Page 131 - Carbon Nanotubes

ELECTRONIC PROPERTIES OF CARBON NANOTUBES:
EXPERIMENTAL RESULTS

J.-P. ISSI,’ L. LANGER,’ J. HEREMANS? and C. H. QLK~
‘Unite PCPM, Universitt Catholique de Louvain, Louvain-la-Neuve, Belgium
2General Motors Research and Development Center, Warren, MI 48090, U.S.A.
(Received 6 February 1995, accepted 10 February 1995)
Abstract-Band structure calculations show that carbon nanotubes exist as either metals or semiconduc-
tors, depending on diameter and degree of helicity. When the diameters of the nanotubes become com-
parable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning
tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show
evidence of onedimensional behavior: the current-voltage characteristics are consistent with the functional
energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly
with the inverse nanotube diameter. Electrical resistivity and magnetoresistance measurements have been
reported for larger bundles, and the temperature dependence of the electrical resistance of z single micro-
bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the for-
mation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly
diamagnetic behavior. The susceptibility at fields above the value at which the magnetic length equals the
tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sari.-
ple the effect of the Finite tube diameter, the susceptibility has a much more pronounced temperature
dependence.
Key W70rds--Carbon nanotubes, scanning tunneling microscopy, spectroscopy, magnetoresistance, elec-
trical resistivity, magnetic susceptibility.
1. INTRODUCTION perimental challenges result from the fact that tubes
are often produced in bundles, so that obtaining data
The existence of carbon nanotubes with diameters on single, well-characterized tubes has not yet been
small compared to the de Broglie wavelength has been achieved. We review here some experimental observa-
described by Iijima[l,2,3] and others[4,5]. The energy tions relevant to the electronic structure of individ-
band structures for carbon nanotubes have been cal- ual nanotubes or on bundles of nanotubes: combined
culated by a number of authors and the results are scanning tunneling microscopy and spectroscopy,
summarized in this issue by M.S. Dresselhaus, 6. Dres- temperature-dependent resistivity, magnetoresistance
selhaus, and R. Saito. In short, the tubules can be (MR), and magnetic susceptibility.
either metallic or semiconducting, depending on the
tubule diameter and chirality[6,7,8]. The calculated
density of states[8] shows I/(KEj)’’’ singularities 2. SCANNING TUNNELING
characteristic of one-dimensional (1D) systems. The SPECTROSCOPY STUDIES
separation between the singularities around the Fermi Scanning tunneling spectroscopy (STS) can, in prin-
energy is the energy gap for the tubes that are semi- ciple, probe the electronic density of states of a single-
conducting, and scales linearly with the inverse of the wall nanotube, or the outermost cylinder of a multi-wall
tube outer diameter[7,8]. This contrasts with the case tubule, or of a bundle of tubules. With this technique,
of a rod-shaped quantum wire, for which the gap is it is further possible to carry out both STS and scan-
expected to scale with the inverse square of the diam- ning tunneling microscopy (STM) measurements at the
eter. The relevant energy scale for the gap in carbon same location on the same tubule and, therefore, to
nanotubes is the nearest-neighbor overlap integral in measure the tubule diameter concurrently with the STS
graphite (3.14 eV)[9]. This makes room-temperature spectrum. No reports have yet been made of a deter-
observation of the quantum size effects, in principle, mination of the chiral angle of a tubule with the STM
possible in nanotubes with diameters in the nm range, technique. Several groups have, thus far, attempted
because the sublevel energy separations are of the or- STS studies of individual tubules.
der of 1 eV. The first report of current-voltage (I-V) measure-
Experimental measurements to test these remark- ments by Zhang and Lieber[lO] suggested a gap in the
able theoretical predictions of the electronic structure density of states below about 200 MeV and semicon-
of carbon nanotubes are difficult to carry out because ducting behavior in the smallest of their nanotubes
of the strong dependence of the predicted properties (6 nm diameter). The study that provides the most de-
on tubule diameter and chirality. Ideally, electronic or tailed test of the theory for the electronic properties
optical measurements should be made on individual of the 1D carbon nanotubes, thus far, is the combined
single-wall nanotubes that have been characterized STM/STS study by Olk and Heremans[ 111, even though
with regard to diameter and chiral angle. Further ex- it is still preliminary. In this study, more than nine

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