Chia-Fu Chen

Fabrication and CO Sensing Properties of Mesostructured ZnO Gas Sensors

This study presents the synthesis of mesostructured ZnO using a template replication method and the fabrication of a carbon monoxide sensor using the synthesized ZnO using the dielectrophoresis process. The mesoporous carbon, CMK-3, was employed for the template and zinc nitrite was used as the precursor for synthesizing the ordered porous ZnO. The mesostructured ZnO was analyzed using X-ray diffraction XRD patterns, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and N2 adsorption–desorption isotherms. The XRD patterns indicated that the ZnO exhibited a highly ordered structure after the template was removed. The morphology of the ZnO was randomly oriented and the structure was polycrystalline. The surface area, pore volume, and pore size of the porous ZnO were 61.3 m2 g−1, 0.31 cm3 g−1, and 5.3 nm, respectively. When CO gas was injected, the optimum sensitivities at 250°C were 12.1, 14.6, 18.4, 26.0, and 60% when the CO concentrations were 10, 20, 30, 50, and 70 ppm, respectively.

Label free sub-picomole level DNA detection with Ag nanoparticle decorated Au nanotip arrays as surface enhanced Raman spectroscopy platform.

Label free optical sensing of adenine and thymine oligonucleotides has been achieved at the sub-picomole level using self assembled silver nanoparticles (AgNPs) decorated gold nanotip (AuNT) arrays. The platform consisting of the AuNTs not only aids in efficient bio-immobilization, but also packs AgNPs in a three dimensional high surface area workspace, assisting in surface enhanced Raman scattering (SERS). The use of sub-10 nm AgNPs with optimum inter-particle distance ensures amplification of the chemically specific Raman signals of the adsorbed adenine, thymine, cytosine and guanine molecules in SERS experiments. High temporal stability of the Raman signals ensured reliable and repeatable DNA detection even after three weeks of ambient desk-top conservation. This facile architecture, being three dimensional and non-lithographic, differs from conventional SERS platforms.

Morphology control of silicon nanotips fabricated by electron cyclotron resonance plasma etching

Formation of well-aligned silicon nanotips etched monolithically from a silicon substrate has been demonstrated. The effect of the process temperature on the physicochemical etching of silicon and subsequent fabrication of these nanotips has been investigated. 2.2-μm-long nanotips were formed at the process temperature of 250°C and then decreased in length with increasing process temperature. Above 800°C, the formation of the silicon nanotips was inhibited. Spectroscopic evidence attributes this fact to the efficient formation of silicon carbide thin film at higher process temperatures, instead of discontinuous nanomasks at lower process temperatures that prevent etching of the substrate. Another reason for this inhibited formation of nanotips is the reduced etching rate of the silicon by agents such as atomic H at higher process temperatures.

Fabrication of Mesostructured Cobalt Oxide Sensor and Its Application for CO Detector

The paper presents a method for mesostructured cobalt oxide sensor fabrication by the template replication method and immobilization using dielectrophoresis process. The method consists of enriching the mesostructured cobalt oxide particles in a microelectrode gap that drove mesostructured cobalt oxide particles to a high electric field region and then exposed to CO gas. The cobalt oxide sensor has improved sensing performance in carbon monoxide (CO) detection at concentration as low as 10 ppm and a very short recovering time, due to the higher surface area.

Geometrically tuned and chemically switched wetting properties of silicon nanotips

The wetting properties of silicon nanotips (SiNTs) are discussed. SiNTs were prepared by single step dry etching of silicon wafers in an electron cyclotron resonance plasma of silane, methane, argon and hydrogen and water contact angles were measured as a function of their aspect ratio (α) and the inter-tip distance. The hydrophilic nature of the SiNTs is tunable with α and the inter-tip distance. Super-hydrophilicity with water contact angles close to 2° was observed with α>12 (length ∼1500xa0nm). Upon coating a 1500xa0nm long SiNT with TiO(2), the water contact angle jumped from 2° to ∼140°, demonstrating a switchover from super-hydrophilic to hydrophobic surface properties.

Cation-Mediated Effects on Zinc Oxide Films Formed by Chemical Bath Deposition

Crystalline ZnO films with a densely packed morphology were grown on a silicon oxide (SiOx) glass substrate by chemical bath deposition (CBD) in an aqueous-solution bath containing magnesium nitrate [Mg(NO3)22H2O] used as a cation-mediation compound, zinc nitrate [Zn(NO3)26H2O], and dimethylamineborane (DMAB) at 65 °C. Grains of ZnO films grown in nonmediated solution preferentially grew on the (002) plane. In the cation-mediated-solution bath, the films were compressed into a compact morphology by the increased tendency for films to grow in the (002) prefer orientation. The crystalline ZnO film exhibits good optical transmittance of over 90%. Additionally, the direct band-gap value was 3.22 eV, which is less than 3.38 eV, which was obtained using a nonmediated-solution bath. This was due to the lattice constant variation caused by cation mediation.

Electron field emission from well-aligned GaP nanotips

Field emission of electrons from single crystal gallium phosphide (GaP) nanotips has been investigated. GaP nanotip arrays were fabricated using silane-methane-argon-hydrogen based plasma using the self-masking dry etching technique in an electron-cyclotron-resonance microwave plasma enhanced chemical vapor deposition system. These nanotips have an average of 2 and 80 nm in apex and bottom diameters, respectively. They are 900 nm in height, which makes them the perfect electron emission source for their high aspect ratio topography. A nanosized silicon carbide (SiC) cap on each GaP nanotip in the array has been found. The SiC core has a heterointerface with GaP crystal that was observed using a high resolution transmission electron microscope. Field emission analysis shows low turn-on fields of 8.5–9u2002V/μm. Cold electron emissions in Fowler–Nordheim type current-voltage were observed from such GaP nanotip arrays.