A Scanning Tunneling Microscopy and X-ray Photoelectron

Electrochemically Induced Atom-by-Atom Growth of ZnS Thin Films: A New Approach for ZnS Co-deposition. Tuba Öznülüer, İbrahim Erdoǧan, and Ümit ...
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5654

Langmuir 1999, 15, 5654-5661

A Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Study of Electrochemically Grown ZnS Monolayers on Au(111) Anthony Gichuhi and Curtis Shannon* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312

Scott S. Perry Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received December 21, 1998. In Final Form: May 6, 1999 The structure and chemical composition of electrosynthesized ZnS thin films on Au(111) substrates was studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The films were grown by alternating underpotential deposition and oxidative adsorption cycles of S and Zn from solution precursors (electrochemical atomic layer epitaxy, EC-ALE). Oxidative adsorption of the first layer of S results in a (x3 × x3)R30° adlattice on Au(111). In the initial Zn monolayer, XPS indicates the presence of SO42- derived from the supporting electrolyte. The coadsorbed SO42- can be chemically or electrochemically displaced by H2S(aq) or HS(aq)-, leading to the formation of ZnS. Atomically resolved STM images show the ZnS/Au(111) monolayer to be 6-fold symmetric with an interatomic spacing of 0.37 ( 0.01 nm. The structure of the deposit on the micrometer scale was also investigated by STM. The first complete EC-ALE cycle results in the formation of nanocrystallites of ZnS randomly distributed across Au(111) terraces. The average diameter of the crystallites is 10 ( 5 nm, and the apparent coverage of ZnS is 0.38.

Introduction The sequential electrosorption of well-ordered atomic layers of the elemental constituents of a target compound is the principle behind electrochemical atomic layer epitaxy (EC-ALE). In EC-ALE, a surface-limited electrochemical reaction such as underpotential deposition (upd)1 is used to synthesize a binary compound by successive deposition of each element from its respective solution precursor.2 The widespread application of II-VI compound semiconductors in optoelectronics,3 solar energy conversion,4 and, more recently, thin film electroluminescent display (TFELD)5 technologies has spawned a renewed interest in these materials within the semiconductor community. EC-ALE is an attractive electrosynthetic alternative to conventional deposition methods because it is inexpensive, operates at ambient temperature and pressure, and provides precise film thickness control. This technique promises to overcome problems associated with other electrosynthetic approaches, such as the formation of highly polycrystalline deposits and interfacial interdiffusion.6 There are, however, a number of challenges associated with synthesizing quality thin films using upd. More specifically, it has been shown that the structure of upd layers is strongly influenced by the extent of repulsive interactions arising from anions coadsorbed with the metal * Corresponding author. Telephone: 334-844-6964. Fax: 334844-6959. E-mail: [email protected]. (1) (a) Kolb, D. M. Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11, p 125. (b) Ju¨ttner, K.; Lorenz, W. J. Z. Phys. Chem. N. F. 1980, 122, 163. (2) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (3) Gebhart, W. Mater. Sci. Eng. 1992, B11, 1. (4) (a) Licht, S.; Forouzan, F.; Longo, K. Anal. Chem. 1990, 62, 1356. (b) Banerjee, A.; Das, S. R.; Thakoor, A. P.; Randhawa, H. S.; Chopra, K. L. Solid-State Electron. 1979, 22, 495. (c) Bonnet, D. Phys. Status Solidi A 1972, 11, K-35. (5) Bhargava, R. N. J. Cryst. Growth 1982, 59, 15.

adatom layer.7 Surface-sensitive techniques such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and surface X-ray scattering (SXRS) have been employed to elucidate the role of the supporting electrolyte on the resulting upd structures in systems such as Cu/Au(111),8 Bi/Au(111),9 Ag/Au(111),10 Hg/Au(111),11 and, more recently, Cd/Au(111).12 (6) (a) Chen, J. H.; Wan, C. C. J. Electroanal. Chem. 1994, 365, 87. (b) Das, S. K.; Morris, G. C. Sol. Energy Mater. Sol. Cells 1993, 30, 107. (c) Bouroushian, M.; Loı¨zos, Z. R.; Spyrellis, N.; Maurin, G. Thin Solid Films, 1993, 29, 101. (d) Mattei, R. C.; Feigelson, R. S. In: “Electrochemistry of Semiconductors and Electronics: Proceses and Devices”, McHardy, J., Ludwig, F. Eds.: Noyes Publications: New Jersey, 1992; p. 1. (e) Shearson, P. C. Sol. Energy Mater. Sol. Cells, 1992, 27, 377. (f) Cataldi, T. R. I.; Blackham, I. G.; Briggs, G. A. D.; Pethica, J. B.; Hill, H. A. O. J. Electroanal. Chem. 1990, 290, 1. (g) Lockhande, C. D.; Pawar, S. H. Phys. Status Solidi A 1989, 111, 17. (h) Falop, G. F.; Taylor, R. M.; Maurin, G. Thin Solid Films 1991, 204, 139. (i) Sahn, S. N.; Sanchez, C. Solid State Commun. 1990, 73, 597. (7) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1992, 339, 299. (8) (a) Manne, S.; Hansma, P. K.; Massie, J.; Eilings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (b) Manne, S.; Massie, J.; Eilings, V. B.; Hansma, P. K.; Gewirth, A. A. J. Vac. Sci. Technol., B 1991, 9, 950. (c) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (d) Magnussen, O. M.; Hotlos, J.; Beitel, G.; Kolb, D. M.; Behm, R. J. J. Vac. Sci. Technol., B 1991, 9, 969. (e) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gorden, J. G.; Melroy, O. R. Phys. Rev. Lett. 1995, 75, 4472. (f) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (9) (a) Sayed, S. M.; Ju¨ttner, K. Electrochim. Acta 1983, 28, 1635. (b) Chen, C.; Kepler, K. D.; Gewirth, A. A.; Ocko, B. M.; Wang, J. J. Phys. Chem. 1993, 97, 7290. (10) Chen, C.-h.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451. (11) (a) Li, J.; Abrun˜a, H. D. J. Phys. Chem. B 1997, 101, 244. (b) Li, J.; Abrun˜a, H. D. J. Phys. Chem. B 1997, 101, 2907. (c) Chen, C.-h.; Gewirth, A. A. Phys. Rev. Lett. 1992, 68, 1571. (d) Inukai, J.; Sugita, S.; Itaya, K. J. Electroanal. Chem. 1996, 403, 159. (e) Zeng, X.; Prasad, S.; Bruckenstein, S. Langmuir 1998, 14, 2535. (12) (a) Niece, B. K.; Gewirth, A. A. Langmuir 1997, 13, 6302. (b) Bondos, J. C.; Gewirth, A. A.; Nuzzo, R. G. J. Phys. Chem. 1996, 100, 8617. (c) Von Shultze, J. W.; Koppitz, F. D.; Lohrengel, M. M. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 693. (d) Colleti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145.

10.1021/la981743p CCC: $18.00 © 1999 American Chemical Society Published on Web 06/19/1999

Electrochemically Grown ZnS Monolayers

EC-ALE has so far been successfully applied to the growth of relatively polar II-VI Cd-13 and Zn-14 based compound semiconductors. In a series of earlier papers on CdS growth on Au, for example, we reported on the kinetics of CdS monolayer formation15 and on the influence of the Au(111)16 and Au(100)17 crystallographic orientations on the structure of the final deposit. Our findings indicated that when Cd was deposited on S-modified Au surfaces, the characteristic thermodynamic product (i.e., wurtzite CdS) was formed. More recently, in an attempt to improve the overall CdS film quality,18 we investigated the effect of the deposition order on film structure. Interestingly, when S (from aqueous H2S) is deposited on a Cd-modified Au surface, the characteristic hexagonal structure of wurtzite CdS was not observed until the deposition of the third complete monolayer. The more complex adatom structures associated with this approach were attributed to the strong positive interaction between Cd adatoms and the Au substrate, an interaction that extended beyond the first monolayer. Stickney and coworkers have recently demonstrated that stable deposits of ZnTe, ZnSe and ZnS can be electrochemically grown on low-index planes of single-crystal Au.14 Thermodynamics appeared to play a role in the formation of the initial monolayer: the differences in the upd potentials of these materials was consistent with their free energies of formation, ZnTe > ZnSe > ZnS. They found that Zn was hardest to deposit on Te and easiest to deposit on S. Structural information on these systems, however, was not provided due to the difficulty of imaging those surfaces. In this work, we report a scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS) characterization of ZnS monolayer films grown on Au(111) by ECALE. Specifically, we examined the structure and chemical composition of an atomic layer of Zn underpotentially deposited at (x3 × x3)R30°-S/Au from SO42- containing electrolyte. Atomic resolution STM reveals a hexagonal structure with an interatomic spacing that was not characteristic of any ZnS spacing in either its wurtzite or cubic forms. The presence of coadsorbed SO42- in addition to the S2- species is confirmed by XPS. Previous studies have shown that SO42- ions do, indeed, play a role in determining the structure of upd layers on bare metal electrodes.8-12 Postdeposition surface modifications, carried out under potential control, in which aqueous H2S or HS- was employed to chemically displace SO42-, were successful in converting the SO42- terminated monolayer to ZnS. Our postelectrolysis treatments were, in principle, similar to those frequently undertaken in ALE,19 in which self-limiting chemical reactions of gas-phase precursors with specific terminating atoms are used to ensure highquality epitaxial films. After dosing the surface with aqueous H2S or HS-, a 6-fold symmetric overlayer with an interatomic spacing characteristic of wurtzite ZnS is observed. In addition, XPS confirms the displacement of (13) (a) Hayden, B. E.; Nandhakumar, I. S. J. Phys. Chem. B 1998, 102, 4897. (b) Colleti, L. P.; Flowers, B. H., Jr.; Stickney, J. L. J. Electrochem Soc. 1998, 145, 1442. (c) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 362. (d) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 375, (e) Stickney, J. L.; Villegas, I.; Gregory, B. W.; Suggs, D. W. J. Vac. Sci. Technol., A 1992, 10, 886. (f) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (g) Villegas, I.; Stickney, J. L. J. Electrochem Soc. 1992, 139, 686. (h) Gregory, B. W.; Suggs, D. W.; Stickney, J. L. J. Electrochem Soc. 1992, 138, 1279. (14) Colleti, L. P.; Thomas, S.; Wilmer, E. M.; Stickney, J. L. Mater. Res. Soc. Symp. Proc. 1997, 451, 235. (15) Demir, U.; Shannon, C. Langmuir 1996, 12, 6091. (16) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (17) Demir, U.; Shannon, C. Langmuir 1996, 12, 594. (18) Gichuhi, A.; Boone, B. E.; Demir, U.; Shannon, C. J. Phys. Chem. B 1998, 102, 6499.

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SO42- by S2-. Finally, micrometer-scale STM images indicate the formation of nanometer-scale ZnS crystallites which appears to be driven by lattice-mismatch induced strain. Rubinstein and Hodes have observed similar morphologies when CdSe is electrodeposited on Au(111) from a solution of CdSO4 and elemental Se.20 In addition, Resch et al. reported in situ studies of ZnS grown by the successive ionic layer adsorption and reaction (SILAR)21 method showing similar islandlike features.22 Experimental Section Au(111) Preparation. Single-crystal Au(111) substrates were prepared according to previously published literature methods.23 Briefly, a 0.2-1.0 mm polycrystalline Au wire (Alfa-Johnson Matthey, 99.999%) was flameannealed into a microbead in an Ar-sheathed H2-O2 flame to reveal several elliptical (111) facets whose major and minor axis measured approximately 1000 mm and 500 mm, respectively. Immediately following removal from the flame, the Au microbead was submerged in ultrapure water to protect the surface from contamination. These substrates can easily be aligned for STM imaging using a low-magnification optical microscope. Chemicals. Na2S‚9H2O (crystals, Fisher Scientific), NaClO4 (HPLC grade, Fisher Scientific), and KOH (pellets, Fisher Scientific), ZnSO4‚7H2O (99%, Aldrich), Na2SO4 (anhydrous, Fisher Scientific), HClO4 (70%, reagent A. C. S., Fisher Scientific) were used as received without further purification. All solutions were made using Millipore Q 18.2 MΩ resistance water and purged for 20 min with ultrahigh purity (UHP) Ar to remove dissolved O2. Electrochemistry. Cyclic voltammetry was performed using a Pine AFRDE-5 bipotentiostat and an HP-7055 X-Y recorder. The flow-through electrochemical cell consisted of a three-electrode configuration: the Au microbead as the working electrode, a Pt wire as the auxiliary electrode, and a Ag/AgCl (3 M NaCl) as the reference electrode to which all potentials are referred. All depositions were carried out from pressurized solution reservoirs made of Teflon or Kel-F. The electrochemical cell was directly connected to the solution-handling manifold that allowed the electrolytes to be changed without the electrode being exposed to the laboratory ambient. The underpotential deposition of S was carried out from a 1 mM Na2S‚9H2O in a 0.1 M NaClO4/0.01 M KOH electrolyte (pH 11.4). Following the deposition of S on naked Au, the electrode was rinsed under potential control with the neat HS- electrolyte to remove any unreacted HS-. Next, the S-modified electrode was rinsed with a (19) (a) Luo, Y.; Slater, D.; Han, M.; Moryl, J.; Osgood, R. M., Jr.; Langmuir 1998, 14, 1493. (b) Yarmoff, J. A.; Shuh, D. K.; Durbin, T. D.; Lo, C. W.; Lapiano-Smith, D. A.; McFeely, F. R.; Himpsel, F. J. J. Vac. Sci. Technol., A 1992, 10, 2303. (c) Jow, M. Y.; Maa, B. Y.; Morishita, T.; Dapkus, P. D. J. Electron. Mater. 1995, 24 (1), 25. (d) Yoshikawa, A.; Okamoto, T.; Yasuda, H.; Yamaga, S.; Kasai, H. J. Cryst. Growth 1990, 101, 86. (e) Yoshikawa, A.; Kobayashi, M.; Tokita, S. Appl. Surf. Sci. 1994, 82/83, 316. (f) Suntola, T.; Hyvarinen, J. Annu. Rev. Mater. Sci. 1985, 15, 177. (g) Wu, Y.; Toyoda, T.; Kawakami, Y.; Fujita, Sz.; Fujita, Sg. Jpn. J. Appl. Phys. 1990, 29 (5), L727-730. (20) (a) Golan, Y.; Margulis, L.; Rubinstein, I.; Hodes, G. Langmuir 1992, 8, 749. (b) Golan, Y.; Hatzor, A.; Hutchison, J. L.; Rubinstein, I.; Hodes, G. Isr. J. Chem. 1997, 37, 303. (c) Golan, Y.; Hodes, G.; Rubinstein, I. J. Phys. Chem. 1996, 100, L633. (21) (a) Nicolau, Y. F. Appl. Surf. Sci. 1985, 22/23, 1061. (b) Nicolau, Y. F.; Menard, J. C. J. Cryst. Growth 1988, 92, 128. (c) Nicolau, Y. F. U.S. Patent 4,675,207A, 1987. (d) Nicolau, Y. F.; Dupuy, M.; Brunel, M. J. Electrochem. Soc. 1990, 137, 2915. (22) Resch, R.; Friedbacher, G.; Grasserbauer, M.; Kanniainen, T.; Lindroos, S.; Leskela¨, M.; Niinisto¨, L. Fresnius J. Anal. Chem. 1997, 358, 80. (23) Hsu, T. Ultramicroscopy 1988, 11, 167.

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blank Zn2+ electrolyte before the deposition of Zn. The underpotential deposition of Zn was carried out from 1 mM ZnSO4‚7H2O in a 0.1 M Na2SO4 electrolyte (pH 5.7). After the deposition of Zn at its underpotential, the electrode was once again rinsed with the blank electrolyte (pH 5.7) to remove unreacted Zn2+. The chemical postdeposition rinses, which followed electrochemical deposition cycles, involved the use of one of the following HS- containing electrolytes to displace any electrostatically bound HSO4- species from the Zn layer: 1 mM Na2S‚9H2O in 0.1 M NaClO4/0.01 M HClO4 electrolyte at pH 2.4 or a 1 mM Na2S‚9H2O in 0.1 M NaClO4/0.01 M KOH electrolyte at pH 11.4. Finally, the electrode was rinsed with a HS- free electrolyte to remove unreacted H2S or HS-. This posttreatment procedure was performed to ensure that the lattice was completely terminated with S and not HSO4- ions. As before, all rinses were performed under potential control. The substrate was then emersed, rinsed with deionized water, dried with Ar or N2, and stored under UHP Ar prior to imaging or XPS analysis. The electrochemical analogue of the posttreatment procedure described above was carried out as follows: Zn was underpotentially deposited on the Au microbead from a 1 mM ZnSO4‚7H2O contained in a 0.1 M Na2SO4 electrolyte (pH 5.7). Next, S was deposited on the Znmodified Au surfaces from 1 mM Na2S‚9H2O in a 0.1 M NaClO4/0.01 M KOH electrolyte (pH 11.4). Scanning Tunneling Microscopy (STM). All scanning tunneling microscopy experiments were performed under ambient conditions using a model SA-1 STM (Park Scientific Instruments, Sunnyvale, CA). Atomic- and micrometer-scale images were acquired using both constant height and constant current modes; the exact tunneling conditions are given in the figure captions. W tips, used in the atomic scale images, were prepared by etching a 0.5-mm diameter wire in 1 M KOH solution using a model TE-100 STM tip etcher (Park Scientific Instruments). Pt:Ir (90:10) tips, cut at a 45° angle, were used for the micrometer-scale images. In all cases, the sample was biased positive relative to the tip. The x-y plane calibration was performed using two different standards: highly oriented pyrolytic graphite (HOPG, donated by Dr. Arthur Moore, Union Carbide, Parma, OH) and a Au(111) single crystal in which the interatomic distance of Au is 0.29 nm. The calibration of the piezo in the z-direction (i.e., normal to the plane of the surface) was carried out using the Au atomic step height (0.24 nm). Unless otherwise stated, all images presented are unfiltered. X-ray Photoelectron Spectroscopy (XPS). Corelevel photoelectron spectra were collected with a PHI 5750 XPS system using a 04-548 dual anode source and a PHI 10-360 hemispherical analyzer. The Al source (1486.6 eV) was operated at 400 W, and spectra were collected with a 45° takeoff angle. These measurements were made using a pass energy of 46.95 eV in order to enhance sensitivity to the inherently weak sulfur 2p signal. Samples were briefly exposed to atmospheric conditions while being loaded into the XPS system; however, no evidence for surface contamination was observed in any of the measurements. All XPS measurements were performed on larger Au beads prepared in manners identical to those used in the preparation of the microball samples for microscopy studies.

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Figure 1. Cyclic voltammetric behavior of S in the underpotential region at a naked Au electrode. The scan rate was 0.100 V s-1, and the electrode area was 0.092 cm2.

S layer on Au is shown in Figure 1. An atomic layer of S was deposited from 1 mM Na2S‚9H2O in a 0.1 M NaClO4/ 0.01 M KOH electrolyte at pH 11.4. The electrode potential was scanned between -1.10 and -0.700 V. Two sets of oxidative adsorption and reductive desorption peaks are observed with E0′ values of -0.960 and -0.840 V, respectively. The origin of the two peaks is still a matter of debate and is complicated by the fact that the voltammetric behavior is pH- and electrolyte-dependent. Previous workers have postulated a variety of mechanisms for the electrooxidation of HS- on Au electrodes. On the basis of in situ visible reflectance measurements, for example, Arvia suggested a two-step mechanism involving an initial adsorption of HS- followed by a two-electron oxidation to form adsorbed S.24 Weaver and co-workers have investigated the behavior of HS- on Au(111) electrodes using cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS).25 Although not explicitly mentioned by the authors, there appear to be two waves between -1.00 and -0.800 V in cyclic voltammograms recorded in basic electrolyte. SERS data clearly indicate the adsorption of S on the Au surface as well as the protonation-deprotonation of adsorbed S as the potential is scanned through the more positive of the two waves. Finally, these workers investigated the structure of the S adlayer using in situ STM, albeit under acidic conditions. In the anodic adsorption region, a (x3 × x3)R30° overlayer of S is formed on Au(111).26 These findings are consistent with the behavior of S on most fcc metals in electrolyte and in ultrahigh vacuum. At the emersion potential used in this study, i.e., -0.700 V, a stable (x3 × x3)R30° S structure is always observed by STM. Coulometry indicates that a total charge of approximately 148 ( 18 µC cm-2 is passed under the two anodic waves. Assuming two electrons are transferred overall, this charge density corresponds to a surface coverage of about 0.33, which is in excellent agreement with the known coverage of the (x3 × x3)R30° S/Au(111) adlattice.

Results and Discussion Deposition of S on Au(111). A representative cyclic voltammogram for the oxidative adsorption of the initial

(24) Arvia, A. J. J. Electroanal. Chem. 1990, 283, 319. (25) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992, 8, 668. (26) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156.

Electrochemically Grown ZnS Monolayers

Further insight into the nature of these two waves can be obtained by comparison to the behavior of HS- on Ag(111). In a series of recent papers, White and co-workers convincingly have shown that the electrooxidation of HSon Ag(111) electrodes proceeds by a two-step mechanism.27 The first step is a one-electron oxidation leading to the formation of adsorbed HS. The second step corresponds to the one-electron oxidation of adsorbed HS to form a monolayer of Ag2S and H+(aq). Interestingly, the first (adsorptive) step of this reaction is thought to proceed via the formation of distinct structural phases, a low-density and a high-density phase. This interpretation is supported by the fact that the adsorptive step can be resolved into two distinct voltammetric waves and by comparison to the behavior of electrochemically driven self-assembly of alkanethiolate monolayers on Ag(111). A similar two-step mechanism is likely operative in the case of HS- oxidative adsorption on Au at basic pHs. Interestingly, however, there is no evidence, from the voltammetry at least, that indicates the formation of a low-density phase of HS(ad) on the Au surface. When the S/Au(111) surface is imaged on the micrometer-scale numerous surface depressions or pits are always observed by STM (Figure 2B). The morphology of a clean Au(111) surface is shown in Figure 2A for comparison. These features are characterized by an average depth of 0.25 nm and range in diameter from about 10 to roughly 25 nm. In addition, the pits occupy a fractional area of approximately 0.08. On the basis of these findings, we believe that these features correspond to Au vacancy islands formed as a result of the lifting of the (23 × x3) reconstruction of Au(111) upon adsorption of HS-.28 Although the vacancy island fractional area and average depth agree well with what has been reported in the literature for alkanethiolate self-assembled monolayers, the average diameter of the vacancy islands we observe is significantly larger than what is typically seen in the alkanethiolate case. For this reason, we believe that an etching process may also contribute to the formation of these pits. Historically, basic sulfide solutions have been used to solubilize Au.29 In addition, similar corrosion features were recently reported by Gorman and Touzov following the dosing of Au(111) with gas-phase H2S.30 Electrochemistry of Zn Deposition on S/Au(111). A representative cyclic voltammogram for the deposition of Zn on S/Au (111) is shown in Figure 3. The underpotential deposition of Zn was carried out from 1 mM ZnSO4‚7H2O contained in a 0.1 M Na2SO4 electrolyte (pH 5.7). The electrode potential was swept between 0.200 (immersion) and -0.550 V (emersion). The deposition peak is centered at -0.450 V and is shifted 0.150 V positive of its position on naked Au. Likewise, the stripping peak, centered at -0.190 V, is shifted positive by 0.330 V with respect to its value on naked Au. This strongly suggests an increased stability for the deposition of Zn on S/Au compared to its deposition on bare Au. The charge densities (27) (a) Stevenson, K. J.; Gao, X.; Hatchett, D. W.; White, H. S. J. Electroanal. Chem. 1998, 447, 43. (b) Hatchett, D. W.; White, H. S. J. Phys. Chem. 1996, 100, 9854. (c) Hatchett, D. W.; Gao, X.; Catron, S. W.; White, H. S. J. Phys. Chem. 1996, 100, 331. (d) Stevenson, K. J.; Hatchett, D. W.; White, H. S. Isr. J. Chem. 1997, 37, 173. (e) Aloisi, G. D.; Cavallini, M.; Innocenti, M.; Foresti, M. L.; Pezzatini, G.; Guidelli, R. J. Phys. Chem. B 1997, 101, 4774. (f) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506. See also: Foresti, M. L.; Pezzatini, G.; Cavallini, M.; Aliosi, G.; Innocenti, M.; Guidelli, R. J. Phys. Chem. B 1998, 102, 7413. (28) Poirier, G. E. Langmuir 1997, 13, 2019 and references therein. (29) (a) Eggelston, T. A. I. M. E. Trans. 1880-1881, 97, 639. (b) Mellor, J. W. A Comprehensive Treatise on Inorganic and Theoretical Chemistry; Longmans, Green and Co.: London, 1923; Vol. 3. (c) Ogryzlo, S. P. Econ. Geol. 1935, 30, 400. (d) Smith, F. Econ. Geol. 1943, 38, 561. (30) Gorman, C. B.; Touzov, I. Langmuir 1997, 13, 4850.

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Figure 2. Micrometer resolution (0.4 µm × 0.4 µm) scanning tunneling microscopy. (A) Naked Au(111) showing atomically flat terraces separated by monatomic steps; Vt ) 0.19 V, It ) 10 nA, and scan rate ) 2 Hz. (B) The first monolayer of S on Au(111) formed upon emersion of the Au electrode at -0.700 V; Vt ) 0.37 V, It ) 1.5 nA, and scan rate ) 1 Hz. Both images were acquired in constant current mode.

under the cathodic and anodic peaks are 84 ( 9 and 87 ( 10 µC cm-2, respectively. Previous workers31 have shown that the electrosorption valency for Zn upd on Au is approximately 1. Assuming an electrosorption valency of 1 in our experiments, these charge densities correspond to a Zn coverage of approximately 0.33. Following the upd of Zn on S/Au, the XPS spectrum of the S 2p region (Figure 4) clearly reveals the presence of (31) (a) Nakamura, M.; Aramata, A.; Yamagishi, A.; Taniguchi, M. J. Electroanal. Chem. 1998, 446, 227. (b) Moniwa, S.; Aramata, A. J. Electroanal. Chem. 1994, 376, 203. See also: Quaiyyum et al.34

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Figure 3. Cyclic voltammetric response of Zn in the underpotential region at a S-modified Au electrode. The scan rate was 0.100 V s-1, and the electrode area was 0.092 cm2.

Figure 4. X-ray photoelectron spectrum of the S 2p region following the underpotential deposition of Zn on the S-modified Au surface.

two predominant S-containing species. The feature centered at ca. 162 eV is assigned to the convoluted 2p doublet of a S2- species. Although this is in the range of reported sulfide species,32 it is significantly lower than the 164 eV binding energy previously reported for ZnS. It is thought that the proximity of this sulfur species to the gold substrate leads to the observed shift. The feature at higher binding energy (ca. 169 eV) is assigned to the convoluted 2p doublet of a SO42- species. The presence of other minority species cannot be excluded due to the limited resolution of the spectra. A representative atomically resolved STM image of the S/Au(111) following Zn upd is shown in Figure 5. This image reveals a 6-fold symmetric structure with an interatomic spacing of 0.32 ( 0.014 nm that we observe following the emersion of the electrode at -0.550 V). This spacing does not correspond to any of the characteristic lattice parameters of wurzite or cubic ZnS. This is not particularly surprising considering that XPS indicates the presence of SO42- on the surface. Although our XPS data (32) Svensson, J.-E.; Johansson, L.-G. J. Electrochem. Soc. 1995, 452, 1484.

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Figure 5. A 4.0 nm × 4.0 nm scanning tunneling microscopy image of the first complete monolayer of ZnS immediately after emersing the electrode at -0.550 V; Vt ) 0.26 V, It ) 0.50 nA, and scan rate ) 22 Hz. This image was acquired in constant height mode.

also indicate that no significant amount of surface oxide is formed, it should be noted that the spacing observed in this image closely resembles that found in hexagonal zincite, ZnO (a0 ) 0.325 nm).33 The presence of small amounts of ZnO, formed either electrochemically or by atmospheric oxidation of the terminal Zn layer cannot be completely ruled out. Postelectrolysis Surface Modification. A series of chemical treatments was investigated in order to convert the lattice termination from SO42- to S2-. Following the deposition of Zn on S/Au(111), the electrode was briefly rinsed with either 1 mM Na2S‚9 H2O in 0.10 M NaClO4/ 0.01 M HClO4 (H2S, pH 2.4) or 1 mM Na2S‚9 H2O in 0.10 M NaClO4/0.01 M KOH (HS-, pH 11.4) at the Zn emersion potential, i.e., - 0.560 V. After rinsing and drying the H2S or HS- dosed electrode in a stream of Ar, atomic- (Figure 6) and micrometer-scale (Figure 7) STM images of the posttreated surfaces were obtained. The image shown in Figure 6 is representative of the 6-fold symmetric structure of the monolayer we observed after either postelectrolysis treatment. The S-S distances in this image were determined to be 0.37 ( 0.01 nm which is within experimental error of the bulk spacing of ZnS (a ) 0.382 nm)33 in the wurtzite form, indicating that both surface treatment procedures resulted in quantitative exchange of S2- for SO42-. The XPS data to be discussed in the next section confirm this structural assignment. As an aside, we note that this finding also supports our earlier contention that the majority species present on the surface following emersion of the Zn upd monolayer is sulfateterminated Zn. (33) Kittel, C. Introduction to Solid State Physics, 5th ed.; John Wiley and Sons Inc.: New York, 1976; p 28. Our XPS data indicates that the majority of oxygen-containing species is SO42- not ZnO. The binding energy and peak shape of the O1s peak was the same in all our spectra and is assigned as arising from a sulfate species on the basis of the observed binding energy (532 eV). Neferdov has shown that ZnO and ZnSO4 can be distinguished on the basis of binding energy. Oxygen 1s binding energies are 530.4 eV for ZnO and 532.5 eV for ZnSO4. See: Nefedov, V. I.; Firsov, M. N.; Shaplygin, I. S. J. Electron. Spectrosc. Phenom. 1982, 26, 65 and Nefedov, V. I. J. Electron. Spectrosc. Phenom. 1982, 25, 29.

Electrochemically Grown ZnS Monolayers

Langmuir, Vol. 15, No. 17, 1999 5659 Scheme 1. Proposed Structure of Wurtzite ZnS on Au(111)a

Figure 6. A 4.0 nm × 4.0 nm scanning tunneling microscopy image of the first complete monolayer of ZnS after postelectrolysis treatment with HS-; Vt ) 0.017 V, It ) 2.2 nA, and scan rate ) 18 Hz. This image was acquired at constant height mode. Images of surfaces treated with H2S were identical. See text for details.

a Key: (a) Stacking of Zn and S layers in hexagonal ZnS showing the repeating c-spacing and (b) schematic representation of ZnS depicting the 2:x7 ZnS/Au supercell. The lattice mismatch is +0.13%.

Figure 7. A micrometer-resolution (0.4 µm × 0.4 µm) scanning tunneling microscopy image of ZnS crystallites formed following postelectrolysis surface modifications with HS- (surfaces treated with H2S gave identical results); Vt ) 0.22 V, It ) 0.35 nA, and scan rate ) 1.5 Hz. This image was acquired using constant current mode.

Investigation of the deposit morphology at the micrometer-scale reveals a complex surface structure (Figure 7). In particular, two unique structural features can be identified in this image. First, as in the case of the S/Au monolayer, about 8% of the surface consists of Au vacancy islands (pits). Second, we observe numerous protrusions measuring 10 ( 5 nm in diameter and 0.35 ( 0.06 nm in height. The fractional coverage of these features is 0.38, in excellent agreement with the known amount of Zn and S atoms electrodeposited. Furthermore, we note that the height of the features is approximately one-half the

c-spacing in wurtzite ZnS. The height of a S-Zn-S sandwich depositing as wurtzite ZnS with the S-Zn-S axis oriented along the surface normal is expected to be approximately c/2. Taken together, our data suggests that the ZnS deposits epitaxially on Au(111) with the ZnS c-axis parallel to the Au(111) surface normal (Scheme 1). The azimuthal orientation of the crystallites cannot be established unambiguously from the STM data alone. The proposed unit cell was chosen on the basis of lattice matching with Au(111). In this structure, the a- and b-axes of ZnS are rotated 19.1° with respect to those of Au(111). The resulting 2:x7 supercell is characterized by a lattice mismatch of +0.13%. Similar structures have been observed in the electrodeposition of CdSe nanocrystals on Au(111) surfaces.20 The crystallites we observe are narrowly dispersed in diameter. Thus, it is thought that the driving force for formation of similarly sized crystals is lattice-mismatch-induced strain. We can compare the present results to earlier results from our laboratory on the deposition of CdS on Au(111). In that case, STM revealed a locally ordered (3 × 3) CdS adlattice on Au.16 On the micrometer scale, however, the surface appeared to be atomically smooth, although we were not able to image the grain structure of these films. The lattice mismatch (-4.53%) was much worse in the case of CdS/ Au(111) than in the case of ZnS/Au(111). We believe that the CdS formed a highly polycrystalline deposit in which the crystallite size was too small to resolve at the micrometer scale in the STM, thus giving the appearance

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Figure 9. Cyclic voltammetric behavior of Zn in the underpotential region at a naked Au electrode. The scan rate was 0.100 V s-1, and the electrode area was 0.092 cm2.

Figure 8. X-ray photoelectron spectra of the S 2p region following the posttreatment of the SO42- containing Zn adlattice with HS- (top) and H2S (bottom) containing electrolytes. See text for details.

of an atomically flat surface. Due to the higher degree of lattice matching in the present case, it is possible to resolve epitaxial, oriented ZnS crystallites in the STM experiments. In Figure 8, we present an XPS spectrum (S 2p region) that is representative of the postelectrolysis modified surfaces. The HS- (pH 11.4) and H2S (pH 2.4) treatments of the Zn layer lead to the elimination of the SO42- (168.7 eV) binding energy peak as shown in Figure 8A,B, respectively. Taken together with the STM data discussed earlier, this result demonstrates conclusively the quantitative conversion of the adlayer to ZnS. Electrochemistry of Zn Deposition on Au(111). For comparison, we synthesized a monolayer of ZnS by depositing Zn on Au followed by the oxidative adsorption of an atomic layer of S; this approach ensured that the terminating species was S and not SO42-. Figure 9 shows a cyclic voltammogram for the upd of Zn on Au. The underpotential deposition of Zn was, as before, carried out from 1 mM ZnSO4‚7H2O contained in a 0.1 M Na2SO4 electrolyte (pH 5.7). The electrode potential was scanned between -0.100 (immersion) and -0.800 V (emersion). The underpotential shift, (∆Ep), between the bulk stripping and upd stripping peaks of Zn on Au (at pH 5.7) is less than 0.350 V, strongly suggesting a strong positive interaction between Zn and Au. For this reason, the negative cathodic limit had to be judiciously chosen so as to mitigate against bulk Zn deposition. Cathodic

Figure 10. STM (4.0 nm × 4.0 nm), of the (3 × 3) Zn adlattice on Au(111) immediately after emersing the electrode at -0.800 V; Vt ) 0.32 V, It ) 1.1 nA, and scan rate ) 20 Hz. This image was acquired at constant-height mode. A low-pass (Wiener) filter was applied to this image to reduce high-frequency noise components.

and anodic peaks are observed at -0.640 and -0.520 V, respectively. The average charge passed under the anodic and cathodic peaks was determined to be 93 ( 13 µC cm-2 (corresponding to a coverage of 0.42 assuming an electrosorption valency of unity). An STM micrograph for the 6-fold symmetric (3 × 3) Zn upd structure on Au(111) is shown in Figure 10. The interatomic spacing was determined to be 0.36 ( 0.02 nm. The 0.44 fractional coverage estimated from the STM agrees well with that determined from coulometry calculations. We believe the identity of the spots shown in this image are those of either Zn or SO42- ions that may be adsorbed on top of the Zn atoms. Although Zn is known to form strong alloys with Au,34 even at its underpotential, we saw no morphological evidence (such as increased roughening) from our STM images of alloying. Cd is also known to alloy with Au and

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Langmuir, Vol. 15, No. 17, 1999 5661

has recently been shown by Gewirth et al. to likewise form stable upd layers on Au(111).12 Electrochemistry of S Deposition on Zn/Au(111). S deposition on the Zn-modified Au surface was carried out from 1 mM Na2S‚9H2O in a 0.1 M NaClO4/0.01 M KOH at pH 11.4. The electrode potential was scanned between -1.10 (immersion) and -0.500 V (emersion). Figure 11 represents the typical voltammetric response we observe for S on Zn/Au(111). Oxidative adsorption and reductive stripping peaks appear at -0.830 and -0.940 V, respectively. The charge passed under the stripping peak was calculated to be roughly 185 µC cm-2 (θ ≈ 0.42). We observed a residual cathodic current under these conditions that we attribute to competing H2 evolution at these negative potentials. XPS analysis of electrochemically posttreated samples confirms the electrochemical exchange of SO42- by S2-; only the convoluted S 2p doublet characteristic of a sulfide species (ca. 162 eV) is observed. The appearance of the spectra were identical to the spectra shown in Figure 8. Conclusions In summary, we have investigated the structure of the initial ZnS monolayer electrochemically grown on Au(111) by EC-ALE. We have found that in the initial stages of growth, the structure of the monolayer formed is influenced by components of supporting electrolyte. We have shown, however, that after a brief exposure of the SO42- terminated lattice to H2S (pH 2.4) or HS- (pH 11.4) the target product (i.e., wurtzite ZnS) is recovered. We believe that this exposure leads to the complete displacement of weakly bound SO42- ions with S2-. In addition, we have shown that, as a result of lattice-mismatchinduced strain between ZnS and Au(111), nanocrystals (34) (a) Quaiyyum, M. A.; Aramata, A.; Moniwa, S.; Taguchi, S.; Enyo, M. J. Electroanal. Chem. 1994, 373, 61. (b) Tadjeddine, A.; Tourillon, G. Elektrokhimiya 1993, 29, 63. (c) Despic, A. R.; Pavlovic, M. G. Electrochim. Acta 1982, 27, 1539. (d) Nasar, A.; Shamsuddin, M. Thermochim. Acta 1992, 205, 157. (e) Chu, M. G.; McBreen, J.; Adzic, G. J. Electrochem. Soc. 1981, 128, 2281.

Figure 11. Cyclic voltammetric behavior of S in the underpotential region at a Zn-modified Au electrode. The scan rate was 0.100 V s-1, and the electrode area was 0.092 cm2.

narrowly dispersed in diameter form randomly across the entire Au(111) terraces. Acknowledgment. The financial support of this work by the National Science Foundation (OSR-9557748), the Society of Analytical Chemists of Pittsburgh, and Auburn University is gratefully acknowledged. This work made use of MRSEC/TCSUH Shared Experimental Facilities supported by the National Science Foundation under Award Number DMR-9632667 and the Texas Center for Superconductivity at the University of Houston. LA981743P