Shao-Liang Cheng

Superhydrophilicity to superhydrophobicity transition of CuO nanowire films

The surface of CuO is known for its hydrophilicity and exhibits superhydrophilic nature as nanowires are present. When exposed in the air at room temperature or treated by low temperature annealing, however, transition from superhydrophilicity to superhydrophobicity of the CuO nanowire films are observed. Since the chemical structure of the films after treatment remains the same as CuO according to x-ray photoelectron spectroscopy spectra, the superhydrophobicity may be attributed to partial deoxidation of the upmost layer of CuO surfaces into Cu2O-like hydrophobic surfaces. Nonetheless, superhydrophilicity is recovered if the superhydrophobic CuO film is subject to high temperature annealing.

Tiny bubble removal by gas flow through porous superhydrophobic surfaces: Ostwald ripening

The fact that tiny bubbles are easily formed on the rough, hydrophobic surface results in difficulties in bubble detachment and removal. We show that bubbles captured by porous superhydrophobic surfaces merge into larger ones, which can detach by buoyancy. The responsible mechanism is convective Ostwald ripening because networklike pores in the superhydrophobic film remain nonwetted and provide passage for gas flow between adhered bubbles. A large bubble grows spontaneously by absorbing all small adhered bubbles due to capillary pressure differences. Our results demonstrate that porous hydrophobic film can be an efficient, passive way of bubble removal in microfluidic systems.

Reduction-assisted sintering of micron-sized copper powders at low temperature by ethanol vapor

The low temperature sintering of micron-sized Cu powders is achieved by ethanol vapor annealing. A Cu-pancake is formed and has enough mechanical strength to sustain the gravitational pull. The electrical resistivity of the Cu-pancake formed by flaky powders is lower than that by spheroidal ones because more contact area of the former facilitates the sintering process. The resistivity of the Cu-pancake grows with decreasing the annealing temperature but it is still about 10−3 Ω cm at 120 °C. The sintered Cu-pancake is characterized by X-ray diffraction and X-ray photoelectron spectroscopy to investigate the sintering mechanism. The low temperature sintering is attributed to the reduction of the native oxide on surfaces of Cu powders by the ethanol vapor. The reduced Cu is very active and tends to sinter with each other to lower the surface energy. This reduction-assisted sintering may be useful in the fabrication of conductive patterns on flexible substrates. The prepared Cu pattern on polyethylene naphthalate exhibits repeatable flexibility and acceptable conductivity.

Conversion from self-assembled block copolymer nanodomains to carbon nanostructures with well-defined morphology

We developed a method to fabricate arrays of nanostructured carbon materials of high quality, via direct pyrolysis of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP) as thin films without small organic molecules incorporated as carbon resources. Prior to pyrolysis, the nanodomains were cross-linked with UV radiation under nitrogen (UVIN). As upon pyrolysis thermal energy imparts mobility to nanodomains to overcome the constraints imposed by cross-linked chains, the free surface is inevitably covered with the PS block that has a smaller surface energy; unless favorable interactions with the P2VP block exist at the free surface, the outer layer of carbonaceous films is predominantly composed of the pyrolyzed PS block. To overcome this problem, films were capped with a SiO2 layer after UVIN. The capping layer appears to have two advantages – increased areal yields and an improved morphological fidelity. As a result, arrays of nanostructured carbon of great quality were fabricated even at temperatures far above the decomposition temperatures of PS and P2VP blocks.

Growth kinetics and wettability conversion of vertically-aligned ZnO nanowires synthesized by a hydrothermal method

We report here the first study of the growth kinetics of vertically-aligned ZnO nanowire arrays grown on Al-doped ZnO (AZO) seed layer-coated substrates by a hydrothermal method. The as-synthesized vertical ZnO nanowires possess a single-crystalline wurtzite structure and a preferred growth orientation along the [0001] direction. The ZnO nanowires were found to grow following the reaction-controlled process and their lengths could be tuned from several to over ten micrometers by adjusting the hydrothermal temperature and time. By measuring the growth rates at different synthesis temperatures, the activation energy for the linear growth of vertical ZnO nanowires on AZO-seeded substrates derived from the slope of the Arrhenius plot was around 35 kJ mol−1. The obtained value is smaller than that of ZnO nanowires grown in bulk solution, which can be explained by the different nucleation mechanisms. From water contact angle measurements, it is found that the as-synthesized ZnO nanowires are hydrophilic in nature, and their surface wettability can be adjusted by the storage time and heat treatment conditions. Furthermore, the reversible switching of the surface wettability of ZnO nanowires has been accomplished by alternate annealing in vacuum and oxygen atmospheres. The ZnO nanowires with switchable surface wettability will find promising applications in surface engineering.

Tailoring Carbon Nanostructure with Diverse and Tunable Morphology by the Pyrolysis of Self-Assembled Lamellar Nanodomains of a Block Copolymer

The pyrolysis of a block copolymer thin film, the free surface of which was in contact with air or a capping layer of SiO2, produced four carbon nanostructures. Thin films of a diblock copolymer having perpendicularly oriented lamellar nanodomains served as carbon and nitrogen precursors. Before pyrolysis, the lamellar nanodomains were cross-linked with UV irradiation under nitrogen gas (UVIN). Without a capping layer, pyrolysis caused a structural transformation from lamellar nanodomains to short carbon nanowires or to dropletlike nanocarbons in a row via Rayleigh instability, depending on the duration of pyrolysis. When capped with a layer of SiO2 followed by pyrolysis, the lamellar nanodomains were converted to pod-like, spaghetti-like, or long worm-like carbon nanostructures. These carbon nanostructures were driven by controlling the surface or interface tension and the residual yield of solid carbonaceous species.

Fabrication of periodic arrays of needle-like Si nanowires on (001)Si and their enhanced field emission characteristics

Well-ordered arrays of needle-like and cone-like Si nanostructures have been successfully produced on (001)Si substrates. The morphology and dimensions of the tapered Si nanostructure arrays can be readily controlled by adjusting the Ag-nanoparticle catalytic etching/removal (ACER) cycles. All the produced vertical tapered Si nanostructures were single crystalline and their axial crystallographic orientation was the same as that of the (001)Si substrate. The produced long needle-like Si nanowires exhibited superior field emission properties compared to as-produced and short cone-like Si nanowires. The enhanced field emission performance of the long needle-like Si nanowire array can be attributed to the good vertical alignment, sharp tips, high aspect ratio, single-crystalline structure, and well-ordered arrangement. The experimental results present the exciting prospect that with appropriate control, the combined approach proposed here promises to be applicable for fabricating other highly-ordered, vertically-aligned tapered Si-based nanowire arrays and may offer potential applications in constructing various high efficiency Si-based field emission nanodevices.