Deposition of iron sulfide thin films by AACVD from

Journal of Crystal Growth 346 (2012) 106–112

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Deposition of iron sulfide thin films by AACVD from single source precursors Masood Akhtar, Ahmed Lutfi Abdelhady, M. Azad Malik, Paul O’Brien n The School of Chemistry and The School of Materials, The University of Manchester, Manchester M13 9PL, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 October 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Communicated by D.W. Shaw Available online 24 February 2012

The unsymmetrical [Fe(S2CNEtiPr)3] (1), [Fe(S2CNEtMe)3] (2) and symmetrical [Fe(S2CN(Hex)2)3] (3), [Fe(S2CN(Et)2)3] (4) tris(dialkyldithiocarbamato)iron(III) complexes were used as single source precursors for the deposition of iron sulfide thin films by the aerosol assisted chemical vapor Deposition (AACVD) method. The unsymmetrical complexes deposited the mixed phases (pyrite and marcasite) at all deposition temperatures except the complex (2) which deposited pyrite and pyrrhotite at 400 1C. The symmetrical complex (3) with longer alkyl groups produced a mixture of pyrite and pyrrhotite phases at 350 and 450 1C but pyrite and mackinawite at 400 1C whereas the complex (4) with shorter alkyl groups deposited a mixture of pyrite and marcasite at 350 1C but a pure pyrrhotite phase at 400 and 450 1C. & 2012 Elsevier B.V. All rights reserved.

Keywords: A1. Crystallites A1. Substrates A3. Aerosol assisted chemical vapor deposition (AACVD) B1. Cubic pyrite B1. Hexagonal pyrrhotite B1. tris(dialkyldithiocarbamato)iron(III)

1. Introduction There is a growing demand for non-toxic, abundant, and cheap solar cell materials even if they have lower efficiencies than those of commonly used ones. Many of the materials used so far are either toxic and/or non-abundant such as cadmium, lead, indium or selenium, which means that these materials have limitations especially in large scale solar energy generation. Recent estimates of the annual electricity potential as well as material extraction costs and environmental friendliness led to the identification of materials that could be used in photovoltaic applications on a large scale [1]. The most promising materials include copper and iron sulfide. The iron sulfide (Fe-S) system has a complex phase diagram with several phases. These phases of iron sulfide are pyrite (cubicFeS2), marcasite (calcium chloride structure-FeS2), pyrrhotite-IT (Fe1 xS), pyrrhotite-4 M (Fe7S8), Fe9S10, greigite (cubic spinelFe3S4), troilite-2H (FeS) and mackinawite (Fe1 þ xS) [2–5]. Pyrite is an interesting phase of iron sulfide because of its potential application as an absorber material for thin film solar cells due to its band gap (Eg¼0.8–0.95 eV), high absorption coefficient ( 105 cm 1) and perceived low toxicity [6]. In the Fe-S system the stoichiometric ratio of iron:sulfur is important in determining the structure. Stoichiometric iron sulfide (FeS) adopts the trollite structure, with antiferromagnetic

n

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0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2012.02.013

properties at room temperature and undergoes transition into the NiAs-type structure above 120 1C, which includes the pyrrhotites (Fe1 xS) and Fe7S8 [7,8]. Due to Fe vacancy ordering, many pyrrhotites (Fe1 xS) give a range of compositions with superstructures and interesting magnetic and electrical properties [9,10]. The magnetic behavior of the pyrrhotites is very sensitive to changes in composition [11]. FexS samples with x ¼0.87–0.88 are Weiss type ferromagnetic. The non-stoichiometric Fe1 xS shows different morphologies, including whiskers [12], nanowires [13], and U-shaped microslots [13,14]. Iron sulfide thin films have been prepared by atmospheric- or low-pressure metal–organic chemical vapor deposition (AP or LP MOCVD; FeS2) [15–18]. Schleigh and Chang [15] deposited FeS2 thin films using iron pentacarbonyl [Fe(CO)5], hydrogen sulfide, and tert-butyl sulfide as precursors by LPCVD. Other techniques include sulfurization of iron oxides to FeS2 [19,20], ion beam and reactive sputtering (FeS2) [21], plasma assisted sulfurization of iron (FeS2) [22], flash evaporation (FeS2) [23], vacuum thermal evaporation (FeS2) [24], vapor transport (FeS2) [25], and chemical spray pyrolysis (FeS2) [26]. Different phases of iron sulfide nanoparticles were produced by high-energy mechanical milling combined with mechanochemical processing for FeS and FeS2 [27], dendrimer-stabilized FeS [28], solvothermal synthesis of Fe3S4 [29], sulfur-reducing bacteria for Fe1 xS and Fe3S4 [30,31], laser pyrolysis of iron complexes for FeS [32], polymer-stabilized wet chemical synthesis of FeS [33], reverse micelles for FeS2 [34], and the decomposition of single-source precursors for FeS2 [35]. Meester et al. [36] prepared iron disulfide (FeS2) using iron(III) acetylacetonate [Fe(acac)3], tert-butyl disulfide, and hydrogen.

M. Akhtar et al. / Journal of Crystal Growth 346 (2012) 106–112

2. Experimental All syntheses were performed under an inert atmosphere of dry nitrogen using standard Schlenk techniques. All reagents were purchased from Sigma-Aldrich and used as received. Solvents were distilled prior to use. Elemental analysis was performed by the University of Manchester micro-analytical laboratory. Mass spectra were recorded on a Kratos concept 1S instrument. Infrared spectra were recorded on a Specac single reflectance ATR instrument (4000–400 cm 1, resolution 4 cm 1). Melting points were recorded on a Barloworld SMP10 Melting Point Apparatus. p-XRD studies were performed on an Xpert diffractometer using Cu-Ka radiation. The samples were mounted flat and scanned between 201 and 651 with a step size of 0.051 and various count rates. AFM analysis was carried using a Veeco CP2 instrument. Films were carbon coated using a Edward’s E306A coating system before carrying out SEM, mapping and EDX analysis. SEM analysis was performed using a Philips XL 30FEG and EDX was carried out using a DX4 instrument. TEM analysis of scratched films was performed using CM200 and tecnai instruments.

argon flow rate was controlled by a Platon flow gage. Six Si substrates (approximately. 1 2 cm2) were placed inside the reactor tube, which is placed in a CARBOLITE furnace. The precursor solution in a round-bottom flask was kept in a water bath above the piezoelectric modulator of a PIFCO ultrasonic humidifier (Model No. 1077). The aerosol droplets of the precursor thus generated were transferred into the hot wall zone of the reactor by the carrier gas. Both the solvent and the precursor were evaporated and the precursor vapor reached the heated substrate surface where thermally induced reactions and film deposition took place.

3. Result and discussion 3.1. Deposition of iron sulfide thin films from [Fe(S2CNEtiPr)3] (1) Deposition was carried out on silicon substrates at temperatures from 350 to 450 1C with an argon carrier gas flow rate of 160 sccm. Reflective dark brown uniform films were deposited at 350 and 450 1C, whereas black uniform films were obtained at 400 1C. The p-XRD patterns of the as deposited films at 300–450 1C (Fig. 1) show films of cubic pyrite (FeS1.96 (ICDD No. 01-0738127)) with a smaller amount of marcasite (FeS2 (ICDD No. 04003-2016)). Diffraction patterns of cubic pyrite (FeS1.96) planes of (111), (200), (210), (220), (311) and (023) were dominant at all deposition temperatures with only a minor peak at 54.631 (2y) corresponding to marcasite. The SEM images (Fig. 2(a)) show the sheet and rod-like crystallites (size 2–5 mm) from deposition at 350 1C. At deposition temperatures of 400 1C sheet-like crystallites (Fig. 2(b)) were deposited. Sheet and cube-like crystallites (Fig. 2(c)) were formed at 450 1C. EDX analysis shows that the films are composed of iron:sulfur ratio 50:50 (350 1C), 50:49 (400 1C), and 78:21 (450 1C). The elemental mapping (Fig. 2(d)) shows the uniform distributions of iron and sulfur. 3D AFM images were used to analyze the surface topography of the films. Fig. 3(a) shows well interconnected crystallites for the films deposited at 400 1C, with average roughness of 147.1 nm (Fig. 3(b)). A TEM image (Fig. 3(c)) was taken of the scratched films showing irregular shaped crystallites, which have a single crystalline nature as determined by the SAED pattern shown in Fig. 3(d).

(200)

(023) (111)

Intensity (a.u)

There have been a very limited number of iron complexes employed as single-source precursors for the deposition of iron sulfide as Fe1 þ xS, FeS2, and Fe1 xS thin films, which include dithiocarbamato complexes [Fe(S2CNRR0 )3] (R, R0 ¼Et, Et,1 Me, iPr) [37] and the sulfur-bridged binuclear iron carbonyl complex [Fe2(CO)6(m-S2)] [38]. Vanitha and O’Brien [39] reported pyrrhotite (Fe7S8) nanocrystals from a single source cubane-type Fe–S cluster by thermolysis in octylamine at 180 1C and greigite (Fe3S4) in dodecylamine at 200 1C. Gao et al. [40] used tris(diethyldithiocarbamato)iron(III) (Fe(S2CNEt2)3) and bis(diethyldithiocarbamato)Fe(II):1,10-phenanthroline [Fe(S2CNEt2)2](phen) to prepare nanosheets of greigite (Fe3S4) and pyrrhotite (Fe7S8) by thermolysis in oleylamine at 240–320 1C. Recently, we used a series of iron(III) thiobiurets complexes as single source precursors for the synthesis of iron sulfide thin films by the aerosol assisted chemical vapor deposition (AACVD) method [41]. Different iron sulfide phases, including FeS hexagonal troilite, cubic pyrite (FeS2) and tetragonal pyrrhotite (Fe1 xS), were deposited depending on the nature of precursor and the deposition temperature. Puthussery et al. [42] prepared colloidal pyrite NCs by injecting a solution of sulfur dissolved in diphenyl ether into a solution of FeCl2 in octadecylamine at 220 1C and stirring for several hours. Most recently we prepared the iron sulfide nanocrystals with greigite and pyrrhotite phases from thermolysis of tris(dialkyldithiocarbamato)iron(III) in oleylamine, hexadecylamine and octadecene [43]. Herein, we report the use of unsymmetrical and symmetrical iron(III) complexes of dialkyldithiocarbamates as single source precursors to synthesize iron sulfide thin films by the aerosolassisted chemical vapor deposition (AACVD) method.

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(210) (211)

(220)

(311) +

2.1. Precursor synthesis tris(dialkyldithiocarbamato)iron(III) compounds (1–4) were prepared as described previously by us [43–45]. 2.2. Deposition of thin films by AACVD

20 In a typical deposition, 0.25 g (0.461 mmol) of the precursor was dissolved in 15 ml toluene in a two-necked 100 ml round-bottom flask with a gas inlet that allowed the carrier gas (argon) to pass into the solution to aid the transport of the aerosol. This flask was connected to the reactor tube by a piece of reinforced tubing. The

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40 45 50 2- Theta (degree)

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Fig. 1. (a) Standard p-XRD patterns of iron sulfide (pyrite FeS1.96 (ICDD No. 01073-8127)) and thin films deposited from complex [Fe(S2CNEtiPr)3] (1) onto silicon at (b) 350, (c) 400, and (d) 450 1C. The (þ ) symbol denotes the marcasite (ICDD No. 04-003-2016).

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3.2. Deposition of iron sulfide thin films from [Fe(S2CNEtMe)3] (2) The p-XRD patterns of the as deposited films at 350 1C (Fig. 4(b)) correspond to the cubic pyrite (FeS1.96 (ICDD No. 01073-8127)) phase and at 400 1C (Fig. 4(c)) a mixture of cubic pyrite (FeS1.96) and hexagonal pyrrhotite-IT (Fe0.95S1.05 (ICDD No. 01-075-0600)) whereas the films deposited at 450 1C (Fig. 4(d)) consist of cubic pyrite (FeS1.96). All three samples show a weak diffraction peak at 54.631 (2y) corresponding to marcasite (ICDD No. 04-003-2016). The SEM images from the films deposited at 350 1C on silicon substrates from complex [Fe(S2CNEtMe)3] (2) show clusters of hexagonal plates with size range of 3–6 mm. The size of individual plates ranges from 0.5 to 1 mm (Fig. 5(a) and its inset). However, the films deposited at growth temperatures 400 and 450 1C show (Fig. 5(b) and (c)) sheet-like crystallites with an average length of

3 mm. EDX analysis showed that the films have iron:sulfur ratio 65:35 (350 1C), 53:44 (400 1C), and 64:35 (450 1C). The high-resolution transmission electron microscopy (HRTEM) images of the scratched films deposited from complex [Fe(S2CNEtMe)3] (2) at growth temperature 400 1C show the ˚ correspondlattice fringes (Fig. 5(d)) with a d-spacing of 3.12 (A) ing to a (111) reflection of cubic pyrite phase. The surface topography of the films analyzed by a 3D AFM image (Fig. 5(e)) of films deposited at 400 1C shows globular crystallites with an average roughness value of 136.7 nm (Fig. 5(f)).

3.3. Deposition of iron sulfide thin films from [Fe(S2CN(Hex)2)3] (3) The reflective black uniform films were deposited at 350, 400 and 450 1C. The p-XRD patterns of the as deposited films at 350

(200)

20µm

50µm

Intensity (a.u)

(023) (220)

*

20 20µm

10µm 10µm

Fig. 2. SEM images of iron sulfide (pyrite FeS1.96) films deposited from complex [Fe(S2CNEtiPr)3] (1) onto silicon at (a) 350, (b) 400, and (c) 450 1C, and (d) elemental mapping at 400 1C.

(311) +

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30

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40 45 50 2-Theta (degree)

*

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Fig. 4. (a) Standard p-XRD patterns of iron sulfide (pyrite FeS1.96 (ICDD No. 01073-8127)). Thin films deposited from complex [Fe(S2CNEtMe)3] (2) on silicon substrate at 350 1C show (b) pyrites, (c) at 400 1C a mixture of pyrite and pyrrhotite (*) (ICDD No. 01-075-0600) and (d) at 450 1C mostly pyrites whereas (þ) shows marcasite (ICDD No. 04-003-2016).

Fig. 3. (a) 3D AFM images of iron sulfide thin film and (b) average roughness and Rms roughness (c) shows TEM images of scratched thin film and (d) shows the SAED pattern of the thin films from precursor [Fe(S2CNEtiPr)3] (1) at 400 1C.


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Fig. 5. SEM images of iron sulfide thin films from precursor [Fe(S2CNEtMe)3] (2) deposited at (a) 350, (b) 400, and (c) 450 1C and (d) HRTEM at 400 1C showing d-spacing ˚ corresponding to (111) plane of pyrite phase, (e) 3D AFM image of iron sulfide thin film and (f) shows average roughness and Rms roughness of thin films from (3.12(A)) precursor (3) at 400 1C.

and 450 1C (Fig. 6(b) and (d)) show cubic pyrite (FeS1.96(ICDD No: 01-073-8127)) as the predominant phase with impurities of hexagonal pyrrhotite-IT (Fe0.95S1.05 (ICDD No. 01-075-0600)). Films produced at 400 1C mainly consist of the cubic pyrite (FeS1.96 (ICDD No. 01-073-8127); Fig. 6(c)) phase with a smaller amount of tetragonal mackinawite (FeS (ICDD No. 04-0036935)) phase. SEM images of films deposited at 350 and 400 1C show sheetlike crystallites (Fig. 7(a) and (b)) with an average size of 3–5 mm. Flower-like clusters were deposited (Fig. 7(c)) at 450 1C with an average size of 5–10 mm. EDX analysis shows that the films have iron:sulfur ratios 65:35 (350 and 400 1C), and 53:44 (450 1C). The elemental mapping (Fig. 7(d)) of the image shows the uniform distributions of iron and sulfur. SAED patterns of the films grown at 400 1C show the characteristic of single crystallites (Fig. 7(b) inset). The 3D AFM image (Fig. 7(e)) shows the growth of closely packed crystallites onto a silicon substrate at 400 1C with an average roughness of 52.74 nm (Fig. 7(f)).

(ICDD No. 04-003-2016). The p-XRD patterns of the as deposited films at 400 and 450 1C (Fig. 8(c) and (d)) show pure hexagonal pyrrhotite- IT (Fe0.95S1.05 (ICDD No. 01-075-0600)). The major diffraction peaks for hexagonal pyrrhotite-IT could be indexed to (100), (101), (102) and (110) planes. SEM images of films deposited at 350, 400 and 450 1C show sheet-like crystallites (Fig. 9(a)–(c)) with an average size of 1–5 mm. The EDX analysis of the films shows the ratio iron:sulfur as 61:39 (350 and 450 1C), and 56:44 (400 1C). The TEM image (Fig. 9(b) inset) from the scratched sample of films grown at 400 1C shows the characteristic of single crystallites. The elemental mapping of the films (Fig. 9(d)) shows almost equal distribution of iron and sulfur. The AFM image of the films deposited at 400 1C (Fig. 9(e)) shows the growth of closely packed crystallites onto a silicon substrate with an average roughness (Fig. 9(f)) of 7.48 nm.

4. Conclusion 3.4. Deposition of iron sulfide thin films from [Fe(S2CN(Et)2)3] (4) The p-XRD patterns of the as deposited films at 350 1C (Fig. 8(b)) show the cubic pyrite (FeS1.96 (ICDD No. 01-073-8127)) phase with major diffraction peaks (200), (220), (311), and (023). The weak diffraction peak at 54.631 (2y) corresponds to the marcasite phase

Iron sulfide minerals are typified by a range of significant temperature induced compositions and phase transformations. Our study involved the use of different deposition temperatures (350, 400, 450 1C) and also different precursors 1–4. The relative stabilities of various phases of iron sulfide are shown in Fig. 10 using a plot

110


(023)

(200)

(200) (311)

* Intensity (a.u)

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+

(100) (101) *

(102)

(110)

Intensity (a.u)

(220)

(023) (311) (220)

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Fig. 6. (a) Standard p-XRD patterns of iron sulfide (pyrite FeS1.96 (ICDD No. 01073-8127)), and iron sulfide thin films deposited from [Fe(S2CN(Hex)2)3] (3) onto silicon at (b) 350, (c) 400 and (d) 450 1C. The asterisk symbol (*) denotes the pyrrhotite phase (ICDD No. 01-075-0600). The major diffraction peaks could be indexed to (200), (220), (201), (311) and (023) planes of cubic pyrite (FeS1.96) and (*) shows pyrrhotite-IT (Fe0.95S1.05) (ICDD No. 01-075-0600).

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+

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Fig. 8. (a) Standard p-XRD patterns of iron sulfide (pyrite FeS1.96 (ICDD No. 01–073-8127)), and iron sulfide thin films deposited from complex [Fe(S2CN(Et)2)3] (4) (b) at 350 1C shows pyrites with (þ ) minor marcasite (ICDD No: 04-003-2016) (c) 400 1C, hexagonal pyrrhotite-IT (ICDD No. 01-075-0600) and (d) 450 1C hexagonal pyrrhotite-IT (ICDD No. 01-075-0600).

Fig. 7. SEM images of the films deposited from complex [Fe(S2CN(Hex)2)3] (3) at (a) 350 (b) 400 and (c) 450 1C. Inset (b) shows the SAED pattern (d) elemental mapping at 350 1C. (e) 3D AFM images of iron sulfide thin film and (f) average roughness and Rms roughness at 400 1C.


111

Fig. 9. SEM images of iron sulfide thin films deposited from precursor (4) at (a) 350, (b) 400 and (c) 450 1C. Inset (b) shows the SAED pattern, (d) elemental mapping at 400 1C, (e) 3D AFM images of iron sulfide thin films and (f) average roughness and Rms roughness from precursor (4) at 400 1C.

Fig. 10. The main phases of iron sulfide thin films from precursors (1)–(4). The relative amount of each phase is represented as the height of the cylinder. These are approximated based on the p-XRD results. Compared to the relative thermodynamic stabilities of the various phases of iron sulfides (the z-axis) and the phases after O’Brien and co-workers, Vaughan and Lennie [3] and Ramasamy et al. [41].

similar to that developed by O’Brien and co-workers, Vaughan and Lennie [3] and Ramasamy et al. [41]. The height of the pyramid on the negative z axis represents the free energy of formation of each

phase. The solid line represents the thermodynamic stability and connects the stable phases FeS (troilite) and FeS2 (pyrite). In this study two main phases were obtained, which were pyrite and pyrrhotite, shown at either end of the solid line in Fig. 10, suggesting that the deposition of the thin films is thermodynamically controlled. The unsymmetrical tris(dialkyldithiocarbamato)iron(III) complexes (1) and (2) deposited thin films with a mixture of pyrite, pyrrhotite and marcasite at all growth temperatures. The symmetrical tris(dialkyldithiocarbamato)iron(III) complexes with longer alkyl groups (3) gave a mixture of pyrite and pyrrhotite at 350 and 450 1C. The same complex at 400 1C gave a different mixture (pyrite and mackinawite). The symmetrical tris(dialkyldithiocarbamato)iron(III) with the shorter alkyl groups (4) gave the pure pyrrhotite phase at higher growth temperatures (400, 450 1C). The same precursor produced pure greigite phase nanocrystals from the thermolysis in oleylamine [43]. The pyrite phase is dominant in all samples deposited from the unsymmetrical complexes, whereas the pyrrhotite phase is dominant in samples deposited from symmetrical complexes. Most of the thin films produced at higher growth temperature are sulfur deficient due to the fact that at higher temperature, sulfur evaporates [46]. The unsymmetrical tris(dialkyldithiocarbamato)iron(III) complexes (1) and (2) produced mixed morphologies (sheet, rod, cube and hexagonal plate clusters), whereas the symmetrical complexes with longer alkyl groups (3) produced sheet and flowerlike clusters. Symmetrical complexes with shorter alkyl groups (4) only produced sheet-like crystallites.

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Acknowledgments MA thanks the EPSRC for funding. POB wrote this manuscript when he was a Visiting Fellow at IAS University of Durham. He would like to thank the university for the fellowship and Collingwood College and its fellows for being gracious hosts. References [1] C. Wadia, A.P. Alivisatos, D.M. Kammen, Environmental Science and Technology 43 (2009) 2072. [2] J.M. Soon, L.Y. Goh, K.P. Loh, Applied Physics Letters 91 (2007). 084105-1. [3] D.J. Vaughan, A.R. Lennie, Science Progress. (Edinburgh) 75 (1991) 371. [4] J.B. Goodenough, Materials Research Bulletin 13 (1978) 1305. [5] C.N.R. Rao, K.P.R. Pisharody, Progress in Solid State Chemistry 10 (1975) 207. [6] V. Bessergenev, Journal of Physics: Condensed Matter 16 (2004) S531. [7] J.R. Gosselin, M.G. Townsend, R.J. Tremblay, Solid State Communications 19 (1976) 799. [8] S. Kar, S. Chaushuri, Materials Letters 59 (2005) 289. [9] H. Nakazawa, N. Morimoto, Materials Research Bulletin 6 (1971) 345. [10] J.L. Horwood, M.G. Townsend, A.H. Webster, Journal of Solid State Chemistry 17 (1976) 35. [11] F. Li, F. Franzen, Journal of Solid State Chemistry 126 (1996) 108. [12] M.J. Almond, H. Redman, D.A. Rice, Journal of Materials Chemistry 10 (2002) 2842. [13] M. Nath, A. Choudhury, A. Kundu, C.N.R. Rao, Advanced Materials (Weinheim, Germany) 15 (2000) 2098. [14] X. Ma, F. Xu, X. Wang, Y. Du, L. Chen, Z. Zhang, Journal of Crystal Growth 277 (2005) 314. [15] D.M. Schleigh, H.S.W. Chang, Journal of Crystal Growth 112 (1991) 737. [16] B. Thomas, C. Hoepfner, K. Ellmer, S. Fiechter, H. Tributsch, Journal of Crystal Growth 146 (1995) 630. [17] B. Thomas, T. Cibik, C. Hoepfner, D. Diesner, G. Ehlers, S. Fiechter, K. Ellmer, Journal of Materials Science 9 (1998) 61. [18] B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Journal de Physique (Paris) 9 (1999). Pr8–613. [19] G. Smestad, E. Ennaoui, S. Fiechter, H. Tributsch, W.K. Hofman, M. Birkholz, Solar Energy Materials 20 (1990) 149. [20] B. Ouertani, J. Ouerfelli, M. Saadoun, B. Bessais, H. Ezzaouia, J.C. Bernede, Materials Characterization 54 (2005) 431. [21] M. Birkholz, D. Lichtenberger, C. Hoepfner, S. Fiechter, Solar Energy Materials and Solar Cells 27 (1992) 243.

[22] S. Bausch, B. Sailer, H. Keppner, G. Willeke, E. Bucher, G. Frommeyer, Applied Physics Letters 25 (1990) 57. [23] I.J. Ferrer, C. Sanchez, Journal of Applied Physics 70 (1991) 2641. [24] B. Rezig, H. Dalma, M. Kanzai, Renewable Energy 2 (1992) 125. [25] A. Ennaoui, G. Schlichtlorel, S. Fiechter, H. Tributsch, Solar Energy Materials and Solar Cells 25 (1992) 169. [26] G. Smestad, A. Da Silva, H. Tributsch, S. Fiechter, M. Kunst, N. Meziani, M. Birkholz, Solar Energy Materials 18 (1989) 299. [27] P.P. Chin, J. Ding, J.B. Yi., B.H. Liu, Journal of Alloys and Compounds 390 (2005) 255. [28] X. Shi, K. Sun, L.P. Balogh Baker, J.R., Jr. Nanotechnology 17 (2006) 4554. [29] F. Cao, W. Hu, L. Zhou, W. Shi, S. Song, Y. Lei, S. Wang, H. Zhang, Dalton Transactions (2009) 9246. [30] J.H.P. Watson, B.A. Cressey, A.P. Roberts, D.C. Ellwood, J.M. Charnock, A.K. Soper, Journal of Magnetism and Magnetic Materials 214 (2000) 13. [31] J.H.P. Watson, D.C. Ellwood, A.K. Soper Charnock, Journal of Magnetism and Magnetic Materials 203 (1999) 69. [32] X.X. Bi, P.C. Eklund, Materials Research Society Symposium Proceedings 286 (1993) 161. [33] K.M. Paknikar, V. Nagpal, A.V. Pethkar, J.M. Rajwade, Science and Tecnology of Advanced Materials 6 (2005) 370. [34] J.P. Wilcoxon, P.P. Newcomer, G.A. Samara, Solid State Communications 98 (1996) 581. [35] X. Chen, Z. Wang, X. Wang, J. Wan, J. Liu, Y. Qian, Inorganic Chemistry 44 (2005) 951. [36] B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chemical Vapor Deposition 6 (2000) 121. [37] P. O’Brien, D.J. Otway, J.H. Park, Materials Research Society Symposium Proceedings 606 (2000) 133. [38] S.G. Shyu, J.S. Wu, C.C. Wu, S.H. Chuang, K.M. Chi, Inorganica Chimica Acta 334 (2002) 276. [39] P.V. Vanitha, P. O’Brien, Journal of the American Chemical Society 130 (2008) 17256. [40] W. Han, M. Gao, Crystal Growth & Design 8 (2008) 1023. [41] K. Ramasamy, M.A. Malik, M. Helliwell, F. Tuna, P. O’Brien, Inorganic Chemistry 49 (2010) 8495. [42] J. Puthussery, S. Seefeld, N. Berry, M. Gibbs, M. Law, Journal of the American Chemical Society 13 (2011) 716. [43] M. Akhtar., J. Akhtar, M.A. Malik, P. O’Brien, F. Tuna, J. Raftery, M. Helliwell, Journal Of Materials Chemistry 21 (2011) 9737. [44] M.A. Malik, N. Revaprasadu, P. O’Brien, Chemistry of Materials 13 (2001) 913. [45] P.S. Nair, T.R.N. Radhakrishnan, G.A. Kolawole, Journal of Materials Chemistry 11 (2001) 1555. [46] M. Chunggaze, M.A. Malik, P. O’Brien, Advanced Materials for Optics and Electronics 7 (1997) 311.