Nuclear Instruments and Methods in Physics Research B 241 (2005) 511–516

www.elsevier.com/locate/nimb

Select gas absorption in carbon nanotubes loadinga resonant cavity to sense airborne toxin gases

Aman Anand a,*, James A. Roberts a, F. Naab a, Jai N. Dahiya b,O.W. Holland a, Floyd D. McDaniel a

a Department of Physics, P.O. Box 311427, University of North Texas, Denton, TX 76203, United Statesb College of Science and Mathematics, Southeast Missouri State University, Cape Girardeau, MO 63701, United States

Available online 24 August 2005

Abstract

A gas-sensing probe has been investigated that involves the use of microwave resonant circuitry to detect frequencyshifts/Q-changes in a resonant microwave cavity caused by absorption of chemical/biological agents onto carbon nano-tubes (CNTs). Tests were made with single walled (SWNTs) to determine maximum selectivity of the probe. The ulti-mate goal is to accumulate a battery of test data for selected gases to establish a comprehensive inventory of potentialgases that can be sensed with the apparatus. Additionally, it is anticipated that this work will result in a significant sim-plification of the design of the probe as well as the determination of an optimum frequency for probing with the cavity.Data presented in this work is offered as a demonstration of the feasibility of the instrument to detect non-toxic gases.As development continues, the device should be able to detect a wider range of gases including airborne toxins. Particleinduced X-ray emission (PIXE) was used to characterize the impurities within the SWNTS to better understand thenature of the coupling of the microwave with the nanostructured material and ultimately to determine how impuritiesaffect the sensitivity of the probe.� 2005 Elsevier B.V. All rights reserved.

PACS: 85.35.Kt; 84.40.Dc; 07.57.Pt; 82.80.Ha; 84.40.�x

1. Introduction

In this experiment, magnetic and electricalinteractions between the applied fields and the

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.nimb.2005.07.062

* Corresponding author. Tel.: +1 940 594 2423; fax: +1 940565 2227.

E-mail address: aman@unt.edu (A. Anand).

sample were measured using a standard perturba-tion technique with a resonant cavity. Resonantcavities are well-known, highly sensitive devicesthat have been used to make measurements of fun-damental properties of matter in all its phases [1].The interaction of the microwave field and thematerial is generally summarized by the relation-ship given by Z = f1(le,E) � f2(lm,H), where

ed.

Fig. 1. A block diagram of the basic microwave apparatus usedto conduct the pressure cycling studies proposed in this work.

512 A. Anand et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 511–516

f1(le,E) is a measure of the electrical interactionand f2(lm,H) is a measure of the magnetic interac-tion of the sample with the applied external electricfield E and the magnetic field H. Each term be-haves in a different fashion, depending on theproperties of the material. In each case the reso-nant frequency shift and Q change (macroscopicproperties) of the cavity are a measure of le andlm (microscopic properties). From these measure-ments, fundamental properties and identificationof the material loading the cavity are made [2].

The interactions between the resonant cavityand the perturbing load is expressed by the equa-tions of Slater [3] and van Bladel [4] as

Dx=x0 ¼1

2W s

� �Zm

l0H1

XH�

1

�

�el0E1

XE�

1

�dV ; ð1Þ

where Ws is the energy stored per cycle, V is thevolume of the sample, l0 is the magnetic suscepti-bility, H1 is the magnetic field strength, e is theelectric susceptibility, and E1 is the electric fieldstrength of the material loading the cavity. Thedominant term is the electric field interaction andthe magnetic field term can be neglected. The elec-tric susceptibility is complex and further simplifi-cation of the second term in Eq. (1) (Slaterperturbation) leads to:

Df =f0 � C1ðe0 � 1Þ=2; ð2ÞDð1=QÞ � C2e

00; ð3Þ

where C1 and C2 are constants. The experimentalmeasurements are reduced to finding the frequencyshift Df, which characterizes the nature of the loadin the resonant cavity. This macroscopic quantitywill be used as a benchmark to establish the load-ing effects of the CNTs within the cavity as a func-tion of gas exposure.

2. Experimental method and results

A block diagram of the apparatus used to makethe preliminary measurements is shown in Fig. 1.The active element of the gas detector is the reso-

nant cavity loaded with a small amount, �0.5 g,of CNTs. The CNTs are placed on the cylindricalaxis of the cavity in a position where the electricfield vector is a maximum. A detailed picture ofthe resonant cavity is shown in the rightmost partof Fig. 2 with a coupling wave-guide and cablesneeded to monitor the cavity loading. In additionto the resonant cavity, the apparatus consists pri-marily of a network analyzer, which is used to scanthe microwave frequency, and to measure andrecord the resonant response of the cavity. Thenetwork analyzer is an IFR model 6845, and hasa frequency range of 10 MHz–47 GHz. A crystal-controlled oscillator was used as a beat frequencyoscillator (BFO) to determine the frequency ofthe microwave oscillator and to provide a set ofreference markers to calibrate the resonant condi-tions and also to provide highly accurate measure-ments. It should be possible to simplify theapparatus when all of the tests of gas samples havebeen completed. The determination of the macro-scopic quantities, Q and resonant frequency, f

will, hopefully, provide a unique signature for par-ticular species of gas (i.e. toxin). Once an inventoryof selected gases is characterized and their signa-ture stored within the computer memory, theprobe can then be used to detect an unknown tox-in simply by comparison with the database.

The simple circuit employed as a sensor consistsof a microwave resonant cavity operating over anarrow range of frequencies with a small amountof CNTs positioned along the cylindrical axis inthe region of most intense electric vector. Themicrowave response to various gases probedshould be greatly enhanced within the resonant

Fig. 2. The network analyzer used to collect data for the resonant cavity, shown in the foreground, as the gas pressure is changed inincreasing and decreasing values with the carbon nanotubes (CNT) placed on the axis of the cylindrical cavity in a position where theelectric vector is maximum. To the right is shown a close up view of the resonant cavity with coupling wave-guide and cables needed tomonitor the loaded cavity.

-27-26-25-24-23-22-21-20

0 2 4 6 8 9 11 13

FREQUENCY (MHz)

AB

SO

RP

TIO

N (

dB

m)

UN

ITS

Fig. 3. A frequency scan showing different responses of themicrowave resonant cavity to changes in the air pressure. Thehorizontal axis represents the frequency channel recorded bythe network analyzer. The vertical axis represents the powerreflected from the cavity. Each of the measurements corre-sponds to a different air pressure starting below atmosphericpressure and increasing to one atmosphere. The response curvesshift progressive to higher frequencies as the pressure isincreased (i.e. the rightmost curve corresponds to the atmo-spheric pressure).

A. Anand et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 511–516 513

cavity by the addition of SWNTs. The increasedsensitivity is anticipated due to the gas–carbonnanotubes interaction, which should be greatly en-hanced due to the large surface area to mass of thecarbon nanotubes. To test this hypothesis, experi-ments were conducted by measuring the pressurewhile sweeping the resonant cavity through anarrow frequency near 9.3 GHz for different gaspressure. The CNT-based apparatus was testedinitially with only air. The range of probing isextended as dictated by the gas being sensed bythe apparatus. Several oscillator circuits have beenused for sensors based on nanotechnology [6–11]they were comprised of multi-walled nanotubes(MWNTs) and/or silicon dioxide. In one experi-ment, the materials were deposited onto a planarinductor–capacitor resonant circuit, which moni-tors the materials and is able to determine, accord-ing to the resonant frequency conditions, if there iscarbon dioxide, oxygen, or ammonia present in thearea of the sensor [4].

The resonant cavity method reported in thispaper is potentially more sensitive than othertechniques. Prakash and Roberts [5] used thismethod (without incorporation of the CNT load-ing) to demonstrate that, when alcohol vapor isadmitted into a resonant cavity, the frequencyshifts with the pressure in the cavity. An interest-

ing aspect of these experiments was the observa-tion that the frequency response of the cavity didnot depend only upon pressure but also uponwhether the cavity was being pressurized ordepressurized.

In the present work, four measurements weremade corresponding to different air pressure inthe resonant cavity. The results of the frequencyscan of the resonant cavity under these differentconditions are shown in Fig. 3. The units of power

514 A. Anand et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 511–516

absorption in Fig. 3 are given in dBm. This unit isoften used in communication systems especially inacoustics, microwaves and fiber optics where dBmis defined as

dBm ¼ 10 logP 1

P 2

� �;

where P1 is the measured power level and P2 is thereference power level [12]. In our case we had setthe reference power level of the analyzer to be�27 dBm. Off resonance, the electric field vectordoes not couple in phase with the microwave field,and as a result much of the input power is re-flected. At resonance, the electric vector is wellcoupled with the external field so that there is littleor no reflected wave and hence no power loss. Thiscan be seen in Fig. 3 where, at the resonant fre-quency, there is a maximum power adsorption(i.e. minimum reflected power).

It is clear in Fig. 3 that the frequency sweep ineach scan encompasses non-resonant modes atboth lower and higher frequencies, as well as a sin-gle resonant condition near the center of the band-width where maximum power is delivered to thecavity. It is also clear that substantial shifts inthe resonant frequency occur when the cavity pres-sure is changed. The scans from left to right inFig. 3 correspond to increasing air pressure inthe resonant cavity beginning with an initial pres-sure of �30 in. of Hg and ending at atmosphericpressure. The pressure increment between eachmeasurement was �10 in. of Hg. As pressure in-creases, the resonant frequency shifts monotoni-cally to higher values. The magnitude of theshifts, �1 MHz /10 in. Hg, is quite large and indi-cates that this method of cavity loading withCNTs may provide an extremely sensitive toolfor gas recognition.

The loading effect of the CNTs in the resonantcavity depends sensitivity upon the effects of gasexposure, as well as the interaction of the CNTswith the microwave field. A model has been pro-posed for transferring microwave energy by para-metric resonance to mechanical energy oflongitudinal and transverse vibrations of the entireCNT [13]. The model provides a basis for under-standing the anomalous heating previously re-

ported [14,15]. Polarization of SWNT under anexternal static electric field has been studied theo-retically [16–18] and experimentally [19]. It hasbeen shown that under a static electric field, adipole moment is induced along the axial directionof a SWNT, which depends on the electronic bandstructure. Gases physically adsorbed onto CNTsby van der Waals forces and chemically adsorbedby bonding to carbon atoms have three main ef-fects on CNTs: (1) changes in the electronic struc-ture causing charge redistribution [20]; (2)increases in the damping of waves excited on theCNTs; (3) changes in the spacing between nano-tubes [21]. The model shows parametric resonanceif damping is below a critical coefficient involvingthe electric field frequency and provides an under-standing of the interaction of CNTs with micro-waves in terms of non-linear dynamics of CNTsmodeled as an elastic string. The damping coeffi-cient j was shown to play a key role in determin-ing the behavior of nanotubes. When the dampingis too large, transverse waves cannot be excited;when j < b but j > 0, electron–phonon interaction(Joule heating) dominates the energy conversionprocess. As j ! 0, phonon–phonon interactionmust be considered. It has been pointed out thatresonant response of CNTs to microwaves resultsin two effects: an increase in electron–phonon scat-tering and the appearance of non-linear effects orphonon–phonon scattering. Since CNTs are notpure in their as-prepared state, it is of interest tomonitor the impurities.

It is clear that the response of the cavity will de-pend critically upon the structure and morphologyof the CNTs, as well as the impurities. It is welldocumented that as-prepared CNTs contain a cer-tain amount of carbonaceous and metallic con-tamination. Metallic impurities are mainly due tothe different catalyst material used by the vendorsduring growth. Elemental analysis of the SWNTswas done using particle induced X-ray emission(PIXE) as shown in Fig. 4. It is clear from inspec-tion of this spectrum that, even though a numberof impurities are present within the SWNTs, thecatalyst metal, Fe, is the most abundant impurity.While contamination of the SWNTs is a concernin the present experiments, previous results havedemonstrated that the coupling of the microwave

50 100 150 200 250 300 350

100

1000

10000

100000

C u-K α

N i-Kα

F e-K β

F e-K α

C r-K α

C a-K β

C a-K α

K-K α

C l-K βC l-K α

S -K α

P -K α

S i-Kα

A l-K αCou

nts

Channel

0 50 100 150 200 250 300 3500

40000

80000

120000

160000

C l-K αFe-Kβ

Fe-K α

Co

un

ts

C hanne l

Fig. 4. PIXE spectrum acquired by irradiation of SWNTs oriented at an angle of 163� with a 1.5 MeV proton beam from the 2.5 MVVan de Graaff accelerator at the University of North Texas. A HPGe X-ray detector was positioned to intercept the X-rays at abackscattering angle of 145�.

A. Anand et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 511–516 515

with CNTs is not strongly dependent upon thepresence of metallic impurities [15].

3. Summary and conclusions

CNTs have been shown to have a markedloading effect upon the response of a microwaveresonant cavity as a function of air pressure. Theresonant frequency shifts by a large amount formodest changes in air pressure. These results dem-onstrate the potential of this method to provide asensitive marker for detection of both the gas spe-cies tested, as well as sensing pressure variations.Further experiments are needed to extend this workto other gases and lower concentrations to deter-mine the ability of the CNT-based resonant cavityto discriminate between the various conditions ofresonance and from these identify toxic gases.

Acknowledgements

The part of our experiment has been sponsoredin part by the NSF, ONR (N00014-03-1-0880),

Texas Advanced Technology Program, and theRobert A. Welch Foundation.

References

[1] K.H. Hong, Microwave properties of liquids and solidsusing a resonant microwave cavity as a probe, Ph.D.dissertation, University of North Texas, 1974;K.H. Hong, Microwave properties of liquids and solidsusing a microwave cavity as a probe, J. Appl. Phys. 45(1974) 2452.

[2] J. Roberts, T. Imholt, Z. Ye, C.A. Dykes, D.W. Price Jr.,J.M. Tour, Electromagnetic wave properties of polymerblends of single wall carbon nanotubes using a resonantmicrowave cavity as a probe, J. Appl. Phys. 95 (April)(2004), and references cited therein.

[3] J.C. Slater, Microwave. Electronics, Rev. Mod. Phys 18(1946) 601.

[4] Van Bladel, The Interior Problem, McGraw-Hill Book Co.,New York, 1964 (Chapter 10).

[5] V. Prakash, J.A. Roberts, Perturbation of a resonantmicrowave cavity by select alcohol vapors, J. MicrowavePower 45–50 (1986).

[6] S. Chopra, A. Pham, J. Gaillard, A. Parker, A.M. Rao,Carbon-nanotube-based resonant-circuit sensor for ammo-nia, Appl. Phys. Lett. 80 (24) (2002) 4632.

[7] M. Penza, G. Cassano, P. Aversa, F. Antolini, A. Cusano,A. Cutolo, M. Giordano, L. Nicolais, Alcohol detection

516 A. Anand et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 511–516

using carbon nanotubes acoustic and optical sensors, Appl.Phys. Lett. 85 (12) (2004) 2379.

[8] S. Chopra, K. McGuire, N. Gothard, A.M. Rao, A. Pham,Selective gas detection using a carbon nanotube sensor, J.Appl. Phys. Lett. 83 (11) (2003) 2280.

[9] A. Kuznetsova, J.T. Yates Jr., J. Liu, R.E. Smalley,Physical adsorption of xenon in open single walled carbonnanotubes: observation of a quasi-one-dimensional con-fined Xe phase, J. Chem. Phys. 112 (21) (2000) 9590.

[10] S. Chopra, K. McGuire, N. Gothard, A.M. Rao, A. Pham,Selective gas detection using a carbon nanotube sensor,Appl. Phys. Lett. 83 (11) (2003) 2280.

[11] Chunyu Li, Tsu-Wei Chou, Mass detection using carbonnanotube-based nanomechanical resonators, Appl. Phys.Lett. 84 (25) (2004) 5246.

[12] Kingfisher Application Note, Decibel dBm Definition.Available from: <http://www.kingfisher.com/au/aapnotes/A01.htm>.

[13] Z. Ye, W.D. Deering, A. Krohin, J.A. Roberts, A shockwave model to explain microwave energy conversion insingle wall carbon nanotubes, Phys. Rev. B, Submitted forpublication.

[14] A. Wadhawan, D. Garrett, J.M. Perez, Nanoparticle-assisted microwave absorption by single-wall carbonnanotubes, Appl. Phys. Lett. 83 (2003) 2683.

[15] F. Naab, M. Dhoubhadel, O.W. Holland, J.L. Duggan,J.A. Roberts, F.D. McDaniel, The role of metallic impu-

rities in the interaction of carbon nanotubes with micro-wave radiation, in: Proceedings of the 10th InternationalConference on Particle Induced X-ray Emission and ItsAnalytical Applications, 2004, p. 601.1. Published elec-tronically at <http://pixe2004.ijs.si/proceedings>).

[16] L.X. Benedict, S.G. Louie, M.L. Cohen, Static polariz-abilities of single-wall carbon nanotubes, Phys. Rev. B 52(1995) 8541.

[17] F. Torrens, Effect of type, size and deformation on thepolarizability of carbon nanotubes from atomic incre-ments, Nanotechnology 15 (2004) S259.

[18] Y. Guo, W. Guo, Mechanical and electrostatic propertiesof carbon nanotubes under tensile loading and electricfield, J. Phys. D: Appl. Phy. 36 (2003) 805.

[19] A. Jorio, A.G. Souza Filho, V.W. Brar, A.K. Swan, M.S.Unlu, B.B. Goldberg, A. Righi, J.H. Hafner, C.M. Lieber,R. Saito, G. Dresselhaus, M.S. Dresselhaus, Polarizedresonant Raman study of isolated single-wall carbonnanotubes: symmetry selection rules, dipolar and multipo-lar antenna effects, Phys. Rev. B 65 (2002) 121402.

[20] B. Reulet, A.Yu. Kasumov, R. DeBlock, I.I. Khodos,Yu.B. Gorbatov, V.T. Volkov, C. Journet, H. Bouchiat,Acoustoelectric effects in carbon nanotubes, Phys. Rev.Lett. 85 (2000) 2829.

[21] M.M. Calbi, A. Mizel, M.W. Cole, Lattice dilation near asingle hydrogen molecule in an interstitial channel within ananotube bundle, Phys. Rev. B 69 (2004) 195408.