  In the present work, Cs aberration corrected and monochromated scanning
transmission electron microscopy electron energy loss spectroscopy STEM-EELS
has been used to explore experimental set-ups that allows bandgaps of high
refractive index materials to be determined. Semi-convergence and -collection
angles in the micro-radian range were combined with off-axis or dark field EELS
to avoid relativistic losses and guided light modes in the low loss range to
contribute to the acquired EEL spectra. Off-axis EELS further suppressed the
zero loss peak and the tail of the zero loss peak. The bandgap of several
GaAs-based materials were successfully determined by direct inspection and
without any background subtraction of the EEL spectra. The presented set-up
does not require that the acceleration voltage is set to below the Cerenkov
limit and can be applied over the entire acceleration voltage range of modern
TEMs and for a wide range of specimen thicknesses.
