Electrochemistry of S adlayers at underpotentially deposited Cd on Au(111): Implications for the electrosynthesis of high-quality CdS thin films

We report an electrochemical, scanning probe microscopic, and Raman spectroscopic investigation of thin CdS films grown by electrochemical atomic layer epitaxy aimed at understanding the role played by the order of deposition on film quality. Using scanning tunneling microscopy (STM), we determine the atomic level structure of the first three monolayers of CdS for Cd-first films. The initial Cd underpotential deposition layer forms a stable ((4)(-2)(0)(1))-Au(111) adlattice in which the two unique nearest neighbor spacings an 0.33 and 0.29 nm. When H2S is electrolyzed at this surface at underpotential, an adlattice is produced in which the S-S interatomic spacing is 0.34 nm. The unit cell of this structure is ((4)(0) (-2)(2) with respect to Au(111). Both the Cd and S monolayers appear to be nearly closest-packed, in contrast to the more open structures observed in the S-first films. Interestingly, the second monolayers of Cd and S were found to have the same structure as in the first monolayer of CdS, implying that the first two CdS monolayers are epitaxial. Only after deposition of the third monolayer of CdS are interatomic spacings characteristic of bulk CdS observed. Micrometer-resolution STM images reveal a significant decrease in pit density, average pit diameter, and average pit depth for the Cd-first films compared to that for the S-first films. Finally, resonance Raman data indicate that S-first films are significantly more polydisperse than Cd-first films. Taken together, these data point to a structural rearrangement in the first monolayer of CdS as the origin of the increased polycrystallinity in the S-first films. In contrast, the closest-packed structures formed when Cd is deposited first lead to films with fewer defects. The implications of these findings on the electrosynthesis of thin film materials is briefly discussed.