Semiconductor Nanoparticles

Semiconductors are key components of devices used everyday, including computers, light emitting diodes, sensors, etc. Semiconductors are a unique class of materials in that they can assume characteristic properties of both metals and insulators, depending on conditions that determine the electronic nature of the valence and conduction bands. In the ground state, the valence band is completely filled and separated from the conduction band by a narrow band gap (E_g). When sufficient energy is applied to a semiconductor, it becomes conducting by excitation of electrons from the valence band into the conduction band. This excitation process leaves holes in the valence band, and thus creates “electron-hole pairs” (EHPs). When these EHPs are in intimate contact (i.e., the electrons and holes have not dissociated) they are termed “excitons.” In the presence of an external electric field, the electron and the hole will migrate (in opposite directions) in the conduction and valence bands, respectively (Figure 2.1).