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Nanojunction effects in multiple ZnO nanowire gas sensor
Sensors and Actuators B 150 (2010) 389–393
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanojunction effects in multiple ZnO nanowire gas sensor R. Khan a , H.-W. Ra a , J.T. Kim a , W.S. Jang b , D. Sharma a , Y.H. Im a,∗ a b
School of Semiconductor and Chemical Engineering and Technology, Chonbuk National University, DeokJinDong 1Ga 664-14, Jeonju 561-756, Republic of Korea Convergence Plasma Research Center, National Fusion Research Institute, Gwahangno 113, Yusung-gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 September 2009 Received in revised form 7 June 2010 Accepted 26 June 2010 Available online 27 July 2010 Keywords: Semiconductor oxide Multiple nanowire Nanojunction Gas sensor
a b s t r a c t The effects of nanojunctions in multiple ZnO nanowire sensors were investigated in terms of the electrical properties, the sensor response, and the rate characteristic of sensor response based on a comparison against single nanowire sensors. Due to the effects of nanojunctions, the multiple nanowire devices showed reduced gating effects as an electrical characteristic and an enhanced sensor response in hydrogen detection as compared to single nanowire device. On the other hand, it was found that the sensor response rate in multiple nanowires was affected by gas diffusion inside the nanojunctions. The effect of the nanojunctions on the rate characteristics of the sensor response was quantified from simplified kineticdiffusion modeling based on the kinetic information of the single nanowire. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting oxide sensors have been widely used as gas sensor material because of its low cost, high sensitivity, fast response and relative simplicity [1,2]. These sensors are based on a change in the equilibrium state of the surface oxygen reaction under reducing gases (CO, H2 , CH4 , etc.), which leads to a change in the resistance of the gas sensing materials. Among the metal oxides based gas sensors, ZnO has great potential in gas sensing applications because of its unique properties such as the wide band gap (3.37 eV), high mobility of conduction electrons and excellent thermal and chemical stability under the operating conditions [2–6]. Additionally, ZnO is one of the most prolific materials for the production of various nanostructures such as nanowires, nanobelts, nanosheets and other complex nanostructures [7,8]. Recently, a great deal of interest has been focused toward using one-dimensional ZnO nanostructures such as nanowires and nanobelts, as gas sensors because of their high surface-to-volume ratio and crystallinity [8]. Many reports have demonstrated the advantage of ZnO nanowire sensors in the field of chemical/gas sensors [2,9]. However, the large-scale fabrication process of these sensors still suffers from the inherent drawback of nanowire alignment. To address the issue, there have been recent developments in alternative nanowire based gas sensors including nanowire thin films and nanowires with nanojunctions between two electrodes [10–13]. Furthermore, previous reports showed that the sensitivities of these sensors could be improved compared to a single metal oxide nanowire sensor because of the potential barrier modulation
∗ Corresponding author. Tel.: +82 063 270 2434; fax: +82 063 270 2306. E-mail address: [email protected] (Y.H. Im). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.052
between nanowires. For example, Zhang et al. demonstrated lower detection limit of NO2 down to 5 parts per billion (ppb) using multiple metal oxide nanowire when the single nanowire had a detection limit of 20 ppb [12]. Recently, the ZnO nanowire-bridge based gas sensor showed higher response compared with different types of sensors including ZnO nanocrystals, metal doped ZnO thin film, or ZnO nanowires [13]. These phenomena could be explained by the contribution of the nanojunctions between nanowires, which is similar to the grain boundary modulations in thin film gas sensors [1,14]. Although these approaches based on the nanojunctions can take advantage on the sensor response, gas diffusion inside the nanojunctions need to be investigated because of one of key parameters to determine sensor performance such as sensor response rate. However, the systematic research for this effect has not yet reported for multiple nanowire gas sensors using nanojunctions between nanowires. In this work, systematic studies were carried out on the role of nanojunctions between nanowires in hydrogen detection through a comparison between single and multiple ZnO nanowire gas sensors. These sensors were fabricated using conventional bottom-up methods and showed different sensor responses and response rates for hydrogen gas. The sensing characteristics for the single nanowire were used as a reference to better understand the characteristics of the multiple nanowires. Finally, simplified kinetic-diffusion modeling was performed to provide better insight into the role of the nanojunctions for the sensor response rate. 2. Experimental details The ZnO nanowires were synthesized on a Si substrate by simple thermal evaporation of Zn powders at 900 ◦ C for 30 min in a horizontal tube furnace, as reported previously [15]. To fabricate
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the ZnO nanowire gas sensors, the as-synthesized ZnO nanowires of 100 mg were uniformly dispersed in an ethanol solution of 50 mL using the ultra sonication method (30 s) and then transferred on to doped Si (1 0 0) wafers with a 300 nm thick layer of thermal oxide. Then using the dielectrophoresis (DEP) process, the ZnO nanowires were aligned between two Ti (20 nm) and Au (20 nm) electrode patterned with the conventional lift off technique. During alignment, a drop of the ZnO nanowire solution of 2 L was dropped onto the defined electrodes and an alternating voltage (20 Vpp , 200 kHz) was applied to the electrodes for the DEP process. For the comparison purposes, the single and multiple nanowire devices were obtained by controlling the DEP force. Finally, a top contact electrode of Ti (20 nm) and Au (20) was deposited on the substrate along with the aligned nanowire for better contact. The distance between the electrodes and diameter of the single and multiple ZnO nanowires were about 10 m and 100–150 nm, respectively. A HP 4145B semiconductor parameter analyzer was used for electrical characterization of the single and multiple ZnO nanowire devices at room temperature. Gas sensing was carried out with a home-built apparatus consisting of a vacuum chamber, gas feeding system and data acquisition system as described in a previous study [6]. Hydrogen sensing of the single and multiple ZnO nanowire gas sensors was measured at 200 ◦ C and atmospheric pressure, with ultrahigh purity nitrogen (99.999%) as the carrier gas. In this work, the sensor response (S) was defined as the rate of the resistance change, S = RH2 /RAir , where RAir and RH2 are the resistances in ambient air and hydrogen gas, respectively. 3. Result and discussions Fig. 1(a) represents a typical scanning electron microscopy (SEM) image of the as-synthesized ZnO nanowires with diameters ranging from 100 to 150 nm. The crystallinity of the ZnO nanowires was characterized by X-ray diffraction (XRD) pattern measured with Cu-K␣ radiation. As shown in Fig. 1(b), the obtained peaks were well matched with the diffraction peak indexed for the wurzite phase of ZnO (JCPDS, 36-1451). Fig. 2(a) and (b) show typical SEM images of the single and multiple ZnO nanowire devices,
Fig. 1. (a) SEM image and (b) XRD pattern of the as-synthesized ZnO nanowires with diameters in the range of 100–150 nm.
Fig. 2. SEM images (a, b) and I–V curves (c, d) of the single and multiple ZnO nanowire FETs, respectively.
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gas sensors. Similar issues could also exist in the nanojunctions of the multiple nanowires as shown in Fig. 4(b). To address this issue, simple modeling was performed to analyze the correlation between the surface kinetics and the internal diffusion of the gas molecules into nanojunctions. In general, the gas sensing mechanism of a metal oxide can be explained by the charge transfer during the adsorption and desorption of an oxygen species at an available surface adsorption site (*). 1 g O2 + ∗ + e − ↔ O − 2
(1)
The indices g and s correspond to the gas phase and surface, respectively. For simplicity, O− was assumed to be the dominant species for oxygen adsorption [16]. As discussed in previous studies, an assumption was made that stated that the rate equation followed only first-order kinetics in terms of the partial pressure of oxygen [16–18]. Fig. 3. The typical response curves of the single and multiple ZnO nanowire for variations in the hydrogen concentration ranging from 50 to 100 ppm at 200 ◦ C. The voltage applied to both the individual nanowire and multiple nanowires was 1.0 V without gate voltage.
where the single and multiple ZnO nanowires bridge between the two electrodes, respectively. For comparison purposes, the distance between the electrodes and the diameter of the ZnO nanowires were almost the same for both devices. Fig. 2(c) and (d) exhibited the typical electrical characteristics for the single and multiple ZnO nanowire field effect transistors (FETs), respectively. The typical ntype semiconductor characteristics were confirmed in both cases by the pronounced back gate effect at source–drain voltage (VDS ) ranging from 0.2 to 1.0 V. Due to increased channels, the multiple nanowire devices showed greatly enhanced conductance, which is one order higher than that of single nanowire devices. Fig. 3 shows the gas sensing characteristics as a function of time for the single and multiple ZnO nanowires with various hydrogen concentrations in the range of 50–100 parts per million (ppm) at 200 ◦ C. The sensor response was higher for the multiple ZnO nanowires than the response for the single ZnO nanowire over the entire range of hydrogen concentration. This result matched previously reported works using multiple metal oxide nanowires well [12,13]. Oxygen adsorbed onto the oxide surface through a charge transfer when the ZnO nanowire surface was exposed to air so that the surface depletion region increased along the longitudinal as shown in Fig. 4(a). For ZnO nanowire exposed in a reducing atmosphere such as hydrogen gas, the surface depletion region decreased due to the surface reactions of the hydrogen molecules and the chemisorbed oxygen species on the nanowire surface, which in turn decreased the ZnO nanowire resistance. On the other hand, the multiple nanowires in Fig. 4(b) were composed of a lot of nanojunctions, which acted as a potential barrier for electron flow. The potential barrier decreased as the nanowire was exposed to the reducing gas, resulting in an increase in the current flow. The potential barrier modulation of multiple nanowires was more efficient than the modulation of the surface depletion of the single ZnO nanowire in gas sensing. Therefore, the potential barrier modulation in the nanojunctions caused the enhanced sensor response for the multiple nanowires in Fig. 3. Meanwhile, the results in Fig. 3 also showed the differences between the rate characteristics of the sensor response for the single and multiple nanowires. The rate characteristics of the multiple nanowires have not been studied in detail to date, whereas the sensor response of the multiple wires can be elucidated. As indicated earlier, the sensor response rate should be one of the key factors in commercial gas sensors and is strongly related to gas diffusion inside the grain boundaries and surface reactions in the thin film
d = {kf no pO2 (1 − ) − kr } dt
(2)
In this equation, is the surface coverage of chemisorbed oxygen [O− s ], pO2 is the partial pressure of oxygen, kf and kr are the oxygen adsorption and desorption rate constants, respectively, and no is the total number of electrons available. The “effectiveness factor” represents the ratio of the observed surface kinetics rate, to surface kinetic rate which would occur in the absence of the internal gas diffusion resistance into the nanojunctions. The surface coverage can be obtained from a simple integration of Eq. (2). =
1 (k no pO2 − (kf no pO2 kf no pO2 + kr f − (kf no pO2 + kr )eq )[e−(kf no pO2 +kr )t ] )
(3)
The term eq is the surface coverage of chemisorbed oxygen at the initial equilibrium state. In this modeling, a single crystalline ZnO nanowire has an effectiveness factor of 1.0 because the gas diffusion effect can be ignored. The surface coverage is proportional to the following normalized sensor response. (t) ∝
SMax − S(t) = a(b − c[e−t/a ]) SMax
(4)
Here, Smax is the maximum sensor response at each hydrogen concentration, and a, b and c are fitting parameters based on Eq. (3). Fig. 5(a) shows that this heuristic approach fit the experimental time evolution of sensor response well for a single nanowire sensor with a hydrogen concentration of 100 ppm. Assuming the same surface kinetics (or the same fitting parameters) for both the single and multiple nanowires, the effectiveness factors of the multiple nanowires was obtained by fitting the data using the following equations. (t) ∝
SMax − S(t) = a(b − c[e−t/a ] ) SMax
(5)
This approach fit the data, with an R square value of more than 0.95. Finally, Fig. 5(b) shows the effectiveness factors as a function of the hydrogen concentration. When the multiple nanowires were exposed to hydrogen gas, the effectiveness factor remained at about 1.0 at the entire hydrogen concentrations in our experiment. Therefore, there was no significant gas diffusion resistance in the nanojunctions of the multiple nanowires. In other words, the response time for the multiple ZnO nanowires was close to that of the single nanowire. On the other hand, the significant change in the effectiveness factor was observed when the hydrogen was withdrawn. As shown in Fig. 5(b), the effectiveness factor for the recovery periods of the sensor was less than that of the
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Fig. 4. Schematic diagram of the (a) single and (b) multiple ZnO nanowire gas sensors. The dashed circles in (b) show the nanojunctions between the nanowires where potential barrier modulation and gas diffusion occur.
response periods up to 1000 ppm hydrogen concentrations. This result indicated that there was a significant diffusion resistance in the recovery period because of the low diffusivity of the oxygen molecules. However, these result shows that the diffusion resistance gradually reduced as the hydrogen concentration decreased most likely because less oxygen was required to reach the surface equilibrium in the nanojunctions. In our experiment, the diffusion resistance disappears completely above the hydrogen concen-
tration of 60 ppm. Therefore, the recovery time of the multiple nanowires got close to that of the single ZnO nanowire. Although more work needs to be done for a complete understanding of gas diffusion with a space charge in the nanojunctions, these results indicated that the sensor response rate in hydrogen sensing using multiple nanowires can be explained by the gas diffusion effects in the nanojunctions. 4. Conclusion The nanojunction effects in multiple ZnO nanowire gas sensors were systematically studied. For this work, single and multiple ZnO nanowire gas sensors were fabricated via conventional bottom-up methods. The multiple nanowire FET showed a negative shift in the threshold voltage compared to the single nanowire FET due to the presence of the nanojunctions. This shift indicated reduced gating effects. In hydrogen sensing, the sensor response in the multiple nanowire gas sensors was enhanced because of the potential barrier modulation in the nanojunctions as reported in previous works. A comparison between the single and multiple nanowire gas sensors revealed that the rate characteristics of hydrogen sensor response in the multiple nanowires were subjected to the effects of the gas diffusion resistance inside the nanojunctions, directly affected the recovery time. These results provided useful information for the fabrication of high performance gas sensors. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (3112006-2-D00411). References
Fig. 5. (a) The normalized sensor response fitted by Eq. (4) using the time dependence data (symbols) of the sensor response. The fitting parameters in the response and recovery curve are (a: 0.186, b: −0.207 and c: −5.12) and (a: 0.69, b: 1.52, and c: 1.44), respectively. (b) The effectiveness factors calculated from Eq. (5) as a function of the hydrogen concentration to represent the effects of the gas diffusion resistance.
[1] G. Sberveglieri, Recent developments in semiconducting thin-film gas sensors, Sens. Actuator B Chem. 23 (1995) 103–109. [2] K.J. Choi, H.W. Jang, One-dimensional oxide nanostructures as gas-sensing materials: review and issues, Sensors 10 (2010) 4083. [3] Z.R. Dai, Z.W. Pan, Z.L. Wang, Novel nanostructures of functional oxides synthesized by thermal evaporation, Adv. Funct. Mater. 13 (2003) 9–24. [4] A.M. Morales, C.M. Lieber, A laser ablation method for the synthesis of crystalline semiconductor nanowires, Science 279 (1998) 208–211. [5] A.B. Djurisic, Y.H. Leung, Optical properties of ZnO nanostructures, Small 2 (2006) 944–961. [6] H.W. Ra, K.S. Choi, J.H. Kim, Y.B. Hahn, Y.H. Im, Fabrication of ZnO nanowires using nanoscale spacer lithography for gas sensors, Small 4 (2008) 1105–1109. [7] H.W. Ra, K.S. Choi, Y.B. Hahn, Y.H. Im, Role of the vapor composition in the evolution from 1D to 2D ZnO nanostructures, J. Phys. Chem. C 112 (2008) 17926–17930. [8] L. Schmidet-Mende, J.L. MacManus-Driscoll, ZnO nanostructures, defects and devices, Mater. Today 10 (2007) 40–48. [9] C.S. Rout, S.H. Krishna, S.R.C. Vivekchand, A. Govindara, C.N.R. Rao, Hydrogen and ethanol sensors based on ZnO nanorods, nanowires and nanotubes, Chem. Phys. Lett. 418 (2006) 586–590.
R. Khan et al. / Sensors and Actuators B 150 (2010) 389–393 [10] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties, Appl. Phys. Lett. 76 (2000) 2011–2013. [11] L. Vayssieres, Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions, Adv. Mater. 15 (2003) 464–466. [12] D. Zhang, Z. Lie, C. Li, T. Tang, X. Lie, S. Han, B. Lei, C. Zhou, Detection of NO2 down to ppb levels using individual and multiple In2 O3 nanowire devices, Nano Lett. 4 (2004) 1919–1924. [13] M.W. Ahn, K.S. Park, J.H. Heo, D.W. Kim, K.J. Choi, J.G. Park, On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity, Sens. Actuator B 138 (2009) 168–173. [14] G. Korotcenkov, The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors, Mater. Sci. Eng. R 47 (2008) 1–39. [15] H.W. Ra, K.S. Choi, C.W. Ok, S.Y. Jo, K.H. Bai, Y.H. Im, Ion bombardment effects on ZnO nanowires during plasma treatment, Appl. Phys. Lett. 93 (2008) 0331126. [16] Y. Zhang, A. Kolmakov, S. Chretien, H. Metiu, M. Moskovits, Control of catalytic reactions at the surface of a metal oxide nanowire by manipulating electron density inside it, Nano Lett. 4 (2004) 403–407. [17] Y. Zhang, A. Kolmakov, Y. Lilach, M. Moskovits, Electronic control of chemistry and catalysis at the surface of an individual tin oxide nanowire, J. Phys. Chem. B 109 (2005) 1923–1929. [18] H. Scott Fogler, Elements of Chemical Reaction Engineering, 4th ed., Prentice Hall PTR, New Jersey, 2006.
Biographies Rizwan Khan received his BSc in chemistry and MSc Degree in analytical chemistry in 2003 and 2005, respectively, from AMU India, and now perusing his PhD from Chonbuk National University, Korea. His current research interests are the chemical functionalization of semiconductor oxide nanowire and their chemical and biosensor applications.
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Hyun-Wook Ra received his BS in chemical engineering and MS Degree in the School of Semiconductor Science and Technology in 2003 and 2005, respectively, from Chonbuk National University, Korea, and now perusing his PhD from Chonbuk National University. His current research interests are semiconductor oxide nanowire devices for chemical and biological applications. Jintae Kim received his BS in chemical engineering and MS Degree in the Department of Hydrogen and Fuel Cells Engineering Specialized Graduate School in 2007 and 2009, respectively, from Chonbuk National University, Korea, and now perusing his PhD from Chonbuk National University, South Korea. His current research interests are semiconductor oxide based biosensor and micro fuel systems. Won-Seok Chang received his BS in Applied physics and MS Degree in the Department of Applied Physics Graduate School in 2005 and 2007, respectively, from Hanyang University, Korea, and now working as researcher in National Fusion Research Institute, Korea. His current research interests are plasma-surface reaction kinetics and architecture of database for plasma properties. Deepti Sharma received her BS in genetics (2001) from Dyal Singh College, MS in biochemistry (2004) from Kurukshetra University, and Ph.D degree in biochemistry (2008) from National Dairy Reasearch Institute (NDRI), India. She is now working as a postdoctoral fellow in Chonbuk National University, Korea. Her current research interests are the bimolecular functionalization of semiconductor oxide nanowire, and cell monitoring for biological applications. Yeon Ho Im is an assistant professor in the School of Semiconductor and Chemical Engineering at the Chonbuk National University. He earned his PhD in 2001 (Chonbuk National University, Korea) in the Department of Chemical Engineering. Previously he was a postdoctoral fellow in the Center for Gigascale Integration at Rensselaer Polytechnic Institute and a senior engineer in the Memory Division at SAMSUNG. His current research interests are chemical and biological sensors, and micro fuel systems.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanojunction effects in multiple ZnO nanowire gas sensor R. Khan a , H.-W. Ra a , J.T. Kim a , W.S. Jang b , D. Sharma a , Y.H. Im a,∗ a b
School of Semiconductor and Chemical Engineering and Technology, Chonbuk National University, DeokJinDong 1Ga 664-14, Jeonju 561-756, Republic of Korea Convergence Plasma Research Center, National Fusion Research Institute, Gwahangno 113, Yusung-gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 September 2009 Received in revised form 7 June 2010 Accepted 26 June 2010 Available online 27 July 2010 Keywords: Semiconductor oxide Multiple nanowire Nanojunction Gas sensor
a b s t r a c t The effects of nanojunctions in multiple ZnO nanowire sensors were investigated in terms of the electrical properties, the sensor response, and the rate characteristic of sensor response based on a comparison against single nanowire sensors. Due to the effects of nanojunctions, the multiple nanowire devices showed reduced gating effects as an electrical characteristic and an enhanced sensor response in hydrogen detection as compared to single nanowire device. On the other hand, it was found that the sensor response rate in multiple nanowires was affected by gas diffusion inside the nanojunctions. The effect of the nanojunctions on the rate characteristics of the sensor response was quantified from simplified kineticdiffusion modeling based on the kinetic information of the single nanowire. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting oxide sensors have been widely used as gas sensor material because of its low cost, high sensitivity, fast response and relative simplicity [1,2]. These sensors are based on a change in the equilibrium state of the surface oxygen reaction under reducing gases (CO, H2 , CH4 , etc.), which leads to a change in the resistance of the gas sensing materials. Among the metal oxides based gas sensors, ZnO has great potential in gas sensing applications because of its unique properties such as the wide band gap (3.37 eV), high mobility of conduction electrons and excellent thermal and chemical stability under the operating conditions [2–6]. Additionally, ZnO is one of the most prolific materials for the production of various nanostructures such as nanowires, nanobelts, nanosheets and other complex nanostructures [7,8]. Recently, a great deal of interest has been focused toward using one-dimensional ZnO nanostructures such as nanowires and nanobelts, as gas sensors because of their high surface-to-volume ratio and crystallinity [8]. Many reports have demonstrated the advantage of ZnO nanowire sensors in the field of chemical/gas sensors [2,9]. However, the large-scale fabrication process of these sensors still suffers from the inherent drawback of nanowire alignment. To address the issue, there have been recent developments in alternative nanowire based gas sensors including nanowire thin films and nanowires with nanojunctions between two electrodes [10–13]. Furthermore, previous reports showed that the sensitivities of these sensors could be improved compared to a single metal oxide nanowire sensor because of the potential barrier modulation
∗ Corresponding author. Tel.: +82 063 270 2434; fax: +82 063 270 2306. E-mail address: [email protected] (Y.H. Im). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.052
between nanowires. For example, Zhang et al. demonstrated lower detection limit of NO2 down to 5 parts per billion (ppb) using multiple metal oxide nanowire when the single nanowire had a detection limit of 20 ppb [12]. Recently, the ZnO nanowire-bridge based gas sensor showed higher response compared with different types of sensors including ZnO nanocrystals, metal doped ZnO thin film, or ZnO nanowires [13]. These phenomena could be explained by the contribution of the nanojunctions between nanowires, which is similar to the grain boundary modulations in thin film gas sensors [1,14]. Although these approaches based on the nanojunctions can take advantage on the sensor response, gas diffusion inside the nanojunctions need to be investigated because of one of key parameters to determine sensor performance such as sensor response rate. However, the systematic research for this effect has not yet reported for multiple nanowire gas sensors using nanojunctions between nanowires. In this work, systematic studies were carried out on the role of nanojunctions between nanowires in hydrogen detection through a comparison between single and multiple ZnO nanowire gas sensors. These sensors were fabricated using conventional bottom-up methods and showed different sensor responses and response rates for hydrogen gas. The sensing characteristics for the single nanowire were used as a reference to better understand the characteristics of the multiple nanowires. Finally, simplified kinetic-diffusion modeling was performed to provide better insight into the role of the nanojunctions for the sensor response rate. 2. Experimental details The ZnO nanowires were synthesized on a Si substrate by simple thermal evaporation of Zn powders at 900 ◦ C for 30 min in a horizontal tube furnace, as reported previously [15]. To fabricate
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R. Khan et al. / Sensors and Actuators B 150 (2010) 389–393
the ZnO nanowire gas sensors, the as-synthesized ZnO nanowires of 100 mg were uniformly dispersed in an ethanol solution of 50 mL using the ultra sonication method (30 s) and then transferred on to doped Si (1 0 0) wafers with a 300 nm thick layer of thermal oxide. Then using the dielectrophoresis (DEP) process, the ZnO nanowires were aligned between two Ti (20 nm) and Au (20 nm) electrode patterned with the conventional lift off technique. During alignment, a drop of the ZnO nanowire solution of 2 L was dropped onto the defined electrodes and an alternating voltage (20 Vpp , 200 kHz) was applied to the electrodes for the DEP process. For the comparison purposes, the single and multiple nanowire devices were obtained by controlling the DEP force. Finally, a top contact electrode of Ti (20 nm) and Au (20) was deposited on the substrate along with the aligned nanowire for better contact. The distance between the electrodes and diameter of the single and multiple ZnO nanowires were about 10 m and 100–150 nm, respectively. A HP 4145B semiconductor parameter analyzer was used for electrical characterization of the single and multiple ZnO nanowire devices at room temperature. Gas sensing was carried out with a home-built apparatus consisting of a vacuum chamber, gas feeding system and data acquisition system as described in a previous study [6]. Hydrogen sensing of the single and multiple ZnO nanowire gas sensors was measured at 200 ◦ C and atmospheric pressure, with ultrahigh purity nitrogen (99.999%) as the carrier gas. In this work, the sensor response (S) was defined as the rate of the resistance change, S = RH2 /RAir , where RAir and RH2 are the resistances in ambient air and hydrogen gas, respectively. 3. Result and discussions Fig. 1(a) represents a typical scanning electron microscopy (SEM) image of the as-synthesized ZnO nanowires with diameters ranging from 100 to 150 nm. The crystallinity of the ZnO nanowires was characterized by X-ray diffraction (XRD) pattern measured with Cu-K␣ radiation. As shown in Fig. 1(b), the obtained peaks were well matched with the diffraction peak indexed for the wurzite phase of ZnO (JCPDS, 36-1451). Fig. 2(a) and (b) show typical SEM images of the single and multiple ZnO nanowire devices,
Fig. 1. (a) SEM image and (b) XRD pattern of the as-synthesized ZnO nanowires with diameters in the range of 100–150 nm.
Fig. 2. SEM images (a, b) and I–V curves (c, d) of the single and multiple ZnO nanowire FETs, respectively.
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gas sensors. Similar issues could also exist in the nanojunctions of the multiple nanowires as shown in Fig. 4(b). To address this issue, simple modeling was performed to analyze the correlation between the surface kinetics and the internal diffusion of the gas molecules into nanojunctions. In general, the gas sensing mechanism of a metal oxide can be explained by the charge transfer during the adsorption and desorption of an oxygen species at an available surface adsorption site (*). 1 g O2 + ∗ + e − ↔ O − 2
(1)
The indices g and s correspond to the gas phase and surface, respectively. For simplicity, O− was assumed to be the dominant species for oxygen adsorption [16]. As discussed in previous studies, an assumption was made that stated that the rate equation followed only first-order kinetics in terms of the partial pressure of oxygen [16–18]. Fig. 3. The typical response curves of the single and multiple ZnO nanowire for variations in the hydrogen concentration ranging from 50 to 100 ppm at 200 ◦ C. The voltage applied to both the individual nanowire and multiple nanowires was 1.0 V without gate voltage.
where the single and multiple ZnO nanowires bridge between the two electrodes, respectively. For comparison purposes, the distance between the electrodes and the diameter of the ZnO nanowires were almost the same for both devices. Fig. 2(c) and (d) exhibited the typical electrical characteristics for the single and multiple ZnO nanowire field effect transistors (FETs), respectively. The typical ntype semiconductor characteristics were confirmed in both cases by the pronounced back gate effect at source–drain voltage (VDS ) ranging from 0.2 to 1.0 V. Due to increased channels, the multiple nanowire devices showed greatly enhanced conductance, which is one order higher than that of single nanowire devices. Fig. 3 shows the gas sensing characteristics as a function of time for the single and multiple ZnO nanowires with various hydrogen concentrations in the range of 50–100 parts per million (ppm) at 200 ◦ C. The sensor response was higher for the multiple ZnO nanowires than the response for the single ZnO nanowire over the entire range of hydrogen concentration. This result matched previously reported works using multiple metal oxide nanowires well [12,13]. Oxygen adsorbed onto the oxide surface through a charge transfer when the ZnO nanowire surface was exposed to air so that the surface depletion region increased along the longitudinal as shown in Fig. 4(a). For ZnO nanowire exposed in a reducing atmosphere such as hydrogen gas, the surface depletion region decreased due to the surface reactions of the hydrogen molecules and the chemisorbed oxygen species on the nanowire surface, which in turn decreased the ZnO nanowire resistance. On the other hand, the multiple nanowires in Fig. 4(b) were composed of a lot of nanojunctions, which acted as a potential barrier for electron flow. The potential barrier decreased as the nanowire was exposed to the reducing gas, resulting in an increase in the current flow. The potential barrier modulation of multiple nanowires was more efficient than the modulation of the surface depletion of the single ZnO nanowire in gas sensing. Therefore, the potential barrier modulation in the nanojunctions caused the enhanced sensor response for the multiple nanowires in Fig. 3. Meanwhile, the results in Fig. 3 also showed the differences between the rate characteristics of the sensor response for the single and multiple nanowires. The rate characteristics of the multiple nanowires have not been studied in detail to date, whereas the sensor response of the multiple wires can be elucidated. As indicated earlier, the sensor response rate should be one of the key factors in commercial gas sensors and is strongly related to gas diffusion inside the grain boundaries and surface reactions in the thin film
d = {kf no pO2 (1 − ) − kr } dt
(2)
In this equation, is the surface coverage of chemisorbed oxygen [O− s ], pO2 is the partial pressure of oxygen, kf and kr are the oxygen adsorption and desorption rate constants, respectively, and no is the total number of electrons available. The “effectiveness factor” represents the ratio of the observed surface kinetics rate, to surface kinetic rate which would occur in the absence of the internal gas diffusion resistance into the nanojunctions. The surface coverage can be obtained from a simple integration of Eq. (2). =
1 (k no pO2 − (kf no pO2 kf no pO2 + kr f − (kf no pO2 + kr )eq )[e−(kf no pO2 +kr )t ] )
(3)
The term eq is the surface coverage of chemisorbed oxygen at the initial equilibrium state. In this modeling, a single crystalline ZnO nanowire has an effectiveness factor of 1.0 because the gas diffusion effect can be ignored. The surface coverage is proportional to the following normalized sensor response. (t) ∝
SMax − S(t) = a(b − c[e−t/a ]) SMax
(4)
Here, Smax is the maximum sensor response at each hydrogen concentration, and a, b and c are fitting parameters based on Eq. (3). Fig. 5(a) shows that this heuristic approach fit the experimental time evolution of sensor response well for a single nanowire sensor with a hydrogen concentration of 100 ppm. Assuming the same surface kinetics (or the same fitting parameters) for both the single and multiple nanowires, the effectiveness factors of the multiple nanowires was obtained by fitting the data using the following equations. (t) ∝
SMax − S(t) = a(b − c[e−t/a ] ) SMax
(5)
This approach fit the data, with an R square value of more than 0.95. Finally, Fig. 5(b) shows the effectiveness factors as a function of the hydrogen concentration. When the multiple nanowires were exposed to hydrogen gas, the effectiveness factor remained at about 1.0 at the entire hydrogen concentrations in our experiment. Therefore, there was no significant gas diffusion resistance in the nanojunctions of the multiple nanowires. In other words, the response time for the multiple ZnO nanowires was close to that of the single nanowire. On the other hand, the significant change in the effectiveness factor was observed when the hydrogen was withdrawn. As shown in Fig. 5(b), the effectiveness factor for the recovery periods of the sensor was less than that of the
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Fig. 4. Schematic diagram of the (a) single and (b) multiple ZnO nanowire gas sensors. The dashed circles in (b) show the nanojunctions between the nanowires where potential barrier modulation and gas diffusion occur.
response periods up to 1000 ppm hydrogen concentrations. This result indicated that there was a significant diffusion resistance in the recovery period because of the low diffusivity of the oxygen molecules. However, these result shows that the diffusion resistance gradually reduced as the hydrogen concentration decreased most likely because less oxygen was required to reach the surface equilibrium in the nanojunctions. In our experiment, the diffusion resistance disappears completely above the hydrogen concen-
tration of 60 ppm. Therefore, the recovery time of the multiple nanowires got close to that of the single ZnO nanowire. Although more work needs to be done for a complete understanding of gas diffusion with a space charge in the nanojunctions, these results indicated that the sensor response rate in hydrogen sensing using multiple nanowires can be explained by the gas diffusion effects in the nanojunctions. 4. Conclusion The nanojunction effects in multiple ZnO nanowire gas sensors were systematically studied. For this work, single and multiple ZnO nanowire gas sensors were fabricated via conventional bottom-up methods. The multiple nanowire FET showed a negative shift in the threshold voltage compared to the single nanowire FET due to the presence of the nanojunctions. This shift indicated reduced gating effects. In hydrogen sensing, the sensor response in the multiple nanowire gas sensors was enhanced because of the potential barrier modulation in the nanojunctions as reported in previous works. A comparison between the single and multiple nanowire gas sensors revealed that the rate characteristics of hydrogen sensor response in the multiple nanowires were subjected to the effects of the gas diffusion resistance inside the nanojunctions, directly affected the recovery time. These results provided useful information for the fabrication of high performance gas sensors. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (3112006-2-D00411). References
Fig. 5. (a) The normalized sensor response fitted by Eq. (4) using the time dependence data (symbols) of the sensor response. The fitting parameters in the response and recovery curve are (a: 0.186, b: −0.207 and c: −5.12) and (a: 0.69, b: 1.52, and c: 1.44), respectively. (b) The effectiveness factors calculated from Eq. (5) as a function of the hydrogen concentration to represent the effects of the gas diffusion resistance.
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Biographies Rizwan Khan received his BSc in chemistry and MSc Degree in analytical chemistry in 2003 and 2005, respectively, from AMU India, and now perusing his PhD from Chonbuk National University, Korea. His current research interests are the chemical functionalization of semiconductor oxide nanowire and their chemical and biosensor applications.
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Hyun-Wook Ra received his BS in chemical engineering and MS Degree in the School of Semiconductor Science and Technology in 2003 and 2005, respectively, from Chonbuk National University, Korea, and now perusing his PhD from Chonbuk National University. His current research interests are semiconductor oxide nanowire devices for chemical and biological applications. Jintae Kim received his BS in chemical engineering and MS Degree in the Department of Hydrogen and Fuel Cells Engineering Specialized Graduate School in 2007 and 2009, respectively, from Chonbuk National University, Korea, and now perusing his PhD from Chonbuk National University, South Korea. His current research interests are semiconductor oxide based biosensor and micro fuel systems. Won-Seok Chang received his BS in Applied physics and MS Degree in the Department of Applied Physics Graduate School in 2005 and 2007, respectively, from Hanyang University, Korea, and now working as researcher in National Fusion Research Institute, Korea. His current research interests are plasma-surface reaction kinetics and architecture of database for plasma properties. Deepti Sharma received her BS in genetics (2001) from Dyal Singh College, MS in biochemistry (2004) from Kurukshetra University, and Ph.D degree in biochemistry (2008) from National Dairy Reasearch Institute (NDRI), India. She is now working as a postdoctoral fellow in Chonbuk National University, Korea. Her current research interests are the bimolecular functionalization of semiconductor oxide nanowire, and cell monitoring for biological applications. Yeon Ho Im is an assistant professor in the School of Semiconductor and Chemical Engineering at the Chonbuk National University. He earned his PhD in 2001 (Chonbuk National University, Korea) in the Department of Chemical Engineering. Previously he was a postdoctoral fellow in the Center for Gigascale Integration at Rensselaer Polytechnic Institute and a senior engineer in the Memory Division at SAMSUNG. His current research interests are chemical and biological sensors, and micro fuel systems.