An SiO2/Si substrate has been widely used to support two-dimensional (2d) flakes grown by chemical vapor deposition or prepared by micromechanical cleavage. The Raman intensity of the vibration modes of 2d flakes is used to identify the layer number of 2d flakes on the SiO2/Si substrate, however, such an intensity is usually dependent on the flake quality, crystal orientation and laser polarization. Here, we used graphene flakes, a prototype system, to demonstrate how to use the intensity ratio between the Si peak from SiO2/Si substrates underneath graphene flakes and that from bare SiO2/Si substrates for the layer-number identification of graphene flakes up to 100 layers. This technique is robust, fast and nondestructive against sample orientation, laser excitation and the presence of defects in the graphene layers. The effect of relevant experimental parameters on the layer-number identification was discussed in detail, such as the thickness of the SiO2 layer, laser excitation wavelength and numerical aperture of the used objective. This paves the way to use Raman signals from dielectric substrates for layer-number identification of ultrathin flakes of various 2d materials.

The electron-phonon coupling (EPC) in a material is at the frontier of the fundamental research, underlying many quantum behaviors. van der Waals heterostructures (vdWHs) provide an ideal platform to reveal the intrinsic interaction between their electrons and phonons. In particular, the flexible van der Waals stacking of different atomic crystals leads to multiple opportunities to engineer the interlayer phonon modes for EPC. Here, in hBN/WS 2 vdWH, we report the strong cross-dimensional coupling between the layer-breathing phonons well extended over tens to hundreds of layer thick vdWH and the electrons localized within the few-layer WS 2 constituent. The strength of such cross-dimensional EPC can be well reproduced by a microscopic picture through the mediation by the interfacial coupling and also the interlayer bond polarizability model in vdWHs. The study on cross-dimensional EPC paves the way to manipulate the interaction between electrons and phonons in various vdWHs by interfacial engineering for possible interesting physical phenomena.

Interfacial coupling between neighboring layers of van der Waals heterostructures (vdWHs), formed by vertically stacking more than two types of two-dimensional materials (2DMs), greatly affects their physical properties and device performance. Although high-resolution cross-sectional scanning tunneling electron microscopy can directly image the atomically sharp interfaces in the vdWHs, the interfacial coupling and lattice dynamics of vdWHs formed by two different types of 2DMs, such as semimetal and semiconductor, are not clear so far. Here, we report the ultralow-frequency Raman spectroscopy investigation on interfacial couplings in the vdWHs formed by graphene and MoS flakes. Because of the significant interfacial layer-breathing couplings between MoS and graphene flakes, a series of layer-breathing modes with frequencies dependent on their layer numbers are observed in the vdWHs, which can be described by the linear chain model. It is found that the interfacial layer-breathing force constant between MoS and graphene, α(I) = 60 × 10 N/m, is comparable with the layer-breathing force constant of multilayer MoS and graphene. The results suggest that the interfacial layer-breathing couplings in the vdWHs formed by MoS and graphene flakes are not sensitive to their stacking order and twist angle between the two constituents. Our results demonstrate that the interfacial interlayer coupling in vdWHs formed by two-dimensional semimetals and semiconductors can lead to new lattice vibration modes, which not only can be used to measure the interfacial interactions in vdWHs but also is beneficial to fundamentally understand the properties of vdWHs for further engineering the vdWHs-based electronic and photonic devices.