Vertically aligned conductive carbon nanotube junctions and arrays for deviceapplicationsSujit K. Biswas, Robert Vajtai, Bingqing Wei, Guowen Meng, Leo J. Schowalter, and Pulickel M. Ajayan Citation: Applied Physics Letters 84, 2889 (2004); doi: 10.1063/1.1702130 View online: http://dx.doi.org/10.1063/1.1702130 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Junction array carbon nanotube bolometer J. Appl. Phys. 113, 164307 (2013); 10.1063/1.4802582 An experimental method to determine the resistance of a vertically aligned carbon nanotube forest in contact witha conductive layer J. Appl. Phys. 112, 044901 (2012); 10.1063/1.4742069 Horizontally-aligned carbon nanotubes arrays and their interactions with liquid crystal molecules: Physicalcharacteristics and display applications AIP Advances 2, 012110 (2012); 10.1063/1.3679155 Sub-ppm NO 2 detection by Al 2 O 3 contact passivated carbon nanotube field effect transistors Appl. Phys. Lett. 94, 183502 (2009); 10.1063/1.3125259 All-around contact for carbon nanotube field-effect transistors made by ac dielectrophoresis J. Vac. Sci. Technol. B 24, 131 (2006); 10.1116/1.2150226

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.204.37.217 On: Mon, 01 Dec 2014 22:27:41

Vertically aligned conductive carbon nanotube junctions and arraysfor device applications

Sujit K. BiswasDepartment of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy,New York 12180

Robert Vajtaia)

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180

Bingqing Weia)

Department of Electrical and Compter Engineering, Louisiana State University, Baton Rouge, LA 70803

Guowen MengDepartment of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180

Leo J. SchowalterDepartment of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy,New York 12180

Pulickel M. Ajayana)

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180

~Received 26 September 2003; accepted 13 February 2004!

Electrical transport through high-density arrays of carbon nanotubes grown within vertical pores ofanodized alumina was measured. Individual nanotubes were studied using conductive tip atomicforce microscopy, with bias applied between the tip and platinum back electrode. Multiwallednanotubes of diameter about 50 nm, with 5 nm thick walls were found to have a resistivity lowerthan 1.431025 V m. A potential barrier was found to exist between the sensing tip and nanotube,resulting in nonlinear current–voltage characteristics. Low-resistance contact was formed bybreaking down this barrier, once the circuit was stressed beyond 1.5 V. ©2004 American Instituteof Physics. @DOI: 10.1063/1.1702130#

The major challenge in using carbon nanotubes as semi-conducting devices or interconnect material in electronics isgetting them to grow at precisely determined locations. Ac-tual development of circuits need controlled growth of nano-tubes with uniform and tailored properties.1 Moreover, thesemethods should be compatible with current microelectronicsfabrication. We describe the electrical characteristics of ver-tically aligned multiwalled nanotubes grown within a nan-oporous template of anodized alumina. The alumina templateprovides a basis for producing a highly uniform array ofnanotubes, which are electrically insulated from each other.The very small sizes of the structures make traditional elec-trical measurements difficult.2 Using contact pads to electri-cally connect multiple vertical nanotubes in parallel yield theaggregate properties of multiple channels. We used scanningsurface potential microscopy~SSPM!, and conductive tipatomic force microscopy~CT-AFM! to measure the proper-ties of each of these structures individually.3–5

Nanoporous alumina templates, 50mm thick, were pre-pared by anodization of alumina substrates.6 Nanotubes weregrown7,8 inside the template using chemical vapor deposition~CVD! with xylene at 800 °C. Electrical back contact to thenanotubes was fabricated by deposition of platinum. Topog-raphy shown in Fig. 1@~a!#, acquired with a tapping modeatomic force microscope~AFM!, using a nanotube enhanced

tip, showed open ended nanotubes inside the template pores.The diameter of the nanotubes ranged between 40 and 60nm. Further structural studies were carried out after etchingoff the alumina template using HF. The etching did not affectthe nanotubes which were analyzed using transmission elec-tron microscopy~TEM!. The TEM images in Fig. 1@~b!#,revealed that the nanotubes had high aspect ratio, were openended, thin walled, and hollow.

Surface potential measurements9 were carried out with aDigital Instruments Nanoscope IIIa AFM and tungsten car-bide coated tips. Topographic information was acquired withthe tip electrically grounded. During the second interleaved

a!Also at: Rensselaer Nanotechnology Center, Rensselaer PolytechnicInstitute, Troy, NY 12180.

FIG. 1. ~a! Tapping mode AFM image of CVD grown open-ended multi-walled nanotubes, within a template of anodized alumina. Superior resolu-tion of the nanotubes was made possible by the use of a nanotube-enhancedtip. The average diameter of the nanotubes was found to be 46 nm, and thecenter-to-center distance was 107 nm. The vertical scale for the image is 60nm. ~b! TEM image of individual multiwalled nanotubes grown inside na-noporous alumina template using chemical vapor deposition, after etchingoff alumina. The tubes are thin walled, hollow, and open ended.

APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 15 12 APRIL 2004

28890003-6951/2004/84(15)/2889/3/$22.00 © 2004 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.204.37.217 On: Mon, 01 Dec 2014 22:27:41

scan, the tip retraces the topographic scan at a preset heightof a few nanometers above the surface. The tip experiences acapacitive force proportional to the potential difference be-tween the tip and local sample region. Consequently, when asinusoidal voltageVac is applied to the tip near the resonantfrequency, the magnitude of the first harmonic of this forcesignalF will be F}(Vsample2Vtip)Vac. The tip voltageVtip isadjusted such that the first harmonic signal is nullified, indi-cating that it is equal to the sample voltageVsample. Thisvoltage is recorded as the tip scans along the surface. As thetip potential is kept nearly equal to the potential on the sur-face, perturbations in measurements using SSPM is verysmall. Surface potential differences as small as 6 mV, limitedby noise, could be measured. The horizontal spatial resolu-tion, determined mainly by the tip shape was about 25 nm.Current measurements10 were done in contact mode withconductive silicon tips with platinum iridium coating. Thesample was biased with a variable voltage source, and thecurrent through the tip was sensed with an inverting modecurrent to voltage converter with variable gains between 106

and 1012. Currents of the order of 0.1 pA could be measured,with a horizontal spatial resolution of 20 nm.

The deposited metal back contact was assumed to beelectrically uniform. The control sample with empty aluminaFigs. 2~a!–2~c! showed only minor potential variations withchange in bias. The structure can be modeled as an infinitecharged plane, with a dielectric media between the conductorand tip. The surface potential variation at a heightz will varyas V(z)5V02(s)/2«0k /z, whereV0 is the applied poten-tial, s is the charge density,k the dielectric permittivity ofalumina. The difference in the voltages measured over alu-mina and holes can be written as:

dV52sz

2«0S 12

1

kD .

This equation explains the nature of potential variations onthe bare alumina template. Surface potential in the nanotubeembedded sample was found to be more responsive to theapplied bias. Figures 2~d!–2~f! shows the surface potentialvariations on the nanotube embedded sample, after the con-stant potential bias has been numerically subtracted from thereadings. The nanotube sites had a potential difference of 56

mV with the surrounding alumina, when biased at23.0 V,and250 mV when biased at13.0 V, as compared to 34 mVfor the bare alumina. This enhancement of the potential atthe nanotube sites gives an indication of the presence of thenanotubes. However potential measurements on these struc-tures do not provide a clear verdict on the contact propertiesof the nanotubes and their conductive nature.2 Direct currentmeasurement on the nanotube sites were performed using ascanning current measurement setup. Current measurementusing the CT-AFM revealed the entire surface to be conduct-ing current for an as-grown sample. An average of610 nAcurrent was measured for potential biases of210 mV, and10 mV, with a gain of 108, translating to a resistance of 1MV. This is the magnitude of the resistance (Rs) which wasput in series with the nanotubes to protect the sensitive elec-tronics. We concluded that the sample surface had a thincoating of residual carbon from the CVD process which hadonly a small resistance and was essentially causing a short tothe back contact.

The sample processing steps were slightly altered to re-move the surface conducting material. The CVD grownsample was etched in 49% HF solution for 15 min, prior tothe back contact deposition. Etching removed a layer of alu-mina from both sides, while leaving the nanotubes un-harmed. This also helped making better back contact to theslightly protruding nanotubes. Topography of the etchedsamples, measured with a metal coated tip is shown in Fig.3~a!. Current measurements clearly showed that the nano-tubes were conducting. The current responded to variation inpotential bias, and reversed polarity as the bias was changedfrom 110 mV to 210 mV as shown in Figs 3~b! and 3~c!.No current was observed in the surrounding alumina. Themaximum current measured in the nanotubes was 20 nA inmagnitude, with a preamplifier gain of 108 and a series re-sistance (Rs) of 100 kV. No current was observed in theempty alumina substrate, probed with a gain of 1012, andbias voltage of magnitude 1.0 V which translates to back-ground noise less than the 1.0 pA.

The nanometer movement flexibility of the CT-AFMwas used to position the tip on specific locations to measurecurrent–voltage characteristics of each vertical nanotubechannel.11 A resistance of 1 MV was placed in series with thenanotubes, as shown in the schematic in Fig. 4. The current–voltage measurements shown in Fig. 4, observed over sev-eral nanotube channels within the range of20.5 to10.5 V,demonstrated repeatable nonlinear characteristics. The resis-tance of the circuit was calculated to be in the order of 20MV at 0.4 V. The nonlinear current–voltage curves suggestthe presence of a potential barrier. Measurement over several

FIG. 2. Surface potential image of bare alumina template~a!–~c!, and atemplate with CVD grown nanotubes~d!–~f!. Measurements were carriedout at24.5, 0.0, and14.5 V bias from left to right. The constant potentialbias was subtracted from the nanotube sample signals showing a potentialenhancement of 56 mV at the sites occupied with nanotubes in comparisonto the bare alumina substrate with potential variation of 34 mV at63.0 Vbias.

FIG. 3. Topography~a!, and conductive tip AFM image of current in nano-tubes, taken at~b! 110 mV, and~c! 210 mV bias with an inverting modepreamplifier gain of 108 and a series resistance of 100 kV. Current range is20 nA, showing responses to change in the applied bias and polarity.

2890 Appl. Phys. Lett., Vol. 84, No. 15, 12 April 2004 Biswas et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.204.37.217 On: Mon, 01 Dec 2014 22:27:41

nanotube sites showed that the nature of the measurementscan drastically change once very high currents were passed.Ramping the bias above 1.5 V resulted in currents in theorder of a few microamperes. The current–voltage character-istics only yielded the value of the limiting resistanceRs

when the circuit was under stress. This indicated that theresistance of the nanotube junction had been lowered far be-low the current limiting resistance of 1 MV. The fact thatthis barrier was present for each measurement suggested thatit was mainly due to the nanotube to tip contact. A highcurrent through the circuit drastically improved the contactproperties. These nanotube circuits could sustain currents inthe order of a few microamperes, limited by the series resis-tance, for about 10 min before the contacts were lost. How-

ever, the measurements conclusively showed that the nano-tubes were electrically conducting channels with a resistivityof 1.431025 V m, and could sustain high current densitiesof at least;105 A/cm2.

In summary, the current measurement with CT-AFMdemonstrated that the vertical nanotubes grown within thepores of alumina templates were electrically conducting. Thecurrent–voltage characteristics were found to be sensitive tothe tip to nanotube junction. The nonlinear nature of thecharacteristics indicated the presence of a barrier, whichcould be broken down when the tubes were stressed with ahigh enough bias to pass microamperes of current. Such uni-form high-density structures can prove useful for three-dimensional contacts or for device fabrication.

This work was supported by the Interconnect Focus Re-search Center, NY and the Nanoscale Science and Engineer-ing Center at Rensselaer Polytechnic Institute. The authorsacknowledge Dr. S. B. Schujman for her assistance in theelectrical measurements.

1A. Bachtold, P. Hadley, T. Nakanishi, and C. Dekker, Science294, 1317~2001!.

2L. Ba, J. Shu, Z. Lu, J. Li, W. Lei, B. Wang, and W. Li, Appl. Phys. Lett.93, 9977~2003!.

3G. Friedbacher and H. Fuchs, Pure Appl. Chem.71, 1337~1999!.4A. Bachtold, M. S. Fuhrer, S. Plyasunov, M. Forero, E. H. Anderson,A. Zettl, and P. L. McEuen, Phys. Rev. Lett.84, 6082~2000!.

5M. Freitag, M. Radosavljevic, W. Clauss, and A. T. Johnson, Phys. Rev. B62, R2307~2000!.

6Y. Li, G. W. Meng, and L. D. Zhang, Appl. Phys. Lett.76, 2011~2000!.7E. J. Bae, W. B. Choi, K. S. Jeong, J. U. Chu, G.-S. Park, S. Song, andI. K. Yoo, Adv. Mater.~Weinheim, Ger.! 14, 277 ~2002!.

8B. Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath, and P. M.Ajayan, Nature~London! 416, 495 ~2002!.

9M. Nonnenmacher, M. P. O’Boyle, and H. K. Wickramasinghe, Appl.Phys. Lett.58, 2921~1991!.

10A. Fujiwara, R. Iijima, K. Ishii, H. Suematsu, H. Kataura, Y. Maniwa,S. Suzuki, and Y. Achiba, Appl. Phys. Lett.80, 1993~2002!.

11J. Li, R. Stevens, L. Delzeit, H. T. Ng, A. Cassell, J. Han, and M. Meyyap-pan, Appl. Phys. Lett.81, 910 ~2002!.

FIG. 4. Comparison of current, in logarithmic scale, measured on individualnanotube ends, at different locations with a conductive tip. The current–voltage characteristics measured are nonlinear, indicating the presence of apotential barrier. Large currents can break this barrier and make better con-tacts to the nanotubes. Under such conditions the measurements only reflectthe current limiting series resistance (Rs51 MV) indicating that the nano-tube channel resistance is much smaller than 1 MV.

2891Appl. Phys. Lett., Vol. 84, No. 15, 12 April 2004 Biswas et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.204.37.217 On: Mon, 01 Dec 2014 22:27:41