Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
. MEASUREMENTS OF TIIE CONDUCTIVITY OF INDIVIDUAL 10 NM CARBON NANOTTJBES GEORGE M. WHITESIDES AND CARL S. WEISBECKER Harvard University, Departmentof Chemistry,12 Oxford St. Cambridge,MA 02138. ABSTRACT Catalytically grown carbonfibers approximately10 nm in diameterand severalmicrons long were characterizedby transmissionelectronmicroscopyand determinedto be multiplewalled nanotubes,and a techniquewas developedto measurethe conductivity of an individual nanotube.Nanotubeswere dispersedin solventsand precipitatedonto lithographically defined gold contactsto make a 'nano-wire' circuit. Non-contactAFM was usedto image the nanowires, and a resistanceo_fLI.4 (+ 1.0 ) MC) was measuredthrough a single nanotubeat23o C. A resistivity of 9.5x10-) Q m was estimatedfor carbonconductingalong the axis of a fiber. Local heatingof nanotubesappearedto occur at high current densities.The nanotubescould sustaincurrentson the order of 10 pA per fiber, but applicationofcurrents on the order of 100 pA per fiber resultedin rapid decompositionin air and breaking of the circuit. INTRODUCTION A circuit was designed,consistingof two gold contactsseparatedby approximately5 pms and connectedby a carbon nanotube,and current was passedthrough the circuit. To our knowledge no other measurementsof the conductivity of individual carbon nanotubeswith comparablysmall diametershave beenreported,althoughsometheoreticalpredictionshave been advancedregardingelectronicpropertiesof nanotubes[-3]. Conductivity measurements of much larger carbon fibers of severaltypes [4-8] and recently of carbon nanotubebundles nanofibersis difficult to [9,10] are also documented.Electrical resistivity in catalytically grown 'nano-wires' were determineby conventionalmeansbecauseof their small size [11]. The imagedby non-contactAFM. Relatedimagesof nanotubesand nanotubebundleshave been reportedusing other scanningprobe techniquesU2-141. Carbon nanotubesare interestingfor the purposeof building nano-scaledevicesbecauseof their size. Our researchgroup has an interestin the developmentof new lithographic techniques for the patterningof surfacesI I 5] , and one of the reasonsfor developingsuch techniquesis to find better ways of fabricating small (lessthan 30 nm ) features.Various non-classical phenomenasuch as single electrontunneling [6], which might be exploited to build new devices,are observedat increasinglyhigh temperaturesas featuresizesdecrease.A complementaryapproachto building nano-electronicdevicesmight be to chemically build up structuresfrom colloidal or molecularpiecessuch as carbonnanotubesin addition to relying on lithographic techniques. EXPERIMENTAL A sampleof catalytically grown carbonnanotubeswas obtainedfrom Hyperion Catalysis International.We imaged the carbonnanotubesby transmissionelectronmicroscopy (TEM), and the carbon in the materialconsistedmostly of fibers (figure 1). TEM was performed on a Phillips EM 420 microscope.The carbonnanotubeswere dispersedin ethanolor xyleneswith sonicationand pipetted onto TEM grids from StructureProbesInc. The fibers had diametersof approximately 10 nm and lengthsof severalpms. 263 MaterialsResearchSociety Mat. Res.Soc. Symp. Proc.Vol.349.@1994 N-dopedsilicon (100) waferswere obtainedfrom Silicon SenseInc. Thesewaferswere oxidizedin air at l0O0 o C for 24 hours.The oxidizedwaferswere mountedin a dual sourceebeamevaporatorcontaininga gold sourceand eithera titanium,nickel,or chromium sourceto be used as an adhesionpromoter. l0 A of adhesionpromoter followed by 200 A of gold were depositedon the '*'afers.The gold coatedwafers were patternedthrough a mask using photolithographl.then the exposedgold was etchedfor 2 secin Gold EtchantTFA suppliedby TranseneInc. The oxide layer insulatedthe gold contactsfrom eachother and from the silicon substrate.The resistremaining on the gold contactswas strippedoff, and PMMA was then spin-coatedonto the wafersfor e-beampatterning. E-beampatterningwas performedon a JEOL JSM-flO ScanningMicroscope.A window parrernwas written in the PMMA resist,and it consistedof a rectangularbox with dimensions of l0 p.m x 20 pm. The window areawas scannedwith a 35 kV 400 pA e-beamusing a line doseof 8 nC/cm for best results.Upon development,the PMMA completely insulatedthe gold contactsexceptinsideof this window. The developmentof thesewindows and the number of nano-wirespresentinside of them were assessedby AFM on a TopometricsScanningProbeMicroscope.The fibers were imaged in air on the AFM using low resonantfrequency 1660-00silicon tips from Topometrics.The tips oscillatedat approximately 150 kHz, and amplitudedetectionwas usedto collect the sign_al. Currentswere applied to the gold contactsusing a PrincetonApplied ResearchModel273 operatedin the galvanostaticmode. The current sourcewas connected PotentiostaUGalvanostat to the gold substratevia wires plantedinto indium solder. RESULTS Figure 1: Typical nanotubeshave diametersof about 10 nm and lengthsof severalpms. The high resolutionimage on the left was recordedby Y. Lu at the Harvard MRL Central Facility. 0 0nm Figure 2: A non-contactAFM image of a nanotubeprecipitatedonto a gold film is depicted. Figure 3: Lithographedgold contactson an oxidized silicon substrateare connectedto wires via indium solder.A rectangularwindow in PMMA is centeredwhere the contactsmake their closestapproachat a separationof approximately5 pm (seearrow). E-Beam exposuresalong the top left contactoccurredduring positioning of the e-beamfor the window exposure.The inset of this visual image is a view through an optical microscope. \ We mounted the carbon fibers on lithographicallydefined gold contacts-sothat we could was used as a measuretheir conductivity, and non-contait a'tomicforce microsco-qY-(AFM) images method for imaging thesefibers on a circuit. Comparisonof the AFM imageswith TEM thicknessof fibers enoneouslyappearedto be approxl-T3jglyten times showedthat the"lat-eral able to wider than the .o.r..iiv i.aled vertical thicknessin the AFM images(figure 2).y" were and tip AFM the between interaclion weak the because scanover individuii;6.;;;;pr"ducibly the fibers did not causethem to move. Dispersionsof carbon nanotubeswere sonicatedin ethanolto make dispersionsof 4xl0-3 or 4xl0-4 me/ml and precipitatedonto thermally oxidized silicon (100) substrates puti..nJ with lithofrupn"a gbtOf6ututes(figure 3). Tha gold featureshad been previoutly ,-_, pmz areawhere coatedwith insulatingpoly(methyl methacrylate)(P.MMAIexcept within a20o pMMA was subsequEn'tfy'rL*oved using e-'beamlithography.The solventwas either allowed to under i streamof argon-'A 1,90pA current was typically dry in air or to "uupoiutJquickly A voltage greaterthan 1-0V betweenthe contactswas contacti. two^gold across uppti.O to the depositionof fibers. The occurrenceof .up".ltatiGrfprior iirr to measuredano ouseri"eJ in a one or more nanotubeslying ucro^r,the gap b6tweenthe two gold contactsresulted dispersion dramatic and sustaineddecffiasein the u"otiug"that was measired, and drops gf 9. The nanotubes was realized. drop potential a substrate"until the onto r;dii;"t ;;;6"t4 them from preciiitated onto an areaof approximately I cm2, but the PMM+ layer prevented alea window The 20O the of i{rside gold except Ttmz.window' making electrical contactwith number of fibers was small enough to be scanneEwith un'efM tip so that we'could observethe bridging the two gold contactsinside of it' fiber' The -ulrti"uf4: The gold contactsin this image were thought to be bridge.dvia a single Figure was diameter inner The hollow nm. 10 8 and beiween was n"ighTof the nanotube and the assumedto be 1/3 of the total diameterbasedupon TEM imagesof similar fibers, length of the cross-sectionalareaof carbonwas thereforeestimatedto be 57 1+ 16 ) nmz' The f i b e rw a s7 . 5 1 + 1 . 0) P m . 266 0 . 15 0 . 10 230C S fo p e : 1 1 . 4 ( t 1 . 0 ) MO 0.05 o ct) 0.00 G = o J -0.05 -0.10 .df ,lf! ."' .4.0 (p,|') - 0 . 15 -1 -5 5 0 Current (nA) 1.5 0 Figure 5: The voltage vs. appliedcurrent plot correspondingto the image in figure 4 is depicted. The d. c. conductivity was measuredat room temperature(23 o C) using a galvanostatto apply currentsin forward and reversedirectionsthrough the nanotubesthat connectedthe gold contacts.In our initial experimentsno attemptwas madeto thermostatthe circuit or correct for self-heatingof the nanotubes.Since the conductivity was derived from the limiting slope of a voltage vs. applied current curve at low currents,self-heatingis not likely to have been a problem. Often, more than one fiber or an aggregateof fibers was observedto be bridging the contacts.Our best exampleof gold contactsbridged by a single fiber is shown in figure 4. Aggregatesof carbon fibers were also present,but only one nanotubewas observedto be bridging the contacts. The voltage vs. applied current plot that we attributeto this single bridging fiber is shown in figure 5. The resistanceof the fiber was obtainedfrom a linear fit to the plot at low magnitudes of applied current, and it is I 1.4 ( t L0 ) MO. This measurementcorrelateswell with some other measurementsthat we have made in which more than one connectingfiber was observed and the measuredresistancewas correspondinglylower. By measuringthe length and crosssectionalareaof the nanotubefrom its AFM image the resistivity of carbon along the fiber axis could be estimatedto be 9.5x10-5 Q m. This estitated resistivity is more than an order of magnitudehigher than reportedresistivitiesin the basalplane of orderedpyrolytic graphite [ 17]. The resistivity in carbon nanotubebundlessynthesizedby arc dischargehas beenbeen estimatedby othersto vary between t0-a and 10-5C)m igl anAto be 6.5xlO-5 O m at 300 K [0]. The room temperatureresistivity of methane-derivedcarbonfibers was shown to be 10-) Q m or less dependingon heat treatmenttemperature[7]. The resistivity of benzene- 267 derived carbon fibers was shown to be on the order of l0-6 C)m after heat treatmentat 3500'C, and it increasedas the diameterof the fiber decreasedbetween34 and 1.6 ttms [8]. At currents above I pA the voltage vs. applied current curve in figure 5 deviates significantly from linearity and the measuredvoltage becomesmore uncertain. The reason for the lowering of the resistanceat high currents may be local heating of the nanotubes.We observed that the nanotubeswere capable of supporting on the order of l0 pA of applied current per fiber without being damaged.Application of currents on the order of 100 pA per fiber resulted in rapid decomposition of the fibers in air and breaking of the circuit. We wish to thank Dr. D. Moy at Hyperion for providing the carbon fibers and for advice. CONCLUSIONS These carbon fibers show promise as materials for building nano-scaleelectronic devices. They can be imaged individually and reproducibly on a circuit by non-contact AFM. Our estimate of the rJsistivity of carbon in a-single nanotubeparallel to its axis is much higher.than resistivitiesthat have be-enreportedfor orderedpyrolytic graphite [ 17]. The resistivity ir similar to resistivity measurementsoTnanotubesof similar diameterin coaxial bundles [9,10]. The resisitivity is an order of magnitudeor more higher than the resistivity of individual vapor grown fibers with diametersgreaterthan I pm [7,8]. The contributionof contactresistanceto 6ur measurementis still unclear,but someinformation about contactresistancemay be A four-point probe measurementof a single obtainablefrom a. c. conductivity measurements. nanotubewould be desirable,bui also difficult to accomplish.We will also attemptin future experiments to better control and detect local heating that is probably occurring when we apply high currentsto the nano-wires. REFERENCES N. Hamada,S. -i. Sawada,and A. Oshiyama,Phys.Rev. ktt. 68 (10), 1579(1992). H. Ajiki, T. Ando, J. Phys.Soc.Jpn.62 (4), 1255-1266( 199_3). J. Appl. Phys.73 (2), 494-5N (1993). M. S. Dlesselhaus, R. Siito, G. Dresselhaus, J.Imai and K. Kaneko,Langmuir, 8, 1695-1691(1992). K. Kuriyama,Phys.Rev. B, 19 (47),12415-12419(1993)-__ C. Olorian, M. Matte, and C. J. Lum, Electrochem.Soc. 138 (8), 2330-2334(1991'). J. Heremans,I. Rahim, and M. S. Dresselhaus,Phys. Rev. B 32 (10), 6742 (1985). and M. Endo,CarbonA (l),67-72 (1986). M.Z. Tahar,M. S. Dresselhaus, L. Langer, L. Stockman,J. P. Heremans,V. Bayot, C. H. Olk, C. Van Haesendonck,Y. Bruynseraede,and J-P. Issi, J. Mater. Res. 9 (q,2n,Q994). 10. S. Ni. Song,X. K. Wang, R. P. H. Chang,and J. B. Ketterson,Phys.Rev. Lett. 72 (5),697 (1994). 700 1 1 .N. M. Rodriquez,J. Mater. Res. 8(12), 3246 (1993). t 2 . C. M. Liebeiand Z.Zhang, Appl.Phys. ktt. 62,2795 (1993)' D. Sarid, L. D. Lamb, F. A. Tinker, J. Jiao, and 1 3 .M. J. Gallagher,D. Chen,-B.P.-Jacobsen, D. R. Huffman, Surf. Sci. Lett. 281,L335-L340 (1993). 14. C. H. Olk and J. P. Heremans,J. Mater. Res.9 (2),259-262 (1994). 1 5 .A. Kumar and G. M. Whitesides,Science263,60 (1994). 1 6 .Q. Niu, Phys.Rev. I-ett.64 (15), 1812(1990);P. Delsing, K. K. Likharev,L. S. Kuzmin, (1989). andT. Claeson,ibid. 63 (17), 1861-1864 1 7 .c. A. Klein, J. Appl. Phys.33, 3346(1962);p. R. Wallace, Phys.Rev. 7l (9),622 (1947). 1. 2. 3. 4. 5. 6. 7. 8. g.