Scanning tunneling microscopy of vapor-phase grown nanotubes of carbon

Scanning tunneling microscopy of vapor-phase grown nanotubes of carbon

J. Php. Pergamon Ckm. Solids Vol. 54, No. 12, pp. 1871-1877, 1993 IF 1994 Elsevier Science Ltd Printed in Grea;Britain. All rights rcszrved 0022-36...

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J. Php.

Pergamon

Ckm.

Solids Vol. 54, No. 12, pp. 1871-1877, 1993 IF 1994 Elsevier Science Ltd Printed in Grea;Britain. All rights rcszrved 0022-3697/93 96.00 + 0.00

SCANNING TUNNELING MICROSCOPY OF VAPOR-PHASE GROWN NANOTUBES OF CARBON MAOHUI GE and KLAUS SATTLER University of Hawaii, Department of Physics and Astronomy, 2505 Correa Road, Honolulu, HI 96822, U.S.A. (Received

19 July 1993)

Abstract-Nanotubes of carbon can be produced by vapor condensation of carbon on a flat graphite surface. Scanning tunneling microscopy revealed the presence of tubes with very small diameters from 10 to 70& and up to several hundred nanometers in length. Most of the tubes are terminated by hemispherical caps and some are well aligned, forming bundles. Atomic resolution images of the tubes show that they have a helical graphitic nature. Keywords:

Fullerene, nanotube, STM.

INTRODUCTION

Since the discovery of C,, tremendous interest has been generated in the physics and chemistry of Cm and other fullerenes. Despite such enormous activity little, however, is known about fullerene growth, and neither the onset of nucleation nor the progression towards the fullerene network are understood. Besides spherical cage structures, tubular carbon structures were recently produced in an arc-discharge arrangement [ 1,2]. The tubes formed at the end of the negative carbon electrode after a D.C. arc-discharge in a partial helium atmosphere. The synthesis of large quantities of nanotubes has also been achieved by applying the same method [3]. Transmission electron microscopy (TEM) and electron diffraction patterns revealed that they are helical concentric graphitic networks. Although various properties of the tubules were measured, their atomic structure could not be directly determined, and their growth mechanism is still not understand. Recently, it has been reported that lead can be inserted into the carbon tubular cages [4], and thinning and opening of carbon tubules during oxidation has also been observed [S, 61. Single shell nanotubes about 1 nm in diameter generated by introduced nanophase metal particles have also been reported [7,81. Theoretical studies predict that carbon tubules will exhibit striking electronic transport properties, ranging from metallic to moderate-band-gap semiconductor behavior, depending on their diameters and helical structures [9-121. Also, such tubules are predicted to shield guest atoms from external electric and

magnetic fields [13]. Other calculations have shown that the tubules should have high strength and rigidity [14]. We have produced carbon tubules by quasi-free vapor condensation [15]. This method is different from the previously reported generation method of carbon nanotubes [l-3]. Using a scanning tunneling microscope @TM), we found tubes with various diameters, lengths, and orientations on the graphite substrates. The images show detailed atomic structure on the tubes and directly prove that the tube surfaces are perfect honeycomb networks. In addition to the atomic lattice we observe a superpattern on the tubes which is due to an incorporated inner tube with a different helicity. The smallest tube imaged in our experiments has a diameter of about lOA, which is roughly the size of the molecule C6,,. Based on our STM analysis, we suggest that tubule growth starts with the formation of a fullerene hemisphere. Besides isolated tubules we also observed tubules aligned parallel to each other, forming bundles. This suggests that the bundle is another growth path for the formation of tubular carbon structures. EXPERIMENTAL We prepared

the samples

METHOD by vapor deposition

of

on highly-oriented pyrolytic graphite carbon (HOPG) in high vacuum. The graphite was freshly cleaved in vacuum and carefully examined by STM before the deposition to ensure that its surface was atomically flat and defect-free over micrometer areas. The carbon vapor was produced by resistively heating a pure carbon foil, and the base pressure for

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deposition was typically N lo-* torr. The deposition rate was monitored by a quartz crystal film thickness monitor, and the average thickness of the carbon adlayers was controlled to be in the range of 20-60 A. The substrate was kept at -30°C during evaporation. After deposition, the samples were transferred to an ultra-high vacuum (UWV) chamber (-IO-‘* torr) without breaking vacuum, and then analyzed by a STM in UHV. The STM imaging was performed at room temperature. The microscope was mostly operated in the constant current mode, in which the tipto-sample distance is kept constant. Bias voltages of 100-800 mV (both positive and negative) and tunneling currents of 0.5-3.0 nA were applied. The samples were also taken out from the UHV chamber after they were imaged by the UHV STM, and examined again by another STM in air to check their stability. RESULTS AND DISCUSSION On different samples prepared under similar conditions, nanotubes of carbon with diameters between 10 and 70 8, and up to a few hundred nanometers in length were found. Two STM images of such tubes are shown in Fig. 1. In the three-dimensional pictures, a cylindrical shape is clearly displayed but whereas Fig. la shows parts of two long tubes, Fig. 1b shows smaller ones. Most of the tubes are terminated by hemispherical caps, and lie almost horizontally on the substrate. Irregular nanostructures were also formed as displayed in the images. We observe in some cases a coaxial arrangement of the outermost and an inner tube (in the middle of Fig. la). The outer tube is terminated in this region and the adjacent inner one is imaged simultaneously. The end of the outermost tube appears to be broader than its body portion. This is due to the dangling bonds present in this region, which cause a higher electronic local density-of-states (LDOS). For such a concentric arrangement, we measure an interlayer spacing of 3.4& which is about the graphite interlayer distance (3.35 A). The coaxial configuration was confirmed by atomic scale imaging. In Fig. 2a we show an atomic resolution image of a carbon tube which is 3.5 nm in diameter. The structure imaged at the upper-right corner of the picture comes from another tube, both tubes being - 100 nm long. A perfect honeycomb surface strueture is displaced. We determined the lattice parameter to be 1.4 A, which is similar to that of graphite (1.42 A), directly proving that the tubular surface is a graphitic network. For the tube in Fig 2a, the honeycomb surface lattice is helical with respect to the tube axis with a measured chiral angle of 5.0 & 0.5”. The chiral angle

is defined as the smallest angle between the tube axis and the C-C bond directions of the honeycomb lattice, and lies between 0” and 30”. The direction of the tube axis was determined from a large-scale image. It is important to clarify whether the observed tubules are scroll-type filaments or perfect cylinders. Scroll-type filaments should have edge overlaps on their surfaces, but from an extensive STM survey along the tube surfaces on the atomic scale we did not observe any edges due to incomplete carbon layers, and we therefore conclude that the tubes are complete graphic cylinders. A graphitic monolayer tube can be visualized as a conformal mapping of a two-dimensional graphene sheet onto the surface of a cylinder, and the lattice needs to match perfectly at the closure line. Choosing the cylinder joint line in different directions on the unfolded graphitic lattice leads to different helicities. A single helicity can only give a set of discrete diameters. In the case of a multilayer concentric tube, in order to obtain a diameter which exactly matches the required interlayer spacing, the tube layers need to adjust their helicities. Therefore, in general, different helicities for different layers in a multilayer tube are expected, and this is, in fact, confirmed by our experimental evidence. In Fig. 2a we observe a zigzag superpattern in addition to the atomic lattice which is extended along the axis at the surface of the tube. The zigzag angle is 120” and the period is N 16 A. The question therefore arises, what is the origin of this superstructure? We recall that giant lattices have been observed on planar graphite by STM [l&21] in rare cases which can be explained by misorientation of the top layer relative to the bulk layers [17-191. This produces a moire pattern, which has the same type of lattice but a much larger lattice parameter. The angle of misorientation determines the lattice constant. Due to the nonequivalent atomic the electronic local density-of-states stacking, (LDOS) at the Fermi level of the top layer is modulated [17J. Therefore the STM can image a giant lattice in addition to the atomic lattice and from the period of this giant lattice, the relative orientation of the first two layers can be determined. For multilayer tubes with different helicities between the top and second shells, one expects such a superpattern to be observed. However, besides the analogy between two misoriented graphene sheets and two concentric graphitic sheets of different helicity, one should also consider their difference. For planar sheets the superpattern is extended throughout the whole plane, while for cylindrical sheets it should appear only within narrow stripes along the tube axis. This is due to the

Scanning tunneling microscopy of carbon

Fig. 1. Two three-dimensional

STM images of carbon tubules on a graphite substrate.

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difference in curvature of the two cylinders which effects the atomic stacking, mainly perpendicular to the tube axis. In Fig. 2b we illustrate a schematic model of such a superpattern, in which two graphene sheets rotated relative to each other are superimposed, resulting in a giant honeycomb lattice. Choosing 9” as the misorientation angle we obtain a superlattice period of 16 A. Generally, for small chiral angles of both cylinders, the tube axis is approximately along the zigzag direction of the giant honeycomb lattice. A zigzag superpattern along the tube axis is actually directly observed in the STM image of Fig. 2a, and the measured period of 16 8, reveals that the second layer of the tube is misoriented by 9” relative to the first layer. Therefore the first and the second tube shells have chiral angles of 5” and -4”, respectively, showing that the tubes are indeed composed of at least two coaxial graphitic cylinders with different helicities. From the previous STM images, we demonstrated that STM reveals the presence of carbon nanotubes

on a flat graphite substrate generated by vapor-phase growth. Now we discuss further the possible growth process of the carbon tubes. For the formation of carbon tubes, spherical-type nucleation seeds seem to be required. In a commonly-used method for the production of carbon fibers, the fiber growth is initiated by submicrometer size catalytic metal particles [22]. Tubes grown from a graphite electrode during arc-discharge might also be related to nanoparticle-like seeds present at the substrate [2]. It was recently reported that growth of single shell buckytubes (close to 1 nm in diameter) can be initiated by iron and cobalt nanophase particles introduced into the carbon-arc [7,8]. Under vapor-phase growth conditions which resemble those for the production of fullerenes, another type of nucleation seed is possible, in which the tubes may grow out of a hollow fullerene cap instead of a compact spherical particle. After half a fullerene is formed, the subsequent growth can proceed to a tubular structure rather than to a spherical cage.

nM Fig. Z(a)

Scanning tunneling microscopy of carbon

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Fig. 2(b) Fig. 2. (a) Atomic resolution STM image of a carbon tubule, 35 A in diameter. In addition to the atomic honeycomb lattice, a zigzag superpattern along the tube axis is displaced. (b) Structural model demonstrating a giant superpattern produced by two misoriented graphene sheets. The carbon atoms in the first layer are filled while the second layer atoms are open. Between the two dashed lines we highlight those first layer atoms with white which do not overlap with second-layer atoms. Due to their higher electronic local density of states (LDOS) at the Fermi level these atoms (/I-type atoms) appear particularly bright in grey scale STM images [17,24]. They produce a zigzag superpattern along the tube axis within the two white dashed lines as indicated.

Indeed, the smallest tube which we found had a diameter of 10 8, which is roughly the diameter of C& and is predicted to be the limiting case of vaporgrown monolayer graphitic tubes [23]. Our observation of hemispherically-capped 10 A tubes suggests that an incomplete &,-cluster is the nucleation seed for these tubes, and the &-derived tube could be the core of possible multilayer concentric graphitic tubes. After the fullerene-bard tube has been formed, further concentric shells can be added by graphitic cylindrical layer growth. In addition to the isolated carbon tubule, we also observed assemblies of nanotubes in the form of

bundles. The bundles are remarkably long, up to a few hundred nanometers and composed of perfectly aligned nanotubes with slightly different diameters over their full length. Their widths are between I5 and 50 nm. The STM image in, Fig. 3 shows part of a bundle which is -20 nm in width with a total of 200nm as determined from a larger scale image, and composed of tubules with different diameters, from 2 to 4 nm. All the tubules are perfectly aligned and closely packed. The bundle is located almost horizontally on the flat graphite substrate, and is separated from other deposited nanostructures. The tubes in the bundle are

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Rev. Lrtr. 6, [email protected]).

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tunneling

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