The oxidative underpotential deposition of sulfur on Ag(111) from alkaline solutions of Na 2 S was investigated by in situ scanning tunneling microscopy (STM), cyclic voltammetry, and chronocoulometry. Proceeding toward more positive potentials, the cyclic voltammetric curve shows three partially overlapping peaks A-C and an isolated and more acute peak D. The STM images of the overlayer of adsorbed sulfur over the potential region between peaks C and D reveal a ( 3 × 3)R30°structure; those at potentials positive to peak D a ( 7 × 7)R19°structure: each lattice site of the latter structure is occupied by a triplet of sulfur atoms. The fractional coverage, 1 / 3 , for the ( 3 × 3)R30°structure is in perfect agreement with the maximum surface concentration, Γ max ) 7.7 × 10 -10 mol cm -2 , obtained from a thermodynamic analysis of the chronocoulometric charge vs potential curves; 2FΓ max is about 10% larger than the charge associated with the combination of peaks A-C. On the other hand, the 2FΓ value corresponding to the fractional coverage, 3 / 7 , for the ( 7 × 7)R19°structure agrees satisfactorily with the charge associated with the sum of peaks A-D, thus suggesting a total electron transfer from sulfide ions to the metal over the range of stability of the latter structure.

We applied the electrochemical atomic layer epitaxy ͑ECALE͒ methodology to obtain deposits of CdS and ZnS on Ag͑111͒ by alternate underpotential deposition of the elements forming the compounds. The amounts of the elements deposited, determined by stripping coulometry, always yielded a stoichiometric 1:1 ratio. The deposits were formed using an automated electrochemical deposition system, described here, making use of a simple distribution valve.Recent work in our group is devoted to the growth of thin-film compound semiconductors on silver single crystals by electrochemical atomic layer epitaxy ͑ECALE͒. Stickney and co-workers developed this method to obtain structurally well-ordered II-VI and III-V compound semiconductors on gold at low cost. 1-3 The method is based on the alternate electrodeposition of atomic layers of the elements making up a compound, at underpotential. Underpotential deposition ͑UPD͒ is a surface-limited electrochemical phenomenon that results in the deposition of an atomic layer. A monolayer of the compound is obtained by alternating the UPD of the metallic element with the UPD of the nonmetallic element in a cycle. The ECALE cycle can be repeated as many times as necessary to obtain deposits of practical thickness, and the thickness of the deposit is determined by the number of cycles.The method requires the definition of precise experimental conditions, such as potentials, reactants, concentrations, supporting electrolytes, pH, deposition times, and the possible use of complexing agents. These conditions are strictly dependent on the compound one wants to form and on the substrate used. We found the conditions to grow practically all II-VI compound semiconductors and are now beginning to study the III-V compounds. The substrate that has been used up to now is Ag͑111͒, a single crystal to ensure the maximum probability for the epitaxial growth.In a previous paper we described the experimental conditions needed to obtain up to five sulfur layers and four cadmium layers of CdS. Sulfur layers were obtained by oxidative UPD from sulfide ion solutions, 4-6 whereas cadmium layers were obtained by reductive UPD from cadmium ion solutions. 7 Both precursors were dissolved in pyrophosphate plus sodium hydroxide of pH 12. The high pH was used to shift the hydrogen evolution toward very negative potentials in order to evidence the whole underpotential oxidation process of sulfide ions which takes place between Ϫ1.35 and Ϫ0.8 V/SCE. A strong complexing agent such as pyrophosphate was used to keep cadmium ions in solution at this high pH. This paper describes the growth of thicker deposits of CdS, up to 150 deposition cycles, obtained using an automated system. The deposit morphology was examined by scanning electron microscopy ͑SEM͒. This paper also describes conditions to form ZnS.The experimental conditions for CdS and ZnS growth on silver are different from those required on gold. 8-10 ExperimentalMerck analytical reagent-grade 3CdSO 4 •8H 2 O and Aldrich analytical reagent-grade Na 2 S were used ...

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 ( 7 × 7)R19.1°, with one atom per lattice site, relative to the Ag(111) substrate.

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