Lattice Dynamics of Hexagonal Boron Nitride (h-BN) by Inelastic X-ray

We aim to address the angle resolved collective electronic excitation spectrum in individualized and aligned single wall carbon nanotubes (SWNT). Preceding studies employing optical absorption spectroscopy (OAS) [1] and angle resolved electron energy loss spectroscopy (AR-EELS) [2]have revealed distinct on-axis and cross-axis polarized electronic excitations. With EELS we have identi¯ed plasmons at 5 eV and 17 eV of the electronic ¼ and ¾ systems, re-spectively. Due to the limitations by multiple scattering and line broadening we were not able to measure the plasmon dispersions above 2.5 ºA¡1. We seek to overcome these limitations by means of inelastic x-ray scattering (IXS). The very special macroscopic SWNT material has a very low bulk density of » 0:05g=cm3 [3], which accounts for the close match of the q = 0 limit from the plasmon dispersions and peak positions from OAS. Addressing the electronic excitation spectrum is a crucial prerequisite for elucidating the exceptional optical properties of the novel dielectric metamaterial[4] formed by the sparse array of individual nanotubes. From the theoretical side we showed that crystal local ¯eld e®ects (cLFE) are essential for the correct description of dielectric properties and hence the loss-function of isolated SWNT [5]. Furthermore, the on-axis plasmon dispersion of SWNT could be identi¯ed as a quantitative reminiscence the electronic in-plane excitations of isolated graphene sheets [2]. Regarding the full tubular structure and the full assessment of the on-axis as well as the cross-axis plasmon(dispersions), there are ongoing activities in the European theoretical spectroscopy