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.

We will begin this account by comparing dynamic excitons in a nanocrystal with two commonly encountered electron–hole states, namely, free carriers (completely unbounded electron and hole) in conventional semiconductors and Frenkel exciton in organic semiconductors. This reveals challenges faced by the current workhorse of optoelectronic devices mostly based on conventional semiconductors and highlight the unique advantages of colloidal nanocrystals as a semiconductor platform for photon/exciton manipulation. Colloidal semiconductor nanocrystals are synthesized and processed in solution, an indispensable advantage not only in terms of economic/environmental cost but also for exciton manipulation.

Solution chemistry coupled with dynamic excitons offer special means for exciton/photon manipulation. Specifically, properties of a dynamic exciton can be synthetically tuned by varying the size/shape/composition/ligands of the nanocrystals to match properties of the involved photons. Namely, these include precisely accurate recombination energy for color accuracy, engineered exciton–phonon coupling for color purity, orientation of anisotropic transition dipole for polarized photon emission, continuous absorption with tunable absorption onset, designed spatial localization of two quasiparticles through monolayer-accurate epitaxial shell growth, largely adjustable energies of top (bottom) of valence (conduction) band for charge transfer in optoelectronics and photochemistry, exciton radiative decay and Auger nonradiative decay lifetimes tunable in the millisecond to picosecond range for photocatalysis and lasing, etc.

The other aspect of exciton/photon manipulation using colloidal semiconductor nanocrystals lays on their widely open field of optical, optoelectronic, and photochemical applications to be discussed in the last part of this account, i.e., photon input and photon output, electricity input and photon output, and photon input and electricity output. Each of them is centered around dynamic excitons. Here, we will not discuss photocatalysis and photosynthesis, which can be considered as a special class of the third category. For each category, we will focus on revealing their unique fundamental principles based on dynamic excitons.