Polytypism and unexpected strong interlayer coupling in two-dimensional layered ReS2.

Anisotropic two-dimensional (2D) van der Waals (vdW) layered materials, with both scientific interest and application potential, offer one more dimension than isotropic 2D materials to tune their physical properties. Various physical properties of 2D multi-layer materials are modulated by varying their stacking orders owing to significant interlayer vdW coupling. Multilayer rhenium disulfide (ReS2), a representative anisotropic 2D material, was expected to be randomly stacked and lack interlayer coupling. Here, we demonstrate two stable stacking orders, namely isotropic-like (IS) and anisotropic-like (AI) N layer (NL, N > 1) ReS2 are revealed by ultralow- and high-frequency Raman spectroscopy, photoluminescence and first-principles density functional theory calculation. Two interlayer shear modes are observed in AI-NL-ReS2 while only one shear mode appears in IS-NL-ReS2, suggesting anisotropic- and isotropic-like stacking orders in IS- and AI-NL-ReS2, respectively. This explicit difference in the observed frequencies identifies an unexpected strong interlayer coupling in IS- and AI-NL-ReS2. Quantitatively, the force constants of them are found to be around 55-90% of those of multilayer MoS2. The revealed strong interlayer coupling and polytypism in multi-layer ReS2 may stimulate future studies on engineering physical properties of other anisotropic 2D materials by stacking orders.

Determining layer number of two-dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrates.

Transition-metal dichalcogenide (TMD) semiconductors have been widely studied due to their distinctive electronic and optical properties. The property of TMD flakes is a function of their thickness, or layer number (N). How to determine the N of ultrathin TMD materials is of primary importance for fundamental study and practical applications. Raman mode intensity from substrates has been used to identify the N of intrinsic and defective multilayer graphenes up to N = 100. However, such analysis is not applicable to ultrathin TMD flakes due to the lack of a unified complex refractive index (ñ) from monolayer to bulk TMDs. Here, we discuss the N identification of TMD flakes on the SiO2/Si substrate by the intensity ratio between the Si peak from 100 nm (or 89 nm) SiO2/Si substrates underneath TMD flakes and that from bare SiO2/Si substrates. We assume the real part of ñ of TMD flakes as that of monolayer TMD and treat the imaginary part of ñ as a fitting parameter to fit the experimental intensity ratio. An empirical ñ, namely, ñ(eff), of ultrathin MoS2, WS2 and WSe2 flakes from monolayer to multilayer is obtained for typical laser excitations (2.54 eV, 2.34 eV or 2.09 eV). The fitted ñ(eff) of MoS2 has been used to identify the N of MoS2 flakes deposited on 302 nm SiO2/Si substrate, which agrees well with that determined from their shear and layer-breathing modes. This technique of measuring Raman intensity from the substrate can be extended to identify the N of ultrathin 2D flakes with N-dependent ñ. For application purposes, the intensity ratio excited by specific laser excitations has been provided for MoS2, WS2 and WSe2 flakes and multilayer graphene flakes deposited on Si substrates covered by a 80-110 nm or 280-310 nm SiO2 layer.

Interface Coupling in Twisted Multilayer Graphene by Resonant Raman Spectroscopy of Layer Breathing Modes.

Raman spectroscopy is the prime nondestructive characterization tool for graphene and related layered materials. The shear (C) and layer breathing modes (LBMs) are due to relative motions of the planes, either perpendicular or parallel to their normal. This allows one to directly probe the interlayer interactions in multilayer samples. Graphene and other two-dimensional (2d) crystals can be combined to form various hybrids and heterostructures, creating materials on demand with properties determined by the interlayer interaction. This is the case even for a single material, where multilayer stacks with different relative orientations have different optical and electronic properties. In twisted multilayer graphene there is a significant enhancement of the C modes due to resonance with new optically allowed electronic transitions, determined by the relative orientation of the layers. Here we show that this applies also to the LBMs, which can be now directly measured at room temperature. We find that twisting has a small effect on LBMs, quite different from the case of the C modes. This implies that the periodicity mismatch between two twisted layers mostly affects shear interactions. Our work shows that ultralow-frequency Raman spectroscopy is an ideal tool to uncover the interface coupling of 2d hybrids and heterostructures.

Layer number identification of intrinsic and defective multilayered graphenes up to 100 layers by the Raman mode intensity from substrates.

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.

Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material.

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.

Resonant Raman spectroscopy of twisted multilayer graphene.

Graphene and other two-dimensional crystals can be combined to form various hybrids and heterostructures, creating materials on demand with properties determined by the interlayer interaction. This is the case even for a single material, where multilayer stacks with different relative orientation have different optical and electronic properties. Probing and understanding the interface coupling is thus of primary importance for fundamental science and applications. Here we study twisted multilayer graphene flakes with multi-wavelength Raman spectroscopy. We find a significant intensity enhancement of the interlayer coupling modes (C peaks) due to resonance with new optically allowed electronic transitions, determined by the relative orientation of the layers. The interlayer coupling results in a Davydov splitting of the C peak in systems consisting of two equivalent graphene multilayers. This allows us to directly quantify the interlayer interaction, which is much smaller compared with Bernal-stacked interfaces. This paves the way to the use of Raman spectroscopy to uncover the interface coupling of two-dimensional hybrids and heterostructures.