Yonghao Xiu

Mechanically robust superhydrophobicity on hierarchically structured Si surfaces

Improvement of the robustness of superhydrophobic surfaces is critical in order to achieve commercial applications of these surfaces in such diverse areas as self-cleaning, water repellency and corrosion resistance. In this study, the mechanical robustness of superhydrophobic surfaces was evaluated on hierarchically structured silicon surfaces. The effect of two-scale hierarchical structures on robustness was investigated using an abrasion test and the results compared to those of superhydrophobic surfaces fabricated from polymeric materials and from silicon that contains only nanostructures. Unlike the polymeric and nanostructure-only surfaces, the hierarchical structures retained superhydrophobic behavior after mechanical abrasion.

UV and thermally stable superhydrophobic coatings from sol-gel processing.

A method for the preparation of inorganic superhydrophobic silica coatings using sol-gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol-gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 degrees C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 degrees C and UV testing times of 5500 h. Contact angles remained >160 degrees with contact angle hysteresis approximately 2 degrees.

Superhydrophobic and low light reflectivity silicon surfaces fabricated by hierarchical etching.

Silicon is employed in a variety of electronic and optical devices such as integrated circuits, photovoltaics, sensors, and detectors. In this paper, Au-assisted etching of silicon has been used to prepare superhydrophobic surfaces that may add unique properties to such devices. Surfaces were characterized by contact angle and contact angle hysteresis. Superhydrophobic surfaces with reduced hysteresis were prepared by Au-assisted etching of pyramid-structured silicon surfaces to generate hierarchical surfaces. Consideration of the Laplace pressure on hydrophobized hierarchical surfaces gives insight into the manner by which contact is established at the liquid/composite surface interface. Light reflectivity from the etched surfaces was also investigated to assess application of these structures to photovoltaic devices.

Silicon surface structure-controlled oleophobicity.

Superoleophobic surfaces display contact angles >150 degrees with liquids that have lower surface energies than does water. The design of superoleophobic surfaces requires an understanding of the effect of the geometrical shape of etched silicon surfaces on the contact angle and hysteresis observed when different liquids are brought into contact with these surfaces. This study used liquid-based metal-assisted etching and various silane treatments to create superoleophobic surfaces on a Si(111) surface. Etch conditions such as the etch time and etch solution concentration played critical roles in establishing the oleophobicity of Si(111). When compared to Youngs contact angle, the apparent contact angle showed a transition from a Cassie to a Wenzel state for low-surface-energy liquids as different silane treatments were applied to the silicon surface. These results demonstrated the relationship between the re-entrant angle of etched surface structures and the contact angle transition between Cassie and Wenzel behavior on etched Si(111) surfaces.

Robust Superhydrophobic Surfaces Prepared With Epoxy Resin and Silica Nanoparticles

When nanoparticles are incorporated into surfaces to generate roughness, adhesion of the particles is critical to achieve a durable superhydrophobic surface. In this investigation, we explored the use of bis-phenol A based epoxy and silica nanoparticles to form a composite layer on substrates. After an plasma treatment of the surface layer, the epoxy was etched away and silica nanoparticles exposed on the surface, thereby generating roughness. The plasma etching time was examined to correlate the resulting surface morphology and water droplet contact angles after a fluoroalkyl silane treatment. Surface mechanical stability was studied by an abrasion test. Water vapor condensation on the surface was also assessed by investigation of the contact angle, which offers insight into the applicability of the surfaces to use under hot and humid conditions where degradation of the superhydrophobic surfaces may occur.

UV-Resistant and Superhydrophobic Self-Cleaning Surfaces Using Sol–Gel Processes

Inorganic superhydrophobic, low-surface energy silica coatings have been prepared using sol–gel processing with tetraethoxysilane and trifluoropropyltrimethoxysilane as precursors. When trifluoropropyltrimethoxysilane was incorporated into silica, hydrophobic trifluoropropyl groups were present on the surface, yielding hydrophobicity. When combined with the rough surfaces from a sol–gel process, superhydrophobic surfaces can be generated. The surfaces were characterized by contact angle measurements, scanning electron microscopy and atomic force microscopy. Thermogravimetric analysis was used to determine the thermal stability of the as prepared silica materials. In order to measure the surface hydrophobicity retention of the silica surfaces under UV irradiation, UV accelerated aging tests were conducted according to ASTM D 4329.

Epoxy/h-BN composites for thermally conductive underfill material

In order to enhance thermal conductivity of underfill materials, hexagonal boron nitride (h-BN) was employed as a thermally conductive filler. The relationship between the filler morphology and its effects on the thermal conductivity is focused in this study. Two different h-BN fillers with different morphologies were applied to generate an efficient thermal transport path through the filler and epoxy resin interfaces. The microstructure, dynamic mechanical properties and thermal conductivity for the h-BN filled underfills were studied and characterized. The h-BN greatly increased the thermal conductivity of the underfill and their moduli were well controlled by an addition of a low filler level which can reduce the thermal stress in flip-chip packaging application.

Optimizing geometrical design of superhydrophobic surfaces for prevention of microelectromechanical system (MEMS) stiction

Due to the surface smoothness of micromachined structures, strong adhesion forces between these fabricated structures and the substrate can be developed. The major adhesion mechanisms include capillary forces, hydrogen bonding, electrostatic forces and van der Waals forces. Once contact is made, the magnitude of these forces is in some cases sufficient to deform and pin these structures to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in MEMS. The basic approaches to prevent stiction are increasing surface roughness and/or lowering solid surface energy by coating with low surface energy materials. Combination of micro- and nano-meter scale roughness can dramatically increase the surface roughness. However, in fabrication process, how to optimally design surface geometry with micro-/nano-meter roughness is still not clear. The objectives of this paper are to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses, and to optimize the surface geometrical design for fabricating stable superhydrophobic surfaces. Two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films, to compare wetting on micro-patterned silicon surfaces with wetting on nano-scale roughness surfaces. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies and to improve the stability of the superhydrophobic surface, formed by plasma enhanced chemical vapor deposition (PECVD). The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on micro-scale rough surfaces. However, the presence of nano-scale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1deg and establishes stable and robust superhydrophobicity, although nano-scale roughness could not increase the apparent contact angle significantly if the micro-scale roughness dominates. The results of the research could guide the optimized designs of the surfaces for prevention of microelectromechanical (MEMS) stiction

A rapid growth of aligned carbon nanotube films and high-aspect-ratio arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we report an efficient process to grow well-aligned CNT films and high-aspect-ratio CNT arrays with very high area distribution density (>1600 µm−2). Chemical vapor deposition (CVD) was invoked to deposit highly aligned CNTs on Al2O3/Fe coated silicon substrates of several square centimeter area using ethylene as the carbon source, and argon and hydrogen as carrier gases. The nanotubes grew at a high rate of ∼100 µm/min. for nanotube films at 800°C, while the nanotube arrays grew at ∼140 µm/min. even at 750°C, due to the base growth mode. The CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs are of high purity and form densely aligned arrays with controllable size and height. The as-grown CNT structures have considerable potential for thermal management and electrical interconnects for microelectronic devices.

A Novel Method to Prepare Superhydrophobic, Self-Cleaning and Transparent Coatings for Biomedical Applications

In this paper, a novel method of preparing nanostructured ultra-thin films for transparent superhydrophobic coatings is reported for the first time by Y. Xiu et al. A solgel process that uses a eutectic liquid solvent was employed. This solvent serves as both a low vapor pressure, low melting point liquid and a templating agent in the solgel process. UV-visible spectra indicate that after coating a glass microscope slide, with the transparent superhydrophobic coating, the light transmittance improved relative to that of a bare glass slide. The superhydrophobic surface is biocompatible because of the extremely low surface energy that results from the combination of a fluoroalkyl chain attached to the surface and the surface nanostructure. Preliminary studies suggest that the superhydrophobic coating may effectively prevent surface contamination by bacteria.