Magneto-optic properties of II-VI semiconductor quantum dots.

摘要

Low dimensional systems of semiconductor quantum dots in glass composites exhibit interesting physical properties arising from spatial confinement effects; an example is the discretization of the energy spectrum. In semiconductor quantum dots, electronic wave functions experience effects of quantum confinement arising from the dot-glass interface acting as an infinite potential barrier. This leads to electronic transitions having higher energies with decreasing dot size. The shift of the electronic energies is generally associated with an increase in the Faraday rotation due to exciton confinement. Magnetooptic measurements of II-VI semiconductor quantum dots in a boro-silicate glass matrix have been studied. The Faraday rotation of the quantum dot glass composite shows an increase over both the bulk semiconductor crystal for the same volume fraction, and the pure boro-silicate glass. The Verdet constant, initially constant, demonstrates an increase to another, higher constant value, at higher magnetic fields. This kink in the slope is achieved at lower fields with smaller quantum dots, and this finding is noted to be invariant, throughout the various samples studied. Kinks in Faraday rotation curves have been found by others in rare earth doped glasses as well. Kinks seen in those glasses occur at lower magnetic fields than for the quantum dots.; Several possible explanations are examined to explain this phenomenon. Greater homogeneity of nanocrystal size and higher degree of sphericity is suggested as an explanation for the higher Verdet Constants seen in the samples more homogeneous in size and sphericity. Given the presence of kinks for both semiconductor quantum dots and rare earth doped glasses; one commonality is suggested to be regions of electron localization. Here, regions of intermediate-range order of 2--4 nm. in rare earth doped glasses are suggested to be associated with electron localization in a manner analogous to quantum dots. Ultimately, it is the localization of a small number of electrons elevated to the conduction band which is suggested to be affected by the incident magnetic field. Magnetically-induced orientation of the spins of those electrons is hypothesized to be the cause of the jumps or kinks in the Faraday rotation seen.