Ab initio study of electronic structures of InAs and GaSb nanowires along various crystallographic orientations

InAs and GaSb nanowires oriented along different crystallographic axes—the [0 0 1], [1 0 1] and [1 1 1] directions of zinc-blende structure—have been studied utilizing a first-principles derived nonlocal screened atomic pseudopotential theory, to investigate the band structure, polarization ratio and effective masses of these semiconductor nanowires and their dependences on the wire lateral size and axis orientation. The band energy dispersion over entire Brillouin zone and orbital energy are determined and found to exhibit different characteristics for three types of wires. There is an explicit dispersion hump in the conduction bands of [0 0 1] nanowires with two larger diameters and [1 0 1] nanowires with the smallest diameter considered. Moreover, the [1 1 1] nanowires are shown to exhibit very different orbital energy for the maximum valence state at the zone-boundary point, compared with [0 0 1] and [1 0 1] nanowires. These differences present significant and detailed insight for experimental determination of the band structure in InAs and GaSb nanowires. Furthermore, we study the polarization ratio of these nanowires for different orientations. Our calculation results indicate that, for the same lateral size, the [1 1 1] nanowires give extraordinarily higher polarization ratio compared to nanowires along the other two directions, and at the same time have larger band-edge photoluminescence transition intensity. Consequently, the [1 1 1] nanowires are predicted to be better suitable for optoelectronic applications. We also significantly find that polarization ratio and transition intensity displays different varying trend of dependence on lateral size of nanowires. Specially, the calculated polarization ratio is shown to increase with the decreasing size, which is opposite to the behavior displayed by the optical transition intensity. The predicted polarization ratios of [1 0 1] and [1 1 1] nanowires for 10.6 Å diameter approach the limit of 100%. In addition, the electron and hole masses for InAs and GaSb nanowires with different crystallographic axes have been calculated. For the [1 0 1] and [1 1 1] oriented nanowires, the hole masses are predicted to be around 0.1–0.2 m₀, which are notably smaller than the values (∼0.5 m₀) along the same direction for their bulk counterparts. Thus, we demonstrates an inspired possibility of obtaining a high hole mobility in nanowires that is not available in bulk. The small hole mobility is interpreted as to be associated with the strong electronic band mixing in nanowires.     

Structural characterization of β-SiC nanowires synthesized by direct heating method

Crystal structure of β-SiC nanowires was investigated using Raman spectroscopy, FT-IR, XRD, transmission electron microscopy and selected area electron diffraction. Cubic β-SiC nanowires were synthesized by heating NiO catalyzed Si substrates with WO₃ and graphite mixed powders in the growth temperature of 1000–1100 °C. HRTEM image showed atomic arrangements of the grown SiC nanowires with a main growth direction of [111]. Raman spectra showed two characteristic peaks at 796 cm^− 1 and 968 cm^− 1, which are corresponding to transversal optic mode and longitudinal optic mode of β-SiC, respectively. Also, FT-IR absorption spectroscopy showed a SiC characteristic absorption band at ∼792 cm^− 1.     

Atomic and electronic properties of realizable size single-crystal GaN nanotubes by first principles

We studied the diameter and wall thickness dependent atomic and electronic properties of practical size single-crystal GaN nanotubes using first principle calculations. Single-crystal GaN nanotubes are similar to the hexagonal GaN nanowires, grown in the [0001] direction with [10-10] facets, except there is an axial hexagonal void in them. We first demonstrated that the atomic and electronic properties of these tubes are mainly determined by the thickness of their wurtzite walls; and their diameters have negligible effects. Then, considering the individual walls of GaN nanotubes in two-dimensional slab calculations we examine the bond distances, formation energy, band gap, effective electron mass and the evolution of electronic density of the states as a function of thickness for unsaturated and hydrogen-saturated slabs of GaN. Calculations revealed that the unsaturated dangling bonds at the surfaces induce defect states in the band gap region of unsaturated tubes. Therefore, regardless of diameter and wall thickness, their band gaps are always smaller than that of the bulk GaN. However, the band gaps of the hydrogen-saturated tubes are found to be amplified with respect to bulk GaN. The amplification in the band gaps as a function of wall thickness in the range of 5.6-16.9 A and 16.9-28.1 A scales with a factor of 1/d(0.9281) and 1/d(1.769), respectively. Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness as small as 28.1 A would be stable and they would have a noticeably larger band gap with respect to the band gap of bulk GaN.     

Study on optical weak absorption of borate crystals

Borate crystal is an important type of nonlinear optical crystals used in frequency conversion in all-solid-state lasers. Especially, LiB₃O₅ (LBO), CsB₃O₅ (CBO) and CsLiB₆O₁₀ (CLBO) are the most advanced. Although these borate crystals are all constructed by the same anionic group-(B₃O₇)⁵⁻, they show different nonlinear optical properties. In this study, bulk weak absorption values of three borate crystals have been studied at 1064 nm by a photothermal common-path interferometer. The bulk weak absorption values of them along [1 0 0], [0 1 0] and [0 0 1] directions were obtained, respectively, to be approximately 17.5 ppm cm⁻¹, 15 ppm cm⁻¹ and 20 ppm cm⁻¹ (LBO); 80 ppm cm⁻¹, 100 ppm cm⁻¹ and 40 ppm cm⁻¹ (CBO); 600 ppm cm⁻¹, 600 ppm cm⁻¹ and 150 ppm cm⁻¹ (CLBO) at 1064 nm. The results showed an obvious discrepancy of the values of these crystals along three axis directions. A correlation between the bulk weak absorption property and crystal intrinsic structure was then discussed. It is found that the bulk weak absorption values strongly depend on the interstitial area surrounded by the B–O frames. The interstitial area is larger, the bulk weak absorption value is higher.     

GaN epilayer on separately deposited buffer layer by hot wall epitaxy

High quality GaN epilayers were grown on a sapphire substrate using a hot wall epitaxy method. We have investigated the crystal, optical, and electrical properties of GaN epilayers grown as functions of the nitridation condition of the substrate and the growth condition of GaN buffer layer. In order to study an effective method to grow a buffer layer for the growth of high quality GaN epilayer, the buffer layers were formed on the nitridated substrate using two different methods. One is separately deposited buffer layer (SDBL), and the other is co-deposited buffer layer (CDBL). It was observed that the growth condition of the buffer layer had a strong influence on the crystal and optical properties of GaN epilayer. A strong band edge emission peak at 3.474 eV was observed from the photoluminescence spectrum measured at 5 K for GaN epilayer grown at the optimum condition of the buffer layer. The carrier concentration and mobility of undoped GaN epilayer grown with a growth rate of 0.5 μm h⁻¹ were 2 × 10¹⁸ cm⁻³ and >50 cm³ V⁻¹ s⁻¹ at room temperature, respectively.     

Multifunctional CuO nanowire embodied structural supercapacitor based on woven carbon fiber/ionic liquid–polyester resin

Novel structural supercapacitors based on CuO nanowires and woven carbon fiber (WCF) has been developed for the first time employing vacuum assisted resin transfer molding (VARTM) process. The growth of CuO nanowires on WCF is an efficient process and can be used in structural capacitors which can trigger the electric vehicle industries toward a new direction. The specific surface area of the carbon fiber was enhanced by NaOH etching (41.36 m² g⁻¹) and by growing CuO nanowires (132.85 m² g⁻¹) on the surface of the WCF. The specific capacitance of the CuO–WCF based supercapacitor was 2.48 F g⁻¹, compared with 0.16 F g⁻¹ for the bare WCF-based supercapacitor. The usage of ionic liquid and lithium salt improved the capacitance to 5.40 and 6.75 F g⁻¹ with lowest ESR and R_p values of 133 and 1240 Ω along with improving mechanical properties within an acceptable range. The energy and power densities were also increased up to 106.04 mW h kg⁻¹ and 12.57 W kg⁻¹. Thus, this study demonstrated that growing CuO nanowires on the surface of WCF is a novel approach to improve multifunctionality that could be exploited in diverse applications such as electric cars, unmanned aerial vehicles (UAVs), and portable electronic devices.     

Synthesis and photoluminescence of zinc sulfide nanowires by simple thermal chemical vapor deposition

We present a synthetic method of zinc sulfide nanowires by a simple and safe reaction of zinc oxide and iron sulfide powders on a gold-coated silicon substrate through chemical vapor transport and condensation. High quality ZnS nanowires with single crystalline wurtzite structures are grown along [0 0 1] direction with diameters in the range of 10–30 nm and lengths up to tens of micrometers. Photoluminescence spectrum shows strong emission near 339 nm. These nanowires with cleaved ends could be a prominent candidate material for a nanoscale cavity as a ultra-violet nanolaser.     

Study of using aqueous NH3 to synthesize GaN nanowires on Si(1 1 1) by thermal chemical vapor deposition

High-quality GaN nanowires (NWs) and zigzag-shaped NWs were grown on catalyst-free Si(1 1 1) substrate by thermal chemical vapor deposition (TCVD). Gallium (Ga) metal and aqueous NH₃ solution are used as a source of materials. Ga vapor was directly reacts with gaseous NH₃ under controlled nitrogen flow at 1050 °C. Scanning electron microscopy (SEM) images showed that the morphology of GaN displayed various densities of NWs and zigzag NWs depending on the gas flow rate, and increased nitrogen flow rate caused density reduction. The GaN NWs exhibited clear X-ray diffraction analysis (XRD) peaks that corresponded to GaN with hexagonal wurtzite structures. The photoluminescence spectra showed that the ultraviolet band emission of GaN NWs had a strong near band-edge emission (NBE) at 361–367 nm. Yellow band emissions were observed at low and high flow rates due to nitrogen and Ga vacancies, respectively. Moderate N₂ flow resulted in a strong NBE emission and a high optical quality of the NWs. This study shows the possibility of low-cost synthesis of GaN nanostructures on Si wafers using aqueous NH₃ solution.     

Density functional theories study on the properties of Ga1 − xCrxAs

The structural, electronic, magnetic and optical properties of Ga_1 − xCr_xAs (x = 0, 6.25%, 12.5%) have been studied by first-principles calculations based on the HSE hybrid density functional theories. The optical properties, including the complex dielectric function, optical refractive index, extinction coefficient and absorption coefficient are discussed for radiation up to 15 eV. The results predicate that the system of Ga_1 − xCr_xAs exhibits typical half-metallic properties, in which Cr forms deep levels in the forbidden band and reduces the energy gap, increases static dielectric constant and obviously red-shifts the absorption edge. With the increase of the fraction of Cr, the material gradually exhibits noticeable anisotropy in the photon energy range of 0–5 eV. In addition, the p-d hybridization reduces the magnetic moment of Cr from its free space charge value of 3 μ_B and a smaller atomic magnetic moments of As and Ga atoms are generated.     

First principle calculation of physical properties of barium based chalcogenides BaM4S7 (M = Ga,Al); a DFT,DFT-D and hybrid functional HSE06 study

The electronic structure, elastic and optical properties have been calculated for the novel nonlinear optical (NLO) crystals BaQ₄S₇ (Q = Ga, Al) using plane wave pseudo-potential density functional theory (DFT) method as implemented in CASTEP and ABINIT codes. In this study we used both hybrid HSE06 and DFT-D functionals with GGA approximation. These NLO compounds, which belong to the mm2 point group, are particularly interesting because of their transparency in the mid-infrared region and wide energy band gap. We present results for electronic structure, elastic tensor coefficients, refractive indices and second order nonlinear optical susceptibilities. The calculated energy band gap and frequency dependent refractive indices as well as the NLO coefficients of BaGa₄S₇ are in good agreement with the experimental values. With no reported theoretical or experimental energy band gap and optical properties of BaAl₄S₇, we present for the first time its electronic structure and above mentioned optical coefficients. This compound has higher direct band gap with 3.74 eV, better optical birefringence and second-order NLO coefficients than most NLO compounds. The second-order NLO coefficients for BaAl₄S₇ have been calculated as d₃₁ = 3.15 pm/V, d₃₁ = 2.20 pm/V, d₃₃ = −6.31 pm/V.     

Size effects of nano-pattern in Si(1 1 1) substrate on the selective growth behavior of GaN nanowires by MOCVD

We report on the size effects of nano-patterned Si(1 1 1) substrates on the selective growth of GaN nanowires (NWs) using metal organic chemical vapor deposition. The nano-patterns on Si(1 1 1) substrates were fabricated by etching process of Au nano-droplets. The size of nano-patterns fabricated on Si(1 1 1) substrates were corresponding to size of Au nano-droplets, and the diameter of GaN NWs grown on nano-patterns was similar to the size of nano-pattern. Dense and well-oriented GaN NWs were grown on Si(1 1 1) substrates corresponding to the nano-patterns with an average diameter of about 50 nm. However, only a few GaN bulk grains, and mixed phase of a few NWs and bulk crystal of GaN were grown on the nano-patterned Si(1 1 1) having too small and large diameter, respectively, compare to the nano-patterns with diameter of 50 nm. Our results suggested that the selective growth of GaN NWs is strongly affected by the size of nano-patterns and its related mechanism.     

Electronic structure of alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4 single crystal: DFT approach

The electronic band structure, total and angular momentum resolved projected density of states for NaSr₃Be₃B₃O₉F₄ are calculated using the all-electron full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method. The calculations are performed within four exchange correlations namely; local density approximation (LDA), general gradient approximation (PBE-GGA), Engel–Vosko generalized gradient approximation (EVGGA) and the recently modified Becke–Johnson potential (mBJ). Calculations suggest that NaSr₃Be₃B₃O₉F₄ is a direct wide band gap semiconductor. The exchange correlations potentials exhibit significant influence on the value of the energy gap being about 4.82 eV (LDA), 5.16 eV (GGA), 6.20 (EVGGA) and 7.20 eV (mBJ). The mBJ approach succeed by large amount in bringing the calculated energy gap closer to the experimental one (7.28 eV). The angular momentum resolved projected density of states shows the existence of a strong hybridization between the various orbitals. In additional we have calculated the electronic charge density distribution in two crystallographic planes namely (1 0 1) and (0 0 −1) to visualized the chemical bonding characters.     

Structure and growth mechanism of CuO plates obtained by microwave-hydrothermal without surfactants

CuO plates were obtained by microwave-hydrothermal processing at 130 °C for 30 min without any surfactant. X-ray diffraction, Rietveld refinement and selected area electron diffraction showed that the CuO plates present a monoclinic structure without secondary phases. The nitrogen adsorption isotherm measurements revealed a specific surface area of approximately 30 m²/g. Field-emission gun scanning electron microscopy and transmission electron microscope micrographs indicated that the growth process of these plates was through Ostwald ripening and aggregation of plates surface by Van der Waals forces along to the two [1 0 0] and [0 1 0] directions.     

Electromechanical properties of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 crystals under uniaxial stress

The electric field and the stress induced strain and polarization responses of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN–0.32PT) relaxor ferroelectric single crystals have been systematically investigated by experimental study. The responses of [0 0 1] oriented single crystal to stress cycle and electric field are explained by polarization rotation and phase transformations mechanism. However, the responses of [0 1 1] and [1 1 1] oriented single crystal should be explained by domain switching. The differences of strain and polarization between the minimum and maximal values of electric field in [0 0 1] orientation firstly increase then decrease with enhancing of compressive stress.     

Single-crystal oxoborate (Pb3O)2(BO3)2WO4: Growth and characterization

An oxoborate, (Pb₃O)₂(BO₃)₂WO₄, has been prepared by solid-state reaction methods below 620 °C. Single-crystal XRD analysis shows that it crystallizes in the orthorhombic group Cmcm with a = 18.480(4) Å, b = 6.3567(13) Å, c = 11.672(2) Å, Z = 4. The crystal structure is composed of one-dimensional 1/∞ [Pb₃O]⁴⁺ chains formed by corner-sharing OPb₄ tetrahedra. BO₃ and WO₄ groups are located around the chains to hold them together via PbO bonds. The IR spectra further confirmed the presence of BO₃ groups. Furthermore we have performed theoretical calculations by employing the all-electron full potential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. Starting from our XRD data we have optimized the atomic positions by minimizing the forces. These are used to calculate the electronic band structure, the atomic site-decomposed density of states, electron charge density and the chemical bonding features. The calculated electronic band structure and densities of states suggest that this oxoborate possesses a wide energy band gap. The valence band maxima and the conduction band minima are located at Y point in the Brillouin zone resulting in a direct energy band gap of 2.3 eV using the local density approximation and 2.6 eV for the Engel–Vosko generalized gradient approximation. This compares well with our experimentally measured energy band gap of 2.9 eV. From our calculated electron charge density distribution, we obtain an image of the electron clouds that surround the molecules in the unit cell of the crystal. The chemical bonding features were analyzed and the substantial covalent interactions are observed between Pb and O, B and O and W and O atoms.     

Ab-initio electronic structure,optical, dielectric and bonding properties of lizardite-1T

The structural, electronic, optical, dielectric and bonding properties of lizardite-1T [Mg₃Si₂O₅(OH)₄] are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the earlier experimental and theoretical results. The direct electronic band gap at the Γ-point is estimated to be 3.34 eV, which is less than the optical band gap of ～4.0 eV measured from the fundamental absorption edge. A remarkable optical anisotropy is observed in the optical spectra. The dielectric properties are consistent with previous theoretical calculations. Analysis of Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the lizardite-1T and the results are also consistent with previous theoretical calculations and experimental results.     

Structural,optical, thermal,photoconductivity, laser damage threshold and fluorescence analysis of an organic material: β-P-amino benzoic acid single crystal

β-P-amino benzoic acid, an organic single crystal was grown by slow evaporation technique. Single crystal X-ray diffraction studies show that the grown crystal has β-polymorph of P-amino benzoic acid [β-PABA] form and the lattice parameters are a = 6.30 Å, b = 8.61 Å, c = 12.43 Å α = γ = 90° and β = 100.20°. FTIR analysis confirms that bands at 1588 cm⁻¹, 1415 cm⁻¹ are assigned to ring skeletal vibrations of title compound. The molecular structure of the grown crystal has been identified by Nuclear Magnetic Resonance spectral study. The optical absorbance spectrum from 200 to 1100 nm shows that there is an edge absorbance in UV region. Optical band gap of the crystal has been assessed from the absorbance spectrum. The thermal properties of crystals were evaluated from TG-DTA analysis, it exhibits that there is no weight loss up to 187 °C. Laser damage threshold indicates that the grown crystal has no surface damage up to 35 mJ. Photoconductivity and fluorescence spectral experiments are also carried out and the results are discussed.     

Electrochemical formation of anisotropic periodic grooves on InP for quantum wire laser growth

We report on an electrochemical process whereby regular, ordered and periodic V-grooves are etched into an InP(0 0 1) surface during constant potential anodization in a 1 mol dm⁻³ KOH solution in dark conditions. Transmission electron microscopy and atomic force microscopy were employed to characterize the morphological changes induced by the etch process. V-grooves with a periodicity of 0.15 μm were observed delineated by the {1 1 1} etch-stop planes and are preferentially elongated in the [1 1 0] direction. X-ray photoelectron and reflection anisotropy spectroscopies show that the terminating faces of the grooves are very ordered and In-rich. It is hoped that such textured surfaces can be employed in the realization of massively parallel quantum wire laser growth.     

Synthesis and nonlinear optical properties of two new two-photon initiators: Triphenylamine derivatives

Two new two-photon polymerization initiators, 1-{(1E)-2-[4-(diphenylamino)phenyl]vinyl}-4-[4-N,N-di-methylamino]benzene (PVMB) and 1-[(1E)-2-(4-{[4-((1E)-2-{4-[4-N,N-di-methylamino]phenyl}vinyl)phenyl]phenylamino}phenyl)vinyl]-4-[4-N,N-di-methylamino]benzene (DPVMB) have been synthesized with solid phase Wittig reaction. The single-photon fluorescence, quantum yields, lifetimes, solvent effects of the initiators were studied in detail and both compounds exhibited solvent-sensitivity. Two compounds are good two-photon absorbing chromophores and operative two-photon photopolymerization initiators. Two-photon absorption (TPA) cross-sections from 730 nm to 870 nm at intervals of 10 nm have been measured with the two-photon-induced fluorescence method, and the maximal values of the TPA cross-sections of PVMB and DPVMB are 1.6 GM and 19.9 GM (1 GM = 1 × 10⁻⁵⁰ cm⁴ s photon⁻¹ molecule⁻¹) at 800 nm, respectively. A microstructure has been fabricated by using DPVMB as initiator.     

Experimental investigation of 3-W class-AB cryogenically-cooled amplifier employing GaN HEMT for front end receivers of mobile base stations

This paper presents experimental results of a 2-GHz band gallium nitride high electron mobility transistor (GaN HEMT) amplifier cryogenically-cooled to 60 K as a part of the cryogenic receiver front end (CRFE) for mobile base station receivers. At a temperature of 60 K, the GaN HEMT amplifier attains the maximum power added efficiency of 62%, the saturation output power of 35 dBm, the gain of 26 dB, and the noise figure of 2.6 dB when operating at class-AB biasing. The results reported herein are the first on the performance of a cryogenically-cooled GaN HEMT amplifier aiming at use in a 2-GHz band CRFE.