SiNW Arrays Research Articles

Novel Insulator Isolated Si NW Sensors Fabricated using Bulk Substrate with Low-cost and High-quality

In this paper, insulator isolated Si NW array were fabricated using self-allied spacer image transfer (SIT), multi-step time division multiplexing (TDM) etching and linear oxidation processes. The insulator isolated Si NWs owe a pentagonal shape, well single crystal morphology and high nanowire quality. The fabricated Si NW array were used to fabricate insulator isolated Si NW sensors. The low-cost insulator isolated Si NW sensor fabricated from bulk silicon substrates exhibits a linearly variation of current response to the laser at this wavelength, exhibiting good repeatability and the prospect of replacing SOI materials. The proposed approach of Si NW materials and sensors fabrication realizes the high efficiency, low cost and small fluctuation of Si NW pattern and device preparation, overcoming the problems of low efficiency for e-beam lithography and the difficulty of large-scale array formation.

Thermally induced evolution of the structure and optical properties of silicon nanowires

In the present paper, we report on the investigation of thermal annealing (TA) effect on structural and optical properties of crystalline silicon nanowires produced by metal-assisted chemical etching approach. In particular, the impact of TA on nanowire length, relative volume and size distribution of voids is described in terms of Lifshitz-Slyozov-Wagner theory considering the TA induced Oswald ripening in the SiNW arrays. It was also found that TA leads to a decrease of the SiNWs total reflection in the wide UV–VIS-IR spectral range. The reported effects can be used for tuning of crystalline SiNWs arrays in view of their further applications in photonics related fields.

Leveraging Preconcentration and Partial Separations with Silicon Nanowire Arrays to Improve the Detection of Low Volatility Trace Explosive Vapors

Trace detection of explosive vapors in real world environments is hindered by strict sensitivity and selectivity requirements that need to be met in order for these techniques to be effective. The physical properties of most explosive molecules limit environmentally relevant vapor concentrations to the part per billion (ppbv) – part per trillion (pptv) range due to their saturated vapor pressures under real world conditions. Additionally, trace explosive vapor detectors must be able to discriminate against a number of potential chemical and biological interferents present in complex real world environments. In this work, we present silicon nanowire arrays (SiNW) as a novel tunable material aimed at providing trace explosive vapor detectors with the sensitivity and selectivity necessary for operational use.Dense, highly ordered silicon nanowires are fabricated through a nanosphere lithography process and topped with a porous gold electrode resulting in a high surface area adsorptive substrate intrinsically capable of rapid, precise temperature control via Joule heating. Explosive vapors are selectively preconcentrated onto the SiNW array by exploiting their affinities for the SiO2 surface of the nanowires. Chromatographic like partial separations of preconcentrated material on the array can be achieved through careful selection of desorption parameters. Using a multichannel detector such as an IMS or MS we demonstrate the preconcentration of a number of explosive vapors including 2,6-DNT, TNT, and RDX above and below their saturated vapor concentrations at room temperature, in addition to exploring the effects of desorption flow rates and temperatures on the partial separation of 2,6-DNT, 2,4-DNT, and TNT vapors. Figure 1

Trapezoidal SiNWs array fabricated by AFM-LAO for Dengue virus DNA oligomer detection

This paper demonstrates an array of SiNWs device was fabricated by Atomic Force Microscope-Local Anodic Oxidation (AFM-LAO) technique for Dengue virus deoxyribonucleic acid (DNA) oligomer detection. The SiNWs were wet etched by using 25 wt% tetramethylammonium hydroxide (TMAH) with addition of isopropyl alcohol (IPA) at various concentrations (10, 20 and 30 vol%). The produced SiNWs were in trapezoidal shape with different dimension and surface roughness. The surface of SiNWs was modified with 3-aminopropyltriethoxysilane and glutaraldehyde followed by the immobilized peptide nucleic acid, then hybridized with Dengue DNA oligomer. The hybridization procedure causes a current change due to accumulation of negative charges introduced on SiNWs surface which determines the performance of the device. The small width and rough surface of SiNWs device has high sensitivity (80.3 μAM−1) and low detection limit (2.22 fM).

ZnO/Si nanowires heterojunction array-based nitric oxide (NO) gas sensor with noise-limited detectivity approaching 10 ppb

We report a ZnO/Silicon nanowire (ZnO/Si NWs) heterojunction array-based NO gas sensor operating at room temperature with an extremely high response (noise limited response ∼10 ppb). The sensor shows very high selectivity towards NO gas sensing and limited perturbation in response due to the presence of moisture. The sensor has been fabricated by using cost-effective chemical processing that is compatible with wafer-level processing. The vertically aligned Si NWs array has been made by an electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The electrical characteristic of the model was to fit the I–V data before and after exposure to gas and essential changes in electrical parameters were obtained. This was then explained based on a proposal for the mechanism of gas sensing. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW.

Silicon nanowires@Co3O4 arrays film with Z‑scheme band alignment for hydrogen evolution

Photocatalytic water splitting for hydrogen evolution provides an attractive strategy for clean, low-cost, and environment-friendly production of hydrogen from solar energy. Here, the film composite of Co3O4 nanoparticles decorated well-defined one-dimensional (1D) silicon nanowires arrays (SiNWs@Co3O4) are constructed by employing SiNWs arrays as a building block via a facile calcination approach. In comparison with bare SiNWs arrays, the resultant SiNWs@Co3O4 arrays composite exhibits distinctly enhanced photocatalytic performance for the evolution of hydrogen. The enhanced photoactivity is attributed to the formation of a direct Z-scheme band alignment in the SiNWs@Co3O4 film composite, which results in an improved spatial separation and migration of photoinduced electron-hole pairs as compared to bare SiNWs under light irradiation. Our work is anticipated to promote ongoing interest in the design of high-performance artificial 1D-based Z-scheme film photocatalysts with enhanced performance.

Tin dioxide nanoparticles as catalyst precursors for plasma-assisted vapor–liquid–solid growth of silicon nanowires with well-controlled density

The fabrication of arrays of silicon nanowires (Si NWs) with well-defined surface coverage using the vapor–liquid–solid process requires a good control of the density and size distribution for the metal catalyst. We report on a cost-effective bottom-up approach to produce Si NWs by a low-temperature deposition technology using plasma-enhanced chemical vapor deposition and tin dioxide (SnO2) nanoparticles as the source of tin catalyst. This strategy offers a straightforward method to select specific particle sizes by conventional colloidal techniques, and to tune the surface coverage using a polyelectrolyte layer to efficiently immobilize the particles on the substrate by electrostatic grafting. After a further step of reduction into tin metal droplets using hydrogen plasma treatment, the catalyst particles are used for the growth of Si NWs. This approach allows the prodcution of controlled Si NWs arrays which can be used as a template for radial junction thin film solar cells.

Comparison of KOH and TMAH Etching on Sinw Arrays Fabricated via AFM Lithography

Silicon nanowire (SiNW) arrays were fabricated by using top-down fabrication approach via atomic force microcopy (AFM) lithography on silicon-on-insulator (SOI) (100) substrate. Local anodic oxidation (LAO) technique carried out by AFM lithography to draw oxide pattern on top of the SOI substrate. The next steps to fabricate silicon nanowires involve chemical etching of the unmasked silicon by wet etching with the addition of organic additive. In this research, isopropyl alcohol (IPA) was used to improve the surface smoothness of silicon nanowires. Two types of etchant, potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) with and without admixture of IPA at different etching time (10s, 20s, and 30s) were studied. After etching, the local oxide pattern was removed using dilute hydrofluoric acid (HF). The relationships between etching time with wall angle and surface roughness of the produced SiNWs were characterized in detail by using AFM and SEM. The result revealed that the wall angle for SiNW arrays that etched using TMAH was higher compare to KOH. Increasing the etching time produced large wall angle (θ) for both etchants. It was also found that the surface roughness for SiNW arrays was decreased with increasing time. SiNW arrays etched using KOH have higher surface roughness compared to TMAH. The obtained results also show that the etching depths increased with increasing etching time.

Co-Fe Prussian Blue Coordination Polymer Modified Silicon Nanowires Array for Efficient Photoelectrochemical Water Oxidation.

The Co-Fe prussian blue coordination polymer modified Silicon nanowires array (SiNW@CoHCF array) exhibits promising behavior in photoelectrochemical (PEC) water oxidation, giving rise to a largely enhanced photocurrent density and stability, much superior to those of SiNW-based photoanodes. The CoHCF shell as effective co-catalyst increases the reaction kinetics, thus depressing the charge recombination rate, as well as protects SiNWs array photoanode from photocorrosion. It is found that the CoHCF significantly decreases the charge transfer impedance (Rct), which means that the CoHCF facilitates majority transport and collection. These advantages may account for the performance enhancement.

Influence of the hard masks profiles on formation of nanometer Si scalloped fins arrays

In this work, the impact of hard masks (HMs) profiles on nanometer scalloped-fins (S-fins) by reactive ion etching (RIE) is extensively investigated. A popular spacer image transfer (SIT) process originated from conventional FinFET mass fabrication is adopted for the formation of the S-fins arrays by using a new developed time division multiplexing (TDM) etching process. It is observed that the higher spacers HMs with sharp corners result in the asymmetric S-fins. In order to explain the phenomenon and achieve the desirable S-fins profiles, a possible mechanism causing the severely asymmetric S-fins is firstly proposed. It suggests that the negative charges accumulated on the non-conducting higher spacers during the RIE process play a significant role on the formation of the asymmetric profile of S-fins. An effective approach based on the proposed scheme is demonstrated by reducing the heights of spacers HMs, which improves the simultaneity of etching processes, achieves the symmetric S-fins profiles with desirable notches and obtains three vertically stacked SiNWs arrays.

Conformal and continuous deposition of bifunctional cobalt phosphide layers on p-silicon nanowire arrays for improved solar hydrogen evolution

Vertically aligned p-silicon nanowire (SiNW) arrays have been extensively investigated in recent years as promising photocathodes for solar-driven hydrogen evolution. However, the fabrication of SiNW photocathodes with both high photoelectrocatalytic activity and long-term operational stability using a simple and affordable approach is a challenging task. Herein, we report conformal and continuous deposition of a di-cobalt phosphide (Co2P) layer on lithography-patterned highly ordered SiNW arrays via a cost-effective drop-casting method followed by a low-temperature phosphorization treatment. The as-deposited Co2P layer consists of crystalline nanoparticles and has an intimate contact with SiNWs, forming a well-defined SiNW@Co2P core/shell nanostructure. The conformal and continuous Co2P layer functions as a highly efficient catalyst capable of substantially improving the photoelectrocatalytic activity for the hydrogen evolution reaction (HER) and effectively passivates the SiNWs to protect them from photo-oxidation, thus prolonging the lifetime of the electrode. As aconsequence, the SiNW@Co2P photocathode with an optimized Co2P layer thickness exhibits a high photocurrent density of–21.9 mA·cm−2 at 0 V versus reversible hydrogen electrode and excellent operational stability up to 20 h for solar-driven hydrogen evolution, outperforming many nanostructured silicon photocathodes reported in the literature. The combination of passivation and catalytic functions in a single continuous layer represents a promising strategy for designing high-performance semiconductor photoelectrodes for use insolar-driven water splitting, which may simplify fabrication procedures andpotentially reduce production costs.

Optimizing Photovoltaic Charge Generation of Hybrid Heterojunction Core-Shell Silicon Nanowire Arrays: An FDTD Analysis.

Development of highly efficient nanowire-based photovoltaic devices requires an accurate modeling of light scattering from interfaces and optical carrier generation inside the cell. A comprehensive study of optical absorption and carrier generation enables us to tap the full potential of nanowire arrays (NWAs). In this study, we have done a systematic study to optimize the core–shell structure of vertically aligned silicon nanowire (Si NW) arrays coated with PTB7:PC71BM by means of finite difference time domain optical simulations to maximize the photon absorption. Initially, the core thickness of hybrid Si NWs has been optimized for the most efficient light absorption. The further improvement of light absorption has been studied by varying the coating thickness of low-band gap organic polymer PTB7:PC71BM on Si NWAs. A delineative analysis shows that NWs with a 150 nm thick silicon core and 60 nm thick coating of PTB7:PC71BM exhibit broad band absorption and the optimum ideal current density of about 34.95 mA/cm2, which are larger than those of their planar counterpart with the same amount of absorbing material and also better than those previously reported for NWs. The basic principle and the physical process taking place during absorption and current generation have been also discussed. The optimization of the hybrid heterojunction Si NW arrays and understanding of their optical characteristics may contribute to the development of economical and highly efficient hybrid solar cells.

Photocatalytic degradation of cationic and anionic dyes in water using hydrogen-terminated silicon nanowires as catalyst

The photocatalytic degradation of cationic (methylene blue (MB)) and anionic (methyl orange (MO)) dyes was investigated using hydrogen-terminated silicon nanowires (H-SiNWs) as photocatalysts. Several silicon nanowires samples with different morphologies were elaborated and the morphology was changed by acting on the silicon nanowires formation parameters such as substrate type, doping level, crystallographic orientation, silver deposition time and etching time. It was shown that the photocatalytic activity strongly depends on the morphology of SiNWs arrays. Indeed, it was found that n-type H-SiNWs elaborated on highly doped Si (100) substrates exhibit the highest photocatalytic activity for the degradation of MB. In addition, it was demonstrated that H-SiNWs are more efficient for the photodegradation of MO than MB in the solutions with pH values higher than the pH of zero charge point of silica (pHpzc).

Research on controllable preparation and antireflection properties of zigzag SiNWs arrays

ABSTRACTIn this work, an alternative metal-assisted chemical etching method is proposed to fabricate large-area uniform zigzag silicon nanowires. The effects of deposition time, etching time, HF concentration and H2O2 concentration on the nanostructure of nanowires were systematically investigated. The results demonstrate the suitable preparation concentrations in the alternating etching process are HF([SI]/[SII] = 9.2 M/2.3 M) and H2O2{[SI]/[SII] = (0.04 M∼0.26 M)/(0.4 M∼0.65 M)}. Moreover, the anti-reflec-tion properties of the resulting zigzag SiNWs arrays have been studied, the average reflectance is almost lower than 10% at wavelength range of 200∼1200 nm. The superior anti-reflection performance of zigzag SiNWs shows a huge potential application in high-efficiency silicon solar cells.

Vertically Aligned Silicon Nanowire Array Decorated by Ag or Au Nanoparticles as SERS Substrate for Bio-molecular Detection

This review article summerises preparation techniques of vertically aligned silicon nanowire (Si NW) arrays through metal-assisted chemical etching (MacEtch) process and plasmonic nanoparticles (Ag and Au) with the perspective of the fabrication of surface-enhanced Raman scattering (SERS)-active substrates which are highly efficient for bio-molecular detection. At first, basic methods and mechanisms for SERS have been introduced and size and shape effects of the nanoparticles (NPs) on plasmonic vibration have been discussed. Comparative discussions on optical and plasmonic characteristics of Ag and Au NPs have also been presented in this section. Potential techniques for the synthesis of Ag and Au NPs with different sizes and shapes have been reported in the following section. Basic processes and mechanism for the fabrication of vertically aligned Si NW arrays on Si by MacEtch of Si wafer have been discussed. Template-assisted fabrication techniques for the vertically aligned Si NW arrays with controlled diameter and number density have also been reported. Finally, multifarious ways for the fabrication of SERS-active substrates by assembling noble metal NPs onto the NW surface have been discussed and their performance for bio-molecular detection has also been reported.

Advanced Optoelectronic Devices based on Si Quantum Dots/Si Nanowires Hetero-structures

Si quantum dots are currently extensively studied since they can be used to develop many kinds of optoelectronic devices. In this report, we review the fabrication of Si quantum dots (Si QD) /Si nanowires (Si NWs) hetero-structures by deposition of Si QDs/SiO2 or Si QDs/SiC multilayers on Si NWs arrays. The electroluminescence and photovoltaic devices based on the formed hetero-structures have been prepared and the improved performance is confirmed. It is also found that the surface recombination via the surface defects states on the Si NWs, especially the ones obtained by the long-time etching, may deteriorate the device properties though they exhibit the better anti-reflection characteristics. The possible surface passivation approaches are briefly discussed.

Fabrication of ultra-low antireflection SiNWs arrays from mc-Si using one step MACE

In this work, SiNWs arrays were fabricated from mc-Si wafer using one step nano-silver catalyzed chemical etching method. The effect of AgNO3 concentration on morphology structure, antireflection property and effective carrier lifetime of textured mc-Si were carefully studied. The results indicate that the length and inter spacing of SiNWs arrays are conducive for improving antireflection property of SiNWs arrays, but the increasing etching seriously deteriorates effective carrier lifetime. Moreover, KOH correction of SiNWs was performed to further reduce the reflectance loss, which is also conducive to reduce the photo-generated carrier recombination. Under preference immersion time of 240 s, a large scale SiNWs arrays with the ultra-low antireflection ability of ~3.4% and excellent effective carrier lifetime of 3.30 μs can be obtained, which shows a great significance for producing high-efficiency polysilicon solar cells.

Multiplexed detection of lung cancer biomarkers in patients serum with CMOS-compatible silicon nanowire arrays

In this work, a real-time assay for highly sensitive, label-free, multiplexed electrical detection of lung cancer biomarkers was developed by using silicon nanowire field-effect (SiNW-FET) devices. Highly responsive SiNW arrays were fabricated using a CMOS-compatible anisotropic self-stop etching technique with mass reproducibility and low cost character. The SiNW nanosensor was integrated with PDMS microfluidic device, which allows rapid analyte delivery, makes the analysis to be conducted using exceedingly small samples and enables potential multiplexed detection. The nanowire arrays allowed highly selective and sensitive multiplexed detection of microRNA (miRNA)−126 and CEA. Due to high surface-to-volume ratio that the nanowire dimensions confer, the detection floor of single molecule was achieved. The potential utility in identifying clinical samples for early diagnosis of cancer was demonstrated by analyzing biomarkers in clinical related samples. The developed nanosensor with capability for multiplexed real-time monitoring of biomarkers with high sensitivity and selectivity in clinically relevant samples is highly attractive for diagnosis and treatment of cancer and other diseases.

Fabrication and photoelectrochemical properties of silicon nanowires/g-C3N4 core/shell arrays

A photoelectrochemical (PEC) cell made of metal-free carbon nitride (g-C3N4) @siliconnanowire(Si NW) arrays (denoted as Si NWs/g-C3N4) is presented in this work. The as-prepared photoelectrodes with different mass contents of g-C3N4 have been synthesized via a metal-catalyzed electroless etching (MCEE), liquid atomic layer deposition (LALD) and annealing methods. The amount of g-C3N4 on the Si NW arrays can be controlled by tuning the concentration of the cyanamide solution used in the LALD procedure. The dense and vertically aligned Si NWs/g-C3N4 core/shell nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In comparison with FTO/g-C3N4 and Si NW samples, the Si NWs/g-C3N4 samples showed significantly enhanced photocurrents over the entire potential sweep range. Electrochemical impedance spectroscopy (EIS) was conducted to investigate the properties of the charge transfer process, and the results indicated that the enhanced PEC performance may be due to the increased photo-generated interfacial charge transfer between the Si NWs and g-C3N4. The photocurrent density reached 45μA/cm2 under 100mW/cm2 (AM 1.5G) illumination at 0V (vs. Pt) in neutral Na2SO4 solution (pH∼7.62). Finally, a systematical PEC mechanism of the Si NWs/g-C3N4 was proposed.

Fabrication of the hierarchical structure photocathode by structuring the surface nanopores on Si nanowires standing on p-Si wafer for the effective photoelectrochemical reduction of Cr(VI) in the aqueous solution

Abstract Herein, we reported a method to inhibit the oxidation passivation of Si nanomaterials (taking Si nanowire array (SiNW, about 100 nm of diameter) as a representative) via structuring nanopores (less than 10 nm) on their surface. SiNW array was prepared through metal-assisted chemical etching. After 30 min of etching, nanopores were formed on the surface of SiNW and the amount of nanopores increased with the prolonging of the etching duration. Due to the generation of the nanopores, the decrease of optical reflection was observed. The photoelectrochemical performance of samples prepared after 30, 60 and 90 min etching were evaluated. The sample etched for 60 min displayed the highest and stable photocurrent and was used in the photoelectrocatalytic testing. The SiNW with nanopores exhibited significant photoelectrocatalytic stability than that of SiNW. The kinetic constant of Cr(VI) reduction over SiNW with nanopores was 0.081 min−1, which was 1.8 times as much as that over SiNW (0.045 cm−1) and maintained above 0.080 min−1 after three repeated experimental runs. These results demonstrate that constructing surface nanopores on the surface of SiNW is an effective approach to realize the use of Si materials as photoelectrodes in the aqueous solution.