Surface Science of Semiconductor Nanocrystals

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Abstract 9505302 Alivisatos Semiconductor nanocrystals are a new type of material exhibiting a wide range of size dependent optical, electrical, and thermodynamic properties. A great deal is now known about the interior composition and structure of nanocrystals, but relatively little is known about their interfaces. The aim of this work is to develop the ability to control the composition and structure of semiconductor nanocrystal interfaces. This is necessary before the physics of quantum confinement can be understood and before the technological promise of this class of materials can be assessed. Two main tools will be used: 1) photoelectron spectroscopy and related techniques, and 2) infrared reflection-absorption spectroscopy. Using an ordinary X-ray source, it will be possible to routinely identify and potentially to manipulate the chemical identity of the nanocrystal surface species. With the Advanced Light Source synchrotron, it will be possible to perform photoemission experiments at higher resolution and with variable photon energy. These experiments will be targeted at measuring average bond lengths, and the density of interior and surface electronic states, both occupied and unoccupied. Infrared reflection-absorption spectroscopy will be used to identify the chemical species present, and the nature of their bonding. If the nanocrystals can be oriented in the chemical deposition process, then orientational information can also be extracted. %%% Semiconductor nanocrystals are a new type of material exhibiting a wide range of size dependent optical, electrical, and thermodynamic properties. The limiting factor scientifically and technologically in the area of nanocrystals is inadequate understanding and control of the nanocrystal surface. In optical spectroscopy, trap levels associated with the interface strongly influence absorption and emission linewidths and lifetimes. In electrical studies, all carriers must pass through the interface. In thermo dynamic studies, the fact that order 50% of the atoms are at the surface strongly influences the phase diagrams. A great deal is now known about the interior composition and structure of nanocrystals, but relatively little is known about their interfaces. The objective of this work is to construct a "surface science" of nanocrystals, just as there is one for plane crystal surfaces.