Observation of Layer-Breathing Mode Vibrations in Few-Layer Graphene through Combination Raman Scattering

We report the observation of layer-breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from two to six layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm^–1. From double resonance theory, we assign the feature as the LO+ZO′ combination mode of the out-of-plane LBM (ZO′) and the in-plane longitudinal optical mode (LO). The LOZO′ Raman band is found to exhibit multiple peaks with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO′-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG

Toward Ferroelectric Control of Monolayer MoS₂

The chemical vapor deposition (CVD) of molybdenum disulfide (MoS₂) single-layer films onto periodically poled lithium niobate is possible while maintaining the substrate polarization pattern. The MoS₂ growth exhibits a preference for the ferroelectric domains polarized “up” with respect to the surface so that the MoS₂ film may be templated by the substrate ferroelectric polarization pattern without the need for further lithography. MoS₂ monolayers preserve the surface polarization of the “up” domains, while slightly quenching the surface polarization on the “down” domains as revealed by piezoresponse force microscopy. Electrical transport measurements suggest changes in the dominant carrier for CVD MoS₂ under application of an external voltage, depending on the domain orientation of the ferroelectric substrate. Such sensitivity to ferroelectric substrate polarization opens the possibility for ferroelectric nonvolatile gating of transition metal dichalcogenides in scalable devices fabricated free of exfoliation and transfer