Exploring Optical Behaviour of All Inorganic Perovskite Nanocrystals through Ultrasensitive Single Particle Spectroscopy and Ultrafast Dynamics

Mandal, Saptarshi (2022) Exploring Optical Behaviour of All Inorganic Perovskite Nanocrystals through Ultrasensitive Single Particle Spectroscopy and Ultrafast Dynamics. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Saptarshi Mandal (14IP023)) 14IP023_DCS.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

In the first chapter of the thesis, quantum dot (QD) has been defined, and PNC has been introduced as a new class of nanomaterial. The effect of quantum confinement in size-dependent optical properties of QDs has been discussed. Defect tolerance nature and halide composition-dependent optoelectronic behaviour of PNCs have been depicted. A brief overview of previous literature work related to PL spectroscopic study of PNCs both at the ensemble and single particle level along with ultrafast exciton dynamics has been provided. In the second chapter of the thesis, the details of Instantaneous, Room Temperature, Open-air Atmosphere (IRTOA) synthesis procedure of all inorganic PNCs have been described. Both the elemental and structural characterization of the PNCs, along with their steady state and time-resolved optical properties, has been depicted. A detailed description of the instrumentations used in the steady state and time resolved measurements at the ensemble and single particle level as well has been provided. A home built total internal reflection fluorescence (TIRF) microscope setup has been used for measurement at the single particle level. Femtosecond-based pump-probe transient absorption (TA) setup has been used for ultrafast charge carrier dynamics measurements. Different statistical analyses related to single particle blinking dynamics and ergodicity in the optical behaviour of the PNCs has been described. In the third chapter of the thesis, excitation wavelength dependent variation of PLQY of CsPbBr3 PNC has been depicted. A decreasing trend in the PLQY value with increasing excitation energy has been obtained. The mechanistic origin of this behaviour in the PNC system has been investigated by employing excitation wavelength dependent ultrafast transient absorption study and excitation wavelength dependent ultrasensitive single particle blinking dynamics study. The decreasing extent of the normalized transient population (ΔA/A) due to slower thermalization and greater extent of trapping at higher energy levels beyond the conduction band minimum (CBM) leads to the decreasing PLQY for higher energy excitations beyond CBM. From the blinking dynamics study, it has been observed that the trapping gets suppressed, and concomitantly the detrapping gets enhanced with increasing the excitation wavelength, thus increasing the PLQY value. Interesting correlations between properties obtained from ultrafast dynamics and steady state as well as ultrasensitive single particle spectroscopy and ensemble measurements have been obtained. While comparing the excitation energy dependent relative drop in PLQY for the PNC system with CdSe and InP based core-alloy-shell (CAS) QD system, it has been observed that the extent of trap states beyond CBM in CsPbBr₃ PNC is significantly less than CdSe and InP based CAS QDs. Thus optical superiority of CsPbBr₃ PNC over CdSe and InP based CAS QDs could be evidenced. The fourth chapter of the thesis deals with the halide content dependent charge carrier dynamics in mixed halide CsPb(Br/Cl)₃ PNCs. For near band edge excitations, an increase in steady state PLQY value has been observed with increasing the relative bromide content in mixed halide CsPb(Br/Cl)₃ PNCs. Furthermore, the relative drop in the PLQY with increasing excess energy from the band edge is less for bromide-rich PNCs. The origin of this interesting observation has been understood in detail by employing ultrafast TA measurement and ultrasensitive single particle spectroscopy. From TA analyses, it has been shown that the magnitude of the normalized band edge population increases more than two-fold because of the reduced trap states with increasing bromide content. With increasing bromide content, the extent of trapping near the band edge decreases and radiative recombination increases and the hot excitons get trapped slower. Measurement at single particle level revealed that as the bromide content increases, most of the particles remain in the ON state. With increasing bromide content, a lower extent of trapping and a greater extent of detrapping of the charge carriers have been observed. Moreover, the magnitude of the Detrapping rate/Trapping rate (RD/T) of the charge carriers increases more than 25 fold with increasing bromide content. In the fifth chapter of the thesis, the ergodicity nature of blinking in the CsPbBr3 PNC system has been explored in detail. A series of multimodal statistical methodologies like the two-sample Kolmogorov Smirnov (KS) test, individual analyses of probability density distribution, Lévy statistical analyses, etc., have been accomplished extensively to understand the presence of near ergodicity and minimal statistical ageing in this PNC system. From KS test analyses, OFF event distributions of CsPbBr3 PNC are found to be statistically more similar than the ON event distributions, unlike CdSe based QD system. Moreover, it has been observed that the OFF → ON switching process is less heterogeneous in nature than ON → OFF switching process. The magnitudes of the blinking dynamics parameters (power law exponents and truncation times) obtained from cumulative analyses matched quite well with the peak value of the normal distribution of these parameters obtained from hundred individual single particles. Time evolution analyses of the fraction of the particles in ON state and total fluorescence intensity of the PNCs have shown a negligible decrease in (a) fraction of the particles in ON state and (b) the total fluorescence intensity of all single particles for an extended period of three minutes. The mean of ON fractions of all the single PNCs has shown a minimal reduction in the value with time. Moreover, the magnitude of relative dispersion (σr) starts from a low value (~0.2) and, with increasing integration time, converges to a near-zero magnitude (0.09). All these values are optically much better than the CdSe QD system indicating that, unlike CdSe QD, the CsPbBr₃ PNC is much robust, exhibiting near ergodicity and minimal statistical ageing. In the sixth chapter, a brief overview of the conclusions drawn in chapters 2, 3, 4, and 5 has been provided. The current status and possible future modifications of PNCs as next-generation material has been delineated. Different optical spectroscopy related unresolved aspects of PNCs have been discussed. Possible ways of investigations to shed light on those aspects have been elaborated. Some potential biological applications with these PNCs have also been proposed.

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