  Gallium is a plasmonic material offering ultraviolet to near-infrared
tunability, facile and scalable preparation, and good stability of
nanoparticles. In our contribution, we experimentally demonstrate the link
between the shape and size of individual gallium nanoparticles and their
optical properties. To this end, we utilize scanning transmission electron
microscopy combined with electron energy loss spectroscopy. Lens-shaped gallium
nanoparticles with a diameter between 10 nm and 200 nm were grown directly on a
silicon nitride membrane using an in-house developed effusion cell operated at
ultra-high vacuum conditions. We have experimentally proved that they support
localized surface plasmon resonances and their dipole mode can be tuned through
their size from ultraviolet to near-infrared spectral region. The measurements
are supported by numerical simulations using realistic particle shapes and
sizes. Our results open the way for future applications of gallium
nanoparticles such as hyperspectral absorption of sunlight in energy harvesting
or plasmon-enhanced luminescence of ultraviolet emitters.
