Organic Film Structure Physisorbed Films ESP in SAMS SP in Molecular Films Reforming Catalysts ECH


Electron-Stimulated Processes in Chemisorbed Self-Assembled Monolayers

The IRRAS system has been designed to allow the IRRAS characterization of surfaces before, during and after exposure to low-energy electrons.  Our studies of the C-H stretching region of the Cn/Au self-assembled monolayers (2800-3000 cm-1) have shown that these features suffered progressive depletions when exposed to electrons of ~7-12 eV electrons; initial energy deposition was shown to proceed by resonant electron attachment.  The methyl terminations of the films were particularly sensitive to electron irradiation. The dissociation yields at this interface were shown to increase rapidly with chain length.  Using semi-empirical potentials for the C-H stretching co-ordinate on excited (repulsive) energy surfaces, we have shown that the dependence of the damage cross-sections on chain-length is the result of quenching of the excited states by proximity to the metallic substrate by dipole-image dipole interactions: excitations to methyl groups farther from the substrate have longer excited state lifetimes, and therefore greater probability to survive long enough to allow dissociation. Application of this quenching model to our experimental cross-sections allowed extraction of chain-length dependent excited state lifetimes, which range from 2-10 fs for the butanethiol-to-hexadecanethiol monolayers studied, respectively.

In addition to the chemical consequences of electron-scattering, we have also found that the excess energy release into the organic film (~6 eV per dissociation event) causes significant physical disordering of the organic phase, as evidenced by an overall broadening of the IRRAS peaks and the introduction of a broad background to the spectra.  Thermal cycling of the film cannot eliminate this disorder.  However, by performing the irradiation at ~50 K, we find that the physical disorder is essentially eliminated; the irradiated sample can be heated to 300 K without the introduction of physical order; the onset of permanent damage during irradiation appears to be ~175 K.  We have interpreted this in terms of the ability of the film to dissipate the local temperature jump accompanying dissociation without exceeding local temperatures of ~400 K.  Supporting evidence was obtained by comparison of the electron-stimulated damage in 2D and 3D assemblies.  This has important consequences for the possible use of organic monolayer films as electron resists in electronic devices, as well as for the long-term stability of these systems in hostile environments.

We have recently studied the chemical consequences that follow the initial bond-rupture events.  Molecular hydrogen is produced by low-energy electron scattering in n-alkane SAMs, but the mechanism of its formation has remained controversial;  both unimolecular and bi-molecular formation channels have been proposed in the literature.  By using isotopically substituted h33-C16 and d33-C16 films, we have quantitatively shown that the yield of the unimolecular channel is negligible, and that the molecular hydrogen is formed by the reactive scattering of a primary dissociation fragment (H, H-) abstracting another hydrogen from a spatially distinct adsorbed molecule.


Where to from here ?

The next phase of this work is to study electron-induced processes in self-assembled monolayers bound to Hg and Ga surfaces.

In this way the role of the substrate in equilibrating excess energy release can be directly probed via the control of the substrate temperature across the melting transition of the metal.  Dissipation of excess energy is a fundamental problem in these films that limits their resistance to high temperatures as well as to external excitation sources.  Another avenue of research is the exploration of the effective range of reactive fragments in the film prior to the second abstraction process that leads to molecular hydrogen production.


Relevant Instrumentation (click on image to see apparatus)