Дисертації з теми "Chalcogenide quantum dots"

Dans cette thèse, la synthèse, les propriétés et les applications de verres contenant des quantum dots (QDs) de chalcogénure ont été étudiées. Des verres contenant des QDs à base de chalcogénure de plomb (PbSe ou PbS) ont été préparés. Leurs propriétés optiques et leurs applications potentielles ont été explorées en combinaison avec le co-dopage aux ions Tm3+. De plus, sur la base de ces résultats, des verres contenant des QDs de ZnS ou de ZnSe, sans plomb, ont été préparés avec succès. Leurs performances luminescentes ont été encore améliorées par dopage avec des ions de métaux de transition représentés ici par le nickel. Ces résultats jettent les bases pour l’amélioration des propriétés optiques de verres contant des QDs à base de chalcogénure de plomb et aussi pour le développement de verres aux QD sans métaux lourds et donc plus respectueux de l’environnement. Bien que des améliorations futures soient possibles et nécessaires pour des applications réelles, ces verres aux QDs de chalcogénure, développés dans ce travail, présentent un potentiel d'applications dans les domaines des concentrateurs solaires luminescents, de l'anti-contrefaçon optique, de l'éclairage à semi-conducteurs et de la mesure optique de la température
In this dissertation, the synthesis, properties and applications of glasses containing chalcogenide quantum dots (QDs) have been studied. Multicomponent lead chalcogenide QDs glasses (containing PbSe or PbS QDs) were successfully prepared, and their optical properties and potential applications were explored in combination with rare earth Tm3+ ion doping. In addition, based on the results, lead-free and environmentally friendly chalcogenide QDs glasses (containing ZnS or ZnSe QDs) were successfully prepared, and its luminescent performance was further improved by doping with transition metal nickel ions. These results lay the foundation for the improvement of optical properties of lead-based chalcogenide QDs and for the development of environmentally friendly heavy metal-free chalcogenide QDs glasses. Although future improvements are possible and necessary for practical applications, these chalcogenide QDs glasses developed in this work have application potential in the fields of luminescent solar concentrators, optical anti-counterfeiting, solid-state lighting, and optical temperature sensing

Quantum dots (QDs) are the foundation of many optoelectronic devices because their optical and electronic properties are synthetically tunable. The inherent connection between synthetically controllable physical parameters, such as size, shape, and surface chemistry, and QD electronic properties provides flexibility in manipulating excited states. The properties of the ligands that passivate the QD surface and provide such synthetic control, however, are quite different from those that are beneficial for use in optoelectronic devices. In these applications, ligands that promote charge transfer are desired. To this end, significant research efforts have focused on post-synthetic ligand exchange to shorter, more conductive ligand species. Surface ligand identity, however, is a physical parameter intimately tied to QD excited state behavior in addition to charge transfer. A particularly interesting group of ligands, due to the extraordinarily thin ligand shell they create around the QD, are the chalcogenides S2-, Se2-, and Te2-. While promising, little is known about how these chalcogenide ligands affect QD photoexcited states. This dissertation focuses on the impact of chalcogenide ligands on the excited state dynamics of cadmium chalcogenide QDs and associated implications for charge transfer. This is accomplished through a combination of theoretical (Chapters 2, 3, and 6) and experimental (Chapters 2, 4, 5 and 6) methods. We establish a theoretical foundation for describing chalcogenide capped QD photoexcited states and measure the dynamics of these excited states using transient absorption spectroscopy. The presented results highlight the drastic effects surface modification can have on QD photoexcited state dynamics and provide insights for more informed design of optoelectronic systems.

Quantum dots (QDs) are promising materials for photovoltaic (PV) and light-emitting diode (LED) applications due to their unique properties: photostability, size-tunable absorptivity, and narrow line-width emission. These properties are tailored by surface passivations by ligands. However, ligands used in the synthesis of colloidal QDs need to be exchanged with ligands designed for specific applications. The mechanism behind ligand exchange is not well understood. Density functional theory (DFT) is utilized to gain fundamental understanding of ligand exchange (LE) and the resulting effect on the photophysics of QDs. Experimental studies show that phenyldithiocarbamates (PTCs) derivatives can improve the photocurrent of QD-based PVs. Our calculations show that the PTC undergoes decomposition on the CdSe QD surface. Decomposed products of PTCs strongly interact with the surface of QDs, which could cause unforeseen challenges during the implementation of these functionalized QDs in PVs. Secondly, we studied the mechanism of photoluminescence (PL) enhancement by hydride treatment. In experiments, the PL increases by 55 times, but the mechanism is unclear. We found that hydride can interact with surface Se2- producing H2Se gas and passivate surface Cd2+. These interactions result in optically active QDs. Thiol derivatives can also improve PL when LE results in low surface coverage of thiols. The PL is quenched if LE is performed at high concentrations and acidic environments. DFT simulations reveal three scenarios for the thiol interacts with QDs: coordination of thiol, networking between surface and/or other ligands, or thiolate formation. It is the last scenario that was found to be responsible for PL quenching. Lastly, PbS(e)/CdS(e) core/shell QDs are investigated to obtain relaxation rates of electron and hole cooling via interactions with phonons. The band structure of the core/shell QDs facilitates carrier multiplication (CM), a process that generates multiple charge carrier pairs per one absorbed photon. It is thought that CM is facilitated because there are interface associated states that reduce carrier cooling. Non-Adiabatic Molecular Dynamics (NAMD) simulations show that this hypothesis is correct and PbSe/CdSe carrier cooling is about two times slower compared to PbS/CdS due to weaker coupling to optical phonons.

Quantum dots (QDs) have the potential to produce more than one exciton per incident photon, if the photon energy is greater than twice the band gap energy. This process of multiple exciton generation (MEG) has the potential to lead to a step change in the efficiency of solar panels, by utilising energy commonly wasted as heat in conventional solar cells. A wide range of CdSe/CdTe and CdTe/CdSe quantum dots with and without a CdS shell were synthesised with varying core sizes and shell thicknesses. The excited state dynamics of these samples were studied with transient absorption and photoluminescence studies, with their MEG efficiencies obtained. Record MEG efficiencies were obtained with values reaching 142 ± 9 % achieved. The charge separation afforded by the type-II electronic configuration, allowed the first attractive biexciton interaction for a type-II QD system, with the potential for reducing the creation energy for a second exciton this affords. Efficient surface passivation of QDs was achieved through the reaction of CdCl2 with CdTe QDs, with near unity photoluminescence quantum yields (PLQYs) achieved. The suppression of surface trap states resulted in mono-exponential photoluminescence decay traces, with a resultant increase in exciton lifetime. Further CdCl2 treatment was carried out on CdSe/CdTe quasi-type-II QDs with alternating ‘Cd rich’ and ‘Te rich’ surfaces to elucidate the processes involved in surface treatment. It is shown that Te surface atoms are preferentially etched upon treatment, with the reaction being more aggressive when ‘Te rich’ surfaces are treated. The importance of surface composition is studied with trap states associated with chalcogen dangling bonds more prevalent and hence the increased requirement for their passivation is outlined. Control of the core/shell interface is also shown to be important in reducing trap states and ultimately increasing PLQYs, which is desirable for many optoelectronic applications.

Magister Scientiae - MSc (Chemistry)
Although cadmium and lead chalcogenide quantum dot have excellent optical and photoluminescent properties that are highly favorable for biological applications, there still exists increasing concerns due to the toxicity of these metals. We, therefore, report the synthesis of new aqueous soluble IrSe quantum dot at room temperature utilizing a bottom-up wet chemistry approach. NaHSe and H2IrCl6 were utilized as the Se and Ir source, respectively. High-resolution transmission electron microscopy reveals that the synthesized 3MPA-IrSe Qd are 3 nm in diameter. The characteristics and properties of the IrSe Qd are investigated utilizing, Selected Area electron diffraction, ATR- Fourier Transform Infra-Red Spectroscopy, Energy Dispersive X-ray spectroscopy, Photoluminescence, Cyclic Voltammetry and chronocoulometry. A 3 fold increase in the optical band gap of IrSe quantum dot in comparison to reported bulk IrSe is observed consistent with the effective mass approximation theory for semiconductor materials of particles sizes < 10 nm. The PL emission of the IrSe quantum dot is at 519 nm. Their electro-activity is studied on gold electrodes and exhibit reduction and oxidation at - 107 mV and +641 mV, with lowered reductive potentials. The synthesized quantum dot are suitable for low energy requiring electrochemical applications such as biological sensors and candidates for further investigation as photoluminescent biological labels.

Pour que l'énergie photovoltaïque devienne compétitive, les coûts de production doivent être baissés et l'efficacité des cellules augmentée. Les cellules solaires à base de nanocristaux semi-conducteurs constituent une approche prometteuse pour remplir ces objectifs combinant une mise en œuvre par voie liquide avec la possibilité d'ajuster précisément la largeur de bande interdite et les niveaux électroniques. Aujourd'hui, les rendements de conversion des cellules constituées de nanocristaux de sulfure de plomb approchent les 7%. Seulement, à cause des normes européennes destinées à l'affranchissement du plomb du fait de ses risques pour la santé et l'environnement, de nouveaux matériaux doivent être trouvés. Cette thèse concerne la synthèse de nouveaux types de nanocristaux semi-conducteurs et leur application dans des cellules solaires. La synthèse des nanocristaux de CuInSe2 et de SnS de taille et de forme contrôlées a été effectuée, notamment par des voies de synthèses reproductibles dont le passage à grande échelle est facilement possible. Une analyse approfondie de la structure des nanocristaux de SnS par spectroscopie Mössbauer a montré que ces nanocristaux avaient une forte tendance à s'oxyder, ce qui limite leur utilisation dans des dispositifs électroniques après exposition à l'air. La constitution de couches minces continues ayant de bonnes propriétés électriques a été effectuée par le dépôt contrôlé de nanocristaux ainsi que l'échange de leurs ligands de surface. En particulier, un nouveau type de ligand inorganique a été utilisé qui a montré une augmentation de la conductivité des films multiplié par quatre ordres de grandeurs par rapport aux ligands initiaux. Enfin, la préparation de cellules solaires basées sur ces couches minces de nanocristaux a montré des résultats encourageants et notamment un clair effet photovoltaïque lorsque le dépôt est effectué sous atmosphère inerte
In order to be cost-effective, photovoltaic energy conversion needs to improve the solar cell efficiencies while decreasing the production costs. Nanocrystal based solar cells could fulfil these requirements through solution-processing, band gap and energy level engineering. PbS nanocrystal thin films already proved their potential for use as solar cell active materials with power conversion efficiencies approaching 7%. However, since lead based compounds are not compatible with European regulations and present high risks for health and environment, semiconductor nanocrystals of alternative materials have to be developed. This thesis focuses on novel types of semiconductor nanocrystals and their application in photovoltaics. The first part of the study deals with the synthesis of size- and shape-controlled CuInSe2 and SnS nanocrystals. An in-depth investigation of the structure of SnS nanocrystals using Mössbauer spectroscopy revealed their high oxidation sensitivity, which limits their usability in optoelectronic devices after air exposure. The second part deals with the thin film preparation and the surface ligand exchange of the obtained nanocrystals. Using a fully inorganic nanocrystal-surface ligand system, the deposited films exhibited a current density improved by four orders of magnitude as compared to the initial ligands. Finally, solar cell devices based on nanocrystal thin films were fabricated, which showed encouraging results with a clear photovoltaic effect when processed under inert atmosphere

Abhijit Bera_Ph.D thesis
The work incorporated in this thesis is mainly focused on various single source metal precursors like metal thiolates and metal dithiocarbamate complexes. Herein, several simple and general methods have been developed for the synthesis of various such single source metal precursors, which comprising the main two constituents of metal chalcogenide nanocrystals (NCs), namely, the tiny inorganic metal chalcogenide complex as core and an organic molecule as shell. Specially, both binary metal thiolates and bimetallic (ternary) thiolates have been prepared and both of them turned out to be excellent precursors for the synthesis of metal sulfide/selenide NCs. The methods used to prepare metal chalcogenide NCs included a direct-heating (solvo-thermal decomposition) method or solid state grinding method. First, the large scale synthesis of various 2D molecular precursors like metal thiolates and metal dithiocarbamate complexes (M-C8DTCA) have been developed and studied their thermal decomposition to metal sulfide NCs via solution based methods. We observed that some of the metal thiolates like Pb-thiolate requires very high temperature to decompose into PbS resulting in particles bigger than their Bohr exciton radius and hence displayed poor optical properties. In the next, to reduce the decomposition temperature an active sulfur precursor called octyl ammonium octyldithiocarbamate (C8DTCA) has been utilized for the synthesis of various metal sulfide NCs (including most challenging PbS NCs, with tunable optical properties) by solution based method (hot injection) or solid state grinding method. We also show that the size of the nanocrystals could be controlled by changing the reaction temperature or metal: chalcogenide precursor ratio. Interestingly, we have also been successful in establishing that these newly developed solid state grinding methods are scalable without compromising their structural and optical properties. The binary or ternary materials synthesized by these solid state routes could be re-dispersed as desired in non-polar organic solvents allowing them to be solution processible. The optical properties of the metal chalcogenide nanocrystals could further be improved by post synthetic surface passivation.
CSIR-NCL
AcSIR

Bright emitters with photoluminescence in the spectral region of 800–1600 nm are increasingly important as optical reporters for molecular imaging, sensing, and telecommunication and as active components in electrooptical and photovoltaic devices. Their rational design is directly linked to suitable methods for the characterization of their signal-relevant properties, especially their photoluminescence quantum yield (Φf ). Aiming at the development of bright semiconductor nanocrystals with emission >1000 nm, we designed a new NIR/IR integrating sphere setup for the wavelength region of 600–1600 nm. We assessed the performance of this setup by acquiring the corrected emission spectra and Φf of the organic dyes |trybe, IR140, and IR26 and several infrared (IR)-emissive Cd₁₋ₓHgₓTe and PbS semiconductor nanocrystals and comparing them to data obtained with two independently calibrated fluorescence instruments absolutely or relative to previously evaluated reference dyes. Our results highlight special challenges of photoluminescence studies in the IR ranging from solvent absorption to the lack of spectral and intensity standards together with quantum dot-specific challenges like photobrightening and photodarkening and the size-dependent air stability and photostability of differently sized oleate-capped PbS colloids. These effects can be representative of lead chalcogenides. Moreover, we redetermined the Φf of IR26, the most frequently used IR reference dye, to 1.1 × 10⁻³ in 1,2-dichloroethane DCE with a thorough sample reabsorption and solvent absorption correction. Our results indicate the need for a critical reevaluation of Φf values of IR-emissive nanomaterials and offer guidelines for improved Φf measurements.

Pour que l'énergie photovoltaïque devienne compétitive, les coûts de production doivent être baissés et l'efficacité des cellules augmentée. Les cellules solaires à base de nanocristaux semi-conducteurs constituent une approche prometteuse pour remplir ces objectifs combinant une mise en œuvre par voie liquide avec la possibilité d'ajuster précisément la largeur de bande interdite et les niveaux électroniques. Aujourd'hui, les rendements de conversion des cellules constituées de nanocristaux de sulfure de plomb approchent les 7%. Seulement, à cause des normes européennes destinées à l'affranchissement du plomb du fait de ses risques pour la santé et l'environnement, de nouveaux matériaux doivent être trouvés. Cette thèse concerne la synthèse de nouveaux types de nanocristaux semi-conducteurs et leur application dans des cellules solaires. La synthèse des nanocristaux de CuInSe2 et de SnS de taille et de forme contrôlées a été effectuée, notamment par des voies de synthèses reproductibles dont le passage à grande échelle est facilement possible. Une analyse approfondie de la structure des nanocristaux de SnS par spectroscopie Mössbauer a montré que ces nanocristaux avaient une forte tendance à s'oxyder, ce qui limite leur utilisation dans des dispositifs électroniques après exposition à l'air. La constitution de couches minces continues ayant de bonnes propriétés électriques a été effectuée par le dépôt contrôlé de nanocristaux ainsi que l'échange de leurs ligands de surface. En particulier, un nouveau type de ligand inorganique a été utilisé qui a montré une augmentation de la conductivité des films multiplié par quatre ordres de grandeurs par rapport aux ligands initiaux. Enfin, la préparation de cellules solaires basées sur ces couches minces de nanocristaux a montré des résultats encourageants et notamment un clair effet photovoltaïque lorsque le dépôt est effectué sous atmosphère inerte.

Nanometer-sized semiconductor crystals, termed ‘quantum dots’, are of fundamental interest because of their size-tunable properties. Three-dimensional quantum confinement of charge carriers by the small crystal size results in discrete atomic-like electronic states. This dissertation describes the synthesis and in-depth characterization of lead chalcogenide colloidal quantum dots for forthcoming applications as near-infrared single photon emitters. An efficient single photon source that operates at telecommunication wavelengths (between 1.3 and 1.6 µm) is a basic requirement for many photonic quantum technologies, such as quantum computing and quantum cryptography. Chapters 1 and 2 of this work provide an introduction to colloidal quantum dots and their use as single photon emitters. It includes a description of photonic crystal microcavities and their ability to enhance the spontaneous emission rate of quantum dots. The synthesis and basic characterization of PbSe and PbS quantum dots is then discussed in chapter 3. In particular, a new synthetic method for the preparation of highly photoluminescent PbS quantum dots is presented. PbSe/CdSe core/shell quantum dots prepared by a cation exchange reaction are also described and a significant improvement in photo-stability is shown. Chapter 3 concludes with a description of three different surface modification techniques. PbSe core and PbSe/CdSe core/shell materials are investigated further in chapter 4 by advanced characterization techniques that include high-angle annular dark field (HAADF) imaging, energy-filtered transmission electron microscopy (EF-TEM) imaging, energy-dependent X-ray photo-electron spectroscopy (XPS), small angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). The information obtained from these techniques is combined to form a structural model of the PbSe core and PbSe/CdSe core/shell quantum dots with greater complexity than previously reported. In chapter 5, the temperature-dependent photoluminescence from PbSe and PbSe/CdSe core/shell quantum dots is discussed and a thermal model is presented that accounts for the large (non-trivial) temperature dependence of the Stokes shift and photoluminescence lineshape over the entire temperature range (4.5 to 295 K). Chapter 6 examines two scalable methods to integrate the colloidal quantum dots into silicon two-dimensional photonic crystal slab microcavities (a requirement for efficient single photon emission). Finally, conclusions and possible future work are discussed in chapter 7.
Graduate

Semiconductor nanocrystals, or quantum dots, have attracted significant interest for use in solid state lighting, biological imaging, photovoltaics, catalysis, and displays such as televisions or tablets. Quantum dots excel in these applications because of their narrow emission profiles, high absorptivity at high energies, and optoelectronic properties that can be easily tuned using colloidal chemistry. The last point in particular has driven the development of new synthetic methods for producing a range of semiconducting materials on the nanoscale. Academically, interest in the synthesis of quantum dots has also extended to the mechanism of their formation and its implications for the growth of nanoscale crystals more generally. This thesis addresses facets of both points above, first by developing several novel syntheses for indium and gallium phosphide nanocrystals, and second by leveraging the synthetic control it allows to study the mechanisms of homogeneous crystal growth. Chapter 1 provides a brief overview of the colloidal syntheses, optoelectronic properties, and formation mechanisms of quantum dots. Emphasis is placed on the development of new chemical syntheses for nanoscale materials and how the size, size distribution, and morphology can be carefully controlled by thoughtful reaction design. The progression of quantum dot synthesis is presented and specific innovations to the precursor and surfactant design are highlighted. Next, a brief discussion about nanocrystal surface chemistry and its impact on the photophysical properties of the inorganic core is described along with its proposed influence on the kinetics of nanocrystal growth. Finally, classical theories of homogeneous crystal growth are presented and used to explain the origin of the exceptionally narrow size distributions accessible in a wide range of materials. Chapter 2 introduces two novel synthetic pathways to InP nanocrystals. The first describes a small library of substituted aminophosphines that can control the precursor conversion reactivity by over an order of magnitude. Leveraging the collection of aminophosphines, it is demonstrated that at sufficiently high temperatures, the rate of precursor conversion can be used to vary the final nanocrystal size—disputing previous findings for InP nanocrystals. We show that the reactivity of the phosphine is governed by a pre-equilibrium between the precursor and an intermediate (P(NHR)3) that goes on to form InP. Variations to the initial aminophosphine substitution pattern change the position of the pre-equilibrium, thereby allowing the rate of [InP]i deposition to be controlled. The second synthetic method leverages metal phosphonate salts as a surfactant to synthesize large samples of InP. We find that the nanocrystals grow via a ripening mechanism and display excellent crystallinity as determined by powder X-ray diffraction and pair distribution function analysis. Finally, we demonstrate that the final nanocrystals are bound by both phosphonates and phosphines through the use of 31P nuclear magnetic resonance spectroscopy. Chapter 3 expands on the syntheses of InP in the previous chapter by developing methods to form GaP, InxGa1-xP, and InP-based core-shell structures. At the onset, two distinct syntheses of GaP are introduced, one similar to the metal phosphonate route used to form InP, and one that used a mixture of amines to stabilize GaP colloidally. The phosphonate method results in small GaP with somewhat indistinct scattering patterns, while the amine method results in large GaP whose morphology can be varied depending on the solvent selected. Leveraging the newly developed InP and GaP syntheses we demonstrate that InxGa1-xP alloys could be directly synthesized from mixtures of In3+ and Ga3+ salts. We also show that InxGa1-xP can be accessed indirectly via cation exchange of Zn3P2 or Cd3P2, however attempts at synthesizing alloys via cation exchange with phosphonate bound GaP were found to be largely unsuccessful. Finally, the chapter contains initial attempts at synthesizing GaP/InP core-shells with the intention of producing GaP/InP/GaP spherical quantum well architectures. Preliminary data show that InP can be deposited using several different methods, though it remains unclear whether the optical properties will be suitable for integration in solid state lighting applications. Chapter 4 examines the crystal growth processes that precede the formation of monodisperse ensembles of InP, PbS, and PbSe nanocrystals. Surprisingly, we find that nucleation persists for a substantial portion of the total reaction time—a stark departure from the canonical “burst” of nucleation proposed originally by Victor LaMer. We go on to measure the nucleation period for a variety of different reaction conditions and find that the fraction of reaction time nucleation extends over is sensitive to both the material and reaction temperature. This is consistent with a mechanism where faster kinetics of monomer attachment reduce the duration of crystal nucleation—a conclusion that can be surmised by nucleation mass balance models that show a clear material and temperature dependence on the rate of nanocrystal growth. We also interrogate the claim that solute molecules accumulate prior to the formation of mature nanostructures. In situ X-ray experiments clearly corroborate the appearance of solute-like species at early reaction times that build up prior to the appearance of crystals with extended structure. Finally, we propose a novel size-focusing mechanism predicated on a size dependent growth rate. Using population mass balance modeling we show that the measurements of size and size distribution are qualitatively consistent with a growth rate inversely proportional to nanocrystal size.

Copper iron chalcogenides constitute a promising class of optoelectronic materials courtesy of their narrow bandgaps and earth abundant constitution. However, they are yet to receive the attention they deserve due to the lack of easy synthetic protocols and poorly understood material properties. Discordant narratives in the literature regarding their optoelectronic properties has also prevented them from being used for device-based applications. This thesis is aimed at rectifying a few of these issues. The objective of this thesis is to synthesize and study the properties of copper iron chalcogenide nanocrystals viz., CuFeS2 and CuFeSe2, and to explore their utility in the context of optoelectronic devices. Chapter 1 provides a brief introduction to the fundamental concepts related to the work described in this thesis. The chapter further discusses the scope and motivation behind the work carried out in this thesis. Chapter 2 describes our efforts to assign the nature of a feature in the optical absorption spectrum of CuFeS2 nanocrystals occurring at ~500 nm. Using a combination of steady-state and time-resolved optical spectroscopy as well as transport measurements we assign the feature to be a localized surface plasmon resonance and attribute the peculiar properties exhibited by CuFeS2 nanocrystals to this feature. Further, the transport measurements revealed that films of these nanocrystals can support a photoresponse. Chapter 3 describes the fabrication and characterization of a broadband photodetector based on CuFeS2 nanocrystals. Briefly, we fabricated heterojunctions of CuFeS2 nanocrystals with bulk n type silicon and demonstrated a broadband photoresponse from 460 nm-2200 nm with response time of the order of microseconds. The photodetector was further found to possess a photothermal response that is bolometric in nature, which allows the device to sense hot objects at room temperature. Chapter 4 describes our efforts to synthesize and study the optoelectronic properties of CuFeSe2 and CuFeSe2-CdS core-shell nanocrystals. We synthesized CuFeSe2 nanocrystals and studied their properties using structural, optical and electrical characterization techniques. The nanocrystals were found to have a very narrow bandgap of 0.11 eV and were also found to exhibit a plasmon resonance at ~410 nm. We further found that the films of these nanocrystals exhibited a photoresponse in the MIR, thus making them a promising candidate for infrared photodetection. We further synthesized highly luminescent CuFeSe2-CdS core-shell nanocrystals and found that the energetic position of their emission is greatly dependent on the sequence in which the shell growth precursors are added to the reaction mixture. Using optical and structural characterization techniques, we find that there are two different core-shell variants that result from the synthesis and their formation is determined by which one of the shell growth precursors is added to the reaction mixture first. The key difference between the two variants were found to be the presence of an interfacial CdSe layer which occurs whenever the cation precursor is added to the reaction mixture first. Chapter 5 describes the synthesis of CuFexGa1-xS2 nanocrystals, a hitherto unknown composition of nanocrystals. Using alloying as a strategy, we synthesized CuFexGa1-xS2 nanocrystals corresponding to different Fe:Ga ratios. The properties of the resulting nanocrystals were found to be greatly dependent on their composition.

碩士
中興大學
奈米科學研究所
99
We study copper sulfide (Cu2-xS), a low-cost and non-toxic light absorbing material and apply to the quantum dot-sensitized solar cells (QDDSC). The copper sulfide quantum dots (QDs) were synthesized on a nanoporous TiO2 electrode by the successive ionic layer adsorption and reaction method (SILAR). To improve efficiency, passivation treatments including a TiO2 under layer ,a ZnS coating and additional treatments including annealing, a TiO2 scattering layer and an Au counterelectrode were used. The best cell yields a short-circuit current of 22.9 mA/cm2, an open circuit voltage of 0.14 V, a fill factor of 20.2% and a power conversion efficiency of 0.65%. By replacing the platinum count erelectrode with a gold electrode, the performance improves to conversion efficiency 0.90%, open circuit voltage 0.17V, short-circuit current 28.1mA/cm2 and fill factor 18.9%. The efficiency of gold-electrode cells are ~ 38% higher than that of the Pt electrode cells. The crystallinity and morphology were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The optical properties of these Cu2-xS QDs were characterized by UV-vis spectroscopy.

博士
國立中興大學
物理學系所
99
We present a new photosensitizer – Ag2S quantum dots (QDs) – for solar cells. The QDs were grown by the successive ionic layer adsorption and reaction deposition method. The assembled Ag2S-QD solar cells yield a best power conversion efficiency of 1.70% and a short-circuit current of 1.54 mA/cm2 under 10.8% sun. The solar cells have a maximal external quantum efficiency (EQE) of 50% at λ=530 nm and an average EQE of ~ 42% over the spectral range of 400–1000 nm. For the family of silver chalcogenide system-Ag2Se quantum dots (QDs), the external quantum efficiency (EQE) spectrum of the assembled cells covers the entire solar power spectrum of 350–2500 nm with an average EQE of ~ 80% in the short-wavelength region (350–800 nm) and 56% over entire solar spectrum. The effective photovoltaic range of Ag2S and Ag2Se were ~ 2-4 and 7–14 times, respectively broader than that of the cadmium calcogenide system—CdS and CdSe. The photocurrent that Ag2Se generates is four times higher than that of N3 dye. The best solar cell yields power conversion efficiencies of 1.76% and 3.12% under 99.4% and 9.7% sun, respectively. We also have demonstrated of Ag2S/Ag2Se co-sensitized solar cells with polysulfide redox couple. Our best efficiency at one sun is 1.27% featuring CuS counterelectrode, which is higher than single QDs under the same kind of electrolyte and an average EQE entire solar spectrum ~ 68%. A higher photocurrent than that of single QDs can be generated from this double-layered QDs is almost five times compared with N3 dye. The results show that silver chalcogenide element can be used as a highly efficient broadband sensitizer for solar cells.