Inelastic electron tunneling process

In contrast to electron-beam lithography, the mechanism of STM lithography is generally an inelastic electron tunneling process, with energy quanta of an electron volt. STM can also be used for single-atom or single-molecule manipulations for the fabrication of individual nanostructures. 

Figure 7.3 Schematic model of the inelastic electron tunneling process. An electron can tunnel inelasticaUy (denoted as e (inel)) in addition to the elastic channel (denoted as e"(el)) when the electron energy eV is larger than a specific vibration energy (A 2)...

In addition, the tautomerization cannot be induced with bias voltages that correspond to the HOMO (-1.7 V). Since direct tunneling into or from a molecular orbital can thus be excluded as the origin, tautomerization might be induced through inelastic electron tunneling process. However, the precise mechanism has not been clarified. 

While not specifically using time-resolved excitation. Silly and Charra applied time-autocorrelated two-photon counting measurements to STL emission of bare gold surfaces. They found that because of the strong local nature of the inelastic electron tunneling process that is at the basis of STL, photons emitted show strong bunching behavior on the nanosecond timescale. Such measurements should also be elucidative when applied to quantum nanoemitters such as quantum dots. 

A short review on electron tunnelling contains references to the development of electron-transfer theory, and to some more recent experimental work. A review on inelastic atom-atom collisions includes the theory of gas-phase electron-transfer processes. Developments within the Marcus adiabatic theory are considered under the first two headings. 

Thus far the discussion has centered on elastic tunneling, but consideration of inelastic processes may offer additional analytical opportunities. An energy scale of the relevant phenomena is presented in Table 2. Inelastic tunneling was first observed in metal-oxide-metal junctions. It was immediately developed as a technique for photon-free vibrational spectroscopy (lETS) where the tunneling electrons dissipate energy by coupling to vibra-... 

An STM manipulation mechanism related to the adsorption and desorption processes of single atoms and molecules is known as vertical manipulation (Fig. 10). This process involves transfer of single atoms or molecules between the tip and substrate and vice versa (Fig. 10(a)). An atomic switch realized by the repeated transfer of a Xe atom between the STM tip and a Ni(110) substrate is the first example of vertical manipulation [22]. The atom/molecule transfer process can be realized by using an electric field between the tip and sample, or by multiple excitations with inelastic tunneling electrons, or by making mechanical contact between the tip and atom/molecule. This transfer mechanism can be modeled by using a double potential well as shown in Fig. 10(b). At an imaging distance, approximately 6 A between tip and surface, the atom/molecule has two possible... 

The observations illustrate that inelastic and thermally activated tunnel channels may apply to metalloproteins and large transition metal complexes. The channels hold perspectives for mapping protein structure, adsorption and electronic function at metallic surfaces. One observation regarding the latter is, for example that the two electrode potentials can be varied in parallel, relative to a common reference electrode potential, at fixed bias potential. This is equivalent to taking the local redox level up or down relative to the Fermi levels (Fig. 5.6a). If both electrode potentials are shifted negatively, and the redox level is empty (oxidized), then the current at first rises. It reaches a maximum, convoluted with the bias potential between the two Fermi levels, and then drops as further potential variation takes the redox level below the Fermi level of the positively biased electrode. The relation between such current-voltage patterns and other three-level processes, such as molecular resonance Raman scattering [76], has been discussed [38]. 

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