INTRODUCTION Sensors A sensor is an instrument that responds to a physical stimulus. The sensor is responsible for converting some type of physical phenomenon into a quantity measurable by a data acquisition (DAQ) system. Why Metal oxide nano-crystals for sensing?? Metal oxides possess a broad range of electronic, chemical, and physical properties that are often highly sensitive to changes in their chemical environment. The sensing properties of semiconductor metal oxide (nano-belts, nano-wires or nano-ribbons) assures improved selectivity due to there crystalline nature. Functional one-dimensional (1D) nanomaterials have been attracted as promising elements for environmental sensing applications. Therefore, many researchers have had huge efforts to synthesize  and characterize  various 1D nanostructure materials. 3
Introduction cont.. In particular, ZnO NWs are synthesized by hydrothermal process and chemical vapor deposition CVD for the application of gas sensor to detect various gases such NO2, NH3, CO, H2, H2S, and C2H5OH. However, those methods require additional processes for the integration of nanostructures into a device. Furthermore, they tend to show low reliability and poor electrical robustness . Additionally, the low selectivity and sensitivity of gas sensor based on ZnO NWs remain to be important issues. Here, they have developed a novel fabrication method of ZnO NWs based on localized hydrothermal synthesis. And simple surface modification process with Pt NPs for gas sensing applications, which can address the aforementioned issues simultaneously. Motivation Improve the reliability and robustness of the device by designing new fabrication method. Improve sensing performance of ZnO NW-based sensor by surface modification with Pt NP’s. 4
HYDROGEN SENSING MECHANISM AND MEASUREMENT: In nanowires network, there are many overlapping wires that connect each other. The electric current has to pass through these connecting nodes, which are surrounded by an electron depletion layer that is affected by the surface charge. In air, it is well known that oxygen is adsorbed on a ZnO NWs surface as O2, O, and O2 ions by extracting electrons from the conduction band . When ZnO NWs sensor is exposed to H2 gas, chemical reaction between the negatively charged oxygen adsorbed on the ZnO surface and H2 gas is generated. (O-)ZnO + 2H H2O (g) + e- (1) 2H2 + O2- 2H2O + e- (2) Hydrogen atoms react with these chemisorbed oxygen ions and produce H2O molecules. This reaction consumes chemisorbed oxygen from the nanowire surface and releases free electrons to ZnO. Potential barrier at the nodes becomes lower through such chemical reaction. These mechanism leads to change in the resistance of the ZnO NW network between sensing electrodes. 5
CATALYTIC EFFECT OF PT NPS : It is supposed that the sensitivity of ZnO NW’s with Pt NP’s increases because Pt NP’s help in converting the H2 molecules into 2 H+ ions there by enhancing the reaction between H+ ions and adsorbed oxygen molecules. 6
FABRICATIONPhotoresist was spin-coated on Si/SiO2 substrate.Au deposition and lift-off process.In order to promote the adhesion of Au,thin Cr layer was deposited before Au deposition.Next silicon dioxide layer was deposited by PECVDprocess as a passivation layer between the microheaterand sensing electrode layers.To fabricate interdigitated electrodes on the passivationlayer, photoresist-patterning, Cr/Au deposition, andlift-off process was repeated.Finally, the passivation layer was selectively removed onthe contact pad area using buffered oxide etchant. 7
SYNTHESIS Nanostructure Synthesis by Local Heating In the first step, the substrate was coated with ZnO nanoparticle seed solution and heated at 150℃ for 20 min to form a thin layer of ZnO nanoparticle seeds.  For the growth of ZnO NWs, the aqueous ZnO precursor solution(zinc nitrate (25 mM), HTMA (25 mM), PEI(6 mM)) was put into PDMS well placed on the chip, and then they supplied microheater arrays with the voltages 0.8V to create localized joule heating (as shown in Figure 4). Required growth temperature of NW’s (95℃) was regulated by microheaters. 8
Numerical Simulation of Microheater Arrays :COMSOL Multiphysics TM software was used to confirm the steady-state temperature distribution generatedby microheaters. 9
Nanostructure Synthesis by Local Heating Figure 5 (a-b) shows the SEM images of ZnO NWs synthesized between interdigitated electrodes. This result indicates that ZnO NWs were selectively grown on the hot spots created by microheaters, while no production of nanowires elsewhere. The ZnO NWs synthesized locally have the diameters and lengths of 50~100 nm and 1~2 μm, respectively and form a network structure. (Figure 5 (c-d)). 10
Nanostructure Modification via Local Heating In order to improve the sensitivity of the device, the surface of ZnO NWs was selectively coated with Pt NPs by applying local heating while the device was exposed to aqueous Pt precursor solution (Figure 6). In the presence of thermal energy, Pt NPs could be coated on the surface of ZnO NWs due to reduction of Pt precursor. Since conventional methods for metal deposition like sputtering ,electrochemical deposition can only be used for coating large areas so here we use another technique to deposite Pt nanoparticles selectively on nanowires. For the surface modification of ZnO NWs, aqueous Pt precursor solution was put in PDMS well placed on the device with locally grown ZnO NWs and the device was heated by using microheaters. This process was maintained at approximately at 90℃ for 25 min. 11
RESULTS AND DISCUSSIONS Hydrogen sensing characteristics for bare ZnO NWs and Pt NPs coated-ZnO NWs based on gas sensors were investigated. Rair is the resistance of the sensor in air . RH2 is the resistance after exposing it in H2. Bare ZnO NW sensor could detect H2 in a wide range of concentrations from 100 to 5000 ppm at 350℃. As presented in figure, the magnitude of sensitivity to H2 went downward from 350℃ to 250℃. 12
RESULTS AND DISCUSSIONS In addition, the sensor performed low reliability at low temperature. Thus it shows that there is a suitable operating temperature for ZnO gas sensor. Also its clearly visible that the sensitivity of bare ZnO NW sensor to H2 shows an increase when it is coated with Pt nanoparticles. 13
CONCLUSION Local synthesis of ZnO NWs is reported so it eliminates the additional process required for integration of nanostructures onto the sensing device. Thus improving the reliability and robustness of the device. The sensitivity of the gas sensors is improved by surface modification of ZnO NW’s with Pt nanoparticles . Also this method of fabrication can lead to high performance multiplexed chemical sensors array. 14
REFERENCES  T. Y. Wei, P. H. Yeh, S. Y. Lu, and Z. L. Wang,  V. Sysoev, J. Goschnick, T. Schneider, E. Strelcov, and “Gigantic enhancement in sensitivity using schottky A. Kolmakov, “A gradient microarray electronic nose contacted nanowire nanosensor”, J. Am. Chem. Soc., vol. based on percolating SnO2 nanowire sensing elements”, 131, pp. 17690-17695, 2009. Nano Lett., vol. 7 pp. 3192-3188, 2007.  T. Gao, and T. H. Wang, “Synthesis and properties of  A. R. Raju, and C. N. R. Rao, “Gas-sensing multipod-shaped ZnO nanorods for gas-sensor characteristics of ZnO and copper-impregnated ZnO”, applications”, Appl. Phys. A, vol. 80, pp. 1451-1454, 2005. Sensors and Actuators B, vol. 3, pp, 305-310, 1991.  I. Park,Z. Li, A. P. Pisano, and R. S. Williams, “Top-  S. J Ippolitoa, S. Kandasamya, K. Kalantar-zadeha, down fabricated silicon nanowire sensors for real-time and B. W. Wlodarskia, “Hydrogen sensing characteristics chemical detection”, Nano technology, vol. 21, pp. 015501, of WO3 thin film conductometric sensors activated by Pt 2010. and Au catalysts”, Sensors and actuators B, vol. 108, pp. 154-158, 2005.  C. Pacholski, A.Komowski, and H. Weller, “Self- assembly of ZnO: from nanodots to nanorods,” Angew. Chem. Int. Ed., vol. 41, pp. 1188-1191, 2002. 15
Pt precursor solution was prepared by adding sodium citrate solution (30 mM) into the aqueous solution of K2PtCl4 (0.1 mM). Sodium citrate was used as a reducing agent and a stabilizer. Also, aqueous sodium hydroxide solution (0.1 M) was added to Pt precursor solution to prevent the surface etching of ZnO NWs. 17
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