Oxide electron tunneling

Another technique that has proved useful in establishing chemical bonding of coupling agents at interfaces is inelastic electron tunneling spectroscopy (ITES). For example. Van Velzen [16] examined 3-(trimethoxysilyl)propanethiol by this technique. Approximately monolayer quantities of this silane were adsorbed on the barrier oxide of an aluminum-aluminum oxide-metal tunneling junction two metals were investigated, lead and silver. It was concluded that the silane is... 

Electron tunnelling through the stable oxide film to the adsorbed oxygen which sets up a potential and causes ion drift, thus resulting in logarithmic oxide growth. 

Whereas a film formed in dry air consists essentially of an anhydrous oxide and may reach a thickness of 3 nm, in the presence of water (ranging from condensed films deposited from humid atmospheres to bulk aqueous phases) further thickening occurs as partial hydration increases the electron tunnelling conductivity. Other components in contaminated atmospheres may become incorporated (e.g. HjS, SO2, CO2, Cl ), as described in Sections 2.2 and3.1. 

Page CC, Moser CC, Chen X, Dutton PL. 1999. Natural engineering principles of electron tunneling in biological oxidation-reduction. Nature 402 47-52. 

Despite the enormous impact that scanning probe methods have had on our understanding of reactions at oxide surfaces, both STM and AFM suffer from the lack of chemical specificity. The application of STM-inelastic electron tunneling spectroscopy is a potential solution as it can be used to measure the vibrational spectrum of individual molecules at the surface [69, 70]. 

Another conductivity mechanism could be suggested for LB films of this polymer with Ag+ cations. Such cations can accept or release electrons easily, so in the layer of such cations the conductivity could be caused by electron transitions between the ions with different degrees of oxidation. With tunneling microscopy an anomaly in the dl/dV(V) curves near zero bias was discovered for the LB films in Ag form with an odd number of layers there was a conductivity peak some 150-200 mV wide (Figure 7.4, Curves 1, 3) but no anomaly for these same films with an even number of layers (Figure 7.4, Curve 2). For LB films with an odd number of layers the ordered superstructure of the scale 11.5 x 11.5 x lO cm has been found in a conductivity dl/dV (x,y) measurement regime. The scale of such a structure corresponds to 3 x 2 surface reconstruction (Figure 7.5). 

The oxide film may be very thin, involving only a few monolayers, so that electron tunneling through the film may occur. 

The next example of molecular tunneling near absolute zero was the rebinding of ligands to heme proteins that I have already mentioned. The typical distances of electron tunneling in various (e.g., radiation-chemical) oxidation reduction processes in solid state at 100 to 140°K are of the order of tens of angstroms. Distances of molecular tunneling at about 4°K are equal to 0.3 to 0.5 A. 

For the last few years numerous electron tunneling processes on the surface of highly dispersed (specific surface area of about 100-200m2g x) oxides have been discovered and investigated [67-70]. The processes studied represent reactions of recombination of radiation defects. In highly dispersed oxides a considerable portion of these defects is known to be stabilized on the surface. 

With increasing extent of coverage of the surface with N20 molecules, the rate of decay of F+ -centres at 77 K has been found to rise. Since reaction (20) proceeds even at T = 4.2 K and no direct contact of the F+-centre, located in the bulk of MgO, with the adsorbed N20 seems to be possible, the most probable mechanism of the reaction in question is electron tunneling from the F -centres in the bulk of the MgO to the adsorbed molecules of nitrous oxide. The distance of the transfer at 77 K within the time of 105s has been estimated to exceed 15 A. 

Thus the data obtained so far indicate that electron donor and electron acceptor centres on the surface of highly dispersed oxides, including adsorbed molecules, may undergo long-range electron tunneling reactions with centres of the opposite type located both on the surface and in the bulk of the oxides. 

Experimental research on the role of electron tunneling in biological systems is typically carried out on subchromatophore and subchloroplast fragments at low temperatures. The operation of photosynthesizing objects at low temperatures was first discovered by Chance and Nishimura [28] who reported the oxidation of cytochrome c under the action of light in photosyn-... 

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