Photoluminescence-Excitation Mapping of Single Walled Carbon

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Transcript Photoluminescence-Excitation Mapping of Single Walled Carbon

Photoluminescence-Excitation Mapping of
Single Walled Carbon Nanotubes
Adrian Nish, Robin J Nicholas
Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK
e-mail: [email protected]
PLE Mapping
has become one of the most useful
methods for characterising different
samples of SWNTs in terms of their
Some of the background, experimental
method and current work on analysing
these spectra is presented here.
Single-Walled Carbon Nanotubes
• The intensity of PL is very sensitive to the wavelength of excitation
• Two systems are currently used for excitation:
-75W Xenon lamp & 0.3m monochromator with a linewidth of <1nm4
{ 300-950nm }
-Tunable Ti:Sapphire
{ 700-990nm }
• A Si-CCD {800-1100nm} and InGaAs PDA {900-1600nm} are used for
PLE Map taken with the
system in the above
• Hexagonal network of carbon atoms rolled up to
make a seamless cylinder
• Different ‘cuts’ have
different helicities as
defined by the ‘chiral
• Structure and diameter
are given by integer
values (n,m) from C =
na1 + ma2 where a1 & a2
are lattice unit vectors
and C is circumference
• Photoluminescence excitation mapping yields
distinct peaks which allow unambiguous identification
of nanotube species present in a sample3
Intensity Distribution
• PL intensity of the different nanotube
species (n,m) in a map is determined by
their relative concentration and their
fluorescence efficiency
Electronic Structure
• Using empirically determined positions, a
simulated PLE map was produced to model
dominant tube species found in SWNTs
grown by the ‘HiPCO’ method
• Carbon nanotubes
possess large π-electronic
systems similar to planar
• Reduced dimensionality
around the circumference
of the SWNT causes a
quantization of the allowed
• Since no sample of known (n,m) concentration exists, we must
guess the distribution of nanotube species
Tube Axis
• Applying a Gaussian function to the diameter and chiral angle5
distributions localises the strong emission to a few species, similar
to the experimental spectra
• One third of nanotubes have wavevectors
passing through the degenerate K point resulting
in metallic behaviour. The remaining two thirds
are narrow gap semi-conductors
• Sharp features in the
density of electronic states
lead to strong optical
• The diameter distribution is thought to be a function of the
SWNT growth process
• Photoluminescence from
SWNTs can be observed in
the near infra red2
• However, the chiral angle dependency is possibly due to both
the growth process and a strong dependency of the fluorescence
yield on this angle6
• To a first approximation the energy gap in
SWNTs is inversely proportional to diameter
• Trigonal warping of the graphene energy
surfaces close to the K point results in a further
chiral angle dependency
• Thus distinct energy gaps exist for all species
(n,m) which may then be characterised using PL
[1] Reich, Carbon Nanotubes (Wiley) 2004
[4] Campbell R., MPhys. Project Report
[2] O’Connell et al., Science 297, 593 (2002)
[5] Meyer et al., Ultramicroscopy 106, 176-190 (2006)
[3] Bachilo et al., Science 298, 2361 (2002)
[6] Oyama et al., Carbon 44, 873-879 (2006)