Si Nanowire Arrays Research Articles

Averaging effect on improving signal reproducibility of gap-based and gap-free SERS substrates based on ordered Si nanowire arrays

A gap-free SERS substrate was found to achieve both high reproducibility and high enhancement against the reproducibility/enhancement trade-off in gap-based substrates.

High performance transparent in-plane silicon nanowire Fin-TFTs via a robust nano-droplet-scanning crystallization dynamics.

High mobility, scalable and even transparent thin-film transistors (TFTs) are always being pursued in the field of large area electronics. While excimer laser-beam-scanning can crystallize amorphous Si (a-Si) into high mobility poly-Si, it is limited to small areas. We here demonstrate a robust nano-droplet-scanning strategy that converts an a-Si:H thin film directly into periodic poly-Si nano-channels, with the aid of well-coordinated indium droplets. This enables the robust batch-fabrication of high performance Fin-TFTs with a high hole mobility of >100 cm2 V-1 s-1 and an excellent subthreshold swing of only 163 mV dec-1, via a low temperature <350 °C thin film process. More importantly, precise integration of tiny poly-Si channels, measuring only 60 nm in diameter and 2 μm apart on glass substrates, provides an unprecedented transparent Si-based TFT technology to visible light, which is widely sought for the next generation of high aperture displays and fully transparent electronics. The successful implementation of such a reliable nano-droplet-scanning strategy, rooted in the strength of nanoscale growth dynamics, will enable eventually the batch-manufacturing and upgrade of high performance large area electronics in general, and high definition and scalable flat-panel displays in particular.

Control over Alignment and Growth Kinetics of Si Nanowires through Surface Fluctuation of Liquid Precursor

Control over alignment and growth kinetics of vertically aligned Si nanowire (v-SiNW) arrays, which were grown using chemical vapor deposition (CVD) via a metal catalyst-assisted vapor–liquid–solid (VLS) mechanism, was demonstrated by introducing a homemade bubbler system containing a SiCl4 solution as the Si precursor. Careful control over the bubbler afforded different amounts of SiCl4 supplied to the reactor. By varying the dipping depth (Dd) and tilting angle (Ta) of the bubbler, the SiCl4 precursor concentration would fluctuate to different degrees. The different SiCl4 concentrations afforded the fine-tuning of v-SiNW array properties like alignment and growth kinetics. The degree of alignment of v-SiNWs could be increased with large amounts of SiCl4, which was caused by slight shallow depth or gentle tilting of the SiCl4 solution in the bubbler due to an increasing degree of fluctuation and fluctuation area. The ability to control alignment and growth kinetics of v-SiNW arrays could be employed in a...

Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M-B (M = Ni, Co) Catalysts on Silicon Nanowire Arrays.

Modification of p-type Si surface by active and stable earth-abundant electrocatalysts is an effective strategy to improve the sluggish kinetics for the hydrogen evolution reaction (HER) at p-Si/electrolyte interface and to develop highly efficient and low-cost photocathodes for hydrogen production from water. To this end, Si nanowire (Si-NW) array has been loaded with highly efficient electrocatalysts, M-B (M = Ni, Co), by facile and quick electroless plating to build M-B catalyst-modified Si nanowire-array-textured photocathodes for water reduction to H2. Compared with the bare Si-NW array, composite Si-NWs/M-B arrays display evidently enhanced photoelectrochemical (PEC) performance. The onset potential (Vphon) of cathodic photocurrent is positively shifted by 530-540 mV to 0.44-0.45 V vs RHE, and the short-circuit current density (Jsc) is up to 19.5 mA cm-2 in neutral buffer solution under simulated 1 sun illumination. Impressively, the half-cell photopower conversion efficiencies (ηhc) of the optimized Si-NWs/Co-B (2.53%) and Si-NWs/Ni-B (2.45%) are comparable to that of Si-NWs/Pt (2.46%). In terms of the large Jsc, Vphon, and ηhc values, as well as the high Faradaic efficiency, Si-NWs/M-B electrodes are among the top performing Si photocathodes which are modified with HER electrocatalysts but have no buried solid/solid junction.

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.

Nanodicing Single Crystalline Silicon Nanowire Arrays.

Here, we demonstrate a novel method for the production of single-crystal Si nanowire arrays based on the top-down carving of Si-nanowall structures from a donor substrate, and their subsequent controlled and selective harvesting into a sacrificial solid material block. Nanosectioning of the nanostructures-embedding block by ultramicrotome leads to the formation of size, shape, and orientation-controlled high quality nanowire arrays. Additionally, we introduce a novel approach that enables transferring the nanowire arrays to any acceptor substrate, while preserving their orientation, and placing them on defined locations. Furthermore, crystallographic analysis and electrical measurements were performed, proving that the quality of the sectioned nanowires, which derive from their original crystalline donor substrate, are remarkably preserved.

Hierarchical Branched Vanadium Oxide Nanorod@Si Nanowire Architecture for High Performance Supercapacitors

Vanadium oxide (VOx ) nanorods are uniformly synthesized on dense Si nanowire arrays. This 3D hierarchical nanoarchitecture offers a novel high-performance supercapacitor electrode design with significantly improved specific capacitance and high-rate capability.

Vertical Si nanowire arrays fabricated by magnetically guided metal-assisted chemical etching

In this work, vertically aligned Si nanowire arrays were fabricated by magnetically guided metal-assisted directional chemical etching. Using an anodized aluminum oxide template as a shadow mask, nanoscale Ni dot arrays were fabricated on an Si wafer to serve as a mask to protect the Si during the etching. For the magnetically guided chemical etching, we deposited a tri-layer metal catalyst (Au/Fe/Au) in a Swiss-cheese configuration and etched the sample under the magnetic field to improve the directionality of the Si nanowire etching and increase the etching rate along the vertical direction. After the etching, the nanowires were dried with minimal surface-tension-induced aggregation by utilizing a supercritical CO2 drying procedure. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed the formation of single-crystal Si nanowires. The method developed here for producing vertically aligned Si nanowire arrays could find a wide range of applications in electrochemical and electronic devices.

Enhanced Electroluminescence From Si Quantum Dots-Based Light-Emitting Devices With Si Nanowire Structures and Hydrogen Passivation

Enhanced electroluminescence (EL) has been achieved from the light-emitting devices containing Si quantum dots/SiO2 multilayers deposited on Si nanowire arrays because of the good antireflection characteristics of Si nanowire structures, which improves the light extraction efficiency. However, it is found that the EL is first enhanced with increasing the depth of the Si nanowires and then reduced to further increase the depth, though it exhibits the lowest reflectance (∼3%), which may be due to the increased surface defect states after long time etching, as revealed by electron spin resonance measurements. It is demonstrated that the posthydrogen plasma annealing treatments can passivate the surface defect states which, in turn, improve device performance. The best device shows the turn-on voltage as low as 3.5 V, and the EL intensity of devices on Si nanowire arrays is enhanced 16-fold, compared with that of flat one.

Inherent formation of porous p-type Si nanowires using palladium-assisted chemical etching

Porous silicon (Si) nanowire arrays were directly fabricated from lightly p-doped Si substrates using a palladium (Pd)-assisted chemical etching at room temperature. The mechanistic studies indicated that anodic dissolution of Si was established by the accumulated positive charges at Pd/Si schottky interfaces in the presence of H2O2 oxidants. In addition to the primary etching direction vertically to the substrate planes, the additional sidewall etching was stimulated by the separated Pd nanoparticles during reaction that constitutes the porous features covering on the nanowires surfaces thoroughly. These combined effects lead to the distinct etching characteristics and remarkable photoluminescent properties of resulted nanostructures.

Experimental Demonstration of Directive Si3N4 Optical Leaky Wave Antennas With Semiconductor Perturbations

Directive optical leaky wave antennas (OLWAs) fabricated in CMOS-compatible semiconductor planar waveguide technology have the potential to provide high directivity with electrical tunability promising for modulation and switching capabilities. We experimentally demonstrate directive radiation from a silicon nitride (Si3N 4) waveguide-based OLWA. The OLWA design comprises a crystalline silicon (Si) nanowire array buried inside a Si 3N4 waveguide. Each Si nanowire has a width of 260 nm and a height of 150 nm. The OLWA is designed to exhibit a directive radiation pattern at telecom wavelengths. The measured radiation pattern at the wavelength 1550 nm has its maximum emission intensity at the angle of 85.1° relative to the waveguide axis and a half-power beam width of approximately 5.0°, which are consistent with our theoretical predictions. The results indicate that the TM mode is more radiative than the TE mode in our fabricated devices. Also, the radiation pattern of the measured OLWA shows dependence on wavelength, which is typical of leaky wave antennas. The device is promising in chip-to-space optical interconnect applications.

Fabrication of metallic nanodisc hexagonal arrays using nanosphere lithography and two-step lift-off

Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.

Directed Assembly of Nanoparticle Catalysts on Nanowire Photoelectrodes for Photoelectrochemical CO2 Reduction.

Reducing carbon dioxide with a multicomponent artificial photosynthetic system, closely mimicking nature, represents a promising approach for energy storage. Previous works have focused on exploiting light-harvesting semiconductor nanowires (NW) for photoelectrochemical water splitting. With the newly developed CO2 reduction nanoparticle (NP) catalysts, direct interfacing of these nanocatalysts with NW light absorbers for photoelectrochemical reduction of CO2 becomes feasible. Here, we demonstrate a directed assembly of NP catalysts on vertical NW substrates for CO2-to-CO conversion under illumination. Guided by the one-dimensional geometry, well-dispersed assembly of Au3Cu NPs on the surface of Si NW arrays was achieved with facile coverage tunability. Such Au3Cu NP decorated Si NW arrays can readily serve as effective CO2 reduction photoelectrodes, exhibiting high CO2-to-CO selectivity close to 80% at -0.20 V vs RHE with suppressed hydrogen evolution. A reduction of 120 mV overpotential compared to the planar (PL) counterpart was observed resulting from the optimized spatial arrangement of NP catalysts on the high surface area NW arrays. In addition, this system showed consistent photoelectrochemical CO2 reduction capability up to 18 h. This simple photoelectrode assembly process will lead to further progress in artificial photosynthesis, by allowing the combination of developments in each subfield to create an efficient light-driven system generating carbon-based fuels.

Morphology Transformation of Au Nanoparticles for Vertically Aligned Si Nanowire Arrays

Morphology Transformation of Au Nanoparticles for Vertically Aligned Si Nanowire Arrays

Development of a facile block copolymer method for creating hard mask patterns integrated into semiconductor manufacturing

Our goal is to develop a facile process to create patterns of inorganic oxides and metals on a substrate that can act as hard masks. These materials should have high etch contrast (compared to silicon) and so allow high-aspect-ratio, high-fidelity pattern transfer whilst being readily integrable in modern semiconductor fabrication (FAB friendly). Here, we show that ultra-small-dimension hard masks can be used to develop large areas of densely packed vertically and horizontally orientated Si nanowire arrays. The inorganic and metal hard masks (Ni, NiO, and ZnO) of different morphologies and dimensions were formed using microphase-separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin films by varying the BCP molecular weight, annealing temperature, and annealing solvent(s). The self-assembled polymer patterns were solvent-processed, and metal ions were included into chosen domains via a selective inclusion method. Inorganic oxide nanopatterns were subsequently developed using standard techniques. High-resolution transmission electron microscopy studies show that high-aspect-ratio pattern transfer could be affected by standard plasma etch techniques. The masking ability of the different materials was compared in order to create the highest quality uniform and smooth sidewall profiles of the Si nanowire arrays. Notably good performance of the metal mask was seen, and this could impact the use of these materials at small dimensions where conventional methods are severely limited.

Location-Controlled Growth of Vertically Aligned Si Nanowires using Au Nanodisks Patterned by KrF Stepper Lithography.

The location-controlled epitaxial growth of vertically aligned Si nanowire (v-SiNW) arrays over large surface area was investigated with Au nanodisks (AuNDs) patterned by KrF stepper lithography. There are two steps for synthesizing v-SiNWs from an AuND pattern: annealing and growth. The annealing process induces the formation of a single Au nanoparticle (AuNP) from an AuND pattern, which consists of several cracked AuNPs. Here, the oxide layer between the AuNDs and Si substrate is necessary for impeding the diffusion of Si atoms into the AuNPs. However, the oxide layer must be removed for properly aligned epitaxial SiNW growth. These SiNW arrays in large area can contribute highly to improve a nanowire-based engineering by controlling the location of SiNWs with consistent pitch.

Oxide-coated silicon nanowire array capacitor electrodes in room temperature ionic liquid

Improved performance of Si nanowire arrays for capacitor electrodes in ionic liquid [Bmim][NTf2], is obtained by spin-on-doping the nanowires followed by hot, concentrated nitric acid oxidation. n- and p-type Si nanowire arrays are fabricated via a 2-step metal-assisted chemical etch process to increase the effective surface area. Spin-on-doping increases the doping density of the nanowires, enhancing the current by a factor of more than 3. The well-controlled HNO3 oxidation defines a thin, dense oxide layer on the Si nanowires increasing chemical stability, both expanding the electrochemical window and increasing the current further by a factor >2. Specific capacitances of 238μFcm−2 (∼0.4Fg−1, 159mFcm−3) and 404μFcm−2 (∼0.7Fg−1, 269mFcm−3) are obtained for n- and p-type Si nanowire arrays, respectively.

High efficiency n-Si/p-Cu2O core-shell nanowires photodiode prepared by atomic layer deposition of Cu2O on well-ordered Si nanowires array

A highly efficient n-Si/p-Cu2O core-shell (C-S) nanowire (NW) photodiode was fabricated using Cu2O grown by atomic layer deposition (ALD) on a well-ordered Si NW array. Ordered Si nanowires arrays were fabricated by nano-sphere lithography to pattern metal catalysts for the metal-assisted etching of silicon, resulting in a Si NW arrays with a good arrangement, smooth surface and small diameter distribution. The ALD-Cu2O thin films were grown using a new non-fluorinated Cu precursor, bis(1-dimethylamino-2-methyl-2-butoxy)copper (C14H32N2O2Cu), and water vapor (H2O) at 140°C. Transmission electron microscopy equipped with an energy dispersive spectrometer confirmed that p-Cu2O thin films had been coated over arrayed Si NWs with a diameter of 150 nm (aspect ratio of ∼7.6). The C-S NW photodiode exhibited more sensitive photodetection performance under ultraviolet illumination as well as an enhanced photocurrent density in the forward biasing region than the planar structure diode. The superior performance of C-S NWs photodiode was explained by the lower reflectance of light and the effective carrier separation and collection originating from the C-S NWs structure.

Fabricating vertically aligned sub-20 nm Si nanowire arrays by chemical etching and thermal oxidation

Silicon nanowires (SiNWs) are appealing building blocks in various applications, including photovoltaics, photonics, and sensors. Fabricating SiNW arrays with diameters <100 nm remains challenging through conventional top-down approaches. In this work, chemical etching and thermal oxidation are combined to fabricate vertically aligned, sub-20 nm SiNW arrays. Defect-free SiNWs with diameters between 95 and 200 nm are first fabricated by nanosphere (NS) lithography and chemical etching. The key aspects for defect-free SiNW fabrication are identified as: (1) achieving a high etching selectivity during NS size reduction; (2) retaining the circular NS shape with smooth sidewalls; and (3) using a directional metal deposition technique. SiNWs with identical spacing but variable diameters are demonstrated by changing the reactive ion etching power. The diameter of the SiNWs is reduced by thermal oxidation, where self-limiting oxidation is encountered after oxidizing the SiNWs at 950 °C for 1 h. A second oxidation is performed to achieve vertically aligned, sub-20 nm SiNW arrays. Si/SiO2 core/shell NWs are obtained before removing the oxidized shell. HRTEM imaging shows that the SiNWs have excellent crystallinity.

Flexible Near-Infrared Photovoltaic Devices Based on Plasmonic Hot-Electron Injection into Silicon Nanowire Arrays.

The development of flexible near-infrared (NIR) photovoltaic (PV) devices containing silicon meets the strong demands for solar utilization, portability, and sustainable manufacture; however, improvements in the NIR light absorption and conversion efficiencies in ultrathin crystalline Si are required. We have developed an approach to improve the quantum efficiency of flexible PV devices in the NIR spectral region by integrating Si nanowire arrays with plasmonic Ag nanoplates. The Ag nanoplates can directly harvest and convert NIR light into plasmonic hot electrons for injection into Si, while the Si nanowire arrays offer light trapping. Taking the wavelength of 800 nm as an example, the external quantum efficiency has been improved by 59 % by the integration Ag nanoplates. This work provides an alternative strategy for the design and fabrication of flexible NIR PVs.