Ting-Jen Hsueh

Highly sensitive ZnO nanowire ethanol sensor with Pd adsorption

The authors report the growth of high-density single crystalline ZnO nanowires on patterned ZnO:Ga∕SiO2∕Si templates, Pd adsorption on nanowire surfaces, and the fabrication of ZnO nanowire-based ethanol gas sensors. With Pd adsorption, it was found that measured sensitivities of the ethanol gas sensors increased from 18.5% to 44.5% at 170°C and increased from 36.0% to 61.5% at 230°C.

Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes

The investigation explores the fabrication and characteristics of ZnO nanowire (NW)/p-GaN/ZnO NW heterojunction light-emitting diodes (LEDs). Vertically aligned ZnO NWs arrays were grown on the p-GaN substrate. The n-p-n heterojunction LED was fabricated by combining indium tin oxide/glass substrate with the prepared ZnO NWs/p-GaN substrate. The symmetrical rectifying behavior demonstrates that the heterostructure herein was formed with two p-n junction diodes and connected back to back. The room-temperature electroluminescent emission peak at 415 nm was attributed to the band offset at the interface between n-ZnO and p-GaN and defect-related emission from ZnO and GaN. Finally, the photograph indicated the LED clearly emitted blue light.

Highly Sensitive ZnO Nanowire Acetone Vapor Sensor With Au Adsorption

In this study, the growth of high-density single-crystalline ZnO nanowires on patterned ZnO:Ga/ SiO2/Si templates was reported. We also adsorbed Au onto nanowire surfaces and fabricated ZnO nanowire acetone vapor sensors. With 200-ppm acetone vapor concentration, it was found that we could enhance the device sensitivities at 300deg C from 18.5% to 82.5% by Au adsorption. It was also found that measured responses at 300degC were around 52%, 61%, 71%, 77%, and 82% when the accumulative acetone vapor concentration reached 5, 10, 50, 100, and 200 ppm, respectively, for the ZnO nanowire sensor with Au adsorption.

A ZnO-Nanowire Phototransistor Prepared on Glass Substrates

The fabrication of a phototransistor via the bridging of two prefabricated electrodes with a laterally grown ZnO nanowire is reported. It was found that the fabricated device is an n-channel enhancement-mode phototransistor with a dark carrier concentration of 6.34 × 10(17) cm(-3) when the gate voltage is biased at 5 V. With an incident-light wavelength of 360 nm and a zero gate bias, it was found that the noise equivalent power and normalized detectivity (D*) of the fabricated ZnO phototransistor were 6.67 × 10(-17) W and 1.27 × 10(13) cm Hz(0.5) W(-1), respectively. It was also found that the current in the device can be modulated efficiently by tuning the wavelength of the excitation source.

ZnO Nanowire-Based Oxygen Gas Sensor

We report growth of vertically well-aligned ZnO nanowires on ZnO:Ga/glass templates and the fabrication of resistive ZnO nanowire-based oxygen gas sensor. It was found that the ZnO nanowires are grown preferred oriented in the (002) direction with a small X-ray diffraction full-width-half-maximum. From high resolution transmission electron microscopy, scanning electron microscopy and micro-Raman measurements, it was found that the ZnO nanowires prepared in this study are single crystalline with good crystal quality. It was also found that measured sample resistance increased logarithmically as the oxygen gas pressure in the chamber was increased. Such a relationship suggests that the device is potentially useful for resistive oxygen gas sensing at room temperature.

Water- and Humidity-Enhanced UV Detector by Using p-Type La-Doped ZnO Nanowires on Flexible Polyimide Substrate

High-density La-doped ZnO nanowires (NWs) were grown hydrothermally on flexible polyimide substrate. The length and diameter of the NWs were around 860 nm and 80-160 nm, respectively. All XRD peaks of the La-doped sample shift to a larger angle. The strong PL peak of the La-doped sample is 380 nm, which is close to the 3.3 eV ZnO bandgap. That PL dominated indicates that the La-doped sample has a great amount of oxygen vacancies. The lattice constants ~0.514 nm of the ZnO:La NW were smaller when measured by HR-TEM. The EDX spectrum determined that the La-doped sample contains approximately 1.27 at % La. The La-doped sample was found to be p-type by Hall Effect measurement. The dark current of the p-ZnO:La NWs decreased with increased relative humidity (RH), while the photocurrent of the p-ZnO:La nanowires increased with increased RH. The higher RH environment was improved that UV response performance. Based on the highest 98% RH, the photocurrent/dark current ratio was around 47.73. The UV response of water drops on the p-ZnO:La NWs was around 2 orders compared to 40% RH. In a water environment, the photocurrent/dark current ratio of p-ZnO:La NWs was 212.1, which is the maximum UV response.

ZnO Nanotube Ethanol Gas Sensors

This investigation reports on the growth of ZnO nanotubes on patterned Au/Al 2 O 3 /Au/RuO 2 templates by reactive evaporation and the fabrication of a ZnO nanotube ethanol gas sensor. The as-grown ZnO nanotubes were polycrystalline and had holes at their terminals and sidewalls. Injection of ethanol gas reduced the resistivity of the fabricated sensor. Introducing 100 ppm ethanol gas increased the measured sensitivities of the ethanol gas sensors from 34 to 87% as the temperature increased from 90 to 230°C. Additionally, the sensitivity of the sensor increased with the concentration of injected ethanol gas.

Fabrication of Humidity Sensor Based on Bilayer Graphene

Bilayer graphene was synthesized by chemical vapor deposition, for which the optical transmission is ~95%-96%. XPS detected that a few oxygen molecules were absorbed on the surface of graphene. The Raman and optical transmission spectra verified that the graphene sample is bilayer. The responses of the bilayer graphene sample increased with increased relative humidity.

Vertical p-Type Cu-Doped ZnO/n-Type ZnO Homojunction Nanowire-Based Ultraviolet Photodetector by the Furnace System with Hotwire Assistance

Vertical p-ZnO:Cu/n-ZnO homojunction nanowires (NWs) and whole ZnO:Cu NWs were synthesized on a ZnO thin film/glass substrate by a furnace at 600 °C with 1700 °C hotwire assistance. According to the ZnO:Cu NW investigation, the energy-dispersive X-ray (EDX) spectrum indicates that the Cu content is 3.01 atomic %. The X-ray diffraction (XRD) peaks of ZnO:Cu NWs shift toward larger angles with increasing amounts of doped Cu. The Cu dopant enhanced the photoluminescence (PL) green-band peak and decreased the conductivity of the NWs, as measured by I-V. The gas sensing measurement and Hall effect verified that all ZnO:Cu NWs were p-type. In this study, transmission electron microscopy (TEM) and EDX mapping images revealed that the majority of the Cu element is located at the top of the p-ZnO:Cu/n-ZnO NW. The high-resolution transmission electron microscopy (HRTEM) image of the p-ZnO:Cu region shows that the NWs are [0001] growth-oriented, with lateral surfaces enclosed by (1̅101) planes. The I-V curve of p-ZnO:Cu/n-ZnO NWs displays the characteristics of normal rectifying diodes. The photocurrent under ultraviolet (UV) exposure was around 6 times higher than the dark current at the reverse bias of -5 V.

p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes

This study reports the deposition of cuprous oxide [Cu2O] onto titanium dioxide [TiO2] nanowires [NWs] prepared on TiO2/glass templates. The average length and average diameter of these thermally oxidized and evaporated TiO2 NWs are 0.1 to 0.4 μm and 30 to 100 nm, respectively. The deposited Cu2O fills gaps between the TiO2 NWs with good step coverage to form nanoshells surrounding the TiO2 cores. The p-Cu2O/n-TiO2 NW heterostructure exhibits a rectifying behavior with a sharp turn-on at approximately 0.9 V. Furthermore, the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratios and fast responses.