Dissertations / Theses on the topic 'Semiconductor nanocrystals'
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1
Yerci, Selcuk. "Spectroscopic Characterization Of Semiconductor Nanocrystals." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/12608177/index.pdf.
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Semiconductor nanocrystals are expected to play an important role in the development of new generation of microelectronic and photonic devices such as light emitting diodes and memory elements. Optimization of these devices requires detailed investigations. Various spectroscopic techniques have been developed for material and devices characterization. This study covers the applications of the following techniques for the analysis of nanocrystalline materials: Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, X-Ray Diffraction (XRD) and X-Ray Photoelectron (XPS). Transmission Electron Microscopy (TEM) and Secondary Ion Mass Spectrometry (SIMS) are also used as complementary methods. Crystallinity ratio, size, physical and chemical environment of the nanostructures were probed with these methods. Si and Ge nanocrystals were formed into the oxides Al2O3 and SiO2 by ion implantation, magnetron sputtering and laser ablation methods. FTIR and XPS are two methods used to extract information on the surface of the nanocrystals. Raman and XRD are non destructive and easy-to-operate methods used widely to estimate the crystallinity to amorphous ratio and the sizes of the nanocrystals.
In this study, the structural variations of SiO2 matrix during the formation of Si nanocrystals were characterized by FTIR. The shift in position and changes in intensity of the Si-O-Si asymmetric stretching band of SiOx was monitored. An indirect metrology method based on FTIR was developed to show the nanocrystal formation. Ge nanocrystals formed in SiO2 matrix were investigated using FTIR, Raman and XRD methods. FTIR spectroscopy showed that Ge atoms segregate completely from the matrix at relatively low temperatures 900 oC. The stress between the Ge nanocrystals and the matrix can vary in samples produced by magnetron sputtering if the production conditions are slightly different. Si and Ge nanocrystals were formed into Al2O3 matrix by ion implantation of Si and Ge ions into sapphire matrix. Raman, XRD, XPS and TEM methods were employed to characterize the formed nanocrystals. XRD is used to estimate the nanocrystal sizes which are in agreement with TEM observations. The stress on nanocrystals was observed by Raman and XRD methods, and a quantitative calculation was employed to the Si nanocrystals using the Raman results. XPS and SIMS depth profiles of the sample implanted with Si, and annealed at 1000 oC were measured. Precipitation of Si atoms with the heat treatment to form the nanocrystals was observed using XPS. The volume fraction of the SiOx shell to the Si core in Si nanocrystals was found to be 7.9 % at projection range of implantation.
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Kudera, Stefan. "Formation of Colloidal Semiconductor Nanocrystals." Diss., lmu, 2007. http://nbn-resolving.de/urn:nbn:de:bvb:19-77315.
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Finlayson, C. E. "Optical characterisation of semiconductor nanocrystals." Thesis, University of Cambridge, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599029.
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The achievement of luminescent nanocrystalline solid films, with good optical quality, will be crucial to the development of opto-electronic devices based on such materials. Although (CdSe)ZnS "core-shell" nanocrystals are typically found to have solution photoluminescence (PL) efficiencies in excess of 60%, the values associated with solid films are found to be an order of magnitude lower. Care of surface chemistry and control of nanocrystal/matrix interactions are of paramount importance. Furthermore, the PL efficiency exhibits a dependence on nanocrystal concentration consistent with a semi-quantitative model describing the effects of Förster energy transfer between nanocrystals and the associated trapping at surface sites. In addition to the ability to control optical properties by variation of the nanocrystal dimensions, it is also possible to alter the optical environment in which the nanocrystals are situated. By placing films of nanocrystals into high-Q, planar microcavities, it is possible to produce significant alteration of photoluminescence into very narrow resonant modes of the cavity. This is an important technical step towards the realisation of a nanocrystal laser. The combination of robust semiconductor emitters with the convenience of solution processing also offers considerable advantages over conventional molecular beam epitaxy (MBE) techniques. Finally, the PL emission from close-packed core-shell nanocrystalline thin films under intense picosecond UV excitation is studied. Strong, stable line-narrowing features are observed as the excitation intensity is increased, both at 77K and at room temperature; these are attributed to waveguiding and amplified spontaneous emission (ASE) in the films. Such behaviour would usually be considered as the signature of optical gain. Lasing from microcavities based on these films has yet to be observed, however, and a semi-empirical model of line-narrowing threshold intensities and cavity-photon lifetimes suggests that higher gain, lower losses or greater cavity finesse may be required for this.
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Fairclough, Simon Michael. "Carrier dynamics within semiconductor nanocrystals." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:857f624d-d93d-498d-910b-73cce12c4e0b.
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This thesis explores how the carrier dynamics within semiconductor nanocrystals can be directly engineered through specific core-shell design. Emphasis is placed on how material characteristics, such as strain or alloying at a core-shell interface, can influence the exciton energies and the recombination dynamics within semiconductor nanocrystals. This study synthesises type-II heterojunction ZnTe/ZnSe core-shell nanocrystals via a diethyl zinc-free synthesis method, producing small size distributions and quantum yields as high as 12%. It was found that the 7% lattice mismatch between the core and shell materials places limitations on the range of structures in which coherent growth is achieved. By developing compositional and strained atomistic core-shell models a variety of physical and optical properties could be simulated and has led to a clear picture of the core-shell architecture to be built. This characterisation provides evidence that the low bulk modulus ZnTe cores are compressed by the higher bulk modulus smaller lattice constant ZnSe shells. Further studies show how strain is manifested in structures with 'sharp' core-shell interfaces and how intentional alloying the interface can influence the growth and exciton energies. A (2-6)-band effective mass model was able to distinguish between the as-grown 'sharp' and 'alloyed' interfaces which indicated that strain accentuates the redshift of the excitonic state whilst reduced strain within an alloyed interface sees a reduced redshift. Single nanocrystal spectroscopy investigations of brightly emitting single graded alloyed nanocrystals and of a size series of commercially available CdSe/ZnS nanocrystals showed almost no fluorescence intermittency (nearly 'non-blinking'). These investigations also identified trion recombination as the main mechanism within the blinking 'off' state. Ultimately this thesis adds to the growing understanding of how specific core-shell architectures manipulate the electronic structure and develops techniques to identify specific material characteristics and how these characteristics influence the physical and optical properties within semiconductor nanocrystals.
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Lau, Pick Chung. "Novel Applications of Semiconductor Nanocrystals." Diss., The University of Arizona, 2013. http://hdl.handle.net/10150/297024.
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We have investigated ways of modifying a common water soluble CdTe NCs to become non-photobleaching. Such NCs are capable of responding reversibly to an inter-switching of the oxygen and argon environments over multiple hours of photoexcitation. They are found to quench upon exposure to oxygen, but when the system is purged with argon, their photoluminescence (PL) revives to the original intensity. Such discovery could potentially be used as oxygen nanosensors. These PL robust CdTe NCs immobilized on glass substrates also exhibit significant changes in their PL when certain organic/bio molecules are placed in their vicinity (nanoscale). This novel technique also known as NC-organic molecule close proximity imaging (NC-cp imaging) has found to provide contrast ratio greater by a factor of 2-3 compared to conventional fluorescence imaging technique. PL of NCs is recoverable upon removal of these organic molecules, therefore validating these NCs as potential all-optical organic molecular nanosensors and, upon optimization, ultimately serving as point detectors for purposes of super-resolution microscopy (with proper instrumentation). No solvents are required for this sensing mechanism since all solutions were dried under argon flow. Furthermore, core graded shell CdSe/CdSeₓS(1-x)/CdS giant nanocrystal (g-NCs) were found to have very robust PL temperature response. At a size of 10.2 nm in diameter, these g-NCs undergo PL drop of only 30% at 355K (normalized to PL intensity at 85K). In comparison, the core step shells CdSe/CdS g-NCs at the same diameter exhibit 80% PL drop at 355K. Spectral shifting and broadening were acquired and found to be 5-10 times and 2-4 times smaller respectively than the standard CdSe core and CdSe/CdS core shell NCs. It is also discovered that these core graded shell g-NCs are largely nonblinking and have insignificant photoluminescence decay even after exciting the samples at very high irradiance (44 kW/cm²) for over an hour. These types of g-NCs have great potential to be used as the active medium for temperature insensitive laser devices in the visible range or temperature insensitive bioprobes for bioimaging applications.
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Radovanovic, Pavle V. "Synthesis, spectroscopy, and magnetism of diluted magnetic semiconductor nanocrystals /." Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/8494.
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Sher, Pin-Hao. "Carrier Dynamics in Single Semiconductor Nanocrystals." Thesis, University of Oxford, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.514991.
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Hewa-Kasakarage, Nishshanka Niroshan. "Charge Separation in Heterostructured Semiconductor Nanocrystals." Bowling Green State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1275766369.
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Klinkova, Anna. "Cation Exchange Reactions in Semiconductor Nanocrystals." Bowling Green State University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1308392960.
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Archer, Paul I. "Building on the hot-injection architecture : giving worth to alternative nanocrystal syntheses /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/8520.
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Eley, Clive William. "The rational design of photocatalytic semiconductor nanocrystals." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:ee29c922-857c-432a-8316-a7e04c822b1d.
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This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb2O5 nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5+ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.
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Lees, Emma E. "Preparation and characterisation of biocompatible semiconductor nanocrystals." Connect to thesis, 2009. http://repository.unimelb.edu.au/10187/7084.
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Semiconductor nanocrystals exhibit unique optical and physical properties that make them an attractive alternative to organic dyes for fluorescent bioapplications. Although significant advances have been made since their first reported use in biology a decade ago, it still remains a challenge to prepare high quality, biocompatible semiconductor nanocrystals.In this thesis, studies are described with the aim to prepare robust, biocompatible semiconductor nanocrystals that exhibit each of the properties necessary for their implementation in biological applications. Two different approaches were investigated: ligand exchange and polymer encapsulation, and advances in each are presented. A heterobifunctional ligand suitable for bioconjugation, carboxyl terminated dihydrolipoic acid poly(ethylene glycol) (DHLA-PEG-COOH), was synthesised and characterised to prepare water-soluble, biocompatible semiconductor nanocrystals via ligand exchange. It was found that nanocrystals transferred into water using DHLA-PEG-COOH exhibit the same optical properties and colloidal stability as those prepared using DHLA-PEG. It was demonstrated that the surface charge of the nanocrystals may be controlled by altering the ratio of DHLA-PEG:DHLA-PEG- COOH ligands. In a different approach, colloidally stable, biocompatible nanocrystals were prepared via polymer encapsulation. It was found that by employing a low molecular weight polymer, biocompatible nanocrystals that exhibit a small hydrodynamic diameter could be realised.
Experimental results are presented on the conjugation of biocompatible nanocrystals to protein targets. It was found that while standard coupling chemistries yield protein-dye conjugates, these chemistries did not result in protein-nanocrystal conjugates. In order to overcome the drawbacks of standard coupling chemistries, which are susceptible to hydrolysis, a novel conjugation scheme utilising copper-free click chemistry is proposed.
Finally, the success of nanocrystals in bioapplications depends on the ability to characterise nanocrystal-protein conjugates. By means of analytical ultracentrifugation, data on the sedimentation properties of nanocrystals and nanocrystal-protein conjugates was obtained. Analysis of these data provided information on fundamental physical properties of biocompatible nanocrystals and nanocrystal-protein conjugates, in particular the core crystal size, hydrodynamic size, number of surface ligands and nanocrystal:protein stoichiometry. Such a precise, comprehensive characterisation of nanocrystals in general, and nanocrystal-protein conjugates in particular, will greatly facilitate their use in bioapplications.
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Geyer, Scott Mitchell. "Science and applications of infrared semiconductor nanocrystals." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/62053.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2010.Vita. Cataloged from PDF version of thesis.
Includes bibliographical references (p. 149-158).
In this work we study several applications of semiconductor nanocrystals (NCs) with infrared band gaps. In the first half, we explore the physics of two systems with applications in NC based photovoltaics. The physics of mixed films of CdTe and CdSe NCs is studied in chapter 2 as a model for NC based bulk heterojunction photovoltaics. We demonstrate that the presence of an active electron trap on the CdTe dramatically reduces the electron mobility in mixed films. The trapping state is linked to oxidation of the CdTe NCs. A cadmium oleate treatment is shown to reduced the oxidation rate. In chapter 3, we present a method to switch the carrier type of InAs NCs deposited in a thin film from p-type to n-type by the addition of cadmium. This provides a stable pre-deposition technique to control the NC carrier type and is a step towards pn homojunction based NC devices. We discuss the role that surface passivation and substitution doping may play in determining the carrier type. The second half explores the use of NCs for photodetector applications. Chapter 4 presents our efforts to move from a single pixel, proof of principle PbS NC infrared detector to a large area infrared imaging camera. A method to control the resistivity of the NC film through oxidation and re-treatment with ethanedithiol is presented. This allows for integration of our NC film with existing read out technology. The noise spectrum is shown to be dominated by 1/f noise and the dependence of the noise on the bias and channel length is determined. The detectivity is found to be determined by the carrier lifetime and dark current carrier density. In chapter 5, we demonstrate efficient UV-IR dual band detectors based on luminescent down conversion. In this design, NCs absorb UV light and re-emit the light in the infrared band of an InGaAs detector. The high quantum yields of infrared nanocrystals and unique absorption profile are shown to provide a significant advantage over organic dyes. The bandwidth of the detectors is measured and the effect of the down conversion layer on the spatial resolution is characterized.
by Scott Mitchell Geyer.
Ph.D.
14
Nirmal, Manoj. "Photophysics of cadmium selenide (CdSe) semiconductor nanocrystals." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/10715.
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Rengers, Christin, Sergei V. Voitekhovich, Susann Kittler, André Wolf, Marion Adam, Nikolai Gaponik, Stefan Kaskel, and Alexander Eychmüller. "3D assembly of silica encapsulated semiconductor nanocrystals." Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-188584.
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Non-ordered porous networks, so-called aerogels, can be achieved by the 3D assembly of quantum dots (QDs). These materials are well suited for photonic applications, however a certain quenching of the photoluminescence (PL) intensity is observed in these structures. This PL quenching is mainly attributed to the energy transfer mechanisms that result from the close contact of the nanoparticles in the network. Here, we demonstrate the formation of a novel aerogel material with non-quenching PL behaviour by non-classical, reversible gel formation from tetrazole capped silica encapsulated QDs. Monitoring of the gelation/degelation by optical spectroscopy showed that the optical properties of the nanocrystals could be preserved in the 3D network since no spectral shifts and lifetime shortening, which can be attributed to the coupling between QDs, are observed in the gels as compared to the original colloidal solutions. In comparison with other QD-silica monoliths, QDs in our gels are homogeneously distributed with a distinct and controllable distance. In addition we show that the silica shell is porous and allows metal ions to pass through the shell and interact with the QD core causing detectable changes of the emission properties. We further show the applicability of this gelation method to other QD materials which sets the stage for facile preparation of a variety of mixed gel structures.
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Khon, Elena. "Synthesis and Applications of Heterostructured Semiconductor Nanocrystals." Bowling Green State University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1374512926.
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Murphy, James Edward. "Semiconductor nanocrystals and nanocrystal arrays: Synthesis, characterization, and time-resolved terahertz spectroscopy photoconductivity measurements." Diss., Connect to online resource, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3207726.
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Tavenner, Kruger Sasha Dawn. "Homogeneous linewidth and spectral diffusion in semiconductor nanocrystals /." view abstract or download file of text, 2006. http://proquest.umi.com/pqdweb?did=1251884311&sid=3&Fmt=2&clientId=11238&RQT=309&VName=PQD.
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Thesis (Ph. D.)--University of Oregon, 2006.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 164-170). Also available for download via the World Wide Web; free to University of Oregon users.
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Zhao, Lijuan. "Chemical syntheses and characterizations of II-VI semiconductor nanocrystals /." View abstract or full-text, 2007. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202007%20ZHAO.
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Kriegel, Ilka. "Near-infrared plasmonics with vacancy doped semiconductor nanocrystals." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-164558.
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Plasmonics with heavily doped semiconductor nanocrystals (NCs) is an emerging field in NC science. However, impurity doping of NCs remains far from trivial and is, as yet, dominated by a low chemical control over the incorporated dopant atoms. An appealing alternative is vacancy doping, where the formation of vacancies in the structure is responsible for an increased carrier density and elegantly circumvents the issues related to impurity doping. Due to high carrier densities of around 10^21cm^(-3) localized surface plasmon resonances (LSPRs) in the near infrared (NIR) are expected, and as such highlighted to close the gap between conventionally doped NCs and noble metal nanoparticles. Copper chalcogenide NCs, namely copper sulfide (Cu2-xS), copper selenide (Cu2-xSe), and copper telluride (Cu2-xTe), are an attractive example of vacancy doped semiconductor NCs, with spectra dominated by intense NIR resonances. Within this study thorough experimental evidence has been given to prove the plasmonic nature of those NIR resonances. By presenting typical plasmonic characteristics, such as refractive index sensitivity of the LSPR, its intrinsic size dependence, plasmon dynamics, or interparticle plasmon coupling, the LSPRs in copper chalcogenide NCs have unambiguously been identified. The chemical nature of vacancy doping turns out to deliver an additional, highly attractive means of control over the LSPR in vacancy doped copper chalcogenide NCs. Through chemical tailoring of the copper vacancy density via controlled oxidation and reduction, as shown in this study, a reversible tuning of the LSPR over a wide range of frequencies in the NIR (1000-2000 nm) becomes feasible. This highlights copper chalcogenide NCs over conventional plasmonic materials. Notably, the complete suppression of the LSPR uncovers the excitonic features present only in the purely semiconducting, un-doped NCs and reveals the unique option to selectively address excitons and highly tunable LSPRs in one material (bandgap Eg~1.2 eV). As such, copper chalcogenide NCs appear to hold as an attractive material system for the investigation of exciton plasmon interactions. Indeed, a quenching of the excitonic transitions in the presence of the developing LSPR is demonstrated within this work, with a full recovery of the initial excitonic properties upon its suppression. A theoretical study on the shape dependent plasmonic properties of Cu2-xTe NCs reveals a deviation from the usual Drude model and suggests that the carriers in vacancy doped copper chalcogenide NCs cannot be treated as fully free. On the other hand, the Lorentz model of localized oscillators appears to account for the weak shape dependence, as observed experimentally, indicating an essential degree of localization of the carriers in vacancy doped copper chalcogenide NCs.
Taken together, this work delivers a huge step toward the complete optical and structural characterization of plasmonic copper chalcogenide NCs. The advantages of semiconductor NC chemistry have been exploited to provide access to novel plasmonic shapes, such as tetrapods that have not been feasible to produce so far. A precise size, shape and phase control presents the basis for this study, and together with a thorough theoretical investigation delivers important aspects to uncover the tunable plasmonic properties of vacancy doped copper chalcogenide NCs.
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Xu, Shu. "Synthesis of High Quality Low-toxic Semiconductor Nanocrystals." Thesis, University of East Anglia, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.502562.
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New approaches for the synthesis of highly luminescent InP and InP/ZnS nanocrystals were developed by stepwise systematic investigation of the parameters during the reaction stages. The parameters, including solvents, precursors, ligands, capping agents, protic agents, Lewis acids and bases and temperature were discussed in detail in different chapters. The investigation processes helped increase understanding of understand the reaction and surface passivation mechanisms and to develop convenient synthesis approaches. Highly luminescent InP NCs were prepared with an in-situ indium chloride complex in the presence of zinc carboxylates or zinc dithiocarbamates - convenient nucleation initiators and stabilisers. The nanocrystals prepared covered a wide photoluminescence emission range from blue to the near infra-red. This synthesis method also allowed the in-situ growth of highly luminescent InP/ZnS core-shell nanocrystals as well. The principles of selection of reagents are also applicable in the synthesis of other nanocrystals. The as-prepared high-quality InP and InP/ZnS nanocrystals have been exploited in applications in the fields of light emitting diodes (LEOs) and bioimaging.
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Tyrrell, Edward J. "Effective mass modelling of excitons in semiconductor nanocrystals." Thesis, University of Oxford, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665169.
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This thesis investigates the properties of exciton states in semiconductor nanocrystals using effective mass models. We focus on type-I core-shell nanocrystals in which the staggered band alignments of the core and shell material mean the lowest energy states for electrons and holes lie in different spatial regions, giving rise to spatially indirect excitons. The technological potential of type-I! nanocrystals provides motivation for understanding single exciton states which determine many important optical properties. In the first research chapter we study CdTe/CdSe and CdSe/CdTe nanocrystals using a single-band and (2,6)-band effective mass model. The (2,6)-band model is based on a multiband k·p theory previously developed for spherical quantum dot heterostructures. We calculate exciton energies as a function of the core radius a and shell width as of the heterostructure, and assign six exciton transitions in the experimental absorption spectra of CdTe/CdSe nanocrystals. The second research chapter is concerned with strained ZnTe/ZnSe nanocrystals. The (2,6)-band model is modified to incorporate strain using a continuum elasticity model. Exciton energies from absorption spectra are compared with the predictions of the strained and unstrained nanocrystal models, showing that they only describe the lowest exciton energy of one of the three size series. Improved agreement is found for the change in exciton energy due to a particular shell width, with the strained nanocrystal model giving much better fits to the as-dependence. The as-dependence of nanocrystals with alloyed heterointerfaces is better described by the unstrained nanocrystal model, indicating alloying relaxes the strain in this system. In the final chapter we model spatial correlations between the electron and hole in CdTe/CdSe and CdSe/CdTe nanocrystals using a configuration interaction approach developed in the framework of the (2,6)-band k·p theory. We find that the single-particle basis can, be restricted without changing t~e resulting exciton energies significantly; using this decoupled configuration ihter~ction approach we calculate exciton energy shifts due to correlation as a function of aand as. Dielectric confinement increases correlation for many heterostructure designs by shifting carrier wavefunctions away from the surface, and the interparticle Coulomb interaction leads to large changes in radial probability density for the uniform dielectric constant case. Dielectric confinement affects the correlated hole more than the electron so that excitons in the CdSe/CdTe heterostructure are more affected by the dielectric environment than those in the CdTe/CdSe heterostructure. The overall behaviour of the correlated charge density is due to the net effect of the type-I! spatial confinement, interparticle Coulomb attraction, dielectric confinement and single-particle electronic structure.
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Liptay, Thomas J. (Thomas John). "Spectral properties of semiconductor nanocrystals and their applications." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/40502.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 174-177).
The ability to engineer the optical properties of the semiconductor nanocrystals by controlling their growth - size, shape, materials, coatings, etc - makes them appealing for many optical applications. Despite the impressive development of nanocrystal manufacturing capabilities, there are still many basic questions about how to model nanocrystals that have yet to be adequately answered. This thesis investigates three important optical properties: 1) the temperature dependence of the bandedge absorption energy Eabs(T), 2) the temperature dependence of the Stokes shift, and 3) the homogeneous linewidth. We relate these properties to various nanocrystal applications with particular focus on nanocrystal based microbead barcodes. We present measurements of the temperature dependence of the absorption and emission spectra from 5 sizes of CdSe/ZnS nanocrystal ensembles. Our measurements show that dEabs(T)/dT is similar to the value for bulk CdSe for all sizes of nanocrystals, in contrast with previous experiments. We develop a model that can explain measured values of dEabs(T)/dT in both epitaxial quantum dots and colloidal nanocrystals of different materials. We interpret our measurements of the temperature dependence of the Stokes shift and linewidth, along with single nanocrystal fluorescence, from the perspective of two models based on different physical processes: 1) the fine structure of the bandedge exciton and 2) exciton-acoustic phonon scattering. We find that neither theory is able to adequately explain our measurements in isolation. We conclude that a comprehensive model that includes both physical mechanisms is required to explain our experimental results.
(cont.) We present a detailed analysis of nanocrystal based microbead barcodes for high throughput biological screening. We make design decisions for how such a system would operate, develop a Monte Carlo simulation of the expected noise, and investigate different coding architectures. We investigate this system from the perspective of information and coding theory. We develop a Monte Carlo code generation algorithm to evaluate the information capacity of this system.
by Thomas J. Liptay.
Ph.D.
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Razgoniaeva, Natalia Razgoniaeva. "Photochemical energy conversion in metal-semiconductor hybrid nanocrystals." Bowling Green State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1465822519.
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Chen, Dechao. "Colloidal Quasi-one-dimensional Zinc-based Semiconductor Nanocrystals." Thesis, Curtin University, 2020. http://hdl.handle.net/20.500.11937/79908.
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In the recent decades, colloidal nanocrystals, which also termed as quantum dots, have witnessed an extraordinary boom on the new synthesised structures and improved performances in relation to their applications. This thesis focuses on developing facile synthesis strategies, new properties and applications of the emerging colloidal quasi-one-dimensional zinc-based semiconductor nanocrystals. The novel structures have shown interesting properties with relevance to diverse applications.
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Li, Wenhua. "Shape Control and Functional Properties of Copper Chalcogenide Colloidal Nanocrystals." Doctoral thesis, Universitat de Barcelona, 2013. http://hdl.handle.net/10803/131051.
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The high quality CuxS nanocrystals were synthesized (Chapter 3) and the profound understanding and skills to prepare colloidal nanocrystals has been obtained and improved. It revealed a very simple synthetic route not only for the systematic investigation on the size control of the copper sulfide nanodisks but also for studying the influence of different stoichiometric ratios on the shape of copper sulfide nanocrystals. An increase of the precursor concentration in the growth solution resulted in the formation of tetradecahedral and dodecahedral nanocrystals. XRD results showed these nanodisks had a similar composition close to Cu1.78S as spherical nanocrystals, however, the tetradecahedral and dodecahedral nanocrystals were characterized with a composition close to Cu1.96S as deduced from their djurleite crystal phase. An oriented attachment was proposed as growth mechanism for polyhedrons growth and the slow nucleation rate allows an accurate control of the size and morphology of CuxS nanocrystals, from spheres and disks to tetradecahedrons and dodecahedrons by tuning the precursor concentration from 0.05 M to 1.0 M and reaction conditions. Dodecahedrons with different size can be easily prepared by elongating the reaction time. These nanocrystals can be used as cathodes in all-vanadium redox flow batteries and showed a significant improvement of the cathodic reaction reversibility, especially the dodecahedrons.
The CuxSe nanocubes with mean edge length of 17 nm±0.9 nm were synthesized (Chapter 4). The role of various metal ions playing on shape of CuxSe nanocrystals was discussed during the synthesis. The underlying mechanism was illustrated by preparing copper selenide nanocubes in the presence of Al ions whereas there was no any Al detected on the surface or within the final cubes. The morphology control is proved to be thermodynamically directed during the ripening regime and it exemplified the shape-direction of semiconductor nanocrystals by metal ions for the first time. It is a platform to produce cubic nanoparticles with different composition by cation exchange such as Ag2Te nanocubes. The plasmonic properties of the obtained nanocubes were further characterized and it demonstrated the strong plasmonic absorption peak at 950 nm.
A reproducible procedure to prepare highly monodisperse copper telluride nanocubes, nanoplates and nanorods was presented in Chapter 5. The procedure is based on the reaction of a copper salt with trioctylphosphine telluride (TOP-Te) in the presence of Lithium bis(trimethylsilyl) amide (LiN(SiMe3)2), trioctylphosphine (TOP), trioctylphosphine oxide (TOPO) and oleylamine (OLA). The high reaction temperature as 220 °C was found to be necessary to obtain cube-shaped NPs with narrow size distributions. By tuning the precursor ratio of Cu:Te, the size of these nanocubes could be controlled in the range between 10 and 20 nm. When decreasing the reaction temperature to 190 °C and the growth time to 15 min, highly homogeneous copper telluride nanoplates were produced. An increase of the TOP concentration from 0.125 ml to 0.75 ml resulted in the formation of nanorods. It was proposed the LiN(SiMe3)2 to activate the formation of a Cu-oleylamido complex and it is the actual species reacting with TOP-Te. The Cu-oleyamido complexes and/or lithium oleylamine may play a key role stabilizing the NP surface during growth. Copper telluride nanocubes and nanoplates display a strong near-infrared optical absorption at 900nm associated to localized surface plasmon resonances. This plasmon resonance can be exploited for the design of surface-enhanced Raman scattering (SERS) sensors for unconventional optical probes such as nile red containing oxygen based functional groups. This is the first time using Cu-chalcogenide as probes for SERS application and demonstrates its potential interest in future. Preliminary analysis of the use of copper telluride nanocubes as cytotoxic and photothermal agents is also discussed herein.Inicialment vam establir les condicions per preparar Cu(x)S. Com a mecanisme de creixement es va proposar el que es coneix amb el nom d’oriented attachment, en el qual els nanocristalls s’uneixen en una determinada orientació per formar altres formes més complexes. Establint les condicions en les quals es donava aquest mecanisme podíem produir nanocristalls de Cu2-xS amb un acurat control sobre la seva composició i/o forma, des de partícules esfèriques fins a nanopartícules en forma de disc o bé acanat amb partícules amb forma tetradecaèdríca o dodecaèdríca. Aquest control es va aconseguir simplement variant la concentració del precursor i les condicions de reacció. El segon sistema que es va estudiar va ser la producció de nanocristalls de Cu(x)Se. En el nostre treball preteniem descobrir nous procediments per sintetitzar nanocristalls de Cu(x)Se controlant la seva morfologia. Es va descubrir que es podia controlar la forma final dels nanocristalls de Cu(x)Se simplement introduint ions metàl•lics a la solució. En particular, en presència d’ions d’alumini es van produir nanocubs amb una longitud lateral de 17 nm ± 0.9 nm. Addicionalment es van estudiar les propietats plasmòniques d’aquests nanocubs. També es van utilizar aquests cubs de seleniur de coure com a base per produir cubs d’altres semiconductors a travès de l’intercanvi catiònic. Com a exemple es van produir cubs de Ag(2)Te. Finalment, es va estudiar el calcogenur binari, Cu(x)Te. Es va desenvolupar un mètode de síntesi per produïr nanocubs, nanoplaques i nanorods altament monodispersos. Es va observar que els paràmetres clau per controlar la forma eren la temperatura i la quantitat de surfactants. En canvi, per controlar el tamany es va observar que el paràmetre més important era la proporció entre Cu i Te present a la solució. Aquests nanocristalls posseïen propietats plasmòniques amb un pic d’absorpció al voltant dels 900 nm.
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Serincan, Ugur. "Formation Of Semiconductor Nanocrystals In Sio2 By Ion Implantation." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/3/12605016/index.pdf.
Full textAbstract:
In this study, we used ion implantation technique to synthesize semiconductor
(Ge, Si) nanocrystals in SiO2 matrix. Ge and Si nanocrystals have been
successfully formed by Ge and Si implantation and post annealing. Implanted
samples were examined by characterization techniques such as TEM, XPS, EDS,
SAD, SIMS, PL, Raman and FTIR spectroscopy and the presence of Ge and Si
nanocrystals in the SiO2 matrix has been evidenced by these measurements. It
was shown that implantation dose, implantation energy, annealing temperature,
annealing time and annealing ambient are important parameters for the formation
and evolution of semiconductor nanocrystals embedded in SiO2 matrix. The
size and size distribution of Ge and Si nanocrystals were estimated successfully
by fitting Raman and PL spectra obtained from Ge and Si implanted samples, respectively. It was demonstrated that Si implanted and post annealed samples
exhibit two broad PL peaks at ∼
625 and 850 nm, even at room temperature. Origin of these peaks was investigated by temperature, excitation power and excitation wavelength dependence of PL spectrum and etch-measure experiments and it was shown that the peak observed at &
#8764
625 nm is related with defects (clusters or chain of Si located near the surface) while the other is related to the Si nanocrystals. As an expected effect of quantum size phenomenon, the peak observed at &
#8764
850 nm was found to depend on the nanocrystal size. Finally, the formation and evolution of Ge and Si nanocrystals were monitored by FTIR spectroscopy and it was shown that the deformation in SiO2 matrix caused by ion implantation tends to recover itself much quicker in the case of the Ge implantation. This is a result of effective segregation of Ge atoms at relatively low temperatures.
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Anderson, Kevin E. H. "Semiconductor nanocrystals : synthesis, characterization, and instrumentation for femtosecond spectroscopy." Thesis, McGill University, 2006. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=99317.
Full textAbstract:
Semiconductor nanocrystals exhibit strongly size-dependent absorption spectra and consequently are of great interest for use in nanoscale devices. As the excited states of these crystals are short-lived, spectroscopic investigation of the nature and size dependence of their energy levels requires femtosecond laser pulses. We have synthesized cadmium selenide nanocrystals and constructed a femtosecond instrument around a commercial laser system from Coherent to study them.The instrument is capable of measuring the autocorrelation and crosscorrelation of laser pulses and can measure the transient absorption dynamics of nanocrystals via pump-probe spectroscopy. A novel means of simultaneously measuring the dynamics associated with two different excitation wavelengths has also been employed.
I have written a LabVIEW program for data acquisition from this instrument, capable of reducing measurement noise through averaging and rejection of bad measurements. We have successfully used this software in probing specific transitions in cadmium selenide nanocrystals.
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McLaurin, Emily J. (Emily Jane). "Phosphorescent semiconductor nanocrystals and proteins for biological oxygen sensing." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/62726.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.Vita. Cataloged from PDF version of thesis.
Includes bibliographical references.
Oxygen is required for cellular respiration by all complex life making it a key metabolic profiling factor in biological systems. Tumors are defined by hypoxia (low pO2), which has been shown to influence response to radiation therapy and chemotheraphy. However, very little is known about spatio-temporal changes in P0 2 during tumor progression and therapy. To fully characterize and probe the tumor microenvironment, new tools are needed to quantitatively assess the microanatonical and physiological changes occurring during tumor growth and treatment. This thesis explores the design and construction of new oxygen sensors as tools for monitoring the tumor microenvironment in real-time. Semiconductor nanocrystals or quantum dots (QDs) are the basis of these tools. Previously, most imaging applications of QDs have used them as indicators of position; they have lacked a response to their local environment. Tethering a phosphorescent complex to a QD enables fluorescence resonance energy transfer to be exploited as a signal transduction mechanism, sensitizing the QD to oxygen. The mechanism for oxygen sensing involves kinetic quenching of the emission of the energy accepting phosphor in the presence of oxygen, while the emission of the energy donating QD remains stable. This mechanism was chosen owing to the unique ability of oxygen to quench emission from a phosphorescent compound, but not fluorescence from a QD. Phosphors such as osmium polypyridines (Chapter 2), Pd or Pt porphyrins (Chapters 3 and 4), or phosphorescent proteins (Chapters 5 and 6) may all be employed. An additional benefit of FRET excitation includes very large one- and two-photon excitation cross-sections of QDs. Together, these properties make the probes ideal candidates for 02 sensing applications in biological microenvironments, where probe concentrations may vary, and where the use of multiphoton excitation in microscopy presents significant advantages in imaging thick samples and in limiting extraneous tissue damage.
by Emily J. McLaurin.
Ph.D.
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Almeida, António José Sousa de. "Magnetic resonance studies of spin systems in semiconductor nanocrystals." Doctoral thesis, Universidade de Aveiro, 2016. http://hdl.handle.net/10773/18636.
Full textAbstract:
Doutoramento em FísicaEsta tese apresenta um estudo experimental de sistemas de spins fornecidos por dopantes electrónicos e por defeitos capturadores de carga em nanocristais (NCs) semiconductores, por meio de técnicas de ressonância magnética. Aqui, investigámos problemas que têm efeitos limitadores de performance nas propriedades de NCs semiconductores para o seu uso em aplicações tecnológicas. Nomeadamente, estudámos a dopagem electrónica de NCs semiconductores. A dopagem é crítica para controlar o comportamento de semiconductores, que de outra forma seriam isoladores. Investigámos também defeitos capturadores de carga, que podem ter um impacto negativo na conductividade de NCs semiconductores ao capturar portadores de carga em estados electrónicos deslocalizados de NCs. Para além disso, abordámos a origem da anisotropia magnética em NCs de materiais diamagnéticos. Nesta tese, reportamos investigações usando medidas de ressonância paramagnética electrónica (RPE) quantitativa, dizendo respeito à eficiência de dopagem electrónica de Si NCs com átomos de P e à sua dependência com o ambiente envolvendo os NCs. Das medidas de RPE quantitativas, estimamos eficiências de dopagem nos NCs que são consistentes com a incorporação da maioria dos dopantes P como dadores substitucionais nos NCs. Observamos também que a eficiência de dopagem dos NCs varia em várias ordens de grandeza dependendo do ambiente envolvendo os NCs, devido a uma forte compensação dos dadores por moléculas absorbidas na superfície dos NCs. Usando espectroscopia RPE dependente da temperatura, mostramos também que a energia de ionização dos dopantes P em Si NCs aumenta relativamente ao seu correspondente cristal macroscópico devido a confinamento. Usamos espectroscopia RPE dependente da temperatura para estudar a interacção entre múltiplos dopantes incorporados num único Si NC e o seu impacto na estrutura electrónica destes NCs. Monitorizámos experimentalmente a interacção de troca em pares de dadores P (dímeros de dadores) em Si NCs através de um desvio da ressonância magnética do seu estado tripleto em relação ao paramagnetismo de Curie. Mostrámos que a interacção de troca entre dadores próximos entre si pode ser bem descrita pela teoria de massa efectiva, permitindo o cálculo de muitas configurações de dopantes e permitindo a consideração de efeitos estatísticos cruciais em conjuntos de nanocristais. Descobrimos que dímeros de dadores induzem estados discretos num NC, e que a sua separação energética difere em até três ordens de grandeza para dímeros colocados aleatoriamente num conjunto de NCs devido a uma enorme dependência da energia de troca na configuração do dímero. Investigámos também sistemas de spins induzidos por defeitos capturadores de carga e como estes defeitos podem afectar a dopagem de NCs. Identificamos a presença de dois estados de carga de um defeito em NCs de CdSe usando espectroscopia RPE combinada com a afinação electrónica de NCs através de dopagem com Ag induzida quimicamente. A partir de de RPE foto-induzido, mostramos que estes defeitos têm um papel central na fixação do nível de Fermi em conjuntos de NCs. Através da análise da dependência do sinal de RPE dos defeitos com a concentração de dopantes de Ag, mostramos também que os defeitos actuam como capturadores efectivos de electrões nos NCs. Do RPE dependente da temperatura, estimamos um limite inferior para a energia de ionização dos defeitos estudados. Com base nas características do espectro RPE dos defeitos observados, propomos que está associado a lacunas de Se com o estado paramagnético sendo o estado positivo do defeito. Para além disso, mostramos que as interacções magnéticas entre spins associados a defeitos nos NCs podem induzir efeitos de anisotropia magnética em conjuntos de NCs que não são esperados acontecer no cristal macroscópico correspondente. Usando espectroscopia de ressonância ferromagnética (RFM) com dependência angular, medimos a anisotropia magnética em conjuntos de aleatórios de NCs de CdSe através da gravação do espectro de ressonância magnética para várias orientações do campo magnético externo. As dependências angulares do campo ressonante são diferente para conjuntos aparentemente similares de NCs de CdSe. Mostramos que a forma e amplitude da variação angular do RFM pode ser bem descrita po um modelo simples que toma em consideração as interacções dipolo-dipolo entre dipolos localizados na superfície dos NCs. Os dipolos na superfície podem originar de ligações pendentes em sítios da superfície que não estão passivados por ligantes. Dos nossos cálculos, descobrimos que para diferentes conjuntos aleatórios de NCs a força da anisotropia magnética induzida por interacções dipolo-dipolo pode tomar valores abrangendo quatro ordens de grandeza, dependendo do arranjo específico dos NCs no conjunto e da distribuição específica dos dipolos na superfície de cada NC. Esta enorme variabilidade pode justificar a disparidade de anisotropias magnéticas observada nas nossas experiências.
This thesis presents an experimental study of systems of spins provided by electronic dopants and by charge trapping defects in semiconductor NCs, by means of magnetic resonance spectroscopy techniques. Here, we have investigated issues that have performance-limiting effects on the properties of semiconductor NCs for their use in technological applications. Namely, we have studied the electronic doping of semiconductor NCs. Doping is critical to control the behavior of semiconductors, which would otherwise be electrically insulating. We have further investigated charge trapping defects in semiconductor NCs, which can have a negative impact on the conductivity of semiconductor NCs by capturing charge carriers from delocalized electronic states of the NCs. Moreover, we addressed the origin of magnetic anisotropy in NCs of diamagnetic materials. In this thesis, we report investigations using quantitative electron paramagnetic resonance (EPR) measurements concerning the efficiency of electronic doping of Si NCs with P atoms and its dependence on the environment surrounding the NCs. From quantitative EPR measurements, we estimate doping efficiencies in the NCs that are consistent with the incorporation of most P dopants as substitutional donors in the NCs. We further observe that the doping efficiency of the NCs varies by several orders of magnitude depending on the NCs surrounding environment due to a strong compensation of donors by molecules adsorbed to the NCs surface. Using temperature-dependent EPR spectroscopy, we further show that the ionization energy of P dopants in Si NCs increases with respect to their bulk counterpart due to confinement. We use temperature-dependent EPR spectroscopy to study the interaction between multiple P dopants incorporated in a single Si NC and its impact on the electronic structure of these NCs. We experimentally probe the exchange interaction in pairs of P donors (donor dimers) in Si NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We showed that the exchange coupling of closely spaced donors can be well described by effective mass theory, enabling the calculation of many dopant configurations and allowing the consideration of statistical effects crucial in NC ensembles. We find that donor dimers induce discrete states in a NC, and that their energy splitting differs by up to three orders of magnitude for randomly placed dimers in a NC ensemble due to an enormous dependence of the exchange energy on the dimer configuration. We also investigate systems of spins induced by charge trapping defects and how these defects can affect the doping of NCs. We identify the presence of two charge states of a defect in CdSe NCs using EPR spectroscopy, combined with electronic tuning of NCs via chemically induced Ag doping. From light-induced EPR, we show that these defects have a central role on Fermi level pinning of NC ensembles. By analyzing the dependence of the EPR signal of the defects on the concentration of Ag dopants, we further demonstrate that the defects act as effective electron traps in the NCs. From temperaturedependent EPR, we estimate a lower limit for the ionization energy of the studied defects. Based on the characteristics of the EPR spectrum of the observed defect, we propose that it is associated to Se vacancies with the paramagnetic state being the positively charged state of the defect. Moreover, we show that magnetic interactions between spins associated to defects in NCs can induce magnetic anisotropy effects in NCs ensembles that are not expected to occur in their bulk counterpart. Using angulardependent ferromagnetic resonance (FMR) spectroscopy, we measure the magnetic anisotropy in different random ensembles of CdSe NCs by recording magnetic resonance spectra for various orientations of the external magnetic field. The observed angular dependencies of resonant field are different for apparently similar CdSe NC ensembles. We show that the shape and amplitude of the FMR angular variation can be well described by a simple model that considers magnetic dipole-dipole interactions between dipoles located at the NCs surface. The surface dipoles may originate from dangling bonds on surface sites that are not passivated by ligands. From our calculations, we find that for different random ensembles of NCs the strength of the magnetic anisotropy induced by dipole-dipole interactions may take values spanning four orders of magnitude, depending on the specific arrangement of the NCs in the ensemble and the specific distribution of the surface dipoles in each NC. This huge variability may justify the disparity of magnetic anisotropies observed in our experiments.
31
Sayevich, Uladzimir. "Synthesis, Surface Design and Assembling of Colloidal Semiconductor Nanocrystals." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-209074.
Full textAbstract:
The work presented in the thesis is focused on the synthesis of diverse colloidal semiconductor NCs in organic media, their surface design with tiny inorganic and hybrid capping species in solution phase, and subsequent assembling of these NC building units into two-dimensional close-packed thin-films and three-dimensional non-ordered porous superstructures.
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Zbydniewska, Ewa. "Electronic properties of coupled semiconductor nanocrystals and carbon nanotubes." Thesis, Lille 1, 2016. http://www.theses.fr/2016LIL10010/document.
Full textAbstract:
Ce travail de thèse décrit les propriétés électroniques de nanodispositifs couplés entre transistors à nanotubes de carbone (CNTFETs) et nanocristaux semiconducteurs colloïdaux CdSe/ZnS individuels en régime de détection de charge unique à température ambiante. Les transferts de charges élémentaires entre nanotubes et nanocristaux sont mis en évidence par les fluctuations temporelles du courant des transistors à tension de grille fixée, et font apparaître un signal à deux niveaux (bruit télégraphique ou RTS), observé sur des échelles de temps entre 1s et 0.1 ms. Les temps d’occupation τ des niveaux de courant suivent une loi de puissance P(τ)~τ-α où l’exposant α varie entre 1.5 et 4 (typiquement proche de 2.8). Cette observation suggère que les fluctuations de charges observées sont à la base des phénomènes de "clignotement optique" des nanocristaux colloïdaux étudiés. L’analyse spectroscopique des dispositifs permet d’attribuer ce clignotement à des pièges dans la bande interdite des nanocristaux, avec une énergie de chargement Ec de l’ordre de 200 meV. L’approche présentée dans ce travail peut être étendue à des mesures électro-optiques, et donc permettre une meilleure compréhension des phénomènes physiques contrôlant les propriétés optoélectroniques de nanodispositifs à base de nanocristaux semiconducteursWe study the electronic properties of coupled semiconductor nanocrystals and carbon nanotubes. We report measurements of single electron transfers between single CdSe colloidal nanocrystal coupled to a carbon nanotube field effect transistor at room temperature in ambient conditions. The measurements consist of nanotube current level monitoring as a function of time for fixed gate voltage. We observe a sequence of high - low currents (random telegraph signal) on time scales up to several seconds with ms sampling time. We attribute the two level current fluctuations to the transfer of single electron onto the nanocrystal. The probability of the occupation time τ at the high or low current state follows a power law of the form P(τ)~τ-α where exponent α lies between 1.5 and 4 (typically close to 2.8). The observation suggests that the two-level current switching is similar to the fluorescence intermittency (optical blinking) observed in individual quantum dots. The spectroscopic analysis of the devices based on coupled semiconductor nanocrystals and carbon nanotubes is consistent with the charging of nanocrystal defect states with a charging energy of Ec ~ 200 meV. The approach developed here enables to probe the trap state dynamics in quantum dots in ambient air and room temperature from a purely electrical approach, and therefore to better understand the physics at hand in (opto)electronic devices based on quantum dots
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Chen, Wei. "Zn–based chalcogenide 1D semiconductor nanocrystals: synthesis and applications." Thesis, Curtin University, 2020. http://hdl.handle.net/20.500.11937/81226.
Full textAbstract:
This research work reports a series of new colloidal approaches for the synthesis of uniform Zn-based chalcogenide semiconductor nanocrystals, including 0D and 1D ZnS, ZnSe, ZnSxSe1-x nanocrystals. The influence of precursor activity on nanoparticles growth was investigated by experiment and theoretical calculation. Novel Au-ZnSe hybrid structures with controlled Au domain growth were obtained and used for photocatalytic H2 generation.
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Chang, Jin. "Controlled synthesis of inorganic semiconductor nanocrystals and their applications." Thesis, Queensland University of Technology, 2013. https://eprints.qut.edu.au/63960/1/Jin_Chang_Thesis.pdf.
Full textAbstract:
This thesis is a comprehensive study of the synthesis of nanomaterials. It explores the synthetic methods on the control of the size, shape and phase of semiconductor nanocrystals. A number of important conclusions, including the mechanism behind crystal growth and the structure-relationship, have been drawn through the experimental and theoretical investigation. The synthesized nanocrystals have been tested for applications in gas sensing, photocatalysis and solar cells, which exhibit considerable commercialization potential.
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IMRAN, MUHAMMAD. "Synthesis and Post-synthesis Transformations of Colloidal Semiconductor Nanocrystals." Doctoral thesis, Università degli studi di Genova, 2018. http://hdl.handle.net/11567/945513.
Full textAbstract:
The present PhD thesis focuses on two main classes of semiconductor colloidal nanocrystals, i.e. lead halide perovskite and copper chalcogenides. The former class of semiconductor NCs are promising materials for many high performance optoelectronics applications, as they exhibit a tunable band gap in the range of 1.4 to 2.9 eV and an efficient photoluminescence characterized by narrow emission linewidths and have been explored the most in the last years. Following the standard hot injection based synthesis and selecting a combination of short chain acid (octanoic acid or hexanoic acid) together with alkyl amines (octylamine and oleylamine) we prepared strongly fluorescent CsPbBr3 perovskite nanowires with tuneable width, in the range from 20 nm (exhibiting no quantum confinement, hence emitting in the green) to 3 nm (in the strong quantum-confinement regime, emitting in the blue) for the first time. However the main limitation of the colloidal synthesis protocols that was followed in aforementioned case including the ligand assisted reprecipitation routes which is the second most frequently used method for preparation of LHPs, is that they employ PbX2 (X= Cl, Br, or I) salts as both lead and halide precursors which consequently limit the precise tunability of the amount of reaction species such as metals or halides precursors and are not applicable to entire family of APbX3 (A=FA, MA and Cs; X=Cl, Br, I). To overcome this issue we developed benzoyl halide based colloidal synthesis route i.e broadly applicable to the entire family of LHP NCs and not only ensures the independent tunability of reaction precursors but also maintain the overall integrity of the NCs such as phase purity and high PLQY. Despite the significant advances in synthesis procedures, the control over size monodispersity, shape and phase purity remains another long standing challenge. This is in fact due to the tendency of primary alkyl amine in the form of alkylammonium ions that could compete with Cs+ ions and leads to the anisotropic growth such as NPLs or their use in excess permotes the Pb-depleted Cs4PbX6 phases. We develop here a strategy to achieve size, shape and phase pure CsPbBr3 nanocubes by substituting primary alkyl amines with secondary alkyl amines. We attributed this excellent control over the shape and phase purity to the inability of secondary amines to find the right steric conditions at the surface of the nanocrystals which consequently limits the formation of low dimensional structures. The shape purity and narrow size distribution leads to their ease of self-assembly in superlattices reaching up to 50 microns in lateral dimensions, which are the largest dimensions reported to date for superlattices of LHP NCs. The second class of materials studied here, i.e. copper chalcogenides, are mainly attractive due to their tunable composition via post synthesis chemical transformations, plasmonic properties, low toxicity and environmental friendliness. Taking the advantage of colloidal synthesis and using Cu2S as a template we develop a strategy to obtain novel AuCuS-Cu2S heterostructure through cation exchange, which cannot be realized through conventional synthesis approaches. We further investigated the stability of Cu2S NCs with different dimensionalities and their thermal evolution subsequent to the metal decoration. Interestingly the presence of additional metallic NCs, such as Au and Pt not only improves their thermal stability but also leads to the formation of bi-metallic alloys semiconductor heterostructure.
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Spina, Carla. "Zinc oxide semiconducting nanocrystals : scaffolds for intrinsic and extrinsic defects." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=115869.
Full textAbstract:
As a material whose applications are many and growing, zinc oxide still remains a complex system whose photoluminescent (PL), structural, electrical, and photocatalytic properties have not been fundamentally understood. The luminescent properties of zinc oxide (ZnO) nanocrystals (NCs) are very sensitive to crystal structure, and defect states in zinc oxide, which in turn is very sensitive to preparation methods, post-synthesis workup, and thermal treatments. Understanding and managing this rich defect chemistry is critical to controlling ZnO properties. As the surface-to-volume ratio of ZnO increases as materials enter the quantum regime, the surface defects play a stronger role. The exact role of the defect states and their contribution to the physical and chemical properties of ZnO has been studies in great lengths yet still remains controversial.
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Smith, Andrew Michael. "Engineering semiconductor nanocrystals for molecular, cellular, and in vivo imaging." Diss., Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/37124.
Full textAbstract:
Biomedicine has recently exploited many nanotechnology platforms for the detection and treatment of disease as well as for the fundamental study of cellular biology. A prime example of these successes is the implementation of semiconductor quantum dots in a wide range of biological and medical applications, from in vitro biosensing to in vivo cancer imaging. Quantum dots are nearly spherical nanocrystals composed of semiconductor materials that can emit fluorescent light with high intensity and a strong resistance to degradation. The aim of this thesis is to understand the fundamental physics of colloidal quantum dots, to engineer their optical and structural properties for applications in biology and medicine, and to examine the interaction of these particles with biomolecules and living cells. Toward these goals, new synthetic strategies for colloidal nanocrystals have been developed, implementing a cation exchange method for independent tuning of size and fluorescence, and a bandgap engineering technique that utilizes mechanical strain imposed by coherent shell growth. In addition, stable nanocrystals have been prepared with ultrathin coatings (< 2 nm), 'amphibious' solubility, and broadly tunable bioaffinity, induced by self-assembly with polyhistidine-sequences on recombinant proteins. Finally, colloidal quantum dots have been studied in biological fluids and living cells in order to elucidate their interactions with biological systems. It was found that these interactions are strongly dependent on the size of the nanocrystal, and cytotoxic effects of these particles are largely independent of their composition of heavy metal atoms, demonstrating that the rule book for toxicology must be rewritten for nanomaterials.
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Yuan, Chunze. "The Study of II-VI Semiconductor Nanocrystals Sensitized Solar Cells." Licentiate thesis, KTH, Teoretisk kemi och biologi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-93752.
Full textAbstract:
Semiconductor nanocrystals, also referred to as quantum dots (QDs), have been the focus of great scientific and technological efforts in solar cells, as a result of their advantages of low-cost, photostability, high molar extinction coefficients and size-dependent optical properties. Due to the multi-electron generation effect, the theoretically maximum efficiency of quantum dots-sensitized solar cells (QDSCs) is as high as 44%, which is much higher than that of dye-sensitized solar cells (DSCs). Thus QDSCs have a clear potential to overtake the efficiency of all other kinds of solar cells. In recent years, the efficiency of QDSCs has been improved very quickly to around 5%. It is however still much lower than that of DSCs. The low efficiency is mostly caused by the high electron loss between electrolyte and electrodes and the lack of an efficient electrolyte. In this thesis, we have been working to enhance the performance of QDSCs with II-VI group nanocrystals by increasing the electron injection efficiency from QDs to TiO2 and developing new redox couples in electrolyte. To increase the electron injection, firstly, colloidal ZnSe/CdS type-II QDs were synthesized and applied for QDSCs for the first time, whose photoelectron and photohole are located on CdS shell and ZnSe core, respectively. The spatial separation between photoelectron and photohole can effectively enhance the charge extraction efficiency, facilitating electron injection, and also effectively expand the absorption spectrum. All these characteristics contribute to the high photon to current conversion efficiency. Furthermore, a comparison between the performances of ZnSe/CdS and CdS/ZnSe QDs shows that the electron distribution is important for the electron injection of the QDs in QDSCs. Secondly, colloidal CdS/CdSe quantum rods (QRs) were applied to a quantum rod-sensitized solar cell (QRSCs) that showed a higher electron injection efficiency than analogous QDSCs. It is concluded that reducing the carrier confinement dimensions of nanocrystals can improve electron injection efficiency of nanocrystal sensitized solar cells. In this thesis, two types of organic electrolytes based McMT-/BMT and TMTU/TMTU-TFO were used for QDSCs. By reducing the charge recombination between the electrolyte and counter electrode, fill factor (FF) of these QDSCs was significantly improved. At the same time, the photovoltages of the QDSCs were remarkably increased. As a result, the overall conversion efficiency of QDSCs based on the new electrolytes was much higher than that with a commonly used inorganic electrolyte. In addition, CdS QDSCs on NiO photoelectrode were studied which shows a n-type photovoltaic performance. This performance is attributed to the formation of a thin Cd metal film before CdS QDs formation on NiO. Since the CB edge of CdS sits between the Fermi level and the CB edge of Cd metal, a much strong electron transfer between Cd and CdS QD is obtained, resulting in the observed n-type photovoltaic performance of these CdS/NiO QDSCs.QC 20120425
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Nair, Gautham Padmanabhan. "Many-body processes in the photophysics of colloidal semiconductor nanocrystals." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/54219.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Vita. Cataloged from student submitted PDF version of thesis.
Includes bibliographical references (p. 135-143).
In this work we have experimentally studied several aspects of two Coulomb processes that change the number of electrons and holes in colloidal semiconductor nanocrystals (NCs). Carrier Multiplication (CM) is the production of additional electron-hole pairs by collision of a highly excited carrier with valence electrons. Efficient CM would improve the performance of solar energy conversion devices, but it is weak in the bulk. Recent reports by several groups suggested highly efficient CM in semi-conductor NCs. We describe here our assessment of CM using transient photoluminescence in CdSe and lead chalcogenide NCs. Biexciton radiative and nonradiative rates were determined. In our study, no detectable CM was found in CdSe NCs photoexcited at a photon energy of up to 5.9 eV, and the CM yields observed for PbSe NCs at 3.1 eV were found consistent with bulk values. Reasons for the strong disagreement with prior measurements are discussed, and the low yields are theoretically accounted for. The second part of the thesis describes two studies of the "Auger" nonradiative recombination process whereby an electron-hole pair recombines while transferring its energy to a third particle. This mechanism is responsible for the short multiexciton lifetimes in NCs. In one study, we demonstrate a direct method for determining biexciton quantum yields in single nanocrystals by photon cross-correlation (antibunching) measurements. We find significant inhomogeneity in these values, indicating a previously obscured variation in Auger recombination rates.
(cont.) Another set of experiments tests the conventional charging model of NC fluorescence intermittency ("blinking") which attributes off-state quenching to Auger decay, by studying single NCs with relatively long multiexciton Auger lifetimes. We find that off-state exciton quantum yields are significantly lower than the quantum yield of a biexciton and we demonstrate that multiexciton emission also shows strong intermittency. Both of these findings contradict the standard charging model. Alternatives are discussed.
by Gautham Padmanabhan Nair.
Ph.D.
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Allen, Peter M. (Peter Matthew). "Semiconductor nanocrystals : synthesis, mechanisms of formation, and applications in biology." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/58464.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2010."April 29, 2010." Vita. Cataloged from PDF version of thesis.
Includes bibliographical references.
The primary focus of this thesis is the synthesis and applications of semiconductor nanocrystals, or quantum dots (QDs). Novel synthetic routes to ternary 1-III-VI QDs are presented, and we report the first highly luminescent Cu-In-Se QDs spanning the red to near-infrared region. The synthetic method is modular and is extended to Ag-In-Se, Cu-In-Zn-S, and Ag-In-Zn-S QDs, luminescent from the blue to near-infrared. The development of new core-shell InAs(ZnCdS) QDs is discussed in the context of making highly fluorescent, stable biological probes in the near-infrared region. Applications in biological systems from cellular labeling to sentinel lymph node mapping are demonstrated. In addition, we present new methods for doping InAs QDs in order to control carrier type through the introduction of acceptor defects such as cadmium. The synthesis and characterization of n and p type InAs QDs is discussed. In order to understand the differences in size distributions with current III-V QD synthetic procedures and II-VI and IV-VI QD syntheses we have explored the molecular mechanisms that lead to the formation of InP and InAs QDs. We find that current III-V QD syntheses result in the depletion of molecular precursors immediately following nucleation, preventing growth from molecular precursors, thus failing to meet the a key criterion for a monodisperse colloidal synthesis in the Dinegar and LaMer model. In the conclusion of this thesis, we explore the electrically controlled solution-liquid-solid (EC-SLS) synthesis of InP nanowires. Using the EC-SLS method, we are able to controllably place n type InP nanowires into field effect transistor geometries.
by Peter M. Allen.
Ph.D.
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La, Rosa Marcello. "Development of luminescent semiconductor nanocrystals (Quantum Dots) for photoinduced applications." Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0591/document.
Full textAbstract:
Cette thèse s’est focalisée sur le développement de nanocristaux semi-conducteurs luminescents, i.e. des boîtes quantiques (quantum dots, QDs), pour des applications photo-induites. Ces nanomatériaux possèdent des propriétés optiques très intéressantes qui dépendent de leur taille et sont prometteurs pour des applications dans divers domaines.Les QD sont généralement hydrophobes mais de nombreuses applications intéressantes requiert une compatibilité avec l'eau ou du moins avec un environnement polaire. Cela, nécessite donc un traitement post-synthétique afin de modifier leur solubilité.Au cours de cette thèse, une nouvelle méthode pour transférer les QDs d'un solvant apolaire vers un solvant polaire a été développée en les fonctionnalisant avec l'acide lipoïque, un tensioactif complexant.L'acide lipoïque est une molécule chirale générant donc un effet de dichroïsme induit qui a pu être étudier, ainsi que sa dépendance vis-à-vis la taille des nanocristaux.Un objectif clef de ces recherches était le développement de QDs présentant un transfert d'énergie électronique réversible (REET). Il s’agit d’un transfert d'énergie bidirectionnel entre les QDs photoexcités et des unités chromophores appropriées fixées sur leur surface, dont la conséquence la plus importante est l'allongement de la durée de vie de luminescence du QD.Enfin, un nouveau protocole pour le dépôt de QDs chargés sur un substrat vitreux localement polarisé a été développé en collaboration avec le Dr Marc Dussauze de l'Université de BordeauxThis thesis focuses on the development of luminescent semiconductor nanocrystals quantum dots (QDs) for photoinduced applications. QDs are promising nanomaterials with size-dependent optical properties and are attractive for applications in several fields.However, QDs are commonly hydrophobic and many interesting applications require their compatibility with water or at least with a polar environment, meaning a post-synthetic treatment is required to confer a different solubility.During these studies, a new method for transferring QDs from an apolar solvent to another one polar has been successfully developed, by exploiting lipoic acid, as a versatile surface capping agent. Moreover, lipoic acid is a chiral molecule so a possible induced dichroism effect has been also investigated, as well as its dependence on the size of nanocrystals.A major aim of this research was the development of QDs exhibiting reversible electronic energy transfer (REET). Such a process is a bidirectional energy transfer between the photoexcited QDs and suitable chromophoric units attached on their surface, whose most important consequence is the elongation of the luminescence lifetime of the QD. Strong experimental evidence for REET and accompanying modifications of the photophysical properties has been obtained. Such a process to our knowledge has never been observed in QD-based systems.Finally, a novel protocol for depositing charged QDs on a locally polarized glassy substrate has been developed in collaboration with Dr. Marc Dussauze of the University of Bordeaux
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Jalilikashtiban, Reza. "Structural and compositional properties of semiconductor quantum dots and nanocrystals." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/structural-and-compositional-properties-of-semiconductor-quantum-dots-and-nanocrystals(ceb22d89-7ed7-4c54-9d4c-612a9331404d).html.
Full textAbstract:
The research carried out here employed analytical and imaging transmission electron microscopy and scanning transmission electron microscopy to gain a good understanding of local structure and composition of semiconductor nanocrystals and quantum dots for electronics and optoelectronics applications. One of the world's most advanced analytical scanning transmission electron microscopes in the field, the Daresbury SuperSTEM, was used to scrutinise the structure and composition of the samples. Three nanostructure systems are investigated in this thesis: 1. Structures consisting of Ge-nanocrystals (NCs) in alumina. Here HRTEM suggests relaxed and twinned smaller NCs grown annealed at lower temperature compared to elongated non-faulty bigger NCs annealed at higher temperature. HRTEM also suggests a polycrystalline structure of the matrix. 2. With regards to the InAs/GaAs quantum dots (QD) the study aims in particular at elucidating QD formation by investigating samples grown with and without growth interrupt (GI). Diffraction contrast TEM shows formation of buried dots in the sample prepared with GI whereas for the sample without GI the immediate growth of GaAs after InAs inhibits diffusion and segregation of In adotoms, and no footprint of buried dots has been observed. HRTEM and HAADF show coherent QDs in the sample with GI and abrupt InAs/GaAs interfaces in the sample without GI. In executing energy electron loss spectroscopy (EELS) and geometric phase analysis (GPA) the distribution of In in InGaAs/GaAs QDs has been obtained in samples grown in the critical thickness regime for quantum dot formation. The highest In percentage achieved in the dots grown with a nominal fraction of 100% was ~70%. EELS shows variations in the In concentration within the QD structure and wetting layer 3. In the case of Er-doped Si-NCs in silica this research tries to provide an understanding of structure, composition and position of excess Si and Er in the silica matrix of materials prepared under different growth conditions and to correlate this information with the PL emission, all with the aim to find preparation routes for optimum optical efficiency for applications of this materials system in silicon photonics. High spatial correlation between Si-NCs, Er and O in the Er and Si co-implanted sample with strong indication of an Er-oxide/Si core-shell structure had been found. The lack of an Er-oxide plasmon indicates, however, that the shell structure and its interface with the SiNCs is highly defective and a likely cause for non-radiative recombination. The sample with similar excess Er and Si concentrations but prepared in a two-stage implantation and annealing process shows a 10 times improvement in the optical emission. Here no spatial correlation between Er and Si-NCs was found in core loss EELS. EELS and HAADF evidenced more highly, near-atomically dispersed Er in the matrix with no formation of a core-shell structure as compared to the co-implanted sample. No footprint of Er-silicide plasmon was observed by low loss valence band EELS investigation in the co-implanted sample.
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Shcheglov, Kirill V. Atwater Harry Albert. "Synthesis, optical and electronic properties of group IV semiconductor nanocrystals /." Diss., Pasadena, Calif. : California Institute of Technology, 1997. http://resolver.caltech.edu/CaltechETD:etd-01172008-081522.
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La, Rosa Marcello <1989>. "Development of Luminescent Semiconductor Nanocrystals (Quantum Dots) for Photoinduced Applications." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amsdottorato.unibo.it/8059/1/LaRosa_Marcello_Tesi.pdf.
Full textAbstract:
This thesis focuses on the development of luminescent semiconductor nanocrystals quantum dots (QDs) for photoinduced applications. QDs are promising nanomaterials with size-dependent optical properties and are attractive for applications in several fields.
However, QDs are commonly hydrophobic and many interesting applications require their compatibility with water or at least with a polar environment, meaning a post-synthetic treatment is required to confer a different solubility.
During these studies, a new method for transferring QDs from an apolar solvent to another one polar has been successfully developed, by exploiting lipoic acid, as a versatile surface capping agent. Moreover lipoic acid is a chiral molecule so a possible induced dichroism effect, which has been investigated, as well as its dependence on the size of nanocrystals.
A major aim of this research was the development of QDs exhibiting reversible electronic energy transfer (REET). Such a process is a bidirectional energy transfer between the photoexcited QDs and suitable chromophoric units attached on their surface, where the most important consequence is the elongation of the luminescence lifetime of the QD. Strong experimental evidence for REET and accompanying modifications of the photophysical properties has been obtained. Such a process to our knowledge has never been observed in QD-based systems.
Finally, a novel protocol for depositing charged QDs on a locally polarized glassy substrate has been developed in collaboration with Dr. Marc Dussauze of the University of Bordeaux.
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Darugar, Qusai A. "Surface effects on the ultrafast electronic relaxation of some semiconductor and metallic nanoparticles." Diss., Available online, Georgia Institute of Technology, 2006, 2006. http://etd.gatech.edu/theses/available/etd-06272006-160645/.
Full textAbstract:
Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2007.Zhang, John, Committee Member ; Wang, Zhong, Committee Member ; El-Sayed, Mostafa, Committee Chair ; Orlando, Thomas, Committee Member ; Lyon, Andrew, Committee Member.
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Warner, Jamie. "Colloidal lead sulphide nanocrystals for quantum technology applications /." [St. Lucia, Qld.], 2004. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe18436.pdf.
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Pal, Anuushka. "Investigation of fluorescence quenching of quantum dots and its application in photocatalytic organic transformations." Thesis, IIT Delhi, 2017. http://localhost:8080/iit/handle/2074/7235.
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Urgessa, Zelalem Nigussa. "Growth and characterization of ZnO nanorods using chemical bath deposition." Thesis, Nelson Mandela Metropolitan University, 2012. http://hdl.handle.net/10948/d1021124.
Full textAbstract:
Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.
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Norberg, Nicholas S. "Magnetic nanocrystals : synthesis and properties of diluted magnetic semiconductor quantum dots /." Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/8625.
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Kowerko, Danny. "Dynamic Processes in Functionalised Perylene Bisimide Molecules, Semiconductor Nanocrystals and Assemblies." Doctoral thesis, Universitätsbibliothek Chemnitz, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-64194.
Full textAbstract:
Funktionalisierte organische Perylenbisimidfarbstoffe (PBI) und aus Cadmiumselenid bestehende Halbleiternanokristalle werden hinsichtlich physikalischer sowie chemischer Wechselwirkungsprozesse miteinander und mit ihrer Umgebung mittels zeitaufgelöster optischer Spektroskopie untersucht. Im Mittelpunkt der Studien an diesem organisch/anorganischen Modellsystem nanoskopischer Größe steht die Aggregatbildungskinetik und die Identifikation und Quantifizierung von Transferpozessen. Die Anbindung der gut löslichen PBI-Farbstoffe an die Oberfläche solcher Halbleiternanokristalle mittels spezieller Ankergruppen wird durch Selbstorganisation in Lösung realisiert. Die Kombination von Absorptions- und zeitaufgelöster Fluoreszenzspektroskopie zeigt einen unterschiedlich starken Einfluss von Liganden und Farbstoffen auf die Fluoreszenzlöschung der Nanokristalle und belegt, dass Resonanzenergietransfer zum Farbstoff nur in sehr geringem Maße die physikalische Ursache der Fluoreszenzlöschung ist. Die Anzahl adsorbierter Farbstoffe und die Stärke der Fluoreszenzlöschung eines einzelnen Farbstoffmoleküls werden aus zeitaufgelösten Einzelmolekülexperimenten an immobilisierten Emittern gewonnen, welche den direkten spektroskopischen Zugang zur Verteilung gebundener und freier Farbstoffe/Nanokristalle erlaubt. Darüber hinaus werden ankergruppen- und umgebungsspezifische Einflüsse auf die Konformations- und Orientierungsdynamik von Perylenbisimidmolekülen dargestellt. Abschließend werden photo-physikalische Gemeinsamkeiten chemisch unterschiedlich hervorgerufener Fluoreszenzlöschungsprozesse herausgearbeitet und im Kontext von Einzelkristall-Blinkprozessen diskutiert.