Anisotropic Physical Properties Research Articles

Tailoring a Two-Dimensional Halide Perovskite Composed of the Secondary Amine Cation for Anisotropic X-ray Responses.

Two-dimensional (2D) metal-halide perovskites have shown broad application prospects in the field of optoelectronic detection. The presence of the natural quantum-well structure results in strong anisotropy of physical properties, while studies on anisotropic X-ray responses remain insufficient. Here, we present an intriguing anisotropy of X-ray-responsive behaviors in a 2D halide perovskite, (t-ACH)2(DMA)Pb2Br7 (1, where t-ACH is trans-4-(aminomethyl)cyclohexanecarboxylate and DMA is dimethylamine), in which the secondary amine DMA+ cation with a large ionic radius locates inside the perovskite cage to form inorganic frameworks. The alternative alignment of inorganic slabs and organic bilayers creates a typical quantum-well architecture, which accounts for the generation of photoelectronic anisotropy. High-quality crystals of 1 exhibit notable semiconducting properties with a large μτ product (1.9 × 10-4 cm2 V-1). Intriguingly, 1 has better X-ray detection sensitivity (∼569.9 μC Gyair-1 cm-2) along the in-plane direction, which is attributed to its excellent charge carrier transport performance in this direction. Conversely, the higher resistance stemming from the organic barrier results in a lower detection limit along the out-of-plane direction (∼78.1 nGyair s-1), much lower than the medical diagnostic criteria (∼5.5 μGyair s-1). This work might open up new possibilities for the creative use of hybrid perovskites in direct X-ray detection.

Near-Field Radiative Heat Transfer between Layered β-GeSe Slabs: First-Principles Approach.

The group-IV monochalcogenide monolayers, GeSe, are interesting and novel two-dimensional (2D) semiconductor materials due to their highly anisotropic physical properties. Monolayers of the different GeSe polymorphs have already had their physical properties and potential applications extensively investigated. However, few-layer homostructures, which can also be approximated as 2D systems in many cases, have not received the same attention. For this reason, in this work, we investigate the optical properties of a free-standing few-layer β-GeSe system and use this information to investigate their performance in the near-field radiative heat transfer (NFRHT). The required optical conductivity of the few-layer 2D material is calculated by using density functional theory (DFT), including spin-orbit coupling. The band structure is investigated for up to five layers, and the effective electron masses are calculated correspondingly. Using this information, both the intraband transitions due to the presence of free electrons introduced by doping and the interband transitions are considered. The contribution of the ionic vibrations is also included in calculating the optical properties because of its relevance to NFRHT through the resulting active optical phonons. With all these contributions included, more realistic predictions of the NFRHT between the layered 2D β-GeSe materials can be obtained. It is found that the heat transfer attainable with the layered system is similar to that of a single layer of β-GeSe we have obtained previously.

Investigating the causes of permeability anisotropy in heterogeneous conglomeratic sandstone using multiscale digital rock

Heterogeneous conglomeratic sandstone exhibits anisotropic physical properties, rendering a comprehensive analysis of its physical processes challenging with experimental measurements. Digital rock technology provides a visual and intuitive analysis of the microphysical processes in rocks, thereby aiding in scientific inquiry. Nevertheless, the multiscale characteristics of conglomeratic sandstone cannot be fully captured by a single-scale digital rock, thus limiting its ability to characterize the pore structure. Our work introduces a proposed workflow that uses multiscale digital rock fusion to investigate permeability anisotropy in heterogeneous rock. We use a cycle-consistent generative adversarial network to fuse computed tomography scan data of different resolutions, creating a large-scale and high-precision digital rock that comprehensively represents the conglomeratic sandstone pore structure. Subsequently, the digital rock is partitioned into multiple blocks, and the permeability of each block is simulated using a pore network. Finally, the total permeability of the sample is calculated by conducting an upscaling numerical simulation using the Darcy-Stokes equation. This process facilitates the analysis of the pore structure in conglomeratic sandstone and provides a step-by-step solution for permeability. From a multiscale perspective, this approach reveals that the anisotropy of permeability in conglomeratic sandstone stems from the layered distribution of grain sizes and differences in grain arrangement across different directions.

Electrical anisotropy and its mitigation in conductive polymers printed by vat photopolymerization

In most additive manufacturing (AM) technologies, objects are realized layer by layer. This layer-by-layer construction leads to inherent anisotropic physical properties. Controlling, understanding and sometimes mitigating such anisotropy is a critical issue in the development of AM. Electrical anisotropy in conductive nanocomposites processed by Vat photopolymerization is demonstrated and quantified in the present work. In the used method, which enjoys high resolution and high speed, layers of an acrylate based resin, are successively cross-linked by UV irradiation of 2D patterns. Carbon nanotubes are used as conductive fillers for their low percolation threshold that allows realizing conductive and still sufficiently transparent materials for UV polymerization. Conductivity parallel to the layers of 3D objects is found to be much greater than conductivity perpendicular to the layers. This electrical anisotropy is explained by the high contact resistance between printed layers. High contact resistance results from the slow diffusion of carbon nanotubes from the uncured material towards the interface of the cured object. It is found that implementing a delay time before curing successive layers, or decreasing the matrix viscosity with temperature, to promote diffusion of the conductive particles allow substantial reduction of the contact resistance between layers. As a result, conductivity anisotropy can be reduced by almost two orders of magnitude. This control and mitigation of conductivity anisotropy allows reconciliation of the high resolution of the Vat photopolymerization technology with the possibility to realize uniform 3D materials.

Anisotropic Mechanical Behaviors of Shale Rock and Their Relation to Hydraulic Fracturing in a Shale Reservoir: A Review

Shale gas is an important supplement to the supply of natural gas resources and plays an important role on the world’s energy stage. The efficient implementation of hydraulic fracturing is the key issue in the exploration and exploitation of shale gas. The existence of bedding structure results in a distinct anisotropy of shale rock formation. The anisotropic behaviors of shale rock have important impacts on wellbore stability, hydraulic fracture propagation, and the formation of complex fracture networks. This paper briefly reviews previous work on the anisotropic mechanical properties of shale rock and their relation to hydraulic fracturing in shale reservoirs. In this paper, the research status of work addressing the lithological characteristics of shale rock is summarized first, particularly work considering the mineral constituent, which determines its physical and mechanical behavior in essence. Then the anisotropic physical and mechanical properties of shale specimens, including ultrasonic anisotropy, mechanical behavior under uniaxial and triaxial compression tests, and tensile property under the Brazilian test, are summarized, and the state of the literature on fracture toughness anisotropy is discussed. The concerns of anisotropic mechanical behavior under laboratory tests are emphasized in this paper, particularly the evaluation of shale brittleness based on mechanical characteristics, which is discussed in detail. Finally, further concerns such as the effects of bedding plane on hydraulic fracturing failure strength, crack propagation, and failure pattern are also drawn out. This review study will provide a better understanding of current research findings on the anisotropic mechanical properties of shale rock, which can provide insight into the shale anisotropy related to the fracture propagation of hydraulic fracturing in shale reservoirs.

TiN-Au/HfO2 -Au Multilayer Thin Films with Tunable Hyperbolic Optical Response.

Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self-assembled HfO2 -Au/TiN-Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO2 and TiN layers forming the alternative layers of HfO2 -Au VAN and TiN-Au VAN. The HfO2 and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi-layer thickness and the number of the bi-layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three-phase VAN design shows great potential for tailorable optical components in future integrated devices.

Generation of highly anisotropic physical properties in ferromagnetic thin films controlled by their differently oriented nano-sheets

We fabricated ferromagnetic nano-crystalline thin films of Co, Fe, Co–Fe and Co-rich and Fe-rich, Co–MT and Fe–MT (MT = transition metal), constituted by nano-sheets with a controlled slant. Visualization of these nano-sheets by Scanning Tunneling Microscopy and High-Resolution Transmission Electron Microscopy (HRTEM) showed typically tilt angles ≈56° with respect to the substrate plane, and nano-sheets ≈3.0–4.0 nm thick, ≈30–100 nm wide, and ≈200–300 nm long, with an inter-sheet distance of ≈0.9–1.2 nm, depending on their constitutive elements. Induced by this nano-morphology, these films exhibited large uniaxial magnetic anisotropy in the plane, the easy direction of magnetization being parallel to the longitudinal direction of the nano-sheets. In the as-grown films, typical values of the anisotropy field were between Hk ≈ 48 and 110 kA/m depending on composition. The changes in the nano-morphology caused by thermal treatments, and hence in the anisotropic properties, were also visualized by HRTEM, including chemical analysis at the nano-scale. Some films retained their nano-sheet morphology and increased their anisotropies by up to three times after being heated to at least 500 °C: for example, the thermal treatments produced crystallization processes and the growth of CoV and CoFe magnetic phases, maintaining the nano-sheet morphology. In contrast, other annealed films, Co, Fe, CoZn, CoCu… lost their nano-sheet morphology and hence their anisotropies. This work opens a path of study for these new magnetically anisotropic materials, particularly with respect to the nano-morphological and structural changes related to the increase in magnetic anisotropy.

Features of Anisotropic Drug Delivery Systems.

Natural materials are anisotropic. Delivery systems occurring in nature, such as viruses, blood cells, pollen, and many others, do have anisotropy, while delivery systems made artificially are mostly isotropic. There is apparent complexity in engineering anisotropic particles or capsules with micron and submicron sizes. Nevertheless, some promising examples of how to fabricate particles with anisotropic shapes or having anisotropic chemical and/or physical properties are developed. Anisotropy of particles, once they face biological systems, influences their behavior. Internalization by the cells, flow in the bloodstream, biodistribution over organs and tissues, directed release, and toxicity of particles regardless of the same chemistry are all reported to be factors of anisotropy of delivery systems. Here, the current methods are reviewed to introduce anisotropy to particles or capsules, including loading with various therapeutic cargo, variable physical properties primarily by anisotropic magnetic properties, controlling directional motion, and making Janus particles. The advantages of combining different anisotropy in one entity for delivery and common problems and limitations for fabrication are under discussion.

Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. Knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of their anisotropic thermo-mechanical properties and exfoliation mechanism towards the preparation of important single-layer materials like hexagonal boron nitride nanosheets and phosphorene. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane and volumetric thermal expansion coefficients and compressibility parameters by subjecting these materials to hydrostatic conditions and by collecting a large number of data points in small pressure and temperature increments. In addition, based on the anisotropic behavior of bP, we propose the use of this material as a sensor for the simultaneous determination of pressure and temperature in the range of 0-5 GPa and 298-1700 K, respectively.

Two‐dimensional anisotropic acoustic wave modelling using the support operator method

AbstractWe developed an algorithm to simulate two‐dimensional frequency domain acoustic‐wave response in a transversely isotropic medium with a tilted symmetry axis. The algorithm employs a support operator finite‐difference method for modelling. This method constructs a nine‐point stencil finite‐difference scheme for second‐order elliptic equations for generalized anisotropic physical properties. The medium's properties are described as P‐wave velocity on the symmetric axis, density, Thomsen's anisotropic parameters (epsilon and delta) and the tilt angle. The benchmarking analysis of the modelled amplitude is illustrated using an isotropic whole‐space model. Several synthetic experiments are conducted to evaluate the accuracy of the scheme for anisotropic models. The results suggest that the developed algorithm simulates the P‐wave solution and the fictitious S‐wave mode as reported in the literature. Simulation for a heterogeneous model with a spatially varying tilt angle of the medium symmetry axis is performed to ascertain the algorithm's robustness. The outcomes of the numerical experiments demonstrate that the developed algorithm can accurately simulate the frequency domain response of acoustic waves in the tilted anisotropic media.

Thermodynamic Origins of Structural Metastability in Two-Dimensional Black Arsenic.

Two-dimensional (2D) materials have aroused considerable research interest owing to their potential applications in nanoelectronics and optoelectronics. Thermodynamic stability of 2D structures inevitably affects the performance and power consumption of the fabricated nanodevices. Black arsenic (b-As), as a cousin of black phosphorus, has presented extremely high anisotropy in physical properties. However, systematic research on structural stability of b-As is still lacking. Herein, we demonstrated the detailed analysis on structural metastability of the natural b-As, and determined its existence conditions in terms of two essential thermodynamic variables, hydrostatic pressure and temperature. Our results confirmed that b-As can survive only below 0.7 GPa, and then irreversibly transforms to gray arsenic, consistent with our theoretical calculations. Furthermore, a thermal annealing strategy was developed to precisely control the thickness of the b-As flake, and it sublimates at 300 °C. These results could pave the way for 2D b-As in many promising applications.

Gold Nanorod-Induced Effects in a Mesogenic Compound 4-(trans-4-n-Hexylcyclohexyl) isothiocyanatobenzene.

Gold nanorods (GNRs) have a capsule-like structure with different optical properties than spherical gold nanoparticles due to surface plasmon resonance. Liquid crystals (LCs) are mesogenic compounds having crystal-like orientation and liquid-like fluidity. They are important materials from a technological point of view. Both GNRs and LC compounds are anisotropic in shape and properties. Different nano entities show interesting results when dispersed in different liquid crystalline materials which are instrumental from the application point of view. In the present work, GNRs have been dispersed in nematic liquid crystalline materials, namely 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT). Calorimetric, texture, spectroscopic, and dielectric studies were carried out for a pure 6CHBT and its composites with GNRs. Different calorimetric and dielectric parameters such as transition temperature, enthalpy, heat flow, permittivity, dielectric strength, dielectric anisotropy, and relaxation frequency have been determined, and the effect of GNRs has been explored. This article gives an insight into the influence of GNRs on the morphology and anisotropic physical properties of the nematic liquid crystalline material.

Utilization of Physical Anisotropy in Metal-Organic Frameworks via Post-Synthetic Alignment Control with Liquid Crystal.

Metal-organic frameworks (MOFs) represent crystalline materials constructed from combinations of metal and organic units to often yield anisotropic porous structures and physical properties. Postsynthetic methods to align the MOF crystals in bulk remain scarce yet tremendously important to fully utilize their structure-driven intrinsic properties. Herein, we present an unprecedented composite of liquid crystals (LCs) and MOFs and demonstrate the use of nematic LCs to dynamically control the orientation of MOF crystals with exceptional order parameters (as high as 0.965). Unique patterns formed through a facile multidirectional alignment of MOF crystals exhibit polarized fluorescence with the fluorescence intensity of a pattern dependent on the angle of a polarizer, offering potential use in various optical applications such as an optical security label. Further, the alignment mechanism indicates that the method is applicable to numerous combinations of MOFs and LCs, which include UV polymerizable LC monomers used to fabricate free-standing composite films.

Transversally isotropic elastic material applicable for permafrost rocks: а case study

In this paper we present the principles for a new method of quaternion factorization of the equilibrium equations for the transversally isotropic elasticity. Natural and artificial materials have anisotropy of physical properties. Many of them have transversal isotropy of elastic properties. Transversally isotropic materials are used in many technologies and industries, for example, in rock mechanics under permafrost conditions. Mathematical description of such materials involves the model of a transversally-isotropic material with 5 independent elastic constants. The equations of this model are more complicated than those for isotropic elasticity, and their analysis causes much more difficulties. One of the methods for analyzing such equations is factorization, i.e. reduction to the solution of simpler first-order equations.

Tunable Magnetic and Optical Anisotropy in ZrO2‐Co Vertically Aligned Nanocomposites

AbstractMetamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO2‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm−3, strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.

Single-Crystal Growth of a Cubic Laves-Phase Ferromagnet HoAl2 by a Laser Floating-Zone Method

The successful growth of single crystals of a cubic Laves-phase material HoAl2 with the space group Fd-3m is reported in this study. The crystals were grown by a floating-zone method with five laser diodes as a heat source. Al-rich feed rods were prepared as compensation for heavy evaporation during the growth. The nominal ratio for the feed rods was optimized as Ho:Al = 1:2.5. Single crystals of HoAl2 with a length of 50 mm were first grown in this technique. Obtaining the large-sized crystal by the floating-zone method enabled us to systematically explore the physical properties using the same batch crystal. The crystal possessed a second-ordered ferromagnetic transition at 29 K and a first-ordered spin-reorientation transition at 20 K. The bulk physical properties, such as specific heat, magnetic susceptibility, isothermal magnetization, and thermal expansion measurements, were measured. Additionally, a magnetocaloric effect was evaluated by the magnetic entropy change. We demonstrate that anisotropic physical properties along the principal axes ([100], [110], and [111]) emerged below the magnetically ordered states, in contrast to the isotropic behavior in the paramagnetic state.

Anisotropic properties of a noncentrosymmetric NbReSi superconducting single crystal

A single crystal of NbReSi which crystallizes in the hexagonal structure with the noncentrosymmetric space group $P\overline{6}2m$ (No. 189) has been grown by the Czochralski method using a tetra-arc furnace. The anisotropic physical properties have been studied by measuring the magnetic susceptibility and electrical transport. The superconducting transition temperature $({T}_{\mathrm{c}})$ is around 6.1 K, and the measured upper critical field $({H}_{\mathrm{c}2})$ values are 8.39 T and 11.65 T for applied field parallel to [0001] and $[11\overline{2}0]$, respectively. The anisotropy ratio $\mathrm{\ensuremath{\Gamma}}={H}_{\mathrm{c}2}^{a}/{H}_{\mathrm{c}2}^{c}$ is close to 1.38. The GL parameters ${\ensuremath{\kappa}}_{c}$ and ${\ensuremath{\kappa}}_{a}$ are much larger than $1/\sqrt{2}$, indicating that NbReSi is a strong type-II superconductor. The lower critical field $({H}_{\mathrm{c}1})$ values are very small (1.65 mT for $H\phantom{\rule{0.16em}{0ex}}||\phantom{\rule{0.16em}{0ex}}[0001]$ and 1.22 mT for $H\phantom{\rule{0.16em}{0ex}}||\phantom{\rule{0.16em}{0ex}}[11\overline{2}0]$) compared to ${H}_{\mathrm{c}2}$. From the magnetization hysteresis loops, calculated critical current at low field is of the order ${10}^{4}$ $\mathrm{A}/{\mathrm{cm}}^{2}$ which indicates the presence of strong pinning in the sample.

Single‐Orientation Epitaxy of Quasi‐1D Tellurium Nanowires on M‐Plane Sapphire for Highly Uniform Polarization Sensitive Short‐Wave Infrared Photodetection

AbstractTellurium (Te), an elemental van der Waals semiconductor, has intriguing anisotropic physical properties owing to its inherent quais‐1D crystal structure. Synthesizing ultrathin Te crystal with uniform orientation is important to its large‐scale device applications, but that remains a great challenge. Herein, the nanoscale grooves‐induced unidirectional epitaxy growth of 1D Te nanowires via physical vapor deposition on the annealed m‐plane sapphire is demonstrated. By enhancing the annealing temperature from 1000 to 1300 °C, nanoscale grooves on m‐plane sapphire arising along the [100] direction and gradually distinct, and the corresponding Te nanowires grown on them turns from random to uniform, finally achieving nearly 95% unidirectional Te nanowires. The as‐grown Te nanowires possess high crystallinity with clearly chiral helical chains along the c‐axis direction and reveal thickness‐tunable bandgap with prominent linear‐dichroic. As results, the Te nanowire‐based photodetectors demonstrate a broadband photoresponse from visible (532 nm) to short‐wave infrared (2530 nm), with high responsivity of 327 A W−1 as well as strong and uniform polarization sensitivity (anisotropic ratio = 2.05) to 1550 nm light. The high crystallinity and superior anisotropy of Te nanowires, combined with the orientation‐controlled preparation endows it with great feasibility for constructing chip‐scale multifunctional optoelectronic devices.

Quasi-One-Dimensional ZrS3 Nanoflakes for Broadband and Polarized Photodetection with High Tuning Flexibility.

Two-dimensional (2D) layered materials with low crystal symmetries have exhibited unique anisotropic physical properties. Here, we report systematic studies on the photoresponse of field effect transistors (FETs) fabricated using quasi-one-dimensional ZrS3 nanoflakes. The as-fabricated phototransistors exhibit a broadband photocurrent response from ultraviolet to visible regions, where the responsivity and detectivity can be enhanced via additional gate voltages. Furthermore, benefiting from the strong in-plane anisotropy of ZrS3, we observe a gate-voltage and illumination wavelength-dependent polarized photocurrent response, while its sub-millisecond-time response speed is also polarization-dependent. Our results demonstrate the flexible tunability of photodetectors based on anisotropic layered semiconductors, which substantially broadens the application of low symmetry layered materials in polarization-sensitive optoelectronic devices.

Conformational and Supramolecular Aspects in Chirality of Flexible Camphor-Containing Schiff Base as an Inducer of Helical Liquid Crystals.

The experimental and theoretical study of influence of the conformational state and association on the chirality of the stereochemically nonrigid biologically active bis-camphorolidenpropylenediamine (CPDA) and its ability to induce the helical mesophase of alkoxycyanobiphenyls liquid-crystalline binary mixture was carried out. On the basis of quantum-chemical simulation of the CPDA structure, four relatively stable conformers were detected. A comparison of the calculated and experimental electronic circular dichroism (ECD) and 1H, 13C, 15N NMR spectra, as well as specific optical rotation and dipole moments, allowed to establish the most probable trans-gauche conformational state (tg) of dicamphorodiimine and CPDA dimer with a predominantly mutually parallel arrangement of molecular dipoles. The induction of helical phases in LC mixtures based on cyanobiphenyls and bis-camphorolidenpropylenediamine was studied by polarization microscopy. The clearance temperatures and the helix pitch of the mesophases were measured. The helical twisting power (HTP) was calculated. The decrease in HTP with increasing dopant concentration was shown to be connected with the CPDA association process in the LC phase. The effect of camphor-containing chiral dopants of various structures on nematic LCs was compared. The values of the permittivity and birefringence components of the CPDA solutions in CB-2 were measured experimentally. A strong effect of this dopant on the anisotropic physical properties of the induced chiral nematic was established. A significant decrease in the dielectric anisotropy was associated with the 3D compensation of the LC dipoles during the formation of the helix.