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Thesis (Ph. D.)--University of Wisconsin--Madison, 1961. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The thesis describes the synthesis of PbS nano-, micro-structures by colloidal and hydrothermal techniques. Size and morphology dependent luminescence studies were carried out in detail. Application oriented studies like ion sensing and modulation of luminescence are carried out on colloidal PbS QDs. Photoelectrical studies are carried out on various morphologies of PbS microstructures. We observe the persistent photoconductivity, growth and quenching of photocurrent, and a few novel phenomena in flower shaped PbS microstructures. This work is presented in eight chapters inclusive of summary and directions for future work. CHAPTER 1 provides a brief introduction to optical and photoelectrical properties of semiconductor quantum dots and hydrothermal technique in preparation of quantum structures. A review of PbS nanostructures and its technological applications are discussed. CHAPTER 2 provides the experimental techniques used in this work. First, the synthesis of PbS nano-, micro-structures by various methods, and characterization tools used in this work are briefly presented. CHAPTER 3 deals with the synthesis of PbS quantum dots in poly vinyl alcohol with various precursor concentrations to identify the surface states by temperature dependent photoluminescence (PL) measurements. Average bandgap value calculated from absorption measurements was 2.1 eV. We have observed that high-energy PL bands (>1.3 eV) are due to electron traps (Pb dangling bonds) and low-energy bands (<1 eV) are due to hole traps (S dangling bonds). By capping with thiol compounds (mercaptoethanol-C2 H5OSH), absence of the 1.67 eV band indicates the passivation of Pb dangling bonds. To explain above observed results, we propose a band diagram with distributed shallow to deep states and attributed them to the specific surface related defects (Pb or S). CHAPTER 4 discusses the ion sensing applications of PbS quantum dots. We found that the sulfur related dangling bonds are quite sensitive to different metallic ions (since mercaptoethanol passivates only Pb atoms). Sulfur related PL band (~ 1 eV) have shown an order of magnitude improvement in its intensity for Hg, Ag ions and relatively low enhancement for Zn, Cd ions at 1 µmol concentrations. However Cu quenches the luminescence. An important distinction may have to be made between PbS and Cd related quantum structures. The PbS QDs can distinguish between Cu & Hg, however Cd related QDs couldn’t distinguish between these two ions. Photo-brightening and photo-darkening is an interesting phenomena indicative of photo-induced ionic migration that either helps in enhancing the emission of sulfur related defect emission or degrades the emission properties depending on the ion concentration. This report is the first of its kind in ion sensing applications using PbS QDs. CHAPTER 5 discusses the results of duel beam excitation on trap luminescence of PbS QDs. By using different lasers simultaneously (514 nm and 670 nm), we have observed the reversible luminescence quenching of trap emission. The high-energy PL band (1.67 eV) has double the quenching effect compared to low-energy PL band (1.1 eV). The luminescence quenching mechanism is attributed to the re-emission of the charge carriers from the traps (photo-ionization) due to the simultaneous excitation with the second beam. The dependence of the temperature, the effect of two beam excitation intensities and modulation frequency dependent quenching mechanism are primarily focused in this chapter. The quenching mechanism is considered to be quite useful in the optical modulation devices. CHAPTER 6 discusses the PL results on various morphologies of PbS nano-, microstructures. Interestingly, after protecting the surface with organic ligands such as mercaptoethanol (C2 H5OSH), dendrite structures have shown high-energy bands (~ 1.0 eV) in the PL spectra, which indicate the existence of various quantum confinement regimes in different branches of dendrites. The anomalous temperature dependent behavior of PL intensity is attributed to the size distribution. CHAPTER 7 discusses the results of photoconductivity measurements on various morphologies of PbS nano-, micro-structures. Flower shaped structures have shown persistent photoconductivity (PPC). This observed PPC is attributed to the presence of potential barriers, which are created by the different confinement regimes or due to the lattice relaxation, which occurs due to the carrier trapping at surfaces. In PPC, the estimated time constants of both build up and decay transients using the stretched exponentials are of the order of few tens of seconds. In PPC measurements, we observe the PC quenching below 40 K and growth above this temperature. PC quenching is attributed to the transfer of photo-excited carriers to a metastable state. The presence of metastable state is supported by the dark conductivity measurements in flower shaped structures. CHAPTER 8 presents the summary and directions for the future work.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

Thesis (M.S.)--University of Missouri, School of Mines and Metallurgy, 1928. / The entire thesis text is included in file. Typescript. Illustrated by author. Title from title screen of thesis/dissertation PDF file (viewed October 21, 2009) Includes bibliographical references (p. 103).

No description available.

The use of size and shape tunable quantum confinement nanocrystals has many potential applications for use in semiconductors, optics and sensors. The synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanoparticles are of particular interest for use in semiconductor, optoelectronics and bio-medical applications. The continuous synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanocrystals was undertaken in this work. Quality colloidal nanocrystal synthesis requires three components: precursors, organic surfactants and solvents. The synthesis of the nanocrystals can be thought of as a nucleation event, followed by a subsequent growth period. Both the nucleation and growth rates were found to be dependent upon factors such as temperature, growth time, and precursor concentration. For a continuous flow system the residence time (at nucleation and growth conditions) was also found to be important. In order to separate the nucleation and growth events, injection techniques were employed to achieve rapid nucleation of nanocrystals with final size dictated by the growth temperature and/or residence time through the growth zone of the reaction system. Experimental parameters to investigate the size, shape, and composition of synthesized nanocrystals included injection temperature, growth temperature, residence time, and concentration of organic surfactants. Size tunability was accomplished for both PbS and CuInSe��� nanocrystals where particle sizes less than 10 nm were achieved and the resulting nanocrystal compositions were found to be at the approximate stoichiometric ratios for both PbS and CuInSe���. The materials used for the process tubing and pumps were found to be important as chlorinated reaction byproducts were found to react with the stainless steel tubing and pump heads. Post processing was also found to be important in order to remove any possible reaction by-products and residual precursors from the surface of synthesized nanocrystals. When at least one dimension of the nanocrystal approaches the exciton Bohr radius, the bandgap for the nanocrystal increases. UV-VIS spectroscopy was used to optically characterize synthesized PbS nanocrystals from our continuous flow synthesis. The absorption spectra for the particles demonstrated an absorption onset showing a large blueshift compared to that of bulk PbS. The blueshift matches closely with literature reports of the quantum confinement effect that would be desired when synthesizing PbS nanoparticles at diameters that are less than the PbS exciton Bohr radius of 18 nm. / Graduation date: 2013

In this thesis, a facile methodology for the synthesis of CdS and PbS particles using novel thiobisphthalimide (D1) and dithiobisphthalimide (D2) organic sulfur precursors is proposed. A slightly different approach was followed for the synthesis of CuS particles. The effect on the size and morphology of the particles of a variety of reaction parameters such as the nature of sulfur precursor, the reducing agent concentration, the metal-to-sulfur ratio and the reaction temperature was investigated. The phase and composition identifications of the CdS and PbS particles were done by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques were employed to determine the morphologies of the particles. The optical properties of the CdS, PbS and CuS particles were examined by UV-Vis and fluorescence spectroscopy techniques. The phase identification results showed that pure cubic CdS and PbS, and hexagonal CuS particles were successfully synthesized with both D1 and D2 precursors. The synthesized CdS and PbS particles possess various morphologies depending on the precursor type and the reaction parameters mentioned previously. Control of the size of the PbS particles in one dimension in a quantum confinement regime was possible, whereas the CdS particles exhibited faster growth and 3D morphologies. The CuS particles, on the other hand, were produced with high monodispersity in a surfactantless environment.