Chemical Sensing with ZnO Nanowires - IEEE Xplore

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Zhiyong Fan and Jia G. Lu. Department of Chemical Engineering and Materials Science & Department of Electrical Engineering and Computer. Science ...
Chemical Sensing with ZnO Nanowires Zhiyong Fan and Jia G. Lu Department of Chemical Engineering and Materials Science & Department of Electrical Engineering and Computer Science, University of California-Irvine, Irvine, CA 92697, USA this method, ZnO nanowires with a broad diameter distribution was obtained. The detail of nanowire FETs fabrication was reported elsewhere [10] and can be briefly described as following. The as-synthesized ZnO nanowires were suspended in alcohol and deposited onto p++ Si substrates capped with SiO2 layer. Photolithography or electron beam lithography was conducted on such chips to define contact electrode patterns, followed by metallization using ebeam evaporation. The electrical transport study showed that the synthesized ZnO nanowires are typical ntype semiconductors owing to the presence of oxygen vacancy and Zn interstitials. The measured room temperature quasi-one-dimensional electron concentration is on the order of 107 cm-1 [10], corresponding to a volume concentration of ~1019cm-3, and the electron mobility is 20~80 cm2/ V.s.

Abstract—Zinc oxide nanowires were configured as nchannel field-effect transistors. These transistors were implemented as chemical sensors for detection of various gases. The ammonia sensing behavior of nanowires was observed to switch from oxidizing to reducing when temperature increased from 300 to 500 K. This effect is attributed to the temperature dependent chemical potential shift. Carbon monoxide was found to increase the nanowire conductance in the presence of oxygen. In addition, nanowire detection sensitivity dependence on the diameter was investigated.

I.

INTRODUCTION

In the recent decade, semiconducting quasi-onedimensional (Q1D) structures have received enormous attention for their great potential in building nanoscale electronics and optoelectronics, and much progress has been achieved towards these objectives [1-4]. In fact, scaling down of material dimension does not only imply an increase of the device packing density in integrated circuit, but also render material with higher sensitivity to surface adsorption related chemical processes, leading to superior chemical sensing properties of the Q1D systems. As it has been reported, chemical sensing with nanowires and nanotubes is usually realized by monitoring their conductance change upon exposure to the target chemicals [5-7]. Compared with bulk and thin film materials, the enhanced sensitivity of the Q1D systems to the chemical environment is usually interpreted as a result of large surface-to-volume ratio. One should also consider that when the diameter becomes comparable to the Debye screening length, chemisorption induced surface states effectively alter the electronic structure of the entire system.

III.

In order to conduct chemical sensing experiments, nanowire FETs were loaded into a vacuum chamber with electrical feedthroughs. The chamber was then pumped down to 10-3 torr and the sample was baked at 200 ˚C for half hour to remove the surface adsorbents such as O2 and H2O. After the system was cooled down to room temperature, it was filled with 760 torr target gas mixed with pure Ar using mass flow controllers. The concentration of target gas was controlled by setting Ar and target gas at desired flow rate. Various gases have been used for chemical sensing study, such as O2, NO2, NH3 and CO. The monitoring of these gases in environment is of paramount importance. As is known, chemisorbed gas molecules on metal-oxide surface withdraw or donate electrons to material, depending on whether the adsorbent is oxidizing or reducing, giving rise to a conductance change. Experiments demonstrated that nanowire exposure to O2, NO2 and NH3 at room temperature show decreased conductance. As shown in Fig. 1a, nanowire conductance decreased when it was exposed to different concentration of NO2 varying from 1 ppm to 20 ppm, and the nanowire showed faster response to higher concentration gas. Fig. 1b plots the time domain response of nanowire conductance to 1% NH3 at room temperature. In this case, a refresh process is also demonstrated. As reported in ref. 11, a large negative gate field could desorb the surface chemisorption, thus contribute to the nanowire conductance recovery. It can be seen from Fig. 1b that a -30 V gate

In this paper, we report chemical sensing studies with zinc oxide (ZnO) nanowires configured as field effect transistors (FETs). It is noteworthy that this device configuration can not only facilitate the monitoring of conductance change, but also utilize the transverse electric field to modulate the chemical sensing behavior of the nanowire sensors since an external field can regulate the surface chemisorption activity of nanowire via varying chemical potential [8]. II.

NANOWIRE SYNTHESIS AND DEVICE FABRICATION

Single crystalline ZnO nanowires were synthesized via a vapor trapping chemical vapor deposition method [9]. Using

0-7803-9056-3/05/$20.00 © 2005 IEEE.

CHEMCIAL SENSING EXPERIMENTS AND RESULTS

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voltage was applied for 2 minutes to recover the nanowire conductance.

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Figure 2. (a) ZnO nanowire reducing sensing to NH3 at 500K. (b) Schematic of chemical potential shift from 300 to 500K and the resulted reversal of electron transfer direction.

temperature, but μn μNH3 at room 1000

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Figure 1. (a) ZnO nanowire sensing response to different concentration of NO2. (b) Nanowire sensing response to 1% NH3, and the negative gate field induced a surface refresh process.

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Figure 3. The response of nanowire conductance to 0.5% CO in 20% O2 at 500K. The drain-source voltage is 0.5 V.

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form CO2 [6]:

IV.

In conclusion, ZnO nanowires FETs were implemented as chemical sensors for detection of various gases including O2, NH3, CO and O2. It was observed that nanowire FET demonstrated oxidizing sensing property to NO2, NH3 and O2 at room temperature. However, it showed reducing sensing property for CO, and also for NH3 at 500K. This behavior was attributed to the reduction of chemical potential with increasing temperature. Finally, sensitivity dependence on nanowire diameter was discussed using O2 sensing as an example. The result manifests that thinner nanowire achieves higher chemical sensitivity.

CO + O − → CO2 + e − . And an electron will be released back to the nanowire conduction band, resulting in an increase in conductance. As mentioned above, the enhanced sensitivity of the Q1D systems to the chemical environment can be interpreted as a result of large surface-to-volume ratio which is determined by the aspect ratio of the Q1D systems. Therefore, the radius of nanowires should significantly affect their chemical sensitivity. Experimentally, the dependence of the ZnO nanowire sensitivity to O2 on the radius was studied [10] and plotted in Fig. 4. In fact, this dependence can be simply derived using the Drude model. The conductance G of nanowire with radius r and length l is expressed as:

G =

πr

2

neeµ e

l

ACKNOWLEDGEMENT

(1)

where ne and µe is the electron concentration and mobility. According to ref. 15,

2α N S ne = n0 − r

[2] [3]

[4]

(3)

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Apparently, the obtained 1/r relationship resembles the plot in Fig. 4 and clearly demonstrates that as the diameter of nanowire shrinks, it becomes more sensitive to chemisorption on the nanowire surface.

[6]

[7]

0.7

Sensitivity

REFERENCES

(2)

where n0 is electron concentration before nanowire exposure to O2, Ns and α represent the surface density of chemisorbed O2 molecules and the charge transfer coefficient, respectively. Therefore, upon chemisorption,

∆G /G0

This work is supported by National Science Foundation Grants ECS 0306735 and EEC-0210120. Device Fabrication is performed at the Integrated Nanosystem Research Facility at UC, Irvine. The authors thank D. Wang and P. Chang for technical assistance and helpful discussion.

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2 αN S = ⋅ r n0 .

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Nanowire radius (nm) Figure 4. Nanowire radius dependence of the sensitivity to O2

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