Vertically-aligned Silicon Nanowires Research Articles

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Real-time impedimetric detection of cardiac troponin I using ITO-coated vertically aligned silicon nanowires

This research focuses on developing a sensitive nanoelectronic device (NED) with impedimetric detection of human cardiac troponin I (cTnI) for the real-time diagnosis of acute myocardial infarction. The sensing area of NED was composed of indium tin oxide (ITO)-coated vertically aligned silicon nanowires (VASiNWs). ITO-coated-VASiNWs were examined by X-ray diffraction and scanning electron microscope for the identification of crystalline structure and morphological observation, respectively. After cTnI antibody (cTnIAb) modification, various concentrations of cTnI in the human blood sera were then impedimetrically detected by cTnIAb-modified-ITO-coated-VASiNWs via microfluidic systems. Our device exhibited a linear range from 1.76011 to 17601.1 ng/mL with good sensitivity for the detection of cTnI.

Recent Advances in Structuring and Patterning Silicon Nanowire Arrays for Engineering Light Absorption in Three Dimensions.

Vertically aligned silicon nanowire (VA-SiNW) arrays can significantly enhance light absorption and reduce light reflection for efficient light trapping. VA-SiNW arrays thus have the potential to improve solar cell design by providing reduced front-face reflection while allowing the fabrication of thin, flexible, and efficient silicon-based solar cells by lowering the required amount of silicon. Because their interaction with light is highly dependent on the array geometry, the ability to control the array morphology, functionality, and dimension offers many opportunities. Herein, after a short discussion about the remarkable optical properties of SiNW arrays, we report on our recent progress in using chemical and electrochemical methods to structure and pattern SiNW arrays in three dimensions, providing substrates with spatially controlled optical properties. Our approach is based on metal-assisted chemical etching (MACE) and three-dimensional electrochemical axial lithography (3DEAL), which are both affordable and large-scale wet-chemical methods that can provide a spatial resolution all the way down to the sub-5 nm range.

Band gap modification for enhancement in optoelectronic properties with silicon nanowire

Here, by varying the etching time of Si wafer with metal-assisted chemical etching process, we have formed the vertically aligned silicon nanowire (VA-SiNW) by etching the Si wafer for the duration of 40 min with metal-assisted chemical etching method. In this study Field-effect scanning electron microscopy has been used for the characterization of VA-SiNW (FESEM). Here the etching duration of 40 min has been used, the band gap modification has been analyzed by the use of photoluminescence spectroscopy. The study of photoluminescence spectroscopy (PL) and XRD offers proof of our findings and appears as supporting information. These 1-D nanostructures are quite influential in sensing, biological applications anti-reflection coating (ARC) properties and have broad use in electronic field.

Influence of wet etching in KOH on defects in silicon nanowires formed by cryogenic dry etching

The work is devoted to exploration of arrays of vertical aligned silicon nanowires (SiNWs) obtained by cryogenic dry etching in ICP mode with height of 4.5-5.5 μm and aspect ratio of 7. It was shown that geometry of nanowires has crucial influence on rate of wet etching in KOH since it is higher for 3D objects than for planar wafer, and the diameter should be the same along the nanowire for controlled wet etching. Wet etching in 4% KOH solution during 30 s allowed to save array of uniformity vertical aligned SiNWs with height of 4 μm and diameter of 500 nm. Such treatment reduced concentration of defects detected by deep-level transient spectroscopy, particularly, it drops as minimum in two times for deep level with £0=0.68-0.74 eV placed near to surface of wafer.

Fabrication of silicon nanowire arrays by near-field laser ablation and metal-assisted chemical etching

We present an elegant route for the fabrication of ordered arrays of vertically-aligned silicon nanowires with tunable geometry at controlled locations on a silicon wafer. A monolayer of transparent microspheres convectively assembled onto a gold-coated silicon wafer acts as a microlens array. Irradiation with a single nanosecond laser pulse removes the gold beneath each focusing microsphere, leaving behind a hexagonal pattern of holes in the gold layer. Owing to the near-field effects, the diameter of the holes can be at least five times smaller than the laser wavelength. The patterned gold layer is used as catalyst in a metal-assisted chemical etching to produce an array of vertically-aligned silicon nanowires. This approach combines the advantages of direct laser writing with the benefits of parallel laser processing, yielding nanowire arrays with controlled geometry at predefined locations on the silicon surface. The fabricated VA-SiNW arrays can effectively transfect human cells with a plasmid encoding for green fluorescent protein.

Fully Tunable Silicon Nanowire Arrays Fabricated by Soft Nanoparticle Templating.

We demonstrate a fabrication breakthrough to produce large-area arrays of vertically aligned silicon nanowires (VA-SiNWs) with full tunability of the geometry of the single nanowires and of the whole array, paving the way toward advanced programmable designs of nanowire platforms. At the core of our fabrication route, termed "Soft Nanoparticle Templating", is the conversion of gradually compressed self-assembled monolayers of soft nanoparticles (microgels) at a water-oil interface into customized lithographical masks to create VA-SiNW arrays by means of metal-assisted chemical etching (MACE). This combination of bottom-up and top-down techniques affords excellent control of nanowire etching site locations, enabling independent control of nanowire spacing, diameter and height in a single fabrication route. We demonstrate the fabrication of centimeter-scale two-dimensional gradient photonic crystals exhibiting continuously varying structural colors across the entire visible spectrum on a single silicon substrate, and the formation of tunable optical cavities supported by the VA-SiNWs, as unambiguously demonstrated through numerical simulations. Finally, Soft Nanoparticle Templating is combined with optical lithography to create hierarchical and programmable VA-SiNW patterns.

Maximizing Transfection Efficiency of Vertically Aligned Silicon Nanowire Arrays

Vertically aligned silicon nanowire (VA‐SiNW) arrays are emerging as a powerful new tool for gene delivery by means of mechanical transfection. In order to utilize this tool efficiently, uncertainties around the required design parameters need to be removed. Here, a combination of nanosphere lithography and templated metal‐assisted wet chemical etching is used to fabricate VA‐SiNW arrays with a range of diameters, heights, and densities. This fabrication strategy allows identification of critical parameters of surface topography and consequently the design of SiNW arrays that deliver plasmid with high transfection efficiency into a diverse range of human cells whilst maintaining high cell viability. These results illuminate the cell‐materials interactions that mediate VA‐SiNW transfection and have the potential to transform gene therapy and underpin future treatment modalities.

Versatile Particle-Based Route to Engineer Vertically Aligned Silicon Nanowire Arrays and Nanoscale Pores.

Control over particle self-assembly is a prerequisite for the colloidal templating of lithographical etching masks to define nanostructures. This work integrates and combines for the first time bottom-up and top-down approaches, namely, particle self-assembly at liquid-liquid interfaces and metal-assisted chemical etching, to generate vertically aligned silicon nanowire (VA-SiNW) arrays and, alternatively, arrays of nanoscale pores in a silicon wafer. Of particular importance, and in contrast to current techniques, including conventional colloidal lithography, this approach provides excellent control over the nanowire or pore etching site locations and decouples nanowire or pore diameter and spacing. The spacing between pores or nanowires is tuned by adjusting the specific area of the particles at the liquid-liquid interface before deposition. Hence, the process enables fast and low-cost fabrication of ordered nanostructures in silicon and can be easily scaled up. We demonstrate that the fabricated VA-SiNW arrays can be used as in vitro transfection platforms for transfecting human primary cells.

Fabrication of Hierarchical ZnO/Si Nanowire Structure for Photoelectrochemical Cells

We demonstrate the fabrication of three dimensional hierarchical ZnO/Si nanowire core-shell structure, which is used as a high surface area photoelectrode in photoelectrochemical cells for solar water splitting. This structure is fabricated via a two-step solution process: fabricating vertical aligned silicon nanowires via metal assisted etching method and then growing ZnO nanowires on the silicon nanowires via hydrothermal method. Transmission electron microscopy, scanning electron microscopy, spectrometer and X-ray diffractmeter are employed to characterize the hierarchical structure. The surface area of the hierarchical structure increases significantly compared to the simplex nanowire structure, which can substantially promote the photoelectrochemical reactions. The core-shell characteristic is beneficial to the charge carrier separation and the silicon nanowire backbones can work as the charge transfer channels, which contribute to the efficiency improvement. The experimental results indicate that the photocurrent density and the maximum conversion efficiency of the hierarchical structure photoelectrode have increased obviously, confirming that this hierarchical structure is more suitable for the application of photoelectrode than the ZnO nanowires/planar silicon structure.

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission.

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.

Manipulating the antireflective properties of vertically-aligned silicon nanowires

Vertically-aligned silicon nanowires (VA-SiNWs) realized with vacuum metal-assisted chemical etching are demonstrated to be the better candidate for optical manipulation due to their superior physical and optical properties. The length-dependent volume filling factor of as-grown VA-SiNWs agrees with the theoretically optimal conditions which lead to the lowest surface reflectance in visible spectral region. Growing a thin thermal oxide with an appropriate thickness on the surface of VA-SiNWs could further improve the antireflective performance in near-infrared wavelengths. We also demonstrated that the surface reflectance could be adjusted without sacrificing the bandwidth of its antireflective window by tuning the bundle filling factor of two-dimensional VA-SiNW bundle array. Such a flexible surface reflectance cannot be obtained by simply adjusting the physical parameters of planar VA-SiNWs. For implementing SiNW-based solar cells, a pre-patterning method is conducted to enable the formation of antireflective VA-SiNW surfaces at energy-harvesting areas while maintaining planar silicon surfaces at the metallization areas to avoid high electrical contact resistivity over SiNW surfaces. Nanowire-based solar cell shows an 18% increased short-circuit current and a 10% improved overall efficiency compared to the reference cell with a planar silicon top surface.

Relaxing the electrostatic screening effect by patterning vertically-aligned silicon nanowire arrays into bundles for field emission application

Top-down fabrication strategies are proposed and demonstrated to realize arrays of vertically-aligned silicon nanowire bundles and bundle arrays of carbon nanotube–silicon nanowire (CNT–SiNW) heterojunctions, aiming for releasing the electrostatic screening effect and improving the field emission characteristics. The trade-off between the reduction in the electrostatic screening effect and the decrease of emission sites leads to an optimal SiNW bundle arrangement which enables the lowest turn-on electric field of 1.4V/μm and highest emission current density of 191μA/cm2 among all testing SiNW samples. Benefiting from the superior thermal and electrical properties of CNTs and the flexible patterning technologies available for SiNWs, bundle arrays of CNT–SiNW heterojunctions show improved and highly-uniform field emission with a lower turn-on electric field of 0.9V/μm and higher emission current density of 5.86mA/cm2. The application of these materials and their corresponding fabrication approaches is not limited to the field emission but can be used for a variety of emerging fields like nanoelectronics, lithium-ion batteries, and solar cells.

Controlled Fabrication of Silicon Nanowires via Nanosphere Lithograph and Metal Assisted Chemical Etching

We investigated the controlled fabrication of uniform vertical aligned silicon nanowires with desired length, diameter and location by combining nanosphere lithograph and metal assisted chemical etching techniques. The close-packed polystyrene nanospheres array was obtained by self-assemble technique, followed by reactive ion etching to acquire a non-close-packed monolayer template. Subsequently, the template was used to create a metal film with nanoholes array, which enable the controlled fabrication of ordered silicon nanowires via metal assisted chemical etching technique. By adjusting the monolayer of polystyrene nanospheres and the conditions for the metal assisted chemical etching, we obtained uniform distributed silicon nanowires with desired morphology. The aspect ratio of the silicon nanowires can reach to about 86:1. Furthermore, we have obtained the double-layer silicon nanowires by slight modifying the process. The influences of various conditions during etching were also discussed for improving the controlled fabrication.

Silicon Nitride Passivation of Silicon Nanowires Solar Cell

Vertical aligned silicon nanowires were synthesized using chemical etching of silicon wafer. Influence of a silicon nitride layer on the top of the silicon nanowires solar cell has been investigated. The optical properties of a Si NWs array with and without silicon nitride passivation layer are examined in terms of optical reflection property. In the presence of silicon nitride layer, 1% reflection ratio in the spectral range (250-1000nm) is achieved. In addition, the solar cell characteristics have been significantly improved, which exhibits high short circuit current as well high efficiency. Based on the current-voltage measurements and morphology results, we show that the silicon nitride layer can passivate the defects generated by wet etching processes.

Selectively grown vertical silicon nanowire p–n+ photodiodes via aqueous electroless etching

A facile method to selectively grow vertically-aligned silicon nanowires (SiNWs) which can inherit the doping concentration from its mother wafer, with controllable length, is demonstrated using the combination of photolithography and aqueous electroless etching. The use of SU-8-2002, a chemically and mechanically robust photoresist (PR) material, provided a high selectivity for the etching reaction on the exposed surface of 1-μm-thick n+ doped p-type (100) Si substrate, resulting in the fabrication of ∼30-μm-long vertically-aligned SiNW photodiode arrays on the desired locations, while the areas covered with SU-8-2002 remained unreacted. Optical and field emission scanning electron microscope analyses confirmed that SiNWs were selectively grown while retaining the shape of the PR patterns. The electrical and optical measurements of the fabricated p–n+ junction SiNW photodiodes were compared to those of reference planar p–n+ junction Si photodiodes: the current density of the p–n+ junction SiNW photodiodes was approximately 3 times greater than that of the planar counterpart at the forward bias of 5V, which can be attributed to the high density of defect states on the rough surfaces of the synthesized SiNWs, leading to the increased recombination efficiencies for the injected carriers. In addition, the photoresponse of the p–n+ SiNW photodiode arrays was 3.4 times higher than that of the planar device at −3.5V due to the increase in the light scattering.

Mutiscale substrates based on hydrogel-incorporated silicon nanowires for protein patterning and microarray-based immunoassays

Here, protein micropatterns were prepared on micropatterned nanostructures for potential applications in microarray-based multiplex bioassays with enhanced protein-loading capacity and detection sensitivity. Vertically-aligned silicon nanowires (SiNWs) that were about 8μm in height and 150nm in diameter were prepared using an etching process and were surface-modified with aminopropyltriethoxysilane (APTES) to allow them to covalently immobilize proteins. The SiNW substrate was then overlaid with a micropattern of poly(ethylene glycol) (PEG) hydrogel to create defined arrays of microwells consisting of APTES-modified SiNW on the bottom of the wells, with hydrogel on the walls of the wells. Due to the non-adhesiveness of PEG hydrogels toward proteins, proteins were selectively immobilized on the surface-modified SiNW regions to create protein micropatterns. The increase in surface area increased the protein loading capacity of the SiNWs by more than 10 times the capacity of a planar silicon substrate. Immunobinding assays between IgG and anti-IgG and between IgM and anti-IgM that were performed on micropatterned SiNWs emitted stronger fluorescent signals and showed higher sensitivity than assays performed on planar silicon substrates. Finally, microfluidic channels were successfully integrated into the micropatterned SiNWs to enable the simultaneous performance of multiple immunoassays on a single microarray platform.

Formation of mesa-type vertically aligned silicon nanowire bundle arrays by selective-area chemical oxidation and etching processes

A novel top-down scheme for post patterning of vertically aligned silicon nanowires (VA-SiNWs) is proposed and demonstrated by means of chemical oxidation, photolithographic patterning, and chemical etching processes. The authors discovered that only ∼1.4 nm chemical oxide layer is sufficient to protect SiNWs from the attack of KOH etchant. VA-SiNWs can simultaneously serve as an optical antireflective layer between photoresist and silicon substrate for eliminating the backreflection during photolithographic patterning due to the strong optical absorption and light trapping behavior at ultraviolet spectral region. After selective-area SiNW removal with chemical etching procedures, SiNWs at unmasked areas are drastically shortened with a decreased wire number density while oxide masked SiNWs are still vertically aligned with high wire number density and good uniformity. The patterned SiNWs preserves the superior antireflective performance of original SiNWs which makes every SiNW bundle to have dark appearance. Contrary to prepatterning scheme which creates embedded VA-SiNW patterns, this post patterning scheme enables the synthesis of patterned mesa-type SiNWs on silicon surface. The patterned VA-SiNW bundles can be employed for applications in arrayed sensors, arrayed photodiodes, or arrayed field emitters.

A reliable method to grow vertically-aligned silicon nanowires by a novel ramp-cooling process

We have grown silicon nanowires (SiNWs) on Si (111) substrates by gold-catalyzed vapor–liquid–solid (VLS) process using tetrachlorosilane (SiCl4) in a hot-wall chemical vapor deposition reactor. Even under the optimized conditions including H2 annealing to reduce the surface native oxide, epitaxial SiNWs of 150–200nm in diameter often grew along all four 〈111〉 family directions with one direction vertical and three others inclined to the surface. Therefore, the growth of high degree ordered SiNW arrays along [111] only was attempted on Au-coated Si (111) by a ramp-cooling process utilizing the liquid phase epitaxy (LPE) mechanism. The Au-coated Si substrate was first annealed in H2 at 650°C to form Au–Si alloy nanoparticles, and then ramp-cooled at a controlled rate to precipitate epitaxial Si seeds on the substrate based on LPE mechanism. The substrate was further heated in SiCl4/H2 to 850°C for the VLS growths of SiNWs on the Si seeds. Thus, almost 100% vertically-aligned SiNWs along [111] only could be reproducibly grown on Si (111), without using a template or patterning the metal catalyst. The high-density vertically-aligned SiNWs have good potentials for solar cells and nano-devices.

Silicon nanowire-array-textured solar cells for photovoltaic application

In this paper, a vertical-aligned silicon nanowires (Si NWs) array has been synthesized and implemented to the Si NW-array-textured solar cells for photovoltaic application. The optical properties of a Si NWs array on both the plane and pyramid-array-textured substrates were examined in terms of optical reflection property. Less than 2% reflection ratio at 800 nm wavelength was achieved. Using leftover monocrystalline Si (c-Si) wafer (125×125 mm2), a 16.5% energy conversion efficiency, with 35.4% enhancement compared to the pyramid-array-textured c-Si solar cells, was made by the Si NW-array-textured solar cells due to their enhanced optical absorption characteristics. However, without SiNx passivation, the short circuit current reduced due to the increased surface recombination when using Si NWs array as surface texturing, indicating that an optimum surface passivation was prerequisite in high-efficiency Si NW-array-textured solar cells.