Si Nanowire Arrays Research Articles

2D Fractal Arrays of Ultrathin Silicon Nanowires as Cost‐Effective and High‐Performance Substrate for Supercapacitors

Silicon is the most diffused material in the industry; thus, considering its high capacity for energy storage, silicon‐based materials are well studied as battery anodes and supercapacitors. Si nanowires (NWs) emerge due to the high surface to volume ratio, its compatibility with a wafer processing typical of microelectronics, and are studied as anodes for lithium batteries as well as coupled with other materials for supercapacitor application. In this article, the synthesis and application are reported as a lithium anode of 2D fractal arrays of ultrathin Si NWs obtained by a thin‐film metal‐assisted chemical etching (MACE). These Si NWs exhibit a density of about 1012 NWs cm−2, maximizing the surface to volume ratio compared to silver‐salts MACE and other NW fabrication approaches. By using 2.7 μm long NWs, a pseudo‐capacitor behavior with a specific capacitance of about 274.2 μF cm−2 at a scan rate of 50 mV s−1 is obtained. This specific capacitance is two orders of magnitude higher than the one obtained in the same condition by using NWs synthesized by silver‐salt MACE. In this result, the route is opened toward the application of these fractal arrays of ultrathin Si NWs as substrate for supercapacitors with improved efficiency.

Silicon Nanowire Array Weaved by Carbon Chains for Stretchable Lithium-Ion Battery Anode.

To manufacture flexible batteries, it can be a challenge for silicon base anode materials to maintain structural integrity and electrical connectivity under bending and torsion conditions. In this work, 1D silicon nanowire array structures combined with flexible carbon chains consisting of short carbon nanofibers (CNFs) and long carbon nanotubes (CNTs) are proposed. The CNFs and CNTs serve as chain joints and separate chain units, respectively, weaving the well-ordered Si nanowire array into a robust and integrated configuration. The prepared flexible and stretchable silicon array anode exhibits excellent electrochemical performance during dynamic operation. A high initial specific capacity of 2856mAhg-1 is achieved. After 1000 cycles, a capacity retention of 60% (1602mAhg-1) is maintained. Additionally, the capacity attenuation is less than 1% after 100 bending cycles. This excellent cycling stability is obtained with a high Si loading of 6.92mgcm-2. This novel approach offers great promise for the development of high-loading flexible energy-storage devices.

Facile construction of nanowires via etching process to increase heterostructure area for efficient photocatalysis

Semiconductor photocatalysts can eliminate organic pollutants completely, but the poor utilization of solar energy and rapid recombination of photogenerated electron-hole pairs have become a vital obstacle for their widespread application. Herein Si/TiO2 nanowire arrays with core–shell structures were fabricated by metal-assisted chemical etching and atomic-layered deposition. The physical properties and photocatalytic performance of the specimens were tested and compared with pristine Si nanowire arrays. The specimens showed perfect light absorption. The photoluminescence was generally depressed by the shell layer, and the photocurrent and photocatalytic efficiency of Si/TiO2 were improved by 297 and 2 times respectively in contrast to Si. The magnitude of the photocurrent and photocatalytic activity could be modulated by the etching period, in which an ideal etching time of 2 h produced the highest photocatalytic performance. The high efficient photocatalysis of the heterostructure was well illustrated by the Z-scheme band alignment and large specific surface area.

Silicon nanowires array capped with two shells as light-absorption antenna for self-driven broadband photodetectors

Silicon (Si) is one of the most important semiconductor materials, and Si in nanoscale has a direct bandgap; therefore, it can overcome the issues of poor light absorption for its bulk counterpart [Lu et al., Appl. Phys. Lett. 91, 263107 (2007)]. Currently, much interest is focusing on Si nanowires array (Si-NWA) for its unique characteristics, such as the enhanced light absorption and the superior electronic mobility, for photodetectors and solar cells [Ko et al., Mater. Sci. Semicond. Process. 33, 154–160 (2015) and Xie et al., ACS Nano 8, 4015–4022 (2014)]. Si-NWA or pyramid Si based photodetectors usually show higher performance than those based on the Si wafer due to the enhanced light absorption and the radial heterojunction [Coskun et al., Physica B 604, 412669 (2021) and Xiao et al., Adv. Mater. 30, 1801729 (2018)]. However, the light absorption spectrum of Si-NWA is limited within the near-infrared region, and its surface defects reduce the carriers' lifetime.

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

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

High‐Efficiency Silicon Nanowire Array Near Infrared Photodetectors via Length Control and SiOx Surface Passivation

AbstractSilicon (Si) nanowire (NW) array is a promising light‐trapping platform due to the strong interaction between light and nanostructure. A photodetector benefits from the improved optical absorption in the Si NW array. Although the optical absorption increases with the NW length, the large NW length is not always preferable owing to the large surface area. Herein, the systematic study on the Si NW array photodetectors with varied NW lengths is investigated. It is revealed that the photodetectors with 1 µm length provide a highest responsivity of 0.65 A W−1 and a specific detectivity of 1.40 × 109 cm Hz1/2 W−1 at the wavelength of 1000 nm, including the dark current of 54 µA at 1 V. In addition, the silicon oxide (SiOx) surface passivation is introduced to induce the high photogain. As a result, the responsivity is improved by 13 times (0.55 A W−1) at 1100 nm. This work proposes high‐efficiency Si NW array photodetectors by the NW array length control and the SiOx surface passivation.

Laser-Activated Second Harmonic Generation in Flexible Membrane with Si Nanowires.

Nonlinear silicon photonics has a high compatibility with CMOS technology and therefore is particularly attractive for various purposes and applications. Second harmonic generation (SHG) in silicon nanowires (NWs) is widely studied for its high sensitivity to structural changes, low-cost fabrication, and efficient tunability of photonic properties. In this study, we report a fabrication and SHG study of Si nanowire/siloxane flexible membranes. The proposed highly transparent flexible membranes revealed a strong nonlinear response, which was enhanced via activation by an infrared laser beam. The vertical arrays of several nanometer-thin Si NWs effectively generate the SH signal after being exposed to femtosecond infrared laser irradiation in the spectral range of 800-1020 nm. The stable enhancement of SHG induced by laser exposure can be attributed to the functional modifications of the Si NW surface, which can be used for the development of efficient nonlinear platforms based on silicon. This study delivers a valuable contribution to the advancement of optical devices based on silicon and presents novel design and fabrication methods for infrared converters.

Thickness dependent field emission study of LaB6 coated Si nanowire arrays

A simple physical metal mask is efficiently employed to obtain an array of silicon (Si) nanowires (Si-NWs) on a Si substrate grown using the hot filament chemical vapor deposition route. Well adhered and uniform coating of different thicknesses (20 and 50 nm) of lanthanum hexaboride (LaB6) on Si-NWs was obtained using electron beam evaporation technique. The thickness of LaB6 coating was estimated from ellipsometry measurement. Structural, morphological, and chemical properties of the LaB6 coated Si-NWs (LaB6@Si-NWs) arrays were revealed using x-ray diffraction, field emission scanning electron microscope, transmission electron microscope, Raman spectroscopy, and x-ray photoelectron spectroscopy. Field electron emission characteristics of pristine Si-NW array and LaB6 coated Si-NWs array emitters were studied in planar diode configuration at a base pressure of 1 × 10−8 mbar. The values of turn-on field (current density ∼1 μA/cm2) were observed as ∼2.2, 1.2, and 1.6 V/μm for pristine Si-NWs, LaB6@Si-NWs_20, and LaB6@Si-NWs_50 array emitters, respectively. Furthermore, maximum emission current densities of ∼1276.81, 2763.64, and 2231.81 μA/cm2 have been extracted from the pristine Si-NWs, LaB6@Si-NWS_20, and LaB6@Si-NWS_50 array emitters at an applied field of 3.1, 2.7, and 2.7 V/μm, respectively. The LaB6@Si-NWS_20 array emitter demonstrated superior FEE properties as compared to the pristine Si-NWs and LaB6@Si-NWS_50 emitters. Furthermore, LaB6@Si-NWS_20 emitter depicted very good emission current stability tested at a preset value of 1 μA over a duration of 3 h. The enhanced FEE performance exhibited by the LaB6@Si-NWs_20 array emitter is attributed to reduction in effective work function and enhanced electron tunneling probability across the LaB6–Si interface.

Solar light-driven selective photoelectrochemical CO2 reduction to CO in aqueous media using Si nanowire arrays decorated with Au and Au-based metal nanoparticles

To address recent energy and environmental issues, such as global warming and resource depletion, significant interest has been shown in carbon dioxide (CO2) fixation based on photoelectrochemical processes under solar light irradiation. The present paper describes the applicability of gold nanoparticles-decorated silicon nanowire arrays (Au/SiNW) as photoelectrodes to promote CO2 reduction. The decoration with Au nanoparticles of SiNW was performed by an electroless plating utilizing surface hydrogen-terminated silicon groups, generated during the nanowire formation process. Au/SiNW exhibits efficient photoelectrochemical performance for CO2 reduction to produce CO selectively in an aqueous medium under simulated solar light irradiation owing to its vertically aligned nanowire structure and Au nanoparticles as cocatalysts. The former provides high specific surface area and light trapping effect, and the latter induces selective interaction with CO2. Moreover, a unique two-steps method for Au decoration that consists of photo-assisted deposition of copper nanoparticles and the following electroless plating to replace Cu atoms to Au ones achieves more uniform decoration of SiNW with highly dispersed core–shell structured Cu@Au nanoparticles. The resulting photoelectrode, termed Cu@Au/SiNW, shows improved selectivity toward CO production and gives a good Faradic efficiency of 72% in an aqueous medium.

Future Prospects of Luminescent Silicon Nanowires Biosensors.

In this paper, we exploit the perspective of luminescent Si nanowires (NWs) in the growing field of commercial biosensing nanodevices for the selective recognition of proteins and pathogen genomes. We fabricated quantum confined fractal arrays of Si NWs with room temperature emission at 700 nm obtained by thin-film, metal-assisted, chemical etching with high production output at low cost. The fascinating optical features arising from multiple scattering and weak localization of light promote the use of Si NWs as optical biosensing platforms with high sensitivity and selectivity. In this work, label-free Si NW optical sensors are surface modified for the selective detection of C-reactive protein through antigen-gene interaction. In this case, we report the lowest limit of detection (LOD) of 1.6 fM, fostering the flexibility of different dynamic ranges for detection either in saliva or for serum analyses. By varying the NW surface functionalization with the specific antigen, the luminescence quenching of NW biosensors is used to measure the hepatitis B-virus pathogen genome without PCR-amplification, with an LOD of about 20 copies in real samples or blood matrix. The promising results show that NW optical biosensors can detect and isolate extracellular vesicles (EV) marked with CD81 protein with unprecedented sensitivity (LOD 2 × 105 sEV/mL), thus enabling their measurement even in a small amount of blastocoel fluid.

The Array of Si Nanowires Covered with Ag Nanoparticles by ALD: Fabrication Process and Optical Properties

In this work, we proposed a method for creating an Ag/Si composite structure consisting of an array of vertical silicon nanowires (SiNWs) decorated with silver nanoparticles (AgNPs). A two-stage metal-assisted chemical etching of Si was used to obtain the SiNW array, and atomic layer deposition was used to fabricate the AgNPs. A uniform distribution of AgNPs along the SiNW height was achieved. The measured characteristics by spectroscopic ellipsometry directly established the presence of AgNPs deposited on the SiNWs. The height of the sublayers and the fractions of Si and Ag in them were determined using the multilayer model and the effective Bruggeman medium approximation in the interpretation of the experimental data. For AgNP layers deposited on an Si wafer surface, the thickness (from 2.3 to 7.8 nm) and complex dielectric functions were verified within the framework of the Drude–Lorentz model. The optical properties of Ag/SiNW structures with complex spatial geometry were simulated in the COMSOL Multiphysics software. The expected localization of the electric field on the surface and near the AgNP was observed as a result of the plasmon resonance excitation. The calculated enhancement factor reached 1010, which indicates the possibility of using such structures as substrates for surface-enhanced Raman scattering.

Freestanding Metal Nanomembranes and Nanowires by Template Transfer with a Soluble Adhesive.

The fabrication of nanostructures usually involves chemical processes that have in certain steps. Especially, it is necessary to use the chemical etching method to release the as-patterned structures from the substrate in most of the transfer techniques. Here, a novel scheme of template transfer as developed for the fabrication of freestanding Au nanomembranes and nanowires by using a soluble PVP adhesive. The nanomembranes feature the periodic nanohole arrays with high uniformity. Without the substrates, these plasmonic nanohole arrays show symmetric and antisymmetric resonance modes with bright and dark spectral features, respectively, in transmission. Through the spectral analysis for reflection, we have disclosed that the usual dark mode in transmission is not really dark, but it reveals a distinct feature in reflection. Two coupling modes present distinct spectral features in transmission and reflection due to their different loss channels. To show their versatility, the freestanding nanomembranes were also employed as secondary templates to form Si nanowire arrays by the metal-assisted chemical etching method.

(Invited, Digital Presentation) Photovoltaic Applications Using Energy Transfer Characteristics from Quantum Dots

Silicon-related materials are most commonly used in solar cells and become now an invaluable material. However, a reported maximum energy conversion efficiency of Si solar cell is close to reaching its theoretical limits. To further improve the cell performance and create new functions, it will become increasingly important to functionalize Si materials using nanostructures. One- and zero-dimensional Si nanostructures called Si nanowires (SiNWs) and Si quantum dots (Si QDs) are increasingly being used as new solar cell materials. Our group has shown that the conversion efficiency of solar cells can be increased by the non-radiation energy transfer (NRET) from Si QDs [1]. Recently, research has been developed on compound semiconductor QDs and perovskite QDs, and the conversion efficiency has been successfully increased [3-7].n-type SiNW arrays were fabricated by electroless etching and Bosch & nanoimprint lithography followed by CVD process. The CVD process was performed to form p-type Si layer for pn homojunction. Hybrid heterojunction cells of PEDOT:PSS and n-SiNWs were also fabricated. Passivation by ozone and hydrogen were applied to improve the solar cell properties [2]. After the cell fabrications, the solar cells were coated with Si QDs, CdZnS/ZnS QDs, CdZnSe/ZnS QDs and perovskite QDs. The fabrication methods and conditions of QDs have been reported elsewhere [1-7]Energy transfer effects such as NRET are new ways of increasing solar cell efficiency. To effectively use the NRET effect, the surface of Si QDs should be fully passivated by ligand molecules. The NRET process is a highly distance-dependent phenomenon, and its dependence on the length of the passivation ligands clearly showed this. NRET efficiency was increased by shortening the ligand length from 1-octadecene to 1-octene, resulting in higher JSC, ultimately providing higher energy conversion efficiency. The efficiency was increased about 1-2 % by adding SiQDs. We also observed the same effect for CdZnS/ZnS QDs, CdZnSe/ZnS QDs and perovskite CsPbCl3 QDs. In these cases, in addition to the NRET effect, the radiative energy transfer (RET) effect also contributes to the increase in conversion efficiency.

Self-Assembled Au Nanoparticle Monolayers on Silicon in Two- and Three-Dimensions for Surface-Enhanced Raman Scattering Sensing.

Gold nanoparticle/silicon composites are canonical substrates for sensing applications because of their geometry-dependent physicochemical properties and high sensing activity via surface-enhanced Raman spectroscopy (SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized via wet-chemistry on functionalized flat silicon (Si) and vertically aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective approach to prepare such substrates. Herein, we report on the critical parameters that influence nanoparticle coverage, aggregation, and assembly sites in two- and three-dimensions to prepare substrates with homogeneous optical properties and SERS activity. We show that the degree of AuNP aggregation on flat Si depends on the silane used for the Si functionalization, while the AuNP coverage can be adjusted by the incubation time in the AuNP solution, both of which directly affect the substrate properties. In particular, we report the reproducible synthesis of nearly touching AuNP chain monolayers where the AuNPs are separated by nanoscale gaps, likely to be formed due to the capillary forces generated during the drying process. Such substrates, when used for SERS sensing, produce a uniform and large enhancement of the Raman signal due to the high density of hot spots that they provide. We also report the controlled self-assembly of AuNPs on VA-SiNW arrays, which can provide even higher Raman signal enhancement. The directed assembly of the AuNPs in specific regions of the SiNWs with a control over NP density and monolayer morphology (i.e., isolated vs nearly touching NPs) is demonstrated, together with its influence on the resulting SERS activity.

Photo-assisted decoration of Ag-Pt nanoparticles on Si photocathodes for reducing overpotential toward enhanced photoelectrochemical water splitting

Developing new catalysts to decorate photoelectrodes has been widely used to enhance the performance of photoelectrochemical (PEC) cells. However, the high cost, complex synthesis, and poor stability of catalyst decoration strongly hinder its practical application. Here, we report a facile and low-cost decoration of Ag-Pt nanoparticles (Ag-Pt NPs) on Si photocathodes with TiO2/Ti sacrificial overlayers. Such a decoration does not rely on any metallic-ion precursor solution since it is formed automatically via galvanic replacement reactions during PEC measurements; that is, Ti is displaced by Ag+ and Pt2+ ions, which are from the employed reference and counter electrodes, respectively. The as-decorated Ag-Pt NPs are verified to significantly enhance the hydrogen evolution reduction kinetics without substantially degrading the optical performance of Si photocathodes. Owing to optoelectronic advantages, the overpotential required to maintain a photocurrent density of 10 mA cm−2 (under AM 1.5G illumination) is reduced from −0.8 VRHE (for the bare planar Si photocathode) to −0.1 VRHE (for the planar Si photocathode with Ag-Pt NP decoration). Moreover, a further anodic shift (to 0 VRHE) is visible for the Si nanowire array photocathode with Ag-Pt NP decoration, along with high long-term stability of the PEC response in acidic and neutral electrolytes. This study opens a new opportunity for the photo-assisted decoration of various alloy NPs on the morphology-varying photoelectrodes with different applications.

Light Trapping of Inclined Si Nanowires for Efficient Inorganic/Organic Hybrid Solar Cells.

Light/matter interaction of low-dimensional silicon (Si) strongly correlated with its geometrical features, which resulted in being highly critical for the practical development of Si-based photovoltaic applications. Yet, orientation modulation together with apt control over the size and spacing of aligned Si nanowire (SiNW) arrays remained rather challenging. Here, we demonstrated that the transition of formed SiNWs with controlled diameters and spacing from the crystallographically preferred <100> to <110> orientation was realized through the facile adjustment of etchant compositions. The underlying mechanism was found to correlate with the competing reactions between the formation and removal of oxide at Ag/Si interfaces that could be readily tailored through the concentration ratio of HF to H2O2. By employing inclined SiNWs for the construction of hybrid solar cells, the improved cell performances compared with conventional vertical-SiNW-based hybrid cells were demonstrated, showing the conversion efficiency of 12.23%, approximately 1.12 times higher than that of vertical-SiNW-based hybrid solar cells. These were numerically and experimentally interpreted by the involvement of excellent light-trapping effects covering the wide-angle light illuminations of inclined SiNWs, which paved the potential design for next-generation optoelectronic devices.

Large-scale flexible membrane with resonant silicon nanowires for infrared visualization via efficient third harmonic generation

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale. Indeed, various nanophotonic designs have been developed to demonstrate high efficiencies for third harmonic generation (THG). However, fabrication approaches for these designs usually involve high-cost lithography-based technological steps. This work reports on the efficient THG in a free-standing Si nanowire array encapsulated into a flexible polymer membrane. High nonlinearity of Si material χ1111(3)≈2.62×10−19m2/V2 and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (1200–2000 nm) to visible (400–666 nm) upconversion. The fabricated membranes demonstrate high flexibility and transparency, which make them convenient to use as infrared light visualizers.

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.

Silicon nanowire/polymer membrane for infrared visualization via third-harmonic generation

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale.his work reports on the efficient third-harmonic generation in a free-standing Si nanowire array encapsulated into a polymer membrane. High nonlinearity of Si material X 31112.45 ≈ 10 –19m2 /V 2 and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (1200 — 2000nm) to visible (400 — 666nm) upconversion. The fabricated membranes demonstrate high flexibility and semitransparency, which makes them convenient to use as visualizers.

Mesoporous Ultrathin In2O3 Nanosheet Cocatalysts on a Silicon Nanowire Photoanode for Efficient Photoelectrochemical Water Splitting.

Vertical Si nanowire (NW) arrays are a promising photoanode material in the photoelectrochemical (PEC) water splitting field because of their highly efficient light absorption capability and large surface areas for PEC reactions. However, Si NW arrays always suffer from high overpotential, low photocurrent density, and low applied bias photon-to-current efficiency (ABPE) due to their low surface catalytic activity and intense charge recombination. Here, we report an efficient oxygen evolution cocatalyst of optically transparent, mesoporous ultrathin (2.47 nm thick) In2O3 nanosheets, which are coupled on the top of Si NW arrays. Combined with a conformal TiO2 thin film as an intermediate protective layer, this Si NW/TiO2/In2O3 (2.47 nm) heterostructured photoanode exhibited an extremely low onset potential of 0.6 V vs reversible hydrogen electrode (RHE). The Si NW/TiO2/In2O3 (2.47 nm) photoanode also showed a high photocurrent density of 27 mA cm-2 at 1.23 V vs RHE, more than 1 order of magnitude higher than that of the Si NW/TiO2 photoanodes. This improvement in solar water splitting performance was attributed to the significantly promoted charge injection efficiency as a result of the In2O3 nanosheet coupling. This work presents a promising pathway for developing efficient Si-based photoanodes by coupling ultrathin 2D cocatalysts.