The Auger Recombination And Fine Structure Of Bright Excitons In Single CsPbI₃ Nanocrystals

After nearly a decade of development,perovskite solar cells have achieved an increment from 3.8%to 24.2%in the effiency and a working stability up to thousands of hours.Moreover,the blue,green and red perovskite LEDs have reached an external quantum efficiency of 5.7%,-20%and-20%respectively,together with the high fluorescence quantum yield.In addition,perovskite lasers with low threshold and high quality factor have been implemented and the X-radiation detectors made by halide perovskites possess lots of excellent performances,such as large area,low dose and high sensitivity.These series of breakthroughs of halide perovskites have profound physical connotations actually,such as the Rashba effect,the formation of large polarons and the ferroelectric domains,which may be related to the long diffusion length of charge carriers.Combining the quantum confinement with the high quantum yield,tunable wavelength and low defect concentration in their bulk materials,all-inorganic perovskite nanocrystals(NCs)have emerged from 2015 as a new structure of halide perovskites.In addition to the traditional optoelectronic field,they also show great potentials in the field of quantum optics and quantum information.At present,coherent single photon emissions without fluorescence blinking and spectral diffusion have been observed in single perovskite NCs.The stable fine-structure splittings and high-yield negative trions have also been revealed at low temperature.On the other hand,they have provided a valuable platform for studying the physical connotation of halide perovskites.In this paper,we study the underlying mechanisms of nonblinking fluorescence at room temperature and suppressed spectral diffusion at low temperature,the origin of fine-structure splittings of bright excitons and the effect of quantum confinement on the energy-level structuresAlthough traditional CdSe NCs can emit single photons at room temperature,they can be affected by the constant switching of fluorescence intensity.Researchers have devoted great efforts for more than a decade to achieve suppressed fluorescence blinking in CdSe NCs.Although the fluorescence blinking has been almost completely eliminated in large-sized CdSe/CdS core/shell NCs,the photoluminescence energy at low temperatures still varies frequently,which is also known as spectral diffusion.In the second chapter,we mainly introduce the fluorescence properties of single CsPbI3 NCs at room temperature and low temperature(4K).When excited at low laser power,single NCs exhibit blinking-free fluorescense at room temperature.When the laser power is increased,there exist changes in the fluorescence intensity,corresponding to'bright state','grey state' and 'dark state',respectively.The bright state mainly comes from the contribution of neutral excitons and negative trions,while the lifetime of neutral excitons is twice that of negative trions,indicating that the Auger recombination of negative trions is almost suppressed.What's more,the gray state may originate from positive trions or negative doubly charged excitons,while the dark state may be derived from higher order excitons.At cryogenic temperature and with low-power excitation,a fluorescence linewidth of-200?eV is observed in single CsPbI3 NCs,which has reached the limit of our system resolution.In addition,the spectral diffusion has been effectively suppressed.Self-assembled quantum dots are widely used in the field of quantum optics and quantum information due to their stable energy-level splitting and rich exciton physics at low temperature,while traditional CdSe NCs have been highly anticipated in both fundamental physics and device applications due to their free-standing properity However,severe spectral diffusion at low temperature limits their application and development.In the third chapter,we mainly introduce the stable bright exciton fine-structure splitting in single CsPbI3 NCs.The neutral excitons in single CsPbI3 NCs exhibit two orthogonally and linearly polarized states with an energy separation as large as hundreds of ?eV.When the laser power is gradually increased,the fluorescence peaks of negative trions,neutral biexcitons,negative biexcitons and doubly negative charged excitons can be additionlly seen.The energy-level splittings of the above various excitons have provided useful information for the isotropic and anisotropic electron-hole exchange interactions.Moreover,these fingdings have also opened the prologue for the application of single CsPbI3 NCs in quantum optics and quantum informationBased on strong spin-orbit coupling,two possible origins with or without the Rashba effect have been suggested to account for this bright-exciton fine-structure splittings manifested as either doublet or triplet PL peaks in single caesium lead-halide NCs.Thus,it is necessary to further depict its physical image based on the experimental results and theoretical analysis.In the fourth chapter,we mainly introduce the influence of changed quantum-confinement effect on the energy-level structure.The size of CsPbI3 NCs decreases slowly in weakly polar solvents,causing by a blue shift in the position of fluorescence peak.The energy-level structure of neutral excitons also change from the initial doublet splitting to final triplet splitting with a larger energy separation.As the non-radiative Auger recombination gradually dominates,the quantum yield of negative trions also gradually decreases.Based on the above findings,we propose a theoretical model where the enhancement of quantum confinement leads to the transition of the neutral excitons from doublt splitting to triplet splitting.Until now,this quantum confinement effect has been largely nelegected in describing the electronic energy level structure of single perovskite NCs,which should play an important role in their future studies by the research community.