Sputtering of hexadecanethiol monolayers self-assembled onto Ag(111)

Abstract

The process of supttering self-assembled hexadecanethiol (HDT) CH 3(CH 2) 15SH monolayers on Ag(111) has been studied using 800 eV He + ion bombardment with current densities of 0.25 and 0.11 μA cm −2. The sputtering kinetics were monitored by ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable-induced electron spectroscopy (MIES). The dose dependencies of the substrate ISS signal and the work function demonstrate different behaviour in the ranges below and above an ion dosage of about 3.5 × 10 15 cm −2. Accordingly, two stages can be distinguished in the sputtering kinetics. The predominant process in the first stage is assumed to be high-rate scission and ioninduced fragmentation of standing hydrocarbon chains, while the second stage is mainly associated with a slow sputter removal of the sublayer of adsorbed sulfur atoms due to binary collisions. A model explaining the evolution of the Ag ISS signal in the initial stage of sputtering by the creation of “holes” within the layer is proposed. Sputtering of the sulfur sublayer from Ag(111) in the second stage has been determined to occur with a desorption cross-section of about 2.8 × 10 −17 cm 2 and an initial sputtering yield of about 0.018.