In general the colour of a given crystal does not depend on its size or shape. For instance, the typical pink colour of ruby, the first laser crystal, is given by the energy level diagram of Cr³⁺ dopant ions in the Al₂O₃ matrix, and so independent of the crystal size or shape.¹  However the fluorescence properties of semiconductor crystals, that have their absorption edge in the visible or near-infrared spectral region, are strongly modified when they are confined to the nanoscale, in such a way that the emission colour becomes size-dependent. For instance CdSe bulk crystals have a typical grey colour, which is mostly due to their energy gap at 1.74 eV (712 nm).²  However when these crystals are synthesized as spheres of nanometric diameter (quantum dots, QDs) the optical properties of CdSe change dramatically.³  As an example, Figure 4.1 shows how the emission colour of a solution containing QDs of CdSe gradually shifts from red to blue as the dot diameter is reduced from 8 to 2 nm, respectively. This amazing size-induced change in the emission colour provides a variety of applications in different fields, such as laser technology, display devices, highly efficient solar cells and high-brightness and high-resolution fluorescence imaging.^4–6  In particular, the ability of QDs to generate size-controlled well-defined emission colours makes it possible to use these dots as highly sensitive fluorescent thermal nanosensors.