Sphalerite-type Structure Research Articles

Structure and optical properties of ZnxCd1-xS and Cu:ZnxCd1-xS templated on DNA molecules

One-dimensional ZnxCd1−xS and Cu: Zn x Cd1−x S nanostructures were prepared using DNA as a template to promote growth along the molecular axis. The formation of homogeneously alloyed nanocrystals with cubic zinc blende-type structures was verified using x-ray diffraction and Raman spectroscopy. X-ray photoemission spectra revealed the presence of Cu(I) in the doped Cu: Zn x Cd1−x S nanocrystals. The effectiveness of the DNA template to direct the semiconductor growth in one dimension was demonstrated by AFM and TEM. The nanostructures displayed a granular morphology comprising nanoparticles with an average diameter of 14 nm composed of assemblies of smaller crystallites of 2.0 nm in size. Rope-like assemblies with an average diameter of 48 nm and extending in length to several hundred micrometres were obtained by evaporation-induced self-assembly. UV-Vis absorption and emission spectra indicated that the optical bandgaps (2.89–4.00eV) and photoluminescence peaks (608–819 nm) of the DNA-templated nanocrystals could be precisely controlled by modifying the molar ratios of their Zn/Cd precursors. Doping with Cu(I) gave an increase in photoluminescence intensity and a composition-independent red-shift of 0.23 eV. The preparation of DNA-templated Zn x Cd1−x S and Cu: Zn x Cd1−x S provides a simple, low-temperature route to aqueous dispersions of inorganic materials with controlled optical gap.

Comparative high-pressure investigations of Ag2ZnSnS4 and Ag2CdSnS4 compounds

Quaternary kesterite-type (KS) compounds have attracted worldwide attention from the scientific community as promising materials for solar cells. On the route to optimizing their performance, the effect of stress and strain constitutes a critical factor when it comes to thin film applications. Following a recent theoretical study, we report here joint experimental and computational high-pressure investigations on the KS Ag2ZnSnS4 and wurtz–kesterite (WZ–KS)-type Ag2CdSnS4 compounds. Our results reveal that both materials undergo successive transformations, first into a GeSb-type and then toward a CrN-type modification at ambient temperature. Our theoretical calculations predict a metallic character for all Ag2ZnSnS4 and Ag2CdSnS4 high-pressure phases. In addition, structural disorder is observed in KS Ag2ZnSnS4 upon moderate compression, prior to its KS → GeSb-type transition. Decompression leads to the recovery of a disordered zinc blende-type structure in the latter, whereas Ag2CdSnS4 retains the disordered GeSb-type modification. The similarities and deviations from the archetypical KS Cu2ZnSnS4 are discussed.

Study on the decomposition of CuI films due to defect state instability

Cuprous(Ⅰ) iodide (CuI) is widely used as a conducting material in various optoelectronic devices. However, defect properties and instabilities lead to changes in the semiconductor properties of CuI, which seriously affects the stability of related optoelectronic devices. Therefore, in this paper, the decomposition and defect state properties of CuI films are investigated in depth. CuI was prepared by a simple iodination method and belongs to the zinc blende type structure, preferentially crystallizing along the (111) direction. Microscopic tests reveal that grains in CuI film are tightly bonded without obvious cracks or holes, and the grain size reaches 1.231 μm. In the subsequent spectral analysis, multiple emission peaks (free exciton and defect emission peak) were obtained by Gaussian fitting. The two major point defects (Cu vacancy (VCu) and I vacancy (VI)) in CuI were analyzed theoretically, and the calculated defect energy levels matched the energies of the emission peaks. The results of Gaussian waveform fitting and density functional calculations indicate the presence of VCu defects and VI defects in CuI. Significant variations in emission and electrical properties were observed in successive tests, indicating the relatively poor long-term stability of the CuI samples. The changes in the emission characteristics are mainly manifested by the weakening of the 416 nm emission and the enhancement of the 720 nm emission, while the changes in the electrical characteristics are mainly manifested by the increasing resistance. The above changes are attributed to the concentration of defect states, mainly due to the increase of VI defects and the decrease of VCu defects, hence the severe iodine decomposition occurs in the CuI samples. To inhibit the decomposition, three schemes (multiple iodination, multilayer iodination, and encapsulation) were proposed, and relevant experiments were conducted.

Growth Kinetics and Optical Proprties of CdSe Nanocrystals

We were synthesized CdSe nanocrystals (NC) at a temperature of 260 oC and controlling their growth kinetics follows different reaction times from 1 minute to 180 minutes. Base on their optical properties and estimated size to track their growth kinetics follow the Lamer model. Structure and morphology characterized were investigated by XRD pattern and transmission electron microscopy (TEM). All samples show cubic zinc blende type structures. The average NCs size can be calculated by absorption spectra, XRD and TEM, these results lie in close vicinity with each other. The energies of photoluminescence (PL) peaks and band gap of CdSe NCs can be tuned within the regions of 1.99 eV to 2.13 eV and 1.92 eV to 2.12 eV with increasing reaction time. The Stokes shifts and PL emission peaks are narrow, thus confirming the formation of uniformly distributed NCs.

Structural, Optical, and Photocatalytic Properties of ZnSe Nanoparticles Influenced by the Milling Time

ZnSe nanoparticles (NPs) were prepared by combining both hydrothermal and mechanical milling methods. Transmission electron microscopy images show that fabricated ZnSe NPs with a sphere-like shape have an average size (d) in the range of 20–100 nm, affected by changing the milling time from 10 to 60 min. All the samples crystalize in zincblende-type structure without impurities, as confirmed by analyzing X-ray diffraction patterns, Raman spectra, and energy-dispersive X-ray spectroscopy. Carefully checking Raman spectra, we have observed the broadening and redshift of vibration modes as decreasing NP size, which are ascribed to extra appearance of disorder and defects. The photoluminescence study has found a blue emission at 462 nm attributed to the excitonic near-band edge and a broad defect-related emission around 520–555 nm. Increasing milling time leads to the decrease in the exciton-emission intensity, while the defect-related emissions increase gradually. Interestingly, as decreasing d, we have observed an improved photodegradation of Rhodamine B under UV irradiation, proving application potentials of ZnSe NPs in photocatalytic activity.

First Principles Study of High-Pressure Phases of ScN

We report the results of first-principles total-energy calculations for structural properties of scandium nitride (ScN) semiconductor compound in NaCl-type (B1), CsCl-type (B2), zincblende-type (B3), wurtzite-type (B4), NiAs-type (B81), CaSi-type (Bc), B-Sn-type (A5), and CuAu-type (L10) structures. Calculations have been performed with the use of the all-electron full-potential linearized augmented plane wave FP-LAPW method based on density-functional theory (DFT) in the generalized gradient approximation (GGA) for the exchange correlation energy functional. We predict a new phase transition from the most stable cubic NaCl-type structure (B1) to the B-Sn-type one (A5) at 286.82 GPa with a direct band-gap energy of about 1.975 eV. Our calculations show that ScN transforms from the orthorhombic CaSi-type structure (Bc) to A5 at 315 GPa. In agreement with earlier ab initio works, we find that B1 phase transforms to Bc, L10, and B2 structures at 256.27 GPa, 302.08 GPa, and 325.97 GPa, respectively. The electronic structure of A5 phase shows that ScN exhibits a direct band-gap at X point, with Eg of about 1.975 eV.

Synthesis and structural investigation of stoichiometric iron mononitride thin films

We report the synthesis of iron mononitride (FeN) thin films having zinc blende (ZB) type structure, deposited using a direct current magnetron sputtering process on amorphous quartz substrates. The effect of substrate temperature (Ts) on the structure of FeN thin films was systematically investigated by varying the Tsfrom 300 to 500 K(in close intervals of 25K). The long-range as well as the local structure of samples was studied using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at N K-edge, Fe L and K-edge and extended x-ray absorption fine structure (EXAFS) at Fe K-edge. The lattice parameter (LP) obtained from XRD and the presence of an intense pre-edge feature in Fe K-edge XANES confirms the ZB-structure of FeN samples. It was found in this work that the stoichiometric FeN with a LP close to its theoretical value can only be obtained in a narrow temperature regime due to minimization of stresses generated during the growth near the thermodynamic equilibrium conditions. These findings were further validated by EXAFS results yielding Fe–N and Fe–Fe bond distances, in conformity with the theoretically predicted values for FeN in ZB type structure.

Structure, binding energy and optoelectrical properties of p-type CuI thin films: The effects of thickness

Ultra-high transparent p-type copper iodide (CuI) thin films were fabricated by solid iodization of evaporated Cu precursor layers at room temperature. The effect of the thickness on microstructure, binding energy and optoelectrical properties is systematically studied. X-ray diffraction measurements show the polycrystalline nature of the CuI thin films with zincblende type structure. The X-ray photoelectron spectroscopy (XPS) analysis indicates that the oxidation state of Cu is +1 and the estimated value of [Cu]/[I] at 100 nm is 0.87. Excess iodide ions trap considerable holes, causing CuI thin films to exhibit the p-type conductivity, which is consistent with the results of the Hall effect measurement and the non-linear characteristics of the CuI/ITO structure. Moreover, the CuI thin films with thickness of 100 nm exhibits an ultra-high optical transmittance of 95.5% in the wavelength of 380–780 nm and an excellent conductivity of 34 S/cm. These results prove the great potential of CuI as a promising p-type optoelectronic material.

Study of ZnO1-xSx synthesis by non-alcoxide sol-gel method employed in the photocatalytic degradation of a persistent organic pollutant in water

Non-Alcoxide Sol-Gel method was used as synthesis route to obtain the ZnO1-xSx compounds at calcination temperatures of 250 ° C, 300 ° C and 400 ° C, doped with Sulfur at mole fractions of x = 0.05, 0.50, 0.80. The samples obtained were characterized structural and optically by both X-ray diffraction (XRD) and diffuse reflectance spectrophotometry. The XRD analysis showed the formation of the Wurtzite type structure proper of ZnO up to Sulfur mole fraction of x = 0.50. It was observed that this phase disappears at a mole fraction of x = 0.80 and converts to Cubic Zinc blende type structure proper of Zinc Sulfide. Furthermore, was observed that increasing calcination temperature higher crystallinity was achieved for all set of samples. The Rietveld refinement method was used to obtain lattice parameters for Wurtzite structure as well as cell volume and the ratio c/a. These parameters show slight incremental changes in relation to S doped ZnO crystalline structure. On the other hand, it was determined optically, that increasing the dopant concentration to a mole fraction x = 0.50, a maximum reduction of the band gap from 3,29 eV to 2,70 eV was reached, while increasing the calcination temperature shows a general tendency to band gap reduction. Photocatalytic activity of Sulfur doped and pure ZnO powders were tested for degradation of p, p’-DDT under visible light, measuring the p, p’-DDT time-dependent concentration by gas chromatography-μECD coupling. The results revealed that the sulfur modified ZnO material showed an improved photocatalytic activity towards p, p’-DDT under visible light illumination respect to pure ZnO.

Tunable dual emission in type-I/type-II CdSe/CdS/ZnSe nanocrystals

We have used wet chemical methods to fabricate colloidal CdSe/CdSm/ZnSen core/intermediate/shell (C/I/S) nanocrystals (NCs), where m = 1 and 2 and n = 1–4 are the number of monolayers. The growth was monitored by using Raman and UV–vis spectroscopy, which demonstrated the formation of correct CdSe/CdSm/ZnSen C/I/S NCs. X-ray diffraction studies proved single-phase NCs crystallized in the zincblende-type structure. Photoluminescence (PL) studies have indicated that after photoexcitation C/I/S NCs generate simultaneously type-I and type-II emissions, namely EI and EII, associated with CdSe/CdSm and CdSm/ZnSen structures, respectively. For a specific value of m = 2, we have found the redshift of both EI and EII emissions when n is increased from 1 to 4. The PL studies versus the laser-excitation power (Pex) up to about four orders of magnitude allow us to identify the origin of two emissions. A large blueshift of the EII peak is ascribed to the band bending effect resulting from the spatially separated photoexcited carriers in type-II NCs. It appears that the dependence of the EII peak on the cube root of Pex (Pex1/3) is linear while that of the EI peak on Pex1/3 is almost constant. In this work, we also point out that the emission intensity (I) of EI and EII can be tuned by changing the values of n and Pex, where I obeys a power law I∝Pexk, with k = 0.7–1.0 dependent on the emission type. The nature of these phenomena is discussed in comparison with previous studies on C/S and C/I/S nanoheterostructures.

The particle size-dependent optical band gap and magnetic properties of Fe-doped CeO2 nanoparticles

Fe-doped CeO2 (0%–0.08%) nanoparticles have been synthesized by chemical precipitation method. The nanoparticles have been investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscope (TEM), UV–Vis spectrometer and Vibrating sample magnetometer (VSM) techniques. XRD data exhibited the cubic zincblende type structure for Fe-doped CeO2 nanoparticles. The size of nanoparticles was found to decrease from 20 nm (0%) to 14 nm (8%) with increasing Fe concentration which is confirmed by TEM micrographs. The optical absorption spectra of the nanoparticles show the blue-shift of the wavelength of absorption maximum from 550.2 nm to 336.4 nm with increasing Fe concentration owing to decreasing in particle size. The energy band gap was found to increase slightly with increasing the Fe doping level which was attributed to the decrease in the particle size with Fe content originated from quantum confinement effect. The doped Fe-doped CeO2 nanoparticles revealed room temperature ferromagnetism.

Optical and Ferromagnetic Properties of Ni-Doped CdTeSe Quantum Dots

A chemical method was used to prepare Cd1−xNixTe0.5Se0.5 (Cd1−xNixTeSe, x = 0–0.1) quantum dots (QDs) with particle sizes of 3–4 nm. Structural analyses of x-ray diffraction patterns indicate that all QDs are single-phase and crystallize in the zincblende-type structure. The lattice constant gradually decreases with increasing x in Cd1−xNixTeSe. This is due to a partial replacement of Ni (a smaller ion) for Cd2+ (a larger ion). Our study also indicates that the Ni doping causes the red shift of the longitudinal optical mode, the blue shift of the excitonic absorption edge and photoluminescence (PL) peak, and a gradual decrease of the PL quantum yield. When the excitation power increases, the PL peak of CdTeSe (x = 0) is almost unchanged, while that of Cd1−xNixTeSe QDs (x > 0) shifts linearly towards high energies, which is related to the state-filling effect caused by Ni2+ dopants. Comparing with pure CdTeSe, Ni-doped QDs have longer PL decay times, up to ∼ 580 ns. Particularly, all QDs exhibit weak ferromagnetic (FM) order at room temperature generated from defect-mediated exchange interactions of Ni2+ ions. Such results proved ternary Cd1−xNixTeSe QDs having simultaneously the optical and FM properties. Together with very long decay times, they are considered as potential materials for biosensing, photovoltaic and photocatalytic applications.

Electronic Structure and Ferromagnetism in Zincblende Zn1−xCoxS Nanoparticles

We have prepared Zn1−xCoxS nanoparticles (NPs) and studied their local-geometric, electronic, and magnetic properties. X-ray diffraction analyses have indicated that NPs with average crystallite sizes of 2.2 ∼ 2.9 nm are single phase and crystallize in the zincblende-type structure. Cobalt ions with oxidation state + 2 (Co2+) replaced for Zn2+ and almost unchanged the zincblende-type structure of the ZnS host lattice. Interestingly, all samples show room-temperature ferromagnetism and ferromagnetic order increases with increasing Co content (x) in Zn1−xCoxS NPs. With the results obtained from analyzing the local-geometric structures at Zn and Co K-edges and photoluminescence spectra, we believe that ferromagnetism in Zn1−xCoxS NPs is related to intrinsic defects and exchange interactions between Co2+ ions. We have also reviewed the recent studies on ZnS-based dilute-magnetic materials.

Structure and Surface Morphology of Cd1–xMn x Se Epitaxial Films

Cd1–xMn x Se (х = 0.03) epitaxial films are produced by the molecular beam condensation method in a vacuum chamber with a residual pressure of 10–4 Pa on mica and glass substrates. It is established that at room temperature and at a substrate temperature of T = 573 K films of polycrystalline structure grow on the mica substrates, but films of both polycrystalline and amorphous structure grow on the glass substrates. It is shown that the polycrystalline Cd1–xMn x Se (х = 0.03) films, unlike the bulk crystals, have a sphalerite-type structure with a lattice parameter of a = 6.05 A. Increasing the substrate temperature to 673 K leads to epitaxial growth with the direction [111]. Dark aggregates, observed on the film surface, are removed using a source of compensating Se vapors during the growth process. The optimal conditions for the production of structurally perfect epitaxial films are defined.

Specific Features of the Growth, Structure, and Main Physicochemical Properties of FeGa2Se4 Single Crystals

FeGa2Se4 single crystals belonging to the promising class of diluted magnetic semiconductors of the AB2X4 type (A is Mn, Fе, Co, or Ni; B is Ga or In; and X is S, Sе, or Te) are investigated; this compound is currently used for designing solid-state magnetically controlled devices. Optically homogeneous bulk FeGa2Se4 single crystals ~14 mm in diameter and ~50 mm long are obtained for the first time from melt by the modified Bridgman method. The elemental composition of the crystals is determined by X-ray spectral microprobe analysis; it corresponds to the specified composition in the initial charge. It is established by X-ray diffraction analysis that this compound is crystallized into a cubic sphalerite-type structure with unitcell parameter a = 5.498 ± 0.005 A. The melting and crystallization temperatures of the grown FeGa2Se4 single crystals are determined by differential thermal analysis. The melting temperature is 1283 K. The microhardness of the single crystals is measured for the first time, the density is determined by the pycnometric method, and the parameters of hyperfine interaction of iron ions are found on the basis of the Mossbauer spectra.

A tool for automatic recognition of [110] tilt grain boundaries in zincblende-type crystals

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

Identification of trapping and recombination levels, structure, morphology, photoluminescence and optical absorption behavior of alloyed ZnxCd1−xS quantum dots

Cubic ZnxCd1−xS nanoparticles (NPs) synthesized by chemical precipitation method with average crystallite size of 2.9 ± 0.2 nm were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), high-resolution transmission electron microscope (HRTEM), UV–vis absorption, Fourier transform infrared red (FTIR), photoluminescence (PL) emission and Raman spectroscopy. XRD analysis demonstrated systematic shift of diffraction peaks to higher diffraction angles accompanied by a decrease in the lattice parameters with increasing Zn content (x); this confirmed the formation and homogeneity of ZnxCd1−xS nanoalloys. In addition, the dependence of lattice parameters and Raman shift on x showed linear behavior and good agreement with Vegard's law. TEM images of ZnxCd1−xS NPs revealed nearly spherical shape NPs, relatively narrow particle size distribution and standard deviation in the range 1.8–3.4%.; as well as HRTEM images showed well resolved diffraction crystalline planes of zinc-blende cubic type structure. Analysis of optical absorption spectra showed blue shift of both the direct optical band gap from 3.25 to 4.05 eV and excitonic absorption shoulder from 2.56 to 3.88 eV with increasing x, confirming the homogeneity of ZnxCd1−xS alloyed semiconductor NPs. At excitation wavelength 325 nm, the deconvoluted structural defects related PL emission bands are broadened and revealed stronger PL intensity than that at 370 nm. Furthermore, increasing x resulted in PL enhancement accompanied by blue shift of green emission band centered at 515 nm–445 nm. To explain composition dependent PL emission process; trapping and recombination localized levels in ZnxCd1−xS alloyed NPs were identified quantitatively and an energy band diagram was suggested.

Copper iodide synthesized by iodization of Cu-films and deposited using MOCVD

We report the thin film deposition of copper iodide with zincblende type structure (γ-CuI) via metal-organic chemical vapor deposition (MOCVD). Single crystals and thin films of Cu and γ-CuI could be formed on various substrates from cyclopentadienylcopper triethylphosphine (CpCuPEt3) and ethyliodide (C2H5I) as precursors for copper and iodine, respectively. Additionally, the chemical reaction behavior of thermally evaporated and MOCVD deposited elemental copper films with C2H5I is examined. SiO2 glass and various single crystalline oxides and semiconductors were used as substrates. For all cases X-ray diffraction measurements revealed polycrystalline γ-CuI with zincblende type structure; other CuI phases could not be detected. Photoluminescence measurements show a broadened peak at 420nm due to donor-acceptor pair recombination. Intense peaks at shorter wavelengths are attributed to excitonic emissions.

Effective Hamiltonian for electron waves in artificial graphene: A first-principles derivation

We propose a first principles effective medium formalism to study the propagation of electron waves in semiconductor heterostructures with a zero-band gap. Our theory confirms that near the K-point the dynamics of a two-dimensional electron gas modulated by an external electrostatic potential with honeycomb symmetry is described by the same pseudospinor formalism and Dirac massless equation as a graphene monolayer. Furthermore, we highlight that even though other superlattices based on semiconductors with a zincblende-type structure can have a zero band-gap and a linear energy-momentum dispersion, the corresponding effective medium Hamiltonian is rather different from that of graphene, and can be based on a single-component wavefunction.

Structural and optical characterization of CdTe quantum dots thin films

Cadmium telluride quantum dots (CdTe QDs) have been synthesized using hot-injection chemical technique. The CdTe QDs thin films were deposited onto optical flat fused quartz substrates using thermal evaporation technique. The CdTe QDs powder and the as deposited films were characterized using X-ray diffraction and high resolution transmission electron microscope (HRTEM). The X-ray analysis shows that both CdTe QDs powder and the as deposited films crystallize in cubic zinc-blende type structure with lattice parameter 6.46Å and 6.45Å, respectively. The X-ray calculation shows that the average crystallite size of the as deposited CdTe QDs films varied from 1.1nm for the powder to 2.3nm for the thin film. The HRTEM examination of the as deposited films shows that the average particle size vary from 2.5nm for the powder to 2.7nm for the thin film. For the as deposited films, the dependence of (αhν)2 on the incident photon energy indicates that the optical transitions within the film are allowed direct with energies observed at Eg1≅2eV and Eg2≅2.3eV which attributed to quantum confinement effect. The optical band gap increases from 1.5eV for microstructure CdTe to 2eV for nanostructure quantum dots which corresponding to wavelength(620nm) so it is a great benefit to use CdTe quantum dots as solar harvesting devices application in solar spectrum region (400–800nm). Urbach energy is calculated and found to be 360meV which is higher than microstructure CdTe. The refractive index and refractive index dispersion of the as deposited CdTe QDs film has been calculated from transmission and reflection spectra. It has been found that the refractive index is reduced from (2.66) for microstructure CdTe to be (1.7) for CdTe quantum dots. The variation of the refractive index at normal dispersion is well described by Wemple–DiDomenico single oscillator model. The dielectric constant at infinite frequency ε∞ and the ratio of free carriers’ concentration to the effective mass (N/m∗) are calculated and discussed. Finally, the non linear optical parameters are calculated using some empirical relation.