Observation Of Interlayer Phonons in Transition Metal Dichalogenide Atomic Layers and Heterostructures – Rui He

Date: Mon. May 2nd, 2016, 12:30 pm-1:30 pm
Location: Rockefeller 221

Interlayer phonon modes in atomically thin transition metal dichalcogenide (TMD) heterostructures were observed for the first time. We measured the low-frequency Raman response of MoS2/WSe2 and MoSe2/MoS2 heterobilayers. We discovered a distinctive Raman mode (30 – 35 cm-1) that cannot be found in any individual monolayers (see Fig. 1). By comparing with Raman spectra of bilayer (2L) MoS2, 2L MoSe2 and 2L WSe2, we identified the new Raman mode as the layer breathing mode (LBM) arising from the perpendicular vibration between the two TMD layers. The LBM only emerges in bilayer regions with atomically close layer-layer proximity and clean interface. In addition, the LBM frequency exhibits noticeable dependence on the relative orientation between the two TMD layers, which implies a change of interlayer separation and interlayer coupling strength with the layer stacking. We also investigated the ultralow-frequency Raman response of atomically thin ReS2, a special type of TMD with unique distorted 1T structure. We found that the two shear modes in bilayer ReS2 are nondegenerate and clearly resolved in the Raman spectrum (see Fig. 2), in contrast to the doubly degenerate shear modes in other two-dimensional materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes and determine the stacking order in bilayer ReS2. Few-layer ReS2 exhibits rich Raman peaks at frequencies below 50 cm-1, where a panoply of interlayer shear and breathing modes are observed. Download the abstract