Electron Energy Loss Spectroscopy Solutions for Nanoscale Materials Science Problems

Abstract

In the Transmission Electron Microscope (TEM), an incident electron suffers both elastic and inelastic scattering by the solid state thin sample that is being characterised. In the event of inelastic scattering, the incident electron gives a part of its energy to the electrons in the sample. The amount of lost energy can then be measured by a magnetic filter at the end of the column, and a plot displaying how many electrons have lost what amount of energy will give us an Electron Energy Loss (EEL) Spectrum. Thus, in an EEL Spectrum the ordinate axis corresponds to the number of electrons, or counts, and the abscise corresponds to the Energy Loss.

Notice that most electrons shall not suffer any inelastic scattering whatsoever. As a consequence, the greatest contribution to the spectrum is due to these electrons having lost zero energy, giving rise to the so-called zero loss peak (ZLP). As for those electrons having lost a certain amount of energy, they may lose it to ionization of specimen electrons, transitions from occupied core states to unoccupied core states or to conduction band states, to interband transitions or excitations of collective vibrations of conduction band electrons.

Incident electrons carry a given momentum, and it is worth keeping in mind that in an inelastic scattering event not only energy, but also momentum, may be transferred. In fact, this is the reason why it is not straightforward to compare EELS results with those obtained by means of optic spectroscopies.

EELS detectors can provide an energy resolution down to the order of the 0.1 eV. In addition, incident electrons can be tuned by TEM optics, making it possible to get spectroscopic information from an extremely constrained area, and to combine EEL Spectroscopy with TEM imaging.

En el microscopi electrònic de transmissió (TEM), un electró incident sofreix tant xocs elàstics com inelàstics en travessar la mostra prima d’estat sòlid que s’està caracteritzant. En cas de xoc inelàstic, l’electró incident cedeix part de la seva energia als electrons de la mostra. La quantitat d’energia perduda es pot mesurar amb un filtre magnètic situat al final de la columna, i un gràfic que indiqui quants electrons han perdut quina quantitat d’energia ens donarà un espectre de pèrdua d’energia dels electrons, o espectre EELS. Així, en un espectre EELS l’ordenada correspon al número d’electrons, o comptes, i l’abscissa, a la pèrdua d’energia.

Avui en dia l’EELS s’ha convertit en un instrument crucial a la ciència de materials, per causa de la progressiva reducció de l’escala característica implicada en el desenvolupament d’aquesta disciplina, i també gràcies a la millora instrumental que ha tingut lloc en els darrers anys tant en la microscòpia electrònica en general com en l’EELS en particular.

En aquesta tesi, s’han explorat les capacitats de l’EELS com a eina de caracterització de mostres d’estat sòlid a la nanoescala, i s’han aplicat a diversos problemes de ciència de materials.