The study of nanometer-sized semiconductor crystals has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical and chemical properties can be systematically tuned by variation of the size, according to increasingly well-established scaling laws. This article describes colloidal semiconductor nanocrystals belonging to the II-VI and III-V families, and outlines strategies for obtaining electrical access to such dots.
If an inorganic cluster exceeds a certain size—generally in the 10s of unit cells—then it will likely possess a bonding geometry characteristic of a bulk phase. Above this critical size, the nature of the chemical bonds in the cluster remains fixed as a function of the size, but the total number of atoms—or the surface to volume ratio—changes smoothly. This leads to a slow extrapolation of the properties for ideal nanocrystals toward bulk values with increasing size, according to the scaling laws. The ability to control systematically the properties of inorganic materials by variation of size and shape is an important development with many implications for how materials should be processed and assembled.
Many scaling laws have been investigated, including the size variation of bandgap, charging energy, magnetization reversal, and melting. Study of the scaling laws reveals lessons for how to make nanocrystals. This article focuses on the properties of colloidal semiconductor nanocrystals, how to make them, and ways of gaining electrical access to them. Advances in metal, magnetic, and structural nanomaterials are also occurring. Semiconductor dots produced by other processing techniques in the articles by Bimberg, Gammon, and Tarucha in this issue.