Katharina Arnold

FTIR-luminescence mapping of dispersed single-walled carbon nanotubes

We have applied the FTIR-luminescence/FT-Raman technique to map the near-infrared photoluminescence?(PL) of water?surfactant dispersions of single-walled carbon nanotubes (SWNTs) in broad excitation (250?1500?nm) and emission (800?1700?nm) ranges. The excitation wavelength was scanned by using the monochromatized light of standard xenon and tungsten halogen lamps. The PL maps are presented for SWNTs with a mean diameter of ~1.3?nm prepared by the pulsed laser vaporization method. When dispersed by powerful ultrasonic agitation and separated by ultracentrifugation, these nanotubes show structured absorption bands and a PL quantum yield as high as ~10-3. This indicates a large fraction of individual nanotubes in the dispersion. Electronic interband transition energies of nanotubes derived from the PL data correspond reasonably to the energies calculated in the modified tight-binding model of Ding et al.

Intersubband decay of 1-D exciton resonances in carbon nanotubes.

We have studied intersubband decay of E22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n,m) can be mapped onto a connectivity grid with curves of constant (n - m) and (2n + m). Moreover, the global behavior of E22 line widths is best characterized by a strong increase with energy irrespective of their (n-m)mod(3) = +/-1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E22 line widths are dominated by phonon assisted coupling to higher momentum states of the E11 and E12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.

Detection Of Single Carbon Nanotubes in Aqueous Dispersion via Photoluminescence

We report on the ‘on‐line’ photoluminescence (PL) detection of individual single‐walled carbon nanotubes (SWNTs) in water‐surfactant dispersions. This was achieved by using a near‐infrared confocal laser luminescence microscope specially optimized for the PL spectroscopy of SWNTs . A detection of single nanotubes was possible by using dispersions with a relatively low concentration of nanotubes and reducing the PL acquisition time so that a ‘switch‐on‐switch‐off’ behavior of emission peaks was observed, which reflected a random diffusion of different nanotubes through the laser focus volume. These results demonstrate the analytical potential of PL spectroscopy using SWNTs as luminescent markers.

Interband Transition Energy Shifts in Photoluminescence of Single‐Walled Carbon Nanotubes under Hydrostatic Pressure

Photoluminescence (PL) spectroscopy was applied to determine shifts of electronic interband transition energies, E11 and E22, of semiconducting single‐walled carbon nanotubes (SWNTs) under hydrostatic pressure of several kbar in a diamond anvil cell. Our results show that the energy shifts depend not only on the (n,m) structure (helicity) of nanotubes, but also on nanotube aggregation and interaction with the surrounding medium. The latter factors are also affected by application of pressure. This can explain a discrepancy between the experimental data and theoretical predictions for ‘ideal’ (non‐interacting) SWNTs under hydrostatic pressure deformation.Photoluminescence (PL) spectroscopy was applied to determine shifts of electronic interband transition energies, E11 and E22, of semiconducting single‐walled carbon nanotubes (SWNTs) under hydrostatic pressure of several kbar in a diamond anvil cell. Our results show that the energy shifts depend not only on the (n,m) structure (helicity) of nanotubes, but also on nanotube aggregation and interaction with the surrounding medium. The latter factors are also affected by application of pressure. This can explain a discrepancy between the experimental data and theoretical predictions for ‘ideal’ (non‐interacting) SWNTs under hydrostatic pressure deformation.

Strain‐Induced Shifts of the Photoluminescence of Single‐Walled Carbon Nanotubes in Frozen Aqueous Dispersions

Significant shifts of photoluminescence (PL) emission‐excitation resonances have been observed by freezing and cooling of water‐surfactant dispersions of single‐walled carbon nanotubes (SWNTs) down to 16 K. The PL resonances correspond to E11S, E22S electronic energies of specific (n,m) nanotubes. The shifts occur mainly in the interval of ∼150–200 K, are reversible and similar for SWNT dispersions with different surfactants and viscosity‐increasing additives. The sign of the shifts is determined by the (n‐m) mod 3 rule, whereas the shift magnitude depends on a chiral angle, being the smallest for the large angles. These results are in agreement with tight‐binding model calculations of Yang et al. for SWNTs under uniaxial compression (apparently caused by thermal contraction of the ice matrix in our case). This indicates a high sensitivity of electronic properties of SWNTs to mechanical strain and suggests an extended, ‘rod’‐like configuration of nanotubes in frozen dispersions.

Photoluminescence of Single‐Walled Carbon Nanotubes: Estimate of the Lifetime and FTIR‐Luminescence Mapping

We have applied the efficient FTIR‐Luminescence technique to map the near‐infrared photoluminescence (PL) of individual micelle‐isolated single‐walled carbon nanotubes (SWNTs) in a broad UV‐visible‐NIR excitation range. Monochromatic light of xenon and tungsten halogen lamps was used for PL excitation. Such PL maps provide a rich information about characteristic electronic transitions in SWNTs. The PL intensity shows a nonlinear behavior, when excited with nanosecond laser pulses. This is likely due to quenching interactions between multiple electronic excitations generated and propagating in nanotubes. This effect is similar for different, selectively excited nanotubes and allows us to estimate the PL lifetime (electronic relaxation time) in micelle‐isolated nanotubes as ∼100 ps.