Inelastic electron tunneling spectroscopy is used to compare the adsorption onto aluminium oxide of poly(ethy1ene glycol) of relative molar mass 400 (PEG,,,) with poly(ethy1ene glycol) that has been coordinated with zinc chloride. This work carried out on a low-molecular-weight analogue of a polymer electrolyte reveals that the steric hinderance brought about by the coordination of zinc chloride with poly(ethy1ene glycol) changes the mechanism by which the polymer bonds to an aluminium oxide surface. The zinc bonds both to atmospheric water and to the ether oxygens in the polymer, which restricts the conformational freedom of the polymer. This restriction causes the attachment of the polymer to the aluminium oxide substrate to be primarily by hydrogen bonding, which reveals itself through the appearance of an extra peak in the inelastic electron tunneling spectrum at -440 meV. The implication is that when monolayer coatings of PEG,,,/ZnCl, are adsorbed on aluminium oxide, the Coordination of the zinc with the polymer imposes a structure upon the latter.
The shift in position on bias reversal of the 450 meV OH mode in tunnel junctions comprised of aluminium/aluminium oxide/M where M is Pb, Ag and Sn is investigated using inelastic electron tunnelling spectroscopy. Measurements presented here reveal that the peak shift is independent of M within the experimental error of this work. This is consistent with a simple superposition of the three electric fields acting on the hydroxyl group due to the tunnel barrier, the image charge and the external bias respectively. The implication is that as far as the subtle effect of peak shifts on reverse bias is concerned, the tunnel junction environment of the adsorbed species is essentially benign. For certain electrode thicknesses and geometries, peak shifts can arise due to finite electrode resistance even when four-point probe conditions pertain. A simple procedure for treating these is presented.
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