Abstract
Lead-halide perovskite nanocrystals (NCs) are attractive nano-building blocks for photovoltaics and optoelectronic devices as well as quantum light sources. Such developments require a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. In this review, we give an overview of recent magneto-optical spectroscopic studies revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of their inhomogeneities in morphology and crystal structure. We highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton–phonon coupling, which provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. We also decipher the spectroscopic characteristics of other charge complexes whose recombination contributes to photoluminescence. With the rich knowledge gained from these experiments, we provide some perspectives on perovskite NCs as quantum light sources.
Highlights
Since the pioneering works of the group of Kovalenko [1,2] on facile synthesis of colloidal lead-halide perovskite NCs with precise sizes and composition control, the field of research on these materials has grown rapidly in recent years
Using magneto-optical spectroscopy of single perovskite NCs at cryogenic temperatures, a wealth of information has been unraveled on the optical properties of their exciton and other charges complexes
The band-edge exciton fine structure of inorganic as well as hybrid lead halide perovskite NCs consists of a bright triplet and a ground dark singlet located several meV below as a result of electron-hole exchange interaction, which is enhanced by the combined effects of quantum and dielectric confinements
Summary
Since the pioneering works of the group of Kovalenko [1,2] on facile synthesis of colloidal lead-halide perovskite NCs with precise sizes and composition control, the field of research on these materials has grown rapidly in recent years. Due to the electron-hole exchange interaction, the band-edge exciton of an NC with a cubic shape and a cubic crystal structure is split into a low-lying dark singlet state with total angular momentum J = 0 and a threefold degenerate optically active triplet state with J = 1 and projection Jz = 0, ±1 along the z-axis. The thermal broadening of the emission line and the evolution of the PL decay with temperature give insights into the nature of the exciton–phonon coupling and the exciton relaxation dynamics They explain why perovskite NCs exhibit a bright photoluminescence at low temperature and in zero magnetic field even though the ground exciton state is dark. These findings are put into perspective for a potential use of single perovskite NCs as quantum light sources for next-generation quantum technology devices
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