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.     

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.     

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.     

Fabrication of silver-coated silicon nanowire arrays for surface-enhanced Raman scattering by galvanic displacement processes

Silver-coated silicon nanowire (SiNW) arrays were prepared utilizing galvanic displacement processes consisting of three steps: galvanic displacement deposition of silver particles using a HF-AgNO₃ or NH₄F-AgNO₃ aqueous solution; formation of SiNW arrays by a silver-assisted chemical etching process conducted in the HF-H₂O₂ aqueous solution; deposition of silver particles on the SiNW arrays from the NH₄F-AgNO₃ aqueous solution. The effects of the morphology of pre-deposited silver particles and deposition solution on the formation of silver-coated SiNW arrays were discussed. Surface-enhanced Raman scattering (SERS) performances have been studied using Rhodamine 6G (R6G) probe molecules on the silver-coated SiNW substrates.     

Electrical and photocatalytic properties of Na2Ti6O13 nanobelts prepared by molten salt synthesis

Single-crystalline Na₂Ti₆O₁₃ nanobelts were prepared on large-scale by molten salt synthesis at 825 °C for 3 h. The obtained nanobelts have typical width of less than 200 nm and thickness of 10-30 nm, and length up to 10 μm. The growth direction of the nanobelts was determined to be along [0 1 0]. Electrical transport property of an individual nanobelt was measured at room temperature and ambient atmosphere, and results showed that the nanobelts are semiconductor. Na₂Ti₆O₁₃ nanobelts exhibited good photocatalytic efficiency for the degradation of RhB under UV irradiation.     

Static susceptibility and heat capacity studies on V3O7·H2O7 nanobelts

V₃O₇·H₂O nanobelts were prepared by a hydrothermal method at 190 °C using V₂O₅·nH₂O gel and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 40 to 70 nm with lengths up to several micrometers. Measurements of the static magnetic susceptibility and the specific heat show a discontinuous phase transition at around T=145 K, which separates two regions of paramagnetic behavior.     

Formation of V2O3 nanocrystals by thermal reduction of V2O5 thin films

We report the formation of homogeneous and stable V₂O₃ nanocrystals, directly from V₂O₅ thin films, at 600 °C, as observed by using in situ electron microscopy experiments. Thermally-induced reduction of V₂O₅ thin films in vacuum is remarkably different when compared to reduction of V₂O₅ single crystals and results in the formation of nanophase V₂O₃. Thermally grown V₂O₃ nanocrystals exhibit hexagon or square shape and are stable at higher temperature as well as room temperature. The formation of stable nanocrystals through the reduction process in a non-chemical environment (vacuum) could provide a basis for understanding the complex processes of vanadium oxide phase transitions and for controlling the chemical processes to produce oxide nanocrystals.     

Metal nanorod production in silicon matrix by electroless process

Metal filled Si nanopores, that is, metal nanorods in an Si matrix, are produced by an electroless process that consists of three steps: (1) electroless displacement deposition of metal nanoparticles from a metal salt solution containing HF; (2) Si nanopore formation by metal-particle-enhanced HF etching; and (3) metal filling in nanopores by autocatalytic electroless deposition. Ag nanoparticles produce Si nanopores whose sizes are a few tens of nm in diameter and ca. 50 nm deep. Au nanoparticles produce finer and straighter nanopores on Si than the Ag case. These nanopores are filled with a Co or a Co-Ni alloy by autocatalytic deposition using dimethylamine-borane as a reducing agent. Phosphinate can be used as a reducing agent for the Au-deposited-and-pore-formed Si. The important feature of this process is that the metal nanoparticles, that is, the initiation points of the autocatalytic metal deposition, are present on the bottoms of the Si nanopores.     

Characterization of a thin CeO2-ZrO2-Y2O3 films electrochemical deposited on stainless steel

In this paper, we report for the first time formation of a thin CeO₂-ZrO₂-Y₂O₃ films electrodeposited on a stainless steel substrate. The samples have been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD and XPS data indicate formation of a solid solution and additional existence of Ce³⁺ states near the surface. After annealing, SEM examination has shown a microstructure formed by dispersed spherical agglomerates having a size between 20 and 60 nm.     

Silicon etching in Cl2 environment

The ion-beam-assisted etching of silicon in Cl₂ environment is considered. The theoretically calculated dependences of silicon etching rate on the flux of Cl₂ molecules at different ion current densities are compared with experimentally measured. The composition of the adsorbed layer is determined. It is found that SiCl₂ molecules prevail in the adsorbed layer. The reciprocal of relative concentration of SiCl₂ molecules in the adsorbed layer linearly depends on the ion-to-neutral flux ratio.     

Microfabrication of freestanding porous silicon particles containing spectral barcodes

A method to microfabricate micron‐scale freestanding porous silicon photonic crystal particles is described. An electrochemically prepared film of porous silicon on a crystalline silicon substrate is patterned with an SU8‐25 photoresist, and the unmasked porous silicon is removed with a chlorine plasma reactive ion etch. Porous microparticles are then removed from the substrate by electropolishing. Scanning electron microscopy and microscopic reflection spectroscopy are used to characterize the geometry and optical properties of the freestanding particles. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrochemical and quantum chemical studies of some Schiff bases on the corrosion of steel in H2SO4 solution

The efficiency, as steel-corrosion inhibitors in 0.1 M and 1 M H₂SO₄, of two Schiff bases, 2-{[(4-methoxyphenyl)imino]methyl}phenol and 1-{[(4-methoxyphenyl)imino]methyl}-2-naphthol, (abbreviated SB-1 and SB-2, respectively) was investigated by Tafel extrapolation and linear polarization methods. Corrosion parameters and adsorption isotherms were determined from current-potential curves. It was found that the percent inhibition efficiencies (η%) and surface coverage (θ) increase with an increases in the concentrations of inhibitors. The results showed that these compounds act as good corrosion inhibitors especially at high concentrations. The adsorption of used compounds on the steel surface obeys Langmuir's isotherm. Obvious correlation was found between corrosion inhibition efficiency and quantum chemical parameters obtained by B3LYP/6-31g(d) method. The obtained theoretical results have been compared with the experimental findings.     

Catalysis of dispersed silver particles on directional etching of silicon

Silver particles are dispersed on silicon by magnetron sputtering and post-annealing to investigate the catalytic effects of individual silver particles on wet etching of silicon surface. According to scanning electron microscopy, dispersed deep holes are present and the major etching direction is vertical to the surface of a Si(1 0 0) wafer or inclined to that on a Si(1 1 1) wafer. Our experiments indicate that the effect of the anisotropy of Si on directional etching is fundamental and the wafer resistivity and experimental process have important influence on the etching results. In addition, aggregation of silver particles and random horizontal etching on the surface of the wafer are caused by the local imbalance between the oxidant and HF. Our results enable better understanding of the catalytic effects of metal particles on silicon and are helpful to the preparation new silicon nanostructures.     

Effect of gas residence time on the morphology of silicon surface etched in SF6 plasmas

This paper describes the effect of the SF₆ gas residence time on the morphology of silicon (1 0 0) samples etched in a reactive ion etching system. Profilometry and atomic force microscopy techniques were used to characterize the etching process focusing attention on the evolution of the surface morphology. Under the condition of variable pressure and gas flow rate, the decrease of the residence time leads to an increase of the silicon etch rate concomitantly with an increase of the surface roughness. Contrary fact is observed when the gas flow is fixed and the pressure is varied. Here, the increasing of residence time leads to a constant increase of silicon etch rate with small variations in final surface roughness. To better understanding this resident time effect, mass spectrometry analyses were realized during the discharge for both gas flow conditions.     

Structural, optical and photoelectrochemical characterization of CdS nanowire synthesized by chemical bath deposition and wet chemical etching

Nanocrystalline thin films of CdS have been grown onto flexible plastic and titanium substrates by a simple and environmentally benign chemical bath deposition (CBD) method at room temperature. The films consist of clusters of CdS nanoparticles. The clusters of CdS nanoparticles in the films were successfully converted into nanowire (NW) networks using chemical etching process. The possible mechanism of the etching phenomenon is discussed. These films were examined for their structural, surface morphological and optical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectrophotometry techniques, respectively. Photoelectrochemical (PEC) investigations were carried out using cell configuration as n-CdS/(1 M NaOH + 1 M Na₂S + 1 M S)/C. The film of nanowires was found to be hexagonal in structure with the preferential orientation along the (0 0 2) plane. The nanowires have widths in the range of 50-150 nm and have lengths of the order of a few micrometers. Optical studies reveal that the CdS nanowires have value of band gap 2.48 eV, whereas it is 2.58 eV for nanoparticles of CdS. Finally, we report on the ideality of junction improvement of PEC cells when CdS nanoparticles photoelectrode converted into nanowires photoelectrode.     

Ion beam etching of high resolution structures in Ta2O5 for grating-assisted directional coupler applications

An investigation on thin Ta₂O₅ films patterning using argon ion beam etching (IBE) is presented. The etch rates are characterised by varying the angle of incidence of the beam onto the substrate. Ta₂O₅ gratings with a period of 2.2 μm (1.1 μm linewidth) and 0.25 μm thickness are fabricated using an angle of incidence of 0°. The resulting Ta₂O₅ grating cross sectional profiles are analysed using AFM and SEM imaging. A fabrication method is thus demonstrated which could be used to implement wavelength selective gratings in applications such as grating-assisted directional couplers (GADCs).     

Laser-induced forward transfer technique for maskless patterning of amorphous V2O5 thin film

A laser-induced forward transfer technique has been applied for the maskless patterning of amorphous V₂O₅ thin films. A sheet beam of a frequency doubled (SHG) Q-switched Nd:YAG laser was irradiated on a transparent glass substrate (donor), the rear surface of which was pre-coated with a vacuum-deposited V₂O₅ 180 nm thick film was either in direct contact with a second glass substrate (receiver) or a 0.14 mm air-gap was maintained between the donor film and the receiving substrate. Clear, regular stripe pattern of the laser-induced transferred film was obtained on the receiver. The pattern was characterized using X-ray diffraction (XRD), optical absorption spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), atomic force microscopy (AFM), etc.     

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.     

Time-resolved transmission study of fused silica during laser-induced backside dry etching

Laser-induced backside dry etching (LIBDE) is a promising technique for micro- and nanomachining of transparent materials. Although several experiments have already proved the suitability and effectiveness of the technique, there are several open questions concerning the etching mechanism and the concomitant processes. In this paper time-resolved light transmission investigations of etching process of fused silica are presented. 125 nm thick silver coating was irradiated through the carrying 1 mm thick fused silica plate by single pulses of a nanosecond KrF excimer laser. The applied fluences were 0.38, 0.71 and 1 J/cm². During the etching process the irradiated spots were illuminated by an electronically delayed nitrogen laser pumped dye laser. The delay between the pump and probe pulses was varied in the range of 0 ns and 20 μs. It was found that the transmitted probe beam intensity strongly depends on the applied delays and fluences. Scanning electron microscopy and energy dispersive X-ray spectrometry of the etched surface showed the existence of silver droplets and fragments on the illuminated surfaces and silver atoms built into the treated surface layer influencing the transmission behavior of the studied samples.     

The high quality ZnO growth on c-Al2O3 substrate with Cr2O3 buffer layer using plasma-assisted molecular beam epitaxy

High quality epitaxial ZnO films were grown on c-Al₂O₃ substrates with Cr₂O₃ buffer layer by plasma-assisted molecular beam epitaxy (P-MBE). The hexagonal crystalline Cr₂O₃ layer was formed by oxidation of the Cr-metal layer deposited on the c-Al₂O₃ substrate using oxygen plasma. The epitaxial relationship was determined to be ZnO//Cr₂O₃//Cr//Al₂O₃ and ZnO//Cr₂O₃//[0 0 1]Cr//Al₂O_3. The Cr₂O₃ buffer layer was very effective in improving the surface morphology and crystal quality of the ZnO films. The photoluminescence spectrum showed the strong near band-edge emissions with the weak deep-level emission, which implies high optical quality of the ZnO films grown on the Cr₂O₃ buffer.