LOW-ENERGY ELECTRON-INDUCED DAMAGE IN HEXADECANETHIOLATE MONOLAYERS

Low-energy electron-induced damage in hexadecanethiolate (HDT) monolay ers on gold substrates has been investigated using infrared reflection -absorption spectroscopy (IRAS), angle-resolved near edge X-ray absorp tion fine structure spectroscopy (NEXAFS), and advancing water contact angle measurements. HDT films were exposed to electrons of energies 1 0-100 eV and doses between 30 and 14000 mu C/cm(2). The induced damage was monitored both ''in situ'' by NEXAFS measurements interleaved wit h electron irradiations and ''ex-situ'' by NEXAFS, IRAS, and contact a ngle measurements after exposure of the irradiated samples to air. A p rogressive film damage was observed with increasing electron energy an d dose of irradiation. This damage was found to occur during irradiati on in UHV and was not induced by chemical reactions with airborne mole cules during subsequent exposure of the irradiated films to air. The d amage starts in the region of the terminal methyl groups of the HDT fi lms and propagates into the bulk of the film. An analysis of the IRAS and NEXAFS data shows that the conformational and orientational order within the HDT film are most sensitive to low-energy electron irradiat ion. Electron-induced cleavage of C-H and C-C bonds resulting in a par tial desorption of the film constituents also occurs and leads to form ation of C=C double bonds in the film as inferred from the appearance of a pi-resonance in the C 1s NEXAFS spectra. The obtained results ar e of importance for both the optimization of self-assembled-monolayers -based lithography processes and for the general understanding of irra diation-induced changes in organic films.