Low energy electron interactions with 1-decanethiol self-assembled monolayers on Au(111)

Abstract

Understanding the interaction of low energy electrons with organic thin films is important for the development of a wide range of technological applications. In this study, the interaction of 80 eV electrons with self-assembled monolayers (SAMs) of 1-decanethiol grown on Au(111) via vapor phase deposition was explored for both the lying down (striped) phase and the standing up phase. Low-energy electron diffraction measurements performed at 100 K show that the SAM loses its crystalline structure within about 3 min for the lying down phase and approximately 30 s for the standing up phase. For the standing up phase, temperature programed desorption measurements reveal two desorption features for the hydrocarbon fragments of the SAM, one centered around 130 °C and a second near 220 °C. For the lying down phase, only the higher temperature desorption feature is observed. For both phases, desorption peaks for S and H2S that are centered around 250 °C were observed, suggesting that there is a high probability for the alkane chain of the 1-decanethiol molecule to detach from the sulfur head group before desorbing from the surface. For the standing up phase, exposing the SAM to the electron beam results in a near complete attenuation of the two peaks associated with the cracking fragments of the alkane chain. However, for the lying down phase, the intensities and positions of all of the desorption peaks were similar to the unexposed SAMs, which indicates that the cross section for electron beam damage for the lying down phase is much lower than that for the standing up phase. Ex situ x-ray photoelectron spectroscopy reveals a chemical shift of almost 0.5 eV for the C-1s emission after electron exposure for the standing up phase, whereas the shift for the lying down phase was less than 0.1 eV. These results indicate that exposure of alkanethiol SAMs to 80 eV electrons results in both disordering of the SAM and decomposition of the alkanethiol molecule SAMs. For the standing up phase, the rate of decomposition is much higher than the lying down phase. The lower decomposition rate for the lying down phase is primarily attributed to the quenching of excess charge in this phase since the entire molecule is in direct contact with the metallic substrate.