Diameter control of tungsten oxide nanowires as grown by thermal evaporation

Tungsten oxide nanowires with controllable diameter were synthesized on Si substrates by thermal evaporation of tungsten trioxide powder in a tube furnace. Depending on the temperature of the source (900-1000?°C), tungsten oxide W(18)O(49) nanowires with diameters ranging from 10 to 100?nm are obtained with high yield. The exponential dependence of the nanowire diameter on the source temperature leads to an energy of about 2.0?eV. The growth process is discussed; it is believed to be a kinetic effect.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Thermodesorption of samarium from oxidized tungsten surface

Thermodesorption of samarium (Sm) atoms and samarium oxide (SmO) molecules from samarium films deposited in vacuum onto tungsten substrates covered with thin (<1 nm thick) layers of tungsten oxides has been studied. It is established that samarium reduces tungsten from its oxides. The parameters of Sm and SmO desorption from an oxidized tungsten surface are determined.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

The catalyst-free synthesis of large-area tungsten oxide nanowire arrays on ITO substrate and field emission properties

Tungsten oxide nanowire arrays have been grown on indium tin oxide coated glass substrate using tungsten trioxide powders as source by thermal evaporation approach without any catalysts. When the O₂/Ar flow rate ratio was 1/100, large-scale, high-density and uniformly distributed tungsten oxide nanowire arrays were obtained. The morphology and structure properties of the tungsten oxide nanowires were characterized by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The influences of the oxygen concentration on the growth, density, shape and structure of the nanowires were investigated. The possible growth mechanism which governs the various types of nanowire arrays as the O₂/Ar flow rate ratios changed is also discussed. Field emission properties of tungsten oxide nanowire arrays were studied at a poor vacuum condition. The remarkable performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application as electron emitters.     

Synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties

In the present work, we report the solvothermal synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties. Uniform, well-defined Co-doped tungsten oxide nanoblocks consisting of ultrathin nanowires, lenticular bundled Zn-doped tungsten oxide nanorods, and plate-shaped or flower-like Fe-doped tungsten oxide particles have been successfully prepared with the addition of different dopants. The doping of Co and Zn ions may prohibit the oriented growth of the tungsten oxide nanowires and favor their self-assembly, leading to the formation of bundled nanoblocks and nanorods. The Fe-doped tungsten oxide flowers may result from the ordered arrangement of single nanoplates. The band gaps of undoped, Fe-, Zn-, and Co-doped tungsten oxides are found to be 2.92, 2.78, 2.62, and 2.52 eV, respectively.     

An approach for synthesizing various types of tungsten oxide nanostructure

Various types of tungsten oxide nanostructures, including nanowires, nanobundles, and three-dimensional nanowire networks, have been synthesized on large-area silicon substrates by simply heating an array of tungsten filaments in a vacuum chamber. The fine structure and components of as-prepared products can been controlled by changing the tungsten filament temperature. This approach is free of catalysts and templates, and provides an economical method for large-scale preparation of various types of tungsten oxide nanostructures for applications.     

HFCVD and CSVT techniques working together to produce nanostructured tungsten oxide

Hot Filament Chemical Vapour Deposition (HFCVD) was used to deposit tungsten oxide clusters on a graphite surface using a tungsten filament as source. These clusters were subsequently transported onto quartz and Si substrates where they transformed into WO_3 − X nanowires. The transport of tungsten species from source to substrate was made through the Close Spaced Vapour Transport (CSVT) technique. The synthesized nanostructures were characterized by means of SEM, EDS, XRD techniques as well as by Raman spectroscopy. The results show that the nanowires have a monoclinic structure, a diameter about 100 nm and they were extended many tenths of micrometers.     

Sublimation growth of titanium nitride crystals

The sublimation-recondensation growth of titanium nitride crystals with N/Ti ratio of 0.99 on tungsten substrates is reported. The growth rate dependence on temperature and pressure was determined, and the calculated activation energy was 775.8 ± 29.8 kJ/mol. The lateral and vertical growth rates changed with the time of growth and the fraction of the tungsten substrate surface covered. The orientation relationship of TiN (001) || W (001) with TiN [100] || W [110], a 45° angle between TiN [100] and W [100], occurs not only for TiN crystals deposited on (001) textured tungsten but also for TiN crystals deposited on randomly orientated tungsten. This study demonstrates that this preferred orientational relationship minimizes the lattice mismatch between the TiN and tungsten.     

Thermal oxidation of chemical vapour deposited tungsten layers on silicon substrates for embedded non-volatile memory application

This research is targeted to enhance the functionality of bipolar complementary metal-oxide-semiconductor by innovative concepts of embedded resistive random access memory (RRAM) cells integration in the back-end-of-line (BEOL) region. The material of our interest is tungsten oxide as an insulator in RRAM cells and we focussed on the growth and characterisation of closed tungsten oxide layers. In this materials science study, we investigated the tungsten oxidation process under BEOL constraints (< 450 °C). Thin films of tungsten oxide (6-50 nm) were prepared by oxidising, under an atmosphere of one bar oxygen, the chemical vapour deposited tungsten layers on TiN covered silicon wafers. The X-ray photoelectron spectroscopy investigations indicate that the stoichiometric WO₃ grows after oxidation at 300 °C for an hour. The tungsten oxide layers prepared above 300 °C for longer than 15 min were non-stoichiometric. The X-ray diffraction investigations reveal the crystallisation of the WO₃ layers in monoclinic phase above 350 °C when oxidised for longer than 30 min; above 400 °C the (001) growth texture becomes dominant.     

Oxidation of aluminium nitride substrates

The growth of oxide films on two types of aluminium nitride substrates of different origin has been studied as a function of temperature. At a given set of oxidation reaction parameters, the oxide layers grown on substrates with a relatively large grain size and high concentrations of Y-Al-O-based liquid sintering aid phases (type I substrates) were observed to be thicker and more diffuse than those obtained on substrates with an average particle size of approximately 3 m and low liquid sintering aid concentrations (type II substrates). The effects of the oxygen partial pressure variation on the oxide film growth have been investigated for the oxidation of type II AIN substrates. The kinetics of the growth of oxide films on such substrates were analysed and determined to fit best to a linear rate law. This type of rate law indicates that the rate-limiting step in the growth of oxide films on high-quality type II aluminium nitride substrates is an interface reaction-controlled process.     

基于第一性原理计算的纳米氧化钨研究进展

纳米氧化钨作为一种具有独特物理化学性质的半导体功能材料,已被广泛应用于环境、能源、生命科学、信息技术等领域。本文基于第一性原理计算在纳米氧化钨中的应用进展,概述了量子力学基础上的第一性原理及密度泛函理论的发展历程及基本理论,介绍了该领域常用的MS (Materials studio)、VASP (Vienna ab initio simulation package)等模拟计算软件,并分类阐述了第一性原理计算对氧化钨的微观电子结构、物质相互作用、分子热动力学等方面的研究成果。最后提出了第一性原理计算在纳米氧化钨这类半导体材料研究中存在的问题及未来发展趋势。     

Structural and magnetic properties of nickel catalyzed-tungsten oxide nanosheets synthesized using e-beam rapid thermal annealing

Nanosheets of nickel catalyzed tungsten oxide have been grown on Si (100) substrate by electron beam rapid thermal annealing (ERTA) process. The thin films of W and Ni were deposited in a conventional electron beam evaporation system under high vacuum conditions and then subjected to ERTA. Scanning electron microscopy and atomic force microscopy were used to study the systematic growth of the nanosheets. Nanosheets with a uniform thickness of 200 nm were obtained for the beam current of 9 mA with a voltage of 5 kV ERTA for 60 s. X-ray diffractograms indicate that the formation of multiple phases of nickel, nickel oxide and tungsten oxide with respect to the variation in the beam current. Vibrating sample magnetometer studies indicate that the magnetic properties of this system vary with the beam current.     

Growth and characterization of stoichiometric tungsten oxide nanorods by thermal evaporation and subsequent annealing

Stoichiometric tungsten oxide (WO(3)) nanorods are synthesized on tungsten (W) substrates by a high-temperature, catalyst-free, physical deposition process and by subsequent annealing in oxygen atmosphere. Tungsten oxide nanorods are grown by thermal evaporation of WO(3) powder at elevated temperature in a tube furnace. XRD, TEM and XPS analysis shows that the as-grown nanorods are single crystalline and non-stoichiometric (WO(x)). Annealing of WO(x) nanorods at 700?°C under oxygen atmosphere has led to the formation of stoichiometric WO(3) as evidenced by XRD, XPS and Raman analysis.     

PAA influence on chitosan membrane calcification

This work discusses the influence of polyacrylic acid (PAA) on chitosan in vitro calcification. Calcium phosphate deposition on porous and dense (non-porous) chitosan membranes treated or not with PAA has been studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction, potentiometric titration and attenuated total reflectance infrared spectroscopy (ATR-IR). The experiments were carried out by soaking chitosan membranes in simulated body fluids (SBF), 1.0 and 1.5 times the human serum salt concentration, at 36.5 °C during 7 days at pH 7.4 or 7.8. SEM micrographs showed that the use of these growth solutions resulted in substrates with significant coverage with calcium phosphate. PAA led to a higher coverage even at lower pH (7.4), but the morphological and chemical nature of deposits was remarkably different if compared to deposits on pure chitosan membranes. Although PAA has shown to promote a more intense deposition, probably by offering ion-binding sites adequate for nucleation, it does not necessarily determine a well-defined crystal growth orientation.     

A hot-wire chemical vapor deposition (HWCVD) method for metal oxide and their alloy nanowire arrays

A concept for synthesizing nanowire arrays of transition metal oxides and their alloys using hot wire chemical vapor deposition (HWCVD) is discussed. Here, unlike conventional HWCVD, the hot filaments act as the source of the metal for the synthesis of one dimensional nanostructures. In the present concept, the chemical vapor transport of metal oxides generated by heating the filaments in low amounts of oxygen, onto substrates maintained at lower temperatures leads to the formation of metal oxide nanowires. Experiments performed using tungsten and molybdenum filaments showed that the nucleation density of the resulting metal oxide nanowires could be varied by varying the substrate temperature. Experiments performed using a magnesium source inside the reactor, in addition to tungsten filaments, resulted in the formation of MgWO₄ nanowires. This clearly indicates the possibility of either doping the metal oxide nanowires or alloying during synthesis.     

Optimization of organized silicon nanowires growth inside porous anodic alumina template using hot wire chemical vapor deposition process

A Hot Wire assisted Chemical Vapor Deposition (HWCVD) process has been developed for producing high-density arrays of parallel, straight and organized silicon nanowires (SiNWs) inside vertical Porous Anodic Alumina (PAA) templates, exploring temperatures ranging from 430 °C to 600 °C, and pressures varying between 2.5 and 7.5 mbar. In order to prevent parasitic amorphous silicon (a-Si) deposit and to promote the crystalline SiNWs growth, we used a tungsten hot wire to partially crack H₂ into atomic hydrogen, which acts like a selective etchant regarding a-Si. Here we describe the optimization route we followed to limit the deposit of a-Si onto the surface of the porous membrane and on the walls of the pores, which led to the possibility to grow SiNWs inside the PAA membranes. Such an approach has high potentialities for device realization, like PIN junctions, FETs or electrodes for Li-ion batteries.     

The effect of processing parameters on the synthesis of tungsten oxide nanomaterials by a modified plasma arc gas condensation technique

In the present study, tungsten oxide nanomaterials were synthesized by a modified plasma arc gas condensation technique. The effects of processing parameters (plasma current ranged from 70-90 A and chamber pressure ranged from 200-600 torr) on the preparation of tungsten oxide nanomaterials were investigated. X-ray diffraction results showed that all of the nanomaterials synthesized in the present study exhibited W5O14 phase. Field emission scanning electron microscopy and transmission electron microscopy examinations revealed that the tungsten oxide nanomaterials were equiaxed when prepared at a relatively low plasma current of 70 A, and turned into rod-like nanoparticles with increasing plasma current (80 or 90 A). Generally, the relative amount, diameter, and length of tungsten oxide nanorods increased with increasing plasma currents or chamber pressures. The aspect ratio of the as-prepared tungsten oxide nanorods reached a maximum of 12.7 when a plasma current of 90 A and a chamber pressure of 400 torr were used. A growth mechanism for tungsten oxide nanorods was proposed.     

Direct growth and photoelectrochemical properties of tungsten oxide nanobelt arrays

A single-step route was developed for the direct growth of tungsten oxide nanobelt arrays by heating a tungsten sheet without additional catalysts or reactants. X-ray diffraction and Raman analysis indicated that the tungsten oxide nanobelts were monoclinic. The surface photovoltage signal and photocurrent density of the tungsten oxide nanobelt arrays clearly suggested a high photoconversion ability. Further investigation demonstrated that the photoelectrocatalytic activity of the nanobelt arrays was higher than that of a tungsten oxide film when using phenol as the probe molecule.     

电子束蒸发制备氧化钨、氧化镍薄膜的电致变色性能

电致变色(EC)材料在外加电压作用下能可逆地改变其光学性能.其中氧化钨与氧化镍是典型的电致变色用材料.本文用电子束蒸发的方法在ITO玻璃基片上制备了此两种薄膜,研究了热处理工艺对薄膜结构与电致变色性能的影响.电致变色性能由电化学方法测试.封装的半固态智能窗器件具有很好的电致变色性能.