Electron Energy Loss Spectroscopy in a Transmission Electron Microscope can probe with a high degree of spatial resolution the electronic excitation spectra of nanomaterial over an extended optical spectral range, typically from 2 to 50 eV. Here we describe a derived technique, called Near Field EELS, in which a subnanometer probe of high-energy electrons is positioned at controlled distances from the surface of an individual nano-object. It can then probe the locally induced electromagnetic field at the nanometer scale, which depends on the detailed nature, shape and dielectric response of the investigated nanostructure. This will be demonstrated on different kinds of nanotubes, the atomic structure of which being simultaneously determined by imaging. The obtained spectra can be well modeled within the framework of the so-called continuum dielectric theory. The importance of the coupling between inner and outer surfaces of the nanotubes will be pointed out. Most recent data concern the measurements of the optical gap for Boron Nitride nanotubes of different structural characteristics. Preliminary results on exploration of the electromagnetic fields on individual silver particles with different sizes and shapes, bringing therefore a complementary contribution to plasmonic studies at an unprecedented level of spatial resolution, is presented.
One of the most versatile formalism for the study of the electrodynamic response of solids, surfaces and interfaces or nanoparticles is the continuum dielectric model. In this contribution, we develop an application of this dielectric approach to nanocylinders and more particularly to the simulation of near-field electron energy loss (EEL)spectra of nanotube bundles. On the experimental side, EELS in a Scanning Transmission Electron Microscope (STEM) combines both spatial and energy resolutions in the plasmonic energy range and then permits the spectroscopic analysis of the surface and volume excitations of nanoparticles.
Amongst the challenges brought about by the discovery of carbon nanotubes, one can cite the understanding of their optical properties. In this contribution, pursuing this goal within a dielectric continuum model, we focus on the dispersion and coupling of surface plasmon excitations of hollow nanocylinders and on the near-field EELS of nanotube nanocrystals (bundles). Experimental EELS in a STEM have also been obtained on bundles of carbon nanotubes. The interpretation in terms of effectif medium theory is successfuly performed both for surface and bulk losses associated with the σ plasmon.
A short review of electron-energy-loss spectroscopy (EELS) experiments of carbon nanotubes and onions is presented. The dielectric response function of these nanostructures is derived from electrodynamics. Loss spectra computed with the dielectric theory are compared with spatially-resolved experimental spectra. The main features of the loss spectra obtained with non-penetrating electrons can be attributed to surface plasmon excitations (π plasmon at 6 eV and π + σ plasmon at 15 and 17-18 eV).
Carbon nanotubes may constitute the ultimate conducting wires for nano-electronics, with their diameters as small as a few tens of atoms and their length of order one micrometer. Because of the particular band structure of graphite, nanotubes have at most two conducting channels, which makes them a one dimensional conductor with very exotic properties. Experimental investigations have indeed shown non conventional features, such as non-ohmic behavior, superconductivity and an ability to carry a huge current density.
We have carried out shot noise measurements on nanotubes which are suspended between metallic electrodes. One consequence of the suspended geometry is a very low 1/f noise, thereby enabling the extraction of shot noise. In bundles of nanotubes, we find a reduction of shot noise by more than a factor 100 compared to the full noise 2.e.I expected for uncorrelated electrons. A low noise is also found in an isolated single wall nanotube.
In a simple non-interacting-electron picture, such a low shot noise implies that the electrical conduction through a bundle of nanotubes is concentrated in a few ballistic tubes. Another interpretation however would be that a substantial fraction of the tubes conduct with a strong reduction of the effective charge (more than a factor 50) due to electron-electron interaction.