Electrochemical Atomic Layer Epitaxy Deposition of CdS on Ag (111

Marcus D. Lay, Kris Varazo, and John L. Stickney ... Melissa S. Sander, Marisol S. Martín-González, Ronald Gronsky, Timothy Sands, and Angelica M. S...
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J. Phys. Chem. B 1998, 102, 7413-7420

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Electrochemical Atomic Layer Epitaxy Deposition of CdS on Ag(111): An Electrochemical and STM Investigation M. L. Foresti, G. Pezzatini, M. Cavallini, G. Aloisi, M. Innocenti, and R. Guidelli Dipartimento di Chimica, UniVersita’ di Firenze, Via G. Capponi, 9 50121 Firenze, Italy ReceiVed: February 17, 1998; In Final Form: May 26, 1998

The electrochemical atomic layer epitaxy methodology was employed to obtain CdS deposits on Ag(111) by alternate underpotential deposition of up to five layers of sulfur and four layers of cadmium. The charge involved in each layer was determined by cyclic voltammetry. With the exception of the first sulfur layer, the charge involved in the deposition of each sulfur and cadmium layer was the same, indicating the achievement of a stoichiometric, 1:1 ratio, deposit from the second layer. The first layer, which consisted of sulfur on the bare silver surface, involved a higher charge and is to be regarded as an interface between the metal and the compound. The measured charge is in good agreement with that estimated on the basis of STM images. The structure revealed by STM for all but the first layer was a (x7 × x7)R19.1°, with one atom per lattice site, relative to the Ag(111) substrate.

1. Introduction Electrochemical atomic layer epitaxy (ECALE) was proposed by Stickney and co-workers1,2 as a low-cost procedure for producing structurally well-ordered ultrathin layers of compound semiconductors such as CdS, CdSe, CdTe, and ZnS on polycrystalline as well as on single-crystal gold electrodes. The ECALE method is based on the alternate underpotential deposition of atomic layers of both elements forming the compound semiconductor. Underpotential deposition (upd) is the well-known phenomenon whereby the potential necessary to deposit one element onto a second element occurs before that necessary to deposit the element on itself. The resulting deposit is generally limited to an atomic layer: this circumstance is easily confirmed by the independence of the height of the corresponding voltammetric peak upon the bulk concentration of the depositing element and, thus, ensures atomic-level control in the deposit. Moreover, when upd is performed on a single-crystal face, the resulting deposit is generally epitaxial, namely it is strongly affected by the crystallographic orientation of the substrate. In the ECALE method, the underpotential electroreduction of the metallic element is alternated with the underpotential electrooxidation of the nonmetallic element. The epitaxial growth of the resulting compounds makes them particularly suitable for use as nanometric materials in modern technological applications. Colletti et al.2 adapted the ECALE procedure to grow CdS on polycrystalline gold electrodes. The surface coverage, as measured relative to the number of surface Au atoms, was found to be approximately equal to 0.45 for upd of either S or Cd. The coverage values, as a function of the number of upd cycles, were determined by anodic stripping of both elements, with the Cd deposit regenerating Cd2+ and the S deposit being oxidized to sulfate. The latter oxidative stripping took place at potentials as positive as +0.7 V/SCE and can, therefore, only be observed on metals with a double-layer region extending toward positive potentials, such as gold. In the present investigation, the ECALE methodology was employed to obtain deposits of CdS on Ag(111) by the alternated

upd of S and Cd. The use of silver in place of gold as a substrate for compound semiconductors may be advantageous in view of potential applications in the fabrication of devices due to its lower cost. Another advantage is related to the upd voltammetric peaks, which on silver are often better defined than on gold. On the other hand, silver is less noble than gold and its surface is, therefore, more reactive. As a consequence, the double-layer region in the positive direction is narrower than on gold, so that different experimental conditions must be adopted not only for the compound growth, but also for its electrochemical characterization. Moreover, silver surface reactivity requires more severe treatments before use: a chemical polishing followed by the application of a potential sufficiently negative to produce mild hydrogen evolution was used and sometimes required repetition. Sulfur overlayers on the low-index faces of silver were first investigated by White and co-workers.3 The cyclic voltammograms of Ag(111) in 0.2 M NaOH solutions containing Na2S obtained by these authors show three peaks (herein denoted by A, C, and D) occurring at -1.21, -1.12, and -0.90 V vs Ag/ AgCl (3 M NaCl), respectively. On the basis of coulometric measurements of interfacial charge and of electrochemical quartz crystal microbalence measurements of mass, Hatchett and White3b hypothesize a (x7 × x7)R10.9° structure, with three sulfur adatoms per primitive unit cell. According to these authors, between peaks C and D, a AgSHads layer is formed, resulting from the transfer of one electron per adsorbed SHion to the metal, while peak D marks the transfer of a further electron to the metal and the release of a proton to the solution, with formation of a Ag2Sads layer. Oxidative S upd on Ag(111) from alkaline solutions of Na2S was subsequently investigated in the present laboratory by in situ scanning tunneling microscopy (STM), cyclic voltammetry, and chronocoulometry.4 Our cyclic voltammograms are very similar to those reported by White and co-workers,3 apart from the presence of a very small peak (peak B) between peaks A and C, which is usually observed with freshly cut Ag(111) electrodes and seems to be related to the surface state of the electrode.4 The STM images reveal a (x3 × x3)R30° structure

S1089-5647(98)01177-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/29/1998

7414 J. Phys. Chem. B, Vol. 102, No. 38, 1998 at potentials between peaks C and D and a (x7 × x7)R19.1° structure at more potentials positive than peak D: in the latter structure, each lattice site is occupied by a triplet of sulfur atoms. The fractional coverage, 1/3, for the (x3 × x3)R30° structure is in perfect agreement with the maximum surface concentration, Γmax ) 7.7 × 10-10 mol cm-2, obtained from a thermodynamic analysis of chronocoulometric charge vs potential curves. However, the corresponding charge, calculated assuming that the oxidation of one HS- ion involves two electrons, is 148 µC cm-2 and, therefore, slightly greater than 130 µC cm-2 estimated from the area under the three voltammetric peaks A, B, and C. In addition, a charge of 125 µC cm-2 was obtained directly from chronocoulometric measurements. The lower value of the experimental charges can be explained by assuming that electron transfer is not complete. Conversely, in the potential range of stability of the (x7 × x7)R19.1° structure, characterized by a fractional coverage of 3/7, the corresponding calculated charge, 185 µC cm-2, coincides with both the voltammetric and the chronocoulometric experimental values, thus pointing to complete electron transfer from the HS- ion to the metal in the more positive peak D. Incidentally, the latter structure is that realized in the first step of the ECALE procedure adopted in the present work, since the first atomic layer is obtained by oxidative S upd on Ag(111) at -0.8 V/SCE. This is the first of a series of papers aiming at extending the ECALE technique, which Stickney and co-workers have developed on gold substrates,1 to silver single-crystal and polycrystalline electrodes. The main goal is the epitaxial electrodeposition of group II-VI semiconductor nanomaterials of practical interest on silver by the low-cost, lowtemperature ECALE method. This requires an extensive investigation of the optimum conditions for the alternated deposition of atomic layers of an element of group VI and a metal of group II on silver substrates. This paper describes convenient electrochemical conditions for the electrodeposition of CdS layers on Ag(111) and reports an in situ STM investigation of the structure of the first atomic layers deposited. 2. Experimental Section Merck Suprapur NaF was baked at 700° C to remove organic impurities. Merck Suprapur NaOH, Merck analytical reagentgrade 3CdSO4‚8H2O, and Aldrich analytical reagent-grade Na2S were used without further purification. Merck KClO4 and Na4P2O7‚10H2O were recrystallized twice from twice distilled water and then dried. Fabrication, polishing, and treatment of the Ag(111) electrode are described in ref 4. All potentials are referred to the saturated calomel electrode (SCE). Both electrochemical and STM measurements were carried out after verifying the quality of the electrode surface by recording a cyclic voltammogram in a solution of KClO4 or NaF of a concentration low enough (10-2 M) to exclude the specific adsorption of the salt. Electrochemical Measurements. The ECALE procedure on polycrystalline electrodes is usually performed using a thinlayer electrochemical cell,5 since the high ratio of electrode surface to cell volume makes it optimal for quantitative coulometric determinations of deposited films and permits a significant decrease in surface contamination. However, the shape of the single crystals (cylinders ∼5 mm in diameter and 3 cm long with the face of interest not exactly perpendicular to the cylinder axis) imposed the use of a conventional thick-layer electrochemical cell and the hanging solution method6 in order to avoid exposure of the polycrystalline sides. The cell was a glass cylinder, ∼2.5 cm in diameter, with the inlet for the solutions positioned at the bottom and the outlet

Foresti et al. ∼1 cm above to keep the volume of the solution at a constant value of ∼5 cm3. The supporting electrolyte was forced to flow by gravity from a flask connected to the inlet through a stopcock. The proper concentrations of HS- or cadmium ion in the cell were then realized by adding deaerated portions of more concentrated solutions with a microsyringe. A gold wire was used as a counter electrode, and an external saturated calomel electrode (SCE) served as the reference. The four-electrode potentiostatic system by Herrmann et al.7 was employed to minimize the noise, and a positive feedback circuitry was utilized to correct for the uncompensated resistance. The solution was deaerated with argon, bubbled into the solution before measurements, and flowed over the solution during measurements. The cylindrical single-crystal electrode was held by a silver wire sealed into a glass tube that was secured to a movable stand. The ECALE procedure requires the separate upd of both elements, with each deposition being followed by rinsing the cell with an inert electrolyte to prevent CdS precipitation, which would result if both HS- and cadmium ions were present in the same solution. To this purpose, a volume of inert electrolyte alone must flow through the cell after each upd to remove any traces of the last deposited element from the solution. The rinsing procedure was performed while controlling the electrode potential. Despite the relatively small volume of the solution contained in the cell, an accurate rinsing procedure requires a relatively large volume of inert electrolyte. This rinsing was found to cause a partial dissolution of the sulfur deposit at pH values 0.7 mM. Scanning the potential more negative than -0.70 V resulted in further Cd deposition. Figure 2 shows a plot of the charge for the oxidative stripping of Cd upd as a function of the potential for Cd upd when adopting a deposition time of 1 min and a 1 mM cadmium ion solution. It is apparent that the deposition time of 1 min is insufficient for completion of the reductive Cd upd at potentials more positive than -0.55 V, whereas further Cd deposition starts to occur at potentials more negative than -0.70 V. The charge for stripping Cd upd was also estimated by a chronocoulometric procedure analogous to that adopted in ref 4 for S upd. To that end, the applied potential was stepped from a variable initial value E, ranging from -0.58 to -0.1 V so as to cover the whole potential region of Cd upd, to a fixed final value Ef ) -0.1 V, where the deposited Cd is instantaneously reoxidized to cadmium ion. The rest time of the electrode at E was made long enough to attain equilibrium for

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Figure 3. Plot of the chronocoulometric charge Q0 for the oxidative stripping of Cd upd on Ag(111) against E, obtained as described in the text from a solution of 1 × 10-3 M CdSO4 + 0.05 M Na4P2O7 + 0.01 M NaOH.

Cd upd. Figure 3 shows a plot of the charge density Q0(E), as obtained by linear extrapolation to t ) 0 of the charge vs time curves following each potential step, against E. The slope of the foot of the sigmoidal curve of Q0(E) vs E is a measure of the capacity of a Ag/S/Cd structure, whereas the slope of the corresponding plateau measures the capacity of a S-coated silver electrode. By assuming a gradual change in capacity along the rising portion of the Q0(E) vs E curve, the self-explanatory graphical procedure in Figure 3 was adopted to estimate the charge involved in the oxidative stripping of Cd upd. This amounts to 78 µC cm-2 and is, therefore, slightly greater than the value obtained by cyclic voltammetry. The procedure for obtaining the first Cd layer on top of the first S can be simplified by just keeping the electrode at E ) -0.8 V for 1 min in the HS- solution, washing the cell, shifting the applied potential to -0.58 V, injecting the cadmium ion solution, holding at the latter potential for 1 min, and finally washing the cell again. Recording a cyclic voltammogram for upd of the second S layer on top of the Ag/S/Cd structure is prevented, as the first S layer would be reductively stripped at potentials more negative than -0.8 V. Before proceeding further, the voltammetric behavior of S upd on Cd was, therefore, investigated on a Cd layer deposited by upd directly on Ag(111). Cd upd on Ag(111), from a solution of 0.05 M Na4P2O7 + 0.01 M NaOH, gives rise to a concentration-independent cathodic peak at a potential, -0.61 V, more negative than that on S-coated Ag(111). The cyclic voltammogram obtained after depositing Cd by upd at -0.7 V, washing the cell at the same potential, shifting the potential to -1.3 V, and injecting the HS- solution depends on the volume of supporting electrolyte employed for the washing at -0.7 V. With a minimum washing volume, voltammogram 1 in Figure 4 is obtained; this shows three rounded peaks that correspond to oxidative S upd on the Ag/ Cd structure at potentials more negative than on bare Ag(111) (see Figure 1a). Thus, the less negative peak in Figure 4, curve 1, lies at ca. -1.0 V, while the typical sharp peak D for S upd on Ag at -0.86 V (see Figure 1a) is lacking. This behavior is explained by the free energy of formation of CdS being more negative than that of Ag2S. Use of progressively increasing volumes of the supporting electrolyte for the washing process at -0.7 V causes the cyclic voltammogram for S upd to evolve gradually from curve 1 to curve 3 in Figure 4. It is apparent that the anodic peak D for S upd on bare silver starts to appear in curve 2 of Figure 4 and becomes the predominant feature in

Foresti et al.

Figure 4. Cyclic voltammograms for the oxidative S upd from a solution of 5 × 10-4 M Na2S + 0.05 M Na4P2O7 + 0.01 M NaOH on top of an atomic layer of Cd deposited by upd on Ag(111) as descibed in the text. Curves 1-3 were obtained upon using progressively increasing volumes of the washing solution before the injection of the HS- solution.

curve 3. This indicates that excessive washing tends to dissolve the Cd deposit on Ag(111), causing the gradual appearance of the typical features for S upd on Ag and the concomitant disappearance of those for S upd on the Ag/Cd structure. The lack of stability of the Cd deposit is also confirmed by STM experiments, in which the tip current seems to destroy it, thus preventing imaging of the deposit with atomic resolution. The relatively low stability of a Cd layer in direct contact with Ag(111), together with the risk of possible alloy formation between Cd and Ag as reported in the literature,11,12 is in favor of sulfur as the first layer. Nonetheless, the fact that oxidative S upd on a Cd layer takes place at potentials more negative than on bare Ag(111) allowed us to maintain the value of -0.8 V for the upd of further S layers. In addition, the fact that reductive Cd upd on a S layer takes place at potentials more positive than on bare Ag(111) allowed us to maintain the value of -0.58 V for the upd of further Cd layers. The procedure used to obtain a second S layer on the Ag/S/ Cd structure is identical with that used to obtain the first S layer. It consists of keeping the electrode at -0.8 V in a HS- solution for 1 min, washing the cell, shifting the potential to -0.58 V, injecting the cadmium ion solution, holding the electrode at the latter potential for 1 min, washing the cell, shifting the potential to -0.8 V, and once again injecting the HS- solution; all these operations make use of the same supporting electrolyte, i.e., aqueous 0.05 M Na4P2O7 + 0.01 M NaOH. The second S layer is also surface-limited and, hence, results from a S upd. Thus, the charge involved in its reductive stripping is independent of the HS- bulk concentration and of the rest time at -0.8 V. It is obtained by first stripping the Cd layer anodically (see curve 1 in Figure 5); the subsequent reductive stripping gives rise to a first, less-negative, reduction peak that merges partially with the typical peak D for the reductive stripping of the first S layer (see curve 2′ in Figure 5). The charge associated with the lessnegative peak was estimated by subtracting the charge, 185 µC cm-2, for the stripping of the first S layer from the overall charge of all stripping peaks. This charge amounts to ∼72 µC cm-2 and is, therefore, very close to that for the oxidative stripping of the first Cd layer. The procedure used to obtain further alternate Cd and S layers is identical with that used for the Ag/S/Cd/S structure. The above procedure was used to obtain up to five S layers and

An Electrochemical and STM Investigation

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Figure 5. Linear-sweep voltammograms for the oxidative stripping of 1-4 Cd layers (labeled 1-4) and for the reductive stripping of 1-5 S layers (labeled from 1′-5′).

Figure 7. STM image obtained on Ag(111) after depositing a S layer by upd at -0.8 V, washing the STM cell, shifting the potential to -0.55 V, and injecting a solution of 1 × 10-3 M CdSO4 + 0.05 M Na4P2O7 + 0.01 M NaOH. Tunneling conditions: bias voltage ) 0.1 V, tunneling current ) 3 nA.

Figure 6. Plots of the charge involved in the stripping of Cd (b) and S (O) as a function of the number of the ECALE cycles. The slopes yield 75 µC cm-2 for the stripping of one Cd layer and 72 µC cm-2 for that of one S layer, other than the first S layer.

four Cd layers. The overall charge involved in each different number of layers was determined by oxidative stripping of Cd and subsequent reductive stripping of S. Figure 5 shows the oxidation peaks for stripping from 1 to 4 Cd layers, as well as the reduction peaks for stripping from 1 to 5 S layers. In all cases, the linear sweep voltammograms were recorded from -0.8 to -0.1 V for Cd stripping and from -0.1 to -1.3 V for S stripping. A sweep rate of 10 mV s-1 was used, which was low enough to ensure complete dissolution of the deposits. Note that the peaks for Cd stripping shift toward more positive potentials with an increase in the number of S and Cd layers, indicating that the presence of an increasing number of these layers makes the Cd more difficult to strip. Once all the Cd layers have been stripped anodically, the remaining S layers, other than the first one, behave like bulk S; hence, during the subsequent reductive stripping, they are reduced to HS- at more positive potentials. With the exception of the first S layer, the charge involved in each layer of either S or Cd is approximately equal to that involved in the first Cd layer. This is apparent from plots of the charge for S and Cd stripping as a function of the number of layers (Figure 6), whose slopes are equal to 72 and 75 µC cm-2, respectively. The linear behavior observed supports layer-by-layer growth. It should be noted that if the reductive S stripping is made to precede the oxidative Cd stripping by first scanning in the negative direction, the S stripping becomes much more difficult and many potential scans are required for complete dissolution of the deposit.

Preliminary results of an ex situ XPS analysis of a sample of 15 alternate Cd and S layers deposited on Ag(111) confirm the presence of cadmium and sulfur in the expected 1:1 stoichiometric ratio, within the limits of reliability of the reported atomic sensitivity factors. The Cd Auger parameter is in agreement with that reported for CdS,13 and S is in the sulfide chemical state. The oxidative Cd stripping and the reductive S stripping carried out on the above sample yielded a charge per atomic layer of about 73 µC cm-2, in agreement with the results in Figure 6. This confirms the CdS nature of the deposited film. Analogous conclusions were drawn by Colletti et al.14 with CdS deposits on polycrystalline gold by using grazing incident-angle X-ray diffraction. STM Measurements. Apart from the much smaller volumes of the solutions employed, the procedure followed to deposit the different layers of S and Cd in the STM cell is identical with that previously described. As expected, the first S layer on Ag(111) in aqueous NaOH at -0.8 V, both in the absence and in the presence of pyrophosphate, shows a (x7 × x7)R19.1° unit cell, where each lattice site is occupied by a triplet of sulfur atoms.4 Figure 7 shows an STM image of the first Cd layer on top of the first S layer and reveals a hexagonal lattice of Cd atoms. The spacing between the spots, 7.6 ( 0.8 Å, and the angles, 60° between the rows, match the values expected for a (x7 × x7)R19.1° structure relative to the silver substrate. To better investigate S atoms in the underlying, first S layer, the STM image in Figure 8 was produced. A high-resolution, constantheight mode image was filtered by a two-dimensional Fourier transform, and the current scale was inverted chromatically, in such a way that the most protruding parts of the STM image are darker while the deepest parts are brighter. That is, the brightest spots appear to represent the deepest S atoms while the dark spots are Cd atoms that occupy a slighly decentralized position inside hexagonal structures that form a kind of honeycomb. Three alternate vertexes of the hexagons are decidedly brighter and correspond to the deepest S atoms; the other three vertexes, which can only be detected by a very careful inspection of the STM image, are less bright and point

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Figure 8. STM image obtained on Ag(111) from the same solution as in Figure 7 at -0.52 V by filtering with a two-dimensional Fourier transform and inverting the chromatic scale. Tunneling conditions: bias voltage ) 0.1 V, tunneling current ) 2 nA.

to the presence of three slightly higher S atoms. The above image suggests that the first Cd layer induces a rearrangement in the underlying S atoms. We may tentatively assume that the (x7 × x7)R19.1° sulfur structure, with each lattice site occupied by a triplet of sulfur atoms, is converted into a structure with a more homogeneous distribution of S atoms within the (x7 × x7)R19.1° structure. This structure closely resembles that previously reported by Rovida and Pratesi15 for a S overlayer on Ag(111) in uhv by LEED. In the latter structure, along any given row the S atoms are adsorbed over a succession of two 3-fold hollow sites and one top site. This overlayer can be thought of as hexagons of sulfur atoms. Of the six sulfur atoms making up a hexagon, three S atoms sit on 3-fold hollow sites, alternated with three sitting on top sites. Still another S atom sits on the 3-fold hollow site at the center of the hexagon. In the arrangement suggested by Figure 8, a slight shift of the latter S atom from the center makes room for a Cd atom to sit in a 2-fold site, between two S atoms. The Cd atom is thus itself shifted slightly off center of the hexagon of S atoms. An outline of this tentative hexagonal arrangement is drawn on the upper left side of the STM image in Figure 8. A top view of the proposed arrangement of Cd and S atoms on the Ag(111) face is depicted schematically in Figure 9, with hexagonal and primitive unit cells. Assuming that the upd of 1 mol of Cd involves 2 F, the charge density associated with the proposed (x7 × x7)R19.1° structure of the first Cd layer amounts to 63.7 µC cm-2. This charge is in good agreement with the experimental charge, once a factor of 1.15 is used to account for the roughness of the silver substrate. The second S layer appears to reproduce the structure of the underlying Cd layer, as can be deduced from the STM image in Figure 10. Here, the S atoms are arranged in a hexagonal lattice, with interatomic distances of 7.6 ( 0.8 Å and angles of 60° between the rows. The linear scans along the a and b directions in the figure yield the tunneling current vs distance profiles reported in the same figure. The smaller peaks in the profile along the b direction are attributed to the Cd atoms, which lie below the S atoms of the second S layer. These peaks are absent in the profile along the a direction, suggesting that no underlying Cd atoms are present along this direction. These

Foresti et al.

Figure 9. Schematic illustration of the proposed structure of the first S layer on Ag(111), with a Cd layer on top. The smallest circles are Ag atoms and the largest are S atoms, whereas the intermediate and darkest ones are Cd atoms. The S atoms on the top sites are more heavily shaded than those on the hollow sites.

Figure 10. STM image obtained on Ag(111) after depositing by upd first a S layer and then a Cd layer, washing the STM cell, and injecting a solution of 1 × 10-4 M Na2S + 0.05 M Na4P2O7 + 0.01 M NaOH at -0.78 V. Tunneling conditions: bias voltage ) 0.26 V, tunneling current ) 3 nA. Figures a and b are tunneling current vs distance profiles along the corresponding directions as drawn in the STM image.

results indicate that the second S layer sits on 2-fold sites in the first Cd layer. Thus, the second S layer has a (1 × 1) structure with respect to the first Cd layer and continues to

An Electrochemical and STM Investigation

Figure 11. Schematic illustration of the structure of the first Cd layer and the second S layer on Ag(111). For the sake of clarity, the structure of the first S layer in direct contact with the silver substrate was omitted. The smallest circles are Ag atoms and the largest are S atoms, whereas the intermediate and darkest ones are Cd atoms.The primitive twodimensional cells of the S and Cd layers are also drawn.

display a (x7 × x7)R19.1° unit cell with respect to the Ag(111) substrate. This structure is depicted schematically in Figure 11, where the first S layer is omitted for the sake of clarity. The figure also shows two equivalent overlapping primitive unit cells for the second S layer and the Cd layer. The charge density associated with the second S layer should again be 63.7 µC cm-2, once we assume that the oxidative upd of 1 mol of S involves 2 F. A further layer of cadmium was also examined. The STM images are less clear and are not reported here. At any rate, this second Cd layer reproduces the structure of the first, suggesting no rearrangement of the underlying S layer. It is interesting to compare the above Ag(111)/S/Cd structure with the Au(111)/S/Cd structure reported by Demir and Shannon16 on the basis of an STM investigation. These authors report a (x3 × x3)R30° structure with a S-S distance of 5.0 Å for the S layer electrodeposited on Au(111) and a (3 × 3) structure with a Cd-Cd distance of 4.3 Å for the Cd layer on top of the S layer. The difference between the two structures cannot be ascribed to a difference in the lattice constants of Ag and Au, since they are practically identical. A (x3 × x3)R30° structure is also shown by S upd on Ag(111) at potentials between peak C and D in Figure 1a, but an order-order twodimensional phase transition to the more compressed (x7 × x7)R19.1° takes place along peak D.4 No such a transition was reported on Au(111). This may possibly be ascribed to a higher affinity of S for Ag than for Au. Evidently, the different structure of the first S layer on Ag(111) at potentials more positive than peak D with respect to that on Au(111) influences differently the epitaxial growth of the first Cd layer, which on Ag(111) has a (x7 × x7)R19.1° structure with a Cd-Cd distance of 7.6 ( 0.8 Å. 4.Conclusions The electrochemical measurements indicate that with only the exception of the first S layer, the charge associated with each layer of either Cd or S has the same average value of 73 µC cm-2, corresponding to 0.165 monolayers, when referred to one monolayer of the Ag(111) substrate. Moreover, the STM images of the second S layer and the first two Cd layers point to a (x7 × x7)R19.1° unit cell structure with a single atom per lattice site, which yields a 1/7 fractional coverage, i.e., 0.143 monolayers. These values are significantly lower than the 3/7 fractional coverage of the first S layer. The first S layer is,

J. Phys. Chem. B, Vol. 102, No. 38, 1998 7419 however, in direct contact with the silver substrate and, hence, can be regarded as an interface between the metal and the compound semiconductor electrodeposited on it. The near unit value of the S/Cd ratio for all subsequent atomic layers as well as a preliminary XPS analysis of the deposit attest to the formation of stoichiometric CdS. Upd is generally influenced by the surface structure of the metal substrate. Thus, the Ag(111) single-crystal face is expected to impart to the first layer its own surface order, which then propagates to the successive layers, thus producing an epitaxial compound. These expectations are actually confirmed by the STM images of the first four atomic layers, whereas the unit value of the S/Cd ratio for the successive layers strongly suggests that the epitaxial growth is maintained in these further layers. The stability of the CdS deposit on Ag(111) was checked as follows. The electrode covered by CdS was first rinsed with water while under potential control, dried in an argon atmosphere, and then exposed to air for more than 1 h. The treatment was not found to affect the charge involved in the subsequent stripping of Cd and S by the procedure previously described, when the uppermost layer was a S layer; conversely, when the uppermost layer was a Cd layer, most of this layer was lost prior to the subsequent stripping analysis. This suggests that the S layer stabilizes the deposit, in agreement with the previous observation that Cd is stabilized to oxidation when bound to S. The extremely small thicknesssa few Angstromssof the CdS deposits so far obtained excludes any possibility of characterizing them with respect to their semiconductor properties. However, the procedure previously described can be repeated as many times as is required to obtain semiconductor nanomaterials of practical interest through a suitable automation and computerization of the procedure.17 Work in this direction is in progress. Acknowledgment. The financial support of the Ministero della Ricerca Scientifica e Tecnologica (MURST) and of the Consiglio Nazionale delle Ricerche (CNR) is gratefully acknowledged. The authors thank Prof. John L. Stickney (University of Georgia, Athens) for very fruitful discussions and for providing them with his manuscript on CdS formation on Au by ECALE prior to publication. They also thank Prof. Gianfranco Rovida (University of Firenze, Italy) for the XPS analysis. References and Notes (1) (a) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (b) Rhee, C. K.; Huang, B. M.; Wilmer, E. M.; Thomas, S.; Stickney, J. L. Mater. Manuf. Processes. 1995, 10, 283. (c) Gregory, B. W.; Suggs, D. W.; Stickney, J. L. J. Electrochem. Soc. 1991, 138, 1279. (2) Colletti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (3) (a) Hatchett, D. W.; Gao, X.; Catron, S. W.; White, H. S. J. Phys. Chem. 1996, 100, 331. (b) Hatchett, D. W.; White, H. S. J. Phys. Chem. 1996, 100, 9854. (c) Stevenson, K. J.; Hatchett, D. W.; White H. S. Isr. J. Chem. 1997, 37, 173. (4) Aloisi, G. D.; Cavallini, M.; Innocenti, M.; Foresti, M. L.; Pezzatini, G.; Guidelli, R. J. Phys. Chem. 1997, 101, 4774. (5) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1990, 293, 85. (6) Dickertmann, D.; Schultze, J. W.; Koppitz, F. D. Electrochim. Acta 1976, 21, 967. (7) Herrmann, C. C.; Perrault, G. G.; Konrad, D.; Pilla, A. A. Bull. Soc. Chim. Fr. 1972, 12, 4468. (8) Gigginback, W. Inorg. Chem. 1971, 10, 1333. (9) Hamelin, A.; Foresti, M. L.; Guidelli, R. J. Electroanal. Chem. 1993, 346, 251. (10) Aloisi, G. D.; Funtikov, A. M.; Guidelli, R. Surf. Sci. 1993, 296, 291.

7420 J. Phys. Chem. B, Vol. 102, No. 38, 1998 (11) Schmidt, E.; Christen, M.; Beyeler, P. J. Electroanal. Chem. 1973, 42, 275. (12) Bort, H.; Juttner, K.; Lorenz, W. J.; Staikov, G. Electrochim. Acta 1983, 28, 993. (13) Gaarenstroom, S. W.; Winograd, N. J. J. Chem. Phys. 1977, 67, 3500.

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