Layer-breathing Mode Research Articles

Phonon and Exciton Properties between WS2 and MoS2 Layers via Inversion Heterostructure Engineering.

Recently, two-dimensional (2D) van der Waals heterostructures (vdWHs) have exhibited emergent electronic and optical properties due to their peculiar phonons and excitons, which lay the foundation for the development of photoelectronic devices. The dielectric environment plays an important role in the interlayer coupling of vdWHs. Here, we studied the interlayer and extra-layer dielectric effects on phonon and exciton properties in WS2/MoS2 and MoS2/WS2 vdWHs by Raman and photoluminescence (PL) spectroscopy. The ultralow frequency (ULF) Raman modes are insensitive to atomic arrangement at the interface between 1LW and 1LM and dielectric environments of neighboring materials, and the layer breathing mode (LBM) frequency follows that of WS2. The shift of high-frequency (HF) Raman modes is attributable to interlayer dielectric screening and charge transfer effects. Furthermore, the energy of interlayer coupling exciton peak I is insensitive to atomic arrangement at the interface between 1LW and 1LM and its energy follows that of MoS2, but the slight intensity difference in inversion vdWHs means that the substrate's dielectric properties may induce doping on the bottom layer. This paper provides fundamental understanding of phonon and exciton properties of such artificially formed vdWHs structures, which is important for new insights into manipulating the performances of potential devices.

Different optical characteristics between monolayer and bilayer WS2 due to interlayer interaction

In this research Raman spectroscopy and photoluminescence (PL) spectroscopy measurements reveal different optical characteristics between monolayer (1 L) and bilayer (2 L) WS2 due to the interlayer interaction. First, ultralow frequency (ULF) Raman spectra indicate that 2 L WS2 shows additional shear mode (S) and layer-breathing (LB) mode in comparison with 1 L WS2. The 2 L WS2 has the in-plane vibration mode E2g1 red-shifted while the out-plane vibration mode A1g blue-shifted. Then, both direct transition (DT) and indirect transition (IT) emission are observed in the PL spectra for 2 L WS2 while only DT for 1 L WS2. The PL signal corresponding to localized excitons, that is measured as a low energy side tail with the DT emission, is more prominent for 1 L WS2 at low temperature of 7 K. The DT emission and IT emission show opposite variation trend as the temperature increases from 7 K to 300 K, while the average phonon energy in 2 L WS2 is estimated to be almost twice of that for 1 L WS2. These observations illustrate that the interlayer interaction not only change the phonon characteristics for WS2, but also impact the energy band structure and the exciton dynamics, including exciton radiative recombination and exciton-phonon interaction.

Vertical van der Waals Heterostructure of Single Layer InSe and SiGe

We present a first-principles investigation on the stability, electronic structure and mechanical response of ultra-thin heterostructure composed of single layers of InSe and SiGe. First, by per- forming total energy optimization and phonon calculations, we show that single layers of InSe and SiGe can form dynamically stable heterostructures in 12 different stacking types. Valence and conduction band edges of the heterobilayers form a type-I heterojunction having a tiny bandgap ranging between 0.09-0.48 eV. Calculations on elastic-stiffness tensor reveal that two mechanically soft single layers form a heterostructure which is stiffer than constituent layers due to relatively strong interlayer interaction. Moreover, phonon analysis show that the bilayer heterostructure has highly Raman active modes at 205.3 and 43.7 cm −1 , stemming from out-of-plane interlayer mode and layer breathing mode, respectively. Our results show that, as a stable type-I heterojunction, ultra-thin heterobilayer of InSe/SiGe holds ...

Direct observation of interlayer coherent acoustic phonon dynamics in bilayer and few-layer PtSe2

This work reports the real-time observation of the interlayer lattice vibrations in bilayer and few-layer PtSe2 by means of the coherent phonon method. The layer-breathing mode and standing wave mode of the interlayer vibrations are found to coexist in such a kind of group-10 transition metal dichalcogenides (TMDCs). The interlayer breathing force constant standing for perpendicular coupling (per effective atom) is derived as 7.5 N/m, 2.5 times larger than that of graphene. The interlayer shearing force constant is comparable to the interlayer breathing force constant, which indicates that PtSe2 has nearly isotropic interlayer coupling. The low-frequency Raman spectroscopy elucidates the polarization behavior of the layer-breathing mode that is assigned to have A1g symmetry. The standing wave mode shows redshift with the increasing number of layers, which successfully determines the out-of-plane sound velocity of PtSe2 experimentally. Our results manifest that the coherent phonon method is a good tool to uncover the interlayer lattice vibrations, beyond the conventional Raman spectroscopy limit. The strong interlayer interaction in group-10 TMDCs reveals their promising potential in high-frequency (∼terahertz) micro-mechanical resonators.

Physical Fingerprints of the 2O-tαP Phase in Phosphorene Stacking.

The 2O-tαP phase is a bilayer phosphorene stacking twisted by ∼70.5° standing out from all the potential candidates predicted by our previous work. Here, by linear response theory, we directly verified that the 2O-tαP phase preserves the intrinsic features of phonon spectrum of the existing AB phase, reflecting a stable thermodynamic behavior. Then we provided three distinct fingerprints to help finding this new phase: upon comparison to the existing shifting bilayer phosphorene, the in-plane elastic constants showed a much weaker anisotropic response, providing a characteristic mechanical criterion; the calculated Raman spectrum revealed forthe low frequency rang thelayer-breathing modeand theout-of-plane twisted mode, L-A1 and L-A2, both of which together stabilize the twisted structure; in particular, thesimulated scanning tunneling microscope image presented recognizable cross stripes, which should withstand an examination of exfoliated bilayer and few-layer black phosphorus.

Effect of humidity on the interlayer interaction of bilayer graphene

The lubricating ability of graphite largely depends on the environmental humidity, essentially the amount of water in between its layers. In general, intercalated molecules in layered materials modify their extraordinary properties by interacting with the layers. To understand the interaction of intercalated water molecules with graphene layers, we performed Raman measurements on bi-layer graphene at various humidity levels and observed an additional peak close to that of the low-frequency layer breathing mode between two graphene layers. The additional peak is attributed to the vibration between an intercalated water layer and the graphene layers. We further propose that the monolayer coverage of water increases between bilayer graphene with increasing environmental humidity while the interaction between the water layer and graphene layers remains approximately unchanged, until too much water is intercalated to keep the monolayer structure, at just over 50\% relative humidity. Notably, the results suggest that unexpectedly humidity could be an important factor affecting the properties of layered materials, as it significantly modifies the interlayer interaction.

Dependence of characteristic interlayer vibration modes on interlayer spin arrangement in stacked graphene nanofragments

The freedom of interlayer spin arrangement could emerge when assembling small-sized graphene-based nanostructures through stacking. Its possible influence on interlayer vibrations could be significant for developing Raman probes towards small complexes and device design involving intermolecular vibronic process. In this paper, using density-functional-theory calculations on bilayer and trilayer rhombic graphene fragments, we compare finite-size analogues to interlayer shear and breathing modes regarding to ferromagnetic and antiferromagnetic interlayer spin arrangements. For bilayer units, the antiferromagnetic configuration further separate two shear mode components by around 10 cm−1, inducing a further Raman peak splitting. This response is robust regarding to slightly enlarging the fragment and increasing layer number to three. Layer breathing mode is spin-arrangement robust in bilayers but show different arrangement-sensitivity in ABA-like and ABC-like trilayers. Visualization of interface electron density reveals the dependence of anisotropic interface environment to interlayer spin arrangement and unambiguously correlates the arrangement style with shearing responses. Further application of this analysis clarifies the arrangement-based phase-cooperation in trilayer shearing. Results and analysis in this work may contribute to the characterization and design of graphene-based nanostructures as well as understanding the interfacial nature beneath their intermolecular varieties in dynamic processes.

Anharmonicities in phonon combinations and overtones in bilayered graphene: A temperature-dependent approach

This paper studies phonon anharmonicities related to the phonon combination ${\mathrm{LOZO}}^{\ensuremath{'}}$ and phonon overtone 2ZO in a $AB$-stacked bilayer graphene (2LG). The results explain in detail the rule of the ${\mathrm{ZO}}^{\ensuremath{'}}$ layer breathing mode in the 2LG electron and phonon relaxations, especially at temperatures above 543 K, where anomalous behaviors are observed for the ${\mathrm{LOZO}}^{\ensuremath{'}}$ frequencies, linewidths (and therefore, lifetimes), and integrated areas. Surprisingly, the 2ZO frequencies and linewidths do not show any dependence with temperature (ZO is the out-of-phase vibration of the layers). This result is explained via nonsymmetric lattice distortions and via the almost null $\mathrm{Gr}\stackrel{\ifmmode \ddot{}\else \"{}\fi{}}{\text{u}}\mathrm{neisen}$ parameter associated to the ZO mode. Recently, the correct assignments for the phonon combination and overtone modes studied here have been put in debate once again in a theoretical work by Popov [Carbon 91, 436 (2015)]. This work shows how temperature-dependent Raman spectroscopy is used to propose a solution for these recent assignment problems. Finally, although 2LG is the system used here, the measurements and discussions to approach electron and phonon relaxations have the potential to be extended to any other multilayered structure that presents ${\mathrm{ZO}}^{\ensuremath{'}}$- and ZO-like phonon modes.

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus.

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

Raman scattering and anomalous Stokes-anti-Stokes ratio in MoTe2 atomic layers.

Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.

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.

Stacking-Dependent Interlayer Coupling in Trilayer MoS₂ with Broken Inversion Symmetry.

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin-orbit coupling (SOC) and interlayer coupling in different structural symmetries. Such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS2 blocks.

Electronic Raman scattering in graphite and single-layer and few-layer graphene

We investigated polarization-resolved electronic Raman scattering in different graphitic structures, including bulk graphite and single-layer and few-layer graphene. For all investigated samples, the broad continua of interband electronic transitions were detected at an energy $\ensuremath{\sim}0.35$ eV, while they were expected to be at $\ensuremath{\sim}6$ eV [Phys. Rev. B 88, 085416 (2013)]. The symmetry of the observed excitations corresponds to the ${A}_{2g}$ irreducible representation. A quasilinear behavior of the Raman response is observed at low energies in all cases at room temperature, in agreement with performed tight-binding calculations. High-energy features at $\ensuremath{\sim}0.8$ eV are detected in the spectra of graphite and few-layer graphene. They are attributed to interband transitions in the vicinity of the $K$ point, which involve electronic bands split by interlayer interaction. The effects of the substrate type, defect amount, and doping on the continuum line shape and symmetry are discovered. The silent layer-breathing mode is observed in a single-layer graphene. The results evidence that the electronic light scattering in graphitic structures without an external magnetic field is a powerful tool, which provides a variety of data on the structure and symmetry of low-energy electronic excitations.

Observation of interlayer phonon modes in van der Waals heterostructures

We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS2/WSe2 and MoSe2/MoS2 heterobilayers as well as twisted MoS2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30 ~ 40 cm-1) that arise from the layer-breathing mode (LBM) vibrations between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and atomically clean interface. Its frequency also evolves systematically with the relative orientation between of the two layers. Our research demonstrates that LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.

Temperature-activated layer-breathing vibrations in few-layer graphene.

We investigated the low-frequency Raman spectra of freestanding few-layer graphene (FLG) at varying temperatures (400-900 K) controlled by laser heating. At high temperature, we observed the fundamental Raman mode for the lowest-frequency branch of rigid-plane layer-breathing mode (LBM) vibration. The mode frequency redshifts dramatically from 81 cm(-1) for bilayer to 23 cm(-1) for 8-layer. The thickness dependence is well described by a simple model of coupled oscillators. Notably, the LBM Raman response is unobservable at room temperature, and it is turned on at higher temperature (>600 K) with a steep increase of Raman intensity. The observation suggests that the LBM vibration is strongly suppressed by molecules adsorbed on the graphene surface but is activated as desorption occurs at high temperature.

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.

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.