Electromagnetic quantum-size effects in directional near-field EELS of nanocrystals

Pronounced electromagnetic quantum-size effects are observed in directional near-field electron energy loss spectroscopy of rectangular nanocrystals. A theoretical analysis, exploiting a unique coincidence of all relevant length parameters in this spectroscopy and the momentum space filtering imposed by the exponentially decaying electron-plasmon interaction in the vacuum, reveals strong sensitivity of the EEL signal to the size and shape of the nanocrystal. The breakdown of momentum conservation along the beam direction, associated with the finite size of the nanocrystal, drastically alters the EEL signal pattern.