A combined surface-enhanced Raman-X-ray photoelectron spectroscopic study of 2-mercaptobenzothiazole monolayers on polycrystalline Au and Ag films

The monolayers of 2-mercaptobenzothiazole (MBT) on polycrystalline Au and A g films have been studied: by surface-enhanced Raman (SERS) and X-ray photo electron (XPS) spectroscopies. Whereas MBT adsorbs with its molecular plane hat on Ag, its plane is perpendicular on Au, This difference in adsorbate geometry is manifested as differences in the intensities of certain vibrati onal peaks in the Raman spectra. Ab initio molecular orbital (MO) calculati ons suggest that MBT adsorbs on Au in the thione form and on Ag in the thio l form The monolayers are stable up to a temperature of 473 K, much higher than most of the self-assembled monolayers (SAMs) studied so far. The adsor ption geometry does not undergo any significant change when the monolayers are heated, although minor changes are observed in the SER spectra of the A g monolayer. Raman intensities of all the bands increase with heating, and the enhanced intensity is preserved even after cooling. This is attributed to monolayer-induced coalescence: of gold islands leading to the formation of an extended self-assembly, XPS studies confirm chemisorption, although s tructural details are not manifested. (C) 1999 Academic Press.