Electrical Studies of Semiconductor-Nanocrystal Colloids
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g. Semiconductor dots produced by other processing techniques in the articles by Bimberg, Gammon, and Tarucha in this issue. Processing on the Nanoscale Many routes have been developed for preparing nanostructures, ranging from single-atom manipulation to organic synthesis (see the article by Nozik and Micic in this issue). The tremendous strength of the colloidal preparation routes is that they yield large amounts of monodisperse nanocrystals with easy size tuning. Further the presence of the organic molecules on the surface of a nanocrystal enables extensive chemical manipulation after the synthesis. One interesting conclusion from this work is that the preparation of nanocrystals may in many ways be far more forgiving than the fabrication of two- or three-dimensional materials: 1) Nanocrystals can be prepared at comparatively modest temperatures. One of the
most famous scaling laws relates the variation of melting temperature with size and shows that it drops as 1/r.1314 The large drop in melting temperature toward small sizes facilitates the creation of highly crystalline and faceted nanoparticles at temperatures compatible with wet chemical processing. From a kinetic perspective, a great deal of control for chemical processes can be achieved in solution. For this reason, extremely highquality inorganic nanoparticles can be prepared as colloids. Recent successes in the preparation of II-VI1516 and III-V1720 nanocrystals illustrate the strengths of the colloidal-preparation techniques. Nanocrystal precursor molecules are injected into a hot surfactant at 300°C;
nucleation and growth of the nanoparticles occur. No aggregation takes place because the particles are terminated by a monolayer of surfactant. At the end of the preparation, nanocrystals are isolated with a monolayer surfactant coat. The surfactant can be exchanged with other organic molecules, allowing the environment of the nanoparticles to be controlled. 2) Nanocrystals act as single structural do-
mains. Structurally, nanocrystals are at a crossover point between molecules— in which the number and location of each atom can be exactly specified—and bulk matter, which requires statistical descriptions. Both kinetkally and thermodynamically, nanocrystals tend to exclude high-energy defects such as grain boundaries. (Lower energy defects such as stacking faults are common however.) It is far easier to anneal out a defect from a very small crystal than from a large one. Assuming the barrier to defect migration is the same as in the bulk solid (certainly an overestimate because the melting temperature is lower), then the defect simply has less distance to travel and will be annealed out faster in a smaller crystal. Thermodynamically, defects are present in a large crystal of atoms at finite temperature. However if the free energy of defect formation is relatively constant with size, then a very small solid is less likely to contain equilibrium defects. The high quality of colloidal dots is confirmed by experiments on the kinetics of pressure-induced solid
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