Silicon nanowires as chemical sensors

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Silicon nanowires as chemical sensors X.T. Zhou, J.Q. Hu, C.P. Li, D.D.D. Ma, C.S. Lee, S.T. Lee * Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong SAR, China Abstract Chemical sensitivity of silicon nanowires bundles has been studied. Upon exposure to ammonia gas and water vapor, the electrical resistance of the HF-etched relative to non-etched silicon nanowires sample is found to dramatically decrease even at room temperature. This phenomenon serves as the basis for a new kind of sensor based on silicon nanowires. The sensor, made by a bundle of etched silicon nanowires, is simple and exhibits a fast response, high sensitivity and reversibility. The interactions between gas molecules and silicon nanowires, as well as the effect of silicon oxide sheath on the sensitivity and the mechanisms of gas sensing with silicon nanowires are discussed. Ó 2003 Elsevier Science B.V. All rights reserved. The measurement of NH 3 is needed in indus- trial, medical and living environments [1], and the detection of humidity is important in many areas, including meteorology, domestic environment, medicine, food production, industry, and agricul- ture [2]. The most common gas sensing devices at low-cost are solid-state-encompassing catalytic and metal oxide semi-conductor types as well as electrochemical devices [2]. For nanosized materi- als, surface properties become paramount due to their large surface-to-buck ratio. This makes nanoscale materials particularly appealing in the applications where such properties are exploited, such as gas and biomedical sensors. Indeed, Mar- tinelli et al. [3] and Williams and Coles [4] reported that the properties of gas sensing materials could be improved by the use of nanosized semi-con- ducting oxide powders. Kong et al. [5] reported that an individual semi-conducting single-walled carbon nanotube exhibited high sensitivity to NH 3 and NO 2 at room temperature. However, some properties of carbon nanotubes, such as difficulty in producing pure semi-conducting carbon na- notubes and in modifying the surface of the carbon nanotubes, could pose as problems in their devel- opment as sensor [6]. Silicon nanowires, which have attract much attention in recent years for their potential appli- cations in mesoscopic research and nanodevices [7–9], appear to be immune from the above limi- tations. In addition, the massive knowledge for doping and surface modification of bulk Si should be readily extendable to Si nanowires. In fact, LieberÕs group [6] reported that as-prepared, oxide coated B-doped Si nanowires can be used for highly sensitive, real-time electrically based sensor for chemical and biological species in aqueous solution. Recently, we have developed a new Chemical Physics Letters 369 (2003) 220–224 www.elsevier.com/locate/cplett * Corresponding author. Fax: +852-27844696. E-mail address: apannale@cityu.edu.hk (S.T. Lee). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)02008-0 method, called oxide-assisted growth [9], that is capable of producing high-purity Si nanowires in large quantity, which makes silicon nanowires potentially possible for use as low-cost sensors. In addition, it is much easier to prepare a device of chemical sensors by using a bundle of Si nanowires than using a single Si nanowire. Here, we report the chemical sensitivity of electrical resistance of bundles of Si nanowires to NH 3 and water vapors, and their capability of detecting small concentra- tions of other gases. Si nanowires were prepared by the oxide-as- sisted growth technique [9]. The as-prepared Si nanowires were etched in a water solution of 5% HF in volume for 2 min. Then they were washed in water and dried in air at room temperature. The HF-etched and non-etched Si nanowires were characterized by transmission electron microscopy (TEM, Philips CM 200 FEG). Bundles of etched and non-etched Si nanowires were made by pressing wires of about 0.4 mg in weight onto the surface of insulating glasses. Two electrodes were made by applying silver glue at the two ends of each bundle of Si nanowires. The distance between the two electrodes was 5 mm. Fig. 1 shows the picture of such a silicon nanowire device. After drying the silver glue in air at room temperature, the Si nanowire devices were put into a vacuum chamber (70 L in volume and pumped by a me- chanical pump) and a dc source was connected between the two silver electrodes. After evacuating the chamber and introducing the gases into the chamber, the electric resistance of the Si nanowires devices was measured. The voltage of the dc source was fixed at 10 V and the current was measured by a pico-ammeter (Keithley 485). Figs. 2a and b shows respectively the TEM im- ages of the non-etched and HF-etched Si nano- wires. The average diameter of the non-etched Si nanowires is about 20 nm, while the average di- ameter of the etched nanowires is a little smaller than that of the non-etched ones due to the removal of the amorphous silicon oxide sheath. The high-resolution TEM image of the non-etched Si nanowires (the inset of Fig. 2a) shows clearly an amorphous silicon oxide sheath covering the single crystal silicon core; while the amorphous silicon oxide sheath was almost completely re- moved from the surface of the HF-etched wires (inset of Fig. 2b). Fig. 3 shows the electrical response of the Si nanowire bundles when different gases were in- troduced into the vacuum chamber. When the etched Si nanowire device (Fig. 3a) was exposed to a mixture of ammonia and nitrogen (ammonia concentration: 1000 ppm), the resistance de- creased very fast at the beginning, falling by three orders after 10 min of exposure to ammo- nia and nitrogen. The resistance continued to decrease but slowly with increasing time. The sluggish response of resistance was due to the slowly changing pressure of ammonia, as it took about 70 min to fill the large size of the cham- ber with 760 T of NH 3 ð0:1%Þ=N 2 (flowrates of NH 3 and N 2 were 1 and 1000 sccm, respectively). The resistance of the etched Si nanowires de- creased from $1 Â 10 13 X in vacuum (2 Â 10 À2 T) to $1 Â 10 9 X in the mixture gases of NH 3 ð0:1%Þ=N 2 . In contrast, Fig. 2a shows less than one order of magnitude decrease in the resistance of the same sample when the chamber was filled with pure N 2 (flowrate of nitrogen: 1000 sccm). The observation shows that the resistance of Si Fig. 1. Optical micrograph of a silicon nanowire sensor. X.T. Zhou et al. / Chemical Physics Letters 369 (2003) 220–224 221 nanowires is extremely sensitive to NH 3 . Upon venting the chamber to air (relative humidity: 60%), the resistance of the etched sample de- creased rapidly from $ 3:5 Â 10 12 X in a vacuum of 2 Â 1 À2 Tto$ 5 Â 10 9 X (Fig. 3a) in air in 1 min as the flowrate of air was very large and the chamber could be completely filled in 1 min during venting. After venting to air, the resis- tance of the sample increased slowly with time. We then used a dehumidifier to reduce the rela- tive humidity in air from 60% to 40%, and found that the resistance of the etched sample increased nearly by one order of magnitude (Fig. 3a). These results indicate that it was the water vapor in air that was primarily responsible for the re- sistance change in the Si nanowire device. The sensitivities (defined as the ratio of the resistance of the nanowire device before and after the gas exposure) for the 1000 ppm NH 3 and the air with a relative humidity of 60% are about 10,000 and 100, respectively. As the gases (NH 3 ð0:1%Þ=N 2 or air) are re- moved by pumping, the resistance of the etched Si Fig. 2. Transmission electron micrographs (TEM) of non- etched (a) and HF-etched Si nanowires (b). The insets show the high-resolution TEM of a single non-etch and HF-etched Si nanowire, respectively. Fig. 3. Electrical responses of the Si nanowire bundle of to N 2 , a mixture of N 2 ,NH 3 (NH 3 concentration: 1000 ppm), and air with a relative humidity of 60%; (a) when the gases were in- troduced into the chamber and (b) when the gases were pumped away. 222 X.T. Zhou et al. / Chemical Physics Letters 369 (2003) 220–224 nanowire sample (Fig. 3b) increased rapidly at the beginning, then slowly to the original value before the sample was exposed to the gases. Comparing with the resistance in N 2 , we conclude that the resistance decrease of etched Si nanowire device was due to NH 3 or air. This means that the resis- tance of Si nanowires recovered totally after the gases were removed and the typical recovery times for the NH 3 and water vapor are 5 and 0.5 h, re- spectively. The recovery rate of the Si nanowire seems to be much faster than that of carbon nanotube sensor [5]. Although a more quantitative evaluation is desirable, a simple estimate puts the response time of the resistance change to be con- siderably less than 1 min if a smaller vacuum chamber is used. Nevertheless, it is clear that the electrical sensitivity of the Si nanowires to both NH 3 and water vapor is fast and reversible, which suggests that the Si nanowires sensor is reusable after gas exposure. We performed similar experiments on the non- etched Si nanowires sample and found their re- sistance to be rather insensitive to either NH 3 or water vapor. The resistance of the non-etched sample in vacuum (2 Â 10 À2 T) was almost the same as that of the etched sample, but the re- sistance of the non-etched sample showed very little change upon exposure to NH 3 and water vapor. The gas molecules may affect the resistance of the Si nanowire sample in two possible ways: (1) the contact resistance across two nanowires and (2) the surface resistance along the individual nanowire. We note that the length of nanowires ranges from several micrometer to several tens micrometer, and the distance between the cathode and the anode is 5 mm. Thus, the charge carriers have to transport across hundreds of contacts of nanowires between the two electrodes. The contact resistance between nanowires should therefore play a very important role in determining the electrical current because hundreds of wire con- tacts exist in the 5 mm circuit, although experi- mental data indicated that there is little tunneling barrier at the junction of the two-crossed Si nanowires [12]. On one hand, the NH 3 gas and the water vapor may act as a chemical gate, which shifts the Fermi levels of the Si nanowires and reduces the resistance of the sample. Indeed, the conductance of a single Si nanowire could be modulated by an applied gate [10–12], such as by a chemical gate in a solution via protonation and deprotonation [6]. Kong et al. [5] also suggested that NH 3 had the molecular gating effect, which effectively shifted the valence band of a single semi-conductive carbon nanotube away from the Fermi level, resulting in hole depletion and de- creasing conductance of carbon nanotube. On the other hand, through charge exchange the gas molecules adsorbed on the surface of the Si nanowires could cause a decrease in the potential barrier height between two contacting nanowires, thus a decrease in the contact resistance. This is similar to the model of polycrystalline semicon- ductor SnO 2 sensors. For instance, Shimizu and Egashira [13] suggested that the gas molecules decreased the potential barrier of the grain boundary. For non-etched sample, the silicon nanowires were sheathed with an amorphous silicon oxide shell with a thickness more than 1 nm (inset of Fig. 2a). As a result, gas molecules are adsorbed only on the surface of the relatively thick amorphous silicon oxide sheath instead onto the crystalline Si core. The extremely high resistance of the oxide sheath is likely to be negligibly affected by the ef- fect of the adsorbed gases. For the etched sample, although native oxide is invariably formed on Si nanowires upon exposure to air, the native oxide layer is extremely thin and not continuous, and is much thinner than the oxide sheath on the as- grown wires formed at high temperature during growth. The existence of the thick oxide sheath, acting as a shielding barrier, is responsible for the large sensitivity difference in resistance between the etched and non-etched Si nanowire samples to the NH 3 gas and water vapor. In summary, we report the high chemical sensitivity of the resistance of HF-etched Si nanowires to NH 3 and water vapor exposure. The removal of the amorphous silicon oxide sheath is responsible for the significant improve- ment of the chemical sensitivity of Si nanowires. Si nanowires are potentially a good candidate for gas sensing applications, which warrants further exploration. X.T. Zhou et al. / Chemical Physics Letters 369 (2003) 220–224 223 Acknowledgements This work was supported in part by a Central Allocation Grant [Project No. CityU 3/01C (8730016)], a CERG Grant [Project No. CityU 1063/01P (9040637)] of the Research Grants Council of Hong Kong SAR, and the Chinese Academy of Sciences. References [1] Y. Takao, K. Miyazaki, Y. Shimizu, M. Egashira, J. Electrochem. Soc. 141 (1994) 1028. [2] J. Watson, K. Ihokura, MRS Bull. 49 (June) (1999). [3] G. Martinelli, M.C. Carotta, E. Traversa, G. Ghiotti, MRS Bull. 30 (June) (1999). [4] G. Williams, G.S.V. Coles, MRS Bull. 25 (June) (1999). [5] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622. [6] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [7] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [8] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [9] S.T. Lee, N. Wang, Y.F. Zhang, Y.H. 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