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Porous silicon nanowires (pSi NWs) have attracted considerable interest due to their unique structural, optical properties and biocompatibility. The most common method for their top-down synthesis is metal-assisted chemical etching (MACE) of crystalline silicon (c-Si) wafers using silver nanoparticles as a catalyst. However, the replacement of silver with bioinert gold nanoparticles (Au NPs) markedly improves the efficiency of pSi NWs in biomedical applications. The present study demonstrates the fabrication of porous pSi NWs arrays using Au NPs as the catalyst in MACE of c-Si wafers with a resistivity of 1–5 mOhm·cm. Using scanning electron microscopy (SEM), the formation of arrays of porous nanowires with a diameter of 50 nm that consist of small silicon nanocrystals (nc-Si) and pores was observed. Raman spectroscopy analysis determined the size of nc-Si is about 4 nm. The pSi NWs exhibit effective photoluminescence (PL) with a peak in the red spectrum, which is attributed to the quantum confinement effect occurred in small 4 nm nc-Si. In addition, the pSi NWs exhibit low toxicity towards MCF-7 cancer cells, and their PL characteristics allow them to be used as contrast agents for bioimaging

Hydrogen is an active substitute for current commercial fuels as a green energy source. However, the progress toward hydrogen economy is stalled due to a lack of efficient and economical storage systems usually restrained by the material used. Some specific issues still need to improve in recent practical hydrogen storage methods: low surface area, limited capacity to attract gas adsorbate, high-temperature medium for hydrogen desorption, and extreme reactivity towards Oxygen and air. The paper presents a study on the surface modification of silicon nanostructures (SiNSs) synthesized by chemical etching, followed by structure optimization to enhance the hydrogen storage capacity. The standard anodization method prepares porous silicon (PS) while metal-assisted chemical etching fabricates Si and Porous Si nanowires (PSiNWs) on Si and PS substrates. Morphological features like surface area and porous nature were evaluated using SEM, AFM, and BET to interpret the gas-surface interaction. The modified surface area and average roughness of SiNSs increase maximum to 437 m2 g−1 and 501 nm, respectively. The PS shows the highest gas-surface interaction (up to 86.6%) and surface energy due to uniform pore distributions and high surface area. The NMR and FTIR investigate the hydrogen termination on the surface, whereas TG-DSC indicates the desorption of surface hydrides at ∼290 °C. A rank matrix is constructed considering various adsorption-desorption and fabrication parameters to select optimized SiNS, whereas PS is selected as a suitable candidate for hydrogen storage. • Electrochemical anodization and metal-assisted chemical etching fabricate porous silicon and porous Si nanowires. • The gas physisorption improves in nanopores due to the overlap of attractive fields from adjacent pore walls. • High resistivity substrate Si nanostructures exhibit uniform and homogeneous pore size distribution. • Continuous water rinsing on the hydrogenated surface of Si nanostructures causes hydrogen desorption from the surface. • Porous silicon stands prominently and has a high potential for hydrogen storage based on the rank matrix. [ABSTRACT FROM AUTHOR]

—Rapid and accurate bacterial identification plays a crucial role across diverse sectors, offering valuable applications. While traditional culture and molecular techniques maintain a high standard, the contemporary demand for diagnostics centers on swiftly delivering dependable insights into bacterial infections, directly at the site, within a short timeframe. In this study, we present a novel approach utilizing surface-enhanced Raman scattering (SERS)-active nanostructured substrates in the form of gold- and silver-coated porous silicon nanowires (AuAg@pSiNWs) for rapid and highly sensitive bacterial detection. The porous silicon nanowires are fabricated using a straightforward method known as metal-assisted chemical etching. Subsequently, silver and gold decoration is achieved through chemical reduction of metal salts, imbuing the substrates with SERS-active properties. Scanning electron microscopy data reveals that upon incubation with AuAg@pSiNWs, bacteria are localized amidst clusters and on the surface of the nanowires, particularly in the vicinity of bimetallic gold and silver nanoparticles. Illustrated through the utilization of Listeria innocua bacteria as a model, the SERS efficacy of AuAg@pSiNWs is highlighted, enabling rapid label-free bacterial diagnosis with a limit of detection of 1.14 × 104 CFU/mL. [ABSTRACT FROM AUTHOR]

Abstract Porous silicon nanowires (PSiNWs) surfaces were grafted with organic functional groups. The obtained hybrid structure was tested as probe electrode for the electrochemical detection of mercury (Hg(II)) in solution. An electrochemical study of the Glycyl-Glycyl-Glycine-modified PSiNWs electrode is achieved in the presence of mercury ions. The recorded cyclic voltammograms show a quasi-irreversible process corresponding to the Hg(II)/Hg couple. These results demonstrate the potential role of peptides grafted on porous silicon nanowire in developing strategies for simple and fast detection of metal ions in solution. [ABSTRACT FROM AUTHOR]

Graphical abstract The marriage of porous silicon nanowires and OTS provides a barrier to water molecules in gas detection process. Highlights • The interferences of different humidity in gas detection progress were studied for SiNWs-based gas sensors. • OTS was utilized to modify partially insensitive porous SiNWs surface with the Si-O bond for improving humidity resistance. • The ultra-low concentration NO 2 detection and excellent reversibility of the OTS/SiNWs sensor was observed in high humidity. • Possible mechanism for enhanced performance of OTS/SiNWs was analyzed. Abstract Porous silicon nanowires (SiNWs) is considered as one of promising candidates for high-performance NO 2 sensor application in human health and quality life owing to its fast response and stable operation at room temperature. However, the development of SiNWs-based gas sensors for stable detection of ppb-level NO 2 in practical high-humidity environments remains a challenge. In this study, octadecyltrichlorosilane (OTS) was utilized to modify partially insensitive SiNWs surface with the Si-O bond, yielding an immunity to high humidity in gas detection process. The modified OTS provides an umbrella-like barrier blocking water molecules attraction on the nanowire surface which disinhibits the main adsorption sites and sensing channels of the sensor. The OTS-modified porous SiNWs (OTS/SiNWs) sensor exhibited a practical capability to detect 5 ppb NO 2 at humidity of 25%–55% RH and a response of 1.8% to 50 ppb NO 2 at 75% RH at room temperature. Moreover, the fast response/recovery characteristics and good reversibility of the OTS/SiNWs sensor was observed even under high humidity conditions. In addition, the OTS/SiNWs sensor displayed excellent stability to humidity variations. The enhanced gas sensing performance is attributed to a marriage of porous structure of SiNWs and OTS. The simple modification for SiNWs-based gas sensor will broaden new avenues, not only in chemical trace detection, but also in high humidity health monitoring. [ABSTRACT FROM AUTHOR]

Here, the non-specific interaction of the H1N1 influenza virus with a porous layer of silicon nanowires (PSi NWs) was studied by transmission and scanning electron microscopy (TEM, SEM, respectively) and optical spectroscopy. PSi NW layer with a thickness of about 200 nm was fabricated by metal-assisted chemical etching of p-type highly doped crystalline silicon wafers, and consist of porous nanowires with a diameter of 50–200 nm, and a distance between the nanowires of 100–200 nm. It was shown that during the adsorption of viruses, viral particles with a diameter of about 100 nm bind to the porous surface of the nanowires. This interaction was revealed using TEM, SEM, and causes wavelength shifts in the Fabry–Perot fringes in the reflection spectrum of visible light from the PSi NW layer. The results show that thin layers of PSi NWs are a promising nanomaterial for creating filters and sensors for binding and detection of viruses.

As a most promising anode candidate, silicon has the shortcomings of low electron conductivity and high volume expansion of 300%, hindering its applications in lithium ion batteries (LIBs). In this study, one-dimension porous silicon nanowires (pSi-NWs) were prepared through a simple metal-assisted chemical etching process by using metallurgical silicon as raw material. To consolidate the structural integrity of pSi-NWs, a crossed carbon skeleton (c-Cs) was introduced into pSi-NWs via in-situ polymerization and carbonization process. The resultant pSi-NWs@c-Cs composite delivered the high capacity of 1253 mAh g −1 with good cycling stability as well as the notable rate capability (476 mAh g −1 at 4 A g −1 ), much superior to those of pSi-NWs and pSi-NWs@reduced graphene oxide composite. The enhanced electrochemical performance of pSi-NWs@c-Cs composite is attributed to the crossed carbon skeleton in constructing the advanced Si/C interface to more effectively improve the electron conductivity of pSi-NWs and acting as the protective shell to keep the structure integrity of pSi-NWs. As the additive of commercial graphite anode, 28% of capacity augment (460 mAh g −1 at 0.2 A g −1 ) was realized by adding 20 wt% of pSi-NWs@c-Cs composite into graphite, demonstrating its promising applications in LIBs. [ABSTRACT FROM AUTHOR]

Porous silicon nanowires (PSiNWs) array is a promising material for development of integrated gas sensors operating at room temperature. This work reports the fabrication of PSiNWs assembly with different structural features and its effect on gas‐sensing performance. Bundling and well separating PSiNWs arrays are fabricated by MACE method, respectively, based on the effective modulation of surface wettability of the initial Si substrate. The HF pretreatment creates a hydrophobic surface favorable for deposition of irregular Ag nanoflakes and then for the formation of bundling PSiNWs array. In contrast, the PSiNWs with well lateral separation are formed based on the predeposited uniform Ag nanoparticles on a hydrophilic Si surface. The PSiNWs array featured by tip‐clusters is proved to be highly effective in achieving highly sensitive and rapid response to NO2 gas at room temperature. Satisfying dynamic characteristic and selectivity are meanwhile observed for the bundling array. The formation of the bundling or separating of PSiNWs is discussed in terms of the force balance of individual nanowire, which is further correlated with non‐uniform distribution of Ag nanoclusters caused by H‐termination. Meanwhile, high sensing performance of bundling nanowires is analyzed based on the structural promotion of the unique configuration of tip‐cluster to sensing response. [ABSTRACT FROM AUTHOR]

Porous silicon nanowire arrays are shown to be used for optical detection of molecular oxygen (O2). The samples are produced by metal assisted chemical etching of heavily boron‐doped wafers of crystalline silicon. It is found that the photoluminescence signal from porous silicon nanowires quenched in O2 ambience but restored again in N2 atmosphere, which repeated in several cycles. Electron paramagnetic resonance data demonstrate that number of silicon dangling bonds (Pb‐center) increases after O2 adsorption, but then drops in vacuum. The experimental results are explained by a microscopic model taking into account the reversible charging/recharging of surface defects (Pb‐centers) due to the oxygen adsorption/desorption. The obtained results indicate that porous silicon nanowires are prospective for applications as reversible optical gas sensor. [ABSTRACT FROM AUTHOR]

Porous materials provide a large surface-to-volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure-property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single-crystalline silicon nanowires using a novel electron-beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m−1 K−1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

Abstract Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A “red-shift” of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices.

In this paper, we report, for the first time, an investigative study involving the engineering of lightly doped porous silicon nanowire arrays (pSiNWs) by exposing solid silicon nanowire arrays (SiNWs) to an acid vapor emanating from HF/HNO 3 hot solution. SEM and TEM images exhibit vertically distributed SiNW arrays on the whole silicon (Si) surface with relatively smooth surface sidewalls. By submitting the SiNW arrays to Acid Vapor Etching (AVE), they become porous with a substantial decrease in their densities and lengths. Increasing etching duration leads to a higher porosity without affecting the wire diameter which remains almost constant nearly 100 nm. Exceeding a critical etching duration, a porous structure is observed superseding the SiNW structure. The morphological characterizations have been correlated to the optical properties. We note a blue shift of the strong visible photoluminescence (PL) bands after AVE treatment due to the decrease of the silicon quantum dots diameter (Si-QDs). UV–Visible measurement shows a decrease of the total reflectivity by 5% after AVE treatment. [ABSTRACT FROM AUTHOR]

In this paper, we report a comparative study between porous silicon (pSi) and porous silicon nanowires (pSiNWs). Acid Vapor Etching (AVE) treatment has been used to perform porous structure on flat Si and SiNWs array substrates respectively. SiNW structure is prepared by the widely used Silver catalyzed etching method. SEM and TEM images show that AVE treatment induces porous structure in the whole Si wafer and the SiNW sidewall. Comparatively to pSi, pSiNWs exhibit a low reflectivity in the whole spectral range which decreases with etching duration. However, the reflectivity of pSi changes with porous layer thickness. Both pSi and pSiNWs exhibit a significant PL peak situated at 2 eV. PL peaks are attributed to the quantum confinement effect in the silicon nanocrystallites (SiNCs). We discussed the significant enhancement in the peak intensities and a shift toward lower energy displayed in Raman spectra for both pSi and pSiNWs. We reported a correlative study of the AVE treatment effect on the minority carrier life time of flat silicon and SiNW arrays with the passivation effect of chemical induced silicon oxides highlighted by FTIR spectra. [ABSTRACT FROM AUTHOR]

Extremely cheap mesoporous silicon nanowires by metal‐assisted chemical etching of inexpensive metallurgical silicon have a higher porosity and thus volume expansion possibility than simple silicon nanowires. Metallurgical Si nanowires (MG‐SiNWs) show a reversible capacity of about 2111 mAh g−1 at a current rate of 0.2 C, an encouraging stability of over at least 50 cycles, and a good rate capability. [ABSTRACT FROM AUTHOR]

In this paper, the moderately and lightly doped porous silicon nanowires (PSiNWs) were fabricated by the 'one-pot procedure' metal-assisted chemical etching (MACE) method in the HF/HO/AgNO system at room temperature. The effects of HO concentration on the nanostructure of silicon nanowires (SiNWs) were investigated. The experimental results indicate that porous structure can be introduced by the addition of HO and the pore structure could be controlled by adjusting the concentration of HO. The HO species replaces Ag as the oxidant and the Ag nanoparticles work as catalyst during the etching. And the concentration of HO influences the nucleation and motility of Ag particles, which leads to formation of different porous structure within the nanowires. A mechanism based on the lateral etching which is catalyzed by Ag particles under the motivation by HO reduction is proposed to explain the PSiNWs formation. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the simple pre-oxidization process is firstly used to treat the starting silicon wafer, and then MPSiNWs are successfully fabricated from the moderately doped wafer by one-step MACE technology in HF/AgNO3 system. The PL spectrum of MPSiNWs obtained from the oxidized silicon wafers show a large blue-shift, which can be attributed to the deep Q. C. effect induced by numerous mesoporous structures. The effects of HF and AgNO3 concentration on formation of SiNWs were carefully investigated. The results indicate that the higher HF concentration is favorable to the growth of SiNWs, and the density of SiNWs is significantly reduced when Ag+ ions concentrations are too high. The deposition behaviors of Ag+ ions on oxidized and unoxidized silicon surface were studied. According to the experimental results, a model was proposed to explain the formation mechanism of porous SiNWs by etching the oxidized starting silicon. [Copyright &y& Elsevier]

Highlights: [•] The metal-assisted chemical etching of c-Si leads to the formation of porous SiNWs. [•] The SiNWs formation is a result of a series of redox reactions in AgNO3/HF solution. [•] The formed silver particles regulated the sizes of SiNWs and their orientations. [•] The size, the porosity, and the reflectivity of the SiNWs depend on etching time. [•] The QSSPC measurement reveals a detorioration of the minority carrier lifetime. [ABSTRACT FROM AUTHOR]

Highlights: [•] Measured reflectance of porous silicon nanowires films inferior to 0.1%. [•] Modeling using the transfer matrix formalism. [•] Absorbance of porous silicon nanowires superior to that obtained with silicon nanowires. [•] Ultimate efficiency equal to 25% for normal incidence angle. [Copyright &y& Elsevier]

Abstract: One-step synthesis of lightly doped porous silicon nanowire arrays was achieved by etching the silicon wafer in HF/AgNO3/H2O2 solution at room temperature. The lightly doped porous silicon nanowires (pNWs) have circular nanopores on the sidewall, which can emit strong green fluorescence. The surface morphologies of these nanowires could be controlled by simply adjusting the concentration of H2O2, which influences the distribution of silver nanoparticles (Ag NPs) along the nanowire axis. A mechanism based on Ag NPs-induced lateral etching of nanowires was proposed to explain the formation of pNWs. The controllable and widely applicable synthesis of pNWs will open their potential application to nanoscale photoluminescence devices. [Copyright &y& Elsevier]

Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A 'red-shift' of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices. [ABSTRACT FROM AUTHOR]

Porous silicon nanowires (PSi NWs) are a promising material for applications in sensorics, photovoltaics, and biomedicine. However, the structural and optical properties of PSi NWs are extremely sensitive to the concentrations of chemical solutions used in their manufacture. In this study, PSi NWs were fabricated by a two step metal-assisted chemical etching method from highly doped crystalline silicon wafers with a resistivity of <0.005 Ω cm. It was shown that the structural and optical properties of the obtained samples depend on the concentration of hydrogen peroxide (H 2 O 2), which in our experiments varied from 5 to 30%. As a result of etching, a layer of NWs with a diameter of 50–200 nm is obtained from above, and a thin layer of porous silicon is obtained from below. The ratio of the thicknesses of these two layers can be tuned by choosing the required concentration of H 2 O 2. The porosity of the samples, calculated in the approximation of the effective Bruggeman medium from the IR reflection spectra, was 40–50%. Electron microscopy demonstrated the presence of a large amount of silicon nanocrystals in the volume of PSi NWs, which exhibited efficient visible photoluminescence due to quantum confinement effect. It was shown that the size of the nanocrystals can be also tunable on the used concentration of H 2 O 2. [ABSTRACT FROM AUTHOR]