We present a fast, high yield, low cost method for the production of scanning probes with aligned carbon nanotubes protruding from the ends. The procedure is described and images of undercut films are used to demonstrate the improved probe quality for topography measurements. A magnetophoretic model of the attachment and alignment processes is discussed

Falvo, M. R.

Hall, A.

Matthews, W. G.

Superfine, R.

Washburn, S.

Carolina Digital Repository

APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 15 14 APRIL 2003DownSimple and efficient method for carbon nanotube attachment to scanningprobes and other substratesA. HallDepartment Physics and Astronomy, University of North Carolina at Chapel Hill,Chapel Hill, North Carolina 27599W. G. MatthewsCystic Fibrosis Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599R. Superfine, M. R. Falvo, and S. Washburna)Department of Physics and Astronomy, University of North Carolina at Chapel Hill,Chapel Hill, North Carolina 27599~Received 12 June 2002; accepted 17 February 2003!We present a fast, high yield, low cost method for the production of scanning probes with alignedcarbon nanotubes protruding from the ends. The procedure is described and images of undercutfilms are used to demonstrate the improved probe quality for topography measurements. Amagnetophoretic model of the attachment and alignment processes is discussed. ©2003 AmericanInstitute of Physics.@DOI: 10.1063/1.1567049#ane-icsopeeMigsisaeuerchngfoon-aitndhahedeofthetip1FMips0m-ledtip,tiposttipventoaninsetThere have been several attempts to use carbon ntubes ~CNT! as scanning probe sensors. Their high aspratio and mechanical resilience1,2 make them ideal candidates for force measurements, and their ideal electrproperties3 imply utility in scanning potentiometry. Previouattempts have included chemical vapor deposition growtha CNT directly onto commercial atomic force microsco~AFM! tips made of Si or one of its derivatives,4 and themechanical attachment of a CNT onto an AFM tip.5–8 Find-ing these methods somewhat arduous, we have developmethod based on the liquid deposition of a CNT onto AFtips. Recognizing the anisotropic tendency of a CNT to alalong magnetic field lines,9,10 we have used magnetic fieldto align the CNT along the AFM tip axis. In addition, thmethod may be amenable to attaching the CNT to almostconductive material.An apparatus was assembled in order to introducmagnetic field onto a single AFM probe and a nanotube spension, as shown in Fig. 1. Here, a custom-made beakpositioned above a solenoid (;340 turns) containing a highpermeability, type I, low carbon magnetic iron core~Scien-tific Alloys, Westerly, RI!. The beaker was designed in sua way as to minimize the distance between the AFM tip athe solenoid core. This was accomplished by attachinglass cover slip with a shelf, which serves as a platformthe probe chip, to a glass cylinder (;20 mm diameter). Thispositioned the AFM tip about 400mm from the solenoid tip.Into the beaker was introduced 5 mL of carbarc-synthesized11 multiwalled CNTs suspended in dichloromethane. Prior to this introduction, the suspension wsonicated for 10–15 min to ensure maximum homogeneAn AFM probe was sputter coated with 60 nm of gold aintroduced, tip-side down, onto the submerged platform. Tprobe tip was visually aligned above the pole piece. Anternating current of 7 A at 60 Hz was then applied to ta!Electronic mail: sean@physics.unc.edu2500003-6951/2003/82(15)/2506/3/$20.00loaded 15 Jul 2011 to 152.13.249.254. Redistribution subject to AIP licenso-ctalfd annyas-isdarsy.el-solenoid, resulting in a measured magnetic field amplituB0'0.1 T. With this apparatus, the anisotropic propertiesthe CNT cause the tubes that come into contact withprobe tip to be preferentially oriented parallel to thedirection9 and hence protruding down from the end.Using this method, with the solenoid energized formin, we obtain potentially useful CNT-tipped probes~i.e.,probes with a nanotube protruding past the end of the Atip! in about 50% of the trials. Examples of processed tc n be seen in Fig. 2~a!. In total, we have produced over 6tips with a CNT extending from them. We have further deonstrated that tips from unsuccessful trials can be recycby etching away the gold and reprocessing the sameincreasing the overall yield without the expense of a newchip. In general, the length of attached tubes varied, but mwere found to protrude between 100 and 500 nm from thesurface. This is a range conducive to scanning over efairly large features. The protrusion angles with respectthe cone axis of these useful tips were found to haveFIG. 1. Schematic of the experimental apparatus atop a solenoid. Theshows the area through which the changing flux goes.6 © 2003 American Institute of Physicse or copyright; see http://apl.aip.org/about/rights_and_permissionstireadumtincfuuswathhethdsrage-ceeoneocedbe,meatehinnd,ipsheandtheertingotheiconthetedthementat0°.ofomofe ofsig.d-entoughF-ntal2507Appl. Phys. Lett., Vol. 82, No. 15, 14 April 2003 Hall et al.Downaverage value of about 35°, where 0° is parallel with theaxis and ideal for imaging. Population data of measuangle ranges for eight samples are shown in Fig. 2~b! andthey are consistent with our expectations of an approximcosu dependence of alignment with the field.Control experiments involving simple introduction anremoval of the tips into and from the suspension withoenergizing the magnetic coil were shown to attach someterial to the probe surface. In these samples, however,majority of the attached material was amorphous contamtion. Few CNTs were found to lay flat against the surfarather than protruding from the end of the tip. No useCNT-tipped probes were obtained in 20 control trials. Ththe magnetic field appears to be necessary to obtain tipsa CNT extending from them. Similarly, the introduction ofdirect current onto the solenoid failed to attract a CNT toAFM tip and did not result in useful probes.A plausible explanation for the effectiveness of tmethod may be the attraction between dipoles induced inCNT and the metal film on the AFM. Superposing fielfrom slices of the conical tip, we find the highest concenttion of force at the end of the AFM tip. Given that a chanin flux is proportional toAfluxBov ~Ref. 12! ~see Fig. 1!, weexpect an induced current encircling the tipI'(AfluxAcurrentBOv)/(rL), wherev is the frequency,r is theresistivity of the gold coating,L is the average circumferential path length around the tip,Acurrent is the total cross-sectional area of the gold coating on the tip, andAflux is thearea through which the magnetic flux threads. The indudipole of the probe is on the order of 10211 A m2. The av-erage induced dipole on the multiwall CNT, is on the ordof 10223 A m2. This diamagnetic effect can be up to tworders of magnitude larger than the paramagnetic compoexpected from published susceptibility data,13,14allowing theattractive force to be dominant. Modeling the gold coatingthe AFM probe as parallel rings~Fig. 1, inset!, we can evalu-FIG. 2. ~a! Examples of nanotube functionalized probes prepared withmethod and~b! population densities of protrusion angle ranges for eiprocessed tips.loaded 15 Jul 2011 to 152.13.249.254. Redistribution subject to AIP licenspdteta-hea-el,ithee-drntfate the the potential energy between the two indudipoles:12 U(r )5(2mo/2pr3) mtubemprobe, where r is thedistance between the probe tip and the CNT, andmtube andmprobe are the dipole moments of the tube and the prorespectively. Assuming that the attraction must overcotranslational thermal energy@;(3/2)kBT# of the CNT in thesuspension, we find that the dipole attraction can dominfor separations on the order of 1mm. Considering an esti-mated tube density in the suspension of 104 tubes per mm3,we suppose that a CNT may be found occasionally witthis distance due to convection and Brownian motion athat in agreement with our results, we might form useful tin a modest fraction of the time. Thus, it is plausible that tsuspended CNTs are both aligned by the magnetic fieldattracted to the tip by magnetophoresis. Upon reachingtip surface, the CNT is probably held in place by van dWaals interactions after the oscillating field is removed.In order to demonstrate the effectiveness of the resulprobes, we recorded atomic force microscopy images bwith a CNT-modified tip and with a virgin probe of the sambrand. The surface was an undercut metal lead on a silsubstrate. Thus, an ideal cross-sectional AFM image ofedge of the metal would show a 90° angle. In the presenexperiments, edge angle measurements were taken onsame area of the undercut sample before and after attachof a CNT to the tip. The virgin probe yielded a 78° edgebest, which is consistent with the listed probe spec of 8The CNT-modified tip achieved a consistent value87° – 88°. This shows a decrease in the profile error fr13° – 20° to about 3° or a relative increase in precisionabout 80%. We also noted that the mechanical responsthe CNT-modified AFM tips differed from the virgin tips, ais apparent from feedback oscillations in the image in F3~b!. We suppose this is due to our failure to optimize feeback parameters for the modified tips and to the inherrtFIG. 3. AFM images and a typical graph of the cross section of an Hundercut metal surface taken by~a! a conventional tip and~b! a nanotube tipprepared with our method. Shown is the angle of the line with the horizotaken at the indicated point.e or copyright; see http://apl.aip.org/about/rights_and_permissionstsivMioaneisnganreooInchsoulwnce.ci.y,, J..J.M.2508 Appl. Phys. Lett., Vol. 82, No. 15, 14 April 2003 Hall et al.Downcomplexity of feedback control for two spring constan~cantilever and CNT bending! in series.We have developed a simple, reliable, and inexpenstechnique for the high yield assembly of a CNT onto AFprobes. The procedure involves only two steps: Metallizatof scanning probes and placement of the probes in a ntube suspension accompanied by an alternating magnfield. It requires little time and no demanding control. Itpossible that in addition to aligning the CNT, the oscillatimagnetic field provides an attraction between the CNTthe probe tip. Images of chemically undercut metal structu~Fig. 3! provide evidence of the increased effectivenesstips prepared using this method. The ‘‘cone angle’’ effectthe tip shape for the virgin AFM probe is about 70° – 77°.contrast, the artifact for the CNT tips is 87° – 88°, or mucloser to the ideal form factor of 90°. A concomitant increain detail is apparent in the lateral resolution of the topgraphic images taken with the latter probes.The authors would like to thank Dr. J. P. Lu for helpfdiscussions on magnetic interactions of CNTs and acknoloaded 15 Jul 2011 to 152.13.249.254. Redistribution subject to AIP licenseno-ticdsffe-l-edge the Office of Naval Research and the National ScieFoundation for financial support of this work.1M. R. Falvo, G. J. Clary, R. M. Taylor II, V. Chi, F. P. Brooks Jr., SWashburn, and R. Superfine, Nature~London! 389, 582 ~1997!.2E. W. Wong, P. E. Sheehan, and C. M. Lieber, Science277, 1971~1997!.3H. Dai, E. W. Wong, and C. M. Lieber, Science272, 523 ~1996!.4C. L. Cheung, J. H. Hafner, and C. M. Lieber, Proc. Natl. Acad. SU.S.A. 97, 3809~2000!.5H. J. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. SmalleNature~London! 384, 147 ~1996!.6S. Akita, H. Nishijima, Y. Nakayama, F. Tokumasu, and K. TakeyasuPhys. D32, 1044~1999!.7H. Nishijima, S. Kamo, S. Akita, Y. Nakayama, K. I. Hohmura, S. HYoshimura, and K. Takeyasu, Appl. Phys. Lett.74, 4061~1999!.8E. W. Wong, P. E. Sheehan, and C. M. Lieber, Appl. Phys. Lett.73, 3465~1998!.9J. P. Lu, Phys. Rev. Lett.74, 1123~1995!.10B. W. Smith, Z. Benes, D. E. Luzzi, J. E. Fischer, D. A. Walters, M.Casavant, J. Schmidt, and R. E. Smalley, Appl. Phys. Lett.77, 663~2000!.11T. W. Ebbesen and P. M. Ajayan, Nature~London! 358, 16 ~1992!.12D. J. Griffiths, Introduction to Electrodynamics, 3rd ed.~Prentice–Hall,Englewood Cliffs, NJ, 1999!.13A. P. Ramirez, R. C. Haddon, O. Zhou, R. M. Fleming, J. Zhang, S.McClure, and R. E. Smalley, Science265, 84 ~1994!.14F. Tsui, L. Jin, and O. Zhou, Appl. Phys. Lett.76, 1452~2000!.e or copyright; see http://apl.aip.org/about/rights_and_permissions

Simple and efficient method for carbon nanotube attachment to scanning probes and other substrates

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Simple and efficient method for carbon nanotube attachment to scanning probes and other substrates

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