Hexagonal Lattice Constants Research Articles

Space-charge-controlled field emission analysis of current conduction in amorphous and crystallized atomic-layer-deposited Al2O3 on GaN

As previously reported, postdeposition annealing at 800 °C and higher simultaneously crystallizes atomic-layer-deposited (ALD) Al2O3 films and reduces the current in Al/ALD-Al2O3/(0001) GaN capacitors by two orders of magnitude. This current reduction is caused by the enhancement of conduction band offset from 1.4 to 1.8 eV, as revealed by the space-charge-controlled field emission analysis. Selected area electron diffraction (SAED) patterns demonstrate that the crystallized films consist of twinned (111)-oriented cubic γ-Al2O3 with an epitaxial relation of Al2O3 ⟨01¯1⟩∥ GaN ⟨21¯1¯0⟩. The SAED patterns additionally include spots that are specific to triaxially tripled γ-Al2O3. The aforementioned epitaxy is due to the similarity of hexagonal close-packed sublattices between oxygen on a (111) γ-Al2O3 plane and nitrogen on a (0001) GaN plane. However, the hexagonal close-packed lattice constant of γ-Al2O3 is 12% smaller than that of GaN, necessitating domain matching epitaxy. The thickness of the interfacial transition layer caused by the large misfit is estimated to be thinner than four monolayers of oxygen sublattice, by using the methodology developed here. Based on these results, the effect of Al2O3 crystallinity on the characteristics of Al2O3/GaN capacitors, such as conduction current, dielectric breakdown, interface states, and bias instability, was comprehensively captured. According to x-ray diffraction analyses, Al2O3 films crystallize at 700 °C, which is ∼100 °C lower than the threshold temperature estimated by transmission electron microscope observations. This difference was possibly caused by locally crystallized Al2O3 films, as confirmed by the slightly reduced current. These findings form a basis for improving ALD-Al2O3 films as gate insulator.

Lu-atom-ordered oxonitridoaluminosilicate Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1.

A single crystal of Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1 (barium cerium lutetium aluminosilicate nitride oxide) was obtained by heating a mixed powder of Ba3N2, Si3N4, Al, Lu2O3, and CeO2 at 2173 K for 1 h under N2 gas at 0.85 MPa. X-ray single-crystal structure analysis revealed that the title oxynitride is hexa-gonal (lattice constants: a = 6.0378 (5) Å, c = 9.8133 (9) Å; space group: P63 mc) and isostructural with BaYbSi4N7. (Ba,Ce) and Lu atoms occupy twelvefold and sixfold coordination sites, respectively.

Graphite thermal expansion coefficient measured by in-situ x-ray diffraction

Precision temperature measurement of a nano system with high sensitivity and fast response is still a challenge. The marvelous thermal and mechanical properties of graphite will allow the creation of superior nanoscale temperature sensors. In-situ x-ray diffraction was employed to determine the graphite hexagonal crystal lattice dimensions and the coefficient of thermal expansion based on the calculation of its interatomic distance. The energy of graphite was mapped over the first Brillouin zone in the temperature range of 50 °C–1200 °C at intervals of 50 °C. Energy-based comparative studies between the quantum free electron approach obtained by an inelastic scattering and an harmonic oscillator are introduced by the principal quantum number associated with the excitation level. The hexagonal lattice constants, interlayer distance and interatomic distance of graphite crystals are investigated analytically with consideration given to their temperature dependence and the carbon peak (002), where the 2θ value decreases slightly with increasing temperature. The coefficient of thermal expansion of graphite-based interatomic distance is negative and tends toward zero with increasing temperature, which is in very good agreement with experiments. Moreover, the energy probability distributions enclosed by reciprocal lattice vectors of the hexagonal lattice are defined and interpreted based on lattice dimensions with varying temperature. Linear changes of the temperature-driven unit cell lattice dimensions and analysis of the kinetic energy of the electron in graphite may both be utilised for the advanced temperature interpretation model and preliminary design of a precise nanothermometer.

Fabricating ZnSnN2 with cosputtering

The deposition of ZnSnN2 by co-sputtering was studied systematically. Different deposition parameters, including substrate temperatures, N2 flow rates and different work pressure etc. were tried and the structural, optical and electrical properties were studied. The results show that the deposition parameters to fabricate ZnSnN2 are relatively wide. The suitable substrate temperature to obtain polycrystalline ZnSnN2 is around 350 °C when other parameters are kept constant. Relatively larger N2 flow rate can suppress the formation of metallic phase such as Sn and favour the deposition of ZnSnN2 with relatively pure phase, with other parameters to be constant. The addition of the substrate bias voltage or a relatively lower power of the Sn target is harmful to the film crystallization of ZnSnN2. The samples are considered to be wurtzite due to the absence of the orthorhombic (110) and (011) peaks at 20°–22°. The hexagonal lattice constants a and c are about 3.36–3.40 Å and 5.49 Å, respectively. The sputtering pressure can affect the structural and electrical properties of ZnSnN2. ZnSnN2 prepared at larger work pressure such as 7.0 Pa has a lower electron density (6.72 × 1019 cm−3) and a higher mobility (24.3 cm2 V−1 s−1) as compared with those prepared at 1.0 and 3.0 Pa.

Strain engineering of antimonene by a first-principles study: Mechanical and electronic properties

In this work, we investigate the mechanical and electronic properties of monolayer antimonene in its most stable beta-phase using first-principles calculations. The upper region of its valence band is found to solely consist of lone pair p-orbital states, which are by nature more delocalized than the d-orbital states in transition metal dichalcogenides, implying superior transport performance of antimonene. The Young's and shear moduli of beta-antimonene are observed to be ~25% higher than those of bulk antimony, while the hexagonal lattice constant of the monolayer reduces significantly (~5%) from that in bulk, indicative of strong inter-layer coupling. The ideal tensile test of beta-antimonene under applied uniaxial strain highlights ideal strengths of 6 GPa and 8 GPa, corresponding to critical strains of 15% and 17% in the zigzag and armchair directions, respectively. During the deformation process, the structural integrity of the material is shown to be better preserved, albeit moderately, in the armchair direction. Interestingly, the application of uniaxial strain in the zigzag and armchair directions unveil direction-dependent trends in the electronic band structure. We find that the nature of the band gap remains insensitive to strain in the zigzag direction, while strain in the armchair direction activates an indirect-direct band gap transition at a critical strain of 4%, owing to a band switching mechanism. The curvature of the conduction band minimum increases during the transition, which suggests a lighter effective mass of electrons in the direct-gap configuration than in the free-standing state of equilibrium. The work function of free-standing beta-antimonene is 4.59 eV and it attains a maximum value of 5.07 eV under an applied biaxial strain of 4%.

A top-down approach for the synthesis of nano-sized Ba-doped hydroxyapatite

Nano-sized barium-doped hydroxyapatite (n-BaHA) was successfully synthesized by a facile one-pot mechanochemical process. The effect of dopant loading on phase composition and morphological features as well as hexagonal lattice constants was assessed to study structural evolution of the milled particles. From the XRD-Rietveld refinement, the a- and c-axis directions and unit cell volume showed an increasing trend with the increase of the dopant content. From the TEM observations, the doped nanopowders were composed of nanospheroids with an average particle size of 36 ± 2 nm. This novel top-down approach provides an outline for the future design of nanostructured bioceramics.

Mechanochemical preparation and structural characterization of Ta-doped chlorapatite nanopowders

Nanocrystalline tantalum-doped chlorapatite (Ta-doped ClA) was successfully synthesized using a facile mechanochemical method. In the absence of the dopant, milling for 3h led to the formation of a poorly crystalline hydroxyapatite, while in its presence of the Ta dopant, Ta-doped ClA nanopowders were produced as a result of an oncoming reaction. The results indicated that lattice micro-strain, crystallite size, crystallinity level, phase percentage and hexagonal lattice constants of the substituted apatite nanopowders were dramatically affected by the doping concentration. The a-axis and unit cell volume increased with the increase in the doping concentration, owing to the ionic radius difference of Cl− and OH− ions. From the TEM observations, the doped powders consisted of nanoneedles with a mean size of 60±20nm in length and 14±4nm in width.

A new Bi-based visible-light-sensitive photocatalyst BiLa1.4Ca0.6O4.2: crystal structure, optical property and photocatalytic activity.

A new compound of BiLa1.4Ca0.6O4.2 is synthesized through solid state reaction, where the Ca substitutes, in part, the La site in a stable BiLa2O4.5 phase. The structure of the BiLa1.4Ca0.6O4.2 crystallizes in space group R3mH with a hexagonal lattice constants of a = 3.893(1) Å, c = 9.891(1) Å. Its optical absorption edge is about 2.05 eV, which just spans the visible light region. The photocatalytic activity of the BiLa1.4Ca0.6O4.2 powder to degradation of RhB under visible light irradiation is measured and improved more than 7 times by annealing in nitrogen ambient, indicating that annealing in nitrogen can effectively improve the photocatalytic activity by producing oxygen vacancy. Although the absolute photocatalytic activity obtained is low, there is great potential for enhancing the activity such as nanoscaling, doping, and coupling with other compounds.

A new type photodiode: p-Si/GaN pn junction in series with GaN/Ag Schottky diode

Large quantities of gallium nitride (GaN) nanoparticles were successfully synthesized via a facile sol-gel approach. X-ray diffraction analysis confirms the polycrystalline nature of the GaN with hexagonal wurtzite structure and lattice constants a = 0.3189 nm and c = 0.5185 nm. The morphology of the GaN film was investigated by field emission scanning electron microscopy. The obtained results indicate that the synthesized GaN nanorods have an average length of around 60 nm and an average diameter of 23 nm. The optical band gap of the GaN film was obtained to be 3.4 eV. The gallium nitride/p-Si Schottky diode was fabricated by thermal evaporation technique on p-silicon. The current–voltage (I–V) characteristics of the fabricated diode was tested under dark and various light intensities. T The diode ideality factor and barrier height were computed using forward bias I–V characteristics of the diode and are found to be 1.66 and 0.53 eV, respectively. The obtained results suggest that the film preparation by sol gel method is fast and simple to prepare GaN based photodiode by according to metal organic deposition methods.

Surfactant-induced phases in water-supported alkane monolayers: II. Structure.

The structure of the Langmuir-Gibbs films of normal alkanes C(n) of length n = 12-21 formed at the surface of aqueous solutions of C(m)TAB surfactants, m = 14, 16, and 18, was studied by surface-specific synchrotron X-ray methods. At high temperatures, a laterally disordered monolayer of mixed alkane molecules and surface-adsorbed surfactant tails is found, having thicknesses well below those of the alkanes' and surfactant tails' extended length. The mixed monolayer undergoes a freezing transition at a temperature T(s)(n,m), which forms, for n ≤ m + 1, a crystalline monolayer of mixed alkane molecules and surfactant tails. For n ≥ m + 2, a bilayer forms, consisting of an upper pure-alkane, crystalline monolayer and a lower liquidlike monolayer. The crystalline monolayer in both cases consists of hexagonally packed extended, surface-normal-aligned chains. The hexagonal lattice constant is found to decrease with increasing n. The films' structure is discussed in conjunction with their thermodynamic properties presented in an accompanying paper.

Monoclinic deformation of the crystal lattice of hematite α-Fe2O3

Lineshape analysis of high resolution synchrotron radiation diffraction patterns of polycrystalline α-Fe2O3 powders show distortions from trigonal (space group R-3c) symmetry. The symmetry lowering is observed for α-Fe2O3 polycrystalline powders from several commercial vendors. The experimental observations can be explained by assuming a monoclinic symmetry (space group C2/c). The relative differences between the monoclinic and the hexagonal lattice constants are of the order of 10−4. The monoclinic symmetry of the crystal structure of α-Fe2O3 (C2/c) is in agreement with magnetic space group analysis [American Mineralogist 95 (2010) 974].

Preparation of ZnS thin films by using photoassisted MOCVD

Zinc sulfide thin films were grown by using both normal metal-organic chemical vapor deposition (MOCVD) and photoassisted MOCVD with diethylzinc and t-butylthiol as sources for the zinc and the sulfur, respectively. The effects of UV irradiation and reactor chamber pressure on the film growth were investigated. The basic physical properties of the ZnS thin films grown by using MOCVD were also investigated with X-ray diffraction, optical absorption, and Raman spectroscopy. The growth rate of the ZnS thin films grown by using photoassisted MOCVD were improved by about 28% compared to that of the films grown by using normal MOCVD. The growth rate also increased with increasing reactor chamber pressure. The energy band gap obtained from the optical absorption spectrum was 3.58 eV. MOCVD-grown ZnS thin films were hexagonal (lattice constants: a = 0.382 nm and c = 0.626 nm). In the Raman spectrum, the A1 and the E1 transverse peaks (A1(TO) and E1(TO)) were observed at 277 cm−1, and the A1 and the E1 longitudinal peaks (A1(LO) and E1(LO)) were observed at 354 cm−1.

An Anionic Phospholipid Enables the Hydrophobic Surfactant Proteins to Alter Spontaneous Curvature

The hydrophobic surfactant proteins, SP-B and SP-C, greatly accelerate the adsorption of the surfactant lipids to an air/water interface. Previous studies of factors that affect curvature suggest that vesicles may adsorb via a rate-limiting structure with prominent negative curvature, in which the hydrophilic face of the lipid leaflets is concave. To determine if SP-B and SP-C might promote adsorption by inducing negative curvature, we used small-angle x-ray scattering to test whether the physiological mixture of the two proteins affects the radius of cylindrical monolayers in the inverse hexagonal phase. With dioleoyl phosphatidylethanolamine alone, the proteins had no effect on the hexagonal lattice constant, suggesting that the proteins fail to insert into the cylindrical monolayers. The surfactant lipids also contain ∼10% anionic phospholipids, which might allow incorporation of the cationic proteins. With 10% of the anionic dioleoyl phosphatidylglycerol added to dioleoyl phosphatidylethanolamine, the proteins induced a dose-related decrease in the hexagonal lattice constant. At 30°C, the reduction reached a maximum of 8% relative to the lipids alone at ∼1% (w/w) protein. Variation of NaCl concentration tested whether the effect of the protein represented a strictly electrostatic effect that screening by electrolyte would eliminate. With concentrations up to 3 M NaCl, the dose-related change in the hexagonal lattice constant decreased but persisted. Measurements at different hydrations determined the location of the pivotal plane and proved that the change in the lattice constant produced by the proteins resulted from a shift in spontaneous curvature. These results provide the most direct evidence yet that the surfactant proteins can induce negative curvature in lipid leaflets. This finding supports the model in which the proteins promote adsorption by facilitating the formation of a negatively curved, rate-limiting structure.

Effects of the magnetoelastic anisotropy in Ni nanowire arrays

Nanoporous anodic alumina films on Al substrates have been used as templates for the growth of electrodeposited Ni nanowires. The nanowire diameter ranges between 35 and 65nm, and the hexagonal symmetry lattice constant is 105nm. The magnetization curves for these nanowire arrays with and without the Al substrate have been measured in a temperature range from 5to300K, from which the effective magnetic anisotropy, coercivity, and remanence have been determined. The effective easy magnetization axis changes from parallel to perpendicular to the Ni nanowire axis as temperature decreases, as a result of the magnetocrystalline and the magnetoelastic anisotropy terms, arising from the different thermal expansion coefficients of the ferromagnetic nanowires, the alumina and Al.

Boron-doped ZnO films grown by successive chemical solution deposition

ZnO films are deposited on various substrates from zincate solutions (0.1 M) containing boron with a B/Zn atomic ratio of 5%. These films show an optimum n-type resistivity of 40–60 Ω cm. The composition, structure and morphology of the films are studied using x-ray photoelectron spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy. The optical properties of the films are investigated by transmittance and photoluminescence (PL) spectroscopy. The XRD results reveal that the films consist of only hexagonal ZnO with no other crystalline trace compounds. Boron enters the film with almost the same ratio as added into the solution. Other impurities such as Cl and N can also enter the film from the bath solution. Films contain, most likely, zinc hydroxide but in the amorphous phase. The films' hexagonal lattice constants match those for un-doped films. The as-grown films have optical transmittance ≈80% (800 nm) and a direct band gap energy of 3.35 ± 0.05 eV, which reduces to 3.24 ± 0.04 eV upon thermal annealing. The room-temperature PL properties of the films prepared from different bath solutions are presented. The effect of thermal annealing and the bath composition on the UV and visible emissions are discussed.

ZnO films grown by successive chemical solution deposition

ZnO films were grown from 0.1-M zincate solutions on stainless-steel and aluminosilicate glass substrates by the successive chemical solution deposition method. The structure, morphology, composition, and optical emission properties of the films were studied by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and photoluminescence techniques. Results revealed that the as-grown film contains a substantial amount of amorphous zinc hydroxide (15–25%), at least on its surface. This can be reduced to 7% by annealing the film in argon (350 °C, 1 h). Despite the presence of the hydroxide phase, the films hexagonal lattice constants match the standard values. The films surface texture and the grains shape and preferential orientation depend on the type of the substrate and its surface conditioning. The UV photoluminescence emission from as-grown films at 3.22±0.04 eV (380–390 nm) and its suppression due to the effect of chlorine are addressed.

Thermoelectric and structural properties of a new Chevrel phase: Ti 0.3Mo 5RuSe 8

The new Chevrel phase Ti 0.3Mo 5RuSe 8 has been synthesized and characterized by quantitative microprobe analysis, powder X-ray diffraction, and high-temperature thermoelectric properties measurements. The thermoelectric properties of this compound are compared to the previously reported data for other related Chevrel phases. We report also the results of Rietveld analysis of powder X-ray diffraction data for Ti 0.3Mo 5RuSe 8. This compound adopts the rhombohedral Chevrel phase structure (space group R 3 ¯ , Z=3) with hexagonal lattice constants a=9.75430(25) Å and c=10.79064(40) Å. The low level of incorporation and low scattering power of Ti precluded the identification of the Ti positions, and Rietveld refinement was carried out only for the Mo 5RuSe 8 framework of Ti 0.3Mo 5RuSe 8 ( R p=10.5%, R wp=14.6%). Rietveld analysis was also used to refine the structure of the unfilled phase Mo 5RuSe 8 ( R 3 ¯ , Z=3, a=9.63994(8) Å, c=10.97191(11) Å, R p=8.0%, R wp=10.5%). Comparisons between the two structures are made.

Structural Variants in Attempted Heteroepitaxial Growth of B12As2 on 6H–SiC (0001)

Boron sub-arsenide, B12As2, is based on twelve-atom clusters of boron atoms and two-atom As–As chains. By contrast, SiC is a tetrahedrally bonded covalent semiconductor. Despite these fundamental differences, the basal plane hexagonal lattice constant of boron sub-arsenide is twice that of SiC. This coincidence suggests the possibility of heteroepitaxial growth of boron sub-arsenide films on properly aligned SiC. However, there are a variety of incommensurate alignments by which heteroepitaxial growth of B12As2 on (0001) 6H–SiC can occur. In this study, we first used geometrical crystallographic considerations to describe the possible arrangements of B12As2 on (0001) 6H–SiC. We identified four translational and two rotational variants. We then analyzed electron backscattered diffraction and transmission electron microscopy images for evidence of distinct domains of such structural variants. Micron-scale regions with each of the two possible rotational alignments of B12As2 icosahedra with the SiC surface were seen. On a finer length scale (100–300 nm) within these regions, boron-rich boundaries were found, consistent with those between pairs of the four equivalent translational variants associated with a two-to-one lattice match. Boron-carbide reaction layers were also observed at interfaces between SiC and B12As2.

Theoretical investigation of stoichiometric lithium monoboride

Structural energies around two recently reported diffraction results of nominally stoichiometric LiB samples have been investigated using full potential electronic structure methods. It is found that one of the reported structures is unstable to large relaxations of the hexagonal lattice constants. Calculations of a pseudodiamond structure and other B-chain derived structures indicate that this ambient pressure, stoichiometric lithium-monoboride system is indeed characterized by strongly double-bonded B chains. However, the structure is subject to very low-energy motions of Li atoms along the c axis, and also to relative displacements of B chains along the c direction. The B chains themselves are robust, with the boron bond-stretching mode at 138 meV. Although the deformation potential for this phonon mode is substantial, nevertheless the electron-phonon coupling is rather small and superconductivity can be expected at best only at low temperature. The lowest-energy geometry within this class of structures is identified, but due to only tiny differences in total energy, samples are likely to contain several types of structural imperfection as suggested by diffraction data.

Influence of carbon content on the crystallographic structure of boron carbide films

Boron carbide thin films were synthesised by laser-assisted chemical vapour deposition (LCVD), using a CO2 laser beam and boron trichloride and methane as precursors. Boron and carbon contents were measured by electron probe microanalysis (EPMA). Microstructural analysis was carried out by Raman microspectroscopy and glancing-incidence X-ray diffraction (GIXRD) was used to study the crystallographic structure and to determine the lattice parameters of the polycrystalline films. The rhombohedral–hexagonal boron carbide crystal lattice constants were plotted as a function of the carbon content, and the non-linear behaviour observed was interpreted on the basis of the complex structure of boron carbide.