Quantum dots are nanometre-sized semiconducting crystals. While chemists tend to interrogate molecular structure to understand properties, and materials physicists might interrogate the composition and bulk structure of larger crystals, scientists must pay close attention to the size of quantum dots to understand their behaviour. This is because of quantum confinement effects, which can be used to tune the electronic structure of the quantum dots and thus their properties, in particular their optical properties, such as absorbance and fluorescence; for example, small dots emit short-wavelength light while larger ones emit longer-wavelength light.