Carbon Material with a Highly Ordered Linear-Chain Structure - Polyynes: Synthesis, Properties, and Applications - page 230

FIGURE 11.14 Temperature dependence of conductivity of oriented carbon films of
different thickness.

where n is determined by the dimension of the sample and is equal to 1/4
for three-dimensional and 1/3 for two-dimensional samples. If n 1/2, the

Coulomb repulsion of electrons becomes importent. It is thought that the
electron states localized near the Fermi level can be formed as a result of

¼

the state of the chain kink ( Figure 11.1(g) ) .

The temperature dependence of the d.c. electric conductivity of the sp 1
films in the direction parallel to the chain axis shows that it is characterized
by the thermally activated behavior and the activation energy is equal to
0.02 eV in the temperature interval 160-300K. This is in good agreement
with the activation energy of solitons in polyyne chains.
As a result of doping the sp 1 -hybridized carbon films with H, N, Se,
Te, and S, a decrease of electrical resistivity in the direction parallel to the
film surface and an increase in the direction normal to the film surface by
the factor of 100 is observed. At the same time the optical measurements
show that the effective optical gap of the films increases up to 3 eV (see

Section 11.4.7 ) .

It was found by ESCA and TEM studies that some doping atoms (H and
N) create strong chemical bonds with carbon chains during doping, whereas
other atoms, for example S, do not form chemical bonds and are situated
among the carbon chains.