Atom-by-Atom Growth of CdS Thin Films by an Electrochemical Co

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Atom-by-Atom Growth of CdS Thin Films by an Electrochemical Co-deposition Method: Effects of pH on the Growth Mechanism and Structure I4 lkay S¸ is¸ man, Murat Alanyalıogˇ lu, and U 2 mit Demir* Department of Chemistry, Arts and Sciences Faculty, Atatu¨rk UniVersity, 25240 Erzurum, Turkey ReceiVed: September 28, 2006; In Final Form: December 11, 2006

Thin films of cadmium sulfide (CdS) were prepared on Au(111) by a new electrochemical co-deposition method. The appropriate electrodeposition potentials based on the underpotential deposition potentials (upd) of Cd and S have been determined by the cyclic voltammetric measurements. The films were grown at room temperature from an aqueous solution of CdSO4, ethylenediaminetetraacetic acid (EDTA), and Na2S at various pHs. The thin films were characterized by atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and UV-visible spectroscopy techniques. The Cd/S ratio of electrodeposited thin films was found to be 1:1 determined by electron dispersive spectroscopy (EDS). XRD, STM, and AFM results revealed that the electrosynthesized thin films deposited at -0.520 V (vs Ag/AgCl) have a highly preferential orientation along (110) directions for hexagonal crystal and (111) directions for cubic crystal in the solutions with pH ) 4 and pH ) 5, respectively. The phase transition from cubic to hexagonal structure was observed with decreasing pH below 5. UV-visible absorption measurements as a function of deposition time indicated that the band gap of the CdS film increases as the deposition time decreases.

1. Introduction Cadmium sulfide (CdS) is one of the most investigated IIVI semiconductor materials with a direct optical band gap of 2.4 eV. Thin films of CdS have many potential applications in various optoelectronic devices.1-4 These thin films must be highly crystalline and have a particular single crystallographic orientation for the most thin-film device applications. Devices based on thin film are fabricated using vacuum deposition techniques such as vacuum evaporation,5 chemical vapor deposition (CVD),6 molecular beam epitaxy (MBE),7 and sputtering.8 However, electrochemical synthesis of thin films is an alternative to vacuum-based methods due to their low cost and the ability to work at ambient temperature and pressure. Electrochemical atomic layer epitaxy (ECALE), developed by Stickney and co-workers, is used to grow structurally wellordered compound semiconductors with an atomic level control under ambient temperature and pressure.9-14 A layer of the compound is deposited by alternating the underpotential deposition of the metallic element with the underpotential deposition of the nonmetallic element in a cycle. The growth of singlecrystal thin films of CdS thin films have been achieved by ECALE.15-17 However, this method is very time-consuming and produces a large amount of dilute wastewater due to the substrate being rinsed after each deposition. Automated flow electrodeposition systems for ECALE were developed to overcome these disadvantages.18 Another electrochemical approach is to deposit both species from the same solutions at the same time by the induced co-deposition mechanisms,19-21 which means that the reduction of the more noble component induced the reduction of the less noble component at a constant potential.22 We have developed a new electrochemical method23 which is based on co-deposition from the same solution containing Pb2+, S2-, and EDTA2- (EDTA ) ethylenediaminetetraacetic acid) complexing * To whom correspondence should be addressed. Phone: +90-442-231 4434. Fax: +90-442-236 0948. E-mail: [email protected].

agent, at a constant potential, which is determined from the underpotential deposition (upd) of Pb and S. This method, a new electrochemical co-deposition method, is the combination of upd and co-deposition. We showed that this method could also be used for atom-by-atom growth of highly crystalline thin films of PbS23 and ZnS24. In the present study, we illustrated the detailed growth process of CdS thin films on single crystalline Au(111) electrodes by using atomic force microscopy (AFM), X-ray diffraction (XRD), electron dispersive spectroscopy (EDS), and UV-vis-near-IR spectroscopy techniques. We found out that the highly preferentially orientated hexagonal wurtzite-structured and cubic (zinc blende) CdS thin films could be selectively synthesized depending on the pH of the solution. 2. Experimental Section All electrochemical experiments were performed with a BAS 100B/W electrochemical workstation connected to a threeelectrode cell (C3 Cell Stand, BAS) at room temperature. The Au(111) working electrode similar to a ball-shaped droplet was (111)-oriented single-crystal gold (Alfa-Johnson Matthey, 99,995%) prepared as previously described by Hamelin.25 An indium tin oxide (ITO)-coated quartz substrate with sheet resistance of 10 Ω cm-2 was used as working electrode for optical measurements. In all electrochemical experiments, the reference electrode was an Ag|AgCl (3 M NaCl) (Bioanalytical Systems, Inc., West Lafayette, IN) and a platinum wire used as counter electrode. Deionized water (i.e., >18 MΩ) was used in all experiments. CdS deposition was carried out from a 0.001-0.005 M Na2S (Na2S·9H2O, Merck), 0.05 M CdSO4 (CdSO4‚8/3H2O, Merck), and 0.15 M EDTA2- (C10H14O8N2Na2‚2H2O, Merck) solution at different pH’s adjusted by the buffer solution of potassium phthalate. Scanning probe microscopy measurements were performed in ambient conditions, with a Molecular Imaging Model PicoScan instrument, equipped so that it could be operated in

10.1021/jp066393r CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

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Figure 3. Transmission UV-visible spectra of CdS films deposited on ITO-coated quartz substrates at various times.

Figure 1. Cyclic voltammograms of the Au(111) substrate recorded at 100 mV/s in solution containing (a) 0.005 M Na2S and 0.15 M EDTA, (b) 0.050 M CdSO4, and 0.1 M KCl, and (c) 0.050 M CdSO4 and 0.15 M EDTA.

Figure 4. Plot of (Rhν)2 vs hν of CdS thin films with various deposition times.

Figure 2. Overlapped voltammograms shown in Figure 1a,c.

AFM or STM modes. All AFM images were acquired in contact mode using etched silicon probes (Pointprobe) having fundamental resonance frequencies (ω0) of 13 kHz. These 450 µm long cantilevers were used to record topographic images at a scan rate of 2 Hz. For STM, Pt/Ir wires (90:10) were employed as tunneling tips. X-ray diffraction patterns were acquired using a Rigaku Advance powder X-ray diffractometer with use of Cu KR radiation (λ ) 1.54050 Å). Absorption measurements were performed using a Shimadzu UV-3101 UV-vis-near-IR spectrometer at room temperature. 3. Results and Discussion (3.1) Electrochemical Deposition. A typical cyclic voltammogram for an Au(111) substrate in 0.15 M EDTA2- and 0.005 M S2- solutions at pH 5.0, recorded at the upd region, is shown in Figure 1a. The voltammogram is characterized by a broad anodic peak (deposition), labeled AS, as well as a cathodic (stripping) peak, labeled CS, that corresponds to S electrodeposition/electrodesorption on/from the Au(111) surface at a submonolayer level. The voltammetric behavior of S upd is essentially identical to that reported for aqueous solutions previously.26,27 Figure 1c shows a cyclic voltammetric curve of Cd on Au(111) at the underpotential range in the solutions

Figure 5. EDS spectrum of the CdS thin film deposited for 2 h at pH ) 5.

containing 0.05 M Cd2+ ions and 0.15 M EDTA solutions. For comparison, the cyclic voltametry of Cd deposition and stripping in an EDTA-free solution on Au(111) electrode is shown in Figure 1b at the same pH. The voltammograms for Cd upd from aqueous solution, without EDTA, corresponds well to that observed in previous electrochemical studies of this system28,29 and is characterized by an adsorption (CCd)/desorption (ACd) peaks pair. In the presence of EDTA, as a complexing agent for Cd2+, the voltammograms of CdEDTA2- on the Au(111) surface contain a broad deposition (peak CCd) and broad stripping (peak ACd) and are quite different from those obtained on Au(111) electrodes in aqueous solution. Compared to an

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Figure 6. STM (a, b) and AFM (c) images of CdS films deposited at -520 mV for various deposition times (pH ) 4): (a) 15 s, (b) 10 min, and (c) 2 h.

EDTA-free Cd upd system, both deposition and stripping peak potentials are shifted toward more negative potentials. A similar shift has been observed for the deposition of Pb on Au(111) electrodes coated with a self-assembled monolayer (SAM) of alkanethiols due to the blocking effect of the SAM30 and for the upd deposition of Zn on Au(111) electrodes due to specific adsorption of anions.31 ∆Ep (difference between cathodic and anodic peak potentials) is about 293 mV and much higher than in EDTA-free solutions (40 mV), which is characteristic of a kinetically slow chemical step coupled to a rapid electrontransfer process. Presumably, this is related to the dissociation of CdEDTA-complex ions upon reduction. The voltammograms of S upd and Cd upd in EDTA solutions are shown in the same figure (Figure 2). If we keep the potential of the electrode constant at a potential that is in the middle of the co-deposition region, as shown in Figure 2, which stands for a region between the reductive upd wave of Cd and the oxidative upd wave of S, theoretically, Cd and S will be deposited simultaneously at the electrode at this potential. Since the value of this potential is not enough for the bulk deposition of Cd and S, deposition of Cd2+ on Cd or S2- on S will not occur. Therefore, it should promote the atom-by-atom growth of CdS at the substrate. This method, based on simultaneous underpotential deposition of Cd and S from the same solution containing EDTA, Cd2+, and S2- at a constant potential, allows us to deposit thin films of CdS with various thicknesses by simply using different deposition times, as we have shown before.23,24 (3.2) Characterization of CdS Thin Films. (3.2.1) Electronic Properties of CdS Thin Films. Figure 3 shows the UV-visible absorption spectra of CdS thin films deposited on ITO-coated quartz substrate at different time intervals at a constant potential of -520 mV. The spectrum in the 460-510 nm spectral window reveals that the absorption edges around 465 nm become more pronounced and shift to lower energy with increasing time of the deposition, which is quite similar to spectra reported for the thin film of CdS. This shift in the absorption edge as a function of thicknesses (deposition time) is thought to be caused by quantum confinement effects in the CdS layer. The band gap values of the films were calculated by plotting (Rhν)2 vs hν and extrapolating the linear portion of graph to the energy axis (Figure 4). The dependence of the band gap shift of CdS films on the deposition time and the number of coulombs passed during the electrodeposition (pH ) 5) are shown in Table 1. It

TABLE 1: Dependency of Band Gap on the Deposition Time and the Number of Coulombs Passed (pH ) 5) deposition time (min)

charge (mC/cm2)

Eg (eV)

2 8 15 20 30

2.18 3.60 5.43 7.16 8.77

2.61 2.54 2.49 2.44 2.40

can be seen that the band gap decreases from 2.61 to 2.40 as the number of coulombs passed, which is related with the amount of CdS deposits on the Au(111) surface, increases from 2.18 to 8.77 mC‚cm-2. We obtained similar results for the band gap measurements at both pH 5 and pH 4 for the same amount of coulombs passed during the electrodeposition. These obtained band gap values agree with the previous values obtained by the other groups for CdS thin films.17,32 EDS measurement (shown in Figure 5) on a 7 nm thick CdS film indicate that the thin film is composed of Cd and S. The molecular ratio of Cd/S of the thin film calculated from the EDS quantitative analysis data is almost 1:1 (Cd, 50.53%; S, 49.47%), which is close to that of a bulk CdS crystal. (3.2.2) Morphological and Structural Characterization of CdS Thin Films. To characterize these CdS deposits using scanning probe microscopy, STM and contact mode AFM were employed. Parts a,b (STM images) and c (AFM) of Figure 6 show the topographical image representing the time evolution of layer growth of the CdS thin film on an atomically flat Au(111)

Figure 7. Powder XRD diffractograms of the CdS films grown at pH ) 4 for 2 h (a) and 2 min (b).

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Figure 8. STM (a, b) and AFM (c) images of CdS films deposited at -520 mV for various deposition times (pH ) 5): (a) 2 min, (b) 10 min, and (c) 2 h.

surface electrodeposited at pH ) 4 for 15 s, 10 min, and 2 h, respectively. Cross-sectional profiles along a line of these images is also displayed under the image. The STM image and crosssectional profile of the CdS film (Figure 6a) reveal that the thin film is almost flat with a thickness of 0.5-0.7 nm corresponding to a monolayer of CdS. The Au(111) terraces are distinctly visible in the Figure 6a. Figure 6b shows the STM image and the cross-sectional profile of CdS obtained after 10 min deposition, indicating that the film is almost complete and the second monolayer formed on top of the first monolayer. The AFM image in Figure 6c, recorded for a 2 h deposition time, reveals a surface containing numerous spherical features, with rather uniform size variation between 700 and 900 nm with an average diameter around 800 nm. Similar circular domains in hexagonal structure were observed in acidic chemical bath for electrochemical deposition19 and for solution synthesis.33 AFM and STM images recorded as a function of deposition time clearly indicate that the atomically flat uncompleted multiple layer of CdS formed simultaneously at pH 4. After a curtain amount of CdS deposition, the growth mode abruptly changes to the three-dimensional (3D) island growth, and eventually circular domains form (Figure 6c). The XRD spectrum obtained for the deposition times of 2 h and 2 min are shown in Figure 7a,b. From the diffraction profiles it can be seen that a strong peak at 2θ )38.2 corresponds to a (111) crystal face of Au substrate and a weak peak at 2θ ) 43.7 corresponds to a hexagonal wurtzite-structured CdS, having the preferential orientation along the (110) plane (JCPDS Card, File No. 411049). The presence of small amount of Au(200) diffraction planes at 2θ ) 44.39 were also observed for the substrate used for 2 h deposition (Figure 7a). Parts a and b of Figure 7 show that there is no shift in peak position while the intensity of the peak at 43.7 increases, indicating that there is no phase transition during the film growth. However, when the pH changed to 5.0, the STM image of the prominent topographic features recorded are circular protrusions on atomically smooth regions of the Au(111) surface, as shown in Figure 8a. The heights and diameters of these protrusions, which are evident from the line scans shown in Figure 8a, are 0.5 nm and 15-25 nm, respectively. This image clearly shows that the CdS nanoparticles form on the Au(111) surface from well-separated crystal nuclei rather than from a saturated chemisorbed monolayer as in Figure 6a. Increases in the apparent diameter and density of these CdS nanoparticles

Figure 9. Powder XRD diffractograms of the CdS films grown at pH ) 5 for (a) 2 h, (b) 0.5 h, and (c) 2 min.

with increasing deposition time are also evident from the image recorded for 10 min deposition (Figure 8b). As the time for deposition increases to 2 h, the circular nanoparticles disappear and rather flat square crystallites appear, which cover the entire surface, appearing as shown in the AFM image (Figure 8c). These square crystallites become larger as the deposition time increases to 2 h, and they are aligned with each other, which is consistent with the in-plane orientation of the film. All the electrodeposition experiments performed for the values above pH 5 result in the square crystallites. The XRD spectra of these films deposited at pH ) 5 for different times are shown in Figure 9a-c. The broad diffraction peak at 26.5° (2θ scale) arising from (111) reflections from CdS is assigned to the cubic (zinc blende) form (JCPDS Card, File No. 10-0454). As the time increases, the peak corresponding to CdS becomes more intense and narrower, while the peak at 38.2 corresponding to the (111) reflections of the Au substrate becomes smaller in intensity and finally disappears. 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 CdS was formed in the form of a cubic phase under various techniques.34,35 No peaks due to other phases, such as CdO, Cd, and S, were detected, indicating the high purity of the products. These results suggest that the pH of solutions during the electrochemical depositions have great influence on the growth of the CdS thin films. The preferred orientations of CdS along (110) and (111) planes at different pHs are due to controlled nucleation processes associated with the low deposition rate. It

2674 J. Phys. Chem. C, Vol. 111, No. 6, 2007 has been shown that the current density plays an important role on the film growth rate, which determines the structure of the film.36 It is reported that the electrochemical deposition leads to a wurtzite structure at low pH.19,36 The cubic structure of CdS was observed for a chemical bath deposited CdS at high pH.37-39 4. Conclusions CdS thin films have been successfully prepared by electrochemical co-deposition from the same solution. We were able to obtain well-crystallized materials with good optical properties by a one-step process. The amount of CdS deposits of the thin films can be controlled by variation electrochemical deposition time. The band gap values of CdS thin films obtained in this study were between 2.4 and 2.6 eV. In addition, the resulting deposit structure can be controlled by using different pH values for deposition. SPM and XRD data indicated that the films have a hexagonal phase with a preferred orientation in the (110) plane at pH 4 and below while cubic phase with a preferred orientation in the (111) plane at pH 5 and above. Acknowledgment. Atatu¨rk University is gratefully acknowledged for the financial support of this work. References and Notes (1) Mcclean, I. P.; Thomas, C. B. Semicond. Sci. Technol. 1992, 7, 1394. (2) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (3) Wang, Z. L. AdV. Mater. 2000, 12, 1295. (4) Pavaskar, N. R.; Menezes, C. A.; Sinha, A. P. B. J. Electrochem. Soc. 1977, 124, 743. (5) Romeo, N.; Sberveglieri, G.; Tarricone, L. Appl. Phys. Lett. 1978, 32, 807. (6) Fainer, N. I.; Kosinova, M. L.; Rumyantsev, Yu. M.; Salman, E. G.; Kuznetsov, F. A. Thin Solid Films 1996, 280, 16. (7) Cheon, J.; Zink, J. I. J. Am. Chem. Soc. 1997, 119, 3838. (8) Fraser, D. B.; Melchior, H. J. Appl. Phys. 1972, 43, 3120. (9) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (10) Villegas, I.; Stickney, J. L. J. Electrochem. Soc. 1992, 139, 686.

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