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


Electron-Stimulated Processes in Molecular Films

We have recently extended our studies of electron-stimulated processes to the molecular films described in section (B) above.  In the case of Fe(CO)5 films, we find that the elimination of CO vibrational bands (1950-2060 cm-1) can be effected on the amorphous (as deposited) films with astonishing ease with electrons of 0.5 to 10 eV; the extremely large cross-sections for this process (100-200 Å2/electron), the non-exponential behaviour of the damage processes as a function of electron dose, and the new spectral features observed using IRRAS (~2070 cm-1) all suggest that the de-carboxylation of the film proceeds by a multi-step polymerization process, such that one primary electron-induced elimination of CO event can lead to the loss of as many as 8-10 CO molecules from the film, and the formation of intermediate Fex(CO)~2-3x polymeric species.  Continued irradiation can readily eliminate the IRRAS signal of surface CO, indicating that atomic or metallic Fe is being deposited on the surface.  Although the as-deposited amorphous films are extremely sensitive to electron-irradiation, transformation to the crystalline phase by sample heating to ~135 K renders the film totally insensitive to irradiation, due to caging phenomena.

The ability to induce CO elimination using electrons <5 eV of energy (i.e. below the damage threshold for Cn/Au films) suggests that this technique may be useful for the non-destructive deposition of metals on organic films for applications in molecular electronics.  This has been confirmed by examining the IRRAS spectra of the organic substrates during irradiation of Fe(CO)5 / Cn/Au systems : no damage of the organic substrate is observed for irradiation doses sufficiently elevated to completely remove the CO bands from the film.  In conjunction with the deposition of Ni and Co complexes on organic substrates, it is possible that ultrathin magnetic films can be deposited without the introduction of magnetically 'dead' layers that are often produced by evaporation onto strongly-interacting substrates. 


Where to from here ?

The next phase of this work will compare the electron-driven processes in these thin-films with the photolysis processes, and to study charge+ energy transfer processes at interfaces.

Photolysis of adsorbates often proceeds by generation of substrate photoelectrons, but without the direct comparison with the controlled electron beam results, the mechanism and exact range of electron energies involved is difficult to know or control. In a collaboration with Prof. Erik Jensen (Physics, UNBC), we are studying the electron- and laser stimulated processes in adsorbed Ni(CO)4.  The goal is to isolate the photo-electron generated decarbonylation yields, and to determine the range of photo-electron energies that are most effective in these processes.  Initial IRRAS and direct electron-impact work in our laboratory are very encouraging, and show a highly effective CO-elimination channel for Ei<2 eV. Photo-stimulated work began in Jensen’s laboratory in July 2005, using the Ni(13CO)4 that we have synthesized.

One fundamental shortcoming of photon- and electron-stimulated processes in condensed phase systems is the lack of site-selectivity in the initial particle-molecule interaction; this is particularly relevant in the case of Cn SAMs, where the electronic structure of the saturated organic molecule is relatively constant across the methylene and methyl sites, leading to similar excitation and dissociation dynamics.  We have recently developed a mechanism by which electron-stimulated processes are induced preferentially at the chain terminations of a C16 SAM (normally, damage is relatively uniformly distributed at C sites more than ~ 1nm from the metal surface).  Our approach involves the electron-induced creation of anionic excitons in Xe overlayers at precisely Ei=7.7 eV; the diffusion of these (charge+energy) carriers leads to the direct interaction with the methyl terminations of the substrate SAM, where they selectively induce bond rupture.  Whereas conventional electron bombardment causes similar damage to the CH2 and CH3 components of the film, our new approach precludes damage to the subsurface CH2 groups.  This has important consequences for the use of SAMs in molecular electronics; whereas excess electron conduction through Cn SAMs is relatively efficient and well understood, these results show that excess electron conduction with an associated excited state is essentially forbidden.  The molecular, energetic and fragmentation channel selectivity of this process will be explored to determine if this could be a generally useful method to promote selective lithographic modifications of surfaces.


Relevant Instrumentation (click on image to see apparatus)