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

Abstract

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.

Electronic and Optoelectronic Applications Based on 2D Novel Anisotropic Transition Metal Dichalcogenides

“…In this special 1T′ crystalline structure, the extra electron in the d orbital of the rhenium atom promotes the construction of a strong metallic Re—Re bond (Figure 1b) 32, 38, 66, 96. The distance between these dimerized Re atoms can be even shorter than that in rhenium single crystals97; thus, the total energy and symmetry of the system are reduced 40.…”

Advent of 2D Rhenium Disulfide (ReS2): Fundamentals to Applications

“…[20] However, the existence of lacked interlayer coupling was put into debate when several groups reported the existence of polytypism and strong interlayer coupling in ReS2 layers. [53][54][55] More extensive study is needed to resolve this.…”

“…There are three pronounced Raman peaks at 13, 16.5 and 28 cm -1 observed for two-layer ReS2 (see Figure 7c). [53,54] However, these peaks are absent in monolayer, implying that the peaks originated from the vibrations between the two layers. [55] The Raman peak at 28 cm -1 was assigned to breathing mode, while the peaks at 13 and 16.5 cm -1 were asigned to parallel shear modes (S||) and vertical shear modes (S┴),…”

“…In particular the observation of the shear modes indicates a well-defined layer stacking order in ReS2 crystal. [21,[53][54][55] This is because generation of shear mode vibrations require good atomic registration between neighbouring layers.…”

Interlayer bond polarizability model for stacking-dependent low-frequency Raman scattering in layered materials

“…7,9,33 They can be categorized into two types: in-plane shear and out-of-plane breathing vibration modes. Due to their greater sensitivity to interlayer coupling, LF Raman modes can directly probe the interfacial coupling, and they have been found as more effective indicators of the layer thickness [34][35][36][37][38][39][40][41][42][43][44][45][46] and stacking 24,[47][48][49][50][51][52][53][54][55] for diverse 2D materials. LF Raman spectroscopy is a rapidly developing field of research and has accelerated due to recent development of volume Bragg gratings for ultra-narrow optical filters with bandwidth about 1 cm -1 , which allows efficient cut-off of the excitation laser light without employing expensive triple monochromators for LF Raman measurements.…”