Optical Studies Of The Phase-segregation Effect In Mixed-halide Perovskite Nanocrystals

Semiconductor mixed-halide perovskites have attracted a lot of research attention very recently,owing to their large absorption coefficients and tunable energy bandgaps across almost the whole visible-light spectral range,which are beneficial for the potential applications in solar-cell and light-emitting devices.However,the mixed bromide-iodide perovskites would be divided into the bromide-and iodide-rich regions under continuous light illumination.The iodide-rich region,with a lower energy bandgap,is capable of capturing the photo-generated charge carriers to reduce the open-circuit voltage and to yield red-shifted fluorescence in the respective solar-cell and light-emitting applications.For the further advancement of mixed-halide perovskites in the above optoelectronic devices,it is imperative for the researchers to figure out the underlying mechanisms of this phase-segregation effect and to design feasible routes for its complete suppression.It should be noted that the phase segregation effect widely reported in bulk mixed-halide perovskites normally occurs at the nanometer scale.From the perspective of their low-dimensional counterpart of colloidal nanocrystals(NCs),we can provide a more comprehensive view for understanding the phase separation effects.In this dissertation,we have synthesized mixed-halide Cs Pb(Br_1-xI_x)₃(0<x<1)NCs and characterized them at both room and cryogenic temperatures by means of single-particle and ultrafast spectroscopic techniques,with emphasizes not only on understanding the phase-segregation effect but also revealing novel photophysical properties.1.We synthesize Cs Pb(Br_1-xI_x)₃ NCs and perform transient absorption and time-resolved fluorescence measurements on their high-density films,showing how the variation in the mixed-halide ratio influences the phase segregation process.According to the time-resolved fluorescence measurements,we observe a blue shift of the PL peak due to the sublimation of I₂ from the NCs with a variation of the x values from 0.2-0.8.And as the concentration of Br increases(the value of x decreases),the phase segregation rate would be accelerated.For the samples with the x?0.5,we observe two photo-induced bleaching peaks from the transient absorption measurement,implying that Cs Pb(Br_1-xI_x)₃ NCs films have already been separated into the pure-bromide and mixed bromide-iodide regions.Upon light illumination to provide the driving force of either local electric field or large polaron,the phase segregation process of the mixed bromide-iodide regions would be accelerated.2.We utilize the light illumination to change the energy bandgap of mixed-halide Cs Pb(Br_1-xI_x)₃ NCs,and the time-delayed fluorescence from the Cy3 dye molecules can be observed due to the F(?)rster-type energy transfer.Under the circumstance of phase segregation,the emission spectrum of donor Cs Pb(Br_1-xI_x)₃ NCs would overlap with the absorption spectrum of acceptor Cy3 dye molecules,so that the latter would accept the exciton energy from the former and emit fluorescence.When the light illumination is removed,the bromide and iodide ions in the Cs Pb(Br_1-xI_x)₃ NCs would remix to restore the original emission spectrum,which can be manipulated to realize reversible energy transfer processes of multiple cycles.By the special design of the distribution of dye molecules,we can utilize illumination to realize the dynamic display of some special patterns,which provides a unique technical means for the design of anti-counterfeiting.3.By investigating the optical properties of single Cs Pb(Br_1-xI_x)₃ NCs at the room temperature and the cryogenic temperature,respectively,we have revealed the existence of multiple quantum dots due to the aggregation of iodide ions.At the room temperature and before phase segregation,a single Cs Pb(Br_1-xI_x)₃ NC emits low-purity single photons with the PL flickering effect(wihout on-off states);after phase segregation,the resulting single Cs Pb Br₃ NC starts emitting high-purity single photons with the PL blinking effect(with on-off states).At the cryogenic temperature,multiple sets of PL spectra can be simultaneously observed from a single Cs Pb(Br_1-xI_x)₃ NC with correlated variations in their intensities and wavelengths.The above observations strongly suggest that multiple potential-energy minima could be present inside a single Cs Pb Br_1.2I_1.8 NC to behave like closely-packed single quantum dots for the respective localization and recombination of photo-generated excitons.To summarize,we have employed single-particle and ultrafast spectroscopic techniques to characterize single and ensemble Cs Pb(Br_1-xI_x)₃ NCs at both room and cryogenic temperatures,in order to provide a deeper understanding of the underlying mechanisms for phase segregation.Although this seemingly annoying effect should be mitigated in the relevant device operations,we point out that it can be positively utilized to realize time-controlled display of some image patterns combined with the energy transfer process,thus providing a unique approach for the design of counterfeiting icon and secure-information protection.We have also obtained solid evidences for the existence of multiple quantum emitters inside a single Cs Pb(Br_1-xI_x)₃ NC,which not only is related to the origin of the phase segregation effect but also will promote the development of a potent platform for manipulating the mutural interactions of multiple quantum bits.