Plasmons in layered nanospheres and nanotubes investigated by spatially resolved electron energy-loss spectroscopy

We present an extensive electron energy loss spectroscopy study of the low- loss energy region, recorded on multishell carbon and boron-nitride nanotub es and carbon hyperfullerenes. Collections of spectra were recorded in a sc anning transmission electron microscope by scanning a subnanometer probe fr om vacuum into the center of the nano-objects. This experimental technique provides the unique ability of disentangling and identifying the different excitation modes of a nanoparticle. We concentrate on the study of surface modes excited in a near-field geometry where the coupling distance between the electron beam and the surface of the nano-objects is accurately monitor ed. Similarities between surface collective excitations in the different la yered nanostructures (cylindrical or spherical, boron nitride, or carbon co nstituted) are pointed out. Two surface modes at 12-13 eV and 17-18 eV are experimentally clearly evidenced. We show that these modes are accurately d escribed by a classical continuum dielectric model taking fully into accoun t the anisotropic character and the hollow geometry of the nanoparticles. T hese two modes are shown to be directly related to the in-plane and out-of- plane components of the dielectric tensor. The higher-energy mode (in-plane mode) is shown to shift to higher energy with decreasing impact parameter, as a result of an increase in the weights of the high-order multipolar mod es while reaching the surface of the nano-objects.