An important parameter of a semiconductor material is the width of the energy gap that separates the conduction from the valence energy bands (Figure 1.1a, left). In semiconductors of macroscopic sizes, the width of this gap is a fixed parameter, which is determined by the material’s identity. However, the situation changes in the case of nanoscale semiconductor particles with sizes less than ~10 nm (Figure 1.1a, right). This size range corresponds to the regime of quantum confinement for which electronic excitations “feel” the presence of the particle boundaries and respond to changes in the particle size by adjusting their energy spectra. This phenomenon is known as the quantum size effect, whereas nanoscale particles that exhibit it are often referred to as quantum dots (QDs).