Formation of the first monolayer of CdSe on Au(111) by electrochemical ALE

Electrochemical ALE (ECALE), the electrochemical analog of atomic layer epitaxy (ALE), is being developed. Central to ALE is the formation of atomic layers of the elements using surface limited reactions, in a cycle. In electrochemistry another name for a surface limited electrodeposition reaction is underpotential deposition (UPD). In ECALE, compound thin films are formed an atomic layer at a time by using UPD, in a cycle. In the study presented here the formation of the first monolayer of CdSe on Au(111) is described. The first step in the ECALE cycle is the formation of an atomic layer of Se via UPD. UPD of Se resulted in a surface composed of domains of domains of two structures: a (√3 × √3)R30° structure at 13 coverage, and a close packed layer of square eight membered Se rings. The ring structure consists of three domains, each rotated by 120°. Formation of a single structure, homogeneously covering the whole surface proved difficult due to the kinetics of the UPD process. Subsequent deposition of a Cd atomic layer resulted in formation of a CdSe structure with a (3 × 3) unit cell, and 49 coverage of both Cd and Se. A structure proposed to account for the unit cell and coverages, consists of a slice out of the bulk wurtzite structure for CdSe laid onto the Au(111) substrate, where twice the surface lattice constant for CdSe matches up with three times the Au interatomic spacing.