Shu-Jen Chen

ZnO nanopencils: Efficient field emitters

ZnO nanopencils were synthesized on a silicon wafer without catalysts at a low temperature of 550 ° C through a simple two-step pressure controlled thermal evaporation. Penholders were well-hexagonal faceted and the diameter of pen tips on the nanopencils was in the range of 20–30 nm. High-resolution transmission electron microscopy shows that the nanopencils were single crystals growing along the [0001] direction and the pen tips subtend a small angle with multiple surface perturbations. Field-emission measurements on the nanopencils show a low turn-on field of 3.7V∕μm at a current density of 10μA∕cm2. The emission current density reached 1.3mA∕cm2 at an applied field of 4.6V∕μm. The emission at the low field is attributed to the sharp tip and surface perturbations on the nanopencils.

Single-crystalline AlZnO nanowires/nanotubes synthesized at low temperature

Single-crystalline AlZnO nanomaterials were synthesized through a proposed alloy-evaporation deposition method at the low temperature of 550°C by thermal chemical vapor deposition. Transmission electron microscopy images show that AlZnO nanowires, or nanowire/nanotube junction structures, can be synthesized where the Al∕(Al+Zn) atomic ratio is determined to be about 2.5 and 12at.%, respectively, by electron energy loss spectrometry. Room-temperature cathodoluminescence measurements show that the AlZnO nanowires exhibit a strong ultraviolet emission, which shifts to a higher energy from 3.29to3.34eV due to Al incorporation.

ZnO symmetric nanosheets integrated with nanowalls

Diverse ZnO integrated nanostructures, constructed by epitaxial nanowalls and symmetric single-crystalline nanosheets, were successfully synthesized via a strain-assisted self-catalyzed process at a low temperature of 500°C. The nanostructures started with the growth of ZnO nanowires, nucleated on a rugged ZnO single-crystalline film via a strain-assisted self-catalyzed growth mechanism. The nanowalls were then formed by the interconnection of the nanowires. Finally, the nanosheets were grown from the edges of the nanowalls. The growth mechanisms were supported by direct experimental evidence. Room-temperature cathodoluminance spectra show a relatively strong and sharp ultraviolet emission as well as a weak and broad green emission. The integrated nanostructure may be applied to develop self-inclusive nanoelectronics.

ZnO hexagonal arrays of nanowires grown on nanorods

ZnO single-crystalline nanowire-type nanostructures were synthesized on silicon by thermal chemical vapor deposition without catalysts through a two-step pressure-controlled vapor-reflected process at a low temperature of 550 °C where self-organized hexagonal crystalline or porous nanowire arrays were grown on nanorods. The nanowire diameter is around 20 nm and number of nanowires is selected by the nanorod size. Cathodoluminescence spectra exhibit strong green emissions, indicative of high oxygen-vacancy density, which sheds a light on further applications for multichannel nanoconductors in nanodevices.

Growth and field-emission properties of single-crystalline conic ZnO nanotubes

Single-crystalline conic ZnO nanotubes were synthesized on Si(001) without catalysts by thermal chemical vapour deposition at 475 °C. The nanotubes grown along the ZnO [0001] direction are sharp open-ended tips consisting of planar defects revealed by transmission electron microscopy and scanning electron microscopy. The nanotubes were formed by self-assembly of numerous six-radiated branches of hexagram nanosheets. The diameters and the wall thicknesses of the nanotubes are in the ranges 30–100 and 15–30 nm, respectively. The lengths and areal densities of the nanotubes are around 0.5 µm and 1–9 × 108 cm−2, respectively. Field-emission measurements on the conic nanotubes show a low turn-on field of 3.5 V µm−1 at 10 µA cm−2. The field-emission properties related to the nanotube density and geometry and the nonlinearity in the Fowler–Nordheim (FN) plot at lower field region are discussed.

The evolution of well-aligned amorphous carbon nanotubes and porous ZnO/C core–shell nanorod arrays for photosensor applications

Well-aligned amorphous carbon nanotube (a-CNT) and porous ZnO/C core-shell nanorod (NR) arrays were fabricated for the first time by a proposed deposition-etching-evaporation (DEE) route. The arrays were prepared by deposition of carbon on the surface of well-aligned ZnO NR arrays by thermal decomposition of acetone followed by spontaneous etching and evaporation of core-ZnO. By utilizing the decomposition of acetone as well as distinct degrees of interaction between intermediate products and ZnO, well-aligned nonporous ZnO/C core-shell NR, porous ZnO/C core-shell NR, and a-CNT arrays were separately prepared by varying the working temperature from 400 to 700 °C. Scanning electron microscopy and high-resolution transmission electron microscopy show that the thickness of carbon shells increases from 3 to 10 nm with the increase in working temperature. Raman spectra demonstrate slight sp(2) bonds of carbon, indicating small graphite regions embedded in amorphous carbon nanoshells. The E(2) peaks of ZnO reduce with the increase in substrate temperature. Photoresponse measurements of ZnO/C NR arrays shows enhancement of both photoresponsivity and response velocity, and the interference of humidity with regard to photosensing is effectively reduced by the capping of carbon nanoshells. The work not only provides an effective route to improve the photosensing of semiconductor nanomaterials for practical applications, but also sheds light on preparing various hollow carbon and porous ZnO/C core-shell nanostructures with distinct morphologies by employing the routes presented in the paper on diverse ZnO nanostructures for optoelectrochemical applications.