Confined electronic systems

The neutral insulator TMTSF, which shows field-effect conduction with /Th — 0.2 cm s (Nam et al, 2003), when transformed into a Bechgaard salt also becomes superconducting, but at lower temperatures. In this case the perfect segregation of organic and inorganic molecular planes leads to confined electronic systems, which in the normal state are quasi ID. Organic superconductors based on the BEDT-TTF molecule represent the case of pure 2D electronic systems. 

Confined electronic systems are quantum systems in which carriers, either electron or holes, are free to move only in a restricted number of dimensions. In the confined dimension, the sizes of the structural elements are of the order of a few de Broglie wavelengths of the carriers or less. Depending on their dimensionality, these structures can be quantum dots (0-D), quantum wires (1-D), or quantum wells (2-D). Quantum wells are typically produced by the alternate epitaxial growth of two or more different semiconductors. Quantum wires are less commonly encountered, since their fabrication procedures are much more complicated (Sect. 5.3.4). 

Olah and Watkins (187) correlated l3C chemical shifts in crowded phenyl-ethanes with bond-electron polarizations brought about by van der Waals interactions. They found that these effects cannot be confined to one single C7-H bond but operate throughout the whole molecule and produce shielding of ortho and deshielding of a- and meta carbon atoms. The para carbon atoms are unaffected, which is taken as evidence that only the o-electron systems of the phenyl groups are involved in these steric interactions (187). 

Confined quantum systems of a finite number of electrons bound in a fabricated nano-scale potential, typically of the order of 1 100 nm, are... 

As is the case with any scientific development with important technological consequences, basic research plays a fundamental role whereby appropriate models are designed to explore and predict the physical and chemical behavior of a system. A confined quantum system is a clear example where theory constitutes a cornerstone for explanation and prediction of new properties of spatially limited atoms, molecules, electrons, excitons, etc. Theoretical study of possible confined structures might also suggest and stimulate further experimental investigations. In essence, the design of novel materials with exceptional properties requires proper theoretical modeling. 

Faulkner et al. performed surface-confined electrochemistry at high pressures to probe the structure of the transition state during the oxidation of a tethered ferrocene probe (analogous to System 4) [139]. In these studies, the ferrocene-containing SAMs on gold were subjected to pressures between 1 and 6000 atm. The pressure dependence of the anodic peak potential reveals a positive volume of activation for oxidation, which is consistent with a solvent reorganization in the transition state, which allows ion complexation. This study demonstrates the importance of structural and environmental effects on surface-confined electron-transfer processes. 

For oxidized nc-Si there exist surface states at the Si/Si02 interface in which photogenerated electron-hole pairs can be localized when the optical band gap has increased enough in small crystallites (Figs 5a + 5b). From these states radiative recombination of the excitons can occur on a time scale of tens of microseconds. Ab initio calculations for one sided oxidized Si planar sheets show that there is a direct-allowed transition of 1.66 eV at the F point [7] In TTSS and PDS the sharp PL- and absorption bands together with the results from the microwave absorption indicate that the origin of the luminescence is of a molecular nature caused by localization in the Si backbone Those mplecular systems, small clusters, nanocrystallites or strongly confined artificial systems may have a potential for a fully Si-based optoelectronic in the future. 

The term nanocatalysis was introduced by Somorjai in 1994 when he used confined electrons of an STM tip to induce an electrochemical process. Earlier experiments on free clusters pointed towards the possibility of using small clusters with intrinsically confined valence electrons as catalysts to tune the properties atom-by-atom. These two completely different pioneering ideas have become further sophisticated during the last few years. It has become possible to use size-selected clusters on surfaces to catalyze simple chemical reactions and to tune the catalytic properties with size as well as using the tip of an STM to control every step of a chemical reaction on a local scale. With these examples a deeper understanding of nanocatalytic factors is now emerging and such studies will have profound impact on the catalysis of systems at the ultimate size limit. 

As has been mentioned in Chapter 3, the total wave function of the yr-electron system is constructed from atomic orbitals that are antisymmetric with respect to the principal plane of the molecule. We shall confine ourselves to bases formed by the it valence orbitals of the unsaturated atoms of a molecule, e.g. the 2px orbitals of doubly linked... 

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