Rong-Ming Ko

The influence of oxygen content in the sputtering gas on the self-synthesis of tungsten oxide nanowires on sputter-deposited tungsten films

The self-synthesis of tungsten oxide (W18O49) nanowires on sputter-deposited W films prepared under different O2/Ar flow rate ratios (OAFRRs) in the sputtering gas is reported. After thermally annealing at 700?850??C in N2 ambient for 15?min, dense and well crystalline W18O49 (010) nanowires or nanobelts were obtained depending on the oxygen content in the sputtering gas. Experimental results show that the annealing temperature required for the full growth of W18O49 nanowires reduced when the OAFRR in the sputtering gas was increased. It is found that the oxygen absorbed in the surface region is responsible for the growth of nanowires. As the OAFRR was increased to (8?sccm)/(24?sccm), which resulted in a saturated oxygen content of about 55 at.% inside the W film, large-scale nanobelts or nanosheets of W18O49 were grown. The possible growth mechanism which governs the evolution from nanowires to nanobelts as the OAFRR was changed is also discussed.

Preparation of p-SnO/n-ZnO Heterojunction Nanowire Arrays and Their Optoelectronic Characteristics under UV Illumination

In this work, the use of a ZnO-nanowire (NW)-based heterojunction array structure for application to UV sensors is proposed. Nano-heterojunction arrays (NHAs) were formed via the oblique-angle sputtering deposition of p-type tin monoxide onto vertically aligned ZnO-NWs grown by hydrothermal growth (HTG). The current density–voltage (J–V) curve in darkness shows that the prepared SnO/ZnO-NW NHAs have rectifying current–voltage characteristics. They also exhibit a superior response to UV (254 nm) light illumination. The optoelectronic properties of the SnO/ZnO-NW NHAs with different SnO thicknesses (50–1000 nm) under different UV light intensities (2–6 mW/cm2) were investigated and discussed. UV sensitivity (IUV/Idark) as high as 8.5 was obtained.

Preparation of tungsten oxide nanowires from sputter-deposited WCx films using an annealing/oxidation process

The self-synthesis of tungsten oxide (W18O49) nanowires on sputter-deposited WCx films using a simple annealing/oxidization process was reported. It was found that thermal annealing of WCx films at 680°C for 30min in nitrogen followed by oxidation at 450°C for 30min in pure oxygen would yield dense and well-crystallized monoclinic W18O49 (010) nanowires with a typical length/diameter of about 0.15–0.2μm∕10–20nm. The formation of W18O49 nanowires is attributed to the nuclei of immature W2C nanowires experiencing a regrowth process, accompanied by carbon depletion and the oxidization of tungsten during the subsequent oxidization process.

The evolution of tungsten oxide nanostructures from nanowires to nanosheets

The self-synthesis of tungsten oxide (W18O49) nanowires/nanosheets on sputtering-deposited tungsten films was obtained by thermal annealing in nitrogen under the thermal budgets of 750 °C for ≥2 h or ≥850 °C for 0.5 h. Experimental evidence of the nanomorphology transformation of tungsten oxide nanostructures from nanowires to nanosheets was presented, which can be attributed to the formation and re-crystallization of an amorphous interface layer between two neighboring parallel-growth nanowires linked together with a low angle misalignment along their growth directions.

Development of Gas Sensors Based on Tungsten Oxide Nanowires in Metal/SiO2/Metal Structure and Their Sensing Responses to NO2

In this work, the fabrication of gas sensors utilizing self-synthesized tungsten oxide nanowires (TONWs) and their response to NO2 are reported. The gas sensor is based on a sputter-deposited WCx/SiO2/WCx triple-layer structure with the periphery of the SiO2 layer etched chemically. Self-synthesized TONWs with crystalline W18O49(010) were grown by simple thermal annealing in nitrogen ambient, which linked, in parallel, the upper and lower WCx electrodes for gas sensing. The TONW-based sensors increased in resistance in NO2 because the surface of TONWs comprised oxygen adsorbates and the adjacent space charge region was electron-depleted. The amount of enlargement in resistance increased with increasing temperature. To improve the detectability of the parallel-connected TONW-based sensor, a connection of several individual sensors in series was proposed to enlarge the number of TONWs for gas sensing. For the 8-series-connected sensor, a sensitivity as high as 9.3, a response time as low as about 9 s, and a detectability as low as 2 ppm for NO2 were obtained.

Improvement of field emission characteristics of tungsten oxide nanowires by hydrogen plasma treatment

The use of hydrogen plasma (H-plasma) treatment to improve field emission (FE) characteristics of self-synthesized tungsten oxide nanowires (TONWs) is reported. With a H-plasma treatment under a working power of 200 W and a pressure of 500 mtorr for 20 s, improved FE characteristics with a turn-on field (4.7 V/μm at 10 μA/cm2) lower than those of the as-grown case by 23% and a reduction in the effective emission barrier of 0.72 eV were obtained, which is attributed to the reduction in oxygen adsorption, decrease in the wire length and density, and transition of TONWs surfaces from well crystalline into the amorphous phase.

From metastable to stable: possible mechanisms for the evolution of W18O49 nanostructures

The evolution of nanoscale W18O49 from nanowires into nanosheets is investigated. FESEM and HRTEM images of self-synthesized W18O49 nanostructures obtained from sputter-deposited tungsten films after thermal annealing at various temperatures (550–900 °C) and times (0.5–3 h) are presented and discussed. W18O49 nanostructures obtained from annealed samples with sufficient thermal budgets exhibit Moire fringes with various periodicities along the growth direction. Experimental results confirm that the evolution of nanostructures from nanowires into nanosheets can be attributed to two mechanisms. One is the formation of an amorphous buffer layer on the (02) planes between two neighboring nanowires that facilitates twisting and/or rotation of the two wires and re-crystallization of the buffer layer. The other is the assembly of two neighboring nanowires into a quasi-single crystal by coherent their (010) planes, followed by the rolling of subgrain boundaries to form single-crystalline nanosheets.

Preparation and optoelectronic properties of NiO/ZnO heterostructure nanowires

This study proposes the use of a ZnO-nanowire (ZnO-NW)-based heterojunction structure for applications of nano optoelectronic sensors and photovoltaic devices. Nano heterojunctions (NHJs) were formed via e-beam deposition of ptype nickel oxide (NiO) onto the vertical-aligned ZnO-NWs grown by hydro-thermal growth method. The dark J-V curve shows that the prepared NiO/ZnO-NWs NHJ has a diode-like behavior with a forward threshold voltage (Vth) of 1.2 V and a leakage current (Jr at -1V) of 0.02 μA/cm2, respectively. It also exhibits a superior response to UV (366 nm) and AM 1.5G light illuminations. The Vth and the photocurrents (i.e., Jr at -1V) under UV (366 nm @ 6 mW/cm2) and AM 1.5G light were 0.7 V/0.06 μA/cm2 and 0.5 V/ 3.2 μA/cm2, respectively, revealing an increase in the diode current of about 3× and 160×, respectively.

Low voltage operable field emission triodes with high transconductance based on laterally grown ZnO nanowires

laterally grown ZnO nanowires Tseng Hsing Lin, Fu-Shou Tsai, Yung-Chun Tu, Rong-Ming Ko, and Shui-Jinn Wang* 1 Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan. 2 Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan. *Phone: +886-6-2757575-62351, Fax: +886-6-2763882, E-mail: sjwang@mail.ncku.edu.tw

Tip engineering of hydrothermally grown ZnO nanorods and its application in low-voltage operable field emitters

Recently, one-dimensional (1D) n-type ZnO-nanorods and nanowires have attracted considerable interest due to their unique large excition binding energy of 60 meV and wide bandgap energy of 3.37 eV at room temperature[1]. In addition, the excellent properties of ZnO material such as thermal stability, mechanical strength, negative electron affinity, and superior oxidation resistance[2], which make much potential application in electron field emission. In our previous work, we used a simple two electrode pattern with a bi-layer of platinum (Pt) and an aluminum-doped zinc-oxide (AZO) layer for the lateral growth of ZnO nanorods by hydrothermal (HT) growth method for the fabrication of two-terminal field emission (FE) devices with an emitter-to-emitter structure[3]. Though encouraging experimental results on FE performance were obtained, there is much room for improvement of the FE properties of the ZnO nanorods. In this study, a two-step HT method is proposed to sharpen the tip portion of the grown ZnO nanorods. The influence of HT growth parameters on the shape of the tip of ZnO nanorods are examined and discussed. Applications of ZnO nanorods with the proposed tip engineering in FE emitter with an emitter-to-emitter structure are demonstrated. Comparison of the FE characteristics between emitters with and without tip engineering are also presented and discussed.