Point defects experimental investigations

If the coupling of the electrons to certain centers is strong, their spectra may be distinguished from that of the crystal as a whole (point defect color centers in ionic crystals, polarons in semiconductors). The spectra of defects can therefore be used for analytical or even kinetic investigations. In principle, it should be possible to construct devices which have, under favorable conditions, a sufficient spatial resolution to experimentally determine the basic kinetic quantity c,( , t). 

The quantum chemical modeling is a very useful supplement to spectroscopic experimental methods for investigation of properties of point defects, however, until recently it was used mainly for calculations of vertical excitation energies. The modeling of structural transformation in excited electronic states is still a rather complicated task, which requires state-of-the-art quantum chemical calculations. In this chapter, we first describe theoretical methods applied in ab initio and vibronic theory calculations and then demonstrate their applications in theoretical studies of various point defects in silica and germania. 

It is well known that defects play an important role in determining material properties. Point defects play a major role in all macroscopic material properties that are related to atomic diffusion mechanisms and to electronic properties in semiconductors. Line defects, or dislocations, are unquestionably recognized as the basic elements that lead to plasticity and fracture (Fig. 20.1). Although the study of individual solid-state defects has reached an advanced level, investigations into the collective behavior of defects under nonequilibrium conditions remain in their infancy. Nonetheless, significant progress has been made in dislocation dynamics and plastic instabilities over the past several years, and the importance of nonlinear phenomena has also been assessed in this field. Dislocation structures have been observed experimentally. 

Investigating the atomic defects is important in tailoring the electronic properties of SWCNTs. Recent experimental study reported a method to selectively modify the electronic properties of semiconductor SWCNTs by the creation and annihilation of point defects on their surface with the tip of a scanning tunneling microscope (STM) (Berthe et al. 2007). Such experimental study motivates theoreticians to explore the structures, energetics, reactivities, and electronic properties of SWCNTs containing different types of defects. 

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