Computing the energy dissipation of suspended and supported acoustic cavities in nanoscale MoS2 films.

The confinement of elastic waves in nanoscale cavities is important for creating ultrahigh-frequency (terahertz) mechanical resonances. We present molecular dynamics simulations to compute the energy dissipation of these resonances in supported and suspended nanoscale films. We used MoS2 as a representative film, which is a layered crystal whose thickness can be scaled down to a few atoms. We found that coupling the film to a substrate significantly modifies its energy dissipation, and thus the fundamental limits on its quality factor (Q). Changing the energy dissipation pathway from in-plane to cross-plane increases the timescale for heat to leave the film by ∼10× due to slow thermal conduction across the film-substrate interface. As such, the energy lost from the elastic wave can be dominated by faster phononic equilibration rather than slower thermal equilibration. This suggests that manipulating the phononic properties of the material may be the more practical route to improving Q. The frequencies and ringdown times from the simulations are found to match well with Raman scattering and pump-probe experiments. This work is valuable for understanding the impact of substrates on the energy dissipation of films resonating at terahertz frequencies and to better inform their design.