Metal-assisted Chemical Etching Research Articles

Cicada wing-inspired artificial nanopatterns with antifouling properties for clay adhesion

To clarify the antifouling properties of cicada-inspired nanostructured surfaces and search for a long-term usable antifouling material, the wings of two types of cicadas in Japan, Graptopsaltria nigrofuscata (GN) and Cryptotympana facialis (CF), were examined by surface wettability and surface free energy. Furthermore, considering the complexity and mechanical stability of natural nanostructured surfaces, we mimicked both cicada wing surfaces by combining nanosphere lithography and metal-assisted chemical etching (MacEtch) on Si wafer, which is low-cost and large-area pattern-fabricable. We obtained artificial nanopatterns with pillar pitches of 200, 500, and 1000 nm (naming 200, 500, and 1000 series). As a result, comparing artificial nanopatterns with cicada wings indicated that nanopillar structures with high roughness factors or structural aspect ratios are likely to have excellent hydrophobic properties and antifouling features. Cryptotympana facialis wings and 200 series Si samples showed less clay particle adhesion than other patterns. These results indicate smaller structured surfaces where air may get trapped, resulting in hydrophobic surfaces contributing to self-cleaning or fouling-release properties. Further, reducing the contact area between nanostructures and clay can be used as an effective strategy to prevent clay adhesion has been proposed, but it has targeted limitations. Herein, we present a novel insight that more fine and high-structural-density nanostructural patterns are universal to prevent clay adhesion.

Enhancement of photothermal effect using a hierarchical plasmonic structure

The photothermal effect exhibited by plasmonic structures has garnered considerable attention because of its versatile application prospects. However, the conversion efficiency of broadband light sources and large area fabrication still pose challenges. Herein, a hierarchical plasmonic structure was proposed as a photothermal substrate, which integrated a two-dimensional periodic microstructure with a disordered nanostructure. The microstructure and nanostructure were obtained through litho-etching and metal-assisted chemical etching technologies, respectively. Titanium nitride was deposited on the hierarchical structure as the mediator between the irradiation and generation of heat. The metallic nanostructure on the hierarchical structure played a crucial role in generating the thermoplasmonic effect. Moreover, the type (vertical and tapered shapes) of the microstructure and morphology (porous texture and deep trench) of the nanostructure were also altered to investigate the photothermal effect. The addition of deeper nanotrenches on the tapered microstructure exhibited a synergistic effect on the photothermal efficiency. The hierarchical plasmonic structures were experimentally demonstrated to exhibit a 3.6 times greater photothermal efficiency (change in temperature) compared to a titanium nitride film. These engineered hierarchical structures have remarkable potential for use as non-classical photothermal devices.

Ag-Coated Si nanowire arrays: A new route for the precise detection of fungicides by surface enhanced Raman scattering

In this study, we present an efficient method based on vertically oriented silicon nanowire arrays coated with silver nanoparticles to achieve significantly surface enhanced Raman scattering (SERS) signals. The method is applicable for the accurate detection of fungicides and related compounds. The substrates were obtained by a two-step process consisting of the growth of Si nanowire arrays on a silicon wafer by metal-assisted chemical etching and subsequent coating with Ag nanoparticles by the sputtering method. The effect of sputtering time on the growth of nanowires, their morphology and enhancement factor of the signals obtained from the SERS sensor were studied in detail. The final enhancement factor of Raman shift is 1.1 × 107, allowing the limit of trace detection for carbendazim to be as low as 0.1 ppm, with a relative standard deviation of less than 2.07%. This result opens up the potential for direct application of prepared substrates in ultra-fast chemical analysis.

Microarray biochip fabricated on silicon nanowires/carbon dots heterostructures for enhanced viral DNA detection

Recent studies have shown that human oncoviruses are responsible for 10% of cancer cases worldwide. While PCR assays are highly sensitive and specific, microarrays offer outstanding performance for detecting multiple targets simultaneously. However, in microarray systems, accurate detection of DNA viruses relies on the type of microarray platform, while obtaining high-quality, fluorescent-labelled single-stranded DNA bares an equal contribution.Our study demonstrates a proof-of-concept of a microarray platform for enhanced DNA detection. First, we assembled 3D microarray chips on silicon nanowires (SiNWs) substrates using a one-step metal-assisted chemical etching (1-MACE) technique, coated them with SU-8 polymer, and ornated with carbon quantum dots (CDs) in either poly(diallyldimethylammonium) PDDA or poly(ethyleneimine) PEI solutions. Second, we obtained high-quality labelled HPV 16 target ssDNA through classic PCR and asymmetric PCR with a nested primer labelled with Cy5, then hybridised the ssDNA on the SiNWs/SU-8/CDs platforms.The fluorescent signal of hybridised DNA was strongest on the SiNWs/SU-8 substrates, and adding CDs further enhanced the signal. The best signal intensities (4.303, 3.7) and coefficients of variation (3.12%, 2.59%) were achieved by combining HPV 16 probes co-immobilised with CDs functionalised with PDDA. Our study conclusively demonstrates the vast potential of this microarray platform for improved DNA detection.

Etching-Induced Longitudinal Phase Inhomogeneity in Fractal Silicon: Identification through Depth-Profiled Raman Mapping

The coexistence of intermediate amorphous-like and nanocrystalline phases in an etched silicon wafer fabricated using metal-assisted chemical etching is being reported here. In addition, the long-pending origin of amorphous-like features in the Raman spectrum from porous silicon is also being established using spatial Raman spectroscopy and validated using longitudinal Raman spectro-microscopy. Raman spectro-microscopy data have been used here as a bridge to get the complete picture about the presence of coexisting a-Si/nanocrystalline Si phases, about which SEM, TEM, and XRD provide information at different scales. Raman spectra do not show any variation spatially when taken from different parts on the surface, whereas when recorded from different depths by defocusing the laser, they show amorphous and nanocrystalline phases in different proportions. Quantification of the disorder present in terms of medium- or short-range order and estimation of the nanocrystalline size have been done using a modified phonon confinement model and bond polarizability model. Overall, a consolidated picture using three different extraordinary techniques, XRD, SEM, and TEM, that probe microstructures at different scales has been analyzed in unison with Raman spectro-microscopy to reveal structural variation in fractal Si obtained because of porosification.

Reduction of Nitroaromatics by Gold Nanoparticles on Porous Silicon Fabricated Using Metal-Assisted Chemical Etching.

In this study, we investigated the use of porous silicon (PSi) fabricated using metal-assisted chemical etching (MACE) as a substrate for the deposition of Au nanoparticles (NPs) for the reduction of nitroaromatic compounds. PSi provides a high surface area for the deposition of Au NPs, and MACE allows for the fabrication of a well-defined porous structure in a single step. We used the reduction of p-nitroaniline as a model reaction to evaluate the catalytic activity of Au NPs on PSi. The results indicate that the Au NPs on the PSi exhibited excellent catalytic activity, which was affected by the etching time. Overall, our results highlighted the potential of PSi fabricated using MACE as a substrate for the deposition of metal NPs for catalytic applications.

CMOS-compatible electrochemical nanoimprint: High throughput fabrication of ordered microstructures on semiconductor wafer by using a glassy carbon mold

Metal assisted chemical etching (MACE), as an emerging wet chemical etching method for fabricating semiconductor micro/nano-structures (MNS), has attracted wide attentions because of the advantages of simple processes, high accuracy and no damages. However, the conventionally used noble metal catalysts (such as Au, Pt,) trapped in the micro/nano-structures are difficult to be removed or reused, not only harming the electronic performances of the CMOS devices but also increasing the fabrication cost. Here, we propose an electrochemical nanoimprint strategy by using a glassy carbon (GC) imprint mold. To overcome the poor electrocatalytic activity of GC, the GC imprint mold was connected to a Pt sheet in the cathodic chamber. To avoid the contaminations of metal and metallic cations in the fabricated MNS, the cathodic chamber was separated from the anodic one by a proton exchange membrane. By optimizing the technical parameters, including the contact pressure between GC imprint mold and GaAs wafer, the concentration of electron acceptor in the cathodic chamber, the imprinting time, etc., a remarkable corrosion rate of about 130 nm/min was achieved, comparable with that of platinum electrocatalyst. Furthermore, by using a GC imprint mold in the spatially separated corrosion cell, the contaminations caused by both the metal catalysts and metallic ions can be eliminated. Thus, this work provides a potential approach to the high throughput fabrication of CMOS-compatible semiconductor microdevices.

Construction of Si nanowires/ZnFe2O4/Ag photocatalysts with enhanced photocatalytic activity under visible light and magnetic field

The Si nanowires/ZnFe2O4/Ag composite photocatalyst is prepared by metal-assisted chemical etching and hydrothermal methods. Photoelectric conversion and photodegradation experiments show that the composite structure offers high photocatalytic performance under visible light. The photocurrent density of Si nanowires/ZnFe2O4/Ag is 1.76 mA/cm2, and the Methyl orange photodegradation rate is 19 % within 90 min. Furthermore, the photocatalytic activity of composite photocatalysts was studied under the magnetic field, which further improves the photocatalytic efficiency significantly. Energy band structure studies revealed that the interfacial contact in the Si nanowires/ZnFe2O4/Ag increases carrier transfer and charge separation, thereby improving photocatalytic performance. It can be seen from the I-V results that the Si nanowires/ZnFe2O4/Ag has a negative magnetoresistance. Therefore, in the presence of a magnetic field, photogenerated carriers can be more effectively transferred to the electrode surface to participate in the photocatalytic reaction with improved photocatalytic performance.

Comparing metal assisted chemical etching of N and P-type silicon nanostructures

Comparing metal assisted chemical etching of N and P-type silicon nanostructures

Impacts of Wafer Doping Type on Structural and Optical Properties of Black Silicon Fabricated by Metal-Assisted Chemical Etching

In this work, the impacts of wafer doping type on structural and optical properties of black silicon (b-Si) fabricated by metal-assisted chemical etching (MACE) process are investigated. P-type and n-type mono-crystalline silicon (mono c-Si) wafers are etched in an aqueous solution of hydrofluoric acid (HF), silver nitrate (AgNO3) and deionised water (DI H2O) at room temperature and various durations from 5-20 minutes. Surface morphological results demonstrate the formation of b-Si nanowires (NWs) with average lengths of 0.4-0.8 μm for p-type wafers and 0.8-3.0 μm for n-type wafers. The higher length of the NWs for the n-type wafers is due to the minority charge carriers, which lead to a higher etching rate during the MACE process. Within the 300-1100 nm wavelength region, weighted average reflection (WAR) for the p-type and n-type wafers decreases to 6.6% and 6.4%, respectively, after 20 minutes of etching. The corresponding improvement in broadband light absorption results in maximum potential short-circuit current density (Jsc (max)) of 38.2 and 38.8 mA/cm2 for the p-type and n-type b-Si, respectively, which is an of enhancement of 39.9% and 42.1% when compared to the Jsc (max) of planar c-Si reference.

Free-standing nanowire printed surfaces with high variability in substrate selection for boiling heat transfer enhancement

Nanowires (NWs) constitute a promising solution to the overheating of high-heat-load surfaces in multiphase heat transfer. Typical nanostructure fabrication methods such as complex photolithography and metal-assisted chemical etching (MACE) that utilize patterned templates are limited by high manufacturing costs and type of substrate materials. These limitations significantly hinder the industrial applicability of NW structures in high heat flux units. In this study, we first examine the effect of the morphology of Zinc oxide (ZnO) NW printed substrates, fabricated via microcontact printing (µCP) and solution based growing, on the heat transfer characteristics in pool boiling. Unlike advanced photolithography, µCP offers greater variability and convenience in terms of the substrate selection and large area creation for NW fabrication. Compared with the silicon substrate, the underlying mechanisms for enhancing both the critical heat flux and heat transfer coefficient of ZnO NW substrates are explored by analyzing the surface morphology, bubble, and wicking characteristics of the test surfaces, in order to determine the applicability of boiling heat transfer using ZnO NWs in industrial fields.

Localization of the E. coli Dps protein molecules in a silicon wires matrix according to scanning electron microscopy and X-ray photoelectron spectroscopy

The work is related to the study of the morphological features of silicon wires arrays combined with a nanomaterial of natural origin, a bacterial ferritin-like protein Dps, and their relationship with the composition of the surface and interior. A silicon wires array was formed by metal-assisted wet chemical etching. To obtain recombinant protein, Escherichia coli BL21*(DE3) cells were used as producers, and purification was carried out by the chromatography method. The combination of silicon wires with protein molecules was carried out by layering under laboratory conditions, followed by drying. The resulting hybrid material was studied by scanning electron microscopy and X-ray photoelectron spectroscopy. The initial silicon wires array had sharp boundaries on the surface. The diameter of the silicon wires was about 100 nm, while the distances between the wires can vary widely, reaching several hundred nanometres or be less than 100 nanometres, depending on the formation conditions, in the absence of noticeable transition layers. The pores formed in this way are available for filling with protein during deposition. The effectiveness of using the scanning electron microscopy method to study the morphology of the hybrid material “silicon wires – bacterial protein Dps” as well as X-ray photoelectron spectroscopy method together with ion etching for the investigation of the composition and physico-chemical of the hybrid material was demonstrated. Complementary results have shown that the molecular culture, which is a solution of oligomers of the recombinant Dps protein of E.coli bacterial cells, can penetrate deep into the pores of the silicon wires array with an extremely developed surface. The possibility of the control of the filling of silicon wires arrays by varying the pore morphology and other modes of formation of structuresand their surface has been demonstrated. The obtained data can be used to study the possibilities of the functionalization of the developed surface of silicon wires by their driven coating with controlled delivery of biohybrid material.

Bubble Effects on Manufacturing of Silicon Nanowires by Metal-Assisted Chemical Etching

Abstract Micro/nano-textured Si wafers manufactured using metal-assisted chemical etching (MACE) have been the focus of several studies, but the mechanism of bubble generation during the MACE process affecting textured surfaces has rarely been reported. This study investigated the bubble effect due to the different placement patterns of the Si wafer (face-up, stirred face-down, and face-down). The results indicated that the placement pattern of the Si wafer notably influences the uniformity of outward appearance. At 2 h of etching, the outward appearance uniformity of face-up etching was more homogeneous than that of stirred face-down and face-down patterns, and the Si nanowires (SiNWs) processed through face-up etching were longer (41 μm) than those subjected to stirred face-down etching (36 μm) and face-down etching (32 μm). Therefore, the placement pattern of Si wafer can affect the uniformity and properties of SiNWs because of bubbles trapped inside cavities or between SiNWs.

SERS Nanowire Chip and Machine Learning-Enabled Classification of Wild-Type and Antibiotic-Resistant Bacteria at Species and Strain Levels.

Antimicrobial resistance (AMR) is a major health threat worldwide and the culture-based bacterial detection methods are slow. Surface-enhanced Raman spectroscopy (SERS) can be used to identify target analytes in real time with sensitivity down to the single-molecule level, providing a promising solution for the culture-free bacterial detection. We report the fabrication of SERS substrates having tightly packed silver (Ag) nanoparticles loaded onto long silicon nanowires (Si NWs) grown by the metal-assisted chemical etching (MACE) method for the detection of bacteria. The optimized SERS chips exhibited sensitivity down to 10-12 M concentration of R6G molecules and detected reproducible Raman spectra of bacteria down to a concentration of 100 colony forming units (CFU)/mL, which is a thousand times lower than the clinical threshold of bacterial infections like UTI (105 CFU/mL). A Siamese neural network model was used to classify SERS spectra from bacteria specimens. The trained model identified 12 different bacterial species, including those which are causative agents for tuberculosis and urinary tract infection (UTI). Next, the SERS chips and another Siamese neural network model were used to differentiate AMR strains from susceptible strains of Escherichia coli (E. coli). The enhancement offered by SERS chip-enabled acquisitions of Raman spectra of bacteria directly in the synthetic urine by spiking the sample with only 103 CFU/mL E. coli. Thus, the present study lays the ground for the identification and quantification of bacteria on SERS chips, thereby offering a potential future use for rapid, reproducible, label-free, and low limit detection of clinical pathogens.

Photoelectrocatalysis Synthesis of Ammonia Based on a Ni-Doped MoS2/Si Nanowires Photocathode and Porous Water with High N2 Solubility

The synthesis of ammonia through photocatalysis or photoelectrochemistry (PEC) and nitrogen reduction reaction (NRR) has become one of the recent research hotspots in the field, where the catalyzed materials and strategies are critical for the NRR. Herein, a Ni-doped MoS2/Si nanowires (Ni-MoS2/Si NWs) photocathode is prepared, where the Si NWs are formed on the surface of a Si slice by the metal-assisted chemical etching method, and the hydrothermally synthesized Ni-MoS2 nanosheets are then cast-coated on the Si NWs electrode. Porous water with high solubility of N2 is prepared by treating a hydrophobic porous coordination polymer with hydrophilic bovine serum albumin for subsequent aqueous dispersing. The relevant electrodes and materials are characterized by electrochemistry, UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller method, and zeta potential method. The uses of the Ni-MoS2/Si NWs photocathode and the porous water with high nitrogen solubility for PEC-NRR give a yield of NH3 of 12.0 mmol h-1 m-2 under optimal conditions (e.g., at 0.25 V vs RHE), and the obtained apparent Faradaic efficiency higher than 100% is discussed from the inherent photocurrent-free photocatalysis effect of the photoelectrodes and the suggested classification of three kinds of electrons in PEC, which may have some reference value in understanding and improving other PEC-based processes.

Heterogeneous Photocatalysis using Electroless Deposition of Ni/NiO Nanoparticles on Silicon Nanowires for the Degradation of Methyl Orange

Aims: This work uses the MACE method to synthesize SiNWs- NiNPs/NiONPs to degrade organic pollutants by photocatalysis. Background: Photocatalytic degradation has been applied as an attractive solution to remove several organic pollutants. Heterostructured nanomaterials have become an interesting platform for investigation. Metal-assisted chemical etching (MACE) stands out as a promising technique because it is simple, low cost, and fast. Objective: Attain the degradation of methyl orange (MO) in the presence of silicon nanowires (SiNWs) in heterojunction with Nickel/Nickel Oxide nanoparticles (NiNPs-NiONPs). Methods: SiNWs were synthesized by metal (Ag) assisted chemical etching (MACE) of monocrystalline silicon wafers. NiNPs were non-electrolytically deposited on the SiNWs (electroless method). The morphology of the SiNWs- NiNPs/NiONPs was observed by SEM. Results: Heterogeneous photocatalytic degradation of methyl orange (C14H14N3NaO3S) in an aqueous solution at a concentration of 20 ppm had an efficiency of 66.5% after 180 min under UV irradiation. The MO degradation percentage was determined using UV-visible spectrophotometry. Conclusion: The SiNWs-NiNPs/NiONPs were obtained composed mainly of Si covered by SiO2 decorated on the tips with Ni (II) in the form of NiO and a small amount of nickel metal. The removal efficiency obtained at 180 min of light exposure was 66.5%. After the photocatalysis tests, further oxidation of the NiNPS into NiONPS, was attributed to the reactive oxygen species in the aqueous medium based on the changes of the oxygen and Ni2p3/2 peaks by XPS. Other: Through XPS, the oxidation state of the SiNWs- NiNPs/NiONPs was analyzed.

Metal-Assisted Etching of n-Type and p-Type Silicon Using Patterned Platinum Films: Spatial Distribution of Mesoporous Layer and Open Circuit Potential of Silicon

Platinum (Pt) is one of the interesting catalysts in metal-assisted etching (metal-assisted chemical etching) of silicon (Si). The Pt-assisted etching induces not only the dissolution of Si under the Pt catalysts but also the formation of mesoporous layer on the Si surface away from them. In this work, we etched n-Si and p-Si by using patterned Pt films with a diameter of 5 μm and an interval of 50 μm. For both the cases, the Si surface under the Pt catalysts was selectively etched and macropores with a diameter of 5 μm were formed. The macropores formed on n-Si were deeper than those formed on p-Si. The mesoporous layer was observed only around the macropores on n-Si, while it was observed over the entire surface of p-Si. We also measured the open circuit potential of Si in the etching solution. The positive shift of potential of n-Si by the Pt deposition was smaller than that of p-Si except for the initial stage of etching, which can be explained by the polarization characteristics. We discussed the etching behavior of n-Si and p-Si on the basis of the results of structure observation and electrochemical measurements.

Effect of High-Temperature Annealing on Raman Characteristics of Silicon Nanowire Arrays

We demonstrate two distinct experimental processes involving the large-area growth of ordered and disordered silicon nanowire arrays (SiNWs) on a p-type silicon substrate using the metal-assisted chemical etching method. The two processes are based on the etching of monocrystalline silicon wafers by randomly distributed Ag films and ultra-thin Au films with ordered nano-mesh arrays, respectively, wherein the growth of SiNWs is implemented using a specific proportion of a HF-containing solution at room temperature. In this study, the microstructural change mechanisms for the two morphologically different arrays before and after annealing were investigated using Raman spectra. The effects of various mechanisms on the observed Raman scattering peak’s deviation from symmetry, redshift and broadening were analyzed. The evolution of the unstable amorphous structures of nanoscale materials during the high-temperature annealing process was observed via high-resolution scanning electron microscope (SEM) observations. The scattering peak parameters determined from the Raman spectra led to conclusions concerning the various mechanisms by which high-temperature annealing influences the microstructures of the two morphologically different SiNWs fabricated on the p-type silicon substrate. Therefore, the deviation of SiNWs from the monocrystalline silicon scattering peak at 520.05 cm−1 when changing the diameter of the nanowire columns was calculated to further analyze the effect of thermal annealing on Raman characteristics.

Fabrication of superhydrophobic surfaces for applications in total internal reflection effects

Superhydrophobic surfaces have attracted significant attention in the applied science community. This paper presents three different methods for fabricating superhydrophobic surfaces, which can be applied in liquid light guides with the total internal reflection effect playing a central role. The selective bi-polymer etching only produces a hydrophobic porous poly(methyl methacrylate) surface but is simple and versatile for use on an arbitrary surface. In metal-assisted chemical etching, wet etching of a silver layer on a silicon sample creates porous silicon with superhydrophobicity, characterized by a water contact angle of 160°. The metal-assisted chemical etching method modified with the presence of polystyrene nanoparticles further improved the water contact angle to 164° by creating a nanopillar silicon structure. The metal-assisted chemical etching methods are more complicated but can produce superhydrophobic surfaces with very high water contact angles. These results show that superhydrophobic surfaces fabricated by methods in this study can be used for total internal reflection effect at the interface between water with huge potential applications in liquid light guides.

Single Layer Lift-Off of CSAR62 for Dense Nanostructured Patterns

Lift-off processing is a common method of pattern transfer for different nanofabrication applications. With the emergence of chemically amplified and semi-amplified resist systems, the possibilities for pattern definition via electron beam lithography has been widened. We report a reliable and simple lift-off process for dense nanostructured pattern in CSAR62. The pattern is defined in a single layer CSAR62 resist mask for gold nanostructures on silicon. The process offers a slimmed down pathway for pattern definition of dense nanostructures with varied feature size and an up to 10 nm thick gold layer. The resulting patterns from this process have been successfully used in metal assisted chemical etching applications.