Materials Chemistry and Physics 121 (2010) 555–560 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Effect of complexing agent on growth process and properties of nanostructured Bi2 S3 thin ﬁlms deposited by chemical bath deposition method A.U. Ubale ∗ Thin Film Physics Laboratory, Department of Physics, Govt. Vidarbha Institute of Science and Humanities, VMV Road, Amravati 444604, Maharashtra, India a r t i c l e i n f o Article history: Received 14 August 2009 Received in revised form 25 January 2010 Accepted 14 February 2010 Keywords: Bi2 S3 EDTA Electrical and structural properties a b s t r a c t Nanostructured Bi2 S3 thin ﬁlms have been prepared onto amorphous glass substrates by chemical bath deposition method at room temperature using bismuth nitrate and sodium thiosulphate as cationic and anionic precursors with EDTA as complexing agent in aqueous medium. The X-ray diffraction study reveals that the ﬁlms deposited without the complexing agent are amorphous in nature and becomes nanocrystalline in the presence of EDTA. The resistivity for the ﬁlms prepared from EDTA complexed bath is decreased due to the improvement in grain structure. The decrease in optical bandgap and activation energy is observed as the thickness of the ﬁlm varies from 45 to 211 nm on account of the variation of the volume of complexing agent in reaction bath. Studies reveal that the growth mechanism of Bi2 S3 gets affected in the presence of complexing agent EDTA and shows impact on structural, electrical and optical properties. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor nanostructured thin ﬁlms are always important in materials science due to their outstanding electronic and optical properties and extensively useful applications in various optoelectronic devices. To manufacture an electronic device needs a lot of information about the deposition processes, patterning and etching of various nanostructured semiconducting materials. There are number of chemical and physical methods such as chemical vapour deposition, spray pyrolysis, screen printing, electrodeposition, chemical bath deposition, pulsed laser deposition, sol–gel, electron beam evaporation, and metal organic chemical vapour deposition. Every technique has its own advantages and disadvantages. One of the greatest disadvantages is that some of them need very sophisticated instruments along with vacuum which increases the production cost of the material. But solution-based deposition method, i.e. chemical bath deposition of semiconducting materials offers the possibility of depositing thin ﬁlms at low temperature under atmospheric conditions at low fabrication cost. Binary semiconductors of the type AV BVI have been receiving considerable attention in recent years because of the possibility of their utilization in the fabrication of solar energy converters. Hence, it is interesting to study in detail the various properties of some of the members of this family. Bismuth-sulphide, which has a large bandgap, appears to be a suitable material for solar applications and has been used in liquid junction solar cells [1–3]. ∗ Tel.: +91 0721 2531706; fax: +91 0721 2531705. E-mail address: [email protected] 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.021 Different workers have reported chemical deposition of Bi2 S3 on different types of substrates with characterization such as chemical deposition , interface gas–solution , electrodeposition , and spray pyrolysis . Krishnamurthy and Shivkumar  deposited Bi2 S3 ﬁlms using the hot wall chemical deposition technique. Benramdane et al.  deposited Bi2 S3 ﬁlms onto glass substrates by spray pyrolysis method using bismuth chloride and thiourea having bismuth and sulphur source respectively. Pawar et al. [10,11] prepared amorphous Bi2 S3 and Sb2−x Bix S3 ﬁlms by the solution–gas interface method. Electrodeposition method was used by Lokhande and Bhosale  to prepare polycrystalline Bi2 S3 thin ﬁlms. Krishna Moorthy  has prepared polycrystalline stoichiometric Bi2 S3 ﬁlms by physical deposition technique. Pramanik and Bhattacharya  have also deposited amorphous Bi2 S3 thin ﬁlms from an alkaline bath using TEA as a complexing agent. Biswas et al.  have prepared thin ﬁlms of Bi2 S3 by solution growth technique using triethanolamine (TEA) as the complexing agent. Lokhande et al. [16,4] have deposited thin ﬁlms of Bi2 S3 from an alkaline as well as acidic bath using EDTA complexing agent. Ubale et al.  have prepared Bi2 S3 thin ﬁlms by modiﬁed chemical bath deposition at room temperature and reported their electrical and optical properties. The non-aqueous chemical deposition of the Bi2 S3 thin ﬁlms has been reported by Desai and Lokhande  using bismuth nitrate and thiourea in acetic acid and formaldehyde solvents respectively. In this paper we report on the growth mechanism of nanostructured Bi2 S3 thin ﬁlms deposited by chemical bath deposition method. Any insoluble surface, metallic or nonmetallic of any shape and size kept in contact with the solution may be deposited. The uniqueness of chemical bath deposition method is the low deposition temperature which avoids oxidation or corrosion of metallic 556 A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560 substrates. The deposition parameters can be easily controlled in order to deposit nanostructured material on the substrate. The main purpose of this work is to describe how the growth process of Bi2 S3 gets affected in the presence of the complexing agent EDTA (ethylenediaminetetraacetic acid) and also to study its impact on the structural, electrical and optical properties. 2. Experimental It is important to take proper precautions before depositing the ﬁlms. One of these is the careful cleaning of the substrates as it helps adhere the ﬁlms on the substrates most effectively. Initially, the slides were washed with liquid detergent and then, boiled in conc. chromic acid (0.5 M) for 2 h and kept in it for 48 h. The substrates were then washed with double distilled water. Finally the substrates were dried using AR grade acetone before using it for further procedure. For deposition of Bi2 S3 thin ﬁlms, bismuth nitrate was used as Bi3+ and sodium thiosulphate as S2− ion source in acidic medium. For this, 25 ml solutions of 0.05 M bismuth nitrate, and 25 ml of 0.05 M sodium thiosulphate were mixed together (pH = 2.5) and kept at 303 K temperatures for further deposition. The chemical deposition was based on the reaction between the EDTA complex of Bi3+ ions and sodium thiosulphate in acidic media. If the ionic product (IP) of Bi3+ and S2− exceeds the solubility product (SP) of Bi2 S3 , the formation of nuclei in the solution takes place. Thus to study the effect of complexing agent, six different bath compositions were carried out by changing the volume of EDTA. The thickness of the ﬁlm was measured by the gravimetric method. The twopoint dc probe method of dark electrical resistivity was used to study the variation of resistivity with temperature. The copper block was used as a sample holder and chromel–alumel thermocouple was used to measure the temperature. The area of the ﬁlm (0.5 cm2 ) was deﬁned and silver paste was applied to insure good ohmic contact to Bi2 S3 ﬁlms. For the measurement of resistivity, a constant voltage was applied across the sample and the current was noted using a digital nanometer. The structural studies were carried out using Philips PW 1710 diffractometer, with Cu-Ka radiation having wavelength = 1.5405 Å. Using JSM-6360 scanning electron microscope carried out the microstructural studies. The optical properties of Bi2 S3 were studied by using Systronics-119 spectrophotometer. 3. Results and discussion 3.1. Bi2 S3 growth mechanism To deposit bismuth-sulphide (Bi2 S3 ) thin ﬁlm a slow release of Bi3+ and S2− ions are required in an aqueous medium. These ions then get condensed on the substrate placed in the solution when its ionic product (I.P.) exceeds the solubility product (S.P.). The deposition process follows an ion-by-ion or cluster-by-cluster condensation on the substrate. In the ion-by-ion growth method for deposition of thin ﬁlms, the adsorption of metal ions on the surface of the substrate is an important step which forms the nucleation centers. However, in cluster-by-cluster growth process, the relatively large numbers of nuclei are formed in the solution as well as on the surface of the substrate which produces large number of small particles. As a result, there are large number of centers are formed upon which growth process can take place; none of the, particles grow very large and a colloidal suspension is formed. The process of precipitation of a substance from the solution onto a substrate depends mainly on the formation of a nucleus and subsequent growth of a ﬁlm. The rate of precipitate formation in the solution depends on the ionic concentration of bismuth, sulphur and complexing agent and deposition temperature. A schematic diagram that illustrates the CBD Bi2 S3 growth mechanism based on our current understanding is given in Fig. 2. As mentioned earlier, there are two major competing reactions in a CBD Bi2 S3 growth process: ion-by-ion and cluster-by-cluster precipitate formation. The bismuth salt (Bi(NO3 )3 ·5H2 O) produces free bismuth ions (Bi3+ ) through a dissociation reaction. Sodium thiosulphate releases free sulphide ions through an equilibrium hydrolysis reaction. Free bismuth ions then react with free sulphide ions to form Bi2 S3 particles in the bulk solution. However, in the presence of EDTA, bismuth ions form a complex, controlling the concentration of free bismuth ions. When EDTA is not added in the reaction bath, the ﬁlm growth rate is very high giving minimum terminal ﬁlm thickness indicating cluster-by-cluster deposition process. However, addition of EDTA suppresses the reaction rate indicating ion-by-ion deposition which gives higher ﬁlm thickness. Thus, the addition of EDTA in the reaction bath was increased from 0 to 25 ml. When EDTA content was small, the main species of bismuth are (Bi3+ ) non-chelated. In the presence of EDTA the Bi3+ ions form a complex, as trivalent bismuth has high afﬁnity to form complex with organic ligands, which inﬂuences availability of Bi3+ ions in the solution. The complexation of organic ligands may also inﬂuence the activity of organic ligands. Low quantity of EDTA in reaction bath can increase the solubility of many metals, and thereby, may increase their availability. For bismuth, which has high solubility, the added EDTA does not Fig. 1. Schematic diagram of CBD Bi2 S3 growth mechanism. A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560 557 Table 1 Variation of Bi2 S3 ﬁlm thickness with EDTA volume. Film A B C D E F Bath composition (ml) Thickness (nm) 0.5 M bismuth nitrate 0.2 M EDTA 0.2 M sodium thiosulphate 25 25 25 25 25 25 00 05 10 15 20 25 25 25 25 25 25 25 45 81 111 139 191 201 likely increase the bismuth stimulative effect in the growth process. In the growth process, the availability of Bi3+ ions for S2− is higher than [Bi(EDTA)]+ . Consequently, bismuth chelated with EDTA, would not increase its stimulative effect. In this way, EDTA is an amino derived organic compound known to be a strong hexdentate chelating agent. It forms a complex with metal ions and dissociates reversibly at low rate. The ion-by-ion and cluster-bycluster growth mechanisms of Bi2 S3 are schematically represented in Fig. 1. In present process, it was observed that without EDTA, the rate of dissociation is high which causes fast precipitation in the bath. However, when EDTA is added, it reduces the rate of dissociation by forming complex. The rate of dissociation decreases with EDTA volume in reaction bath which gives more terminal thickness. The variation of ﬁlm thickness with EDTA volume for 4 h deposition time is shown in Table 1. 3.2. Structural characterization X-ray diffraction is a powerful non-destructive method for material characterization, by which the crystal structure, orientation, and grain size can be determined. Structural identiﬁcation of Bi2 S3 ﬁlms was carried out with X-ray diffraction in the range of angle 2 between 20 and 60◦ . Fig. 2 shows the XRD patterns of Bi2 S3 thin ﬁlms. The observed broad hump in XRD pattern is due to amorphous glass substrate. The ﬁlm having thickness 45 and 81 nm is amorphous in nature and show nanocrystalline nature with orthorhombic structure for higher thickness. Table 2 summaries the crystallographic data of these ﬁlms compared with ASTM data ﬁle (JCPDS 170320) . Well deﬁned (2 2 0), (1 3 0), (2 1 1), (3 1 1) and (2 3 1) peaks are observed in the XRD pattern. These results are in good agreement with that obtained by Benramdane et al. , and Mizogushi et al. . The ﬁlms are oriented in the (1 3 0) direction. The crystallite size of the ﬁlm was determined from the line (1 3 0) by using Scherrer formula: d= ˇ cos Fig. 2. X-ray diffraction patterns of Bi2 S3 thin ﬁlm of thickness: (A) 45 nm; (B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm. (1) where is the wavelength used (1.54 Å); ˇ is the angular line width at half maximum intensity in radians; is the Bragg’s angle. It was found that crystallite size increases from 11 to 26 nm as ﬁlm thickness increases from 111 to 201 nm. This signiﬁcant improvement in crystallite size is due to controlled slow release of Fig. 3. Variation of log vs. 1/T × 103 (K−1 ) for Bi2 S3 ﬁlms of thickness: (A) 45 nm; (B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm. Table 2 Comparison of XRD data for Bi2 S3 thin ﬁlms with the JCPDS card. Standard data JCPDS card 170320 Observed data Bi2 S3 ﬁlm Thickness 201 nm Thickness 191 nm Thickness 139 nm Thickness 111 nm hkl d (Å) d (Å) d (Å) d (Å) d (Å) 220 130 211 311 231 3.967 3.569 3.118 2.641 2.305 3.977 3.566 3.112 2.634 2.318 3.965 3.559 3.120 2.648 2.311 – 3.561 3.110 2.639 – – 3.570 3.116 – – 558 A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560 Fig. 4. Variation of resistivity of Bi2 S3 ﬁlm with reciprocal of thickness at temperature 373 K. bismuth ions from its complex [Bi(EDTA)]+ in the solutions which give probability of growth of larger particles. 3.3. Electrical resistivity The dark electrical resistivity of Bi2 S3 ﬁlms was studied in the temperature range 333–453 K using dc two-point probe method. Fig. 3 shows the variation of log of resistivity (log ) with reciprocal of temperature (1/T) × 103 . It is seen that resistivity decreases with temperature indicating semiconducting nature of ﬁlms. The resistivity variation with ﬁlm thickness at temperature 373 K is given in Fig. 4. The resistivity of Bi2 S3 is of the order of 104 cm and it decreases from 24.08 × 104 to 4.33 × 104 cm as the ﬁlm thickness increases from 45 to 201 nm. The high value of resistivity is due to large number of grain boundaries and discontinues in the ﬁlm. The thermal activation energy was calculated using the relation: = 0 exp E 0 KT , (2) where is resistivity at temperature T, 0 is a constant, K is Boltzmann constant (8.62 × 10−5 eV K−1 ) and E0 is the activation energy required for conduction. Fig. 5. Variation of ﬁlm thickness and activation energy of Bi2 S3 with EDTA volume in reaction bath. The activation energy was found to be decreased from 0.85 to 0. 41 nm as ﬁlm thickness varies from 45 to 201 nm. Fig. 5 shows variation of activation energy and ﬁlm thickness with volume of EDTA in reaction bath. As per reaction mechanism, the addition of EDTA slows down the growth rate giving more terminal thickness with better grain growth. 3.4. Surface morphology The SEM of the Bi2 S3 thin ﬁlms of thickness 45, 81, 139 and 191 nm deposited on the glass substrate was examined (Fig. 6). The surface of the ﬁlm is smooth, well covering the glass substrate. The nucleation centers formation rate of Bi2 S3 depends on the rates of release Bi3+ and S2− ions. Hence, only a large number of the ions are utilized for ﬁlm formation by cluster-by-cluster, resulting in a lower ﬁnal thickness with smaller grains as seen in sample (A). However in the presence of EDTA, complex formation takes place and due to slow release of bismuth ions few nucleation centers are formed by ion-by-ion growth resulting in a higher thickness with larger grains as seen in sample (B), (D) and (E). The improvement in grain growth Fig. 6. SEM images of Bi2 S3 ﬁlm of thickness: (A) 45 nm; (B) 81 nm; (D) 139 nm and (E) 191 nm. A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560 559 Fig. 9. Variation of ﬁlm thickness and optical bandgap energy of Bi2 S3 with EDTA volume in reaction bath. by using the relation: n Fig. 7. Variation of optical absorption vs. wavelength of Bi2 S3 ﬁlms of thickness: (A) 45 nm; (B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm. is observed with percentage of EDTA in reaction bath. The grain is quite irregular in shape for the sample deposited without EDTA and shows somewhat circular in nature for the samples deposited with EDTA (sample E) which conﬁrms ion-by-ion growth of Bi2 S3 . 3.5. Optical properties The optical properties of the chemically deposited Bi2 S3 ﬁlms were characterized by optical absorption measurements. The absorption spectra were recorded with a Systronics-119 spectrophotometer typically in the wavelength range of 300–1100 nm at room temperature (Fig. 7). The nature of transition involved in semiconductors can be determined on the basis of dependence of absorption coefﬁcient (˛) of the material on incident photon energy (h). The nature of the transition (direct or indirect) is determined ˛= A(hv − Eg) , hv (3) where h is the photon energy, Eg is the bandgap energy, A and n are constants. For allowed direct transitions n = 1/2 and for allowed indirect transitions n = 2. Fig. 8 shows the spectral dependence of ˛, in the form of (˛h)1/2 against h. The values of the optical bandgap were determined for Bi2 S3 ﬁlms of different thicknesses and are shown in Fig. 9. It is clear from this ﬁgure that the energy gap increases from 1.84 eV to 2.66 eV with increasing thickness of the ﬁlms from 201 to 45 nm. The presence of defects in the nanostructured ﬁlms produces discrete states in the band structure which is responsible for the high value of the energy gap in the case of ﬁlm of thickness 45 nm. However, for higher thickness, the ﬁlms are more homogeneous and reduce the number of defects and disorder which decreases the density of localized states in the band structure and consequently decreases the optical energy gap. Also the result of the conﬁnement of the exciton, essentially as the particle size is reduced, the hole and the electron are forced closer together, and the separation between the energy levels changes. 4. Conclusions In the present investigation, the effect of complexing agent EDTA on growth process was studied. EDTA is an amino derived organic compound known to be a strong hexdentate chelating agent. It forms a complex with metal ions and dissociates reversibly at low rate. It is also observed that without EDTA the rate of dissociation is high which causes fast precipitation in the bath giving clusterby-cluster deposition. However when EDTA is added, it reduces the rate of dissociation by forming a complex. 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