Sphalerite Research Articles

InGaN/GaN QWs on tetrahedral structures grown on graphene/SiC

In recent years, flexible nitride LEDs have been developed for applications in fields such as lighting, displays but also medicine and biology. Said nanowire-based flexible LEDs are made of InGaN/GaN core-shell nanowires encapsulated in a polymer matrix, which is peeled off and constitutes the final flexible LED structure. However, the current peel off process is complex and hardly compatible with industrial applications. One solution to this issue could be the Van Der Waals epitaxy of the LED structures on graphene, as this approach is known to ease the separation from the original substrate. However, to realize Van der Waals epitaxy and prevent nucleation of GaN on defects located within the graphene layer, the quality of the graphene needs to be optimized. In this work we employ graphene grown by Chemical Vapor Deposition on SiC, known to be notably less defective than graphene grown on metals or sapphire. In this work, tetrahedral structures of GaN have been grown on monolayer and multilayer graphene on SiC. X-Ray Diffraction measurements confirmed that the GaN followed the crystallographic directions of SiC, implying growth through remote-epitaxy mechanism thanks to the presence of graphene in both cases. Last, InGaN/GaN structures were realized by growth of a 5 period of InGaN/GaN quantum wells on these GaN tetrahedra. Cathodoluminescence as well as photoluminescence measurements revealed the presence of stacking faults and Zinc Blende inclusion, as well as a broad visible emission near 480 nm of quantum wells.

Bandgap energy prediction of senary zincblende III–V semiconductor compounds using machine learning

Two complimentary machine learning (ML) models are developed to estimate the bandgap energy (Eg) of senary zincblende (ZB) III–V semiconductor compounds Alx1Gax2Inx3Py1Asy2Sby3 from molar fractions. The models work – as well – for any compound with smaller number of constituent elements from binaries to quinaries. Some of these compounds exhibit transition between a direct and an indirect Eg as a function of composition. This is due to the fact that the bandgap for this class of materials is either the direct gap at Γ point (EgΓ) or the indirect one between the maximum valance band at Γ point and the minimum conduction band around X point (EgX). Hence, to capture precisely the Eg and its state, the ML models estimate separately the EgΓ and EgX where the lowest energy gap is the effective Eg. For model training, 18 ternary ZB III–V semiconductor compounds are used, which contain all possible ternary configurations of III-type cations (Al, Ga, In) and V-type anions (P, As, Sb). The achieved testing accuracy using cubic regression with ridge regularization – in terms of root mean square error (RMSE) – are 7.583 and 0.021 meV for EgΓ and EgX, respectively. These correspond to R2 of 0.99995 and 1.00000 (8 nines), respectively. To validate the models, they are benchmarked against experimental measurements of four different quaternary ZB III–V semiconductor compounds. The agreement is very noticeable demonstrating the validity and the excellent accuracy of the developed models.

Temperature-dependent study of the fabricated ZnS/p-Si heterojunction

In the present work, ZnS thin film was deposited onto a p-type Si-substrate using the chemical bath deposition method, and subsequently Ag/n-ZnS/p-Si/Ag heterojunction device was fabricated to investigate its diode parameters in the temperature range of 0–60 °C. Cubic zinc blende phase of the prepared ZnS thin film was confirmed using grazing incidence X-ray diffraction (GIXRD); Scherrer analysis revealed a lower limit of crystallite size as 2.4 nm. Scanning electron microscopy (SEM) revealed the surface morphology of the prepared ZnS thin film, which exhibited good homogeneity and density. UV–Visible absorption spectroscopy revealed the good UV-region absorbance and visible-region transmittance of the deposited ZnS thin film. The corresponding energy band gap was obtained from Tauc analysis and found to be 3.88eV. Photoluminescent emission (PLE) spectroscopy showed the defect mediated radiative transitions in the deposited ZnS thin film. Current-voltage (I–V) measurements of the fabricated heterojunction device exhibited Schottky diode behaviour. Diode parameters of reverse saturation current, Schottky barrier height, ideality factor and series resistance were obtained from the I–V measurements in the temperature range of 0–60 °C; barrier height was obtained in the range of 0.87–0.74eV, and ideality factor was obtained close to unity for I–V measurements made at 30 °C and 60 °C.

Doping effects on the ferroelectric properties of wurtzite nitrides

Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a β-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Influence of Capping Ligands, Solvent, and Thermal Effects on CdSe Quantum Dot Optical Properties by DFT Calculations.

Cadmium selenide nanomaterials are very important materials in photonics, catalysis, and biomedical applications due to their optical properties that can be tuned through size, shape, and surface passivation. In this report, static and ab initio molecular dynamics density functional theory (DFT) simulations are used to characterize the effect of ligand adsorption on the electronic properties of the (110) surface of zinc blende and wurtzite CdSe and a (CdSe)33 nanoparticle. Adsorption energies depend on ligand surface coverage and result from a balance between chemical affinity and ligand-surface and ligand-ligand dispersive interactions. In addition, while little structural reorganization occurs upon slab formation, Cd···Cd distances become shorter and the Se-Cd-Se angles become smaller in the bare nanoparticle model. This originates mid-gap states that strongly influence the absorption optical spectra of nonpassivated (CdSe)33. Ligand passivation on both zinc blende and wurtzite surfaces does not induce a surface reorganization, and thus, the band gap remains nonaffected with respect to bare surfaces. In contrast, structural reconstruction is more apparent for the nanoparticle, which significantly increases its highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap upon passivation. Solvent effects decrease the band gap difference between the passivated and nonpassivated nanoparticles, the maximum of the absorption spectra being blue-shifted around 20 nm by the effect of the ligands. Overall, calculations show that flexible surface cadmium sites are responsible for the appearance of mid-gap states that are partially localized on the most reconstructed regions of the nanoparticle that can be controlled through appropriate ligand adsorption.

Homojunction-Structured Cadmium Sulfide Enhances Photocatalytic Coproduction of Hydrogen and Kraft Lignin Conversion

Producing hydrogen energy and aromatic compounds simultaneously from kraft lignin under solar irradiation is a promising strategy for alleviating the energy crisis. However, the achieved photorefining process is not only limited by the complex structure of kraft lignin but also by the lack of efficient photocatalysts. Herein, we employed a simple strategy to prepare homojunction-structured cadmium sulfide with rich sulfur vacancies. The perfect lattice matching of the cubic zinc blende phase and hexagonal wurtzite phase realized the spatial separation of photogenerated electrons and holes. With the assistance of rich sulfur vacancies, the well-designed photocatalyst exhibited an unprecedented hydrogen production ability (835.8 μmol·g–1 h–1) and value-added phenolic compounds were also generated by decomposing kraft lignin. This work will provide a mild and sustainable pathway to produce clean energy and value-added chemicals from kraft lignin.

Gallium Phosphide Nanowires Grown on SiO2 by Gas-Source Molecular Beam Epitaxy

GaP as one of the III–V semiconductors has an indirect band gap in its natural zinc-blend (ZB) crystal phase, limiting its applications in optoelectronics. The atomic arrangements of the ZB GaP, however, can be changed by adding energy to the system, for example, using strain and defects. In such a way, GaP can be crystallized in the wurtzite (WZ) phase with a direct band gap in the yellow–green range and promising new optical properties. GaP nanostructures offer the great possibility to induce strain, and hence, one can expect to obtain the WZ phase by modifying the geometry and dimensionality of GaP. In this work, we present GaP nanowires (NWs) grown on SiO2 substrates by gas-source molecular beam epitaxy. Raman measurements on individual GaP NWs indicate that NWs are poly-type crystal structures with the starting growth of the WZ phase, transforming into the ZB phase, and ending as the WZ phase. Photoluminescence at 9 K from an ensemble of NWs shows emissions at 2.09–2.14 eV, which are related to the direct band gap of the WZ phase and peaks between 2.26 and 2.3 eV due to the ZB phase. The emission of the WZ GaP phase is observable up to 160 K. Cathodoluminescence at 83 K shows directly the emission between 2.09 and 2.14 eV along the single NWs, indicating the presence of the WZ phase. Our results demonstrate the realization of poly-type, ZB, and WZ GaP NWs on SiO2 by gas-source molecular beam epitaxy.

Profiling the off-center atomic displacements in CuCl at finite temperatures with a deep-learning potential

Cuprous halides ($\mathrm{Cu}X$, $X=\mathrm{Cl}$, Br, or I) have been extensively investigated in the literature, but there still exist debates on whether the ground-state structures of $\mathrm{Cu}X$ are zinc blende. By performing molecular dynamics simulations at finite temperatures with a newly developed accurate deep-learning potential for CuCl, we find that, in the absence of accurate exchange interactions, there exist collective off-center displacements of Cu, leading to the formation of large complex Cu cluster structures. However, these cluster structures are unstable once exchange interactions are properly included. Nevertheless, due to strong anharmonicity, thermal fluctuations could also lead to sizable off-center Cu displacements, resulting in instantaneous small Cu clusters. Still, these cluster configurations are not stable or metastable in the ground state. We thus unambiguously demonstrate that, although anharmonic off-center displacements of Cu are present in CuCl at finite temperatures, zinc blende is still the thermodynamically most favorable phase. These insights are critical to the understanding of liquidlike anharmonic behavior of Cu atoms in various Cu-containing compounds, which impacts a number of important properties such as thermal conductivity, ferroelectricity, and superconductivity.

Effect of MPA capping on the structural, optical and thermal properties of Zn0.96Ni0.04S nanoparticles

Chemical precipitation method was used to prepare of Zn0.96Ni0.04S nanoparticles. XRD showed that ZnS:Ni2+ nanoparticles crystallized in zinc blend structure with preferential orientation by the side of (111) plane. Scanning electron microscope (SEM) images of Zn0.96Ni0.04S nanoparticles portray substantial agglomeration of particles with potholed spherical morphology and energy dispersive spectrum (EDAX) measurements incorrigible that the presence of Ni in ZnS lattice. High resolution tunneling electron microscope (HRTEM) divulge that Zn0.96Ni0.04S particle size were ranges from ∼ 15 to 20 nm with good crystalline character and SAED pattern were good concurrence with X-ray diffractive (XRD) results. UV–Vis spectroscopy was engage to evaluate optical band gap of Zn0.96Ni0.04S nanoparticle, and which further disclose that when 3-mercaptopropionic (MPA) concentration increase guide the absorption edge shifted to lower wavelength. Peak at 411 °C with matching weight loss of 39.2 wt% in DTA reveals that Ni2+ ions are get away from the ZnS matrix.

Binding energy and photoionization cross section of piezoelectric core/shell quantum dots

We have studied the binding energy and photoionization cross section of a donor impurity in core/shell piezoelectric quantum dots within the framework of the effective-mass approximation. A size-dependent internal piezoelectric field along the axis of the quantum dot caused by the lattice-mismatch induced strain has been described by a full analytical model. The results show that the photoionization cross section and binding energy become largest when the impurity is at the center of QD. The photoionization cross section in wurtzite structure is much larger than that in zinc blende structure, the optimal ratio of Rc/Rs to maximize the peak value of photoionization cross is between about 0.6 and 0.7.

Combinatorial study to the physical properties of Cd1-xZnxTe thin films as budding absorber for solar cell applications

Cadmium telluride (CdTe) has long been recognized as promising absorber for single-junction solar cells, yet it suffers from snag of apposite contact. CdZnTe provides band gap tunability and can be used as absorber layer in tandem devices concerned. The impact of Zn content on physical properties of Cd1-xZnxTe thin films is indubitable and hence, present work demonstrates concentration evolution to physical properties of Cd1-xZnxTe thin films. Structural characterization depicted zinc blende cubic phased (220) plane as dominant reflection. The optical energy band gap (Eg) of films is attained within range 1.70–1.92 eV and electrical properties portrayed Ohmic character of films. Photoluminescence spectra showed single peak and topographical analysis demonstrated hill like-topographies. Field emission scanning electron microscopy (FESEM) maps exposed compact morphologies and compositional studies validated presence of constituent elements. The present study unveiled that maximum grain size, apt optical energy band gap and higher conductivity of Cd0.9Zn0.1Te films make these as budding absorbers for solar cell applications.

Tunability of optical properties of InSb films developed by pulsed laser deposition

The InSb thin films were successfully prepared from binary alloy target by Pulsed Laser Deposition (PLD) onto heated Si/SiO2 substrates at various deposition temperatures (Td) ranging from room temperature to 400 °C. The evolutions of structural (XRD, XPS), microstructural (AFM, SEM) and optical (XPS, FTIR, RAMAN) properties as a function of deposition temperature were systematically taken into account. The analyzed results indicate that the films are well crystallized with Zinc Blende (ZB) structure for Td ≥ RT. Increasing Td induces significant signature of preferentially crystal orientations of (h11) accompanied by distinctive microstructural evolution from continuous fashion (Td ≤ 300 °C) to almost isolated islands feature (Td ≥ 350 °C). Derivations of crystallite sizes, lattice strain, dislocation density, Lotgering orientation factor, and root mean square roughness were correlated with Raman shift and XPS analysis, and optical properties. The developed thin films allow to observe the flexibility of optical band gap energy (from 0.18 eV to 0.50 eV), which is directly related to the quantum confinement effect coupled with the band-filling effect of Burstein–Moss shift.

Study of structural and spectroscopic characterization of co-doped ZnS Nanoparticles capped with L-Arginine

The aim of the present study was to prepare ZnS nanoparticles co-doped with Li+-Cs+, were synthesized using a chemical co-precipitation method. The ZnS-(Li+-Cs+) nanoparticle’s were capped with L- Arginine and reduced using sodium sulphide. The main advantage of this method is to synthesized semiconductor nanomaterial with wide band gap and nanoparticles are chemically stable over a long time. The as prepared nanoparticles were characterised by X-Ray Diffraction technique for phase analysis of the sample. Particle size is found in the range of 2 to 4 nm calculated by Debye Scherer method. XRD confirms the evolution of stable cubic zinc blend phase of ZnS nanoparticles. The strong interaction between the capping agent L- arginine, Zn and S is revealed from the Fourier Transform Infrared (FTIR) spectrum.

CdSe quantum dots enhancing blue emission of nematic liquid crystals

Herein, we report the enhancement in blue emission of nematic liquid crystals (NLCs) doped with CdSe quantum dots (QDs) at room temperature. CdSe QDs were synthesized by a high temperature wet chemical method. X-ray diffraction pattern suggest zinc blend crystal structure of CdSe QDs without impurity phase formation. The absorption peak and PL emission of QDs is observed at 503 nm and 526 nm respectively. The QDs size is found to be 2.3 nm calculated by excitonic peak. The polarization states of the QD-based NLCs were studied by using polarising optical microscopic (POM) images under crossed geometry of polarizer and analyser. The uniform colour distribution throughout the cell indicates uniform cell thickness with a planar alignment of liquid crystal molecules. Moreover, dark, and bright states of POM images are used to analyse the defects in alignment of NLC molecules. It is worthwhile to note here that the incorporation of QDs in NLCs helps in reducing defects and light leakage centres which further increases the absorption and hence emission of NLCs. The significant enhancement in the PL intensity of the NLC of about 60% upon CdSe QDs doping is observed at room temperature which attributed to the increase in molecular alignment of NLCs composite.

Synthesis of a new class of high-entropy nitride ceramics and the effects of nitrogen vacancies on their magnetic properties

This research prepared a new type of (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)N0.956 high-entropy nitride (HEN) ceramic powder with a cubic single-phase zinc blende structure using the (Co,Cr,Fe,Mn,Ni)3O4 high-entropy spinel oxide as raw material through ammonia nitridation. Nitrogen vacancies are the main feature of HENs, considerably influencing the HEN their structure and magnetic properties. (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)N0.864 HENs with nitrogen vacancies were synthesized through hydrogenation. Theoretical calculations and experimental characterization results show that the samples obtained by heat treatment at 300 °C for 30 min in Ar/H2 atmosphere have the most nitrogen vacancies. Nitrogen vacancies affect the interactions among metal ions in the crystal, resulting in the spin tilt of some divalent metal ions requiring more energy; thus, coercivity (Hc) decreases from 411.98 to 251.40 Oe, and saturation magnetization (Ms) increases from 5.71 to 8.09 emu/g. We offer a fresh viewpoint to explain the structural origin of magnetic characteristics in high-entropy ceramics based on the theoretical calculations of the association between defects, structure, and magnetism as well as the characterization data.

Anisotropic electronic transport properties in two-dimensional ferroelectric In[formula omitted]Se[formula omitted] monolayer

We investigate the electronic structure and electronic transport properties of two-dimensional (2D) In2Se3 monolayer, using density functional theory combined with the nonequilibrium Green’s function (NEGF) method. Two ferroelectric phases of In2Se3 monolayer with zincblende (ZB) and wurtzite (WZ) structures are included, along the zigzag (ZZ) and armchair (AC) directions, respectively. The current–voltage (I–V) characteristics show that the ZB In2Se3 monolayer along the ZZ direction is better for electronic transport. The voltage drop contour reveals that along the ZZ direction, the Se atom has 3 paths to the adjacent Se atoms, while along the AC direction, it reduces to 2 paths. The momentum-dependent transmission reveals that in the process of transformation from the left electrode to the right electrode, there are momentum exchanges among electrons, which is of importance to electronic transport.

Mechanical and lattice vibrational investigations of nanostructured AlxIn1-xPySbzAs1-y-z alloys

The compositional dependence of the polarities, elastic constants, elastic moduli, Poisson's ratio, and linear compressibility has been computed for zinc blende AlxIn1-xPySbzAs1-y-z alloys with composition y ranging from 0 to 1. Also, the compositional influence of the bond-stretching force constant, bond-bending force constant, internal strain parameter, Cauchy ratio, effective charge, the Born ratio, anisotropy factor, and sound velocity has been determined. The experimental data and the studied properties for the novel studied alloy lattice matched to the InP substrate show a good level of agreement. The empirical pseudo-potential approach and the virtual crystal approximation have been employed in this work. The obtained results are predicted to help understand the elastic characteristics of AlxIn1-xPySbzAs1-y-z and for applications in optoelectronics.

(Cd,Mn)S in the Composite Photocatalyst: Zinc Blende and Wurtzite Particles or Integrowth of These Two Modifications?

In this study, the crystalline structure and particle shape of Cd1-xMnxS (x~0.3) in the composite photocatalysts prepared by hydrothermal synthesis at different temperatures (T = 80, 100, 120, and 140 °C) were analyzed. Along with mixed Cd-Mn sulfide, the catalysts contain a small amount of β-Mn3O4. XRD patterns of (Cd,Mn)S have features inherent to both cubic zinc blende and hexagonal wurtzite structure. Moreover, XRD peaks are anisotropically broadened. First, the heterogeneous (or two-phased) model was considered by the commonly used Rietveld method. Phase ratio, average crystallite sizes, and strains for both phases were formally determined. However, it was shown that this model is not correct because relatively narrow and broad peaks cannot be fitted simultaneously. Then, the homogeneous model was tested by Debye Function Analysis. This model assumes that particles are statistically homogeneous, but each particle contains lamellar intergrowth of zinc blende and wurtzite modifications. The probability of stacking faults, as well as the average radii of spherical and ellipsoidal particles, were varied. It was shown that nanocrystalline Cd0.7Mn0.3S particles have an ellipsoidal shape. Ellipsoids are elongated along the direction normal to the plane of defects. An increase in the hydrothermal synthesis temperature from 80 °C to 140 °C leads to an enlargement of particles and a gradual decrease in the probability of stacking faults in the wurtzite structure from 0.47 to 0.36. Therefore, with increasing temperature, the structure of (Cd,Mn)S nanoparticles transforms from almost random polytype cubic/hexagonal (ZB:WZ = 47:53) to a preferably hexagonal structure (ZB:WZ = 36:64). Mn2+ ions facilitate CdS phase transformation from zinc blende to wurtzite structure. There is no direct correlation between the structure and photocatalytic activity.

Effects of electric field and hydrostatic pressure on the exciton states in strained zinc-blende In[formula omitted]Ga[formula omitted]N-GaN coupled double quantum wells

Based on the effective mass approximation, the variational theory study of the exciton properties in a strained zinc blende GaN/InxGa1−xN/GaN/InyGa1−yN/GaN coupled double quantum well is presented considering the combined effects of hydrostatic pressure, strain, and external electric field. The results show that the exciton binding energy first increases, it achieves a maximum value, and then decreases with the reduction of the well width. Contrarily, the exciton binding energy decreases slightly first, it reaches a minimum value, and then increase gradually until a constant with the enhancement of the middle barrier thickness. Meantime, the pronounced dependencies of the exciton binding energy, interband transition energy, emission wavelength, oscillator strength, and radiation decay time of the coupled double quantum well on hydrostatic pressure are analyzed systematically. As the hydrostatic pressure increases, the exciton binding energy increases linearly, and the interband transition energy (emission wavelength) is proportional (inversely) to hydrostatic pressure. In addition, the variation of the exciton binding energy and Stark shift with the external electric field is explored detailedly.

Theoretical study of second and third order elastic constants of β-SiC

First principles calculations of the second and third order elastic constants of the cubic silicon carbide β-SiC have been accomplished using the density functional theory within the local density approximationand the pseudopotential plane-wave method with the Hartwigsen Goedecker Hutter scheme. The secondorder elastic constants Cij compared fairly well with those previously measured by one of us using theBrillouin light scattering technique (P. Djemia) [1]. The ab-initio calculations include parameterspressure dependence determinations in relation with the structural transformation from a zinc Blende toa rocksalt structure. The transition pressure at which arises elastic instabilities is found to be Pt = 113GPa. Also, we investigate the effects of anharmonicity by calculating the third order elastic constantsC_ijk . The experimental values are still unknown and comparisons with our calculations are suitable in thefuture.