Cheng-Liang Hsu

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.

Ultraviolet photodetectors with ZnO nanowires prepared on ZnO:Ga/glass templates

Vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors were fabricated by spin-on-glass technology on ZnO:Ga/glass templates. With 2V applied bias, it was found that dark current density of the fabricated device was only 2.0×10−7A∕cm2. It was also found that UV-to-visible rejection ratio and quantum efficiency of the fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.

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.

Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates

Vertical single-crystal ZnO nanowires with uniform diameter and uniform length were selectively grown on ZnO:Ga/glass templates at 600/spl deg/C by a self-catalyzed vapor-liquid-solid process without any metal catalyst. 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. Photoluminescence, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements also confirmed good crystal quality of our ZnO nanowires. Field emitters using these ZnO nanowires were also fabricated. It was found that threshold field of the fabricated field emitters was 14 V//spl mu/m. With an applied electric field of 24 V//spl mu/m, it was found that the emission current density was around 0.1 mA/cm/sup 2/.

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.

Well-Aligned, Vertically Al-Doped ZnO Nanowires Synthesized on ZnO : Ga ∕ Glass Templates

High-density, single-crystal, vertically aligned, Al-doped ZnO nanowires with an Al content of 1.05 atom % were synthesized on ZnO:Ga/glass templates at 550°C. Although introducing Al did not change the physical dimensions of the ZnO nanowires, the lattice constant increased by 0.25% and the optical properties of the ZnO nanowires were degraded. However, the experimental results also showed that the threshold emission field can be significantly decreased from 16 to 10 V/μm, and the work function, Φ, can also be reduced from 5.3 to 3.39 eV by introducing Al atoms into the ZnO nanowires.

A Lateral ZnO Nanowire Photodetector Prepared on Glass Substrate

We report the fabrication of a lateral ZnO nanowire UV photodetector on a glass substrate using tungsten as the growth barrier to suppress the vertical growth of the ZnO nanowires. Upon UV illumination, the detector current increased by more than 1 order of magnitude. Furthermore, the noise equivalent power and the normalized detectivity D * of the fabricated lateral ZnO nanowire photodetector were 7.89 × 10 -11 W and 1.9 × 10 8 cm Hz 0.5 W -1 , respectively.

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.