Understanding Optical Behaviour of CdSe based Core/Gradient-Alloy-Shell/Shell Quantum Dots through Ultrafast Dynamics and Single Particle Spectroscopy

Roy, Debjit (2018) Understanding Optical Behaviour of CdSe based Core/Gradient-Alloy-Shell/Shell Quantum Dots through Ultrafast Dynamics and Single Particle Spectroscopy. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Optically superior (high photoluminescence quantum yield (PLQY), high molar extinction coefficient) and chemically robust (high photochemical stability) semiconductor quantum dots (QDs) which can be synthesized in large scale with easier and reproducible chemical synthesis route, are of very high demand for potential applications like single particle tracking and single photon emission. These specialized applications requires stable emission at the single particle level for significantly longer durations i.e. PL blinking, blueing and bleaching needs to be stopped or significantly suppressed. An otherwise bright and photostable core/shell (CS) QDs at the ensemble level could not serve the purpose at the single particle level. Very recently core/gradiently alloyed shell/shell (CAGSS) QDs shows tremendous potential as a superior optical probe at the ensemble level. The optical properties (molar extinction coefficient, PLQY, photostability) are at par (if not superior) with conventional core/shell QDs like CdSe/CdS or CdSe/ZnS at the ensemble level. It has been argued in literature that, for CGASS QDs, smoothening of lattice parameters at the interface results in reduced population of defect traps and softening of confinement potential causes enhanced confinement for the photogenerated charge carriers as well as reduction of non-radiative Auger recombination (AR) processes. These arguments suggest that CGASS QDs should in principle be able to suppress PL blinking, blueing and bleaching processes, however no such detailed study existed in literature. Moreover ‘one-pot one-step’ synthesis scheme for these CGASS QDs offers opportunity for large scale synthesis in a much reproducible manner which is not possible for conventional SILAR based synthesis technique for core/shell QDs. Very little is known about the excited state PL decay dynamics (electron-hole recombination dynamics) of these CGASS QDs. For core-shell QDs, compositional similarity do not ensure similar optical behaviour. There exists discrepancies in understanding the PL decay dynamics for core-shell QDs. Proper understanding of PL decay dynamics of CGASS QDs is clearly lacking which is very important for optoelectronic and photovoltaic applications. It is also necessary to correlate spectral and temporal optical properties of CGASS QDs at the ensemble as well as at the single particle level. At the single particle level efficacy of gradient alloying towards suppression of undesired processes (blueing, bleaching and blinking) is worth probing. Effect of gradiently alloyed shelling on PL blinking dynamics has not been explored in details too. Apart from that in literature there are very few reports regarding correlation between the optical behaviour at the ensemble and at the single particle level. It would also be interesting to probe the effect of varying degree of radiative recombination on PL blinking dynamics with CGASS QDs having different PLQY. Thus, at the beginning of this thesis work, the quest for understanding the photophysics of CGASS QDs in detail has been indispensable, so that the potential of this material in practical applications can be fully explored. In the first chapter of the thesis QD has been introduced, with the description of quantum confinement effect. The development of core to core/shell to core/shell/shell QDs has been discussed and a brief overview of previous literature work related to photoluminescence spectroscopy of QDs both at ensemble and single particle level along with ultrafast exciton dynamics. Basic introduction about CGASS QDs has been provided. In the second chapter, details of one-step one-pot synthesis scheme has been described along with the characterisation of the materials. The instrumentation details of steady state and time resolved optical measurements at the ensemble level and single particle level have been provided. Analyses and fitting of the intensity decay data at the ensemble level and analysis of the single particle blinking dynamics also have been described. For single particle measurement a home build Confocal and TIRF setup have been used. For ultrafast measurements a femtosecond based pump-probe transient absorption setup has been used. In the third chapter, correlation between spectral and temporal optical behaviour of differently emitting CdSe based CGASS QDs have been shown. Highly luminescent (PLQY as high as 0.96) QDs have been synthesized in “one pot” using the reactivity difference between Cd & Zn precursors and Se & S precursors. All differently emitting (blue to green to orange to red) CGASS QDs exhibit similar spectral and temporal optical behaviour in a reproducible manner. The excited-state decay dynamics has been explained with a simple three-level model. Pico-nano second excited state dynamics and steady state optical behaviour (PLQY) could be successfully correlated and from a multi-exponential decay trace, the role of different time constants of the PL decay at the ensemble level can be understood reasonably well. In the fourth chapter, optical behaviour of different colour emitting single CGASS QDs have been shown. These CGASS QDs could successfully suppress undesired processes at the single particle level. Elimination of blueing and bleaching have been achieved. Different colour emitting CGASS QDs are shown to be extremely photostable at the single particle level for 1 h or even longer even under air atmosphere. Significantly suppressed PL blinking behaviour with average size less than 7 nm and emitting in entire range of visible spectrum i.e. from blue to green to orange to red could be achieved in CGASS QD systems. Single particle level investigation has been successfully correlated with the ensemble level property. The enhancement of radiative recombination rate and the suppression of non-radiative Auger recombination rate have been observed to be consonant with each other. The orange emitting CGASS QD has the maximum near unity PL QY, i.e. with highest degree of radiative recombination rate and most suppressed non-radiative Auger recombination rate. In the fifth chapter, excitation energy dependent variation of PL efficiency (PLQY) for CGASS QDs have been shown. With increasing excitation energy above band edge the PLQY decreases. The reasons behind this apparent violation of ‘Vavilov’s rule’ have been investigated employing ultrafast dynamics at the ensemble level in combination with PL blinking dynamics at the single particle level. Decreasing extent of hot electron trapping, and increasing extent of hot hole trapping towards the band edge lead to enhanced PLQY for lower energy excitations. While comparing excitation energy dependent variation of PL efficiency for CGASS QDs and CS QDs, it could be shown that for similar variation of excitation energy the variation in PLQY is only ~ 1.6 fold (from ~ 0.90 to ~ 0.56) for CGASS QDs in comparison to 9 fold (from ~ 0.92 to ~ 0.10) for CdSe/ZnS CS QDs. This observation indicates that CGASS QD could provide improved confinement for the photogenerated charge carriers from trapping throughout the excitation energy landscape in comparison to CS QDs and thus, optical superiority of CGASS QD over CS QDs could be evidenced. In the sixth chapter a brief overview of the conclusions drawn in chapter third, fourth and fifth have been mentioned. In addition, a few still unresolved questions have been delineated. Possible experiments to shed light in the directions from which answers for those questions can be obtained have been elaborated.

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