Collective Excitations in Monolayer Graphene on Metals: Phonons and Plasmons

Abstract

In this chapter, we report on experiments on collective excitations in graphene/metal interfaces by means of electron energy loss spectroscopy. Many of the peculiar graphene’s properties are related to its electronic collective excitations, even if their understanding is still missing. We have found a linear dispersion of the sheet plasmon in metals and such behaviour is attributed to the nonlocal screening of the plasmon mode of graphene caused by the underlying metal substrate. Due to its low energy and its linear dispersion, the sheet plasmon is expected to play an important role in graphene dynamics. This could be especially relevant for future graphene-based nano-optical devices, since the rippled, nanostructured surface of graphene on metal substrates provides an interesting scenario to couple acoustic surface plasmons and light. Measurements on plasmon modes also indicate that π electrons in graphene/metal interfaces behave as an interacting electron gas. We also report on collective modes due to the lattice dynamics of the graphene sheet. From experimental phonon dispersions, therefore it is possible to draw conclusions about the interaction strength in graphene/metal interfaces. Such investigation is of fundamental importance for graphene-based devices as both electronic and optical excitations can be scattered by phonon states or decay into vibrational excitations.