Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Phases

Abstract

AbstractWe review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 Å in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ∼0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.