Colloidal Nanocrystals: A Promising Semiconductor Platform for Photon/Exciton Manipulation

The invention of single-crystalline semiconductors and related devices allows us to manipulate electrons (or holes) as free charge carriers with ease. Photons and electrons are two types of fundamental particles for electromagnetic interaction, and optical/optoelectronic devices are thus likely as important as semiconductor electronic devices. Photons themselves have negligible direct interactions with each other, and manipulating photons─controlling their color purity and color accuracy, phase coherency and polarity, conversion from/to other forms of energy, etc.─is primarily achieved through their interactions with matter. Different from dealing with a single type of quasiparticle (electrons or holes) in a specific spatial region for electron manipulation, either absorbing or emitting a photon by matter, always involves a colocalized electron–hole pair as the transient state. In this sense, the key for manipulating photons is manipulating electron–hole pairs that are often called excitons. Similar to the corresponding bulk semiconductor, the binding energy is insufficient to stably bond a Wannier–Mott exciton in a typical semiconductor nanocrystal. However, two dynamic quasiparticles (electron and hole) are spatially confined within a nanocrystal by the energy barriers provided by the surrounding ligands/solvents, leading to formation of a special type of exciton, i.e., dynamic exciton.