Oleg Gogolin

The temperature dependence of exciton peak energies in large CuI quantum dots embedded in a glass matrix has been measured. The gap shrinkage effect for both cubic and hexagonal CuI phases was observed. The temperature coefficients of the energy gaps and the effective phonon energies are deduced.

The temperature dependence of exciton peak energies in large CuI quantum dots embedded in a glass matrix has been measured. The gap shrinkage effect for both cubic and hexagonal CuI phases was observed. The temperature coefficients of the energy gaps and the effective phonon energies are deduced.

The results on the selective photoluminescence (PL), photoluminescence excitation (PLE) and the degree of linear polarization (DLP) for the PL spectrum in AgI nanocrystals (NC) embedded in a glass host matrix are presented. The measurements indicate a fine structure of the exciton ground state due to the electron–hole exchange interaction: the PL originates from the lowest triplet state (“dark” exciton) when the higher singlet state (“bright” exciton) is excited. The obtained negative DLP confirms that the emitting state is not directly excited by the pump light, but is created through the virtual interaction with a higher excitonic state of different polarization features and subsequent relaxation.The corresponding red shift of the PL spectrum with respect to the PLE spectrum has been studied in dependence on the NC size. The exchange splitting shows a pronounced size dependence strongly increasing with a restriction of the dot radii. The value of the exchange energy of the exciton for AgI bulk crystals is determined. Both the short- and long-range contributions to the exchange splitting are discussed.

In linear optics, we report on measurements of the absolute external quantum efficiency of bulk ZnO and powders using an integrating sphere. At low temperature the near band edge emission efficiency can reach 0.15 in the best samples. For deep center luminescence this value may be even higher. When going to room temperature (RT) the quantum efficiency drops by about one order of magnitude. From time resolved luminescence measurements we deduce the lifetime of the free and bound excitons to be in the sub ns regime and find for the latter a systematic increase with increasing binding energy. Concerning lasing, we discuss the role of excitonic processes and the recombination in an inverted electron–hole plasma (EHP). While excitonic processes seem well justified at lower temperatures and densities, doubts arise concerning the concept of excitonic lasing at RT in ZnO. The densities at laser threshold at RT are frequently close to the Mott density or above but below the density at which population inversion in an EHP is reached. We suggest alternative processes which can explain stimulated emission in this density regime in an EHP at RT.

We report measurements of dynamic (ac) electrical conductivity in borosilicate glasses doped with the semiconductors CdS x Se 1-x and AgI in a wide range of frequencies and temperatures. The concentrations of homogenously dissolved dopants are governed by the heat treatment conditions of the glass samples leading to a creation of CdSSe and AgI nanocrystals. The ac conductivity rises with increasing average size of the CdSSe nanocrystals, in contrast to the case of the metal halide doped borosilicate glasses.

We report measurements of dynamic (a.c.) electrical conductivity in borosilicate glasses doped with cadmium sulfoselenide and copper and silver iodides in a wide temperature range below the glass transition temperature Tg and at different frequencies. The concentration of the mobile dopant ions is governed by specific heat treatment conditions of the glass samples leading to a creation of the CdSSe, AgI and CuI semiconductor nanocrystals. Investigations include different cases from a full solution of the dopant ions coming from a dissociation of the dopants during the glass preparation to their almost complete incorporation into the nanocrystals in the glass matrix. At temperatures higher than 150 °C–200 °C the a.c. conductivity in all the examined glasses exhibits the Arrhenius behavior. In this temperature range the mixed mobile ion effect is detected: the doped glasses have the low values of the conductivity compared to the undoped ones. In the low temperature range only weak temperature dependence is detected for all the samples. The mixed mobile ion effect is still presented for the CdSSe-doped glasses, whereas the AgI- and CuI-doped glasses exhibit the classical MMIE which is essentially absent at low temperatures.