Interfacial reactions during sputter deposition of Ta and TaN films on organosilicate glass: XPS and TEM results

The evolution of the interface between organosilicate glass (OSG) and sputter deposited Ta or TaN films has been characterized by X-ray phototelectron spectroscopy (XPS). Cross-sectional TEM (XTEM) was also used to analyze Ta/OSG and TaN/OSG/interfaces for samples formed under different deposition conditions. XPS data show that Ta deposition onto OSG results in formation of an interphase between 1 and 2 nm thick composed of oxidized Ta and C. Metallic Ta is then formed on top of the interfacial region. In contrast, Ta-rich TaN formation occurs with some nitridation of the substrate, but with no significant interphase formation. The XPS data are consistent with the XTEM data. The XTEM results for Ta/OSG indicate a spatially irregular interface over a length scale of ∼2 nm, while results for TaN/OSG indicate a spatially abrupt region.     

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.     

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.     

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.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3–x(Na0.5La0.5)TiO3 ceramic system

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–x(Na_0.5La_0.5)TiO₃ ceramic system were investigated. Two-phase system was confirmed by the XRD patterns and the EDX analysis. A co-existed second phase (Mg_0.95Co_0.05)Ti₂O₅ was also detected. The microwave dielectric properties are strongly related to the density and the matrix of the specimen. A new microwave dielectric material 0.88(Mg_0.95Co_0.05)TiO₃–0.12(Na_0.5La_0.5)TiO₃, possessing an excellent combination of dielectric properties: ε_r  22.36, Q × f  110,000 GHz (at 9 GHz), τ_f  2.9 ppm/°C), is proposed as a candidate dielectric for GPS patch antennas.     

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.     

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.     

Gas-phase vibrational spectroscopy and ab initio calculations of Rb(H2O)n and Rb(H2O)nAr cluster ions

The competition between ion–water electrostatic interactions and water–water hydrogen bonding in cluster ions depends on several factors, including charge density of the ion and temperature of the system. Infrared photodissociation spectra of Rb⁺(H₂O)_n=2–5 and Rb⁺(H₂O)_n=1–5Ar are presented here and compared to previous experiments involving potassium and cesium. The temperature, or internal energy, of hydrated rubidium cluster ions is controlled by varying the evaporative path available for cluster formation. Warmer clusters (with effective temperatures of 250–500 K) are formed by the evaporation of water, while colder clusters (40–120 K) can be formed by argon evaporation. Colder cluster ions tend to favor conformers with more hydrogen bonds compared to those cluster ions at warmer temperatures. Previous work from this laboratory has shown significant and dramatic differences between the spectra of hydrated potassium and cesium ions. With a charge density intermediate between that of K⁺ and Cs⁺, Rb⁺ plays an important role in bridging the gap in our previous studies.     

A new method to preserve the c-axis growth of thick YBa2Cu3O7–δ films grown by pulsed laser deposition

YBa₂Cu₃O_7–δ (YBCO) films were prepared on (1 0 0) MgO substrates by pulsed laser deposition (PLD) method. In order to eliminate the a-axis growth, which is commonly observed in the YBCO film thicker than a critical value, we developed a new PLD target that was sintered at a temperature far below YBCO 123 phase formation. The surface analysis made by AFM technique confirmed that very fine particles of around 20 nm size could be ejected from the new target to the substrate. The fine oxide clusters could be easily moved and incorporated into the YBCO phase thus benefited the c-axis growth even in the thick films. For instance, only the c-axis growth in the new film with a thickness of about 650 nm was larger than a critical thickness of the a-axis growth. However, in the standard film of the same thickness, there is 24.5% of the a-axis growth accompanying the main c-axis growth. Therefore, the c-axis growth could be preserved in the very thick YBCO film by a non-superconducting target.     

X-ray characterization and phase transformation kinetics of ball-mill prepared nanocrystalline Mg–Zn-ferrite at elevated temperatures

Nanocrystalline Mg–Zn-ferrite is prepared by ball milling the stoichiometric powder mixture of MgO, ZnO and α-Fe₂O₃. A non-stoichiometric ferrite phase is noticed to form after 3 h of milling when particles of starting materials became nano-sized. After 25 h of milling, stoichiometric ferrite phase is formed with 9 nm particle size. Post annealing study of ball-milled sample reveals that the nanocrystalline ferrite phase is stable up to 873 K and then starts to decompose into individual starting phases. However, heat treatment of unmilled stoichiometric powder mixture even at 1473 K for 1 h duration does not result in formation of stoichiometric Mg–Zn-ferrite phase.     

Solid-state Water-mediated Transport Reduction of Nanostructured Iron Oxides

The Fe²⁺/Fe³⁺ ratio in two-dimensional iron oxide nanosructures (nanolayers with a thickness of 0.3–1.5 nm on silica surface) may be precisely controlled using the transport reduction (TR) technique. The species –O–Fe(OH)₂ and (Si–O–)₂–FeOH forming the surface monolayer are not reduced at 400–600°C because of their covalent bonding to the silica surface, as demonstrated by Mössbauer spectroscopy. Iron oxide microparticles (microstructures) obtained by the impregnation technique, being chemically unbound to silica, are subjected to reduction at T 500°C with formation of metallic iron in the form of -Fe. Transport reduction of supported nanostructures (consisting of 1 or 4 monolayers) at T 600°C produces bulk iron(II) silicate and metallic iron phases. The structural-chemical transformations occurring in transport reduction of supported iron oxide nanolayers are proved to be governed by specific phase processes in the nanostructures themselves.     

Fabrication of well-defined individual dislocations in SiGe as a novel one-dimensional system

We show a new way to fabricate well-defined individual dislocations in SiGe. We started with a fully pseudomorphic but metastable SiGe layer grown on Si(0 0 1) by molecular beam epitaxy. Next, elongated (1 mm) mesa stripes with various widths (0.5–3 μm) were fabricated by a combination of isotropic and anisotropic etching. For smaller stripes, elastic relaxation of the strained SiGe layer can occur, transforming the originally biaxial strained layer into uniaxial strained subsystems. Subsequent strain relaxation caused by high temperature treatments leads to the formation of individual dislocation along the mesa stripes. The number of parallel dislocation can be adjusted by the original strain (Si:Ge ratio and layer thickness) and the mesa widths. We were able to fabricate structures with exactly one dislocation. Finally, contact pads were added to the stripes enabling the electrical characterization of individual dislocation.     

In-situ TiC particle reinforced Ti–Al matrix composites: Powder preparation by mechanical alloying and Selective Laser Melting behavior

Mechanical alloying of Ti–Al–graphite elemental powder mixture was performed to synthesize nanocomposite powder with Ti(Al) solid solution matrix reinforced by in-situ formed TiC particles. The evolutions in phases, microstructures, and compositions of milled powders with the applied milling times were investigated. It showed that with increasing the milling time, the starting irregularly shaped powder underwent a successive change in its morphology from a flattened shape (10 h) to a highly coarsened spherical one (15 h) and, eventually, to a fine, equiaxed and uniform one (above 25 h). The prepared TiC/Ti(Al) composite powder was nanocrystalline, with the estimated average crystallite size of 12 nm and of 7 nm for Ti(Al) and TiC, respectively. Formation mechanisms behind the microstructural development of powders were elucidated. The Ti(Al) solid solution is formed through a gradual and progressive solution of Al into Ti lattice. The formation of TiC is through an abrupt, exothermic, and self-sustaining reaction between Ti and C elements. Selective Laser Melting (SLM) of as-prepared TiC/Ti(Al) composite powder was performed. The TiC particle reinforced TiAl₃ (a major phase) and Ti₃AlC₂ (a minor phase) matrix composite part was obtained after SLM. Although a slight grain growth occurred as relative to as-milled powder, the SLM processed composites still exhibited a refined microstructure.     

Quantum confinement and interface structure of Si nanocrystals of sizes 3–5 nm embedded in a-SiO2

Spectroscopic ellipsometry and Monte Carlo simulations are employed to answer the fundamental question whether the energy gaps of Si nanocrystals with sizes in the range of 3–5 nm, which are embedded in amorphous silica, follow or deviate from the quantum confinement model, and to examine their interfacial structure. It is shown that the optical properties of these nanocrystals are well described by the Forouhi–Bloomer interband model. Analysis of the optical measurements over a photon-energy range of 1.5–5 eV shows that the gap of embedded nanocrystals with a mean size of 3.9 nm follows closely quantum confinement theory. A large band gap expansion (0.65 eV) compared to bulk Si is observed. The Monte Carlo simulations reveal a non-abrupt interface and a large fraction of interface oxygen bonds. This, in conjunction with the experimental observations, indicates that oxygen states and the chemical disorder at the interface have a negligible influence on the optical properties of the material in this size regime.     

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.     

Dielectric constant and current transport for HfO2 thin films on ITO

The dielectric constant and leakage current mechanisms for HfO₂ thin films deposited on indium–tin–oxide using reactive rf sputtering deposition were examined. Indium–tin–oxide was selected as the bottom metal as it is of interest as an electrode in transparent field-effect transistor development. The dielectric constant of HfO₂ films was approximately 20 and did not vary significantly with deposition conditions. Temperature-dependent leakage current measurements indicate that Schottky emission is the dominant transport mechanism in films deposited at low temperature and/or low oxygen pressure. The HfO₂/indium–tin–oxide barrier height was extracted to be 1.1±0.2 eV. Films deposited at high temperature and/or oxygen pressure deviate from the Schottky emission model, presumably due to the formation of polycrystalline material with grain boundary conduction. PACS  73.61.Ng; 73.50.Lw; 77.55.+f     

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.     

Study of structural, optical and electrical properties of ZnO and SnO2 thin films

The investigation of structure, optical and electrical properties of tin and zinc oxide films on glass substrates by using magnetron sputtering are carried out. X-ray data show the formation of textured tin oxides film during deposition and its transformation to SnO₂ polycrystalline film at low temperature (200 C) if the concentration of oxygen in the chamber is high (O₂ — 100%, Ar — 0%). Optimal conditions of SnO₂ polycrystalline film deposition (pressure of Ar–O₂ mixture in chamber — 2.7 Pa, concentration of O₂ — 10%) are determined. Low resistivity of as-deposited ZnO film and increasing ZnO crystallite sizes and phase volume at temperatures higher than the melting point of Zn (419.5 C) are explained by formation of conductive Zn and ZnO particle chains and their destruction, respectively.     

The effect of stoichiometry on the combustion behavior of a nanoscale Al/MoO3 thermite

The effect of stoichiometry on the combustion behavior of the nanoscale aluminum molybdenum trioxide (nAl/MoO₃) thermite was studied in a burn tube experiment by characterizing the propagation velocity and pressure output of the reaction. Changing the stoichiometry affects the combustion through changes in the product temperature, phase, and composition. The mixture ratios of the composites were varied over an extremely wide range (5% nAl (95% MoO₃)–90% nAl (10% MoO₃)). Results revealed three separate combustion regimes: a steady high speed propagation (100 to 1000 m/s) from approximately 10% to 65% nAl, an oscillating and accelerating wave near 70% nAl, and a steady-slow speed propagation (0.1–1 m/s) from approximately 75% to 85% nAl. Propagation was observed to fail both <10% nAl and >85% nAl. This is the first known observation of such limits for a nanoscale thermite in a tube geometry. The instrumented tube tests revealed peak pressures over 8 MPa near stoichiometric conditions in the steady high speed propagation region, no measurable pressure rise at low speed propagation, and building pressures for accelerating waves. The results suggest the propagation mode to be a supersonic convective wave for near stoichiometric mixtures and a conductive deflagration for extremely fuel-rich mixtures. The implications of these results for microscale combustion applications are discussed.     

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.