Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Pha
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ABSTRACT We 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 A 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 longitudinaloptical (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 These studies show that a single nucleation event occurs per nanocrystals under pressure. nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, sixcoordinate high pressure phases may be metastable at STP.
A NANOCRYSTAL EXCITON QUANTUM MECHANICS Semiconductor nanocrystals offer the opportunity to explore the evolution of bulk electronic and structural properties as the size of the system increases from the molecular scale. In addition, their strongly size-dependent optical properties render them attractive candidates as tunable light absorbers and emitters in optoelectronic devices such as light emitting diodes and quantum-dot lasers. New fabrication methods have enabled the synthesis of highly monodisperse (OR < 4%) CdSe nanocrystals with radii tunable between 10 and 50 A, which luminesce with high quantum yield (-0.1 to 0.9 at 10 K).' While the absorption spectra of these dots are now fairly well understood, the nature of the emitting state remains controversial. Even high quality samples exhibit unusually long radiative lifetimes ('tR -1 pts at 10 K) 2' 3 relative to the bulk exciton recombination time ('TR -1 ns).4 Since these radiative rates were expected to be comparable, band edge emission in II-VI nanocrystals was rationalized as a "surface effect" and assigned to the recombination of weakly overlapping surface-localized carriers. 2' 3' 5 Ho
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