Semi-empirical tight binding modelling of CdSTe/CdTe, ZnSSe/ZnSe and ZnSSe/ CdSe heterostructures

We present a semi-empirical sp³s^? tight binding model to calculate the effects of alloy composition and strain on electronic band structure of Cd and Zn based group II-VI heterostructures for photovoltaic devices. The semi-empirical sp³s^? TB model Hamiltonian includes second nearest neighbor interactions and spin-orbit splitting of p-states. Bond lengths and atomic energies of cation and anion forming ternary semiconductors are taken as nonlinear function of composition. The 2NN sp³s^? tight binding model calculations are compared with those of the package program WIEN2k which uses the generalized gradient approximation (GGA) and local spin density approximation (LSDA) based scaling law for the scissor operator for the self energy corrections to the DFT energy band gaps of semiconductors. We found that both the GGA and LSDA corrected WIEN 2k simulations and 2NN sp³s^? TB model accurately reproduces the band gaps and both the valence band and conduction band dispersion curves at Γ, X and L high symmetry points of Brillouin zone, also in good agreement with experiment.     

Fabrication of ZnO/ZnS/ZnSe nanosheets for enhanced photocatalytic activity under simulated solar light

Journal of Materials Science: Materials in Electronics - Ternary composite ZnO/ZnS/ZnSe nanosheets (NSs) with diverse proportions of three modules successfully manufactured by a plain two-step...     

Synthesis and Characterization of Mn:ZnSe/ZnS/ZnMnS Sandwiched QDs for Multimodal Imaging and Theranostic Applications

In this work, a facile aqueous synthesis method is optimized to produce Mn:ZnSe/ZnS/ZnMnS sandwiched quantum dots (SQDs). In this core–shell co‐doped system, paramagnetic Mn²⁺ ions are introduced as core and shell dopants to generate Mn phosphorescence and enhance the magnetic resonance imaging signal, respectively. T1 relaxivity of the nanoparticles can be improved and manipulated by raising the shell doping level. Steady state and time‐resolved optical measurements suggest that, after high level shell doping, Mn phosphorescence of the core can be sustained by the sandwiched ZnS shell. Because the SQDs are free of toxic heavy metal compositions, excellent biocompatibility of the prepared nanocrystals is verified by in vitro MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide) assay. To explore the theranostic applications of SQDs, liposome‐SQD assemblies are prepared and used for ex vivo optical and magnetic resonance imaging. In addition, these engineered SQDs as nanocarrier for gene delivery in therapy of Panc‐1 cancer cells are employed. The therapeutic effects of the nanocrystals formulation are confirmed by gene expression analysis and cell viability assay.     

含量子点纳米青蒿琥酯胶囊的制备与性能研究

通过在纳米青蒿琥酯胶囊内包埋有荧光特性的量子点,探索纳米药物药理研究可视化的方法;通过初生态微晶法制备了具有可检测荧光特性的纳米胶囊,探讨影响纳米胶囊形成和量子点荧光稳定性的主要因素。     

Effect of annealing dynamics on the ion-engineered CdTe/ZnTe solar cells

Journal of Materials Science: Materials in Electronics - Cd1?xZnxTe (CZT)-based matrices have highest absorption capabilities and tunable band gap, so it can be utilized as the central most...     

A transparent photovoltaic device of NiO/MgO quantum dots/TiO2 arrays pn junction with carrier injection of MgO QDs

Journal of Materials Science: Materials in Electronics - A transparent photovoltaic device of NiO/MgO QDs/TiO2 array pn junction was fabricated via a continuous hydrothermal-hydrolysis-sputtering...     

Effect of CdS Layer Thickness on Thermally Evaporated-CdS/CdTe Solar Cell Efficiency

Journal of Materials Science: Materials in Electronics - CdS thin films were deposited with different thicknesses on Fluorine-doped Tin Oxide glass substrates by vacuum thermal evaporation...     

Generation of white light from co-doped (Cu and Mn) ZnSe QDs

White light generation is achieved by single-step co-doping of copper and manganese into the robust ZnSe quantum dots (QDs) which were synthesised using a wet chemical route. Photoluminescence (PL) emission spectra revealed three peaks related to blue (ZnSe), green (copper related) and orange (manganese related). The PL spectra indicated no surface and/or trap state related emission. Photoluminescence excitation (PLE) measurements confirmed co-doping of copper and manganese in the same QD. PLE spectra recorded with emission wavelength fixed at copper and manganese showed a band edge at the same position, indicating the incorporation of both copper and manganese in the same QD. Time-resolved PL measurements suggest an atomic like nature of Mn and Cu in ZnSe QDs.     

Physical investigation of ZnSe QDs synthesized by polyol method at 200 °C

In this paper, zinc selenide nanoparticles powder was successfully synthesized using rapid polyol method. The preparation method was changed by using new Se solvents in the final stage to delete seleniums which have not participated in reaction product. This change in the preparation method increased the purity of final product (92 %); and using selenium’s solvents as detergents caused the production of ZnSe with roughly 100 % purity. X-ray diffractions showed that the samples had a cubic structure with lattice constant equalling 5.6699 Å and with 5.5 nm for crystallite size. Atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) images showed that the particles were almost spherical and well crystallized ZnSe nanoparticles were formed. The average sizes of nanoparticles were 15 and 16.4 nm for AFM and HRTEM, respectively. Absorption Spectra of all samples showed a blue shift in comparison with bulk ZnSe. It showed low absorption in a wide range of wavelengths. Band gap energy of the pure ZnSe nanoparticles was found to be 4.51 eV, which is higher than that of the bulk value of ZnSe (2.67 eV). Photoluminescence spectra of the samples showed emission at 450–500 nm wavelengths at room temperature which are useful for the application of solar cells, quantum dot light-emitting diodes and blue organic light-emitting diodes devices.     

The Effect of Structural Defects on the Thermal Conductivity of ZnS, ZnSe, and CdTe Polycrystals

ZnS, ZnSe, and CdTe polycrystals are experimentally investigated. Mechanisms are treated which restrict the thermal conductivity in samples prepared by recrystallization pressing and by deposition from the vapor phase and subjected to additional strain. Anomalies are observed on the temperature dependences of the thermal resistance of investigated samples, which are due to the special features of their phonon spectra and to the variation of the contribution made by longitudinal and transverse phonon branches to the heat transfer in the Debye temperature region and higher.     

Phase transformation and thermal stability of ZnSe QDs due to annealing: emergence of ZnO

In the present study ZnSe quantum dots (QDs) were synthesized by chemical co-precipitation method using mercaptoethanol as the capping agent. These nanostructures were characterized for structure and surface morphology by using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry respectively. The average size of ZnSe quantum dots from XRD and HRTEM is found to be 4–5 nm having cubic crystalline phase. Effect of annealing temperature, which were determined as phase transformation temperatures (299 and 426 °C) using differential scanning calorimetry has been investigated for structural and thermal stability of QDs. The XRD of annealed samples at temperatures 325 and 442 °C (slightly higher than the temperatures corresponding to two crystallization peaks in the DSC scan) have been carried out to find structural changes corresponding to these annealing temperatures. Sample annealed at 325 °C showed no change in the phase except improvement in intensity of peaks (crystallinity) whereas sample annealed at 442 °C showed transitions from cubic phase of ZnSe to ZnO and orthorhombic phases of ZnSeO₄, ZnSeO₃. Emergence of ZnO peaks in the XRD pattern of annealed samples have been further verified by Raman spectroscopy of the annealed samples. Besides this crystallization kinetics of ZnSe quantum dots has been employed to determine activation energies of these transitions due to oxidation by employing Kissinger, Augis Bennett and Ozawa’s models. Higher activation energy of crystallization corresponding to first crystallization peak shows that the cubic phase is more thermally stable.     

Wet-electrochemical growth of CdTe layers for photovoltaic applications

Journal of Materials Science: Materials in Electronics - Cadmium telluride (CdTe) thin films have been prepared using a low-cost potentiostatic electrochemical technique from an aqueous...     

Synthesis, morphology, and optical properties of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods

We report the synthesis of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods with a CdSe or CdS shell. The shell growth takes place on the lateral faces of the tetrapod legs, whereas the leg length remains essentially unchanged. Both the CdSe and CdS shell growth shifts the luminescence of the nanoheterostructures to the near-IR range, up to 850 nm, with a quantum yield of up to 20%. Analysis of the kinetics of the shift of the excitonic absorption band during nanoheterostructure growth suggests that the growth rate of CdS exceeds that of CdSe. The variation of the luminescence wavelength with shell thickness is compared to numerical modeling results.     

MoO3–x QDs/MXene (Ti3C2Tx) self-assembled heterostructure for multifunctional application with antistatic,smoke suppression,and antibacterial on polyester fabric

Journal of Materials Science - The exploration of novel and multifunctional coatings through composite nanomaterials design is crucial for developing next-generation polyester fabrics. Herein, we...     

Effects of green-, yellow- and red-emitting CdTe QDs on the interaction between 7-hydroxyflavone/quercetin with human serum albumin in vitro

In this study, 7-hydroxyflavone and quercetin were studied for their affinities for human serum albumin (HSA) in the presence and absence of quantum dots (QDs). Three typical CdTe QDs with maximum emissions of 535?nm (green-emitting, G-QDs), 598?nm (yellow-emitting, Y-QDs) and 654?nm (red-emitting, R-QDs) were tested. The fluorescence intensities of HSA decreased remarkably with addition of QDs. Quercetin resulted in obvious blue-shifts of the maximum λ _em of HSA from 340.8 to 332.6?nm and 7-hydroxyflavone hardly changed the maximum λ _em of HSA in the absence of QDs. The extents of shifts of the maximum λ _em of HSA induced by 7-hydroxyflavone or quercetin in the presence of QDs were larger than that in the absence of QDs. When 8.00?µmol/L of 7-hydroxyflavone was added, the fluorescence decreased by 68.1%, 65.7% and 71.8% in the presence of G-QDs, R-QDs and Y-GDs, respectively, from the original G-QDs, R-QDs and Y-GDs that resulted in 42%, 43% and 55%, respectively. When 8.00?µmol/L of quercetin was tested, the fluorescence decreased by 72.4%, 69.5% and 75.6% in the presence of G-QDs, R-QDs and Y-GDs, respectively. G- and R-QDs hardly affected the affinities of 7-hydroxyflavone and quercetin for HSA. However, Y-QDs decreased the affinities of 7-hydroxyflavone and quercetin for HSA by about 13.61% and 6.8%, respectively.     

Effect of a ZnSe buffer layer on the surface, structural, and optical properties of the ZnTe/ZnSe/GaAs heterostructures

Lattice-mismatched ZnTe epilayers on GaAs (1 0 0) substrates with and without ZnSe buffer layers were grown by using molecular beam epitaxy. AFM, XRD, and TEM measurements were performed to investigate the surface and structural properties of the ZnTe thin films. Photoluminescence, Raman scattering, and TEM measurements showed that the crystallinity of a ZnTe epilayer grown on a GaAs substrate was remarkably improved by using a ZnSe buffer layer. Photoreflectance measurements showed that the strain of the ZnTe layer with the ZnSe buffer layer was smaller than that without the ZnSe buffer layer. These results indicate that ZnTe epitaxial films grown on GaAs substrates with ZnSe buffer layers hold promise for potential applications in optoelectronic devices operating in the blue-green spectral region.     

Coalescence and interface diffusion in linear CdTe/CdSe/CdTe heterojunction nanorods

Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS) along the lengths of individual nanorods aged in hot solvent for different amounts of time. The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model, which appears to provide a reasonable estimate of the coalescence rate.     

Synthesis,characterization, and electrical property of CdTe/trioctylphosphine oxide core/shell nanowires

Thick trioctylphosphine oxide (TOPO) layers were controllably coated onto CdTe nanowires. The shell thicknesses were readily tuned by controlling the reaction temperatures in coordinating TOPO solvent or by varying amounts of TOPO in noncoordinating ODE solvent. The shells were coherent and rough if synthesized in the TOPO solvent, while the shells became very uniform and smooth if synthesized in the ODE solvent. The electrically insulating effects of TOPO shells were directly confirmed through nanodevice of individual core/shell nanowire (NW). The present scheme to overcoat TOPO around semiconductor NWs could, in principle, be exploited to be a general approach to encapsulate a variety of colloidal nanocrystals to form novel core–shell nanohybrids.     

Electrical,mechanical and thermal properties of ZnO/SiR composite dielectric

Journal of Materials Science: Materials in Electronics - High-voltage direct current (HVDC) cable accessories are the weakest part of the cable operation, and possess the highest failure rate due...