Doped/Alloyed Perovskite Nanocrystals: Exploring Steady State Optical Behaviour and Exciton Dynamics

Mukherjee, Soumen (2025) Doped/Alloyed Perovskite Nanocrystals: Exploring Steady State Optical Behaviour and Exciton Dynamics. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Soumen Mukherjee (19RS018)) 19RS018.pdf - Submitted Version Restricted to Repository staff only Download (18MB)

Abstract

Perovskite nanocrystals (PNCs) are new class of materials having wide range of applications towards LEDs, solar cells etc. CsPbI₃ PNC has longer range of absorption spectrum than CsPbBr₃ PNC but the former possess low photoluminescence quantum yield (PLQY) and suffers from lack of long term stability. Moreover, there is a strong quest regarding enhancing the brightness of PNCs, and there is no known channel other than enhancing the PLQY. Additionally there were no serious investigations regarding the ultrafast exciton dynamics and ultrasensitive single particle spectroscopic behaviour of CsPbI₃ PNC. Furthermore, application potential of Mn doped PNC, specially towards suitability as resonance energy transfer donor had not been tested. Through Zn alloying and suitable surface passivating ligand modification, enhanced PLQY and stability could be achieved for CsPbI₃ PNC. Through accurate estimations of (i) the concentrations of the ions, employing inductively coupled plasm mass spectrometry (ICP-MS), and (ii) structural information employing transmission electron microscopy (TEM) and (iii) optical information through steady state absorption, extremely high values of molar extinction coefficient (~2 × 10⁸ M⁻¹cm⁻¹), intrinsic absorption coefficient (5.25 × 10⁵ cm⁻¹) and absorption cross-section (7.72 × 10⁻¹³ cm²) have been achieved for Stearic Acid Coordinated Zn Alloyed Perovskite (SACZAP). Additionally this PNC exhibit near unity PLQY (0.98). A combination of highest molar extinction coefficient and near unity PLQY make SACZAP PNC the most super-bright (brightness = ~2 × 108 M⁻¹cm⁻¹) nanomaterial in the entire perovskite and quantum dot family. Employing ultrafast dynamics on this super-bright SACZAP PNC, following interesting observations have been made: with increasing excitation wavelength (λex) from 390 nm to 630 nm, (i) hot carrier cooling time decreased (from 756 fs to 107 fs), (ii) amplitude of charge carrier trapping decreased from 0.74 to 0.41, whereas, (iii) amplitude of radiative recombination increased from 0.26 to 0.59, (iv) normalized bleach amplitude (-λA/A) increased from 0.010 to 0.031. Upon probing SACZAP PNC at the single particle level, with increasing λex from 405 nm to 647 nm, (v) peak maximum of ON fraction increased from 64% to 90%, (vi) ON truncation time increased from 3.6 s to 8.3 s, and (vii) detrapping rate/trapping rate (RD/T) increased from 1.0 to 3.3. Change in the magnitudes of all these parameters could be correlated with excitation wavelength dependent PLQY. Thereby, spectroscopic properties from (a) steady state, (b) ultrafast dynamics and (c) ultrasensitive single particle spectroscopy could be correlated with each other. Such a correlation is quite rare in literature. Significant memory in the single particle blinking behaviour, minimal heterogeneity in the single particle blinking mechanism and nearly ergodic optical behaviour have been observed for SACZAP. Zn alloyed CsPb(Cl/Br)₃ PNC has also been synthesized and used as a model system for Mn doping in order to understand the effect of Mn doping on the exciton dynamics. Zn alloyed Mn doped CsPb(Cl/Br)3 mixed halide PNCs with dual emission (host and Mn dopant emission) have been synthesized. High PLQY (0.62) and narrow FWHM of the host PL (25 nm) could be achieved for the PNC having host PL emission maximum at 450 nm (PNC 450). Upon keeping nearly same PL emission maximum and PLQY for both the PNC samples (with and without Mn doping), radiative decay rate for the host band decreased 7 times whereas non-radiative decay rate increased 3 times, upon Mn doping. Lower value of Huang Rhys factor (4 ± 0.5) and higher value of phonon energy (58 ± 2 meV) in Mn dopant system, in comparison to undoped system indicates lesser electron phonon coupling for the Mn doped PNC. Temperature dependent dopant PL decay exhibited biexponential decay behaviour with the time constants τ1 = 450-540 μs and τ2 = 1.1-1.2 ms. Single particle analyses revealed that upon Mn doping, probability density distribution against time for both ON and OFF event durations could be fitted with simpler power law equation, unlike the truncated power law equation necessary for the undoped one. All these observations clearly demonstrate that Mn doping transforms the exciton dynamics of the PNC very significantly. Upon obtaining extraordinary optical properties of the PNCs it was necessary to explore their application potential as for example suitability as resonance energy transfer donor. External energy transfer (EET) and its associated mechanism, using oxazine 725 perchlorate (OXZ 725) dye (as an acceptor) and Mn dopant emission as donor, for PNC 450, have been investigated. With increasing concentration of OXZ 725, the integrated intensity of the Mn dopant PL band decreased, whereas, the integrated intensity of the PL band at ~670 nm increased due to EET process from Mn dopant (donor) to OXZ 725 (acceptor). Both static and dynamic quenching have been observed for this donor-acceptor pair. The magnitude of the static and dynamic quenching constants have been calculated to be 1.65x 10⁵ M⁻¹ and 5.63 x 10⁴ M⁻¹, respectively. Not only the integrated intensity but also the average lifetime of the Mn dopant in presence of OXZ 725 decreased. PL decay of the acceptor exhibited a rise followed by a decay, clearly confirming the EET phenomenon from the donor to the acceptor. This external energy transfer follows the FRET mechanism and this can be concluded from high value of spectral overlap (6.81 × 10¹⁵ M⁻¹cm⁻¹nm⁴), acceptable Förster distance (5.89 nm) and high RET efficiency (0.71) between donor (Mn dopant) and acceptor (OXZ 725).

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