Highly conductive nanolayers on strontium titanate produced by preferential ion-beam etching

Developing fabrication methods for electronically active nanostructures is an important challenge of modern science and technology. Fabrication efforts for crystalline materials have been focused on state-of-the-art epitaxial growth techniques. These techniques are based on deposition of precisely controlled combinations of various materials on a heated substrate. We report a method that does not require deposition and transforms a nanoscale layer of a complex crystalline compound into a new material using low-energy ion-beam preferential etching (IBPE). We demonstrate this method by transforming a widely used insulator model system, SrTiO3, into a transparent conductor. Most significantly, the resistivity decreases with decreasing temperature as approximately T2.5+/-0.3 and eventually falls below that of room-temperature copper. These transport measurements imply a crystal quality in the conduction channel comparable to that obtained with the highest-quality growth techniques. The universality of low-energy IBPE implies wide potential applicability to fabrication of other nanolayers.     

Characterization of zirconia- and niobia-silica mixture coatings produced by ion-beam sputtering

ZrO2-SiO2 and Nb2O5-SiO2 mixture coatings as well as those of pure zirconia (ZrO2), niobia (Nb2O5), and silica (SiO2) deposited by ion-beam sputtering were investigated. Refractive-index dispersions, bandgaps, and volumetric fractions of materials in mixed coatings were analyzed from spectrophotometric data. Optical scattering, surface roughness, nanostructure, and optical resistance were also studied. Zirconia-silica mixtures experience the transition from crystalline to amorphous phase by increasing the content of SiO2. This also results in reduced surface roughness. All niobia and silica coatings and their mixtures were amorphous. The obtained laser-induced damage thresholds in the subpicosecond range also correlates with respect to the silica content in both zirconia- and niobia-silica mixtures.     

Measurement of lifetimes of thin carbon stripper foils produced by ion-beam sputtering

Thin carbon stripper foils used in high-intensity proton accelerators and heavy-ion accelerators must have long lifetimes. Thin carbon foils were fabricated by ion-beam sputtering using reactive and inert gas ions. The lifetime of the foils was measured using a KEK 650-keV high-intensity DC H⁻ (negative hydrogen ion) beam; changes in the foil thickness and surface deformations during irradiation were investigated. The lifetime of a typical stripper foil fabricated by heavy-ion-beam (Ar and Kr) sputtering was 60-70 times longer than that of the best commercially available foils. This paper reports a fabrication method for carbon stripper foils, along with an investigation of their lifetimes and changes in foil thickness during beam irradiation.     

Surface modification of nanoporous alumina membranes by plasma polymerization

The deposition of plasma polymer coatings onto porous alumina (PA) membranes was investigated with the aim of adjusting the surface chemistry and the pore size of the membranes. PA membranes from commercial sources with a range of pore diameters (20, 100 and 200?nm) were used and modified by plasma polymerization using n-heptylamine (HA) monomer, which resulted in a chemically reactive polymer surface with amino groups. Heptylamine plasma polymer (HAPP) layers with a thickness less than the pore diameter do not span the pores but reduce their diameter. Accordingly, by adjusting the deposition time and thus the thickness of the plasma polymer coating, it is feasible to produce any desired pore diameter. The structural and chemical properties of modified membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray electron spectroscopy (XPS). The resultant PA membranes with specific surface chemistry and controlled pore size are applicable for molecular separation, cell culture, bioreactors, biosensing, drug delivery, and engineering complex composite membranes.     

Measurement of lifetimes of thin carbon stripper foils produced by ion-beam sputtering

Thin carbon stripper foils used in high-intensity proton accelerators and heavy-ion accelerators must have long lifetimes. Thin carbon foils were fabricated by ion-beam sputtering using reactive and inert gas ions. The lifetime of the foils was measured using a KEK 650-keV high-intensity DC H⁻ (negative hydrogen ion) beam; changes in the foil thickness and surface deformations during irradiation were investigated. The lifetime of a typical stripper foil fabricated by heavy-ion-beam (Ar and Kr) sputtering was 60–70 times longer than that of the best commercially available foils. This paper reports a fabrication method for carbon stripper foils, along with an investigation of their lifetimes and changes in foil thickness during beam irradiation.     

Electrically conductive polymer nanocomposites of polymethylmethacrylate and carbon nanofibers prepared by chaotic mixing

Electrically conductive composites of poly(methyl methacrylate) and carbon nanofibers (CNF) were prepared in a low-shear chaotic mixer. These materials showed a percolation threshold of approximately 2 wt.% CNF compared to 6 wt.% for materials prepared in an internal mixer under comparable conditions of mean shear rate. It was found in materials prepared by chaotic mixing that nanofibers were pulled out of the bundles and oriented along the flow directions to produce electrically conductive networks. Electrical conductivity showed great sensitivity to mixing time around percolation threshold and remained almost unchanged with prolonged chaotic mixing above the percolation threshold. Thermal, thermo-mechanical and mechanical properties of the composites were investigated.     

Platinum-mediated healing of defective graphene produced by irradiating glassy carbon with a hydrogen ion-beam

The effect of platinum catalyst on the thermally activated healing of defects produced in a graphene-ribbon network by irradiating glassy carbon with a 15 keV hydrogen-ion beam has been investigated by Raman spectrometry. The platinum has been incorporated into glassy carbon by hydrogen-ion beam irradiation of a thin layer of platinum salt deposited on the glassy carbon surface. The presence of platinum is beneficial because it becomes incorporated by ion-beam mixing and facilitates the structural healing of the amorphous subsurface layer by decreasing the healing temperature from 500 °C to ∼270 °C in comparison to irradiated glassy carbon that contains no platinum. In the case of chemically doped platinum in glassy carbon the in-plane structural ordering, demonstrated by the decreasing I_D/I_G ratio, is a linear function of the platinum added to the phenol-formaldehyde resin as precursor. The results of the density functional theory calculations showed that platinum mediates the reorganization of the bond network and the removal of defects present in the graphene layer.     

Carbon-loaded porous composites produced by matrix carbonization of poly(vinylidene fluoride)

Using poly(vinylidene fluoride) (PVDF) carbonization at 750° C in fine-particle silica and its mixtures with graphite, we have prepared carbon-loaded porous composites which offer benzene absorption from 0.90 to 1.52 ml/g, compressive strength of 6 MPa, and Brinell hardness of up to 18 MPa. We observed the formation of various nanostructures (spheres, spherical segments, and layered platelets) and sizes (several to hundred nanometers). X-ray photoelectron and energy dispersive x-ray spectroscopy data indicate the presence of C-C, C =C, CO, COO, and CHF groups on the carbon surface. X-ray emission spectroscopy data show that the silica matrix composite prepared via PVDF carbonization contains small carbon clusters weakly bonded to the matrix. The silica/graphite matrix composite contains multilayer carbon films strongly bonded to the matrix. The OK _α spectra of both composites are similar to the spectrum of pure SiO₂.     

Studies of self-organization processes in nanoporous alumina membranes by small-angle neutron scattering

We performed studies of the self-organization processes in nanoporous alumina membranes at initial and late stages of aluminum anodization by using scanning electron microscopy (SEM) and small-angle neutron scattering (SANS). SEM observations indicated three stages in the self-organization of nanopores in alumina: (1) nucleation of random nanopores with a broad radius distribution, (2) narrowing the radius distribution and (3) slow evolution of the nanoporous structure towards ordering of nanopores into large domains. SANS studies revealed orientational correlation between ordered domains of nanopores, which is characterized by a small misorientation angle. For the samples with high aspect ratios of nanopores, the SANS patterns showed azimuthal smearing, which was attributed to the redistribution of nanopores between the domains during their growth.     

Fast fabrication of long-range ordered porous alumina membranes by hard anodization

Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.     

Optical properties of TiNx produced by reactive evaporation and reactive ion-beam sputtering

Thin films of titanium nitride are produced by activated reactive evaporation and reactive ion-beam sputtering. The optical constants n and k are calculated over the wavelength region 400–700 nm and vacuum-to-air changes are investigated. It is found that substantial modification of the optical properties occurs as a result of water sorption and oxidation in the bulk of the films.     

以不同聚合物和有机前驱体模板法制备纳米孔炭及其热性能(英文)

以糠醇(FA)、4,4-双马来酰亚胺二苯基甲醇(BM)、BM与二乙烯基苯共聚物(BM-DVB)和蔗糖(S)为炭前驱体,硅胶为硬模板制备了六种纳米孔炭。在氦气气氛中800℃～1 000℃炭化30 min,炭化后利用质量分数40%HF脱除模板。利用元素分析、氮气吸附、热分析和漫反射红外表征产物的孔结构和化学结构及它们对多孔炭热性能的影响。结果表明,前驱体对多孔炭的性能有一定的影响。前驱体的化学结构和热处理条件对炭的孔隙结构,亲水性和热稳定性都有影响。最稳定的产物中含有氮和磷元素。     

规则多孔铜热膨胀性能的研究

在氢气或氢气和氩气的混合高压气氛中,采用定向凝固技术制备了规则多孔铜材料。测试了不同气孔率的规则多孔铜在平行和垂直气孔方向上的热膨胀系数;研究了气孔、气孔方向和气孔率对其热膨胀系数的影响规律,并对其规律做了理论预测。结果表明,规则多孔铜的热膨胀系数随着温度升高先急剧增大到一定值后趋于平稳;温度在40~130℃,气孔中存在闭孔时,规则多孔铜的CTE值随气孔率的增大而缓慢增大,且比纯铜时略大;当气孔主要以通孔形式存在时,气孔率与孔径的比值越大,规则多孔铜的CTE值越低。温度>130℃时,规则多孔铜的热膨胀系数与纯铜的几乎相同,气孔的存在对铜的膨胀无明显影响。     

Fabrication of ordered Cr nanostructures by self agglomeration on porous anodic alumina membranes

In this paper, we propose an efficient and effective method to fabricate highly ordered Cr nanoarrays with sub-gaps less than 15 nm and particle size less than 50 nm on the top surface of a modified porous anodic alumina membrane (PAA). In addition, the factors that influence the structural parameters of the fabricated nanostructures were studied. With the aid of SEM and AFM images, the amount of sputter-coated Cr was tailored to the given PAA surface morphology. The mechanism of formation of the Cr nanostructures was also discussed.     

Characterization of nanoporous structures of polyphenylene oxide derived carbon membranes by means of 129Xe NMR

The 129Xe NMR spectroscopy has become a powerful technique of materials characterization because the xenon atom has a very large polarizability. It is well known that the signal of xenon sorbed in porous media is sensitively affected by the surrounding environments such as the chemistry of material surface. In this study, the pore properties of nanoporous PPO (polyphenylene oxide) derived carbon membranes were characterized by means of the variable temperature (VT)-hyperpolarized Xe NMR. The Xe NMR results showed good agreements with the adsorption results of CO2 for the PPO derived nanoporous carbon membranes. It was clearly shown that the 129Xe NMR could be used as one of the promising characterization methods of nanoporous materials with low surface area and small pore volume.     

Fabrication of metallic nanowire arrays by electrodeposition into nanoporous alumina membranes: effect of barrier layer

Deposition into nanoporous alumina membranes is widely used for nanowire fabrication. Herein using AC electrodeposition ternary Fe–Co–Ni nanowires are fabricated within the nanoscale-pores of alumina membranes. Using an electrodeposition frequency of 1,000 Hz, 15 V_rms, consistently and repeatably yield nanowire arrays over membranes several cm² in extent. Electrochemical Impedance Spectroscopy (EIS) is used to explain the effects of AC electrodeposition frequency. The impedance of the residual alumina barrier layer, separating the underlying aluminum metal and the nanoporous membrane, decreases drastically with electrodeposition frequency facilitating uniform pore-filling of samples several cm² in area. Anodic polarization studies on thin films having alloy compositions identical to the nanowires display excellent corrosion resistance properties.     

Photocatalytic activity of manganese, chromium and cobalt-doped anatase titanium dioxide nanoporous electrodes produced by re-anodization method

Transition metal-doped TiO₂ electrodes were prepared by re-anodization and characterized. The structure of these electrodes was investigated by X-ray diffraction and electron diffraction, which mainly showed typical characteristic anatase reflections without any dopant-related peaks. The amount of transition metal dopant in TiO₂ was kept at approximately 1.0 at.%, as measured by energy dispersive X-ray spectroscopy. The effects of different types of dopants on the photocatalytic activity were revealed by measuring the degradation of an organic aqueous solution containing a dye (acid red G) using a combination of ultraviolet (UV) light energy in the presence of these electrodes. The photocatalytic efficiency was remarkably enhanced by the incorporation of Mn ²⁺ and Cr³⁺. Mn²⁺ showed the most significant enhancement. However, Co²⁺ accelerated the rate of acid red G degradation only slightly. Langmuir-Hinshelwood rate expression was employed for the degradation of acid red G by UV/TiO₂ electrodes system. The adsorption equilibrium constant, the rate constant, and the initial degradation rate were determined for different electrodes. The effect of the concentration of Mn²⁺ on the degradation of acid red G was also investigated and the results showed that there is an optimal value (about 1.0 at.%) of the concentration of Mn²⁺ for inducing faster degradation of the dye. The enhanced photocatalytic degradation rate of acid red G in the presence of transition metals is attributed to the increase of the charge separation in these systems.