Synthesis and Characterization of CdS/CISe Quantum Dots with Core–Shell Structure

Quantum dots have received great interest due to their excellent optoelectronic properties. However, the surface defects of quantum dots affect the carrier transport and ultimately reduce the photovoltaic efficiency. In this paper, a core–shell quantum dot by hot-injection method is prepared to grow a narrow-band semiconductor layer (CuInSe₂ (CISe) quantumdot) on the surface of a broad-band core material (cadmium sulfide (CdS) nanocrystal). The composition, structure, optical properties, and decay lifetime of CdS/CISe core–shells are investigated in more detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), UV–vis spectrophotometry, and fluorescence spectroscopy. The CdS/CISe core–shell structure has a broadened absorption range and still shows CISe-related quantum effects. The increased size of the core–shell and the smaller specific surface area of the CISe shell layer lead to a lower carrier complexation chance, which improves the carrier lifetime.     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Luminescent Detection of ATP in Aqueous Solution Using Positively Charged CdSe–ZnS Quantum Dots

Commercially available CdSe–ZnS Quantum Dots (QDs) have been modified by exchanging the hydrophobic surface ligands with (2-mercaptoethyl)-trimethylammonium chloride. The resulting water soluble conjugate was titrated with solutions of adenosine triphosphate (ATP), adenosine diphosphate, adenosine monophosphate, guanosine triphosphate (GTP), guanosine diphosphate and guanosine monophosphate in 0.01 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (pH 7.4). A strong fluorescence quench of about 80% was observed for ATP, a quench of 25% was observed for GTP while the others had virtually no effect. The quenching effect of ATP and GTP was attributed to the high negative charge density associated with these substrate’s resulting in a strong attraction to the QD surface enabling them to engage in electron transfer with the excited QD. The lack of fluorescence quenching associated with the other nucleotides was most likely due to their reduced charge density resulting in a lower affinity for the QD surface.     

Time-Resolved Photoluminescence Spectroscopy： A Novel Technique for Determination of Luminescence of Quantum Dots

The time-resolved photoluminescence （PL） spectroscopy measured by the gradually increasing start delay time is utilized as a tool for the determination of the luminescence of quantmn dots （QDs）. The luminescence evolution of self-assembled CdSe QDs during the luminescence decay is fully revealed in terms of the experiment technique. The characteristic narrow luminescence lines of self-assembled CdSe QDs are obtained with increasing start delay time.     

Strained and Piezoelectric Characteristics of Nitride Quantum Dots

The deformation potential and piezoelectric field in nitride GaN/AIN quantum dots （QDs） are investigated in the framework of effective mass approximation （EMA） and finite element method （FEM）. The strained fields and piezoelectric characteristics are studied by using FEM for GaN/AIN QDs （GaN embedded in AIN） in the shape of truncated hexagonal pyramids. We presented the calculated results of the electronic states, wave functions, QD strain field distribution and piezoelectric effects in the QDs. Effects of spontaneous and piezoelectric polarization are taken into account in the calculation. The theoretical results are dependent on QD shapes and sizes. Some of them make the GaN/AIN QDs interesting candidates in optoelectronic applications.     

Synthesis and Characterization of Waterborne Epoxy–Acrylic Corrosion-Resistant Coatings

Waterborne acrylate (AC) and epoxy–acrylate (EAC), and the ether of melamine formaldehyde (EM) as cross-linker, were synthesized. The structures of AC and EAC were characterized by Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (¹H NMR) spectroscopy. Their anticorrosive EM-cured AC and EAC coatings (referred to here as AC/EM and EAC/EM, respectively) were applied on mild steel strips and their properties were evaluated by physicomechanical and corrosion resistance tests, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The coating system EAC/EM exhibited superior performance in all aspects of physicomechanical performance, corrosion resistance, and thermal stability when compared with AC/EM.     

Synthesis and Investigation of Magnesium–Aluminum Nanopowders and Ceramics Activated by Copper

Technical Physics - Nanopowders of two crystalline phases, magnesium–aluminum spinel (98 wt %), and cuprite (2 wt %) have been obtained by laser evaporation of a solid target, which...     

Optical Properties of Quantum Dots with a Core–Multishell Structure

JETP Letters - In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine....     

Synthesis of Indium–Zinc Oxide Nanofibers and Investigation of Their Sensitivity to Ultraviolet Radiation

Optics and Spectroscopy - Indium–zinc oxide (IZO) nanofibers are synthesized on Si–SiO2 substrates. The size, morphology, crystal structure, and composition of nanofibers are...     

Photoluminescence of Self－Assembled InAs／GaAs Quantum Dots Covered by InAlAs and InGaAs Combination Strain－Reducing Layer

Self-assembled lnAs/GaAs quantum dots covered by the 1-nm InxAI(1-x)As (x = 0.2, 0.3) and 3-nm In0.2Ga0.8As combination strain-reducing layer are fabricated, whose height can take up to 30-46 nm. The luminescence emission at a long-wavelength of 1.33μm and the energy separation between the ground and the first-excited state of 86meV are observed at room temperature. Furthermore, comparative study proves that the energy separation can increase to 91 meV by multiple stacking.     

Local Structure of Copper Centers Obtained during Solid-Phase Synthesis in Copper–Mordenite Zeolite

Technical Physics - The influence of temperature conditions for synthesis on the nearest-neighbor environment of copper atoms in copper–mordenite zeolites produced by solid-phase ion exchange...     

Synthesis,Characterization, and X-Band Permittivity Studies of Polypyrrole–Zirconium Composite

The zirconium dioxide-doped polypyrrole composites with different weight percentage were prepared by in situ polymerization methods. The prepared composites were subjected for characterization with different tools like Fourier transform infrared spectroscopy and scanning electron microscopy. The Fourier transform infrared spectroscopy spectra indicate a characteristic peaks which confirms the formation of composites. Scanning electron microscopy image indicates that the metal oxides are agglomerated with polypyrrole due to that the size of the composite particles increases to 7 µm. The real part of permittivity (?′) at X-band measurements of all samples first decrease and then increase in the higher frequency range. Meanwhile, the values of ?″ for polypyrrole–zirconia composites changes spontaneously due to the scattering of free charges explained by Kramers–Kronig relations. The reflection loss graph shows that the maximum reflection loss (RL) reaches up to ?54.6 dB at 10.8 GHz and the effective absorption bandwidth (RL ≤ ?10 dB) is 6.7 GHz.     

Butterfly Effect in a System of Quantum Dots in the Sachdev–Ye–Kitaev Model

JETP Letters - A method has been proposed to calculate the out-of-time time ordered correlator in the generalization of the Sachdev–Ye–Kitaev model with a nonzero spatial dimension. The...     

Quantum-Confinement Effects on Binding Energies and Optical Properties of Excitons in Quantum Dots

Quantum-confinement effects on the binding energy and the linear optical susceptibility of excitons in quantum dots are studied. It is found that the binding energy and the linear optical susceptibility are sensitive to the barrier height and the dot size. For an infinite barrier, the binding energy of excitons decreases monotonically with the increasing dot radius, and the dbsorption intensity has almost the same amplitude with the increasing photon energy. For a finite barrier, the binding energy has a maximum value with the increasing dot radius, and the absorption intensity damps rapidly with the increasing photon energy. The effective mass ratio is also found to have an influence on the binding energy. The results could be confirmed by future experiments on excitons in quantum dots.     

One- and Two-Photon Excited Fluorescence of CdSe and CdSe/ZnS Quantum Dots in n-Hexane

One- and two-photon absorption and excited fluorescence of the CdSe and the core-shell structure CdSe/ZnS quantum dots （QDs） in n-hexane is investigated. The linear and nonlinear absorption coefficients are measured and the two-photon-absorption cross sections of the QDs are also obtained. For both one-photon fluorescence and two-photon fluorescence, the emission efficiency of CdSe/ZnS is much higher than that of CdSe, originating from the effective surface passivation of the core-shell structure.     

Improved Luminescence Properties and Thermal Stability of ZnS Quantum Dots by Organic and Inorganic Passivation

ZnS quantum dots(QDs) synthesized in water and ethanol solutions were coated with polystyrene (PS) and SiO2 shells,respectively.The band edge emission was enhanced by nearly five times after PS coating and by about thirteen times after SiO2 coating,because the surface trap states were removed.From the photoluminescence properties of ZnS QDs coated with PS and SiO2 shells we have detected the improvement of thermal stability.This is due to the fact that the surface passivation can prevent the further growth of the ZnS QDs and the diffusion of oxygen on the surface of ZnS QDs during thermal oxidation.     

Synthesis and Characterization of New Organic–Inorganic Hybrid Nanosilica Fillers Prepared by Sol–Gel Reaction

In light of the success of making multicomponent sol–gel glasses, in this study the synthesis of new hybrid nanosilicas with controlled hydrophilic/hydrophobic properties was carried out by incorporating organic based species with appropriate functional groups to inorganic based alkoxides by co-condensation reaction. Furthermore, in this study the acidity and the water content during reaction synthesis was proven to be critical in controlling the structure of the hybrid nanosilicas. These nanosilicas with different hydrophobicity were obtained by using TEOS (tetraethylorthosilicate) and 10 wt% two organic modifiers (tris-hydroxymethylaminomethane — TRIS — and 1,1,1,3,3,3-hexamethyldisilazane-HDMS). The average diameter of the nano-particles ranged between 7 and 10 nm. The addition of the organic modifiers changed the pH of the hybrid nanosilicas, to a different extent depending on the nature of the organic modifier. The hydrophobicity in the hybrid nanosilicas estimated from pH measurements in water and ethanol, and from IR spectroscopy, can be tailored by incorporating different organic modifiers on the network during synthesis. Finally, the measurement of pH of the hybrid nanosilicas by using solvents with different polarity/acidity was a simple and efficient method of assessing the hydrophobicity of the nanosilicas.     

Optimum Indium Concentration for Growth of 1.3μmInAs/InxGa1-xAs Quantum Dots in a Well

Five InAs/InxGa1-xAs quantum dots in a well （DWELL） with different indium concentration x are grown by solid source molecular beam epitaxy. The high quantum dot density is observed in the InAs/In0.3Ga0.7As DWELL. The photoluminescence （PL） experiments indicate that the ground state peaks of InAs/In0.15 Ga0.85As and InAs/In0.22 Ga0.78As DWELLs shift to 1.31 and 1.33μm, respectively. The optical properties are investigated by using the PL and piezoreflectance spectroscope methods. An abnormal blue shift has been observed with the further increase of x from 0.22 to 0.30.     

Fluorescence “Switch on” of Conjugates of CdTe@ZnS Quantum Dots with Al, Ni and Zn Tetraamino-Phthalocyanines by Hydrogen Peroxide: Characterization and Applications as Luminescent Nanosensors

In this study, we have developed a novel nanoprobe for H₂O₂ based on the conjugation of CdTe@ZnS quantum dots (QDs) to different metal tetraamino-phthalocyanine (MTAPc): (M?=?(OAc)Al, {OAc?=?acetate}, Ni and Zn). Chemical coordination of the QDs to the MTAPc resulted in the fluorescence “switch off” of the linked QDs which was associated with Förster resonance energy transfer (FRET). In the presence of varying concentration of H₂O₂, the fluorescence of the linked QDs was progressively “switched on” and the FRET mechanism between the QDs and the MTAPc was disrupted. The sensitivity/limit of detection of the nanoprobe followed the order: QDs-ZnTAPc (2.2 μM)?>?QDs-NiTAPc (4.4 μM)?>?QDs-AlTAPc (9.8 μM) while the selectivity followed the order: QDs-NiTAPc?>?QDs-AlTAPc?>?QDs-ZnTAPc. The varying degree of sensitivity/selectivity and mechanism of detection is discussed in detail.