Semiconductor quantum dots (QDs) are fluorescent nanocrystals (NCs) normally smaller than 20 nm in diameter [1]. Typically, they are composed of an inorganic core, made up of a few hundred to a few thousand atoms, surrounded by an organic outer layer of surfactant molecules (ligands). Bulk semiconductors are characterized by a composition-dependent bandgap (Figure 2.1a), which is the minimum energy required to excite an electron from the valence band into the conduction band. With the absorption of a photon of energy greater than the bandgap energy, the excitation of an electron leaves an orbital hole in the valence band to form an electron–hole pair (i.e., exciton). The relaxation of the excited electron back to the valence band may be accompanied by the emission of a photon, a process 12known as radiative recombination. The exciton has a finite size defined by the exciton Bohr radius. When the size of a semiconductor NC is smaller than the Bohr radius, charge carriers become spatially confined and energy levels are quantized, with values directly related to the QD size (Figure 2.1a and b) [1–4]. In the case of quantum confinement, QDs show a size-dependent bandgap, as described in Equation 2.1 [2]: E = E g + ( ℏ 2 π 2 2 R 2 ) ( 1 m e + 1 m h ) − 1.8 e 2 ε R https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315216317/9134b793-84f1-4ae5-94ae-1958ce4ea481/content/equ2_1.tif"/>