Phase transitions in the electrodeposition of tellurium atomic layers on Au(100)

This paper describes the electrochemical formation of tellurium atomic laye rs on Au(100) and the phase transitions associated with those layers. Volta mmetry in HTeO2+ shows two clear sub-monolayer reduction features prior to the onset of bulk Te deposition. In situ electrochemical scanning tunneling microscopy (STM) has shown that a 0.25 coverage (2 x 2) structure is the f irst Te adlayer formed by scanning into the first reduction feature. The (2 x 2) unit cell was also observed using low energy electron diffraction (LE ED). As the potential was shifted through the first deposition feature, the (2 x 2) converted to a 0.33 coverage (2 x root 10) structure, by the rando m introduction of Te atom chains in the initial (2 x 2) layer. Te-Te distan ces in the chains were consistent with Te atoms at their van der Waals diam eter, 0.44 nm. The chains can be thought of as boundaries between domains o f the (2 x 2), shifted by root 2 from each other. The (2 x root 10) structu re consists of a complete layer of these chains. Further deposition, past t he first reduction feature, resulted in a 0.375 coverage (2 x 4) structure. The (2 x 4) also appears to consist of chains of Te atoms, but with every third atom set in a lower site where they are not easily imaged under some tunneling conditions. Scanning into the second reduction feature, a shoulde r on bulk Te deposition, atoms in the (2 x 4) structure became compressed, converting to a structure with a coverage of 0.67 and a (root 2 x root 5) u nit cell. At this stage, the Te atoms were no longer separated by a distanc e consistent with their van der Waals diameter, but with one closer to that of the underlying Au atoms, 0.288 nm. The Te atoms appear to have covalent ly bonded to each other, simultaneously inducing a roughening transition in the surface. The transition may result from stress induced via bonding bet ween the Au substrate and this covalently bound Te atomic layer. All the st ructures described above are single atomic layers. Three-dimensional nuclea tion progressed at potentials associated with the bulk Te deposition featur e. Needles, or rectangular Te atom domains with high aspect ratios, propaga ting in the [100], were formed in a second layer, apparently on domains of the (root 2 x root 5) structure. (C) 1999 Elsevier Science S.A. All rights reserved.