Combination Of Metal-assisted Chemical Etching Research Articles

Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers.

Metal-assisted electrochemical imprinting (Mac-Imprint) is a combination of metal-assisted chemical etching (MACE) and nanoimprint lithography that is capable of direct patterning 3D micro- and nanoscale features in monocrystalline group IV (e.g., Si) and III-V (e.g., GaAs) semiconductors without the need of sacrificial templates and lithographical steps. During this process, a reusable stamp coated with a noble metal catalyst is brought in contact with a Si wafer in the presence of a hydrofluoric acid (HF) and hydrogen peroxide (H2O2) mixture, which leads to the selective etching of Si at the metal-semiconductor contact interface. In this protocol, we discuss the stamp and substrate preparation methods applied in two Mac-Imprint configurations: (1) Porous Si Mac-Imprint with a solid catalyst; and (2) Solid Si Mac-Imprint with a porous catalyst. This process is high throughput and is capable of centimeter-scale parallel patterning with sub-20 nm resolution. It also provides low defect density and large area patterning in a single operation and bypasses the need for dry etching such as deep reactive ion etching (DRIE).

Dynamic range boosting for electrochemical sensing by morphological optimization of three-dimensional silicon porous framework@Au nanoparticles

Herein we would like to introduce a general method for improving the dynamic range for electrochemical sensing devices based on 3D porous electrodes by morphological optimization. As a showcase, a series of Au nanoparticles integrated three-dimensional silicon-based porous frameworks working electrodes (3D-pSi@AuNP) with the array height ranging between 20 µm and 120 µm are prepared by a combination of metal-assisted chemical etching and surface defects induced isotropic etching in HF/H2O2 mixture and Galvanic displacement, and their electrochemical sensing performance for H2O2 are investigated by voltametric and amperometric methods in detail. The experimental results show that, as the array height of 3D-pSi was increased from 20 µm to 120 µm, the electrochemical active surface area (EASA) was raised by nearly 6 folds, and the upper limit of linear range of detection of H2O2 can be boosted from 4.39 mM to 32.30 mM. With the optimization of Au deposition, it could be further increased to 56.57 mM. We expect this approach to be one of the general methodologies for the improvement of electrochemical sensing devices, especially for those scenarios where the fast detection of high concentration of H2O2 is critical, such as fuel cell and other catalytic oxygen reduction reaction systems.

Anisotropic silicon nanowire arrays fabricated by colloidal lithography.

The combination of metal-assisted chemical etching (MACE) and colloidal lithography allows for the affordable, large-scale and high-throughput synthesis of silicon nanowire (SiNW) arrays. However, many geometric parameters of these arrays are coupled and cannot be addressed individually using colloidal lithography. Despite recent advancements towards higher flexibility, SiNWs fabricated via colloidal lithography and MACE usually have circular, isotropic cross-sections inherited from the spherical templates. Here we report a facile technique to synthesize anisotropic SiNWs with tunable cross-sections via colloidal lithography and MACE. Metal films with an elliptical nanohole array can form from shadows of colloidal particles during thermal evaporation of the metal at tilted angles. The aspect ratio of these anisotropic holes can be conveniently controlled via the deposition angle. Consecutive MACE using these patterned substrates with or without prior removal of the templating spheres results in arrays of anisotropic SiNWs with either elliptical or crescent-shaped cross-sections, respectively. As a consequence of the anisotropy, long SiNWs with elliptical cross-sections preferentially collapse along their short axis, leading to a controlled bundling process and the creation of anisotropic surface topographies. These results demonstrate that a rich library of SiNW shapes and mesostructures can be prepared using simple spherical colloidal particles as masks.

Atomic layer deposition of palladium coated TiO2/Si nanopillars: ToF-SIMS, AES and XPS characterization study

Nanocomposite based on Palladium (Pd) Coated TiO2/Si nanopillars is an attractive candidate for photocatalytic applications because of its outstanding electrochemical and optical characteristics. In this research, Pd/TiO2/Si nanopillars were synthesized by combination of metal-assisted chemical etching and atomic layer deposition, and then the surface was investigated by means of Electron microscopy, Time-of-Flight Secondary Ion Mass-Spectrometry (ToF-SIMS), Auger Electron Spectroscopy (AES) and X-Ray Photoelectron spectroscopy (XPS). The spatial distribution of different chemical components and contaminations on the surface of the produced nanocomposites was evaluated by ToF-SIMS mapping. Depth profiling by AES was carried out to determine the chemical composition and the conformality of Pd and TiO2 layer over the Si pillars. The elemental composition and stoichiometry were determined by XPS analysis. The XPS valence band analysis was performed in order to investigate the modification of TiO2/Si nanopillars electronic structure after Pd deposition. It was found that the Pd coating decreases the concentration of photoactive defects that can reduce the photoelectrochemical efficiency of TiO2.

Large-Scale Synthesis of Highly Uniform Silicon Nanowire Arrays Using Metal-Assisted Chemical Etching.

The combination of metal-assisted chemical etching (MACE) with colloidal lithography has emerged as a simple and cost-effective approach to nanostructure silicon. It is especially efficient at synthesizing Si micro- and nanowire arrays using a catalytic metal mesh, which sinks into the silicon substrate during the etching process. The approach provides a precise control over the array geometry, without requiring expensive nanopatterning techniques. Although MACE is a high-throughput solution-based approach, achieving large-scale homogeneity can be challenging because of the instability of the metal catalyst when the experimental parameters are not set appropriately. Such instabilities can lead to metal film fracture, significantly damaging the substrate and thus compromising the nanowire array quality. Here, we report on the critical parameters that influence the stability of the metal catalyst layer for achieving large-scale homogeneous MACE: etchant composition, metal film thickness, adhesion layer thickness, nanowire diameter and pitch, metal film coverage, Si/Au/etchant interface length, and crystalline quality of the colloidal template (grain size and defects). Our results investigate the origin of the catalyst film fracture and reveal that MACE experiments should be optimized for each Si wire array geometry by keeping the etch rate below a certain threshold. We show that the Si/Au/etchant interface length also affects the etch rate and should thus be considered when optimizing the MACE experimental parameters. Finally, our results demonstrate that colloidal templates with small grain sizes (i.e., <100 μm2) can yield significant problems during the pattern transfer because of a high density of defects at the grain boundaries that negatively affects the metal film stability. As such, this work provides guidelines for the large-scale synthesis of Si micro- and nanowire arrays via MACE, relevant for both new and experienced researchers working with MACE.

Porous Silicon-Zinc Oxide Nanocomposites Prepared by Atomic Layer Deposition for Biophotonic Applications.

In the current research, a porous silicon/zinc oxide (PSi/ZnO) nanocomposite produced by a combination of metal-assisted chemical etching (MACE) and atomic layer deposition (ALD) methods is presented. The applicability of the composite for biophotonics (optical biosensing) was investigated. To characterize the structural and optical properties of the produced PSi/ZnO nanocomposites, several studies were performed: scanning and transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance, and photoluminescence (PL). It was found that the ALD ZnO layer fully covers the PSi, and it possesses a polycrystalline wurtzite structure. The effect of the number of ALD cycles and the type of Si doping on the optical properties of nanocomposites was determined. PL measurements showed a “shoulder-shape” emission in the visible range. The mechanisms of the observed PL were discussed. It was demonstrated that the improved PL performance of the PSi/ZnO nanocomposites could be used for implementation in optical biosensor applications. Furthermore, the produced PSi/ZnO nanocomposite was tested for optical/PL biosensing towards mycotoxins (Aflatoxin B1) detection, confirming the applicability of the nanocomposites.

Combined metal-assisted chemical etching and anisotropic wet etching for anti-reflection inverted pyramidal cavities on dendrite-like textured substrates

Abstract A simple and low-cost method using the combination of metal-assisted chemical etching (MacEtch) and anisotropic wet etching was performed to fabricate anti-reflection inverted pyramidal cavities on dendrite-like textured silicon substrates. To achieve this, a thin Ag film was deposited on an n-type (100) silicon substrate to form agglomerated Ag particles and MacEtch was performed to obtain vertically aligned etching holes on Si substrate. Subsequently, anisotropic wet etching was conducted and the etchant would penetrate the porous structure to form inverted pyramidal cavities on the dendrite-like structure. Using this two-step etching, excellent anti-reflection behavior was obtained for our textured substrates.

Broadband luminescence in defect-engineered electrochemically produced porous Si/ZnO nanostructures

The fabrication, by an all electrochemical process, of porous Si/ZnO nanostructures with engineered structural defects, leading to strong and broadband deep level emission from ZnO, is presented. Such nanostructures are fabricated by a combination of metal-assisted chemical etching of Si and direct current electrodeposition of ZnO. It makes the whole fabrication process low-cost, compatible with Complementary Metal-Oxide Semiconductor technology, scalable and easily industrialised. The photoluminescence spectra of the porous Si/ZnO nanostructures reveal a correlation between the lineshape, as well as the strength of the emission, with the morphology of the underlying porous Si, that control the induced defects in the ZnO. Appropriate fabrication conditions of the porous Si lead to exceptionally bright Gaussian-type emission that covers almost the entire visible spectrum, indicating that porous Si/ZnO nanostructures could be a cornerstone material towards white-light-emitting devices.

A systematic study of the impact of etching time to the sensitivity of SiNW sensor fabricated by MACEtch process

In this paper, silicon nanowires (SiNWs) was fabricated by a combination of metal-assisted chemical etching (MACEtch) and nanosphere lithography. We get the silicon nanowires with different specific surface area by changing the etching time. The microscopic structure of the silicon nanowires is observed by field emission scanning electron microscope (FESEM). The gas sensing performances of the SiNWs with different specific surface area have been systematically examined by measuring the resistance change towards the concentrations of NO2 in the range of 1–5ppm at room temperature (RT, 300K), the gas sensor composed of SiNWs showed perfect gas sensitive property and possessed a short response–recovery time. The main reason of these excellent attributes is quite likely that high specific surface area of the SiNWs, and NO2 sensing mechanism of the SiNWs was also further explained, which can be attributed to the oxygen in the air and detected NO2 extract electrons from the surface of the SiNWs, and the resistivity of SiNWs changed with the changing of space-charge layer under the of SiNWs surface.

Ordered arrays of patterned nanoporous silicon

The fabrication of ordered arrays of patterned nanoporous Si with two-level nanostructures using a combination of metal-assisted chemical etching and nanoimprint lithography is presented. One nanostructure is the periodic arrays of holes and pillars with a period of several hundred nanometers and another nanostructure is the nanoporosity with a pore size of about 10 nm. The etching rate of (1 0 0) oriented Si is clearly higher than that of (1 1 0) and (1 1 1) oriented Si under the same conditions. More effective pore formation occurs in (1 1 1) oriented Si than in (1 0 0) and (1 1 0) oriented Si. Cathodoluminescence was measured and the patterned array of nanoporous Si shows visible luminescence with an overlap of two red emission bands at room temperature.