Metal-assisted Chemical Etching Research Articles

Development of surface-texturized black silicon through metal-assisted chemical etching and its application in the thermophotovoltaic field: a review and recommendation

Abstract The Shockley-Queisser limit poses a significant challenge in solar technology research, limiting the theoretical efficiency to around 30%. Thermophotovoltaic (TPV) systems have emerged as a solution by incorporating a thermal absorber in traditional solar cell setups to achieve total efficiency beyond the limits. The efficiency of the overall system heavily depends on the performance and quality of the thermal absorber, which absorbs photons from the heat source and transfers them to the TPV cell. However, complex and expensive fabrication processes have hindered widespread adoption of TPV technology. The well-established metal assisted chemical etching (MACE) method could be the best choice to mitigate these as it is a cost-effective, scalable, and mass-production-friendly process, which is widely used for surface texturization, creating nanostructures like nanopores, pyramids, and nanowires. MACE
technique is also suitable for producing highly efficient silicon-based thermal absorbers with over 90% absorption rate, which can contribute to increased total conversion efficiency. However, it does not come without challenges such as maintaining control over the etch rate in order to achieve uniformity. This paper comprehensively reviews the utilization of MACE for fabricating silicon-based thermal absorbers in TPV systems with the range of effective wavelengths of 600 – 2000 nm which corresponds to the energy level of 0.55 – 1.85 eV. The advantages and challenges of MACE, along with characterization techniques, are extensively discussed. By providing valuable insights, this paper aims to support researchers interested in advancing TPV technology.

Photonic sensor based on surface imprinted polymers for enhanced point-of-care diagnosis of bacterial urinary tract infections

Effective bacterial detection is crucial for health diagnostics, particularly for the detection of pathogenic species like Escherichia coli (E. coli), which is responsible for up to 90% of urinary tract infections (UTIs), is especially crucial. Current detection methods are time-consuming, often delaying diagnosis and treatment. This study introduces an innovative approach for rapid E. coli detection using porous silicon (PSi) substrates combined with Surface Imprinted Polymers (SIPs) for photoluminescence-based (PL-based) E. coli detection. The PSi/SIP substrates offer high sensitivity, selectivity, and a low limit of detection (LOD) without the need for natural recognition elements. These substrates, fabricated via metal-assisted chemical etching (MACE) and PDMS-based E. coli imprinting, demonstrate reliable repeatability and a fast detection. Real-time detection experiments in phosphate-buffered saline (PBS) and urine showed consistent stair-like quenching of the PL signal with increasing E. coli concentrations, achieving theoretical LODs of approximately 13 ± 2 CFU/mL in PBS and 17 ± 3 CFU/mL in urine. The substrates exhibited excellent selectivity, differentiating E. coli from other species such as Cronobacter sakazakii (C. sakazakii) and Listeria monocytogenes. The high sensitivity and reproducibility of PSi/SIP substrates, along with the ease of use and rapid detection capabilities of the resulting sensor, highlight the potential of this novel platform for point-of-care (PoC) applications in clinical diagnostics.

Modified Si Oxidation Behavior by Ultrathin Ni Catalyst Enabling Oxidant-Less Metal-Assisted Chemical Etching.

Metal-assisted chemical etching (MACE), a wet-based anisotropic etching process for semiconductors, has emerged as an alternative to plasma-based etching. However, using noble metal catalysts in MACE limits the implementation of complementary metal-oxide-semiconductor (CMOS) processes. This study explores Si etching using an ultrathin Ni catalyst as a novel approach for MACE. The thickness of the Ni catalyst emerges as a critical parameter, with 1 nm of Ni proving to be the optimal thickness to achieve smooth and deep etching. Unlike conventional MACE methods, the ultrathin Ni catalyst enables Si etching without strong oxidants. Wafer-scale Si etching demonstrates the versatility of the ultrathin Ni catalyst in producing various microstructures. It is found that the ultrathin Ni/Si interfacial state plays a crucial role in influencing the Si reactivity, lowering the barrier for Si oxidation. CMOS-compatible and cost-efficient ultrathin Ni makes MACE a promising alternative for semiconductor nanofabrication. This study pioneers MACE using an ultrathin non-noble metal catalyst, offering valuable insights for researchers in this field.

Recent Advances in Black Silicon Surface Modification for Enhanced Light Trapping in Photodetectors

The intricate nanostructured surface of black silicon (BSi) has advanced photodetector technology by enhancing light absorption. Herein, we delve into the latest advancements in BSi surface modification techniques, specifically focusing on their profound impact on light trapping and resultant photodetector performance improvement. Established methods such as metal-assisted chemical etching, electrochemical etching, reactive ion etching, plasma etching, and laser ablation are comprehensively analyzed, delving into their mechanisms and highlighting their respective advantages and limitations. We also explore the impact of BSi on the emerging applications in silicon (Si)-based photodetectors, showcasing their potential for pushing the boundaries of light-trapping efficiency. Throughout this review, we critically evaluate the trade-offs between fabrication complexity and performance enhancement, providing valuable insights for future development in this rapidly evolving field. This knowledge on the BSi surface modification and its applications in photodetectors can play a crucial role in future implementations to substantially boost light trapping and the performance of Si-based optical detection devices consequently.

Hyperspectral Analysis of Silicon Nanowires Manufactured Through Metal-Assisted Chemical Etching

Abstract This study used hyperspectral imaging to analyze localized near-field interactions between incident electromagnetic waves and silicon nanowire (SiNW) arrays manufactured through catalytic etching of Si wafers for different durations. The results revealed that the unetched upper surface area on Si wafers and reflection of incident light decreased with increasing etching time. A light reflection band peaking at approximately 880 nm was generated from arrays etched for more than 1 h. We used six separate hyperspectral images to analyze the wavelength-dependent spatial optical responses of the fabricated SiNW arrays. The images revealed hot spots of light reflection from unetched Si surfaces in the wavelength range of 470–750 nm and a resonant peak at 880 nm for a photonic crystal derived from a random SiNW array. Accordingly, hyperspectral imaging enables the assessment of localized optical responses of SiNW arrays, which can then be optimized to cater to various applications.

Ruthenium catalyst processing and oxidation for scalable complementary metal-oxide-semiconductor-compatible metal-assisted chemical etch

Ruthenium catalyst processing and oxidation for scalable complementary metal-oxide-semiconductor-compatible metal-assisted chemical etch

Enhancing the anti-wear performance of graphene sheets embedded carbon films through interface nanotexture fabricated on the substrate

The effect and mechanism of surface texture on friction have been extensively explored, such as storing the abrasive and reducing the real contact area. However, studying the impact of the interface texture remains crucial, especially as the texture size approaches the nanoscale. In the present work, the silicon substrate with different parameters nanotexture was fabricated using a metal-assisted chemical etching process, and the graphene sheets embedded carbon (GSEC) film was deposited on the nanotextured substrate via a low-energy electron irradiation process to produce the interface nanotextured carbon films. After the addition of the interface nanotexture, the tribological performance of carbon films exhibited significant enhancement, with the degree of improvement being influenced by the parameters of the interface nanotexture. A super-low wear rate was achieved, with a value of 6.54 ± 0.70 × 10−8 mm3/N m. The improved tribological performance is due to the enhancement of film-substrate adhesion strength facilitated by interface nanotexture, and its role in promoting the rapid formation of a stable transfer film on the counter-grinding ball. The carbon film with nanotexture displays a small size of the abrasive particles, which contributes to the stabilization of the friction process.

Understanding the impact of porosity on Li-ion diffusion enhancement in micro-sized silicon particles for advanced batteries

Lithium-ion batteries (LIBs) require advanced and practical anode materials, such as porous silicon (PSi), which can meet these expectations due to their rapid Li-ion diffusion rates and structural relaxation. Several studies have focused on the structural design of PSi or its composites to improve Li-ion diffusion; however, no systematic and quantitative evaluations of the impact of porosity on Li-ion diffusion and battery performance have been reported. Therefore, to understand the impact of porosity on Li-ion diffusion, we developed micro-sized porous silicon (m-PSi) particles with various porosities via metal-assisted chemical etching (MACE) method using different concentrations of AgNO3 (0.02–0.08 M). Among the as-prepared samples, m-PSi-0.06 (prepared using 0.06 M AgNO3) with ∼70 % porosity achieved a maximum Li-ion diffusion rate of 2.35 × 10−9 cm2 s−1. This led to a high-rate capability, enhanced Li-ion storage, and better cyclic retention of 61.4 % compared to the non-porous micro-sized Si (m-Si) (5.8 %) sample at a current density of 0.1 A g−1. Furthermore, the optimal porosity of m-PSi-0.06 resulted in an impressive rate capability of 760 mAh g−1 at a high current density of 4 A g−1 with a recovery rate of 80 %. The improved efficiency of m-PSi-0.06 is attributed to its optimized mesoporosity, which ensures sufficient space for Li-ion storage and shortens the effective transmission path to accelerate Li-ion diffusion. Moreover, the mesopores provide a greater number of active sites for the reversible adsorption of Li ions, thereby improving the capacitive behavior of the anode. In contrast, the highly nanoporous m-PSi-0.08, with a pore diameter of 1–3 nm, impeded Li-ion movement and restricted diffusion. These results reveal that a high nano porosity hinders Li-ion diffusion, whereas an optimal mesoporosity ensures swift diffusion in m-PSi anodes. These insights could guide the design of Si-based anode materials for advanced LIBs.

(Invited) Selective Wet Etching of Semiconductor Materials by Novel Catalyst-assisted Modes

Since reports in the mid-1990s on metal-induced pitting on a Si surface in microelectronics, much attention has been paid to the positive use of corrosion of a semiconductor surface loaded with a metallic catalyst in solutions, known as metal-assisted chemical etching. This paper describes two novel modes of catalyst-assisted etching. The first involves the formation of nano trenches along the atomic step edges of a Si(111) surface with the help of self-assembled metallic nanowires, enabling the separation of neighboring terraces. The second discusses the science and application of nanocarbon-assisted etching, free from noble metals, of a Ge surface in water.

Silicon Nanowires via Metal-Assisted Chemical Etching for Energy Storage Applications.

Silicon nanowires (SiNWs) have demonstrated great potential for energy storage due to their exceptional electrical conductivity, large surface area, and wide compositional range. Metal-assisted chemical etching (MACE) is a widely used top-down technique for fabricating silicon micro/nanostructures. SiNWs fabricated by MACE exhibit significant surface areas and diverse surface chemistry. Since the material composition and surface chemistry have a significant impact on the electrochemical energy storage performance, integrating SiNWs with diverse materials like porous carbon, metal oxides/sulfides, and polymers, can establish composites with excellent properties. Hence, it is imperative to meticulously fabricate SiNW-based materials with customizable morphologies and enhanced electrochemical energy-storage performance. This review provides an in-depth study of recent advancements in SiNW-based materials with enhanced performance for energy storage systems, such as supercapacitors (SCs) and lithium-ion batteries (LIBs). It includes a concise overview of the history, MACE synthesis, and characteristics of SiNWs. Further, it also explores the key elements that influence the MACE process of SiNWs and delves into structural engineering. Additionally, we introduce recent advances in SiNW-based materials for the design of high-performance energy-storage devices, namely SCs and LIBs. Finally, we present the crucial future prospects of SiNW-based materials for energy-storage applications.

A Reactor based on P‐N Heterojunction of Polypyrrole and Porous Silicon for Photocatalytic and Electro‐assisted Photocatalytic Performance

AbstractIn this work, a photoelectrochemical reactor with an anode based on hybrid structure between conducting polymer polypyrrole (PPy) and porous silicon (PSi) was fabricated and utilized to evaluate the photocatalysis and electro‐assisted photocatalysis for degradation of rhodamine B (RhB) solution. The PSi was made of n‐type silicon single‐crystal wafer by the metal‐assisted chemical etching (MACE). The anode PPy/PSi formed following a polymerization of the PPy on the PSi from the vapour‐phase route using oxidant FeCl3. Typical material characterizations of the PPy, PSi and PPy/PSi were investigated via scanning electron microscope (SEM), and spectroscopies of Fourier‐transform infrared (FTIR), Raman scattering and UV‐vis. Under visible light radiation (halogen light with the wavelength region of 400–700 nm), photocatalytic and electro‐assisted photocatalytic processes of the reactor were conducted by degradation of the RhB solution. A heterojunction (p‐n) of the PPy/PSi was proposed to explain the efficient catalytic performance of the reactor.

Real-time SERS sensing of highly toxic seawater contaminants using plasmonic silver assembled pyramidal/nanowire heterostructures

Hierarchically oriented plasmonic nanostructures have attracted an advanced molecular sensing platform for multifaceted applications. In the present work, we have attempted to fabricate ultrasensitive and cost-effective pyramidal/nanowire hetero arrays, hosted with silver nanoparticles for the effective surface-enhanced Raman spectroscopy (SERS) assisted detection of toxic water contaminants. Three-dimensionally oriented pyramidal/nanowire hetero arrays with enhanced surface area were fabricated by combining wet etching and metal-assisted chemical etching techniques. The regulation of structural features was achieved by controlling the process parameters during fabrication. Particularly, the evolution of morphological properties of the hybrid sensor was investigated in terms of nanowire aspect ratio and further analyzed in terms of SERS properties. The hetero arrays exhibited significant enhancement in Raman signal for sensing organic pollutants. Moreover, these hetero arrays with a nanowire length of ∼1 µm exhibited the highest signal enhancement. Further, the excellent reproducibility and reusability characteristics were also demonstrated for the fabricated sensors. Different cationic and anionic organic pollutants were employed for efficacy studies which showed detection limits ranging from femtomolar to picomolar levels. Finally, the real-time sensing of organic contaminants such as aniline was also investigated from seawater with an estimated enhancement factor of 8 × 108, indicating that as-fabricated hetero arrays can perform as outstanding SERS sensors for environmental remediation applications.

Enhanced photoconductivity via photon down-conversion by incorporation of solution-processed 3C-SiC QDs on nanostructured black silicon

Colloidal quantum dots (CODs) have attracted attention towards the next-generation optoelectronic devices capable of tuning the bandgap to capture photons at the UV region which is the major impediment of silicon (Si) for optoelectronic applications. However, CODs convert higher-energy photons into lower-energies photons through spectral down-conversion to UV visible. This study describes the photoconductivity effects of colloidal 3C-SiC QDs onto the underlying black silicon (b-Si) for spectral down-conversion effect. The Si showed a remarkable decrease in broadband reflectance after being etched to b-Si via metal-assisted chemical etching (MACE) over a broad spectral wavelength range of 300–1100 nm. Incorporating QDs onto underlying b-Si enhanced the device responsivity from 0.034 A/W to 0.53 A/W with the formation of space charge through the down-conversion effect. Furthermore, the photovoltaic measurements demonstrate the superior performance of hybrid colloidal 3C-Si QDs/b-Si with a power conversion efficiency (PCE) of ∼7.28 % compared to b-Si without QDs (5.57 %) photovoltaic cells. Our research provides insight into the down-conversion effects of colloidal 3C-SiC QDs for photovoltaic and photodetector applications.

A strategy to reutilize silver nanoparticles on silicon nanowires via photocathodic activation for enhanced and sustainable hydrogen evolution

The development of electrodes for hydrogen evolution via water splitting in a neutral electrolyte is highly desirable, especially considering that natural water resources such as seawater typically have a neutral pH. In this study, silicon nanowires (SiNWs) arrays with uniform silver (Ag) decoration are prepared as a cathode material, which exhibits excellent and stable electrochemical performance for neutral water electrolysis. Ag ions, utilized in the preparation of SiNWs through metal-assisted chemical etching technique, are retained and decorated within the SiNWs array without undergoing a dissolution process, referred to as Native SiNWs. Remained Ag+ ions on the SiNWs are activated photocathodically and reduced to Ag0, resulting in a significantly enhanced performance as an electrocatalyst for hydrogen evolution reaction (HER). Hence, this photocathodic activation of Native SiNWs (PCA-Native SiNWs) eliminates the need for additional cocatalyst deposition for HER, utilizing the Ag ions employed in the SiNWs preparation process. This enhancement in HER activity is exclusively observed by photocathodic activation, not by cathodic activation without light irradiation, indicating the crucial role of photoelectrons in SiNWs under light irradiation for the activation. Additionally, the nanowire structure serves as an ideal platform for Ag nanoparticles, providing sufficient surface area, resulting in excellent dispersion and an increased number of active sites. Furthermore, PCA-Native SiNWs demonstrates outstanding electrochemical stability for over 60 h, with no significant reduction in current density or structural degradation in a neutral electrolyte. Several characterizations and electrochemical analyses have been conducted to elucidate the in-situ decoration of Ag nanoparticles on SiNWs and investigate the surface chemistry for the activation mechanism. This work introduces a new perspective on using Ag-decorated SiNWs directly as an electrocatalyst for effective hydrogen evolution in a neutral media.

Gravity-assisted gradient size exclusion separation of microparticles by gap-modifiable silicon nanowire arrays

The separation and detection of microparticles within complex samples pose substantial challenges due to the intricate variations in size and concentration. A strategy employing gravity-assisted gradient size exclusion principle based on controllable gap sizes on the surface of silicon nanowire arrays (SiNWAs) has been established to achieve the separation of microparticles with diverse sizes. The formation of gradient gap sizes was accomplished by meticulously investigating the impact of oxidation-reduction reactions through metal-assisted chemical etching. Particles of different sizes were initially aggregated at the accumulation base, followed by a sequential size exclusion process within the finely regulated 0.9–12.5 μm gradient-gap-sized separation region facilitated with gravity-assisted, leading to a comprehensive separation of microparticles based on their respective size differences, progressing from small to large. The effective separation of four model-sized microparticles demonstrated a separation degree of ≥2.7, purity of ≥96.1 %, RSDs of ≤4.6 %, and a separation capacity of up to 107 particles. The separation efficacy of this gradient-sized chip was verified by evaluating the more complex atmospheric particulates with varying sizes, which exhibited separation degree ranging between 2 and 10. This method offers a precise separation range, easily adjustable separation sizes, and simple operation, rendering it a versatile tool for separating complex samples.

Detection of Thiocholine for Pesticide Qualification Using Indium-Tin-Oxide-Coated Vertically Aligned Silicon Nanowires Extended Gate Field-Effect Transistor

Thiocholine is the hydrolysis of acetylthiocholine (ATCh) by acetylcholinesterase (AChE). The detection of thiocholine can be utilized to assess the activity and inhibitors of AChE, making it a valuable recognition element for constructing biosensors used in pesticide detection[1, 2]. The enzymatic hydrolysis of ATCh is catalyzed by AChE. First, high-aspect-ratio vertically aligned silicon nanowires (VASiNWs) were successfully fabricated through metal-assisted chemical etching (MACE). To investigate the impact of nanostructures on enhancing the sensitivity of the extended gate of the field-effect transistor (EGFET), the gate terminal was connected to 3-mercaptopropyl trimethoxysilane (MPTMS) modified indium-tin-oxide (ITO)-coated VASiNWs for the electrical detection of ATCh-catalyzed thiocholine, as illustrated in Figure 1. Various solutions of ATCh-catalyzed thiocholine were collected by applying different concentrations of ATCh (ranging from 2 mM to 10 mM) to the AChE-coated wells independently for 10 minutes. Subsequently, these solutions were introduced to the MPTMS modified ITO-coated VASiNWs EGFET, and the change in drain current was measured using a semiconductor parameter analyzer (HP 4145B). The thiocholine current demonstrated a sensitivity of 29.39 uA/concentration and a linearity of 0.88. Exploring the potential of pesticides to inhibit AChE hydrolysis represents a significant breakthrough, paving the way for the advancement of biosensors in pesticide detection. References Liu, G., et al., Sensitive electrochemical detection of enzymatically generated thiocholine at carbon nanotube modified glassy carbon electrode. Electrochemistry Communications, 2005. 7(11): p. 1163-1169. Pandey, P.C., et al., Acetylthiocholine/acetylcholine and thiocholine/choline electrochemical biosensors/sensors based on an organically modified sol–gel glass enzyme reactor and graphite paste electrode. Sensors and Actuators B: Chemical, 2000. 62(2): p. 109-116. Figure 1

Application of silicon nanowires in sensors of temperature, light and humidity

Resistive and capacitive types of sensors based on silicon nanowires (SiNWs) for measuring of physical quantities have been manufactured. Silicon nanowires were obtained through a two-step metal-assisted chemical etching (MACE) method. The surface morphology of the silicon nanowire array was investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The impact of SiNWs synthesis parameters on sensor sensitivity has been investigated. In particular, the influence of deposition time of AgNPs and etching time of silicon, as well as the content of H2O2 in the etchant, preliminary texturing of the substrate, and additional post-processing in isotropic/anisotropic etchant after the MACE process, on the characteristics of physical quantity sensors has been determined. Static and dynamic parameters were calculated for the obtained sensors. To compare the obtained sensors, physical sensors without SiNWs were manufactured. The highest response value for temperature sensors was 132, which is two orders of magnitude better than for sensors without SiNWs. The presence of SiNWs on the surface of the sensor led to an increase in the response value by about two orders of magnitude for all types of sensors. The speed of humidity sensors improved by 3–4 times, and for temperature and light sensors by an order of magnitude. Further device applications include of multi-parameter sensors, human breath sensors, and solar radiation power sensors.

Metal assisted chemical etching derived reusable and scalable production of large-area Ag nanowire-based flexible transparent electrodes for electrochromic Zn-ion battery

To advance polydimethylsiloxane (PDMS)-supported silver nanowires (Ag NWs) based flexible and transparent electrodes (AgNWs/PDMS), innovative patterning techniques are essential for enabling large-area fabrication with cost-effective reusability features. Here, we introduce a metal assisted chemical etching (MACE) protocol for patterning Si wafers, allowing the repetitive fabrication of large-sized AgNWs/PDMS electrodes with controlled penetration depth for the first time. To the best of our knowledge, MACE technology has not previously been employed for fabricating AgNWs/PDMS electrodes. Through the careful selection of etchant, etching time, and suitably doped Si wafers (n-type and p-type), the resulting AgNWs/PDMS electrodes offer favorable optical, electrical and flexiblity characteristics. The electrodes deliver a sheet resistance of 18 Ω‧sq−1 at 88 % transmittance (550 nm) while retaining 93.18 % transmittance with only a 7 Ω‧sq−1 resistance increase after 10,000 bending cycles (3 mm). The penetration depth control offered by this method ensures impressive mechanical durability without additional post-processing. Moreover, the etched Si wafers can be reused multiple times, reducing overall costs. The sizes of AgNWs/PDMS electrodes produced using this method depend entirely on the Si wafer size, allowing scalability by employing larger wafers. As a proof-of-concept, we also demonstrate the fabrication of a robust, flexible, electrochromic zinc ion battery utilizing the AgNWs/PDMS electrodes developed in this study.

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Enhanced Capability of Hydrogen Evolution Photocathode by Laminated Interface Engineering of Co/MoS2 QDs/pyramid-black Si.

We present a novel and stable laminated structure to enhance the performance and stability of silicon (Si) photocathode devices for photoelectrochemical (PEC) water splitting. First, by utilizing Cu nanoparticle catalysts to work on a n+p-black Si substrate via the metal-assisted chemical etching, we can achieve the black silicon with a porous pyramid structure. The low depth holes on the surface of the pyramid caused by Cu etching not only help enhance the light capture capability with quite low surface reflectivity (<5%) but also efficiently protect the p-n junction from damage. To improve the charge migration efficiency and mitigate parasitic light absorption from cocatalysts at the same time, we drop casted quantum dots (QDs) MoS2 with the size of nanometer scale as the first layer of catalyst. Hence, we then can safely electrodeposit cocatalyst Co nanoparticles to further enhance interface transfer efficiency. The synergistic effects of cocatalysts and optimized light absorption from the morphology and QDs contributed to the overall enhancement of PEC performance, offering a promising pathway for an efficient, low cost, and stable (over 100 h) hydrogen production photocathode.