Electrochemical and spectroscopic study of the self-assembling mechanism of normal and chelating alkanethiols on copper

The self-assembly of aliphatic thiol (RSH), dithiol (R(SH) ₂), and dithiocarboxylic acid (RS ₂H) onto mildly oxidized and highly oxidized copper was studied in real time by in situ electrochemical impedance spectroscopy (EIS). Ex situ characterization of the films was carried out using linear sweep voltammetry (LSV), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). In situ EIS studies found a very fast adsorption of RSH, R(SH) ₂, and RS ₂H (within 10-15 s). This fast adsorption step is followed by the long-term additional adsorption and consolidation of SAM. However, the self-assembly of RS ₂H passes through an intermediate step of molecule rearrangement for around 10 to 30 min after around 2 to 7 min of self-assembly. The binding of both sulfur moieties of R(SH) ₂ with Cu happens simultaneous. The oxide reduction capacity of RSH, R(SH) ₂, and RS ₂H was good. However, the XPS studies showed the decomposition of RS ₂H-based SAMs to Cu ₂S. Monolayers prepared on both mildly oxidized and heavily oxidized Cu with R(SH) ₂ had the highest stability. Monolayers of RS ₂H showed the least stability on both mildly oxidized and heavily oxidized Cu. Although RSH-based SAMs had good organization on both mildly oxidized and highly oxidized Cu, R(SH) ₂-based SAMs did not show good organization in either case. The RS ₂H monolayer had good organization only on mildly oxidized Cu. © 2012 American Chemical Society.