Electron transport, self-assembly, and electroluminescence of nanocrystal superlattices
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In order to assess the potential applications of nanotechnology, the fundamental properties of nanocrystals and the self-assembled arrays they form must be studied in detail. The electrical conductivity of monodisperse and polydisperse Ag nanocrystal superlattices was measured as a function of temperature. A fundamental difference between polydisperse and monodisperse nanocrystal superlattices was found. Polydisperse superlattices displayed insulating behavior throughout the entire temperature range. Monodisperse superlattices displayed a metal-insulator transition that shifted to lower temperatures for larger Ag nanocrystals. At temperatures above the metalinsulator transition, the monodisperse superlattices exhibited a positive temperature coefficient of resistance, characteristic of a metal. Below the metalvii insulator transition, the temperature coefficient of resistance was negative, characteristic of an insulator. The ability to control the formation of complex, self-assembled nanocrystal superlattices is very important for potential electrical and optical applications. With the correct concentration and size ratio, nanocrystals with a bimodal size distribution can self-assemble into LmSn structures. The formation of 2D monolayers of these complex structures was studied by performing random sequential adsorption (RSA) simulations of tethered hard disks that are able to undergo limited Monte Carlo surface diffusion. Nanocrystal size ratios of 0.155, 0.414, and 0.533 were examined. Melting simulations of perfect LmSn structures reveal that RSA kinetics frustrate superlattice ordering, creating a kinetic bottleneck to ordered LmSn structures. One of the more promising applications for nanocrystals is light emitting diodes (LEDs). Si nanocrystals synthesized by thermal decomposition of phenylsilane precursors in a supercritical hexane solution with and without the addition of octanol, which serves as a capping ligand, were used as the emitting layer in an LED. Electroluminescence from these Si nanocrystal LEDs was reddish-orange or white depending on the reaction conditions of the nanocrystal synthesis. The current-voltage behavior was characteristic of space-charge limited current, and the devices exhibited relatively low turn-on voltages (∼ 6-7 V). External quantum efficiencies varied between 10-5 and 10-4 %.