| Abstract : |
The electronic properties of carbon nanotubes
(CNT) are determined by the way, in which
the graphene sheet is rolled into a cylinder.
In its spectroscopic mode, scanning tunneling
microscope (STM) is a unique tool for the
investigation of electronic properties of a CNT.
The interpretation of experimental data is made
difficult by the complexity of the tunneling
system. We use our recently developed computer
code [1] to get insight into the tunneling
process through the STM tip-CNT-support system.
The tunneling problem is regarded as a problem
in potential scattering theory. The current
density is determined as a statistical average
of probability current densities of wave packets
(WPs) scattered on an effective potential.
Effects of tip geometry, potential barrier
between CNT and support, tunneling vs. point
contact conduction, WP incidence angle and
energy, and STM bias voltage are investigated
in detail. The results are compared with
experiments. As long as the electronic
structure of the CNT and of its support are
similar, the major distortion of the STM
image arises from the geometric convolution
of the tip shape with the tube shape.
When their electronic structure is different,
further distortions arise [1]. The existence
of the second tunnel gap [2], between the CNT
and its support may introduce complications
in the interpretation of STS data. The magnitude
of the tunneling current is determined by the
CNT-tip tunneling gap. The asymmetry in the
tunneling current [3] will be determined
by the contact between the free CNT and the
support [4]. If this contact is a low resistance,
ohmic contact, the I(V) spectrum will be
symmetric, if this contact is dominated by
tunneling than asymmetry may show up in the
I(V) curves. Atomic resolution imaging of CNTs
is possible in point contact mode of STM.
Recent tight-binding calculations of atomic
resolution STM images [5] compared to
experimentally measured atomic resolution images,
and to diameter values inferred from STS
measurements indicate anomalously small gap
values between the CNT and the tip [6].
The different tunneling situations have a
characteristic influence on the tunneling
probability as a function of the WP incidence
angle and energy [7].
1. G. I. Márk, L. P. Biró, and J. Gyulai, Phys. Rev. B 58, 12645 (1998).
2. L. P.Biró, J. Gyulai, Ph. Lambin, J. B. Nagy, S. Lazarescu, G. I. Márk, A. Fonseca, P. R. Surján, Zs. Szekeres, P. A. Thiry, and A. A. Lucas, Carbon 36, 689 (1998).
3. L. P. Biró, P. A. Thiry, Ph. Lambin, C. Journet, P. Bernier, and A. A. Lucas, Appl. Phys. Lett. 73, 3680 (1998).
4. G. I. Márk, L. P. Biró, J. Gyulai, P. A. Thiry, and Ph. Lambin, in Electronic Properties of Novel Materials -- Science & Technology of Molecular Nanostructures (in press, 1999).
5. V. Meunier, and Ph. Lambin, Phys. Rev. Lett. 81, 5888 (1998).
6. L. C. Venema, V. Meunier, Ph. Lambin, and C. Dekker, (submitted to PRB).
7. G. I. Márk, L. P. Biró, J. Gyulai, P. A. Thiry, A. A. Lucas, and Ph. Lambin, (to be published). |
