Potential and kinetic sputtering of alkanethiol self-assembled monolayers by impact of highly charged ions

Abstract

Highly charged ions have been used to study the sputtering of positive molecular fragments from mercaptoundecanoic acid and dodecanethiol self-assembled monolayers on gold surfaces. The samples were bombarded with ${\mathrm{Ar}}^{q+}$ $(4\ensuremath{\leqslant}q\ensuremath{\leqslant}10)$ ions with kinetic energies from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}18\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The main fragments detected were ${\mathrm{H}}^{+}$, ${\mathrm{C}}_{n}{\mathrm{H}}_{2n}^{+}$, and ${\mathrm{C}}_{n+1}{\mathrm{O}}_{2}{\mathrm{H}}_{2n+1}^{+}$ from mercaptoundecanoic and ${\mathrm{H}}^{+}$, ${\mathrm{C}}_{n}{\mathrm{H}}_{2n}^{+}$, and ${\mathrm{C}}_{n+1}{\mathrm{H}}_{2n+3}^{+}$ from dodecanethiol. The proton yields were increased with larger charge state $q$ of the highly charged ion (HCI) in both samples, scaling as ${q}^{\ensuremath{\gamma}}$, with $\ensuremath{\gamma}\ensuremath{\sim}5$. The charge state dependence is discussed in terms of electron transfer to the HCI. The final yield of protons depends on molecular functional group characteristics, orientation on the surface, and reneutralization phenomena.