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Structural and morphological studies of chemical bath-deposited nanocrystalline CdS films and its alloys
Materials Chemistry and Physics 93 (2005) 368–375
Structural and morphological studies of chemical bath-deposited nanocrystalline CdS films and its alloys S.N. Sharma a,∗ , R.K. Sharma a , K.N. Sood b , S. Singh b b
a Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Electron Microscope Section, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Received 22 January 2005; accepted 21 March 2005
Abstract The microstructure and surface morphology of chemical bath-deposited (CBD) nanocrystalline CdS films, its alloys (CdS:Ag and CdS:Mn) and the influence of growth conditions on their properties have been investigated by means of X-ray diffraction (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies, respectively. CBD nanocrystalline CdS films exhibit the coexistence of hexagonal and cubic phases. This is related to the polymorphic tendency of the CdS structure and indicates a strong influence of local parameters in the deposition conditions. Microscopic studies indicated that CdS films are formed from coalescence of individual crystallites with a high degree of crystallinity. The preferred orientation of CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. In CdS:Ag alloys, presence of Ag2 S monoclinic species is indicated and presence of uniform, monodispersive and spherical crystallites is evident. In CdS:Mn alloys, presence of well-defined hexagonal structures with size distribution of the order of 0.2–2.0 m corresponding to monocrystalline MnS species is indicated. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Alloys; Precipitation; Microstructure
1. Introduction Extensive research has been done on the deposition and characterization of semiconducting cadmium sulphide (CdS) thin films owing to their potential application in the area of optoelectronic device fabrication [1–3]. Polycrystalline CdS thin films have good optical transmittance, wide band-gap and electrical properties suitable for their application to photovoltaics [4]. CdS-based solar cell structure exhibits better optical confinement towards higher efficiencies [5]. In recent years, priority has been given to develop low-cost deposition technique for CdS thin films [6]. Though various deposition techniques such as electrodeposition [6], screen printing [7], sputtering [8], spray pyrolysis [9] have been reported, chemical bath-deposition (CBD) method has attracted much attention since it is confirmed as a simple and promising technique ∗ Corresponding author. Tel.: +91 11 25742609–14x2409; fax: +91 11 25726938/52. E-mail address: [email protected] (S.N. Sharma).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.03.022
to obtain device quality films [10,11]. It is a low-temperature, low-cost and scaleable technique, which has proven to yield very thin, but pinhole-free films of CdS, which have been used in the fabrication of CdTe/CdS solar cells having 15.8% efficiency [5]. In the CBD technique, CdS deposition is carried out in an alkaline solution (pH > 9) containing thiourea [SC(NH2 )2 ] and cadmium salt (CdCl2 ). When ammonia is used as the complexing agent, the overall reaction for CdS deposition is likely to be: CdCl2 + SC(NH2 )2 + 2H2 O → CdS + 2NH4 Cl + CO2 The formation of CdS can take place heterogeneously on the substrate surface, depositing CdS or homogeneously in solution, producing CdS precipitate. The homogeneous process is highly undesirable since the adsorption of CdS particles on the substrate surface yields powdery and non-adherent films. The homogeneous process may be suppressed by using conditions for the formation of CdS at low-rates, such as low-
S.N. Sharma et al. / Materials Chemistry and Physics 93 (2005) 368–375
concentration of CdCl2 and (NH2 )2 CS, high concentrations of NH3 and NH4 Cl, low-temperature, etc. The heterogeneous deposition takes place more readily on crystalline substrates. In the case of amorphous substrates, the process can be promoted by chemical etching of the substrate, which creates nucleation sites [12]. Despite extensive studies that have been performed on CdS thin films, the microstructure and surface morphology of CdS nanoclusters, its alloys CdS:Ag and CdS:Mn and the influence of growth conditions on their properties are not well understood. To our knowledge, no detailed structural and morphological studies have been done on CdS:Ag and CdS:Mn alloys, respectively. Questions remain about the internal structure of the films for, e.g., the attribution of the X-ray diffraction peaks to hexagonal, cubic or mixed structures being reported as a function of the deposition conditions is controversial. Another controversial aspect being the growth mechanism itself, i.e., growth from individual atoms (ion-by-ion or heterogeneous process) in solutions or to a cluster-by-cluster growth (homogeneous) mechanism in solutions. For this reason, it is imperative to study the influence of deposition variables on the structural properties of chemical bath-deposited nanocrystalline CdS films and its alloys. Here, we report the results of our structural and morphological studies of nanocrystalline CdS films and its alloys by means of X-ray diffraction (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies, respectively.
2. Experimental The deposition of CdS films has been carried out under a wide range of reactants concentration. Aqueous solutions containing (i) CdCl2 in the concentration range of 0.03–0.5 M, (ii) NH4 Cl in the concentration range of 0.03–0.5 M, (iii) (NH2 )2 CS in the concentration range of 0.03–0.5 M and (iv) liquid ammonia (∼12N) was used to raise the pH of the reaction mixture to ∼12. For silver alloying, the deposition conditions were as follows: [CdCl2 ] ∼ 0.03 M; [(NH2 )2 CS] ∼ 0.033 M; [NH4 Cl] ∼ 0.033 M and [AgNO3 ] ∼ 0.003 M, while for manganese alloying, we used [MnCl2 ] ∼ 0.003 M, instead of AgNO3 with the rest of the conditions being the same. Films were deposited on glass substrates and the temperature of the solution was maintained at ∼85 ◦ C. The substrate was
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placed vertically in the bath and the solution was magnetically stirred. Prior to the deposition, the substrates were cleaned and etched with 2% HF for about 5 min. The overall deposition rate was in the range of 10–15 nm min−1 . Multiple deposition runs, each from a fresh solution, were used for the preparation of the thicker films (d ≥ 300 nm). During the deposition the approximate starting times of the homogeneous and heterogeneous reactions were observed. Homogeneous reaction was marked by a turbid and opaque reaction bath. A predominant heterogeneous reaction was characterized by a clear reaction bath, while yellowish CdS film formed on the substrate. After each deposition, the coated substrate was ultrasonically cleaned in deionized water and dried. The film thickness was measured by the stylus method. The film structure was studied by X-ray diffraction using a Siemens ˚ In order to carry out the TEM D500 (Cu K␣ ∼ 1.5406 A). studies, carbon films deposited on copper grids were used as substrates. TEM and SEM studies were carried out using a JEOL (JEM-200CX) and LEO 440 electron microscope, respectively.
3. Results and discussion 3.1. Thickness dependence From the dependence of optical properties on the deposition conditions of CBD nanocrystalline CdS films, it is imperative to propose a set of suitable deposition conditions for CdS thin film applications as a window layer for polycrystalline solar cells. In this study, the standard conditions (S1) being: [CdCl2 ] ∼ 0.031 M, [(NH2 )2 CS] ∼ 0.031 M, [NH4 Cl] ∼ 0.031 M and NH3 ∼ 25.0 cm3 with bath temperature ∼85 ◦ C. Under these conditions, a maximum transparency (T > 80%) with a crystallite size ∼6 nm is obtained. The composition of the solution used for growing CdS samples and its alloys (CdS:Ag; CdS:Mn) and their physical properties are summarized in Table 1. The time dependence of the film thickness of nanocrystalline CdS films is shown in Fig. 1. From Fig. 1, two different regions can be observed, a first region, where the film thickness varies linearly with deposition time, indicating a constant growth rate, and a second region, where a film growth reaches saturation and the film growth virtually stops here. Initially, the film growth is negligible due to an “incubation” period required for the formation of critical nuclei from a
Table 1 Composition of the bath and physical properties of nanocrystalline CdS films (pH ∼ 12, temperature ∼ 85 ◦ C) System
Thickness (nm)
[CdCl2 ] (M)
[(NH2 )2 CS] (M)
[NH4 Cl] (M)
Grain size from XRD (nm)
CdS S0 CdS S1 CdS S2 CdS S1 (powder) CdS:Ag S3 (powder) CdS:Mn S3 (powder)
150 300 500 – – –
0.031 0.031 0.5 0.031 0.03 0.03
0.031 0.031 0.5 0.031 0.033 0.033
0.031 0.031 0.5 0.031 0.033 0.033
4 6 8 18 21 26
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Fig. 1. Time dependence of film thickness (d) for nanocrystalline CdS films.
homogeneous system onto a substrate. Once the nuclei are formed by a homogeneous nucleation process, the rate rises rapidly until the rate of deposition equals the rate of dissolution. Consequently, the film growth stops and one obtains a terminal thickness as shown in Fig. 1. The behavior observed
Fig. 2. SEM micrographs of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) d ∼ 150 nm and (b) d ∼ 300 nm.
in the saturation region can be explained by taking into account the CdS homogeneous precipitation, which takes place simultaneously to film deposition. The CdS precipitation produces a fast consumption of the Cd2+ - and S2− -free ions and within a short time of few minutes, the ion precursors reach an equilibrium situation for a given deposition condition, which avoids process evolution (thin film deposition and homogeneous precipitation). Despite this, parametric variations allow some continuation of the process. The rate of deposition and terminal thickness depends on the number of nucleation centers, supersaturation of the solution and stirring. The larger the number of nucleation centers, the more rapid is the deposition and the larger is the terminal thickness. Thus, for a given set of conditions (reagent concentrations, pH, temperature, etc.), we can expect a terminal film thickness, normally between 200 and 300 nm. Dipping the coated surfaces again in a fresh solution causes further deposition to take place. Thick and multilayer films can thus be deposited by sequential dippings. Fig. 2a and b shows SEM micrographs of nanocrystalline CdS films prepared under standard conditions with different thicknesses. The thicker films (d ∼ 300 nm) are deposited either by continuous dip, single dip or multiple dips. About the characteristic energy of CdS, they all exhibit similar optical properties. In the 150-nm-thick sample (Fig. 2a), needleshaped grains (size ∼ 100–200 nm) are seen. With increase in film thickness from 150 to 300 nm, the grains group together to form domains in thicker films as shown in Fig. 2b. Here, more dense and uniform needle-shaped crystallites
Fig. 3. XRD spectra of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) d ∼ 150 nm and (b) d ∼ 300 nm.
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(size ∼ 150–350 nm) are seen as compared to Fig. 2a. Formation of clusters are initiated at higher thicknesses (Fig. 2b). Comparison of Fig. 2a and b indicates that the domain size increases with the film thickness. Thus, for lower thickness CdS film, the presence of regular-shaped grains indicated that CdS films are formed from coalescence of individual crystallites with a high degree of crystallinity. The preferred orientation of CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. The crystallographic properties of CBD grown nanocrystalline CdS films have been investigated by the X-ray diffraction technique. CdS exists in two crystalline modifications: the hexagonal (H) (wurtzite phase) and the cubic (C) (zincblende) phase [13]. Fig. 3a and b shows the XRD pattern of the CdS films (d ∼ 150 and 300 nm) deposited on a glass
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substrate prepared under standard conditions. From Fig. 3a, corresponding to d ∼ 150 nm, the XRD pattern shows mainly an amorphous structure with some signatures of weak crystalline features. Due to size effect, the XRD peaks are broadened and their widths become larger. However, for thicker film (d ∼ 300 nm), the XRD pattern (Fig. 3b) indicates higher crystallinity. Here, diffraction peaks associated with both cubic and hexagonal phases are obtained with a strong peak at 2θ = 26.6◦ associated with the (0 0 2) reflection of hexagonal CdS or the (1 1 1) reflection of the cubic modification, which indicates a strong preferred orientation [13]. The preferred orientation of this CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. Besides this, two additional weak diffraction peaks at 2θ = 44.3◦ and 53◦ associated with the (1 1 0) and (1 1 2) reflections of
Fig. 4. TEM micrographs and the corresponding ED patterns of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) t ∼ 5 min; (b) t ∼ 10 min. (c) ED pattern of nanocrystalline CdS film prepared under S2 condition: [CdCl2 ] ∼ 0.5 M; [(NH2 )2 CS] ∼ 0.5 M and [NH4 Cl] ∼ 0.5 M.
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the hexagonal modification or the (2 0 0) and (3 1 1) reflections of the cubic CdS structure are also observed. It has been argued that the “ion-by-ion” process (heterogeneous reaction) results in compact films, which have pure hexagonal or mixed hexagonal or cubic phase, whereas the “cluster-bycluster” process (homogeneous reaction) gives rise to porous layers with pure cubic phase [13,14]. We therefore conclude that our CdS films, which consist of mixed hexagonal and cubic phases, are grown dominantly by the “ion-by-ion” process. From Table 1, it is evident that with increase in thickness from 150 to 300 nm, grain size (as evaluated from XRD studies) increases from 4 to 6 nm, respectively. Fig. 4a shows the TEM micrograph and the corresponding electron diffraction pattern of a CdS film obtained under standard conditions with a deposition time (t) of 5 min. Here, the grains are well shaped though sparsely populated, suggesting a well-defined crystalline structure and their size is in the range 20–60 nm. The corresponding electron diffraction (ED) pattern (inset of Fig. 4a) is composed of discontinuous rings, as can be expected for polycrystalline samples with good crystallinity [15]. Here, the ring pattern corresponds to fine grain and randomly oriented crystallites. However, upon increasing the deposition time to 10 min, the micrograph presents a dense and closed structure and the coalescence of the crystallites with size range 70–110 nm could be observed
(Fig. 4b). The corresponding ED pattern (inset of Fig. 4b) is more intense than that in Fig. 4a and it exhibits well-defined rings with spotty features, thus indicating large crystallite sizes as a result of coalescence of smaller crystallites. 3.2. Effect of stoichiometry As shown in Table 1, with increase in the concentration of the reactants from standard condition S1 [CdCl2 ∼ 0.031 M, (NH2 )2 CS ∼ 0.031 M and NH4 Cl ∼ 0.031 M] to S2 [CdCl2 ∼ 0.5 M, (NH2 )2 CS ∼ 0.5 M and NH4 Cl ∼ 0.5 M], the thickness of the films increases from 300 to 500 nm with concurrent increase in the crystallite size from 6 to 8 nm. Fig. 4c shows the electron diffraction pattern for the CdS film deposited under S2 condition. Here, the rings are much more diffuse, thus corresponding to a small crystallite size as compared to the electron diffraction pattern of S1 condition (Fig. 4b). Although the XRD data (Table 1) show an increase in crystallite size from 6 to 8 nm upon S1 –S2 condition, but TED pattern indicates that the colloids are made by the agglomeration of many smaller individual crystallites as compared to the crystallites formed under S1 condition. Under S2 condition, thick, powdery and rough films are obtained. The plausible explanation for the above behavior is as follows: with increase in concentration of the reactants
Fig. 5. XRD pattern of (a) CdS powders prepared under standard conditions (S1 ). (b and c) CdS:Ag and CdS:Mn powders prepared under S3 conditions, respectively.
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(S1 –S2 ), the growth rate increases and correspondingly, the grain size increases (Table 1) since the onset of colloidal formation takes place and the mode of the reaction now changes from heterogeneous-type to homogeneous-type, and hence the formation of non-adherent and powdery films. The homogeneous reaction leads to increase in defect density of the CdS films since it is known that the defect density of a crystalline particle increases simultaneously to the increase of the rate of formation of the particle [16]. The poorly resolved diffraction spectrum (Fig. 4c) of the colloids is a testimonial to this. This underlines the importance of our selection of standard conditions (S1 ), where nanocrystalline CdS films obtained were transparent, uniform and adherent with the mode of the reaction being heterogeneous-type. 3.3. Effect of alloying The nanocrystalline CdS films were prepared under standard conditions (S1 ). The CdS:Ag and CdS:Mn alloys were prepared under S3 conditions, the conditions were as follows: [CdCl2 ] ∼ 0.03 M; [(NH2 )2 CS] ∼ 0.033 M; [NH4 Cl] ∼ 0.033 M and [AgNO3 ]/MnCl2 ∼ 0.003 M, re-
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spectively. Fig. 5a shows XRD pattern of CdS powders from solution prepared under standard conditions. From Fig. 5a, it is evident that the XRD spectrum of the colloidal powder is different from the XRD pattern of CdS thin film (Fig. 3b). The film on glass substrate shows a simple spectrum (Fig. 3b), while the CdS powder precipitated in the solution shows many diffraction peaks associated mainly with both cubic and hexagonal phases; however, presence of orthorhombic phases is also indicated (Fig. 5a). Using Scherrer’s formula, the grain size of CdS powders from solution is ∼18 nm, while that calculated for CdS thin film is ∼6 nm (Table 1). The increase in the grain size from 6 to 18 nm as we go from CdS thin film to CdS powders from solution can be attributed to the change in the mode of reaction from heterogeneous-type to homogeneous-type due to the onset of colloidal formation. Fig. 5b shows the XRD pattern of CdS:Ag powder material, which indicates the existence of mainly monoclinic Ag2 S in addition to Ag cubic, AgS and CdS (cubic and hexagonal) phases, respectively. The diffraction peaks are indexed as shown in Fig. 5b according to the data from JCPDS file nos. 14-0072, 41-1402, 83-0674, 41-1049, 10-0454, respectively. From Fig. 5b, it could be concluded that incorporation of
Fig. 6. TEM micrographs and the corresponding ED patterns of: (a and b) CdS:Ag and CdS:Mn alloys prepared under S3 conditions, respectively. (c) TEM micrograph of a large hexagonal structure of size ∼2.0 m corresponding to CdS:Mn alloy.
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small amounts of Ag in the film implies changes in the crystal structure of CdS. Using the Scherrer’s formula and using the breadth of the peak at values of 2θ ∼ 27◦ , the size of the CdS–Ag composites is ∼21 nm, which is in good agreement with the size from TEM ∼20 nm. The XRD pattern of the CdS:Mn powder material shows peaks typical of particles in the nanosize regime (Fig. 5c). The main peaks associated with cubic MnS2 being (2 0 0), (2 1 0), (2 2 1) and (2 2 2) with the co-existence of (1 1 1) ␣MnS, (5 2 1) Mn (cubic), (1 1 1) CdS (cubic) and (0 0 2), (1 1 0) and (1 1 2) CdS (hexagonal) phases, respectively. The diffraction peaks are indexed as shown in Fig. 5c according to the data from JCPDS file nos. 3-1062, 25-549, 6-518, 6-314 and 100454, respectively. Using Scherrer’s formula, the size of the crystallites corresponding to CdS:Mn alloys are estimated to be ∼26 nm in accordance with TEM results. From Fig. 5c, the shift of the peaks associated with CdS lattice particularly at 2θ ∼ 27◦ could be ascribed to the partial substitution of Cd by the Mn as also observed by others [17]. Thus, sufficient incorporation of Mn into CdS lattice has occurred resulting in a mixed phase of cubic MnS2 and CdS structure. These results are different from others as inefficient manganese incorporation into CdS lattice has been reported since Mn has the tendency to segregate at the surface or forms MnS precipitates [18]. In our case, if segregation of Mn would have occurred then it would have rendered our CdS:Mn alloys unstable over longer periods of time, but our films were quite stable for several weeks. Fig. 6a–c shows the electron micrographs and the corresponding electron diffraction patterns of CdS:Ag and CdS:Mn alloy films, respectively. Fig. 6a shows the TEM micrograph and the corresponding TED pattern of CdS:Ag alloy film deposited under S3 conditions. The resulting CdS–Ag nanosize (size ∼ 20 nm) crystallites are essentially monodisperse and nearly spherical in shape. This is confirmed by the diffraction pattern as shown in the inset of Fig. 6a, which indicates a random distribution of fine grains. Fig. 6b shows the TEM micrograph and the corresponding TED pattern of CdS:Mn alloy film deposited under S3 conditions. Here, small particles (size ∼ 15–25 nm) are seen in the background of larger particles (size ∼ 100 nm) and a tendency to form droplet-like features are observed. Here, presence of well-defined hexagonal structures with size distribution of the order of 0.2 m is indicated (Fig. 6b). The diffraction pattern (inset of Fig. 6b) shows the presence of diffuse rings along with spotty pattern. The diffused rings indicate the presence of nanometric particles, while the spotty pattern indicate the presence of larger particles (Fig. 6b). Fig. 6c shows a grain having regular hexagon-like shape (size ∼ 2.0 m), which could correspond to a monocrystalline species [15]. This fact indicates that CdS:Mn alloy films are formed from coalescence of individual microscopic monocrystals with a high degree of crystallinity. A regular hexagonal shape can appear only when the crystallites have a hexagonal structure with their c-axis or a cubic structure with their [1 1 1] axis, parallel to the electron beam [15]. This structure could
correspond to that of cadmium hydroxide or a complex of MnS [19] (as also seen from XRD pattern Fig. 5c) arising probably at the liquid–air interface during the reaction process.
4. Conclusions Nanocrystalline CdS films and its alloys (CdS:Ag and CdS:Mn) have been successfully deposited by CBD technique. From the time dependence study of the film thickness of nanocrystalline CdS films, a constant and saturation growth regimes are observed and CdS films with the terminal thickness between 200 and 300 nm are obtained. Microscopic studies indicated that nanocrystalline CdS films are fairly uniform and composed of domains, which are formed by coalescence of smaller crystallites. The nanocrystalline CdS films exhibited both hexagonal and cubic phases and the mode of reaction for CdS formation proceeds via an ion-toion (heterogeneous process) rather than cluster coagulation (homogeneous) mechanism. The CdS:Ag alloys shows the presence of mainly monoclinic Ag2 S species and the resulting crystallites are uniform, monodispersive and spherical in shape. The CdS:Mn alloys show the co-existence of both nanometric and larger crystalline particles. Here, the presence of large (∼0.2–2.0 m) structure indicates the formation of MnS monocrystalline species.
Acknowledgment We thank the Director, National Physical Laboratory, New Delhi, India, for permission to publish the work.
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S.N. Sharma et al. / Materials Chemistry and Physics 93 (2005) 368–375 [15] B.E.P. Beeston, R.W. Horne, R. Markman, in: A.M. Glauert (Ed.), Electron Diffraction and Optical Diffraction Techniques, NorthHolland Pub. Co., Cambridge, 1972. [16] A.G. Walton, The Formation and Properties of Precipitates, R.E. Kreiger Publishing Company, Huttington, NY, 1979.
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[17] D.-S. Chuu, Y.-C. Chang, C.-Y. Hsieh, Thin Solid Films 304 (1997) 28. [18] G. Counio, T. Gaeoin, J.P. Boilot, J. Phys. Chem. B 102 (1998) 5237. [19] X.K. Zhao, S. Xu, J.H. Fendler, Langmuir 7 (1991) 520.
Structural and morphological studies of chemical bath-deposited nanocrystalline CdS films and its alloys S.N. Sharma a,∗ , R.K. Sharma a , K.N. Sood b , S. Singh b b
a Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Electron Microscope Section, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Received 22 January 2005; accepted 21 March 2005
Abstract The microstructure and surface morphology of chemical bath-deposited (CBD) nanocrystalline CdS films, its alloys (CdS:Ag and CdS:Mn) and the influence of growth conditions on their properties have been investigated by means of X-ray diffraction (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies, respectively. CBD nanocrystalline CdS films exhibit the coexistence of hexagonal and cubic phases. This is related to the polymorphic tendency of the CdS structure and indicates a strong influence of local parameters in the deposition conditions. Microscopic studies indicated that CdS films are formed from coalescence of individual crystallites with a high degree of crystallinity. The preferred orientation of CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. In CdS:Ag alloys, presence of Ag2 S monoclinic species is indicated and presence of uniform, monodispersive and spherical crystallites is evident. In CdS:Mn alloys, presence of well-defined hexagonal structures with size distribution of the order of 0.2–2.0 m corresponding to monocrystalline MnS species is indicated. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Alloys; Precipitation; Microstructure
1. Introduction Extensive research has been done on the deposition and characterization of semiconducting cadmium sulphide (CdS) thin films owing to their potential application in the area of optoelectronic device fabrication [1–3]. Polycrystalline CdS thin films have good optical transmittance, wide band-gap and electrical properties suitable for their application to photovoltaics [4]. CdS-based solar cell structure exhibits better optical confinement towards higher efficiencies [5]. In recent years, priority has been given to develop low-cost deposition technique for CdS thin films [6]. Though various deposition techniques such as electrodeposition [6], screen printing [7], sputtering [8], spray pyrolysis [9] have been reported, chemical bath-deposition (CBD) method has attracted much attention since it is confirmed as a simple and promising technique ∗ Corresponding author. Tel.: +91 11 25742609–14x2409; fax: +91 11 25726938/52. E-mail address: [email protected] (S.N. Sharma).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.03.022
to obtain device quality films [10,11]. It is a low-temperature, low-cost and scaleable technique, which has proven to yield very thin, but pinhole-free films of CdS, which have been used in the fabrication of CdTe/CdS solar cells having 15.8% efficiency [5]. In the CBD technique, CdS deposition is carried out in an alkaline solution (pH > 9) containing thiourea [SC(NH2 )2 ] and cadmium salt (CdCl2 ). When ammonia is used as the complexing agent, the overall reaction for CdS deposition is likely to be: CdCl2 + SC(NH2 )2 + 2H2 O → CdS + 2NH4 Cl + CO2 The formation of CdS can take place heterogeneously on the substrate surface, depositing CdS or homogeneously in solution, producing CdS precipitate. The homogeneous process is highly undesirable since the adsorption of CdS particles on the substrate surface yields powdery and non-adherent films. The homogeneous process may be suppressed by using conditions for the formation of CdS at low-rates, such as low-
S.N. Sharma et al. / Materials Chemistry and Physics 93 (2005) 368–375
concentration of CdCl2 and (NH2 )2 CS, high concentrations of NH3 and NH4 Cl, low-temperature, etc. The heterogeneous deposition takes place more readily on crystalline substrates. In the case of amorphous substrates, the process can be promoted by chemical etching of the substrate, which creates nucleation sites [12]. Despite extensive studies that have been performed on CdS thin films, the microstructure and surface morphology of CdS nanoclusters, its alloys CdS:Ag and CdS:Mn and the influence of growth conditions on their properties are not well understood. To our knowledge, no detailed structural and morphological studies have been done on CdS:Ag and CdS:Mn alloys, respectively. Questions remain about the internal structure of the films for, e.g., the attribution of the X-ray diffraction peaks to hexagonal, cubic or mixed structures being reported as a function of the deposition conditions is controversial. Another controversial aspect being the growth mechanism itself, i.e., growth from individual atoms (ion-by-ion or heterogeneous process) in solutions or to a cluster-by-cluster growth (homogeneous) mechanism in solutions. For this reason, it is imperative to study the influence of deposition variables on the structural properties of chemical bath-deposited nanocrystalline CdS films and its alloys. Here, we report the results of our structural and morphological studies of nanocrystalline CdS films and its alloys by means of X-ray diffraction (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies, respectively.
2. Experimental The deposition of CdS films has been carried out under a wide range of reactants concentration. Aqueous solutions containing (i) CdCl2 in the concentration range of 0.03–0.5 M, (ii) NH4 Cl in the concentration range of 0.03–0.5 M, (iii) (NH2 )2 CS in the concentration range of 0.03–0.5 M and (iv) liquid ammonia (∼12N) was used to raise the pH of the reaction mixture to ∼12. For silver alloying, the deposition conditions were as follows: [CdCl2 ] ∼ 0.03 M; [(NH2 )2 CS] ∼ 0.033 M; [NH4 Cl] ∼ 0.033 M and [AgNO3 ] ∼ 0.003 M, while for manganese alloying, we used [MnCl2 ] ∼ 0.003 M, instead of AgNO3 with the rest of the conditions being the same. Films were deposited on glass substrates and the temperature of the solution was maintained at ∼85 ◦ C. The substrate was
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placed vertically in the bath and the solution was magnetically stirred. Prior to the deposition, the substrates were cleaned and etched with 2% HF for about 5 min. The overall deposition rate was in the range of 10–15 nm min−1 . Multiple deposition runs, each from a fresh solution, were used for the preparation of the thicker films (d ≥ 300 nm). During the deposition the approximate starting times of the homogeneous and heterogeneous reactions were observed. Homogeneous reaction was marked by a turbid and opaque reaction bath. A predominant heterogeneous reaction was characterized by a clear reaction bath, while yellowish CdS film formed on the substrate. After each deposition, the coated substrate was ultrasonically cleaned in deionized water and dried. The film thickness was measured by the stylus method. The film structure was studied by X-ray diffraction using a Siemens ˚ In order to carry out the TEM D500 (Cu K␣ ∼ 1.5406 A). studies, carbon films deposited on copper grids were used as substrates. TEM and SEM studies were carried out using a JEOL (JEM-200CX) and LEO 440 electron microscope, respectively.
3. Results and discussion 3.1. Thickness dependence From the dependence of optical properties on the deposition conditions of CBD nanocrystalline CdS films, it is imperative to propose a set of suitable deposition conditions for CdS thin film applications as a window layer for polycrystalline solar cells. In this study, the standard conditions (S1) being: [CdCl2 ] ∼ 0.031 M, [(NH2 )2 CS] ∼ 0.031 M, [NH4 Cl] ∼ 0.031 M and NH3 ∼ 25.0 cm3 with bath temperature ∼85 ◦ C. Under these conditions, a maximum transparency (T > 80%) with a crystallite size ∼6 nm is obtained. The composition of the solution used for growing CdS samples and its alloys (CdS:Ag; CdS:Mn) and their physical properties are summarized in Table 1. The time dependence of the film thickness of nanocrystalline CdS films is shown in Fig. 1. From Fig. 1, two different regions can be observed, a first region, where the film thickness varies linearly with deposition time, indicating a constant growth rate, and a second region, where a film growth reaches saturation and the film growth virtually stops here. Initially, the film growth is negligible due to an “incubation” period required for the formation of critical nuclei from a
Table 1 Composition of the bath and physical properties of nanocrystalline CdS films (pH ∼ 12, temperature ∼ 85 ◦ C) System
Thickness (nm)
[CdCl2 ] (M)
[(NH2 )2 CS] (M)
[NH4 Cl] (M)
Grain size from XRD (nm)
CdS S0 CdS S1 CdS S2 CdS S1 (powder) CdS:Ag S3 (powder) CdS:Mn S3 (powder)
150 300 500 – – –
0.031 0.031 0.5 0.031 0.03 0.03
0.031 0.031 0.5 0.031 0.033 0.033
0.031 0.031 0.5 0.031 0.033 0.033
4 6 8 18 21 26
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Fig. 1. Time dependence of film thickness (d) for nanocrystalline CdS films.
homogeneous system onto a substrate. Once the nuclei are formed by a homogeneous nucleation process, the rate rises rapidly until the rate of deposition equals the rate of dissolution. Consequently, the film growth stops and one obtains a terminal thickness as shown in Fig. 1. The behavior observed
Fig. 2. SEM micrographs of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) d ∼ 150 nm and (b) d ∼ 300 nm.
in the saturation region can be explained by taking into account the CdS homogeneous precipitation, which takes place simultaneously to film deposition. The CdS precipitation produces a fast consumption of the Cd2+ - and S2− -free ions and within a short time of few minutes, the ion precursors reach an equilibrium situation for a given deposition condition, which avoids process evolution (thin film deposition and homogeneous precipitation). Despite this, parametric variations allow some continuation of the process. The rate of deposition and terminal thickness depends on the number of nucleation centers, supersaturation of the solution and stirring. The larger the number of nucleation centers, the more rapid is the deposition and the larger is the terminal thickness. Thus, for a given set of conditions (reagent concentrations, pH, temperature, etc.), we can expect a terminal film thickness, normally between 200 and 300 nm. Dipping the coated surfaces again in a fresh solution causes further deposition to take place. Thick and multilayer films can thus be deposited by sequential dippings. Fig. 2a and b shows SEM micrographs of nanocrystalline CdS films prepared under standard conditions with different thicknesses. The thicker films (d ∼ 300 nm) are deposited either by continuous dip, single dip or multiple dips. About the characteristic energy of CdS, they all exhibit similar optical properties. In the 150-nm-thick sample (Fig. 2a), needleshaped grains (size ∼ 100–200 nm) are seen. With increase in film thickness from 150 to 300 nm, the grains group together to form domains in thicker films as shown in Fig. 2b. Here, more dense and uniform needle-shaped crystallites
Fig. 3. XRD spectra of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) d ∼ 150 nm and (b) d ∼ 300 nm.
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(size ∼ 150–350 nm) are seen as compared to Fig. 2a. Formation of clusters are initiated at higher thicknesses (Fig. 2b). Comparison of Fig. 2a and b indicates that the domain size increases with the film thickness. Thus, for lower thickness CdS film, the presence of regular-shaped grains indicated that CdS films are formed from coalescence of individual crystallites with a high degree of crystallinity. The preferred orientation of CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. The crystallographic properties of CBD grown nanocrystalline CdS films have been investigated by the X-ray diffraction technique. CdS exists in two crystalline modifications: the hexagonal (H) (wurtzite phase) and the cubic (C) (zincblende) phase [13]. Fig. 3a and b shows the XRD pattern of the CdS films (d ∼ 150 and 300 nm) deposited on a glass
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substrate prepared under standard conditions. From Fig. 3a, corresponding to d ∼ 150 nm, the XRD pattern shows mainly an amorphous structure with some signatures of weak crystalline features. Due to size effect, the XRD peaks are broadened and their widths become larger. However, for thicker film (d ∼ 300 nm), the XRD pattern (Fig. 3b) indicates higher crystallinity. Here, diffraction peaks associated with both cubic and hexagonal phases are obtained with a strong peak at 2θ = 26.6◦ associated with the (0 0 2) reflection of hexagonal CdS or the (1 1 1) reflection of the cubic modification, which indicates a strong preferred orientation [13]. The preferred orientation of this CdS film is due to the controlled nucleation process associated with the low-formation rate of CdS. Besides this, two additional weak diffraction peaks at 2θ = 44.3◦ and 53◦ associated with the (1 1 0) and (1 1 2) reflections of
Fig. 4. TEM micrographs and the corresponding ED patterns of nanocrystalline CdS films prepared under standard conditions (S1 ): (a) t ∼ 5 min; (b) t ∼ 10 min. (c) ED pattern of nanocrystalline CdS film prepared under S2 condition: [CdCl2 ] ∼ 0.5 M; [(NH2 )2 CS] ∼ 0.5 M and [NH4 Cl] ∼ 0.5 M.
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the hexagonal modification or the (2 0 0) and (3 1 1) reflections of the cubic CdS structure are also observed. It has been argued that the “ion-by-ion” process (heterogeneous reaction) results in compact films, which have pure hexagonal or mixed hexagonal or cubic phase, whereas the “cluster-bycluster” process (homogeneous reaction) gives rise to porous layers with pure cubic phase [13,14]. We therefore conclude that our CdS films, which consist of mixed hexagonal and cubic phases, are grown dominantly by the “ion-by-ion” process. From Table 1, it is evident that with increase in thickness from 150 to 300 nm, grain size (as evaluated from XRD studies) increases from 4 to 6 nm, respectively. Fig. 4a shows the TEM micrograph and the corresponding electron diffraction pattern of a CdS film obtained under standard conditions with a deposition time (t) of 5 min. Here, the grains are well shaped though sparsely populated, suggesting a well-defined crystalline structure and their size is in the range 20–60 nm. The corresponding electron diffraction (ED) pattern (inset of Fig. 4a) is composed of discontinuous rings, as can be expected for polycrystalline samples with good crystallinity [15]. Here, the ring pattern corresponds to fine grain and randomly oriented crystallites. However, upon increasing the deposition time to 10 min, the micrograph presents a dense and closed structure and the coalescence of the crystallites with size range 70–110 nm could be observed
(Fig. 4b). The corresponding ED pattern (inset of Fig. 4b) is more intense than that in Fig. 4a and it exhibits well-defined rings with spotty features, thus indicating large crystallite sizes as a result of coalescence of smaller crystallites. 3.2. Effect of stoichiometry As shown in Table 1, with increase in the concentration of the reactants from standard condition S1 [CdCl2 ∼ 0.031 M, (NH2 )2 CS ∼ 0.031 M and NH4 Cl ∼ 0.031 M] to S2 [CdCl2 ∼ 0.5 M, (NH2 )2 CS ∼ 0.5 M and NH4 Cl ∼ 0.5 M], the thickness of the films increases from 300 to 500 nm with concurrent increase in the crystallite size from 6 to 8 nm. Fig. 4c shows the electron diffraction pattern for the CdS film deposited under S2 condition. Here, the rings are much more diffuse, thus corresponding to a small crystallite size as compared to the electron diffraction pattern of S1 condition (Fig. 4b). Although the XRD data (Table 1) show an increase in crystallite size from 6 to 8 nm upon S1 –S2 condition, but TED pattern indicates that the colloids are made by the agglomeration of many smaller individual crystallites as compared to the crystallites formed under S1 condition. Under S2 condition, thick, powdery and rough films are obtained. The plausible explanation for the above behavior is as follows: with increase in concentration of the reactants
Fig. 5. XRD pattern of (a) CdS powders prepared under standard conditions (S1 ). (b and c) CdS:Ag and CdS:Mn powders prepared under S3 conditions, respectively.
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(S1 –S2 ), the growth rate increases and correspondingly, the grain size increases (Table 1) since the onset of colloidal formation takes place and the mode of the reaction now changes from heterogeneous-type to homogeneous-type, and hence the formation of non-adherent and powdery films. The homogeneous reaction leads to increase in defect density of the CdS films since it is known that the defect density of a crystalline particle increases simultaneously to the increase of the rate of formation of the particle [16]. The poorly resolved diffraction spectrum (Fig. 4c) of the colloids is a testimonial to this. This underlines the importance of our selection of standard conditions (S1 ), where nanocrystalline CdS films obtained were transparent, uniform and adherent with the mode of the reaction being heterogeneous-type. 3.3. Effect of alloying The nanocrystalline CdS films were prepared under standard conditions (S1 ). The CdS:Ag and CdS:Mn alloys were prepared under S3 conditions, the conditions were as follows: [CdCl2 ] ∼ 0.03 M; [(NH2 )2 CS] ∼ 0.033 M; [NH4 Cl] ∼ 0.033 M and [AgNO3 ]/MnCl2 ∼ 0.003 M, re-
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spectively. Fig. 5a shows XRD pattern of CdS powders from solution prepared under standard conditions. From Fig. 5a, it is evident that the XRD spectrum of the colloidal powder is different from the XRD pattern of CdS thin film (Fig. 3b). The film on glass substrate shows a simple spectrum (Fig. 3b), while the CdS powder precipitated in the solution shows many diffraction peaks associated mainly with both cubic and hexagonal phases; however, presence of orthorhombic phases is also indicated (Fig. 5a). Using Scherrer’s formula, the grain size of CdS powders from solution is ∼18 nm, while that calculated for CdS thin film is ∼6 nm (Table 1). The increase in the grain size from 6 to 18 nm as we go from CdS thin film to CdS powders from solution can be attributed to the change in the mode of reaction from heterogeneous-type to homogeneous-type due to the onset of colloidal formation. Fig. 5b shows the XRD pattern of CdS:Ag powder material, which indicates the existence of mainly monoclinic Ag2 S in addition to Ag cubic, AgS and CdS (cubic and hexagonal) phases, respectively. The diffraction peaks are indexed as shown in Fig. 5b according to the data from JCPDS file nos. 14-0072, 41-1402, 83-0674, 41-1049, 10-0454, respectively. From Fig. 5b, it could be concluded that incorporation of
Fig. 6. TEM micrographs and the corresponding ED patterns of: (a and b) CdS:Ag and CdS:Mn alloys prepared under S3 conditions, respectively. (c) TEM micrograph of a large hexagonal structure of size ∼2.0 m corresponding to CdS:Mn alloy.
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small amounts of Ag in the film implies changes in the crystal structure of CdS. Using the Scherrer’s formula and using the breadth of the peak at values of 2θ ∼ 27◦ , the size of the CdS–Ag composites is ∼21 nm, which is in good agreement with the size from TEM ∼20 nm. The XRD pattern of the CdS:Mn powder material shows peaks typical of particles in the nanosize regime (Fig. 5c). The main peaks associated with cubic MnS2 being (2 0 0), (2 1 0), (2 2 1) and (2 2 2) with the co-existence of (1 1 1) ␣MnS, (5 2 1) Mn (cubic), (1 1 1) CdS (cubic) and (0 0 2), (1 1 0) and (1 1 2) CdS (hexagonal) phases, respectively. The diffraction peaks are indexed as shown in Fig. 5c according to the data from JCPDS file nos. 3-1062, 25-549, 6-518, 6-314 and 100454, respectively. Using Scherrer’s formula, the size of the crystallites corresponding to CdS:Mn alloys are estimated to be ∼26 nm in accordance with TEM results. From Fig. 5c, the shift of the peaks associated with CdS lattice particularly at 2θ ∼ 27◦ could be ascribed to the partial substitution of Cd by the Mn as also observed by others [17]. Thus, sufficient incorporation of Mn into CdS lattice has occurred resulting in a mixed phase of cubic MnS2 and CdS structure. These results are different from others as inefficient manganese incorporation into CdS lattice has been reported since Mn has the tendency to segregate at the surface or forms MnS precipitates [18]. In our case, if segregation of Mn would have occurred then it would have rendered our CdS:Mn alloys unstable over longer periods of time, but our films were quite stable for several weeks. Fig. 6a–c shows the electron micrographs and the corresponding electron diffraction patterns of CdS:Ag and CdS:Mn alloy films, respectively. Fig. 6a shows the TEM micrograph and the corresponding TED pattern of CdS:Ag alloy film deposited under S3 conditions. The resulting CdS–Ag nanosize (size ∼ 20 nm) crystallites are essentially monodisperse and nearly spherical in shape. This is confirmed by the diffraction pattern as shown in the inset of Fig. 6a, which indicates a random distribution of fine grains. Fig. 6b shows the TEM micrograph and the corresponding TED pattern of CdS:Mn alloy film deposited under S3 conditions. Here, small particles (size ∼ 15–25 nm) are seen in the background of larger particles (size ∼ 100 nm) and a tendency to form droplet-like features are observed. Here, presence of well-defined hexagonal structures with size distribution of the order of 0.2 m is indicated (Fig. 6b). The diffraction pattern (inset of Fig. 6b) shows the presence of diffuse rings along with spotty pattern. The diffused rings indicate the presence of nanometric particles, while the spotty pattern indicate the presence of larger particles (Fig. 6b). Fig. 6c shows a grain having regular hexagon-like shape (size ∼ 2.0 m), which could correspond to a monocrystalline species [15]. This fact indicates that CdS:Mn alloy films are formed from coalescence of individual microscopic monocrystals with a high degree of crystallinity. A regular hexagonal shape can appear only when the crystallites have a hexagonal structure with their c-axis or a cubic structure with their [1 1 1] axis, parallel to the electron beam [15]. This structure could
correspond to that of cadmium hydroxide or a complex of MnS [19] (as also seen from XRD pattern Fig. 5c) arising probably at the liquid–air interface during the reaction process.
4. Conclusions Nanocrystalline CdS films and its alloys (CdS:Ag and CdS:Mn) have been successfully deposited by CBD technique. From the time dependence study of the film thickness of nanocrystalline CdS films, a constant and saturation growth regimes are observed and CdS films with the terminal thickness between 200 and 300 nm are obtained. Microscopic studies indicated that nanocrystalline CdS films are fairly uniform and composed of domains, which are formed by coalescence of smaller crystallites. The nanocrystalline CdS films exhibited both hexagonal and cubic phases and the mode of reaction for CdS formation proceeds via an ion-toion (heterogeneous process) rather than cluster coagulation (homogeneous) mechanism. The CdS:Ag alloys shows the presence of mainly monoclinic Ag2 S species and the resulting crystallites are uniform, monodispersive and spherical in shape. The CdS:Mn alloys show the co-existence of both nanometric and larger crystalline particles. Here, the presence of large (∼0.2–2.0 m) structure indicates the formation of MnS monocrystalline species.
Acknowledgment We thank the Director, National Physical Laboratory, New Delhi, India, for permission to publish the work.
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