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Doping and Charging in Colloidal Semiconductor Nanocrystals

Moonsub Shim, Congjun Wang, David J. Norris, and Philippe Guyot-Sionnest Introduction Modern semiconductor technology has been enabled by the ability to control the number of carriers (electrons and holes) that are available in the semiconductor crystal. This control has been achieved primarily with two methods: doping, which entails the introduction of impurity atoms that contribute additional carriers into the crystal lattice; and charging, which involves the use of applied electric fields to manipulate carrier densities near an interface or junction. By controlling the carriers with these methods, the electrical properties of the semiconductor can be precisely tailored for a particular application. Accordingly, doping and charging play a major role in most modern semiconductor devices. In view of the importance of these two processes in bulk materials, it is interesting to consider their potential impact on nanometer-scale semiconductor structures. Such structures have been extensively studied over the last two decades to investigate the influence of size on physical properties. If the structure is sufficiently small, its boundaries squeeze the carriers and modify their behavior. One example is the semiconductor quantum dot (QD), where the electrons and holes are threedimensionally confined inside a nanometerscale “quantum box.” The confinement causes a discrete atomic-like energy-level structure for the carriers, and a variety of new physical phenomena are observed.1,2 Thus, more recent efforts have started to explore the possibility of combining quantum confinement with the introduction of extra carriers by means of doping and charging to obtain a new set of properties in these materials.

MRS BULLETIN/DECEMBER 2001

In this short review, we briefly outline recent efforts to explore this issue in colloidal QDs (also referred to as nanocrystals).3–5 Unlike surface-grown QDs,6 which are deposited directly on a semiconductor substrate, colloidal QDs are chemically synthesized in solution.7 While this approach makes them less amenable than surface-grown QDs to incorporation into conventional technology, the chemical synthesis offers much more control over their chemical composition, shape, and size. Further, since colloidal QDs can be dispersed as homogeneous solutions in a variety of liquid or solid matrices or arranged into close-packed solids8 or hierarchical assemblies,9 their advantage lies in their potential as a highly adaptable building block of future nanotechnology, where self-assembly is expected to play a major role.

Impurity Doping While impurity doping in bulk semiconductors is now routine, the extension to semiconductor nanocrystals is not trivial. The nanometer size of the colloidal QD leads to new difficulties not encountered in bulk materials. For example, while a heavily doped bulk semiconductor typically has about 1 dopant for every 105 atoms, a colloidal QD with a 5-nm diameter has about a thousand atoms that are not at the surface.