The fabrication of ordered arrays of oriented and bent carbon nanotube on a patterned silicon surface with a micron scale spacing extending over millimeter size surface areas, was presented. A combination of multistep purification and oxidation procedures were used to generate bundles containing 3-20 nanotubes of high purity. The patterned surface was composed of alternating hydrophilic and hydrophobic stripes. The results suggest that the patterning is controlled by the hydrodynamic behavior of a fluid front and orientation and bending mechanisms are facilitated by the pinned carbon nanotubes trapped by the liquid-solid-vapor contact line.open849

Ko, Hyunhyub

Peleshanko, S

Tsukruk, VV

Physical Review Letters

ScholarWorks@UNIST

P H Y S I C A L R E V I E W L E T T E R S week ending13 FEBRUARY 2004VOLUME 92, NUMBER 6Nanotube Surface Arrays: Weaving, Bending, and Assembling on Patterned SiliconVladimir V. Tsukruk,* Hyunhyub Ko, and Sergiy PeleshankoMaterials Science & Engineering Department, Iowa State University, Ames, Iowa 50011, USA(Received 6 August 2003; published 12 February 2004)065502-1We report the fabrication of ordered arrays of oriented and bent carbon nanotube on a patternedsilicon surface with a micron scale spacing extending over millimeter size surface areas.We suggest thatthe patterning is controlled by the hydrodynamic behavior of a fluid front and orientation and bendingmechanisms are facilitated by the pinned carbon nanotubes trapped by the liquid-solid-vapor contactline. The bending of the pinned nanotubes occurs along the shrinking receding front of the dryingmicrodroplets. The formation of stratified microfluidic layers is vital for stimulating periodic insta-bilities of the contact line.DOI: 10.1103/PhysRevLett.92.065502 PACS numbers: 61.46.+w, 81.07.–bFIG. 1. (a) Schematic of the surface patterning through mi-crocontact printing routine. (b) Casting of nanotube solution onrays over millimeter size surface areas. This approach tilted patterned substrate. (c) Dip coating.Highly ordered arrays of carbon nanotubes assembledon solid surfaces are considered as a critical element forfabricating nanoscale electronic and electromechanicaldevices with a high density of organized functioningelements [1]. The unique shape of the carbon nanotubesand their bundles with the aspect ratio exceeding 1:104are exploited for the fabrication of one-dimensionalnanostructures such as randomly oriented monolayerfilms [2], oriented nematic arrays [3], arrays on patternedsurface areas [4], precisely positioned individual nano-tubes [5], and interelectrode functional networks [6]. Theordered growth of vertically oriented densely packednanotubes on patterned surfaces has been observed [7,8].A high level of sophistication has been demonstratedwith a variety of complex networks composed of straightcarbon nanotubes. In-plane orientation was achieved by avariety of wet-chemistry approaches such as controlleddirectional fluidic flow, variable liquid-air contact line,selective adsorption on functionalized surfaces, as well asby the application of electric and magnetic fields [2–6,9,10]. However, the fabrication of complex multidimen-sional networks requires the ability to control nanotubeshapes considering a strong dependence of local elec-tronic properties upon the bending state [11]. For ex-ample, the formation of the ring structure of biologicalmacromolecules such as DNA is an efficient way to or-ganize the bioinformation transfer [12].It has been recently observed that nanotube rings canbe formed by the nanotubes and their bundles duringcatalytic synthesis or oxidation process [13,14]. It hasbeen suggested that the bending mechanism is based onthermal motion/bending or bubble cavitation. The result-ing shape is stabilized by van der Waals interactions orchemical bonding between the tube ends. However, in allcases, only random collections of rings deposited on asolid substrate were observed.Here we report a first successful result to produce amassive amount of straight and looped nanotubes (bentand folded nanotubes, open and closed loops) assembledwith orientational order within positionally ordered ar-0031-9007=04=92(6)=065502(4)$22.50 provides not only uniform orientation but also well-defined spacing in array positioning on a micron scalecontrolled by the dimensions of the alternating hydro-philic and hydrophobic surface areas. We suggest that theorientation and bending mechanism are facilitated by thepinned carbon nanotubes subjected to hydrodynamic pat-terned microflow followed by trapping at the liquid-solid-vapor contact line undergoing the surface instability.To facilitate the approach proposed, we used a combi-nation of multistep purification and oxidation proceduresto generate bundles containing 3–20 nanotubes of highpurity. Selective adsorption was conducted on the surfaceof patterned silicon wafers. An oriented nanotube arraywas formed by casting or dip-coating techniques asshown in Fig. 1. The patterned surface was composed ofalternating hydrophilic and hydrophobic stripes: amine-terminated self-assembled monolayers (SAMs) with theability for strong grafting of carbon nanotubes andmethyl-terminated SAMs with weak adsorbing ability.Firm tethering can be caused by strong interaction be-tween amine groups and sulfate and carboxylic groups of2004 The American Physical Society 065502-1FIG. 2. AFM topographical [(a), (c)–(f)] and phase (b) im-ages of different types of patterned surface arrays of woven andcombed nanotube arrays formed by casting [(a), (b), (c), (f)]and by dip coating [(d), (e)]. (a) Woven nanotube arrays on par-allel stripes, 50 50 m; top inset shows microstructure of asingle stripe (10 10 m); bottom inset shows 2D Fourier-transform demonstrating perfect spacing and vertical ordering;Z scale is 40 nm. (b) High resolution (2 2 m) phase imageof a highly oriented straight carbon nanotube array within asingle stripe; inset shows 2D Fourier transform demonstrat-ing a high level of orientation nematic ordering; Z scale is 10.(c) Image of an individual stripe (10 10 m) with a com-plex chevron type of oriented nanotubes; Z scale is 20 nm.(d) Combed nanotube monolayer showing long-range orienta-tion of bent tubes uniformly oriented within stripes; Z scale is20 nm. (e) Selected surface areas with tethered carbon nano-tubes in ‘‘noose’’ shapes. Marked areas show identically loopednanotubes; Z scale is 20 nm. (f) Woven nanotube arrays on arectangular pattern, 50 50 m; Z scale is 80 nm. The bottominset shows 2D Fourier transform demonstrating perfect posi-tional ordering of arrays. The top inset shows the ‘‘nest’’ shapeof an individual array with nanotube bundles predominantlybent and aligned along the edges.P H Y S I C A L R E V I E W L E T T E R S week ending13 FEBRUARY 2004VOLUME 92, NUMBER 6sodium dodecyl sulfate (SDS) molecules used to stabilizesolution and nanotube ends [15,16].The variation of the withdrawing and casting condi-tions results in the fabrication of a variety of orderedarrays of carbon nanotubes anchored to hydrophilic,amine-terminated surface SAM areas represented bydarker stripes of 7 m width (Fig. 2(d)). White stripesof about 3 m across on atomic force microscopy (AFM)topographical images represent thicker hydrophobicmethyl-terminated areas (about 1.2 nm difference inthickness). The types of local orientation and orderingof the carbon nanotubes confined to the ‘‘sticky’’ stripesrange from high density array of ‘‘woven’’ nanotubemonolayer with different textures (chevron or more com-plicated), oriented straight nanotubes, bent nanotubesstretched along the stripes, and bent ‘‘combed’’ nanotubes(see some examples in Figs. 2 and 3). Orientation orderingwithin individual stripes resembles that achieved by uni-directional fluidic flow and can be described as nematictype with uniform local orientation and no longitudinalpositional order as indicated by narrow arcs in a 2DFourier transform image [Fig. 2(b)]. These oriented arrayareas with virtually an identical repetition of orientedtexture within stripes are regularly spaced over millime-ter size surface areas as confirmed by sharp, pointlikerows of corresponding Fourier transforms from fabri-cated arrays [Fig. 2(a)]. These nanotube arrays are firmlytethered to functionalized silicon surfaces through a com-bination of chemical bonding and strong van der Waalsinteractions [5,17] and cannot be removed with intensiveultrasonic treatment.Careful analysis of the local configuration of carbonnanotubes within the surface arrays of low surface cover-age unexpectedly discovered consistent repetition ofsimilar, nonlinear shapes of individual nanotubes, whichare very different from rodlike shapes of nanotubesaligned by uniform flow [Figs. 2(d), 2(e), and 3]. Depend-ing upon specific fabrication conditions (rate of with-drawal, rinsing, and drying), sharply folded ‘‘needleeyes,’’ elongated ‘‘hooks,’’ single and double ‘‘nooses,’’and ‘‘arches’’ are consistently observed for nanotubesand their bundles on a micron scale (Fig. 3). The bentnanotubes are uniformly oriented along one directionalthough their positions within stripes are not correlated.On average, up to 90% of nanotubes are uniformly ori-ented and up to 40% of all aligned nanotubes under givenconditions are folded in a similar manner with a highdegree of shape persistence over large surface areas withlinear density of about one nanotube per micron. This is arelatively high yield considering a stochastic nature of theprocess. This ordering could be further enhanced by, e.g.,creating a rectangular grid of hydrophilic/hydrophobicsurface areas. In fact, our preliminary results demon-strated that in this case, indeed, a regular array of‘‘nest’’-shaped carbon nanotubes predominantly alignedalong the edges of hydrophilic squares can be created[Fig. 2(f)].065502-2Inspection of the bent nanotubes at high magnificationrevealed that loops of different types formed by bendingonly a portion of the nanotube, typically less than half ofits total length. A significant portion of the backwardbent tube is in close contact with the straight, unbentportion. The radius of curvature of the coiled portion is065502-2Pinned endTrapped nanotubeReceding frontAdvancing frontcontact lineGravitational forcesContinuous bendingTrapped nanotubeFIG. 4. Schematic of the suggested mechanism of pinnednanotube bending by a moving receding front of a dryingdroplet.FIG. 3. High-resolution 3D images of looped nanotubes andcross-sectional profile along the bent nanotube (red marksindicate depth of 2.0 nm). Z scale is 20 nm.P H Y S I C A L R E V I E W L E T T E R S week ending13 FEBRUARY 2004VOLUME 92, NUMBER 6within 200–300 nm. It is clear that bending and anchor-ing of the nanotubes under such confined conditions resultin the development of significant local stresses, whichlead to either the splitting of bundles or the sectioningof nanotubes (e.g., see 3D image of nanotube ‘‘arch’’ andsurface profile in Fig. 3). In the last case, the flat surfacesections of the nanotubes are separated by 0.3–0.6 nmgrooves indicating a wrinkle formation. Overall, thisresults in one-dimensional roughness along the nanotubeof 0.2–0.3 nm, a characteristic of atomic scale corruga-tion of the nanotube walls caused by sharp wrinkles ofthe graphitic layers during bending as was already ob-served [18]. No single point buckling is found for the bentnanotubes indicating a uniform bending stress distributedover the entire nanotube length.Considering the features of the phenomenon observed,we suggest the moving contact line to be responsible forthe formation of woven or looped nanotubes (Fig. 4). Infact, the liquid drying under the unidirectional gravita-tional force inevitably leads to the directional flow withinrelatively thick layers followed by the formation of agradient molecularly thick microfluidic layer, which is asubject of the contact line instability [6,19,20]. Suchinstabilities cause the formation of an array of periodi-cally spaced microdroplets left behind the moving triplecontact line [21].To envision how this process could interact with carbonnanotubes, we should emphasize that the carbon nano-tubes adsorbed from solution should be tethered to thehydrophilic surface [22]. The pinned nanotubes can betrapped by the moving contact line of drying microdrop-lets and initially align themselves along the droplet cir-cumference in straight form (Fig. 4). At the next stage ofdrying, the decreasing of the overall length of the contactline along with increasing of its curvature force thetrapped carbon nanotubes to follow the overall diminish-ing shape of the droplets by gradual bending (Fig. 4). Thenanotube loops are fixed in this strained state by stronginteractions with functionalized substrate and van derWaals interaction between the two portions of the nano-tube in a close contact. The variation of the initial trap-ping scenario and size/location of a microdroplet causes avariety of looped shapes observed under different prepa-065502-3ration conditions. In addition, preorientation of carbonnanotubes along the hydrophilic stripes caused by hydro-dynamic forces should play a significant role in the tex-ture formation as will be discussed in a separatepublication [23].The mechanism of nanotube looping suggested herecould be valid only if the surface tension acting along theshrinking contact line outweighs strain forces arisingfrom nanotube bending. The evaluation of the surfacetension, which arises from the shrinking contact line[24], leads to the conclusion that only elastic rods witha spring constant higher than 1–5 N=m could sustain thepressure across the curved contact line. On the other hand,the evaluation of the bending properties required fornanotube folding can be conducted using the known valueof the elastic modulus, E 1 Tpa [25,26]. A spring con-stant K for the nanotube portion participating in continu-ous bending can be determined within the solid rodapproximation by using the equation K  3R4E=4L3,where R is the bundle radius and L is the length of thecurved portion [27]. Estimation for L  0:3–1 m andR  1–2:5 nm gives values close to 102 N=m, which iswell below the critical stiffness of 1–5 N=m required toprevent tube bending. Thus, carbon nanotubes alignedalong the fluid-air-solid interface can be easily bent bythe shrinking contact line.In fact, alignment, stretching, and bending of stiffrodlike molecules such as DNA aligned along the contactline were observed and used for the combing process andthe evaluation of the nanomechanical elastic properties ofthe biological molecules [28,29]. On the other hand, themechanism of trapping carbon nanotubes at the surfaceof microbubbles in ultrasonically treated solution andtheir bending during cavitation and bubble collapse wassuggested as an explanation for the ring formation.065502-3P H Y S I C A L R E V I E W L E T T E R S week ending13 FEBRUARY 2004VOLUME 92, NUMBER 6Finally, we would like to emphasize the role of the surfacepatterning in organized coiling of the carbon nanotubes.Obviously, the initial formation of stripped microfluidiclayers moving along the hydrophilic microscopic stripesis critical in stimulating the periodic instabilities of thecontact line with spacing controlled by the dimensions ofthe hydrophilic and hydrophobic areas.We believe that the combination of this newly observedphenomenon with proper hydrophilic-hydrophobic pat-terning may lead to directed assembly of uniformly ori-ented and looped nanotubes (carbon or other origin) inregular surface arrays. Controlled bending of nanotubesresulting in tunable electronic and optical properties canbe thought as a means for the fabrication of large-scalenanosensor arrays.Purification.—The single walled carbon nanotube(SWNT) raw material, produced by the arc dischargemethod, was purchased from MER (Arizona, U.S.A.).The purification procedure involved oxidation in nitricacid, centrifugation, and filtration [30]. SDS surfactant(Aldrich) was used to make stable colloidal suspension ofSWNTs [31]. The suspension was filtered with polycar-bonate membranes (pore size 0:4 m).Substrate patterning.—Microcontact printing (CP)was used for patterning of the silicon oxide surface [32].The poly(dimethylsiloxane) (PDMS) stamp was curedand released from a patterned mold. Octadecyltri-chlorosilane (OTS) solution was used as the ink forCP with an aminopropyltriethoxysilane (APTS) layerformed on the unreacted regions. Procedures were con-ducted under Cleanroom-100 conditions according tousual procedure [33,34].Nanotube adsorption.—Casting was conducted bydrop drying of nanotube solution on tilted substrates(Fig. 1). For the dip-coating process, patterned substrateswere immersed in a nanotube solution for about 24 h.After that, the substrates were withdrawn, dried verti-cally in air, and then rinsed.Surface characterization.—AFM scanning was per-formed in the light tapping mode on a Dimension 3000(Digital Instruments, Inc.) microscope under ambientconditions according to the usual experimental procedureadapted in our laboratory [35,36].This study is supported by NASA-CNDE Center atIowa State University under Contract No. 102098, NSF-CMS-0099868, and AFOSR Grant No. F496200210205.06550*Author to whom correspondence should be addressed.Electronic address: vladimir@iastate.edu[1] H. G. Craighead, Science 290, 1532 (2000).[2] T.V. Sreekumar et al., Chem. Mater. 15, 175 (2003).2-4[3] K. Q. Jian, H.-S. Shim, A. Schwartzman, G. P. Crawford,and R. H. Hurt, Adv. Mater. 15, 164 (2003).[4] H. Shimoda et al., Adv. Mater. 14, 899 (2002).[5] M. Burghard, G. Duesberg, G. Philipp, J. Muster, andS. Roth, Adv. Mater. 10, 584 (1998).[6] Y. Huang, X. Duan, Q. Wei, and C. M. Lieber, Science291, 630 (2001).[7] A. Cao et al., Appl. Phys. Lett. 81, 1297 (2002).[8] J. Li, C. Papadopoulos, J. M. Xu, and M. Moskovits, Appl.Phys. Lett. 75, 367 (1999).[9] A. Maiti, Nature Mater. 2, 440 (2003).[10] A. Ural, Y. M. Li, and H. J. Dai, Appl. Phys. Lett. 81, 3464(2002).[11] S.V. Rotkin and Yu. Gogotsi, Mater. Res. Innov. 5, 191(2002).[12] G. M. Whitesides, J. P. Mathias, and C. T. Seto, Science254, 1312 (1991).[13] R. Martel, H. R. Shea, and P. Avouris, Nature (London)398, 299 (1999).[14] M. Sano, A. Kamino, J. Okamura, and S. Shinkai,Science 293, 1299 (2001).[15] J. Liu et al., Chem. Phys. Lett. 303, 125 (1999).[16] L. T. Cai, J. L. Bahr, Y. X. Yao, and J. M. Tour, Chem.Mater. 14, 4235 (2002).[17] V. N. Bliznyuk, M. P. Everson, and V.V. Tsukruk,J. Tribol. 120, 489 (1998).[18] Progress in Transmission Electron Microscopy 2. Appli-cations in Materials Science, edited by X.-F. Zhang andZ. Zhang, Springer Series in Surface Sciences (Springer,New York, 2001), Vol. 39.[19] S. M. Troian, X. L. Wu, and S. A. Safran, Phys. Rev. Lett.62, 1496 (1989).[20] J. A. Diez and L. Kondic, Phys. Rev. Lett. 86, 632 (2001).[21] O. Kathaus, L. Gracjo, N. Maruyama, and M. Shimo-mura, Chaos 9, 308 (1999).[22] B. Kim and W. M. Sigmund, Langmuir 19, 4848 (2003).[23] H. Ko, S. Peleshanko, and V.V. Tsukruk, J. Phys. Chem.(to be published).[24] A.W. Adamson, Physical Chemistry of Surfaces (Wiley,New York, 1990), 5th ed.[25] E.W. Wong, P. E. Sheehan, and C. M. Lieber, Science 277,1971 (1997).[26] D. A. Walters et al., Appl. Phys. Lett. 74, 3803 (1999).[27] F. P. Beer and E. R. Johnston, Mechanics of Materials(McGraw-Hill, New York, 1992).[28] A. Bensimon et al., Science 265, 2096 (1994).[29] D. Bensimon, A. J. Simon, V. Croquette, and A. Ben-simon, Phys. Rev. Lett. 74, 4754 (1995).[30] J. Liu et al., Science 280, 1253 (1998).[31] J.-M. Bonard, Adv. Mater. 9, 827 (1997).[32] N. L. Jeon, K. Finnie, K. Branshaw, and R. G. Nuzzo,Langmuir 13, 3382 (1997).[33] V.V. Tsukruk and V. N. Bliznyuk, Langmuir 14, 446(1998).[34] V.V. Tsukruk, Adv. Mater. 13, 95 (2001).[35] V.V. Tsukruk and D. H. Reneker, Polymer 36, 1791(1995).[36] V.V. Tsukruk, Rubber Chem. Technol. 70, 430 (1997).065502-4

Nanotube surface arrays: Weaving, bending, and assembling on patterned silicon

Abstract

Similar works

Full text

Available Versions

ScholarWorks@UNIST