Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Spectroscopy of carbon nanostructure in high magnetic fields
Using an InGaAs detector array we investigated the high-field optical and electronic properties of carbon
nanotubes. For the photoluminescence (PL) experiments we used a titanium-sapphire Laser and various laser
diodes. With this method we were able to show the effects of magnetic field on the electronic properties of
carbon nanotubes, i.e. adding an Aharonov-Bohm phase on the band structure and modifying the PL spectra in
fields up to 77 T [1]. The high-field PL spectroscopy of carbon nanotubes also revealed an increase in intensity
of the PL peaks, the so-called magnetic brightening [2,3]. By using highly aligned films of carbon nanotubes, we
have demonstrated that the rather weak brightening observed under the influence of a perpendicular magnetic
field reflects only the residual disorder in the film. The brightening thus relates exclusively to the magnetic field
component parallel to the tube axis [4]. We also were able to determine quantitatively the zero field splitting of
the two lowest exciton states, responsible for the magnetic field induced brightening [4]. This has been achieved
by fitting a theoretical model to the available high-field data and extrapolating the exciton energies to B=0.
30
Fig. 5: PL peak evolution for highly aligned
samples perpendicular and parallel to the magnetic
field. The brightening for the field perpendicular to
the sample reflects only the residual disorder in the
film [4].
The band structure of graphene yields to unconventional effects such as electrons behaving like massless Dirac
fermions. Thus the Landau level structure is different from other 2-D semiconductors. At high fields and high
energies theoretical models predict an asymmetry between electrons and holes. The absorption of circular
polarized light should reveal it. The analysis of our NIR measurements of this effect is still in progress.
[1] S. Zaric et al, Phys. Rev. Lett. 96 (2006) 016406, [2] J. Shaver et al, Nano Lett. 7 (2007) 1851, [3]
I.B.Mortimer et al, Phys. Rev. B (2007) 085404, [4] J. Shaver et al, Phys. Rev. B 78, (2008) 081402.
Dynamic alignment of single walled carbon nanotubes in aqueous suspensions
The main purpose of this work is to investigate the dynamic properties of carbon nanotubes in different aqueous
suspensions. Due to different magnetic susceptibilities, for metallic and semiconducting tubes, along the tube
long axis and perpendicular to it, carbon nanotubes will align parallel to an external magnetic field.
To quantify this effect we analyse the field induced linear dichroism : a valuable method to detect magnetic
anisotropies and to quantify it. A theoretical model based on the Smoluchovski equation for rigid rods were
developed in order to determine quantitatively other effects governing the alignment such as temperature,
viscosity, bundling or length distribution of nanotubes. [1]
Fig.6 Comparison between calculated (black) and
measured (purple) linear dichroism (LD). The LD
is following the magnetic pulse (green) [1].
[1] Shaver et al, ACS nano 3 (2009), 1