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.

wileyonlinelibrary.comconventional heterostructures is strongly dictated by lattice mismatch which determines the interface quality and thus, the heterostructure performance. Beyond the traditional group IV, III-V, or II-VI semiconductors, 2D layered crystals (e.g., graphene, [ 3 ] transition metal dichalcogenides, [ 4 ] hexagonal boron nitride ( h -BN), [ 5 ] phosphorene, [ 6 ] etc.) have emerged as promising candidates for next generation electronics and optoelectronics due to their unique properties. These 2D layered materials can be artifi cially combined to fabricate various van der Waals (vdW) heterostructures without the lattice match limitation. Novel physical properties of these vdW heterostructures have been investigated theoretically and experimentally, and devices based on those new heterostructures such as tunnel transistors and sensors have already been demonstrated. [7][8][9][10][11] This far, these vdW heterostructures have mainly been fabricated by a top-down process of manual transfer or a bottomup method of chemical vapor deposition (CVD) growth. The fi rst demonstration of vdW heterostructures were realized by vertically stacking different 2D materials (graphene/ h -BN, [ 8,12 ] MoS 2 /graphene, [ 10 ] graphene/WS 2 , [ 9 ] etc.) using conventional polymethyl-methacrylate-mediated transfer method. [ 7 ] The physical properties of these heterostructures are signifi cantly infl uenced by relative orientation of the layers and interfacial quality between them. However, the stacking style and crystal orientation cannot be easily controlled by mechanical transfer method. In addition, such strategies cannot ensure good interfacial quality. Compared to manual transfer, CVD epitaxial growth is a powerful approach for fabricating 2D vdW heterostructures with controlled stacking style, crystal orientation, and clean interface. Indeed, using this strategy, some vertical heterostructures have already been successfully grown, including graphene/ h -BN, [ 13 ] MoS 2 / h -BN, [ 14 ] MoSe 2 /graphene, [ 15 ] MoS 2 / graphene, [ 16 ] WS 2 /MoS 2 , [ 17 ] and MoS 2 /SnS 2 . [ 18 ] This far, the components of the reported 2D-vertical vdW heterostructures have been restricted to layered materials with planar crystal structures. However, many non-layered materials such as cadmium sulfi de (CdS) also exhibit attractive optoelectronic properties. [ 19 ] Combination of such non-layered functional semiconductors with layered materials (e.g., MoS 2 ) could create a new type of vdW heterostructure to provide novel

Angle-resolved polarized Raman (ARPR) spectroscopy can be utilized to assign the Raman modes based on crystal symmetry and Raman selection rules and also to characterize the crystallographic orientation of anisotropic materials. However, polarized Raman measurements can be implemented by several different configurations and thus lead to different results. In this work, we systematically analyze three typical polarization configurations: 1) to change the polarization of the incident laser, 2) to rotate the sample, and 3) to set a half-wave plate in the common optical path of incident laser and scattered Raman signal to simultaneously vary their polarization directions. We provide a general approach of polarization analysis on the Raman intensity under the three polarization configurations and demonstrate that the latter two cases are equivalent to each other. Because the basal plane of highly ordered pyrolytic graphite (HOPG) exhibits isotropic feature and its edge plane is highly anisotropic, HOPG can be treated as a modelling system to study ARPR spectroscopy of twodimensional materials on their basal and edge planes. Therefore, we verify the ARPR behaviors of HOPG on its basal and edge planes at three different polarization configurations. The orientation direction of HOPG edge plane can be accurately determined by the angle-resolved polarization-dependent G mode intensity without rotating sample, which shows potential application for orientation determination of other anisotropic and vertically standing two-dimensional materials and other materials.