|M.Sc Student||Gokhberg Kirill|
|Subject||Electron (Hole) Paramagnetic Resonance of Nearly Spherical|
|Department||Department of Chemistry||Supervisor||Professor Emeritus Tsofar Maniv|
The wealth of physical phenomena occurring in
semiconductor crystallites of nanometer size has been a subject of intensive
research for the last decade both from the purely academic interest and in view
of their application in modern technologies. Optically detected magnetic
resonance (ODMR) is one of the techniques used for investigation of these
systems. In the ODMR experiment, carried out on spherically shaped nanocrystals
of CdSe, a series of unusual broad peaks was repeatedly found. The broad energy
distribution of these peaks was suggestive of the delocalized nature of the
excited electron and hole in the interior of nanocrystals, and, as we have
shown, these peaks can be assigned to a paramagnetic resonance.
In the ODMR technique observation of the magnetic resonance is carried out by measuring the luminescence that results from the electron-hole pair recombination. Analysis of luminescence spectra showed that in the initial state of this recombination process the carriers undergoing the magnetic resonance move nearly freely in the entire space of the nanocrystal, while in the final state they are trapped at the surface. We have demonstrated, that in order to explain the positions of the observed peaks, one should detect magnetic resonance of a free carrier in the excited energy states within the conduction or valence bands. The lifetime of such excited states in the bulk solids is known to be several orders of magnitude smaller than the precession period of the carrier at resonance. In nanocrystals, however, dramatic increase in the lifetime of excited energy states make possible observation of the magnetic resonance in the excited levels. This is due to the discretization of energy levels in nanocrystals, unlike the continuous energy bands in bulk solid, and discretization of the phonon spectrum, which stem from the quantum confinement effect. For the same reason the characteristic rate of the spin-lattice relaxation processes is reduced, and the spin relaxation times become larger than the pair recombination times, which makes magnetic resonance of free carriers observable in ODMR experiment. In addition we have demonstrated that results obtained for spherical nanocrystals remained valid in uniaxially distorted nanocrystals.
To provide solid support for the qualitative picture described above, we have performed in this work calculations based on a theoretical model of nanocrystals which exploits the k×p perturbation theory. We have found calculated results to be in very good quantitative agreement with the corresponding values documented in the literature.