Electron tunneling in crystals

Tezcan FA, Crane BR, Winkler JR, Gray HB. Electron tunneling in protein crystals. Proc Natl Acad Sci USA 2001 98 5002-6. 

Synthesis of a trigonal Cu(II) complex, a blue copper synthetic analogue Electron tunneling in heme-protein single crystals... 

The electron transport properties described earlier markedly differ when the particles are organized on the substrate. When particles are isolated on the substrate, the well-known Coulomb blockade behavior is observed. When particles are arranged in a close-packed hexagonal network, the electron tunneling transport between two adjacent particles competes with that of particle-substrate. This is enhanced when the number of layers made of particles increases and they form a FCC structure. Then ohmic behavior dominates, with the number of neighbor particles increasing. In the FCC structure, a direct electron tunneling process from the tip to the substrate occurs via an electrical percolation process. Hence a micro-crystal made of nanoparticles acts as a metal. 

Figure 7.10 The principle of field ionization. Left the potential for a helium atom near a metal without field, and (right) in the presence of an electric field of strength F (V/cm). Field ionization by electron tunneling becomes possible when the He Is level (ionization potential /) is above the Fermi level of the metal. Tunneling increases when the He atom is closer to the surface. This, however, requires high local fields, which are present at the edges of crystal facets or at adsorbed atoms.

The crystals of akaganeite are not microporous. Micropores observed by TEM are considered to be due to irradiation in the electron beam (Galbrait et al., 1979 Naono et al., 1982). Open ended, cylindrical, interparticular micropores have been reported these arose as a result of alignment of the rod-like crystals into parallel arrays (Paterson and Tait, 1977). Akaganeite does possess a potential structural microporosity arising from the presence of 0.21-0.24 nm across tunnels in the structure. At room... 

If the localized electron tunnels out through the barrier (state 1 in Fig. 12 b) a certain amount of f-f overlapping is present. States like 1 in Fig. 12 b are called sometimes resonant states or "virtually bound" states. In contrast with case 2 in Fig. 12b, which we may call of full localization , the wave function of a resonant state does not die out rapidly, but keeps a finite amplitude in the crystal, even far away from the core. For this reason, overlapping may take place with adjacent atoms and a band may be built as in ii. (If the band formed is a very narrow band, sometimes the names of localized state or of resonance band are employed, too. Attention is drawn, however, that in this case one refers to a many-electron, many-atoms wave function of itinerant character in the sense of band theory whereas in the case of resonant states one refers to a one-electron state, bound to the central potential of the core (see Chap. F)). 

Ideas about the tunneling mechanism of the recombination of donor acceptor pairs in crystals seem to be first used in ref. 51 to explain the low-temperature of photo-bleaching (i.e. decay on illumination) of F-centres in single crystals of KBr. F-centres are electrons located in anion vacancies and are generated simultaneously with hole centres (centres of the Br3 type which are called H-centres) via radiolysis of alkali halide crystals. 

---