Carbon Material with a Highly Ordered Linear-Chain Structure - Polyynes: Synthesis, Properties, and Applications - page 229

it (below). The STM image of the film was obtained by a Nanoscope-3

microscope and is shown in Figure 11.13(c) . For comparison The STM

image of the golden supporting layer is also shown in Figure 11.13(c) for
comparison. According to AFM studies the surface of the film is extremely
smooth in contrast to that obtained by STM. The STM image reveals the
features, which are relevant to the underlying gold substrate with gold
particles on it. This means that the STM image is explained by tunneling
transparency of the sp 1 -hybridized carbon film but not by the surface
corrugation. This effect presumably has the same origin as the tunneling
conductivity of long-chain hydrocarbon films deposited on conductive
substrates.

11.4.6 E LECTRIC C ONDUCTIVITY

According to the proposed model of atomic structure of the chain carbon
films the conductivity of the films should be high in the direction of carbon
chains and low in the direction perpendicular to the chain axis, because the
distance between the chains is much higher than that between the carbon
atoms in the chain. Therefore, electron hopping among the chains is quite
small.
The value of electric conductivity of the film was measured using a four-
point system of gold contacts. The film thickness was 80 nm. The room
temperature electric conductivity in the vertical direction was found to be
17.7

10 6 m. The

ratio between the electric conductivity in the vertical and horizontal
directions is equal to 2

m while that in the horizontal direction was 3.2

10 5 . This means that the oriented sp 1 -hybridized
carbon films indeed belong to anisotropic conductors.
The dark d.c. electric conductivity

ðÞ
direction parallel to the film surface depends on both the temperature and
film thickness. There are two different regions in the

T of the films ( Figure 11.14 ) in the

ðÞ

T curves plotted in

ðÞ =

the activation coordinates log T. The first one at temperatures of

150K<T<300K is linear and is characterized by a slope, which depends on
the film thickness. The mechanism of conductivity changes at lower tem-
peratures in the range of 70K<T<140K and the

1

dependence becomes
nonlinear in this temperature region. The most appropriate mechanism of
conductivity of the carbon films at high temperatures is their thermal
activation from gap state into the states above mobility edge. The activation
energy decreases from 0.16 eV to 0.06 eV when the thickness of the films
increases from 20 nm to 100 nm.
The temperature behavior of conductivity follows the variable length
hopping conductivity via electron states localized near the Fermi level and is
described by the expression

ðÞ T

n

ðÞ¼ T exp ð ð T o = T Þ Þ ,