ARTICLE pubs.acs.org/JPCC
Junction-Tuned SnO2 Nanowires and Their Sensing Properties Jae Young Park, Sun-Woo Choi, and Sang Sub Kim* School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea ABSTRACT: On the basis of a selective growth of SnO2 nanowires by the vaporliquidsolid growth method, networked SnO2 nanowire sensors were fabricated. Then, their sensing properties were systematically investigated in terms of NO2. The density of junctions was controlled by altering the spacing of the patterned-interdigital electrodes (PIEs), on which SnO2 nanowires selectively grew and touched the nanowires grown on neighboring PIEs, eventually producing junctions. The sensing mechanism was attributed to the change not only in the width of the space charge region along the length direction of each nanowire but also in the height of the built-in potential at the junctions during adsorption and desorption of gaseous species. Narrower spacings of PIEs led to an increasing the density of junctions projected to the plane and, consequently, superior properties for gas sensing. Importantly, a general principle to prepare networked nanowires of superior sensing capabilities was suggested from the point of view of nanowire shape and electrode configuration.
1. INTRODUCTION In recent years, various one-dimensional (1D) oxide nanomaterials in the forms of nanorods, nanowires, nanofibers, and nanotubes have attracted great attention because of fundamental scientific interest in the nanomaterials and also due to their potential applications in a variety of functional devices.14 Particularly, for the chemical sensor application, 1D nanomaterials of metal oxides usually show better performance compared with their thin-film or bulk counterparts because of their enhanced surface-to-volume ratio, which allows for very sensitive transduction of the gas/surface interaction into a change in electrical conductivity.5 SnO2 is a well-known oxide semiconducting material that can be used for sensors which detect oxidizing or reducing gaseous species due to its high sensitivity and stability at a relatively low operating temperature.6 Therefore, many research efforts have been made to develop a novel method of sensor fabrication based on 1D SnO2 nanomaterials to improve sensitivity, reliability, and response time.710 Although chemical sensors using a single nanowire are very promising for fast detection and high sensitivity,1113 its practical application still has considerable challenges due to the following hurdles: first, the fabrication of chemical sensors based on a single nanowire usually requires an extremely careful lithography process, which demands a series of fabrication steps which are expensive and tedious; second, the current change in a single nanowire during the interaction process of gaseous species on its surface is infinitesimal, consequently necessitating an expensive measurement system; third, the measured current values generally show a significant variation even in the same sample due to not only slightly different sizes and morphologies of each nanowire but also due to the different natures of the electrical contacts. These problems strongly indicate that securing reliable chemical sensors based on a single nanowire is difficult. In more recent years, two types of approaches have been attempted to overcome the shortcomings of single nanowire gas r 2011 American Chemical Society
sensors. One approach is the use of vertically aligned nanowire arrays,1419 in which metal electrodes were deposited either on the top or on the bottom of nanowire arrays, consequently leading to the participation of multiple nanowires in the sensing process. This circumvents the shortcomings caused by the use of a single nanowire. A literature survey reveals that hydrothermally grown, vertically aligned ZnO nanotube arrays were used for NO2 sensors at room temperature.14 In addition, vertically aligned ZnO,15,16 TiO2,17 SnO2,18 and Fe2O319 nanowire arrays were fabricated as sensors to detect various gaseous species present at ppm levels. The other approach is to use randomly networked nanowires, in which multiple nanowires are also involved in the sensing process.2025 In this case, the change in resistance during the adsorption and desorption of gaseous species is likely to be caused by the alteration both in the width of the surface depletion layer of each nanowire and in the height of the potential barriers built in the junctions of the networked nanowires. These 2-fold effects may be used together to facilitate a faster response and superior sensitivity to a certain chemical species. Accordingly, some research groups have launched in order to fabricate sensors based on randomly networked nanowires.2025 For instance, electrospinning-synthesized networked SnO2 and TiO2 nanofibers were used as sensors for ethanol,20 toluene,21 and NO2.22 Networked SnO2 and ZnO nanowires grown by a vapor-phase growth method exhibited good responses to hydrogen23 and NO2,24 respectively. Chemical gas-sensing properties of networked nanowires are likely to be associated with the nature of networking, particularly with the density of junctions. However, as far as we know, no study has addressed the effects of the density of junctions in Received: March 5, 2011 Revised: June 2, 2011 Published: June 06, 2011 12774
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Figure 1. Schematic illustrating the typical fabrication process of networked SnO2 nanowire sensors used in this study.
networked nanowires on their sensing properties. In the present work, we first controlled the density of junctions in networked SnO2 nanowires and investigated the resulting effects on the sensing properties with regard to NO2. Then we suggest a general principle to prepare networked nanowires of outstanding sensing properties.
2. EXPERIMENTAL SECTION First, prior to growing SnO2 nanowires, patterned-interdigital electrodes (PIEs) were made on SiO2-grown Si (100) substrates using a conventional photoilothographic process. This includes depositing a photoresit (PR) layer by a spinning method, exposing the PR layer to ultraviolet light using a photomask of an interdigital electrode pattern, removing the exposed area and depositing trilayers of Au (3 nm)/Pt (100 nm)/Ni (50 nm) in sequence by a sputtering method, and finally removing the remaining PR layer by a lift-off process. In particular, to improve the adhesion between the SiO2 and the Pt layers, a 50-nm-Ni layer was inserted between them. Networked SnO2 nanowires were synthesized by the wellknown vaporliquidsolid (VLS) growth method. The substrates on which PIEs had been made were introduced into a horizontal quartz tube furnace, in which an alumina crucible containing Sn powders (Aldrich, 99.9%) was placed. Then the furnace was evacuated by a rotary pump down to a pressure of 1 103 Torr and heated up to 900 °C for 10 min. During the VLS growth of SnO2 nanowires, N2 and O2 flowed through the quartz tube at rates of 300 and 10 standard cubic centimeter per minute (sccm), respectively. SnO2 nanowires selectively grown on PIEs tangled with each other between the electrode spacing, consequently producing the junctions as schematically shown in Figure 1. We
have deliberately designed PIEs of different electrode spacings of 10, 20, 50, 70, and 100 μm. The number of junctions in the networked SnO2 nanowires grown on the PIEs with different spacings has been investigated to understand the nanowire’s sensing properties with regard to NO2. The procedure of fabricating the networked SnO2 nanowire sensors studied in this work is provided in Figure 1. The microstructure and phase of networked SnO2 nanowires were investigated using field-emission scanning electron microscopy (FE-SEM, Hitachi/S-4200), high resolution transmission electron microscopy (HR-TEM, Jeol/JEM-2100F), and X-ray diffraction (XRD, Philips/X’pert MPD). Photoluminescence (PL) spectra were obtained at room temperature via a 325-nm HeCd laser as an excitation source. For the purpose of confirming the formation of ohmic contacts between the electrodes and the SnO2 nanowires, currentvoltage (IV) measurements were carried out as a function of O2 pressure, while the fabricated sensors were in a vacuum chamber. The base pressure of the vacuum chamber, which was connected to a turbomolecular pump, was typically ∼5 106 Torr. The response of the networked SnO2 nanowire sensors to NO2 was measured using a homemade gas dilution and sensing system. The sensitivity (S) was estimated according to the following formula: S = Rg/Ra for NO2, where Ra is the resistance in the absence of NO2 and Rg is the resistance in the presence of NO2.
3. RESULTS AND DISCUSSION SnO2 nanowires were grown selectively on PIEs thanks to the Au catalytic layer. Figure 2a shows a plan-view low-magnified FESEM image taken from the sample of 20-um-spacing PIEs, which surely indicates a selective growth of SnO2 nanowires. The inset 12775
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Figure 2. Typical FE-SEM images of networked SnO2 nanowires with 20-um-spacing PIEs. (a) Low-magnification, (b) high-magnification, and (c) cross-sectional images. The inset in (a) shows an enlarged image taken from a single SnO2 nanowire.
shows an enlarged image of a single SnO2 nanowire, which is about 80 nm in diameter. A magnified image taken from the spacing area is shown in Figure 2b. Importantly, individual SnO2 nanowires grown on neighboring PIEs tangle with each other and thus eventually form junctions. To confirm the formation of junctions more clearly, a cross-sectional view is shown in Figure 2c. Evidently, SnO2 nanowires exist densely on the PIEs and they form junctions in the middle of the spacing between two PIEs. The microstructure and phase of VLS-grown SnO2 nanowires on the Au catalytic layer were investigated. Figure 3a shows a high-resolution lattice image taken from a single SnO2 nanowire, revealing its defect-free, single-crystalline nature without any noticeable dislocations or stacking faults. The spacing of 0.47 nm is in a good agreement with the lattice constant of SnO2 along the [001] direction. A selected area electron diffraction pattern obtained from a region of the nanowire, shown in the inset of Figure 3a, is a clear spotty pattern, again confirming its single crystallinity. Figure 3b shows a typical XRD pattern of SnO2 nanowires. All the peaks can be indexed as a tetragonal rutile structure of SnO2 with lattice constants of a = 0.473 nm and c = 0.318 nm (JCPDS card, No 88-0287). A typical PL spectrum
Figure 3. (a) High-resolution TEM image taken from a single SnO2 nanowire. The inset is an SAED pattern, indicating the single-crystalline nature of the nanowire. (b) XRD θ-2θ pattern of SnO2 nanowires. (c) PL spectrum obtained at room temperature from SnO2 nanowires.
of SnO2 nanowires measured at room temperature upon photoexicitation at a wavelength of 325 nm is presented in Figure 3c. The strong yellow emission at 2.12 eV is attributed to the electron transition at the defect levels, such as O vacancies and Sn interstitials to the band gap. This strong yellow emission is often observed for SnO2 nanowires grown by various methods.25 All these results suggest that the VLS-grown SnO2 nanowires in this work were of high crystallinity. Figure 4 illustrates the cross-sectional FE-SEM images of the networked SnO2 nanowires synthesized on PIEs with different spacings of 10, 20, 50, 70, and 100 um. It is evident in Figure 4a that the SnO2 nanowires grown on 10-μm-spacing PIEs contact each other and form junctions between the neighboring PIEs. Importantly, nanowires whose growth angles are greater than a critical angle (θc) do not touch those grown on the neighboring PIEs. In contrast, nanowires whose growth angles are less than θc are likely to touch those on the neighboring PIEs, eventually 12776
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Figure 4. Cross-sectional FE-SEM images of the networked SnO2 nanowires synthesized on PIEs with different spacings of (a) 10, (b) 20, (c) 50, (d) 70, and (e) 100 um. The white arrows indicate the critical angles of junctions.
Figure 5. Geometry of junctions in networked SnO2 nanowire sensors.
producing the junctions. The critical angle increases as the PIE spacing decreases. For 10 min, which was the growth time of SnO2 nanowires in this work, the nanowires grown on 100-μm-spacing PIEs do not touch those grown on neighboring PIEs, indicating that no junctions form, as shown in Figure 4e.
The nature of networking highly influences the capability of detecting gaseous species. In this work, we propose a parameter to quantify the nature of networking. For the parameter, the density of junctions projected to the plane (F) generated in the networked SnO2 nanowires was calculated based on the following reasonable assumptions. First, the diameter (Dnw) of each SnO2 nanowire was assumed to be identical as 80 nm, which was obtained from observation of the microstructure, although there were slight differences in the diameter of each nanowire. Second, only SnO2 nanowrires grown at angles less than θc were assumed to form the junctions with the nanowires grown at the same angles on neighboring PIEs. This assumption is also reasonable because the measured values of θc from the microstructure observations, denoted as the white arrows in Figure 4, are similar to the theoretical ones of θc. Third, it was assumed that SnO2 nanowires grew in a close-packed manner on PIEs. The geometry of a junction based on these assumptions is schematically represented in Figure 5. Then, F can be calculated using the following equation F ¼ ðπ=2ÞðDE =Dnw Þr
ð1Þ
where DE is the diameter of electrode (in this work DE was 20 μm), Dnw is the diameter of nanowires (80 nm), and r is the ratio of θc to 90°, that is, θc/90°. From the geometrical consideration in Figure 5, the relationship of S = 2Lnw cos θc between the electrode 12777
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Table 1. Summary of the Critical Angle (θc) and the Density of Junctions Projected to the Plane (G) in Relation to the Electrode Spacing (S) electrode spacing (S, μm) critical angle (θc, deg) density of junctions (F) 10
72
157
20
66
144
50
57
124
70
25
55
100
0
0
Figure 7. (a) Dynamic response of the networked SnO2 nanowire sensor with 50-um-spacing PIEs to NO2. (b) Sensitivity as a function of NO2 concentration measured at 150400 °C.
fixed temperature26,27 σ ¼ APO2 1=m Figure 6. (a) IV behavior of a networked SnO2 sensor with 50-μmspacing PIEs, measured at different O2 concentrations. (b) Conductivity as a function of O2 concentration.
spacing (S) and the critical angle (θc) is satisfied. With these relationships, F is finally expressed as F ¼ ðπ=2ÞðDE =Dnw Þ cos1 fS=ð2LnwÞ g=90°
ð2Þ
In the specific growth condition used in this work, θc and F in relation to S are summarized in Table 1. The 10-μm-spacing PIE forms 157 junctions. This gradually decreases as the spacing of the PIEs increases. The effects of the density of junctions in the networked SnO2 nanowires on sensing properties were investigated. Figure 6a shows the results of currentvoltage (IV) measurement at various oxygen pressures for the networked SnO2 nanowire sensor with 50-μm-spacing PIEs. The slopes representing the conductivity are linear over the measured range of oxygen pressures, indicating that ohmic contacts were successfully formed between the nanowires and the electrode layers. The electrical conductivity of oxide semiconductors generally exhibits a strong dependence on O2 pressure, following the following relationship at a
ð3Þ
where σ is the electrical conductivity, A is the proportional coefficient, PO2 is the pressure of O2, and m is the exponent. The value of m depends on the type of dominant defects generated in the oxide semiconductors at a particular temperature, consequently revealing the conductivity mechanism operating in the nanowires. Figure 6b is the plot of log σ vs log PO2 for the sensor. It exhibits a linear relationship. The value of 1/m is 1.397, indicating that the n-type conduction behavior operates in the sensor during adsorption and desorption of O2. Very similar IV results were obtained for the sensors with different PIEs spacings, which are not presented here to avoid repetition. Sensing properties of the networked SnO2 sensors with regard to NO2 were systematically investigated. Figure 7a shows the change in resistance as a function of NO2 concentration ranging from 1 to 70 ppm at 300 °C for the sensor fabricated from the networked SnO2 nanowires with 50-μm-spacing PIEs. The sensor response clearly tracks NO2. The resistance of the sensor increases upon exposure to NO2, whereas it decreases upon the removal of NO2. It is noteworthy that the sensor responses were stable and reproducible for several testing cycles. It is well-known that NO2 molecules adsorbed on the surface of networked SnO2 nanowires extract electrons from them, eventually leading to the surface depletion of each SnO2 nanowire. On the other hand, the 12778
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Figure 9. (a) Schematic illustrating a sensing mechanism of a networked nanowire sensor to NO2. (b) Built-in potential as a function of temperature at an NO2 concentration of 1 ppm. (c) Schematic illustrating the equivalent circuit of the total resistance of one networked nanowire.
Figure 8. (a) Variation of sensitivity as a function of NO2 concentration for the sensors with different electrode spacings. (b) Sensitivity and response time as a function of the density of junctions, measured at 50 ppm NO2.
release of electrons occurs upon desorption of NO2 molecules. This charge transfer accounts for the resistance change observed in the sensor. The sensitivity of the sensor has been investigated as a function of temperature under various NO2 concentrations and the results are summarized in Figure 7b. At a given NO2 concentration, the sensitivity increases as the temperature increases until 300 °C, and then it tends to decrease with further temperature increases until 400 °C. The temperature showing the best sensitivity for each NO2 concentration is around 300 °C. This is most likely related to the change in the chemical reaction of adsorbed/desorbed NO2 molecules on the surface of SnO2 nanowires at that temperature. The specific mechanism for the chemical reaction is not clear at present; however, this is similar to the results in the literature.28,29 For the purpose of investigating the effects of the density of junctions on the sensing properties with regard to NO2, the
responses of the sensors with different PIE spacings at 300 °C were measured. The results are summarized in Figure 8. As the spacing increases, the sensitivity tends to decrease. For a NO2 concentration of 50 ppm, denoted as the dotted line in Figure 8a, the sensing properties, such as sensitivity, response, and recovery times, are plotted as a function of the number of junctions (N) in Figure 8b. Clearly, superior sensing properties are obtained with a larger number of junctions. It is also of note that the response time obtained from the sensor from the 10-um-spacing PIEs is much shorter than those of previously reported gas sensors based on SnO2 and In2O3SnO2 thin films,30 nanostructured SnO2 films,31 and SnO2 nanobelts and nanowires.32,33 Two factors usually determine the sensing properties of networked nanowire sensors.34,35 One is the change in width of the depletion layer along the length direction of each nanowire. This is the only mechanism operating in single nanowire sensors. The other mechanism exists in the networked nanowire sensors in which the junctions are present. A change in the built-inpotential around the junctions of networked nanowires arises during adsorption and desorption. These two types of change in resistance when NO2 molecules adsorb on a networked SnO2 nanowire are schematically described in Figure 9a, in which the 12779
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surface depletion layer and the potential barrier built in the networked junction are described. The height of the built-in potential at a particular concentration of NO2 can be estimated by plotting ln σ vs 1/T, assuming that the surface depletion is insensitive to change in temperature since the built-in potential is usually described as σ ¼ σ 0 expð eVs =kTÞ
ð4Þ
where σ0 is a constant that depends on the kind of materials, contact area, image force, and mobility and k and T are the Boltzmann constant and absolute temperature, respectively. eVs is the built-in potential. To estimate the built-in potential, the conductivity of the sensor with the 50-μm-spacing PIEs was measured as a function of temperature at an NO2 concentration of 1 ppm. The result is summarized in Figure 9b. From the slope, the height of the built-in potential at that NO2 concentration is estimated to be 0.81 eV. This potential barrier plays the role of a resistor, which is connected in series with the resistance component of the nanowire itself. The total resistance of one networked nanowire can be expressed by a series resistance and its equivalent circuit is schematically described in Figure 9c. According to our results, the density of junctions greatly influences the sensing properties of networked SnO2 nanowire sensors. As the density of junctions increases, superior sensing properties were shown. This is reasonable because a large ensemble of networked SnO2 nanowires may give better sensing properties. Equation 2 gives us a general principle to prepare networked nanowires of outstanding sensing properties. One approach to obtain a bigger F value is associated with control of the shape of nanowires. Slender and long nanowires are more favorable. The other approach is to control the configuration of catalytic electrode layers, that is, PIEs. PIEs with a narrow distance and large pad area are more favorable. In spite of the outstanding detection ability of single nanowire sensors, the fabrication of such devices hinders their practical applications due to a fortuitous process to obtain devices connected with single nanowires. For cost-conscious chemical sensing applications, it is of utmost importance to develop a process involving minimal fabrication steps and yet produces high quality devices with a high yield. Our approach may solve the above-mentioned problems, boasting advantages such as simplified fabrication and enhanced sensitivity.
4. CONCLUSIONS By use of the selective growth of SnO2 nanowires and their entanglement, multiple-networked SnO2 nanowire sensors have been successfully fabricated. The density of junctions was deliberately controlled via changing the PIE spacing. We found that the density of junctions greatly influenced the sensing properties including sensitivity, response, and recovery times. As the PIE spacings decreased, the density of junctions increased, consequently revealing superior sensing properties with regards to NO2. In addition, we suggested a general principle of how to prepare networked nanowires of superior sensing capabilities from the point of view of nanowire shape and electrode configuration. The novel method for fabricating chemical gas sensors using vaporphase grown nanowires in this work may circumvent the drawbacks of single nanowire chemical sensors.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Nuclear R&D program through the National Research Foundation of Korea. ’ REFERENCES (1) Tian, B.; Zheng, X.; Kemp, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885–890. (2) Henini, M. Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics; Elsevier Publication: Amsterdam, the Netherlands, 2008. (3) Nam, S. W.; Jiang, X.; Xiong, Q.; Ham, D.; Lieber, C. M. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21035–21038. (4) Ahmad, U. Metal Oxide Nanostructures and Their Applications; American Scientific Publication: Valencia, California, USA, 2010. (5) Vander, Wal, R. L.; Hunter, G.; W.; Xu, J. C.; Kulis, M. J.; Berger, G. M.; Ticich, T. M. Sens. Actuators B 2009, 138, 113–119. (6) Hagen, W.; Lambrich, R. E.; Jagois, J. Adv. Solid State Phys. 1983, 23, 259–274. (7) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z. W.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 1869–1871. (8) Kolmakov, A.; Moskovits, M. Annu. Rev. Mater. Res. 2004, 34, 151–180. (9) Comini, E.; Baratto, C.; Faglia, G.; Ferroni, M.; Vomiero, A.; Sberveglieri, G. Prog. Mater. Sci. 2009, 54, 1–67. (10) Kolmakov, A. Int. J. Nanotechnol. 2008, 5, 450–474. (11) Fan, Z.; Lu, J. G. Appl. Phys. Lett. 2005, 86, 123510–123512. (12) Wang, W.; Xiong, H. D.; Edelstein, M. D.; Gundlach, D.; Suehle, J. S.; Richter, C. A. J. Appl. Phys. 2007, 101, 044313–044317. (13) Andrei, P.; Fields, L. L.; Zheng, J. P.; Cheng, Y.; Xiong, P. Sens. Actuators B 2007, 128, 226–234. (14) Wang, J. X.; Sun, X. W.; Yang, Y.; Wu, C. M. Nanotechnology 2009, 20, 465501–465504. (15) Park, J. Y.; Song, D. E.; Kim, S. S. Nanotechnology 2008, 19, 105503–105507. (16) Park, J. Y.; Kim, S. S. Nanoscale Res. Lett. 2010, 5, 353–359. (17) Francioso, L.; Taurino, A. M.; Forleo, A.; Siciliano, P. Sens. Actuators B 2008, 130, 70–76. (18) Huang, H.; Lee, Y. C.; Chow, C. L.; Tan, O. K.; Tse, M. S.; Guo, J.; White, T. Sens. Actuators B 2009, 138, 201–206. (19) Hwang, S. O.; Kim, C. H.; Myung, Y.; Park, S.-H.; Park, J.; Kim, J.; Han, C.-S.; Kim, J.-Y. J. Phys. Chem. C 2008, 112, 13911–13916. (20) Zhang, Y.; He, X.; Li, J.; Miao, Z.; Huang, F. Sens. Actuaors. B 2008, 132, 67–73. (21) Qi, Q.; Zhang, T.; Liu, L.; Zheng, X. Sens. Actuators B 2009, 137, 471–475. (22) Kim, I.-D.; Rothschild, A.; Lee, B. H.; Kim, D. Y.; Jo, S. M.; Tuller, H. L. Nano Lett. 2006, 6, 2009–2013. (23) Wang, B.; Zhu, L. F.; Yang, Y. H.; Xu, N. S.; Yang, G. W. J. Phys. Chem. C 2008, 112, 6643–6647. (24) Ahn, M.-W.; Park, K.-S.; Heo, J.-H.; Park, J.-G.; Kim, D.-W.; Choi, K. J.; Lee, J.-H.; Hong, S.-H. Appl. Phys. Lett. 2008, 93, 263103– 263105. (25) Agekyan, V. T. Phys. Stat. Solidi. (a) 1977, 43, 11–42. (26) Moseley, P. T.; Crocker, A. J. Sensor Materials; IOP Publishing: Bristol, U.K., 1996. (27) Moseley, P. T. Sens. Actuators B 1992, 6, 149–156. (28) Ihokura, K.; Watson, J. The stannic Oxide Gas Sensor: principle and application; CRC Press: Boca Raton, Florida, USA, 1994. (29) Madou, M. J.; Morrison, S. R. Chemical Sensing with Solid State Devices; Academic Press: NewYork, USA, 1988. 12780
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