Molecular conduction

The last decade of the twentieth century has been revolutionary in the study of molecular electron transfer processes. For the preceding century scientists have investigated three types of such processes transfer between a donor and an acceptor species, transfer between two sites on the same molecule and transfer between a molecular species in solution and a metal or a semiconductor electrode. The main observable in such studies is the electron transfer rate, though in studies of photoinduced electron transfer processes the quantum yield, defined as the number of electrons transfeiTed per photon absorbed, is also a useful observable. The invention of the tunneling microscope and later experimental developments have now made it possible to investigate another manifestation of electron transfer electronic conduction by a molecule connecting two bulk metal or semiconductor electrodes. In this 

In Fig. 17.5(a) the junction is at equilibrium the Fermi eneigies on the two sides are equal, fl =, and the molecular levels are arranged so that the 

Several comments should be made at this point. First, the simple independent electron picture discussed above is only useful for qualitative descriptions of electron transport in such systems electron-electron interactions and electronic correlations should be taken into account in realistic treatments. Second, the discussion above is appropriate at zero temperature. For finite temperature the Fermi energies should be replaced by the corresponding electron chemical potentials on the two sides, and the energy thresholds will be broadened by the 

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For turbulent flow of a fluid past a solid, it has long been known that, in the immediate neighborhood of the surface, there exists a relatively quiet zone of fluid, commonly called the Him. As one approaches the wall from the body of the flowing fluid, the flow tends to become less turbulent and develops into laminar flow immediately adjacent to the wall. The film consists of that portion of the flow which is essentially in laminar motion (the laminar sublayer) and through which heat is transferred by molecular conduction. The resistance of the laminar layer to heat flow will vaiy according to its thickness and can range from 95 percent of the total resistance for some fluids to about I percent for other fluids (liquid metals). The turbulent core and the buffer layer between the laminar sublayer and turbulent core each offer a resistance to beat transfer which is a function of the turbulence and the thermal properties of the flowing fluid. The relative temperature difference across each of the layers is dependent upon their resistance to heat flow. 

Laminar Flow Normally, laminar flow occurs in closed ducts when Nrc < 2100 (based on equivalent diameter = 4 X free area -i-perimeter). Laminar-flow heat transfer has been subjected to extensive theoretical study. The energy equation has been solved for a variety of boundaiy conditions and geometrical configurations. However, true laminar-flow heat transfer veiy rarely occurs. Natural-convecdion effects are almost always present, so that the assumption that molecular conduction alone occurs is not vahd. Therefore, empirically derived equations are most rehable. 

Fig. 22 Phosphorus porphyrin arrays with molecular conducting tetrathiophene wires...

Most of the available data have been recorded under conditions such that only the terms for eddy transport and conduction through the solid are significant. Equation 12.7.19 requires that /c increase with particle diameter, mass velocity, and the conductivity of the solid. It is consistent with data for low conductivity solids, but some discrepancies arise for very high conductivity solids (108). At Reynolds numbers greater than 40, the contribution of the molecular conduction term is negligible. 

Effect of Halogen-Halogen Interactions in Molecular Conducting Magnets... 

Nitzan A (2001) A relationship between electron-transfer rates and molecular conduction. J Phys Chem A 105(12) 2677-2679... 

Wang K, Rangel NL, Kundu S, Sotelo JC, Tovar RM, Seminario JM, Liang H (2009) Switchable molecular conductivity. J Am Chem Soc 131(30) 10447-10451... 

Galperin M, Ratner MA, Nitzan A (2009) Raman scattering from nonequilibrium molecular conduction junctions. Nano Lett 9(2) 758-762... 

Damle P, Ghosh AW, Datta S (2002) First-principles analysis of molecular conduction using quantum chemistry software. Chem Phys 281 (2—3) 171—187... 

Landau A, Kronik L, Nitzan A (2008) Cooperative effects in molecular conduction. J Comput Theor Nanosci 5 535-544... 

Selzer Y, Cai L, Cabassi MA, Yao Y, Tour JM, Mayer TS, Allara DL (2004) Effect of local environment on molecular conduction isolated molecule versus self-assembled monolayer. 

He J, Fu Q, Lindsay S, Ciszek JW, Tour JM (2006) Electrochemical origin of voltage-controlled molecular conductance switching. J Am Chem Soc 128 14828-14835... 

Tao et al. [32] pioneered a technique based on the formation of single molecular junctions between the tip of an STM and a metal substrate. The method was adapted by other groups, modified and applied to a large number of molecular conductance studies at (electrified) solid/liquid interfaces [33, 113-119]. For details we refer to Sect. 2.3. 

The dependence of the single-molecular conductances on the torsion angle (p between the two phenyl rings was addressed by choosing a series of five additional BPDN derivatives (Figs. 16 and 20a) [287]. All conductance histograms exhibited only one dominant single molecular junction-related feature, which is rather sharp... 

Solomon GC, Andrews DQ, Hansen T, Goldsmith RH, Wasielezski MR, van Duyne RP, Ratner MA (2008) Understanding quantum interferences in coherent molecular conduction. J Chem Phys 129 054701... 

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