Transition from Doublet to Triplet Excitons in Single Perovskite Nanocrystals

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr3 perovskite nanocrystals

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr₃ perovskite nanocrystals (NCs) ensembles. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies.

Many-Body Correlations and Exciton Complexes in CsPbBr3 Quantum Dots

All-inorganic lead-halide perovskite (LHP) (CsPbX₃ , X = Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light-emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser), as well as for devices exploiting strong light-matter interaction at room temperature. Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, for example, by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity in quantum light sources. Here, by combining cryogenic single-QD photoluminescence spectroscopy with configuration-interaction (CI) calculations, the size-dependent trion and biexciton binding energies are addressed. Trion binding energies increase from 7 to 17 meV for QD sizes decreasing from 30 to 9 nm, while the biexciton binding energies increase from 15 to 30 meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. The findings here provide a deep insight into the multiexciton properties in all-inorganic LHP QDs, essential for classical and quantum optoelectronic devices.

Degradation mechanisms of perovskite nanocrystals in color-converted InGaN micro-light-emitting diodes

The metal halide perovskite nanocrystals (NCs) have attracted much attention because of their excellent optical properties and potential for application in optoelectronic devices. However, their photo- and thermostability are still practical challenges and need further optimization. Here, we have studied the degradation behaviors of CsPbI₃ NCs utilized as optical conversion layer in InGaN based blue micro-LEDs in situ. Furthermore, the effects of temperature and light irradiation on perovskite NCs were investigated respectively. The results indicate that both blue light irradiation and high temperature can cause the increased nonradiative recombination rate, resulting in the degradation of perovskite NCs and reduction of the photoluminescence quantum yield (PLQY). Especially in high-temperature condition, both the single-exciton nonradiative recombination rate and the biexciton nonradiative recombination rate are increased, causing the significant reduction of PLQY of perovskite NCs in high temperature environment than blue light irradiation. Our work provides a detailed insight about the correlation between the light irradiation and temperature consequences for CsPbI₃ NCs and may help to pave the way toward optoelectronic device applications.

Quantized Exciton Motion and Fine Energy-Level Structure of a Single Perovskite Nanowire

The quantum-confinement effect profoundly influences the exciton energy-level structures and recombination dynamics of semiconductor nanostructures but remains largely unexplored in traditional one-dimensional nanowires mainly due to their poor optical qualities. Here, we show that in defect-tolerant perovskite material of highly luminescent CsPbBr₃ nanowires, the exciton's center-of-mass motion perpendicular to the axial direction is severely confined. This is reflected in the two sets of photoluminescence spectra emitted from a single CsPbBr₃ nanowire, each of which consists of doublet peaks with linear polarizations perpendicular and parallel to the axial direction. Moreover, different exciton states can be mixed by the Rashba spin-orbit coupling effect, resulting in two single photoluminescence peaks with linear polarizations both along the nanowire axis. The above findings mark the emergence of an ideal platform for the exploration of intrinsic one-dimensional exciton photophysics and optoelectronics, thus bridging the long-missing research gap between the well-studied two- and zero-dimensional semiconductor nanostructures.

Optical Probing of Crystal Lattice Configurations in Single CsPbBr3 Nanoplatelets

Quantum-confined nanostructures of CsPbBr₃ with luminescence quantum efficiencies approaching unity have shown tremendous potential for lighting and quantum light applications. In contrast to CsPbBr₃ quantum dots, where the fine structure of the emissive exciton state has been intensely discussed, the relationship among lattice orientation, shape anisotropy, and exciton fine structure in lead halide nanoplatelets has not yet been established. In this work, we investigate the fine structure of the bright triplet exciton of individual CsPbBr₃ nanoplatelets by polarization-resolved micro- and magnetophotoluminescence spectroscopy at liquid helium temperature and find a large zero-field splitting of up to 2.5 meV. A unique relation between the crystal structure and the photoluminescence emission confirms the existence of two distinct crystal configurations in such nanoplatelets with different alignments of the crystal axes with respect to the nanoplatelet facets. Polarization-resolved experiments eventually allow us to determine the absolute orientation of an individual nanoplatelet on the substrate purely by optical means.

Exciton linewidth broadening induced by exciton-phonon interactions in CsPbBr3 nanocrystals

Quantum dephasing of excitonic transitions in CsPbBr₃ nanocrystals has been studied using two-dimensional electronic spectroscopy at cryogenic temperatures. The exciton-phonon interactions for acoustic and optical modes exhibit different effects on the coherent dynamics of excitonic transitions. The homogeneous linewidth shows a proportional dependence on the temperature, suggesting the primary dephasing channel of the elastic scattering between exciton and acoustic modes. The exciton-optical mode interaction is manifested as the beatings of off-diagonal signals in the population time domain at the frequencies of 29 and 51 cm^-1, indicating phonon replicas of excitonic transitions arising from coherent exciton-phonon interaction. The insight information of exciton homogeneous broadening in perovskite nanocrystals is essential for the potential application of quantum light sources.