Tuesday, 23 July 2013

Light-emitting nanotubes get brighter with zero-dimensional states

Light-emitting nanotubes get
brighter with zero-dimensional
states
When an exciton (blue spot) moving
along a nanotube collides with a
zero-dimensional state (red spot),
the exciton decays radiatively by
emitting a photon. Here, the
scientists generated local zero-
dimensional states by doping the
nanotubes with oxygen atoms. Credit:
Yuhei Miyauchi, et al. ©2013
Macmillan Publishers Limited
Carbon nanotubes have the potential
to function as light-emitting devices,
which could lead to a variety of
nanophotonics applications. However,
nanotubes currently have a low
luminescence quantum yield,
typically around 1%, which is
restricted by their one-dimensional
nature. In a new study, scientists
have demonstrated that artificially
modifying the dimensionality of
carbon nanotubes by doping them
with zero-dimensional states can
increase their luminosity to 18%. The
findings could lead to the
development of nanophotonics
devices such as a near-infrared
single-photon emitter that operates
at room temperature.
The researchers, Yuhei Miyauchi, et
al., have published their paper on
modifying the dimensionality of
carbon nanotubes in a recent issue
of Nature Photonics.
Under an applied electric current or
light irradiation , excited electrons and
holes (positively charged locations
where electrons are missing) are
created, and carbon nanotubes emit
near-infrared light. In this process,
excited electrons and holes form
bound states called excitons, and a
photon is emitted due to the
recombination of an electron and a
hole during this process.
As the researchers explain, a
nanotube's brightness, or
luminescence quantum yield, is
determined by the balance between
the radiative and non-radiative decay
rates of its excitons. In nanotubes,
non-radiative decay dominates,
resulting in low luminescence.
Previous research has shown that
this non-radiative decay is mainly
due to the rapid collision between
excitons and nanotube defects, which
quench, or suppress, the excitons.
Efforts have been made to reduce
the defect quenching of the excitons,
with varying success.
However, not all defects quench
excitons. As the scientists explain,
defects with certain electronic
structures can capture excitons and
convert them into photons with a
very high radiative decay rate,
possibly even higher than the
excitons' intrinsic rate. These
beneficial defects function as zero-
dimensional states, and the scientists
saw them as an opportunity to
improve nanotube luminescence.
In experiments, the researchers
sparsely doped the carbon nanotubes
with oxygen atoms, which act as
zero-dimension-like states embedded
in the one-dimensional nanotubes.
They found that, at room
temperature, excitons in the zero-
dimension-like states can achieve a
luminescence quantum yield of 18%,
an order of magnitude larger than
the 1% value of those in one-
dimensional nanotubes. The
researchers attribute this
improvement to mechanisms that
reduce the non-radiative decay rate
and enhance the radiative decay rate ,
and predict that the luminescence
could be further improved.
"We think that the luminescence can
be further increased if we can find a
better local atomic structure of an
artificial zero-dimensional state,"
Miyauchi, a researcher at Kyoto
University and the Japan Science and
Technology Agency, told Phys.org . "At
this point, our zero-dimensional
state has a lower lying dark state just
below the bright state, which results
in about 50% reduction of the
quantum yield at room temperature.
If one can find a better local
structure, we expect that it may be
possible to remove this dark state
below the bright state. Then, we
expect further increase of the
luminescence yield of excitons in the
local state."
In the future, the researchers hope
that the results will stimulate further
investigation of zero-dimensional—
one-dimensional hybrid systems,
regarding applications as well as the
fundamental physics behind the
systems.
"We plan to develop a more
sophisticated technique to generate
only one zero-dimensional state in a
single suspended carbon nanotube
connected to electrodes, which is
necessary to develop a real near-
infrared single-photon emitter
operable at room temperature using
carbon nanotubes," Miyauchi said.
"We also plan to try to achieve lasing
using this material. Although it has
been considered to be very difficult
to achieve lasing using carbon
nanotubes as gain media because of
the very rapid non-radiative decay
due to rapid collisions between
excitons under a strong excitation
regime, we believe that it would be
possible using zero-dimensional
states in carbon nanotubes , because
excitons in zero-dimensional states
would avoid collision with other
excitons.
"Our findings could also lead to the
fabrication of all-carbon near-
infrared LEDs or lasers. Near- infrared
light sources are very important for
telecommunications using optical
fibers. One usually needs minor
metals such as In, Ga, and As, to
fabricate light emitters for this
wavelength range. If one can make
efficient light sources using only
abundant carbon and without any
minor metals, it would be very nice
from the viewpoint of the resource
problem.
"We are also very interested in the
fundamental physics in these nice
hybrid low-dimensional
nanostructures, and we will explore
another more interesting physics in
them that possibly emerges from the
interactions between the states with
different dimensions in the same
nanostructures."
More information: Yuhei Miyauchi,
et al. "Brightening of excitons in
carbon nanotubes on dimensionality
modification." Nature Photonics. DOI:
10.1038/NPHOTON.2013.179

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