Metal-to-semiconductor transition in nanocrystals: size and temperature dependence

The electrical conductivity of nanocrystals is investigated by using a hybrid approach that treats the electronic structure of a nanocrystal quantum mechanically, and the transport of electrons semiclassically. The simplicity and its analytical nature of the present approach allow one to gain unique insights into the size dependent metal-to-semiconductor transition in nanocrystals. The analytical expression for the electrical conductivity of nanocrystals as a function of size and temperature demonstrates that for a given temperature, there is a critical size at which the metal-to-semiconductor transition occurs which decreases with increasing temperature. For a given sized nanocrystal, a critical temperature for the occurrence of the metal-to-semiconductor transition is also demonstrated. The critical transition temperature is found to decrease with the increasing nanocrystal size. Although the predicted size and the temperature dependence of electrical conductivity for nanocrystals is shown to be remarkably consistent with the recent experimental observations, the effect of the size distribution of nanocrystals must be further investigated. (C) 2000 Elsevier Science Ltd. All rights reserved.