Silicon nanowires as chemical sensors

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Silicon nanowires as chemical sensors

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 ofsilicon nanowires bundles has been studied Upon exposure to ammonia gas and water vapor, the electrical resistance ofthe 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 ofetched 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 ofsilicon oxide sheath on the sensitivity and the mechanisms ofgas sensing with silicon nanowires are discussed

Ó 2003 Elsevier Science B.V All rights reserved

The measurement ofNH3 is needed in

indus-trial, medical and living environments [1], and the

detection ofhumidity 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 ofgas sensing materials could

be improved by the use ofnanosized

semi-con-ducting oxide powders Kong et al [5] reported that an individual semi-conducting single-walled carbon nanotube exhibited high sensitivity to NH3

and NO2 at room temperature However, some properties ofcarbon 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

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 ofproducing 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 ofSi nanowires

than using a single Si nanowire Here, we report

the chemical sensitivity ofelectrical resistance of

bundles ofSi nanowires to NH3and water vapors,

and their capability ofdetecting small

concentra-tions ofother gases

Si nanowires were prepared by the

oxide-as-sisted growth technique [9] The as-prepared Si

nanowires were etched in a water solution of5%

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 ofetched

and non-etched Si nanowires were made by

pressing wires ofabout 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 ofSi nanowires The distance between

the two electrodes was 5 mm Fig 1 shows the

picture ofsuch 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 ofthe Si nanowires devices was measured The voltage ofthe 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 ofthe non-etched and HF-etched Si nano-wires The average diameter ofthe non-etched Si nanowires is about 20 nm, while the average di-ameter ofthe etched nanowires is a little smaller than that ofthe non-etched ones due to the removal ofthe amorphous silicon oxide sheath The high-resolution TEM image ofthe non-etched

Si nanowires (the inset ofFig 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 ofFig 2b)

Fig 3 shows the electrical response ofthe 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 ofammonia 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 ofresistance was due to the slowly changing pressure ofammonia, as it took about 70 min to fill the large size ofthe cham-ber with 760 T ofNH3 ð0:1%Þ=N2 (flowrates of

NH3 and N2 were 1 and 1000 sccm, respectively) The resistance ofthe etched Si nanowires de-creased from 1  1013Xin vacuum (2 102 T)

to 1  109 X in the mixture gases ofNH3

ð0:1%Þ=N2 In contrast, Fig 2a shows less than one order ofmagnitude decrease in the resistance ofthe same sample when the chamber was filled with pure N2 (flowrate ofnitrogen: 1000 sccm) The observation shows that the resistance ofSi

Fig 1 Optical micrograph ofa silicon nanowire sensor.

nanowires is extremely sensitive to NH3 Upon

venting the chamber to air (relative humidity:

60%), the resistance ofthe etched sample

de-creased rapidly from  3:5  1012 Xin a vacuum

of2 12 T to  5  109 X (Fig 3a) in air in

1 min as the flowrate ofair was very large and

the chamber could be completely filled in 1 min

during venting After venting to air, the resis-tance ofthe 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 ofthe etched sample increased nearly by one order ofmagnitude (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 ofthe resistance

of the nanowire device before and after the gas exposure) for the 1000 ppm NH3 and the air with

a relative humidity of60% are about 10,000 and

100, respectively

As the gases (NH3 ð0:1%Þ=N2 or air) are re-moved by pumping, the resistance ofthe etched Si

Fig 2 Transmission electron micrographs (TEM)

ofnon-etched (a) and HF-ofnon-etched Si nanowires (b) The insets show the

high-resolution TEM ofa single non-etch and HF-etched Si

nanowire, respectively.

Fig 3 Electrical responses ofthe Si nanowire bundle ofto N 2 ,

a mixture ofN 2 , NH 3 (NH 3 concentration: 1000 ppm), and air with a relative humidity of60%; (a) when the gases were in-troduced into the chamber and (b) when the gases were pumped away.

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 N2, we conclude that the

resistance decrease ofetched Si nanowire device

was due to NH3or air This means that the

resis-tance ofSi nanowires recovered totally after the

gases were removed and the typical recovery times

for the NH3and water vapor are 5 and 0.5 h,

re-spectively The recovery rate ofthe 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 ofthe resistance change to be

con-siderably less than 1 min ifa smaller vacuum

chamber is used Nevertheless, it is clear that the

electrical sensitivity ofthe Si nanowires to both

NH3and 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 NH3 or

water vapor The resistance ofthe non-etched

sample in vacuum (2 102 T) was almost the

same as that ofthe etched sample, but the

re-sistance ofthe non-etched sample showed very

little change upon exposure to NH3 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 ofnanowires

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 ofcontacts 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 ofwire

con-tacts exist in the 5 mm circuit, although

experi-mental data indicated that there is little tunneling

barrier at the junction ofthe two-crossed Si

nanowires [12] On one hand, the NH3gas and the

water vapor may act as a chemical gate, which

shifts the Fermi levels of the Si nanowires and

reduces the resistance ofthe sample Indeed, the conductance ofa 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 NH3 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 ofcarbon 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 ofpolycrystalline semicon-ductor SnO2 sensors For instance, Shimizu and Egashira [13] suggested that the gas molecules decreased the potential barrier ofthe 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 ofFig 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 ofthe 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 ofthe 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 NH3gas and water vapor

In summary, we report the high chemical sensitivity ofthe resistance ofHF-etched Si nanowires to NH3 and water vapor exposure The removal ofthe amorphous silicon oxide sheath is responsible for the significant improve-ment ofthe chemical sensitivity ofSi nanowires

Si nanowires are potentially a good candidate for gas sensing applications, which warrants further exploration

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)] ofthe Research Grants

Council ofHong Kong SAR, and the Chinese

Academy ofSciences

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 Tang, MRS Bull 36 (August) (1999).

[10] J.Y Yu, S.W Chung, J.R Heath, J Phys Chem B 104 (2000) 11864.

[11] S.W Chung, J.Y Yu, J.R Heath, Appl Phys Lett 76 (2000) 2068.

[12] X Duan, J Hu, C Lieber, Y Cui, J Phys Chem B 104 (2000) 5213.

[13] Y Shimizu, M Egashira, MRS Bull 18 (June) (1999).