Coupling and Stacking Order of ReS₂ Atomic Layers Revealed by Ultralow-Frequency Raman Spectroscopy

Abstract

We investigate the ultralow-frequency Raman response of atomically thin ReS 2 , a special type of twodimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS 2 exhibit rich Raman spectra at frequencies below 50 cm -1 , where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS 2 layers are coupled and stacked orderly, in contrast to the general belief that the ReS 2 layers are decoupled from one another. While the interlayer breathing modes can be described by a linear chain model as in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS 2 are non-degenerate and well separated in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes, and determine the stacking order in bilayer ReS 2 .Interlayer coupling and stacking order are crucial factors to define the physics in few-layer two-dimensional (2D) materials, such as the electronic structure [1], band gap tunability [2] and quantum Hall phases [3]. These interlayer properties are closely related to the intralayer lattice structure of the crystals. Knowledge of such correlation would allow us to tailor the material properties and possibly realize new quantum phases. Experimental approach to this topic is, however, challenging due to the lack of suitable materials with contrasting structure. For instance, few-layer graphene and the common transition metal dichalcogenides (TMDs), such as MoS 2 and WSe 2 , possess similar stacking order due to their similar hexagonal in-plane lattice structure. To reveal the subtle influence of intralayer lattice on the interlayer coupling, it is imperative to explore new 2D materials with differing lattice structure.Rhenium disulfide (ReS 2 ) is a new type of 2D semiconductors with intriguing properties [4][5][6][7][8][9][10][11][12]. In contrast to the widely studied group-VI TMDs, such as MoS 2 and WSe 2 that possess 1H or 1T structure, ReS 2 monolayers exhibit unique distorted 1T structure as the stable phase [ Fig. 1(a)]. This is because the rhenium atom possesses one extra valence electron, leading to the formation of additional Re-Re bonds in ReS 2 . A superlattice structure of rhenium chains is thus formed to distort the monolayer crystal from the more symmetric 1T phase [ Fig. 1(a)]. As a consequence, the ReS 2 crystal exhibits strong in-plane anisotropy in the electronic, vibrational and mechanical properties [4][5][6][7][8][9].The in-plane distortion of ReS 2 lattice is expected to affect profoundly the interlayer coupling in few-layer ReS 2 crystals. A recent study [4], for instance, reports that the band gap of ReS 2 remains direct from single layer (1L) to the bulk, in contrast to the direct-to-indirect band gap transition i...

Booming Development of Group IV–VI Semiconductors: Fresh Blood of 2D Family

“…Furthermore, polarized Raman characterization was implemented to explore the polarization dependence due to the reduced symmetry of the orthorhombic phase. The authors inspected that the orthorhombic phase SnS nanosheets exhibit strong anisotropic Raman response similar with black phosphorous150 and ReS 2 151, 152, 153, 154. The parallel‐polarization configuration strongly suggests that the A 1g mode (190.7 cm −1 ) can be employed to detect crystallographic orientation of the SnS flakes because of the A 1g mode reaches the maximum as illumination light polarization is parallel to armchair direction of the SnS flakes, which may also exist in other group IV orthorhombic phases such as SnSe, GeS, and GeSe.…”

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

“…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┴),…”

“…[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.…”

“…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.…”