Mechanisms of Acoustic Desorption of Atomic Clusters and Exfoliation of Graphene Multilayers

The phenomenon of acoustic desorption is studied in a series of atomistic simulations that account for the nonlinear evolution of the acoustic wave profiles during the wave propagation through the substrates. The simulations performed for atomic clusters demonstrate a nonthermal nature of the desorption from surfaces exposed to the acoustic waves. The acoustic desorption is characterized by a sharp threshold-like dependence of the desorption probability on the binding energy of the adsorbates and a nearly linear increase of the threshold binding energy for the desorption on the mass of the adsorbates. The equivalent temperature required for reproducing the acoustic desorption through the thermal activation is estimated to be more than 20 times higher than the actual substrate temperature. The extension of the investigation to the acoustic exfoliation of graphene multilayers from Cu(111) substrate demonstrates the ability of strong nonlinear acoustic pulses to cause the ejection of multilayers of various thicknesses while keeping a single graphene layer on the substrate. For both the atomic clusters and thin graphene multilayers, the nonlinear sharpening of the acoustic waves during their propagation through the substrates plays a key role in creating the conditions for acoustic desorption/exfoliation. A simple analytical model based on the consideration of resonant coupling of high-frequency harmonics of the nonlinear acoustic waves with the vibrational modes of the adsorbates is formulated and is shown to provide a reliable semiquantitative estimate of the conditions for the desorption of atomic clusters and thin graphene multilayers. For a thicker graphite overlayer, with a thickness exceeding that of the shock front of the wave, the acoustically driven ejection (or spallation) of the overlayer is described at the continuum level, by considering the reflection, transmission, and interaction of stress waves in the surface region of the system. The computational predictions have practical implications for interpretation of the results of laser-induced acoustic desorption mass spectrometry experiments and provide ideas for the development of new approaches utilizing the acoustic energy in thin film growth, transfer of 2D materials, and regeneration of the catalytic activity of metal surfaces.