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Electrochemically Induced Atom-by-Atom Growth of ZnS Thin Films: A New Approach for ZnS Co-deposition Tuba O ¨ znu¨lu¨er, I˙ brahim Erdogˇan, and U ¨ mit Demir* Atatu¨rk UniVersity, Arts and Sciences Faculty, Department of Chemistry, 25240 Erzurum, Turkey ReceiVed September 3, 2005. In Final Form: February 27, 2006 Ultrathin films of ZnS were grown on Au (111) substrates using a novel, simple co-deposition method and characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy. Cyclic voltammograms were used to determine approximate deposition potentials for co-deposition. XRD shows that the material growth is highly preferential with (111) orientation. Both AFM and XRD data indicate that the ZnS growth mechanism starts by the formation of rounded nanoparticles at the surface and then continues by lateral and vertical growth to form flat square crystallites of ZnS. UV-vis spectra taken for the ZnS thin films with various thicknesses, which is related to deposition time, shows that the band gap of the ZnS decreases as the film thickness increases.
1. Introduction Size-quantized semiconductor thin films, also referred to as quantum wells, have been used in optoelectronic, luminescence, and heterojunction devices.1,2 ZnS is a II-VI semiconductor with a large band gap and many potential application in thin-film device technology.3-5 Because the properties of semiconductors depend both on the dimensions and the superlattice structure, the film must be highly crystalline and have a particular single crystallographic orientation. Electrochemical atomic layer epitaxy (EC-ALE) is an attractive electrosynthetic method of depositing thin films of II-VI compound semiconductors,6,7 which has been developed by Stickney.8 In EC-ALE based on layer-by-layer growth, the underpotential deposition (upd) was used to form the sequential atomic layers of each element of a compound semiconductor. Separate solutions and potentials are used to deposit atomic layers of each element electrochemically in a cycle in this method. The problems with this technique are that electrode is required to be rinsed after each upd deposition, which may result in the loss of potential control, deposit reproducibility problems, and waste of solution. Automated deposition systems by EC-ALE were developed to overcome these problems.9 Another particularly promising method is the successive ionic layer adsorption and reaction (SILAR), which is based on a sequential immersing of the substrates in the solutions of each element with rinsing between with water.10 The electrodeposition of inorganic compound semiconductors has been also proposed to occur by the so-called induced co-deposition mechanisms, which means that the reduction of the more noble component * Corresponding author. E-mail:
[email protected]. Tel: +90-4422314434. Fax: +90-442-236 0948. (1) Britt, J.; Ferekides, C. Appl. Phys. Lett. 1993, 62, 2851-2852. (2) Tran, N. H.; Hartmann, A. J.; Lamb, R. N. J. Phys. Chem. B 2000, 104, 1150-1152. (3) Torimoto, T.; Obayashi, A.; Kuwabata, S.; Yasuda, H.; Mori, H.; Yoneyama, H. Langmuir 2000, 16, 5820-5824. (4) Gichuhi, A.; Shannon, C.; Perry, S. S. Langmuir 1999, 15, 5654-5661. (5) Colletti, L. P.; Slaughter, R.; Stickney, J. L. Proc. Electrochem. Soc. 1997, 97, 1-10. (6) Demir, U ¨ .; Shannon, C. Langmuir 1994, 10, 2794-2799. (7) Gichuhi, A.; Boone, B. E.; Demir, U ¨ .; Shannon, C. J. Phys. Chem. B 1998, 102, 6499-6506. (8) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543561. (9) Flowers, B. H., Jr.; Wade, T. L.; Garvey, J. W.; Lay, M.; Happek, U.; Stickney, J. L. J. Electroanal. Chem. 2002, 524-525, 273-285. (10) Nicolau, Y. F. Appl. Surf. Sci. 1985, 22, 1061-1074.
induces the reduction of the less noble component at a constant potential.11 Pioneering work has been done on the co-deposition of CdTe from an aqueous solution of CdSO4 and TeO2 by Kro¨ger et al.12 Recently, we have developed a simple, convenient method of electrochemical deposition, which based on the co-deposition of Pb and S precursors from the same solution containing EDTA2and HS- at the upd of Pb and S.13 Results indicated that the highly crystalline deposits of PbS thin films could be synthesized by this method. In this article, we report on the growth of ZnS thin films deposited on single-crystalline Au (111) by new electrochemical co-deposition through a one-step process. To determine the characteristics of morphology, composition, and structure, the deposited films were examined by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) techniques. This study also provides information on the optical and electronic properties of the electrodeposited thin films of ZnS. 2. Experimental Section 2.1. Electrochemistry. The voltammetric and coulometric measurements were performed with a BAS 100B/W electrochemical workstation connected to a three-electrode cell (C3 Cell Stand, BAS). The working electrode was (111)-oriented single-crystal gold (Johnson Matthey, 99.999%) prepared as previously described by Hamelin.14 In all cases, an Ag|AgCl (3 M NaCl) electrode served as the reference electrode, and a platinum wire was used as the counter electrode. All of the electrolyte solutions used in this study were prepared from deionized water (i.e., >18 MΩ) from a Milli-Q system. ZnS deposition was carried out from 0.01 M Na2S (Na2S‚9H2O, Reagent grade, Aldrich), 0.3 M Zn2+ (Zn(CH3COO)2‚2H2O, Merck), and 0.32 M EDTA2- (C10H14N2Na2O8‚2H2O, Merck) solutions at pH 4.6. 2.2. Instrumentation. Absorption spectra of ZnS deposits on the ITO-coated quartz plates were recorded using a Shimadzu UV-3101 UV-vis-NIR spectrometer at room temperature. The powder X-ray diffraction patterns of the deposits were recorded using a Rigaku (11) Kro¨ger, F. A. J. Electrochem. Soc. 1978, 125, 2028-2034. (12) Panicker, M. P.R.; Knaster, M.; Kro¨ger, F. A. J. Electrochem. Soc. 1978, 125, 566-572. (13) O ¨ znu¨lu¨er, T.; Erdogˇan, I˙ .; S¸ is¸ man, I˙ .; Demir, U ¨ . Chem. Mater. 2005, 17, 935-937. (14) Hamelin, A. Mod. Aspects Electrochem. 1985, 16, 1.
10.1021/la052404g CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006
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Figure 2. Transmission UV-visible spectra of ZnS films deposited on ITO-coated quartz substrates at different times.
Figure 1. (a-c) Cyclic voltammogram of a Au (111) substrate in solutions containing (a) 0.3 M Zn2+, (b) 0.01 M Na2S and 0.32 M EDTA, and (c) Zn2+ in 0.32 M EDTA solution. Advance Powder X-ray diffractometer (using Cu KR, λ ) 1.54050 Å radiation). AFM imaging was performed in air using a Molecular Imaging model picoscan instrument. All images were acquired in contact mode using etched silicon probes (Pointprobe) having a fundamental resonance frequency (ωo) of 13 kHz. These 450-µmlong cantilevers were used to record topographic images at a scan rate of 2 Hz. A Kartos ES 300 electron spectrometer with nonmonochromatic Mg K X-rays was used to record the XPS spectra.
3. Results and Discussion 3.1. Electrochemical Deposition. A representative cyclic voltammogram for the oxidative adsorption of the initial S layer on Au (111) substrates containing 0.32 M EDTA2- and 0.01 M Na2S solutions at pH 4.6 is shown in Figure 1b. The voltammetric behavior of the S upd is essentially identical to that reported for aqueous solutions.15,16 Broad anodic deposition peak A (around -545 mV) is associated with desorption peak C (around -570 mV) for one atomic layer of S. The cyclic voltammetric curves for the Zn upd on Au (111) in aqueous solution containing 0.3 M Zn2+ ions are shown in Figure 1a at the same pH. The main features in these voltammograms are a sharp cathodic (denoted C, -276 mV) and a sharp anodic peak (denoted A, -205 mV) that correspond to upd waves of Zn, consistent with earlier studies of this system.17 In the presence of EDTA as a complexing agent for Zn2+, the voltammogram of Zn under the same conditions as in aqueous solution contains only one redox couple corresponding to the deposition (CZn, -488 mV) and stripping (AZn, -398 mV) of Zn atomic layers. The shape and potentials of these peaks are quite different from those of the peak obtained on Au (111) electrodes in aqueous solution (Figure 1c). Both cathodic deposition and anodic stripping peak potentials shift negatively by 210 and 190 mV, respectively. A similar shift has been observed for the deposition of Pb on Au (111) electrodes (15) Alanyalioglu, M.; Cakal, H.; O ¨ ztu¨rk, A. E.; Demir, U ¨ . J. Phys. Chem. B 2001, 105, 10588-10593. (16) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156-4159. (17) Moniwa, S.; Aramata, A. J. Electroanal. Chem. 1994, 376, 203-206.
coated with a self-assembled monolayer (SAM) of alkanethiols due to the blocking effect of SAM18 and for the upd deposition of Zn on Au (111) electrodes due to the specific adsorption of anions.19 Moreover, ∆Ep (the difference between cathodic and anodic peak potentials) is larger than the Zn upd in aqueous solutions, which is characteristic of a kinetically slow chemical step coupled to a rapid electron-transfer process. Presumably, this is related to the dissociation of ZnEDTA complex ions upon reduction. The charge densities under the cathodic and anodic peaks are 90 ( 10 µC cm2, which correspond to a coverage of approximately 0.33 and are independent of ZnEDTA2- concentration. Therefore, these peaks are considered to result from the upd of Zn. When we added 0.32 M EDTA2-, 0.3 M Zn2+, and 0.01 M Na2S to a single solution, no precipitation was observed, indicating that the solution involves both S species (in the form of S2-, HS-, or H2S depending on pH) and Zn2+ (complexed as ZnEDTA2-) ions. The cyclic voltammograms of S upd and Zn upd in EDTA solutions are shown in the Figure 1b and c. If the potential of the electrode is kept constant at a potential in the middle of the co-deposition region as shown in Figure 1, which stands for a region between the reductive upd wave of Zn and the oxidative upd wave of S, then Zn and S will be deposited simultaneously at the electrode. Because the value of this potential is not enough for the bulk deposition of either Zn or S, there was no deposition of Zn2+ on Zn or S2- on S. In other words, the potential between the dotted lines shown in Figure 1 is limited for the upd deposition of both Zn and S atomic layers but not for the bulk deposition. Therefore, it should promote the atom-by-atom growth of ZnS at the substrate while keeping Zn complexed and avoiding the formation of ZnS particles in the solution phase so that they do not contribute to the film growth. On the basis of this idea, we have attempted to use this method to synthesize ZnS thin films with various thicknesses by simply using different deposition times from the same solution. 3.2. Electronic Properties of ZnS Thin Films. UV-visible absorption spectra of ZnS thin films on ITO-coated quartz substrates at different time intervals are shown in Figure 2. Electrodeposition was carried out at a constant potential of -520 mV, which was determined from Figure 1. An examination of the absorption spectrum in the 250-400 nm spectral windows, which is quite similar to spectra reported for colloids20 and thin film of ZnS,21 indicates that the absorption edges (appearing as (18) Oyamatsu, D.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1999, 473, 59-67. (19) Takahashi, S.; Aramata, A.; Nakamura, M.; Hasabe, K.; Taniguchi, M.; Taguchi, S.; Yamgishi, A. Surf. Sci. 2002, 512, 37-47. (20) Sooklal, K.; Cullum, B. S.; Angel, S. M.; Murphy, C. J. J. Phys. Chem. 1996, 100, 4551-4555.
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Figure 4. Dependence of the band gap of ZnS films on the thickness. Figure 3. Plots of (Ahν)2 vs hν for ZnS thin films with various thicknesses.
a shoulder) around 280 nm becomes more pronounced and shift to lower energy with increasing deposition time. At a deposition time of 12 h, there was a 29 nm red shift in its absorption edge as compared to that for the deposition time of 2 h. It has been reported that ZnS is a direct band gap semiconductor and therefore plots of (Ahν)2 versus hν should be straight lines with intercepts on the energy axis giving the band gaps of the films.22 Such a plot for ZnS thin films electrodeposited at various times is shown in Figure 3. The band gap of a thin film electrodeposited for 12 h was found to be 3.60 eV, which is similar to the value for the band gap energy of bulk ZnS.23 The band gap of a ZnS thin film deposited for 2 h was 4.00 eV. These obtained values agree with the previous values obtained by the other groups for ZnS thin films and quantum dots.3,24 Because the thickness of the ZnS thin film will be dependent on the electrodeposition time, we expect to have thicker ZnS films at longer times and thinner films at shorter times. The thicknesses of the ZnS films deposited at different times shown in Figure 3 are calculated from the charge obtained by coulometric measurements during the deposition time and the surface area of the electrode. A plot of the experimentally determined band gaps as a function of the thickness of the ZnS thin films is shown in Figure 4. The expected increase in the band gap as the film thickness decreases is observed. These results are consistent with the observation of other workers that increasing the thickness of the ZnS3 and PbS25 thin films gives rise to a spectral red shift. 3.3. Morphological and Structural Characterization of ZnS Thin Films. Typical AFM images of ZnS deposited films at short times (0.5 h) are shown in Figure 5a. The step edge separating two terraces is roughened compared to naked Au (111), indicating that step edges are more stable sites for ZnS nucleation and growth. It has been reported that S atoms become progressively bonded to step edges.26,27 Evenly distributed rounded features of approximately the same diameter (25 ( 5 (21) Kovtyukhova, N. I.; Buzaneva, V. E.; Waraksa, C. C.; Martin, R. B.; Mallouk, E. T. Chem. Mater. 2000, 12, 383-389 (22) Herrero, J.; Guillen, C. J. Appl. Phys. 1991, 69, 429-432. (23) Lokhande, C. D.; Jadhav, M. S.; Pawar, S. H. J. Electrochem. Soc. 1989, 136, 2756-2757. (24) Li, Y.; Ding, Y.; Zhang, Y.; Qian, Y. J. Phys. Chem. Solids 1999, 60, 13-15. (25) Dutta, K. A.; Ho, T.; Zhang, L. Stroeve, P. Chem. Mater. 2000, 12, 1042-1048. (26) Salvarezza, R. C.; Vela, M. E.; Andreasen, G.; Vericat, C. J. Phys. Chem. B 2000, 104, 302-37. (27) Lay, D. M.; Varazo, K.; Stickney, J. L. Langmuir 2003, 19, 8416-8416.
nm) on the flat Au (111) terraces can also be seen in this image. These data suggest that the ZnS nanoparticles form on the Au (111) surface from well-separated crystal nuclei rather than from a saturated chemisorbed monolayer, as is normally the case in the surface sol-gel technique21 or in EC-ALE4 films nucleated by the underpotential deposition of one ion. As the time for deposition increases to 2 h, the diameter and the density of these structures increase as indicated in Figure 5b. Similar structures have been observed in the electrodeposition of ZnS on Au (111).4 Although the surface is covered with rounded crystals measuring 40 ( 5 nm in diameter and 0.8 nm in height, several depressions of approximately 0.8 nm in depth are also seen. These depressions may originate from the etching of the Au surface. It is known that the adsorption of sulfur and n-alkanethiols causes depressions in the underlying Au (111) surface.28 The surface morphology of the ZnS grown at 10 h is quite different from that described above. The film consists of extended (width approximately 100 nm) and rather flat square crystallites of different thicknesses, which cover the entire surface (Figure 5c). These square crystallites are aligned with each other, which is consistent with the in-plane orientation of the film. The XRD spectrum of electrochemically grown ZnS thin film on the Au (111) substrate at shorter times consist of two major peaks (Figure 6a) and two minor peaks. The broad diffraction peak at 28.6° (2θ scale) arising from (111) reflections from ZnS is assigned to the cubic sphalerite form (JCPDS card, file no. 50566). The obviously weaker diffractions at 47.50 and 56.30 correspond to (220) and (311) reflections of cubic sphalerite ZnS, respectively. The other strong peak at 38.2° arises from the (111) reflections of Au, which is used as a single-crystalline substrate in this study. As the deposition time increases as shown in Figure 6b and c, the intensity of Au (111) reflections decreases and the intensity of ZnS (111) increases, whereas diffractions along (220) and (311) becomes almost invisible. This indicates that the crystallites in the film have a single preferred (111) orientation, which is the same as that reported previously in which the ZnS was formed in the form of a cubic phase by various techniques.3,24,29-33 No matter how long the reaction time is, all of these diffractions result from the sphalerite form (28) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (29) Hsu, Y.-J.; Lu, S.-Y. Langmuir 2004, 20, 194-201. (30) Meng, X. M.; Liu, J.; Jiang, Y.; Chen, W. W.; Lee, C. S.; Bello, I.; Lee, S. T. Chem. Phys. Lett. 2003, 382, 434-438. (31) Chae, W.-S.; Yoon, J.-H.; Yu, H.; Jang, D.-J.; Kim, Y.-R. J. Phys. Chem. B 2004, 108, 11509-11513. (32) Wang, W.; Germanenko, I.; El-Shal, S. M. Chem. Mater. 2002, 14, 30283033. (33) Lee, E. Y. M.; Tran, H. N.; Russel, J. J.; Lamb, R. N. J. Phys. Chem. B 2003, 107, 5208-5211.
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Figure 6. XRD patterns of the ZnS sample electrodeposited onto single-crystal Au (111) at various times: (a) 2, (b) 4, and (c) 10 h.
Figure 7. X-ray photoelectron spectrum of a ZnS film grown on Au (111).
Figure 5. AFM images of ZnS formed on the Au (111) electrode after potential-controlled deposition at -520 mV.
(zinc blende) of ZnS. The more intense and narrow the XRD peak for ZnS obtained for the deposits at longer times, the
periodicity of the individual domains indicates that the electrodeposited films at longer times consist of larger crystallites. From the half-width of the XRD peak of ZnS (111) in Figure 6c, the average particle size is estimated to be 60-80 nm on the basis of the Scherrer equation,34 which is smaller than the particle size observed from AFM images. Although the mechanism of ZnS film growth by this technique is still not understood in detail, these results supports the fact that the first stage of growth is nucleation. Time-dependent AFM images and XRD spectra make us believe that, after the initial nucleation stage, the growth involves continuous particle growth in the lateral direction from 3D particles formed at the beginning of deposition. The surface chemical compositions of the ZnS thin films were determined by XPS analysis. The peak areas of Zn and S cores were used to calculate the Zn-to-S ratio, and it is found to be Zn(1.0)/S(1.2 ( 0.2). A slight excess of sulfur may be present, but it is outside of the experimental uncertainty. The XPS analysis results of the ZnS thin film (5 nm in thickness) in Figure 7 that show that the binding energy of Zn 2p3/2 is at 1021.8 eV and that of S 2p is at 162.3 both illustrate the existence of ZnS.35 Figure (34) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; Addison-Wesley: Reading, MA, 1978. (35) Chen, S.; Liu, W. Langmuir 1999, 15, 8100-8104.
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7 reveals that the ZnS surface contains only trace amounts of Na and Cl. No other impurities are detected in the film.
4. Conclusions This work clearly shows that the electrochemical deposition based on the upd potentials of Zn and S from the same solution containing EDTA, Zn2+, and S2- is an experimentally simple technique for the synthesis of thin films of semiconductor materials. In addition, the results obtained in this study suggest that the thickness of the ZnS films can be controlled by the deposition time. We were able to obtain well-crystallized materials with good optical properties by a one-step process. Increasing
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the time for electrodeposition of ZnS leads to an increase in both the thickness and the absorption wavelength, which causes a red shift in the band gap of semiconducting materials. AFM and XRD results confirm the superior quality of thin film semiconductors prepared by this technique, in comparison to alternative electrochemical procedures that do not offer the same structural control. Acknowledgment. Atatu¨rk University is gratefully acknowledged for financially supporting this work. We are thankful to Dr. Sefik Su¨zer (Bilkent University) for the XPS analysis. LA052404G
