Formation of aligned silicon-nanowire on silicon in aqueous HF/(AgNO3 + Na2S2O8) solution

Highly oriented silicon nanowire (SiNW) layer was fabricated by etching Si substrate in HF/(AgNO₃ + Na₂S₂O₈) solution at 50 °C. The morphology and the photoluminescence (PL) of the etched layer as a function of Na₂S₂O₈ concentration were studied. The SiNW layers formed on silicon were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). It was demonstrated that the morphology of the etched layers depends on the Na₂S₂O₈ concentration. Room-temperature photoluminescence (PL) from etched layer was observed. It was found that the utilisation of Na₂S₂O₈ decreases PL peak intensity. Finally, a discussion on the formation process of the silicon nanowires is presented.     

The electrical properties and phase transformation of PLZST 2/85/13/2 antiferroelectric thin films on different bottom electrode

Pb_0.97La_0.02(Zr_0.85Sn_0.13Ti_0.02)O₃ (PLZST 2/85/13/2) antiferroelectric thin films were deposited on Pt(111)/Ti/SiO₂/Si and LaNiO₃(LNO)/SiO₂/Si substrates through a modified sol-gel process. The phase structure and microstructure of PLZST 2/85/13/2 antiferroelectric thin films were analysed by x-ray diffraction (XRD), scanning electron microcopy (SEM) and field-emission SEM (FE-SEM). The antiferroelectric nature of the PLZST 2/85/13/2 thin films on two electrodes was demonstrated by the C-V (capacitance-voltage) and P-E (polarization-electric field) measurement. The maximum polarizations for PLZST 2/85/13/2 films on Pt and LNO electrodes were 42 and 18 μC/cm², respectively. The temperature dependence of the dielectric property of the PLZST 2/85/13/2 films was measured under different dc electric fields. Also, the phase transformation of the PLZST 2/85/13/2 films was studied in detail as a function of temperature and dc electric field.     

Formation of aligned silicon nanowire on silicon by electroless etching in HF solution

It was demonstrated that the etching in HF-based aqueous solution containing AgNO₃ and Na₂S₂O₈ as oxidizing agents or by Au-assisted electroless etching in HF/H₂O₂ solution at 50 °C yields films composed of aligned Si nanowire (SiNW). SiNW of diameters ∼10 nm were formed. The morphology and the photoluminescence (PL) of the etched layer as a function of etching solution composition were studied. The SiNW layers formed on silicon were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and photoluminescence. It was demonstrated that the morphology and the photoluminescence of the etched layers strongly depends on the type of etching solution. Finally, a discussion on the formation process of the silicon nanowires is presented.     

Fabrication of Ru(bpy)3-titanate nanotube nanocomposite and its application as sensitive solid-state electrochemiluminescence sensor material

A novel and facile method for effective immobilization of Ru(bpy)₃²⁺ within titanate nanotubes (TiNTs) and its application as a sensitive solid-state electrochemiluminescence (ECL) sensor material was studied. The process involved the formation of Ru(bpy)₃²⁺-titanate nanotube nanocomposite (Ru-TiNTs) via electrostatic interactions by mixing TiNTs and Ru(bpy)₃(ClO₄)₂ in aqueous medium. Then Ru-TiNTs were attached to the surface of a Pt electrode to form an ECL sensor. Characterization of Ru(bpy)₃²⁺-titanate nanotube nanocomposite was accomplished by transmission electron microscopy, X-ray photoelectron spectrum, and field emission scanning electron microscope. The electrochemistry and ECL behavior of Ru(bpy)₃²⁺ immobilized on TiNTs were studied with tripropylamine as a coreactant. As-prepared Ru-TiNTs exhibited very good stability and Ru(bpy)₃²⁺ species contained showed excellent ECL behavior. Therefore, the as-prepared Ru(bpy)₃²⁺-titanate nanotube nanocomposite exhibited great promise as new luminescent materials for solid-state ECL detection.     

Effects of organic acids on the size-controlled synthesis of rutile TiO2 nanorods

Size-controlled synthesis of pure rutile-phase TiO₂ nanorods was carried out by a hydrothermal method using different organic acids as modifiers, and metatitanic acid and concentrated sulfuric acid as raw materials. The synthesized rutile TiO₂ nanorods were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of organic acid modifiers on the sizes of rutile TiO₂ nanorods were investigated. It was found that the steric effect occurred by the organic modifiers and non-polarity of organic acids were beneficial to the formation of small-sized rutile TiO₂ nanorods. The strongly coordinative interaction between the carboxyl (or hydroxyl) group of the modifier and the surface of TiO₂ nanoparticles effectively inhibited the crystal growth.     

Formation of alumina-ceria mixed oxide in model systems

Interaction of aluminium with cerium oxide was studied by photoelectron spectroscopy of Al/CeO₂(1 1 1) and CeO₂/Al(1 1 1) model systems. It was found in both cases that metallic aluminium was immediately oxidized, CeO₂ was partially reduced and a mixed oxide with cerium present as Ce³⁺ was formed. The compound is probably cerium aluminate CeAlO₃ mixed with Al₂O₃ or Ce₂O₃. In both cases the intermixing was limited by the diffusion of aluminium into ceria. The excess of deposited material above this limit formed AlO_x and CeO₂ overlayers on the top of the mixed oxide + aluminate/CeO₂ and mixed oxide + aluminate/Al films, respectively.     

Preparation and characterization of cerium oxide doped TiO2 nanoparticles

A mixed oxide consisting of TiO₂ as the major phase and CeO_2−y (0<y<0.5) as the dopant phase was prepared via the sol-gel reaction of Ti(i-OC₃H₇)₃ in an aqueous solution of Ce(NO₃)₃. The resulting oxide powders with different CeO_2−y contents were all composed of nano-sized spheres. The CeO_2−y phase was identified to have retarding effect on the phase transition from anatase TiO₂ to rutile TiO₂ at calcinations temperature as high as 800 °C, which would otherwise be a thorough conversion. The CeO_2−y-TiO₂ powders could apparently shift the UV-absorption band of TiO₂ toward visible range, and there was an optimal CeO_2−y content in association with the maximum absorbance. This effect is interpreted as the existence of an n-type impurity band, due to the substitution of Ti⁴⁺ for Ce^3+/4+ at the interface between the two oxides, in the gap of TiO₂. According to X-ray photoelectron spectroscopy (XPS) investigation, the Ti element mainly existed as the chemical state of Ti⁴⁺ and the Ce oxide doping did not affect the peak position of Ti 2p. The Ce 3d spectrum of CeO_2−y-doped TiO₂ sample basically denotes a mixture of Ce^3+/4+ oxidation states giving rise to a myriad of peaks.     

Effect of CeO2 buffer layer thickness on the structures and properties of YBCO coated conductors

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on inclined-substrate-deposited (ISD) MgO-textured metal substrates by pulsed laser deposition. CeO₂ was deposited as a buffer layer prior to YBCO growth. CeO₂ layers of different thickness were prepared to evaluate the thickness dependence of the YBCO films. The biaxial alignment features of the films were examined by X-ray diffraction 2θ-scans, pole-figure, ?-scans and rocking curves of Ω angles. The significant influence of the CeO₂ thickness on the structure and properties of the YBCO films were demonstrated and the optimal thickness was found to be about 10 nm. High values of T_c = 91 K and J_c = 5.5 × 10⁵ A/cm² were obtained on YBCO films with optimal CeO₂ thickness at 77 K in zero field. The possible mechanisms responsible for the dependence of the structure and the properties of the YBCO films on the thickness of the CeO₂ buffer layers are discussed.     

VPO catalysts synthesized on substrates with modified activated carbons

VPO catalysts were prepared on oxidized and unoxidized activated carbons differing in initial porous structure. Carbons were oxidized under relatively soft (30% H₂O₂, 200 °C) and hard (50% H₂O₂, 350 °C) conditions. Carbon modification was carried out hydrothermally in a traditional autoclave (HTT) or a microwave reactor (MWT). The synthesis was also carried out under hydrothermal (HTS or MWS) conditions. V₂O₅ and NH₄VO₃ were used as precursors. The samples are characterized by diversified porous structure at S_BET = 732-1617 m²/g and V_por = 0.44-0.90 cm³/g, as well as various degree of VPO crystallinity. Possibility of preparation of the VPO catalysts under ecologically appropriate conditions, i.e. in aqueous solutions, was shown.     

Oxidizing agent concentration effect on metal-assisted electroless etching mechanism in HF-oxidizing agent-H2O solutions

The mechanism of metal-assisted electroless etching of silicon in HF-oxidizing agent-H₂O etching system as a function of oxidizing agent concentration was studied. Three types of oxidizing agent were experimented namely Na₂S₂O₈, K₂Cr₂O₇ and KMnO₄. Their concentrations were varied from 0.05 M to 0.3 M. The layers formed on silicon were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX). It is shown that an insoluble solid-phase film (K₂SiF₆) form on silicon surface when concentration of K₂Cr₂O₇ or KMnO₄ increases in chemical solutions. On other hand, when Na₂S₂O₈ concentration increases, the surface roughness decreases without any chemical complex formation.     

Molecular beam epitaxy of GaN on a substrate of MoS2 layered compound

The feasibility of MoS₂ layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS₂ by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS₂ was compared with that on Al₂O₃. For GaN grown on MoS₂ substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al₂O₃. These results suggest the layered compound such as MoS₂, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 26 May 1999     

Influence of ZrO2 in HfO2 on reflectance of HfO2/SiO2 multilayer at 248 nm prepared by electron-beam evaporation

Influence of ZrO₂ in HfO₂ on the reflectance of HfO₂/SiO₂ multilayer at 248 nm was investigated. Two kinds of HfO₂ with different ZrO₂ content were chosen as high refractive index material and the same kind of SiO₂ as low refractive index material to prepare the mirrors by electron-beam evaporation. The impurities in two kinds of HfO₂ starting coating materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum (GDMS) technology and secondary ion mass spectrometry (SIMS) equipment, respectively. It showed that between the two kinds of HfO₂, either the bulk materials or their corresponding films, the difference of ZrO₂ was much larger than that of the other impurities such as Ti and Fe. It is the Zr element that affects the property of thin films. Both in theoretical and in experimental, the mirror prepared with the HfO₂ starting material containing more Zr content has a lower reflectance. Because the extinction coefficient of zirconia is relatively high in UV region, it can be treated as one kind of absorbing defects to influence the optical property of the mirrors.     

Effect of NaAlO2 concentrations on microstructure and corrosion resistance of Al2O3/ZrO2 coatings formed on zirconium by micro-arc oxidation

In this study, Al₂O₃/ZrO₂ composite coatings were prepared on Zr substrates by micro-arc oxidation (MAO) in the NaAlO₂-containing electrolytes, and the effect of NaAlO₂ concentration on the microstructure, bond strength, microhardness and corrosion resistance of coatings was systematically investigated. The study reveals that the adequate NaAlO₂ in the electrolyte (>0.2 M) is essential to the formation of needle-like α-Al₂O₃ in the coatings, and the amount of α-Al₂O₃ rises with the increase of the NaAlO₂ concentration. m-ZrO₂ and t-ZrO₂ are present in all of the coatings, but their relative amount largely depends on the amount of Al₂O₃. It is also found that as the NaAlO₂ concentration increases from 0.2 to 0.3 M, the coating becomes denser and thicker, and its bond strength, maximum microhardness and corrosion resistance increases as well. The coating formed at 0.3 M NaAlO₂ demonstrates the highest bond strength of 52 MPa, the maximum microhardness of 1600 Hv_0.2N and the superior corrosion resistance. However, the overhigh concentration of NaAlO₂ (0.35 M) is found harmful to the coating's microstructure and properties.     

Post-annealing effect upon optical properties of electron beam evaporated molybdenum oxide thin films

Molybdenum oxide (MoO₃) thin films were deposited by electron beam evaporation. The chemical composition, microstructure, optical and electrical properties of MoO₃ thin films depend on the annealing temperature and ambient atmosphere. X-ray diffraction (XRD) shows that crystalline MoO₃ films can be obtained at various post-annealing temperatures from 200 to 500 °C in N₂ and O₂. X-ray photoelectron spectroscopy (XPS) results reveal that the O-1s emission peak was shifted slightly toward lower binding energies as the annealing temperature in N₂ was increased. The oxygen vacancies and conductivity of MoO₃ film increased with the annealing temperature. However, when the MoO₃ films were annealed in an atmosphere of O₂, the optical transmission, the O/Mo ratio and the photon energy increased with the annealing temperature. The results differ from those for films annealed in a N₂ atmosphere.     

Effect of alkali leaching on the surface structure of Ni3Al catalyst

The surface structure of the alkali-leached single-phase Ni₃Al powder was investigated by X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area analysis, electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. It was found that fine Ni particles of several nm in diameter were formed on the outer surface layer of the Ni₃Al powder after the alkali leaching process. The surface of the Ni particles was covered with a thin layer of Ni oxides and hydroxide, Ni₂O₃, NiO and Ni(OH)₂, and these Ni oxides and hydroxide can be easily reduced by hydrogen to the metallic nickel that is catalytically active. The inside of the Ni₃Al powder remained as the original Ni₃Al ordered structure after alkali leaching. Having heat resistant properties, the Ni₃Al phase can serve as a support of the fine Ni particles and provide the structural and thermal stabilities to the fine Ni particles.     

Preparation of nitrogen-doped titanium dioxide with visible-light photocatalytic activity using a facile hydrothermal method

Nitrogen-doped TiO₂ (N-TiO₂) nanoparticles have been successfully prepared via a direct and simple hydrothermal reaction of a commercial Degussa P25 with triethanol amine as solvent and nitrogen source. As-prepared N-TiO₂ was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible light (UV-vis) absorption spectra, electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) techniques. The results confirm that hydrothermal reaction is an effective way to incorporate nitrogen into the TiO₂ lattice, especially nitrogen substitute for titanium. The nitrogen concentration in TiO₂ can be as high as 21% (molar ratio), which is described as Ti_1−yO_2−xN_x+y (in this paper, x=0.36, y=0.27, i.e., Ti_0.73O_1.64N_0.63). The chemical statuses of N have been assigned to N-Ti-O and O-N-O in the TiO₂ lattice as identified by XPS. Photocatalytic degradation of methyl orange has been carried out in both UV-vis (simulated solar light) and the visible region (λ>400 nm). N-TiO₂ exhibits higher activity than the Degussa P25 TiO₂ photocatalyst, particularly under visible-light irradiation. This study has developed a promising and practical pathway to new nitrogen-doped photocatalysts.     

Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite

The effect of anions such as Cl⁻, SO₄²⁻, and HPO₄²⁻ on the phase stability of FeOOH (α or γ) during precipitation is investigated. Oxidation of Fe(OH)₂·xH₂O from FeCl₂ solution with high Cl⁻ concentration ([Cl⁻]/[Fe]=R_Cl≥8) or (NH₄)₂Fe(SO₄)₂ (FAS) with [HPO₄²⁻]/[Fe]=R_P≥0.02 yields phase-pure γ-FeOOH. In the medium ranges of R_Cl and R_P, mixed phases of α-FeOOH and γ-FeOOH are obtained. Replacement of OH⁻ by Cl⁻ with the bridging cations or strongly bonded HPO₄²⁻ ions in the matrix of the intermediate phase (Fe_x²⁺Fe_y³⁺(OH)_2x+2y−nz·xH₂O(A)_zⁿ⁻, where A is anions such as Cl⁻, SO₄²⁻, HPO₄²⁻, etc.), promoted the lower density γ-FeOOH. However, the particles are less developed and have poor crystallinity as evidenced from transmission electron microscope and thermogravimetry-differential thermal analysis of the precipitates. Whereas, monophasic, uniformly sized, nano-lath shaped particles with high aspect ratio >10 are obtained when morphology-controlling cation additives such as Pt⁴⁺, Pd²⁺ or Rh³⁺ are present in FeCl₂ (R_Cl≥8) solution. Preferential adsorption of additives on (0k0) and (h00) planes limits the growth in the perpendicular directions leading to high aspect ratios. The effect of these additives are suppressed by the phosphate ion, a strong complexing ligand, giving rise to fibrous aggregate with the length of individual particles as small as 10-30 nm. While most of the Cl⁻ ion is removed from the final precipitates on washing, phosphate remained as HPO₄²⁻ as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating γ-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mössbauer spectroscopy. This leads to the low σ_s (44-48 emu/g) and H_c (120-130 Oe) for γ-Fe₂O_3−δ particles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of σ_s (73 emu/g) and H_c (320 Oe), which decreases to 30 emu/g and 75 Oe, respectively, for nanoparticles obtained from phosphate stabilized lepidocrocite. The mobility of iron ions and counter mobility of vacancies during the topotactic transformation of γ-FeOOH to magnetite to γ-Fe₂O_3−δ renders the particles pore-free.     

Y2O3 stabilized ZrO2 thin films deposited by electron beam evaporation: Structural, morphological characterization and laser induced damage threshold

Four kinds of Y₂O₃ stabilized ZrO₂ (YSZ) thin films with different Y₂O₃ content have been prepared on BK7 substrates by electron-beam evaporation method. Structural properties and surface morphology of thin films were investigated by X-ray diffraction (XRD) spectra and scanning probe microscope. Laser induced damage threshold (LIDT) was determined. It was found that crystalline phase and microstructure of YSZ thin films was dependent on Y₂O₃ molar content. YSZ thin films changed from monoclinic phase to high temperature phase (tetragonal and cubic) with the increase of Y₂O₃ content. The LIDT of stabilized thin film is more than that of unstabilized thin films. The reason is that ZrO₂ material undergoes phase transition during the course of e-beam evaporation resulting in more numbers of defects compared to that of YSZ thin films. These defects act as absorptive center and the original breakdown points.     

Enhancement of carrier mobility in pentacene thin-film transistor on SiO2 by controlling the initial film growth modes

Pentacene thin-film transistors (TFTs) were fabricated on thermally grown SiO₂ gate insulator under the conditions of various pre-cleaning treatments. Initial nucleation and growth of the material films on treated substrates were observed by atomic force microscope. The performance of fabricated TFT devices with different surface cleaning approaches was found to be highly related to the initial film morphologies. In contrast to the three-dimensional island-like growth mode on SiO₂ under an organic cleaning process, a layer-by-layer initial growth occurred on the SiO₂ insulator cleaned with ammonia solution, which was believed to be the origination of the excellent electrical properties of the TFT device. Field effect mobility of the TFT device could achieve as high as 1.0 cm²/Vs on the bared SiO₂/Si substrate and the on/off ratio was over 10⁶.     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.