Photoelectronic Properties of C-based Nanostructures

1
Photoelectronic Properties of C-based
Nanostructures C. Zhang, J.C. Chao, X.G. Guo and
Feng Liu Dept. of Materials Science and
Engineering, University of Utah, Salt Lake City,
UT
We have recently developed a tight-binding
method 1 that enabled us to calculate the photo
adsorption rate of carbon-based nanostructures,
such as nanotubes and graphene nanoribbons, due
to impurity and defect scattering. We formulated
a quantum transport equation in the presence of
both electron-electron and electron-impurity
scattering, to calculate photon absorption rate
and photoconductivity of nanostructures. The
electron-impurity scattering is treated in the
most general manner by a random potential within
the tight-binding formalism. Physically, the
random potential will induce indirect and
intraband transitions that are forbidden in an
ideal system, by adding an arbitrary momentum to
the electrons Figure 1 shows our calculations
1 of the optical properties for three (10,0),
(15,0), and (20,0) zigzag SWNTs. In Fig. 1a, we
plot the imaginary part (eI) of the dielectric
functions of the three SWNTs, which are
essentially their photon adsorption spectra
without impurity scattering. For semiconductor
(10,0) and (20,0) tubes, electrons are excited
from the valence to the conduction bands of the
same momentum. The peaks in the eI (Fig. 1a)
correspond to various transition energies. There
exists a threshold excitation energy, which is
defined by the band gap for the two semiconductor
tubes. For the metallic (15,0) tube, the
intraband transition channel is also open and the
threshold energy is much lower. The eI changes
drastically with the frequency, reflecting both
resonant and non-resonant coupling of electrons
with photons.
2
In Fig. 1b, we show the photo absorption rates
of the same three SWNTs as a function of photon
energy, taking into account the impurity
scattering. They are plotted against the
background of the solar energy spectrum to
indicate their frequency range of adsorption.
Common to all the SWNTs is the unusually high
value of absorption rate induced by impurity
scattering, reflected by peaks followed by a
large additional continuum in the spectra. In
contrast, without impurity scattering, the
absorption spectra contain only discrete peaks
separated by small tails (Fig. 1a). There is a
striking difference between the absorption rate
of semiconducting (10,0) and (20,0) tubes and
that of the metallic (15,0) tube. The
semiconductor tubes (10,0) and (20,0) tubes
have a threshold absorption energy below which no
absorption takes place, in accordance with their
respective band gaps. In general, the smaller
semiconductor tubes have a higher absorption rate
than larger tubes. So, the smaller tube has a
high threshold but also a larger output voltage.
The absorption of the metallic (15,0) tube shows
an additional absorption peak in the far IR range
that is due to the intraband plasmon excitation.
(a)
(b)
Fig. 1. The imaginary part (eI) of the dielectric
functions of the three zigzag SWNTs, as a
function of photon energy.
1 Impurity mediated absorption continuum in
single-walled carbon nanotubes, C. Zhang, J.C.
Chao, X.G. Guo and Feng Liu, Appl. Phys. Lett.
90, 023106 (2007).