ECSTM imaging of alkanethiol electrodesorption: an evidence for Au adatoms in the SAM

M. E. VELA; C. VERICAT; P. CARRO; R. SALVARE

Encuentro; 61st ISE Annual Meeting; 2010

The structure of the S-head Au interface in alkanethiol self assembled monolayers remains acontroversial point. Several works have been published with new experimental data and theoreticalmodels, both for the √3×√3 R30° lattice and the c(4×2) lattice, that would indicate that thioladsorption promotes a strong reconstruction of the Au(111) surface, with Au vacancies and/or Auadatoms playing a key role. In this communication we report results on alkanethiol SAM removalfrom Au(111) surfaces using ECSTM in alkaline solution using an electroreduction procedure. Wehave performed in situ STM imaging of Au(111) substrates covered by hexanethiolate selfassembled monolayers (HT-SAM) in 0.1 M NaOH solution before and after thiol desorption Wecorrelate the final coverage of islands with the starting molecular surface structure. Molecularresolution of the HT-SAM taken at an applied potential E = -0.50 V shows different rotated domainsconsisting in zig-zag and rectangular c(4×2) surface structures. Then, the potential was scanned from -0.50 V to -0.99 V, just at the reductive desorption peak of the hexanethiolate. The charge density (q) involved in this peak corresponds to the complete removal of the chemisorbed thiolates with a charge density consistent with 1/3 surface coverage assuming one electron transfer for the reductive process. The STM image obtained at -0.99 V shows that the rows of the c(4×2) structure are no longer observed, while a large number of nanometre- sized islands with average height 0.24± 0.03 nm are formed. The fraction of the area covered by the islands in the STM images under electrochemical control results in θadat= 0.30-0.40. This value is much larger than the θadat =0.17 expected for adatom model involving two thiols per Au adatom and also slightly higher than the θadat=0.33 value expected for the those models involving one thiol per Au adatom. Thus, the possibility that the islands shown in the STM images after desorption experiments could be assigned to RSH aggregates that remain physisorbed on the Au surface, or, in the case of reductive desorption in aqueous electrolytes, to the formation of RS- micelles, can not be ruled out.