Anisotropic Layered Materials Research Articles

A Unified Prediction Strategy for Angle‐Resolved Polarized Raman Response of Black Phosphorus

AbstractPredicting the angle‐resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) is the ultimate goal in the field of ARPRS research. So far, multiple physical mechanisms are studied on the representative black phosphorus (BP) to understand the intricate ARPRS response. However, the lack of a complete physical picture of the response modulation has hindered progress in response prediction. Herein, using BP as an example, a unified strategy for predicting the thickness‐dependent ARPRS response of ALMs is proposed. Crucially, with only one ARPRS measurement of a bulk ALM of interest, the response of nanoflakes of any thickness can be predicted. This is achieved based on the core concept of intrinsic Raman anisotropy (RA). Integrated experiments, analysis, and calculations reveal that the response of bulk BP is reshaped by the rarely noticed anisotropic absorption effect, which leads to an unique thickness‐independent constant modulation. Therefore, the interference‐free bulk counterpart of ALMs can serve as an ideal platform for accurately accessing intrinsic RA. This work provides a new paradigm for ARPRS research and paves the way for profound study of intrinsic RA.

Unveiling the Mechanism of Phonon-Polariton Damping in α-MoO3.

Phonon polaritons (PhPs), light coupled to lattice vibrations, in the highly anisotropic polar layered material molybdenum trioxide (α-MoO3) are currently the focus of intense research efforts due to their extreme subwavelength field confinement, directional propagation, and unprecedented low losses. Nevertheless, prior research has primarily concentrated on exploiting the squeezing and steering capabilities of α-MoO3 PhPs, without inquiring much into the dominant microscopic mechanism that determines their long lifetimes, which is key for their implementation in nanophotonic applications. This study delves into the fundamental processes that govern PhP damping in α-MoO3 by combining ab initio calculations with scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy measurements across a broad temperature range (8-300 K). The remarkable agreement between our theoretical predictions and experimental observations allows us to identify third-order anharmonic phonon-phonon scattering as the main damping mechanism of α-MoO3 PhPs. These findings shed light on the fundamental limits of low-loss PhPs, which is a crucial factor for assessing their implementation into nanophotonic devices.

Modelling structural colour from helicoidal multi-layer thin films with natural disorder.

A coupled mode theory based on Takagi-Taupin equations describing electromagnetic scattering from distorted periodic arrays is applied to the problem of light scattering from beetles. We extend the method to include perturbations in the permittivity tensor to helicoidal arrays seen in many species of scarab beetle and optically anisotropic layered materials more generally. This extension permits analysis of typical dislocations arising from the biological assembly process and the presence of other structures in the elytra. We show that by extracting structural information from transmission electron microscopy data, including characteristic disorder parameters, good agreement with spectral specular and non-specular reflectance measurements is obtained.

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Strong-anisotropy-induced instability and discussion on kink band under complex loads

Compressive instability, such as the occurrence of kink bands, is common in composite materials and other highly anisotropic layered materials, limiting their use. Most previous research has focused on the microstructure and heterogeneity in studying the compressive instability of composites. In this paper, to highlight and quantify the influence of anisotropy, we investigate the deformation instability of effective homogeneous solids with strong anisotropy. Firstly, we re-write the local instability criterion from an energy perspective and verify its applicability. Even when the heterogeneity of the composites is eliminated, the derived criterion can reasonably predict the kink band instability and can be reduced to the well-known Rosen’s estimate. The strong stiffness anisotropy plays a dominant role in kink band instability. For unidirectional fiber-reinforced composites, increasing the modulus of the hard phase contributes little to an increase in the compressive strength because, as the anisotropy increases, the critical strain decreases and becomes approximately inversely proportional to the anisotropy degree. The kink band instability is also investigated under complex loads. We find that lateral tension increases the critical kink band load, while lateral compression or shear decreases the critical kink band load. To estimate the instability, we develop a series of instability phase diagrams that can be used as a guideline for predicting the compressive instability of composite materials. We have found that reducing the anisotropy of the material is the main way to increase the critical load of kink band instability, which is important for material design and engineering applications.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3.

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO3 as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO3 that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO3 is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO3 as an example, although it is generally applicable to all transparent birefringent ALMs.

An anisotropic localizing gradient damage approach for failure analysis of fiber reinforced composites

This article presents an anisotropic gradient-enhanced continuum damage model developed within the finite element method framework to address complex fracture phenomena in anisotropic layered materials with unidirectional fiber-reinforced composites as the primary material examples. The main objective of the work is to model damage anisotropy due to progressive intra-laminar fracture at mesoscale in transversally isotropic composite laminae using distinct damage variables associated with different in-plane failure modes. Departing from the conventional gradient enhancements, the model adopts an improved spatial nonlocal description to ensure correct localized damage bandwidths using a single internal length scale. The coupled system of equations is decoupled using an operator-split (staggered) methodology to ensure a robust and straightforward computational implementation without compromising accuracy using lower order finite elements. The proposed damage model is tested on experimental results of fracture response in a single-edge notched tension, center notched tension, and open-hole tension fiber-reinforced composite laminae, where the numerical results were consistent with experimental observations.

Fabrication of Textured Porous Ti<sub>3</sub>SiC<sub>2</sub> by Slip Casting under High Magnetic Field and Microstructural Evolution through High Temperature Deformation

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti3SiC2 polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200°C. Dense textured Ti3SiC2 sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400°C for 1 h. The porosity of the textured Ti3SiC2 was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti3SiC2 as a spacer media. The highly textured Ti3SiC2 polycrystals with porosity of 8.4 vol% and 16.7 vol%, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti3SiC2 showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

Domain-Decomposition Localized Method of Fundamental Solutions for Large-Scale Heat Conduction in Anisotropic Layered Materials

Domain-Decomposition Localized Method of Fundamental Solutions for Large-Scale Heat Conduction in Anisotropic Layered Materials

Twisted van der Waals Josephson Junction Based on a High-Tc Superconductor.

Stacking two-dimensional van der Waals (vdW) materials rotated with respect to each other show versatility for studying exotic quantum phenomena. In particular, anisotropic layered materials have great potential for such twistronics applications, providing high tunability. Here, we report anisotropic superconducting order parameters in twisted Bi2Sr2CaCu2O8+x (Bi-2212) vdW junctions with an atomically clean vdW interface, achieved using the microcleave-and-stack technique. The vdW junctions with twist angles of 0° and 90° showed the maximum Josephson coupling, comparable to that of intrinsic Josephson junctions. As the twist angle approaches 45°, Josephson coupling is suppressed, and eventually disappears at 45°. The observed twist angle dependence of the Josephson coupling can be explained quantitatively by theoretical calculation with the d-wave superconducting order parameter of Bi-2212 and finite tunneling incoherence of the junction. Our results revealed the anisotropic nature of Bi-2212 and provided a novel fabrication technique for vdW-based twistronics platforms compatible with air-sensitive vdW materials.

Fabrication of Textured Porous Ti<sub>3</sub>SiC<sub>2</sub> by Slip Casting under High Magnetic Field and Microstructural Evolution through High Temperature Deformation

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti3SiC2 polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200℃. Dense textured Ti3SiC2 sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400℃ for 1 h. The porosity of the textured Ti3SiC2 was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti3SiC2 as a spacer media. The highly textured Ti3SiC2 polycrystals with porosity of 8.4 vol％ and 16.7 vol％, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti3SiC2 showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

Programmable hyperbolic polaritons in van der Waals semiconductors.

Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range associated with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of subdiffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe2). We used photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the observed polaritonic response.

Lighting a route for hyperbolic dispersion

Quantum Materials The propagation of light within a material is usually well defined, with the propagation described by scattering and dispersion. In artificially designed metamaterials and in anisotropic layered materials, the dispersion can be hyperbolic, giving rise to subwavelength confinement of the light. Sternbach et al. show that the hyperbolic dispersion can be optically switched on and off on demand in the layered transition metal dichalcogenide tungsten diselenide (see the Perspective by Deng and Chen). Illuminating the material with ultrafast pulses of sub-bandgap light creates a transient waveguide, resulting in hyperbolic dispersion in the material. The ability to tune the dispersion characteristics on demand using optical pumping is an effective approach for developing ultrafast switching photonic devices and controlling the propagation of light on the nanoscale. Science , this issue p. [617][1]; see also p. [572][2] [1]: /lookup/doi/10.1126/science.abe9163 [2]: /lookup/doi/10.1126/science.abf7933

A novel flexible, layered, recoverable SiO2 fiber skeleton and aerogel composites material prepared by papermaking process

The anisotropic ceramic fiber skeleton and its aerogel composite materials have important applications in the fields of high temperature insulation and aerospace. However, the traditional ceramic fiber skeleton has been limited in its application due to the complexity of its preparation process. Here, referencing the traditional papermaking technology, a flexible and recoverable SiO2 fiber skeleton with anisotropic layered structure was prepared through papermaking process and low-temperature sintering (170 °C) technology. Complete heteromorphic skeleton could be prepared by this simple process. On this basis, we impregnated the fiber skeleton with silica aerogel to further improve the composite properties of the pure fiber block. Compared with SiO2 fiber skeleton, the SiO2 aerogel/fiber skeleton composite exhibited lower thermal conductivity (0.023 W/(m·k)), higher compressive strength (2.19 MPa) and excellent high temperature thermal insulation. The recoverable and excellent thermal insulation properties of anisotropic layered composite materials provide an effective way for the manufacture of low-cost aerogel composite materials. At the same time, it has potential application value in high-temperature thermal insulation materials.

Surface Passivation by Excess Sulfur for Controlled Synthesis of Large, Thin SnS Flakes

Tin sulfide (SnS) is a part of the group IV chalcogenides (SnX, GeX; X:S, Se), a family of anisotropic layered materials considered for thin-film photovoltaics, optoelectronics, and valleytronics a...

Understanding angle-resolved polarized Raman scattering from black phosphorus at normal and oblique laser incidences

The selection rule for angle-resolved polarized Raman (ARPR) intensity of phonons from standard group-theoretical method in isotropic materials would break down in anisotropic layered materials (ALMs) due to birefringence and linear dichroism effects. The two effects result in depth-dependent polarization and intensity of incident laser and scattered signal inside ALMs and thus make a challenge to predict ARPR intensity at any laser incidence direction. Herein, taking in-plane anisotropic black phosphorus as a prototype, we developed a so-called birefringence-linear-dichroism (BLD) model to quantitatively understand its ARPR intensity at both normal and oblique laser incidences by the same set of real Raman tensors for certain laser excitation. No fitting parameter is needed, once the birefringence and linear dichroism effects are considered with the complex refractive indexes. An approach was proposed to experimentally determine real Raman tensor and complex refractive indexes, respectively, from the relative Raman intensity along its principle axes and incident-angle resolved reflectivity by Fresnel’s law. The results suggest that the previously reported ARPR intensity of ultrathin ALM flakes deposited on a multilayered substrate at normal laser incidence can be also understood based on the BLD model by considering the depth-dependent polarization and intensity of incident laser and scattered Raman signal induced by both birefringence and linear dichroism effects within ALM flakes and the interference effects in the multilayered structures, which are dependent on the excitation wavelength, thickness of ALM flakes and dielectric layers of the substrate. This work can be generally applicable to any opaque anisotropic crystals, offering a promising route to predict and manipulate the polarized behaviors of related phonons.

Experimental measurement of thermal conductivity along different crystallographic planes in graphite

In this work, the time-domain thermoreflectance (TDTR) method is used to measure the thermal conductivity of graphite along different crystallographic planes at room temperature for the first time and the thermal conductivities along the non-principal axes of graphite are obtained. A focused ion beam is used to cut graphite samples along different crystallographic planes for the TDTR measurement. Then, a thermal model is developed to extract the thermal conductivity of graphite along different crystallographic planes from the measured signals of the TDTR method. The measured thermal conductivities along different crystallographic planes in graphite agree well with the anisotropy model, revealing that the traditional TDTR method can be used to measure the non-principal axis thermal conductivity of anisotropic layered materials. Moreover, the experimental results demonstrate that once the crystallographic plane deviates from the cross-plane direction, the in-plane phonon modes will dominate the heat transfer in graphite.

Identifying the Crystalline Orientation of Mechanically Exfoliated Anisotropic Layered Materials through Their Morphologies

AbstractThe unique orientation‐dependent properties of in‐plane anisotropic 2D materials allow for designing novel electronic and optoelectronic devices that are hardly realized in isotropic materials. All such anisotropic studies and relevant applications require identification of the crystal orientation in advance. Although many methods have been developed to characterize the crystal orientation such as polarization‐resolved optical techniques, they usually suffer from the complex and time‐consuming process. Here, a convenient yet effective method is reported that the crystal orientation of mechanically exfoliated anisotropic 2D materials can be determined just through their morphologies. Using GeSe as an example, it is shown that the exfoliated flakes from bulk crystals naturally form long‐straight edges perpendicular to the direction with the lowest Young's modulus stemming from its in‐plane mechanical anisotropy, thus enabling the crystal orientation identification through the cleaved edges. This proposed method based on the facilely obtained anisotropic mechanical properties provides a fast, convenient, and non‐destructive approach for crystal orientation identification.

Nonlinear optical properties of anisotropic two-dimensional layered materials for ultrafast photonics

Abstract The discovery of graphene has intrigued the significant interest in exploring and developing the two-dimensional layered materials (2DLMs) for the photonics application in recent years. Unlike the isotropic graphene, a number of 2DLMs possess the in-plane anisotropic crystal structure with low symmetry, enabling a new degree of freedom for achieving the novel polarization-dependent and versatile ultrafast photonic devices. In this review article, we focus on the typical anisotropic 2DLMs including BP, ReS2, ReSe2, SnS, and SnSe and summarize the recent development of these anisotropic 2DLMs in the pulsed laser and the optical switch applications. First, we introduce the fabrication methods as well as the material characterization of the anisotropic 2DLMs by analyzing the polarized Raman configuration. Second, we discuss the anisotropic nonlinear optical properties of the anisotropic 2DLMs and concentrate on the anisotropic nonlinear absorption response. Next, we sum up state of the art of the anisotropic 2DLMs in the application of pulse lasers and optical switches. This review ends with perspectives on the challenge and outlook of the anisotropic 2DLMs for ultrafast photonics applications.

GaSe saturable absorber for mode-locked Yb-doped fiber laser at 1.04 µm

Transition-metal dichalcogenides, such as tungsten disulfide (WS2) and molybdenium disulfide (MoS2), are highly anisotropic layered materials and have attracted growing interest from basic research to practical applications due to their exotic physical property that may complement graphene and other semiconductor materials. GaSe nanosheets are found to exhibit broadband nonlinear saturable absorption property, and saturable absorbers are fabricated by depositing GaSe nanosheets on side-polished fibers. Attributing to the weak evanescent field and long interaction length, the GaSe nanosheets are not exposed to large optical intensity, which allows the saturable absorber to work at the high-power regime. The saturable absorbers are used to mode locked dispersion-managed soliton fiber lasers.