Emission processes of molecule-metal cluster ions from self-assembled monolayers of octanethiols on gold and silver

In this contribution, we focus on the emission processes of molecule-metal cluster ions from self-assembled monolayers (SAMs) of octanethiols CH3(CH2)(7)SH on gold and silver. To improve our understanding of these complex phenomena, mass spectra and kinetic energy distributions (KEDs) of these two systems have been measured and compared using time-of-flight-SIMS under 15 keV Ga+ bombardment. First, the spectra obtained from SAMs/Ag exhibit positive (M-H)(m)Ag-m+1(+) and negative (M-H)(m)Ag-m-1(-) cluster ions that are generally more intense than the (M-H)(m)Au-n(-) observed for SAMs/Au. This trend is attributed to the electronegativity difference between S and these two metals resulting in a more ionic Ag-S bond. Second, our results show that, like for the SAM/Au system already investigated, unimolecular dissociation of Ag-thiolate clusters in the acceleration section of the spectrometer is an important formation mechanism. The fraction of the (M-H)(m)Ag-n(+,-) aggregates formed in the vacuum via this process is even significantly higher than that of the (M-H)(m)Au-n(-) cluster ions. This suggests that the cluster ions ejected from SAMs/Ag are less stable than those ejected from SAMs/Au. It is also observed that the high energy parts of the KEDs are steeper than for gold, which is probably due to the same phenomenon. (C) 2004 Elsevier B.V. All rights reserved.