Metal-assisted Chemical Etching Process Research Articles

Fabrication of humidity monitoring sensor using porous silicon nitride structures for alkaline conditions

Porous silicon nitride structures were fabricated for a humidity sensor. The porous silicon structures were fabricated by the metal-assisted chemical etching process, and the conformal silicon nitride thin film was deposited by the atomic layer deposition process. The optimized porous sensor with the 10 nm-thick silicon nitride thin film had a hydrophilic surface and compared to other sensors, had an excellent humidity sensing response. Especially, it showed a superior humidity sensing response at 1 kHz with fast response and recovery times of 13.3 s and 12.4 s, respectively, were observed. Based on the electrochemical impedance spectroscopy results, the equivalent circuits and humidity sensing mechanism were discussed. The chemical stability of the silicon nitride was characterized using Tafel analysis in alkaline electrolytes. Additionally, the sensor's humidity sensing capabilities were tested under cement-embedded conditions.

Controllable Fabrication of Silicon Solar Cells with Inverted or Upright Pyramid Structures by a One-Step Copper Catalysis Method

Silicon has garnered significant attention as the primary material for solar cell preparation. Traditional alkaline etching solutions are limited to creating an upright pyramid structure on monocrystalline silicon surfaces. However, research indicates that an inverted pyramid structure exhibits superior light-trapping properties compared to the upright pyramid structure. In this study, we employed a one-step copper ion metal-assisted chemical etching process to fabricate an inverted pyramid structure on monocrystalline silicon wafers. This method allows for the customization of either inverted or upright pyramid structures by adjusting the concentration of specific solution components. Characterization of the textured silicon wafers reveals that the inverted pyramid structure exhibits lower reflectivity than both the upright pyramid structure and polished silicon. By integrating this texturing technique into the solar cell production line, we successfully produced solar cells with both inverted and upright pyramid structures. Evaluation of various solar cell parameters demonstrates that the inverted pyramid structure outperforms the upright pyramid structure, showcasing lower reflectivity and higher photoelectric conversion efficiency.

Highly sensitive photoelectrochemical sensors for multichannel detection and uniform bioimaging based on fireworks-like silicon nanowires and its heterojunctions fabricated by MACE and sputtering processes

Photoelectrochemical (PEC) sensors have emerged as a promising candidate for biochemical detection. However, PEC sensors based on bulk semiconductors are limited by sensitivity and detection varieties, though they can provide uniform imaging; PEC sensors based on nanomaterials can meet the requirements of various analytes and sensitivity, but the inhomogeneity of their surface poses a challenge to imaging uniformity. In this work, metal-assisted chemical etching (MACE) and sputtering processes were used to directly fabricated PEC sensors with fireworks-like silicon nanowires (F-SiNWs) and their heterojunctions with excellent performance. Through regulating the etching and sputtering times, the structure and nanomaterials attachment of F-SiNWs can be well controlled. The photoelectric and sensing properties of F-SiNWs with different morphologies were investigated and optimized. Then, the sensor fabricated by optimal F-SiNWs can directly detect H2O2 and indirectly detect glucose under negative bias, while directly detect dopamine (DA) under positive bias. The detection of these three analytes do not interfere with each other, contributing to an excellent multichannel detection system. Subsequently, the multichannel detection of various secretions (H2O2, DA and glucose) of PC-12 cells was achieved by the sensor. Finally, the imaging uniformity of the sensor was demonstrated in different solutions. PEC bioimaging was successfully applied to monitor enzyme activity by bioimaging gradients and cell survival by bioimaging distribution. These results demonstrate the high sensitivity in multichannel detection and excellent stability and homogeneity in bioimaging of the sensor. Therefore, MACE and sputtering process can be a promising tool for constructing PEC sensor.

Surface modification of silicon nanowires via drop-casting for high-performance Li-ion battery electrodes: SiNWs decorated with molybdenum oxide nanoparticles

The functionalization of molybdenum oxide (MoO3) nanoparticles is presented as a method to significantly enhance the cycling stability of lithium-ion battery (LIB) anodes based on silicon nanowire (SiNW) arrays. Transition-metal oxides have emerged as promising candidates for advanced anode materials in modern Li-ion batteries. In this study, we explore a novel approach involving the deposition of MoO3 nanoparticles via unique drop-casting technique onto pre-fabricated SiNW arrays, fabricated using a straightforward one-step metal-assisted chemical etching (MACE) process. The primary objective is to assess their potential suitability as anode materials for Li-ion batteries. Our methodology entails the top-down synthesis of binder-free hybrid electrodes, achieved by depositing Mo oxides nanostructures onto SiNW arrays through a combination of drop-casting and thermal annealing processes. The resulting MoO3@SiNWs hybrid structure exhibits distinctive and specialized attributes, including exceptional structural resilience, diminutive particle dimensions, and a porous configuration. These features effectively enhance electron and ion accessibility at the electrode-electrolyte interface.Electrochemical assessments reveal that the MoO3@SiNWs hybrids exhibit superior lithium storage performance compared to bare SiNW electrodes. Particularly under high current densities, MoO3 nanoparticles deposited via drop-casting technique demonstrate improved cycling stability and increased capacity. The enhanced electrochemical characteristics are primarily ascribed to the synergistic effects between the MoO3 nanoparticles and SiNW arrays. The findings of this study strongly suggest that the MoO3@SiNWs hybrid structure holds substantial promise as anode materials for high-performance energy storage devices.

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.

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.

Cone Angle Modulation of Silicon Nanocones through Double Etching Processes in Metal-Assisted Chemical Etching

All-wet metal-assisted chemical etching (MACE) has been demonstrated as a simple and effective method to fabricate silicon nanocones (SiNCs). The properties of SiNCs strongly depend on their structure parameters such as the cone angle and can be optimized through modulating the cone angle. However, cone angle modulation of SiNCs in all-wet MACE processes has not been achieved. Here, we report the fabrication of SiNCs with different cone angles through double etching processes in all-wet MACE. Large-size silver nanoparticles (AgNPs) were first obtained by solution deposition. After a common etching process, a double etching process with diluted etching solutions was introduced. In the etching step, AgNPs gradually reduced with the Si etching, which resulted in the formation of SiNCs. The cone angles of SiNCs depend on the changing rate of AgNP sizes. Based on this mechanism, SiNCs with different cone angles from 5°-20° were successfully fabricated by changing the concentration of the diluted etching solution in the double etching process. The contact angle of SiNCs with different cone angles was also investigated. It is found that the wettability of the SiNCs becomes poorer with increasing cone angles. Cone angle modulation of SiNCs in all-wet MACE will benefit the property control and the wide applications of SiNCs.

Chemical Excitation of Silicon Photoconductors by Metal-Assisted Chemical Etching

The chemical transformations taking place during many of the reactions of the Si surface have been well documented, but the in situ dynamics of the reactions remain largely unexplored even for widely used electrochemical processes such as metal-assisted chemical etching (MACE). In this work, we design both n- and p-type Si photoconductors covered with silver nanoparticles to demonstrate photoconductors’ sensitivity to the MACE process and their ability to provide in situ information about the dynamics of the reactions occurring during MACE. The experimental results show that both n- and p-type photoconductors exhibit a response to MACE that is much stronger than their response to light with an intensity of 2 mW/cm2. The observations are further explained by a thermodynamic analysis of the relevant energy levels of the system, showing how both electron and hole injection into the conduction and valence bands by the Si/etching solution interface contribute to the photoconductors’ response and excite Si. These results clearly demonstrate a new chemical operating mode for photoconductors, showing that in addition to being sensitive to excitation by light, a photoconductor can also be excited by chemical reactions providing means to monitor the dynamics of the reactions in situ and thus also for chemical sensing.

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation.

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction-oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificialvascularization.

Room-Temperature Sub-ppm Detection and Machine Learning-Based High-Accuracy Selective Analysis of Ammonia Gas Using Silicon Vertical Microwire Arrays

The potential applications of silicon microwire materials in monitoring gases have not been fully exploited. Uniform silicon vertical microwire arrays (Si VMWA) are fabricated using a metal-assisted chemical etching process after optimizing the conditions. The characteristics and responses of Si VMWA-based sensors with different diameters to ammonia gas (NH3) are investigated in both air and nitrogen environments. The sensing mechanism of the sensor to NH3 is discussed to clarify the response in different environments. The sensor exhibits a linear response to a wide range of NH3 concentrations (4%@2 ppm–122%@500 ppm) at room temperature and even shows a distinct response at 200 ppb of NH3. In addition, it demonstrates great repeatability/reversibility and moderate selectivity to ammonia gas against other gases (nitrogen dioxide, toluene, and isobutane). Furthermore, machine learning-based principal component analysis and random forest algorithms enable us to discriminate NH3 from other possible interfering gases and predict gas concentration with an accuracy of over 95%. Thus, our approach using the Si VMWA-based sensor with machine learning-based data analysis represents a significant step toward the environmental sensing of specific chemical analytes in the household and industries.

Sensitive and Specific Detection of Estrogens Featuring Doped Silicon Nanowire Arrays.

Estrogens and estrogen-mimicking compounds in the aquatic environment are known to cause negative impacts to both ecosystems and human health. In this initial proof-of-principle study, we developed a novel vertically oriented silicon nanowire (vSiNW) array-based biosensor for low-cost, highly sensitive and selective detection of estrogens. The vSiNW arrays were formed using an inexpensive and scalable metal-assisted chemical etching (MACE) process. A vSiNW array-based p-n junction diode (vSiNW-diode) transducer design for the biosensor was used and functionalized via 3-aminopropyltriethoxysilane (APTES)-based silane chemistry to bond estrogen receptor-alpha (ER-α) to the surface of the vSiNWs. Following receptor conjugation, the biosensors were exposed to increasing concentrations of estradiol (E2), resulting in a well-calibrated sensor response (R 2 ≥ 0.84, 1-100 ng/mL concentration range). Fluorescence measurements quantified the distribution of estrogen receptors across the vSiNW array compared to planar Si, indicating an average of 7 times higher receptor presence on the vSiNW array surface. We tested the biosensor's target selectivity by comparing it to another estrogen (estrone [E1]) and an androgen (testosterone), where we measured a high positive electrical biosensor response after E1 exposure and a minimal response after testosterone. The regeneration capacity of the biosensor was tested following three successive rinses with phosphate buffer solution (PBS) between hormone exposure. Traditional horizontally oriented Si NW field effect transistor (hSiNW-FET)-based biosensors report electrical current changes at the nanoampere (nA) level that require bulky and expensive measurement equipment making them unsuitable for field measurements, whereas the reported vSiNW-diode biosensor exhibits current changes in the microampere (μA) range, demonstrating up to 100-fold electrical signal amplification, thus enabling sensor signal measurement using inexpensive electronics. The highly sensitive and specific vSiNW-diode biosensor developed here will enable the creation of low-cost, portable, field-deployable biosensors that can detect estrogenic compounds in waterways in real-time.

Properties of PEDOT:PSS on Black Silicon and Hybrid Textured Surfaces

This work investigates properties of Poly (3,4-ethylenedioxythiophene)–poly (styrene sulfonate) (PEDOT:PSS) on black silicon (nanotextured) and hybrid textured (nanotextured/microtextured) surfaces. The black silicon (b-Si) surface is fabricated using two-step metal-assisted chemical etching (MACE) process on crystalline silicon (c-Si) while the hybrid textures are fabricated using two-step MACE process on microscale pyramids. With PEDOT:PSS, weighted average reflection (WAR) reduces from 9.2% to 7.7% for b-Si and from 7.2% to 5.2% for hybrid textures. This is due to the anti-reflective (AR) property of the polymer. Electrical characterizations of the PEDOT:PSS layer reveal higher sheet resistance (Rs), lower hole concentration (nh) and improved mobility (μh) with the presence of the surface textures on c-Si, in comparison to the results from planar c-Si reference.

Fabrication of high-performance microfluidic SERS substrates by metal-assisted chemical etching of silicon scratches

Surface-enhanced Raman spectroscopy (SERS) substrate-based microfluidic systems are applied extensively in diverse fields. For affordable detection and diagnosis, we propose a novel strategy for flexible, low-cost, and rapid fabrication of microfluidic SERS substrates by metal-assisted chemical etching of scratches on a silicon surface. A silicon substrate was spin-coated with polyketone resin (PK), and patterned by scratching with a diamond tip. Notably, defects created by diamond tip scratching on silicon substrate promoted subsequent metal deposition. A micro/nano nested structure was prepared by metal-assisted chemical etching process based on combined effect of scratching and metal catalysis. The PK layer served as a stable mask during metal deposition and etching. The prepared SERS-active detection sites with micro/nano nested structures exhibited substantial enhancement effects and good stability. Taking rhodamine 6G as a probe molecule, the microfluidic SERS substrate exhibited a high detection capability, with nanomolar detection limits (10−9 M) and high long-term stability (at least 120 days). The micro/nano nested structure exhibited an enhancement factor of 2.725 × 105 compared to a gold film deposited on a flat silicon surface. The proposed method is promising for chemical and biological detection applications.

Structure and electrical behavior of silicon nanowires prepared by MACE process

We report on the structure and electrical characteristics of silicon nanowire arrays prepared by metal assisted chemical etching (MACE) method, investigated by cross-sectional scanning electron microscopy (SEM) and high resolution X-ray diffraction (HR-XRD) methods. SEM micrographs show arrays of merged parallel nanowires, with lengths of 700 nm and 1000 nm, resulted after 1.5 min and 5 min etching time, respectively. X-ray reciprocal space maps (RSMs) around Si (004) reciprocal lattice point indicate the presence of 0D structural defects rather than of extended defects. The photoluminescence spectra exhibit emission bands at 1.70 eV and 1.61 eV, with intensity significantly higher in the case of longer wires and associated with the more defected surface. The transient photoluminescence spectroscopy reveals average lifetime of 60 ôs and 111 ôs for the two SiNW arrays, which correlate with a larger density of defects states in the latest case. The I-V characteristics of the nanowires, show a memristive behavior with the applied voltage sweep rate in the range 5–0.32V/s. We attribute this behavior to trap states which control the carrier concentration, and model this effect using an equivalent circuit. Photogeneration processes under excitation wavelengths in visible domain, 405 nm - 650 nm, and under light intensity in the range 20–100 mW/cm2 provided a further insight into the trap states.

Nanoengineered nanochannels for thermally ionic nanofluidic energy harvesting

This work demonstrates thermal-to-electric energy conversion based on ionic nanofluidic transport in nanochannels inducted by a temperature gradient. Two types of highly periodic and high aspect ratio nanochannels have been fabricated in a silicon (Si) substrate and in an aluminium oxide (Al2O3) membrane. Silicon nanochannels with diameter of 100 nm and height of 300 μm have been produced by metal-assisted chemical etching process (MACE), while nanochannels with the dimensions of the 10 nm and 3 μm respectively, were fabricated in a Al2O3 membrane by the anodic aluminum oxidation (AAO) process. Moreover, a novel approach of thermally nanofluidic energy harvesting was proposed and conducted. The performance of the thermally ionic nanofluidic energy harvesting system in cases with and without nanochannels has been evaluated and compared. The advantage of the presence nanochannels on energy conversion has been confirmed. The electrolyte concentration dependence on the output power has been determined. Also, the effect of nanochannel materials, including alumina and silicon materials, on the power density has been verified. The ionic Seebeck coefficient was enhanced by 7.3 times when the nanochannels were present. The silicon substrate with nanochannels demonstrated the highest performance for energy harvesting. Its power density reaches approximately 1.47 mW/m2, which was 13.3 times larger than the case without nanochannels. This investigation may open new opportunities for the future thermoelectric generators based on ionic transport in nanochannels.

Silicon nanowire arrays synthesized using the modified MACE process: Integration into chemical sensors and solar cells

In this work, the influence of the technological process for metal-assisted chemical etching on surface morphology and electrophysical properties of obtained nanostructures has been investigated. It has been demonstrated that the obtained structures with a high aspect ratio could be used both in sensors and solar cells. It has been shown that application of the metal-assisted chemical etching (MACE) process enables to significantly improve the short-circuit current density in silicon solar cells (up to 29 mA/cm2). Also, the possibility of detection of hydrogen peroxide and glucose (via enzymatic reaction) by resistor-like sensors with nanostructured silicon as the sensitive area has been demonstrated with the sensitivity up to 2.5…2.75 mA/V•%.

A systematically integrated recycling and upgrading technology for waste crystalline silicon photovoltaic module

Efficient resource recovery and reutilization are important for overcoming the immense challenges associated with waste photovoltaic module management and the limited raw material supplies, but these processes are significantly impeded by the low recycling efficiency and poor profitability of current technologies. Here, an integrated crystalline silicon cell regeneration technique is proposed by skillfully combining the sequential processes of nondestructive silicon cell recovery, wafer prepurification, one-step ultrapurification and texturing using metal-assisted chemical etching (MACE), and intrasystem reutilization of recovered materials. High-purity and intact silicon wafers are successfully recovered, and various anti-reflection textures including the intriguing dual-scale micro/nano structure are controllably constructed simultaneously by applying a single-step MACE process. The favorable thickness (165 μm), resistivity (1.02–2.28 Ω•cm) and carrier lifetime (1.12–2.47 μs) of the recovered silicon wafers, along with their ultralow reflectivity (5–15%) compared with commercial silicon wafers, make them excellent viable options for high-efficient photovoltaic module production. A rough economic assessment reveals that this integrated strategy presents a lower production cost than that of a conventional recovery process or the price of silicon wafer from industrial production process, and the fully recovered Al frame, tempered glass, Cu ribbons, and high-purity Ag and Al powders can be reused within the system for solar cell regeneration and as catalysts for MACE, resulting in economic viability and high resource sustainability.

A potential method to induce hydrophobicity on glass surface while retaining its anti-reflective abilities

Fabrication of anti-reflective and hydrophobic glass surfaces has been an active area of research. It has been reported that most of the coatings developing methods produces fragile and thin coatings which consequently have limited practical usage because of their abrasion-sensitivity. However, with thicker coatings, transparency of glass gets compromised, rendering it inoperable to be used in applications requiring transparency. To overcome the problem there is a need to have a solution that can practically produce glass substrates having hydrophobic properties without significant loss in transmittance, making them useful in anti-reflective hydrophobic applications. The presented work focuses on utilizing the effects of surface roughness and surface chemistry on commercial grade glass substrates to produce robust hydrophobic surfaces without appreciable compromise on transmittance. Metal Assisted Chemical Etching (MACE) was used to develop controlled structured surface roughness on the glass substrate whereas the surface chemistry was modified via silane treatment. The results demonstrated surface roughness and surface chemistry are both equivalently important to achieve an optimum surface having hydrophobic properties without any major transmittance lost. Successful implementation of the silver (Ag) particle assisted MACE process and chemistry modification of the glass surface resulted in contact angles above 100° with nearly 2–3% transmission lost in reference to the transmittance of parent glass substrates.

Wafer-Scale Fabrication of Silicon Nanocones via Controlling Catalyst Evolution in All-Wet Metal-Assisted Chemical Etching.

All-wet metal-assisted chemical etching (MACE) is a simple and low-cost method to fabricate one-dimensional Si nanostructures. However, it remains a challenge to fabricate Si nanocones (SiNCs) with this method. Here, we achieved wafer-scale fabrication of SiNC arrays through an all-wet MACE process. The key to fabricate SiNCs is to control the catalyst evolution from deposition to etching stages. Different from conventional MACE processes, large-size Ag particles by solution deposition are obtained through increasing AgNO3 concentration or extending the reaction time in the seed solution. Then, the large-size Ag particles are simultaneously etched during the Si etching process in an etching solution with a high H2O2 concentration due to the accelerated cathode process and inhibited anode process in Ag/Si microscopic galvanic cells. The successive decrease of Ag particle sizes causes the proportionate increase of diameters of the etched Si nanostructures, forming SiNC arrays. The SiNC arrays exhibit a stronger light-trapping ability and better photoelectrochemical performance compared with Si nanowire arrays. SiNCs were fabricated by using n-type 1–10 Ω cm Si(100) wafers in this work. Though the specific experimental conditions for preparing SiNCs may differ when using different Si wafers, the summarized diagram will still provide valuable guidance for morphology control of Si nanostructures in MACE processes.

Optimized Hole Injection, Diffusion, and Consumption for Efficient Metal-Assisted Chemical Etching Depending on the Silicon Doping Type and Metal Catalyst Area

Even though metal-assisted chemical etching (MACE) has been widely used for silicon (Si) nano/micromachining due to its low cost and high applicability, it is still difficult to fabricate elaborate Si nano/microstructures for high-value applications. In particular, further studies about effective factors in the MACE process with systematic approaches are highly required to obtain explicitly controlled MACE products. In this study, how the Si doping type and gold (Au) catalyst area influence MACE products in terms of the hole behavior during MACE is investigated. The type-dependent etching rate of MACE, amount of reaction products, and lateral etching/porosity of the MACE product are characterized with hole injection, diffusion, and consumption scenarios, respectively. Moreover, it is demonstrated that the dependence of the etching rate of MACE and stability/porosity of the MACE product on the Au catalyst area can be understood from the point of view of hole injection per interfacial contact plane area of the Au catalysts. As a result, elaborately controlled Si nano/microstructures can be obtained through an effective and accurate MACE process due to the optimized hole behavior. These findings could increase the accessibility and commercial availability of MACE products in many fields.