Kinetic energy distributions of molecular and cluster ions sputtered from self-assembled monolayers of octanethiol on gold

Abstract

Self-assembled monolayers (SAMs) of alkanethiols are an ideal model system to study the mechanisms that lead to emission of organic species under keV ion bombardment. In this contribution, we focus on the emission processes of gold-molecule cluster ions, which are not fully understood yet. To gain insights into these processes, monolayers of octanethiol CH 3(CH 2) 7SH adsorbed on gold are investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). First, the static SIMS conditions are verified using the degradation of the SAM signals as a function of ion fluence. Second, the kinetic energy distributions (KEDs) of fragment, parent and cluster ions ejected upon 15 keV Ga + ion bombardment are measured. The peak maxima, FWHMs and high energy tails of the distributions are analyzed for Au-thiolate clusters, as well as thiol fragments. After calibration of the energy spectra using monoatomic ions, we find that the KEDs of all the clusters containing the thiolate molecule peak at about 1.2 eV. Besides, the distributions of the gold-molecule cluster ions including Au(M–H) 2 −, the most intense cluster peak in the spectrum, are significantly narrower than that of the hydrocarbon fragments.