Strong temperature dependence of irradiation effects in organic layers

Radiation damage of self-assembled monolayers (SAMs), which are prototypes of thin organic layers and highly organized biological systems, has been investigated with synchrotron radiation at variable temperature. For the prototypes alkanethiolates on polycrystalline Au(1 1 1), and alkyls on monocrystalline Si(1 1 1), we have measured X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and stimulated desorption of ions and neutrals at 50 and at 300 K. We find that the damage induced has a strong dependence on temperature. Two limiting cases can be identified. Reactions involving movement or transport of single atoms (e.g., desorption of H+) and local rearrangement (C=C bond formation) proceed nearly independently of temperature. On the other hand, reactions requiring transport of heavy fragments (desorption of large neutral chain fragments, total material loss, formation of disulfide species requiring movement of large chains) are efficiently quenched by cooling. Probable mechanistic reasons will be discussed. In our opinion, these findings have broader implications as well. We foresee the combined use of temperature and irradiation by electrons or photons for advanced tailoring of self-assembled monolayers on surfaces. In addition, our results have direct bearing on cryogenic approaches in advanced electron and X-ray microscopy and spectroscopy of biological macromolecules and cells. (c) 2005 Elsevier B.V. All rights reserved.