CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

0272-8842/$ - sehttp://dx.doi.org/

nCorrespondinE-mail addre

Ceramics International 40 (2014) 8305–8310www.elsevier.com/locate/ceramint

Gas sensing properties of multiple networked GaN/WO3 core-shellnanowire sensors

Sunghoon Park, Hyunsung Ko, Soohyun Kim, Chongmu Leen

Department of Materials Science and Engineering, Inha University, Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea

Received 19 November 2013; received in revised form 8 January 2014; accepted 8 January 2014Available online 17 January 2014

Abstract

GaN nanowires and GaN-core/WO3-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO3 and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses ofapproximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO2 gases, respectively. These responses are comparable to thoseobtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and5 ppm NO2 gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO3

thin film. The improvement in the response of GaN nanowires to NO2 gas by encapsulation is attributed to the modulation of electron transport atGaN–WO3 heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrierheight, resulting in an enhanced sensing property of the core-shell nanostructures.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: GaN; WO3; Nanowires; Gas sensors

1. Introduction

GaN is very stable thermally and chemically, and hardlyetched by wet chemical etchants. This makes it very suitablefor gas sensors operating in harsh environments such as hightemperature and corrosive ambients. It can be used forapplications including fuel leak detection in spacecraft andrelease of toxic or corrosive gases [1,2]. Since the sensitivity ofPt/GaN Schottky diodes to hydrogen and hydrocarbons attemperatures up to 400 1C was first reported by Luther et al. in1999 [3], studies on the detection of toxic gases includingNO2, NH3, and CO in a high temperature environment usingGaN gas sensors have been reported [4–6]. GaN gas sensorshave a unique advantage of integration with GaN-based solar-blind UV photodetectors or high power and high temperatureelectronic units on the same chip. Therefore, most GaN gassensors were reported to utilize Schottky contacts [3,7–10]made up of catalytic metals such as Pt or Pd, and to measurethe change of effective Schottky barrier height. On the other

e front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2014.01.035

g author. Tel.: þ82 32 860 7536; fax: þ82 32 862 5546.ss: cmlee@inha.ac.kr (C. Lee).

hand, very few studies have been reported on the multiplenetworked GaN 1D nanostructure gas sensors. Heterostructureformation has been reported as a technique to enhance thesensing performance, detection limit and operation temperatureof 1D nanostructure sensors [11–13]. In this study, theheterostructure formation technique was adopted to enhancethe sensing properties of the GaN 1D nanostructure-basedsensors. Multiple networked GaN-core/WO3-shell nanowiresensors were fabricated and their NO2 gas sensing propertieswere examined.

2. Experimental

2.1. Synthesis of GaN nanowires and GaN-core/WO3-shellnanowires

GaN nanowires were grown on Si substrates by evaporatingGaN powders at 1000 1C in flowing ammonia gas. C-planesapphire substrates sputter-coated with �3 nm of Au wereplaced downstream the GaN powders. After the system waspumped down, the furnace was ramped to 1000 1C andmaintained at this temperature for 1 h. Ammonia gas with a

ghts reserved.

S. Park et al. / Ceramics International 40 (2014) 8305–83108306

flow rate of 20 cm3/min and nitrogen gas with a flow rate of100 cm3/min were introduced to the system throughout thereaction process. After synthesis, the gas supply was turnedoff and the furnace was cooled to room temperature. Subse-quently, WO3-coated GaN nanowires were fabricated by thesputter-deposition of WO3 on GaN nanowires. GaN nanowireswere coated with a thin WO3 layer by sputtering in a conven-tional radio frequency (RF) magnetron sputtering system with5.0� 10�6 Torr base vacuum. During the coating process theworking pressure and N2 gas flow rate were maintained at

Fig. 1. (a) SEM image of GaN/WO3 core-shell nanowires. (b) XRD patterns o(d) HRTEM image of a typical GaN/WO3 core-shell nanowire. (e) Corresponding

2.0� 10�2 Torr and 20 sccm, respectively. The RF sputteringpower and sputtering time were 50 W and 10 min, respectively.The substrate temperature was kept at room temperature.

2.2. Characterization of GaN nanowires andGaN-core/WO3-shell nanowires

The morphology and structure of the products were character-ized by field emission scanning electron microscopy (FESEM,Hitachi S-4200) and transmission electron microscopy (TEM,

f GaN/WO3 core-shell nanowires. (c) Low magnification TEM images andSAED pattern.

S. Park et al. / Ceramics International 40 (2014) 8305–8310 8307

JEOL 2100F). The crystallographic structure was determined byglancing angle X-ray diffraction (XRD) using Cu Kα radiation(0.1542 nm) at a scan rate of 4 1/min and at a glancing angle of0.51 with a rotating detector.

2.3. Fabrication of nanowire sensors and measurementof their sensing properties

Ni (�200 nm in thickness) and Au (�50 nm) thin filmswere deposited sequentially by sputtering to form electrodesusing an interdigital electrode mask. The GaN nanostructureswere dispersed ultrasonically in a mixture of deionized water(5 ml) and isopropyl alcohol (5 ml), and dried at 90 1C for30 min. A 200 nm thick SiO2 film was grown thermally onsingle crystalline Si (1 0 0). A slurry droplet containing theGaN nanostructures (10 ml) was dropped onto the SiO2-coatedSi substrates equipped with a pair of interdigitated (IDE)Ni (�200 nm)/Au (�50 nm) electrodes with a gap of 20 μm.Gas sensing experiments were carried out in a conventionalgas-flow apparatus. The sensor element set inside a glasschamber was heated to a designate temperature of 300–500 1Cexternally by an electrical furnace. Test gas atmospheres withppm concentrations were produced by mixing two streams ofsynthetic air. Whereas the first stream consisted of drysynthetic air (20% O2/80% N2), the second one consisted ofsynthetic air containing 1000 ppm of NO2. The desired NO2

concentration levels were obtained by adjusting the relativeflow rates of the first and the second gas stream whilemaintaining a total flow rate of 100 cm3/min. A NationalInstruments LabviewTM program was used to control the massflow controllers. NO2 containing gas stream was passedthrough silica gel to reduce the effects of humidity on thetarget gas before it was allowed to enter the test chamberthrough the MFCs. The real concentration of the NO2 targetgas in the test chamber was controlled using a gas detector(model: ToxiRAE-II, maker: RAE system), but the level oforganic contaminants was not checked.

The reference air was introduced to the system through astainless steel pipe inserted into the quartz tube in a flow mode.The gas was spouted out toward the IDE pattern (or thenanowires) that was attached at the tip of the electrode insertedinto the quartz tube on the opposite side of the steel pipe. Thedistance between the gas outlet and the IDE pattern was�10 cm. A Keithley sourcemeter-2612 was used to acquirethe impedance data. The sourcemeter was hooked to acomputer via a GPIB cable and this data was also acquiredusing the LabView™ software. The response was defined asthe ratio of sensor resistance in the hydrogen gas containing airRg to that in air Ra.

3. Results and discussion

Fig. 1(a) shows a SEM image of the GaN-core/WO3-shell 1Dnanostructures synthesized in this study. The nanowires were afew tens to a few hundreds of nanometers in diameter and upto a few hundreds of micrometers in length. An enlarged SEM

image of a typical 1D nanostructure showed that the nano-structure had a wire-like morphology (inset in Fig. 1(a)).Fig. 1(b) shows XRD patterns of the as-synthesized GaN-

core/WO3-shell nanowires. The main reflection peaks in thepattern of the as-synthesized core-shell nanowires (Fig. 1(b))can be indexed to an wurtzite structure, which is in goodagreement with the reported data for bulk GaN crystals(JCPDS Card no. 88-2361, a¼0.3162 nm and c¼0.5142 nm),indicating that the nanomaterial is GaN. In addition to the mainreflections from the (1 0 0), (0 0 2), (1 0 1), and (1 0 2) latticeplanes of GaN, reflections from the (2 0 0), (2 1 2), (2 2 2), and(3 2 1) lattice planes of WO3 were also identified, suggestingthat WO3 shells are also crystalline.Fig. 1(c) shows a low-magnification TEM image of a typical

as-synthesized core-shell nanowire, indicating that the dia-meter of the core and the thickness of the shell layers in a core-shell nanowire were approximately 50 nm and 10 nm, respec-tively, and the shell thickness uniformity along the lengthdirection was excellent, despite that WO3 was deposited by asputtering technique. Fig. 1(d) presents the local high-resolution TEM (HRTEM) image enlarging the core-shellinterface region of the nanowire. The resolved spacingbetween two neighboring parallel fringes in the core regionwas approximately 0.25 nm, which is in good agreement withthe interplanar distance of bulk wurtzite GaN (0 0 2) planes.On the other hand, the resolved spacing between twoneighboring parallel fringes in the shell region was approxi-mately 0.27 nm, which is in good agreement with the inter-planar distance of bulk simple tetragonal WO3 (1 1 2) planes.The corresponding selected area of the electron diffraction(SAED) pattern, which was recorded perpendicular to the longaxis, can be indexed for the [0 1 0] zone axis of GaN. Thestrong reflection spots in the corresponding selected areaelectron diffraction (SAED) pattern (Fig. 1(e)) were assignedto the (0 0 2) and (1 0 1) reflections of wurtzite-structuredGaN, indicating that the GaN nanowire in the TEM image wasa single crystal. In addition to the spotty pattern, dimconcentric rings assigned to (2 1 2) and (3 2 1) WO3 reflec-tions were also observed, indicating that the WO3 shells werepolycrystalline. Briefly, the HRTEM image and SAED patternrevealed that the core comprised wurtzite-structured singlecrystal GaN, whereas the shells comprised polycrystallineWO3.Fig. 2(a) presents the dynamic responses of pristine GaN

nanowires and GaN-core/WO3-shell nanowires, respectively,at 300 1C to a typical oxidizing gas NO2. The resistanceincreased upon exposure to NO2 and recovered completely tothe initial value upon the removal of NO2. The sensorresponses to NO2 gas were also quite stable and reproduciblefor the repeated test cycles. Fig. 2(b) and (c) shows theenlarged parts of the data in Fig. 2(a) measured at a NO2

concentration of 5 ppm for pristine GaN nanowires and GaN-core/WO3-shell nanowires to reveal the moments of gas inputand gas stop. Fig. 2(d) shows that the responses of pristineGaN nanowires to 1, 2, 3, 4, and 5 ppm NO2 gases wereapproximately 125%, 140%, 146%, 159%, and 183%, respec-tively. In contrast, the responses of GaN-core/WO3-shell nanowires

Fig. 2. (a) Dynamic responses of pristine GaN nanowire and GaN-core/WO3-shell nanowire sensors to 5 ppm NO2 gas. (b) Enlarged part of the response curve ofthe core-shell nanowire sensor. (c) Enlarged part of the response curve of the pristine WO3 nanorod sensor. (d) Responses of pristine GaN nanowire and GaN-core/WO3-shell nanowire sensors as a function of the NO2 gas concentration.

S. Park et al. / Ceramics International 40 (2014) 8305–83108308

to 1, 2, 3, 4, and 5 ppm NO2 gases were 165%, 194%, 228%,271%, and 329%, respectively. Therefore, the responses of thenanowires to 1, 2, 3, 4, and 5 ppm NO2 gases were improved1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, through theencapsulation of GaN nanowires with WO3. Table 1 showsthat the responses of the core-shell nanowires to NO2 gas arecomparable to those of typical metal oxide semiconductorpristine and core-shell 1D nanostructures reported previously,even though a somewhat higher NO2 gas concentration rangeused in this study was taken into consideration. The GaN-core/WO3-shell nanowires show a higher response to NO2 gas thanall other nanostructures except SnO2 hollow spheres and In2O3

nanowires [14–24]. Actually, the effect of temperature on thesensitivity of gas sensors must be considered in this compar-ison. The magnitude of the substrate temperature has a verystrong influence on the NO2 sensitivity of metal oxidematerials [25]. In general, the sensing properties are signifi-cantly enhanced with increasing substrate temperature anddifferent temperatures could easily produce sensitivity changeswhich by far outweigh any materials-related differences.

The gas-sensing mechanism of GaN can be explained usinga surface depletion model and the resistance change dependson the species and the amount of chemisorbed oxygen on thesurface as that of metal oxides. The adsorbed oxygenmolecules transform into oxygen ions (O� , O2� and O2

�)depending on temperature in air atmosphere by capturing freeelectrons from the nitride. The stable oxygen ions are O2

�

below 100 1C, O� between 100 1C and 300 1C, and O2�

above 300 1C [26]. When the GaN nanowires are exposed toNO2 gas, NO2 gas reacts with the adsorbed O� ions as well asadsorb directly on the surface of GaN nanowires according tothe following reactions [27,28]:

NO2 (gas)þe�-NO2� (ads) (1)

NO2� (ads)þO� (ads)þ2e�-NO (gas)þ2O2� (ads). (2)

Since GaN is an n-type semiconductor, the oxidizing NO2

molecules adsorbed on the nitride surface may captureelectrons from the conduction band and form NO2

�.A depletion layer is formed on the surface of GaN, resultingin the increase in resistance.On the other hand, the improvement in the response of GaN

nanowires to NO2 gas by encapsulating them with WO3 can beexplained by the space-charge model [29,30] and formation ofdefects in the WO3 shell layer [31]. Upon exposure to NO2

gas, NO2 gas is adsorbed by the core-shell nanowire sensorand electrons are released from the WO3 shell layers, andattracted to the adsorbed NO2 molecules because an oxidizinggas, such as NO2, acts as an electron acceptor in the reaction.This reaction will result in an increase in the depletion layerwidth, and an increase in the resistance of the nanowire sensor.On the other hand, trapped electrons are released to theWO3 shell layer by NO2 gas after stopping the supply ofNO2 gas, leading to a decrease in the depletion layer width andresistance. The electron exchange between the surface statesand the WO3 shell layer occurs within the space charge region,

Table 1Comparison of the responses of pristine GaN nanowires and GaN-core/WO3-shell nanowires to NO2 gas with those of metal oxide semiconductor 1Dnanostructures.

Nanomaterial Temperature (1C) NO2 conc. (ppm) Response (%) Ref.

GaN nanowires 300 5 183 Present workGaN-core/WO3-shell nanowires 300 5 329 Present workZnO nanorods 300 0.1 35 14ZnO fibers 100 0.4 50 15In-doped SnO2 nanoparticles 250 500 100 16SnO2 nanoribbon Room temperature 3 116 17SnO2 hollow spheres 160 5 1150 18In2O3 nanowires 400 50 360 19In2O3 nanowires 250 50 200 20WO3 nanorods 300 1 200 21Au-doped WO3 powders 150 10 350 22SnO2-core/ZnO-shell nanofibers 300 70–2000 20–320 23ZnGa2O4-core/ZnO-shell nanowires 250 1 260 24

Fig. 3. Schematic diagram of the cross-section of a typical GaN nanostruc-ture with a diameter of 50 nm to show the depletion layer and gas sensingmechanism.

S. Park et al. / Ceramics International 40 (2014) 8305–8310 8309

i.e. the depletion layer. In principle, the depletion layer widthcan vary over several orders of magnitude depending on theresidual doping of the WO3 layer and its defect density. TheWO3 shell layer contains a relatively high density of defectsgenerated during the WO3 sputter-deposition process whichwithdraw charge carriers from the GaN core and thus reduceits effective doping, leading to an increase in the depletionlayer width (Fig. 3). Lower doping and stronger depletion ofthe GaN core will result in a higher gas sensitivity [31].

4. Conclusions

The multiple networked pristine GaN nanowires showedresponses of 125–183% to 1–5 ppm NO2 gases. These responsesare comparable or higher than those obtained previously usingmetal oxide one-dimensional nanostructures. The responses ofthe nanowires to NO2 gases were improved further through theencapsulation of GaN nanowires with a WO3 thin film. Theimprovement in the response of GaN nanowires to NO2 gas byencapsulation can be accounted for based on the space-chargemodel and formation of defects in the WO3 shell layer. Theelectron exchange between the surface states and the WO3 shelllayer occurs within the space charge region, i.e. the depletionlayer. The depletion layer width can vary over several orders ofmagnitude depending on the residual doping of the WO3 layerand its defect density. The WO3 shell layer contains a relativelyhigh density of defects generated during the WO3 sputter-deposition process which withdraw charge carriers from theGaN core and thus reduce its effective doping. Lower dopingand stronger depletion of the GaN core will result in a higher gassensitivity.

Acknowledgment

This work was supported financially by Inha University.

References

[1] J. Johnson, A. Baca, R. Briggs, R. Shul, C. Monier, F. Ren, S. Pearton,A. Dabiran, A. Wowchack, C. Polley, P. Chow, Effect of gate length onDC performance of AlGaN/GaN HEMTs grown by MBE, Solid-StateElectron. 45 (2001) 1979–1985.

[2] E.J Connolly, G.M O’Halloran, H.T.M. Pham, P.M. Sarro, P.J. French,Relative humidity sensors using porous SiC membranes and Al electro-des, Sens. Actuators A 100 (2004) 25–31.

[3] B.P. Luther, S.D. Wolter, S.E. Mohney, High temperature Pt Schottkydiode gas sensors on n-type GaN, Sens. Actuators B 56 (1999) 164–168.

[4] J. Schalwig, G. Müller, M. Eickhoff, O. Ambacher, M. Stutzmann, GroupIII-nitride-based gas sensors for combustion monitoring, Mater. Sci. Eng.B 93 (2002) 207–214.

S. Park et al. / Ceramics International 40 (2014) 8305–83108310

[5] J. Schalwig, G. Müller, M. Eickhoff, O. Ambacher, M. Stutzmann, Gassensitive GaN/AlGaN-heterostructures, Sens. Actuators B 87 (2002)425–430.

[6] G. Zhao, W. Sutton, D. Pavlidis, E.L. Piner, J. Schwank, S. Hubbard,A novel Pt-AlGaN/GaN heterostructure Schottky diode gas sensor on Si,IEICE Trans. Electron. E86-C (2003) 2027–2031.

[7] J. Schalwig, G. Muller, L. Gorgens, G. Dollinger, Hydrogen responsemechanism of Pt–GaN Schottky diodes, Appl. Phys. Lett. 80 (2002)1222–1224.

[8] J. Kim, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, Reversiblebarrier height changes in hydrogen-sensitive Pd/GaN and Pt/GaN diodes,Appl. Phys. Lett. 82 (2003) 739–741.

[9] J. Kim, B.P. Gila, C.R. Abernathy, G.Y. Chung, F. Ren, S.J. Pearton,Comparison of Pt/GaN and Pt/4H-SiC gas sensors, Solid-state Electron.47 (2003) 1487–1490.

[10] O. Weidemann, M. Hermann, G. Steinhoff, H. Wingbrant, A. Lloyd-Spetz, M. Stutzmann, M. Eickhoff, Influence of surface oxides onhydrogen-sensitive Pd:GaN Schottky diodes, Appl. Phys. Lett. 83 (2003)773–775.

[11] M.A. Sanchez-Castillo, C. Couto, W.B. Kim, J.A. Dumestic, Gold-nanotube membranes for the oxidation of CO at gas–water interfaces,Angew. Chem. 116 (2004) 1160–1162.

[12] G. Jágerszki, R.E. Gyurcsányi, L. Höfler, E. Pretsch, Hybridization-modulated ion fluxes through peptide-nucleic-acid-functionalized goldnanotubes. A new approach to quantitative label-free DNA analysis,Nano Lett. 7 (2007) 1609–1612.

[13] Y. Oshima, A. Onga, K. Takayanagi, Helical gold nanotube synthesizedat 150 K, Phys. Rev. Lett. 91 (2003) 205503–205506.

[14] E. Oh, H.Y. Choi, S.H. Jung, S. Cho, J.C. Kim, K.H. Lee, S.W. Kang,J. Kim, J.Y. Yun, S.H. Jeong, High-performance NO2 gas sensor basedon ZnO nanorod grown by ultrasonic irradiation, Sens. Actuators B 141(2009) 239–243.

[15] C. Baratto, G. Sberveglieri, A. Onischuk, B. Caruso, S. di Stasio, Lowtemperature selective NO2 sensors by nanostructured fibres of ZnO, Sens.Actuators B 100 (2004) 261–265.

[16] J. Kaur, R. Kumar, M.C. Bhatnagar, Effect of indium-doped SnO2

nanoparticles on NO2 gas sensing properties, Sens. Actuators B 126(2007) 478–484.

[17] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Photochemical sensing ofNO2 with SnO2 nanoribbon nanosensors at room temperature, Angew.Chem. Int. Ed. 41 (2002) 2405–2408.

[18] J. Zhang, S. Wang, Y. Wang, Y. Wang, B. Zhu, H. Xia, X. Guo,S. Zhang, W. Huang, S. Wu, NO2 sensing performance of SnO2 hollow-sphere sensor, Sens. Actuators B 135 (2009) 610–617.

[19] A. Vomiero, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, G. Sberveglieri,Controlled growth and sensing properties of In2O3 nanowires, Cryst.Growth Des. 7 (2007) 2500–2504.

[20] P. Xu, Z. Cheng, Q. Pan, J. Xu, Q. Xiang, W. Yu, Y. Chu, High aspectratio In2O3 nanowires: Synthesis, mechanism and NO2 gas-sensingproperties, Sens. Actuators B 130 (2008) 802–808.

[21] Z. Liu, M. Miyauchi, T. Yamazaki, Y. Shen, Facile synthesis and NO2

gas sensing of tungsten oxide nanorods assembled microspheres, Sens.Actuators B 140 (2009) 514–519.

[22] H. Xia, Y. Wang, F. Kong, S. Wang, B. Zhu, X. Guo, J. Zhang, S. Wu,Au-doped WO3-based sensor for NO2 detection at low operatingtemperature, Sens. Actuators B 134 (2008) 133–139.

[23] S.W. Choi, J.Y. Park, S.S. Kim, Synthesis of SnO2-ZnO core-shellnanofibers via a novel two-step process and their gas sensing properties,Nanotechnology 20 (2009) 465603–465608.

[24] I.C. Chen, S.S. Lin, T.J. Lin, C.L. Hsu, T.J. Hsueh, T.Y. Shieh, Theassessment for sensitivity of a NO2 gas sensor with ZnGa2O4/ZnO core-shell nanowires—a novel approach, Sensors 10 (2010) 3057–3072.

[25] B. Ruhland, Th. Becker, G. Müller, Gas-kinetic interactions of nitrousoxides with SnO2 surfaces, Sens. Actuators B 50 (1998) 85–94.

[26] D. Manno, G. Micocci, R. Rella, A. Serra, A. Taurino, A. Tepore,Titanium oxide thin films for NH3 monitoring: structural and physicalcharacterizations, J. Appl. Phys. 82 (1997) 54–59.

[27] T.V. Belysheva, L.P. Bogovtseva, E.A. Kazachkov, N.V. Serebryakova,Gas-sensing properties of doped In2O3 films as sensors for NO2 in air,J. Anal. Chem. 58 (2003) 583–587.

[28] R. Ferro, J.A. Rodriguez, P. Bertrand, Peculiarities of nitrogen dioxidedetection with sprayed undoped and indium-doped zinc oxide thin films,Thin Solid Films 516 (2008) 2225–2230.

[29] R.W.J. Scott, S.M. Yang, G. Chabanis, N. Coombs, D.E. Williams,G.A. Ozin, Tin dioxide opals and inverted opals: near-ideal microstruc-tures for gas sensors, Adv. Mater. 13 (2001) 1468–1472.

[30] H. Ogawa, M. Nishikawa, A. Abe, Hall measurement studies and anelectrical conduction model of tin oxide ultrafine particle films, J. Appl.Phys. 53 (1982) 4448–4455.

[31] N. Yamazoe, Toward innovations of gas sensor technology, Sens.Actuators B 108 (2005) 2–14.