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2009 CARBON ALLOTROPESInvestigation of the dynamic alignment of single walled carbon nanotubesin high pulsed magnetic fieldsThe different carbon allotropes have attracted renewed attentionin recent years with the successful fabrication of thelower dimensional forms of graphite e.g. fullerenes, carbonnanotubes, graphene and graphene nano-ribbons. Carbonnanotubes are unique nano-objects with highly anisotropicelectrical, magnetic and optical properties. Over the lastfew years our understanding of the physics of carbon nanotubeshas made considerable progress and the detailedcomprehension of the complex physical properties of carbonnanotubes has significantly increased. Depending onthe diameter, and the helical arrangement of the hexagonallyarranged carbon atoms, carbon nanotubes can be eithermetallic or semiconducting.In this work we have investigated the magnetic propertiesof single walled carbon nanotubes. Semiconducting nanotubesare diamagnetic both along and perpendicular totheir long axis, however, the magnitude of the perpendicularsusceptibility is higher. Metallic nanotubes are paramagneticalong their long axis and diamagnetic perpendicularto it. This constrains both types of single walled carbonnanotubes to align parallel to a magnetic field. The samplesinvestigated here are composed of individually suspendedsingle walled carbon nanotubes in an aqueous solution. Theaim is to study the dynamic alignment of the single walledcarbon nanotubes during the application of a pulsed magneticfields via the absorption of polarized light.difference between the measured and the simulated lengthdistribution.Further measurements with length sorted samples are beingundertaken. The ultimate aim is to identify the role ofthe single walled carbon nanotubes in the dynamic processand determine the influence of the chirality on the magneticanisotropy.Figure 1: The time and field dependent alignment of a carbonnanotubes suspended in aqueous solution.To measure the alignment of single walled carbon nanotubeswe make use of the fact that nanotubes absorb lightonly if it is polarized parallel to the axis of the nanotube.We thus refer to linear dichroism spectroscopy: The absorptionratio between light polarized parallel and perpendicularto the applied magnetic field directly reflects the degree ofalignment of the ensemble on nanotubes under the influenceof this magnetic field. Measuring the integrated absorptionspectra of a white light source at different times during themagnetic field pulse allows us to determine the time dependentalignment of the single walled carbon nanotubes(figure 1).Our data can be understood with the aid of a theoreticalmodel based on rotational diffusion of rigid rods [Shaveret al., ACS Nano 3,1 131 (2009)]. A typical result of thesimulation is shown in figure 2. The length distribution ofthe single walled carbon nanotubes in the sample was determinedusing an atomic force microscope. The simulationwhich best describes our experimental data shows a slightFigure 2: Time dependent alignment and best match fit (red).Insert: The matching length distribution (red) is larger than themeasured distribution (blue).N. Ubrig, S. George, O. PortugallJ. Kono, M. Pasquali (Rice University)5