Wuxing Lai

Carbonization-assisted integration of silica nanowires to photoresist-derived three-dimensional carbon microelectrode arrays

We propose a novel technique of integrating silica nanowires to carbon microelectrode arrays on silicon substrates. The silica nanowires were grown on photoresist-derived three-dimensional carbon microelectrode arrays during carbonization of patterned photoresist in a tube furnace at 1000 °C under a gaseous environment of N(2) and H(2) in the presence of Cu catalyst, sputtered initially as a thin layer on the structure surface. Carbonization-assisted nucleation and growth are proposed to extend the Cu-catalyzed vapor-liquid-solid mechanism for the nanowire integration behaviour. The growth of silica nanowires exploits Si from the etched silicon substrate under the Cu particles. It is found that the thickness of the initial Cu coating layer plays an important role as catalyst on the morphology and on the amount of grown silica nanowires. These nanowires have lengths of up to 100 µm and diameters ranging from 50 to 200 nm, with 30 nm Cu film sputtered initially. The study also reveals that the nanowire-integrated microelectrodes significantly enhance the electrochemical performance compared to blank ones. A specific capacitance increase of over 13 times is demonstrated in the electrochemical experiment. The platform can be used to develop large-scale miniaturized devices and systems with increased efficiency for applications in electrochemical, biological and energy-related fields.

Growth of highly bright-white silica nanowires as diffusive reflection coating in LED lighting

Large quantities of silica nanowires were synthesized through thermal treatment of silicon wafer in the atmosphere of N(2)/H(2)(5%) under 1200 °C with Cu as catalyst. These nanowires grew to form a natural bright-white mat, which showed highly diffusive reflectivity over the UV-visible range, with more than 60% at the whole range and up to 88% at 350 nm. The utilization of silica nanowires in diffusive coating on the reflector cup of LED is demonstrated, which shows greatly improved light distribution comparing with the specular reflector cup. It is expected that these nanowires can be promising coating material for optoelectronic applications.

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor–liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.

Pyrolysis-assisted graphene exfoliation from graphite particles deposited on photoresist pillars

A simple and cost-effective route is described to exfoliate graphene through deposition of graphite particles on SU-8 pattern followed by the high-temperature pyrolysis process. During the pyrolysis process, the SU-8 photoresist pillars were slowly converted to desired glassy-like carbon posts and various gases were released such as carbon oxides, water vapor, methane and hydrogen. The splitting phenomenon of graphite into graphene sheets was observed. The rapid ramping rate of temperatue and the released gases intercalated inside the graphite are thought to be the predominant mechanism driving the exfoliation. The introduced approach can be applied to fabricate graphene-integrated carbon MEMS for a variety of electrochemical applications.

FABRICATION OF LARGE-AREA SILICON NANOWIRE ARRAYS FOR SOLAR CELLS

In this paper, a simple, low-cost and efficient method was adopted to fabricate large-area Si nanowire (SiNW) arrays by inserting the p-type (100) silicon wafer into aqueous HF + AgNO3 solution for a certain time at room temperature. Surface of the silicon wafer with high aspect ratio SiNW shows the characteristics of low-reflection as low as 5% in the 450–800 nm wavelength range, especially less than 1% after etching for 60 min. The surface also exhibits super-hydrophobicity with water contact angle up to 150°. We investigated the relationship between the etching duration and aspect ratio of the SiNW systematically and demonstrated that the aspect ratio of the SiNW can be controlled. The antireflection surface shows a potential implication in increasing the conversation efficiency for solar cells, and the self-cleaning properties will further enhance the resistance to environment conditions for a long-life work.

Wet chemical etching silicon wafer with low-reflection and super-hydrophobicity

Successful etching silicon wafer for solar cells has been already prepared. In this work, wet chemical etching process at room temperature was introduced to fabricate large-area growth of Si nanowire (SiNW) arrays. The process was carried out on p-type (100) silicon wafer in aqueous HF+AgNO3 etching solution at room temperature. The wafer surface consisting of SiNW arrays shows the characteristics of low-reflection as low as less than 5% in the 450–790 nm wavelength range. The surface also exhibits super hydrophobicity with a high water contact angle up to 150°. Si wafers with large-area SiNW arrays on the surfaces displaying properties of low-reflection and super-hydrophobicity will have potential applications for fabricating high-efficiency and self-cleaning silicon solar cells.

Bulk synthesis of long silicon nitride nanowires on silicon wafer

A Nickel-catalyzed synthesis method was developed to grow large-scale Si<inf>3</inf>N<inf>4</inf> nanowires on silicon wafer in forming gas of N<inf>2</inf>/H<inf>2</inf>(5%) under 1200°C. These nanowires consisted of uniform diameter ranging from 50 to 100 nm with length up to 500 µm. Vapor-liquid-solid mechanism was proposed to interpret the nanowire growth procedure. It was characterized that the nanowires were mainly composed of single-crystal α-Si<inf>3</inf>N<inf>4</inf>, growing along the [120] direction.

Si nanowires arrays fabricated by wet chemical etching for antireflection and self-cleaning

Here we report a simple and cost effective fabrication technique, which created large area vertical Si nanowires (diameter in ~200 nm) by means of silver induced wet chemical etching on single crystalline Si substrates. By this technique, Si nanowires were fabricated on single crystalline in aqueous 5M HF and 0.02M AgNO3 solution at room temperature. The scanning electron microscope (SEM) images indicate that etched silicon wafers consist of dense and nearly vertically aligned one-dimensional nanostructures. Length of Si nanowires was found to increase linearly with etching time (0-300 min). The mechanism of vertical nanowires formation can be understood as being a self-assembled Ag induced selective etching process based on the localized microscopic electrochemical cell model. A low reflectivity averaged ~1.7% from 450 to 790 nm was observed. The nanometer scale rough surface can make water droplet either in the so-called Wenzel or the Cassie regime, which can increase contact angle (CA). High CA makes the surface hydrophobicity and self-cleaning. Water CA (150°) was observed on the etched Si surface. Such antireflection (AR) and self-cleaning surface may have potential applications for silicon solar cells.

Carbon-assisted growth and high visible-light optical reflectivity of amorphous silicon oxynitride nanowires

Large amounts of amorphous silicon oxynitride nanowires have been synthesized on silicon wafer through carbon-assisted vapor-solid growth avoiding the contamination from metallic catalysts. These nanowires have the length of up to 100 μm, with a diameter ranging from 50 to 150 nm. Around 3-nm-sized nanostructures are observed to be homogeneously distributed within a nanowire cross-section matrix. The unique configuration might determine the growth of ternary amorphous structure and its special splitting behavior. Optical properties of the nanowires have also been investigated. The obtained nanowires were attractive for their exceptional whiteness, perceived brightness, and optical brilliance. These nanowires display greatly enhanced reflection over the whole visible wavelength, with more than 80% of light reflected on most of the wavelength ranging from 400 to 700 nm and the lowest reflectivity exceeding 70%, exhibiting performance superior to that of the reported white beetle. Intense visible photoluminescence is also observed over a broad spectrum ranging from 320 to 500 nm with two shoulders centered at around 444 and 468 nm, respectively.

Optimal design of 3-D carbon micro-electrode array for dielectrophoretic manipulation nanoparticles in fluids

The 3-D carbon micro-electrode array, have been designed and simulated for dielectrophoretic manipulation of nanoparticles in fluids. The effects of electrode shape, applied voltage, electrode spacing and geometric size of the electrodes on the gradient in the electric field intensity is considered. Results show that the magnitude in the gradient in the electric field intensity produced by square column electrodes is relatively larger compared to other two electrode shapes, which is favorable to realize dielectrophoretic manipulation of particles; the dependence on the applied voltage is found to be on the order of the applied voltage squared, and as the spacing and the width is reduced, the magnitude of the gradient increases exponentially. With the increment of the height of electrode, the electric field is extended into wider space, which is beneficial to the improvement of manipulation efficiency and throughput.