Layer Breathing Research Articles

Stability, electronic and phononic properties of β and 1T structures of SiTex (x = 1, 2) and their vertical heterostructures

By performing first-principles calculations, we predict a novel, stable single layer phase of silicon ditelluride, 1T-, and its possible vertical heterostructures with single layer β-SiTe. Structural optimization and phonon calculations reveal that 1T- structure has a dynamically stable ground state. Further analysis of the vibrational spectrum at the point shows that single layer 1T- has characteristic phonon modes at 80, 149, 191 and 294 . Electronic-band structure demonstrates that 1T- phase exhibits a nonmagnetic metallic ground state with a negligible intrinsic spin–orbit splitting. Moreover, it is shown that similar structural parameters of 1T- and existing β-SiTe phases allows construction of 1T-β heterostructures with a negligible lattice mismatch. In this regard, it is found that two energetically favorable stacking orders, namely AA and B, have distinctive shear and layer breathing phonon modes. It is important to note that the combination of semiconducting β-SiTe and metallic 1T- building blocks forms ultra-thin Schottky barriers that can be used in nanoscale optoelectronic device technologies.

Raman spectroscopic characterization of stacking configuration and interlayer coupling of twisted multilayer graphene grown by chemical vapor deposition

Multilayer graphene (MLG) grown by chemical vapor deposition (CVD) is a promising material for electronic and optoelectronic devices. Understanding the stacking configuration and interlayer coupling of MLGs is technologically relevant and of importance for the device applications. Here, we reported a kind of twisted MLGs (tMLGs), in which only one twist angle was revealed from the twist-related modes, R and R'. With increasing the total layer number of N (N > 2), the observed interlayer shear modes in the tMLG flake always follow those of AB-stacked (N-1)LG, while the observed interlayer breathing modes always follow those of AB-stacked NLG, independent of its twist angle. The tMLGs are identified as t(1+n)LGs, which are formed by stacking one graphene monolayer on the top of AB-stacked n layer graphenes by rotating a certain angle between them. The layer breathing coupling of the t(1+n)LGs is almost identical to that of mechanically-exfoliated tMLGs, which demonstrates the high quality of MLGs grown by CVD. This study provides an applicable approach to probe the stacking configuration and interlayer coupling of MLGs grown by CVD or related methods. This work also demonstrates the possibility to grow MLG flakes with a fixed stacking configuration, e.g., t(1+n)LG, by the CVD method.

Physical origin of Davydov splitting and resonant Raman spectroscopy of Davydov components in multilayerMoTe2

We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) MoTe2. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML MoTe2 based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe2. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe2 (NL MoTe2; N is an integer). Under the resonant excitation condition, we observe N-dependent Davydov components in ML MoTe2 , originating from the Raman-active A'1(A21g) modes at ~172 cm-1. More than two Davydov components are observed in NL MoTe2 for N larger than 4 by Raman spectroscopy. The N-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency A'1(A21g) modes. The different resonant profiles for the two Davydov components in 3L MoTe2 indicate that proper excitation energy of ~1.8-2.2 eV must be chosen to observe the Davydov splitting in ML MoTe2 . Our work presents a simple way to identify layer number of ultrathin MoTe2 flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe2 and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

Effect of Uniaxial Strain on Low Frequency Raman Modes in Few Layers Molybdenum Disulfide

The effect of uniaxial tensile strain on the shear mode (C mode) and layer breathing mode (LB mode) of few layers MoS2 were studied. The C mode is doubly degenerated into two sub-peaks as applied strain increases. No measurable Raman shift for the LB mode is observed, while its Raman intensity decreases gradually as the strain increases. In addition, the C and LB modes exhibit distinguished thickness dependence. The C mode shows a clear blueshift as the strain increases, while the LB mode is softened in the process. Moreover, the evolution of C mode with thickness follows an enhanced linear-chain model plus a linear component very well. It implies that the C mode is a better thickness navigator as compared to the L mode.

Observation of anisotropic interlayer Raman modes in few‐layer ReS2

ReS2 has recently emerged as a new member in the rapidly growing family of two‐dimensional materials. Unlike MoS2 or WSe2, the optical and electrical properties of ReS2 are not isotropic due to the reduced symmetry of the crystal. Here, we present layer‐dependent Raman measurements of ReS2 samples ranging from monolayers to ten layers in the ultralow frequency regime. We observe layer breathing and shear modes which allow for easy assignment of the number of layers. Polarization‐dependent measurements give further insight into the crystal structure and reveal an energetic shift of the shear mode which stems from the in‐plane anisotropy of the shear modulus in this material. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Low wavenumber Raman spectroscopy of highly crystalline MoSe2grown by chemical vapor deposition

Transition metal dichalcogenides (TMDs) have recently attracted attention due to their interesting electronic and optical properties. Fabrication of these materials in a reliable and facile method is important for future applications, as are methods to characterize material quality. Here we present the chemical vapor deposition of MoSe2 monolayer and few layer crystals. These results show the practicality of using chemical vapor deposition to reliably fabricate these materials. Low frequency Raman spectra and mapping of shear and layer breathing modes of MoSe2 are presented for the first time. We correlate the behavior of these modes with layer number in the materials. The usefulness of low frequency Raman mapping to probe the symmetry, quality, and monolayer presence in CVD grown 2D materials is emphasized. Raman map of maximum intensity position for low wavenumber shear and layer breathing modes of MoSe2.

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.

ChemInform Abstract: Phonon and Raman Scattering of Two‐Dimensional Transition Metal Dichalcogenides from Monolayer, Multilayer to Bulk Material

AbstractReview: 195 refs.

Substrate-free layer-number identification of two-dimensional materials: A case of Mo0.5W0.5S2 alloy

Any of two or more two-dimensional (2D) materials with similar properties can be alloyed into a new layered material, namely, 2D alloy. Individual monolayer in 2D alloys is kept together by van der Waals interactions. The property of multilayer alloys is a function of their layer number. Here, we studied the shear (C) and layer-breathing (LB) modes of Mo0.5W0.5S2 alloy flakes and their link to the layer number. The study reveals that the disorder effect is absent in the C and LB modes of 2D alloys, and the monatomic chain model can be used to estimate the frequencies of the C and LB modes. We demonstrated how to use the frequencies of C and LB modes to identify the layer number of alloy flakes deposited on different substrates. This technique is independent of the substrate, stoichiometry, monolayer thickness, and complex refractive index of 2D materials, offering a robust and substrate-free approach for layer-number identification of ultrathin flakes of 2D materials, such as 2D crystals and 2D alloys.

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.

Quantification of the Relative z-Polarized Electromagnetic Field Contribution and Associated Investigation of Asymmetric Shape of Layer Breathing Mode from Au Nanoparticle–Graphene–Au Thin Film Junctions

Simultaneous utilization of both in-plane and out-of-plane phonon vibration of graphene sandwiched between a Au nanoparticle (NP) and Au TF junction can have a significant role in quantifying the z-polarized electromagnetic (EM) field contribution. The layer breathing mode (LBM), which involves the relative displacement of the individual graphene layers in the graphene sheet in the normal directions, can be exclusively observed at Au NP–Au TF junctions. By employing the ILBM/I2D value at each junction, the author can show that a nearly 50–70-times stronger z-polarized EM contribution is present at Au NP–Au TF junctions. Additionally, the author can also determine that a Au NP with a truncated shape shows a relatively ∼1.3-times higher z-polarized EM contribution than a Au NP with a spherical shape because of the higher density of the pointed part from the truncated Au NP. Furthermore, the author reveals the evolution of the asymmetric shape of the LBM peak with increasing z-polarized EM strength. The increase of the full width at half-maximum (fwhm) from the major LBM and the correlated blue shift of the minor LBM with increasing z-polarized EM contribution imply that the degree of coupling between highly localized electrons and low-energy phonons from graphene along the z axis might be reflected and that of the additional n-doped status of graphene stemming from the relatively higher electron density at truncated sites within Au NP might also be reflected. The author is currently developing this surface-enhanced-Raman-scattering- (SERS-) based nanometrology as a more facile optical characterization tool, especially for exploring out-of-plane phonon vibration of various 2D materials.

Raman spectroscopy of twisted bilayer graphene

Twisted bilayer graphene (tBLG) is a two-graphene layers system with a mismatch angle θ between the two hexagonal structures. The interference between the two rotated layers generates a superlattice with a θ-dependent wavevector that gives rise to van Hove singularities in the electronic density of states and activates phonons in the interior of the graphene Brillouin zone. Here we review the use of Raman spectroscopy to study tBLG, exploring the θ-dependent effects, corroborated by independent microscopy analysis. The phonon frequencies give a Raman signature of the specific tBLG, while their linewidths provide a straightforward test for tBLG structural homogeneity. Rich resonance effects, including single and multiple-resonances, intra- and inter-valley scattering events make it possible to accurately measure the energy of superlattice-induced van Hove singularities in the electronic joint density of states, as well as the phonon dispersion relation in tBLG, including the layer breathing vibrational modes.

Observation of Low Energy Raman Modes in Twisted Bilayer Graphene

Two new Raman modes below 100 cm(-1) are observed in twisted bilayer graphene grown by chemical vapor deposition. The two modes are observed in a small range of twisting angle at which the intensity of the G Raman peak is strongly enhanced, indicating that these low energy modes and the G Raman mode share the same resonance enhancement mechanism, as a function of twisting angle. The ~94 cm(-1) mode (measured with a 532 nm laser excitation) is assigned to the fundamental layer breathing vibration (ZO' mode) mediated by the twisted bilayer graphene lattice, which lacks long-range translational symmetry. The dependence of this mode's frequency and line width on the rotational angle can be explained by the double resonance Raman process that is different from the previously identified Raman processes activated by twisted bilayer graphene superlattice. The dependence also reveals the strong impact of electronic-band overlaps of the two graphene layers. Another new mode at ~52 cm(-1), not observed previously in the bilayer graphene system, is tentatively attributed to a torsion mode in which the bottom and top graphene layers rotate out-of-phase in the plane.

Raman scattering study of the phonon dispersion in twisted bilayer graphene

Bi-layer graphene with a twist angle \theta\ between the layers generates a superlattice structure known as Moir\'{e} pattern. This superlattice provides a \theta-dependent q wavevector that activates phonons in the interior of the Brillouin zone. Here we show that this superlattice-induced Raman scattering can be used to probe the phonon dispersion in twisted bi-layer graphene (tBLG). The effect reported here is different from the broadly studied double-resonance in graphene-related materials in many aspects, and despite the absence of stacking order in tBLG, layer breathing vibrations (namely ZO' phonons) are observed.

Raman spectroscopy of shear and layer breathing modes in multilayer MoS2

We study by Raman scattering the shear and layer breathing modes in multilayer MoS2. These are identified by polarization measurements and symmetry analysis. Their positions change with the number of layers, with different scaling for odd and even layers. A chain model explains the results, with general applicability to any layered material, and allows one to monitor their thickness.

Measurement of layer breathing mode vibrations in few-layer graphene

Layer-layer coupling plays a critical role in defining the physical properties of few-layer graphene (FLG). With respect to vibrations, the interlayer coupling is predicted to create a set of N-1 finite frequency, out-of-plane interlayer modes in N-layer graphene. Unlike the widely studied in-plane vibrations, the properties of these layer-breathing modes (LBMs) are defined by the layer-layer interactions. Here we report direct observation of the distinct LBMs for graphene of thicknesses from 2 to 20 layers through electronically resonant overtone Raman bands observed in the range of 80 - 300 cm-1. The Raman bands exhibit multi-peak features that are unique for graphene samples of each layer thickness up to 20 layers. The frequencies of the set of layer breathing modes for all samples can be described within a simple model of nearest plane coupling.

Observation of Layer-Breathing Mode Vibrations in Few-Layer Graphene through Combination Raman Scattering

We report the observation of layer-breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from two to six layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm(-1). From double resonance theory, we assign the feature as the LO+ZO' combination mode of the out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The LOZO' Raman band is found to exhibit multiple peaks with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO'-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG.

Observation of new Raman lines in GeSe and SnSe at low temperatures

Polarized Raman spectra are investigated in GeSe and SnSe at low temperatures. New Raman lines which can not be expected by a group theoretical analysis for the known crystal structure of the orthorhombic D 16 2 h have been observed typically in the ( a, a) and ( b, b) polarization configurations. With decreasing temperature, three lines in GeSe grow weakly at 89,201 and strongly at 226 cm -1 below 150 K, accompanied by enhancement of layer breathing mode (175 cm -1 at 273 K) intensity. One line in SnSe grows weakly at 193 cm -1 below 50 K. The appearance of the new Raman lines as well as a resistivity anomaly suggests a novel structural phase transition in this system.