Tailoring nanomaterial products through electrode material and oxygen partial pressure in a mini-arc plasma reactor

Nanomaterials with controllable morphology and composition are synthesized by a simple one-step vapor condensation process using a mini-arc plasma source. Through systematic investigation of mini-arc reactor parameters, the roles of carrier gas, electrode material, and precursor on producing diverse nanomaterial products are revealed. Desired nanomaterial products, including tungsten oxide nanoparticles (NPs), tungsten oxide nanorods (NRs), tungsten oxide and tin oxide NP mixtures and pure tin dioxide NPs can thus be obtained by tailoring reaction conditions. The amount of oxygen in the reactor is critical to determining the final nanomaterial product. Without any precursor material present, a lower level of oxygen in the reactor favors the production of W₁₈O₄₉ NRs with tungsten as cathode, while a high level of oxygen produces more round WO₃ NPs. With the presence of a precursor material, amorphous particles are favored with a high ratio of argon:oxygen. Oxygen is also found to affect tin oxide crystallization from its amorphous phase in the thermal annealing. Results from this study can be used for guiding gas phase nanomaterial synthesis in the future.     

Structure and interconversion of oxygen-vacancy-related defects on amorphous silica

Atomic structure and structural stability of neutral oxygen vacancies on amorphous silica are investigated using combined Monte Carlo and density functional calculations. We find that, unlike their bulk counterparts, the Si-Si dimer configuration of surface oxygen vacancies is likely to be unstable due to the high tensile strains induced, thereby undergoing thermally activated transformation with a moderate barrier into other stable configurations including dicoordinated silicon, silanone, or a subsurface Si-Si dimer, depending on the local surface structure. Pathways for the interconversion between these oxygen-vacancy-related defects are presented with a discussion of their viability.     

Monte Carlo simulation of growth of porous SiOx by vapor deposition

A random network model containing defects has been developed and applied to the deposition of amorphous SiOx films on a flat substrate. A new Monte Carlo procedure enables dangling bonds to migrate and annihilate. The degree of porosity in the films is found to increase with oxygen content. As the oxygen content increases a larger fraction of pore surfaces is covered with oxygen, and the density of dangling bonds on pore surfaces decreases. Oxygen plays the role of a surfactant, lowering the energies of pore surfaces and enhancing the porosity of amorphous SiO2 compared to amorphous Si.     

Ferromagnetismus von amorphem Eisen

In a previous paper a method has been described to produce thin films of amorphous iron by simultaneous condensation of iron and small additions of oxygen, silicon, or germanium onto a substrate at 20 °K. During annealing the amorphous films crystallize within a narrow range of temperature. In this paper the magnetic properties of these films are investigated. By a new lowtemperature ?gnetometer the magnetization curves can be registered for the different states of annealing. It results that even in the amorphous state iron is ferromagnetic. Below a critical concentration of the oxygen, silicon, or germanium admixtures the magnetic moment of the iron atoms is smaller and the coercive force is greater in the amorphous than in the crystalline structure. Above that concentration the magnetic moments in the two structures are equal, whereas the coercive force of the amorphous films is smaller than that of the crystalline ones. The results are explained by a concentration depending short range order in the amorphous films.     

Excitation of erbium in the heterogeneous nanocrystalline matrix of amorphous silicon

The erbium photoluminescence decay kinetics at a wavelength of 1.54 μm in amorphous hydrogenated silicon films obtained at high oxygen concentrations in a magnetron gas discharge is investigated. Optically active erbium is found to exist both in the semiconducting matrix of amorphous silicon and in dielectric nanocrystals of erbium silicate, which are formed in this case. The concentration ratio of excited erbium in amorphous silicon and in the nanocrystals is determined, as well as the time of excitation transfer from erbium in amorphous silicon to erbium in the nanocrystals. The mechanism of erbium excitation in this heterogeneous system is considered. The external quantum yield of erbium photoluminescence measured at a wavelength of 1.54 μm and room temperature is found to be 0.3–0.4%.     

First-principles investigation of low energy E' center precursors in amorphous silica

We show that oxygen vacancies are not necessary for the formation of E' centers in amorphous SiO? and that a single O deficiency can lead to two charge traps. Employing molecular dynamics with a reactive potential and density functional theory, we generate an ensemble of stoichiometric and oxygen-deficient amorphous SiO? atomic structures and identify low-energy network defects. Three-coordinated Si atoms appear in several low-energy defects both in stoichiometric and O-deficient samples where, in addition to the neutral oxygen vacancy, they appear as isolated defects.     

The Mechanisms of Oxygen Accelerated Low-Temperature Bonding by Ag Nanoparticles

Low-temperature bonding (≤300 ^o C) using Ag nanoparticles (Ag NPs) is considered to be the new generation of bonding technology in power electronics. The oxygen-accelerated sintering has been observed by many researchers which is attributed to the decomposition of organics covered on Ag NPs. In this work, organic-free Ag NPs are fabricated to eliminate the influence of organics, and it is found that the accelerated bonding process by oxygen is strongly correlated to the self-confined amorphous Ag-O compound shell on the surface of Ag NPs. In experiments, the sintering process is apparently accelerated by the elevating oxygen content, and the amorphous shell is observed after sintering, which do not grow thicker even in pure oxygen ambient for a long time while performing active chemical evolutions. In simulations, the results match well with the experiments and indicate that the amorphous shell performed the dynamic oxidation and decomposition process. This dynamic equilibrium is caused by the instability of silver oxides, which would enable the amorphous shell to activate the mobility of the surface mass flow and promote the surface diffusion. The shear strength of SiC chip increased by 354% when bonding in pure oxygen, targeting a broad variety of applications in electronic packaging.     

Low loss niobium oxides film deposited by ion beam sputter deposition

Thin films of niobium oxides are deposited by ion beam sputtering with a Kaufman-type ion source. The deposition rate is function of the oxygen partial pressure. There is an optimum oxygen pressure at 7 × 10^–5 Torr to deposite a stoichiometric film. The as-deposited films are amorphous. The optical parameters, including refractive index, extinction coefficient, and homogeneity, of the oxide films are influenced by post-baking temperature. The surface morphology measured by an atomic force microscope (AFM) shows that there is a certain range of optimum baking temperature which yields a smooth film and a good optical quality.     

Electron-irradiation-induced radiolytic oxygen generation and microsegregation in silicon dioxide polymorphs

The first direct in situ observations of the production and microsegregation of radiolytic interstitial oxygen resulting from electron beam irradiation of crystal and amorphous oxygen deficient SiO2 polymorphs has been made using cathodoluminescence (CL) microanalysis (spectroscopy and microscopy). Previously unreported near-infrared CL emission is observed at 0.968+/-0.003 eV from crystal alpha-SiO2 (quartz) and at 0.971+/-0.003 eV from amorphous a-SiO2 (fused quartz and silica glasses) at 290 K. The energy and width of the near-infrared CL emission from electron-irradiated alpha-SiO2 polymorphs is consistent with the O2 (1)Delta(g)-->(3)Sigma(-)(g) transition associated with molecular oxygen.     

Electrical and optical properties of thin film of amorphous silicon nanoparticles

Electrical and optical properties of thin film of amorphous silicon nanoparticles (a-Si) are studied. Thin film of silicon is synthesized on glass substrate under an ambient gas (Ar) atmosphere using physical vapour condensation system. We have employed Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) to study the morphology and microstructure of this film. It is observed that this silicon film contains almost spherical nanoparticles with size varying between 10 and 40 nm. The average surface roughness is about 140 nm as evident from the AFM image. X-ray diffraction analysis is also performed. The XRD spectrum does not show any significant peak which indicates the amorphous nature of the film. To understand the electrical transport phenomena, the temperature dependence of dc conductivity for this film is studied over a temperature range of (300-100 K). On the basis of temperature dependence of dc conductivity, it is suggested that the conduction takes place via variable range hopping (VRH). Three-dimensional Mott's variable range hopping (3D VRH) is applied to explain the conduction mechanism for the transport of charge carriers in this system. Various Mott's parameters such as density of states, degree of disorder, hopping distance, hopping energy are estimated. In optical properties, we have studied Fourier transform infra-red spectra and the photoluminescence of this amorphous silicon thin film. It is found that these amorphous silicon nanoparticles exhibits strong Si-O-Si stretching mode at 1060 cm⁻¹, which suggests that the large amount of oxygen is adsorbed on the surface of these a-Si nanoparticles. The photoluminescence observed from these amorphous silicon nanoparticles has been explained with the help of oxygen related surface state mechanism.     

Raman scattering and infrared absorption in bulk amorphous red phosphorous

Raman scattering and infrared absorption measurements in bulk amorphous red P indicate considerable spectral detail for an elemental amorphous solid, including a distinct separation of acoustic and lower lying optic-like bands. The results suggest appreciable structural correlations and weak interlayer-like bonding. Increased covalency of amorphous red P relative to amorphous As is indicated.     

Effect of electrochemical reduction on the structural and electrical properties of anodic TiO2 nanotubes

The effect of electrochemical reduction on the structural and electrical properties of amorphous as well as annealed TiO₂ nanotubes (TNTs) is investigated under ambient conditions. TNTs were prepared by anodizing titanium sheet in ethylene glycol electrolyte containing NH₄F and de-ionized water at 40 V for 6 h. Electrochemical reduction is carried out in 1 M aqueous KOH solution for ∼15 s at 3 V. TNTs are characterized by SEM, XRD, XPS and impedance spectrometer. XRD results confirm an increase in d-spacing for (101) and (200) planes, after electrochemical reduction. XPS data reveal that electrochemical reduction produced prominent shifts of ∼0.7–1.0 eV in the binding energies of TNTs. Interestingly, these shifts recover completely (in case of amorphous TNTs) and partially (in case of anatase TNTs) within ∼7 days after reduction process due to oxygen uptake. Partial recovery in the binding energies of anatase TNTs is due to the fact that the oxygen vacancies are thermodynamically more stable as compared to amorphous TNTs. Similarly, the electrochemical reduction process decreases the impedance values of TNTs by more than three orders of magnitudes (from MΩ to kΩ). The impedance values also recover to the similar values before reduction in a span of ∼7days.     

Quick Microwave Assisted Synthesis and <Emphasis Type="Italic">In Vitro</Emphasis> Imaging Application of Oxygen Doped Fluorescent Carbon Dots

In this paper, a fast and simplest one-pot tactic was used to synthesis fluorescent oxygen doped carbon dots from Tween-20 (TTO-CDs) is reported. The TTO-CDs were microwavically synthesized by using Tween-20 as both the carbon precursor and the oxygen dopant as well. The surface morphology, crystalline and/or amorphous nature, composition and optical assets of synthesized TTO-CDs were studied by means of existing techniques. From the results, it was confirmed that the as-synthesized TTO-CDs are amorphous in nature, monodispersed, sphere-shaped and the typical particle size range is 5?±?1.5 nm. The synthesized TTO-CDs emits strong blue fluorescence at 390 nm under excitation of 335 nm. Most interestingly, the excitation dependent emission property of synthesized TTO-CDs was exposed from fluorescence results. The synthesized TTO-CDs have quantum yield of about 14% against quinine sulfate as reference standard. The biotoxicity of synthesized TTO-CDs on HeLa cells was assessed through cytotoxicity assay. These results implied that the fluorescent TTO-CDs showed less biotoxicity, and further which was efficaciously applied as a multicolor staining and bioimaging probe for the confocal imaging of HeLa cells.     

57Fe Mössbauer study of La0.67Ca0.33Mn1 − x Fe x O3 CMR system

The structural changes of near-equiatomic α-FeCr alloys, ground in a vibratory mill in vacuum and in argon, were followed as a function of milling time. An amorphous phase forms in both cases but at a much faster rate when milling in argon than when milling in vacuum. Amorphisation by ball-milling of α-FeCr alloys is deduced to be an intrinsic phenomenon which is however speeded-up by oxygen. The amorphous phase crystallizes into a bcc Cr-rich phase and a bcc Fe-rich phase when annealed for short times.     

Effect of metal oxide and oxygen on the growth of single-walled carbon nanotubes by electric arc discharge

The effect of oxygen on the growth of single-walled carbon nanotubes was studied with Ni–Co alloy powder as catalyst under helium atmosphere of 500 Torr by electric arc discharge. The oxygen included in nickel or (and) cobalt oxides was added in catalyst. The content of oxygen in atmosphere was controlled by changing vacuum degree inside furnace before inputting buffer gas. The examinations of TEM and Raman scattering showed that oxygen in metal oxide as catalyst promotes the nucleation of SWCNT by taking effect on the metal catalyst particles. However, O₂ in atmosphere has the role of oxidizing amorphous particles along with nanotubes. When its molar proportion is higher than 0.22 ppm (Parts per million), the carbon nanotubes produced are oxidized and their purity decreases. The diameter of single-walled carbon nanotube obtained under different condition has a narrow distribution around 1.28 nm.     

Shapes and shifts in the oxygen Auger spectra

The oxidation of silicon and platinum single crystal faces, of polycrystalline supported catalysts and of some alloy surfaces has been studied by AES and as far as possible by LEED. A comparison of the oxygen Auger spectra obtained during the oxidation process with those found on oxides has been made; it shows that the modification of the fine structure of the oxygen Auger peaks gives some information about the binding state of oxygen. Two different structures, which compete one with the other, are described. In one case, a spectrum where three lines dominate is obtained; in the other case, a “quasi-atomic” spectrum characterized by five features is observed: multiplet splitting in the two-hole final state is predominant. Besides these differences in the fine structure of the Auger spectra one can notice shifts of several eV for the main feature. They have been correlated with the various observed LEED patterns. Physisorption, chemisorption, solution of oxygen in the metal lattice, growth of ordered or amorphous oxides are the different possibilities which are discussed.     

Development of structural and optical properties of WOx films upon increasing oxygen partial pressure during reactive sputtering

WO_x films were prepared by reactive dc magnetron sputtering using tungsten target. Sputtering was carried out at a total pressure of 1.2 Pa using a mixture of argon plus oxygen in an effort to determine the influence of the oxygen partial pressure on structural and optical properties of the films. The deposition rate decreases significantly as the surface of the target is oxidized. X-Ray diffraction revealed the amorphous nature of all the films prepared at oxygen partial pressures higher than 1.71×10⁻³ Pa. For higher oxygen partial pressures, fully transparent films were deposited, which showed a slight increase in optical band gap with increasing oxygen partial pressure, while the refractive index was simultaneously decreased.     

In situ purity enhancement/surface modification of single-walled carbon nanotubes synthesized by induction thermal plasma

A simple, cost-effective and energy-efficient approach was developed for in situ purity enhancement and surface modification of single-walled carbon nanotubes (SWCNTs) produced using an induction thermal plasma process. In this process, SWCNT-containing materials are thermally treated with oxygen flow inside a filtration chamber, while they are assembled into the sheets during the synthesis process. Owing to selective thermal oxidation, the amount of amorphous carbon was significantly reduced in the final product resulting in higher purity SWCNT-containing materials. Parametric study indicated that the amorphous carbon content was noticeably diminished in the product at an oxygen volume concentration of 10% in the synthesis system. Raman analysis indicated a decrease in the population of the SWCNTs with diameters smaller than 1.3 nm after in situ exposure to 10 vol.% of oxygen. In addition to the successful reduction of amorphous carbon content, the oxygen-functionalized SWCNTs were also observed in the final product using this process.     

Optical characteristics of amorphous V2O5 thin films colored by an excimer laser

The V₂O₅ films were prepared by an RF sputtering method, and the amorphous films were colored by an UV excimer laser. The crystallinity of the as-grown V₂O₅ film was degenerated greatly by laser irradiation, as determined by X-ray diffraction (XRD) and Raman studies. The transmission and complex refractive index spectra of the V₂O₅ film were affected by variations in the microstructure, including the surface morphology, crystalline structure, and substoichiometry with an oxygen deficiency. Considerable emissions due to oxygen vacancies and band transition of photoluminescence (PL) peaks were observed, and the peaks were significantly changed after laser irradiation. The variations in the optical properties in both films may be attributed to oxygen deficiency induced by laser irradiation.     

AES and LEED study of the activation of GaAsCsO negative electron affinity surfaces

Gallium arsenide epitaxial layers have been activated to negative electron affinity by a two-stage process. This consisted of a heat clean in vacuum and activation with caesium and oxygen, followed by a second heating, to partially desorb the caesium, and re-activation with caesium and oxygen. AES measurements during the first activation indicated that the oxygen/caesium ratio increased as the photoemission increased to a maximum. This ratio was always less than the nearly constant ratio during the second activation which gave greater photoemission. After both the first and second activations desorption studies showed that, whereas the caesium Auger peak height decreased monotonically over a wide range of temperature, the oxygen peak height passed through a maximum as the temperature was increased, before decreasing rapidly to zero as the caesium desorption was completed. This temperature of desorption was slightly higher after the second activation. LEED showed that the caesium-oxygen layer was amorphous.