Metal-assisted Chemical Etching Technique Research Articles

Facile synthesis of RGO/TiO2 heterojunction on SiNWs on Si chip for near infrared detector

Here, a ternary nanostructure-based heterojunction was developed for the detection of infrared (IR) radiation in near wavelength range. During materials synthesis process, silicon nanowires (SiNWs) were developed using metal assisted chemical etching (MACE) technique whereas reduced graphene oxide (RGO) sheets and TiO2 nanoparticles were synthesized by chemical vapour deposition (CVD) and solvothermal method, respectively. Thereafter, RGO/TiO2 nanostructure was deposited over SiNWs to form a heterojunction. To analyse the stoichiometry, morphology, and internal structure, various characterization techniques were performed such as scanning electron microscope (SEM), transmission electron microscope (TEM), Energy dispersive spectrometer (EDS), X-Ray Diffractometer (XRD), UV–Vis spectrophotometer and X-Ray Photoelectron Spectrometer (XPS). The RGO/TiO2@SiNWs nanostructure-based heterojunction was used for the detection of near IR radiation where the responsivity (R) was found to be 17.22 × 10−3 A/W and detectivity (D) was measured to be 0.23 × 1011 Jones with response time < 1 s and recovery time < 3 s. These results were found comparatively better than pristine RGO@Si and RGO@SiNWs nanostructure-based sensors used for the near IR detection under same conditions. The photocurrent response was also recorded with 5 ON/OFF cycles which depicted a good reproducibility of the prepared detector. Hence, the SiNWs on Si chip based highly sensitive and selective photodetector can be used to set a new milestone for various near IR applications.

Passivation-free high performance self-powered photodetector based on Si nanostructure-PEDOT:PSS hybrid heterojunction

Hybrid heterojunction between organic PEDOT:PSS and inorganic Silicon (Si) nanostructures is promising for high-performance self-powered photodetectors due to their favourable band energetics and ease of processing. Traditionally, Si nanostructures require additional passivation layers to reduce surface defect states, which adds complexity and involves the use of harmful organic solvents. Here in, we demonstrate a simple chemical polishing process to reduce defects and fabricate a conformable heterojunction between Si nanostructures and PEDOT:PSS. Si nanostructures fabricated by metal-assisted chemical etching (MACE) technique and subsequently treated by the chemical polishing process are spin-coated with PEDOT:PSS to form heterojunction and employed as self-powered broadband photodetector. The optimized device shows superior performance, such as high responsivity of 555.34 mA/W, quick rise/fall times of 79 ms/81 ms, high External Quantum Efficiency (EQE) of 0.8 at zero bias (0 V) and high photostability up to 500 illumination cycles. Dark I-V characteristics and carrier lifetime measurements reveal that the enhanced performance of chemically polished devices is attributed to the formation of a conformable heterojunction and reduction in defects in the Si nanostructures. Given the scalability and simplicity of the demonstrated passivation-free approach, this work may aid the fabrication of high-performance hybrid Si nanostructured photodetectors.

MoSe2 nanosheets/SiNWs heterojunction-based photocathode for efficient photoelectrochemical water splitting applications

The efficiency of a photoelectrochemical (PEC) device is governed by the solar response of the photoelectrode and kinematics of the hydrogen evolution reaction (HER). In this study, we illustrate a simple and low-cost method for the fabrication of photocathode (PC) via spray coating of MoSe2 nanosheets (prepared by liquid exfoliation) on p-SiNWs (p-type Silicon nanowires (NWs) prepared using metal-assisted chemical etching technique) and studied their integration as a PC for photoelectrochemical-hydrogen evolution reaction performance. The colossal photocurrent density of the resultant p-SiNWs-MoSe2 PC is −14.56 mA cm−2 at −1.2 V, which is appreciably larger in comparison to the bare p-SiNWs. Moreover, the charge transfer resistance (R ct) corresponding to the p-SiNWs-MoSe2 electrode is 339 Ω, which is about two orders of magnitudes lower than that of p-SiNWs is attributed to the high donor density of MoSe2 nanosheets. The improvement in PEC performance is attributed to the strain at the p-SiNWs-MoSe2 interface promoted charge separation and thus reduces the kinetic barrier in water splitting process and enhances the overall HER. These results open up a scalable and effective route for implementing solar-fuel conversion at a large scale.

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.

Cross-fertilized biomimetic structures achieved through nanosphere lithography on an ultrathin wafer for flexible black c-Si SHJ solar cells

The millions of years of history of evolution are practically a history of optimization of all the biological designs and structures so as to cope up with nature and to get the best out of it. These natural designs can be exhaustive sources of readily available solutions for engineering and technological problems. Moth eye-like nanostructured anti-reflection coatings are an innovative concept for the fabrication of advanced and efficient solar cells due to its low reflection attributes when applied on the top of a cell. However, owing to its high surface area and low aspect ratio, the minority carrier life time and wide-angle light collection may be compromised, and these are treated as the major barriers of using such moth eye-like nanostructured anti-reflection coatingsin real cases. To overcome this problem, in this work, it has been aimed to introduce a ‘cross fertilized’ biomimetic structure between the ‘moth eye’ and ‘cacti’, so that maximum light can be utilized without much compromising the electrical properties. Detailed experimental and theoretical analyses on the optical and structural properties of such cross-fertilized nanostructures were carried out, giving emphasis on some crucial parameters like wide angle light collection, unified photon-electron harvesting capabilities, and surface recombination velocity. Finally, practical implementation of this optimized geometry has been carried out by the formation of ‘nano-cone’-based black c-Si structures through a metal-assisted chemical etching technique, followed by the fabrication of black p-c-Si/n-a-Si:H flexible solar cells.

Ag and Cu metal-assisted chemical etching for diamond-wire-sawn single-crystalline silicon solar cell

In the present study, Ag and Cu metal-assisted chemical etching (MACE) technique was used for etching single-crystalline silicon at 15–25 °C, followed by post-processing with the HF/HNO3/H2O solution. Further, the relationship between the shape of the cross-section of the pore and the quality of passivation was deciphered. The parameters Jo1, minority carrier lifetime, and surface recombination velocity were measured, and the correlation between these three parameters and the open-circuit voltage (Voc) value was determined for all solar cells. The changes in the reflectance, solar flux density, and external quantum efficiency with changing wavelength were analyzed, and the relationship of these changes with short-circuit current density was determined. The Voc of all Ag-MACE solar cells is greater than that of all Cu-MACE solar cells, although the efficiency of almost all Cu-MACE solar cells is higher than that of the Ag-MACE solar cells. Furthermore, the electroluminescence image, the adhesion between the silver electrode and the pore, the quality of the aluminum back surface field, the metal residues on the surface, and the bulk of the wafer were systematically investigated. The best solar cell efficiency obtained for Ag-MACE and Cu-MACE PERC structures was 18.68% and 19.32%, respectively.

Fabrication of Black Silicon via Metal-Assisted Chemical Etching—A Review

The metal-assisted chemical etching (MACE) technique is commonly employed for texturing the wafer surfaces when fabricating black silicon (BSi) solar cells and is considered to be a potential technique to improve the efficiency of traditional Si-based solar cells. This article aims to review the MACE technique along with its mechanism for Ag-, Cu- and Ni-assisted etching. Primarily, several essential aspects of the fabrication of BSi are discussed, including chemical reaction, etching direction, mass transfer, and the overall etching process of the MACE method. Thereafter, three metal catalysts (Ag, Cu, and Ni) are critically analyzed to identify their roles in producing cost-effective and sustainable BSi solar cells with higher quality and efficiency. The conducted study revealed that Ag-etched BSi wafers are more suitable for the growth of higher quality and efficiency Si solar cells compared to Cu- and Ni-etched BSi wafers. However, both Cu and Ni seem to be more cost-effective and more appropriate for the mass production of BSi solar cells than Ag-etched wafers. Meanwhile, the Ni-assisted chemical etching process takes a longer time than Cu but the Ni-etched BSi solar cells possess enhanced light absorption capacity and lower activity in terms of the dissolution and oxidation process than Cu-etched BSi solar cells.

Semi-insulating GaAs surface modifications and their influence in the response of THz devices

In this work, we use the Metal-Assisted Chemical Etching (MACE) technique together with an NH3 plasma treatment using a Remote Plasma-Enhanced Chemical Vapor Deposition (RPECVD) equipment to modify the surface of semi-insulating GaAs (SI-GaAs) substrates. Using our modified SI-GaAs, we fabricate photoconductive antennas for THz emission to study the modification's influence on the optical and electrical antenna response. We found substantial gaps in the frequency-domain spectra of the devices, whose origin could reside in a change of the substrate's optical absorption due to: (a) a surficial chemical change or (b) a substrate topographic modification or a combination of both. We showed experimentally that slight and reproducible substrate surface modifications could lead to significant variations in the electrical behavior and THz response of SI-GaAs devices.

Hydrogen sensing behaviour of platinum and palladium functionalized silicon nanowalls

In a chemi-resistive gas sensor, the sensing properties are essentially improved through surface functionalization. In the present work, silicon nanowalls (SiNWs) were carved on silicon wafers by metal-assisted chemical etching technique. Over the SiNWs, platinum (Pt) and palladium (Pd) nanoparticles were deposited through a fluoride-free galvanic displacement deposition technique. The surface-decorated SiNWs were then evaluated for their sensing behaviour towards hydrogen (H2) gas. The morphological investigation revealed vertically aligned SiNW architecture with Pt and Pd nanoparticles sprinkled over the wall-tips. The crystallographic analysis indicated that the SiNWs has the original single-crystalline nature of the silicon wafer even after surface modification. Comparison of H2 gas sensing efficacies indicates an ~ 8 and ~ 4-fold corresponding increase in Pd and Pt functionalized SiNWs, at 200 °C, with respect to pristine SiNWs. A plausible sensing mechanism is suggested with a suitable parallel resistance model. The combinatorial effect of single-crystalline SiNWs and catalytic properties of Pt and Pd resulted in enhanced sensing characteristics. Moreover, the economically scalable aptitude of the fabrication technique makes them suitable for real-time and facile industrial applications.

Near Room Temperature Sensing by In₂O₃ Decorated Silicon Nanowires for Sensitive Detection of Ethanol

The role of indium trioxide (In2O3) decorated Si nanowires (SiNWs) based resistive sensor for selective detection of ethanol vapors at near room temperature has been successfully demonstrated. SiNWs samples were synthesized using metal assisted chemical etching technique and these were decorated by a thin film of indium followed by annealing. The sensing response was captured by measuring the change in resistance of the sensing layer using a Cr-Au inter-digitated-electrode (IDE) structure formed on top of the sensing layers. All sensors were tested for ethanol, acetone, iso-propanol (IPA), xylene, benzene and toluene vapours in the wide concentration range of 5–500 ppm and at different temperatures. Sensors based on SiNWs alone had displayed higher response towards acetone vapours whereas after heterojunction formation with In2O3, significant sensitivity to ethanol was depicted. In2O3 decorated SiNWs resulted in significant enhancement of the sensor response% towards ethanol at near room temperature. Minimum detection of ethanol at 50 ppm and 10 ppm was portrayed by SiNWs and In2O3/SiNWs based sensors respectively. It was concluded that sensing behaviour was a consequence of combinatory effect produced by the presence of both SiNWs and In2O3. A simple explanation with device schematic and band diagrams of the material are proposed to describe the sensing mechanism. This study demonstrates the significance of surface treatment of SiNWs and the role of heterostructures for tuning the sensing properties and development of wafer scalable sensors.

Silicon nanowires/reduced graphene oxide nanocomposite based novel sensor platform for detection of cyclohexane and formaldehyde

The present work provides a new reduced graphene oxide (rGO) modified silicon nanowires (SiNW) nanocomposite-based platform for the fabrication of a sensor. The fabricated sensor was used for the detection of biomarkers of infectious diseases in an easy, inexpensive, and non-invasive way. SiNW have been synthesized using metal-assisted chemical etching technique (MACE) and were casted using GO at room temperature. Subsequently, SiNW/GO was reduced to SiNW/rGO nanocomposite under an inert atmosphere using argon gas and characterized using a scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS), fourier transform infrared (FTIR), X-ray diffractometer (XRD) and Raman spectroscopy. This film was fabricated in the form of an electrode and utilized for the sensitive and selective detection of two volatile organic compounds (VOCs) namely cyclohexane and formaldehyde, found in the exhaled breath of patients with certain infections. The SiNW/rGO nanocomposite sensor showed a contrary response of 1.07 and 0.99 for 1 ppm concentration of cyclohexane and formaldehyde, respectively at room temperature. It showed a limit of detection (LOD) of 1 ppm and exhibits a fast response time of 30 s at ambient conditions. The recovery time of the sensor was 1–2 min and 3 min for cyclohexane and formaldehyde, respectively. Thus, the reported sensing approach using SiNW/rGO nanocomposite could exhibit a new nanocomposite for the fabrication of sensors having high sensitivity, selectivity, repeatability, and stability for target analytes.

Highly Efficient Silicon Nanowire Surface Passivation by Bismuth Nano-Coating for Multifunctional Bi@SiNWs Heterostructures.

A key requirement for the development of highly efficient silicon nanowires (SiNWs) for use in various kinds of cutting-edge applications is the outstanding passivation of their surfaces. In this vein, we report on a superior passivation of a SiNWs surface by bismuth nano-coating (BiNC) for the first time. A metal-assisted chemical etching technique was used to produce the SiNW arrays, while the BiNCs were anchored on the NWs through thermal evaporation. The systematic studies by Scanning Electron Microscopy (SEM), energy dispersive X-ray spectra (EDX), and Fourier Transform Infra-Red (FTIR) spectroscopies highlight the successful decoration of SiNWs by BiNC. The photoluminescence (PL) emission properties of the samples were studied in the visible and near-infrared (NIR) spectral range. Interestingly, nine-fold visible PL enhancement and NIR broadband emission were recorded for the Bi-modified SiNWs. To our best knowledge, this is the first observation of NIR luminescence from Bi-coated SiNWs (Bi@SiNWs), and thus sheds light on a new family of Bi-doped materials operating in the NIR and covering the important telecommunication wavelengths. Excellent anti-reflectance abilities of ~10% and 8% are observed for pure SiNWs and Bi@SiNWs, respectively, as compared to the Si wafer (50–90%). A large decrease in the recombination activities is also obtained from Bi@SiNWs heterostructures. The reasons behind the superior improvement of the Bi@SiNWs performance are discussed in detail. The findings demonstrate the effectiveness of Bi as a novel surface passivation coating, where Bi@SiNWs heterostructures are very promising and multifunctional for photovoltaics, optoelectronics, and telecommunications.

Nanomaterials and nanotechnology for composites: synthesis, structure, properties and new application opportunities

Nanomaterials and nanotechnology for composites: synthesis, structure, properties and new application opportunities

Superhydrophobic diamond-coated Si nanowires for application of anti-biofouling’

The effect of the surface wettability of plasma-modified vertical Si nanowire array on the bio-fouling performance has been investigated. The Si nanowires prepared by a metal-assisted chemical etching technique exhibit a super-hydrophilic surface. The treatment in CH4/H2 gas plasma environment leads to the decoration of graphite and diamond nanoparticles around Si nanowires. The detailed interface between graphite/diamond and Si nanowire was characterized by HRTEM technique. These surface-modified nanowire samples show an increased water contact angle with ultrananocrystalline diamond decorated ones being superhydrophobic. The immersion test in chlorella solution reveals that the diamond-coated Si nanowires possess the least attachment of chlorella in comparison with other Si nanowires. This result confirms that the coating of Si nanowires with diamond nanoparticles shows the best behavior in anti-biofouling. Importantly, this work provides a method fabricated super-hydrophobic surface for the application of biofouling prevention.

Metal Nanoparticle-Decorated Silicon Nanowire Arrays on Silicon Substrate and their Applications.

Herein, we report an efficient method to produce silver (Ag) nanoparticle-decorated silicon (Si) nanowire (NW) arrays on a pyramidal Si (P-Si) substrate by using a pure chemical method and rapid thermal annealing in different atmospheres. A metal-assisted chemical etching technique was used to produce vertical Si NW arrays on pyramidal Si. The etching was observed to be heavily dependent on the substrate type. On planar Si (100), the etching was observed to occur in a uniform manner. However, the etching rate was observed to increase from the top to the base of the Si pyramid. The Si NWs produced from P-Si have zig-zag sidewalls as observed from high-resolution transmission electron microscopy images. However, for the same oxidant concentration, Si NWs produced from planar Si (100) consist of straight and amorphous sidewalls. Local variation of oxidant concentration is responsible for the formation of different sidewalls. The substrates are both surface-enhanced Raman scattering (SERS) active and hydrophobic. The hydrophobicity is due to the dual scale of roughness contributed to by both pyramidal and NW structures. Finite-difference time-domain simulation shows that the gap between two Ag spheres and also the gap between Si NWs and Ag spheres contributed to SERS enhancement.

The use of MACE technique on amorphous silicon-rich silicon nitride thin films for the formation of spherical silica nanoparticles

We present a method that employs the metal-assisted chemical etching technique and a subsequent NH3 plasma treatment for obtaining silica nanoparticles (SNPs) uniformly distributed on an amorphous silicon-rich silicon nitride (SRN) film. These particles ranging from 50 to 300 nm are formed directly from the SRN films, and their density is related to the flow rate of the precursor gas (SiCl4). We determined that the SNPs are a SiOx compound (x > 1) with amorphous structure. Furthermore, the chemical composition of the bulk of the remaining SRN film after the etching in a HF/H2O2-based solution and plasma treatment maintain the properties of a pristine one if the initial thickness of the film is large enough (≥ 1 µm). This method enables the formation of silica nanoparticles directly on a silicon-rich silicon nitride film that could have potential optoelectronic applications, implicit in the optical measurements.

Plasmon resonances in coupled Babinet complementary arrays in the mid-infrared range.

A plasmonic structure with transmission highly tunable in the mid-infrared spectral range is developed. This structure consists of a hexagonal array of metallic discs located on top of silicon pillars protruding through holes in a metallic Babinet complementary film. We reveal with FDTD simulations that changing the hole diameter tunes the main plasmonic resonance frequency of this structure throughout the infrared range. Due to the underlying Babinet physics of these coupled arrays, the spectral width of these plasmonic resonances is strongly reduced, and the higher harmonics are suppressed. Furthermore, we demonstrate that this structure can be easily produced by a combination of the nanosphere lithography and the metal-assisted chemical etching technique.

Large-area MACE Si nano-inverted-pyramids for PERC solar cell application

Silicon (Si) nanostructures are regarded as a competitive candidate for the front texture of future high-efficiency Si solar cells owing to their excellent light-trapping properties. In this paper, we present nano-inverted-pyramids (NIPs) textures on the surface of Cz Si wafer with an area of 156 × 156 mm2 fabricated by employing the metal-assisted chemical etching (MACE) technique. The optical properties of the surface are investigated in detail before and after the application of a SiOx/SiNx antireflection coating. A passivated emitter and rear contact (PERC) solar cell with NIPs (NIPs-PERC) is analyzed at the cell as well module level using simulation. The simulation results indicate that the NIPs-PERC solar cell possesses a higher efficiency (η) of 1.4% relatively than that of the traditional PERC solar cell, benefiting from a significant better reflection performance in both the short as well as long wavelength range. Furthermore, simulations show that a 60-cell module with NIPs-PERC solar cells can yield a peak power of 310 W which is 8 W higher than a traditional PERC module. The novel NIPs-PERC solar cell shows high potential for mass production and opens a broad way for the application of NIPs textures to other high-performance solar cells.

Review of the Preparation and Structures of Si Nanowires, Ge Quantum Dots and Their Composites

Because the motion of charge carriers in nanowires and quantum dots is restricted within nanoscale in two and three dimensions, respectively, both nanowires and quantum dots exhibit many excellent optoelectronic properties. Particularly, with the advantage of being compatible with Si integrated circuits, Silicon nanowires (SiNWs) and germanium quantum dots (GeQDs) have been extensively studied in the past few decades. In order to explore novel physical properties, the integration of SiNWs and GeQDs has attracted great attention recently. In this paper, recent researches on the preparation methods and structures of SiNWs, GeQDs and their composites are reviewed, respectively. The synthesis of SiNWs with random distribution and ordered arrays by using vapor–liquid–solid growth mechanism and metal-assisted chemical etching technique is firstly summarized. Some special structures of SiNWs are also discussed. Furthermore, the development of some novel structures of GeQDs for further improving their optical properties is reviewed. Finally, the growth mechanism and structure evolution of SiNWs/GeQDs composites are illustrated from the view of theory and experiment. The strain in Ge shell layers and SiNWs, the relationship between Ge growth mode and SiNW diameter, and the distribution of GeQDs on the radial and axial directions of SiNWs are discussed in detail. The research about the growth of SiNWs/GeQDs composite structures is in its early stage, so there are many questions that need to be resolved in future.

Fabrication of low reflective nanopore-type black Si layer using one-step Ni-assisted chemical etching for Si solar cell application

Nanotextured Si fabricated through metal-assisted chemical etching (MACE) technique exhibits a promising potential for producing antireflective layer for photovoltaic (PV) application. In this study, a novel single-step nickel (Ni) assisted etching technique was applied to produce an antireflective, nonporous Si (black Si) in an aqueous solution containing hydrofluoric acid (HF), hydrogen peroxide (H2O2) and NiSO4 at 40 °C. Field emission scanning electron microscope was used to characterize different morphologies of the textured Si. Optical reflection measurements of samples were carried out to compare the reflectivity of different morphologies. Results indicated that vertical as well as horizontal pores with nanosized diameters were bored in the Si wafer after 1 h treatment in the etching solution containing different molar ratios of H2O2 to HF. Increasing H2O2 concentration in electrochemical etching solution had a considerable influence on the morphology due to higher injection of positive charges from Ni atoms onto the Si surface. Optimized concentration of H2O2 led to formation of an antireflective layer with 2.1% reflectance of incident light.