Low energy surface phonon dispersion of MoS2

Abstract: The thermal and electronic performance of atomically thin semiconductors, a promising group of materials for electronics, is underpinned by their vibrational (phonon) dynamics. However, surface phonons in layered materials remain poorly understood due to limitations in conventional experimental techniques. We employ helium-3 spin-echo (HeSE) spectroscopy to measure the lowest energy (<10meV) dispersions of surface phonons on molybdenum disulfide (MoS2), a prototypical 2D semiconductor. We reveal that the widely studied E_2g and A_2u modes possess mixed acoustic-optical character, displaying unexpected quartic and quadratic dispersion relations, and finite-layer confinement to the surface of 4-5 and 6 layers, respectively. Crucially, we find no evidence of any pure acoustic mode at the surface of MoS2. This suppresses anharmonic phonon decay processes, thereby limiting thermal transport and providing a mechanistic explanation for poor thermal conductivity in few-layer transition metal dichalcogenides.