Spectroscopy and Exciton Dynamics of Toxic-Metal-Free CuInS₂ Core and Core/Alloy-Shell QDs

Ghosh, Swarnali (2024) Spectroscopy and Exciton Dynamics of Toxic-Metal-Free CuInS₂ Core and Core/Alloy-Shell QDs. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Swarnali Ghosh (17RS041)) 17RS041.pdf - Submitted Version Restricted to Repository staff only Download (18MB)

Abstract

Quantum dots (QDs), emitting across the whole visible to NIR region of the electromagnetic spectrum, possess near unity photoluminescence quantum yield (PLQY), high molar extinction coefficient, superior photostability as well as ambient stability both at the ensemble and single particle level. Such QDs which can be synthesized with easier, low cost and reproducible procedure, and most importantly which are toxic and heavy metal free, are of topical interests as a light harvester and light emitter in modern day research in various fields of sustainable applications such as light emitting diodes (LEDs), solar cells, bioimaging, catalysis, single particle tracking etc. As a choice of toxic metal free QD, CuInS2 based ternary I-III-VI QDs hold the promise to be the most suitable one due to its large Stokes shifts, longer PL lifetime, low toxicity of the constituent materials and low cost synthesis. However, PLQY of core CuInS2 QD is very less which can be attributed to the exciton trapping at the surface defects. By making Core/Shell (CS) or Core/Shell/Shell (CSS) QD, although PLQY could be enhanced to a high value but near unity PLQY could not be achieved. Therefore, achieving near unity PLQY with extremely stable and toxic metal free QD is quite urgent and highly challenging too. Although suppressed PL blinking has been achieved with CdSe based QDs, but there is a strong quest of simultaneously achieving near unity PLQY as well as suppressed blinking in a small sized, extremely stable, toxic metal free QDs. Furthermore, suitability of QDs for several applications have mostly been investigated from electron trapping−detrapping points of view. Although intrinsic (i.e., without adding any external hole trapping material) hole-trapping phenomenon could be investigated employing transient absorption technique for CdSe QD system due to the presence of suitable energy levels, however, such measurements could not be performed for other QDs owing to the absence of suitable energy levels. In recent studies ultrasensitive single particle spectroscopy has been considered as a highly suitable technique to study the intrinsic hole-trapping dynamics for CdSe or InP based QD systems. However, the extent of intrinsic hole-trapping phenomenon in CdSe and InP QDs has been reported to be quite minimal (less than 10%). It is quite important to investigate the question whether, unlike detrimental electron trapping, the intrinsic hole-trapping phenomenon could be beneficial or not for increasing PLQY. How significant is the extent of intrinsic hole trapping and its amplitude variation are quite important to investigate. All these concerns need to be explored in detail. Thus, at the beginning of this thesis work, the detailed understanding regarding the photophysics of the QD is quite necessary so that the potential of such material in practical applications can be understood reasonably well. As a model QD system, CuInS2 based core and Core/Alloy-Shell (CAS) QDs have been chosen. In the first chapter of this thesis, the introduction of QDs and the effect of quantum confinement on the electronic structure of QDs have been described. The gradual development from core to CS to CSS to unique CAS QDs has been discussed. A brief overview of previous literature works related to the optical spectroscopy of QDs both at the ensemble and the single particle level along with ultrafast exciton dynamics, have been delineated. In the second chapter, the detailed “hot-injection one-pot” synthesis procedure (using Schlenk line) have been described. Elemental and structural characterizations of the synthesized CuInS2 core and CuInS2/ZnSeS CAS QDs have been elaborated. Detailed descriptions of the instruments and techniques employed in the work incorporated in the thesis, specially the steady-state, ultrafast and fast time-resolved optical spectroscopic studies at the ensemble level, and total internal reflection fluorescence (TIRF) based single particle spectroscopic investigations have been described. Different kinds of analyses and fitting of the intensity decay data at the ensemble level and analyses of the single particle spectroscopic data have also been elaborated. For ultrafast measurements at the ensemble level, a femtosecond pump-probe transient absorption setup has been used and for the ultrasensitive single particle measurement, a home build TIRF setup has been used. In the third chapter, ultrafast exciton dynamics of CuInS2 core QD has been explored in detail addressing the discrepancies in the ultrafast dynamical investigations reported in literature. From fast pico-nanosecond as well as slow microsecond based time resolved emission spectra and time resolved area normalized emission spectra (TRES-TRANES) analyses, presence of isoemissive point indicates towards the presence of more than one excited states and biexponential nature of the excited state interband dynamics measured employing spectral shift correlation function describes the two different decay channels from the initial Franck Condon excited state to the relaxed excited state. Detailed exciton dynamics (i.e. hot exciton cooling, hot exciton trapping etc.) as well as the position of the intermediate states in between valence band (VB) and conduction band (CB) and the time scales for various related excitonic processes have been explained from the ultrafast pump-probe transient absorption spectroscopic analyses. Position of the intermediate states were further supported from the optical spectroscopic studies at the cryogenic temperature. In the fourth chapter, near-unity PLQY (96%) and highly suppressed photoluminescence blinking (>80% ON fraction) have been achieved simultaneously in ultra-small (size ~3.3 nm), extremely stable (under 24 hr continuous UV illumination as well as in ambient atmosphere for more than one year without significant reduction of PLQY), toxic metal free CuInS2 based CAS QD. Suitable schematic models explaining all these unique properties have also been described. In the fifth chapter, for CuInS2 based CAS QD system, both intrinsic hole trapping as well as hole detrapping phenomena have been reported. Such a report is first of its kind for any QD or perovskite nanocrystal (PNC) system. Very significant amplitude variation of both hole trapping (~16 to ~42 %) as well as hole detrapping (~44 to 23%) have been observed. Unlike detrimental electron trapping, hole trapping has been shown to be quite beneficial towards increasing the magnitude of PLQY to 96%. Simultaneous trapping of both electron and hole leads to the long ON time (~130 s) without blinking for a toxic metal free QD. Such a value is the highest ON time reported so far for any non-toxic metal based nanomaterials (QDs or perovskites). Suitable schematic models explaining all these unique observations have been described. Fabrication of highly bright, colour pure, highly stable (retention of ~95% of emission upon 12 hr continuous illumination) yellow and white QD-LEDs using low-cost “Arduino-Uno” (micro-controller board) based method have been depicted. In the sixth chapter, a brief overview of the conclusions drawn in third, fourth and fifth chapters have been mentioned. In addition, a few still unresolved questions regarding different optical spectroscopic aspects have been delineated. Possible methods or experiments to be carried out to shed light on the pathways from which answers for those unresolved questions can be obtained have been elaborated. Some potential and technical biological applications that can be performed in recent future with this highly luminescent toxic metal free CuInS2 based CAS QD have been proposed.

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