Optical Properties Of Single And Clustered Perovskite Nanocrystals

Owing to the fast developments of both quantum theories and nanotechnologi-cal methods,semiconductor nanocrystals(NCs)with the spatial confinements along all three dimensions have attracted a lot of research attention even since the 1980s.By means single-particle spectroscopy and single-photon detection techniques,as well as the external control knobs of temperature,electric field and magnetic field,the energy-level structures and recombination dynamics of band-edge excitons have been fundamentally studied.Meanwhile,semiconductor NCs are now being widely used in a variety of optoelectronic devices,such as light-emitting diodes,solar cells,photo detectors,lasers and quantum information technologies.Inspired by the superior per-formance of semiconductor lead-halide perovskite films in the solar cell devices,their low-dimensional nanostructures were successfully synthesized in 2015 as a novel type of semiconductor NCs.These perovskite NCs possess large absorption cross sections,high photoluminescence quantum yields and tunable light wavelengths across the vis-ible range,and thus have become a potent competitor of traditional colloidal NCs in both fundamental research and device applications.In this dissertation,we first review recent progress made on the optical studies of semiconductor perovskite NCs at both the ensemble and single-particle levels.Then,after discussing the exciton photophysics and the intrinsic mechanisms for the energy-level fine structures,we will focus on the following three aspects of experimental works.1.We have synthesized mixed-cation(FA/Cs)Pb I₃NCs,and performed single- particle characterization of their optical properties.At room temperature,a sin- gle(FA/Cs)Pb I₃NC demonstrate high-purity single-photon emission(2)⁽²⁾(0)?0) and suppressed PL blinking behavior,together with an absorption cross section of 1.70×10^-13cm²that is much larger than that of traditional colloidal NCs.At the cryogenic temperature of 4 K,the Auger recombination of charged excitons is strongly suppressed in a single(FA/Cs)Pb I₃NC.Meanwhile,their exciton fine- structure splittings can be more easily observed as compared to the Cs Pb X₃NCs, suggesting the important role played by the mixed cations in reducing the symmetry of underlying crystal structures.2.After understanding the optical properties of single perovskite NCs,we have utilized the single cluster structure to explore how they would interact with each other. Such an individual cluster is prepared by the atomic force microscopy(AFM) nanoxerography,which shows discrete PL bands from the composing Cs Pb Br₃ NCs.Interestingly,the short-and long-wavelength PL bands exhibit different optical behaviors,with the sub-and super-linear dependences on the laser excitation power, respectively.This is explained by the generation of charged excitons at the high laser excitation powers,which can migrate from smaller-to larger-sized Cs Pb Br₃ NCs through a Dexter-type energy transfer process.3.In addition to the optical characterization of single perovskite NCs in the free space, we have investigated how their optical properties are modulated by an optical cavity. By combining a metallic mirror and one layer of dielectric materials with a thickness of 800 nm,we show that single perovskite NCs on top of this half-open nanocavity have a decreased PL lifetime and a blue-shifted PL spectrum.We propose that the electromagnetic modes have been changed near the nanocavity,resulting in a Purcell effect that can effectively modify the emission characteristics of the NC dipoles.