Impurities in Semiconductor Nanostructures

As in bulk semiconductors, impurities play an important role in semiconductor nanostructures. Solid-state reactions such as silicide formation are used to make low-resistance electrical contacts in transistor applications, and cation-exchange reactions can be used to completely (or partially) transform a nanostructure from one material to another. Likewise, impurities are used to store electrochemical energy in devices such as Li-ion batteries. In confined semiconductor nanostructures, even single atomic impurities can have dramatic effects on optical, electrical, and magnetic properties. For example, the addition of only 2–3 Ag atoms to a CdSe nanocrystal can lead to an order-of-magnitude increase in fluorescence efficiency. Controlled impurity incorporation and characterization pose significant challenges in these systems. Here, we will discuss the effects of impurity incorporation in semiconductor nanostructures, as well as how in situ transmission electron microscopy (TEM) can be used to study the dynamics of solid-state reactions and impurity diffusion on the nanoscale. Dramatic morphological changes can be observed due to the volume change associated with silicide formation in Si nanowires, and in some systems, the rate of impurity diffusion can be orders of magnitude different than what is observed in the bulk.