Acoustic Phonons from Brillouin Spectroscopy of Chemically Tunable 2D Materials*

Brillouin scattering is a non-invasive, laser-based scattering technique capable of determining the acoustic phonon properties of 2D materials at GHz frequencies. Here, we use Brillouin spectroscopy to map the entire angular dispersion curves of multiple acoustic phonon branches of 2D layered MoO3 and V2O5 - directly probing the effects of phonon quantum confinement within a 2D layered material. Measurements provide longitudinal and transverse sound velocities, refractive indices, and acoustic attenuations. Since acoustic phonons dictate elasticity, we can take advantage of Lamb theory to optically derive the complete elastic stiffness tensor of each 2D layered material as well as the thickness to within less than a monolayer. We demonstrate how the intercalation of metallic Sn, Co, and Cu can chemically tune quantized acoustic phonons and elasticity of 2D layered metal materials by substantially stiffening the van der Waals gap, with longitudinal moduli increasing by 20% or more with a complex effect on the stiffness of the 2D material.  This work establishes methodology to extract precise elastic constants from complex Brillouin scattering of 2D materials. It takes advantage of phonon confinement to capture the complete phonon response with minimal scattering geometries.