are widely used in the optoelectronics industry and play important roles in various applications.

Recently, polymer-ZnO nanocomposites have been greatly studied to enhance the performance of optical devices. Several studies were conducted about preparing a composite of polymer with zinc oxide nanoparticles.Most of the studies concentrated on specific characteristics, such as surface morphology, structural, dielectric, magnetic, electrical and optical properties. However, only a few of them had studied the nonlinear optical properties.

2.6.1


In a study by Kulyk et al. (2009) ZnO /polymethylmethacrylate (PMMA) nanocomposite films were prepared using spin coating method.The second and third harmonic generation (SHG and THG) of the ZnO /PMMA nanocomposites as thins film with different concentrations of ZnO nanoparticles were studied using the Z- scan

27

technique at wavelength 1.064 μm. In addition, the second and third order nonlinear susceptibilities of nanocomposites were estimated.

Sreeja et al.(2010) prepared ZnO/PMMA composites using different particle sizes of ZnO. Thin films with a thickness of 1 μm were produced on a glass plate.The optical absorptive nonlinearity of the ZnO/PMMA nanocomposites was analyzed using the open aperture Z-scan technique. Optical limiting type nonlinearity was observed due to the two photon absorption in ZnO. The efficiency of limiting was found to increase as the band gap decrease. Hence the samples can be best used for optical limiting applications. The samples showed a negative value for nonlinear refractive index (n2); and the magnitude of n2 increased as the band gap decreased.The stability and mechanical properties of the nanoparticles embedded in the PMMA matrix make it more suitable for device fabrication as compared to ZnO nanoparticles dispersed in a solution.

The films of ZnO/polymethyl methacrylate(PMMA)nanocomposites for two concentrations of ZnO were fabricated using spin coating technique (Haripadmam et al. 2012). The Z-scan technique was used to investigate the optical limiting property of ZnO nanoparticles colloids and nanocomposite films of the polymer-ZnO nanoparticles. By exposing the nanocolloids to a pulsed nanosecond laser, the two photon absorption coefficient was enhanced with the increase of applied fluence and particle size.The efficiency of limiting was found to enhance the nanocomposite films, opening a way towards optoelectronic device fabrication.

Thin film of PS/ZnO nanocomposites were prepared by (Jeeju et al. 2012). The spin coating technique was used to prepare the samples with a thickness of 1um.The Z-scan technique was used to investigate the nonlinear optical properties. The results showed negative refractive index and two photon absorption .In addition, the optical limiting was observed in the films.

Jeeju et al. (2013) deposited ZnO/ poly (styrene) - poly (methyl methacrylate) (PS-PMMA) nanocomposite films on glass substrates using spin coating technique.The composite films were characterized by X-ray diffraction, FTIR

28

spectroscopy, atomic force microscopy and UV-Vis-NIR spectroscopy. From the UVVis-NIR spectra, it was observed that the ZnO/PS-PMMA nanocomposite films with 10 wt % ZnO content exhibited excellent shielding property in the UV region and, high transparency in the visible region. The Z-scan technique (open aperture) was used to investigate the optical absorptive nonlinearity of the nanocomposite films. The results indicated that the optical limiting type nonlinearity in the films was due to the two photon absorption. A minimum transmittance of around 0.25 was observed in the ZnO/PS-PMMA nanocomposite films which was much lower compared to ZnO/PMMA and ZnO/PS nanocomposite films. The ZnO/PS-PMMA nanocomposite films also showed a self-defocusing type negative nonlinear refraction, when the closed aperture Z-scan experiment was used. These nanocomposite films extended the ample scope of its applications as excellent optical limiters and efficient UV protectors.

The optical limiting performance of PVA/ZnO nanocomposites was described by Thankappan et al. (2013). By using open aperture Z scan technique, the nonlinear absorption was studied. Besides, the values of the imaginary part of the third-order susceptibility and the optical limiting threshold at a wavelength of 532 nm were measured.

Viswanath et al. (2014) prepared PVA/ZnO nanocomposite.The nonlinear absorption coefficient and nonlinear refractive index of the films were evaluated and then the third order nonlinearity was calculated. The results showed that the nanocomposite films have low threshold optical limiting applications.

In another study, Jeeju, et al. (2014)prepared PVA /ZnO as thin film using spin coating technique.A two photon absorption and a negative refractive index were indicated through the evaluation of nonlinear optical properties of the samples using Z-scan technique.The results showed that the sample which contained 10 wt % ZnO nanoparticles can act as an efficient optical limiter.

29

2.6.2


An up-to-date review on nanocomposites which composed of inorganic nanoparticles and polymer matrix have been reported for optical and magnetic applications (Li et al. 2010). Optical or magnetic characteristics can change as the particle sizes decreases to very small dimensions, which in general is the major interest in the area of nanocomposite materials. The use of inorganic nanoparticles and polymer matrix can produce high-performance novel materials that can be applied in many industrial fields. ZnO nanoparticles of different particle sizes were synthesized by (Anžlovar et al. 2011a). Then, they prepared ZnO nanocomposites with different concentrations and PMMA. The optical properties of the samples showed high absorption at wavelengths between 290 nm and 370 nm in the UV region even at low concentration and highest transmittance at the visible region through the reduction of ZnO concentration.

A review on synthetic methods of ZnO nanoparticles of various shapes, particle sizes, and surface modifications were reported by Anžlovar et al. (2011b) in addition to the procedure of PMMA/ZnO nanocomposites preparation. Moreover, unmodified surface and modified nano ZnO were synthesised and then were used to prepare PMMA/ZnO nanocomposite. The UV-Vis absorption spectra of the samples showed that they were absorbed more than 80% of the incident UV light.

Thin films of poly [2-methoxy-5(2’-ethyl hexyloxy)-phenylenevinylene] (MEH-PPV) containing different weight percentages of ZnO nanoparticles, deposited by spin coating have been studied previously (Zayana et al. 2011; Yahya and Rusop 2012; Nurul and Mohamood 2012).Their optical properties were investigated.The optical characterization of the nanocomposite thin films was performed using UV-Vis spectrophotometer and PL. The thickness of the thin films was measured using a surface profiler. The UV-visible absorption spectra of MEH-PPV: ZnO films showed a small red shift as compared to pure MEH-PPV. The UV-visible absorption band

30

increased while the optical bandgap decreased when the amount of ZnO nanoparticles in the nanocomposite was increased.

Different concentrations of

ZnO nanoparticles were used as fillers in

polyvinyliden fluoride (PVDF) as a polymer matrix to prepare nanocomposite PVDF/ZnO (Indolia and Gaur 2013).By studying the optical properties, it was observed that the increase of ZnO nanoparticles in the nanocomposite increased the absorption of UV light, while showing high transparency for visible light.

Sangawar and Golchha (2013) prepared PS/ZnO nanocomposites using solution cast technique.The effect of ZnO concentrations on the optical properties of nanocomposites was studied. The results revealed that all optical properties parameters examined were affected by the increase of ZnO nanoparticles percentage.

PMMA/ZnO nanocomposites thin films were prepared by (Khan et al. 2014). X-ray, FESEM, UV–Vis absorption, and PL were used to investigate the effect of ZnO concentration on the physical properties of PMMA matrix.

A previous study by Kumar et al. (2014) has prepared PVA nanocomposites and ZnO nanoparticles with different concentrations. The morphology of the samples was studied using X-ray and FTIR and the values of Eg were calculated.

PVDF/ZnO nanocomposites thin films were prepared by Bhunia et al. (2014) using spin coating method with low spin.The thickness of the samples was around 10 µm .The structure of the samples was studied using FESEM and FTIR. The transmittance and reflectance spectra were explored, then the linear absorption coefficient and Eg were calculated. Besides, the PL measurements were recorded. A strong ultraviolet PL emission band at around 383nm was observed.

PVA/ZnO nanocomposites were prepared in previous study using solution casting method (Mansour et al. 2015). X-ray, FTIR and UV-Vis spectroscopy were used to characterize the nanocomposites films. The optical properties such as band gap Eg, refractive index (n) and extinction coefficient (k) of the films were determined.

31

2.7

Z-SCAN TECHNIQUE

By coincidence, similar to any inventions or discoveries, when Mansoor Shiek-Bahae a fresh postdoctoral researcher, and Ali Said, a graduate student, studied the effect of optical limiting through self-focusing in the IR region, they observed that the transmittance through a far-field aperture in different materials, such as CS2, dependent dramatically on the position of the sample along the Z axis with considering the focus of the laser beam. A self-defocusing nonlinearity, for some positions, has caused an increase in the transmittance due to the aperture. At first, these remarks were considered as trivial appearances of self-lensing. Then, they realized that their results were very important to measure the nonlinear absorption as well as the sign and magnitude of nonlinear refraction. That coincidence led to the discovery of a new technique that can determine nonlinear optical properties. It was beautifully simple but extremely useful and was named Z-scan technique (Stryland 2007).

That discovery was published in a brief letter (Sheik-Bahae et al. 1989). Then it was followed by a full article ( Sheik-Bahae et al. 1990) and was named the most cited paper (IEEE J. Quantum Electronics) in history. It describes the analysis of Gaussian input beam and complex third-order nonlinear response, in addition to the fifth order response as the beginning.

All in all, a series of nine original papers by Sheik-Bahae and co-workers (Eric van Stryland & Ali Said) have been published describing and analyzing the technique.The mathematical analysis was also presented.

2.7.1

Literatures in Relation to the Z-scan

A new simple experimental technique that determines the magnitude and sign of nonlinear refractive index (n2) was presented by Sheik-Bahae et al. (1989).That method was named single beam method and it describes high sensitivity and simplicity. All apparatus used were shown and the technique was tested on some materials using two pulse laser (Co2 & Nd: YAG) as the source.

32

Sheik-Bahae et al. (1990) describing that technique in more detail. The applied Z-scan has been explained for different materials and theoretical analysis of the measurements has been given for a thin nonlinear medium. Also, the demonstration about how to determine the nonlinear absorption and possibility of separating the evaluation of nonlinear refraction and nonlinear absorption by the second part of Z-scan (open aperture) were described. The results of the method for ZnSe represented two-photon absorption and negative nonlinear refractive.

The Z-scan technique was extended to measure the value of n2 for thick samples. Their thickness was greater than the focus depth of the input beam; it was named ‘thick sample Z-scan’ which has the ability to measure the magnitude and determine the sign of nonlinear refractive index of those samples (Sheik-bahae et al.1991). Also, the limiting behaviour of the thick limiters observed as a function of zposition was explained.

A new method named position dispersion was derived from the Z- scan technique (Jianguo et al. 1991). It was used to measure the magnitude and to obtain the sign of thermooptic coefficient and beam waist radius of the laser source. Then, that new method was applied to measure the thermooptic coefficient of Chinese tea.

Chappie et al. (1994) studied CS2 samples using the Z-scan technique.The thicknesses of the samples ranged from thin to thick. The results indicated that for a self-focusing optical power limiter, the available thickness was six times the Rayleigh length in the medium.

Petrov et al.(1994) modified the Z-scan technique based on the reflection of a Gaussian beam.The new modification was used for a high‐absorbing materials.The geometrical‐optics approach of that modification was described.

A simple theoretical analysis of beam propagation and optical limiting for Zscan technique were shown in thick nonlinear media (Tian et al. 1995). A thick medium was considered as a stack of thin media.

33

Bridges et al.(1995) demonstrated a new Z-scan measurement technique. It allowed the use of non-Gaussian beam as the laser source and it used thick and thin samples.That new technique can be applied without the knowledge of the characteristics of the laser pulse used.

30 times sensitivity enhancement of nonlinear effect was obtained (Martinelli et al. 1997) when the reflection of Z-scan technique was modified. The incidence angle from the perpendicular angle was changed to be closer to the Brewster angle.

Stryland and Sheik-bahae (1998) explained the Z-scan technique in detail. In addition, the simple relations between its parameters were demonstrated. Also, it was analyzed for thin and thick nonlinear medi. Then, the data were presented and the measurements of Z-scan technique on different organic samples were described.

Samad and Vieira (1998) have developed the Z-scan technique. The HuygensFresnel principle was used to obtain an analytical solution for the propagation of a Gaussian laser beam electric field. Then, the theoretical expression of the two colour Z-scan was obtained. Also, their results were applied on a thick sample.

A two dimensional Z -scan technique was developed using an arbitrary incident beam and sample thickness (Chen et al. 1999). The nonlinear refraction and nonlinear absorption indices of both thin and thick samples were determined.That new development provided information for beam profile advancement inside the nonlinear medium. In addition, a simple and perfect way to improve the design of optical limiting devices was offered.The experimental results observed agreed with the calculation.

Instead of applying open and closed aperture in the Z-scan technique, only closed aperture was used and symmetric feature of the Z-scan curve was syudied to determine the nonlinear refraction index and nonlinear absorption coefficient of the materials (Yin et al. 2000). Also, a quick method was demonstrated to determine the TPA coefficient without knowing the exact focal point.

34

The features of closed aperture Z-scan techniqu were studied in details when both nonlinear refraction and nonlinear absorption were presented simultaneously(Liu et al. 2001). The dependence of ρ, which represents the ratio of the imaginary part to real part of the third order nonlinear susceptibility χ

(3)

, on some parameters that

describe a closed aperture was studied systemically. Such parameters studied were the distance between peak position and valley position, the difference between peak transmittance and valley transmittance, and the ratio of peak height to valley depth.

Yang et al. (2003) studied the distortion in the curve’s measurements of the closed aperture Z-scan caused by misalignment, sample imperfections and intensity fluctuation during the measurements. Therefore, the Z-scan technique became unsuitable for inhomogeneous samples.The I- scan method was suggested as a better method to determine the third - order nonlinear susceptibility for nonhomogeneous thin samples.

A new Z-scan technique suitable for elliptic Gaussian beam was proposed by Tsigaridas et al. (2003).It was based on the direct measurement of the lengths of the beam’s semiaxes through a CCD camera in the far field instead of circular beam that based on the measurement of irradiance transmitted through an aperture .That method was considered unique in determining the optical nonlinearity accurately.

A theoretical analysis of the Z-scan characteristics of thick media with nonlinear refraction and nonlinear absorption was presented (Wei-Ping et al. 2004). It was based on the combination of Gaussian decomposition method and a model of distributed lens. It will be beneficial in the design of optical limiters and determination of nonlinear indices of thick media.

By using high repetition rate lasers, a simple method that can manage the cumulative thermal effects and measure the nonlinear refractive index using Z-scan technique have been reported (Gnoli et al.2005). The effect of cumulative thermal lens from other contributions was able to be separated. That method was demonstrated to determine the third order nonlinearities of different samples .In addition, the contribution of thermal effect cannot be ignored.

35

The numerical analysis of the effect of temporal pulse shape on Z-scan measurement was studied by (Dement and Jovai 2005). Its influence on thin samples was studied for the case of long pulse and short pulse.Then, the pulse shape which caused different value from the real value was investigated, so the suitable pulse shape that should be taken into account can be decided.

The analytical formulae of Z-scan technique were developed by (Tsigaridas et al. 2006).The new development allowed the calculation of nonlinear absorption parameter directly from the transmittance measurements. Those formulae were suitable for large values nonlinear absorption coefficient obtained from the experimental data of both cases, circular and elliptical Gaussian beams. The effect of noise on the results was studied and it was proven that the new method was not affected by high noise.

A modified Z-scan technique which can measure slow response nonlinear refraction of absorbers with brilliant sensitivity was introduced (Guedes et al. 2007). Scheme of detection capable of reducing linear noise was included in the modification.

A new improvement was done on the Z-scan technique by (Gomes et al. 2007). It was named thermally managed eclipse Z-scan. The measurements sensitivity of the third order susceptibility of photonics and biophotonics materials was improved when laser at low intensity was used. Also, thermal and non-thermal contribution to nonlinearity can be distinguished.

Theoretical and experimental study on Z-scan were done by (Yan et al. 2009).The dependence of Z-scan measurements on polarization was studied. Also, the normalized transmittance formula of closed-aperture Z-scan for linearly, elliptically and circularly polarized incidence beams on samples was measured and analyzed theoretically.

36

A new Z-scan technique was presented to determine the nonlinear absorption and refraction when they are presented simultaneously; by measuring the beam dimensions in the far field directly(Tsigaridas et al.2009). The new method can study the influence of nonlinear absorption on Z-scan technique by measuring of the beam dimension which is represented by the production of induces asymmetry in the Z-scan plots of circular and elliptical Gaussian beams. The asymmetry rises exponentially with nonlinear phase shift.In addition, the nonlinear refractive index was deduced by multiplying the radius of Z-scan curve with the opening Z-scan curve. That new technique has high accuracy and efficient in numerical calculations.

In order to develop the Z-scan technique, two new techniques were presented to determine the nonlinear refractive index of materials (Rativa et al. 2009).Firstly,by using a collimated setup and analyzing the transmittance wave front of a collimated laser beam through a sample with a Hartmann- Shack sensor . Secondly, by using a Hartmann- Shack sensor instead of conventional detectors in the Z-scan setup to measure the wave front distortion and the change in transmittance beam simultaneously (second new technique named Hartmann Shack Z-scan).

Identification and separation of

refractive nonlinearities and high-order

absorptive have been demonstrated theoretically and numerically (Shi et al. 2009). The formulae of nonlinear imaginary phase shift for multiphoton absorption were given, and so the normalized transmittance of open aperture Z-scan.Also, two empirical formulae with significant accuracy were given to determine the two-photon and three-photon absorptions directly that only need nonlinear normalized transmittance at the focus.

Yan et al. (2010) suggested a modification for the normal elliptically polarized light Z-scan method.A quarter-wave plate and an analyzer were added earlier before the detector. The results showed an enhancement in the sensitivity of determining the third order nonlinear susceptibility when nonlinear absorption was negligible.

Shi-Fang and Qiang (2010) used an elliptic Gaussian beam and introduced complex beam parameters.This method which was applied to calculate the normalized

37

on-axis transmittance function in the Z-scan technique was found to be simpler than other methods used for calculating high-order nonlinearities. The results showed decrease in the distortion

a

which was observed in the central of the curve when the

ellipticity increased. Also, the variation of normalized peak-valley differences decreased with decreasing ellipticity.

A new technique, named photoacoustic Z-scan has been introduced by (Yelleswarapu and Kothapalli 2010). It is used to determine the nonlinear absorption coefficient in non-transparent, saturable absorber, reverse saturable absorber and highly absorptive materials. Its measurement is based on recording photoacoustic signal that is generated when the sample is translated through a focused laser beam. This new technique has brought together the advantages of high sensitive detection of photoacoustic and original optical Z-scan technique.

A development on the Z-scan technique has been done which in theory stated that it has the ability to apply the open and closed-aperture Z-scan with the existence of third and fifth order nonlinear refraction and saturable absorption simultaneously (Liu and Tomita 2012). The original Z -scan theory can only be applied to the case with the existence of third-order nonlinear refraction with two photons absorption or fifth order nonlinear refraction without two photon absorption. It showed that it can be used experimentally in determining the nonlinearity of nanocomposites containing CdSe quantum/polymer, third and fifth order nonlinear refraction and saturable absorption simultaneosly.

Wang et al. (2012) presented an improvement for the Z-scan technique. The nonlinear absorption processes were divided into two parts, saturable absorption (SA) and reverse saturable absorption (RSA), then the solution of each phenomenon was derived individually. That new model carried out on gold nanorods. A Modification on the Z-scan technique was done by (Kolkowski and Samoc 2014). In the original Z-scan technique, the sample moved along the z-direction; but in this modification, the electrical focus-tunable lens was used to change the focal length f, while the position of the sample is fixed.The new modification was named f-scan.

38

The measurements for both closed and open aperture obtained by the f-scan were confirmed to be equivalent to those obtained using Z-scan .One of the advantages offered by this method is the measurements can be done faster which may be suitable to investigate samples that are not stable for a long time. Another benefit, the sample stays fixed, thus open the possibility of investigating samples that require special conditions such as vacuum or cryogenic temperatures.

Table 2.1

Summary of the values of β, n2, χ(3) in literatures review β cm/W _

n2 cm2/W 13.1 x 10-10

χ(3) e s u 14.7 x 10-8

4.99 x 10-7

4.51 x 10-14

1.27 x 10-6

1.34 x 10-8

6.62 x 10-13

41.67 x 10-13

_

_

13.41 x 10-12

ZnO-Cu colloid

29.38 x 10-8

12.3 x 10-13

_

ZnO-cds colloid

17.3 x 10-9

11 x 10-13

_

ZnO-TiO2 colloid

18 x 10-8

10 x 10-13

_

Sample ZnO: Au Thin Film ZnO: Au ZnO dispersed in water ZnO: Al Thin Film

ZnO : Ag Thin film

_

_

6.2 x 10-11

PS/ZnO Thin Film

2.8 x 10-7

1.33 x 10-13

1.4 x 10-11

PVA/ ZnO Thin Film

_

4.32 x 10-14

_

Ref. Ryasnyanskiy et al. (2007) Ning et al. (2008) Chang and Song (2007) Bahedi et al. (2009) Irimpan et al. (2010) Irimpan et al. (2010) Irimpan et al. (2010) Torres-Torres et al. (2012) Jeeju et al. (2012) Jeeju et al. (2014)

39

CHAPTER III

METHODOLOGY

3.1

INTRODUCTION

This chapter presents the preparation of polymer-ZnO and polymer-ZnO-CuO nanocomposites as stated in the objectives. Then, it presents the measurements and calculations of linear and nonlinear optical properties were done. The proposed methodology is summarized in a block diagram and is shown in Figure 3.1, Figure 3.2 and Figure 3.3. Figure 3.1 and Figure 3.2 represent the preparation of polymer/ZnO nanocomposites and polymer/ZnO/CuO nanocomposites respectively,while Figure 3.3 represents the characterization methods of the samples by, FESEM, EDX, EDS mapping, FTIR, PL, UV-Vis spectroscopy, and Z-scan technique. Besides, the study of surface morphology, measurements, and calculations include the linear and nonlinear optical properties. Then, the third order nonlinearity is calculated and the suitability of the samples as an optical limiting device is explored by applying the transmittance technique.

40

Figure 3.1 Preparation of polymer/ZnO nanocomposites

Figure 3.2 Preparation of polymer/ZnO/CuO nanocomposites by using cast method

41

Figure 3.3

Characterization of the as-prepared samples

3.2

PREPARATION METHOD

3.2.1

Casting Method

Casting is an industrial simple process in which a liquid material is usually poured into a template, and forms the needed shape.It is then allowed to harden in an oven or at room temperature. Finally, the solidified material is removed from the template. This method was used to prepare the nanocomposites as foils, where the film was pulled out from the ceramic dish that was used as the template, as a flexible foil.Similarly, it was used to prepare the film on quartz of PVDF/ZnO nanocomposites, where the film was left on the quartz that was used as the template, after the film harden.

42

3.2.2

Spin Coating

A simple process that has been used for the preparation of homogeneous thin film. A typical process that includes depositing a small volume of solution on the centre of a substrate and then spinning the substrate in order to spread the solution to all area by centrifugal force. The speed and time of the spin affect the thickness of the film besides other parameters related to the nature of the fluid such as viscosity, concentration, solvent, and surface tension. This method was used to prepare PMMA/ZnO nanocomposites as a thin film on quartz.

Figure 3.4

3.3

Spin coating method

AGGLOMERATION AND AGGREGATION

The greatest problem during the preparation of polymer nanocomposites, is the homogenous and completion of a uniform filler distribution inside the polymer matrix(Agrawal et al. 2010; Khan et al. 2014). Usually, the preparation method of nanocomposites and the interaction between nanoparticles and polymer matrix are considered as the main effect of parameters on dispersion other than the nature of solvent used to dissolve the polymer( Dan et al. 2008). Nanoparticles of any materials have high surface energy, so it has the tendency to agglomerate or aggregate. This known from the literature review which has mentioned the difficulty in achieving a good distribution of inorganic nanoparticles in polymer matrix(Agrawal et al. 2010; Li et al. 2010; Kos et al. 2014). Many methods have been applied to avoid the

43

agglomeration and aggregation (Khan et al. 2014; Li et al. 2010). One of the methods is preparing the nanoparticles material independently and then mixed it with a monomer solution; finally, the in situ polymerization is completed (Tang et al. 2006; Hong et al. 2006). The second way is representing the desperation of nanoparticles and dissolving the polymer in the same suitable solvent separately, or in two different solvents but are soluble in each other. After that, the two solutions are mixed together.This method requirs the modification of nanoparticle surface (Anžlovar et al. 2011a ; Hong et al. 2006 ; Liu et al. 2006 ; Khrenov et al. 2005) which can be done by applying different methods such as using organic carboxylic acids as modifiers; i.e. oleic (Zhu et al. 2007). In addition, the modification can be done when nanoparticles are coated with and modified by polymers (Xiong 2010 ; Nia et al. 2015). Lastly, the nanoparticles are precipitated in the polymer matrix which is in the form of bulk polymer or monomer (Kos et al. 2014; Hong et al. 2006; Anžlovar et al.2011a ; Paramo et al. 2010). In this method, the polymer matrix cannot act as a suitable fluid for the aggregation of nanoparticles due to kinetics reasons. Furthermore, ultrasound treatment can be used to break the nanoparticles aggregation or agglomeration. However, the duration of sonication should be chosen carefully (Nia et al. 2015 ; Mandzy et al. 2005; Sygouni and Chrysikopoulos 2015). Previous literature has revealed that the sonication is very effective in breaking up ZnO aggregates. Hence ZnO nanoparticles cannot re-agglomerate again after sonication is stopped due to the entrapment in PMMA (Anžlovar et al. 2011a).So during the preparation of nanocomposites in this work, ultrasound treatment was used with a specific duration time which was determined based on previous literature and lab experimental works to avoid agglomeration and aggregation.

3.4

MATERIALS

3.4.1

Polymers

Four different types of polymers were used as polymer matrix; poly (methyl methacrylate) (PMMA) average Mw=96.700, poly (vinylidene fluoride) (PVDF) average Mw=534.000 polyvinyl alcohol (PVA) average Mw= 85000, and polystyrene (PS) average Mw=192.000. All of them were supplied by Sigma-Aldrich.

44

3.4.2

Zinc Oxide (ZnO)

Zinc Oxide (ZnO) was purchased from Sigma-Aldrich. It was used as a filler in polymer/ZnO and polymer/ZnO/CuO nanocomposites. The size of the nanoparticles was 50 < size < 100 nm. This particle size was chosen because nonlinear optical properties are enhanced as the particle size of nanoparticles that are used as fillers in nanocomposite increases (Sreeja et al. 2010; Jeeju et al. 2012 ; Haripadmam et al. 2012).

3.4.3

Copper Oxide (CuO)

Copper oxide (CuO) was purchased from Sigma-Aldrich; the size of the nanoparticles was 25 < size < 50 nm. It was added to polymer-ZnO nanocomposites as foils to enhance the linear and nonlinear optical properties of the samples. The ZnO nanoparticles were also modified to avoid aggregation and agglomeration that may happen during their dispersion in the polymer matrix.

3.4.4

Solvents

Chloroform (CHCl3), was used as a solvent for PMMA. Its density is equal to 1.489 gm.cm-3. It is considered(Qian et al. 2010; Wagner 1987) as the greatest soluble limit for PMMA compared to other solvents like acetone or toluene. It also has the greatest evaporation rate, less viscosity, and least chemical hazards compared to other solvents. In addition, studies on nonlinear optical properties have suggested (Jaffar 2013) it as a promising solvent when it is used for PMMA doped dye. Moreover, it is a nonpolar solvent.

Dimethylformamide (DMF) (C3H7NO) was used as a solvent for PVDF. Its density is equal to 0.948 gm.cm-3.It is usually considered as a common solvent for this type of polymer (Indolia and Gaur 2013) . Acetone (C3H6O) with a density of 0.791 gm.cm-3 was used as a solvent for PVDF to obtain a particular phase of this polymer,α-phase (Tan et al. 2014) . It is a polar aprotic solvent.

45

Distilled water (H2O) was used as a solvent for PVA. It is a common solvent for this type of polymer (Thankappan et al. 2013). It is a polar protic solvent which structurally has a long hydrocarbon chain. Toluene (C7H8) with a density of 0.87 gm.cm-3 is a common solvent for PS (Jeeju et al. 2012) . It is also a nonpolar solvent.

3.5 PREPARATION OF POLYMER-ZnO NANOCOMPOSITES

To prepare polymer-ZnO nanocomposites one of the four polymers (PMMA, PVDF, PVA, and PS) was used as the polymer matrix and ZnO nanoparticles were used as fillers.

The weight percentage of ZnO in the nanocomposite (wt %) was calculated based on the equation:

𝑤(𝑤𝑡%) = 𝑤

𝑤𝑓

𝑝 + 𝑤𝑓

𝑋 100

(3.1)

Where wt % is the weight percentage of ZnO in the nanocomposite, wp is the weight of the polymer and wf is the weight of the filler.

The ratio of polymer preparation in a polymer/solvent solution is calculated based on the equation below: wt % = 𝑚

𝑚2

2 + 𝑚1

𝑋 100

(3.2)

Where m1 is the mass of solvent and m2 is the mass of solute. The volume of solvent (v) is calculated from the equation below: 𝑣=

𝑚 µ

Where m is the mass of solvent and µ is the density of solvent.

(3.3)

46

The preparation process of pure polymer and polymer-ZnO nanocomposites as foils is shown in figure 3.5. The steps of preparation require basic safety procedures. So, protection procedures were applied such as working in fume hood and donning lab gloves and lab face mask.

Figure 3.5

3.5.1

Preparation method of pure polymer and polymer-ZnO nanocomposites as foil

PMMA/ZnO Nanocomposites

Nanocomposites PMMA/ZnO with different concentrations of ZnO were prepared in two steps; firstly, the PMMA solution was prepared by adding chloroform (CHCl3) to the (PMMA). As in Figure 3.5, to prepare 10 wt % of PMMA /chloroform solution,80 mg of PMMA was dissolved in 1.024 mL of chloroform using a magnetic stirrer (angular velocity of 400 rpm for duration of two hours at room temperature). Secondly, Zinc oxide (ZnO) with concentrations of 1, 3, 5, 10 and 15 wt % were added to the mixture of PMMA/chloroform. Then, the sonicator was used for 15 min

47

to disperse the nanoparticles in the solution. After that, the solution was stirred at room temperature for two hours by a magnetic stirrer (angular velocity 400 rpm) to get a homogeneous solution.

To compare the effect of preparation method on linear and nonlinear optical properties, PMMA/ZnO nanocomposites were prepared as foils and thin films by two different techniques.

Casting technique was used to prepare foils like pure PMMA and PMMA/ZnO. As in Figure 3.5, each one of the solutions of PMMA and PMMA/ZnO with different concentrations was casted uniformly on a glass petri dish at room temperature. After few hours the film was pulled out easily as flexible foil. Next, the foil was kept at room temperature for one day for it to solidify. Later, the foils like pure PMMA and PMMA/ZnO nanocomposites of different concentrations were collected.

The spin coating technique (angular velocity of 800 rpm and time duration of 30 sec) was used to prepare thin films on quartz substrates like pure PMMA and PMMA/ZnO. Substrate cleaning has been considered as the first important step to prepare a good thin film. The quartz substrate (2 cm × 2 cm) was washed with detergent and then cleaned with methanol. After that, it was washed with acetone using a sonicator for 10 min. afterwards, the substrates were rinsed with distilled water, and then dried in hot air. Both solutions of PMMA and PMMA/ZnO were used separately. The solvent was then allowed to evaporate inside the lab at room temperature until dried thin films of pure PMMA and PMMA/ZnO nanocomposites with different concentrations of ZnO nanoparticles were obtained as shown in Figure 3.6.

48

Figure 3.6

3.5.2

Preparation method of pure PMMA and PMMA-ZnO nanocomposites as thin films.

PVDF/ZnO Nanocomposites

To compare the effects of preparation method and solvent on linear and nonlinear optical properties, PVDF/ZnO nanocomposites were prepared as foils and films on quartz by two different techniques.

PVDF/ZnO nanocomposites were prepared as foils using the same procedures applied for the preparation of PMMA/ZnO nanocomposites as foils. Firstly, acetone was failed when used as the solvent to prepare PVDF as foils. So, the PVDF solution was prepared by adding DMF to the PVDF. As shown in Figure 3.5,to prepare 3 wt % of PVDF /DMF solution,90 mg of PVDF was dissolved in 3..06 mL of DMF using a magnetic stirrer (angular velocity of 400 rpm and time duration of two hours at 60 oC). Secondly, zinc oxide (ZnO) with concentrations of 1, 3, 5, 8 and 10wt % were added

49

to the mixture of PVDF/DMF. Then a sonicator was used for 15 min to disperse the nanoparticles in the solution. After that, the solution was stirred at room temperature for two hours by a magnetic stirrer (angular velocity 400 rpm) at room temperature to get a homogeneous solution. By using casting technique, each one of the solutions of PVDF/DMF and PVDF/ZnO was casted uniformly on a ceramic dish at room temperature. Then the samples were kept in an oven at 60 oC for one day. Then, the film was removed easily from the ceramic dish. Next, the foils of pure PVDF and PVDF/ZnO nanocomposites of different concentrations of ZnO nanoparticles were collected.

The preparation of PVDF/ZnO nanocomposites as films on quartz substrate was done as follows; Firstly, the PVDF solution was prepared by adding a suitable solvent, i.e. acetone. To prepare 3 wt. % of PVDF /acetone, 8 mg of PVDF was dissolved in 0.492mL of acetone. The PVDF/acetone solution was stirred by a magnetic stirrer at an angular velocity of 400 rpm and at room temperature for 60 min. Secondly, to prepare PVDF/ZnO nanocomposites with different concentrations of ZnO (1 %, 3 %, 5 %, 8 %, and 10 %), ZnO were added to a mixture of PVDF/ acetone. Then, a sonicator was used for 15 min to disperse the nanoparticles in the solution. After that, the solution was stirred at room temperature for two hours with an angular velocity of 400 rpm by a magnetic stirrer to a get homogeneous solution. The deposition of the solution was done by using casting method which was used earlier to prepare pure PVDF and PVDF/ZnO films by pouring the prepared solution on the quartz substrate as shown in Figure 3.7. The solvent was then allowed to evaporate inside the lab at room temperature until dried samples of pure PVDF and (PVDF/ZnO) nanocomposites with different concentrations were obtained.

50

Figure 3.7

3.5.3

Preparation method of pure PVDF and PVDF/ZnO nanocomposites as film on quartz

PS/ZnO Nanocomposites

PS/ZnO nanocomposites and pure PS were prepared by applying the same procedures used to prepare PMMA/ZnO nanocomposites as foils. Firstly, PS solution was prepared by adding toluene to the PS.As in Figure 3.5, to prepare 10 wt % of the PS/toluene solution, 420 mg of PS was dissolved in 4.326 mL of toluene. Then it was stirred using a magnetic stirrer (angular velocity of 400 rpm and time duration of two hours at 60 oC). Secondly, zinc oxide (ZnO) with concentrations of 1, 3, 5, 10 and 15wt % were added to the mixture of PS/toluene. Then, the sonicator was used for 15 min to disperse the nanoparticles in the solution. After that, the solution was stirred at room temperature for two hours using a magnetic stirrer (angular velocity of 400 rpm) to get the homogeneous solution. By using casting technique, each one of the solutions

51

of PS/toluene and PS/ZnO was casted uniformly on the ceramic dish at room temperature. Then the samples were kept in the oven at 60 oC for one day. Then the film was pulled out easily from the ceramic dish as flexible foil.Next, the foils of pure PS and PS/ZnO nanocomposites with different concentrations of ZnO nanoparticles were collected.

3.5.4

PVA/ZnO Nanocomposites

PVA/ZnO nanocomposites and pure PVA were prepared by the same procedures used to prepare PMMA/ZnO nanocomposites as foils. Firstly, PVA solution was prepared by adding distilled water (H2O) to the PVA. As shown in Figure 3.5, to prepare 10 wt % of PVA/water solution, 60 mg of PVA was dissolved in 1.14 mL of water. After that, a magnetic stirrer (angular velocity of 400 rpm and duration time two hours at 70 o

C) was used to help it dissolve. Secondly, zinc oxide (ZnO) with concentrations of 1,

3, 5, 10 and 15wt % were added to the mixture of PVA/water. Then, the same steps used in the preparation of PMMA/ZnO and PS/ZnO as foils were applied to complete the preparation of PVA/ZnO as foils. Then the samples were kept in the oven at 60 oC for one day. After that, the film was pulled out easily from the ceramic dish as a flexible foil. Next, the foils of pure PVA and PVA/ZnO nanocomposites with different concentrations of ZnO nanoparticles were collected.

3.6

PREPARATION OF POLYMER/ZnO/CuO NANOCOMPOSITES

Copper oxide (CuO) nanoparticles were added to the polymer/ZnO nanocomposites as shown in Figure 3.8. The PMMA-ZnO, PS/ZnO and PVA/ZnO nanocomposites with 10% of ZnO, and PVDF/ZnO with 8% of ZnO were chosen to be added 1% CuO. These nanocomposites with ZnO and CuO concentrations were prepared as the requirements of the study because they have suitable transmittance that can be used in the Z-scan technique with a laser source at a wavelength of 532nm. After preparing polymer-ZnO nanocomposites as mentioned in section 3-5-1, 3-5-2, 3-5-3 and 3-5-4, CuO nanoparticles (1 wt %) were added to these nanocomposites. A sonicator was

52

used for 15 min to disperse the CuO nanoparticles in the solution. After that, the solution (polymer/ZnO/CuO) was stirred at room temperature for two hours using a magnetic stirrer (angular velocity of 400 rpm) to get a homogeneous solution. By using casting technique, each one of the solutions (PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO) was casted uniformly on a ceramic dish at room temperature, whereas PMMA/ZnO/CuO was casted on a glass dish using the same procedure. Then, the first three nanocomposites were kept in the oven at 60 oC for one day, while PMMA/ZnO/CuO samples were kept at room temperature for one day. After that, the film was pulled out easily from the substrate as a flexible foil.

Figure 3.8

Preparation method of nanocomposites as foils.

pure

polymer

and

polymer/ZnO/CuO

53

3.7

CHARACTERIZATION METHODS

3.7.1

Field Emission Scanning Electron Microscope (FESEM)

The surface morphologies of the as-prepared nanocomposites were characterized using field emission scanning electron microscope (FESEM, Carl Zeiss, Merlin Compact, Oxford Instrument, 55VP) with accelerating voltage of 3.0 kV. Prior to the analysis, the samples were coated with a fine layer of platinum in order to reduce the charging effect.

3.7.2

Energy dispersive X-ray (EDX)

Energy dispersive X-ray (EDX) was used to analyze the compositional and purity of the as -prepared nanocomposites materials.

3.7.3

EDS Mapping

An EDS mapping is an image used to show the spatial distribution of elements in asprepared samples. EDX analysis and EDS mapping were obtained from the plug in the analysis of FESEM.

3.7.4

Photoluminescence (PL)

The optical properties of the as-prepared samples were examined using PL spectroscopy (Edinburgh Instruments FLS920).The PL spectra were obtained using a 450 W Xenon lamp, model Xe 900, as the excitation source. The excitation wavelength used in this work was 325nm.The measurement was conducted at room temperature.

54

3.7.5

Fourier Transform Infrared (FTIR)

The as-prepared samples were checked by using Fourier transform infrared (FTIR) spectroscopy (Brand: Perkin Elmer, range: 4000cm-1 - 650cm-1, resolution: 4cm-1).The FTIR spectra were caused by the transmittance mode.

3.7.6

Ultraviolet-Visible Spectroscopy

UV-Vis spectrophotometer (PerkinElmer instruments- Lambda 900 UV/VIS Spectrometer) was used to measure the linear transmittance, linear absorbance and reflectance spectra of the as-prepared samples.The reflectance was measured in a separate parameter using UV-Vis spectroscopy itself which has the features to measure the R values.

3.7.7

Z-scan Technique

The setup of single beam Z-scan technique is shown in Figure 3-9. A Q-switched Nd: YAG pulse laser, (Beijing Mini laser Technology Co., Ltd.) which gives second harmonic at 532 nm (7 ns, 5 Hz) was used as the light source. A 10 cm lens was used to focus the laser beam. The transmitting light energy in the far field, which passed through the aperture, was recorded by energy meter (OPHIR Photonics, A Newport Company).

In the single beam Z-scan technique, the sample moved by the translated stage along the Z-axis that represents the focus of the Gaussian laser beam, (Irimpan 2008).Meanwhile, the transmittance, as a function of the sample position relative to the beam focus, was recorded by a detector through the aperture in the far field. The sample, which acted as a thin lens due to the nonlinear refractive changed the beam dimensions. These changes were translated into various transmittance energy by the aperture. Then, the information provided was used to determine nonlinear refractive index of the sample. Moreover, when the aperture was removed, the differences of transmittance energy due to the nonlinear absorption would provide sufficient information to determine nonlinear absorption coefficient of the sample. One of the

55

important features of thier technique is the easy separation between nonlinear refraction and nonlinear absorption.

Figure 3-9

Z-scan experiment setup

56

CHAPTER IV

RESULTS AND DISCUSSION

4.1

INTRODUCTION

In this chapter, the characterization of as-prepared samples that contain linear and nonlinear optical properties is studied along with the surface morphology.The composition and distribution of the existence element on the samples are confirmed.The FTIR, FESEM, EDX,

EDS mapping, and PL are evaluated. The

linear transmittance, absorbance, and reflectance have been measured by UV-Vis spectroscopy. The linear absorption coefficient, energy gap, extinction coefficient and refractive index are calculated. The nonlinear refractive index, n2, and nonlinear absorption coefficient, β, are assessed using the single beam Z-scan technique, and then the third order susceptibility (3) is calculated. 4.2

FTIR ANALYSIS

The analysis of FTIR spectra of the as-prepared samples with a transmittance mode in the range of 650 cm–1- 4000 cm–1 is shown in the Figures 4.1, 4.2, 4.3 and 4.4.

4.2.1

PMMA/ZnO

The FTIR spectra of PMMA/ZnO as thin films and foils are shown in Figure 4.1. The bands at 2992 cm–1 and 2951 cm–1 are assigned to the CH2, C–O–CH3, and CH stretching vibrations ( Ramesh et al. 2007). A strong peak at 1727 cm-1 appeared due to the presence of ester carbonyl group stretching vibration of PMMA (Gowri et al. 2010). While the bands at 1433 cm-1 appeared due to the stretching

57

vibrations of C=O and–O–CH3 groups of PMMA( Pawde and Deshmukh2009). A small band at 1346 cm-1 may be due to the –CH3 group. The peaks at the range between 1260 cm-1 and1146 cm-1 can be explained by the C–O (ester band) stretching vibration. There were bands at 978 cm-1 and 737 cm-1 that may be due to the bending of C–H (Gowri et al. 2010). Besides that, previous literature has indicated that ZnO nanoparticles have stretching and bending bands. However, they appeared to be suppressed and one of them was revealed between 360cm

-1

and 420 cm-1 (Chilvery

et al. 2014), or near 420 cm-1 (Latif et al. 2012) , or at 468cm-1 and 480 cm-1 (Rajagopalan and Khanna 2013) or near 438cm-1 (Gowri et al. 2010).

Figure 4.1

FTIR of PMMA/ZnO nanocomposites

58

4.2.2

PVDF/ZnO

The FTIR spectra, which was the transmittance mode of PVDF/ZnO are shown in Figure 4.2. According to previous works, the peaks at 766 cm–1, 795 cm–1 and 1402 cm–1 were identified as α phase, while 840cm-1 was identified as β phase(Boccaccio et al. 2002; Bormashenko et al.2004). The peaks 875cm-1, 1070cm-1 and 3021cm-1 also corresponded to β phase (Boccaccio et al. 2002).The IR vibration modes due to γphase were 1171 cm-1 and 1233 cm-1.Vibrational bands at 766cm-1 are CF2 and skeletal bending modes, while at 795cm-1 are CH2 rocking mode.Similarly, the peaks at 840 cm-1 are CH2 rocking mode (Yousefi 2011a ; Rawat et al. 2012). The 840 cm-1 band is common to β and γ phases; a sharp and well-resolved band indicates β phase, whereas a broadband indicates γ phase. The broadband is due to imbrication (Bormashenko et al. 2004 ; Gregorio and Capitao 2000). 1402 cm-1 represents a typical vibrational band which corresponds to the deformed vibration of CH2 group (Junlin et al. 2010). Besides that, literature has indicated that ZnO nanoparticles have stretching and bending bands but they appear to be suppressed. Literature has also revealed that the bands are between 360cm-1 and 420 cm-1 (Chilvery et al. 2014) or near 420 cm-1 (Latif et al. 2012), or at 468 cm-1 and 480 cm-1 (Rajagopalan and Khanna 2013),or near 438cm-1 ( Gowri et al. 2010) .

Figure 4.2

FTIR of PVDF/ZnO nanocomposite

59

4.2.3

PVA/ZnO

The FT-IR spectra of pure PVA and PVA/ZnO nanocomposites as foils are shown in Figure 4-3. A broad and strong absorption band at 3334cm-1 is attributed to O–H stretching vibration. Two peaks at 2912 and 2940cm-1 are assigned to C–H stretching vibration of –CH and –CH2, respectively(Gong et al. 2014) . The band at 1722cm-1 may be due to the carbonyl group (Hemalatha et al. 2014) . The peak at 1422cm-1 is designated as CH2 scissoring mode. Whereas, the peaks at 1374cm-1 and 1329 cm-1 are attributed to CH2 deformation(Gong et al. 2014). The band at 1235cm-1 is due to the C–O–C vibration in vinyl acetate group, and the bands at 1093cm-1 and 916cm-1 are due to the C–O and C–C stretching vibrations(Gong et al. 2014) (Hemalatha et al. 2014).The band at 850cm-1 is due to the CH2 rocking mode(Gong et al. 2014). Previous literature has indicated that ZnO nanoparticles have a main peak in the PVA/ZnO nanocomposite spectra which is at 435cm-1 and is assigned to the stretching vibrations of Zn–O bond (metal–oxygen bond) (Hemalatha et al. 2014);however they appeared to be suppressed.

Figure 4.3

FTIR of PVA/ZnO nanocomposites

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4.2.4

PS/ZnO

FTIR spectra, as the transmittance mode, of foils like PS and PS/ZnO nanocomposites are shown in Figure 4.4.The absorption peaks shown in the PS spectrum as pure polymer were also shown exactly in the spectra of PS/ZnO nanocomposites.It means that they corresponded clearly only to polymeric groups. Hence, the absorption bands did not detect any shift or sharpening even at high concentration of ZnO (15 wt%). Consequently, there was no chemical interaction between PS and ZnO nanoparticles in the resulted nanocomposites. These results agreed with Jeeju (2012) and with Chae and Kim (2005). Both spectra of PS and PS/ZnO nanocomposites showed peaks at 1452 cm-1,1493 cm-1 and 1601 cm-1 that represent the contribution of vibration bands of aromatic C=C of styrene units. The peaks centred at 2924 cm-1 and 2850 cm-1 which were observed in all spectra correspond to asymmetric and symmetric stretching vibrations of –CH2, respectively (Jeeju 2012 ; Chae and Kim 2005). The bands at 758 cm-1 and 700cm-1 indicate the presence of a monosubstituted ring(Chae and Kim 2005). Previous literature has indicated that ZnO nanoparticles have main peaks in the PS-ZnO nanocomposite spectra at 538 cm-1 and 453cm however they appeared to be suppressed.

Figure 4.4

FTIR of PS/ZnO nanocomposites

-1

(Jeeju 2012) ;

61

4.3

FIELD EMISSION SCANNING ELECTRON MICROSCOPE (FESEM)

The surface morphologies of the as-prepared nanocomposites were characterized by studying the top view images obtained from FESEM. Basically, four types of nanocomposite were prepared in this study; (i) thin film and foil like PMMA/ZnO, (ii) Foil and film casted on quartz substrate like PVDF/ZnO, (iii) foil like PVA/ZnO and (iv) foil like PS/ZnO. Polymer/ZnO nanocomposites represented nanocomposites with high concentration of ZnO nanoparticles which was used in the preparation method. In order to modify the optical properties of pristine nanocomposite materials, CuO was selected as the dopant, it was incorporated into a polymeric matric network and later denoted as foil like PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO. Besides, the side view of FESEM cross section was used to measure the thickness of a thin film like PMMA/ZnO and film on quartz like PVDF/ZnO.

4.3.1


Figure 4.5 (a) and (b) show the typical FESEM images of pristine PMMA and PMMA/ZnO nanocomposites prepared as flexible foils and thins film. It was observed that a uniform PMMA film was successfully formed in the thin film based PMMA and foil like PMMA. From figure 4.5 (c) and (d), under the incorporation of ZnO nanoparticle into PMMA solution, it can be clearly seen that a homogeneous dispersion of ZnO nanoparticle was successfully prepared in polymeric matrixPMMA in terms of both thin film and foil methods. The effective size of the ZnO nanoparticles was calculated via image J software and was found to be highly dependent on the preparation method. It is interesting to note that three dimensional (3D) flake-like ZnO nanoflakes ranged from 89nm to 228nm were formed on the thin film-like sample. However, an irregular-shaped ZnO nanoparticle ranged 82nm to 184nm was observed in the foil-like sample. This finding reveals the dependency of film deposition quality on surface energy of the substrate used.

62

To confirm the thin film thickness of the as-prepared sample, the cross-section view of thin film based PMMA and PMMA/ZnO sample in Figure 4.5 (e) and (f) was evaluated. Overall, the sample showed average thickness of 781nm and 820nm for thin film like PMMA and PMMA/ZnO nanocomposite, respectively.

Figure 4.5

4.3.2

Top view FESEM images of (a) thin film based PMMA (b) foil like PMMA (c) thin film based PMMA/ZnO (d) foil like PMMA/ZnO (e) cross section view of thin film based PMMA and (f) cross section view of thin film based PMMA/ZnO


Figure 4.6 (a) and (b) present the top-view FESEM images of the surface morphology of films on quartz and foil-like PVDF as pure polymer, while (c) and (d) present the

63

image of film and foil like PVDF/ZnO nanocomposites. It can be observed that the solvent used to dissolve the polymer affectes the quality of the film and foil. A homogeneous dispersion of ZnO nanoparticle can be observed in polymeric matrix PVDF under both film on quartz and foil. Also, the effective size of the ZnO nanoparticles was calculated via imageJ software and was found to be dependent on the preparation method. It is interesting to note that three dimensional (3D) flake-like ZnO nanoflakes ranged from 73 nm to 161 nm were formed on the film-like sample. However, an irregular-shaped ZnO nanoparticle ranged from 58 nm to 82 nm was observed in the foil-like sample.

Figure 4.6 (g) and (h) show the FESEM cross section images of the asprepared film on quartz like PVDF and PVDF/ZnO nanocomposite. So, the film thickness was confirmed by that images; it was around 9.6 µm -9.9 µm.

Figure 4.6

Top view FESEM images of (a) film based PVDF (b) foil like PVDF (c) film based PVDF/ZnO (d) foil like PVDF/ZnO (e) cross section view of film based PVDF, and (f) cross section view of film based PVDF/ZnO

64

4.3.3


Figure 4.7 (a) and (c) present the top-view FESEM images of the surface morphology of foil-like PVA as pure polymer, while (b) and (d) present the image of foil like PVA/ZnO nanocomposites, where two different magnifications; were used(10K and 30K).A homogeneous dispersion of ZnO nanoparticles can be observed in polymeric matrix-PVA. Also, the effective size of the ZnO nanoparticles was calculated via image J software. It is interesting to note that irregular-shaped ZnO nanoparticles ranged from 64 nm to 87 nm were formed.

Figure 4.7

4.3.4

Top view FESEM images with magnification of 10 k for (a) PVA and (b) PVA/ZnO; and magnification of 30 k for (c) PVA and (d)PVA/ZnO


Figure 4.8 (a) and (c) present the top-view FESEM images of the surface morphology of foil-like PS as pure polymer, while (b) and (d) present the image of foil like PS/ZnO nanocomposites; where two different magnifications; were used(10K and 30K). A homogeneous dispersion of ZnO nanoparticles can be observed in polymeric matrix-PS. Also, the effective size of the ZnO nanoparticle was calculated via imageJ

65

software. Interestingly, it is noted that an irregular-shaped ZnO nanoparticles ranged from 79 nm to 190 nm were formed.

Figure 4.8

4.3.5

Top view FESEM images with magnification of 10 k for (a) PS and (b) PS/ZnO; and magnification of 30 k for (c)PS and (d) S/ZnO

Polymer/ZnO/CuO Nanocomposites

Figure 4.9 presents the top-view FESEM images of the surface morphology of foil like polymer/ZnO/CuO nanocomposites for four types of polymer where two different magnifications; were used (10K and 50K) each type of nanocomposites. Figure (a) and (b) represent PMMA/ZnO/CuO nanocomposites, (c) and (d) represent PVDF/ZnO/CuO

nanocomposites,

(e)

and

(f)

represent

PVA/ZnO/CuO

nanocomposites and (g) and (h) represent PS/ZnO/CuO nanocomposites. A homogeneous dispersion of ZnO and CuO nanoparticles can be observed in the polymeric matrix, however that dispersion was better than the desperation obtained in polymer/ZnO which indicate that ZnO nanoparticles were modified when CuO nanoparticles were added to the nanocomposites. Also, the effective size of the ZnO and CuO nanoparticles was calculated via imageJ software. Interestingly, it is noted that irregular-shaped ZnO nanoparticles and CuO nanoparticles were formed. For PMMA/ZnO/CuO, the size ranged from 74nm to 289nm, For PVDF/ZnO/CuO, it

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ranged from 75 nm to 102 nm, For PVA ZnO/CuO, it ranged from 37 nm to 62 nm and for PS/ZnO/CuO it ranged from 78 nm to 107 nm.

Figure 4.9

Top view FESEM images of : (a) and (b) PMMA/ZnO/CuO nanocomposites; (c) and (d) PVDF/ZnO/CuO nanocomposites; (e)and(f) PVA/ZnO/CuO nanocomposites; (g) and(h) PS/ZnO/CuO nanocomposites

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4.4

ENERGY DISPERSIVE X-RAY (EDX) AND EDS MAPPING

To further confirm the composition and distribution of the elements existed in the samples, EDX survey scan and EDS mapping analysis were performed on the asprepared samples. The polymer/ZnO nanocomposites represented nanocomposites with high concentration of ZnO nanoparticles (15 wt %) which were used in preparation method.

4.4.1


Figure 4.10 shows the PMMA and PMMA/ZnO prepared as thin film and foil; (a – d) for pure PMMA and (e-h) for PMMA/ZnO nanocomposites. There were two elements detected in the pure samples, which were denoted as carbon (C) and oxygen (O); which arose from the carbon chain in the PMMA structure. In addition, silicon element (Si) was also noticed in the thin film sample which arose from quartz substrate as shown in Figure (a). This result suggests that the as-prepared samples were in high-purity condition. Whereas, there were three elements detected in the PMMA/ZnO samples, which were denoted as carbon (C), zinc (Zn) and oxygen (O). These elements arose from the carbon chain in PMMA structure and dopant source. In addition, a coarse film with uneven distribution of PMMA decorated ZnO nanoflakes was also observed via mapping analysis as shown in Figure (g). However, films with even distribution of PMMA were formed on the ZnO nanoparticle embedded foil sample as shown in Figure (h). This occurrence further supports the hypothesis mentioned earlier.

4.4.2


To check the composition and distribution of the elements existed in the as-prepared samples of film on quartz and foil like PVDF and PVDF/ZnO nanocomposites. EDX survey scan and EDS mapping analysis were performed on these samples. The results were shown in Figure 4.11 (a – d) for pure PVDF and (e-g) for PVDF/ZnO nanocomposites. Figure (a) and (b) showed the two elements detected in the pure polymer samples as film on quartz and foil respectively. They were carbon (C) and

68

fluorine (F) which arose from the PVDF structure. Whereas, the EDX spectra of PVDF/ZnO nanocomposites in Figure (e) and (f) show four elements: carbon (C) and fluorine (F) that represent the presence of PVDF structure; and, oxygen (O) and zinc (Zn) in the ZnO compound. This result also suggests that the as-prepared samples were in high-purity condition. Figure (c), (d), (g) and (h) show the EDS mapping of pure PVDF and PVDF/ZnO nanocomposite. The good spatial distributions shown were obviously from carbon, fluorine, oxygen, and zinc.

Figure 4.10

(a) EDX of thin film based PMMA (b)EDX of foil like PMMA, (c) EDS mapping of thin film based PMMA,(d)EDS mapping of foil like PMMA,(e)EDX of thin film based PMMA/ZnO, (f) EDX of foil like PMMA/ZnO, (g)EDS mapping of thin film based PMMA/ZnO, (h)EDS mapping of foil like PMMA/ZnO

69

Figure4.11

4.4.3

(a) EDX of film based PVDF, (b) EDX of foil like PVDF, (c) EDS mapping of film based PVDF, (d) EDS mapping of foil like PVDF, (e) EDX of film based PVDF/ZnO, (f) EDX of foil like PVDF/ZnO,(g) EDS mapping of film based PVDF/ZnO (h) EDS mapping of foil like PVDF/ZnO


Figure 4.12 (a) shows the EDX spectra of foil like PVA as pure polymer. Two elements shown in the figure which are carbon element (C) and oxygen element (O) represent the PVA structure.

70

Figure (b) shows the EDX spectra of foil like PVA/ZnO nanocomposites. The elements shown in the figure are carbon (C) element that represents PVA structure, oxygen element (O) which presents in the form of polymer (PVA) and ZnO compound and zinc element (Zn) which presents in the compound of ZnO.

Figure (c) and (d) show the EDS mapping of foil like PVA as pure polymer and foil like PVA/ZnO nanocomposites as foil respectively. Figure (c) shows good spatial distributions of carbon and oxygen in pure polymer sample. Whereas, the good spatial distributions shown in Figure (d) were obviously from carbon, oxygen, and zinc.

Figure 4.12

4.4.4

(a) EDX of PVA, (b) EDX of PVA/ZnO, (c) EDS mapping of PVA (d) EDS mapping of PVA/ZnO


Figure 4.13 (a) shows the EDX spectra of foil like PS as pure polymer. One element is shown in the figure which is carbon element (C) that represents PS structure. Figure 413 (b) shows the EDX spectra of foil like PS/ZnO nanocomposites. Three elements are shown in the figure which are carbon (C) element that represents PS structure and the presence of oxygen element (O) and zinc element (Zn) in the compound of ZnO.

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Figure 4.13 (c) and (d) show the EDS mapping of foil like PS as pure polymer and foil like PS/ZnO nanocomposites respectively. Figure (c) shows good spatial distribution of carbon in pure polymer sample. However,the good spatial distributions shown in Figure (d) were obviously from carbon, oxygen, and zinc.

Figure 4.13

4.4.5

(a) EDX of PS, (b) EDX of PS/ZnO, (c) EDS mapping of PS, (d) EDS mapping of PS/ZnO


To check the composition and distribution of the element existed in the as-prepared foil like polymer/ZnO/CuO nanocomposites, EDX survey scan and EDS mapping analysis were performed on these samples. Figure 4.14 (a) represents the EDX of PMMA/ZnO/CuO nanocomposites which showed the four elements detected in the sample which were carbon (C) that arose from the PMMA structure, oxygen(O) that also arose from the PMMA structure, zinc(Zn) and copper(Cu) from the dopant ZnO and CuO. Figure 4.14 (b) represents the EDX of PVDF/ZnO/CuO nanocomposites which showed the five elements detected in the sample which were carbon (C) and fluorine (F) that arose from the PVDF structure and oxygen(O), zinc(Zn), and copper(Cu) that arose from dopant ZnO and CuO. Figure 4.14 (e) represents the EDX

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spectrum of PVA/ZnO/CuO nanocomposite which showed the four elements detected which were carbon (C) that arose from the PVA structure, oxygen(O) that arose from the PVA structure and dopant ZnO and CuO, and zinc(Zn) and (Cu)elements from the compounds (ZnO) and (CuO).Figure 4.14 (f) represents the EDX spectrum of PS/ZnO/CuO nanocomposite that showed four elements detected in the sample which were carbon (C) that arose from the PS structure, and oxygen(O), zinc(Zn) and copper(Cu) which arose from the dopant ZnO and CuO. This result suggests that the as-prepared samples were in high-purity condition. Figure 4.14 (c), (d), (g) and (h) show the EDS mapping of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO, and PS/ZnO/CuO nanocomposites respectively. The good spatial distribution was shown obviously from the same elements in the EDX spectra of every nanocomposite.

Figure 4.14

(a) EDX of PMMA/ZnO/CuO,(b) EDX of PVDF/ZnO/CuO, (c) EDS mapping of PMMA/ZnO/CuO, (d) EDS mapping of PVDF/ZnO/CuO,(e) EDX of PVA/ZnO/CuO,(f) EDX of PS/ZnO/CuO,(g) EDS mapping of PVA/ZnO/CuO(h) EDS mapping of PS/ZnO/CuO

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4.5

PHOTOLUMINESCENCE (PL)

The PL spectra results of PMMA/ZnO, PVDF/ZnO, PVA/ZnO and PS/ZnO nanocomposites in the wavelength of 350 nm to 800 nm are shown in Figure (4.15). Excitation Xenon lamp at 325 nm was used. There was no peak in the curves of pure polymer (0 wt. %) for four polymers. But narrow and strong peaks, especially, at high concentration of ZnO nanoparticles were observed in the UV region at around 380 nm for the PMMA/ZnO and PVDF/ZnO nanocomposites and at 383nm and 385 nm for the PVA/ZnO and PS/ZnO nanocomposites respectively. The UV emissions were attributed to the exciton recombination of the ZnO nanoparticles(Cai et al. 2010) which increased with the increasing of ZnO contents. Besides that, there was a broad peak in the visible region which obviously shown at high concentration of ZnO nanoparticles (10 wt % and 15 wt %). The visible emissions are due to some defects such as oxygen vacancies(Liu et al. 2004) .15 wt% ZnO nanocomposite produced maximum emission due to the transferring of high energy from nanoparticles to polymers (Yahya and Rusop 2012) .For PVA/ZnO nanocomposites, the emission peak in the visible region was not clear compared to other three nanocomposites where the emission peak in the visible region was clear relative to the sharp peak in the UV region. Thus, a modification on the ZnO nanoparticles surface by the polymer matrix was indicated. The surface modification removed the defect states within ZnO (Sajimol 2012). The intensity at the UV peak was higher than that found at visible peak for all samples. All of the emission bands for polymer/ZnO nanocomposites were above the pure polymer bands. Those results corresponding to PL spectrum of ZnO generally showed narrow UV emissions from the exciton states and one or more broad visible emissions from the defects(Kulyk et al. 2009; Djurišić and Leung 2006; Sun et al. 2008).

74

Figure 4.15

PL of polymer/ZnO nanocomposites (a) PMMA/ZnO; (b) PVDF/ZnO; (c) PVA/ZnO; and (d) PS/ZnO

4.6

TRANSMITTANCE

4.6.1

Polymer/ZnO Nanocomposites

The measurements of linear transmittance spectra of as-prepared samples are shown in figure 4.16.The UV-Visible transmittance spectra of PMMA/ZnO nanocomposites as thin film is shown in Figure 4.16 (a). The transmittance of pure PMMA (0 wt %) as thin film was high in both UV and visible range, but the transmittance of PMMA/ZnO nanocomposites was proportionally inversed with the concentration of ZnO. Low

75

transmittance spectra was noticed in the UV region, especially, at high concentration of ZnO nanoparticles.

Figure 4.16 (b) shows the UV- Visible transmittance spectra of PMMA/ZnO nanocomposites as foils. The pure PMMA (0 wt %) as foil showed high transparency in the visible range, but relatively lower in the UV range. The effect of adding ZnO nanoparticles in PMMA was obvious; they have very low transmittance in the UV region and it decreased with increasing ZnO nanoparticles in the nanocomposites. Although ZnO nanoparticles have lower absorption spectrum in the visible region, the transmittance of PMMA/ZnO in this region decreased as ZnO concentration increased. This is due to the smoothness of the surface which causes relatively high reflectivity and leads to low transmittance in visible range. In addition, the scattering of ZnO nanoparticles could cause the loose of intensity in this spectral range(Junlin et al. 2010).

The behaviour of nanocomposites as thin films and foils were found to be similar. However, the values of transmittance of foils were less than the transmittance of thin films for the same concentration of ZnO nanoparticles. Those results are due to the thickness of foil (about 70μm) that was much greater than the thickness of thin film (800nm).

Figure 4.16 (c) and (d) demonstrate the transmittance of PVDF/ZnO nanocomposites as film on quartz and foil respectively. The transmittance of pure PVDF (0 wt %) was higher than that of PVDF/ZnO nanocomposites. The transmittance of pure polymer and nanocomposites showed more decrement in the UV region than in the visible region. The effect of adding ZnO nanoparticles was clear in the transmittance spectra of PVDF/ZnO nanocomposites. They have a very low transmittance in the UV region that decreased with increasing content of ZnO in nanocomposite. The transmittance of pure PVDF and PVDF/ZnO nanocomposites as film on quartz was lower than that of foil since the porosity formed in the film was higher than in foil because the evaporation of acetone that was used as a solvent happened at room temperature. So, the high porosity was acquired (Magalhães et al. 2010) and caused scattering of incident light.

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Figure 4.16 (e) and (f) show the transmittance spectra of PVA/ZnO and PS/ZnO nanocomposites as foil respectively. The behaviour of their spectra was similar to the behaviour of PMMA/ZnO and PVDF/ZnO. The transmittance of pure polymer (0 wt %) was higher than the transmittance of polymer/ZnO nanocomposites with different concentrations of ZnO. Besides the effect of increasing the content of ZnO nanoparticles was clear.

Overall, pure PMMA showed high transparency in the visible region (approximately 90%) but relatively lower in the UV region (around 80%).Meanwhile pure PVA showed transparency around 85% in the visible region and lower in the UV region. Pure PS showed transparency approximately 80% in the UV and visible regions. However pure PVDF showed the lowest transparency in the UV and visible regions for all nanocomposites, it was around 15 %. The effect of adding ZnO nanoparticles to all nanocomposites was clear. High concentration of ZnO nanoparticles showed the lowest transmittance in all four nanocomposites. These results are in accordance with the studies by Anžlovar et al. (2012);and Khan et al. (2014) for PMMA/ZnO, Indolia and Gaur (2013); and Bhunia et al. (2014) for PVDF/ZnO, Kumar et al. (2014); and Mallika et al. (2015) for PVA/ZnO ,and Jeeju et al. (2012)for PS/ZnO.

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Figure 4.16

4.6.2

Transmittance spectra of nanocomposites (a) thin film based PMMA/ZnO (b) foil like PMMA/ZnO (c) film on quartz based PVDF/ZnO (d) foil like PVDF/ZnO (e) foil like PVA/ZnO and (f)foil like PS/ZnO

Polymer/ZnO /CuO Nanocomposites

Figure 4.17 (a), (b), (c) and (d) represent the transmittance spectra of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO that were prepared as foil respectively. The effect of adding CuO nanoparticles in nanocomposites

was

observed

clearly.

The

transmittance

spectrum

of

polymer/ZnO/CuO nanocomposite showed lower value than nanocomposite that

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contained only ZnO nanoparticles for four different nanocomposites. The highest effect was noticed in PS/ZnO/CuO nanocomposites, while the lowest effect was noticed in the PVA/ZnO/CuO nanocomposite.

Figure 4.17

Transmittance spectra of nanocomposites (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO and (d) PS/ZnO/CuO

4.7

ABSORPTION

4.7.1

Polymer/ZnO Nanocomposites

The measurements of linear absorption spectra of the as-prepared samples are shown in Figure 4.18. The UV-Visible absorption spectra of PMMA/ZnO nanocomposites as thin film and foils are shown in Figure 4-18 (a) and (b) respectively. Pure PMMA (0 wt%) has low absorption in both regions, UV as well as visible, but PMMA/ZnO

79

nanocomposites have high absorbance in the UV region, in which the absorption peak was 375 nm for thin film samples and 377 nm for foil samples .The absorption peak increased when the content of ZnO nanoparticles increased. This happened in both types of nanocomposites. Besides that, the peak for foil samples was higher than thin film samples even with same content of ZnO. This means the absorption in foil samples was higher than film samples.

Figure 4.18 (c) and (d) demonstrate the UV-Vis absorption spectra of PVDF/ZnO nanocomposites as film on quartz and foil respectively. In both samples, film and foil, the absorption spectrum of pure PVDF (0 wt. %) was limited in the UV region, whereas the absorbance of PVDF/ZnO nanocomposites was enhanced by adding ZnO nanoparticles. The curves of nanocomposites with high ZnO concentration showed a clear absorption peak in the UV region and less absorption in the visible region. A broad absorption peak at approximately 370-374nm was observed in the nanocomposites as film, while a sharp absorption peak at 376 nm observed in the nanocomposites as foil belongs to ZnO nanoparticles behaviour. These result is in accordance with the result obtained by Indolia and Gaur (2013).

Figure4.18 (e) and (f) show the UV-Vis absorption spectra of PVA/ZnO and PS/ZnO nanocomposites as foil, respectively. The behaviour of their spectra was similar to the behaviour of previous nanocomposites. The absorption of pure polymer (0 wt%) was lower than the absorbance of polymer/ZnO nanocomposites with other concentrations of ZnO. Bsides, effect of increasing the content of ZnO nanoparticles became clear. The absorption peak of PVA/ZnO and PS/ZnO nanocomposites was at 377nm.

Overall, The effect of adding ZnO nanoparticles to all nanocomposites was clear due to the nano –size of the ZnO which increases the surface area, so a strong absorption is obtained (Yahya and Rusop 2012).High concentration of ZnO nanoparticles showed the highest absorption in all nanocomposites. The absorption peaks of all nanocomposites were observed at the range of 370-377nm which indicate the effect of polymer matrix due to the variation in the energy band gap(Abdelrazek and Holze 2011). PVA/ZnO nanocomposites with highest concentration(15 wt%)

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showed the highest peak out of the four nanocomposites as foils, it showed the highest decrement in optical

energy band

gap (Figure 4.25).Meanwhile PS/ZnO

nanocomposites showed the lowest peak of them, and PMMA/ZnO as thin film showed the lowest absorption peak for all nanocomposites.

Figure 4.18

Absorption spectra of nanocomposites (a) thin film based PMMA/ZnO, (b) foil like PMMA/ZnO, (c) film on quartz based PVDF/ZnO, (d) foil like PVDF/ZnO, (e) foil like PVA/ZnO, and (f) foil like PS/ZnO

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4.7.2


Figure 4.19 (a), (b), (c) and (d) represent the absorbance spectra of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO that were prepared as foil, respectively. The effect of adding CuO nanoparticles was clearly seen in all nanocomposites in which the absorption peak seem to be higher and the blue shift happened. The absorption of nanocomposites with CuO nanoparticles was greater than the nanocomposites that contained only ZnO nanoparticles in the UV and visible regions.

Figure 4.19

Absorption spectra of nanocomposites (a) PMMA/ZnO/CuO,(b) PVDF/ZnO/CuO, (c) PVA/ZnO/CuO and (d) PS/ZnO/CuO

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4.8

REFLECTANCE

4.8.1

Polymer/ZnO nanocomposites

Reflectance is measured by UV-Vis spectroscopy. (i.e., the reflectance is not derived from the equation T+A+R=1 because it is measured as a separate parameter using UV-Vis spectroscopy which has the feature to measure values of R). The measurements of the reflectance spectra are shown in Figure 4.20.The reflectance spectra of PMMA/ZnO nanocomposites as thin film and foil were shown in Figure 4.20 (a) and (b), respectively. The reflectance of pure PMMA (0 wt%) is the same in the UV and visible regions for both thin film and foil samples, and it has approximately the same value which is around 5%. The reflectance of PMMA/ZnO nanocomposites increased with the increase of ZnO nanoparticles in the nanocomposites in the UV and visible regions for thin film samples whereas for foil samples, the reflectance increased in the visible region and decreased in the UV region. In addition, the foil samples showed higher reflectance compared to thin film samples in the visible region due to thin film having a rougher surface than foil samples.

Figure 4.20 (c) and (d) show the reflectance spectra of PVDF/ZnO nanocomposites as film on quartz and foil respectively. Similar to the reflectance spectrum of pure PMMA, pure PVDF (0 wt%) has the same value in the UV and visible regions in both film and foil samples, with a value around 5%. The increase of the concentration of ZnO nanoparticles in the nanocomposites caused the reflectance spectra in film samples in both regions to increase, however in foil samples the reflectance decreased in the UV region and increased in the visible region. It can be clearly noticed that the reflectance of foil samples are higher than film samples that is due to the roughness of the film sample surface as observed in FESEM images.

Figure 4.20 (e) and (f) demonstrate the reflectance spectra of PVA/ZnO and PS/ZnO nanocomposites as foil, respectively. The reflectance spectra of pure polymer (0 wt %) of both types of polymer show a lower value than their nanocomposites. Generally, all nanocomposites show high reflectance in the visible region which

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decreased sharply in the UV region. All four nanocomposites as foil have the same behaviour in that their reflectance decreased sharply in the UV region, whereby the reflectance decreased with the increase of the concentration of ZnO nanoparticles. However, an opposite behaviour was noticed in the visible region whereby the reflectance increased as the concentration of ZnO nanoparticles in the nanocomposites increased.

Figure 4.20

Reflectance spectra of nanocomposites; (a)thin film PMMA/ZnO, (b) foil like PMMA/ZnO, (c) film on quartz based PVDF/ZnO, (d) foil like PVDF/ZnO, (e) foil like PVA/ZnO, (f) foil like PS/ZnO

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4.8.2


Figure 4.21(a), (b), (c), and (d) represent reflectance spectra of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO that were prepared as foil, respectively. The reflectance of nanocomposites containing CuO nanoparticles is shown to be lower than the reflectance of nanocomposites containing only ZnO nanoparticles in all four nanocomposites in the visible region. That decrement is due to the colour of the nanocomposites which changed, their colour attend to black colour. The PMMA/ZnO/CuO nanocomposites showed a lowering effect in reflectance, while the same decrement effect is observed in the other three nanocomposites. Meanwhile, an opposite behaviour is observed in the UV region, whereby the reflectance of nanocomposites increased when CuO was added.

Figure 4.21

Reflectance spectra of nanocomposites; (a) PMMA/ZnO/CuO (b) PVDF/ZnO/CuO (c) PVA/ZnO/CuO (d) PS/ZnO/CuO

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4.9

LINEAR ABSORPTION COEFFICIENT (α)

4.9.1

Polymer/ZnO nanocomposites

The linear absorption coefficient (α) for thin film samples was calculated using Yahya and Rusop (2012) and Abdullah et al. (2011) : 1

1

𝛼 = 𝑑 𝑙𝑛 𝑇

(4.1)

The linear absorption coefficient (α) for foil samples was calculated according to Hamdalla et al. (2015) : 1

𝛼 = 𝑑 𝑙𝑛

(1−𝑅) 𝑇

(4.2)

where d is the thickness of the sample, T is the transmittance ,and R is the reflectance that was obtained from the data of UV-Visible spectroscopy.

The thickness of the PMMA/ZnO thin film and the PVDF/ZnO film on quartz samples were determined by the FESEM image cross section; it was around 800 nm for PMMA/ZnO, and 9.6-9.9 μm for PVDF/ZnO. The thickness of the prepared samples like the flexible foil was determined using a digital micrometre at different places in each film and an average was determined; they were around 70 μm for PMMA/ZnO, 30µm for PVDF/ZnO,80-90 µm for PVA/ZnO, and 100-120 µm for PS/ZnO. Figure 4.22 (a) and (b) show the linear absorption coefficient (α) spectra of PMMA /ZnO nanocomposites as thin films and foils, respectively. The values of linear absorption coefficient (α )of thin film samples are much higher than the values for foil samples, it is around a 1000 times higher, which is due to α being proportionally inversed to the thickness of the sample whereby the thickness of the foil equals to around a 100 times of the thickness of a thin film. The highest value of α was shown by thin film samples in the UV region, but a lesser value was observed at

86

the visible region. Meanwhile, foil samples indicated a small deference in the values of α between the two regions. The values of α decreased with increasing wavelengths in all samples, however it seemed to be gradual in foils and faster in thin films. Figure 4.22 (c) and (d) show the linear absorption coefficient (α) spectra of PVDF /ZnO nanocomposites as film on quartz and foils, respectively. The values of the linear absorption coefficient (α) of film samples are lower than the values for foil samples, which is due to the difference of thickness of the samples whereby the thickness of the film is less than the thickness of the foil. Both samples of nanocomposites,film and foil show decreasing

value of

(α) with increase the

wavelength to visible region, but it can be observed that the α value of the foil samples decreased faster than film samples in the visible region. Figure 4.22(e) and (f) show the linear absorption coefficient α spectra of PVA/ZnO and PS/ZnO nanocomposites as foil, respectively. In both types of nanocomposites, the value of α in the UV region was higher than in the visible region. Generally in all nanocomposites, the results showed that α increased with the increase of ZnO nanoparticles contained, especially in short wavelengths. This means that there is a high probability of the transition to be directly allowed to take place (Nwanya et al. 2014) . The presence of absorption edges was observed, which is an indication of a good degree of crystallinity of nanocomposite samples (Al-Hussam and Jassim 2012).In addition, α depends on the wavelength of light that is actually being absorbed.

4.9.2

Polymer/ZnO/CuO nanocomposites

Figure 4.23 (a), (b), (c) and (d) represent linear absorption spectra of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO that are prepared as foil, respectively. The thickness of the prepared samples like the flexible foil are determined using a digital micrometre at different places in each film and an average was determined, the thickness was around 75 μm for PMMA/ZnO/CuO, 35µm for PVDF/ZnO/CuO, 90 µm for PVA/ZnO/CuO, and 120 µm for PS/ZnO/CuO.

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The effect of CuO nanoparticles is observed clearly in which the addition of the nanoparticles increased the linear absorption coefficient of the nanocomposites. The highest effect is noticed in the PS/ZnO/CuO nanocomposite.

Figure 4.22

Linear absorption coefficient spectra of nanocomposites, (a) thin film based PMMA/ZnO, (b) foil like PMMA/ZnO, (c) film on quartz based PVDF/ZnO, (d)foil like PVDF/ZnO, (e) foil like PVA/ZnO, (f) foil like PS/ZnO

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Figure 4.23

4.10

Linear absorption coefficient spectra of nanocomposites, (a) PMMA/ZnO/CuO, (b) PVDF/ZnO/CuO, (c) PVA/ZnO/CuO, (d) PS/ZnO/CuO

ENERGY GAP (Eg)

4.10.1 Polymer/ZnO nanocomposites

The value and nature of the energy gap (Eg) depends on the linear absorption coefficient(α) .To determine Eg , the equation from Al-ammar et al. (2013) and AlHussam and Jassim (2012) can be used : (αhν)m = A (hν −Eg)

(4.3)

where α is the absorption coefficient, ν is the frequency, h is Planck’s constant and Eg is the optical energy band gap between the valence band and the conduction band. A is a constant which depends on the transition probability whereas m is an index that describes the optical absorption process, theoretically, it is equal to 2 for direct allowed, 2/3 for direct forbidden , 1/2 for indirect allowed, and 1/3 for indirect

89

forbidden transition(Al-ammar et al. 2013; Sangawar and Golchha 2013). The value of m decides the nature of the Eg or transition involved. Few literatures(Mustafa 2013) revealed that the determination of the index m depends on the value of the absorption coefficient. Another literature (Dorranian et al. 2012) indicated that the estimation of m is obtained from the slope of the graph between log (α) and log (hυ). However, in general, the graph between (αhυ)m and photon energy (hυ) is first plotted, then the value of index m which gives the best linear graph is chosen (Alammar et al. 2013). By using Tauc’s plot, the value of the band gap (Eg) is calculated from the graph of (αhυ) 2 versus (hυ) .The value of Eg is determined by extrapolating the linear portion of the curve to the hυ - axis. Figures 4.24 (a) and (b) demonstrate the relationship between (αhυ)2 and photon energy (hυ) of PMMA/ZnO nanocomposites as thin films and foils, respectively. The optical energy gap of pure PMMA (0 wt% ZnO) is equal to 5.33 eV and 5.08 eV for thin film and foil samples, respectively. Obviously, it depends on the thickness of the sample which agrees with it (Švorčík et al.2008). It can be clearly seen that the values of the optical energy gap depend on the weight percentage of ZnO nanoparticles. There is a decrease in the optical energy gap of nanocomposites for both foils and thin films samples, with the increase of ZnO weight compared to pure PMMA, which is shown in Figure 4.25 .The proportion of the energy gap with the weight percentage of ZnO nanoparticles is inversed. This is due to the increase in the shift of the valence and the conduction band. In addition, the enhancement of carriercarrier interaction due to the high concentration of carrier in valence and conduction bands leads to a reduction in the bandgap (Irimpan 2008). Besides that, the presence of unsaturated defects, especially in foil samples, caused an increase in the density of localized states in the bandgap and then decreased the optical energy gap (El-Zahed et al. 2003).The decrement of the energy gap has the same value in thin film and foil samples that is equal to 0.5 eV, where energy gap equals to 4.83 eV in the thin film nanocomposite with higher weight percentage of ZnO (15wt%). In contrast, its value equals to 4.58 eV in the foil sample.

90

Figures 4.24 (c) and (d) show the relationship between (αhυ)2 and photon energy (hυ) of PVDF/ZnO nanocomposites as film on quartz and foil, respectively. The optical energy gap of pure PVDF (0 wt% ZnO) is equal to 5.5 eV and 5.88 eV for film and foil samples, respectively. As in PMMA/ZnO nanocomposites, the energy band gap is directly proportional with the increase of the weight percentage of ZnO nanoparticles, but the decrement of the energy gap has a different value in both film and foil samples with higher weight percentage of ZnO (15wt %) in nanocomposites. Their values equal to 4.5 eV and 5.1 eV in the film and foil samples respectively. These results correspond to the studies by Indolia and Gaur (2013) and Bhunia et al.(2014). Figures 4.24 (e) and (f) demonstrate the relationship between (αhυ)2 and photon energy (hυ) of PVA/ZnO and PS/ZnO nanocomposites as foil. The optical energy gap of pure PVA and pure PS (0 wt% ZnO) is equal to 5.65 eV and 4.5 eV, respectively while the values of the higher weight percentage of ZnO (15wt %) in nanocomposites equal to 4.75 eV and 3.9 eV respectively.

It was noticed that the optical band gap of the samples were red shifted, and the comparison between all nanocomposites is shown in Figure 4.25, The highest energy gap of pure polymer (0 wt% ZnO) is seen in pure PVDF as foil which equals to 5.88 eV, while the lowest energy gap is shown by pure PS as foil which equals to 4.5 eV. The highest decrement is shown by PVDF/ZnO nanocomposites as film on quartz, while the lowest decrement is shown by PMMA/ZnO as thin film and foil.

91

Figure 4.24

Energy gap of nanocomposites; (a)thin film based PMMA/ZnO; (b) foil like PMMA/ZnO; (c) film on quartz based PVDF/ZnO; (d) Foil like PVDF/ZnO; (e) Foil like PVA/ZnO; (f) Foil like PS/ZnO

92

Figure 4.25

Energy gap of polymer-ZnO nanocomposites versus ZnO nanoparticles %

4.10.2 Polymer/ZnO/CuO nanocomposites

Figure 4.26 (a), (b), (c) and (d) represent the energy band gap of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO prepared as foil, respectively. It was noticed that the optical band gap of the samples were red shifted. The adding of CuO nanoparticles to nanocomposites of polymer/ZnO caused a decrease in the energy gap. The decrement is shown clearly in all four nanocomposites. The values of the energy gap of polymer/ZnO and polymer/ZnO/CuO nanocomposites are tabulated in Table 4.1.

93

Table 4.1 Energy gap of polymer/ZnO (10 wt %) and polymer/ZnO (10 wt %) /CuO (1 wt %)

Nanocomposites

PMMA

PVDF

PVA

PS

Polymer (pure)

(Eg) eV

5.08

5.88

5.65

4.5

Polymer/ZnO

(Eg) eV

4.8

5.15

4.92

4.25

Polymer/ZnO/CuO(Eg) eV

4.5

4.9

4.55

3.85

Figure 4.26

Energy gap of nanocomposites; (a) PMMA/ZnO/CuO; PVDF/ZnO/CuO; (c) PVA/ZnOCuO; (d) PS/ZnO/CuO

(b)

94

4.11

EXTINCTION COEFFICIENT (K)


The extinction coefficient (K) was calculated using (Sangawar and Golchha 2013; Abdul Nabi et al. 2014): 𝐾=

𝛼𝜆 4𝜋

(4.4)

where α is the linear absorption coefficient and λ is the wavelength. Figure 4.27 (a) and (b) show the extinction coefficient of PMMA/ZnO nanocomposites as thin film and foil respectively. The curves of pure PMMA (0 wt %) showed a gradual increment in the UV and visible regions that is observed clearly in the thin film sample. The curves of nanocomposites showed increment in the UV region until the peak absorption of ZnO, and then they decreased sharply until the end of the visible region in the thin film samples with high concentration of ZnO (10 and 15 wt %), but they increased gradually in the visible region with other concentration of ZnO. However the samples of foil showed gradual increment in the visible region. Figure 4.27 (c) and (d) show the extinction coefficient of PVDF/ZnO nanocomposites as film on quartz and foil, respectively. The curves of pure PVDF show a gradual increment in the UV and visible regions in both; film and foil. The curves of PVDF/ZnO nanocomposites showed an increase in the UV region until the peak absorption of ZnO and then decreased sharply in foil samples and they have approximately the same value at the visible region; a simple decrease was shown in film samples, which then gradually increased in the visible region. Figure 4.27(e) and (f) show the extinction coefficient of PVA/ZnO and PS/ZnO nanocomposites respectively. The curve of pure polymer showed the same value in the UV and visible regions for both polymers, but the curves of nanocomposites showed a sharp increase in the UV region until the peak absorption of ZnO, and then decreased sharply in the visible region and gradually decreased, in PVA/ZnO nanocomposites. A gradual increase in the UV region until peak absorption followed by a gradual decrease in the visible region is noticed in the PS/ZnO nanocomposites.

95

Generally, the extinction coefficient depends on the absorption of the sample, so the high absorption for a particular wavelength means high extinction coefficient for that wavelength.

4.11.2 Polymer/ZnO/CuO Nanocomposites

Figure 4.28 (a), (b), (c) and (d) represent the extinction coefficient of PMMA/ZnO/CuO, PVDF/ZnO/CuO, PVA/ZnO/CuO and PS/ZnO/CuO prepared as foil, respectively. The extinction coefficient curves of nanocomposites with CuO nanoparticles are higher than the curves of nanocomposites which contain only ZnO nanoparticles.

Figure 4.27

Extinction coefficient of nanocomposites; (a) thin film based PMMA/ZnO; (b) foil like PMMA/ZnO; (c) film on quartz based PVDF/ZnO; (d) foil like PVDF/ZnO; (e) foil like PVA/ZnO; (f) foil like PS/ZnO

96

Figure 4.28

4.12

Extinction coefficient of nanocomposites; (a) PMMA/ZnO/CuO; (b) PVDF/ZnO/CuO; (c) PVA/ZnO/CuO; (d) PS/ZnO/CuO

REFRACTIVE INDEX (n)

4.12.1 Polymer/ZnO nanocomposites The refractive index (n) was calculated using (Al-ammar et al. 2013;Sangawar and Golchha 2013):

R

(n  1) 2  K 2 (n  1) 2  K 2

(4.5)

where R: is reflectance, n: is the linear refractive index, and K:is the extinction coefficient.

97

When K 2 1.7 Zo

for PVA/ZnO nanocomposites, Figure 4.35, the

occurrence of thermal nonlinearity was clearly shown

and the nonlinear effect

observed represented the third-order process (Mathews et al. 2007; Tamgadge et al. 2014). Usually, the thermal effect has a dominant contribution to the nonlinear optical mechanism when continuous wave laser is used. On the other hand, electronic contribution dominates in nonlinear optical mechanism when pulsed laser is used, as in the current work, except in some limited cases in which thermal contribution will dominate. This limited case took place when it satisfied the condition that the thermal

103

response time of the sample will be in the range of pulse duration of pulsed laser (Boyd 2007). Even when using pulsed laser of short pulses, the cumulative thermooptical effects will arise in the sample in which its refractive index is temperature dependent (i.e. dn/dT ≠0) (Nalda et al. 2002) hence thermal lens is formed. Consequently, it contributes to the result of closed aperture Z-scan.The expression of a nonlinear refractive index n2 of thermal effect is (Boyd 2007; Tamgadge et al. 2015):

𝑛2 = (

𝑑𝑛

) 𝑑𝑇

where (

𝑑𝑛 𝑑𝑇

refractive

𝛼 𝜔𝑜2

(4.10)

𝐾

) is the thermo-optical coefficient which is defined as the variation of the index

with

temperature(Ghosh

1998),α is

the

linear

absorption

coefficient. 𝜔𝑜 is the radius of the laser beam at the focus,and 𝑘 is the thermal conductivity (𝑘 = [𝜌 𝐶𝑝 ]𝐷), where ρ is the density, 𝐶𝑝 is the specific heat at constant pressure, (𝜌 𝐶𝑝 ) represents the heat capacity, and D is the thermal diffusivity. These parameters affected the determination of thermal response time. As ΔZP-V was more than 1.7 Zo, the thermal response time of PVA/ZnO nanocomposite was considered in the range of pulse duration; hence thermal contribution was dominant (Boyd 2007).

By using the variable transmittance values of the closed-aperture Z-scan, the nonlinear phase shift ∆Φ° and the nonlinear refractive index n2 were determined. ∆Φ° can be calculated using (Dorranian et al. 2012; Sheik- Bahae et al. 1990): ∆𝑇

𝑃−𝑉 ∆Φ° = 0.466(1−𝑆) 0.25

(4.11)

ΔTp v : is the change in transmittance between the peak and valley in a closed aperture Z-scan, and is defined as: ∆𝑇𝑃𝑉 = 𝑇𝑃− 𝑇 𝑉

(4.12)

where Tp and Tv are the normalized peak and valley transmittances as seen in Figure 4.31, 4.33, 4.35,and 4.37.

104

The ratio of the light passing through the aperture to the light in front of the aperture was defined as S.

S can also be defined as the linear transmittance of the aperture, which can be calculated according to (Dorranian et al. 2012) : 𝑆 = 1 − exp(−2𝑟𝑎2 /𝑤𝑎2 )

(4.13)

where ra is the radius of the aperture. wa the beam waist on the aperture, wa is calculated as: 𝑤𝑎2 = 𝑤°2 [1+( 𝑧𝑎 / 𝑧° ) 2]

(4.14)

here wo is the beam waist at the focus, Za is the distance between the aperture and focal point,and Zo is the diffraction length of the beam, (Rayleigh range), that was calculated earlier.

Choosing the size of the aperture can help produce better results, with an S value of 0.2, which follows most reported experiments (Stryland and Sheik-Bahae 1998) that used 0.1< S< 0.5. Thus the values of ∆Φ° can be calculated and was used to calculate the nonlinear refractive index n2 using the equation (Irimpan 2008) :

𝑛2 =

𝜆 ∆Φ° 2 𝜋𝐼° 𝐿𝑒𝑓𝑓

(4.15)

where λ is the wavelength of the laser source, Io is the irradiance of the laser beam at the focus, and 𝐿𝑒𝑓𝑓 is the effective length of the sample, which is calculated as ( Sheik- Bahae et al. 1990; Haripadmam et al. 2012):

𝐿𝑒𝑓𝑓 =

1−𝑒 −𝛼𝑑 𝛼

(4.16)

where 𝛼 is the linear absorption coefficient and d is the thickness of the sample.

105

The values of ∆Φ° , 𝐿𝑒𝑓𝑓 and n2 of all polymer/ZnO nanocomposites are listed in the Tables (4.2, 4.3, 4.4, 4.5, 4.6, 4.7). The obtained values of n2 for as-prepared samples are in order 10-12 cm2/W that are larger than values in previous studies which was in order 10-14 cm2/W for PS/ZnO nanocomposite prepared as thin film (Jeeju et al. 2012), 10-13 cm2/W for PMMA/ZnO nanocomposite prepared as thin film (Jeeju 2012), and 10-14 cm2/W for PVA/ZnO nanocomposite prepared as thin film (Jeeju et al. 2014).

Figure 4.31

The normalized transmittance as a function of sample position in the closed aperture Z-scan for thin film based PMMA/ZnO nanocomposites: (a)ZnO(1 wt%) and (b) ZnO(15 wt%),and foil like PMMA/ZnO (c) ZnO(1 wt%)and (d) ZnO(15 wt%)

106

Figure 4.32

Nonlinear refractive index (n2) as a function of ZnO wt% for nanocomposites (a)thin film based PMMA/ZnO (b) foil like PMMA/ZnO

Figure 4.33

The normalized transmittance as a function of sample position in the closed aperture Z-scan for film on quartz based PVDF/ZnO nanocomposites (a)ZnO(1 wt%) and (b) ZnO(10 wt%),and foil like PVDF/ZnO (c) ZnO(1 wt%)and (d) ZnO(10 wt%).

107

Figure 4.34 Nonlinear refractive index (n2) as a function of ZnO wt% for nanocomposites (a)as film on quartz based PVDF/ZnO (b) foil like PVDF/ZnO

Figure 4.35

The normalized transmittance as a function of sample position in the closed aperture Z-scan for foil like PVA/ZnO nanocomposites ;(a) ZnO (1 wt%) ;(b) ZnO (15 wt%)

108

Figure 4.36

Nonlinear refractive index (n2) as a function of ZnO (wt %)for foil like PVA/ZnO nanocomposites

Figure 4.37

The normalized transmittance as a function of sample position in the closed aperture Z-scan for foil like PS/ZnO nanocomposites; (a) ZnO (1 wt %) ; (b) ZnO(15 wt%)

109

Figure 4.38

Nonlinear refractive index (n2) as a function of ZnO (wt%) for foil like PS/ZnO nanocomposites

4.14.2 Polymer/ZnO/CuO Nanocomposites

Figure 4.39 shows the measurements of the normalized transmittance versus the sample position in the closed-aperture Z-scan, (a),(c),(e) and (g) for polymer/ZnO with 10 wt% concentration of ZnO nanoparticles and (b),(d).(f) and (h) for polymer/ZnO/CuO with 10 wt% of ZnO nanoparticles in addition to 1 wt% of CuO nanoparticles. The effect of adding CuO nanoparticles for all nanocomposites is clear whereby, the value of ΔTP-V increased. The highest decrement is shown in the PS/ZnO/CuO nanocomposite, whereas the lowest decrement is shown in the PVDF/ZnO/CuO nanocomposite.

The PVA/ZnO/CuO nanocomposite as shown in Figure 4.39

(e) and (f),

display the same special behaviour which was shown in the PVA/ZnO nanocomposite, (Figure 4.35) that is different from the other three nanocomposites when the ΔZP-V was more than 1.7 Zo that the occurrence of thermal nonlinearity was clearly shown. For other three nanocomposites, ΔZ

P-V

= 1.7 Zo, therefore the presence of pure

electronic third-order nonlinearity was confirmed.

110

The values of ∆Φ° , 𝐿𝑒𝑓𝑓 and n2 of all polymer/ZnO/CuO nanocomposites are calculated using equations 4-11,4-15 and 4-16,and they are listed in Tables (4.8, 4.9, 4.10 and 4.11).

Figure 4.39

The normalized transmittance as a function of sample position in the closed aperture Z-scan for foil like nanocomposites (a) PMMA/ZnO (b) PMMA/ZnO/CuO (c) PVDF/ZnO (d) PVDF/ZnO/CuO (e) PVA/ZnO (f) PVA/ZnO/CuO (g)PS/ZnO (h)PS/ZnO/CuO

111

4.15

NONLINEAR ABSORPTION COEFFICIENT (OPEN APERTURE)

4.15.1 Polymer/ZnO Nanocomposites

The absorption of the material A is intensity dependant ( Mahdi and Dawood 2009): 𝐴 = 𝛼 + 𝛽𝐼

(4.17)

where β is the nonlinear absorption coefficient related to the intensity I and α is the linear absorption coefficient. By removing the aperture, open aperture Z-scan was used to investigate the nonlinear absorption coefficient. During this process, the transmitted beam was collected by the detector without any limitation. Nonlinear absorption may be categorized into four forms: two-photon absorption (TPA), multiphoton absorption, saturable absorption (SA) and reverse absorption (RA).

In order to evaluate the nonlinear absorption coefficient, Figure 4.40 (a) and (b) show the open-aperture Z-scan of PMMA/ZnO nanocomposites as thin film and foil respectively. Figure 4.42 (a) and (b) show the open-aperture Z-scan of PVDF/ZnO nanocomposites as film on quartz and foil respectively. Figure 4.44 and 4.46 show the open-aperture Z-scan of PVA/ZnO and PS/ZnO nanocomposites as foil, respectively. The transmittance was sensitive to nonlinear absorption. The transmittance showed a minimum value at the focus Z=0 and then increased steadily on both sides of the focus, that represented a valley. The valley for PMMA/ ZnO nanocomposite as foil with different concentrations of ZnO nanoparticles is deeper than the nanocomposite as thin film. The valley for PVDF/ ZnO nanocomposites as film on quartz for all concentrations of ZnO nanoparticles is deeper than the valley of nanocomposite as foil .From the resulting data as comparison between four nanocomposites as foil, the valley of PMMA/ ZnO nanocomposites is found to be deeper than the other three nanocomposites, whereas the PVDF/ZnO nanocomposites had the lowest deep valley among all the studies of nanocomposites. This indicates that PMMA/ZnO nanocomposite exhibits stronger nonlinear absorption performance than other nanocomposites. The two-photon absorption caused the change in the transmittance in the samples (Altaify 2009). During the calculated energy gap, it shifted to the lower

112

value as the ZnO concentration was increased. Here, the shift (Vinitha et al.2010) of the energy gap enhanced two photon absorptions. Thus, the results of the open aperture Z-scan showed that the nonlinear absorption coefficient values were enhanced with the increase in the concentration of ZnO nanoparticles, which is clearly shown in Figure 4.41, 4.43, 4.45, and 4.47. These results correspond with the results by Sreeja et al. (2010) and Haripadmam et al. (2012) for PMMA/ZnO although they used nanocomposites prepared as thin film.

From the data, a symmetric valley indicates a positive nonlinear absorption coefficient β for all types of nanocomposites which represents two-photon absorption. To determine the value of nonlinear absorption coefficient β, the following equation (Vinitha et al. 2010) was used:

𝛽=

2√2 Δ𝑡

(4.18)

𝐼𝑜 𝐿𝑒𝑓𝑓.

where Δt is the one valley value obtained from the data of the open Z-scan curve. The values of 𝛽 calculated from equation (4.18) for all types of nanocomposites are listed in Tables (4.2, 4.3, 4.4, 4.5, 4.6, and 4.7). The obtained values of β for as-prepared samples are in order 10-7 cm/W that are larger than values obtained in previous studies which was in order 10-9

cm/W for PS/ZnO

nanocomposite prepared as thin film (Jeeju et al. 2012), and 10-9 PMMA/ZnO nanocomposite prepared as thin film (Jeeju 2012).

cm/W for

113

Figure 4.40

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites (a) thin film based PMMA/ZnO (b) foil like PMMA/ZnO

Figure 4.41

Nonlinear absorption coefficient (β) as a function of ZnO (wt%) for nanocomposites (a) thin film based PMMA/ZnO (b) foil like PMMA/ZnO

114

Figure 4.42

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites: (a) film on quartz based PVDF/ZnO (b) foil like PVDF/ZnO

Figure 4.43

Nonlinear absorption coefficient (β) as a function of ZnO (wt %) for nanocomposites (a) film on quartz based PVDF/ZnO (b) foil like PVDF/ZnO.

115

Figure 4.44

Normalized transmittance as a function of sample position in the open aperture Z-scan for foil like PVA/ZnO nanocomposites

Figure 4.45

Nonlinear absorption coefficient (β) as a function of ZnO (wt%) for foil like PVA/ZnO nanocomposites.

116

Figure 4.46

Normalized transmittance as a function of sample position in the open aperture Z-scan for foil like PS/ZnO nanocomposites

Figure 4.47

Nonlinear absorption coefficient (β) as a function of ZnO (wt%) for foil like PS/ZnO nanocomposites

117

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PMMA/ZnO nanocomposites as thin film for different concentrations of ZnO nanoparticles

Table 4.2

Nanocomposites

as

PMMA/ZnO

PMMA/ZnO

PMMA/ZnO

PMMA/ZnO

PMMA/ZnO

thin film 1wt% Leff ×

10-4

3wt%

5wt%

10wt%

15wt%

cm

7.5840

7.4798

7.4304

7.1365

6.8603

cm/W

1.1188

2.7310

4.2295

6.1652

8.7039

0.1815

0.2178

0.2722

0.3630

0.4765

-2.2518

-2.7398

-3.4476

-4.7861

-6.5347

(e s u)

-1.7694

-2.2769

-3.0255

-4.4290

-6.4330

Im (3) ×10-7 (e s u)

0. 3719

0.9603

1.5705

2.4140

3.6256

 (3) 

0.3724

0.9605

1.5708

2.4144

3.6262

β

×10-8

Δɸ◦ ×10-13

n2

Re

(3)

cm2/W ×10-9

×10-7 (e s u)

Values of Leff , β, Φ° ,n2 , Re χ(3) , Im χ(3) and |χ(3) | for PMMA/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

Table 4.3

Nanocomposites

as

foil

PMMA/ZnO

PMMA/ZnO

PMMA/ZnO

3wt%

5wt%

10wt%

15wt%

5.9454

5.6798

5.1139

4.1394

2.8052

0.4757

0.7747

1.4137

2.1258

4.2572

0.4084

0.6353

0.7261

1.1572

1.4522

-0.6463

-1.0525

-1.3360

-2.6301

-4.8704

1wt%

Leff × 10-3 cm β

PMMA/ZnO

×10-7

cm/W

Δɸ◦ ×10-12

n2

cm2/W

PMMA/ZnO

Re

(3)

×10-8

(e s u)

-0.5868

-1.0905

-1.4592

-3.1127

-6.1438

Im

(3)

×10-6

(e s u)

0. 1827

0.3396

0.6533

1.0645

2.2723

×10-6

(e s u)

0.1828

0.3398

0.6534

1.0650

2.2731



(3)



Table 4.4

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PVDF/ZnO nanocomposites as film on quartz for different concentrations of ZnO nanoparticles

Nanocomposites as

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

film 1wt%

3wt%

cm

3.0405

2.8294

2.4778

2.2489

2.2339

cm/W

1.8604

4.9982

9.3855

13.9738

17.3032

Δɸ◦

0.1429

0.1656

0.1883

0.2269

0.2859

n2×10-13 cm2/W

-4.4232

-5.5076

-7.1507

-9.4920

-12.0403

Re (3) ×10-9 (e s u)

-2.7305

-3.5765

-4.8667

-9.2686

-13.1754

Im (3) ×10-7 (e s u)

0. 4859

1.3733

2.7028

5.7736

8.0119

 (3) 

0.4867

1.3738

2.7033

5.7743

8.0130

Leff × β

10-4

×10-8

×10-7 (e s u)

5wt%

8wt%

10wt%

118

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PVDF/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

Table 4.5

Nanocomposites

as

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

PVDF/ZnO

foil Leff × β

1wt%

3wt%

5wt%

8wt%

cm

1.2926

1.2046

1.0796

1.0515

0.9857

cm/W

0.2431

0.6522

1.1061

1.9427

3.0287

0.0102

0.1248

0.1474

0.1815

0.2042

-0.7432

-0.9747

-1.2852

-1.6242

-1.9490

10-3

×10-7

Δɸ◦ ×10-12

n2

cm2/W

10wt%

Re

(3)

×10-8

(e s u)

-0.4935

-0.6804

-0.9995

-1.4116

-2.0073

Im

(3)

×10-6

(e s u)

0. 0683

0.1926

0.3640

0.7144

1.3199

×10-6 (e s u)

0.0684

0.1928

0.3641

0.7145

1.3200

 (3) 

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PVA/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

Table 4.6

Nanocomposites

as

PVA/ZnO

PVA/ZnO

PVA/ZnO

PVA/ZnO

PVA/ZnO

1wt%

3wt%

5wt%

10wt%

cm

7.5386

6.5690

5.9343

5.0443

4.3396

cm/W

0.2084

0.5740

1.1650

1.7444

2.5346

0.3176

0.4538

0.6807

1.1345

1.4068

-0.3964

-0.6499

-1.0791

-2.1160

-3.0499

-0.2855

-0.4968

-0.9706

-2.0139

-3.8957

s u)

0. 0635

0.1857

0.4438

0.7025

1.3699

(e s u)

0.0635

0.1857

0.4435

0.7028

1.3704

foil Leff × β

10-3

×10-7

Δɸ◦ ×10-12

n2

cm2/W

Re

(3)

×10-8

Im

(3)

×10-6 (e



(3)



(e s u)

×10-6

15wt%

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PS/ZnO nanocomposites as foil for different concentrations of ZnO nanoparticles

Table 4.7

Nanocomposites

as

PS/ZnO

PS/ZnO

PS/ZnO

PS/ZnO

PS/ZnO

1wt%

3wt%

5wt%

10wt%

15wt%

Leff × 10-3 cm

10.6449

10.1404

9.1771

8.9270

5.3114

β ×10-7 cm/W

0.1180

0.3099

0.5992

0.8449

1.7750

Δɸ◦

0.3176

0.3857

0.4538

0.5218

0.5672

-0.2807

-0.3578

-0.4652

-0.5500

-1.0047

(e s u)

-0.2448

-0.3181

-0.4373

-0.5659

-1.3401

Im (3) ×10-6 (e s u)

0. 0435

0.1165

0.2383

0.3678

1.0017

 (3) 

0.0436

0.1166

0.2383

0.3678

1.0018

foil

×10-12

n2

Re

(3)

cm2/W ×10-8

×10-6 (e s u)

119


Figure 4.48 shows the measurements of the normalized transmittance versus the sample position in the open-aperture Z-scan for polymer/ZnO which contains 10 wt% ZnO and polymer/ZnO/CuO which contains 10 wt% ZnO in addition to 1 wt% CuO, (a) represent PMMA/ZnO and PMMA/ZnO/CuO, (b) represent PVDF/ZnO and PVDF/ZnO/CuO,(c) represent PVA/ZnO and PVA/ZnO/CuO and (d) represent PS/ZnO and PS/ZnO/CuO.The effect of adding CuO nanoparticles for all nanocomposites is clear. The valley of the nanocomposite that contains CuO nanoparticles in addition to ZnO nanoparticles is deeper than nanocomposite containing only ZnO nanoparticles that is due to the red shift of energy gap which is clearly noticed in Figure 4.26, it shifted to the lower value as the CuO nanoparticles was added in every one of polymer/ZnO/CuO nanocomposites. Here the shift (Sreeja et al. 2010) of the energy gap enhanced two photon absorptions. Thus, the results of the open aperture Z-scan of the polymer/ZnO/CuO nanocomposites show that the nonlinear absorption coefficient values were enhanced with the addition of CuO nanoparticles. The values of 𝛽 of polymer/ZnO/CuO nanocomposites are calculated using equation (4.18), and they are listed in Tables (4.8, 4.9, 4.10 and 4.11).

Figure 4.48

The normalized transmittance as a function of sample position in the open aperture Z-scan for nanocomposites foil: (a)PMMA /ZnO and PMMA/ZnO/CuO (b)PVDF/ZnO and PVDF/ZnO/CuO (c) PVA/ZnO and PVA/ZnO/CuO (d)PS/ZnOand PS/ZnO/CuO

120

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PMMA/ZnO(10 wt%) and PMMA/ZnO(10 wt%)/CuO(1 wt%) nanocomposites as foil

Table 4.8

Nanocomposites

PMMA/ZnO

PMMA/ZnO/ CuO

Leff × 10-3 cm

4.139

2.864

β

2.125

4.114

1.157

1.588

× 10-7 cm/W

Δɸ◦ n2

× 10-12 cm2/W

Re (3) ×10-8

-2.630

-5.217

(e s u)

-3.1127

-4.5589

Im (3) ×10-6 (e s u)

1.0645

1.5213

1.0650

1.5220

×10-6

 (3) 

(e s u)

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PVDF/ZnO(8 wt%) and PVDF/ZnO(8 wt%)/CuO (1 wt%)nanocomposites as foil

Table 4.9

Nanocomposites

PVDF/ ZnO

PVDF/ZnO/ CuO

Leff × 10-3 cm

1.051

0.862

β

1.942

4.740

0.181

0.294

-1.624

-3.220

Re (3) ×10-8 (e s u)

-1.4116

-2.2535

Im (3) ×10-6 (e s u)

0.7144

1.4037

0.7145

1.4039

× 10-7 cm/W

Δɸ◦ × 10-12 cm2/W

n2

×10-6

 (3) 

(e s u)

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PVA/ZnO(10 wt%) and PVA/ZnO(10 wt%)/CuO(1 wt%) nanocomposites as foil

Table 4.10

Nanocomposites

PVA/ ZnO

PVA/ ZnO/ CuO

Leff × 10-3 cm

5.044

4.904

β

1.744

2.178

1.134

1.996

-2.116

-3.830

Re (3) ×10-8 (e s u)

-2.0139

-3.1275

Im (3) ×10-6 (e s u)

0.7025

0.7527

0.7028

0.7533

× 10-7 cm/W

Δɸ◦ n2

 (3) 

× 10-12 cm2/W

×10-6

(e s u)

121

Values of Leff , β, Φ° ,n2, Re χ(3) , Im χ(3) and |χ(3) | for PS/ZnO(10 wt%) and PS/ZnO(10 wt%)/CuO (1 wt%)nanocomposites as foil

Table 4.11

Nanocomposites

PS/ ZnO

Leff × 10-3 cm

8.927

4.283

β

0.844

3.118

0.521

1.996

-0.550

-4.386

Re (3) ×10-8 (e s u)

-0.5659

-3.8574

Im (3) ×10-6 (e s u)

0.3678

1.1604

0.3678

1.1611

× 10-7 cm/W

Δɸ◦ n2

 (3) 

4.16

PS/ ZnO/ CuO

× 10-12 cm2/W

×10-6

(e s u)

THIRD-ORDER NONLINEAR OPTICAL SUSCEPTIBILITY (3)

(3) is the third order susceptibility that explains processes like third harmonic generation. The nonlinear susceptibility should be practically large as the main step to select materials for nonlinear optical applications. Its value varies with different of nonlinear materials and normally, the third order susceptibility of semiconductors is in the range of 10-13 – 10-10 e s u. The real part of the third order susceptibility is dependent on the nonlinear refraction index n2, while the imaginary part of the third order susceptibility is dependent on the nonlinear absorption coefficient β (Nagaraja et al. 2013).The real and imaginary parts of the third – order nonlinear optical susceptibility (3) can be calculated by using the refractive index n2 and nonlinear absorption coefficient  , according to the following equations (Mathews et al. 2007): 𝑅𝑒 χ(3) (𝑒 𝑠 𝑢) = 10−4

𝐼𝑚 χ(3) (𝑒 𝑠 𝑢) = 10−2

where

o

𝜀○ 𝑐 2 𝑛○2 𝜋

𝜀○ 𝑐 2 𝑛○2 𝜆 4𝜋2

𝑛2 (𝑐𝑚2 ⁄𝑊 )

(4.19)

𝛽 (𝑐𝑚⁄𝑊 )

(4.20)

is the dielectric constant in a vacuum, c is the speed of light in a vacuum, no

is the linear refractive index and λ is the laser wavelength.

122

The absolute value of the third-order nonlinear optical susceptibility χ(3) can be calculated by:  (3) = [(Re ((3)))2 + (Im ((3)) )2]1/2

(4.21)

third order nonlinearity is usually measured in e s u, and sometimes in m2/V2.The values of

Re χ(3) , Im χ(3) and the absolute value of χ(3) for different concentrations

of ZnO nanoparticles in polymer/ZnO are tabulated in Tables 4.2, 4.3, 4.4 , 4.5, 4.6 and 4.7,while the values of polymer/ZnO/CuO nanocomposites are tabulated in Tables 4.8, 4.9, 4.10 and 4.11. The obtained values of third optical susceptibility χ(3) for asprepared samples are in order 10-6 e s u that are larger than the values obtained in previous studies which was in order 10-12 e s u for PS/ZnO nanocomposite prepared as thin film (Jeeju et al. 2012),10-13 and 10-11 e s u for PMMA/ZnO nanocomposite prepared as thin film by Kulyk et al. (2009) and Jeeju (2012) respectively.

4.17

OPTICAL LIMITING

This section presents the optical limiting properties of the as-prepared samples like flexible foil. The optical limiting threshold of PMMA/ZnO nanocomposites with different concentrations of ZnO nanoparticles was measured. The obtained results show that the nanocomposite with the highest concentration has the lowest value of optical limiting threshold, which means it was the best optical limiter. Therefore, the values of optical limiting threshold were measured in four nanocomposites; PMMA/ZnO, PVDF/ZnO, PVA/ZnO, and PS/ZnO with the highest concentration of ZnO nanoparticles. Then, the ability of adding CuO nanoparticles for the nanocomposites to enhance their optical limiting behaviour was evaluated; hence, the optical limiting threshold of these nanocomposites was measured which resulted in the enhancement of the optical limiting property.

123

4.17.1 Optical limiter

The suitability of a sample to be used as an optical limiter depends on the sign and the value of the nonlinear refractive index, n2. Besides that, the presence of a strong nonlinear absorption produces good optical limiting (Kumar et al. 2008). The negative sign of n2 indicates the suitability of the sample to be used as an optical limiter for laser radiation due to self –defocusing, which means it will diverge the radiation. According to the results that were obtained in Chapter 4, Figures (4.31, 4.33, 4.35, 4.37 and 4.39) and Tables (4.2, 4.3, 4.4, 4.5, 4.6 and 4.7), the sign of n2 which were obtained for all as-prepared nanocomposites were negative. Hence, they are considered as promising candidates to be used as optical limiter devices at a wavelength of 532nm (Ryasnyanskiy et al. 2007). The property of optical limiting is mostly found to be absorptive nonlinearity. Hence, it is related to the imaginary part of the third order susceptibility (Mathew et al. 2012).The optical limiting threshold reduced when the size of the nanoparticle increased and the optical limiting performance was enhanced (Hari et al. 2011).

4.17.2 Measurements of optical limiting threshold

The setup of the transmittance technique, which was used to determine the optical limiting threshold of the as-prepared samples, is same as the setup of the Z-scan technique, but the sample should be fixed at the focal point (i.e. it will be constant at this point and will not move along the z-axis as in the Z-scan measurements). Figure 4.49 represents the setup of the transmittance technique. Variable beam splitter (attenuator) was used in the path of the laser to vary the output power of the laser source. The transmittance in the far field was measured using an energy meter and was normalized, and then plotted as a function of input fluence (Mw/cm2). It should be mentioned that the absorption spectra of the as-prepared samples were checked after irradiating them with a laser source and it was found that almost no change happened in the pattern and its intensity, hinting that the as-prepared samples have good photo stability.

124

Figure 4.49

Setup of transmittance technique


Figure 4.50 represents the normalized transmittance as a function of input fluence for the PMMA/ZnO nanocomposites with different concentrations of ZnO nanoparticle. Initially, the output power increased as the input power increased, meanings the output power varies linearly with the input power, and the linear transmittance obeyed Beer's law: 𝐼 = 𝐼𝑜 𝑒 −𝛼𝑑

(4.22)

where I is the incident energy, 𝐼𝑜 is the output energy, α is the linear absorption coefficient and d is the thickness of the sample, until it reaches a certain threshold value when the sample starts to defocus the laser beam, thus causing the cut off of a greater part of the beam cross-section by the aperture (Manshad and Hassan 2012).

The arrow in the figure indicates the approximate fluence in which the normalized transmission begins to deviate from linearity which is defined as the limiting threshold (Hassan et al. 2013). Figure 4.50 clearly reveals that the high concentration of ZnO nanoparticles (15 wt %) showed the lowest value of optical limiting threshold which was 70Mw/cm2 , that meaning it exhibits the strongest optical limiting compared to other concentrations .This concentration showed a deeper

125

valley in the open aperture Z-scan technique (Figure 4.40 ) and the highest TP-V in the closed aperture (Figure 4.31)compared to other PMMA/ZnO nanocomposites of lower ZnO concentration; besides, it showed the lowest energy band gap (Figure 4. 25). Hence, the optical limiting threshold is inversely proportional to the concentration of ZnO nanoparticles in the nanocomposites. It is obvious that the lower optical limiting threshold represents a better optical limiting material (Irimpan and Radhakrishnan 2008).Therefore, the four as-prepared nanocomposites with high concentration of ZnO nanoparticles were chosen to determine and compare the optical limiting threshold of those nanocomposites.

Figure 4.50

Optical limiting threshold of PMMA/ZnO nanocomposites

Figure 4.51 represents the normalized transmittance as a function of input fluence for four types of polymer-ZnO nanocomposites with different polymer matrix and high concentration of ZnO nanoparticles (i.e. 15 wt % for PMMA/ZnO, PVA/ZnO, and PS/ZnO, and 10 wt % for PVDF/ZnO). The lowest optical limiting

126

threshold was 70 Mw/cm2 and was shown by the PMMA/ZnO nanocomposite, which had the highest deep valley in the trace of open aperture Z-scan (nonlinear absorption coefficient) (Figure 4.40) and it had the highest imaginary part of third order susceptibility (Table 4.2), compared to the other three nanocomposites with the highest ZnO concentration. The highest optical limiting threshold was 350 Mw/cm2 and was shown by the PVDF/ZnO nanocomposite, which had the lowest deep valley in the trace of open aperture Z-scan (Figure 4.42). The optical limiting threshold of PVA/ZnO and PS/ZnO was 100 Mw/cm2 and 150 Mw/cm2, respectively.

Figure 4.51

Optical limiting threshold of polymer/ZnO nanocomposites with different polymer matrix

127


Figure 4.52 represents the normalized transmittance as a function of input fluence for four types of polymer/ZnO and polymer/ZnO/CuO nanocomposites that are (a) PMMA/ZnO and PMMA/ZnO/CuO, (b) PVDF/ZnO and PVDF/ZnO/CuO, (c) PVA/ZnO and PVA/ZnO/CuO, and (d) PS/ZnO and PS/ZnO/CuO. From Figure 4.52 (a), it is clear that the addition of CuO nanoparticles reduced the optical limiting threshold from 140 Mw/cm2 for PMMA/ZnO to 60 Mw/cm2 for PMMA/ZnO/CuO. Figure 4.52 (b) shows the reduction of the optical limiting threshold of PVDF/ZnO, which is equal to 350 Mw/cm2 to 300 Mw/cm2 for PVDF/ZnO/CuO. The value of optical limiting threshold of PVA/ZnO is equal to 130 Mw/cm2, while it is reduced to 80 Mw/cm2 for PVA/ZnO/CuO as shown in Figure 4.52 (c).The PS/ZnO/CuO nanocomposite shows the lowest optical limiting threshold which is 50 Mw/cm2, while it is 250 Mw/cm2 for the PS/ZnO nanocomposite, which is clearly shown in Figure 4.52 (d). The PS/ZnO/CuO nanocomposite has the lowest value of optical limiting threshold, so it is considered the best optical limiter, representing the lowest energy band gap of four types of polymer/ZnO/CuO nanocomposites (Figure 4.26), besides, it had a deeper valley in the trace of open aperture Z-scan (nonlinear absorption coefficient) (Figure 4.48) compared to the other three nanocomposites. The highest optical limiting threshold of 300 Mw/cm2 shown by the PVDF/ZnO/CuO nanocomposite which had the highest energy band gap (Figure 4.26).

128

Figure 4.52

Optical limiting threshold of nanocomposites (a) PMMA/ZnO and PMMA/ZnO/CuO (b) PVDF/ZnO and PVDF/ZnO/CuO (c) PVA/ZnO and PVA/ZnO/CuO (d) PS/ZnO and PS/ZnO/CuO

129

CHAPTER V

CONCLUSIONS

5.1

CONCLUSIONS

Thin film based PMMA and PMMA/ZnO nanocomposites, film based PVDF and PVDF/ZnO nanocomposites, a flexible foil-like pure polymer , polymer/ZnO and polymer/ZnO/CuO nanocomposite of four different polymer matrix ,which are PMMA,PVDF,PVA and PS have been prepared successfully with different concentrations of ZnO and CuO nanoparticles which were used as filler.

High linear absorption was shown for UV radiation by all nanocomposites depending on the content of the ZnO nanoparticles and polymer matrix in addition to the preparation method. The nanocomposites have an absorption peak at a wavelength of 370-377 nm which belongs to the ZnO nanoparticles.

The low linear transmittance was observed in the UV region due to the high absorption of ZnO nanoparticles; the same behavior was also noticed in the visible region that high reflectance and scatter caused.

Low reflectance in the UV region and a higher value than that in the visible region were observed. Reflectance of as-prepared like foil samples were inversely proportional with the concentration of ZnO nanoparticles in the UV region, whereas it was directly proportional in the visible region.

Concentration of ZnO nanoparticles influenced the energy band gap and was inversely proportional to each other; hence the energy gap of samples was red shifted.

130

The linear absorption coefficient was shown to be dependent on the concentration of ZnO nanoparticles, besides the thickness of the samples.

The refractive index of as-prepared samples like foil increased with the increase of ZnO nanoparticles in the visible region, while it showed an opposite behavior in the UV region; however its value for polymer/ZnO nanocomposites was more than its value for pure polymer for all nanocomposites.

Extinction coefficient depended on the absorption of the sample; therefore the high absorption for a particular wavelength means a high extinction coefficient for that wavelength.

The adding of CuO nanoparticles showed a clear effect in the linear optical properties of the nanocomposites; the transmittance was lower, the absorption was higher, and the reflectance was highly affected in the visible region, which was reduced. In addition, energy band gap showed red shift, the absorption coefficient increased, the extinction coefficient increased and the refractive index decreased.

The EDX analysis and EDS mapping confirmed the purity of the as-prepared samples. The homogeneous dispersion of ZnO nanoparticles in the polymer matrix was observed in nanocomposites samples through the FESEM test.

The nonlinear refractive index had a negative sign for all as-prepared nanocomposites, and it was in the order of 10-12 cm2/W for nanocomposites asprepared like foil, whereas thin film based PMMA/ZnO and film based PVDF/ZnO were in order of 10-13 cm2/W.The nonlinear absorption coefficient was in the order of 10-7cm/W for nanocomposites prepared as foil, however thin film based PMMA/ZnO and film based PVDF/ZnO were in order of 10-8cm2/W. The absolute value of the third order nonlinear optical susceptibility χ

(3)

was

in the order of 10-6 e s u for samples that were prepared as foil, while thin film based PMMA/ZnO and film based PVDF/ZnO were in order of 10-7 e s u . The effect of the

131

percentage of ZnO nanoparticles and the types of polymer matrix on this parameter was indicated. These obtained values, for n2, β and χ

(3)

, are larger than the values stated in

previous studies.

In the result of closed aperture Z-scan, the pure electronic contribution was dominant in PMMA/ZnO, PVDF/ZnO and PS/ZnO nanocomposites, whereas the thermal contribution was dominant in the PVA/ZnO nanocomposite.

The effect of adding CuO nanoparticles to nanocomposites which were prepared as foil was observed to enhance their nonlinear optical properties. The values of nonlinear refractive index, nonlinear absorption coefficient and third order nonlinear optical susceptibility χ

(3)

of polymer/ZnO/CuO were larger than the values

for polymer/ZnO.

As the nonlinear optical properties showed a negative sign of nonlinear refractive index and strong nonlinear absorptivity, this indicated the suitability of asprepared samples to be used as an optical limiter.

The optical limiting threshold of nanocomposites showed its dependence on the concentration of ZnO nanoparticles .The low optical limiting threshold was shown by high concentration, which indicated a good optical limiting .Besides that,the polymer matrix affected the value of optical limiting threshold, thus PMMA/ZnO with the high concentration of ZnO nanoparticles (15 wt%) showed the lowest value among four different polymer/ZnO nanocomposites, which is 70 Mw/cm2.

The effect of adding CuO nanoparticles enhanced the optical limiting property of nanocomposites; the four different polymer/ZnO/CuO nanocomposites showed lower optical limiting threshold than their values with only ZnO nanoparticles.The lowest optical limiting threshold was obtained by PS/ZnO/CuO which was equal to 50Mw/cm2, therefore it was considered the best optical limiter than other as-prepared nanocomposites.

132

5.2

MAIN CONTRIBUTION

The main contributions of this work are briefly described below: 

From the obtained results, the value of the third order susceptibility had been found to be in order 10-6 e s u, which was considered as a large value. Consequently the main step taken to select materials for nonlinear optical applications had been satisfied.



Pure polymer; polymer/ZnO nanocomposites of four different polymer matrices, which were PMMA, PVDF, PVA and PS had been prepared successfully as flexible foil.



To the best of the author’s knowledge, the preparation of polymer/ZnO/CuO nanocomposites had been considered as the first work, so neither linear nor nonlinear of their optical properties were studied before this.



Studying the nonlinear optical properties of PMMA/ZnO, PVA/ZnO, and PS/ZnO nanocomposites as flexible foils had been considered as the first work, although their nanocomposites as thin film were studied.



Studying the nonlinear optical propertes of PVDF/ZnO nanocomposites had been considered as the first work, as no report had been published for this nanocomposite neither thin film nor flexible foil.



As application for the obtained results of nonlinear optical properties, the evaluation of optical limiting property of as-prepared samples resulted in a good optical limiter device, which is PS/ZnO/CuO nanocomposite as a flexible foil.

133

5.3

RECOMMENDATION FOR THE FUTURE RESEARCH 

Other laser sources with different wavelengths such as 1.06 µm can be used to check the absorptive nonlinearity of the samples. Also, the optical limiting property for this wavelength can be checked.



Mixing of two or three different polymer matrix, which were used in this work, can be mixed together and then the nanocomposite of the blended polymers with ZnO nanoparticles can be prepared.



Adding other metal nanoparticles such as Ag and Au to the nanocomposites that were prepared in this work, to enhance their linear and nonlinear optical properties.



The optical switching property of the as-prepared samples, in this work, can be checked for different wavelengths.

134

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APPENDICES

APPENDEX A

LIST OF PUBLICATIONS

JOURNAL 1. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. 2014. Polymer-ZnO nanocomposites foils and thin films for UV protection. In The 2014 UKM FST Postgraduate Colloquium: Proceedings of the Universiti Kebangsaan Malaysia, Faculty of Science and Technology 2014 Postgraduate Colloquium. AIP Publishing. 1614: 136-141.

2. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. 2015. Measurements of Nonlinear Optical Properties of PVDF/ZnO Using Z-Scan Technique. Brazilian Journal of Physics, 45(5):538-544.

3. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. 2016. Third order nonlinearity of PMMA/ZnO nanocomposites as foils. Optical and Quantum Electronics. 48(1):1-14. 4. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y.2016. Using Z-Scan Technique to Measure the Nonlinear Optical Properties of PMMA/ZnO Nanocomposites. Jurnal Teknologi (Sciences & Engineering) 78:3: 33–38.

5. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y.2016. Investigation of Energy Band Gap in Polymer/ZnO Nanocomposites. Journal of Materials Science: Materials in Electronics. (Accepted).

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6. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y.2016. Influence of polymer matrix on nonlinear optical properties and optical limiting threshold of polymer-ZnO nanocomposites. Journal of Materials Science: Materials in Electronics. (Accepted). 7. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. Influence of CuO nanoparticles on Third Order Nonlinearity and Optical Limiting Threshold of Polymer/ZnO Nanocomposites. Optical and Quantum Electronics. (Under Review).

8. Abdullah, I.Y., Yahaya, M., Haji Jumali, M. H., Shanshool, H.M. Influence of the Substrate on the Crystalline Phase and Morphology of Poly (Vinylidenefluoride) (PVDF) Thin Film. Surface Review and Letters.23 (3): 1650005-1-1650005-8. 9. Abdullah, I.Y., Yahaya, M., Haji Jumali,M. H., Shanshool, H.M. Enhancement piezoelectricity in Poly (Vinylidene Fluoride) by Filler Piezoceramics Lead-Free Potassium Sodium Niobate (KNN). Optical and Quantum Electronics. 48(2):149-1-149-9.

CONFERENCE 1. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. Effect of Zinc Oxide Nanoparticles Concentration on Optical Properties of PMMA/ZnO nanocomposites Thin Film” Proceedings of Tenth TheIIER International Conference, Kuala Lumpur, Malaysia, 12th February 2015, ISBN: 978-93-84209-87-2.

2. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. Optical Properties of PVDF/ZnO Nanocomposites. 7th International Conference on Researches in Engineering, Technology and Sciences (ICRETS),Hotel Dynasty, 218, Jalan Ipoh, 51200 Kuala Lumpur, Wilayah Persekutuan, Kuala Lumpur, Malaysia ,16th-17th July 2015.

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3. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. Using ZScan Technique to Measure the Nonlinear Optical Properties of PMMA/ZnO Nanocomposites. 1st International Laser Technology and Optics Symposium (LATOS 2015) 13th - 14th October 2015, Laser Center, Ibnu Sina Institute for Scientific & Industrial Research (ISI-SIR) UTM.

4. Shanshool, H.M., Yahaya, M., Yunus, W.M.M. and Abdullah, I.Y. PolymerZnO nanocomposites foils and thin films for UV protection. In The 2014 UKM FST Postgraduate Colloquium: Proceedings of the Universiti Kebangsaan Malaysia, Faculty of Science and Technology 2014 Postgraduate Colloquium.10th – 11th April 2014.

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APPENDEX B

CURRICULUM VITAE

Haider Mohammed Shanshool was born on 15th October 1962 in Iraq. He obtained his Bachelor’s degree in Physics from the Physics Department, Faculty of Science, University of Baghdad (Baghdad-Iraq) in 1987. After that, he worked as a co researcher in the Laser Research Centre in the Ministry of Science and Technology since 1988, and continued as a researcher until 2011.He obtained his Master’s degree in Laser Technology from the University of Technology (Baghdad –Iraq) in 2001.Furthermore, he has worked on PhD degree at the School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia (UKM) since 2011.His research interest is in the field of polymer and its nanocomposites for the development of their linear and nonlinear optical properties as well as their application in optical devices.