Добірка наукової літератури з теми "Cadmium Sulfide Nanocrystals"

In this paper, we give a microscopic view concerning influence of the growth conditions on the physical properties of nanocrystals (NCs) thin films made of CdS, prepared using chemical bath deposition CBD technique. We show a crystalline phase transformation of CdS NCs from hexagonal wurtzite (W) structure to cubic zincblende (ZB) when the growth conditions change, particularly the solution pH values. This effect was confirmed using X-ray diffraction (XRD), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) measurements. The optical absorption spectra allow calculation of the bandgap value, [Formula: see text], where significant increase [Formula: see text]200 meV in the CdS bandgap when transforming from Hexagonal to Cubic phase was found.

In this work, a miniature fluidic synthesis platform utilizing millimeter dimension channels yielding highly reproducible batch synthesis of luminescent cadmium sulfide (CdS) quantum dots and nanocrystals is demonstrated.

Density Functional Theory at the generalized-gradient approximation level coupled with large unit cell method is used to simulate the electronic structure of (II-VI) zinc-blende cadmium sulfide nanocrystals that have dimensions 2-2.5 nm. The calculated properties include lattice constant, conduction and valence bands width, energy of the highest occupied orbital, energy of the lowest unoccupied orbital, energy gap, density of states etc. Results show that lattice constant and energy gap converge to definite values. However, highest occupied orbital, lowest unoccupied orbital fluctuates indefinitely depending on the shape of the nanocrystal.

Photoluminescent (PL) properties of undoped nanocrystals of cadmium sulfide were investigated as a function of excitation power intensity. Room-temperature PL spectra of CdS nanocrystals grown from the gas phase revealed two emission bands: with peak positions at 510 nm (near-band-edge emission) and close to 690 nm (deep trap defects). Tunable photoluminescence of CdS nanocrystals with the exchange of the main radiative channel from relaxation through defect levels to direct near-band-edge relaxation with the change of the color was demonstrated. Nonlinear behavior of the intensities of near-band-edge and defect level emission lines as well as the blue shift of the peak of defect level emission are discussed and explained by the finite capacitance of the defect subzone in the forbidden gap. The origin of the red-light emission is due to native defects such as sulfur vacancies or twinning interfaces.

In recent decades, 1D structures, such as Nano whiskers, nanowires, Nano rods, etc., have attracted considerable interest of researcher due to their highly promising application in electronics, photonics, energy conversion and storage systems, medicine and pharmacology, and in modeling interaction with biomolecules and living calls. A prominent place among nanostructures is occupied by 1D nanostructures grown perpendicular to the substrate surface. These nanostructures are called Nano whiskers. Below reported about physicochemical studies of the ensemble of filamentary nanocrystals of cadmium sulfide synthesized by vacuum evaporation and condensation. It is presented the results of technological experiments, the results of electronic microscopy and the results of electron diffraction studies. It is shown that by means of vacuum evaporation and condensation it is possible to synthesize filamentary nanocrystals of diameter from 10 nm to few µm and of the length of few mm. It is revealed technological conditions necessary for the synthesis of filamentary nanocrystals. It is determined relation between growth rate of filamentary nanocrystals and their linear characteristics. It is shown that mechanism of growth of nanocrystals synthesized by used method is in full accordance with model views of classical mechanism vapor-liquid-crystal of Givargizov-Chernov. For revealing of crystalline perfection of filamentary nanocrystals it is used electron diffraction method, at so doing for increasing of the level of analytical signal it is used superposition of diffraction patterns from ensemble of filamentary nanocrystals. The method proposed permitted establish high degree of perfection of filamentary nanocrystals synthesized by vacuum evaporation and condensation.

We introduce new oxygen- and moisture-proof polymer matrixes based on polyisobutylene (PIB) and its block copolymer with styrene [poly(styrene-block-isobutylene-blockstyrene), PSt-b-PIB-b-PSt] for the encapsulation of colloidal semiconductor nanocrystals. In order to prepare transparent and processable composites, we developed a special procedure of nanocrystal surface engineering including ligand exchange of parental organic ligands to inorganic species followed by the attachment of specially designed short-chain PIB functionalized with an amino group. The latter provides excellent compatibility of the particles with the polymer matrixes. As colloidal nanocrystals, we chose CdSe nanoplatelets (NPLs) because they possess a large surface and thus are very sensitive to the environment, in particular in terms of their limited photostability. The encapsulation strategy is quite general and can be applied to a wide variety of semiconductor nanocrystals, as demonstrated on the example of PbS quantum dots. All obtained composites exhibited excellent photostability, being tested in a focus of a powerful white-light source, as well as exceptional chemical stability in a strongly acidic media. We compared these properties of the new composites with those of widely used polyacrylate-based materials, demonstrating the superiority of the former. The developed composites are of particular interest for application in optoelectronic devices, such as color-conversion light-emitting diodes, laser diodes, luminescent solar concentrators, etc.

Les Quantum Dots (QDs) sont des particules cristallines de semi-conducteur ou du métal de forme sphérique et de dimension nanométrique. L'intérêt majeur des QDs réside dans leur grande adaptabilité à de nombreuses applications biologiques.Le but de mon travail était de développer une nouvelle classe de QDs de faible toxicité afin de les utiliser pour la bio-imagerie des cellules cancéreuses. Pour cela, il est nécessaire de préparer des sondes hydrosolubles, photostables, biocompatibles, de luminescence élevée et possédant une faible toxicité. La synthèse des cœurs de type ZnS and ZnSe dopés au manganèse ou au cuivre et stabilisés par l’acide 3-mercapropropionique ou par le 1-thioglycérol a été réalisée par la voie hydrothermale. Les techniques analytiques de caractérisation utilisées sont la spectroscopie UV-visible, la spectroscopie de fluorescence, la diffraction des rayons X (XRD), la spectroscopie photoélectronique de rayon X (XPS), la microscopie électronique à transmission (TEM), la diffusion dynamique de la lumière DLS, la spectroscopie infra-rouge (IR), et la résonance paraélectronique (RPE). La toxicité des QDs a été déterminée sur des cellules cancéreuses. Les différents test de cytotoxicité (MTT, XTT et ferrous oxidation-xylenol orange) ont été réalisés. Finalement, les QDs de type ZnS:Mn conjugués à l’acide folique ont été utilisés pour la bio-imagerie des cellules cancéreuses par le biais d’une excitation biphotonique
Semiconductor QDs are tiny light-emitting crystals, and are emerging as a new class of fluorescent labels for medicine and biology. The aim of this work was to develop a new class of non-toxic QDs probes with essential attributes such as water dispersibility, photostability, biocompatibility, high luminescence and possible excitation with low-energy visible light, using simple processing method. Such nanoprobes could be used for bio-imaging of cancer cells. In the performed studies, I focused on ZnS and ZnSe QDs as they are cadmium-free and might be excited biphotonically.The synthesis protocols of ZnS and ZnSe QDs doped with two ions such as Mn or Cu and stabilized by 3-mercaptopropionic acid or 1-thioglycerol were established, followed by NCs characterization (diameter, surface charge, photophysical properties, …) using analytical techniques such as spectrophotometry UV-vis, fluorimetry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering (DLS), infra-red analysis (FT-IR), thin layer chromatography (TLC) and electron paramagnetic resonance (EPR). The cytotoxicity of synthesized bare and conjugated NPs was evaluated on cancer cell lines using MTT, XTT and ferrous oxidation-xylenol orange assay.Finally, chosen well fluorescent and weakly toxic types of as-prepared and characterized QDs were used for bio-imaging of cancer cells. In these experiments, FA-functionalized NCs were excited biphotonically. The performed experiments showed the potential of QDs as cancer cells fluorescent markers and that they accumulate around the cell nuclei

The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)

Abstract The present Thesis discusses various Titanium dioxide (TiO2) - Cadmium Sulfide (CdS) assemblies for efficient harvesting of the solar photon. Inorganic semiconductor nanocrystals such as CdS have attracted considerable attention in the realm of solar photon harvesting mainly due to beneficial properties such as easy tunability of their optical, electrical, magnetic properties, functional stability i.e. non-degradability under atmospheric conditions, materials synthesis and device fabrication by benchtop methods. However, a major detrimental issue that prevails in semiconductor nanocrystals is charge recombination. Tailored semiconductor assemblies with favourable energetics can significantly alleviate the effect of charge recombination. Improved charge separation in an optimum semiconductor assembly may aid in decrease in charge recombination and hence, result in enhanced photoelectrochemical function. Owing to the band structure, CdS can harvest solar photon and when attached with wide band gap semiconductor TiO2. The photogenerated electron in the CdS conduction band can be injected at ultrafast timescales to the conduction band of the TiO2. The thesis discusses easy and cost-effective synthesis of various TiO2 and CdS assemblies and explores application of them in photovoltaics, photocatalysis and (photo conducting) image sensor. Various interactions and physical properties are also studied including the ultrafast photoinduced electron dynamics from CdS to TiO2. Sun is a great source of alternative energy especially, electrical energy. In this context, nanostructured semiconductor assemblies have demonstrated great potential towards efficient harvest of the solar photon. In Chapter 1, general properties and scope of nanostructured assemblies in the context of few applications namely liquid junction semiconductor sensitized solar cell (for solar photon conversion to electricity), visible light photocatalysis (to degrade pollutants using solar photon) and large area image sensor (sensitive to white light) are discussed. The Chapter also discusses the various characterization and quantification methods which not only provide detailed analysis of properties of the novel semiconductor assemblies but also throw light on the prospects for industrial applications. Chapters 2 to 5 comprises of discussions on the electronic and photovoltaic properties of various shaped semiconductor nanocrystals (average size  30 nm). In Chapter 2, cadmium sulfide (CdS) semiconductor nanocrystals of various shapes (tetrapod, tetrahedron, sphere and rod) obtained using an optimized solvothermal process exhibited a mixed cubic (zinc blende) and hexagonal (wurtzite) crystal structure. The various nanocrystal shapes obtained here are a consequence of the simultaneous presence of wurtzite and zinc blende phases in varying amounts. The simultaneous presence of the two crystal phases in varying amounts is observed to play a pivotal role in not only determining the final nanocrystal shape but also both the electronic and photovoltaic properties of the CdS nanocrystals. Light to electrical energy conversion efficiencies measured in two-electrode configuration laboratory solar cells remarkably decreased by one order in magnitude from tetrapod  tetrahedron  sphere  rod. The tetrapod-CdS nanocrystals, which displayed the highest light to electrical energy conversion efficiency, showed a favourable shift in position of the conduction band edge leading to highest rate of electron injection (from CdS to TiO2) and lowest rate of electron-hole recombination (higher free electron lifetimes). Chapter 2 successfully demonstrated that the photovoltaic (PV) efficiency of a device can be influenced by tuning the shape of the light harvester nanocrystal. While the light to electricity conversion efficiencies varied by one order in magnitude between the various nanocrystal shapes (average size  30 nm), the magnitude of the efficiencies was itself not very high. In Chapter 3, the same nanocrystal shapes are used to sensitize multi-layered Titania films and liquid junction solar cells are then fabricated using them. This optimization of the cell configuration showed tremendous enhancement in the light to electricity conversion efficiency by nearly one order in magnitude compared to the ones discussed in Chapter 2. The semiconductor-electrolyte interface is also studied in detail by performing ac-impedance spectroscopy on the full cell to estimate the electron lifetime of the device. The estimated recombination resistance and the electron lifetime are observed to follow the same trend as of the PV-performances of the cells composed of various shaped nanocrystals in the new configuration. The photoinduced electron transfer processes in a nano-heterostructure semiconductor assembly are complex and depend on various parameters of the constituents of the assembly. Chapter 4 discusses the ultrafast electron transfer characteristics of an assembly comprising of a wide band gap semiconductor, titanium dioxide (TiO2) attached to light harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic phase content. The nanocrystals employed here are the same as that discussed in Chapters 2 and 3. Quantitative analysis of synchrotron high resolution X-ray diffraction data of CdS nanocrystals precisely reveal the presence of both wurtzite and zinc blende phases in varying amounts. The biphasic nature of CdS influences directly the shape of the nanocrystal at long reaction times (as also highlighted in Chapters 2 and 3) as well as the transfer of the photo-excited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. Higher amount of zinc blende phase is observed to be beneficial for fast electron transfer across the CdS-TiO2 interface. The electron transfer rate constant differs by one order in magnitude between the CdS nanocrystals and varies linearly with the fraction of the phases. Chapters 2-4 show that the electron recombination lifetime in a sensitized semiconductor assembly, which has a major impact on the performance in a solar cell, is greatly influenced by the crystal structure and geometric form of the light harvesting semiconductor nanocrystal. In Chapter 5, the final Thesis Chapter related to semiconductor assemblies for liquid junction based semiconductor sensitized solar cells, deals with the influence of downsizing of light harvester nanocrystals on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of the solar cell. The semiconductor (photoanode)-electrolyte interface in a liquid junction semiconductor sensitized solar cell which has a direct impact on the photovoltaic performance is probed here systematically. The light harvesting cadmium sulfide (CdS) nanocrystals (average size  6-12 nm) of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by one order in magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystals sensitized solar cells. In Chapter 6, focus shifts from liquid junction semiconductor sensitized solar cells to visible light photocatalysis. The possibility of harvesting light via a semiconductor assembly of the same chemical compositions (as in Chapters 2-5) however, in a different spatial configuration is again explored. An unprecedented morphology of titanium dioxide (TiO2) and cadmium sulfide (CdS) self-assembly obtained using a ‘truly’ one-pot and highly cost-effective method with a multi-gram scale yield is discussed here. The TiO2– CdS assembly comprised of TiO2 and CdS nanoparticles residing next to each other homogeneously self-assemble into ‘woollen knitting ball’ like microspheres. The microspheres exhibited remarkable potential as a visible light photocatalysts with high recyclability. Finally, in Chapter 7, a semiconductors assembly comprising of titanium dioxide (TiO2) rods sensitized by cadmium sulfide (CdS) nanocrystals for potential applications in large area electronics on three dimensional (3-D) substrates is discussed. Vertically aligned TiO2 rods are grown on a substrate using a 1500C process flow and then sensitized with CdS by SILAR method at room temperature. This structure forms an effective photoconductor as the photo-generated electrons are rapidly removed from the CdS (‘carpet’) via the TiO2 thereby permitting a hole rich CdS. Current-voltage characteristics are measured, and models illustrate space charge limited photo-current as the mechanism of charge transport at moderate voltage bias. With this stable assembly, high speed can be achieved. The frequency response with a loading of 10 pF and 9 M shows a half power frequency of 100 Hz.

The continuous microreactor-assisted solution deposition (MASD) process was used for the deposition of CdS thin films on fluorine-doped tin oxide (FTO) glass. The MASD system, including a T-junction micromixer and a microchannel heat exchanger is capable of isolating the homogeneous particle precipitation from the heterogeneous surface reaction. The results show a dense nanocrystallite CdS thin films with a preferred orientation at (111) plane. Focused-ion-beam was used for TEM specimen preparation to characterize the interfacial microstructure of CdS and FTO layers. The band gap of the microreactor-assisted deposited CdS film was determined at 2.44 eV. X-ray Photon Spectroscopy show the bindings of energies of Cd 3d₃/₂, Cd 3d₅/₂, S 2p₃/₂ and S 2p₁/₂ at 411.7 eV, 404.8 eV, 162.1 eV, and 163.4 eV, respectively. The film growth kinetics was studied by measuring the film thickness deposited from 1 minute to 15 minutes in physical (FIB-TEM) and optical (reflectance spectroscopy) approaches. A growth model that accounts for the residence time in the microchannel using empirical factor (η) obtained from previous reported experimental data. Applying this factor in the proposed modified growth model gives a surface reaction rate of 1.61*10⁶ cm⁴ mole⁻¹s⁻¹, which is considerable higher than the surface reaction rates obtained from the batch CBD process. With the feature of separating homogeneous and heterogeneous surface reaction, the MASD process provides the capability to tailor the surface film growth rate and avoid the saturation growth regime in the batch process. An in situ spectroscopy technique was used to measure the UV-Vis absorption spectra of CdS nanoparticles formed within the continuous flow microreactor. The spectra were analyzed by fitting the sum of three Gaussian functions and one exponential function in order to calculate the nanoparticle size. This deconvolution analysis shows the formation of CdS nanoparticles range from 1.13 nm to 1.26 nm using a residence time from 0.26 s to 3.96 s. Barrier controlled coalescence mechanism seems to be a reasonable model to explain the experimental UV-Vis data obtained from the continuous flow microreactor, with a rate constant k' value of 2.872 s⁻¹. Using CFD, low skewness value of the RTD curve at high flow rate (short τ) suggests good radial mixing at high flow rate is responsible for the formation of smaller CdS nanoparticles with a narrower size distribution. The combination of CdS nanoparticle solution with MASD process resulted in the hindrance of CdS thin film deposition. It is hypothesized that the pre-existing sulfide (S²⁻) ions and CdS nanoparticles changes the chemical species equilibrium of thiourea hydrolysis reaction. Consequently, the lack of thiourea slows down the heterogeneous surface reaction. To test the scalability of the MASD process, a flow cell and reel-to-reel (R2R)-MASD system were setup and demonstrated for the deposition of CdS films on the FTO glass (6" x 6") substrate. The film deposition kinetics was found to be sensitive to the flow conditions within the heat exchanger and the substrate flow cell. The growth kinetics of the CdS films deposited by R2R-MASD process was investigated by with a deposition time of 2.5 min, 6.3 min, and 9 min. In comparison with the continuous MASD process, the growth rate in R2R-MASD is higher, however more difficult to obtain a linear relationship with the deposition time.
Graduation date: 2012
Access restricted to the OSU Community at author's request from Jan. 13, 2012 - Jan. 13, 2013

Cadmium sulfide nanoparticles generally exhibit quantum confinement effects when the particle size is less than 10 nm and approaches the Bohr exciton radius. It is a widely used buffer material in solar cells owing to its wide band transmission of solar light and hence used as a window layer in photovoltaic devices. Sonochemical synthesis permits the rapid heating of reactant baths by acoustic cavitation leading to high local temperatures. In this research, results from batch trials for heating and synthesis are reported. These results were used to design experiments for the continuous synthesis of CdS nanoparticles using a sonochemical reactor consisting of a flow cell and a high intensity horn. By utilizing the continuous synthesis approach a more than hundred fold reduction in processing time over batch synthesis for similar product was reported.

The need for new thermal management technologies to cool electronic components with their ever-increasing density and power requirements has renewed interest in techniques for measuring liquid-phase coolant temperatures, especially nonintrusive techniques with micron-scale spatial resolution. A variety of optical liquid-phase thermometry techniques exploit the changes in the emission characteristics of fluorescent, phosphorescent or luminescent tracers suspended in a liquid-phase coolant. Such techniques are nonintrusive and have micron-scale spatial resolution, but they also require optical access to both excite and image the emissions. Silicon (Si), the leading material for electronic devices, is opaque at visible wavelengths, but is partially transparent in the near-infrared (IR). To date, the only tracers that emit at near-IR wavelengths with reasonable quantum yield are IR quantum dots (IRQD), colloidal nanocrystals of semiconductor materials such as lead sulfide (PbS). Previous work has shown that the intensity of emissions at 1.55 μm from PbS IRQD suspended in toluene are temperature-sensitive, decreasing by as much as 15% as the temperature increased from 20 °C to 60 °C. The accuracy of temperature measurements using PbS IRQD was estimated to be about 5 °C, based on 95% confidence intervals, where the major limit on the accuracy of the technique was the poor photostability of this material [1]. Recently, a new method for creating a cadmium sulfide (CdS) overcoat layer on PbS “cores” has been developed [2]. The experimental results presented here on the temperature sensitivity of these PbS/CdS core-shell infrared quantum dots with an emission peak around 1.35 μm and a diameter of 5.7 nm (with a core diameter of 4 nm) suggest that these new core-shell structures are more temperature-sensitive than the PbS cores. These core-shell quantum dots, when suspended in toluene, were found to have a 0.5% decrease in emission power per °C increase in temperature at suspension temperatures ranging from 20 °C to 60 °C. The uncertainty in the liquid-phase temperatures derived from these emissions was estimated to be less than 0.3 °C based on the standard deviation. Furthermore, the PbS/CdS quantum dots were highly photostable, with a consistent response more than 100 days after suspension. These results imply that that these new IRQD can be used to measure liquid-phase coolant temperatures without disturbing the flow of coolant at an accuracy comparable to commercially available thermocouples in monolithic Si devices.