Enhancement of EELS signal by Electron Beam Precession

Electron energy-loss spectroscopy (EELS) performed in the transmission electron microscope (TEM) provides chemical and electronic information about the considered solid state sample, through the determination of the energy lost by the incident electrons, with an extraordinary spatial resolution.

It is well established that, when a crystal is viewed parallel to the atom columns, the obtained high resolution images show a one-to-one correspondence with the actual atom positions if the balance between the resolution of the instrument and the atomic distance is favourable enough; in this case, the exiting wave function mainly depends on the projected structure, and the physical explanation for this to occur has been proposed to be the channelling of the electrons along the atom columns parallel to the beam direction: through the positive electrostatic potential of the atoms, a column acts as a channel for the electron, within which it can scatter dynamically without leaving the column. This elastic and highly directed incident electron–atom interaction is bound to strongly reduce the EELS signal.

Precession can be considered to overcome the problemsassociated with channelling: if one combines EELS with precession, it is possible to recover an effective two-beam condition while remaining in the zone axis [11–13]. This compensation has been experimentally observed in the case of the Si L2,3 edge in a Si crystal in the [110] zone axis and in the case of the O K and Ti L2,3 edges in a SrTiO3 (STO) crystal in the [001] zone axis

0.481 precessed and precession-less EEL spectra showing the Si L2,3 edge in a Si crystal in zone axis conditions.




Universitat de Barcelona