High Density Suspension of Semiconductor Nanocrystals in Matrices and Their Coherent Cooperative Phenomena among Electronic Excited States

点击翻译按钮获取中文摘要

The aim of this project is to investigate the optical properties of excitons confined in semiconductor nanocrystals together with exciton internal structures, to clarify the mutual interaction among excitons in one nanocrystal, and also to study cooperative phenomena of coherent excitons between nanocrystals both in experiment and theory.1) By means of ion beam sputtering method, we have made CdSe nanocrystals suspended in metrices with high density and also made CdSe, ZnSe and ZnO ultrathin films. We have got the quality of films by X ray diffraction and AFM topography, and the reduction of crystal defects by optical measurement.2) Optical investigation of confined exciton statesa) Picosecond time-resolved luminescence measurement has been perfomed under two-photon resonant excitation. We have measured the size dependence of the ratio between resonant luminescence and resonant scattering and proposed the optical non-Markov processes inherent in nanocrystals.b) By means of single-dot m … More icrospectroscopy of CuCl and ZnCdSe quantum dots, we have succeeded to observe the narrowing, blinking and photofatigue phenomena of exciton luminescence. We have found that the energy of the second lowest confined exciton state is considerably different from that expected for simple exciton confinement model and suggested the possibility of dipole self interaction effect on p-type confined states.c) In CuCl nanocrystals suspended in polymer at high density, we have found a peculiar phenomenon of luminescence enhancement under ultraviolet laser irradiation. Similar to the case of porous Si, photoionization or photo-induced change of surface structure has been considered to play an important role in this phenomenon.d) The optical transition between Rydberg 1S to 2P exciton states has been measured by means of infrared transient spectroscopy. As the size of nanocrystals decreases, the confinement of the 2P exciton deviates from the model of the confinement of exciton translational motion. Theoretical spectra calculated for the 1S to 2P transition is found to coincide qualitatively with the experimental ones.From these results, a progress in the study of confined excitons has been achieved both in experiment and theory and one can expect a possibility of studying the cooperative phenomena of coherent excitons. Less