Investigations into the Optical Properties of Individual, Air-Suspended, Single-Walled Carbon Nanotubes
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Date
2008-09-27T14:37:18Z
Authors
Wilson, Mark
Keyword
nanotube , carbon , photoluminescence , exciton , dynamics , ultrafast , femtosecond excitation correlation , FEC , relative humidity , spectroscopy , single-molecule , saturation , single-walled , fluorescence
Abstract
Single-walled carbon nanotubes are naturally-forming nanostructures that have attracted
considerable recent research interest due to their unique opto-electronic properties
and comparative ease of fabrication. Two-thirds of nanotube species are semiconductors
due to symmetry conditions imposed by their pseudo-one-dimensional tubular structure, and exhibit band-gap photoluminescence when isolated from their environment. Despite their elegant structural simplicity, fundamental properties of carbon nanotubes, such as their intrinsic quantum efficiency, non-linear excitonic recombination mechanisms, and the role of environmental effects, continue to be disputed in the literature.
The design of an apparatus capable of observing nanotube photoluminescence is
presented, along with conclusive proof of the observation of a single (9,8)-chirality
nanotube in the form of spectral, spatial, and polarization-dependent measurements.
The dependance of the excitation and emission spectra of a single nanotube on the
excitation intensity is explored and the emission spectra found to be described by a
Gaussian peak function, in contrast to previously-reported results.
The unexpected ability to cause redshifts in the emission spectrum via the ambient
humidity is discovered, which has consequences on experimental best practices.
Photoluminescence quantum efficiencies are measured to be 4±2% and 13±6% for two different nanotubes. This is at the high end of the range for comparable literature
results, and supports the validity of a recent literature value for the effective atomic absorption coefficient for carbon, AC=1.6×10^−3nm^2, which is ten times greater
than previous literature values.
Pulsed power dependence studies show that the PL emission undergoes ‘hard’
saturation at an excitation intensity of 0.5×10^12photons/pulse/cm2, which is at
least 100 times lower than previous reports and provides insight into non-linear decay
dynamics. A novel theoretical model is developed to explain this saturation process,
which yields an absorption co-efficient of AC=1.2±0.3×10^−3nm^2 as a fit parameter.
Time-resolved photoluminescence dynamics are explored using femtosecond excitation
correlation spectroscopy. Results suggest that the one-body decay processes
are bi-exponential, with time constants of 31±4ps and 313±61ps, but also highlight the limitations of this technique in observing the expected very rapid (~1 ps) two-body Auger recombination process.