Surface tunneling through

In (a), a molecule alights onto a positive electrode surface, its electrons being attracted to the surface and its nuclei repelled. In (b), an electron has tunneled through a barrier onto the electrode, leaving a positive ion that is repelled by and moves away from the positive electrode. 

Ions and protons are much heavier than electrons. While electrons can easily tunnel through layers of solution 5 to 10 A thick, protons can tunnel only over short distances, up to about 0.5 A, and ions do not tunnel at all at room temperature. The transfer of an ion from the solution to a metal surface can be viewed as the breaking up of the solvation cage and subsequent deposition, the reverse process as the jumping of an ion from the surface into a preformed favorable solvent configuration (see Fig. 9.1). 

Fig. 4.5 Schematic projection of the energetics of a reaction. The diagram shows the Born-Oppenheimer energy surface mapped onto the reaction coordinate. The barrier height AE has its zero at the bottom of the reactant well. One of the 3n — 6 vibrational modes orthogonal to the reaction coordinate is shown in the transition state. H and D zero point vibrational levels are shown schematically in the reactant, product, and transition states. The reaction as diagrammed is slightly endothermic, AE > 0. The semiclassical reaction path follows the dash-dot arrows. Alternatively part of the reaction may proceed by tunneling through the barrier from reactants to products with a certain probability as shown with the gray arrow...

Figure 4.27. (a) Schematic of a STM z- Ft injection spectrum (solid curve). The dashed curves represent typical STM tip displacements observed at a clean metal surface, (b) Energy band diagrams for STM tunnelling through a vacuum barrier into the organic thin film and (c) through a Schottky-like barrier with the tip in contact. In both cases, Ft < 0 relative to Ep is shown. Adapted from Muller et al, 2001. 

Fig. 1.6. Tunneling through a controllable vacuum gap. The exponential dependence / V is observed over four orders of magnitude. On clean surfaces, an apparent barrier height of 3.5 eV was observed. (Reproduced from Binnig et al., 1982a, with permission.)...

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

In [4] we have introduced a CA model for the NH3 formation which accounts only for a few aspects of the reaction system. In our simulations the surface was represented as a two-dimensional square lattice with periodic boundary conditions. A gas phase containing N2 and H2 with the mole fraction of t/N and j/h = 1 — j/n, respectively, is above this surface. Because the adsorption of H2 is dissociative an H2 molecule requires two adjacent vacant sites. The adsorption rule for the N2 molecule is more difficult to be described because experiments show that the sticking coefficient of N2 is unusually small (10-7). The adsorption probability can be increased by high energy impact of N2 on the surface. This process is interpreted as tunnelling through the barrier to dissociation [32]. 

It thus appears that even on metal-free SrTi03 conduction-band electrons are the primary reductants. Since similar reaction rates occur on pre-reduced and stoichiometric crystals with disparate depletion layer widths, the electrons do not tunnel through the depletion layer. With no Pt to provide an outlet for electrons at potentials far positive of the flatband potential, strong illumination would flatten the bands almost completely and allow electrons to reach the semiconductor surface. The presence of both electrons and holes at the surface could lead to unique chemistry as well as high surface recombination rates. 

Beginning with composite electrodes the simplest scheme here is to coat the surface of a small gap semiconductor that is well matched to the solar spectrum but which is electro-chemically unstable with a thin film of a wide gap, electrochemically stable semiconductor. To demonstrate the feasibility of using such a composite electrode, the film must be thin enough - of the order of 50-100 A or less - so that at least some of the photogenerated carriers in the small bandgap material can tunnel through to the electrolyte. Furthermore the film must not have any cracks or pinholes since these would... 

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