In a collaborative work between the Universidad Politécnica de Madrid (UPM), the Paul-Drude-Institut (PDI), and the State University of Campinas, we show that SAWs can tune the vibrational properties of graphene. The study was performed during several stays of R. Fandan at PDI in the framework of the European SAWtrain Marie Skłodowska-Curie Innovative Training Network (www.sawtrain.eu). The sample was prepared at the UPM and consists of a piezoelectric substrate holding the graphene sheet and an acoustic resonator for the excitation of high frequency SAWs (400 MHz). The localized deformations of the SAW modulate the hexagonal lattice of graphene, thus inducing a periodic softening/hardening of the graphene phonon modes under the tensile/compressive strain of the SAW, see Fig. 1a. The effect was observed experimentally using a technique specially developed at PDI for the detection of SAW-induced modulation of Raman spectra [Phys. Rev. B 93, 195212 (2016)]. As an example, we observe phonon frequency shifts of up to 10 cm-1 for the G Raman band of graphene for an effective hydrostatic strain of 0.24% at the surface of the substrate, see Fig. 1b.
The modulation mechanism reported here is not limited to single-layer graphene, but can be extended to any other single- or few-layer 2D material and van der Waals heterostructures, where the physics is especially rich and strain engineering opens a whole range of new possibilities.