Natural Single Crystals Research Articles

Hall sign reversal and electrical transport properties of single crystalline SrPd2Ge2

In this study, Hall effects, magnetoresistance, and low temperature resistivity of single crystalline SrPd2Ge2 were measured. The sign change of the Hall coefficient from negative to positive with increasing temperature in the normal state and the violation of Kohler’s rule in magnetoresistance data were observed. These results suggest that electrons and holes would contribute together to the formation of multi bands and that the scattering on the Fermi surface is anisotropic, which should be taken into account for the transport properties of SrPd2Ge2. In addition, we estimated the upper critical field at T = 0 K, Hc2(0) = 6.1 kOe, from the temperature-dependent electrical resistivity measurement down to 0.35 K. Including this, we discuss the nature of SrPd2Ge2 single crystals.

Si primary standards for the calibration of ion-current ratios in the molar-mass measurement of natural Si single crystals

A procedure is described to obtain SI-traceable silicon isotope amount ratios in the BaSiF6 form. On the basis of a gravimetric approach, three synthetic isotope mixtures were made, very similar to natural Si, by blending silicon isotopically enriched with 28Si, 29Si and 30Si. These mixtures served as references to calibrate measurements of (SiF3)+ current ratios. In this way, it was possible to make SI-traceable measurements of silicon isotope amount ratios, without any assumptive correction. The isotopic composition of the natural Si crystal WASO17.2 was remeasured and its molar mass was redetermined.

RAPID AND COST EFFECTIVE SYNTHESIS OF ZnO NANORODS USING MICROWAVE IRRADIATION TECHNIQUE

ZnO nanorods assembled in flower shaped morphology have been successfully synthesized using low power microwave irradiation in a very short duration. The diameter and length of the rods were within 150–190 nm (tip diameter ~15 nm) and 2 μm, respectively, with an aspect ratio of 20–22. The synthesized nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared microscopy (FT-IR), photoluminescence (PL) and magnetization measurements. The XRD and FT-IR results indicate that ZnO nanorods have the pure wurtzite structure with lattice parameters a and c of 3.254 and 5.197 Å, respectively. The selected area electron diffraction (SAED) pattern reveals that the ZnO nanorods are single crystal in nature and grow along [001] plane. Room-temperature PL spectrum of the as-grown ZnO nanorods shows a near-band-edge (NBE) emission peak and defect induced emissions. Magnetization measurements indicate that ZnO nanorods exhibit room temperature ferromagnetism with remanent magnetization (M r ) and coercivity (H c ) about 2.92 × 10-4 (emu/g) and 29.75 Oe , respectively, which may be due to the presence of defects in the ZnO nanorods.

Rationally fabricating hollow particles of complex oxides by a templateless hydrothermal route: the case of single-crystalline SrHfO3 hollow cuboidal nanoshells

Based on the theory of sol-gel science, perovskite SrHfO(3) hollow cuboidal particles with tunable sizes were rationally synthesized by templateless hydrothermal reactions in KOH solutions. The concentrated KOH solution not only elevated the supersaturation of the reactants to promote the grain growth of SrHfO(3) but also controlled the aggregated particle sizes by compressing the electrical double layers of the primary particulates. The following Ostwald ripening process produced hollow particles with sizes ranging from submicrometer to hundred nanometre. The HRTEM image and SAED pattern revealed the single crystal nature of each hollow cuboidal nanoshell. The KOH concentration and reaction time related experiments confirmed that the formation of SrHfO(3) hollow cuboidal nanoshell was driven by the Ostwald ripening process and followed our assumption. The particles experienced solid, core-shell and hollow morphologies as the reaction proceeded. Also, the formation of SrHfO(3) hollow cuboidal nanoshells favored high reaction temperature which initiated and accelerated the ripening process. The as-prepared hollow cuboidal nanoshells displayed blue light emission under UV laser excitation at room temperature. After calcination, the photoluminescence intensity declined due to the improvement of crystallinity.

Perovskite BaZrO3 hollow micro- and nanospheres: controllable fabrication, photoluminescence and adsorption of reactive dyes

Size tunable perovskite-type BaZrO3 hollow micro- and nanospheres have been hydrothermally synthesized in a strong basic medium without any organic or inorganic templates. The products experienced morphology variations of solid, core-shell and hollow nanospheres by alternatively adjusting the base concentration, the reaction temperature or the reaction duration. Investigations on the synthetic parameters revealed that the formation of BaZrO3 hollow nanospheres was driven by the Ostwald ripening process. BaZrO3 hollow nanospheres were produced when the reaction temperature and the base concentration were in a suitable rate due to their functions to elevate the crystal growth speed and the diffusion rate. XRD patterns disclosed the cubic perovskite structure of BaZrO3 hollow spheres. HRTEM and SAED measurements revealed the single crystal nature of a single BaZrO3 hollow nanosphere. The hollow spheres were size tunable by simply adjusting the base concentration at a suitable temperature, which was rationally elucidated by the sol–gel theory. The hollow nanospheres exhibited an intensive green light emission and excellent reactive dye adsorption.

Programmable Growth of Branched Silicon Nanowires Using a Focused Ion Beam

Although significant progress has been made in being able to spatially define the position of material layers in vapor-liquid-solid (VLS) grown nanowires, less work has been carried out in deterministically defining the positions of nanowire branching points to facilitate more complicated structures beyond simple 1D wires. Work to date has focused on the growth of randomly branched nanowire structures. Here we develop a means for programmably designating nanowire branching points by means of focused ion beam-defined VLS catalytic points. This technique is repeatable without losing fidelity allowing multiple rounds of branching point definition followed by branch growth resulting in complex structures. The single crystal nature of this approach allows us to describe resulting structures with linear combinations of base vectors in three-dimensional (3D) space. Finally, by etching the resulting 3D defined wire structures branched nanotubes were fabricated with interconnected nanochannels inside. We believe that the techniques developed here should comprise a useful tool for extending linear VLS nanowire growth to generalized 3D wire structures.

Strength and elastic deformation of natural and synthetic diamond crystals shock compressed along [100

Plane shock wave experiments were conducted to determine the strength and elastic response of natural and synthetic diamond single crystals shocked along [100] to peak elastic stresses of ∼90 and ∼120 GPa. Velocity interferometry was used to measure particle velocity histories and shock velocities in the diamond samples. The maximum elastic wave amplitudes (89±3 GPa) for both crystal types were comparable. This value corresponds to shear stresses of 30 and 35 GPa (∼G/15) for the (111) [11¯0] and (111) [21¯1¯] slip systems, respectively. Surprisingly, the elastic limit (57±3 GPa) was lower for the higher peak stress. The elastic constant C111 was experimentally determined to be −7804±653 GPa.

Synthesis and photocatalytic properties of α-Fe 2O 3 nanoellipsoids

A simple and a novel chemical approach was used for the growth of α-Fe 2O 3 nanostructures. Field emission scanning electron microscopy analysis revealed the monodisperse nanoellipsoid morphology of α-Fe 2O 3 nanostructures. The sizes of the short axis and the long axis of these ellipsoids were in the ranges of 50–60 nm and 40–50 nm, respectively. XRD analysis revealed that the product exhibited α-Fe 2O 3 phase. Comprehensive TEM and HRTEM analysis revealed that α-Fe 2O 3 nanoellipsoids are single crystal in nature. The methylene blue decomposition kinetics was studied for different irradiation time. The methylene blue was totally decomposed by increasing the irradiation time up to 220 min.

Epitaxial Growth of High-Resistivity CdTe Thick Films Grown Using a Modified Close Space Sublimation Method

This paper reports the growth of high-resistivity CdTe thick epitaxial films of single crystal nature using a modified close space sublimation method (MCSS) in a Te-rich environment. We propose that the high Te2 partial pressure results in an increased concentration of TeCd antisites acting as deep donors to produce the high-resistivity CdTe, as well as improved quality of thick films. This is in agreement with the deep-donor model introduced by Fiderele et al. [Cryst. Res. Technol. 38 (2003) 588]. The thick films have a µeτe value in the order 10-4 cm2 V-1 and as expected, the TeCd antisites appeared not to act as electron traps.

IMPROVED MECHANICAL PROPERTY OF HYDROTHERMALLY SYNTHESIZED HYDROXYAPATITE NANORODS REINFORCED WITH POLYETHYLENE

Single crystalline hydroxyapatite nanorods ( HAp - NRs ) were synthesized using a simple hydrothermal method. The synthesized HAp NRs have an aspect ratio of about 8-10. A closer inspection of the nanostructure of a single nanorod revealed a highly regular and defect-free lattice with unique crystallographic plane orientations. X-ray Diffraction (XRD) pattern shows the highly crystalline nature of the product. The Field Emission Scanning Electron Microscopic (FESEM) image shows the uniform size distributed rod like morphology of length 100-150nm and diameter of about 15-20nm. Transmission Electron Microscopic analysis (TEM) and High-Resolution TEM (HRTEM) images provide further insight into the microstructure and morphology of the product. The selected area electron diffraction (SAED) pattern confirms that the nanorods are single crystal in nature. The mechanical strength of hydroxyapatite was studied by reinforcing hydroxyapatite with High Molecular Weight Polyethylene (HMWPE). It clearly shows that the mechanical properties of the nanorods are high when compared to that of bulk HAp . Finally the composites were characterized by means of thermal analysis including thermogravimetry analysis and differential thermal analysis (TGA-DTA) in order to study the thermal stability of the composite and the effect of filler in HMWPE.

Annealing behavior of TiO2-sheathed Ga2O3 nanowires

TiO2-sheathed Ga2O3 one-dimensional (1D) nanostructures were synthesized by thermal evaporation of GaN powders and then sputter-deposition of TiO2. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis results indicate that the Ga2O3 cores are of a single crystal nature with a monoclinic structure while the TiO2 shells are amorphous. Photoluminescence (PL) emission is slightly decreased in intensity by TiO2 coating, but it is significantly increased by thermal annealing in an oxygen atmosphere. The emission peak is also shifted from ~500 to ~550 nm by oxygen annealing. The increase in the green emission is due to the increase in the concentration of the Ga vacancies in the cores by the inflow of oxygen during oxygen annealing. On the other hand, annealing in a nitrogen atmosphere leads to a red shift of the emission to ~700 nm originating from nitrogen doping.

Polarized Raman scattering and infrared spectroscopy of a natural manganocolumbite single crystal

AbstractA well‐ordered natural manganocolumbite single crystal of high quality was used as a prototype for the first determination of the polarized optical phonon modes of materials with the columbite structure. Electron microprobe and X‐ray diffraction characterizations determined the chemical formula as Mn0.60Fe0.40(Nb0.80Ta0.20)2O6, a cationic ordering of 81%, and the crystal structure as belonging to the Pbcn group. Polarized Raman and infrared‐reflectivity spectroscopies on oriented samples allowed us to discern 50 of the 54 predicted gerade (Raman) modes and 31 of the 38 predicted ungerade (infrared) modes for the columbite structure. The selection rules were verified, and polarization leaks only due to slight sample misorientation, confirming the high purity, ordering and quality of the material. From the polar phonon spectra, intrinsic dielectric merit factors < εr> = 29.2 and < Qu × f> of 64 THz were determined, showing adequate values for designing applications in microwave circuitry. Copyright © 2009 John Wiley & Sons, Ltd.

Studies on the synthesis of CdCO 3 nanowires and porous CdO powder

In the present study, we demonstrate the self transformation of aqueous cadmium acetate into CdCO 3 nanowires through hydrothermal reaction. The reaction temperature and the volume ratio of water to ethanol were found to be crucial for the formation of CdCO 3 nanowires. The nanowires are of single crystal in nature having width ∼ 17–30 nm as observed from selected area electron diffraction (SAED) pattern and transmission electron microscopic (TEM) results. The major weight loss found in thermogravimetric analysis (TGA) corresponds to the formation of CdO and CO 2. The powder X-ray diffraction (PXRD) patterns of CdCO 3 and CdO are respectively indexed to pure rhombohedral and cubic phases. The photoluminescence (PL) spectrum of CdO exhibits an emission peak at 483 nm due to the transition between the valence and conduction bands.

Morphological evolution of polished single crystal (100) diamond surface exposed to microwave hydrogen plasma

It is shown that exposure of polished single crystal diamond surfaces to microwave hydrogen plasma at optimized conditions may be applied as a very efficient method for the smoothing out of single crystal diamond surfaces. The effect of microwave (MW) hydrogen plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing scratches or lines. However adverse effects on the surface morphology through the formation of pits were found to occur upon long exposure times and high plasma power. A systematic study of the influence of hydrogen microwave plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub-nano-metric resolution by atomic force microscopy and scanning electron spectroscopy.

Planarization of natural single crystal diamond polishing lines by microwave hydrogen plasma

AbstractThe effect of microwave hydrogenated (MW‐H) plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing lines. A systematic study of the influence of hydrogen MW plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub‐nano‐metric resolution by atomic force microscopy and scanning electron spectroscopy.

Preparation of Dendritic Copper Nanostructures and Their Characterization for Electroreduction

Dendritic copper nanostructures of different morphologies were synthesized by a surfactant-free electrochemical method. Single crystal nature of the nanostructures was revealed from their X-ray diffraction and electron diffraction patterns. Mechanism of dendrite formation was discussed from thermodynamic aspects using the concept of supersaturation. Supersaturation of the copper metal reduced on the surface of the electrode was the crucial factor for the generation of different morphologies. Effects of applied potential, temperature, and the solution concentration on the supersaturation were studied. The NO3− and H2O2 electroreduction ability of the dendritic materials was tested. Use of copper dendrite-modified electrode as NO3− sensor was demonstrated.

Direction dependent mechanical properties of extruded cordierite honeycombs

The description of the texture in an extruded honeycomb structure of cordierite and its effects on mechanical properties is the main goal of this paper. To understand the behaviour of the extruded material, the elastic properties of a natural cordierite single crystal with a transversal isotropic behaviour were determined with different methods. The CTE and Young's modulus in an extruded structure were empirically correlated with the preferred orientation of the grains and single crystal measurements. The correlated properties were compared with the measured properties. The Young's modulus shows a similar behaviour in different directions while the CTE shows a strong anisotropy, and it is thought that microcracking may influence both properties.

Hydrothermal synthesis and optical properties of ZnO single-crystal hexagonal microtubes

ZnO microtubes were successfully synthesized by employing zinc acetate dihydrate as the starting material under hydrothermal condition. The ZnO microtubes were characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that the as-prepared ZnO microtubes were hexagonal and single crystal in nature and grew on Si (1 1 1) substrates along the [0 0 0 2] direction, which have diameters of about 3 μm, lengths of 10–20 μm, and wall thickness of 80 nm. Optical properties of the ZnO microtubes were investigated by temperature-dependent photoluminescence (PL) spectroscopy. The PL spectrum at 82.2 K shows a strong UV emission from the near band-edge with a negligible visible emission from deep level, and donor bound exciton (D 0X), free exciton (FX) and its phonon replica were observed in the UV-region. The PL spectra were dominated by the FX emission throughout the whole temperature range, and the energies of FX transitions and D 0X transitions are close to that of the bulk values of ZnO.

PROTON LOCATION AND HYDROGEN BONDING IN THE HYDROUS LEAD COPPER SULFATES LINARITE, PbCu(SO4)(OH)2, AND CALEDONITE, Pb5Cu2(SO4)3CO3(OH)6

Neutron, time-of-flight, Laue diffraction has been carried out on natural single crystals of linarite, PbCu(SO 4 )(OH) 2 , and caledonite, Pb 5 Cu 2 (SO 4 ) 3 CO 3 (OH) 6 , at 293 K. The structural refinements converged to Rw(F) = 0.085 on the basis of 869 observed reflections for linarite and Rw(F) = 0.066 from 1605 observed reflections for caledonite. The locations of all the hydrogen atoms within these structures were extracted from the refinements, enabling calculation of the hydrogen bonding. Linarite is monoclinic, space group P 2 1 / m , with a 9.682(2), b 5.646(1), c 4.683(6) A, and β 102.66(1)°. It comprises chains of edge-sharing Jahn–Teller-elongated CuO 6 square bipyramids running parallel to the [010] direction, bound together by hydrogen bonds. Between these layers are double PbO 8 and SO 4 layers. The two independent hydrogen atoms have been located and are bound as hydroxyl groups to the oxygen atoms at the corners of the CuO 4 square plane. The H(4) atom has conventional hydrogen bonding. The donor–acceptor distance is 2.924(5) A, and the hydrogen bond is nearly linear, with an angle of 176.1(6)°. The O(4)–H(4) hydroxyl bond distance is 0.952(7) A, and the H(4) ... O(2) hydrogen bond has a length of 1.973(9) A. The H(5) hydrogen atom, however, forms a stronger hydrogen bond, with a donor–acceptor distance of 2.706(5) A and an angle of 171.8(6)°. The O(5)–H(5) hydroxyl bond is lengthened to 1.025(8) A, and the H(5) ... O(4) hydrogen bond is a short 1.687(8) A. Caledonite is orthorhombic, space group Pmn 2 1 with a 20.085(3) A, b 7.141(1) and c 6.563(1) A; like linarite, it comprises chains of edge-sharing Jahn–Teller-elongated CuO 6 square bipyramids, although these chains run parallel to the [001] direction. There are three independent Pb sites, in which Pb is coordinated to nine oxygen ligands, with a range of Pb–O distances of 2.352(4) to 3.571(6) A. Caledonite has three independent hydrogen atoms that similar to those in the linarite structure: they are bound as hydroxyl groups to the oxygen atoms at the corners of the square plane of the axially elongate CuO 6 square bipyramids. The hydrogen bonds in caledonite are all significantly bent, with angles between 160 and 170°; otherwise, the H(1) and H(2) atoms have fairly conventional hydrogen bonding. The donor–acceptor distance for O(1) ... O(4) and O(2) ... O(4) are 2.887(7) and 2.776(6) A, respectively; although the O(1)–H(1) hydroxyl bond at 0.95(1) A is shorter than the O(2)–H(2) hydroxyl bond at 0.98(1) A, the H(1) ... O(4) hydrogen bond, 1.97(1) A in length, is slightly longer than the H(2) ... O(4) hydrogen bond at 1.84(1) A . The O(3)–H(3) hydroxyl bond is 0.946(8) A in length, and the H(3) ... O(8) hydrogen bond is slightly shorter than expected at 1.777(9) A. With the O(3) ... O(8) donor–acceptor distance of 2.711(5) A and the 20° deviation from linearity, this hydrogen bond may be strained.

Anisotropic photocatalytic properties of hematite

Hematite is a semiconducting mineral with a role in natural photoelectrochemical processes, and has been studied from the viewpoint of solar energy utilization. Hematite is an anisotropic conductor, with faster conduction parallel to (001) planes. Flatband potentials and photocurrent onset potentials for natural hematite single crystals and synthetic nanocrystalline hematite films are similar, and show Nernstian behavior within error. At pH7, the flatband potential of single crystals is -0.25 ± 0.1 V vs. Ag/AgCl. Photocurrent onset potential is 0.02 to 0.03 V more negative for crystal faces than for crystal edges. Photocurrent density is a factor of 5 to 10 higher for crystal edges than for crystal (001) faces, presumably because of more rapid charge separation for the edges. Falling photocurrent transients decay more slowly for edges than for faces, consistent with more rapid removal of conduction band electrons into the bulk and therefore reduced availability of such electrons for back reaction. Rising photocurrent transients occur at higher potential, and have the same rise time for both faces and edges. This suggests that the rising transients are due to slow conduction through bulk hematite. The transition from falling to rising transients occurs at a more positive potential for edges than for faces, which is also consistent with more rapid charge transport away from edge surfaces and with Fermi level pinning at edges.