Structural and optical properties of nanocrystalline platinum

Journal of Alloys and Compounds 655 (2016) 415e422

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Structural and optical properties of nanocrystalline platinum octaethylporphyrin (PtOEP) thin films A.A. Abuelwafa a, c, *, A. El-Denglawey a, d, M. Dongol a, M.M. El-Nahass b, T. Soga c a

Nano and Thin Film Lab, Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo 11757, Egypt c Department of Frontier Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan d Physics Department, Faculty of Applied Medical Science, Ain Shams University, Roxy, Cairo 11757, Egypt b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 June 2015 Received in revised form 2 September 2015 Accepted 18 September 2015 Available online 21 September 2015

Thermal evaporation technique was used to prepare the Platinum octaethylporphyrin (PtOEP) thin films. X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM) and Fourier-transform infrared techniques (FT-IR) were used to study the crystal and molecular structure of PtOEP, the results confirmed that the PtOEP thin films have nonostructural features. Extinction coefficient, k and refractive index, n were estimated using spectrophotometric measurements of both transmittance T (l) and reflectance R (l) at normal incidence light in the wavelength range 200e1100 nm. The absorption parameters such as molar extinction coefficient, εmolar, oscillator strength, G and electric dipole strength, q2, the type of electronic transition, optical energy gap and fundamental energy gaps were reported. The normal dispersion (l > 600 nm) of refractive index is discussed in terms of single oscillator model of WempleeDidomenico. The dispersion and the dielectric characterizations of the thin films were investigated and discussed. © 2015 Published by Elsevier B.V.

Keywords: PtOEP thin films Nonostructural Optical properties Normal dispersion

1. Introduction Organic semiconductors are an emerging class of materials whose electronic conductivities lay in the broad range of 109e103 U1 cm1 [1]. At the beginning of the 21st century the organic semiconductors have attracted considerable attention, since they possess a prosperous optoelectronic, electrical and properties for designing and fabrication of electronic and optoelectronic device [2]. The characterizations of these materials in thin film are important for applied sciences because of their potential use in electronics and instrumentation industry. Organic semiconductors have been employed in a variety of thin film photonic devices including as Organic Thin Film Transistors [3,4], Organic Solar Cells (OSC) [5e10], Organic Light-Emitting Diodes (OLED) [11,12]. Porphyrin is one of the promise organic compounds with high stability and efficient light absorption ability in the visible and near-infrared regions of the optical spectrum also it has a conjugated system consisting of 18p electrons so Porphyrin and metalloporphyrins are good candidates for organic semiconductors * Corresponding author. Nano and Thin Film Lab, Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt; Department of Frontier Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. E-mail address: [email protected] (A.A. Abuelwafa). http://dx.doi.org/10.1016/j.jallcom.2015.09.180 0925-8388/© 2015 Published by Elsevier B.V.

and have large potential for applications in optoelectronic devices [13,14]. One of the most a commercial derivatives of Porphyrin is the Platinum octaethylporphyrin (PtOEP) which it recently used as active layer in field effect transistors and solar cells [15e18]. Also, (PtOEP) has been studied as a red phosphorescence material for OLED and it has shown a short phosphorescence lifetime due to mixing of singlet and triplet excited states caused by platinum [11,19]. When the optical constants (n and, k), dispersion and dielectric characterizations in addition to the structure of each layer in the multi-layer devices are known, it is possible to know whether constructive interference occurs between these layers [11]. The clarifications regarding the relation between structural and optical properties have both scientific and technological importance. Few studies have been devoted to the optical properties of PtOEP, focusing mainly on the absorption and emission in liquid solutions and doped films [11,20e24]. However, the structural and optical properties PtOEP thin films are not extensively studied yet. Therefore, in detail study is required to investigate the structural and optical properties of PtOEP thin films. In the present manuscript, high quality PtOEP thin film was prepared by using thermal evaporation technique under high vacuum in order to explore the detail structural and optical properties of that film. The main microstructural characteristics of the film were examined by XRD and TEM, while the molecular structure of the film was investigated

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A.A. Abuelwafa et al. / Journal of Alloys and Compounds 655 (2016) 415e422

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4R k2 ð1 RÞ2

by FTIR. Optical absorption and transitions in the UVeVis region for PtOEP thin films was explored using spectrophotometric method. Also, the dispersion of refractive index was investigated in wide wavelength range and discussed by using single oscillator model.

1þR n¼ þ 1R

2. Experimental

where, a is the absorption coefficient and d is the film thickness. The experimental errors were taken as ±2% for the film thickness also ±1% for T and R and ±3% in computed values of n and k.

(3)

Platinum octaethylporphyrin (PtOEP) powder was obtained from Aldrich Chem Co. The schematic diagram of the molecular structure of PtOEP is shown in Fig. 1. Thin films were prepared by thermal evaporation technique (vacuum coating unit 106 torr e Edwards type E306A) on clean quartz substrate for optical measurements, because the quartz substrate has an extraordinary high optical transmissivity for ultraviolet light. While, the glass substrates using structural measurements. Evaporation of the material was carried out with a quartz crucible heated by a tungsten coil. The evaporation rate as well as the film thickness of the evaporated films was controlled using a quartz crystal monitor FTM5. Thickness was also checked after deposition by interferometry (Tolansky's method) [25]. The deposition rate was controlled at 0.3 nm/s. During the deposition the substrate temperature was kept nearly at room temperature. The XRD patterns of PtOEP in powder form and thin films were performed using (Rigaku RINT 2100 diffractometer) at 40 KV and 30 mA, with CuKa radiation (l ¼ 1.540598 Å). The crystallite shape and size formed using (TEM, JEOL.JEM, 1230) with accelerating voltage 80 KV. The chemical structure characterizations of the powder and as-deposited films were investigated using (FT-IR, Jasco Model 6100) at room temperature with a resolution of ±1 cm1 in the spectral range 400e4000 cm1. Measurements of the transmittance T(l) and reflectance R(l) were measured at normal incidence at room temperature in the spectral range of wavelength (200e1100 nm)using a computerized (ShimadzuUV160A) double beam spectrophotometer with10 nm steps for PtOEP thin films of thickness 170 nm. In order to calculate the optical constant refractive index, n and the absorption index, k at different wavelength, l we used a computer program comprising search technique on the absolute values of the measured T(l) and R(l) at different wavelengths [26,27] and using the following equations:

The X-ray diffraction pattern of PtOEP in its powder form is shown in Fig. 2. This pattern has many diffraction peaks with different intensities indicating that the powder of PtOEP has a polycrystalline nature. Lattice parameters and unit cell volume were calculated using the MAUD program [28,29]. This program fundamentally based on the Rietveld method [28]. The details of crystal data are presented in Table 1. The obtained results from the Rietveld refinement of lattice parameters and unit cell of PtOEP are very close to the results reported in the literature [30e34]. However, the small variations in these values may be attributable to different types of synthesis methods and the experimental conditions. The values of Miller indices and lattice spacing which corresponding to each diffraction line were indexed using CHECKCELL program [35]. The pattern of as-deposited thin films which it shown in Fig. 3. Thin film pattern indicates that as-deposited PtOEP films is partially crystallized also it is seen that a distinct peak at 2q ¼ 8.9 with a preferential orientation in the (001) direction. The average crystallite size, D was calculated using the Scherrer's equation [14]:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 2 1 4ð1 RÞ2 ð1 RÞ4 þ a ¼ ln þ R2 5: d 2T 4T 2

b¼

(1)

3. Results and discussion 3.1. Structural properties of PtOEP

D¼

KS l : b cos q

(4)

where, KS ¼ 0.94 is Scherrer's constant, l is the X-ray wavelength of CuKa (0.15406 nm), q is the Bragg's angle and b is the Full Width at Half Maximum (FWMH) in radian which can be corrected by:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b2f b2P :

(5)

where, bf and bp are FWMH of (001) plane for thin film and powder

al : k¼ 4p

(2)

Fig. 1. Molecular structure of PtOEP.

Fig. 2. Rietveld refinement for XRD pattern of PtOEP in the powder form.


417

Table 1 Crystal data for powder PtOEP. Crystal data Product name Empirical formula Molecular weight Crystal color Crystal system Space group Unit cell Density (g/cm3) The unit cell volume (A3)

Platinum (II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin C36H44N4Pt 727.8 red Triclinic P1 a ¼ 8.193 Å, b ¼ 10.033 Å, c ¼ 10.059 Å,a ¼ 84.53 ,b ¼ 80.95 ,g ¼ 67.15 1.61 752

Fig. 3. X-ray diffraction pattern of PtOEP thin film.

respectively. The computation of the average crystallite size using the Scherrer's equation shows that the crystallite size decreases from 57.1 nm for the powder to 12.94 nm for the as deposited thin films. The nanosize nature of the as deposited thin film is clearly manifested in the broadening or in the FWHM of the diffraction peak of (001) plane in thin film pattern which is larger than those of the source powder of PtOEP. The broadening in diffraction peak of the (001) plane may be owing to small crystallites which leads to large surface scattering so large broadening in diffraction peak occur. The dislocation density, d defined as the length of dislocation lines per unit volume is evaluated from the relation [14]:

d¼

1 : D2

(6)

The micro strain, εs calculated using the following relation [36]:

εs ¼

b cos q 4

(7)

The number of crystallites, Nc per unit surface area could be determined according to [37]:

Nc ¼

d : D3

(8)

where, d is the thickness of the film. The values of dislocation density (d), micro strain, εs and the number of crystallites, Nc for the thin film are 5.95 1015 (line nm)2, 2.79 103and7.83 1016, respectively. Transmission Electron Microscopy (TEM) provides valuable information about size, shape and distribution of nanocrystal samples. Fig. 4(a) shows the TEM image of PtOEP thin films. The TEM image demonstrates the formation of nanocrystalline rods in the thin films. The formation mechanism for the rod shape product was not yet clear. We propose that the nanorod formation

in the present work by the orientation degrees of freedom in organic films, which was allowed to the molecules to deposit on the surface of the substrate in (lying-down) or (standing-up) configurations. If moleculeemolecule interaction (p-stacking) is stronger than moleculeesubstrate interaction, then the nuclei will form in (standing-up) configuration. Otherwise, the nuclei will form in (lying-down) [38e42]. In our sample, the Pt atom is roughly coplanar with the mean plane of the porphyrin macrocycle. Owing to the presence of mirror and rotation symmetries, each asymmetric unit contains one-eighth of the PtOEP molecule. All four pyrrole-ring moieties of each PtOEP molecule simultaneously have strong p-p interactions with its neighbors along the three crystallographic directions (p-stacking) [31]. So we can say the molecule interaction in the present case is stronger than moleculeesubstrate interaction. Therefore, the PtOEP molecules prefer to stack up into nano-rods structure. Fig. 4(b) shows the selected area electron diffraction (SAED) pattern of PtOEP thin films. The electron diffraction pattern indicates to the crystalline nature of PtOEP films. The size distribution of PtOEP nanocrystals was determined by measuring more than 100 particles, which also reveals the good monodispersity of the sample. Fig. 4(c) illustrates the distribution histogram of PtOEP particle size as deduced from TEM image. The average grain size estimated from TEM image is about 12 nm. This value is in close agreement with the results obtained from Scherrer's method and confirms nanostructure property of PtOEP films. The infrared spectrum is a powerful and popular tool for the identification of the molecular structure for organic and inorganic materials in different forms. Fig. 5 shows the FT-IR transmittance spectra of PtOEP powder and the thin films in the finger print region (400e1500 cm1). The peaks positions and assignments are inserted in Table 2 where the various carbon atoms referenced in Table 2 are identified in Fig. 5, a sketch of a repeating segment of the octaethylporphine ligand [43e47]. Although relative peak intensities vary because of the differences in selection rules and orientations [46,47]. The metallooctaethylporphine, MOEP sensitive bands are located near 1230, 995, 922 and 748 cm1 with shift ±3 cm1 for PtOEP powder and thin films. The positions of the Metal-Sensitive Infrared Bands depend on metalpyrrolic nitrogen (M-Np) bond distance [48]. Subsequently, there is good agreement between the peaks positions observed in thin films and PtOEP powder spectrum. This is indicated that PtOEP molecules do not undergo decomposition during substitution process or PtOEP molecules have high chemical stability also the thermal evaporation technique is good one to obtain stoichiometric PtOEP films. 3.2. Optical characterization Spectral distribution of T (l) and R (l) of the as prepared PtOEP films are shown in Fig. 6. It could be noticed that at longer wavelengths (l > 600 nm), all films becomes nearly transparent as non-

418


Fig. 4. (a) TEM image for PtOEP thin film, (b) Electron diffraction pattern for PtOEP thin film, (c) Distributional histogram of PtOEP particle size deduced from TEM image.

Table 2 Peak positions and assignments for PtOEP powder and thin film. Powder

Thin films

Assignment

1468 1372 1272 1230 1151 1113 1054 1020 995 958 922 842 748

1468 1372 1272 1233 1153 1112 1065 1017 992 952 925 844 747

d (CH3) ethyl group ethyl group n (CaN) d (CaCmH) n (CaCm), n (CaN), d (CaCmH) n (CaN), d (CaCmH) ethyl group ethyl group ethyl group n (CaCm), n (CbC1) ethyl group n (CaCm), n (CbC1) л (CmH) d (CbC1H) þ d (C2C1H) ethyl

Values are in cm1.The designation Ca, Cb, Cm, C1, and C2 refer to the carbon atoms adjacent to the nitrogen atom, at the b pyrrole position, the methine bridges and the substituent, respectively. The symbols n, d, and л refer to stretching, bending, and out-of-plane bending [43e47]. See Fig. 5 for clarity. Fig. 5. Infrared spectra of PtOEP powder form, thin film and the sketch of a fragment of octaetylporphyrin indicating the labeling of carbon atoms as they appear in Table 2.

absorbing region (i.e. R þ T ¼ 1) because there is a very small amount of loss energy due to scattering. The inequality (R þ T < 1) at shorter wavelengths (l < 600 nm) known as absorbing region is due to the existence of absorption. The study of the fundamental

absorption edge provides useful complementary information concerning the energy band structure and the type of the optical transition of the charge carriers. The absorption spectra of the metalloporphyrins are well known and their bands in different spectral regions are four orbital model [49] in which the electronic transitions involve excitations from the two highest occupied p


Fig. 6. The optical transmission T(l) and reflection R(l) of PtOEP thin film.

Fig. 8. Molar extinction coefficient (εmolar) for PtOEP thin film.

molecular orbital (HOMOs) into the two lowest unoccupied p* molecular orbital (LUMOs) [50]. Fig. 7 illustrates the absorption spectrum of PtOEP thin film in both UV and Vis. regions. It shows an intense absorption termed Soret (B) band, this band has two peaks Bx and By are observed at 303 nm and 367 nm, respectively for PtOEP thin films, this splitting in the Soret band depends on the distance between the molecules, the angle of their transition dipole moments between neighboring molecules and the number of interacting molecules [20]. There are also two transitions called Q bands (Q1andQ2) are observed at 547 and 510 nm, respectively for PtOEP thin films. It is also noted that there is band called N in the UV region at 239 nm. The Q band is attributed to the metal (Ptþ2) -to-ligand charge-transfer spin-triplet state (MLCT), while the Soret band due to the transition from the singlet ground state to the metal (Ptþ2) -to-ligand charge transfer singlet state [23]. Absorption spectroscopic characterization of PtOEP was studied by other researchers [11,20 and 23]. It is useful to relate the absorption coefficient, a to the molar extinction coefficient, εmolar (is in units of liters per mole-cm), which is often used to describe the absorption of light by nonsolid molecular media by the expression [51]:

a¼

r 3 10 lnð10Þεmolar M

419

(9)

where, r is the solid's mass density and M is the molecular weight. Fig. 8 shows the εmolar, as a function of the wavenumber for PtOEP thin films. Gaussian fitting was performed on each peak of the

spectra. Intensities of all the absorption bands are evaluated by measuring their oscillator strength, G which is found to be proportional to the area under the absorption peak shapes. The oscillator strength and the electric dipole strength, q2 can be calculated by the following mathematical expressions [52]:

Z f ¼ 4:3

εmolar ðyÞdy

(10)

1 y εmolar 2500 Dy

(11)

q2 ¼

where, Dy is the absorption half-band width. The calculated values of G, q2, Dy and y at different peak positions are tabulated in Table 3. It is usual in many organic materials study the type of optical transitions as well as the value of the energy gap at the fundamental absorption edge (a 104 cm1). The energy dependences of the interband absorption coefficient are given by the following expressions [53,54]:

opt r ðahnÞ ¼ b hn Eg

(12)

where, b is a parameter that depends on the transition probability, a is the absorption coefficient, Eopt g is the optical band gap and r is a number which characterize the transition process, where r ¼ 1/2 and r ¼ 3/2 for direct allowed and forbidden transitions, respectively. Also, r ¼ 2 and r ¼ 3 for indirect allowed and forbidden transitions, respectively. The dependence of (ahy) on photon energy (hy) was plotted for different values of, r. It was found that the best fit was obtained at r ¼ 2, so the type of electronic transition is indirect allowed transition, which is shown in Fig. 9. The indirect band gap for PtOEP thin films is evaluated from the x-axis intercepts. The values for the corresponding energies were found to be 2.21, 3.04 and 4.32 eV. The first energy value is the optical gap Eopt g , corresponds to the onset of optical absorption and formation Table 3 Parameters of molar extinction coefficient for the PtOEP thin film.

Fig. 7. Absorption spectra of PtOEP thin film.

Peaks

G

q2(Å)

Dy(cm)1

ymax(cm)1

εmolar (cm2 mol1)

Q1 Q2 BY BX N

0.256 0.011 0.994 0.558 0.378

0.161 0.017 0.349 0.311 0.043

1251.1 231.11 2865.3 4964.3 879.12

18394 20340 27258 31460 39141

5928 3910 8320 4940 4750

420


n2 ¼ 1 þ

Fig. 9. (ahy)1/2 vs. hy for PtOEP thin film.

of a bound electronehole pair, or exciton (Frenkel exciton) [55], but the last energy value is the fundamental energy gap (energy gap between valence band ‘‘p -band’’ and conduction band ‘‘p*-band’’) [55,56], and the values between them may be impurities energy levels. The absolute values of T (l) and R (l) were used to determine the optical constants. The dispersion extinction coefficient, k and reflective index, n of PtOEP film according to Eq. (1) e Eq. (3), see Fig. 10. The dispersion curve was analyzed in terms of the two regions. The first region is Anomalous dispersion at l < 600 nm, in this region the refractive index shows five peaks this behavior of dispersion in this region can be understood using the multioscillator model. While the second region is the normal dispersion at l > 600 nm in this region the refractive index decreases slowly with increasing the wavelength, this behavior can be explained by the single oscillator model [57,58]. T. Tsuboi et al. studied the optical constants of PtOEP emitting layer in single-layer organic light emitting diode (OLED) device and in thin film grown on quartz plate using a phase modulated spectroscopic ellipsometry [11]. At normal dispersion region, the dispersion of reflective index has been analyzed using the single oscillator model developed by Wemple and DiDomenico. This model suggests that the refractive index, n of the investigated film is simply related to the electronic structure of the film material by the following equation [59]:

Fig. 10. Reflective index and Extinction coefficient of PtOEP thin film as a function of wavelength.

Eo Ed Eo2 ðhnÞ2

(13)

where, Eo is the oscillator energy which is the average excitation energy for electronic transitions and Ed is the dispersion energy which measure the strength of the inter-band optical transitions and hn is the photon energy. Fig. 11 shows the variation of (n2 1)1 vs. (hn)2, which is a straight, line verifying Eq. (13). The values of Eo and Ed were determined from the slope and intercept on the vertical axis as 2.22 eV and 3.37eV, respectively. The points of interception with the ordinate at [(hn) 2 ¼ 0] gives the value of the optical dielectric constant at higher frequency, ε∞ ¼ 2.52. The oscillation energy Eo can be correlated with the optical gap acopt cording to the empirical formula Eo z Eg . The single oscillator parameters Eo and Ed can be utilized for calculating the third-order non-linear optical susceptibility c(3) according to Miller's rule in the limit (hy / 0) [60]:

cð3Þ ¼

A ð4pÞ4

ðEd =Eo Þ4

(14)

where A is a quantity that is assumed to be frequency independent and nearly the same for all materials A ¼ 1.7 1010. The estimated values of, c(3) (esu) equals 3.65 1014. The frequency dispersion of complex dielectric, ε characterizes completely the propagation, reflection and loss of light in multilayer structures. It provides information about the electronic structure of the material also the dielectric properties are correlated with electro-optic properties of the crystals so it is an important for the design of highly efficient optoelectronic devices The obtained values of refractive and absorption indices allowed calculation the real part, ε1 and imaginary part, ε2 of the complex dielectric constant through the following relations [13,61]:

9 8 < ε ¼ ε1 þ iε2 = 2 2 ε ¼n k ; : 1 ε2 ¼ 2nk

(15)

The spectrum of real and imaginary parts of the dielectric constant is shown in Fig. 12. As observed in this figure the values of the real part is higher more than the imaginary part. Moreover, the dielectric constant has higher value at lower frequencies and slightly decreases as frequency increases then remains very stable. The magnitude of dielectric constant depends on degree of polarization and the charge displacement in crystal. The decrease in dielectric constants at higher frequencies is attributed to the

Fig. 11. (n21)1 vs. (hy)2 for PtOEP thin film.


Fig. 14. s1 and s2 vs. hy for PtOEP thin film.

Fig. 12. ε1 and ε2 vs. hy for PtOEP thin film.

absence of space charge polarization near the grain boundary interface [62e64]. In transparent region the relation between the real part of dielectric, ε1 function and square of the wavelength, can be given by Ref. [65]:

ε1 ¼ n2 k2 ¼ εL

e2 4p2 εo c2

N :l2 : m*

421

(16)

where, εL is the lattice dielectric constant, e is the elementary charge, c is the speed of light and N/m* is the ratio of carrier concentration to the effective mass. The relation between n2 and l2 is shown in Fig. 13. The values of εL and (N/m*) which determined from the intercept l2 ¼ 0 and the slope of the line for the as deposited PtOEP thin films as 4.21 and 5.16 1057(Kg1 m3), respectively. The obtained data clarify that the value of ε∞ < εL, which may be due to free charge carrier contribution in the polarization process that is occurred inside the material when the light illuminates it [61]. The optical conductivity is one of the powerful tools for studying the electronic states in materials. If a system is subjected to an external electric field, in general, a redistribution of charges occurs and currents are induced. For small enough fields, the induced polarization and the induced currents are proportional to the inducing field. The complex optical conductivity, s is related to the complex dielectric constant, ε by the following relation [66e69]:

8 9 < s ¼ s1 þ is2 = s ¼ uε2 εo : 1 ; s2 ¼ uε1 εo

(17)

where, s1 is the real part of the dielectric constant, s2 is the imaginary part of the optical conductivity, u is the angular frequency and εo is the free space dielectric constant. The real and imaginary parts of the optical conductivity dependence of energy are shown in Fig. 14. It is seen that the optical conductivity increases with increasing photon energy, it has drastically increasing corresponding to absorption edge and optical gap. This suggests that the increase in optical conductivity is due to electrons excited by incident photon energy and the origin of this increasing may be attributed to some changes in the film structure [13]. According to the multi-oscillator model, the energy (eV) corresponding the peaks from n, k, ε1, ε2, s1 and s2 for the PtOEP thin films are presented in Table 4. It's clear that from those values the variation of ε1 and s2 follows the same trend of, n (Dispersion curves). While, the variation of ε2 and s1 follow the behavior of, k which is related to the variation of a with photon energy (Absorption curves). This behavior was observed in NiTPP [13], CoMTPP [66], FeTPPCl [70] and CuTPP [71] thin films. ε2 and s1 curves are always characterized by more than one maxima which correspond approximately to both the onset optical gap, Eopt g and the fundamental energy gap, Eg Refs. [71,72]. 4. Conclusion Thermal evaporation technique was used to prepare PtOEP films. XRD pattern and Rietveld refinement data indicate that PtOEP in powder form has a polycrystalline nature with a triclinic structure. The XRD pattern of thin films and TEM confirmed the nanostructure property of PtOEP films. FT-IR identified the bond type of PtOEP and showed that there are no molecular structural changes. The optical properties were investigated using

Table 4 The energy (eV) corresponding the peaks from n, k, ε1, ε2, s1 and s2 for the PtOEP thin film.

Fig. 13. n2 vs. l2 for PtOEP thin film.

No. peaks

n

ε1

s2

K

ε2

s1

1 2 3 4 5

2.11 2.38 3.34 3.95 4.94

2.09 2.38 3.32 3.97 4.93

2.07 2.26 3.34 3.98 4.95

2.25 2.44 3.34 4.12 4.95

2.25 2.43 3.34 4.13 5.01

2.21 2.45 3.36 4.11 5.06

422


spectrophotometric measurements in the wavelength range 200e1100 nm. The absorption spectra of the PtOEP show different bands the Q band is attributed to the spin-triplet excited state, while the Soret band due to the singlet state. The absorption spectra near the band edge are well fitted using indirect transitions formal and characterizes with three indirect transitions. The first transition is onset energy gap of 2.21eV and optical energy gaps of 3.04 and 4.32 eV. The refractive index dispersion data was analyzed in terms of the two regions. Anomalous dispersion at l < 600 nm and normal dispersion at l > 600 nm. The dispersion parameters of reflective index were analyzed using the single oscillator at normal dispersion region. The disagreement between the values of ε∞ and εL was attributed to the free carrier concentration. The variation of ε1 and s2 follows the same trend of, n, while the variation of ε2 and s1 follow the trend of, k. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

[23] [24] [25] [26] [27] [28] [29]

[30]

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