Highly In‐Plane Optical and Electrical Anisotropy of 2D Germanium Arsenide

Anisotropic 2D materials exhibit unique optical, electrical, and thermoelectric properties that open up possibilities for diverse angle‐dependent devices. However, the explored anisotropic 2D materials are very limited and the methods to identify the crystal orientations and to study the in‐plane anisotropy are in the initial stage. Here azimuth‐dependent reflectance difference microscopy (ADRDM), angle‐resolved Raman spectra, and electrical transport measurements are used to systematically characterize the influence of the anisotropic structure on in‐plane optical and electrical anisotropy of 2D GeAs, a novel group IV–V semiconductor. It is proved that ADRDM offers a way to quickly identify the crystal orientations and also to directly characterize the in‐plane optical anisotropy of layered GeAs. The anisotropic electrical transport behavior of few‐layer GeAs field‐effect transistors is further measured and the anisotropic ratio of the mobility is as high as 4.6, which is higher than the other 2D anisotropic materials such as black phosphorus. The dependence of the Raman intensity anisotropy on the sample thickness, excitation wavelength, and polarization configuration is investigated both experimentally and theoretically. These data will be useful for designing new high‐performance devices and the results suggest a general methodology for characterizing the in‐plane anisotropy of low‐symmetry 2D materials.     

Tuning Molecular Adhesion via Material Anisotropy

Cell adhesion with extracellular matrix depends on the collective behaviors of a large number of receptor‐ligand bonds at the compliant cell‐matrix interface. While most biological tissues and structures, including cells and extracellular matrices, exhibit strongly anisotropic material properties, existing studies on molecular adhesion via receptor‐ligand bonds have been largely limited to isotropic materials. Here the effects of transverse isotropy, a common form of material anisotropy in biological systems, in modulating the adhesion behavior of a cluster of receptor‐ligand bonds are investigated. The results provide a theoretical basis to understand cell adhesion on anisotropic extracellular matrices and to explore the possibility of controlling cell adhesion via anisotropy design in material properties. The combined analysis and simulations show that the orientation of material anisotropy strongly affects the apparent softness felt by the adhesive bonds, thereby altering their ensemble lifetime by several orders of magnitude. An implication of this study is that distinct cellular behaviors can be achieved through remodeling of material anisotropy in either extracellular matrix or cytoskeleton. Comparison between different loading conditions, together with the effects of material anisotropy, yields a rich array of out‐of‐equilibrium behaviors in the molecular interaction between reactant‐bearing soft surfaces, with important implications on the mechanosensitivity of cells.     

Inverse Design of Mechanical Metamaterials That Undergo Buckling

Metamaterials are man‐made materials which get their properties from their structure rather than their chemical composition. Their mesostructure is specifically designed to create functionalities not found in nature. However, despite the broad variety of metamaterials developed in recent years, a straightforward procedure to design these complex materials with tailored properties has not yet been established. Here, the inverse design problem is tackled by introducing a general optimization tool to explore the range of material properties that can be achieved. Specifically, a stochastic optimization algorithm is applied and its applicability to disjoint problems is demonstrated, with a focus on tuning the buckling properties of mechanical metamaterials, including experimental verification of the predictions. Besides this problem, this algorithm can be applied to a large variety of systems that, because of their complexity, would be challenging otherwise. Potential applications range from the design of optomechanical resonators, acoustic band gap materials, to dielectric metasurfaces.     

Wood‐Inspired 3D‐Printed Helical Composites with Tunable and Enhanced Mechanical Performance

Helical fibers are versatile building blocks used by Nature to improve mechanical performance and to tune local behavior of load‐bearing materials. Helicoidal biocomposites are arranged in multiple layers with different fiber orientations. Such heterogeneity, not matched in synthetic materials, provides biological structures with superior properties. This is the case of the multilayer tube‐like structure of the wood cell wall, where each ply features a compliant matrix reinforced by stiff helicoidal microfibrils. Here, 3D polyjet printing and computer simulations are combined to investigate wood‐inspired helix‐reinforced cylinders. Composites with a main layer containing helicoidal fibers, bordered by inner and outer plies having thinner fibrils are considered. It is shown how the mechanical functionalities of the synthetic structures can be programmed by varying fibers/fibrils orientation and matrix compliance. It is demonstrated that failure resistance can be enhanced by enclosing the main helicoidal layer with a minimum amount of thin fibrils oriented perpendicular to the applied load, as observed in wood. Finite element simulations are used to highlight the critical role of the matrix in load‐transfer mechanisms among stiff elements. These structures have the potential to be assembled into larger systems, leading to graded composites with region‐specific properties optimized for multiple functionalities.     

Tough Lessons From Bone: Extreme Mechanical Anisotropy at the Mesoscale

Bone is mechanically and structurally anisotropic with oriented collagen fibrils and nanometer‐sized mineral particles aggregating into lamellar or woven bone.[1] Direct measurements of anisotropic mechanical properties of sublamellar tissue constituents are complicated by the existence of an intrinsic hierarchical architecture. Methods such as nanoindentation provide insight into effective modulus values; however, bulk material properties cannot sufficiently be characterized since such measurements represent properties of near‐surface volumes and are partially averaged over fibril orientations.[2–5] In this study, we focus on the material properties of bone at one single level of hierarchy. By measuring properties of individual parallel‐fibered units of fibrollamellar bone under tension under controlled humidity conditions, an unusually high anisotropy is found. Here, we clearly demonstrate ratios as large as 1:20 in elastic modulus and 1:15 in tensile strength between orientations perpendicular and parallel to the main collagen fiber orientation in native wet bone; these ratios reduce to 1:8 and 1:7, respectively, under dry conditions. This extreme anisotropy appears to be caused by the existence of periodic, weak interfaces at the mesoscopic length scale. These interfaces are thought to be relevant to the proper mechanical and physiological performance of bone.     

Optical and electrical properties of two-dimensional anisotropic materials

Two-dimensional(2D) anisotropic materials, such as B-P, B-As, GeSe, GeAs, ReSe2, KP15 and their hybrid systems, exhibit unique crystal structures and extraordinary anisotropy. This review presents a comprehensive comparison of various 2D anisotropic crystals as well as relevant FETs and photodetectors, especially on their particular anisotropy in optical and electrical properties. First, the structure of typical 2D anisotropic crystal as well as the analysis of structural anisotropy is provided. Then, recent researches on anisotropic Raman spectra are reviewed. Particularly, a brief measurement principle of Raman spectra under three typical polarized measurement configurations is introduced. Finally, recent progress on the electrical and photoelectrical properties of FETs and polarization-sensitive photodetectors based on 2D anisotropic materials is summarized for the comparison between different 2D anisotropic materials. Beyond the high response speed, sensitivity and on/off ratio, these 2D anisotropic crystals exhibit highly conduction ratio and dichroic ratio which can be applied in terms of polarization sensors, polarization spectroscopy imaging, optical radar and remote sensing.     

Giant Phononic Anisotropy and Unusual Anharmonicity of Phosphorene: Interlayer Coupling and Strain Engineering

Phosphorene, an emerging elemental 2D direct band gap semiconductor with fascinating structural and electronic properties distinctively different from other 2D materials such as graphene and MoS₂, is promising for novel nanoelectronic and optoelectronic applications. Phonons, as one of the most important collective excitations, are at the heart of the device performance, as their interactions with electrons and photons govern the carrier mobility and light‐emitting efficiency of the material. Here, through a detailed first‐principles study, it is demonstrated that monolayer phosphorene exhibits a giant phononic anisotropy, and remarkably, this anisotropy is squarely opposite to its electronic counterpart and can be tuned effectively by strain engineering. By sampling the whole Brillouin zone for the monolayer phosphorene, several “hidden” directions are found, along which small‐momentum phonons are “frozen” with strain and possess the smallest degree of anharmonicity. Unexpectedly, these “hidden” directions are intrinsically different from the usually‐studied armchair and zigzag directions. Light is also shed on the anisotropy of interlayer coupling of few‐layer phosphorene by examining the rigid‐layer vibrations. These highly anisotropic and strain‐tunable characteristics of phosphorene offer new possibilities for its applications in thermal management, thermoelectronics, nanoelectronics, and phononics.     

Thermal Conductivity of HfTe5: A Critical Revisit

Hafnium pentatelluride (HfTe₅) has attracted extensive interest due to its exotic electronic, optical, and thermal properties. As a highly anisotropic crystal (layered structure with in‐plane chains), it has highly anisotropic electrical‐transport properties, but the anisotropy of its thermal‐transport properties has not been established. Here, accurate experimental measurements and theoretical calculations are combined to resolve this issue. Time‐domain thermoreflectance measurements find a highly anisotropic thermal conductivity, 28:1:8, with values of 11.3 ± 2.2, 0.41 ± 0.04, and 3.2 ± 2.0 W m^-1 K^-1 along the in‐plane a‐axis, through‐plane b‐axis, and in‐plane c‐axis, respectively. This anisotropy is even larger than what was recently established for ZrTe₅ (12:1:6), but the individual values are somewhat higher, even though Zr has a smaller atomic mass than Hf. Density‐functional‐theory calculations predict thermal conductivities in good agreement with the experimental data, provide comprehensive insights into the results, and reveal the origin of the apparent anomaly of the relative thermal conductivities of the two pentatellurides. These results establish that HfTe₅ and ZrTe₅, and by implication their alloys, have highly anisotropic and ultralow through‐plane thermal conductivities, which can provide guidance for the design of materials for new directional‐heat‐management applications and potentially other thermal functionalities.     

Highly Sensitive Detection of Polarized Light Using Anisotropic 2D ReS2

Due to the novel optical and optoelectronic properties, 2D materials have received increasing interests for optoelectronics applications. Discovering new properties and functionalities of 2D materials is challenging yet promising. Here broadband polarization sensitive photodetectors based on few layer ReS₂ are demonstrated. The transistor based on few layer ReS₂ shows an n‐type behavior with the mobility of about 40 cm² V^?1 s^?1 and on/off ratio of 10⁵. The polarization dependence of photoresponse is ascribed to the unique anisotropic in‐plane crystal structure, consistent with the optical absorption anisotropy. The linear dichroic photodetection with a high photoresponsivity reported here demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways for the applications of 2D materials for light polarization detection.     

Bioinspired Design Criteria for Damage‐Resistant Materials with Periodically Varying Microstructure

Many biological materials, such as bone, nacre, or certain deep‐sea glass sponges, have a hierarchical structure that makes them stiff, tough, and damage tolerant. Different structural features contributing to these exceptional properties have been identified, but a common motif of these materials, the periodic arrangement of structural components with strongly varying stiffness, has not gained sufficient attention. Here we show that the periodicity of the material properties is one of the dominant reasons for the high fracture resistance of these structures and their tolerance to short cracks. If the composite architecture fulfills certain design rules, which are derived in this paper, the stiff structure becomes fracture resistant and, most of all, flaw tolerant. This architectural criterion inspired from nature provides useful guidelines for the design of defect‐tolerant resistant man‐made materials.     

Bioinspired Superwettability Micro/Nanoarchitectures: Fabrications and Applications

Biological systems have evolved over billions of years to develop wetting strategies for advantageous structure–property–performance relations that are crucial for their survival. The discovery of these intriguing relationships has inspired tremendous efforts to investigate the micro/nanoscale features of naturally occurring structures with superwettability. Researchers have since developed new methods and techniques to construct artificial materials that mimic natural structures and functionalities. Here, a brief review of natural hierarchical architectures with liquid repellent properties is presented, and the critical underlying mechanism is summarized with an emphasis on the micro/nanoscopic architectures. The state‐of‐the‐art micro/nanofabrication techniques for creating bioinspired hierarchical superwettability structures that are categorized by random and exquisite features are also reviewed, followed by an overview of their emerging applications, with special attention to biomedical‐related fields. The development of fabrication techniques enhances capabilities relative to those of living systems, paving the way toward advanced structural materials with superior functions and unprecedented characteristics for potential applications.     

Engineering of Graphene Layer Orientation to Attain High Rate Capability and Anisotropic Properties in Li‐Ion Battery Electrodes

Novel carbon films with different graphene layer orientations are investigated as electrode materials for Li‐ion batteries. It is demonstrated that engineering the crystallographic orientation with graphene layers oriented perpendicular to the surface substantially alters stress evolution during Li insertion. With this crystallographic orientation the intercalating/de‐intercalating Li‐ions also have direct access to the graphene interlayer spaces, resulting in higher capacity at faster electrochemical cycling, compared to carbon films with graphene layers parallel to the film surface. Electrodes with perpendicular alignment are prepared by supramolecular synthesis using either spin coating or bar coating of chromonic liquid crystal precursors into precursor organic films followed by in situ carbonization. These materials are compared with in situ stress measurements during lithiation/delithiation cycles, and the bar‐coated films exhibit a highly anisotropic stress which is consistent with long‐range alignment of the graphene layers. In contrast, the in‐plane stresses in the spin‐coated films are isotropic, which is consistent with the presence of randomly oriented domains (still with graphene layers oriented perpendicular to the surface). Overall, the use of thin film graphitic materials with controlled crystallographic orientations provides a valuable platform for investigating the impact of graphene structure on the properties of Li‐ion battery electrode materials.     

Bioinspired Strong and Highly Porous Glass Scaffolds

The quest for more efficient energy-related technologies is driving the development of porous and high-performance structural materials with exceptional mechanical strength. Natural materials achieve their strength through complex hierarchical designs and anisotropic structures that are extremely difficult to replicate synthetically. We emulate nature's design by direct-ink-write assembling of glass scaffolds with a periodic pattern, and controlled sintering of the filaments into anisotropic constructs similar to biological materials. The final product is a porous glass scaffold with a compressive strength (136 MPa) comparable to that of cortical bone and a porosity (60%) comparable to that of trabecular bone. The strength of this porous glass scaffold is ~100 times that of polymer scaffolds and 4-5 times that of ceramic and glass scaffolds with comparable porosities reported elsewhere. The ability to create both porous and strong structures opens a new avenue for fabricating scaffolds for a broad array of applications, including tissue engineering, filtration, lightweight composites, and catalyst support.     

Mo6S3Br6: An Anisotropic 2D Superatomic Semiconductor

Two‐dimensional (2D) van der Waals materials with in‐plane anisotropy are of great interest for directional transport of charge and energy, as exemplified by recent studies on black phosphorus and α‐phase molybdenum trioxide (α‐MO₃). Here, a layered van der Waals semiconductor with in‐plane anisotropy built upon the superatomic units of Mo₆S₃Br₆ is reported. This material possesses robust 2D characteristics with a direct gap of 1.64 eV, as determined by scanning tunneling spectroscopy and first‐principles calculations. Polarization‐dependent Raman spectroscopy measurement and density functional theory calculation reveal strong in‐plane anisotropy. These results suggest an effective strategy to explore anisotropic 2D electronic and optoelectronic properties from superatomic building blocks with multifunctionality, emergent properties, and hierarchical control.     

Complex‐Shaped Cellulose Composites Made by Wet Densification of 3D Printed Scaffolds

Cellulose is an attractive material resource for the fabrication of sustainable functional products, but its processing into structures with complex architecture and high cellulose content remains challenging. Such limitation has prevented cellulose‐based synthetic materials from reaching the level of structural control and mechanical properties observed in their biological counterparts, such as wood and plant tissues. To address this issue, a simple approach is reported to manufacture complex‐shaped cellulose‐based composites, in which the shaping capabilities of 3D printing technologies are combined with a wet densification process that increases the concentration of cellulose in the final printed material. Densification is achieved by exchanging the liquid of the wet printed material with a poor solvent mixture that induces attractive interactions between cellulose particles. The effect of the solvent mixture on the final cellulose concentration is rationalized using solubility parameters that quantify the attractive interparticle interactions. Using X‐ray diffraction analysis and mechanical tests, 3D printed composites obtained through this process are shown to exhibit highly aligned microstructures and mechanical properties significantly higher than those obtained by earlier additively manufactured cellulose‐based materials. These features enable the fabrication of cellulose‐rich synthetic structures that more closely resemble the exquisite designs found in biological materials grown by plants in nature.     

Highly Conductive Organosilica Hybrid Films Prepared from a Liquid‐Crystal Perylene Bisimide Precursor

Significant anisotropic electrical conduction in organosilica films is achieved by long‐range orientation of electroactive perylene bisimide (PBI) moieties in the silica scaffold. A new PBI‐based organosilane precursor is designed with lyotropic liquid‐crystalline properties. The PBI precursor with triethoxysilylphenyl groups exhibits a hexagonal columnar phase in the presence of organic solvents. The lyotropic liquid‐crystalline behavior of the precursor enables the preparation of dip‐coated films consisting of uniaxially aligned columnar aggregates of the PBI precursor on the centimeter scale. The oriented structure is successfully fixed by in situ polycondensation, which yields insoluble, thermally stable PBI–silica hybrid films. The oriented organosilica films doped with hydrazine exhibit high electrical conductivities on the order of 10^?2 S cm^?1, which are at the highest level for organosilica materials, and are comparable to those of all‐organic PBI assemblies. Definite anisotropy of conductivities is also found for these films. The present results suggest that the induction of significant electrical properties in organic molecular assemblies is compatible with the structural stabilization by inorganic–organic hybridization.     

Artificial Helical Screws of 2D Materials with Archimedean Spiral Arrangement

Helix structures, which are frequently observed in nature, act as versatile structural templates for complex functionalities with asymmetry and anisotropy. However, atomically thin 2D materials, including graphene, transition metal dichalcogenides (TMDs), and MXenes, do not have inherent chirality in their planar geometry and cannot easily form such a structure. This study presents the macroscopic self-assembly of 2D materials for helical screws with an Archimedean spiral arrangement. The naturally triggered spontaneous rotation upon the 1D fiber assembly of 2D materials forms helical screws consisting of multiple helices and perversions. For a clear understanding of the morphological evolution of helical screws, variations in the helical pitch and angle are systematically analyzed considering thermodynamic and kinetic conditions. Subsequently, the influence of spontaneous helix formation on the properties of the 2D assembled fibers is investigated in terms of the solvent-driven actuator performance and electrical and electrothermal properties. The suggested approach provides a new perspective on the directed self-assembly of inherently achiral 2D materials toward chiral helix formation.     

Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics

Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low‐cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepared by a confined self‐assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face‐centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addition, the rotational motion and dynamics of such micrometer‐scale clusters suspended in a liquid can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real‐time information about the crystallization pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.     

Dimensionality Concept in Solid‐State Reactions: A Way to Control Synthesis of Functional Materials at the Nanoscale

The concept of dimensionality is fundamental in physics, chemistry, materials science, etc. Low‐dimensional and layered materials are distinguished by their unique physical properties and applications. Concurrently, low‐dimensional reactants, products, and reaction spaces extend the toolbox of materials science considerably. Here, the concept of dimensionality is adapted to solid‐state reactions by counting the basic axes along which the unit cell undergoes significant expansion/shrinking. For illustration, 1D synthesis of layered ternary compounds MA₂X₂ via derivatives of 2D‐Xenes, silicene, and germanene, is demonstrated, and the reaction mechanism and the role of templates are determined. The approach is then extended to 1D synthesis of non‐layered compounds. The 1D nature of the reactions, established with structural studies, is explored by nanoscale confinement. The mutual orientation of the reaction and confinement—parallel (thus preventing the lattice expansion) or orthogonal—controls the reaction pathways and outcome. The work provides a proof‐of‐concept for anisotropic reactivity caused by directional confinement.     

Simulation of Cortical and Cancellous Bone to Accelerate Tissue Regeneration

Different tissues have complex anisotropic structures to support biological functions. Mimicking these complex structures in vitro remains a challenge in biomaterials designs. Here, inspired by different types of silk nanofibers, a composite materials strategy is pursued toward this challenge. A combination of fabrication methods is utilized to achieve separate control of amorphous and beta-sheet rich silk nanofibers in the same solution. Aqueous solutions containing two types of silk nanofibers are simultaneously treated with an electric field and with ethylene glycol diglycidyl ether (EGDE). Under these conditions, the beta-sheet rich silk nanofibers in the mixture responded to the electric field while the amorphous nanofibers are active in the crosslinking process with the EGDE. As a result, cryogels with anisotropic structures are prepared, including mimics for cortical- and cancellous-like bone biomaterials as a complex osteoinductive niche. In vitro studies revealed that mechanical cues of the cryogels induced osteodifferentiation of stem cells while the anisotropy inside the cryogels influenced immune reactions of macrophages. These bioactive cryogels also stimulated improved bone regeneration in vivo through modulation of inflammation, angiogenesis and osteogenesis responses, suggesting an effective strategy to develop bioactive matrices with complex anisotropic structures beneficial to tissue regeneration.