Di Gao

ANTI-ICING SUPERHYDROPHOBIC COATINGS

We use nanoparticle-polymer composites to demonstrate the anti-icing capability of superhydrophobic surfaces and report direct experimental evidence that such surfaces are able to prevent ice formation upon impact of supercooled water both in laboratory conditions and in natural environments. We find that the anti-icing capability of these composites depends not only on their superhydrophobicity but also on the size of the particles exposed on the surface. The critical particle sizes that determine the superhydrophobicity and the anti-icing property are in two different length scales. The effect of particle size on ice formation is explained by using a classical heterogeneous nucleation theory. This result implies that the anti-icing property of a surface is not directly correlated with the superhydrophobicity, and thus, it is uncertain whether a superhydrophobic surface is anti-icing without detailed knowledge of the surface morphology. The result also opens up possibilities for rational design of anti-icing superhydrophobic surfaces by tuning surface textures in multiple length scales.

Super Water- and Oil-Repellent Surfaces on Intrinsically Hydrophilic and Oleophilic Porous Silicon Films

We demonstrate that porous Si films fabricated by a convenient gold-assisted electroless etching process, which possess a hierarchical porous structure consisting of micrometer-sized asperities superimposed onto a network of nanometer-sized pores, are able to induce a superhydrophobic phenomenon on an intrinsically hydrophilic hydrogen-terminated Si surface and a superoleophobic phenomenon on an intrinsically oleophilic self-assembled monolayer-coated Si surface. Through comparison with porous Si films consisting of vertically aligned straight pores, which are hydrophilic and oleophilic, we show that an overhang structure resulting from the hierarchical porous structure is essential to preventing water and oil from penetrating the texture of the films and inducing the observed macroscopic superhydrophobic and superoleophobic phenomena.

Multilayer Assembly of Nanowire Arrays for Dye-Sensitized Solar Cells

Vertically ordered nanostructures synthesized directly on transparent conducting oxide have shown great promise for overcoming the limitations of current dye-sensitized solar cells (DSCs) based on random networks of nanoparticles. However, the synthesis of such structures with a high internal surface area has been challenging. Here we demonstrate a convenient approach that involves alternate cycles of nanowire growth and self-assembled monolayer coating processes for synthesizing multilayer assemblies of ZnO nanowire arrays and using the assemblies for fabrication of DSCs. The assembled multilayer ZnO nanowire arrays possess an internal surface area that is more than 5 times larger than what one can possibly obtain with single-layer nanowire arrays. DSCs fabricated using such multilayer arrays yield a power conversion efficiency of 7%, which is comparable to that of TiO(2) nanoparticle-based DSCs. The ordered structure with a high internal surface area opens up opportunities for further improvement of DSCs.

High-Efficiency Solid-State Dye-Sensitized Solar Cells Based on TiO2-Coated ZnO Nanowire Arrays

Replacing the liquid electrolytes in dye-sensitized solar cells (DSCs) with solid-state hole-transporting materials (HTMs) may solve the packaging challenge and improve the long-term stability of DSCs. The efficiencies of such solid-state DSCs (ss-DSCs), however, have been far below the efficiencies of their counterparts that use liquid electrolytes, primarily due to the challenges in filling HTMs into thick enough sensitized films based on sintered TiO(2) nanoparticles. Here we report fabrication of high-efficiency ss-DSCs using multilayer TiO(2)-coated ZnO nanowire arrays as the photoanodes. The straight channel between the vertically aligned nanostructures combined with a newly developed multistep HTM filling process allows us to effectively fill sensitized films as thick as 50 μm with the HTMs. The resulting ss-DSCs yield an average power conversion efficiency of 5.65%.

A low-temperature CVD process for silicon carbide MEMS

A low-temperature, single precursor CVD process for the realization of SiC-based MEMS and SiC-coated MEMS is described using 1,3-disilabutane. With this deposition method, the fabrication of an all-SiC cantilever beam array is demonstrated using standard microfabrication processes. Also, SiC coating of released Si micromechanical structures is realized using this process. The SiC-coated microstructures are shown to have superior chemical stability when compared to their Si analogs, as well as exhibit highly favorable mechanical properties.

Mechanical elasticity of single and double clamped silicon nanobeams fabricated by the vapor-liquid-solid method

Atomic force microscopy has been used to characterize the mechanical elasticity of Si nanowires synthesized by the vapor-liquid-solid method. The nanowires are horizontally grown between the two facing Si(111) sidewalls of microtrenches prefabricated on a Si(110) substrate, resulting in suspended single and double clamped nanowire-in-trench structures. The deflection of the nanowires is induced and measured by the controlled application of normal forces with the microscope tip. The observed reversibility of the nanowire deflections and the agreement between the measured deflection profiles and the theoretical behavior of single and double clamped elastic beams demonstrate the overall beamlike mechanical behavior and the mechanical rigidity of the clamping ends of the nanowire-in-trench structures. These results demonstrate the potential of the nanowire-in-trench fabrication approach for the integration of VLS grown nanostructures into functional nanomechanical devices.

Fully-differential poly-SiC Lame mode resonator and checkerboard filter

In this paper, we report the first fully-differential, electrostatically transduced RF MEMS resonator. The fully-differential electrode configuration not only cancels capacitive feedthrough between the drive and sense terminals but also eliminates the reduction in electromechanical quality factor (Q) from the large ohmic resistance of the polycrystalline silicon carbide (poly-SiC) suspension and anchor. Using this configuration, we have demonstrated a 173 MHz poly-SiC Lame-mode resonator with a Q of 9,300 in air. By mechanically coupling five Lame-mode resonators into a 2-D checkerboard, we have realized a 173 MHz center frequency band-pass filter with 110 kHz bandwidth and < 2 dB pass-band ripple.

Recent progress toward a manufacturable polycrystalline SiC surface micromachining technology

In this paper, we present results of recent research from our laboratory directed toward a manufacturable SiC surface micromachining technology for microelectromechanical systems (MEMS) applications. These include the development of a low-pressure chemical vapor deposition and in situ doping processes for silicon carbide (SiC) films at relatively low temperatures, as well as the development of selective dry etching processes for SiC using nonmetallic masking materials. Doped polycrystalline SiC films are deposited at 800/spl deg/C by using a precursor 1,3-disilabutane and dopant gas NH/sub 3/, with the minimum resistivity of 26 m/spl Omega//spl middot/cm. Dry etching for SiC and its selectivity toward silicon dioxide and silicon nitride masking materials are investigated using SF/sub 6//O/sub 2/, HBr, and HBr/Cl/sub 2/ transformer coupled plasmas. The etch rate, etch selectivity, and etch profile are characterized and compared for each etch chemistry. By combining the LPCVD and dry etching process with conventional microfabrication technologies, a multiuser SiC MEMS process is developed.

Novel Dimeric Cholesteryl Derivatives and Their Smart Thixotropic Gels

Three novel LS(2)-type dimeric-cholesteryl derivatives (1-3), where S is a steroidal residue and L stands for a linker connecting the two S residues and contains three benzene rings and two amide and two carbamate groups, were designed and prepared. The compounds can gel a wide variety of organic solvents via three different ways, including mixing at room temperature, a heating-cooling cycle, and ultrasound treatment. SEM measurements revealed that the structures and the concentrations of the gelators, the nature of the solvent, and the preparation method employed have a great effect on the morphologies of the gel networks. It was revealed that 1 is a supergelator for DMSO (cgc = 0.04% w/v) and that the 1/DMSO gel can be prepared via any of the three methods mentioned above. Furthermore, the gel possesses excellent mechanical strength and a very smart thixotropic property. FT-IR and temperature- and concentration-dependent (1)H NMR spectroscopy studies revealed that hydrogen bonding and π-π stacking among the molecules of 1 are two important driving forces for the physical gelation of DMSO. In addition, XRD analysis confirmed the layered packing structure of 1 in its DMSO gel.

Transparent superhydrophobic and highly oleophobic coatings

We report a facile process for fabrication of transparent superhydrophobic and highly oleophobic surfaces through assembly of silica nanoparticles and sacrificial polystyrene nanoparticles. The silica and polystyrene nanoparticles are first deposited by a layer-by-layer assembly technique. The polystyrene nanoparticles are then removed by calcination, which leaves a porous network of silica nanoparticles. The cavities created by the sacrificial polystyrene particles form overhang structures on the surfaces. Modified with a fluorocarbon molecule, such surfaces are superhydrophobic and transparent. They also repel liquids with low surface tensions, such as hexadecane, due to the overhang structures that prevent liquids from getting into the air pockets even though the intrinsic contact angles of these liquids are less than 90 degrees.