Graphene/silicon nanowire Schottky junction for enhanced light harvesting

Schottky junction solar cells are assembled by directly coating graphene films on n-type silicon nanowire (SiNW) arrays. The graphene/SiNW junction shows enhanced light trapping and faster carrier transport compared to the graphene/planar Si structure. With chemical doping, the SiNW-based solar cells showed energy conversion efficiencies of up to 2.86% at AM1.5 condition, opening a possibility of using graphene/semiconductor nanostructures in photovoltaic application.     

Influence of Nanopillar Arrays on Fibroblast Motility,Adhesion, and Migration Mechanisms

Surfaces decorated with high aspect ratio nanostructures are a promising tool to study cellular processes and design novel devices to control cellular behavior. However, little is known about the dynamics of cellular phenomenon such as adhesion, spreading, and migration on such surfaces. In particular, how these are influenced by the surface properties. In this work, fibroblast behavior is investigated on regular arrays of 1 µm high polymer nanopillars with varying pillar to pillar distance. Embryonic mouse fibroblasts (NIH‐3T3) spread on all arrays, and on contact with the substrate engulf nanopillars independently of the array pitch. As the cells start to spread, different behavior is observed. On dense arrays which have a pitch equal or below 1 µm, cells are suspended on top of the nanopillars, making only sporadic contact with the glass support. Cells stay attached to the glass support and fully engulf nanopillars during spreading and migration on the sparse arrays which have a pitch of 2 µm and above. These alternate states have a profound effect on cell migration rates. Dynamic F‐actin puncta colocalize with nanopillars during cell spreading and migration. Strong membrane association with engulfed nanopillars might explain the reduced migration rates on sparse arrays.     

Fabrication of flexible and vertical silicon nanowire electronics

Vertical silicon nanowire (SiNW) array devices directly connected on both sides to metallic contacts were fabricated on various non-Si-based substrates (e.g., glass, plastics, and metal foils) in order to fully exploit the nanomaterial properties for final applications. The devices were realized with uniform length Ag-assisted electroless etched SiNW arrays that were detached from their fabrication substrate, typically Si wafers, reattached to arbitrary substrates, and formed with metallic contacts on both sides of the NW array. Electrical characterization of the SiNW array devices exhibits good current-voltage characteristics consistent with the SiNW morphology.     

Nanostructured gold microelectrodes for extracellular recording from electrogenic cells

We present a new biocompatible nanostructured microelectrode array for extracellular signal recording from electrogenic cells. Microfabrication techniques were combined with a template-assisted approach using nanoporous aluminum oxide to develop gold nanopillar electrodes. The nanopillars were approximately 300-400 nm high and had a diameter of 60 nm. Thus, they yielded a higher surface area of the electrodes resulting in a decreased impedance compared to planar electrodes. The interaction between the large-scale gold nanopillar arrays and cardiac muscle cells (HL-1) was investigated via focused ion beam milling. In the resulting cross-sections we observed a tight coupling between the HL-1 cells and the gold nanostructures. However, the cell membranes did not bend into the cleft between adjacent nanopillars due to the high pillar density. We performed extracellular potential recordings from HL-1 cells with the nanostructured microelectrode arrays. The maximal amplitudes recorded with the nanopillar electrodes were up to 100% higher than those recorded with planar gold electrodes. Increasing the aspect ratio of the gold nanopillars and changing the geometrical layout can further enhance the signal quality in the future.     

Fabrication of slantingly-aligned silicon nanowire arrays for solar cell applications

Large-area slantingly-aligned silicon nanowire arrays (SA-SiNW arrays) on Si(111) substrate have been fabricated by wet chemical etching with dry metal deposition method and employed in the fabrication of solar cells for the first time. The formation of SA-SiNW arrays possibly results from the anisotropic etching of silicon by silver catalysts. Superior to the previous cells fabricated with vertically-aligned silicon nanowire arrays (VA-SiNW arrays), the SA-SiNW array solar cells exhibit a highest power conversion efficiency of?11.37%. The improved device performance is attributed to the integration of the excellent anti-reflection property of the arrays and the better electrical contact of the cell as a result of the special slantingly-aligned structure. The high surface recombination velocity of minority carriers in SiNW arrays is still the main limitation on cell performance.     

Realization of thermally durable close-packed 2D gold nanoparticle arrays using self-assembly and plasma etching

Realization of thermally and chemically durable, ordered gold nanostructures using bottom-up self-assembly techniques are essential for applications in a wide range of areas including catalysis, energy generation, and sensing. Herein, we describe a modular process for realizing uniform arrays of gold nanoparticles, with interparticle spacings of 2?nm and above, by using RF plasma etching to remove ligands from self-assembled arrays of ligand-coated gold nanoparticles. Both nanoscale imaging and macroscale spectroscopic characterization techniques were used to determine the optimal conditions for plasma etching, namely RF power, operating pressure, duration of treatment, and type of gas. We then studied the effect of nanoparticle size, interparticle spacing, and type of substrate on the thermal durability of plasma-treated and untreated nanoparticle arrays. Plasma-treated arrays showed enhanced chemical and thermal durability, on account of the removal of ligands. To illustrate the application potential of the developed process, robust SERS (surface-enhanced Raman scattering) substrates were formed using plasma-treated arrays of silver-coated gold nanoparticles that had a silicon wafer or photopaper as the underlying support. The measured value of the average SERS enhancement factor (2?×?10(5)) was quantitatively reproducible on both silicon and paper substrates. The silicon substrates gave quantitatively reproducible results even after thermal annealing. The paper-based SERS substrate was also used to swab and detect probe molecules deposited on a solid surface.     

Integrate silver colloids with silicon nanowire arrays for surface-enhanced Raman scattering

A facile method to prepare uniform and reproducible surface-enhanced Raman scattering (SERS) substrates is presented. Quasi-spherical silver colloids prepared by microwave heating and wafer-scale uniform silicon nanowire (SiNW) arrays fabricated via wet chemical etching were united together as SERS substrates. The novel SERS substrates displayed stronger Raman enhancement than conventional silver colloids as well as outstanding uniformity and reproducibility in our experiments. In addition, it was found that the cross section of SiNW arrays possessed stronger enhancement activity than the front side. The enhancement effects of two adjacent SiNWs (as a simplification of SiNW arrays) were evaluated by the finite difference time domain (FDTD) method.     

Cellular Deformations Induced by Conical Silicon Nanowire Arrays Facilitate Gene Delivery

Engineered cell–nanostructured interfaces generated by vertically aligned silicon nanowire (SiNW) arrays have become a promising platform for orchestrating cell behavior, function, and fate. However, the underlying mechanism in SiNW‐mediated intracellular access and delivery is still poorly understood. This study demonstrates the development of a gene delivery platform based on conical SiNW arrays for mechanical cell transfection, assisted by centrifugal force, for both adherent and nonadherent cells in vitro. Cells form focal adhesions on SiNWs within 6 h, and maintain high viability and motility. Such a functional and dynamic cell–SiNW interface features conformational changes in the plasma membrane and in some cases the nucleus, promoting both direct penetration and endocytosis; this synergistically facilitates SiNW‐mediated delivery of nucleic acids into immortalized cell lines, and into difficult‐to‐transfect primary immune T cells without pre‐activation. Moreover, transfected cells retrieved from SiNWs retain the capacity to proliferate—crucial to future biomedical applications. The results indicate that SiNW‐mediated intracellular delivery holds great promise for developing increasingly sophisticated investigative and therapeutic tools.     

Nanowire arrays in multicrystalline silicon thin films on glass: a promising material for research and applications in nanotechnology

Silicon nanowires (SiNW) were formed on large grained, electron-beam crystallized silicon (Si) thin films of only ～6 μm thickness on glass using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed outstanding structural properties of this nanomaterial. It could be shown that SiNWs with entirely predetermined shapes including lengths, diameters and spacings and straight side walls form independently of their crystalline orientation and arrange in ordered arrays on glass. Furthermore, for the first time grain boundaries could be observed in individual, straightly etched SiNWs. After heat treatment an electronic grade surface quality of the SiNWs could be shown by X-ray photoelectron spectroscopy (XPS). Integrating sphere measurements show that SiNW patterning of the multicrystalline Si (mc-Si) starting thin film on glass substantially increases absorption and reduces reflection, as being desired for an application in thin film photovoltaics (PV). The multicrystalline SiNWs directly mark a starting point for research not only in PV but also in other areas like nanoelectronics, surface functionalization, and nanomechanics.     

Fabrication of a one-dimensional array of nanopores horizontally aligned on a Si substrate

A one-dimensional array of nanopores horizontally aligned on a silicon substrate was successfully fabricated by anodic aluminum oxidation (AAO) using a modified two-step procedure. SEM pictures show clear nanostructures of well-aligned one-dimensional nanopore arrays without cracks at the interfaces of the sandwiched structures. The processes are compatible with the planar silicon integrated circuit processing technology, promising for applications in nanoelectronics. The formation mechanism of a single nanopore array on Si substrates was also discussed.     

High-efficiency si/polymer hybrid solar cells based on synergistic surface texturing of Si nanowires on pyramids

An efficient Si/PEDOT:PSS hybrid solar cell using synergistic surface texturing of Si nanowires (SiNWs) on pyramids is demonstrated. A power conversion efficiency (PCE) of 9.9% is achieved from the cells using the SiNW/pyramid binary structure, which is much higher than similar cells based on planar Si, pyramid-textured Si, and SiNWs. The PCE is the highest reported to-date for hybrid cells based on Si nanostructures and PEDOT.     

A method for the fabrication of sculptured thin films of periodic arrays of standing nanorods

A simple method for the fabrication of sculptured thin films (STFs) of periodic arrays of high-aspect-ratio (20:1) standing nanorods on silicon substrates is described. It is based on oblique evaporation onto periodically arranged arrays of "mini-rod" topography using a rotating sample stage. Recently, it has been shown that these nanostructures can be prepared on relatively large areas and at low cost, making the method suitable for technological applications.     

Fabrication of 3-D gelatin-patterned glass substrates with layer-by-layer and lift-off (LbL-LO) technology

The assembly of multilayer films of gelatin onto glass substrates using layer-by-layer and lift-off (LbL-LO) technology to modify the surface topography and chemistry properties of in vitro cell culture scaffolds is described. The ability to generate such nanoscale systems containing cell-adhesive materials on optically transparent substrates with microscale lateral dimensions, nanoscale vertical dimensions, molecular vertical precision, and flexibility in material selection has important implications for tissue engineering, drug discovery, and basic research in cell biology. Toward this goal, a systematic study on the electrostatic adsorption properties of fluorescein 5-isothiocyanate-gelatin B (FITC-gelatin) was completed. In addition, the integration of protein nanoassembly with microlithographic feature definition was used to pattern three-dimensional FITC-gelatin nanofilms on planar glass substrates. The experimental results indicate that FITC-gelatin is negatively charged at pH 9 and can be alternately assembled with a positively charged polyion, poly(diallyldimethylammonium chloride) (PDDA), to form multilayer films on solid templates with thickness of 5-10 nm per bilayer. Furthermore, images of protein/polymer nanocomposites indicate that LbL-LO is an efficient way to realize the designed substrates. These findings will benefit future research on cell culture and tissue engineering that require methods of generating protein patterns to fabricate novel in vitro cell culture systems.     

Improved sensing characteristics of dual-gate transistor sensor using silicon nanowire arrays defined by nanoimprint lithography

Abstract

This work describes the construction of a sensitive, stable, and label-free sensor based on a dual-gate field-effect transistor (DG FET), in which uniformly distributed and size-controlled silicon nanowire (SiNW) arrays by nanoimprint lithography act as conductor channels. Compared to previous DG FETs with a planar-type silicon channel layer, the constructed SiNW DG FETs exhibited superior electrical properties including a higher capacitive-coupling ratio of 18.0 and a lower off-state leakage current under high-temperature stress. In addition, while the conventional planar single-gate (SG) FET- and planar DG FET-based pH sensors showed the sensitivities of 56.7 mV/pH and 439.3 mV/pH, respectively, the SiNW DG FET-based pH sensors showed not only a higher sensitivity of 984.1 mV/pH, but also a lower drift rate of 0.8% for pH-sensitivity. This demonstrates that the SiNW DG FETs simultaneously achieve high sensitivity and stability, with significant potential for future biosensing applications.     

Synthetically encoded ultrashort-channel nanowire transistors for fast, pointlike cellular signal detection

Nanostructures, which have sizes comparable to biological functional units involved in cellular communication, offer the potential for enhanced sensitivity and spatial resolution compared to planar metal and semiconductor structures. Silicon nanowire (SiNW) field-effect transistors (FETs) have been used as a platform for biomolecular sensors, which maintain excellent signal-to-noise ratios while operating on lengths scales that enable efficient extra- and intracellular integration with living cells. Although the NWs are tens of nanometers in diameter, the active region of the NW FET devices typically spans micrometers, limiting both the length and time scales of detection achievable with these nanodevices. Here, we report a new synthetic method that combines gold-nanocluster-catalyzed vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) NW growth modes to produce synthetically encoded NW devices with ultrasharp (<5 nm) n-type highly doped (n(++)) to lightly doped (n) transitions along the NW growth direction, where n(++) regions serve as source/drain (S/D) electrodes and the n-region functions as an active FET channel. Using this method, we synthesized short-channel n(++)/n/n(++) SiNW FET devices with independently controllable diameters and channel lengths. SiNW devices with channel lengths of 50, 80, and 150 nm interfaced with spontaneously beating cardiomyocytes exhibited well-defined extracellular field potential signals with signal-to-noise values of ca. 4 independent of device size. Significantly, these "pointlike" devices yield peak widths of ～500 μs, which is comparable to the reported time constant for individual sodium ion channels. Multiple FET devices with device separations smaller than 2 μm were also encoded on single SiNWs, thus enabling multiplexed recording from single cells and cell networks with device-to-device time resolution on the order of a few microseconds. These short-channel SiNW FET devices provide a new opportunity to create nanoscale biomolecular sensors that operate on the length and time scales previously inaccessible by other techniques but necessary to investigate fundamental, subcellular biological processes.     

Fabrication of nanostructure and formation of nanocrystal for non-volatile memory

In this work, we have demonstrated that the nanocrystal created by combining the self-assembled block copolymer thin film with regular semiconductor processing can be applicable to non-volatile memory device with increased charge storage capacity over planar structures. Self-assembled block copolymer thin film for nanostructures with critical dimensions below photolithographic resolution limits has been used during all experiments. Nanoporous thin film from PS-b-PMMA diblock copolymer thin film with selective removal of PMMA domains was used to fabricate nanostructure and nanocrystal. We have also reported about surface morphologies and electrical properties of the nano-needle structure formed by RIE technique. The details of nanoscale pattern of the very uniform arrays using RIE are presented. We fabricated different surface structure of nanoscale using block copolymer. We also deposited Si-rich SiNx layer using ICP-CVD on the silicon surface of nanostructure. The deposited films were studied after annealing. PL studies demonstrated nanocrystal in Si-rich SiNx film on nanostructure of silicon.     

Patterning of various silicon structures via polymer lithography and catalytic chemical etching

We demonstrate a facile fabrication of a rich variety of silicon patterns with different length scales by combining polymer lithography and a metal-assisted chemical etching method. Several types of polymer patterns were fabricated on silicon substrates, and silver layers were deposited on the patterned silicon surfaces and used to etch the silicon beneath. Various silicon patterns including topographic lines, concentric rings, and square arrays were created at a micro-?and nanoscale after etching the silicon and subsequent removal of the patterned polymer masks. Alternatively, the arrays of sub-30?nm silicon nanowires were produced by a chemical etching of the silicon wafer which was covered with highly ordered polystyrene-block-polyvinylpyridine (PS-b-PVP) micellar films. In addition, silicon nanohole arrays were also generated by etching with hexagonally packed silver nanoparticles that were prepared using PS-b-PVP block copolymer templates.     

Tunable, high aspect ratio pillars on diverse substrates using copolymer micelle lithography: an interesting platform for applications

We demonstrate the use of copolymer micelle lithography using polystyrene-block-poly(2-vinylpyridine) reverse micelle thin films in their as-coated form to create nanopillars with tunable dimensions and spacing, on different substrates such as silicon, silicon oxide, silicon nitride and quartz. The promise of the approach as a versatile application oriented platform is highlighted by demonstrating its utility for creating super-hydrophobic surfaces, fabrication of nanoporous polymeric membranes, and controlling the areal density of physical vapor deposition derived titanium nitride nanostructures.     

Arrays of silicon micro/nanostructures formed in suspended configurations for deterministic assembly using flat and roller-type stamps

The ability to create and manipulate large arrays of inorganic semiconductor micro/nanostructures for integration on unconventional substrates provides new possibilities in device engineering. Here, simple methods are described for the preparation of structures of single crystalline silicon in suspended and tethered configurations that facilitate their deterministic assembly using transfer-printing techniques. Diverse shapes (e.g., straight or curved edges), thicknesses (between 55 nm and 3 μm), and sizes (areas of 4000 μm(2) to 117 mm(2) ) of structures in varied layouts (regular or irregular arrays, with dense or sparse coverages) can be achieved, using either flat or cylindrical roller-type stamps. To demonstrate the technique, printing with 100% yield onto curved, rigid supports of glass and ceramics and onto thin sheets of plastic is shown. The fabrication of a printed array of silicon p(+) -i-n(+) junction photodiodes on plastic is representative of device-printing capabilities.     

Six-fold hexagonal symmetric nanostructures with various periodic shapes on GaAs substrates for efficient antireflection and hydrophobic properties

We fabricated various periodic nanostructures with a six-fold hexagonal symmetry on gallium arsenide (GaAs) substrates using simple process steps, together with a theoretical analysis of their antireflective properties. Elliptical photoresist (PR) nanopillars, which are inevitably generated by the asymmetric intensity distribution of the laser interference, were converted to rounded lens-like patterns by a thermal reflow process without any additional complex optic systems, thus leading to an exact six-fold hexagonal symmetry. Various shaped periodic nanostructures including nanorods, cones, truncated cones, and even parabolic patterns were obtained under different etching conditions using the rounded lens-like PR patterns formed by the reflow process. For the parabolic structure, the calculated lowest average reflectance of ～ 2.3% was obtained. To achieve better antireflection characteristics, an aluminum-doped zinc oxide (AZO) film was deposited on the GaAs parabolas, which forms an AZO/GaAs parabolic nanostructure. The structure exhibited a low average reflectance of ～ 1.2% over a wide wavelength region of 350-1800?nm and a hydrophobic surface with a water contact angle of θ(c) ～ 115°. The calculated reflectances were reasonably consistent with the measured results.