  Scanning transmission electron microscopy (STEM) combined with electron
energy loss spectroscopy (EELS) has become a standard technique to map
localized surface plasmon resonances with a nanometer spatial and a sufficient
energy resolution over the last 15 years. However, no experimental work
discussing the influence of experimental conditions during the measurement has
been published up to now. We present an experimental study of the influence of
the primary beam energy and the collection semi-angle on the plasmon resonances
measurement by STEM-EELS. To explore the influence of these two experimental
parameters we study a series of gold rods and gold bow-tie and diabolo
antennas. We discuss the impact on experimental characteristics which are
important for successful detection of the plasmon peak in EELS, namely: the
intensity of plasmonic signal, the signal to background ratio, and the signal
to zero-loss peak ratio. We show that the best results are obtained using a
medium primary beam energy, in our case 120 keV, and an arbitrary collection
semi-angle, as it is not a critical parameter at this primary beam energy. Our
instructive overview will help microscopists in the field of plasmonics to
arrange their experiments.
