In-plane Materials Research Articles

Extra-High Mechanical and Phononic Anisotropy in Black Phosphorus Blisters.

Strain is an effective strategy to modulate the electrical, optical, and optoelectronic properties of 2D materials. Conventional circular blisters could generate a biaxial stretching of 2D membranes with notable strain gradients along the hoop direction. However, such a deformation mode cannot be utilized to investigate mechanical responses of in-plane anisotropic 2D materials, for example, black phosphorus (BP), due to its crystallographic orientation dependence. Here, a novel rectangular-shaped bulge device is developed to uniaxially stretch the membrane, and further provide a promising platform to detect orientation-dependent mechanical and optical properties of anisotropic 2D materials. Impressively, the derived anisotropic ratio of Young's modulus of BP flakes is much higher than the values obtained via the nanoindentation method. The extra-high strain-dependent phononic anisotropy in Raman modes along different crystalline orientations is also observed. The designed rectangular budge device expands the uniaxial deformation methods available, allowing to explore the mechanical, and strain-dependent physical properties of other anisotropic 2D materials more broadly.

Highly in-plane anisotropic 2D ReSe2 for polarization-sensitive photodetectors

Here we report a new member of in-plane anisotropic 2D materials Rhenium Diselenide (ReSe2) which holds a stable distorted 1 T’ phase with its relatively narrow bandgap to explore the in-plane anisotropic structural, electrical, and optical properties. The proposed polarization-sensitive photodetectors based on few-layer ReSe2 provides highly anisotropic photodetection behavior with linearly dichroic ratio up to ≈2.62 under 808 nm illumination. We measured carrier transport in space and time by mapping carrier density in a ReSe2 plate using transient absorption microscopy. These results directly determined the carrier diffusion coefficients of about 4100 cm2 s-1 and 2900 cm2 s-1 along the armchair and zigzag directions, respectively, providing fundamental parameters to better understand the anisotropy of ReSe2 and promote future 2D optoelectronic applications in polarization-sensitive detection.

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.

Optical properties and polaritons of low symmetry 2D materials

Low symmetry 2D materials with intrinsic in-plane anisotropic optical properties and high tunability provide a promising platform to explore and manipulate light–matter interactions. To date, dozens of in-plane anisotropic 2D materials with diverse band structures have been discovered. They exhibit rich optical properties, indicating great potential for novel applications in optics, photonics, and optoelectronics. In this paper, we thoroughly review the anisotropic optical properties and polaritons in many kinds of low symmetry 2D materials, aiming to elicit more research interest in this field. First, the optical properties of anisotropic 2D semiconductors, including interband absorption, photoluminescence, excitons, and band structure engineering for tuning optical responses, are introduced. Then fundamentals and advances in experiments of hyperbolic polaritons in anisotropic 2D materials, including phonon, plasmon, and exciton polaritons, are discussed. Finally, a perspective on promising research directions is given.

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.

Flexoelectric effect induced p–n homojunction in monolayer GeSe

Recently, two-dimensional in-plane ferroelectric materials group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) have attracted much attention due to their rich physical properties. Here, we study the flexoelectric effect on the electronic, optical and transport properties in the monolayer germanium selenide (GeSe). We found that bending along the armchair (polarization) direction can separate the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the real space and form the type-II band alignment, which can be used to fabricate p–n homojunctions. This outstanding property hints the potential applications of the bending monolayer GeSe for the exciton transport and solar cell, and provides new routines for advanced two-dimensional semiconductor devices.

Strong In-Plane Anisotropic SiP2 as a IV-V 2D Semiconductor for Polarized Photodetection.

In-plane anisotropic two-dimensional (2D) materials, emerging as an intriguing type of 2D family, provide an ideal platform for designing and fabrication of optoelectronic devices. Exploring air-stable and strong in-plane anisotropic 2D materials is still challenging and promising for polarized photodetection. Herein, SiP2, a 2D IV-V semiconductor, is successfully prepared and introduced into an in-plane anisotropic 2D family. The basic characterizations combined with theoretical calculations reveal 2D SiP2 to exhibit an intrinsically low-symmetry structure, the in-plane anisotropy of phonon vibrations, and an anisotropically dispersed band structure. Moreover, the photodetector based on 2D SiP2 exhibits high performance with a high detectivity of 1012 Jones, a large light on/off ratio of 103, a low dark current of 10-13 A, and a fast response speed of 3 ms. Furthermore, 2D SiP2 demonstrates a high anisotropic photodetection with an anisotropic ratio up to 2. In addition, the polarization-sensitive photodetector presents a dichroic ratio of 1.6 due to the intrinsic linear dichroism. These good characteristics make 2D SiP2 a promising candidate as an in-plane anisotropic semiconductor for high-sensitivity and polarized optoelectronic applications.

Analyzer-free, intensity-based, wide-field magneto-optical microscopy

In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain–boundary contrast is due to the magneto-optical gradient effect. A domain–boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell–Fresnel theory.

2D NbOI2 : A Chiral Semiconductor with Highly In-Plane Anisotropic Electrical and Optical Properties.

Exploring in-plane anisotropic 2D materials is of great significance to the fundamental studies and further development of polarizationsensitive optoelectronics. Herein, chiral niobium oxide diiodide (NbOI2 ) is introduced into the intriguing anisotropic 2D family with the experimental demonstration of anisotropic optical and electrical properties. 2D NbOI2 crystals exhibit highly anisotropic dispersed band structures around the Fermi surface and strong in-plane anisotropy of phonon vibrations owing to the different bonding modes of Nb atoms along the b- and c-axes. Consequently, the anisotropic factors of optical absorbance and photoresponsivity in 2D NbOI2 crystals reach up to 1.75 and 1.7, respectively. These anisotropic properties make 2D NbOI2 an interesting platform for novel polarization-sensitive optoelectronic applications.

Highly In-Plane Anisotropic Two-Dimensional Ternary Ta2NiSe5 for Polarization-Sensitive Photodetectors.

Intriguing anisotropic electrical and optoelectrical properties in two-dimensional (2D) materials are currently gaining increasing interest both for fundamental research and emerging optoelectronic devices. Identifying promising new 2D materials with low-symmetry structures will be rewarding in the development of polarization-integrated nanodevices. In this work, the anisotropic electron transport and optoelectrical properties of multilayer 2D ternary Ta2NiSe5 were systematically researched. The polarization-sensitive Ta2NiSe5 photodetector shows a linearly anisotropy ratio of ≈3.24 with 1064 nm illumination. The multilayer Ta2NiSe5-based field-effective transistors exhibit an excellent field-effective mobility of 161.25 cm2·V-1·s-1 along the a axis (armchair direction) as well as a great current saturation characteristic at room temperature. These results will promote a better understanding of the optoelectrical properties and applications in new categories of the in-plane anisotropic 2D materials.

In-plane anisotropic 2D Ge-based binary materials for optoelectronic applications.

In-plane anisotropic two-dimensional (2D) materials possess unique in-plane anisotropic physical properties arising from their low crystal lattice symmetry. Among these low-symmetry 2D materials, anisotropic Ge-based binary materials have the advantages of simple binary and earth-abundant compositions, good stability, highly anisotropic physical properties along two principle axes, and wide coverage of bandgaps, enabling use in broadband photodetection from the infrared to ultraviolet region. Here, we review recent progress in in-plane anisotropic 2D Ge-based binary materials, focusing on their anisotropic structural, electrical and optical properties. We then discuss demonstrations of optoelectronic applications related to those anisotropic properties including polarization-sensitive photodetection and polarization-based all-optical switches. Finally, we provide further possible opportunities for this relatively new, but quickly expanding family of materials.

In-plane spin-orbit torque magnetization switching and its detection using the spin rectification effect at subgigahertz frequencies

Inplane magnetization reversal of a permalloy/platinum bilayer was detected using the spin rectification effect. Using a sub GHz microwave frequency to excite spin torque ferromagnetic resonance (ST FMR) in the bilayer induces two discrete DC voltages around an external static magnetic field of 0 mT. These discrete voltages depend on the magnetization directions of the permalloy and enable detection of the inplane magnetization reversal. The threshold current density for the magnetization reversal is from 10 to 20 MA/cm^2, the same order as for known spin orbit torque (SOT) switching with in-plane magnetization materials. The magnitude of the signal is the same or larger than that of the typical ST FMR signal; that is, detection of magnetization switching is highly sensitive in spite of deviation from the optimal ST-FMR condition. The proposed method is applicable to a simple device structure even for a small ferromagnetic electrode with a width of 100 nm.

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.

Recent Advances in 2D Metal Monochalcogenides.

The family of emerging low‐symmetry and structural in‐plane anisotropic two‐dimensional (2D) materials has been expanding rapidly in recent years. As an important emerging anisotropic 2D material, the black phosphorene analog group IVA–VI metal monochalcogenides (MMCs) have been surged recently due to their distinctive crystalline symmetries, exotic in‐plane anisotropic electronic and optical response, earth abundance, and environmentally friendly characteristics. In this article, the recent research advancements in the field of anisotropic 2D MMCs are reviewed. At first, the unique wavy crystal structures together with the optical and electronic properties of such materials are discussed. The Review continues with the various methods adopted for the synthesis of layered MMCs including micromechanical and liquid phase exfoliation as well as physical vapor deposition. The last part of the article focuses on the application of the structural anisotropic response of 2D MMCs in field effect transistors, photovoltaic cells nonlinear optics, and valleytronic devices. Besides presenting the significant research in the field of this emerging class of 2D materials, this Review also delineates the existing limitations and discusses emerging possibilities and future prospects.

Plasmonically induced transparency in in-plane isotropic and anisotropic 2D materials.

General two-dimensional (2D) material-based systems that achieve plasmonically induced transparency (PIT) are limited to isotropic graphene only through unidirectional bright-dark mode interaction. Moreover, it is challenging to extend these devices to anisotropic 2D films. In this study, we exploit surface plasmons excited at two crossed grating layers, which can be formed either by dielectric gratings or by the 2D sheet itself, to achieve dynamically tunable PIT in both isotropic and anisotropic 2D materials. Here, each grating simultaneously acts as both bright and dark modes. By taking isotropic graphene and anisotropic black phosphorus (BP) as proofs of concept, we reveal that this PIT can result from either unidirectional bright-dark or bidirectional bright-bright and bright-dark mode hybridized couplings when the incident light is parallelly/perpendicularly or obliquely polarized to the gratings, respectively. Identical grating parameters in isotropic (crossed lattice directions in anisotropic) layers produce polarization-independent single-window PIT, whereas different grating parameters (coincident lattice directions) yield polarization-sensitive double-window PIT. The proposed technique is examined by a two-particle model, showing excellent agreement between the theoretical and numerical results. This study provides insight into the physical mechanisms of PIT and advances the applicability and versatility of 2D material-based PIT devices.

Superior stiffness and vibrational spectroscopic signature of two-dimensional diamond-like carbon nitrides

Abstract Carbon nitride materials are promising for applications in electronics, clean energy production, and heat dissipation. Two-dimensional (2D) diamond-like carbon nitrides α -C 2 N 2 , β -C 2 N 2 , and γ -C 4 N 4 rise as beyond graphene semiconductors. Here, we apply first-principles calculations and group theory to study their structure, mechanical properties, and vibrational signature. The α -C 2 N 2 is the strongest among them, with a 2D Young’s modulus E 2D equal to 616(6) N/m, followed by the γ -C 4 N 4 with an E 2D equal to 632(6) N/m and 581(7) N/m along its zigzag and armchair directions, respectively, and the β -C 2 N 2 with an E 2D equal to 582(9) N/m. These materials are about 2 times stiffer than graphene, and are the stiffest among 2D networks of carbon and nitrogen atoms. The zigzag direction of 2D γ -C 4 N 4 is approximately 8% stronger than its armchair direction, unusual for in-plane anisotropic 2D materials, where the armchair direction is considerably weaker than the zigzag direction. These findings from stress–strain analysis are consistent with the high sound speed and elastic constants values we found by using 2D density-functional perturbation theory framework, suggesting them for mechanical reinforcement. We report the phonon wavenumber, atomic vibrational pattern, and Raman and infrared spectra for all polytypes. The longitudinal and transverse optical modes of the in-plane isotropic polytypes display the breakdown of LO–TO splitting, characteristic of 2D polar crystals. We found that the difference between their phonon wavenumbers can be probed in their unpolarized Raman and infrared spectra. The simulated angular dependency of the Raman intensity under backscattering parallel and cross polarizations show how to assign the A 1 ′ and E ′ modes of the α -C 2 N 2 , the A 1 g and E g modes of the β -C 2 N 2 , and of the A g and B 1 g modes of the γ -C 4 N 4 , being key for polytype identification. These results provide comprehensive information on the emerging 2D diamond-like carbon nitrides, necessary for further developments on their synthesis, characterization, and future device fabrication.

Perpendicularly Magnetized Thin-Film Antidot Arrays for Superparamagnetic Microbead Actuation

We report the directed transport of superparamagnetic microbeads (SPBs) on antidot arrays patterned from multilayered Co/Pt films with perpendicular magnetic anisotropy (PMA). The dynamics of water-suspended SPBs driven across the substrate surface by elliptical rotating magnetic fields is studied experimentally and modeled analytically. Bead motion on PMA substrates is shown to be significantly faster than on similarly patterned films based on in-plane magnetized materials. Furthermore, we show different SPB trajectories on different lattice geometries, suggesting that the magnetic potential distribution plays a crucial role in determining the dynamics. Our findings provide new insights into the enhanced transport of SPBs using PMA thin films as stray field sources that enable dynamic magnetic potential landscapes.

Interface Energy-Level Alignment between Black Phosphorus and F16CuPc Molecular Films

The organic/inorganic semiconductor interfaces play a key role in determining the functionality of charge injection devices. The energy-level alignment at the interfaces between two in-plane organic materials (F16CuPc and CuPc) and single-crystal black phosphorus was systematically studied by in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). In this work, we reveal that the deposition of F16CuPc onto black phosphorus surface leads to a considerable charge transfer across the interface, resulting in the pronounced increment of work function. As for the molecule–substrate interaction, both F16CuPc and CuPc interact with black phosphorus via van der Waals physisorption, as confirmed by the UPS and XPS measurements. Our results demonstrate the versatile use of organic molecules to tailor the energy levels of black phosphorus based hybrid structures, which can be potentially used to optimize its function in future devices.

Recent Developments in Perpendicular Magnetic Anisotropy Thin Films for Data Storage Applications

The incessant demand for higher density, faster access time and lower power consuming memory devices such as random access memories have driven tremendous research and development of materials with out-of-plane magnetization. Magnetic materials with strong out-of-plane magnetization, i.e. perpendicular magnetic anisotropy (PMA), offer superior qualities compared to the in-plane anisotropy materials for hard disk drive and magnetoresistive random access memory (MRAM) devices and have been successfully commercialized in the last decade. With the recent demonstration of spin-transfer torque (STT) magnetic switching, an urge for new materials has emerged for promising STT-MRAM applications. Here, we present a brief overview of PMA materials for two important data storage applications: perpendicular recording and MRAM. We review the various PMA materials developed in recent years for STT-MRAM applications. We discuss the major requirements, challenges and future prospects of these materials for future STT-MRAM devices.

Magnetic Particle Nanosensing by Nucleation of Domain Walls in Ultra-Thin CoFeB/Pt Devices

A novel nanosensor is demonstrated based on domain-wall (DW) nucleation in perpendicular-anisotropy CoFeB/Pt components, with an electrical read-out based on the anomalous Hall effect. Notch-shaped constrictions within perpendicular CoFeB/Pt wires are used to control the local anisotropy. The successful detection of a single NdFeB microbead is demonstrated, based on the variations in DW nucleation in a notched nanowire in the presence of the bead. Maps of the sensor response obtained using scanning gate microscopy measurements are presented. We describe a number of potential advantages of the nucleation-sensing technique as compared with existing DW-based magnetic bead sensors using in-plane magnetized materials.