Sensors and Actuators B 111–112 (2005) 181–186

Gas sensing using single wall carbon nanotubes orderedwith dielectrophoresis

M. Luccia,c, P. Regoliosia,c, A. Realea,c,∗, A. Di Carloa,c, S. Orlanduccib,c, E. Tamburrib,c,M.L. Terranovab,c, P. Luglid, C. Di Natalea,e, A. D’Amico a,e, R. Paolesseb,e

a Department Electronic Engineering, University Rome Tor Vergata, Via del Politecnico, 1 00133 Rome, Italyb Department Chemical Sciences and Technologies, University Rome Tor Vergata, Rome, Italy

c Interdisciplinary Micro and Nano-structured Systems laboratory (MINAS), University Rome Tor Vergata, Rome, Italyd Lehrstuhl fur Nanoelektronik, TU M¨unchen, M¨unchen, Germany

e CNR – Institute Microelectronic Microsystems, Rome, Italy

Available online 8 August 2005

Abstract

We demonstrate efficient NH3 detection in single wall carbon nanotubes (SWCNT) ordered by mean of dielectrophoretical process. Thee eent CNTb Hd hanced alsob sesb theS©

K

1

a[Tnoptbdob

and

s can, andtings oneasyven-bests. IndWC-

NTstrodeund

0d

mployed approach was to disperse the nanotubes, treated following a specific protocol, in CHCl3 and to distribute the suspension betwhe tracks of multifinger Au electrodes (40�m spacing) on SiO2/Si substrates. The control of arrangement and alignment of the SWundles was achieved by applying an alternate voltage (frequency 1 MHz, 10 Vpp) during the solvent evaporation. The sensitivity for N3etection resulted to be strongly enhanced by the degree of SWCNT alignment between the electrodes. The sensitivity resulted eny increasing up to 80◦C the temperature of the devices. We investigated also the effect induced on the NH3 absorption/desorption procesy a gate voltage applied to the Si substrate beneath the interdigitated electrodes on the NH3. The results indicate that the sensitivity ofWCNT-based sensor can be increased applying a negative gate voltage.2005 Elsevier B.V. All rights reserved.

eywords:Carbon nanotubes; Gas sensing; Dielectrophoresys; NH3

. Introduction

Single-walled carbon nanotubes (SWCNT), formed bygraphene sheet wrapped around along a lattice vector

1], represent the last generation of carbon nanomaterials.he increasing interest in studying such tubular graphiticanostructures is motivated by the remarkable propertiesf SWCNT, which make them attractive for a number ofotential technological applications ranging from fuel cells

o nanotransistors[2–3]. A special attention[4–10] has alsoeen devoted to the development of gas sensors, mainly foretection of O2, NO2, or NH3. Gas adsorption on carbon nan-tubes and nanotube bundles is indeed an important issue foroth fundamental research and technical applications, thanks

∗ Corresponding author. Tel.: +39 06 72597372; fax: +39 06 2020519.E-mail address:reale@ing.uniroma2.it (A. Reale).

to the high specific surface area (1580 m2/g2) [11], that allowsa high capability of interaction between gas moleculesSWCNT.

Experiments have shown that the nanotube sensordetect ppm levels of gas molecules at room temperaturethis opens a possibility of developing nanotube operaat room temperature. Chemically induced perturbationthe resistance of nanotubes can give direct informationto read-out, and the system can be interfaced with contional electronic architectures. This may also provide thechances of high integration for lab-on-a-chip applicationparticular, several research groups[7–10]have demonstratethat the electrical conductance of the semiconducting SNTs can change significantly upon exposure to O2, NO2,or NH3 gases. In the present research, purified SWCare used as sensing material in an interdigitated elecplatform for NH3 detection. The sensor response is fo

925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2005.06.033

182 M. Lucci et al. / Sensors and Actuators B 111–112 (2005) 181–186

Fig. 1. Experimental setup.

dependent on different parameters and conditions of opera-tion: (a) the relative orientation of the nanotubes and theirorganization between the electrodes; (b) the temperature ofthe substrate; (c) the voltage applied to a back gate contact.Ordered networks of SWCNT bundles can be achieved usingdielectrophoresis[12,13] to move and position nanotubesalong preferential directions between patterned electrodes.The degree of nanotube aggregation is controlled using dif-ferent solvents and treatments.

2. Sample preparation

In this work, commercially available Carbolex SWCNTs(purity 50–70%) were used. The as-received samples werepurified using HNO3 2M solution. Following this chemicaltreatment, a solid fraction (no-functionalized nanotubes) anda suspension of carbon nanotubes (functionalized nanoma-terials) were obtained. The solid fraction was characterizedby field emission scanning electron microscopy (FESEM)and Raman spectroscopy in order to check phase purity andstructural integrity of the material. The use of infrared spec-troscopy (FTIR) allowed to evidence that the nanotubes werenot functionalised[14]. Finally a controlled amount of SWC-NTs treated following the abovementioned protocols wered reu

itedo des,w is des,w s ofa otiono tricfi thes thed tem-a t ofSo ldh

A back gate contact was prepared on the substrate, toimprove the control of the interaction of gas molecules withthe SWCNTs.

We prepared a specific set up (seeFig. 1) in order to inves-tigate the dependency of sensitivity on different parameters:(a) the degree of order (alignment); (b) temperature; (c) gatevoltage applied.

Both ordered systems obtained by dielectrophoretic pro-cess and randomly placed SWCNTs were used for the detec-tion of NH3. Under the influence of an inhomogeneous elec-tric field the nanotubes in suspension are found to align andmove, with a motion depending upon the relative dielectricconstant of nanotubes and solvent[16,17]. Single nanotubesand bundles moving along a direction parallel to the fieldstick each others, and providing therefore more reliable con-tacts between the gold fingers. The deposition procedureswith and without applied field were repeated several timesin order to check the reproducibility. A FESEM was usedto check the effective difference in the morphological struc-ture obtained following the two different procedures.Fig. 2shows a FESEM image of the SWCNT bundles ordered

F ore-s twoe

ispersed in a CHCl3 solution and sonicated for 30 min befosing.

A controlled volume of this dispersion was deposnto an interdigitated electrode platform (gold electroith 40�m spacing, evaporated on SiO2 layer grown on Substrate). A set of SWCNT coated multifinger electroith different degree of order, were produced by meandielectrophoretic process, where the electrokinetic mf dielectrically polarized materials in non-uniform elecelds is induced by an alternate electrical field applied toolution of SWCNT dispersed in a proper solvent duringeposition and evaporation processes. After many systic studies of the more efficient conditions for alignmenWCNT between our interdigitated electrodes with 40�mf spacing[15], we obtained optimal results with an AC fieaving a frequency of 1 MHz and 10 Vpp.

ig. 2. FESEM image of the SWCNT ordered by mean of dielectrophis. The SWCNT bundles follow the electric field lines between thelectrodes.

M. Lucci et al. / Sensors and Actuators B 111–112 (2005) 181–186 183

Fig. 3. FESEM image of the random placement of SWCNT without electricfield applied between interdigitated electrodes.

and aligned along direction of the applied electric field. Acompletely different feature is obtained when SWCNT aredeposited between electrodes without electric field applied.Fig. 3shows that only a random placement of SWCNT bun-dles can be obtained in this case.

3. Experimental results: detection of NH3

For the gas sensing an aqueous solution with concentration1:10 of NH3 (30% vol.) was put in an bubbler in parallel witha flux of N2. Five different concentration were obtained withfive different N2 fluxes in the bubbler, exactly: 2/200, 4/200,6/200, 8/200 and 10/200 Sccm.

The resistance change was monitored (until the satura-tion condition was reached) varying the NH3 concentration.Fig. 4. shows the resistance increase in our SWCNT basedsensor for three increasing concentrations of NH3, rangingfrom 150 to 750 ppm. The sensitivity of the sensor is calcu-lated as follows: we first determine the adimensional relative

F easingc

resistance changesxi

xi = �R

R0

∣∣∣∣c=ci

(1)

for each ammonia concentrationci , with respect to the equi-librium valueR0.We then plotxi , as a function of the con-centration of NH3. The slope of the linear fit ofxi(ci) expressthe sensitivity of the sensor in (�R/R)/ppm.

The presence of NH3 (electron-donor) influences theSWCNT conductivity, and its reduction reveals their p-typebehaviour[18–21]. Theoretical calculations based on the den-sity functional method[20] show indeed that physical expla-nation of this process can be given in terms of a mechanismof charge transfer from the NH3 molecules to the SWCNT.

It is important to note that the capability of SWCNT tointeract with the environment depends on many physical andchemical parameters, concerning either intrinsic propertiesof the SWCNT (like metallic or semiconductor behaviour,morphology and alignment, functionalisation, etc.), eitherexternal conditions (such as temperature, pressure, etc.). Wehave investigated the effect of some of these properties, thatwe believed where of more stringent interest. In particular weanalyzed the effect of the morphological order of the SWCNTbetween the electrodes through dielectrophoretical alignmentvia ac electric field, the effect of temperature through the useo ect ofp singt

er-a itha fin-g redS hef sedt ce-m arto ules.T

F rdered( entl

ig. 4. Resistance change in the SWCNT based sensor for three incroncentrations of NH3.

f a controlled heater beneath the substrate, and the effolarization of the substrate of the SWCNT through bia

he back gate contact.Fig. 5 shows the different behaviour at room temp

ture, with no gate voltage applied, of the device wligned and disordered SWCNT. The sensitivity of multier with aligned SWCNT is double with respect to disordeWCNT (seeFig. 6). This effect is probably induced by t

act that the ordered SWCNTs are more uniformly expoo the interaction with NH3 molecules, than the case of plaent of SWCNTs in form of a random network, where pf the SWCNT remain inaccessible to the gas moleche morphological analysis shown inFigs. 2 and 3reveals

ig. 5. Relative resistance change of the aligned (diamonds) and disotriangles) SWCNT, as a function of NH3 concentration. Solid lines represinear fits.

184 M. Lucci et al. / Sensors and Actuators B 111–112 (2005) 181–186

Fig. 6. Sensitivity of aligned (right) and of disordered (left) SWCNT.

indeed the striking structural difference of the two cases, thatmight be related to the increased sensitivity of the alignednanotubes.

We investigate also the resistance change varying the tem-perature (23 and 80◦C). The different temperatures wereobtained mounting the multifinger on a controlled heather.Fig. 7shows the better behaviour of the device at the highertemperature for five different NH3 concentration. InFig. 8it ispossible to see the increased sensitivity for the heated device.To investigate the reasons of such behaviour of the sensor,we have also observed how the conductance of the unex-posed SWCNT modifies with temperature. We have foundthat resistance reduces with temperature, as found by otherauthors[21]. These observations suggests that the increasedsensitivity of the sensor at higher temperature is due to a dif-ferent scaling with temperature ofR0 with respect of�Ri . Inother words, the interaction of NH3 with the nanotubes is lessdependent on temperature than the conductance of the bulkSWCNT device, in the temperature range we considered.

We studied also the resistance change obtained by vary-ing the voltage applied between the back gate contact and oneof the electrodes. We checked three different value:−20 V;

F s) andh sr

Fig. 8. Sensitivity of heated (right) and of room temperature (left) SWCNT.

Fig. 9. Relative resistance change induced in the SWCNTs when the backgate voltage is equal to−20 V (squares), 0 V (triangles), +20 V (circles), asa function of NH3 concentration. Solid lines represent linear fits.

0 V; + 20 V. Fig. 9 shows the relative resistance changeinduced in the SWCNTs when the back gate voltage is equalto−20 V (squares), 0 V (triangles), +20 V (circles), as a func-tion of NH3 concentration. Solid lines represent linear fits.In Fig. 10it is shown the sensitivity obtained by varying the

Fig. 10. Sensitivity of SWCNT as a function of the back gate voltage:−20 V(left), 0 V (centre), +20 V (right).

ig. 7. Relative resistance change of the room temperature (diamondeated (triangles) SWCNT, as a function of NH3 concentration. Solid lineepresent linear fits.

M. Lucci et al. / Sensors and Actuators B 111–112 (2005) 181–186 185

voltage applied, and it is visible the different behaviour of thesensor with positive and negative value of the back gate volt-age. The increased sensitivity obtained in the case of negativevoltage applied to the back gate contact might be justified inour opinion by the fact that the adsorption of NH3 (electron-donor) is assisted by the electrostatic interaction controlled bythe gate potential[19]. Moreover, some authors[22,23]haveproven that conductance of random networks of SWCNT canindeed be modulated by a back gate potential, so that a p-typebehaviour of the SWCNT is revealed. The effect of the nega-tive voltage applied to the gate is then twofold, since acts onthe bulk resistanceRo of the SWCNT, and on the resistancechange�Ri .

4. Conclusions

A new strategy to assemble gas sensor based on alignedSWCNTs has been undertaken. We demonstrated that the sen-sitivity of our SWCNT sensor device with respect to NH3 canbe controlled and optimized using aligned SWCNTs, apply-ing negative voltage to the back gate contact, and heating thesample.

Aiming to extend the research to the detection of othergases species (especially NOX) we are currently investigat-i t cano to theo rningt func-t ity,c gicall s.

A

jectM

R

T.

23.J.P.

pl.

00)

ho,

[ 03)

[11] M. Cinke, J. Li, B. Chen, A. Cassell, L. Delzeit, J. Han, M. Meyyap-pan, Chem. Phys. Lett. 365 (2002) 69.

[12] R. Krupke, F. Hennrich, H. Lohneysen, M.M. Kappes, Science 301(2003) 344.

[13] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D: Appl. Phys. 31(1998) L34–L36.

[14] A. Curulli, F. Valentini, S. Orlanducci, M.L. Terranova, S. NunzianteCesaro, G. Palleschi, Proceeding of IEEE-NANO, WE-P9 (Munich-Germany), 2004.

[15] M. Lucci, A Reale, P. Regoliosi, A. Di Carlo, S. Orlanducci, F.Brunetti, M.L. Terranova, P. Lugli, submitted to J. Appl. Phys.

[16] R. Krupke, F. Hennrich, H. Lohneysen, M.M. Kappes, Science 301(2003) 344.

[17] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D: Appl. Phys. 31(1998) L34–L36.

[18] E.S. Snow, J.P. Novak, P.M. Campbell, D. Park, Appl. Phys. Lett.82 (2003) 2145.

[19] J.P. Novak, E.S. Snow, E.J. Houser, D. Park, J.L. Stepnowski, R.A.McGill, Appl. Phys. Lett 83 (2003) 4026–4028.

[20] H. Chang, J.D. Lee, S.M. Lee, Y.H. Lee, Appl. Phys. Lett. 79 (2001)3863.

[21] J. Suehiro, G. Zhou, M. Hara, J. Phys. D: Appl. Phys. 36 (2003)L109–L114.

[22] E.S. Snow, J.P. Novak, P.M. Campbell, D. Park, Appl. Phys. Lett.82 (2003) 2145.

[23] C. Zhou, J. Kong, H. Dai, Appl. Phys. Lett. 76 (2000) 1597.

Biographies

M iver-s lmD usedo tion”a ateri-a tentsp

P rsityo theE ata,w d theirc ts ons easuret

D ica-t tudyo pres-s de thet l ande nd thes oper-t le has2

A me,R ottkyI e isc ineer-i archi essesi

S ion:P oft Mate-

ng the different chemical and physical parameters thaptimise gas sensing response, with a special attentionptimization of the time response of the sensor. Conce

hese issues we are testing various treatments able toionalise SWCNTs in order to improve chemical selectivhecking the temperature range, voltage and morpholoayout of the SWCNT that gives the better performance

cknowledgement

This work has been performed in the frame of the proIUR, FIRB RBNE019TMF.

eferences

[1] S. Iijima, Nature 354 (1991) 56.[2] M.A. Lantz, B. Gotsmann, U.T. Durig, P. Vettiger, Y. Nakayama,

Shimizu, H. Tokumoto, Appl. Phys. Lett. 83 (2003) 1266.[3] S.V. Rotkin, H.E. Ruda, A. Shik, Appl. Phys. Lett. 83 (2003) 16[4] A. Kleinhammes, S.H. Mao, X.J. Yang, X.P. Tang, H. Shimoda,

Lu, O. Zhou, Y. Wu, Phys. Rev. B 68 (2003) 075418.[5] S. Chopra, K. McGuire, N. Gothard, A.M. Rao, A. Pham, Ap

Phys. Lett. 83 (2003) 2280.[6] S. Peng, K. Cho, Nanotechnology 11 (2000) 57.[7] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (20

1801.[8] O.K. Verghese, et al., Sens. Actuators 81 (2001) 32.[9] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. C

H. Dai, Science 287 (2000) 622.10] J. Li, Y. Lu, M. Cinke, J. Han, M. Meyyappan, Nano Lett. 3 (20

929.

. Lucci is PhD student in “Sensorial Systems” at Tor Vergata Unity, Rome (Italy). He works in the “Minas Laboratory” and “Thin Fieposition Laboratory”. Presently his research activity is mainly focn “Carbon Nanotube for Gas Sensing”, “Plastic Solar Cells Applicand “Deposition and Electronic Spectroscopy of Superconductive Mls”. M. Lucci is author of some papers and of 1 patent and of 2 paending.

ietro Regoliosi has taken is degree in Physics in 2002 at Univef Rome “La Sapienza”. Since then he is pursuing his PhD work inlectronic Engineering Department of University of Rome, Tor Verghere he studies the sensing application of carbon nanotubes anomposites. He is also involved in photoconductivity measuremenemiconductor devices (principally GaAs and GaN based ones) to mheir thermal behaviour.

r. A. Reale received the PhD in Microelectronics and Telecommunions in 2001. His main topic of interest include the experimental sf carbon nanotubes for their sensoristic applications for strain andure sensors, as well as for gas sensing. His interests also incluheoretical and experimental analysis of the optical, electro-opticalectrical properties of heterostructure devices based on nitrides, atudy, design, characterization of the linear and non-linear optical pries of active and passive devices for telecommunications. Dr. Rea4 publications on international journals, and 2 patent pending.

ldo Di Carlo received the physics degree from the University of Roome, Italy, in 1991, and the PhD degree from the Walter Sch

nstitute of the Technical University, Munich, Germany, in 1995. Hurrently an associate professor in the Department of Electronic Eng

ng, University of Rome “Tor Vergata,” Rome, Italy, where his resenterests include the theoretical study of optical and transport procn semiconductor nanostructures, devices, and organic materials.

ilvia Orlanducci was born on 5 February 1974 in Rome. EducathD in Chemistry in 2003 at “Tor Vergata” University of Rome. Title

hesis “Synthesis and Characterisation of Nanostructured Carbon

186 M. Lucci et al. / Sensors and Actuators B 111–112 (2005) 181–186

rials” under the supervision of Prof. M.L. Terranova. In 1999 Degreein Chemistry. Research activity: synthesis of diamond films for photoe-mission. Synthesis and characterisation of single wall carbon nanotubes,post-synthesis treatments of CNTs: purification, functionalization, poly-mer composite preparation, self-assembled systems. CNTs based sensorsdevice assembling. She is co-author of 26 papers in international journalswith referee, 12 conference proceedings and more than 70 conferencepresentations.

Emanuela Tamburri studied Chemistry at the “La Sapienza” Universityof Rome (Italy) completing her MSc in 2003 on the area of specializa-tion: Synthesis and Characterization of Materials, Inorganic Chemistry.In the years 2003–2004 she joined scientific activities in the Departmentof Chemical Sciences and Technology at the “Tor Vergata” University ofRome (Italy) where in 2004 she was awarded a research scholarship toundertake a PhD in Chemistry. Her research interests are in: synthesis andcharacterization of electrically conducting and electroactive�-conjugatedpolymers; synthesis and characterisation of polycristalline diamond andsingle wall carbon nanotubes; post-synthesis treatments of carbon nan-otubes: purification, functionalization, polymer composite preparation,self-assembled systems; assembling of carbon nanotubes based sensordevices.

Prof. Maria Letizia Terranova is professor of “Nanostructured Materi-als”, “Lab of Solid State Chemistry” and “General Chemistry for Physics”at Tor Vergata University, Rome (Italy). Chief of the “Laboratory of FilmDeposition” and Coordinator of “Interdisciplinary Micro and Nanostruc-tured System Lab” (MINASlab). Experience in coating technologies andmaterial science. Presently her research activity is mainly focused onsynthesis, post-synthesis treatments, chemical–physical processing andfunctional characterizations of carbon-based materials and nanostructuresf tor of

national projects on production and applications of nanomaterials. Authorof 160 papers and of 3 patents, editor of 2 books.

Paolo Lugli was born in Carpi, Italy, in 1956. He received the Laureadegree in physics from the University of Modena, Modena, Italy, in 1979,and the PhD degree in electrical engineering from Colorado State Univer-sity, Fort Collins, in 1985. He is currently a full professor at the Lehrstuhlfur Nanoelektronik, TU Munchen, Munchen, Germany, and was formerlyfull professor of optoelectronics at the University of Rome “Tor Vergata,”Rome, Italy. His current research interests include the theoretical studyand numerical simulation of semiconductor nanostructures and devices.

Corrado Di Natale is an associate professor of electronics at the Fac-ulty of Engineering of the University of Rome “Tor Vergata”. His mainresearch interests are in the fields of chemical sensors for taste and olfac-tion sensor systems, molecular electronics, and multicomponent analysis.He authored more than 300 papers on peer reviewed journals and inter-national conferences.

Arnaldo D’Amico is full professor of electronics at the Faculty of Engi-neering of the University of Rome “Tor Vergata”. His main researchinterests are in the fields of chemical sensors for taste and olfaction sen-sor systems, micro and nano-systems, and low voltage analog electronics.He authored more than 450 papers on peer reviewed journals and interna-tional conferences. Since 1999 he serves as chairman of the EurosensorsConferences steering committee.

Roberto Paolesseis an associate professor of inorganic chemistry atFaculty of Engineering of the University of Rome Tor Vergata. His mainresearch activity is concerned with the design and the synthesis of pyrrolicmacrocycles and their characterization and application as chemical sen-sors. He authored more than 200 papers on peer reviewed journals andi

or applications in electronics, optoelectronics and sensing. Coordina nternational conferences.