Institut des
NanoSciences de Paris
insp
insp
3.jpg

Low-dimensional oxides

High resolution electron energy loss spectroscopy (HREELS)

Contacts : Jacques Jupille, Rémi Lazzari

HREELS (High Resolution Electron Energy Loss Spectroscopy) consists in analysing as a function of energy and of direction, electrons scattered by a surface irradiated by a mono-energetic electron beam of a few eV with a resolution of a few meV. Diffusion is mainly due to the interaction of the Coulomb potential of the electron with the dipole field of the surface. Measurement of energy losses or gains (anti-Stokes modes) and their dispersion in the wave vector parallel to the surface can probe most low-energy excitations :

  • surface phonons ;
  • vibrational modes of single atoms or molecules ;
  • surface and interface plasmons ;
  • low-energy electronic excitations, in particular due to states in the bandgap.
HREELS experimental set-up

The LK-2000 spectrometer (http://www.lktech.com) consists of two analyzers at 127 ° and a mobile monochromator (Fig. 6). The ultimate resolution (width of the specular peak) is aproximately 5 meV. A manipulator xyz-θ-φ allows cooling the sample down to T 100K. HREELS spectra measurements on insulators are possible with an additional defocused electron gun which allows to compensate the surface charges.

The UHV chamber containing the spectrometer is connected via transfer arms to a preparation chamber (annealing, sputtering, Auger spectroscopy, surface reflectivity) and to a rapid introduction chamber.

JPEG
Schematics of the principles of HREELS

This device has been used to characterize the spectroscopic signatures of the hydroxyls on the surface of bulk crystals (MgO, Al2O3) and also the plasmons in silver nanoparticles.

JPEG JPEG
a) Plasmon resonances (modes parallel and perpendicular to the surface) in Ag nanoparticles deposited on MgO (001) probed by HREELS. b) Dispersion of these resonances as a function of wave vector parallel to the surface.