Carbon Material with a Highly Ordered Linear-Chain Structure - Polyynes: Synthesis, Properties, and Applications - page 231

FIGURE 11.15 UV and visible light adsorption of sp 1 carbon films.

11.4.7 O PTICAL P ROPERTIES

The UV and visible light adsorption of the linear-chain carbon films was
measured by a Perkin Elmer spectrometer in reflection mode. The light
adsorption coefficient, A, was recorded in the region of 250-1100 nm. In this
region the A value is directly proportional to the optical joint density of
states (OJDSs) of the material investigated [15-17]. Figure 11.15 illustrates
the light adsorption results by use of the linear-chain carbon film with
thickness of 200 nm. The maximum at 550 nm (2.26 eV) corresponds to the
film composed of pure linear-chain carbon and at 425 nm (2.92 eV) to the film
of S-doped linear-chain carbon. It can be seen from the figure that the OJDS
of the S-doped film changes compared to the pure sp 1 -carbon film. The
maximum on the dependencies shown in Figure 11.15 corresponds to the
electron p ! p transitions in the valence band. It can be seen that, as a
result of doping, the maximum of adsorption shifts from 2.26 eV to 2.92 eV,
so that the ''effective'' optical gap increases. This is in good agreement with
the fact that the electric resistivity of the films increases after doping.

11.4.8 F IELD E FFECT M EASUREMENTS
Field effect measurements were performed on the sp 1 -hybridized carbon

films doped with S. Figure 11.16(a) shows schematically the cross section of

the sp 1 film deposited on a dielectric substrate, where 2a is the film width
and h is its thickness. If h

2a, the normal component of electric field
created on the surface of the film can be calculated by the equation

1
a 2

E n ¼ p

x 2