Cadmium Telluride Quantum Dots

Among nanomaterials, CdTe holds special technological importance as the only known II-VI material that can form conventional p-n junctions. This makes CdTe very important for the dev

Cadmium Telluride Quantum Dots

Author: John Donegan

Publisher: CRC Press

ISBN: 9814316091

Page: 248

View: 146

In the last two decades, semiconductor quantum dots—small colloidal nanoparticles—have garnered a great deal of scientific interest because of their unique properties. Among nanomaterials, CdTe holds special technological importance as the only known II–VI material that can form conventional p–n junctions. This makes CdTe very important for the development of novel optoelectronic devices such as light-emitting diodes, solar cells, and lasers. Moreover, the demand for water-compatible light emitters and the most common biological buffers give CdTe quantum dots fields a veritable edge in biolabeling and bioimaging. Cadmium Telluride Quantum Dots: Advances and Applications focuses on CdTe quantum dots and addresses their synthesis, assembly, optical properties, and applications in biology and medicine. It makes for a very informative reading for anyone involved in nanotechnology and will also benefit those scientists who are looking for a comprehensive account on the current state of quantum dot–related research.

Synthesis of Cadmium Telluride Quantum Wires and the Similarity of Their Band Gaps to Those of Equidiameter Cadmium Telluride Quantum Dots

The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed.

Synthesis of Cadmium Telluride Quantum Wires and the Similarity of Their Band Gaps to Those of Equidiameter Cadmium Telluride Quantum Dots

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High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range of 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi-nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure, and grow along the [002] direction (parallel to the c axis). The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed.

CdTe ZnTe Quantum Dots Growth and Optical Properties

This paper gives an overview of molecular beam epitaxy (MBE) growth of and the optical properties of Cadmium Telluride (CdTe) quantum dots grown on Zinc Telluride (ZnTe) by self-assembly.

CdTe ZnTe Quantum Dots   Growth and Optical Properties

Author: Sebastian Mackowski

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ISBN:

Page: 11

View: 326

This paper gives an overview of molecular beam epitaxy (MBE) growth of and the optical properties of Cadmium Telluride (CdTe) quantum dots grown on Zinc Telluride (ZnTe) by self-assembly. It is shown that quantum dots in this material system can be obtained either by depositing CdTe at a high substrate temperature or by subjecting CdTe layer to a healing process, up to 70 seconds long before its capping or, eventually, by applying these two methods simultaneously. Moreover, it is found that one can also use the atomic layer epitaxy method to achieve the formation. From optical measurements performed on large quantum dot ensembles it is found that the quantum dot emission is much broader than that of quantum wells, and that it is observable up to much higher temperatures, which indicates strong exciton localization. The latter is also evidenced by an insensitivity of the decay time of the exciton recombination (^3O0 ps) to the temperature. From the presence of a second, very long decay time (^5 ns) and from disappearance of the sharp lines related to recombination in single dots, the acoustic phonon scattering of excitons is found to play an important role in these quantum dot structures. From a magnetic field dependence of the single dot emission energy, the exciton effective g-factor and spatial extension of the exciton wave function are deduced to be equal to -3 and 3 nanometers, respectively. Both the g-factor and the value of the diamagnetic shift are found to be independent of the energy of the quantum dot emission at Beta=Omicron Tau and of the in-plane symmetry of its potential. (11 figures, 35 refs.).

Charge Carrier Processes in Photovoltaic Materials and Devices

Charge separation, transport, and recombination represent fundamental processes for electrons and holes in semiconductor photovoltaic devices.

Charge Carrier Processes in Photovoltaic Materials and Devices

Author: Paul Roland

Publisher:

ISBN:

Page: 108

View: 298

Charge separation, transport, and recombination represent fundamental processes for electrons and holes in semiconductor photovoltaic devices. Here, two distinct materials systems, based on lead sulfide quantum dots and on polycrystalline cadmium telluride, are investigated to advance the understanding of their fundamental nature for insights into the material science necessary to improve the technologies. Lead sulfide quantum dots QDs have been of growing interest in photovoltaics, having recently produced devices exceeding 10% conversion efficiency. Carrier transport via hopping through the quantum dot thin films is not only a function of inter-QD distance, but of the QD size and dielectric media of the surrounding materials. By conducting temperature dependent transmission, photoluminescence, and time resolved photoluminescence measurements, we gain insight into photoluminescence quenching and size-dependent carrier transport through QD ensembles. Turning to commercially relevant cadmium telluride (CdTe), we explore the high concentrations of self-compensating defects (donors and acceptors) in polycrystalline thin films via photoluminescence from recombination at defect sites. Low temperature (25 K) photoluminescence measurements of CdTe reveal numerous radiative transitions due to exciton, trap assisted, and donor-acceptor pair recombination events linked with various defect states. Here we explore the difference between films deposited via close space sublimation (CSS) and radio frequency magnetron sputtering, both as-grown and following a cadmium chloride treatment. The as-grown CSS films exhibited a strong donor-acceptor pair transition associated with deep defect states. Constructing photoluminescence spectra as a function of time from time-resolved photoluminescence data, we report on the temporal evolution of this donor-acceptor transition. Having gained insight into the cadmium telluride film quality from low temperature photoluminescence measurements, we now turn to completed devices, evaluating the influence of back contact transport versus temperature. Cadmium telluride photovoltaic devices are known to form a Schottky junction when simply using a metal back contact. Our group previously reported on the attempted application of iron pyrite nanocrystals as a back contact material due to their high conductivity and doping concentration. These devices, however, exhibited non-ideal current-voltage curves where an S-Kink restricted current collection and reduced efficiency. Here we employ temperature dependent current-voltage measurements to gain insight into the S-Kink behavior and attempt to replicate the current-voltage curves using circuit modeling. We develop a modified diode circuit model where an anti-parallel diode pair serves to limit the current flow at voltages near VOC. This model successfully reproduces the experimental data and provides a means to extract diode parameters from current-voltage plots exhibiting S-Kink behavior.

The Synthesis of CdTe based Quantum Dots

As a multipotent tool for scientific exploration, semiconductor nanoparticles, or quantum dots (QDs), have gained enormous interest in nanoscience in the past two decades.

The Synthesis of CdTe based Quantum Dots

Author: Shohei Taniguchi

Publisher:

ISBN:

Page: 159

View: 574

As a multipotent tool for scientific exploration, semiconductor nanoparticles, or quantum dots (QDs), have gained enormous interest in nanoscience in the past two decades. The research presented here focused on cadmium telluride (CdTe) QDs: novel synthetic methodologies were used to prepare previously inaccessible nanomaterials based on CdTe QDs. -- CdTe/CdSe/ZnSe core/shell/shell QDs were prepared by a one-pot synthesis. The resulting QDs exhibited near infrared emission, were readily dispersed in aqueous media and applied to deep tissue imaging where emission through the skin indicated the gradual transition of the QDs via the lymphatic tract. -- Using a different synthetic approach, CdTe QDs, which were dispersed in organic media, were exposed to mercury cations in a toluene/methanol solution, resulting in CdHgTe nanoalloy formation. The optical characteristics of the resulting materials were substantially red-shifted from those of the original CdTe QDs. Structural changes were also examined and the influence of the addition of metal cations to other colloidal QDs. -- The organometallic compound Cd(TeC6H5)2 was synthesised and used as a single-source precursor for CdTe QDs. Products isolated after thermal decomposition of the single-source precursors showed strong emission of various wavelengths depending on the reaction time. The underlying chemistry on QDs formation was investigated, and CdTe/ZnS QDs were prepared using only single-source precursors. -- To make the QDs useful in biology, the surface of organically synthesised CdTe/ZnS QDs was modified with an amphiphilic protein (hydrophobin) to phase transfer the particles into aqueous solution. The QDs exhibited bright emission after phase transfer, and were applied to cell imaging in order to examine the validity as a fluorophore and the influence on a cell.

Spectroscopic Investigations of Excited Charge Carriers in II VI Nanoparticles

Furthermore, through collaborations with the research group of William E. Buhro, we can also make high quality samples of CdTe QW that enable their quantum-mechanical properties and resultant dynamics to be carefully characterized.

Spectroscopic Investigations of Excited Charge Carriers in II VI Nanoparticles

Author: William Matthew Sanderson

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Page: 253

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The large absorption cross sections and the tunability of the energetic spacings between the states in the conduction (CB) and valence band (VB) within a semiconductor nanoparticle (NP) make them promising media for capturing electromagnetic radiation and converting it into charge carriers, or electricity. In photovoltaic devices that incorporate semiconductor NPs, it would be ideal if every photon could be absorbed by a NP and the carriers could be collected with perfect efficiency and without loss of energy. The relaxation pathways of the carriers within the NPs down to the band edge and their fate at the band edge contribute significantly to this ideal goal. For samples of NPs that are not in a device, but are suspended in solution, the carriers (electrons and holes) would relax to states near the band edge of the CB and VB, and if there are no competing pathways, they would recombine radiatively, and give off light or emission. The specific relaxation mechanisms and their efficiencies depend on many factors including chemical composition, local environment, and the dimensionality of the NP. In this dissertation, I describe the results obtained from a barrage of optical spectroscopy experiments aimed at characterizing the energetics and the relaxation dynamics of photogenerated charge carriers in semiconductor NP samples. Particular focus was placed on samples of one-dimensional (1D) cadmium telluride (CdTe) quantum wires (QWs) as they offer the tunability of their energetics through adjustment of their diameters as well as the length dimensions for efficient charge transport over macroscopic distances. Furthermore, through collaborations with the research group of William E. Buhro, we can also make high quality samples of CdTe QW that enable their quantum-mechanical properties and resultant dynamics to be carefully characterized. In order to probe the role of the densities of the quantum-mechanical states on the relaxation of electrons and holes down to those at the band edge, the dependencies of the efficiency for radiative recombination on the excess energy with which the carriers are prepared was investigated on numerous samples of NPs with varying dimensionality. These samples included cadmium selenide (CdSe) quantum dots (QDs), CdSe quantum platelets (QPs), CdTe QWs, and surface-passivated CdTe QWs. We identified two common trends of the efficiency on excitation energy without distinct differences associated with the dimensionality of the NPs. (1) The overall efficiency of radiative recombination decreases with increasing excitation energy. (2) There are often local minima in the efficiencies when exciting at energies between the spectral features present in the absorption spectra. We were able to conclude that it is not just how high above the band gaps the electrons and holes are prepared, but that at specific energies used to excite these charge carriers, competing, non-radiative relaxation pathways are opened. The relaxation dynamics of the electrons and holes were further investigated in CdTe QWs using time-resolved transient absorption (TA) spectroscopy. In order to untangle the complicated TA signals, we developed a model, termed quantum-state renormalization (QSR), that accounts for the shifting of the quantum-confinement states that occurs due to the change in electron density caused by photoexcitation. The QSR model enabled us to directly probe the population of the electrons and holes through the different states, as well as track their changing energies with time. Several noteworthy results were obtained. The photogenerated holes relax to the band edge on very fast, instrument-limited timescales, and the electrons relax more slowly with a rate of ~0. 6 eV ps−1. The prominent relaxation mechanism was concluded to be a phonon-coupling mechanism, where the carriers relax by converting the kinetic energy associated with quantum confinement to vibrational or phonon modes of the CdTe QW. Lastly, the emission lifetimes of the CdTe QWs were measured as a function of the emission efficiency, or photoluminescence (PL) quantum yield ([phi]PL). The lifetimes for CdTe QW samples with photoluminescence quantum yields > 4% were nearly an order of magnitude greater than the radiative lifetime of CdTe QDs, >̲200 ns versus ~25 ns. We propose these long lifetimes are a consequence of the conservation of momentum that must be maintained during radiative recombination. The photogenerated electron-hole pairs relax to the lowest quantum confinement states, and in these high-quality QWS, the pairs remain bound as 1D excitons. These 1D excitons have a thermal distribution of translational kinetic energy along the long, unconfined dimension of the QWs, and thus they contain significant momentum. This momentum cannot be conserved during radiative recombination, and this channel is closed. Only when the 1D excitons become localized can they emit. These results suggest that long charge carrier lifetimes coupled with the dimensionality of these high-quality semiconductor QWs offer distinct advantages for use in photovoltaics.

Enhanced Specificity of Multiplex Polymerase Chain Reaction Via Cdte Quantum Dots

Proceeds from the sale of this book go to the support of an elderly disabled person.

Enhanced Specificity of Multiplex Polymerase Chain Reaction Via Cdte Quantum Dots

Author: Monterrey Monterrey Press

Publisher:

ISBN: 9781523250813

Page: 36

View: 611

Nanoparticles were recently reported to be able to improve both efficiency and specificity in polymerase chain reaction (PCR). Here, CdTe QDs were introduced into multi-PCR systems. It was found that an appropriate concentration of CdTe QDs could enhance the performance of multi-PCR by reducing the formation of nonspecific products in the complex system, but an excessive amount of CdTe QDs could suppress the PCR. The effects of QDs on PCR can be reversed by increasing the polymerase concentration or by adding bovine serum albumin (BSA). The mechanisms underlying these effects were also discussed. The results indicated that CdTe QDs could be used to optimize the amplification products of the PCR, especially in the multi-PCR system with different primers annealing temperatures, which is of great significance for molecular diagnosis.

CdTe Quantum Dot Fluorescence Thermometry of Rolling Bearing

Temperature is one of the most important parameters affecting the service life and performance of a rolling element bearing component.

CdTe Quantum Dot Fluorescence Thermometry of Rolling Bearing

Author: Ke Yan

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Temperature is one of the most important parameters affecting the service life and performance of a rolling element bearing component. In this paper, a nonintrusive method is developed to monitor the temperature variation of the inner raceway during bearing operation utilizing CdTe quantum dots as the temperature sensors. The CdTe quantum dots were synthesized and were used in constructing a sensor film by means of layer-by-layer electrostatic self-assembly method on an ultrathin glass slice. The peak wavelength shift of the fluorescence spectrum of the sensor film shows a linear and reversible relationship with temperature, and it is used to sense the temperature of the inner raceway. The resolution of the CdTe optothermal sensor is determined to be 0.14 nm/°C. The temperature measurement of rolling element bearing was conducted on a bearing test rig incorporated with an optical fiber fluorescence spectrum detecting system. To verify the accuracy of the temperature obtained by quantum dots sensor film, a thermocouple was used to test the temperature of the inner raceway right before and after the operation. Results show that the temperature obtained by the CdTe quantum dots film sensor is consistent with that by the thermocouple, with an error typically below 10% or smaller.

Probing Electronic and Vibronic States in CdTe Self assembled Quantum Dots and CdS Nanowires Using Room Temperature Resonant Raman Scattering

This is consistent with the interdiffusion of Zn and Cd which makes the confining potential shallower and the average diameter of the quantum dots larger.

Probing Electronic and Vibronic States in CdTe Self assembled Quantum Dots and CdS Nanowires Using Room Temperature Resonant Raman Scattering

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Page: 141

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Resonant Raman scattering (RRS) is a sensitive means to probe electronic and vibrational states in semiconductor nanostructures, even when such states are not accessible either through photoluminescence or transport techniques. In this thesis, resonant Raman scattering is used to probe the electronic and vibronic states of CdTe/ZnTe quantum dots and CdS nanowires at room temperature. In case of the CdTe/ZnTe quantum dots, the sensitivity of the ZnTe longitudinal optical phonon (LO) resonance is used to probe the excited state in the CdTe quantum dots. In addition, this technique is utilized to study the energy shifts due to post growth rapid thermal annealing. Due to annealing, both the ground and excited state energies of the quantum dots move to a higher value. This is consistent with the interdiffusion of Zn and Cd which makes the confining potential shallower and the average diameter of the quantum dots larger. In the case of the CdS nanowires, the 1-LO and 2-LO resonance is readily observed above the broad photoluminescence (PL) background. The lineshape of the integrated intensity of the 1-LO and 2-LO resembles the PL spectrum suggesting that the RRS probes the electronic states in both the ensemble and in single nanowires. In addition, the energy separation between the peaks of the 1-LO and 2-LO resonance of the ensemble CdS nanowires was found to be 34 meV which is comparable to the 37 meV value measured in bulk samples. We also show that spatially-resolved Raman scattering at resonance probes the structural uniformity in a single CdS nanowire and is able to identify the layers of the CdTe/ZnTe quantum dots sample. We found RRS intensity of single CdS nanowire strongly depends on the morphology of the wire. In conclusion, we have shown that RRS at room temperature is a sensitive and nondestructive tool to probe electronic and vibration sates as well as structural uniformity in semiconductor nanostructures.

Effect of Ascorbic Acid on CdTe Quantum Dots and Investigation of Different Chemical Sensors on CdTeHg Quantum Dots

The objective of my thesis is to investigate the effect of ascorbic acid on CdTe quantum dots, which is an antioxidant.

Effect of Ascorbic Acid on CdTe Quantum Dots and Investigation of Different Chemical Sensors on CdTeHg Quantum Dots

Author: Yamini Chitrala

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The objective of my thesis is to investigate the effect of ascorbic acid on CdTe quantum dots, which is an antioxidant. It was observed that the quantum dots recovered their luminescence intensity after the interaction of the ascorbic acid, where the intensity was quenched by the photosensitizer. The luminescence intensity increased gradually for 0.1mM- 0.5mM concentration of ascorbic acid and remained stable with higher concentrations of the antioxidant. Also, it was observed that under some reaction conditions like pH, temperature and concentration, ascorbic acid had a great influence on its photoluminescence. In the second part of the research project, I studied the influence of different chemicals on the luminescence intensity of CdTeHg (NIR) quantum dots. The interaction of certain chemical agents like bleach, dimethyl methyl phosphonate (DMMP), with these quantum dots had quenching effect on the luminescence intensity, whereas with the common background chemicals like ethanol could not quench the luminescence intensity of the quantum dots. Studies were carried out to find out the reasons behind this quenching and whether it is the singlet oxygen generated by these chemicals.