Page 76 - Carbon Nanotubes

SCANNING TUNNELING MICROSCOPY OF CARBON
NANOTUBES AND NANOCONES

KLAUS SATTLER
University of Hawaii, Department of Physics and Astronomy, 2505 Correa Road,
Honolulu, HI 96822, U.S.A.

(Received 18 July 1994; accepted 10 February 1995)
Abstract-Tubular and conical carbon shell structures can be synthesized in the vapor phase. Very hot car-
bon vapor, after being deposited onto highly oriented pyrolytic graphite (HOPG), forms a variety of nano-
structures, in particular single-shell tubes, multishell tubes, bundles of tubes, and cones. The structures
were analyzed by scanning tunneling microscopy (STM) in UHV. Atomic resolution images show directly
the surface atomic structures of the tubes and their helicities. A growth pathway is proposed for fuiler-
enes, tubes, and cones.
Hey Words-Carbon nanotubes, fullerenes, STM, fibers, nanostructures, vapor growth.

1. INTRODUCTION yielding quite realistic three-dimensional images from
tube terminations. Also, besides a difference in rnor-
Hollow carbon nanostructures are exciting new sys- phology, open and closed ends show a difference in
tems for research and for the design of potential nano- electronic structure. Open ends appear with ‘highlighted’
electronic devices. Their atomic structures are closely edges in STM images, which is due to an enhanced
related to their oute: shapes and are described by hex- dangling bond electron state density. Closed ends do
agonal/penitagonal network configurations. The sur- not have such highlighted edges.
faces of such structures are atomically smooth and The growth pathway of various fullerene- and
perfect. The most prominent of these objects are ful- graphene-type nano-objects may be related. They are
lerenes and nanotubes[l]. Other such novel structures synthesized in the vapor phase and often appear simul-
are carbon onions[2] and nanocones[3]. taneously on the same sample. A common growth
Various techniques have been used to image nano- mechanism with similar nucleation seeds may, there-
tubes: scanning electron microscopy (SEM)[ 11, scan- fore, lead to these different structures.
ning tunneling microscopy (STM)[4-71, and atomic
force microscopy (AFM)[S]. Scanning probe micro-
scopes are proximity probes. They can provide three- 2. EXPERIMENTAL
dimensional topographic images and, in addition, can Graphite was used as substrate for the deposition
give the atomic structure of the surface net. They can of carbon vapor. Prior to the tube and cone studies,
also be used to measure the electronic (STM) and elas- this substrate was studied by us carefully by STM be-
tic (AFMJ properties of small structures. STM is re- cause it may exhibit anomalous behavior with unusual
stricted to electrically conducting objects, but AFM periodic surface structures[9,10]. In particular, the
does not have this constraint. cluster-substrate interaction was investigated[l 11. At
STM and AFM images give directly the three- low submonolayer coverages, small clusters and is-
dimensional morphology of tubes and are consistent lands are observed. These tend to have linear struc-
with the structures inferred from SEM. In addition, tures[l2]. Much higher coverages are required for the
atomically resolved STM images make direct helicity synthesis of nanotubes and nanocones. In addition, the
determinations possible[4]. They give information carbon vapor has to be very hot, typically >3O0O0C.
about the nature of stacking of concentric carbon lay- We note that the production of nanotubes by arc dis-
ers within the nanotubes via modifications of their sur- charge occurs also at an intense heat (of the plasma
face density of states. STM is sensitive to such small in the arc) of >30OO0C.
lateral local density of states variations. Contours The graphite (grade-A HOPG) was freshly cleaved
taken from the ends of the tubes show that some of in UHV and carefully examined by STM before the
them are open and others are closed. Many images in- deposition. The HOPC surface was determined to be
dicate that the closed tubes have hemispherical caps. atomically flat and defect-free over micrometer dimen-
Such terminations can be modeled by fullerene hemi- sions. Any defect or adatom would have easily been
spheres with 5/6 networks. detected. The graphite was cooled to -30°C during
IC is not easy to determine detailed properties of the evaporation. The carbon vapor was produced by re-
tube terminations using STM or AFM. These micro- sistively heating a 99.99% purity carbon foil (0.5 mrri
scopes cannot image undercut surfaces and the tip thick) in UHV (base pressure 2 x lo-’ Torr). The de-
shape is convoluted with the cap shape of the nano- position rate of 0.5 A/s was controlled by a quartz
tube. However, the tips may have very sharp edges crystal film thickness monitor. After deposition, the

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