Duy-Thach Phan

Effects of oxygen-functional groups on humidity sensor based graphene oxide thin films

In this work, we showed an effective method to control amount of oxygen-functional groups attached on graphene oxide (GO) by rapid thermal annealing (RTA) process. The GO thin films spraying on SiO2/Si substrates from GO dispersion solution were annealed by RTA at various temperatures from 400 °C to 1200 °C. Resulting in significant reduced GO at 400 °C annealing temperature and removed most of the oxygen-containing functionalities from the annealing GO at 1200 °C. Along with a decrease resistance during increasing annealing temperature, the GO films loss its excellent absorption humidity vapor. The sensitivity (S) of humidity sensors fabricated on these GO films decreased from 35.3 % as-deposited GO to 0.075 % of 1200 °C annealing GO and the response time also increased in annealing samples. The highest sensitivity humidity can be obtained in as-deposited GO, but the humidity sensor-based graphene oxide have to trade-off between sensitivity and long-term stability, which is poor in as-deposited GO films.

Characteristics of ZnO Films Deposited on Poly 3C-SiC Buffer Layer by Sol-Gel Method

This work describes the characteristics of zinc oxide (ZnO) thin films formed on a polycrystalline (poly) 3C-SiC buffer layer using a sol-gel process. The deposited ZnO films were characterized using X-ray diffraction, scanning electron microscopy, and photoluminescence (PL) spectra. ZnO thin films grown on the poly 3C-SiC buffer layer had a nanoparticle structure and porous film. The effects of post-annealing on ZnO film were also studied. The PL spectra at room temperature confirmed the crystal quality and optical properties of ZnO thin films formed on the 3C-SiC buffer layer were improved due to close lattice mismatch in the ZnO/3C-SiC interface.

SYNTHESIS OF ZnO NANOPARTICLES-REDUCED GRAPHENE OXIDE COMPOSITES AND THEIR INTRINSIC GAS SENSING PROPERTIES

A ZnO nanoparticles (NPs)/reduced graphene oxide (rGO) composite was fabricated via a simple one-step solvothermal method with graphene oxide (GO) and Zn(NO3)2 ⋅ 6H2O as the precursors. The morphology, crystal structure and optical properties of the synthesized materials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy. The synthesized composite exhibited rGO layers assorted with tiny ZnO NPs. rGO supposedly acted as a template in the solvothermal process, that may promote the preferential attachment of ZnO NPs and prevented the agglomeration of ZnO NPs in the synthesized composite. It was also found that the electrical properties of the composite improved markedly with bare ZnO NPs, without significantly changing the morphology and crystal structure of the ZnO NPs. The main aim of this research is to develop an efficient sensor and to understand the effect of graphene in sensing characteristics. The synthesized composite was exposed to H2, CO and C2H2 gases to confirm its feasibility for gas sensing, and the results showed preferential detection of reducing gases at low temperature.

P1.7.6 CO Gas Sensing Using Ga Doping ZnO Nanorods by Hydrothermal Method: Effects of Defects-Controlled

We report here the synthesis Ga-doped ZnO nanorods by hydrothermal (HT) method and investigate the effects of Ga-doping on their CO sensing properties. It is found that Ga doping cancel out oxygen-related defects (oxygen interstitial) based on the results of photoluminescence (PL) experiments and further confirmed by the CO sensing experiment. The defect-controlled, which are donor-(shallow donor and zinc interstitial) and acceptor-related (oxygen interstitial) ones, in ZnO nanorods were adjusted by Ga doping level. The CO sensing properties of ZnO nanorods are effectively improved by Ga doping. These can be explained in term of the removal excess oxygen in ZnO nanorods surface, increase shallow donor concentration and Ga-doped ZnO formed active component for CO absorption.

Design and Optimization of Reconfigurable Inset-Fed Microstrip Patch Antennas with High Gain for Wireless Sensor Networks

In this paper, a tunable microstrip patch antenna designed using RF MEMS switches is reported. The design and simulation antenna were performed using high frequency structure simulator (HFSS). The antenna was designed in ISM band and operates simultaneously at 2.4 Ghz and 5.7 Ghz with a -10 dB return-loss bandwidth of 20 Mhz and 180 Mhz, respect- tively. To obtain high efficiency and improve integrated ability, the high resistivity silicon (HRS) wafer was used for the antenna. The antenna achieved high gain with 8 dB at 5.7 Ghz and 1.5 dB at 2.4 Ghz. The RF MEMS DC contact switches was simulated and analysis by ANSYS software.

Characteristics of UV sensors using ZnO nanostructures synthesized by galvanostatic electrochemical deposition

In this study, we demonstrate the ZnO nanorods (NRs) and nanocombs (NCs) by simple galvanostatic electrochemical deposition process. The ZnO nanostructures act as high ultra-violet (UV) sensing material due to high aspect ratio. The photoluminescence (PL) spectra and time-dependent photo response of the ZnO nanostructures grown on Au/Si has exhibited good optical properties. At room temperature, NCs showed good response with change 8% its resistance, few seconds response time and fully recovery. Inversely, in high temperature ZnO NRs indicated better response than NCs with change of 20% of its resistance. The dependence photoresponse on temperature demonstrated clearly how surface-defects affect on UV response of ZnO nanostructures.

Effects of palladium nanocrystal morphologies on hydrogen sensors based on palladium-graphene hydrid

In this work, we demonstrate the effects of palladium (Pd) nanocrystal morphologies on hydrogen (H2) sensors based on Pd-graphene (Gr) hybrid nanomaterials. The Pd nanocrystal with different morphologies of nanocube, nanoporous and core-shell structures were synthesized in colloidal state by a chemical route in two-steps (seed-mediated growth) and then, simply reduced into graphene flakes by hydrazine in a simple one-step process. Resulting in H2 sensors based on Pd-graphene hybrid have such advantages of high sensitivity, good selectivity and stability. Especially, Pd nanoporous and Pt/Pd core-shell structures yield a fast response/recovery time and a large detectable range of H2 gas even at room temperature. The different H2 response properties of various Pd morphologies in hybrid with graphene were investigated and emphasized in this work.

SAW UV sensors using ZnO nanorods grown on AlN/Si structures

In this study, we demonstrate the ZnO nanorods grown on aluminum nitride (AlN)/silicon (Si) layered structure to obtain surface acoustic wave (SAW) UV sensor. The ZnO nanorods act as high UV sensing material due to large surface-to-volume ratio. The X-ray Diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO nanorods grown on AlN/Si had highly (002)-oriented and good optical properties. The piece of ZnO nannorods grown at center of two-port SAW delay line, which based on inter-digital transducer (IDT)/AlN/Si structure was fabricated and exposed under UV light. Resulting in the SAW UV sensor shifted a maximum of 26 kHz under the UV intensity of 0.6 mW/cm2.

High sensitivity SAW UV sensor by using third harmonic based on ZnO/Si structure

In this study, we demonstrate the third harmonic generation in a ZnO/Si layered structure to obtain high sensitivity UV sensor. A ZnO thin film is simultaneously used as an active layer for UV detection and a piezoelectric layer was sputtered by a magnetron sputtering. The X-ray Diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered on Si(100) had highly (002)-oriented and good optical properties. The two-port SAW resonator based on inter-digital transducer (IDT)/ZnO/Si structure was fabricated and exposed under UV light with wavelength of 380 nm. Resulting in the SAW UV sensor shifted extremely of 400 kHz at third harmonic mode compare to frequency shift of 10 kHz in fundamental mode under the same UV intensity of 3 mW/cm2.