Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be 4.32 and 4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit 0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

Superconductivity as a Kosterlitz–Thouless transition in the two-dimensional Hubbard model

We investigate a superconducting Kosterlitz–Thouless transition in the two-dimensional (2D) Hubbard model using auxiliary quantum Monte Carlo method for the ground state. The pair susceptibility is computed for both the attractive and repulsive Hubbard model. The numerical results show that the s-wave pair susceptibility scales as χ  L² for the attractive case, in agreement with previous quantum Monte Carlo studies. The scaling χ  L² also holds for the d-wave pair susceptibility for the repulsive Hubbard model if we adjust the band parameter t′.     

Carbon nanotube–polymer composites for photonic devices

We report the fabrication of high optical quality single wall carbon nanotube polyvinyl alcohol composites and their application in nanotube based photonic devices. These show a broad absorption of semiconductor tubes centred at 1.55 μm, the spectral range of interest for optical communications. The films are used as mode-lockers in an erbium doped fibre laser, achieving 700 fs mode-locked pulses. Raman spectroscopy shows no damage after a long time continuous laser operation.     

Dimensional evolution of silicon nanowires synthesized by Au–Si island-catalyzed chemical vapor deposition

This study explores the nucleation and morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical vapor deposition. The Au–Si islands are formed by Au thin film (1.2–3.0 nm) deposition at room temperature followed by annealing at 700 °C, which are employed as a liquid-droplet catalysis during the growth of the Si-NWs. The Si-NWs are grown by exposing the substrates with Au–Si islands to a mixture of gasses SiH₄ and H₂. The growth temperatures and the pressures are 500–600 °C and 0.1–1.0 Torr, respectively. We found a critical thickness of the Au film for Si-NWs nucleation at a given growth condition. Also, we observed that the dimensional evolution of the NWs significantly depends on the growth pressure and temperature. The resulting NWs are 30–100 nm in diameter and 0.4–12.0 μm in length. For Si (0 0 1) substrates 80% of the NWs are aligned along the 1 1 1 direction which are 30° and 60° with respect to the substrate surface while for Si (1 1 1) most of the NWs are aligned vertically along the 1 1 1 direction. In particular, we observed that there appears to be two types of NWs; one with a straight and another with a tapered shape. The morphological and dimensional evolution of the Si-NWs is significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes.     

Optical properties of GaN nanocrystals embedded into silica matrices

The purpose of this work was to obtain GaN nanocrystals (GaN-nc) embedded into silica-based matrix and to investigate their optical properties by photoluminescence (PL) spectroscopy. GaN-nc have been obtained both by the sol–gel chemistry and by the combustion method (CM). The GaN-nc obtained by CM have been introduced into the silica-based matrix during the formation of the film in the spin-on technique. Strong emission at 3.4 eV has been observed for the films doped by GaN-nc whereas no emission in UV has been observed for GaN-nc obtained by sol–gel chemistry.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.     

Laser gain by electron collisional pumping of Ar VII–V XII

Gain coefficients have been calculated for transitions of singlet levels ns–np of orbital n=4 and n=5 in magnesium-like ions with atomic numbers Z=18, 19, 20, 21, 22 and 23. Population inversions for 4p and 5p levels in these ions were also calculated, via electron collisional excitation, for electron temperature range of 93–231 eV and electron density range of 10¹⁶–10¹⁷ cm⁻³. Under these plasma conditions, the maximum gain that occurred for 4s–4p transition was at electron temperature of 231 eV and electron density of 4×10¹⁷ cm⁻³. Scaling of the maximum gain coefficients with atomic number Z and the plasma parameters is also presented.     

Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.     

Modification of photonic properties in porphyrin-infiltrated opal crystals

Structural, optical and magnetic properties of porphyrin-infiltrated opal hybrid structures were investigated. Bulk samples of synthetic opal were grown by sedimentation technique from colloidal solution of SiO₂ spheres of diameter 250 nm. The structure of the samples was examined by atomic force microscopy. The photonic properties of crystals were investigated by optical measurements in transmission and reflection modes. The stop band was observed in the region 510–550 nm. The photonic properties of synthetic opal crystals were modified by infiltration with aqueous basic solution of iron–porphyrin (FeTPPS) of concentration 1.0 mM. In hybrid samples the absorption bands typical of FeTPPS were observed in the vicinity of the opal stop band. Magnetic properties of FeTPPS-infiltrated opal samples have been studied at 5–300 K in magnetic fields up to 5 T. The FeTPPS-infiltrated opal crystals can be considered as the structures perspective for magnetophotonic devices.     

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range 1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of 3.35–3.68 eV with decreasing thickness in agreement with the calculated values.     

Quasi-distributed displacement sensor for structural monitoring using a commercial OTDR

A quasi-distributed displacement sensor for structural monitoring using an optical time domain reflectometer is demonstrated. Four displacement sensing heads are placed along a standard single mode optical fibre in several locations with different intervals. Their configurations introduce power loss through the decrease of their fibre loop radius when displacement is applied. The decrease of the light intensity with displacement variation is reported. Losses of 9 dB for a 120 mm displacement with a sensitivity of 0.027 dB/mm are reported. The quasi-distributed configuration is able to address sensors with 1 m distance resolution between them.     

Spectroscopic investigation of arsenate and selenate incorporation into hydroxylapatite

The mechanism(s) of arsenate and selenate incorporation into hydroxylapatite (HAP) using extended X-ray absorption fine structure (EXAFS) spectroscopy was investigated for As- and Se-doped HAP samples with concentrations between 200 and 2500 ppm. EXAFS data on As and Se K-edges have shown similar local coordination environments and are similar to that of P in HAP, suggesting the substitution of arsenate or selenate tetrahedra on the phosphate sites. EXAFS best-fitting for As-doped samples shows that the first shell is fitted with approximately 4 O atoms at 1.68 Å, showing As(V) in tetrahedral coordination, and Se K-edge EXAFS data are characterized by the backscattering contributions an oxygen shell at 1.2 Å in the Fourier transform, which can be fit with 4 O atoms at 1.65 ± 0.01 Å. This is characteristic of Se–O distances in SeO₄ tetrahedron. These findings suggest that arsenate and selenate substitute for phosphate groups with local distortions during the incorporation of these metals into the structure of HAP.     

Arsenic-related recombination in MOVPE-grown ZnO/GaAs films

In this paper, ZnO films grown by metalorganic vapour phase epitaxy on various substrates (GaAs, silicon, sapphire) and using different VI /II ratios, are investigated by photoluminescence (PL) spectroscopy. The PL spectra of layers grown on GaAs show significant recombination at 3.320 eV, 3.305 eV and 3.270 eV. These energies are remarkably similar to what have been reported for hybrid beam deposited ZnO:As [Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87] and arsenic-implanted ZnO crystals [T.S. Jeong, M.S. Han, C.J. Youn, Y.S. Park, J. Appl. Phys. 96 (2004) 175], and the lines are ascribed to the incorporation of arsenic, which diffuses from the substrate into the films. Two acceptor levels are deduced at 120 meV and at 140–150 meV.     

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

Effect of ratios of Cd:Se in CdSe nanoparticles on optical edge shifts and photoluminescence properties

This paper addresses the issue related to morphology of CdSe nanoparticles capped with organic molecules. Semiconducting CdSe nanoparticles of 5–16 nm are synthesized using CdO precursor, capped with trioctyl phosphine (TOP)/trioctyl phosphine oxide (TOPO) using different starting precursor ratios of Cd:Se. At an optimum ratio of Cd/Se-2:1, highly luminescent and small sized (5 nm) nanoparticles are obtained. At other Cd/Se precursor ratios (0.5:1, 1:1, 3:1) larger particles are formed with varying photoluminescence (PL) intensity and optical absorption (UV–VIS). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to determine the crystallinity and stoichiometry of the system, respectively. It is shown that the blue shifts of the optical absorption edge concurrent with the CdSe nanocrystal size reduction, for sizes measured by XRD with respect to the bulk semiconductor, agree perfectly with the strong quantum confinement model. The optical edge shifts are significantly higher for CdSe nanocrystallite as measured by transmission electron microscopy (TEM) than the theoretical prediction based on the strong quantum confinement model. This is understood on the basis of agglomeration effects as observed by TEM for CdSe nanocrystallites. The nano-sized CdSe growth island thus formed comprises of several TOP/TOPO passivated nanocrystals.     

The electrodeposition of copper onto UHV-prepared GaAs(0 0 1) surfaces

Synchrotron surface X-ray diffraction has been used to investigate in situ the morphology and epitaxy of monolayer amounts of copper electrodeposited from aqueous electrolyte onto ultra-high vacuum prepared, smooth, Ga- or As-terminated GaAs(0 0 1) surfaces. The fcc lattice of the epitaxial Cu islands is rotated by 5° and tilted by about 9° with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains of Cu islands terminated by {1 1 1} facets.     

One-dimensional excitons in inorganic–organic self-organized quantum-wire crystals [NHC(I)=NH]PbI and [CHSC(=NH)NH]PbI

Optical properties of lead-iodide-based one-dimensional perovskite-type crystals [NH₂C(I)=NH₂]₃PbI₅ and [CH₃SC(=NH₂)NH₂]₃PbI₅ have been investigated theoretically and experimentally. The electronic and excitonic structures are studied based on group theoretical consideration and first-principle band calculation. Strong one-dimensional anisotropy of the optical absorption spectra, large Stokes shifts (1.0 eV) and huge exchange energies (70 meV) indicate that the excitons in these crystals are one-dimensional Frenkel excitons.     

Structure and properties of the one-dimensional cobalt oxide CaCo2O4

We have studied crystal structure and transport properties of the quasi one-dimensional cobalt oxide CaCo₂O₄. The CaCo₂O₄ phase crystallizes in calcium-ferrite type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. Large thermoelectric power (S  150 μV/K at 390 K) with metallic temperature dependence of S, moderate resistivity (ρ  2.9 × 10⁻¹ Ω cm at 390 K) with carrier localization at low temperature, and normal thermal conductivity (κ  6.3 W/Km at 390 K) were observed. The phonon mean-free path was calculated from the observed data, as a function of temperature. The long phonon mean-free path (l  24 Å at 300 K) implies that the thermal conductivity could be suppressed by impurity scattering of phonons with partial element substitution.     

Dependence of flame propagation on pressure and pressurizing gas for an Al/CuO nanoscale thermite

The pressure dependence of flame propagation in an Al/CuO nanoscale thermite was studied. Experiments were performed by loosely packing the Al/CuO mixture in an instrumented burn tube, which was placed in a large volume, constant pressure chamber with optical windows. A high-speed camera was used to take photographic data, and six pressure transducers equally spaced along the length of the burn tube were used to measure the local transient pressure. Ambient pressures were varied between 0 and 15 MPa, and three different pressurizing gases were used: argon, helium, and nitrogen. Three modes of propagation were observed. The pressure at which the mode of propagation changed was similar for argon and nitrogen, however, when pressurized with helium, transition occurred at lower pressures. In the low-pressure regime (0–2 MPa) a constant velocity mode with speeds on the order of 1000 m/s was observed. In this region, a convective mode of propagation was dominant. An accelerating regime was observed for a pressure range of approximately 2–5 MPa in argon and nitrogen, with speeds ranging from 100 to 800 m/s. In helium, however, if an accelerating region existed it occurred over a narrow pressure range which was not observed in the present experiments. An oscillating regime was observed in all three gases, in a pressure range of 5–9 MPa for argon and nitrogen, and a range of 2–4 MPa for helium. Velocities in this region are bimodal, and differ by orders of magnitude, suggesting that the propagation mechanism was oscillating between convective and conductive. At relatively high ambient pressures, a constant velocity mode with speeds on the order of 1 m/s was observed for all three gases. The conductive mode of propagation was likely dominant in this region.     

From Zn microspheres to hollow ZnO microspheres: A simple route to the growth of large scale metallic Zn microspheres and hollow ZnO microspheres

Large scale metallic Zn microspheres and hollow ZnO microspheres are synthesized by thermal evaporation and vapor transport by heating a ZnO/graphite mixture at 1000 °C. Firstly, metallic Zn microspheres are fabricated with diameters in the range of 1–10 μm. The Zn microspheres are then annealed at 600 °C in air, which form hollow semiconducting ZnO microspheres. EDX and XRD spectra reveal that the oxidized material is indeed ZnO. Room temperature photoluminescence spectra of the oxidized material show a sharp peak at 380 nm and a wider broad peak centered at 490 nm. This growth mechanism is discussed and further investigated for other metallic and metal oxide microstructures.