Hybrid materials

Hybrid Microstructures from Aligned CarbonNanotubes and Silica Particles**

Saurabh Agrawal, Ashavani Kumar,Matthew J. Frederick, andGanapathiraman Ramanath*

Carbon nanotubes (CNTs) exhibit fascinating electrical,thermal, and optical properties,[1–4] and remarkable mechan-ical stability,[5–7] which makes these unique one-dimensionalnanostructures promising candidates for use in a variety ofdevices and composites. Recent work has shown that thefabrication of hybrid nanostructures comprised of randomly

[*] S. Agrawal, Dr. A. Kumar, Dr. M. J. Frederick,Prof. Dr. G. RamanathRensselaer Polytechnic InstituteMaterials Science and Engineering Dept.Troy, NY 12180 (USA)Fax: (+1)518-276-8554E-mail: ramanath@rpi.edu

Prof. Dr. G. RamanathMax Planck Institute f8r FestkçrperforschungHeisenbergstrasse 1, 70569 Stuttgart (Germany)

[**] We gratefully acknowledge funding support from the US NationalScience Foundation through ECS 424322, DMI-0304028 NER, andDMR 9984478 CAREER supplement awards, and an Alexandervon Humboldt fellowship (GR).

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oriented CNTs with nanoparti-cles[8–11] or nanowires[12] and incor-porating them into polymers cancreate high-strength compositematerials.[13] Achieving a greaterdegree of control in fabricatinghybrid nanostructures comprisingnanoparticles coupled with orient-ed CNTs or their bundles willenable anisotropic properties andresponses in filled composites.Such an advance will also open upnew ways for organizing CNTs fordevices, through the application ofcolloidal chemistry techniques onthe nanoparticle–CNT hybridbuilding blocks. In order to realizesuch possibilities CNTs or theirbundles must to be grown in anorganized fashion in confined geo-metries, for example, in the micro-meter and submicrometer range.

There has been considerableprogress in growing aligned CNTsin predetermined orientations onplanar substrates by chemicalvapor deposition (CVD).[14–18]

Typical CVD approaches to groworiented CNTs involve lithograph-ic templating and activation ofcatalyst-containing nanoparticles or thin films on the sub-strate, or combining gas-phase catalyst delivery and sub-strate-selective catalyst activation on certain portions of pat-terned surfaces. For example, oriented CNTs can be grownfrom a xylene–ferrocene mixture selectively[14] on silica (inexclusion to silicon) in an orientation normal to the silicasurface, producing CNT bundles that inherit the silica pat-tern topography.[18] The effects of dimensional scaling of thesubstrate patterns on CNT length and orientation, however,are not yet understood.

Here, we demonstrate that CNT–nanoparticle assem-blies with novel aligned morphologies can be obtained byadjusting the size of silica spheres, and qualitatively describethe mechanism of particle-size dependence on CNT nuclea-tion and alignment. Our results open up possibilities for in-tegrating CNTs with micro- and nanoparticles and exploit-ing colloidal chemistry techniques for controlling CNT align-ment and assembly. Further, the results shed light on factorsthat may limit CNT growth and alignment on low-dimen-sional structures.

Densely aligned CNT pillars grow on >4.1-mm-diametersilica microspheres in a direction normal to the silicon sub-strate (see Figure 1a). The morphology is similar to that ob-tained on planar silica substrates, which indicates that theselective CVD process is extendable to growing microme-ter-sized CNT bundles on nanoparticles. The CNTs arewavy, similar to other CVD-grown tubes[17] , with an averageinter-CNT distance such that the CNT density is�1010 cm�2.[19] The size of the CNT bundles scales with in-

creasing sphere diameter. CNTs form a continuous filmwhen grown on closely packed assemblies of the microparti-cles (see Figure 1b). Macroscopic wall-like architecturescomprised of columns of CNT bundles are formed on linearchains of microspheres (see Figure 1c). This feature canconceivably be used to harness organized patterns of silicamicrospheres assembled by combining colloidal chemistryand lithography to create CNT-confined cellular patterns onplanar substrates. While the CNTs grown on isolated micro-spheres are aligned within the bundle, the bundles are notvertically oriented with respect to the substrate in mostcases (see Figure 1d). This observation suggests that supportfrom the bundles from neighboring spheres is important forvertical alignment.

Decreasing the diameter of the silica microspheresbelow �2.4 mm yields CNTs, but without any alignment(see Figure 2a). Further decreases in silica sphere diameterdown to �490 nm result in a sharp decrease in the CNTnumber density (see Figure 2b). CNTs grown on sub-400 nm silica nanoparticles are covered with a large amountof amorphous carbon (see Raman spectroscopy results de-scribed below). The amorphous carbon is present as nano-particles on the CNT surface (see Figure 2c–e), giving riseto a rough morphology. In addition, a large number of Fe-containing nanoparticles are observed (see Figure 2 f). NoCNT growth is observable on nanospheres below 330 nm.

Raman spectroscopy of the CNT–silica sphere hetero-structures shows that the gradual decrease in CNT numberdensity correlates with the increased disorder attributed to

Figure 1. SEM images showing a) CNT pillars grown over 6.84 mm silica spheres; b) a forest ofaligned CNTs formed on a close-packed assembly of spheres; c) a wall formed from aligned CNTsgrown on a chain of closely spaced spheres; d) top view of CNT pillars grown on isolated spheres.

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the formation of sp3-type carbon.[20] Figure 3a shows exam-ple spectra showing an increase in the D-band (1350 cm�1)intensity with decreasing sphere diameter when normalizedwith respect to the G-band (1580 cm�1) intensity. Figure 3bshows the D–G intensity ratio as a function of microspheresize. For CNTs grown on microspheres of 4.1 mm diameterand above, the ratio is nearly constant at �0.3, which corre-sponds to that in multi-walled CNTs grown over planar sub-strates.[21] For smaller sphere diameters up to 490 nm, the

disorder increases gradually to0.39 due to a gradual increase inthe amount of amorphous carbonwith respect to the amount ofCNT deposited. However, below490 nm the disorder increases inan asymptotic manner due to thedeposition of amorphous carbondescribed above (Figure 2e–f).These results can be captured inan empirical relationship wherethe ratio of D-band to G-bandintensity is given byID=IG ¼ 0:29þ 28

d�270 where d is thediameter of the SiO2 sphere innanometers. This relationship isvalid only in the range where d>330 nm. For sphere diameterswhere d<330 nm, the ratio satu-rates at �0.77, which correspondsto the value observed from amor-phous carbon annealed in N2 at800 8C,[22] which is similar to ourCNT growth temperature.

The continuous change in mor-phology from aligned CNT bun-dles to randomly oriented CNTswith decreasing silica particle sizecan be understood as follows:Aligned CNTs grow normal to thesurface of large microspheres dueto the reduction in energy ob-

tained by coordinated van der Waals (vdW) interactions be-tween adjacent CNTs, in a manner similar to that observedin self-assembled molecular layers (SAMs).[23] Although theintertube spacing is �20–30 nm, the waviness of the CNTscauses adjacent tubes to cross each other within the vdW in-teraction distance at different points along their lengths, en-abling a net lateral attractive force between the CNTs. Thesame effect also drives the alignment of bundles on adjacentmicrospheres. The high curvature on smaller microspheresresults in a smaller number of CNTs separated by large an-gular separations. Both factors decrease the number ofcrossing points, and the extent of the lateral reinforcingforce, hence resulting in random CNT growth, which gradu-ally disappears due to the preference for amorphous carbonformation.

In summary, CNT nucleation and morphology on silicamicrospheres are strongly dependent on the particle sizeand packing. By adjusting these parameters, the CNTgrowth and orientation can be controlled. Novel morpholo-gies can be obtained by dispersing the spheres on substratesthat inhibit CNT growth. Geometrical confinement below acritical silica sphere size inhibits the growth of aligned CNTbundles due to the deposition of amorphous carbon and thehigh angular spacing between a smaller number of bundles,which are unfavorable for laterally reinforced alignment ofthe CNTs by van der Waals forces. This lateral size depend-ence of the substrate surface will be an important factor

Figure 2. SEM images showing random CNT growth over silica spheres of different diameters:a) 2.4 mm, b) 490 nm, c) 400 nm. High-magnification SEM images show d) clean CNTs grown on490 nm silica spheres, and e) amorphous C with Fe nanoparticles on 400 nm silica spheresexposed to the CVD precursors. f) TEM micrograph showing Fe nanoparticles (bright due to high-atomic-number contrast) and amorphous carbon.

Figure 3. a) Raman spectra showing D and G bands from CNTs grownon silica spheres of different diameters (shown alongside thecurves). b) Plot of D-band:G-band intensity ratio as a function ofsilica sphere diameter.

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that may limit the growth of highly oriented one-dimension-al nanostructures on nanoscale patterns.

Experimental Section

We drop-coated silica microspheres of chosen diameters be-tween 6.84 mm and 160 nm from a dilute suspension in acetoneonto device-quality Si(001) wafers, pre-cleaned successively inultrasonic baths of trichloroethylene, acetone, and isopropyl al-cohol. The nanoparticle assembly density was controlled by ad-justing the acetone suspension concentration, and substrate tilt-ing. The samples were dried at room temperature for �1 h toremove acetone, and to obtain silica particle assemblies on thesubstrate. We grew CNTs by exposing these samples to axylene–ferrocene mixture in a vacuum tube furnace at 775 8C in100 sccm Ar, known to yield CNT growth selectively on silica inexclusion to silicon.[18] The CNT morphology was characterized bySEM in a JEOL 6330F FESEM microscope operated at 5 kV.Raman spectroscopy was conducted using a Renishaw Raman-scope system with a 514 nm Ar laser.

Keywords:alignment · carbon nanotubes · hybrid structures ·nanoparticles · Raman spectroscopy

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Received: January 20, 2005Published online on May 18, 2005

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