Mat. Res. Soc. S}"lllp_I'roc. Vol. 633 i[) 2001 I\:taterials Rc~carch Society

SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY OF SHORTMULTlWALL CARBON NANOTUBES

A. Hass3nien l.*, A. Mr:r.el2, M. Tokumoto·, X. Zhao:" Y. Ando\ and D. Tomanek~lNanoteclmology research institule, Natlonal1JlslituLe of Advllllced Indusuial Science andTechnology (A 1ST), 1-1-1 Umczono. Tsukuba. Ibaraki, 305-8568, Japan.2Jozef Stefan Institute. 39 Jamova. Ljubljana 1000, Slovenia.JDepartment of Physics, MeijoUniver.sity. Shiogamaguchi, Tempaku-ku, Nagoya, J"lpan.;I Department of Physics and Astronomy. Michigan Stare Universily, East Lansing. Michigan

48824-1116. USA.

ABSTRACT

We reporl on the structural analysis of multiwall c..\rbon nanolubes (MWNTs), producedby DC arc discharge in hydrogen gas. using a scanning LUnneling microscope openued m ambienlconditions. On a microscopic scale the images show tubes condensed in ropes as well asindividual tubes which are separaled from each other. Individual nanotubes exhibit variousdiameters (2.5-6 nm) and chiralitics (0-300). For MWNTs rope, lhe outer portion is composed ofhighly oriented nanO!ubes with ncarly uniform diameter (4-5 nm) and chirality. Strongcon-elation is found between the struclUral parameters and the electronic properties in which theMWNTs span the metallic-semiconductor regime. True atomic-resolution topographic STMil1luges of the OUler shell show hexagonal arrangements of carbon tHoms that are unequallyvisible by STM tip. This suggests thaI the Slacking nature of MWNTs, may effect the electronicband structure of the tube shells_ Unlike other MWNTs produced by arc discharge in helium gas.lhe Icngth of the tubes arc rather short (80-500 nm). which make it feasible to use them as

lengths (80-500nm) and homogeneous diameters (4-5 nm). Because no chemical processing wasused to reduce the lengths of the tubes we expect this method to be very reliable in producingdefect-free short MWCNTs.

Experimental

The MWCNTs samples used for this study were prepared by the usual arc discharge method withH2 as exchange gas. The preparation method is described elsewhere

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samples were collected and sonicated in ethanol for a few minutes prior to being cast on a highlyoriented pyrolytic graphite (HOPG) suhstrate for STM measurements. We have carried out STMmeasurements using a Digital Instruments NanoScopc ][[a operated

tunneling conductance, dl/dV, versus the bias voltage at three different locations on the nanoluberopes. A semiconducting behavior with band gap between 0.4 to 0.9 eV is clearly visible. Thepeaks correspond to Van Have singUlarities at the onsets of one-dimensional energy bands of thecarbon nanotllbe, The peaks are broad due to thermal fiuctuations thm tend to limit the energyresolution to 0.1 eV at room temperature,

300

o

200

100

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5,4 n..

2.7 1'1 ..

0.0 1'1"

n..

Figure 1. STM topographic image of MWCNT ropes. The length of the tubes arc 80-500 nm.

V[mV]

500 1000 1500

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Figure 2. (a) STM topographic image of entangled MWCNT ropes. (b) The STS plot displays asemicoducting behavior with a band gap of 0.4-0.9 eV. For clarity, the curves are offset verticallyby multiples of 0.1 nA/mV, The peaks correspond to Van Hove singularities at the onsets of onedimensional energy bands of carbon nanorube.

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Figure 3 shows an atomic resolution image of a zigzag MWCNT with diameter 2.5 nm. Thedark dots, which represent the centres of the carbon hexagons, show a lattice on a cylindricalsurface, the nanotube wall. The nearest neighbour distance of the dark spots is 0.25 nm whichcompares nicely with the graphite lattice constant.

o 2.5 5.0 7.5 10.0nM

Figure 3. Atomically resolved STM topographic image of a zigzag MWCNT ( the chiral angle is30°). The dark dots arc the centers of carbon hexagons. The inset shows a very high resolution ofthe same tube in the main image. Site asymmetry is clearly visible. The brightest spots (opencircles) indicate atoms with no neighbor in the adjacent layer below, whereas atoms with suchneighbors (solid circles) appear darker. This asymmetry is caused by spatial variations in thelocal electronic density of states.

The hexagon centers appear elongated along the tube circumference due to the geometricaldistortion arising from the locally changing tip-sample arrangement due to the tube morphology.

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On a higher resolution image(see Fig. 3 inset), we have examined the signal intensity at differentatomic sites of MWNTs in Fig. 3 and found tbat some atoms are clearly more visible than others.This effect is not seen in the case of single wall nanoLUbes7, bUI is reminiscent of the sitea

outer shell. STM images of the outer shell show hexagonal arrangements of carbon at m, that areunequally visible by STM Lip. This suggests that the Slacking nature of MW s. may effect theelectronic band structure of the tube shells.

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