Structure of Pt3Co(1 1 0)c(2×4)-O surface studied by scanning tunneling microscopy

A Pt₃Co(1 1 0)c(2×4)-O surface has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. At a very initial oxidation stage exposed at 500°C, creation of missing and/or added row structures running to the [0 0 1] direction on clean Pt₃Co(1 1 0)2×1 surface was imaged from the steps. The surface is fully covered by a well-ordered c(2×4) structure at 2 L oxygen exposure. Atomic-resolution STM image shows the added row-type anti-phase Co-O zigzag chains along the [0 0 1] direction. Based on the images, the structure model for the c(2×4) surface was suggested as a first oxidized layer, which comes from the chemical reaction forming stoichiometric Co monoxide. Further oxygen exposure above 5 L, Co-O clusters imaged to large dots were formed on the surface with the size of about 5-7 Å.     

XPS study of surface chemistry of epiready GaAs(1 0 0) surface after (NH4)2Sx passivation

The results of systematic XPS studies of wet sulfide passivation capabilities to remove the native oxides and excess arsenic at the epiready GaAs(1 0 0) native surfaces are presented. Different procedures of dipping an epitaxy-ready sample (at room and elevated temperatures) in an ammonium polysulfide (NH₄)₂S_x solution combined with a UHV flash annealing were used. The surface chemistry after each processing step was investigated by the inspection of the XPS As3d and Ga3d spectra taken using an enhanced surface sensitivity mode. The analysis revealed that the procedure of sulfidation itself removes native oxides and that both As-S and Ga-S bondings are created, and when combined with subsequent UHV annealing, diminishes excess As efficiently. Moreover, the results are in agreement with our previous work on the surface Fermi level position of (NH₄)₂S_x-treated samples.     

Tl2Ba2CaCu2O8 thin films on 2 in. LaAlO3(0 0 1) substrates

High quality Tl₂Ba₂CaCu₂O₈ (Tl-2212) superconducting thin films are prepared on both sides of 2 in. LaAlO₃(0 0 1) substrates by off-axis magnetron sputtering and post-annealing process. XRD measurements show that these films possess pure Tl-2212 phase with C-axis perpendicular to the substrate surface. The thickness unhomogeneity of the whole film on the 2 in. wafer is less than 5%. The superconducting transition temperatures T_cs of the films are around 105 K. At zero applied magnetic field, the critical current densities J_cs of the films on both sides of the wafer were measured to be above 2 × 10⁶ A/cm² at 77 K. The microwave surface resistance R_s of film was as low as 350 μΩ at 10 GHz and 77 K. In order to test the suitability of Tl-2212 thin films for passive microwave devices, 3-pole bandpass filters have been fabricated from double-sided Tl-2212 films on LaAlO₃ substrates.     

Band structure symmetry effects in Gd multilayers grown on W(1 1 2)

We present evidence of coverage-dependent band structure symmetry effects in scanning tunneling microscopy. We find that the symmetry of the Gd induced band located at about 2 eV below the Fermi energy, at the surface Brillouin zone center, appears to be strongly dependent on the atomic structure in the bilayer regime for Gd on W(1 1 2). Light polarization dependent photoemission from this band indicates more d_z² symmetry character for rectangular Gd structures and rather more d_xzd_yz character for the quasi hexagonal bilayer structure. Band symmetry changes accompanying the structural phase transition from rectangular to quasi-hexagonal Gd bilayer structures are consistent with arguments used to explain the apparent STM corrugation observed for Gd on W(1 1 2) (Surf Sci 520(2002)43).     

Thermal stability and transport studies of (100 − 2x)TeO2-xAg2O-xWO3 (7.5 ≤ x ≤ 30) glass system

Differential scanning calorimetry (DSC), infrared (IR) and direct current (DC) conductivity studies have been carried out on (100 − 2x)TeO₂-xAg₂O-xWO₃ (7.5 ≤ x ≤ 30) glass system. The IR studies show that the structure of glass network consists of [TeO₄], [TeO₃]/[TeO₃₊₁], [WO₄] units. Thermal properties such as the glass transition (T_g), onset crystallization (T_o), thermal stability (ΔT), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl), heat capacity change (ΔC_p) and ratios C_pl/C_pg of the glass systems were calculated. The highest thermal stability (237 °C) obtained in 55TeO₂-22.5Ag₂O-22.5WO₃ glass suggests that this new glass may be a potentially useful candidate material host for rare earth doped optical fibers. The DC conductivity of glasses was measured in temperature region 27-260 °C, the activation energy (E_act) values varied from 1.393 to 0.272 eV and for the temperature interval 170-260 °C, the values of conductivity (σ) of glasses varied from 8.79 × 10⁻⁹ to 1.47 × 10⁻⁶ S cm⁻¹.     

Ordered mixed surface structures formed by coadsorption of dissimilar metal atoms on Cu(0 0 1)

We have found that various ordered mixed surface structures are formed by coadsorption of two dissimilar metal atoms on Cu(0 0 1) at room tepmerature, using low-energy electron diffraction (LEED) I-V analysis. As coadsorbates, we employed Mg, Bi, Li and K, and surface structures formed by the coadsorption systems of (Mg, Li), (Mg, K) and (Mg, Bi) are presented. A tensor LEED analysis provided detailed geometries of the coadsorbates and the substrate surface. It was found that the surface structures in the above three coadsorption sytems exhibit the restructuring of the Cu(0 0 1) surface. The phase separation into individual adsorbates does not take place, implying that some additional stabilization arises. We demonstrate two origins for the stabilization of the ordered mixed surface structures on Cu(0 0 1). Structures and features formed by the individual adsorption of Mg, Bi, Li and K atoms on Cu(0 0 1) are described first, then those of (2√2×√2)R45°-Mg,Li, (√5×√5)R26.7°-Mg,K, c(2×2)-Mg,Bi, and c(6×4)-Mg,Bi structures formed by the coadsorption are presented. We consider on the basis of the determined structural parameters the question why ordered mixed surface structures are formed instead of the phase separation.     

Thermal stability of the Ta (1 1 1) surface covered with Pd

The thermal stability of Ta(1 1 1) face covered with Pd layers of different thickness was examined by the complementary scanning tunnelling microscope (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy and Δφ techniques. It has been found that for thin layers of θ<3 monolayer (ML) heated to 450 K, uplifts appear on the surface that may be treated as an agglomerated form of Pd adsorbate. For layers thicker than θ>3 ML, facets of the {1 1 0} type were observed upon heating to 700 K. Regardless of the conditions of investigation the faceting of the {2 1 1} type did not appear. During the adsorbate desorption the STM images revealed an ordered arrangement of surface atoms in the form of micropyramids composed of 10 atoms each and equally oriented towards the substrate. The appearance of the pyramids on the surface did not affect the observed LEED pattern.     

Epitaxial growth of transparent tin oxide films on (0 0 0 1) sapphire by pulsed laser deposition

The growth of epitaxial SnO₂ on (0 0 0 1) sapphire using pulsed laser deposition is examined. X-ray diffraction analysis shows that the films are highly a-axis oriented SnO₂ with the rutile structure. Three distinct symmetry-equivalent in-plane epitaxial orientations were observed between the film and substrate. With increasing growth temperature, both the growth rate and surface roughness increase with columnar grain formation. Carrier concentration ranged from 10¹⁷ to 10¹⁹ cm⁻³, with mobility of 0.5-3 cm²/V s. The resistivity of the films increases with increasing growth temperature, suggesting a lower density of oxygen vacancy-related defects formed during high temperature deposition.     

Synthesis and electrochemical properties of Ti doped Li3 − xFe2 − xTix(PO4)3/C cathode materials

Li_3 − xFe_2 − xTi_x(PO₄)₃/C (x = 0-0.4) cathodes designed with Fe doped by Ti was studied. Both Li₃Fe₂(PO₄)₃/C (x = 0) and Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C (x = 0.2) possess two plateau potentials of Fe³⁺/Fe²⁺ couple (around 2.8 V and 2.7 V vs. Li⁺/Li) upon discharge observed from galvanostatic charge/discharge and cyclic voltammetry. Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C has higher reversibility and better capacity retention than that of the undoped Li₃Fe₂(PO₄)₃/C. A much higher specific capacity of 122.3 mAh/g was obtained at C/20 in the first cycle, approaching the theoretical capacity of 128 mAh/g, and a capacity of 100.1 mAh/g was held at C/2 after the 20th cycle.     

Enhanced nanoparticle formation by indentation and annealing on 2 MeV Cu ion-implanted SiO2

Enhancement of surface plasmon resonance (SPR) in optical absorption has been found on Cu ion-implanted SiO₂ substrate modified by micro-indentation and post-annealing. Micro-indentation effects on surface plasmon resonance (SPR) in optical absorption have been studied to control nanoparticle formation in Cu ion-implanted SiO₂ substrate. The SiO₂ was firstly implanted with 2 MeV Cu²⁺ ions at an ion flux of 4 μA/cm², up to a fluence of 6 × 10¹⁶ ions/cm². After the ion implantation, dot-array patterns of micro-indents were made by a micro-Vickers hardness tester, and followed by annealing at 600 °C in vacuum for 1 h. The optical absorption spectra of the indented region and the non-indented flat region were measured and compared with each other. After post-annealing at 600 °C, the indented area showed higher absorbance of SPR at 2.2 eV than that of the flat region annealed under the same annealing conditions. The TEM study shows larger and denser Cu precipitates inside the indentation than those in the flat area. The results indicate that the defects produced by indentation enhance the atomic migration in the plastic zone during thermal relaxation process, resulting in promoting the enhanced precipitation of Cu nanoparticles.     

Effects of oxygen pressure on the growth of pulsed laser deposited ZnO films on Si(0 0 1)

ZnO thin films were prepared on Si(0 0 1) substrates using a pulsed laser deposition (PLD) technique and then their growth and properties were investigated particularly as a function of ambient O₂ pressure during film growth. It was found that the microstructure, crystallinity, orientation and optical properties of the films grown are strongly dependent on the O₂ pressures used. Completely c-axis oriented ZnO films are grown in a low O₂ pressure regime (5×10⁻⁴-5×10⁻² Torr), whereas a randomly oriented film with a much lower crystallinity and a rougher grained-surface is grown at an O₂ pressure of 5×10⁻¹ Torr. This deterioration in film quality may be associated with the kinetics of atomic arrangements during deposition. Our results suggest that ambient O₂ pressure is an important processing parameter and should be optimized in a narrow regime in order to grow a ZnO film of good properties in PLD process.     

Optical properties of ZnO nanowire arrays electrodeposited on n- and p-type Si(1 1 1): Effects of thermal annealing

Electrodeposition is a low temperature and low cost growth method of high quality nanostructured active materials for optoelectronic devices. We report the electrochemical preparation of ZnO nanorod/nanowire arrays on n-Si(1 1 1) and p-Si(1 1 1). The effects of thermal annealing and type of substrates on the optical properties of ZnO nanowires electroplated on silicon (1 1 1) substrate are reported. We fabricated ZnO nanowires/p-Si structure that exhibits a strong UV photoluminescence emission and a negligible visible emission. This UV photoluminescence emission proves to be strongly influenced by the thermal annealing at 150-800 °C. Photo-detectors have been fabricated based on the ZnO nanowires/p-Si heterojunction.     

Theoretical investigation of atomic structure and electronic properties of Ca/Si(110)-(2 × 1) reconstruction

We have presented first-principles total-energy calculations for the adsorption of Ca metals onto a Si(110) surface. The density functional method was employed within its local density approximation to study the atomic and electronic properties of the Ca/Si(110) structure. We considered the (1 × 1) and (2 × 1) structural models for Ca coverages of 0.5 monolayer (ML) and 0.25 ML, respectively. Our total-energy calculations indicate that the (1 × 1) phase is not expected to occur. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The Ca/Si(110)-(2 × 1) produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. One filled and two empty surface states were observed in the gap; these empty surface states originate from the uppermost Si dangling bond states and the Ca 4 s states. It is found that the Ca-Si bonds have an ionic nature and complete charge being transferred from Ca to the surface Si atoms. Finally, the key structural parameters of the equilibrium geometry are detailed and compared with the available results for metal-adsorbed Si(110) surface, Ca/Si(001), and Ca/Si(111) structures.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.     

Structure and surface morphology of Mn-implanted TiO2

A study of structure and surface morphology together with magnetic properties of Mn-implanted rutile-type TiO₂ single crystals is performed. Homogenous thin films of about 100 nm with different Mn_xTi_1 − xO₂ (x = 0.03; 0.05 and 0.07) chemical formula were obtained. The Mn ion implanted surface exhibited a dense microstructure with a nano grain size. The dependence of c/a axial ratio on manganese content suggests that Mn³⁺ species substituted tetragonal Ti⁴⁺. The annealing at 873 K caused changes in surface structure, morphology and roughness. A migration of manganese ions into the rutile single crystal takes place and in certain conditions Ti₂O phase occurs. Mn-implanted samples exhibit room temperature ferromagnetism and a Curie temperature of 680 K. Electron spin resonance analysis evidenced that manganese is incorporated by substitution as magnetically isolated Mn⁴⁺, Mn³⁺ and Mn²⁺ species. At 0.07% contents the Mn³⁺ species may enter in interstitial sites contributing to extinction of substitutional magnetic moment.     

Influence of SnO2 addition on the thermoelectric properties of Zn1 − xSnxO (0.01 ≤ x ≤ 0.05)

The grain size and the density of the Zn_1 − xSn_xO (0 ≤ x ≤ 0.05) samples decreased with increasing SnO₂ content. The addition of a small amount of SnO₂ (x ≤ 0.01) to ZnO led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in a significant increase in the power factor. The thermoelectric power factor was maximized to a value of 1.25 × 10⁻³ Wm⁻¹ K⁻² at 1073 K for the Zn_0.99Sn_0.01O sample.     

Ar assisted carbidization of TiO2 (1 1 0) supported Mo nanoparticles by decomposition of C2H4

Ar⁺ assisted carbidization of Mo nanoparticles supported on TiO₂ (1 1 0) is studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In order to activate the diffusion of carbon into the bulk of Mo nanoparticles we applied Ar ions (1 keV) during the exposure of C₂H₄. XPS exhibited that the decomposition of C₂H₄ at 850 K accompanied by ion bombardment results in an almost complete carbidization of nanocrystalline Mo while this treatment performed without ion bombardment results only in the carbidization of the particle surface. The modification of the crystallinity of the Mo-carbide particles was deduced from STM measurements.     

Refractory metal reactivity towards oxide surface: W/TiO2(1 1 0) case

Reactivity of deposited tungsten towards TiO₂(1 1 0) surface was studied using synchrotron radiation photoemission spectroscopy (both core levels, valence band and resonant photoemission) on Materials Science Beamline at ELETTRA. W depositions carried out at room temperature on TiO₂(1 1 0) surface give rise to an interfacial reaction which leads to a metastable situation due to kinetic limitations. Annealing induces chemical changes which are function of the initial coverage; for fractional coverage, annealing induces completion of oxidation of deposit whereas reduction to metallic tungsten occurs for highest coverage. These results demonstrate that interaction of W with TiO₂(1 1 0) surface is, as the one of molybdenum, driven by a balance between W-O interactions and W-W depending on tungsten atoms density on TiO₂ surface.     

Negative thermal expansion and electrical properties of Mn3(Cu0.6NbxGe0.4 − x)N (x = 0.05-0.25) compounds

Bulk materials with the general formula of Mn₃(Cu_0.6Nb_xGe_0.4 − x)N (x = 0.05, 0.1, 0.15, 0.2, 0.25), Mn₃(Cu_0.6Ge_0.4)N and Mn₃(Cu_0.7Ge_0.3)N were fabricated by mechanical ball milling and solid state sintering. Their thermal expansion coefficients and electrical conductivities were investigated in the temperature range of 80-300 K. It is found that the temperature interval of negative temperature expansion behavior is about 95 K in the samples of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N and Mn₃(Cu_0.6 Nb_0.2Ge_0.2)N, which is twice as large as that of Mn₃(Cu_0.7Ge_0.3)N. The negative thermal expansion of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N can reach to − 19.5 × 10⁻⁶ K^− 1 in the temperature range of 165 to 210 K. The electrical conductivity of this series materials is in a level of about 2.5 × 10⁶ (Ω m)^− 1.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.