Page 77 - Carbon Nanotubes

66 K. SATTLER
samples were transferred to a STM operated at 2 x being equally distributed. We note that most of the
lo-'' Torr, without breaking vacuum. Our evapora- nanotubes that we analyzed showed hemispherical ter-
tion and condensation process leads to the formation minations. Therefore, we might assume that the tubes
of various nanostructures, with 70% nanotubes on the start to grow from an incomplete fullerene cap and
average, of the overall products. In some areas, which that the C,, hemisphere is the nucleation seed for the
may be as large as one square micron, we find 100% growth of the 10 A tube. After the C6' hemisphere is
nanotubes. The yield for single-wall tubes varied from formed, growth may continue as an all-hexagon net-
experiment to experiment from a few percent to 80- work, forming a tube, rather than continuing as an al-
90%. Bundles of multiwall tubes were found in some ternating hexagodpentagon network leading to the
areas, but were usually less abundant than isolated C6, sphere. The two caps, on both sides of the 10 A
tubes. Individual nanocones were observed together zigzag tube (C60+18j)[17,1S] are identical, with a total
with tubes, but were quite seldom. The microscope was number of 12 pentagons, following Euler's theorem.
operated in the constant current and in the constant The two caps for the 10 A armchair tube (C60+10j) are
height mode. Atomic resolution images were recorded 36" rotated relative to each other.
in the constant current mode, in which the tip-to- It is interesting that we find the zigzag configura-
sample distance is kept constant by means of an elec- tion for the tube network. The zigzag tube (Fig. 2) is
tronic feedback control. Bias voltages of 100 to 800 the only nonhelical one among all the possible tube
mV (both positive and negative) and tunneling cur- configurations. A cut normal to the C60+18j tube axis
rents of 0.5 to 3.0 nA were applied. A mechanically leaves 18 dangling bonds, compared to 10 dangling
shaped Pt/lr tip was used. bonds for the C60+10j tube. For the armchair tube, it
We did not observe any voltage dependent varia- may be easy to incorporate pentagonal defects lead-
tion of the tube images. Also, the measured heights ing to an early closure because only one additional
of the tubes were comparable with their diameters. atom is required to form a pentagon at the growth pe-
Both of these observations indicate that the tubes have riphery. For the zigzag tube, however, two atoms are
rather metallic than semiconducting properties. required to form a pentagon and the structure might
The tubes were stable over long periods of time. rather continue as a hexagonal network. Therefore,
After several months of being stored in UHV we still the zigzag 10 A single-shell tubes might have a higher
observed the same features as shortly after their prep- probability for growth.
aration. Some of the samples were transferred to an
STM operated in air. Again, we observed similar
structures as seen in UHV. This shows the high stability 4. MULTI-SHELL TUBES
of the tubes. It appears that the vapor-phase growth There is an infinite number of possible atomic
technique produces defect-free tubes, with dangling structures of graphene tubules. Each structure is char-
bonds at the tube edges often being saturated by cap acterized by its diameter and the helical arrangement
terminations. of the carbon hexagons. Presumably, only single-shell
tubes with small diameters of about 10 A are formed
and tubes with larger diameters are multishell tubes.
3. SINGLESHELL TUBES
We produced multilayer tubes with diameters be-
Single-shell tubes are formed from a single layer of tween 20 A and 70 A and up to 2000 A in length[4].
graphite. The surface of the cylinders has a honeycomb- An STM image of such tubes is shown in Fig. 3. The
lattice pattern, just as in a two-dimensional graphite cylindrical shapes are well displayed.
plane. From a theoretical point of view they are inter- We observed in some cases coaxial arrangement of
esting as the embodiment of a one-dimensional (1-D) the outermost and an inner tube. The outer tube may
periodic structure along the tube axis. In the circum- be terminated and the adjacent inner one is imaged si-
ferential direction, periodic boundary conditions ap- multaneously[4]. We measure an interlayer spacing of
ply to the enlarged unit cell. In addition to the chiral 3.4 A, which is about the graphite interlayer distance
structures, there exist two nonchiral configurations, (3.35 A).
zigzag and armchair [ 131. We find that the tubes are placed almost horizontally
Part of a 15-nm long, 10 A tube, is given in Fig. 1. on the substrate. Irregular nanostructures were also
Its surface atomic structure is displayedIl41. A peri- formed, as displayed in the images. However, the high
odic lattice is clearly seen. The cross-sectional profile occurrence of tubes clearly shows that carbon prefers
was also taken, showing the atomically resolved to condense to tubular structures, as opposed to other
curved surface of the tube (inset in Fig. 1). Asymme- nanostructures, under our preparation conditions.
try variations in the unit cell and other distortions in In Fig. 4 we show an atomic resolution image of a
the image are attributed to electronic or mechanical carbon tube. The structure imaged at the upper right
tip-surface interactions[l5,16]. From the helical ar- corner of the picture comes from another tube. Both
rangement of the tube, we find that it has zigzag of them were -1000 A long. A perfect honeycomb
configuration. surface structure is observed. By taking into account
The zigzag and armchair tubes can be closed by the curvature of the tube surface and the STM imaging
hemispherical C6' caps, with 3-fold and 5-fold sym- profile, we find the same lattice parameter as that of
metry, respectively. Both caps contain six pentagons .graphite (1.42 A). This directly proves that the tubu-