Humidity sensing behavior of silicon nanowires with hexamethyldisilazane modification

Sensors and Actuators B 156 (2011) 631–636

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Humidity sensing behavior of silicon nanowires with hexamethyldisilazane modification Xuejiao Chen, Jian Zhang ∗ , Zhiliang Wang, Qiang Yan, Shichao Hui Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electrical Engineering, East China Normal University, Shanghai 200241, China

a r t i c l e

i n f o

Article history: Received 18 November 2010 Received in revised form 2 February 2011 Accepted 4 February 2011 Available online 3 March 2011 Keywords: Humidity Sensing Behavior Hexamethyldisilazane Silicon Nanowires Hydrophobic

a b s t r a c t In this paper, the sensing behavior of the capacitive humidity sensors based on silicon nanowires with and without hexamethyldisilazane (HMDS) modification has been investigated. The sensing mechanism is based on the capacitance variations due to the adsorption/desorption of water molecules among silicon nanowires. The effect of HMDS modification on the sensor’s performance was discussed. The study indicated that after HMDS treatment, the sensor’s surface turns into hydrophobic and the sensor’s performance such as the linearity, hysteresis and response time can be improved remarkably. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Humidity detection is an attractive topic in our daily life. It is of great importance for industry, agriculture and human activities [1,2]. Currently, the development of humidity sensors is mainly based on either resistive or capacitive detection techniques [3–6]. In order to achieve the enhanced performance, many novel humidity sensing materials have been developed such as organic polymers [7–9], inorganic materials [10,11], nanocomposites [12], microchannels [13], porous silicon [14], multi-walled carbon nanotubes [15]. But some technical problems, mainly including large hysteresis (over 5%) [13,16], long response and recovery time (over 100 s) [12,16,17], incompatibility with traditional IC process, narrow humidity detection range (normally 30%–70%) [14], and auxiliary heating elements [17], etc., have restricted their further application. And most commercially available humidity sensors are incapable of working in constantly high humidity environments [18]. Therefore, it is a challenge to develop an ideal humidity sensing material. Recently, silicon nanowires (SiNWs), as a new humidity sensing material, are attracting more and more attention due to their unique electrical properties and the larger surface-to-volume ratio as compared to bulk and thin film counterparts [19,20]. For silicon nanowires based humidity sensors, the high sensitivity can be gained due to the large surface to volume ratio, i.e. more water molecules adsorbed. The surface of silicon nanowires is normally

∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345203. E-mail address: [email protected] (J. Zhang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.02.009

covered by many Si OH chemical bonds. The existence of these hydroxyls enables the hydrophilic surface, which leads to the long response time and the obvious hysteresis. To solve this problem, the modification of silicon nanowires’ surface is necessary, which can enhance the sensor’s performance. Self-assembled monolayer (SAM), a well-ordered monomolecular layer formed spontaneously by the reaction of certain types of molecules with supporting solid materials, has been widely explored to modulate the surface properties in order to enhance or prevent molecule attachment. Two typical families of SAMs are alkanethiol reacting with gold and organosilane reacting with oxidized surfaces [21]. Compared to alkanethiol SAM formation, which needs a special gold deposition procedure, organosilane SAMs can be directly formed on substrates such as glass and oxide silicon, which is especially applicable for biological studies [22]. In our study, Hexamethyldisilazane (HMDS) was used as selfassembled monolayer to modify the silicon nanowires. HMDS, a commonly used photoresist adhesion promoter in micromachining, was self-assembled on the silicon substrate by vapor phase deposition [23]. With hexamethyldisilazane modification, the SiNWs surface turns into hydrophobic due to the methyl termination of the formed HMDS monolayer [24]. This hydrophobic characteristic can effectively prevent water vapor adsorption, reduce the surface conductivity, increase the stability of charge storage, and improve the sensor’s performance. In our work, the SiNWs were first prepared by wet chemical etching and then treated by HMDS. Next, the humidity sensors were constructed by screen printing interdigitated-type electrodes onto the surface of SiNWs or HMDS-modified SiNWs structure. Finally,

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the humidity sensing behavior of silicon nanowires with and without HMDS modification was investigated.

2. Experimental 2.1. Raw materials Single-polished N-type silicon-wafer (with resistivity ∼10–50  cm) was used as the substrate for SiNWs growth. AgNO3 , HF, HNO3 and HMDS were commercially available from SCRC (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). They are all analytical reagents and used without further purification. DI water (18 M), purified with an ultrapure de-ionized (DI) water system, was used for all experiments.

2.2. Preparation of SiNWs and HMDS treatment The silicon nanowires were fabricated by using a chemical etching procedure [25]. Before application, the silicon chips with dimension of ∼0.5 cm × 0.5 cm were first cleaned by RCA standard process, followed by drying with nitrogen. 20 mL AgNO3 solution (35 mM) and 20 mL HF solution (20%) were respectively prepared, then mixed as the etchant at room temperature. The cleaned silicon chips were immersed in the etchant for ∼60 min to get the silicon nanowires. After the silicon nanowires’ preparation, the samples were rinsed with HNO3 solution and DI-water to remove the surface by-products. Then these samples were annealed at 400 ◦ C for 4 min in order to decrease the influence of interface states and surface defects. Afterwards, the silicon nanowires were treated in HMDS ambient at 120 ◦ C for 20 min. Hexamethyldisilazane reacts with surface hydroxyl groups (Si(substrate) OH) and forms the trimethylsiloxy (Si(substrate) O Si (CH3 )3 ) groups through the silylation reaction. 2 Si OH + (CH3 )3 Si NH Si(CH3 )3 → 2Si O Si(CH3 )3 + NH3 ↑ Si OH bonds turned into Si O Si(CH3 )3 bonds [24].

2.3. Sensor and system for humidity measurement The humidity sensors were constructed with an interdigitatedtype electrode (IDE) on the surfaces of both SiNWs and HMDS-modified SiNWs. Fig. 1(a) shows the schematic diagram of as-fabricated humidity sensor and Fig. 1(b) shows the SiNW’s surface with HMDS modification. The electrodes are fabricated by screen printing a ∼5 ␮m silver paste film with 1 mm electrode gaps, followed by baking at 100 ◦ C for 1 h. For measuring the humidity characteristics of SiNWs and HMDS-modified SiNWs based sensors, the measurement system was set up and shown in Fig. 2. The humidity-controlled environments, 11.3, 23, 43, 57, 68, 75, 85, 93% RH, were achieved using saturated aqueous solution of LiCl, CH3 COOK, K2 CO3 , NaBr, KI, NaCl, KCl, KNO3 at room temperature (∼25 ◦ C) [26]. Different humidity environment leads to different effective dielectric constant, i.e. different capacitance. The 555 multivibrator is used to convert the capacitance changes into the output frequency variations. According to f = 1/T = 1/(R1 + 2R2 )C ln 2, the capacitance variation due to the humidity environment will correspondingly lead to the output frequency (f) change, which can be recorded by home-made LabVIEW program in the computer. The entire setup was maintained at room temperature and in a clean environment.

Fig. 1. (a) Schematic diagram of as-fabricated humidity sensor, and (b) Silicon nanowire’s surface modified with HMDS.

3. Result and discussion 3.1. Characterization of as-fabricated humidity sensor The top view photo of as-fabricated sensor is as shown in Fig. 3(a), and the cross-section SEM image is presented in Fig. 3(b), where both silicon nanowires (in the right side) and metal contacts on the nanowires (in the left side) can be observed. The oriented silicon nanowires with length of ∼55 ␮m have been prepared on the bulk silicon substrate. And the length of silicon nanowires can be adjusted by controlling the proper etching time and temperature. 3.2. Measurement of infrared spectra Infrared absorption spectra of silicon nanowires before and after HMDS treatment were obtained using a Fourier transform infrared (FTIR) spectrometer, as presented in Fig. 4. A C H peak appeared after HMDS treatment at 2960 cm−1 . Consequently, it was confirmed that Si OH bonds were turned into Si O Si(CH3 )3 by HMDS modification [24].

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Fig. 2. Schematic diagram of humidity measurement system for sensors.

3.3. Humidity sensing behavior In this study, the humidity sensing behavior of both SiNWs and HMDS-modified SiNWs based sensors were tested under different relative humidity. The parameters including humidity sensitivity and linearity, repeatability and hysteresis, response time and stability were evaluated. 3.3.1. Sensitivity and linearity Fig. 5(a) presents the relationship between the shifted capacitance and the relative humidity (capacitance-RH) of the samples. The reported data are the mean values obtained from several measure cycles. As the relative humidity level increased, the output capacitance of the samples shifted to be higher monotonically. It is indicated that the adsorbed water can lead to the increased dielectric constant and increase the capacitance. For SiNWs based sensor,

Fig. 4. FTIR spectra of silicon nanowires before and after HMDS treatment.

when the relative humidity level changed from 11.3% RH to 93% RH, the capacitance changed from 1.5 nF to 2.68 nF, i.e. the relative capacitance change, C/C = 78.7 %. But for HMDS-modified SiNWs based sensor, the capacitance only changed from 1.1 nF to 1.28 nF, i.e. the relative capacitance change is only 16.4%. The corresponding capacitance changes tended to be suppressed by HMDS treatment, i.e. after being treated with HMDS, less water molecules were adsorbed onto the surface of silicon nanowires. If the sensitivity is defined as the slope of the capacitance-RH response curves, the values of the sensitivity can be calculated from Fig. 5(a). The response curves are linearly fitted and the results are summarized in Table 1, where R is the correlation coefficient. From the table, it was found that the SiNWs based sensor had the bigger sensitivity value, ∼0.0116 nF/% RH. Although the capacitance-RH curve is not linear in the full RH range from 11.3% to 93%, the curves can be divided into two regions: 11.3%–43% RH and 57%–93% RH. In both regions, the sensor exhibits good linearity. For the sensor with HMDS modification, the sensitivity is lowered and the value is 0.0016 nF/% RH, but the linearity is improved greatly. R is close to 1. Fig. 5(b) is the transformed logarithmic capacitance-RH response curves and Fig. 5(c) is the normalized plot by the formula Table 1 Linear fitting results for the capacitance-RH response curves of samples.

Fig. 3. (a) The top view photo of as-fabricated humidity sensor, and (b) The crosssection SEM image of the electrodes.

SiNWs HMDS-modified SiNWs

C = A + B × RH

R

C = 1.2044 + 0.0116 × RH C = 1.0830 + 0.0016 × RH

0.8837 0.9647

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Fig. 6. The capacitance-RH response curve of SiNWs under humidity cycling of 11.3%–57%–11.3% RH and 11.3%–93%–11.3% RH.

and normalized plot synthetically, both SiNWs and HMDS-modified SiNWs are suitable for humidity detection. 3.3.2. Repeatability and hysteresis To study the humidity repeatability, the samples were put into two fixed humidity level environments repeatedly and the output capacitance values were recorded. The sensor responses under two fixed humidity conditions which corresponded to 11.3%–57%–11.3% RH, and 11.3%–93%–11.3% RH, respectively, were tested and the results were shown in Figs. 6 and 7. From the figures, it can be seen that for both samples, the output frequencies are nearly reversible or repeatable when the samples was submitted to a humidity cycling. For example, for 11.3%–57%–11.3% RH cycling, the output capacitance of SiNWs based sensor changes from ∼1.5183 nF (at 11.3% RH) to ∼1.7171 nF (at 57% RH), and then ∼1.5155 nF (back to 11.3% RH), which is very close to the initial value of the cycle. And for HMDS-modified SiNWs based sensor, the output value changes from ∼1.1208 nF (at 11.3% RH) to ∼1.1754 nF (at 57% RH), and then ∼1.1208 nF (back to 11.3% RH). The result indicated that both of the two samples have good repeatability and good response behavior for humidity sensing. The relatively big humidity hysteresis has been a serious problem in the practical humidity sensors. The hysteresis property of as-fabricated sensors was first tested in the humidity range from

Fig. 5. (a) Experimental capacitance-RH response curves of as-fabricated sensors, (b) the transformed logarithmic capacitance-RH response curves corresponding to the data presented by (a), and (c) the normalized plot of the data presented by (a).

C = (C − C11.3 )/(C93 − C11.3 ), where C and C are the capacitances at a certain measuring RH level before and after transformation, C11.3 and C93 represent the capacitances at 11.3% RH and 93% RH respectively. The relative change in capacitance with humidity as well as linearity can also be observed from Fig. 5(b) and (c). So considering the sensitivity and the linearity of logarithmic response curve

Fig. 7. The capacitance-RH response curve of HMDS-modified SiNWs under humidity cycling of 11.3%–57%–11.3% RH and 11.3%–93%–11.3% RH.

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Fig. 8. The hysteresis property of the samples.

11.3% to 93% RH, the ascending direction, and then from 93% to 11.3% RH, the descending direction. The results are as shown in Fig. 8. From the humidity cycle of low-to-high and high-to-low step, it can be seen that the differences between the ascending and descending curves are large for SiNWs based sensor. Its maximum hysteresis rate Emax = m/YFS × 100%, where m is the maximum hysteresis error and YFS is the full scale output, was 8.1%, corresponding to 11.3% RH. For the HMDS-modified SiNWs sensor, the hysteresis is obviously small. The maximum hysteresis was 1.1%, corresponding to 11.3% RH. It is indicated that after HMDS treatment, the hysteresis of the samples can be improved remarkably. Compared with the results in other’s work such as in Ref. [13] and Ref. [16], the hysteresis of our HMDS-modified SiNWs based sensor is relatively low. 3.3.3. Response time and stability Several tests under different humidity levels indicated that generally humidity sensor response can reach stable within 10 min. Therefore, the test period was set to this value, which is similar to the one reported in Ref. [27]. In our work, by defining the time taken to achieve 90% of its total variation of the capacitance as the response time (for RH increasing process) and the recovery time (for RH decreasing process), the experimental response and recovery time of both sensors were extracted and shown in Table 2. After HMDS modification, the response time was obviously improved at high relative humidity level. For example, from 11.3% RH to 93% RH, the response time of HMDS-modified SiNWs based sensor is only 132 s, which is much less than the one of SiNWs based sensor. It was also found for both sensors that the recovery time is longer than the response time at low RH, but vice versa at high RH. This characteristic is mainly attributed to the sensing process of SiNWs and HMDS-modified SiNWs. The adsorption process of water molecule on a solid surface could be classified

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Fig. 9. The stability property of the samples in 30 days.

as chemisorption and physisorption, and it can be understood as firstly a molecule layer of chemisorptions at low RH, and then layer by layer of physisorption with RH increasing gradually. At low RH, dominated by chemisorptions, water molecules are usually doubly bonded to two hydroxyls of the surface and cannot move freely, which leads to the longer recovery time. At high RH, the process is dominated by physisorption rather than chemisoption because the van der Waals interaction leading to physisorption was much weaker than the chemical bonding interaction leading to chemisorptions [28], the recovery time can be much shorter than the response time. In fact, in the instance such as the microsensor based on microchannels [13], silicon nanoporous pillar array [28], it was also reported that the recovery time was shorter than the response time. Stability is an important parameter of humidity sensing properties. The sensors were tested repeatedly under a fixed humidity level (75% RH) for about one month. The results are as shown in Fig. 9. It can be observed that the output capacitances of both samples fluctuate slightly along with the time and the data shows good consistency, which indicated that the samples perform well on stability. In our study, the humidity changes detected by both capacitivetype sensors are obvious. The reason for the capacitance changes can be attributed to the adsorption/desorption process of water molecules on the surface of humidity sensing material. Compared with other capacitive sensors (both commercial and research), the as-prepared sensors in our work, especially the HMDS-modified SiNWs based humidity sensor, exhibit better performance and advantages, including the lower hysteresis, short response time at high RH level, a possible large operating humidity range, good compatibility with traditional IC process, and no need for extra heating equipment.

Table 2 The response time of the samples. SiNWs RH (%)

Response time (s)

RH (%)

Recovery time (s)

HMDS-modified SiNWs RH (%) Response time (s)

RH (%)

Recovery time (s)

11.3 → 23 11.3 → 33 11.3 → 43 11.3 → 57 11.3 → 68 11.3 → 75 11.3 → 85 11.3 → 93

10 12 18 16 108 124 247 350

23 → 11.3 33 → 11.3 43 → 11.3 57 → 11.3 68 → 11.3 75 → 11.3 85 → 11.3 93 → 11.3

20 22 49 15 56 58 70 52

11.3 → 23 11.3 → 33 11.3 → 43 11.3 → 57 11.3 → 68 11.3 → 75 11.3 → 85 11.3 → 93

23 → 11.3 33 → 11.3 43 → 11.3 57 → 11.3 68 → 11.3 75 → 11.3 85 → 11.3 93 → 11.3

33 44 28 23 75 63 96 69

26 31 24 20 64 69 123 132

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4. Conclusion In our study, SiNWs based humidity sensors with and without HMDS modification were fabricated and their humidity sensing behavior was examined. Due to the hydrophobic property of hexamethyldisilazane, the HMDS-modified SiNWs based humidity sensor exhibited the improved humidity sensing behavior, such as linearity, hysteresis and response time compared to the SiNWs based sensor. It is indicated that HMDS can effectively improve the humidity sensing characteristics and is potential for application in humidity detection field. Acknowledgments This work is supported by National Natural Science Foundation of China (grant no. 60672002, 61076070) and by Innovation Program of Shanghai Municipal Education Commission (grant no. 09ZZ46). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University. We deeply appreciate the financial support. References [1] Z.W. Yao, M.J. Yang, A fast response resistance-type humidity sensor based on organic silicon containing cross-linked copolymer, Sens. Actuators B 117 (2006) 93–98. [2] Z.M. Rittersma, A. Splinter, A. Bödecker, W. Benecke, A novel surfacemicromachined capacitive porous silicon humidity sensor, Sens. Actuators B 68 (2000) 210–217. [3] L. Gu, Q.A. Huang, M. Qin, A novel capacitive-type humidity sensor using CMOS fabrication technology, Sens. Actuators B 99 (2004) 491–498. [4] A. Oprea, N. Bârsan, U. Weimar, M.L. Bauersfeld, Capacitive humidity sensors on flexible RFID labels, Sens. Actuators B 132 (2008) 404–410. [5] X. Lv, Y. Li, P. Li, M.J. Yang, A resistive-type humidity sensor based on crosslinked polyelectrolyte prepared by UV irradiation, Sens. Actuators B 135 (2009) 581–586. [6] P.G. Su, C.S. Wang, Novel flexible resistive-type humidity sensor, Sens. Actuators B 123 (2007) 1071–1076. [7] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Humidity sensors based on polymer thin films, Sens. Actuators B 35–36 (1996) 85–90. [8] M. Matsuguchi, T. Kuroiwa, T. Miyagishi, S. Suzaki, T. Ogura, Y. Sakai, Stability and reliability of capacitive-type relative humidity sensor using crosslinked polyimides films, Sens. Actuators B 52 (1998) 53–57. [9] M.R. Yang, K.S. Chen, Humidity sensors using polyvinyl alcohol mixed with electrolytes, Sens. Actuators B 49 (1998) 240–247. [10] E. Traversa, Ceramic sensors for humidity detection: the state-of-the-art and future developments, Sens. Actuators B 23 (1995) 135–156. [11] Y. Zhang, X.J. Zheng, T. Zhang, L.J. Gong, S.H. Dai, Y.Q. Chen, Humidity sensing properties of the sensor based on Bi0.5 K0.5 TiO3 powder, Sens. Actuators B 147 (2010) 180–184. [12] W. Yao, X.J. Chen, J. Zhang, A capacitive humidity sensor based on gold-PVA core-shell nanocomposites, Sens. Actuators B 145 (2010) 327–333. [13] F.J. Miao, B.R. Tao, L. Sun, T. Liu, J.C. You, L.W. Wang, P.K. Chu, Capacitive humidity sensing behavior of ordered Ni/Si microchannel plate nanocomposites, Sens. Actuators A 160 (2010) 48–53. [14] G.D. Francia, A. Castaldo, E. Massera, I. Nasti, L. Quercia, I. Rea, A very sensitive porous silicon based humidity sensor, Sens. Actuators B 111–112 (2005) 135–139. [15] A. Arena, N. Donato, G. Saitta, Capacitive humidity sensors based on MWCNTs/polyelectrolyte interfaces deposited on flexible substrates, Microelectron. J. 40 (2009) 887–890. [16] Y.F. Dong, L.Y. Li, W.F. Jiang, H.Y. Wang, X.J. Li, Capacitive humidity-sensing properties of electron-beam-evaporated nanophased WO3 film on silicon nanoporous pillar array, Physica E 41 (2009) 711–714.

[17] Y. Kim, B. Jung, H. Lee, H. Kim, K.H. Lee, H. Pard, Capacitive humidity sensor design based on anodic aluminum oxide, Sens. Actuators B 141 (2009) 441–446. [18] J.H.-S. Michael, J. Ervin, M. Andersen, Anodic nano-porous humidity sensing thin films for the commercial and industrial applications, IEEE IASI (2004) 1207–1210. [19] X.T. Vu, R. GhoshMoulick, J.F. Eschermann, R. Stockmann, A. Offenhäusser, S. Ingebrandt, Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens. Actuators B 144 (2010) 354–360. [20] H.L. Li, J. Zhang, B.R. Tao, L.J. Wan, W.L. Gong, Investigation of capacitive humidity sensing behavior of silicon nanowires, Physica E 41 (2009) 600–604. [21] V. Chechik, R.M. Crooks, C.J.M. Stirling, Reactions and reactivity in selfassembled monolayers, Adv. Mater. 12 (2000) 1161–1171. [22] J.W. Lussi, C. Tang, P.A. Kuenzi, U. Staufer, G. Csucs, J. Voros, G. Danuser, J.A. Hubbell, M. Textor, Selective molecular assembly patterning at the nanoscale: a novel platform for producing protein patterns by electron-beam lithography on SiO2 /indium tin oxide-coated glass substrates, Nanotechnology 16 (2005) 1781–1786. [23] M.S. Chou, K.L. Chang, UV/ozone degradation of gaseous hexamethyldisilazane (HMDS), Chemosphere 69 (2007) 697–704. [24] T. Kikkawa, S. Kuroki, S. Sakamoto, K. Kohmura, H. Tanaka, N. Hata, Influence of humidity on electrical characteristics of self-assembled porous silica low-k films, J. Electrochem. Soc. 152 (2005) 560–566. [25] L.J. Wan, W.L. Gong, K.W. Jiang, H.L. Li, B.R. Tao, J. Zhang, Preparation and surface modification of silicon nanowires under normal conditions, Appl. Surf. Sci. 254 (2008) 4899–4907. [26] R.C. Weast, CRC Handbook of Chemistry and Physics, CRC press, Boca Raton, FL, 1982. [27] B. Yang, B. Aksak, Q. Lin, M. Sitti, Compliant and low-cost humidity nanosensors using nanoporous polymer membranes, Sens. Actuators B 114 (2006) 254–262. [28] L.Y. Li, Y.F. Dong, W.F. Jiang, H.F. Ji, X.J. Li, High-performance capacitive humidity sensor based on silicon nanoporous pillar array, Thin Solid Films 517 (2008) 948–951.

Biographies Xuejiao Chen was born in Shanghai, China, on December 3, 1984. She received the BS degree from the Microelectronics, East China Normal University (ECNU), Shanghai, China in 2007. She is currently a graduate student in ECNU. Her research interest includes nanomaterials, humidity sensors and biosensors. Jian Zhang received his master degree from Changchun Institute of Optics and Fine Mechanics, and PhD degree from Shanghai Institute of Metallurgy, Chinese Academy of Sciences (CAS), China, in 1994 and 1997, respectively. From 1997 to 1998, he worked as the assistant professor in State Key Laboratory of Transducer, CAS, China. From 1998 to 2000, he worked as a Research Fellow in Nanyang Technological University, Singapore. From August 2000 to December 2003, he worked as a Research Fellow in Institute of Materials Research and Engineering, Singapore. From January 2004, he worked as a professor in Department of Electrical Engineering, East China Normal University. His current research interests include microsensors and arrays, micro-fabrication, gas sensors and biosensors. Zhiliang Wang received his MS degree in Southeast University, China in 2007. From 2009, he has been working towards his PhD degree at the Department of Electronic Engineering, School of Information Science and Technology, East China Normal University. His current research interests include semiconductor materials, devices and nanosensors. Qiang Yan was born in Yancheng, China, on November 3, 1986. He received the BS degree from Nantong University, China, in 2009. He is currently a graduate student in East China Normal University. His research interest includes nanomaterials, humidity sensors and biosensors. Shichao Hui received the BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in electrical engineering from East China Normal University. Her current research interests include semiconductor materials, fuel cells, super-capacitors and nano-sensors.