An estimation on the mechanical stabilities of SAMs by low energy Ar+ cluster ion collision.

The stability of the molecular self-assembled monolayers (SAMs) is of vital importance to the performance of the molecular electronics and their integration to the future electronics devices. Here we study the effect of electron irradiation-induced cross-linking on the stability of self-assembled monolayer of aromatic 5,5'-bis(mercaptomethyl)-2,2'-bipyridine [BPD; HS-CH2-(C5H3N)2-CH2-SH] on Au (111) single crystal surface. As a refence, we also study the properties of SAMs of electron saturated 1-dodecanethiol [C12; CH3-(CH2)11-SH] molecules. The stability of the considered SAMs before and after electron-irradiation is studied using low energy Ar+ cluster depth profiling monitored by recording the X-ray photoelectron spectroscopy (XPS) core level spectra and the UV-photoelectron spectroscopy (UPS) in the valance band range. The results indicate a stronger mechanical stability of BPD SAMs than the C12 SAMs. The stability of BPD SAMs enhances further after electron irradiation due to intermolecular cross-linking, whereas the electron irradiation results in deterioration of C12 molecules due to the saturated nature of the molecules. The depth profiling time of the cross-linked BPD SAM is more than 4 and 8 times longer than the profiling time obtained for pristine and BPD and C12 SAMs, respectively. The UPS results are supported by density functional theory calculations, which show qualitative agreement with the experiment and enable us to interpret the features in the XPS spectra during the etching process for structural characterization. The obtained results offer helpful options to estimate the structural stability of SAMs which is a key factor for the fabrication of molecular devices.

DNA strand breaks induced by low keV energy heavy ions.

We present some results on the interaction of low energy atomic ions with DNA. Experiments consist of irradiation of dried DNA in vacuum with Ar ions at low keV energies for different time intervals. The DNA is placed back in solution and analysed by agarose gel electrophoresis. These experiments demonstrated the production of single and double strand breaks. The induction of these lesions could be due to several processes: direct collisions with DNA constituent atoms resulting in displacements, cascade recoil collisions of the constituent atoms, electron transfer processes between the ion and the DNA as well as breaks induced by molecular excitation and secondary electron interactions. Here we briefly discuss some aspects of direct and recoil collision processes.

Surface channelling and energy losses of 4 keV hydrogen and fluorine ions in grazing scattering on Au(111) and missing row reconstructed Au(110) surfaces.

We present the results of an experimental and theoretical study of surface channelling and energy loss of 4 keV hydrogen and fluorine ions in grazing scattering on a missing row reconstructed Au(110) surface and a Au(111) surface. We performed measurements of energy losses for grazing angle scattering in surface channelling conditions for various azimuthal orientations of the crystal. Experimental results are discussed in the light of trajectory calculations of hydrogen and fluorine ions scattered under grazing incidence conditions on the surface. A nonlinear model is used in order to calculate the ion energy loss in these crystalline systems. Ab initio electronic crystal structure calculations and semi-classical simulations are performed and allow us to delineate various trajectory classes that correspond to different contributions in the energy loss spectra for various azimuthal orientations of the surface.

Role of d electrons in Auger neutralization at metal surfaces.

A generalized theory of Auger electron transfer processes in the interaction of ions with metal surfaces, including the previously ignored role of d electrons is presented. It is shown that a correct and accurate description of Auger neutralization has to account for the contribution of d electrons, as this is illustrated on the case of He+ ion neutralization on Ag, where the neglect of these leads to a strong overestimation of ion survival probabilities. Crystal lattice site specific rates are calculated and allow for a correct description of crystal azimuthal effects in neutralization.

Surface miller index dependence of Auger neutralization of ions on surfaces.

Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.