- Email: [email protected]
Deposition of morphology-tailored PbS thin films by surfactant-enhanced aerosol assisted chemical vapor deposition
Materials Science in Semiconductor Processing 46 (2016) 39–45
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Deposition of morphology-tailored PbS thin films by surfactantenhanced aerosol assisted chemical vapor deposition Malik Dilshad Khan a, Shahid Hameed a, Naghmah Haider b, Adeel Afzal c, Maria Chiara Sportelli d, Nicola Cioffi d, Mohammad Azad Malik e, Javeed Akhtar f,n a
Department of Chemistry, Quaid-i-azam University, Islamabad 45320, Pakistan Geoscience Advance Research Laboratories, Geological Survey of Pakistan, Islamabad, Pakistan c Affiliated Colleges at Hafr Al-Batin, King Fahd University of Petroleum and Minerals, P.O. Box 1803, Hafr Al-Batin 31991, Saudi Arabia d Department of Chemistry, University of Bari, via Orabona 4, Bari 70126, Italy e School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK f Polymers & Materials Synthesis (PMS) Laboratory, COMSATS Institute of Information Technology, Chak Shahzad, Islamabad, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 28 January 2016 Accepted 5 February 2016
In this work, we report the results of deposition of PbS thin films using single molecular precursor, bis(Oisobutylxanthato)lead(II), in the presence of additives namely: sodium dodecyl sulfate (SDS), Tween and Triton x-100, via aerosol assisted chemical vapor deposition (AACVD). The as-deposited PbS thin films are highly crystalline and exhibited superior adhesion to glass substrates. Powder X-ray diffraction (XRD) analysis confirmed the formation of pure cubic phase of PbS. Thin films deposited using 0.4 mM Triton X-100 as additive resulted in wire like structures while 0.8 mM Triton X-100 deposited thin films comprised of predominantly shoe shaped structures. Further, increase in concentration (1.2 mM) of Triton X-100 deposited films having rod like morphology. The scanning electron microscopy (SEM) confirmed that in the presence of SDS, thin films consist of spherical shaped crystallites. Energy dispersive X-ray spectroscopy (EDX) and X-ray photon electron microscopy (XPS) of as-deposited PbS thin films was used to study chemical composition of thin films. & 2016 Elsevier Ltd. All rights reserved.
Keywords: AACVD Surfactants Thin films PbS SDS
1. Introduction Nanostructured semiconductor materials (NSMs) have been extensively studied and explored for potential applications [1–3] in solar cell devices [4], self-cleaning coatings [5,6] and photocatalysis [7,8]. The unusual properties of NSMs are size as well as morphology dependent [9]. It has been observed that a slight variation in shape may have substantial effect on desired properties [4,6]. The efficiency of the dye sensitized solar cell can be increased up to 10 folds using vertically aligned ZnO nanorods [6,8,10]. Similarly, catalytic activity may be enhanced greatly by controlling the morphology of certain facets e.g.; (111) & (100) which are responsible for catalytic activity of the material, making the material a more active catalyst [3,7,8]. Therefore, the development of morphologically-controlled synthetic routes for nanostructured materials is necessary to explore their potential as more efficient and smart materials. A widely adopted approach for morphologically controlled synthesis is the solution based route to manipulate kinetic factors of growth using n
Corresponding author. E-mail address: [email protected] (J. Akhtar).
http://dx.doi.org/10.1016/j.mssp.2016.02.002 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
polymers, surfactants or by altering pH/or growth temperature [11– 14]. Surfactants play an important role in controlling the morphology of nanoparticles. They contain metal coordinating groups as well as solvophilic groups. The metal coordinating groups are generally electron donating which coordinate well with electron poor metal atoms in nanocrystals. This surfactant-metal interaction prevents the aggregation and further growth on surfactant bound facets of nanocrystals. The tail of surfactant extends to solvent and therefore determine the solubility of nanocrystals, which in most cases is hydrophobic in nature. In addition to nanocrystal binding, a surfactant also forms a complex with the reactive monomer species and plays its role in obtaining control over nanocrystal growth [15]. A number of cationic/non-ionic surfactants have been explored extensively to study their role in tailoring the shape of as-prepared nanoparticles [3,11–13]. However, the solution based synthesis involves complex procedures and reaction conditions and relatively longer reaction time is required. Therefore, a simple, fast and cost effective route is required to meet economic and industrial needs. Chemical vapor deposition is a simple technique to grow thin films on a wide variety of substrates [16]. Over the past few decades, this technique has been extensively explored by various
40
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.1. XRD patterns of PbS thin films deposited in the presence of (A) SDS, (B) Triton X-100 and (C) Tween. Samples X1-Z1 were grown without the use of surfactant while X2-Z2, X3-Z3 and X4-X5 are thin films grown using precursor to surfactant mole ratios of 1:1, 1:2 and 1:3 respectively.
2. Experimental
Fig.2. Plot of relative intensity of (200) peak vs. precursor: surfactant mole ratio used in AACVD experiments.
research groups [17–19] (O’Brien et al. Ivan parkin et al. [20,21]. A number of new single molecular precursors have been designed and utilized to grow binary/or ternary phased materials [21]. Further, the efficiency of CVD technique has been improved by engineering new tools to perform CVD process (AACVD, LPCVD, and APCVD) [16,17,22]. One of the huge challenge in CVD deposited thin films is to control the size and morphology of as-deposited crystallites/grains [23,24]. Nature of substrates, deposition temperature and precursor types are key factors that influence the size/shape of particles in CVD deposited thin films [16]. Thus, reproducibility is a key limitation to grow desired thin films by AACVD method. To address this problem, here we report successful deposition of PbS thin films using a single source precursor, bis(O-isobutyldithiocarbonato)lead(II) in the presence of different surfactants, such as SDS (anionic), Triton X-100 and Tween (non-ionic), with the remarkable control over the morphology of as prepared PbS thin films. This approach is unique and may be extended to deposit thin films of other metal chalcogenides as well. Moreover, this approach is equally effective for single/or dual source precursors route. The as-deposited PbS thin films were crystalline and the change in morphology were investigated by field emission scanning electron microscopy (FE-SEM). The purity and chemical composition was studied by energy dispersive X-ray spectroscopy (EDX) and X-ray photon electron spectroscopy (XPS).
The synthetic preparation of ligand was performed in an inert environment of nitrogen. All reagents were purchased from Sigma-Aldrich and were used as received without further purification. 1H NMR spectrum was obtained by Bruker 300 MHz spectrometer. Infrared spectrum was obtained between range of 4000–200 cm 1. Elemental analysis was performed which shows the ratio of different elements present in compound. TGA analysis was carried out at heating rate of 10 °C min 1, under nitrogen environment at flow rate of 50 ml per minute from 30 °C to 500 °C. Thin films of lead chalcogenide deposited on glass substrates were characterized by XRD using Bruker diffractometer (Cu-Kα). Samples were scanned between 10° and 80° with a step size of 0.05. Thin films were carbon coated for SEM and EDX analysis. XPS experiments were performed using a Theta Probe Thermo VG Scientific spectrometer equipped with a micro spot monochromatized AlKα source and a 180° spherical sector analyzer with a two-dimensional electron detector. All the spectra were recorded in Constant Analyzer Energy (CAE) mode using a pass energy of 150 eV for survey and 100 eV for high resolution regions (C1s, O1s, S2p, Na1s, Cl2p, Pb4f). Calibration of the binding energy (BE) scale was performed by taking a suitable signal as internal reference. In particular, the reference peak was the first component of the C1s signal (BE¼ 284.8 eV). The spectra acquisition parameters (channel exposition, number of scans, analyzer parameters, etc.) were selected in order to provide the best energy resolution and signal/ noise ratio. Data analysis and curve-fit procedures were performed by means of Avantage 4.75 commercial software. The same peak lineshape parameters (Gaussian/Lorentzian ratio and full width at half maximum) values were employed for the curve fitting of components belonging to the same high-resolution spectrum. S2p high resolution spectrum was first of all processed removing the doublet minor components (i.e. S2p1/2), exploiting an automatic computational routine provided by the Avantage software. Then, the resulting simplified S2p3/2 spectrum underwent curve-fitting procedures. Effective Attenuation Length (EAL) taken into account was calculated using the software on the basis of the TPP-2 M
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
41
Fig.3. Field emission scanning electron microscope images of PbS thin films deposited by using Triton x-100, (a) without Triton x-100, scale bar 1 mm (b) precursor: Triton x-100 mole ratio 1:1, scale bar 1 mm (c) 1:2, scale bar 1 mm (d) 1:3, scale bar 20 mm, inset scale bar 1 mm and (d) 1:4, scale bar 5 mm, inset scale bar 1 mm respectively.
formalism, allowing an automatic correction of the kinetic energies (KE) of the detected electrons.
3. Synthesis of the precursor The single source precursor (SSP) was prepared by a slight modification of the approach reported in the literature [2]. For this purpose, sodium hydroxide 2.2 g (54 mmol) was dissolved in excess isobutyl alcohol, which also acts as solvent, and stirred for 1 h. The solution was cooled in an ice-bath, carbon disulfide 3.3 ml (54 mmol) added drop-wise and the mixture stirred for another 1 h. The ligand, isobutyl xanthate sodium salt, was filtered, purified and dried. The ligand, 5 g (26 mmol) was dissolved in acetone and drop-wise addition of aqueous lead nitrate 4.4 g, (13 mmol) solution under constant stirring yielded bis(O-isobutyldithiocarbonato)lead(II) complex. Yield 5 g (76%), m.p 142 °C, and elemental analysis: Calc. for C10H18O2PbS4: C, 23.72%, H, 3.58%. Found: C, 23.67%, H, 3.37%. 1HNMR (CDCl3) (δ/ppm): 1.02 (6H, d, CH3), 2.14 (1H, m, CH), 4.42 (2H, d, CH2).
4. Deposition of thin films by AACVD Thin films of lead sulfide were deposited on glass substrates using in-house designed AACVD kit, consisting of carbolite furnace (21-101847, type MTS10/15/130) and a Deurer LB44 humidifier equipped with ultrasonic system. The generalized protocol adopted for deposition of thin films consists of dissolving 0.2 g (0.4 mmol) of SSP in 15 ml toluene in a 100 ml two necked roundbottom flask with a gas inlet attached to one neck to allow flow of
carrier gas (argon) into the solution and assists in transport of the aerosol. A piece of reinforced tubing connected the round bottom flask to the reactor tube. The flow rate of carrier gas, i.e. argon, was controlled by Platon flow gauge. Six borosilicate glass substrates (approx 1 3 cm) were placed inside reactor tube placed in heating zone of carbolite furnace. The round-bottom flask containing precursor solution was placed in water bath above piezoelectric modulator of the ultrasonic humidifier. The aerosol generated by ultrasonic humidifier was transferred to hot wall zone of the reactor with the help of carrier gas. When the solvent and precursor vapors reached the heated surface of the substrates, thermally induced reactions and film deposition took place.
5. Results and discussion Sodium salt of xanthate isobutyl alcohol was prepared followed by the formation of precursor Bis(O-isobutyldithiocarbonato)lead (II). The precursor is air and moisture stable at ambient laboratory conditions. The purity of compound was checked by thin layer chromatography which exhibited a single spot. The as-prepared precursor was characterized by elemental analysis and proton nuclear magnetic resonance. A doublet for two protons was observed at δ 4.42 ppm. This corresponds to protons attached to carbon adjacent to oxygen, hence are most de-shielded. A multiplet for one methine proton at δ 2.14 ppm and a doublet for six protons of two methyl groups at δ 1.02 ppm was observed. The bulk decomposition of SSP was carried out by thermogravimetric analysis (TGA) using nitrogen as the carrier gas (20 ml/min) from 30 to 600 °C at heating rate of 10 °C/min. The TGA shown in supporting information depicts that clean, one step decomposition
42
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.4. Field emission scanning electron microscope images of PbS thin films deposited by using SDS (a) without SDS, scale bar 1 mm (b) Precursor: SDS mole ratio (b) 1:1, scale bar 1 mm (c) 1:2, scale bar 1 mm and (d) 1:3, scale bar 5 mm respectively.
Fig.5. Field emission scanning electron microscope images of PbS thin films deposited by using Tween (a) Precursor: Tween mole ratio 1:1, scale bar 5 mm (b) 1:2, scale bar 5 mm and (c) 1:3, scale bar 5 mm, (d) scale bar 1 mm respectively.
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
43
Fig.6. (a) Pb4f high resolution region. (b) S2p3/2 high resolution region.
of precursor starts at 138 °C and completes at 162 °C with residual mass of 46. 3% against 47.3% theoretical mass. On the basis of TGA profile, it is revealed that the precursor is highly suitable for AACVD process. The deposition of thin films was carried out at 150 °C and 200 °C. Good quality thin films of high crystallinity were deposited at 200 °C while at lower temperature 150 °C, very little deposition took place. Furthermore at temperature of 200 °C, the additives also remain stable and do not decompose, thus providing suitable conditions to study the influence of these additives on as-deposited thin films. Thin films were black in color and showed good adherence to glass substrate (confirmed by scotch-tape test). The average thickness of as-deposited PbS was estimated 1.7 mm from SEM. The structural characterization of as-deposited PbS thin films in the presence of Triton X-100 (0.4, 0.84, 1.2 mM) was performed using powder X-ray diffraction (XRD), that conclusively showed cubic phase (Fig.1) of as-deposited PbS (ICDD 05-0592.). All deposited thin films were remarkably crystalline and highly textured along (200) plane (Fig.1). However, relative intensity of the peaks alters significantly on addition of Triton showing the change in texture or preferential orientation of crystallites. The relative intensity of the peak is maximum when only SSP was used without any additive. On addition of surfactant, the intensity of the peak decreases and this trend becomes more prominent at higher concentrations of surfactant (Fig.2), Plot of relative intensity of (200) peak vs. precursor: surfactant mole ratio used in AACVD experiments. The morphology of PbS thin films deposited in the absence of Triton X-100 comprised spherical shaped like clusters with small aggregate cube like crystallites (average size, 4 mm 7 2) as shown in Fig. 3(a). On addition of Triton x-100 in 1:1 M concentration, the PbS thin films obtained have rod like structures (approx. length 3.8 mm; width, 300 nm) in addition to small spherical crystallites (avg. size, approx. 80 nm) as depicted in Fig. 3(b). The size of crystallites further decreased and spherical shaped particles merged to constitute oblong shoe shaped structures (Fig. 3(c)) when precursor to surfactant ratio of 1:2 was used. Further increase in concentration of Triton X-100 to 1:3 resulted in the formation of curved rod like structures distributed uniformly on the substrate (Fig.3(d)). The inset in Fig. 3(d) shows the perfect cublike morphology of as-deposited PbS thin films by using precursor: surfactant mole ratio of 1:4. SDS surfactant: precursor ratio [1:1] favored mixed morphology with spherical/cube like structures (2 3 mm) of as-deposited PbS thin films at 200 °C (Fig. 4(b)). When amount of surfactant was doubled, the morphology was predominantly spherical crystallites
Table 1 Composition of as-deposited PbS thin film deposited by AACVD using surfactant determined by XPS Peaks
Position (eV)
Surface relative %
Pb (0) Pb-S Pb (II), Pb(IV) S-Pb S-H SOx SOx SO42
136.17 0.2 137.6 7 0.2 138.9 7 0.2 160.5 7 0.2 162.5 7 0.2 164.67 0.2 166.27 0.2 168.17 0.2
2.17 0.2 39.2 7 0.2 58.7 7 0.3 35.57 0.4 15.6 7 0.5 10.5 7 0.4 11.7 7 0.4 26.7 7 0.5
of large size (2 mm) as shown in Fig. 4(c). Perfect spherical crystallites were obtained when surfactant to precursor ratio was increased three time as shown in Fig. 4(d). PbS thin films were also deposited by using Tween, a non-ionic surfactant in different ratios (1:1, 1:2, and 1:3). Lower ratio (1:1, 1:2) produced PbS thin films with mixed spherical/cube like clusters (Fig.5(a–b)), however 1:3 ratio yielded small extended wire like crystallites distributed evenly on the substrate (Fig. 5(c–d)). XPS studies were performed on thin films of PbS deposited by using SDS [1:1] which revealed that lead is found in three states, Pb(0) 136.1 70.2, Pb-S 137.670.2 and oxygenated Pb(II), Pb(IV) 138.9 70.2 eV as shown in Fig. 6(b). XPS analysis of sulfur (S) 2p3/2 region was also performed which indicated that different species of sulfur are present on the surface of thin films (Fig. 6(a)). The data of both scans of lead and sulfur agreed well with literature values [23,24]. Table 1 shows the binding energy (BE) values of lead and sulfur regions, respectively [25,26]. The films used for XPS studies had been exposed to air during XRD measurements and, consequently, significant oxidation of the surfaces had taken place. UV/Vis spectroscopy of precursor solution with and without surfactant was performed (Fig. S1, supplementary information). On the addition of surfactant in precursor solution (e.g; Tween20), a new shoulder appeared around 350 360 nm indicating the formation of an adduct. The surfactants have reactive groups {(OH, C ¼O (Tween-20)), SO42 (SDS), OH (triton x-100)} and have ability to act as Lewis base. Similar type of reports have been described in literature [27,28]. Fig. 7 depicts plausible mechanism of formation of thin films. Precursor and additives when mixed in flask undergo adduct formation. These adducts entities on reaching heated substrates inside the furnace undergo decomposition, thus producing uniform well adhesive thin films. Further work is in
44
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.7. Schematic diagram showing the deposition of PbS thin films in the presence of surfactants, SDS, Tween and Triton x-100.
progress to elucidate the exact mechanism of decomposition of precursor and influence of additives on the morphology of asdeposited PbS thin films by computational chemistry.
[4]
[5]
6. Conclusion A systematic investigation was carried out for the first time to study the effects of surfactants on morphology of PbS thin films using single source precursor in AACVD. The PbS thin films were deposited at 200 °C. Triton X-100, favored rod like morphology of as-deposited PbS while SDS produced spherical shape crystallites predominantly. The change in morphology is achieved by using additives, without changing any other deposition parameter. This approach is unique and can be extended to deposit thin films of other metal chalcogenides as well. Moreover, this approach is equally effective for single/or dual source precursors route to deposit thin films by AACVD.
[6]
[7] [8] [9]
[10]
[11] [12]
[13]
Acknowledgments [14]
JA thanks the COMSATS Institute of Information Technology (CIIT) Islamabad for funding the grant number 16-61/CRGP/CIIT/ IBD/12/943 and also acknowledges financial assistance from the Higher Education Commission for grant number No: PM-IPFP/ HRD/HEC/2011/0585. MA thanks the University of Manchester, school of chemistry for financial assistance.
[15] [16] [17]
[18]
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2016.02.002.
[19]
[20]
References [1] J. Akhtar, M. Afzaal, M. Banski, A. Podhorodecki, M. Syperek, J. Misiewicz, et al., Controlled synthesis of tuned bandgap nanodimensional alloys of PbSxSe1 x, J. Am. Chem. Soc. 133 (2011) 5602–5609. [2] J. Akhtar, M. Afzaal, M.A. Vincent, N.A. Burton, J. Raftery, I.H. Hillier, et al., Understanding the decomposition pathways of mixed sulfur/selenium lead phosphinato complexes explaining the formation of lead selenide, J. Phys. Chem. C 115 (2011) 16904–16909. [3] K. Deori, S.K. Ujjain, R.K. Sharma, S. Deka, Morphology controlled synthesis of
[21]
[22]
[23] [24]
nanoporous Co3O4 nanostructures and their charge storage characteristics in supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 10665–10672. I. Gonzalez-Valls, M. Lira-Cantu, Dye sensitized solar cells based on verticallyaligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime, Energy Environ. Sci. 3 (2010) 789–795. V.A. Ganesh, H.K. Raut, A.S. Nair, S. Ramakrishna, A review on self-cleaning coatings, J. Mater. Chem. 21 (2011) 16304–16322. X. Pu, J. Liu, J. Liang, Y. Xia, W. Feng, Y. Wang, et al., Effective CdS/ZnO nanorod arrays as antireflection coatings for light trapping in c-Si solar cells, RSC Adv. 4 (2014) 23149–23154. A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. F. Xu, L. Sun, Solution-derived ZnO nanostructures for photoanodes of dyesensitized solar cells, Energy Environ. Sci. 4 (2011) 818–841. H. Cölfen, S. Mann, Higher-0rder organization by mesoscale self-assembly and transformation of hybrid nanostructures, Angew. Chem. Int. Ed. 42 (2003) 2350–2365. M. Que, W. Guo, X. Zhang, X. Li, Q. Hua, L. Dong, et al., Flexible quantum dotsensitized solar cells employing CoS nanorod arrays/graphite paper as effective counter electrodes, J. Mater. Chem. A 2 (2014) 13661–13666. B. Weintraub, Z. Zhou, Y. Li, Y. Deng, Solution synthesis of one-dimensional ZnO nanomaterials and their applications, Nanoscale 2 (2010) 1573–1587. J. Akhtar, M. Akhtar, M.A. Malik, P. O’Brien, J. Raftery, A single-source precursor route to unusual PbSe nanostructures by a solution–liquid–solid method, J. Am. Chem. Soc. 134 (2012) 2485–2487. J. Akhtar, M.A. Malik, P. O'Brien, M. Helliwell, Controlled synthesis of PbS nanoparticles and the deposition of thin films by Aerosol-Assisted chemical vapour deposition (AACVD), J. Mater. Chem. 20 (2010) 6116–6124. N. Fuke, L.B. Hoch, A.Y. Koposov, V.W. Manner, D.J. Werder, A. Fukui, et al., CdSe quantum-dot-sensitized solar cell with ∼100% internal quantum efficiency, ACS Nano 4 (2010) 6377–6386. Y. Yin, A.P. Alivisatos, Colloidal nanocrystal synthesis and the organic-inorganic interface, Nature 437 (2005) 664–670. A.C. Jones, P. O'Brien., CVD of compound semiconductors: precursor synthesis development and applications, VCH, Weinheim, 1997. M.A. Ehsan, H.N. Ming, M. Misran, Z. Arifin, E.R.T. Tiekink, A.P. Safwan, et al., Effect of AACVD processing parameters on the growth of Greenockite (CdS) thin films using a single-source cadmium precursor, Chem. Vap. Depos. 18 (2012) 191–200. P. Kevin, D.J. Lewis, J. Raftery, M. Azad Malik, P. O’Brien, Thin Films of SnS by aerosol-assisted chemical vapour deposition (AACVD) using tin(II) dithiocarbamates as single-source precursors, J. Cryst. Growth 415 (2015) 93–99. D.J. Lewis, P. O'Brien, Ambient pressure aerosol-assisted chemical vapour deposition of (CH3NH3)PbBr3, an inorganic–organic perovskite important in photovoltaics, Chem. Commun. 50 (2014) 6319–6321. N. Chadwick, S. Sathasivam, A. Kafizas, S.M. Bawaked, A.Y. Obaid, S. Al-Thabaiti, et al., Combinatorial aerosol assisted chemical vapour deposition of a photocatalytic mixed SnO2/TiO2 thin film, J. Mater. Chem. A 2 (2014) 5108–5116. C.R. Crick, I.P. Parkin, A single step route to superhydrophobic surfaces through aerosol assisted deposition of rough polymer surfaces: duplicating the lotus effect, J. Mater. Chem. 19 (2009) 1074–1076. K. Ahmad, M. Afzaal, P. O’Brien, G. Hua, J. Derek Woollins, Morphological evolution of PbSe crystals via the CVD route, Chem. Mater. 22 (2010) 4619–4624. L.R. Pederson, Two-dimensional chemical-state plot for lead using XPS, J. Electron Spectrosc. Relat. Phenom. 28 (1982) 203–209. K. Laajalehto, I. Kartio, P. Nowak, XPS study of clean metal sulfide surfaces, Appl. Surf. Sci. 81 (1994) 11–15.
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
[25] N.R. Mathews, C. Ángeles–Chávez, M.A. Cortés-Jácome, J.A. Toledo Antonio, Physical properties of pulse electrodeposited lead sulfide thin films, Electrochim. Acta 99 (2013) 76–84. [26] I. Kartio, K. Laajalehto, T. Kaurila, E. Suoninen, A study of galena (PbS) surfaces under controlled potential in pH 4.6 solution by synchrotron radiation excited photoelectron spectroscopy, Appl. Surf. Sci. 93 (1996) 167–177. [27] D.C. Onwudiwe, C.A. Strydom, E.C. Hosten, Effect of methyl substituent in
45
pyridine on the spectral and thermal properties of pyridyl adducts of Zn(II) and Cd(II) dithiocarbamate, Inorg. Chim. Acta 401 (2013) 1–10. [28] A.C. Ekennia, D.C. Onwudiwe, C. Ume, E.E. Ebenso, Mixed ligand complexes of N-Methyl-N-phenyl Dithiocarbamate: synthesis, characterisation, antifungal activity, and solvent extraction studies of the ligand, Bioinorg. Chem. Appl. 2015 (2015) 10.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Deposition of morphology-tailored PbS thin films by surfactantenhanced aerosol assisted chemical vapor deposition Malik Dilshad Khan a, Shahid Hameed a, Naghmah Haider b, Adeel Afzal c, Maria Chiara Sportelli d, Nicola Cioffi d, Mohammad Azad Malik e, Javeed Akhtar f,n a
Department of Chemistry, Quaid-i-azam University, Islamabad 45320, Pakistan Geoscience Advance Research Laboratories, Geological Survey of Pakistan, Islamabad, Pakistan c Affiliated Colleges at Hafr Al-Batin, King Fahd University of Petroleum and Minerals, P.O. Box 1803, Hafr Al-Batin 31991, Saudi Arabia d Department of Chemistry, University of Bari, via Orabona 4, Bari 70126, Italy e School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK f Polymers & Materials Synthesis (PMS) Laboratory, COMSATS Institute of Information Technology, Chak Shahzad, Islamabad, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 28 January 2016 Accepted 5 February 2016
In this work, we report the results of deposition of PbS thin films using single molecular precursor, bis(Oisobutylxanthato)lead(II), in the presence of additives namely: sodium dodecyl sulfate (SDS), Tween and Triton x-100, via aerosol assisted chemical vapor deposition (AACVD). The as-deposited PbS thin films are highly crystalline and exhibited superior adhesion to glass substrates. Powder X-ray diffraction (XRD) analysis confirmed the formation of pure cubic phase of PbS. Thin films deposited using 0.4 mM Triton X-100 as additive resulted in wire like structures while 0.8 mM Triton X-100 deposited thin films comprised of predominantly shoe shaped structures. Further, increase in concentration (1.2 mM) of Triton X-100 deposited films having rod like morphology. The scanning electron microscopy (SEM) confirmed that in the presence of SDS, thin films consist of spherical shaped crystallites. Energy dispersive X-ray spectroscopy (EDX) and X-ray photon electron microscopy (XPS) of as-deposited PbS thin films was used to study chemical composition of thin films. & 2016 Elsevier Ltd. All rights reserved.
Keywords: AACVD Surfactants Thin films PbS SDS
1. Introduction Nanostructured semiconductor materials (NSMs) have been extensively studied and explored for potential applications [1–3] in solar cell devices [4], self-cleaning coatings [5,6] and photocatalysis [7,8]. The unusual properties of NSMs are size as well as morphology dependent [9]. It has been observed that a slight variation in shape may have substantial effect on desired properties [4,6]. The efficiency of the dye sensitized solar cell can be increased up to 10 folds using vertically aligned ZnO nanorods [6,8,10]. Similarly, catalytic activity may be enhanced greatly by controlling the morphology of certain facets e.g.; (111) & (100) which are responsible for catalytic activity of the material, making the material a more active catalyst [3,7,8]. Therefore, the development of morphologically-controlled synthetic routes for nanostructured materials is necessary to explore their potential as more efficient and smart materials. A widely adopted approach for morphologically controlled synthesis is the solution based route to manipulate kinetic factors of growth using n
Corresponding author. E-mail address: [email protected] (J. Akhtar).
http://dx.doi.org/10.1016/j.mssp.2016.02.002 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
polymers, surfactants or by altering pH/or growth temperature [11– 14]. Surfactants play an important role in controlling the morphology of nanoparticles. They contain metal coordinating groups as well as solvophilic groups. The metal coordinating groups are generally electron donating which coordinate well with electron poor metal atoms in nanocrystals. This surfactant-metal interaction prevents the aggregation and further growth on surfactant bound facets of nanocrystals. The tail of surfactant extends to solvent and therefore determine the solubility of nanocrystals, which in most cases is hydrophobic in nature. In addition to nanocrystal binding, a surfactant also forms a complex with the reactive monomer species and plays its role in obtaining control over nanocrystal growth [15]. A number of cationic/non-ionic surfactants have been explored extensively to study their role in tailoring the shape of as-prepared nanoparticles [3,11–13]. However, the solution based synthesis involves complex procedures and reaction conditions and relatively longer reaction time is required. Therefore, a simple, fast and cost effective route is required to meet economic and industrial needs. Chemical vapor deposition is a simple technique to grow thin films on a wide variety of substrates [16]. Over the past few decades, this technique has been extensively explored by various
40
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.1. XRD patterns of PbS thin films deposited in the presence of (A) SDS, (B) Triton X-100 and (C) Tween. Samples X1-Z1 were grown without the use of surfactant while X2-Z2, X3-Z3 and X4-X5 are thin films grown using precursor to surfactant mole ratios of 1:1, 1:2 and 1:3 respectively.
2. Experimental
Fig.2. Plot of relative intensity of (200) peak vs. precursor: surfactant mole ratio used in AACVD experiments.
research groups [17–19] (O’Brien et al. Ivan parkin et al. [20,21]. A number of new single molecular precursors have been designed and utilized to grow binary/or ternary phased materials [21]. Further, the efficiency of CVD technique has been improved by engineering new tools to perform CVD process (AACVD, LPCVD, and APCVD) [16,17,22]. One of the huge challenge in CVD deposited thin films is to control the size and morphology of as-deposited crystallites/grains [23,24]. Nature of substrates, deposition temperature and precursor types are key factors that influence the size/shape of particles in CVD deposited thin films [16]. Thus, reproducibility is a key limitation to grow desired thin films by AACVD method. To address this problem, here we report successful deposition of PbS thin films using a single source precursor, bis(O-isobutyldithiocarbonato)lead(II) in the presence of different surfactants, such as SDS (anionic), Triton X-100 and Tween (non-ionic), with the remarkable control over the morphology of as prepared PbS thin films. This approach is unique and may be extended to deposit thin films of other metal chalcogenides as well. Moreover, this approach is equally effective for single/or dual source precursors route. The as-deposited PbS thin films were crystalline and the change in morphology were investigated by field emission scanning electron microscopy (FE-SEM). The purity and chemical composition was studied by energy dispersive X-ray spectroscopy (EDX) and X-ray photon electron spectroscopy (XPS).
The synthetic preparation of ligand was performed in an inert environment of nitrogen. All reagents were purchased from Sigma-Aldrich and were used as received without further purification. 1H NMR spectrum was obtained by Bruker 300 MHz spectrometer. Infrared spectrum was obtained between range of 4000–200 cm 1. Elemental analysis was performed which shows the ratio of different elements present in compound. TGA analysis was carried out at heating rate of 10 °C min 1, under nitrogen environment at flow rate of 50 ml per minute from 30 °C to 500 °C. Thin films of lead chalcogenide deposited on glass substrates were characterized by XRD using Bruker diffractometer (Cu-Kα). Samples were scanned between 10° and 80° with a step size of 0.05. Thin films were carbon coated for SEM and EDX analysis. XPS experiments were performed using a Theta Probe Thermo VG Scientific spectrometer equipped with a micro spot monochromatized AlKα source and a 180° spherical sector analyzer with a two-dimensional electron detector. All the spectra were recorded in Constant Analyzer Energy (CAE) mode using a pass energy of 150 eV for survey and 100 eV for high resolution regions (C1s, O1s, S2p, Na1s, Cl2p, Pb4f). Calibration of the binding energy (BE) scale was performed by taking a suitable signal as internal reference. In particular, the reference peak was the first component of the C1s signal (BE¼ 284.8 eV). The spectra acquisition parameters (channel exposition, number of scans, analyzer parameters, etc.) were selected in order to provide the best energy resolution and signal/ noise ratio. Data analysis and curve-fit procedures were performed by means of Avantage 4.75 commercial software. The same peak lineshape parameters (Gaussian/Lorentzian ratio and full width at half maximum) values were employed for the curve fitting of components belonging to the same high-resolution spectrum. S2p high resolution spectrum was first of all processed removing the doublet minor components (i.e. S2p1/2), exploiting an automatic computational routine provided by the Avantage software. Then, the resulting simplified S2p3/2 spectrum underwent curve-fitting procedures. Effective Attenuation Length (EAL) taken into account was calculated using the software on the basis of the TPP-2 M
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
41
Fig.3. Field emission scanning electron microscope images of PbS thin films deposited by using Triton x-100, (a) without Triton x-100, scale bar 1 mm (b) precursor: Triton x-100 mole ratio 1:1, scale bar 1 mm (c) 1:2, scale bar 1 mm (d) 1:3, scale bar 20 mm, inset scale bar 1 mm and (d) 1:4, scale bar 5 mm, inset scale bar 1 mm respectively.
formalism, allowing an automatic correction of the kinetic energies (KE) of the detected electrons.
3. Synthesis of the precursor The single source precursor (SSP) was prepared by a slight modification of the approach reported in the literature [2]. For this purpose, sodium hydroxide 2.2 g (54 mmol) was dissolved in excess isobutyl alcohol, which also acts as solvent, and stirred for 1 h. The solution was cooled in an ice-bath, carbon disulfide 3.3 ml (54 mmol) added drop-wise and the mixture stirred for another 1 h. The ligand, isobutyl xanthate sodium salt, was filtered, purified and dried. The ligand, 5 g (26 mmol) was dissolved in acetone and drop-wise addition of aqueous lead nitrate 4.4 g, (13 mmol) solution under constant stirring yielded bis(O-isobutyldithiocarbonato)lead(II) complex. Yield 5 g (76%), m.p 142 °C, and elemental analysis: Calc. for C10H18O2PbS4: C, 23.72%, H, 3.58%. Found: C, 23.67%, H, 3.37%. 1HNMR (CDCl3) (δ/ppm): 1.02 (6H, d, CH3), 2.14 (1H, m, CH), 4.42 (2H, d, CH2).
4. Deposition of thin films by AACVD Thin films of lead sulfide were deposited on glass substrates using in-house designed AACVD kit, consisting of carbolite furnace (21-101847, type MTS10/15/130) and a Deurer LB44 humidifier equipped with ultrasonic system. The generalized protocol adopted for deposition of thin films consists of dissolving 0.2 g (0.4 mmol) of SSP in 15 ml toluene in a 100 ml two necked roundbottom flask with a gas inlet attached to one neck to allow flow of
carrier gas (argon) into the solution and assists in transport of the aerosol. A piece of reinforced tubing connected the round bottom flask to the reactor tube. The flow rate of carrier gas, i.e. argon, was controlled by Platon flow gauge. Six borosilicate glass substrates (approx 1 3 cm) were placed inside reactor tube placed in heating zone of carbolite furnace. The round-bottom flask containing precursor solution was placed in water bath above piezoelectric modulator of the ultrasonic humidifier. The aerosol generated by ultrasonic humidifier was transferred to hot wall zone of the reactor with the help of carrier gas. When the solvent and precursor vapors reached the heated surface of the substrates, thermally induced reactions and film deposition took place.
5. Results and discussion Sodium salt of xanthate isobutyl alcohol was prepared followed by the formation of precursor Bis(O-isobutyldithiocarbonato)lead (II). The precursor is air and moisture stable at ambient laboratory conditions. The purity of compound was checked by thin layer chromatography which exhibited a single spot. The as-prepared precursor was characterized by elemental analysis and proton nuclear magnetic resonance. A doublet for two protons was observed at δ 4.42 ppm. This corresponds to protons attached to carbon adjacent to oxygen, hence are most de-shielded. A multiplet for one methine proton at δ 2.14 ppm and a doublet for six protons of two methyl groups at δ 1.02 ppm was observed. The bulk decomposition of SSP was carried out by thermogravimetric analysis (TGA) using nitrogen as the carrier gas (20 ml/min) from 30 to 600 °C at heating rate of 10 °C/min. The TGA shown in supporting information depicts that clean, one step decomposition
42
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.4. Field emission scanning electron microscope images of PbS thin films deposited by using SDS (a) without SDS, scale bar 1 mm (b) Precursor: SDS mole ratio (b) 1:1, scale bar 1 mm (c) 1:2, scale bar 1 mm and (d) 1:3, scale bar 5 mm respectively.
Fig.5. Field emission scanning electron microscope images of PbS thin films deposited by using Tween (a) Precursor: Tween mole ratio 1:1, scale bar 5 mm (b) 1:2, scale bar 5 mm and (c) 1:3, scale bar 5 mm, (d) scale bar 1 mm respectively.
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
43
Fig.6. (a) Pb4f high resolution region. (b) S2p3/2 high resolution region.
of precursor starts at 138 °C and completes at 162 °C with residual mass of 46. 3% against 47.3% theoretical mass. On the basis of TGA profile, it is revealed that the precursor is highly suitable for AACVD process. The deposition of thin films was carried out at 150 °C and 200 °C. Good quality thin films of high crystallinity were deposited at 200 °C while at lower temperature 150 °C, very little deposition took place. Furthermore at temperature of 200 °C, the additives also remain stable and do not decompose, thus providing suitable conditions to study the influence of these additives on as-deposited thin films. Thin films were black in color and showed good adherence to glass substrate (confirmed by scotch-tape test). The average thickness of as-deposited PbS was estimated 1.7 mm from SEM. The structural characterization of as-deposited PbS thin films in the presence of Triton X-100 (0.4, 0.84, 1.2 mM) was performed using powder X-ray diffraction (XRD), that conclusively showed cubic phase (Fig.1) of as-deposited PbS (ICDD 05-0592.). All deposited thin films were remarkably crystalline and highly textured along (200) plane (Fig.1). However, relative intensity of the peaks alters significantly on addition of Triton showing the change in texture or preferential orientation of crystallites. The relative intensity of the peak is maximum when only SSP was used without any additive. On addition of surfactant, the intensity of the peak decreases and this trend becomes more prominent at higher concentrations of surfactant (Fig.2), Plot of relative intensity of (200) peak vs. precursor: surfactant mole ratio used in AACVD experiments. The morphology of PbS thin films deposited in the absence of Triton X-100 comprised spherical shaped like clusters with small aggregate cube like crystallites (average size, 4 mm 7 2) as shown in Fig. 3(a). On addition of Triton x-100 in 1:1 M concentration, the PbS thin films obtained have rod like structures (approx. length 3.8 mm; width, 300 nm) in addition to small spherical crystallites (avg. size, approx. 80 nm) as depicted in Fig. 3(b). The size of crystallites further decreased and spherical shaped particles merged to constitute oblong shoe shaped structures (Fig. 3(c)) when precursor to surfactant ratio of 1:2 was used. Further increase in concentration of Triton X-100 to 1:3 resulted in the formation of curved rod like structures distributed uniformly on the substrate (Fig.3(d)). The inset in Fig. 3(d) shows the perfect cublike morphology of as-deposited PbS thin films by using precursor: surfactant mole ratio of 1:4. SDS surfactant: precursor ratio [1:1] favored mixed morphology with spherical/cube like structures (2 3 mm) of as-deposited PbS thin films at 200 °C (Fig. 4(b)). When amount of surfactant was doubled, the morphology was predominantly spherical crystallites
Table 1 Composition of as-deposited PbS thin film deposited by AACVD using surfactant determined by XPS Peaks
Position (eV)
Surface relative %
Pb (0) Pb-S Pb (II), Pb(IV) S-Pb S-H SOx SOx SO42
136.17 0.2 137.6 7 0.2 138.9 7 0.2 160.5 7 0.2 162.5 7 0.2 164.67 0.2 166.27 0.2 168.17 0.2
2.17 0.2 39.2 7 0.2 58.7 7 0.3 35.57 0.4 15.6 7 0.5 10.5 7 0.4 11.7 7 0.4 26.7 7 0.5
of large size (2 mm) as shown in Fig. 4(c). Perfect spherical crystallites were obtained when surfactant to precursor ratio was increased three time as shown in Fig. 4(d). PbS thin films were also deposited by using Tween, a non-ionic surfactant in different ratios (1:1, 1:2, and 1:3). Lower ratio (1:1, 1:2) produced PbS thin films with mixed spherical/cube like clusters (Fig.5(a–b)), however 1:3 ratio yielded small extended wire like crystallites distributed evenly on the substrate (Fig. 5(c–d)). XPS studies were performed on thin films of PbS deposited by using SDS [1:1] which revealed that lead is found in three states, Pb(0) 136.1 70.2, Pb-S 137.670.2 and oxygenated Pb(II), Pb(IV) 138.9 70.2 eV as shown in Fig. 6(b). XPS analysis of sulfur (S) 2p3/2 region was also performed which indicated that different species of sulfur are present on the surface of thin films (Fig. 6(a)). The data of both scans of lead and sulfur agreed well with literature values [23,24]. Table 1 shows the binding energy (BE) values of lead and sulfur regions, respectively [25,26]. The films used for XPS studies had been exposed to air during XRD measurements and, consequently, significant oxidation of the surfaces had taken place. UV/Vis spectroscopy of precursor solution with and without surfactant was performed (Fig. S1, supplementary information). On the addition of surfactant in precursor solution (e.g; Tween20), a new shoulder appeared around 350 360 nm indicating the formation of an adduct. The surfactants have reactive groups {(OH, C ¼O (Tween-20)), SO42 (SDS), OH (triton x-100)} and have ability to act as Lewis base. Similar type of reports have been described in literature [27,28]. Fig. 7 depicts plausible mechanism of formation of thin films. Precursor and additives when mixed in flask undergo adduct formation. These adducts entities on reaching heated substrates inside the furnace undergo decomposition, thus producing uniform well adhesive thin films. Further work is in
44
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
Fig.7. Schematic diagram showing the deposition of PbS thin films in the presence of surfactants, SDS, Tween and Triton x-100.
progress to elucidate the exact mechanism of decomposition of precursor and influence of additives on the morphology of asdeposited PbS thin films by computational chemistry.
[4]
[5]
6. Conclusion A systematic investigation was carried out for the first time to study the effects of surfactants on morphology of PbS thin films using single source precursor in AACVD. The PbS thin films were deposited at 200 °C. Triton X-100, favored rod like morphology of as-deposited PbS while SDS produced spherical shape crystallites predominantly. The change in morphology is achieved by using additives, without changing any other deposition parameter. This approach is unique and can be extended to deposit thin films of other metal chalcogenides as well. Moreover, this approach is equally effective for single/or dual source precursors route to deposit thin films by AACVD.
[6]
[7] [8] [9]
[10]
[11] [12]
[13]
Acknowledgments [14]
JA thanks the COMSATS Institute of Information Technology (CIIT) Islamabad for funding the grant number 16-61/CRGP/CIIT/ IBD/12/943 and also acknowledges financial assistance from the Higher Education Commission for grant number No: PM-IPFP/ HRD/HEC/2011/0585. MA thanks the University of Manchester, school of chemistry for financial assistance.
[15] [16] [17]
[18]
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2016.02.002.
[19]
[20]
References [1] J. Akhtar, M. Afzaal, M. Banski, A. Podhorodecki, M. Syperek, J. Misiewicz, et al., Controlled synthesis of tuned bandgap nanodimensional alloys of PbSxSe1 x, J. Am. Chem. Soc. 133 (2011) 5602–5609. [2] J. Akhtar, M. Afzaal, M.A. Vincent, N.A. Burton, J. Raftery, I.H. Hillier, et al., Understanding the decomposition pathways of mixed sulfur/selenium lead phosphinato complexes explaining the formation of lead selenide, J. Phys. Chem. C 115 (2011) 16904–16909. [3] K. Deori, S.K. Ujjain, R.K. Sharma, S. Deka, Morphology controlled synthesis of
[21]
[22]
[23] [24]
nanoporous Co3O4 nanostructures and their charge storage characteristics in supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 10665–10672. I. Gonzalez-Valls, M. Lira-Cantu, Dye sensitized solar cells based on verticallyaligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime, Energy Environ. Sci. 3 (2010) 789–795. V.A. Ganesh, H.K. Raut, A.S. Nair, S. Ramakrishna, A review on self-cleaning coatings, J. Mater. Chem. 21 (2011) 16304–16322. X. Pu, J. Liu, J. Liang, Y. Xia, W. Feng, Y. Wang, et al., Effective CdS/ZnO nanorod arrays as antireflection coatings for light trapping in c-Si solar cells, RSC Adv. 4 (2014) 23149–23154. A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. F. Xu, L. Sun, Solution-derived ZnO nanostructures for photoanodes of dyesensitized solar cells, Energy Environ. Sci. 4 (2011) 818–841. H. Cölfen, S. Mann, Higher-0rder organization by mesoscale self-assembly and transformation of hybrid nanostructures, Angew. Chem. Int. Ed. 42 (2003) 2350–2365. M. Que, W. Guo, X. Zhang, X. Li, Q. Hua, L. Dong, et al., Flexible quantum dotsensitized solar cells employing CoS nanorod arrays/graphite paper as effective counter electrodes, J. Mater. Chem. A 2 (2014) 13661–13666. B. Weintraub, Z. Zhou, Y. Li, Y. Deng, Solution synthesis of one-dimensional ZnO nanomaterials and their applications, Nanoscale 2 (2010) 1573–1587. J. Akhtar, M. Akhtar, M.A. Malik, P. O’Brien, J. Raftery, A single-source precursor route to unusual PbSe nanostructures by a solution–liquid–solid method, J. Am. Chem. Soc. 134 (2012) 2485–2487. J. Akhtar, M.A. Malik, P. O'Brien, M. Helliwell, Controlled synthesis of PbS nanoparticles and the deposition of thin films by Aerosol-Assisted chemical vapour deposition (AACVD), J. Mater. Chem. 20 (2010) 6116–6124. N. Fuke, L.B. Hoch, A.Y. Koposov, V.W. Manner, D.J. Werder, A. Fukui, et al., CdSe quantum-dot-sensitized solar cell with ∼100% internal quantum efficiency, ACS Nano 4 (2010) 6377–6386. Y. Yin, A.P. Alivisatos, Colloidal nanocrystal synthesis and the organic-inorganic interface, Nature 437 (2005) 664–670. A.C. Jones, P. O'Brien., CVD of compound semiconductors: precursor synthesis development and applications, VCH, Weinheim, 1997. M.A. Ehsan, H.N. Ming, M. Misran, Z. Arifin, E.R.T. Tiekink, A.P. Safwan, et al., Effect of AACVD processing parameters on the growth of Greenockite (CdS) thin films using a single-source cadmium precursor, Chem. Vap. Depos. 18 (2012) 191–200. P. Kevin, D.J. Lewis, J. Raftery, M. Azad Malik, P. O’Brien, Thin Films of SnS by aerosol-assisted chemical vapour deposition (AACVD) using tin(II) dithiocarbamates as single-source precursors, J. Cryst. Growth 415 (2015) 93–99. D.J. Lewis, P. O'Brien, Ambient pressure aerosol-assisted chemical vapour deposition of (CH3NH3)PbBr3, an inorganic–organic perovskite important in photovoltaics, Chem. Commun. 50 (2014) 6319–6321. N. Chadwick, S. Sathasivam, A. Kafizas, S.M. Bawaked, A.Y. Obaid, S. Al-Thabaiti, et al., Combinatorial aerosol assisted chemical vapour deposition of a photocatalytic mixed SnO2/TiO2 thin film, J. Mater. Chem. A 2 (2014) 5108–5116. C.R. Crick, I.P. Parkin, A single step route to superhydrophobic surfaces through aerosol assisted deposition of rough polymer surfaces: duplicating the lotus effect, J. Mater. Chem. 19 (2009) 1074–1076. K. Ahmad, M. Afzaal, P. O’Brien, G. Hua, J. Derek Woollins, Morphological evolution of PbSe crystals via the CVD route, Chem. Mater. 22 (2010) 4619–4624. L.R. Pederson, Two-dimensional chemical-state plot for lead using XPS, J. Electron Spectrosc. Relat. Phenom. 28 (1982) 203–209. K. Laajalehto, I. Kartio, P. Nowak, XPS study of clean metal sulfide surfaces, Appl. Surf. Sci. 81 (1994) 11–15.
M.D. Khan et al. / Materials Science in Semiconductor Processing 46 (2016) 39–45
[25] N.R. Mathews, C. Ángeles–Chávez, M.A. Cortés-Jácome, J.A. Toledo Antonio, Physical properties of pulse electrodeposited lead sulfide thin films, Electrochim. Acta 99 (2013) 76–84. [26] I. Kartio, K. Laajalehto, T. Kaurila, E. Suoninen, A study of galena (PbS) surfaces under controlled potential in pH 4.6 solution by synchrotron radiation excited photoelectron spectroscopy, Appl. Surf. Sci. 93 (1996) 167–177. [27] D.C. Onwudiwe, C.A. Strydom, E.C. Hosten, Effect of methyl substituent in
45
pyridine on the spectral and thermal properties of pyridyl adducts of Zn(II) and Cd(II) dithiocarbamate, Inorg. Chim. Acta 401 (2013) 1–10. [28] A.C. Ekennia, D.C. Onwudiwe, C. Ume, E.E. Ebenso, Mixed ligand complexes of N-Methyl-N-phenyl Dithiocarbamate: synthesis, characterisation, antifungal activity, and solvent extraction studies of the ligand, Bioinorg. Chem. Appl. 2015 (2015) 10.