We present local conductance measurements of carbon nanotube networks with nanometer scale resolution and show that there are discrete drops in conductance that correspond to junctions of metallic nanotubes and semiconducting nanotubes. The anomalies of these networks compared to thin films are shown, and a new method of discerning between semiconducting and metallic single-wall carbon nanotubes is demonstrated

Boland, J. J.

Falvo, M. R.

Fridman, Y.

Fu, Q.

Liu, J.

Novak, J.

Papadakis, S. J.

Snow, E.

Stadermann, M.

Superfine, R.

Washburn, S.

English

Carolina Digital Repository

SARAPID COMMUNICATIONSPHYSICAL REVIEW B 69, 201402~R! ~2004!Nanoscale study of conduction through carbon nanotube networksM. Stadermann,1,* S. J. Papadakis,2 M. R. Falvo,3 J. Novak,4 E. Snow,4 Q. Fu,5 J. Liu,5 Y. Fridman,6J. J. Boland,7 R. Superfine,2,3,6 and S. Washburn2,3,6,81Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA2Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA3Curriculum in Applied and Materials Science, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, U4Naval Research Laboratory, Nanostructures Section Code 6876, Washington, DC 20375, USA5Gross Chemistry Laboratory, Duke University, Box 90354, Durham, North Carolina 27708, USA6Department of Computer Science, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA7Department of Chemistry, Trinity College, Dublin 2, Ireland8Department of Biomedical Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA~Received 20 January 2004; published 18 May 2004!We present local conductance measurements of carbon nanotube networks with nanometer scale resolutionand show that there are discrete drops in conductance that correspond to junctions of metallic nanotubes andsemiconducting nanotubes. The anomalies of these networks compared to thin films are shown, and a newmethod of discerning between semiconducting and metallic single-wall carbon nanotubes is demonstrated.DOI: 10.1103/PhysRevB.69.201402 PACS number~s!: 73.63.Fg, 73.63.Rtndelawonroiclt,deoveuahhaiovIncuotueththThoolser-ntoowbescalbyOnthewasasork.lidre-eram-e-hiftfsti-or-le,Onon-sc-theoororkartstainceI. INTRODUCTIONCarbon nanotube field effect transistors~CNT-FETs!1,2have been shown to be very sensitive to ambient conditiocharge transfer due to gas adsorption to the nanotube3 or thecontact4 can drastically change the conductance of thevice. This makes CNT-FETs ideal systems for molecusensing. It has been shown that coating the nanotubespolymers can increase their selectivity in sensapplications,5 and it is hoped that further functionalizatiowill provide sensitivity to specific compounds, such as pteins or sugars, or even larger objects, such as viruses.There are significant challenges associated with fabrtion of devices out of individual CNT. Placement is difficuand CNTs display a wide variety of transport propertiespending on their diameter and chirality. Random networkscarbon nanotubes have been explored as an alternati6,7Nanotube networks take up more space than individsingle wall carbon nanotube~SWNT!, but they are mucheasier to fabricate and show great promise for simple mfabrication. The macroscopic behavior of these networksbeen investigated in Ref. 6. Although it might appear tthese networks could be readily treated by percolattheory,8 in practice the mat sizes for device applications hatoo few elements for this kind of analysis to be instructive.this paper, we perform a nanoscale investigation of therent paths through nanotube networks. We find that the cductance drops sharply at metallic/semiconducting nanojunctions, which we identify by backgating the sample. Walso find that the most important parameter determininglocal conductance at a certain point in the network isnumber of connections at that point.II. DESIGN AND IMPLEMENTATIONTwo different types of nanotube samples were used.first sample consisted of a silicon dioxide surface, upwhich 20 nm of gold were deposited by sputtering. The g0163-1829/2004/69~20!/201402~3!/$22.50 69 2014s:-rithr-a--f.alssastner-n-beeeendis patterned by scratching. Then, tubes synthesized by laablation suspended in 1-Methyl-pyrrolidine were spun othe surface. This sample type provides networks with linterconnectivity. The second type of sample used nanotugrown directly onto silicon dioxide substrates using chemivapor deposition.9 Gold was deposited onto the surfacethermal evaporation and patterned by shadow-masking.samples with back gates, aluminum was deposited onback before the gold deposition, and then the samplesintered for 30 min at 400°C. The oxide thickness w1 mm.A conductance imaging microscope10 was used to takehigh-resolution conductance measurements on the netwThis AFM method uses a tuning fork force probe. A soPt/Ir tip is attached, which allows for simultaneous measument of topography and conductance. A lock-in amplifiwas used to process the current signal. The oscillationplitude of the tuning fork is about 40 nm in all measurments, and scanning is performed with a frequency sD f 52.3 Hz. The sensors have a resonance frequency of 0512.5 kHz. The force between tip and sample was emated from frequency shift/distance curves to be on theder of 80 nN. A voltage of 0.1 V was applied to the sampwhile the tip remained grounded, unless noted otherwise.samples with two contacts, the voltage is applied to one ctact, and the other contact is unconnected.III. RESULTS AND DISCUSSIONThe behavior of current flow through SWNT networkdiffers from that through conductive thin films. The condutance does not decrease linearly with the distance fromelectrode. Instead, we find for nanotube networks with pinterconnections, that the conductance through the netwstays at the same order of magnitude throughout large pof the network, but then decreases dramatically at cerpoints in the network, as shown in Fig. 1. The resistanincrease at these points is on the order of 108 V, and they©2004 The American Physical Society02-1onbthot-nor-ught-asperkaadRAPID COMMUNICATIONSM. STADERMANN et al. PHYSICAL REVIEW B 69, 201402~R! ~2004!coincide with nanotube junctions. We believe that the cductance drops are associated with junctions formedcrossed semiconducting and metallic nanotubes, sincetype of junction is a Schottky barrier11 and is known to havea significantly higher resistance than metallic/metallicFIG. 1. ~Color online! ~a! Topography image of a nanotube nework with low interconnectivity. The network contains bundleswell as individual tubes. These tubes were deposited from sussion and form clusters.~b! Conductance image of the same netwotaken simultaneously. The tubes appear to be thicker in this imthan in~a!. This effect only occurs on insulating substrates and mbe related to charging.~c! Trace through the network, as indicateby the white line in~b!. The electrode is at the top left~not shown!.Conductance decreases coincide with nanotube junctions.20140-yisrFIG. 2. ~Color online! Conductance image of a nanotube nework with high interconnectivity. These tubes were directly growby CVD onto the sample. The electrode is in the bottom right cner. The conductances at pointsA, B, and C are 700, 200, and700 nS.FIG. 3. ~Color online! A back gate voltage blocks current flowthrough semiconducting tubes.~a! Conductance image taken at Vg50 V. ~b! Conductance image taken at Vg510 V. Parts of thenetwork visible in~a! disappear because they are connected throsemiconducting tubes.n-gey2-2atitsnioultipththlyeleththt-wthinhaogrtstaewtheweofent-in athevemi-rry atri-ghws ofde.c-thelec-ish-sedins,plnd PngetR., R.P.RAPID COMMUNICATIONSNANOSCALE STUDY OF CONDUCTION THROUGH . . . PHYSICAL REVIEW B69, 201402~R! ~2004!semiconducting/semiconducting junctions.In networks with high interconnectivity, we again find ththe conductance does not decrease monotonically withcreased distance from the electrode~Fig. 2!. Instead, we canidentify areas of high conductance more than 80mm awayfrom the electrode. Local conductance is more dependenthe number of connections to the specific area: clusternanotubes with many paths to the electrode have sigcantly higher conductances than those parts of the netwconnected to the rest through fewer paths. Not only do mtiple paths reduce resistance through the presence of muparallel resistors, but they also increase the probabilitythere is a chain of nanotubes stretching to the electrodeis purely metallic. Areas with low conductance typically onhave two to three connections to the network, thus it is likthat these connections are dominated by the presenchighly resistive metallic/semiconducting junctions.When a sufficient back gate voltage is applied tosample, current flow through the semiconducting tubes innetwork is suppressed~Fig. 3!. This behavior is consistenwith earlier work on individual SWNT field effect transistors. The bands in the semiconducting tube are shifted dowards in energy by a positive applied voltage, reducingnumber of carriers in the tube, and, more importantly,creasing the width of the Schottky barrier at the contact tdominates the transport through the tube.12 The semiconduct-ing tubes and all parts of the network connected by semicducting tubes are no longer visible in the conductance imaUsing this technique, we view only the purely metallic paof the network and identify their spatial extent. Our dademonstrate that the majority of high-conductance conntions consist of metallic tubes. When connecting the t*Electronic address: staderma@email.unc.edu1S. Tans, A.R.M. Verschueren, and C. Dekker, Nature~London!393, 49 ~1998!.2R. Martel, T. Schmidt, H.R. Shea, T. Hertel, and P. Avouris, ApPhys. Lett.73, 2447~1998!.3J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng, K. Cho, aH. Dai, Science287, 622 ~2000!.4S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, anAvouris, Phys. Rev. Lett.89, 106801~2002!.5P. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. Dai, S. Peand K. Cho, Nano Lett.3, 347 ~2003!.6E. Snow, J. Novak, P. Campbell, and D. Park, Appl. Phys. L20140n-onoffi-rkl-leatatyofeen-e-tn-e.c-oelectrodes on the sample macroscopically and measuringchange in source-drain current with applied gate voltage,find for the sample in Fig. 3, at a source-drain voltageVsd5100 mV applied between the two electrodes, a currof I sd52.9431027A at a gate voltageVg50 V, and I sd52.5431027A at Vg510 V. Other samples show conductance changes between 1% and 50%. This means that,mat of SWNT between two electrodes, a large part ofcurrent is carried by a small number of highly conductimetallic connections, and the connections involving seconducting tubes, even though much more numerous, cacomparable or smaller portion of the current.IV. CONCLUSIONWe have applied a new method for characterizing eleccal networks to perform an analysis of current paths throua nanotube network. It was found that in networks with lointerconnectivity, the conductance decreases in large stepGV magnitude with increasing distance from the electroIn networks with high interconnectivity, the local condutance depends strongly on the number of connections torest of the network, and much less on distance to the etrode. We have demonstrated a new method for distinguing between metallic and semiconducting SWNT, and uthis method to determine the extent of purely metallic chain a network.ACKNOWLEDGMENTThis work was supported by the ONR-MURI programand the Science Foundation Ireland..d.,t.82, 2145~2003!.7J. Novak, E. Snow, E. Houser, D.P. and J.L. Stepnowski, andMcGill, Appl. Phys. Lett.83, 4026~2003!.8A. Miller and E. Abrahams, Phys. Rev.120, 745 ~1960!.9Q. Fu, C. Lu, and J. Liu, Nano Lett.2, 329 ~2002!.10M. Stadermann, H. Grube, J. Boland, S. Papadakis, M. FalvoSuperfine, and S. Washburn, Rev. Sci. Instrum.74, 3653~2003!.11Z. Yao, H.W.C. Postma, L. Balents, and C. Dekker, Nature~Lon-don! 402, 273 ~1999!.12J. Appenzeller, J. Knoch, V. Derycke, R. Martel, S. Wind, andAvouris, Phys. Rev. Lett.89, 126801~2002!.2-3

Nanoscale study of conduction through carbon nanotube networks

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Nanoscale study of conduction through carbon nanotube networks

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