Vertical Silicon Nanowire Arrays Research Articles

Serum metabolomics detected by LDI-TOF-MS can be used to distinguish between diabetic patients with and without diabetic kidney disease.

Diabetic kidney disease (DKD) is an important cause of end-stage renal disease with changes in metabolic characteristics. The objective of this study was to study changes in serum metabolic characteristics in patients with DKD and to examine metabolite panels to distinguish DKD from diabetes with MALDI-TOF-MS. We recruited 40 T2DM patients with or without DKD from a single center for a cross-sectional study. Serum metabolic profiling was performed with MALDI-TOF-MS using a vertical silicon nanowire array. Differential metabolites between DKD and diabetes patients were selected, and their relevance to the clinical parameters of DKD was analyzed. We applied machine learning methods to the differential metabolite panels to distinguish DKD patients from diabetes patients. Twenty-four differential serum metabolites between DKD patients and diabetes patients were identified, which were mainly enriched in butyrate metabolism, TCA cycle, and alanine, aspartate and glutamate metabolism. Among the metabolites, L-kynurenine was positively correlated with urinary microalbumin, urinary microalbumin/creatinine ratio (UACR), creatinine and urea nitrogen content. L-serine, pimelic acid, 5-methylfuran-2-carboxylic acid, 4-methylbenzaldehyde and dihydrouracil were associated with Glomerular filtration rate (eGFR). The panel of differential metabolites could be used to distinguish between DKD and diabetes patients with an AUC value reaching 0.9899-0.9949. Among the differential metabolites, L-kynurenine was related to the progression of DKD. The differential metabolites exhibited excellent performance at distinguishing between DKD and diabetes. This study provides a novel direction for metabolomics-based clinical detection of DKD.

Analysis of synthesized doped vertical silicon nanowire arrays for effective sensing of nitrogen dioxide: As gas sensors

In the present study, the controllable fabrication of silicon nanowires (Si NWs) with vertical alignment was accomplished using metal assisted chemical etching (MACE). The different characteristics, such as structural, morphological, chemical, optical, and dielectric properties were analyzed using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-DRS), and LCR [inductance (L), capacitance (C), and resistance (R)] meter (volume of the gas-sensing chamber is 650 mm3). It was revealed from the morphological study i.e., from the FESEM that p-type Si NWs are smaller in size than n-type Si NWs which is attributable to the energy band gap. The optical band gap (Eg) is observed to increase from 1.64 to 1.89 eV with the decreasing of the crystallite size and the optical reflection spectra of the Si NWs show a shift toward a lower wavelength (blue shift). Moreover, Raman spectra verified the red-shifted, asymmetrically broadened Raman line-shapes, which provides information about the size confinement effect in Si NWs. The MACE approach is excellent for synthesizing nanowire structures for use in gas-sensing applications due to its flexibility. The sensitivity of synthesized Si NWs was tested for NO2 gas. The sensor method is unique based on the testing of the device in the presence of a test gas because the use of the gas-sensing setup has the potential to measure the change in resistance by varying frequency, temperature, and time.

Piezoresistance Characterization of Silicon Nanowires in Uniaxial and Isostatic Pressure Variation.

Silicon nanowires (SiNWs) are known to exhibit a large piezoresistance (PZR) effect, making them suitable for various sensing applications. Here, we report the results of a PZR investigation on randomly distributed and interconnected vertical silicon nanowire arrays as a pressure sensor. The samples were produced from p-type (100) Si wafers using a silver catalyzed top-down etching process. The piezoresistance response of these SiNW arrays was analyzed by measuring their I-V characteristics under applied uniaxial as well as isostatic pressure. The interconnected SiNWs exhibit increased mechanical stability in comparison with separated or periodic nanowires. The repeatability of the fabrication process and statistical distribution of measurements were also tested on several samples from different batches. A sensing resolution down to roughly 1 pressure was observed with uniaxial force application, and more than two orders of magnitude resistance variation were determined for isostatic pressure below atmospheric pressure.

Versatilely tuned vertical silicon nanowire arrays by cryogenic reactive ion etching as a lithium-ion battery anode

Production of high-aspect-ratio silicon (Si) nanowire-based anode for lithium ion batteries is challenging particularly in terms of controlling wire property and geometry to improve the battery performance. This report demonstrates tunable optimization of inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperature to fabricate vertically-aligned silicon nanowire array anodes with high verticality, controllable morphology, and good homogeneity. Three different materials [i.e., photoresist, chromium (Cr), and silicon dioxide (SiO2)] were employed as masks during the subsequent photolithography and cryogenic ICP-RIE processes to investigate their effects on the resulting nanowire structures. Silicon nanowire arrays with a high aspect ratio of up to 22 can be achieved by tuning several etching parameters [i.e., temperature, oxygen/sulfur hexafluoride (O2/SF6) gas mixture ratio, chamber pressure, plasma density, and ion energy]. Higher compressive stress was revealed for longer Si wires by means of Raman spectroscopy. Moreover, an anisotropy of lattice stress was found at the top and sidewall of Si nanowire, indicating compressive and tensile stresses, respectively. From electrochemical characterization, half-cell battery integrating ICP-RIE-based silicon nanowire anode exhibits a capacity of 0.25 mAh cm−2 with 16.67% capacity fading until 20 cycles, which has to be improved for application in future energy storage devices.

Ultrasensitive and High Reproducible Detection of Urinary Metabolites Using the Tip–Contact Extraction Method Coupled with Negative LDI-MS

More and more evidence has proved that urinary metabolites can instantly reflect disease state. Therefore, ultra-sensitive and reproducible detection of urinary metabolites in a high-throughput way is urgently desirable for clinical diagnosis. Matrix-free laser desorption/ionization mass spectrometry (LDI-MS) is a high-throughput platform for metabolites detection, but it is encountered by severe interference from numerous salts in urine samples, because the crystallized urine salt on dried samples could result in poor reproducibility in LDI-MS detection. The present work proposed a tip-contact extraction (TCE) technique to eliminate interference from the urine salt. Vertical silicon nanowire arrays decorated with the fluorinated ethylene propylene film (FEP@VSiNWs) could effectively extract metabolites from the urine sample dropping on its surface. High salt tolerance was observed in the subsequent LDI-MS detection of the metabolites extracted on the tip of FEP@VSiNWs even in the presence of 1 M urea. Stable and reproducible mass spectra for non-target metabolic analysis were obtained in real urine samples with different dilution folds. Urinary metabolites collected from bladder cancer (BC) patients were reliably profiled by the TCE method coupled with negative LDI-MS. Based on this platform, potential metabolic biomarkers that can distinguish BC patients and normal controls were uncovered.

Vertical Tip-to-Tip Interconnection p–n Silicon Nanowires for Plasmonic Hot Electron-Enhanced Broadband Photodetectors

A p–n junction photodetector, formed between the tip-to-tip interconnection of vertical silicon nanowire arrays, was fabricated by using a facile and economic process with remarkable performances. ...

Perovskite/Silicon‐Nanowire‐Based Hybrid Heterojunctions for Fast and Broadband Photodetectors

A hybrid photodetector fabricated with a halide perovskite and silicon nanowire (SiNW) heterojunction for broadband detection ranging from near‐ultraviolet to near‐infrared is reported. The device fabrication is conducted by directly coating chemically etched vertical SiNW arrays with perovskite thin films and nanofibers produced from liquid precursors. The optimized perovskite thin film/SiNW heterojunction‐based device under 532 nm illumination exhibits excellent spectral responsivity of 13 A W−1, high specific detectivity reaching 1013 Jones at 0.3 V external bias, and fast rise/fall times of 22.2/17.6 μs. The device shows very good stability under ambient conditions, even after 30 days of storage. Overall, this simple, all‐solution‐based hybrid photodetector offers great potential for the future integrated optoelectronic devices.

Vertical-Aligned Silicon Nanowire Arrays with Strong Photoluminescence Fabricated by Metal-Assisted Electrochemical Etching

Metal-assisted chemical etching of silicon is a commonly used method to fabricate vertical aligned silicon nanowire arrays. In this report we show that if in the above method the chemical etching is replaced by the electrochemical one, we can also produce silicon nanowire arrays, but with a special characteristic-extremely strong photoluminescence. Further research showed that the huge photoluminescence intensity of the silicon nanowire arrays made by metal-assisted electrochemical etching is related to the anodic oxidation of the silicon nanowires which has occurred during the electrochemical etching. It is most likely that the luminescence of the silicon nanowire arrays made with metal-assisted electrochemical etching is the luminescence of silicon nanocrystallites (located on the surface of silicon nanowire fibers) embedded in a silicon oxide matrix, similar to that in a silicon rich oxide system.

The Facile Implementation of Soft/Tunable Multiband Optical Filters by Stacking Vertical Silicon Nanowire Arrays for Smart Sensing

Multiple band optical filters for optical smart sensing, which is simultaneous multispectral detection of objects, are intensively used in many applications, such as remote monitoring, biomedical imaging, wearable optical sensors, and wireless communication. Typically, periodically stacked dielectric layers are used in multiple band optical filters. However, the use of dielectrics inevitably results in rigidity and inflexibility, which is not suitable for advanced smart optical sensors fabricated on flexible substrates. Therefore, herein, a strategy to implement soft/tunable multiband optical filters made from silicon nanowire array‐based stackable optical filters (Si NW‐SOFs) composed of polymeric membranes containing vertical silicon nanowire arrays that filter light from visible to near‐infrared regions with a narrow bandwidth is proposed. Theoretical and experimental results indicate wavelength selectivity by geometrical parameters (e.g., diameter, period, and height) and the absence of optical disturbance between the layers in the integrated states. Therefore, multiband optical filters with single‐ to triple‐band stops are easily created by changing Si NW‐SOFs. Finally, imaging demonstrations that involve the filtering of monochromatic light sources of three wavelengths (i.e., red, green, and blue) and fluorescent biological tissues show the practical multiple band characteristics of Si NW‐SOFs.

Reflective color filter with precise control of the color coordinate achieved by stacking silicon nanowire arrays onto ultrathin optical coatings

The engineering of structural colors is currently a promising, rapidly emerging research field because structural colors of outstanding spatial resolution and durability can be generated using a sustainable production method. However, the restricted and saturated color range in micro/nano-fabricated structural ‘pigments’ has hindered the dissemination of structural color printing. Here, this article presents a spectral mixing color filter (SMCF), which is the concept of fine-tunable color systems, capable of addressing the current issues in structural color engineering, by stacking a vertical silicon nanowire array embedded in a transparent polymer onto ultrathin optical coating layers. These two photonic structures enable independent tuning the optical resonance of each structure, depending on geometrical parameters, such as the diameter of nanowires and thickness of absorbing medium. Hence, the SMCF facilitates the linear combination of two resonant spectra, thereby enabling fine-tuning and widening of the color gamut. Theoretical studies and experimental results reveal the detailed working mechanisms and extraordinary mechanical feature of the SMCF. Based on the analyses, the concept of flexible optical device, e.g., a reflective anti-counterfeiting sticker, is demonstrated. Successful characterization demonstrates that the proposed strategy can promote the color controllability/purity of structural color and the applicability as flexible optical device.

Tip-Enhanced Photoinduced Electron Transfer and Ionization on Vertical Silicon Nanowires.

Nanostructured semiconductors are one of the most potent candidates for matrix-free laser desorption/ionization mass spectrometric (LDI-MS) analysis of low-molecular-weight molecules. Herein, the enhanced photoinduced electron transfer and LDI on the tip of a vertical silicon nanowire (SiNW) array were investigated. Theoretical simulation and LDI detection of indigo and isatin molecules in negative ion mode revealed that the electric field can be enhanced on the tip end of SiNWs, thereby promoting the energy and electron transfer to the analytes adsorbed on the tip of SiNWs. On the basis of this finding, a tip-contact sampling method coupled with LDI-MS detection was established. In this strategy, the tip of SiNWs can be regarded as microextraction heads for the sampling of molecules when they come in contact with analytes. Impression of skin, tissue, and pericarp on the vertical SiNW array can effectively transfer endogenous metabolites or exogenous substances onto the tip. Upon laser irradiation, the adsorbed molecules on the SiNW tip can be efficiently ionized and detected in negative ion mode because of the tip-enhanced electron transfer and LDI effect. We believe this work may significantly expand the application of LDI-MS in various fields.

Understanding the effect of polydopamine coating on catalytic reduction reactions

The polydopamine (PDOP) layer with varying thicknesses has been deposited on the vertical aligned silicon nanowire (SiNW) arrays by manipulating the PDOP deposition time. It is interestingly found that catalysis ability of PDOP layer does not only depend on its thickness but also the morphology of the support material. In addition to these, by growing gold nanoparticles on PDOP coated SiNWs, catalytic reduction reactions were investigated in the presence of laser illumination having different wavelengths. We observed that, in the presence of laser illumination with a proper wavelength, catalytic conversion is about 12-fold faster than that of the catalytic process which was carried out in the absence of laser illumination.

Optimization and optical characterization of vertical nanowire arrays for core-shell structure solar cells

In this work we study optical properties of vertical silicon nanowire (SiNW) arrays fabricated using metal assisted chemical etching (MACE) coupled with nanosphere lithography (NSL). We have studied optimal configurations minimizing total reflectance of 2µm long SiNW arrays by modeling and compared their performance with experimental data. Fabricated SiNW arrays have shown lower total reflectance than modeled perfectly periodic ones. This has been found to be due to a variation of NW geometry and the presence of domains caused by the self-assembly during NSL process. We have developed a statistical model (based on rigorous coupled wave analysis) describing geometry variations and also demonstrated that different domains have rotated diffraction pattern with respect to their symmetry axes. The statistical model has been successfully validated on total reflectance and normalized Mueller matrix data. Furthermore, we have found that a very high light trapping can be achieved for nanowires only 125nm long (Jsc equivalent of up to 42mA/cm2), which can be exploited in thin and ultra-thin Si solar cells.

Comparison of ordered and disordered silicon nanowire arrays: experimental evidence of photonic crystal modes.

We experimentally compared the reflectance between ordered and disordered silicon nanowires to observe the evidence of photonic crystal modes. For similar diameters, the resonance peaks for the ordered nanowires at a spacing of 400 nm was at a shorter wavelength than the disordered nanowires, consistent to the excitation of photonic crystal modes. Furthermore, the resonant wavelength didn't shift while changing the density of the disordered nanowires, whereas there was a significant shift observed in the ordered ones. At an ordered spacing of 800 nm, the resonance wavelength approached that of the disordered structures, indicating that the ordered structures were starting to behave like individual waveguides. To our knowledge, this is the first direct experimental observation of photonic crystal modes in vertical periodic silicon nanowire arrays.

Silicon Nanowires Promote Neuron-Like Differentiation of Mesenchymal Stem Cells

The manipulation of cell-surface interaction by nanomaterials has been shown to promote various changes in the behavior of mammalian cells, such as cell spreading, migration, and even the cell fate. Here we explore the possible transition of human mesenchymal stem cells (hMSCs) into a neural lineage using vertical silicon nanowire (SiNW) array. In the absence of any differentiation-inducing factor, hMSCs on the longest SiNWs exhibit elongated cell shapes with neurite-like extension. Furthermore, the increased mRNA expression levels of neuron specific genes indicate the neuron-like differentiation of hMSC. To explain the detailed mechanism, we assessed cell-surface interaction by observing the rate of cell migration and the distribution of cytoskeletal actin and focal adhesion. Other characteristics of cellular differentiation, such as slowed growth and proliferation rate were also confirmed. Taken together, our data suggest that neurite-like extensions in hMSCs on SiNWs actively explore the surface with strong adhesion to adjust their morphology, while cell bodies are rather restrained to move.

A vertical tip-tip contact silicon nanowire array for gas sensing.

Novel chemiresistive gas sensors based on a vertical tip-tip contact silicon nanowire (TTC-SiNW) array were constructed. The welding of TTC-SiNWs after joule heating treatment was confirmed by a current-voltage curve (I-V curve). The TTC-SiNW structure not only resolved the problem of electrode contact encountered in conventional nanowire sensors, but also elongated the nanowire length to increase the void space for fast gas diffusion. The TTC-SiNW sensor comprising the same two types of SiNW arrays responded to NO2 very sensitively. The LOD for the p-p and n-n contact SiNW arrays is around 150 ppb and 3 ppb (S/N = 3), respectively. Furthermore, the highly oriented nano-junction formed on the TTC structure provided solid evidence to clarify the contribution of the nanojunction to gas sensing behavior. The TTC-SiNW sensor with a p-n junction displays a significant rectification effect. The sensitive response towards NO2 (LOD is about 18 ppb) was observed at a reverse bias voltage, whereas the response at a forward bias voltage was insignificant. Finally, the mechanism of gas sensing behavior on different types of TTC structures was proposed.

Probing protein complexes inside living cells using a silicon nanowire-based pull-down assay.

Most proteins perform their functions as interacting complexes. Here we propose a novel method for capturing an intracellular protein and its interacting partner out of living cells by utilizing intracellular access of antibody modified vertical silicon nanowire arrays whose surface is covered with a polyethylene glycol layer to prevent strong cell adhesion. Such a feature facilitates the removal of cells by simple washing, enabling subsequent detection of a pulled-down protein and its interacting partner, and further assessment of a drug-induced change in the interacting complex. Our new SiNW-based tool is thus suitable for authentication of protein networks inside living cells.

Selective growth of vertical silicon nanowire array guided by anodic aluminum oxide template

We report on the selective growth of vertical silicon nanowire arrays guided by an anodic aluminum oxide (AAO) template without the introduction of any metallic catalyst. Gas-source molecular beam epitaxy using disilane as a source gas was carried out. The growth conditions such as flow rate and growth temperature were changed to optimize the Si nanowire growth. It was found that the selective growth was promoted at a flow rate of 0.5 sccm, whereas the selective growth was poor at high flow rates of 1 and 2 sccm. One-micrometer-long Si nanowire arrays were obtained at a low flow rate of 0.5 sccm only at the growth temperature of 700 °C. The obtained Si grown at a temperature of 650 °C exhibited conglomerated structures with Si grains piled up inside the nanopores of the AAO template. We found that increasing the growth temperature and decreasing the flow rate are useful for improving the growth selectivity.

Fabrication of vertical silicon nanowire arrays on three-dimensional micro-pyramid-based silicon substrate

Recently, a lot of attention is being paid to combine silicon nanowires (SiNWs) and the conventional micro-pyramid silicon (µT-Si) structures leading to SiNWs/µT-Si binary structures for efficient light harvesting. We report large-area fabrication of vertical SiNWs array over three-dimensional micro-pyramid structured silicon substrate through silver (Ag)-assisted electroless wet chemical etching in aqueous HF and AgNO3 solution. The influence of AgNO3 concentration has been investigated over the formation of the SiNWs. Vertical SiNWs formation takes place for lower AgNO3 concentrations. For higher concentrations, random formation of NWs and depletion of the micro-pyramids is observed. The present study reveals strong influence of local morphology of micro-pyramids over NWs formation. The formation mechanism of such binary structures has been discussed in co-relation with dimensional features of the micro-pyramids and surface free energy. Such hierarchically textured binary structured silicon surfaces exhibit excellent light trapping properties reducing reflectance to as low as <3 % in broad spectral range. Further, such surfaces also have enhanced Raman spectra (~15-fold enhancement) as compared to only micro-pyramid Si surfaces, thereby having potential applications for photovoltaic devices as well as surface enhanced Raman spectroscopic investigations.

Localized absorption in aluminum mask in visible spectrum due to longitudinal modes in vertical silicon nanowire arrays

Localized optical absorption in aluminum masks used for vertical silicon nanowire fabrication is demonstrated experimentally and supported using computer simulations. The mask is in the form of 30 nm thick aluminum nano-disks on top of silicon nanowires arranged in square lattices. The nanowires are 1 μm long, with diameters ranging from 60 nm to 100 nm and spaced 400 nm apart. New spectral features appear in the 500 nm–700 nm wavelengths range and are dependent on both the nanowire diameter and length. The former is due to the excitation of radial modes, whereas the latter stems from longitudinal (Fabry–Perot) resonances. The salient features associated with absorption in the aluminum mask and the role nanowire plays in this connection are discussed.