Investigation of Charge-Carrier Dynamics in Organo-halide Perovskite and Colloidal Quantum Dot Semiconductors

Investigation of Charge-Carrier Dynamics in Organo-halide Perovskite and Colloidal Quantum Dot Semiconductors
Author: Robert Stewart
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Release: 2016
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Crystallite surfaces often dominate the optical and electronic properties of nanocrystalline semiconductors because the fraction of atoms at the surface experience different crystal field environments than bulk atoms, and as a consequence, surfaces influence the energetic landscape of the entire crystallite. Contributing electronic states that are either isoenergetic or below the semiconductor band edges, surface states mediate charge conduction and recombination - two critical processes in optoelectronic device performance. Utilizing a combination of inorganic synthesis, surface characterization, and time-resolved optical spectroscopy, the research presented herein begins to identify the link between charge carrier dynamics and the underlying surface chemistry in two emerging, yet promising, nanocrystal semiconductor systems: organo-halide perovskites and colloidal quantum dots (CQD).Notably, my research provided one of the first reports that charge recombination centers in lead halide perovskite films are localized almost exclusively on the surface of crystallites. Passivation of these nanocrystal surfaces with small molecules that contain strongly coordinating functional groups caused charge-carrier lifetimes in perovskite thin-films to approach the bulk radiative limit reported for single crystal analogues. Likewise, my research contributed to an understanding that surfaces in lead sulfide (PbS) CQDs produce electronic energy levels that are sufficiently delocalized to provide charge conduction pathways in CQD thin-film arrays. Given the strong coupling to the QD surface, charge carrier diffusion lengths were shown to be highly sensitive to the character of surface-bound ligands. My PhD research highlights the importance of understanding the interplay between surface chemistry and nanocrystal semiconductor photophysics as wellivas the importance of selecting surface treatment strategies capable of passivating diversesurfaces to eliminate energetic inhomogeneity while simultaneously allowing strongelectronic coupling across interfaces.

Carrier Dynamics and Lasing Applications of Colloidal Quantum Dots

Carrier Dynamics and Lasing Applications of Colloidal Quantum Dots
Author: Golam Bappi
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Release: 2021
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Colloidal quantum dots have seen progress over the last three decades as an active material for solution processed optoelectronics. Quantum dots offer a tunable optical bandgap from the UV to the mid-IR via control over size and chemical composition. Their optical and electronic properties can be further manipulated through surface engineering and heterostructuring. These materials are processed from solution, enabling low-cost fabrication; and are compatible with a wide range of substrates.In this thesis, I investigate properties of colloidal quantum dots for lasing applications. My findings illuminate fundamental processes that determine their performance in lasing; and point to strategies to overcome present-day limitations. First, I investigate the effects of temperatures reached during continuous-wave excitation on the charge carrier dynamics in CdSe/CdS core/shell QDs, and their effect on the lasing threshold. Modelling and experimental characterization reveal a temperature-activated sub- picosecond electron trapping process that depletes the population of excited QDs. Accordingly, a small decrease in the athermal lasing threshold can yield a large decrease in the continuous- wave lasing threshold due to reduced heat generation. In CdSe/CdS QDs, built-in biaxial strain reduces the valence band-edge degeneracy, lowering the athermal and CW lasing threshold by 30% and 70% respectively. Next I investigate graded CdSe/CdS shells on infrared InAs QDs to suppress non-radiative biexciton Auger recombination. Infrared InAs QDs are promising materials for infrared light emitting devices, but their Auger lifetime is much shorter than those found in more widely explored cadmium and lead chalcogenide materials. The graded CdSe/CdS shells on InAs which I develop herein result in a 2x increase in the Auger lifetime relative to the best value reported in prior InAs QD literature. Finally, I propose a method to achieve nanosecond deep-blue lasing using CsPbCl3 QDs. These perovskite quantum dots suffer from fast biexciton Auger lifetimes, and are consequently able to sustain lasing only under femtosecond pulsed photoexcitation. Forming a superlattice of QDs with aligned dipoles, and coupling them to a high Q-factor distributed feedback grating, is a step toward quasi-CW lasing in this materials system. I design the grating for single mode operation within the gain spectrum of the CsPbCl3 QDs.

Charge Carrier Dynamics in Lead Sulfide Quantum Dot Solids

Charge Carrier Dynamics in Lead Sulfide Quantum Dot Solids
Author: Rachel Hoffman Gilmore
Publisher:
Total Pages: 117
Release: 2017
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Quantum dots, also called semiconductor nanocrystals, are an interesting class of materials because their band gap is a function of the quantum dot size. Their optical properties are not determined solely by the atomic composition, but may be engineered. Advances in quantum dot synthesis have enabled control of the ensemble size dispersity and the creation of monodisperse quantum dot ensembles with size variations of less than one atomic layer. Quantum dots have been used in a variety of applications including solar cells, light-emitting diodes, photodetectors, and thermoelectrics. In many of these applications, understanding charge transport in quantum dot solids is crucial to optimizing efficient devices. We examine charge transport in monodisperse, coupled quantum dot solids using spectroscopic techniques explained by hopping transport models that provide a complementary picture to device measurements. In our monodisperse quantum dot solids, the site-to-site energetic disorder that comes from size dispersity and the size-dependent band gap is very small and spatial disorder in the quantum dot superlattice often has a greater impact on charge transport. In Chapter 2, we show that improved structural order from self-assembly in monodisperse quantum dots reduces the interparticle spacing and has a greater impact than reduced energetic disorder on increasing charge carrier hopping rates. In Chapter 3, we present temperature-dependent transport measurements that demonstrate again that when energetic disorder is very low, structural changes will dominate the dynamics. We find increasing mobility with decreasing temperature that can be explained by a 1-2 Å contraction in the edge-to-edge nearest neighbor quantum dot spacing. In Chapter 4, we study optical states that are 100-200 meV lower in energy than the band gap. Because we work with monodisperse quantum dots, we are able to resolve this trap state separately from the band edge state and study its optical properties. We identify the trap state as dimers that form during synthesis and ligand exchange when two bare quantum dot surfaces fuse. The findings of this thesis point to the importance of minimizing the structural disorder of the coupled quantum dot solid in addition to the energetic disorder to optimize charge carrier transport.

Surface Effects on Charge Carrier Dynamics in Semiconductor Quantum Dots

Surface Effects on Charge Carrier Dynamics in Semiconductor Quantum Dots
Author: Pooja Tyagi
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Release: 2013
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"Due to the large surface-to-volume ratio of quantum dots, their surface conditions play a significant role in determining their electronic and optical properties. In this thesis, we show that the presence of surface states modifies the optical selection rules in quantum dots and enhances the rate of surface charge trapping. These surface-induced effects have profound impact on the measurement of multiexciton recombination and carrier multiplication processes. Specifically, in transient absorption studies, surface states result in additional decay timescales which may be misattributed to multiexciton recombination processes. Additionally, they lead to large "apparent" carrier multiplication yields even under conditions where it is forbidden by energy conservation. The surface-dependent transient absorption studies presented in this work suggest ways to identify and minimize the undesirable surface-induced signals. Interestingly, surface-induced processes also result in significant electrostatic effects. We show that due to the piezoelectric nature of wurtzite CdSe quantum dots, the strong electric field created by surface charge trapping can drive coherent acoustic phonons in these systems. We further show that the amplitude of this piezoelectric response can be controlled by altering the surface conditions of the quantum dot. Finally, we theoretically investigate the effect of multiple surface layers on carrier localization in nanostructures. We find that in a core/barrier/shell configuration, layered nanostructures offer independent control over electron and hole wave functions. These results suggest design principles for wave function engineering in potential quantum dot applications in light emitting devices, photovoltaics and optical amplification." --

Nanocrystal Quantum Dots

Nanocrystal Quantum Dots
Author: Victor I. Klimov
Publisher: CRC Press
Total Pages: 485
Release: 2017-12-19
Genre: Technology & Engineering
ISBN: 1420079271
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A review of recent advancements in colloidal nanocrystals and quantum-confined nanostructures, Nanocrystal Quantum Dots is the second edition of Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, originally published in 2003. This new title reflects the book’s altered focus on semiconductor nanocrystals. Gathering contributions from leading researchers, this book contains new chapters on carrier multiplication (generation of multiexcitons by single photons), doping of semiconductor nanocrystals, and applications of nanocrystals in biology. Other updates include: New insights regarding the underlying mechanisms supporting colloidal nanocrystal growth A revised general overview of multiexciton phenomena, including spectral and dynamical signatures of multiexcitons in transient absorption and photoluminescence Analysis of nanocrystal-specific features of multiexciton recombination A review of the status of new field of carrier multiplication Expanded coverage of theory, covering the regime of high-charge densities New results on quantum dots of lead chalcogenides, with a focus studies of carrier multiplication and the latest results regarding Schottky junction solar cells Presents useful examples to illustrate applications of nanocrystals in biological labeling, imaging, and diagnostics The book also includes a review of recent progress made in biological applications of colloidal nanocrystals, as well as a comparative analysis of the advantages and limitations of techniques for preparing biocompatible quantum dots. The authors summarize the latest developments in the synthesis and understanding of magnetically doped semiconductor nanocrystals, and they present a detailed discussion of issues related to the synthesis, magneto-optics, and photoluminescence of doped colloidal nanocrystals as well. A valuable addition to the pantheon of literature in the field of nanoscience, this book presents pioneering research from experts whose work has led to the numerous advances of the past several years.