Electron tunneling barrier

Willig and co-workers used near-infrared spectroscopy to measure excited-state interfacial electron transfer rates after pulsed light excitation of cis-Ru(dcb)2(NCS)2-Ti02 in vacuum from 20 to 295 K [208]. They reported that excited-state electron injection occurred in less than 25 fs, prior to the redistribution of the excited-state vibrational energy, and that the classical Gerischer model for electron injection was inappropriate for this process. They concluded that the injection reaction is controlled by the electronic tunneling barrier and by the escape of the initially prepared wave packet describing the hot electron from the reaction distance of the oxidized dye molecule. It appears that some sensitizer decomposition occurred in these studies as the transient spectrum was reported to be similar to that of the thermal oxidation product of m-Ru(dcb)2(NCS)2. 

Becka A. M. and Miller C. J. (1992), Electrochemistry at co-hydroxy thiol coated electrodes. 3. Voltage independence of the electron tunneling barrier and measurements of redox kinetics at large overpotentials , J. Phys. Chem. 96, 2657-2668. 

As in the case of solution processed SANDs4, a substantial leakage current (J) reduction in is observed (10 - 10 2 - 10 5 - 10 7 A/cm2 at 2 V) after the capping layer is deposited (FI, F2 - Fl-cap, F2-cap). The leakage current reduction indicates that the capping material functions as an efficient electron tunneling barrier.4... 

Electrons can penetrate the potential barrier between a sample and a probe tip, producing an electron tunneling current that varies exponentially with the distance. 

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... 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

This difference is the irreversible capacity loss (<2jr). Dahn and co-workers [71] were the first to correlate <21R with the capacity required for the formation of the SE1. They found that <2ir is proportional to the specific surface area of the carbon electrode and, assuming the fonnation of an Li2C03 film, calculated an SEI thickness of 45 5 A on the carbon particles, consistent with the barrier thickness needed to prevent electron tunneling [1,2]. They concluded [71] that when all the available surface area is coated with a film of the decomposition products, further decomposition ceases. 

Figure 1. The tunneling of a single electron (SE) between two metal electrodes through an intermediate island (quantum dot) can be blocked of the electrostatic energy of a single excess electron trapped on the central island. In case of non-symmetric tunneling barriers (e.g. tunneling junction on the left, and ideal (infinite-resistance) capacitor on the right), this device model describes a SE box .

On detailed electrical characteristics of a SET transistor utilizing charging effects on metal nanoclusters were reported by Sato et al. [26]. A self-assembled chain of colloidal gold nanoparticles was connected to metal electrodes, which were formed by electron-beam lithography. The cross-linking of the particles as well as their connection to the electrodes results from a linkage by bifunctional organic molecules, which present the tunnel barriers. 

The activation energy of most of the eh reactions, 3.5+0.5 Kcal/mole, is much less than the hydration energy of the electron, -40 Kcal/mole. There are other barriers against reaction, such as repulsion by electrons in molecules. This can only be an accident in the classical mechanism, but not in electron tunneling theory as long as the reaction is exothermic overall. 

When the current in molecular junctions is dominated by electron tunnelling (Fig. 2b), it can be described as a first approximation by Simmons s theory [40,41]. In this model, the current depends on (1) the height O of the potential barrier, which is determined by the interactions of the electron with the medium and (2) the thickness d of the barrier (Fig. 3). 

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