In-plane Anisotropic Materials Research Articles

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons.

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Observation of optical anisotropy and a linear dichroism transition in layered silicon phosphide.

The investigation of in-plane two-dimensional (2D) anisotropic materials has garnered significant attention due to their exceptional electronic, optical, and mechanical characteristics. The anisotropic optical properties and angle-dependent photodetectors based on 2D anisotropic materials have been extensively studied. However, novel in-plane anisotropic materials still need to be explored to satisfy for distinct environments and devices. Here, we report the remarkable anisotropic behavior of excitons and demonstrate a unique linear-dichroism transition of absorption between ultraviolet and visible light in layered silicon phosphide (SiP) through the analysis of polarization photoluminescence (PL) and absorbance spectra. Its high absorption linear dichroism ratio of 1.16 at 388 nm, 1.15 at 532 nm, and 1.19 at 733 nm is revealed, suggesting the brilliant non-isotropic responses. The robust periodic variation of the A1 and A2 Raman modes in 2D SiP materials allows for the determination of their crystal orientation. Furthermore, the presence of indirect excitons with phonon sidebands in the temperature-dependent PL spectra exhibits non-monotonic energy shifts with increasing temperature, which is attributed to an enhanced electron-phonon interaction and thermal expansion. Our findings provide valuable insights into the fundamental physical properties of layered SiP and offer guidelines for designing polarization-sensitive photodetectors and angle-dependent devices based on 2D anisotropic materials.

A new spatial-domain thermoreflectance method to measure a broad range of anisotropic in-plane thermal conductivity

In-plane thermal conductivities of small-scale samples are hard to measure, especially for the lowly conductive ones and those lacking in-plane symmetry (i.e., transversely anisotropic materials). State-of-the-art pump-probe techniques including both the time-domain and the frequency-domain thermoreflectance (TDTR and FDTR) are advantageous in measuring the thermal conductivity of small-scale samples, and various advanced TDTR and FDTR techniques have been developed to measure transversely anisotropic materials. However, the measurable in-plane thermal conductivity (kin) is usually limited to be >10W/(m·K). In this work, a new spatial-domain thermoreflectance (SDTR) method has been developed to measure a broad range of kin of millimeter-scale small samples, including those lacking in-plane symmetry, extending the current limit of the measurable kin to as low as 1 W/(m·K). This SDTR method establishes a new scheme of measurements using the optimized laser spot size and modulation frequency and a new scheme of data processing, enabling measurements of in-plane thermal conductivity tensors of a broad range of kin values with both high accuracy and ease of operation. Some details such as the requirement on the sample geometry, the effect of the transducer layer, and the effect of heat loss are also discussed. As a verification, the kin of some transversely isotropic reference samples with a wide range of kin values including fused silica, sapphire, silicon, and highly ordered pyrolytic graphite (HOPG) have been measured using this new SDTR method. The measured kin agree perfectly well with the literature values with a typical uncertainty of <5%. As a demonstration of the unique capability of this method, the in-plane thermal conductivity tensor of x-cut quartz, an in-plane anisotropic material, has also been measured.

First-principles investigation of in-plane anisotropies in XYTe4 monolayers with X = Hf, Zr, Ti and Y = Si, Ge.

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe4 (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe4 reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe4 monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe4 differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe4 monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.

Stable anisotropic single-layer of ReTe2: a first principles prediction

In order to investigate the structural, vibrational, electronic, and mechanical features of single-layer ReTe2first-principles calculations are performed. Dynamical stability analyses reveal that single-layer ReTe2crystallize in adistorted phase while its 1H and 1T phases are dynamically unstable. Raman spectrum calculations show that single-layer distorted phase of ReTe2exhibits 18 Raman peaks similar to those of ReS2and ReSe2. Electronically, single-layerReTe2is shown to be an indirect gap semiconductor with a suitable band gap for optoelectronic applications. In addition,it is found that the formation of Re-units in the crystal induces anisotropic mechanical parameters. The in-plane stiffnessand Poisson ratio are shown to be significantly dependent on the lattice orientation. Our findings indicate that single-layer form of ReTe2can only crystallize in a dynamically stable distorted phase formed by the Re-units. Single-layer ofdistorted ReTe2can be a potential in-plane anisotropic material for various nanotechnology applications.

Transient and steady-state temperature rise in three-dimensional anisotropic layered structures in pump-probe thermoreflectance experiments

Recent developments of the pump-probe thermoreflectance methods (such as the beam-offset and elliptical-beam approaches of the time-domain and frequency-domain thermoreflectance techniques) enabled measurements of the thermal conductivities of in-plane anisotropic materials. Estimating the temperature rise of anisotropic layered structures under surface heating is critically important to make sure that the temperature rise is not too high to alias the signals in these experiments. However, a simple formula to estimate the temperature rise in three-dimensional (3D) anisotropic layered systems heated by a non-circular laser beam is not available yet, which is the main problem we aim to solve in this work. We first re-derived general formalisms of the temperature rise of a multilayered structure based on the previous literature work by solving the 3D anisotropic heat diffusion equation in the frequency domain. These general formalisms normally require laborious numerical evaluation; however, they could be reduced to explicit analytical expressions for the case of semi-infinite solids. We then extend the analytical expressions to multilayered systems, taking into account the effect of the top layers. This work not only enhances our understanding of the physics of temperature rise due to surface laser heating but also enables quick estimation of the peak temperature rise of 3D anisotropic layered systems in pump-probe thermoreflectance experiments and thus greatly benefits the thermoreflectance experiments in choosing the appropriate heating power intensity for the experiments.

Strain-engineering the in-plane electrical anisotropy of GeSe monolayers.

As a representative in-plane anisotropic two-dimensional (2D) material, germanium monoselenide (GeSe) has attracted considerable attention recently due to its various in-plane anisotropic material properties originating from the low symmetry of a puckered honeycomb structure. Although there have been plenty of reports on the in-plane anisotropic vibrational, electrical and optical properties of GeSe, the strain effect on those appealing anisotropies is still under exploration. Here we report a systematic first-principles computational investigation of strain-engineering of the anisotropic electronic properties of GeSe monolayers. We found that the anisotropic ratio of the effective mass and mobility of charge carriers (electrons and holes) of GeSe along two principle axes can be controlled by using simple strain conditions. Notably, the preferred conducting direction of GeSe can be even rotated by 90° under an appropriate uniaxial strain (>5%). Such effective strain modulation of the electronic anisotropy of GeSe monolayers provides them abundant opportunities for future mechanical-electronic devices.

The In-Plane Anisotropy of WTe2 Investigated by Angle-Dependent and Polarized Raman Spectroscopy.

Tungsten ditelluride (WTe2) is a semi-metallic layered transition metal dichalcogenide with a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various materials properties. We have systemically studied the in-plane anisotropy of Raman modes in few-layer and bulk WTe2 by angle-dependent and polarized Raman spectroscopy (ADPRS). Ten Raman modes are clearly resolved. Their intensities show periodic variation with sample rotating. We identify the symmetries of the detected modes by quantitatively analyzing the ADPRS results based on the symmetry selection rules. Material absorption effect on the phonon modes with high vibration frequencies is investigated by considering complex Raman tensor elements. We also provide a rapid and nondestructive method to identify the crystallographic orientation of WTe2. The crystallographic orientation is further confirmed by the quantitative atomic-resolution force image. Finally, we find that the atomic vibrational tendency and complexity of detected modes are also reflected in the shrinkage degree defined based on ADPRS, which is confirmed by corresponding density functional calculation. Our work provides a deep understanding of the interaction between WTe2 and light, which will benefit in future studies about the anisotropic physical properties of WTe2 and other in-plane anisotropic materials.

Modeling of Material Orientation Effects on AHSS Crush and Fracture Behavior in Axial Crush Tests

Objective: Due to the rolling manufacturing process, most advanced high-strength steels (AHSS) demonstrate in-plane anisotropic material behavior. This study investigates the effects of material orientation on the axial crush behavior and fracture of AHSS with axial crush tests and computer simulations. Methods: Crush simulation models considering material anisotropy and damage evolution were developed in LS-DYNA based on the drop-tower crush test results and coupon characterization test data for DP780 steel. The modified Mohr-Coulomb (MMC) isotropic fracture model was employed in the crush simulation models for fracture prediction. Results: The 12-sided components fabricated in the transverse (T) direction of the sheet exhibited slightly higher crush loads and reduced crush distances compared to those in the longitudinal (L) direction. The crush behavior in each direction was generally proportional to ultimate tensile strength. All of the materials investigated in this study showed some cracking in the crush tests for both component orientations, but only DP780 showed significant anisotropy in fracture behavior with more cracking for the T direction compared to the L direction. Overall, the amount of cracking observed in the tests had little or no significant effect on the axial crush performance. The MMC fracture loci in both the L and T directions were determined using a reverse engineering approach, and the stress–strain curves beyond the uniform elongation point were extended using an optimization method. Both material models MAT103 and MAT224 predicted the crush and fracture behavior with reasonably good accuracy. Conclusions: The predicted fracture mode and force–displacement curves agreed well with the test data for both the L and T directions in axial crush tests of the 12-sided components. The simple isotropic material model MAT224 is adequate for crush simulations to predict material orientation effects on AHSS component crush performance and fracture behavior.

Two-dimensional Green's function tensor and projected local density of states for TM and TE modes in dispersive and anisotropic photonic crystals

A multiple-scattering method for two-dimensional TM modes is extended to calculate the electric Green's function tensor for TE modes of a finite-size photonic crystal consisting of infinite-length cylinders of dispersive and in-plane anisotropic materials. We investigate the frequency and spatial dependence of the projected local density of states for TM and TE modes of the system. The large enhancement factor of the projected local density of states has been observed for dispersive finite photonic crystals.

Static Indentation of Anisotropic Biomaterials Using Axially Asymmetric Indenters—a Computational Study

Indentation has historically been used by biomechanicians to extract the small strain elastic or viscoelastic properties of biological tissues. Because of the axisymmetry of indenters used in these studies however, analysis of the results requires the assumption of material isotropy and often yields an "effective" elastic modulus. Since most biological tissues such as bone and myocardium are known to be anisotropic, the use of conventional indentation techniques for estimating material properties is therefore limited. The feasibility of using an axially asymmetric indenter to determine material directions and in-plane material properties for anisotropic tissue is explored here using finite element analysis. The load versus displacement curves as would be measured by an indenter depend on the orientation of the indenter cross section relative to the in-plane material axes, thus suggesting a method for determining the underlying material directions. Additionally, the stiffness of the tissue response to indentation is sensitive to the values of the in-plane anisotropic material properties and prestretches, and thus test results can be used to back out relevant constitutive parameters.

Focus on Analytical Instruments

Focus on Analytical Instruments