Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals

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.

Watching Excitations in CsPbBr3 Perovskite Nanocrystals Undergo Ultrafast Relaxation to Their Emitting State

“…The increase in line width of the emitting state may be key to part of what distinguishes it from the absorbing state. Based upon single NC spectroscopy and two-dimensional electronic (2DE) spectroscopy, the heterogeneous line widths are expected to be ∼25 meV. If thermal broadening at 300 K is ∼25 meV, then the remaining line width should arise from electron–phonon coupling …”

“…Shown in the schematic are both excitonic absorbing states and emitting states. Included in the emitting states are dark states for which there has been some controversy. ,,, …”

“…The temperature dependent PL measurements reveal strong electron–phonon coupling, as would be expected from Fröhlich theory and material parameters. In contrast, single NC PL experiments reveal weaker coupling by an order of magnitude. , Importantly, these single NC measurements of electron–phonon coupling are probing the same state at 4 K that is reflected in the 500 ps PL decays. The implication is that the state of the system at 4 K is potentially even more exotic than the state of the system at 300 K. This state at 4 K has been proposed as having “giant oscillator strength” due to coherent delocalization over the NC volume. , In addition to phonon effects there may be charging effects to line widths. , While the experimental observation is of fast decay is well-known, the underlying mechanism remains controversial.…”

Uncovering the Influence of Ni2+ Doping in Lead-Halide Perovskite Nanocrystals Using Optically Detected Magnetic Resonance Spectroscopy

“…The halide perovskites in their nanoscale forms have been a focus of scientific interest during the past decade, based on the pioneering development of colloidal synthetic procedures by Kovalenko and co-workers, who stimulated preparation of different morphologies from zero- to three-dimensional structures. − Subsequently, the optical properties of perovskite materials were studied extensively. − Their photoluminescence is characterized by excitonic transitions, uniquely possessing bright triplet emission at low temperatures and dark singlet recombination at room temperature. ,, A few different magneto-optical measurements, monitoring single perovskite nanocubes , or thin films, − revealed inversion symmetry breaking in both 3D and 2D compounds, originating from an internal anisotropy caused by the composition heterogeneity, surface area, or surrounding interfaces. − The lack of inversion symmetry combined with spin–orbit coupling, as often found in these materials, leads to a Rashba effect in both the conduction and the valence band; viz., creation of an effective internal magnetic field that splits band-edge states in k-space into two valleys, each of which accommodates photocarrier spins of opposing polarity. ,,,,, The Rashba field is a source for the bright triplet recombination, as well as for spin-polarized recombination emission, with a typical lifetime of subnanoseconds. ,, These intriguing discoveries stimulated a search for the spin lifetime, spin coherence time, and values of the phenomenological g -factors. ,− Most recent studies report a low-temperature spin-relaxation time ( T 1 ) comparable with the radiative lifetime (∼250 ps) and a spin coherence time varying from ∼4 to ∼70 ps ,− and the exceptional case of 300 ps, all rivaling that of the classic III–V self-assembled quantum dots. , Besides the fast and bright excitonic emission, long emission tails of uncertain origin, up to a tenth of a nanosecond, were re...…”

Single‐Photon Emission from Single Microplate MAPbI3 Nanocrystals with Ultranarrow Photoluminescence Linewidths and Exciton Fine Structures

“…Single-particle optical studies of these organic-inorganic perovskite NCs, mainly in the forms of MAPbX 3 or FAPbX 3 (MA + = CH 3 NH 3 + , FA + = CH(NH 2 ) 2 + , and X -= Bror I -), are thus critical in revealing the intrinsic photophysical processes that are otherwise hidden from the ensemble-averaging measurements. [11,12] In the case of single FAPbX 3 NCs, the photoluminescence (PL) blinking behavior [13,14] and the singlephoton emission feature [13][14][15][16] are now being routinely reported, with the additional demonstrations of the exciton-phonon coupling [16][17][18] and the crystal phase-transition [16,17] effects. Moreover, besides the extraction of charge-exciton and biexciton binding energies from single FAPbX 3 NCs, [18] they have also played a key role to demonstrate that the dark-exciton state is located lower in energy than those of the bright-exciton ones.…”