Raman Spectra of Monolayer, Few-Layer, and Bulk ReSe 2 : An Anisotropic Layered Semiconductor (original) (raw)
Rhenium Dichalcogenides: Layered Semiconductors with Two Vertical Orientations
Nano Letters, 2016
The rhenium and technetium diselenides and disulfides are van der Waals layered semiconductors in some respects similar to more well-known transition metal dichalcogenides (TMD) such as molybdenum sulfide. However, their symmetry is lower, consisting only of an inversion center, so that turning a layer upside-down (that is, applying a C 2 rotation about an in-plane axis) is not a symmetry operation, but reverses the sign of the angle between the two nonequivalent in-plane crystallographic axes. A given layer thus can be placed on a substrate in two symmetrically nonequivalent (but energetically similar) ways. This has consequences for the exploitation of the anisotropic properties of these materials in TMD heterostructures and is expected to lead to a new source of domain structure in large-area layer growth. We produced fewlayer ReS 2 and ReSe 2 samples with controlled "up" or "down" orientations by micromechanical cleavage and we show how polarized Raman microscopy can be used to distinguish these two orientations, thus establishing Raman as an essential tool for the characterization of large-area layers.
Nano letters, 2015
Rhenium disulfide (ReS2) is a semiconducting layered transition metal dichalcogenide that exhibits a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various material properties. Here, we demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS2 through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers.
Layer parity dependent Raman-active modes and crystal symmetry in ReS2
Physical Review B
ReS 2 , a group VII transition metal dichalcogenide (TMD), bears immense prospect in optoelectronic, thermoelectric, catalytic, and energy storage applications. Its distorted structure and significant in-plane anisotropy introduce extra degree of freedom but, on the other hand, make it challenging to determine the absolute characteristics of the material. Here, through experimental and theoretical analysis, we present additional phonon modes in the Raman spectrum of layered ReS 2 which, in literature, are regarded as ambiguous modes arising due to either double resonance or defects. However, we demonstrate that these additional phonon modes arise from the crystal symmetry. This report proves the most followed notion of a layer-independent monolayer (ML)-based unit cell of ReS 2 to be misleading. We further demonstrate that the observed additional phonon modes are driven by unique layer-dependent variations in the crystal symmetry. Moreover, layer-parity-dependent splitting of E 1 g mode and inversion symmetry breaking are discussed in detail. Such layer-dependent features have remained largely ignored in the literature. The results presented here may open new avenues for the application of this material and resolve several contradictions in its properties including the significance of stacking order, layer-dependent crystal symmetry, and shifting of Raman modes.
Monolayer behaviour in bulk ReS2 due to electronic and vibrational decoupling
Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces where the layers are electronically and vibrationally coupled. Isolated monolayers show changes in electronic structure and lattice vibration energies, including a transition from indirect to direct bandgap. Here we present a new member of the family, rhenium disulphide (ReS 2 ), where such variation is absent and bulk behaves as electronically and vibrationally decoupled monolayers stacked together. From bulk to monolayers, ReS 2 remains direct bandgap and its Raman spectrum shows no dependence on the number of layers. Interlayer decoupling is further demonstrated by the insensitivity of the optical absorption and Raman spectrum to interlayer distance modulated by hydrostatic pressure. Theoretical calculations attribute the decoupling to Peierls distortion of the 1T structure of ReS 2 , which prevents ordered stacking and minimizes the interlayer overlap of wavefunctions. Such vanishing interlayer coupling enables probing of two-dimensional-like systems without the need for monolayers.
New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides
Scientific Reports, 2014
Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe 2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe 2 , MoSe 2 , WS 2 and MoS 2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E9, E0 and E 2g modes shift to lower frequencies, and the A9 1 and A 1g modes shift to higher frequencies. Second, new high frequency first order A9 1 and A 1g modes appear, explaining recently reported experimental data for WSe 2 , MoSe 2 and MoS 2 . Third, splitting of modes around A9 1 and A 1g is found which explains those observed in MoSe 2 . Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.
Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy
ACS nano, 2015
Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenide (TMD), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a non-stoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nano-electronic circuits in 2D materials.
Formation and stability of point defects in monolayer rhenium disulfide
Physical Review B, 2014
Recently, rhenium disulfide (ReS 2 ) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ReS 2 crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ReS 2 crystallizes in a stable distorted-1T structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ReS 2 . Experimentally, irradiation with 3-MeV He +2 ions was used to break the strong covalent bonds in ReS 2 flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic α particle irradiation. In order to understand the energetics of possible vacancies in ReS 2 we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1-3μ B . Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.
Synthesis and Characterization of ReS 2 and ReSe 2 Layered Chalcogenide Single Crystals
We report the synthesis of high-quality single crystals of ReS 2 and ReSe 2 transition metal dichalcogenides using a modified Bridgman method that avoids the use of a halogen transport agent. Comprehensive structural characterization using X-ray diffraction and electron microscopy confirm a distorted triclinic 1T′ structure for both crystals and reveal a lack of Bernal stacking in ReS 2. Photo-luminescence (PL) measurements on ReS 2 show a layer-independent bandgap of 1.51 eV, with increased PL intensity from thicker flakes, confirming interlayer coupling to be negligible in this material. For ReSe 2 , the bandgap is weakly layer-dependent and decreases from 1.31 eV for thin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich Raman spectra whose excitation energy dependence was studied. The lower background doping inherent to our crystal growth process results in high field-effect mobility values of 79 and 0.8 cm 2 /(V s) for ReS 2 and ReSe 2 , respectively, as extracted from FET structures fabricated from exfoliated flakes. Our work shows ReX 2 chalcogenides to be promising 2D materials candidates, especially for optoelectronic devices, without the requirement of having monolayer thin flakes to achieve a direct bandgap.
Electronic Band Structure of Rhenium Dichalcogenides
Journal of Electronic Materials, 2018
The band structures of bulk transition metal dichalcogenides ReS 2 and ReSe 2 are presented, showing the complicated nature of interband transitions in these materials, with several close-lying band gaps. Three-dimensional plots of constant energy surfaces in the Brillouin zone at energies near the band extrema are used to show that the valence band maximum and conduction band minimum may not be located at special high symmetry points. We find that both materials are indirect gap materials and that one must be careful to consider the whole Brillouin zone volume in addressing this question.