Investigation Of The Properties Of Multi-excitons In Single Semiconductor Nanocrystals

Semiconductor nanocrystals?NCs?are a kind of nanometer-size semiconductor crystal consisted of approximately 100-10000 atoms with a typical size of 1-10nm.The size of NCs is comparable to or even smaller than the Bohr exciton radius,so NCs are quantum-confined materials.Due to the quantum confinement effect,NCs exhibit intriguing properties,such as spectral blue shift and energy-level quantization.Spectral blue shift is proportional to the level of quantum confinement,which provides great flexibility in the engineering of band structure by controlling the size and geometry of NCs.Because of relaxation of the momentum conservation induced by quantized energy levels,in addition to the enhanced Coulomb interaction and wave function overlap between carriers in nanometer-size NCs,Auger process is extremely efficient in NCs.Fast nonradiative Auger recombination suppresses radiative recombination of multi-excitons,leading to single photon emissions from single NCs.At the same time,efficient Auger effect facilitates impact ionization process,which increases the efficiency of carrier multiplication?CM?in NCs.All the above properties make NCs promising materials for various optoelectronics applications.For LED lightings,color-tunable light can be realized by quantum confinement effect of NCs to produce high quality white-light source;for quantum information applications,semiconductor NCs are a good choice for room temperature single photon source;for solar cell and photon detector,the highly efficient CM in NCs can promote the absorbed photon to electron conversion efficiency.In this thesis,we focus the properties of multi-excitons in single NCs,including studies of single photon emission,photoluminescence?PL?intermittency,spectral diffusion of single CsPbBr₃ NCs and charged two-exciton recombination,CM of single CdSe NCs.In chapter ?,we studied the optical properties of single CsPbBr₃ NCs.We 1 synthesized CsPbBr₃ NCs with average size of 9.4nm by a chemical approach.Since the size is comparable to the Bohr radius,CsPbBr₃ NCs belong to quantum-confined regime.At room temperature,the PL of single CsPbBr₃ NCs switched randomly between the "on" and "off" states,which can be attributed to random formation of charged excitons and the associated nonradiative Auger recombination.The same Auger process also greatly suppressed the radiative recombination of multi-excitons,leading to single photon emission from single NC confirmed by G?2??t?measurement.Owing to the emission wavelength can be tuned by quantum confinement effect and compositional control?mixed halide Cl/Br and Br/I systems?,perovskite NCs are competitive candidates for single photon source.At cryogenic temperature,spectral diffusion was observed in single NCs,which generated a relatively broad 1meV PL linewidth.While the optical properties of CsPbBr₃ NCs are similar to conventional metal-chalcogenide NCs,the absorption cross section of CsPbBr₃ NCs is two orders larger,and much faster radiative recombination of CsPbBr₃ NCs enable higher emission rate of single photon.In chapter ?,we applied time correlated single photon counting?TCSPC?to study charged two-exciton in single CdSe NCs.Due to the Auger process and trapping site,NCs are easy to be charged.When charged NCs absorb two photons simultaneously,charged two-exciton state is formed.Then Auger process governs the sequent recombinations of charged two-exciton and charged exciton.Here we measured PL time trajectories of single elongated CdSe NCs.Under high power laser excitation,an intermediate "grey" level appeared in addition to the normal "on" and"off' ones.This "grey" level is attributed to the formation of two excitons in a single NCs and the accompanied ejection of a hole through the bi-exciton Auger recombination process.When two more excitons are subsequently excited in this single NC,the resulting negatively-charged two-exciton state contributes to the PL of the "grey" level,as supported by the observation of an apparent peak at the zero-time delay of the inter-photon correlation histogram.Moreover,the single exponential PL decay of charged two-exciton implies that lifetime-indistinguishable photon pairs are emitted.Due to the disturbance of charged exciton,the CM efficiency of NCs is still a controversial issue.In chapter ?,we performed TCSPC measurement on single CdSe NCs to clarify CM process.For regular semiconductor NCs,the Auger lifetimes of the charged exciton and the neutral bi-exciton are both at the sub-nanosecond time scale,which is significantly shorter than the radiative lifetime of tens of nanoseconds for the single neutral exciton.Since the routine CM measurements of semiconductor NCs are performed exclusively at the ensemble level,the ultrafast techniques employed are incapable of differentiating between the true and false CM signals associated with the Auger lifetimes of the neutral bi-exciton and the charged exciton,respectively.Meanwhile,for TCSPC measurement of single NCs,the photon emission from the neutral bi-exciton and the charged exciton can be extracted easily from the "on" and"off" level of the PL time trajectories respectively.Under 266-nm laser excitation,whose photon energy is 2.46 times of the energy gap of the CdSe NCs,an average CM efficiency of?20.2%can be reliable estimated from the "on"-level PL decay curves of a statistically large number of single NCs.