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.     

A study of electrodeposition of CuInSe2 thin films with triethanolamine as the complexing agent

In this article, the electrochemistry of CuInSe₂ and its compositional ingredients CuCl₂, InCl₃ and SeO₂ in aqueous solution were investigated. Triethanolamine was added in the single-step electrodeposition of CuInSe₂ from aqueous solution as the complexing agent in order to improve the crystallinity and uniformity of the layer. The stoichiometry, crystal structure and grain sizes of CuInSe₂ thin films of various deposition conditions were compared. The deposition parameters such as the concentration of complexing agent, deposition potential, deposition time and annealing temperature are found to be important factors in the processes of electrical deposition of CuInSe₂ thin films.     

Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array

Silicon nanowire (SiNW) arrays were fabricated on silicon wafers by the metal-assisted chemical etching method. Varied average diameters of SiNW arrays were realized through further treatment in a mixed agent of HF and HNO₃ of certain concentrations. After the treatment, there were more than 93% SiNWs with diameters smaller than 100 nm. The tip of each SiNW was subsequently wrapped with multi-walled carbon nanotubes (MWCNTs) with chemical vapor deposition method. The as-fabricated MWCNT/SiNW arrays were fabricated into electric field emitters, with turn-on field of 2.0 V/μm (current density: 10 μA/cm²), much lower than that of SiNW array (5.0 V/μm). The turn-on electric field of MWCNT/SiNW array decreased with the decreasing of the average diameter of SiNWs, indicating the performance of the field emission is relative to the morphology of SiNWs. As the SiNW array is uniform in height and easy to fabricate, the MWCNT/SiNW array shows potential applications in flat electric display.     

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.     

Modification of alumina barrier-layer through re-anodization in an oxalic acid solution with fluoride additives

A new simple method for modification of the porous alumina barrier-layer is described and characterized by the voltammetric, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) techniques. The method is based on re-anodization of porous alumina under galvanostatic conditions in the anodizing bath that, in addition to conventional anodization solution components, contains fluoride salts: (NH₄)₂SiF₆ or NH₄F. During first few minutes of alumina re-anodization, the sharp drop of anodizing voltage was observed, which is indicative of chemical/electrochemical transformations of the alumina barrier-layer. As a result, the scalloped structure of the barrier-layer changes drastically, becoming smooth and finely grained. Upon re-anodization, a significant loss of insulating ability of the barrier-layer and considerable increase in its capacitance were observed, while the variation of the constant phase element was found to be consistent with the oxide film morphology transformations observed by microscopy techniques. All these changes intensify with fluoride concentration increase. Curiously, (NH₄)₂SiF₆ exhibited about three-fold stronger effect on the barrier-layer properties than NH₄F, thus allowing us to hypothesize about possible chemical break up of SiF₆²⁻ anion and the formation of the AlF₃ phase inside the alumina pores.     

Preparation and study of complex self-assembled film as a super-thin barrier on silver

A self-assembled monolayers (SAMs) of (3-mercaptopropy) trimethoxysilane (3-MPT) chemisorbed on silver surface was chemically modified by 1-octadecanethiol (C₁₈H₃₇SH) (to form self-assembled mixed-monolayer (SAMM)) and the co-polymer of N-vinylcarbazole and methyl methacrylate ester to form complex self-assemblied film (CSAF). The combinative state of interface between SAMs (or SAMM) and co-polymer were characterized by dynamic mechanical thermal analysis (DMTA). The thickness of film on Ag was characterized by X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) measurements in 10% NaOH aqueous solution with the silver surface and covered with film indicated that 3-MPT SAMs modified with C₁₈H₃₇SH and then with co-polymer have higher capability against oxidation.     

High-resolution analytical electron microscopy of catalytically etched silicon nanowires

We report on the characterization of hexagonally ordered, vertically aligned silicon nanowires (SiNW) by means of analytical transmission electron microscopy. Combining colloidal lithography, plasma etching, and catalytic wet etching arrays of SiNW of a sub-50 nm diameter with an aspect ratio of up to 10 could be fabricated. Scanning transmission electron microscopy has been applied in order to investigate the morphology, the internal structure, and the composition of the catalytically etched SiNW. The analysis yielded a single-crystalline porous structure composed of crystalline silicon, amorphous silicon, and SiO _x with x≤2.     

Effect of temperature and silicon resistivity on the elaboration of silicon nanowires by electroless etching

The morphology of silicon nanowire (SiNW) layers formed by Ag-assisted electroless etching in HF/H₂O₂ solution was studied. Prior to the etching, the Ag nanoparticles were deposited on p-type Si(1 0 0) wafers by electroless metal deposition (EMD) in HF/AgNO₃ solution at room temperature. The effect of etching temperature and silicon resistivity on the formation process of nanowires was studied. The secondary ion mass spectra (SIMS) technique is used to study the penetration of silver in the etched layers. The morphology of etched layers was investigated by scanning electron microscope (SEM).     

Realization of controllable etching for ZnO film by NH4Cl aqueous solution and its influence on optical and electrical properties

ZnO films were deposited on c-plane Al₂O₃ substrates by pulsed laser deposition. The etching treatments for as-grown ZnO films were performed in NH₄Cl aqueous solution as a function of NH₄Cl concentration and etching time. It was found that NH₄Cl solution is an appropriate candidate for ZnO wet etching because of its controllable and moderate etching rate. The influence of etching treatment on the morphology, optical and electrical properties of the ZnO films has been investigated systematically by means of X-ray diffraction, atomic force microscope, photoluminescence and Hall effect. The results indicated that the surface morphology and optical properties of the films were highly influenced by etching treatment.     

Fabrication of metal nano-structures using anodic alumina membranes grown in phosphoric acid solution: Tailoring template morphology

The influence of experimental parameters on the morphology of the porous structure and on the formation kinetics has been investigated for anodic alumina membranes (AAM) grown in aqueous H₃PO₄ at 160 V. It was found that pore aspect ratio and membrane porosity on the solution-side surface are influenced by tensiostatic charge, bath temperature and the presence of Al³⁺ ions in solution. Morphological and kinetic data, recorded in different conditions, give useful information on the growth mechanism of pore channels in phosphoric acid solution.Nickel nano-structures have been fabricated using AAM as template. Electroless deposition, performed by adding the reducing agent to a suitable bath in several steps, resulted in the formation of short metal nanotubes (about 5 μm long) in the upper part of the channels. Long Ni nanowires (up to 25 μm) with aspect ratio higher than 100 were obtained by pulsed unipolar electrodeposition from a Watt bath. In this case, both the influence of some experimental parameters on the nanowires growth and the fast current transients during the electrodeposition steps were analyzed.     

Evolution of structural and optical properties of rutile TiO2 thin films synthesized at room temperature by chemical bath deposition method

Nanocrystalline thin films of TiO₂ were prepared on glass substrates from an aqueous solution of TiCl₃ and NH₄OH at room temperature using the simple and cost-effective chemical bath deposition (CBD) method. The influence of deposition time on structural, morphological and optical properties was systematically investigated. TiO₂ transition from a mixed anatase–rutile phase to a pure rutile phase was revealed by low-angle XRD and Raman spectroscopy. Rutile phase formation was confirmed by FTIR spectroscopy. Scanning electron micrographs revealed that the multigrain structure of as-deposited TiO₂ thin films was completely converted into semi-spherical nanoparticles. Optical studies showed that rutile thin films had a high absorption coefficient and a direct bandgap. The optical bandgap decreased slightly (3.29–3.07 eV) with increasing deposition time. The ease of deposition of rutile thin films at low temperature is useful for the fabrication of extremely thin absorber (ETA) solar cells, dye-sensitized solar cells, and gas sensors.     

Surface modification by silver coating for improving electrochemical properties of LiFePO4

LiFePO₄ is a potential candidate for the cathode material of the lithium secondary battery. Fine particle LiFePO₄ was synthesized by the simple co-precipitation method, and aqueous coating on the LiFePO₄ was tried using silver nitrate solution in order to increase electronic conductivity. Highly dispersed silver on the particles enhances the electronic conductivity and increases the capacity. The electrochemical properties of the silver coated LiFePO₄ with the various current densities are analogous to those of highly conductive LiFePO₄. The silver coating can be a promising tool to preserve the capacity even at the high current densities.     

Electrochemical impedance spectroscopic analysis of activation of Al-Zn alloy sacrificial anode by RuO2 catalytic coating

The uniform surface activation characteristics of RuO₂-coated Al-Zn alloy sacrificial anodes were studied in the present work. Electrochemical impedance spectroscopic and SEM/EDAX analyses were carried out to evaluate the galvanic dissolution characteristics of the anodes. The conventional electrochemical methods were also followed. Persistence and retention of catalytic RuO₂ on the anode surface even after a large extent of galvanic dissolution of the anode surface was confirmed based on the EDAX spectroscopy. The anodes showed the same polarization performances even after the anodes were disintegrated to one-third of its original size, revealing the sustained catalytic activity of RuO₂.     

Deposition and wettability of silver nanostructures on Si(1 0 0) substrate based on galvanic displacement reactions

A new route for silver electroless deposition on Si(1 0 0) substrate is developed based on the galvanic displacement process. The basic electroless bath contains NaF and AgNO₃ with different concentrations. The morphologies of electrolessly deposited silver nanostructures, including silver nanowires and nanoparticles, are strongly dependent on the electrolyte composition. Adding an excess dosage of polyvinylpyrrolidone into the basic electrolyte yields final silver films of porous structures composed by multitudinous Ag nanoparticles. The porous silver films possess the surface hydrophobic property after the modification with n-dodecanethiol. Unidirectional wetting and spreading of a water droplet are also demonstrated on the patterned porous Ag films.     

Effect of calcination temperature on the morphology and surface properties of TiO2 nanotube arrays

Well-ordered TiO₂ nanotube arrays were prepared by electrochemical anodization of titanium in aqueous electrolyte solution of H₃PO₄ + NH₄F at a constant voltage of 20 V for 3 h, followed by calcined at various temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) were used to characterize the samples. The results showed that the as-prepared nanotube arrays before being calcined were amorphous and could transform to anatase phase at a heat treatment temperature higher than 400 °C. As the calcination temperatures increased, crystallization of anatase phase enhanced and rutile phase appeared at 600 °C. However, further increasing the calcination temperature would cause the collapse of nanotube arrays. PL intensity of the nanotube arrays annealed at 500 °C was the lowest, which was probably ascribed to better crystallization together with fewer surface defects of the nanotube arrays.     

Au-assisted electroless etching of silicon in aqueous HF/H2O2 solution

The Au-assisted electroless etching of p-type silicon substrate in HF/H₂O₂ solution at 50 °C was investigated. The dependence of the crystallographic orientation, the concentration of etching solution and the silicon resistivity on morphology of etched layer was studied. The layers formed on silicon were investigated by scanning electron microscopy (SEM). It was demonstrated that although the deposited Au on silicon is a continuous film, it can produce a layer of silicon nanowires or macropores depending on the used solution concentration.     

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.     

有序TiO2纳米管阵列结构的可控生长及其物相研究

分别在HF水溶液、含NH₄F和H₂O的乙二醇有机溶液中对Ti箔进行阳极氧化,得到TiO₂纳米管阵列结构.该结构高度有序、分布均匀、垂直取向,且通过阳极氧化工艺条件(如阳极氧化电压、电解液的选择与配比以及氧化时间等)可实现对其结构参数(如管径、管壁厚度、管密度、管长等)的有效控制.利用XRD研究了TiO₂纳米管阵列的物相结构.结果表明：退火前的TiO₂纳米管阵列为无定形结构;分别在真空和氧气氛中50 关键词： 2纳米管阵列')" href="#">TiO₂纳米管阵列 阳极氧化 可控生长     

Field emission from patterned SnO2 nanostructures

A simple and reliable method has been developed for synthesizing finely patterned tin dioxide (SnO₂) nanostructure arrays on silicon substrates. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO₂ nanostructures arrays, a unit area is of ∼500 μm × 200 μm, were synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized SnO₂ nanostructures were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanism of formation of SnO₂ nanostructures was also discussed. The measurement of field emission (FE) revealed that the as-synthesized SnO₂ nanorods, nanowires and nanoparticles arrays have a lower turn-on field of 2.6, 3.2 and 3.9 V/μm, respectively, at the current density of 0.1 μA/cm². This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays, sensor arrays and so on.