Subsequent Electrochemical Etching Research Articles

Pt Island Decorated Au Anode for Formic Acid Oxidation Reaction

For direct formic acid fuel cell (DFAFC), Pt-based catalyst is widely used as an anode. The Pt catalyst shows high activity and stability for formic acid oxidation reaction (FAOR) in acidic condition. For FAOR, it is generally accepted that Pt follows dual pathway including direct and indirect pathway. During the indirect pathway the Pt surface is vulnerable to CO poisoning, which cause the activity degradation. To enhance the CO tolerance, various Pt-based bi/ternary catalysts have been investigated. In recent, Pt–Au alloy catalysts have reported higher current density for direct FAOR pathway compared with pure Pt catalyst. The enhanced FAOR activity is originated from lower CO adsorption energy. However, the alloy structure has low catalyst utilization, which is not applicable to practical device.In this study, we report Pt islands decorated porous Au electrode for anode of the DFAFC. The porous Au transport layer was fabricated on carbon fiber by galvanic displacement and subsequent electrochemical etching process. Using Au transport layer as a substrate, Pt self-terminated electrodeposition (SED) was performed. By adjusting the SED condition, the Pt loading and coverage on Au surface cab be easily controlled. The FAOR activity test of Pt–Au electrodes was performed by cyclic voltammetry (CV). It was revealed that the current density for direct FAOR is linearly increased as the Pt coverage on Au surface is lower. By increasing the CV cycles, the current density for direct FAOR is gradually increased owing to enhanced ensemble effect. Furthermore, the cycled electrode was applicated into practical DFAFC single cell. The single cell demonstrates more than two-order higher Pt mass activity compared with previously reported single cell using Pt–Au anode.

A Pathway to Fabricate Gallium Nitride Embedded 3D High‐Index‐Contrast Optical Structures

AbstractOptical engineering of gallium nitride (GaN) semiconductor material has enabled novel applications and technologies. 3D optical engineering is challenging and mostly accomplished by surface‐level patterning with subtractive or additive means. In this work a pathway, based on epitaxial conductivity control in conjunction with ion implant and subsequent electrochemical etching, to generate complex embedded nanoporous (NP)‐GaN/GaN or air/GaN 3D optical structures is presented. A range of subsurface structures is demonstrated including in‐plane NP‐GaN stripes and curved patterns, air/GaN microchannels, and localized, embedded distributed Bragg reflectors (DBRs) as micromirrors. To further demonstrate the versatility of the technique, adjacent DBR micromirrors, of different thicknesses, embedded depths, and widths, are created in the same fabrication steps. Computer simulations are used to shed light on the limitations of feature sizes of the presented technique. The approach is a simple, scalable, and versatile approach for fabricating embedded 3D optical structures readily extendable beyond GaN semiconductors.

Nanopore arrays in a silicon membrane for parallel single-molecule detection: fabrication

Solid state nanopores enable translocation and detection of single bio-molecules such as DNA in buffer solutions. Here, sub-10 nm nanopore arrays in silicon membranes were fabricated by using electron-beam lithography to define etch pits and by using a subsequent electrochemical etching step. This approach effectively decouples positioning of the pores and the control of their size, where the pore size essentially results from the anodizing current and time in the etching cell. Nanopores with diameters as small as 7 nm, fully penetrating 300 nm thick membranes, were obtained. The presented fabrication scheme to form large arrays of nanopores is attractive for parallel bio-molecule sensing and DNA sequencing using optical techniques. In particular the signal-to-noise ratio is improved compared to other alternatives such as nitride membranes suffering from a high-luminescence background.

Modeling Electrochemical Etching of Proton Irradiated p-GaAs for the Design of MEMS Building Blocks

We model the electrochemical etching rates of p-type GaAs after proton irradiation using a finite element simulation with input from Monte Carlo simulations of the recoil distribution together with defect to conductivity mapping of implantation isolation data taken from literature. The simulations will allow for the design of advanced microelectromechanical systems through proton beam writing and subsequent electrochemical etching using multiple ion fluences as the novel technique for three-dimensional microstructuring.

Controllable Synthesis of Well-aligned ZnO Nanorod Arrays on Varying Substrates via Rapid Electrodeposition

A facile and rapid electrodeposition route was developed to controllably synthesize well-aligned ZnO nanorod arrays on diverse substrates, such as seed-layer pre-formed, pristine indium tin oxide (ITO) and Si, using Zn(NO 3 ) 2 ·6H 2 O and hexamethylenetetramine (HMT) as the precursors. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results indicated that seed-layer pre-modified of ZnO nanorod arrays (ZNRs) possessed single crystalline, a wurtzite crystal structure with preferential growth orientation along [0001] direction. The ZNRs on pre-modified ZnO seed-layer (ZSL) had diameters of 30–50 nm, and aligned vertically to the substrates. ZNRs on ZSL/ITO substrate exhibited a high transmittance (above 80%) in visible wavelength range and the red-shift of band gap energy. An electrochemical reaction model was proposed to explain the growth process of ZnO nanorods. Importantly, the rapid synthesis of ZNRs provided the feasibility of preparation of SERS (surface enhanced Raman scattering) nanocomposite within shorter time by a subsequent electrochemical etching.

3D microstructuring in p-GaAs with proton beam writing using multiple ion fluences

We report on a new method of three-dimensional structuring by means of proton beam writing in p-type gallium arsenide. While up to now vertical features have been created by varying the proton beam energy during irradiation which changes the proton penetration depth and thereby the depth of the material modification, we manufactured 3D structures with a single beam energy but different proton doses supplemented by a subsequent controlled electrochemical etching process. This new approach could simplify 3D structuring in semiconductors and the usage of proton beam writing for the manufacturing of micro electromechanical devices with high aspect ratios and smooth sidewalls.

High-aspect ratio microstructures in p-type GaAs and InP created by proton beam writing

With proton beam writing (PBW) and subsequent electrochemical etching in HF-solution the creation of high-aspect ratio microstructures in p-type InP was performed for the first time. Microstructures with high surface quality as well as high-aspect ratio possessing lateral dimensions down to 1μm were produced. Furthermore, free-standing microstructures were created in this material by a combined irradiation with 2.25MeV protons and 1.125MeV H2+ molecules, were the smallest structure dimension of 0.6μm was achieved for a horizontal needle. The creation of nearly perfect circular microstructures indicates that the crystal structure has little effect on the structuring process by PBW in this material. Moreover, the effect of reduced etching inside of closed irradiation patterns, already known from Si and GaAs, was observed also in InP. In further PBW experiments and subsequent electrochemical etching with KOH-solution p-type GaAs microstructures were produced. By using a 4-fold higher etch current density of 45mA/cm2 compared to former PBW experiments on this material the quality of the microstructures could be improved significantly leading to high aspect-ratio structures with minimum lateral sizes of ∼1μm, nearly vertical side walls as well as circular microstructures. This shows the reduced influence of the crystal structure on the shape of the microstructures compared to experiments with lower etch current density where only flat microstructures with inclined side walls determined by the crystal structure could be created.

Microstructure of the mechanically alloyed and electrochemically etched Ti–6Al–4V and Ti–15Zr–4Nb nanocrystalline alloys

Formation of the Ti–6Al–4V and the Ti–15Zr–4Nb porous nanocrystalline alloys was described. The alloys were prepared by mechanical alloying (MA) followed by pressing, sintering and subsequent anodic electrochemical etching in 1 M H 3PO 4 + 2% HF electrolyte at 10 V for 30 min. The microstructure was investigated by XRD, TEM and SEM. We found that the mechanically alloyed powders are composed from nanostructure grains, whose size increases during the high temperature sintering. The final ultrafine grains improve subsequent etching process. The electrolyte penetrates sinters through the grain boundaries, resulting in effective grains removing and pores formation. The porosity of the Ti–15Zr–4Nb alloy is larger than that of the Ti–6Al–4V alloy. The macropore diameter reaches up to 40 μm, while the average micropore size is in the range from 3.5 to 5.46 nm for the Ti–15Zr–4Nb and the Ti–6Al–4V electrochemically etched alloys, respectively. The prepared porous ultrafine alloys could be possible candidate for a hard tissue implant applications, where the pores in the surface improve bonding of the implant with bone.

An all-silicon, single-mode Bragg cladding rib waveguide

In this paper, we demonstrate a direct method of fabricating an all-silicon, single-mode Bragg cladding rib waveguide using proton beam irradiation and subsequent electrochemical etching. The Bragg waveguide consists of porous silicon layers with a low index core of 1.4 that is bounded by eight bilayers of alternating high and low refractive index of 1.4 and 2.4. Here, the ion irradiation acts to reduce the thickness of porous silicon formed, creating an optical barrier needed for lateral confinement. Single-mode guiding with losses as low as approximately 1 dB/cm were obtained for both TE and TM polarization over a broad range of wavelengths from 1525 nm to 1625 nm. Such an approach offers a method for monolithic integration of Bragg waveguides in silicon, without the need for multiple processes of depositing alternating materials.

Suppression of Pores Formation on a Surface of p-Si by Laser Radiation

The influence of strongly absorbing N¬2 laser radiation on pores formation on a surface of Si single crystal has been investigated using optical microscope, atomic force microscope and photoluminescence. After irradiation by the laser and subsequent electrochemical etching in HF acid solution morphological changes of the irradiated parts of a surface of Si were not observed. At the same time, pores formation on the non-irradiated parts of Si surface took place. The porous part of the Si surface is characterized by strong photoluminescence in red part of spectra with maximum at 1.88 eV and intensity of photoluminescence increases with current density. Suppression of the pores formation by the laser radiation is explained with inversion of Si type conductivity from p-type to n-type. This fact is explained by Thermogradient effect – generation and redistribution of the intrinsic defects in gradient of temperature. It was shown that the depth of n-Si layer on p-Si substrate depends on intensity of laser radiation and it increases with intensity of laser radiation. The results of the investigation can be used for optical recording and storage of information on surface of semiconductors.

Preparation and properties of mechanically alloyed and electrochemically etched porous Ti–6Al–4V

Formation of porous Ti–6Al–4V nanostructure biomaterial was described. The alloy was prepared by mechanical alloying followed by pressing, sintering and subsequent anodic electrochemical etching in 1 M H 3PO 4 + 2% HF electrolyte at 10 V for 30 min. Mechanically alloyed Ti–6Al–4V has nanostructure with grain size of about 35 nm and large grain boundaries volume fraction, which essentially improve etching process. The electrolyte penetrates sintered compacts through the grain boundaries, resulting in effective material removing and pores formation. The pore diameter reaches up to 60 μm, which is very attractive for strong bonding with bone. The anodization of the microcrystalline alloy ingot results in selective etching, revealing of the two-phase structure with relatively flat surface. The corrosion properties were investigated in Ringer’s solution. Mechanically alloyed samples shows worse corrosion resistance than the bulk microcrystalline alloy ingot, but electrochemical etching results in improving corrosion resistance.

Fabrication of microstructures in III–V semiconductors by proton beam writing

While the creation of microstructures in Si and GaAs by irradiation with high energy protons and helium ions and subsequent electrochemical etching has already been reported, the creation of microstructures in InP using this technique is investigated here for the first time. It is demonstrated that proton irradiation of Fe-doped semi-insulating InP(0 0 1) leads to an amplified material removal in the irradiated areas during electrochemical etching in an HF solution. By this way microstructures were produced using a 2.25 MeV proton beam focused down to ∼1 μm. The depth of the etched structures depends on the etching procedure. In addition, amplified etching of non-irradiated material encircled by closed irradiated patterns was observed. Furthermore, irradiation with helium ions leads to the formation of thin freestanding InP layers by simultaneous underetching as well as etching from the sample surface of the irradiated regions. In contrast to the semi-insulating material, the irradiation and electrochemical etching of n-type InP results in an amplified formation of porous material which partly leads to a complete removal of the material in the irradiated areas. A comparison between GaAs and InP concerning the dissolution characteristics shows mainly a similar behavior of the semi-insulating types of these semiconductors and of the n-type materials.

Creation of GaAs microstructures using the nuclear nanoprobe LIPSION

We report on experiments concerning the fabrication of microdimensional structures in p- and n-type GaAs with different orientations using focused beams of high-energy protons and He+ ions and subsequent electrochemical etching. Depending on type and crystal orientation, the irradiated regions show a different etching behaviour resulting in different microstructures despite similar irradiation patterns. Single spot irradiation in (1 0 0) p-type GaAs results in the creation of small, elongated bumps while in ()-oriented, n-type material, a patterned formation of porous material was initiated by the irradiation. The effect of reduced etching of unirradiated material encircled by closed irradiated patterns up to a size of 25 µm, already known from Si, could be observed in GaAs as well. Furthermore, the creation of three-dimensional multilevel structures in (1 0 0)-oriented GaAs is demonstrated.

Submicron structural alteration of polycarbonate surface due to ArF laser irradiation at high doses and the subsequent electro-chemical etching treatment

Excimer lasers at short UV wavelengths create regular surface microstructure on polycarbonate (PC). The effect is strongly dependent on the laser properties such as pulse energy, laser wavelength and energy dose. Among those, the UV dose is dominant. Moreover, the PC microstructure consists of a network of micro cones, whose size and population change drastically at higher UV doses. The surface hardening at lower doses and the laser ablation at higher doses are investigated to explain the phenomenon. The conical microstructure was subsequently treated using an electro-chemical etching process which turns it into an array of micro holes due to the simultaneous chemical etching and electrical breakdown.

Excimer laser beam profile recording based on electrochemical etched polycarbonate

There is no polymeric detector used to register the beam profile of UV lasers. Here, a method is proposed for the measurement of intensive UV beam pattern of the excimer lasers based on the photoablated polycarbonate detector after coherent UV exposure and the subsequent electrochemical etching. UV laser induced defects in the form of self-microstructuring on polycarbonate are developed to replicate the spatial intensity distribution as a beam profiler.

Electrochemical trench etching of silicon triggered via mechanical nanocontacts

We report a method to produce microstructures on silicon wafers using a microscratching technique followed by a subsequent electrochemical trench etching in hydrofluoric-based electrolyte. Micro-scratches are used to trigger macropore formation. We show that mask-less dissolved trenches with aspect ratios up to 1:7 are formed at the scratched regions on (0 0 1)Si surface. The micro-scratches orientate the macropores formation by aligning them in the scratching direction. We propose that dislocations formed during scratching are firstly dissolved leading to the formation of V-shape grooves. The V-shape geometries obtained by this way are used to initiate the macropores nucleation; i.e. due to the geometry, an avalanche current occurs at the grooves base and thus induces local dissolutions of the substrate. High rate local dissolutions are achieved by back-side illumination of the Si wafer.

Electrochemical Preparation of Porous Copper Surfaces in Zinc Chloride-1-ethyl-3-methyl Imidazolium Chloride Ionic Liquid

The preparation of porous copper or copper-zinc surfaces by electrochemical formation of binary alloys on Cu substrate and subsequent electrochemical etching of the zinc was investigated in a zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquid at . Cyclic voltammetry and X-ray diffraction measurements suggested that phase transformation from γ- to β′- alloy occurred during constant potential dealloying. Essentially all the Zn content in the could be removed from the alloy with dealloying at a sufficiently positive potential. Dealloyed materials exhibited well-developed bicontinuous porous structure. The dependence of the surface morphology of the porous Cu film on several experimental parameters, including deposition current and charge, and anodizing potential and temperature, were examined.

Micromachining of Aluminum by Anodizing, Laser Irradiation, and Electrochemical Etching

Microgrooves were fabricated on aluminum surfaces using anodizing, laser irradiation, and electrochemical etching. Aluminum specimens covered with porous type oxide films were irradiated with a pulsed Nd-YAG laser in distilled water to remove the anodic oxide films locally. The subsequent electrochemical etching in CH3COOH / HClO4 solution made the grooves formed by the laser irradiation smoother. The width and the depth of the grooves increased with electrochemical etching time. Mesh-like micro-patterns with 70 µm width and 30 µm depth were fabricated on aluminum surfaces by consecutive processes of anodizing, laser irradiation, and electrochemical etching.

Cross-linked PMMA on Porous Silicon: An Effective Nanomask for Selective Silicon Etching

Combining a porous silicon (PS) buffer layer with cross-linked poly(methyl methacrylate) (PMMA), we have obtained masks that show high resistance to the electrochemical process usually employed to produce PS. PMMA is normally dissolved in an HF/ethyl alcohol mixture, but it becomes resistant to such solution after cross-linking of the polymer. This can be achieved by high-dose electron irradiation, obtaining a mask for the subsequent electrochemical etching. Due to the strong electric field across the masking layer during the electrochemical process, the time duration of such a mask is limited. We demonstrate that the presence of a highly porous silicon thin film lying under the resist is sufficient for an evident improvement of the masking power during the electrochemical process, allowing for the selective formation of thick PS regions. Furthermore, a final PS removal in alkaline solution leads to the formation of silicon micro- and nanostructures in relief, such as microtips and nanomolds.

Three-dimensional micromachining of silicon using a nuclear microprobe

We describe a novel technique for silicon microfabrication based on energetic mega-electron-volt (MeV) helium irradiation and subsequent electrochemical etching. The ion-induced damage in the irradiated regions slows down the porous silicon formation during electrochemical etching, producing a raised microstructure after cleaning in diluted potassium hydroxide solution. The thickness of the porous silicon layer formed depends on the accumulated fluence at each scan point. A relationship between the irradiated fluence and feature height is investigated on a p-type [1 0 0] silicon with a resistivity of 0.03 Ω cm using focused 2 MeV helium beam. We use this relationship to micromachine multilevel structures with a single focused helium beam energy.