Nanobeam propagation and imaging in a FEGTEM/STEM.

Atomic resolution information by EELS can only be obtained by careful control of the propagation and spreading of the beam within the sample. A multislice calculation is used to estimate 3D-intensity distributions in sapphire illuminated with beams of 0.1-0.3 nm diameter, with the focus, Cs-value, and specimen thickness as the variables. The 3D-intensity pattern is then used to predict spatially resolved ELNES signals, interpreted as a convolution of the atomically projected density of states (DOS) with an elastic excitation envelope. The site-and-momentum projected DOS functions are calculated using local density function theory, applied to a rhombohedral grain boundary in sapphire. Finally, experimental difficulties in directly imaging the beam exit wave of a nanobeam-illuminated specimen are demonstrated. Calculation and experiments are for a typical modern high-resolution 300 kV FEGTEM.