Anisotropic Electrical Transport Research Articles

Piperidine and Pyridine Series Lead-Free Dion-Jacobson Phase Tin Perovskite Single Crystals and Their Applications for Field-Effect Transistors.

Two-dimensional (2D) organic-inorganic metal halide perovskites have gained immense attention as alternatives to three-dimensional (3D) perovskites in recent years. The hydrophobic spacers in the layered structure of 2D perovskites make them more moisture-resistant than 3D perovskites. Moreover, they exhibit unique anisotropic electrical transport properties due to a structural confinement effect. In this study, four lead-free Dion-Jacobson (DJ) Sn-based phase perovskite single crystals, 3AMPSnI4, 4AMPSnI4, 3AMPYSnI4, and 4AMPYSnI4 [AMP = (aminomethyl)-piperidinium, AMPY = (aminomethyl)pyridinium] are reported. Results reveal structural differences between them impacting the resulting optical properties. Namely, higher octahedron distortion results in a higher absorption edge. Density functional theory (DFT) is also performed to determine the trends in energy band diagrams, exciton binding energies, and formation energies due to structural differences among the four single crystals. Finally, a field-effect transistor (FET) based on 4AMPSnI4 is demonstrated with a respectable hole mobility of 0.57 cm2 V-1 s-1 requiring a low threshold voltage of only -2.5 V at a drain voltage of -40 V. To the best of our knowledge, this is the third DJ-phase perovskite FET reported to date.

Polarization‐Sensitive Photodetectors Based on Highly In‐Plane Anisotropic Violet Phosphorus with Large Dichroic Ratio

AbstractAnisotropic 2D layered materials with distinctive anisotropic electronic band structures and optical response are promising for practical applications in polarization imaging and sensors. Nonetheless, many of the currently reported polarized photodetectors based on anisotropic 2D materials are constrained by the low polarization responsivity and low linear dichroism ratio. Here a high‐performance polarization‐sensitive photodetector based on the novel in‐plane anisotropy 2D violet phosphorus (VP) crystal is reported. The angle‐resolved polarized Raman spectroscopy (ARPRS) study and the anisotropic electrical transport measurements revealed the highly in‐plane anisotropy phonon vibration and electrical properties of VP crystal. The anisotropic ratio of electron mobility is calculated to be 2.7 at room temperature. Moreover, a polarization‐sensitive photodetector based on the VP nanosheet shows high photoresponsivity of up to 341 A W−1 and a linearly dichroic ratio of up to 3.9, which is much larger than most other anisotropic 2D materials. These results indicate that the anisotropic VP crystal would be suitable for polarization‐sensitive photodetectors, which leads to a promising perspective for future novel electronic and optoelectronic devices.

Origin of the Anomalous Electrical Transports in Quasi-One-Dimensional Ta2NiSe7.

Exploration of exotic transport behavior is of great interest and importance for revealing the properties of the CDW phase of quasi-one-dimensional Ta2NiSe7. We report the anisotropic electrical transport properties of Ta2NiSe7 single crystals in the CDW phase. The anisotropic constant (γ = ρb/ρc) increased rapidly at TCDW = 60 K upon cooling. The results of the Hall resistivity show that both the concentrations and mobilities of carriers change abruptly at TCDW. The out-of-plane AMR exhibits C2 and C4 symmetry components while the in-plane AMR exhibits C2, C4, and C6 at the CDW state. The planar Hall effect is observed in Ta2NiSe7 at low temperature, which is suggested to originate from the anisotropic orbital magnetoresistance. The calculated results show that the Fermi surface of Ta2NiSe7 was slightly reconstructed due to the CDW transition. This work highlights the enhancement of Fermi surface anisotropy during CDW formation and provides a novel approach to study the CDW materials.

A robust and tunable Luttinger liquid in correlated edge of transition-metal second-order topological insulator Ta2Pd3Te5

The interplay between topology and interaction always plays an important role in condensed matter physics and induces many exotic quantum phases, while rare transition metal layered material (TMLM) has been proved to possess both. Here we report a TMLM Ta2Pd3Te5 has the two-dimensional second-order topology (also a quadrupole topological insulator) with correlated edge states - Luttinger liquid. It is ascribed to the unconventional nature of the mismatch between charge- and atomic- centers induced by a remarkable double-band inversion. This one-dimensional protected edge state preserves the Luttinger liquid behavior with robustness and universality in scale from micro- to macro- size, leading to a significant anisotropic electrical transport through two-dimensional sides of bulk materials. Moreover, the bulk gap can be modulated by the thickness, resulting in an extensive-range phase diagram for Luttinger liquid. These provide an attractive model to study the interaction and quantum phases in correlated topological systems.

Magnetocaloric and hydrogen storage multi-functional properties of Eu4Ga8Ge16 compounds

We investigated the anisotropic magnetic, electrical transport, and heat capacity of Eu4Ga8Ge16 single crystals. Magnetization measurements with temperature and magnetic field revealed a complex behavior. Specifically, we observed anisotropic metamagnetic transitions at low magnetic field, and inverse hysteresis at high magnetic fields. These phenomena can be attributed to a Heisenberg ferromagnetic spin chain aligned along the a-axis. Additionally, this ferromagnetic behavior is interspersed with antiferromagnetic interactions between adjacent Heisenberg spin chains. We also noticed an anomalous increase in electrical resistivity at low temperature under magnetic fields, as well as different temperature-exponents of ρ(T) depending on crystal directions, which are attributed to the anisotropic scattering of carriers due to anisotropic magnetic interactions. We also evaluated the magnetocaloric effect and found a substantial magnetic entropy change than those of type-I Eu-based clathrates. The high entropy change (-ΔSM = 13.5 J kg−1 K−1) and adiabatic temperature difference (ΔTad = 7.8 K) near Néel temperature TN = 7.9 K reveal that the Heisenberg spin chain system might be advantageous for inducing a larger magnetic entropy change. Furthermore, we measured the hydrogen storage capacity of Eu4Ga8Ge16 as a multifunctional property because the zeolite-like cage structure can be a good candidate for hydrogen storage applications. Although the hydrogen storage capacity is considerably lower than that of the leading-edge materials, the hydrogen storage capacity can be improved further when we control the Eu-deficiency to place the hydrogen molecule at the cage site, because the hydrogen molecules reside at the (Ga,Ge) framework not in the cage site for fully occupied Eu atomic chain compounds. Hence, Eu4Ga8Ge16 emerges as a versatile substrate for engineering multifunctional energy materials, excelling in both low-temperature magnetocaloric effects and hydrogen storage potential.

Anisotropic Fermi Surfaces, Electrical Transport, and Two-Dimensional Fermi Liquid Behavior in Layered Ternary Boride MoAlB

We report a study of fermiology, electrical anisotropy, and Fermi liquid properties in the layered ternary boride MoAlB, which could be peeled into two-dimensional (2D) metal borides (MBenes). By studying the quantum oscillations in comprehensive methods of magnetization, magnetothermoelectric power, and torque with the first-principle calculations, we reveal three types of bands in this system, including two 2D-like electronic bands and one complex three-dimensional-like hole band. Meanwhile, a large out-of-plane electrical anisotropy (ρbb /ρaa ∼ 1100 and ρbb /ρcc ∼ 500, at 2 K) was observed, which is similar to those of the typical anisotropic semimetals but lower than those of some semiconductors (up to 105). After calculating the Kadowaki–Woods ratio (KWR = A/γ 2), we observed that the ratio of the in-plane A a,c /γ 2 is closer to the universal trend, whereas the out-of-plane A b /γ 2 severely deviates from the universality. This demonstrates a 2D Fermi liquid behavior. In addition, MoAlB cannot be unified using the modified KWR formula like other layered systems (Sr2RuO4 and MoOCl2). This unique feature necessitates further exploration of the Fermi liquid property of this layered molybdenum compound.

Unveiling the Multiradical Character of the Biphenylene Network and Its Anisotropic Charge Transport.

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics.

Transport behavior and thermoelectric properties of SnSe/SnS heterostructure modulated with asymmetric strain engineering

Strain engineering of two-dimensional materials provides specific regulation method for the crystal structure, electric transport behavior and hence thermoelectric properties. Since the layer components of the van der Waals heterojunction exhibit discrepant response to strains, it provides a platform for manipulation of emergent electronic and thermoelectric properties. Here, motivated by the promising thermoelectric materials SnSe and its analogue, we design a specific high-promising thermoelectric candidate based on SnSe-SnS heterostructures, focusing on the strain induced asymmetric bonding-transition and its effect on thermoelectric properties. The compressed SnS/SnSe hetero-bilayer showssignificantly enhanced anisotropic electrical transport properties, due to depressed carrier scattering rate along the robust weak bonding direction. In this armchair direction, extremely high power factor values (3600 μW/cm·K2 for n-type and 4000 μW/cm·K2 for p-type) are predicted at ∼1021 cm−3 at 700 K. We obtain a new state-of-the-art thermoelectric material with extremely high thermoelectric power factor and pave the way for strain engineering of thermoelectric van der Waals heterostructures with robust in-plane weak bonding.

Direction-Dependent Thermoelectric Properties of a Layered Compound In2Te5 Single Crystal

We examine the anisotropic electrical and thermal transport properties in single crystals of In2Te5 belonging to the monoclinic space group C12/c1 with the temperature gradient applied parallel (||) and perpendicular (\(\perp \) ) to the crystallographic c-axis of the crystals. A systematic investigation of structural, electrical, and thermal properties suggests the role of layered structure in this material in guiding its thermoelectric behavior. The thermal conductivity along the c-axis (κ||c) was found to be smaller by a factor of 2 compared to the thermal conductivity along the direction perpendicular to the c-axis (κ\(_{\perp{c}}) \) over the entire temperature range. In contrast, the Seebeck coefficient along the c-axis (S||c) was found to be higher than its value along the direction perpendicular to the c-axis \((S_{\perp{c}}) \). At room temperature, the figure of merit zT||c is found to be four times larger as compared to the figure of merit \(zT_{{\perp}c} \). Our results provide insights into how the resistivity, thermal conductivity, and thermopower depends on the crystalline anisotropy and its impact on the overall zT.

Anisotropic superconducting properties of FeSe0.5Te0.5 single crystals

We investigated the anisotropic electrical transport and magnetic properties of FeSe0.5Te0.5 single crystals grown by the self-flux method. The in-plane resistivity shows a metallic-like temperature dependence, while the out-of-plane resistivity shows a broad hump with a maximum at around 64 K. The magnetization loops for H//c-axis and H//ab-plane are also different, for example, there is a typical second peak for H//c-axis. The in-plane critical current density is larger than the out-of-plane one. The coherence length and penetration depth were estimated by the Ginzburg–Landau theory. The anisotropic parameter γ depends on the applied magnetic field and the temperature. The coupling of superconducting FeSe(Te) layers and the flux pinning mechanism relevant to anisotropy are also discussed.

Highly oriented NiSi2@Si thin-nanocomposite produced by solid state diffusion: Morphological and crystallographic characterization

This article describes a morphological and crystallographic multi-technique characterization of a 2D-nanocomposite consisting of highly oriented NiSi2 nanoplates endotaxially grown inside a single-crystalline Si(001) wafer close to its external surface. This nanostructured material is prepared using a novel procedure which promotes the diffusion of Ni atoms from a deposited Ni-dopped-SiO2 thin film into the volume of a Si(001) flat wafer under controlled thermodynamic conditions. High Resolution Scanning Transmission Electron Microscopy images reveal the formations of well oriented thin hexagonal nanoplates totally buried inside a Si(001) wafer and randomly oriented nearly spherical Ni nanocrystals located inside the Ni-doped SiO2 thin film and also inside a thin layer close to external surface of the Si(001) wafer. The NiSi2 nanoplates formed by endotaxial growth have their large hexagonal flat surfaces parallel to one of the four Si{111} crystallographic planes and exhibit coherent 7A-type interfaces with the host Si matrix. Additional analyses of Grazing-Incidence Small-Angle X-ray Scattering patterns - which probe a high number of nanoparticles - indicated that the average thickness and maximum diameter of NiSi2 nanoplates are 12 nm and 118 nm, respectively, and the average radius of Ni nanocrystals is 1.7 nm. The described process for obtaining 2D-NiSi2@Si nanocomposites opens new possibilities for exploiting the structural features of these materials for use in devices requiring anisotropic electrical transport properties.

Anisotropic thermal and electrical transport properties induced high thermoelectric performance in an Ir2Cl2O2 monolayer.

In recent years, the energy crisis and global warming have been urgent problems that need to be solved. As is known, thermoelectric (TE) materials can transfer heat energy to electrical energy without air pollution. High-throughput calculations as a novel approach are adopted by screening promising TE materials. In this paper, we use first-principles calculations combined with the semiclassical Boltzmann transport theory to estimate the TE performance of monolayer Ir2Cl2O2 according to the prediction that Ir2Cl2O2 has potential as a good TE material via high-throughput calculations. The low thermal conductivities of 1.73 and 4.68 W mK-1 of Ir2Cl2O2 along the x- and y-axes are calculated, respectively, which exhibits the strong anisotropy caused by the difference in group velocities of low-frequency phonon modes. Then, the electronic transport properties are explored, and the figure of merit ZT is eventually obtained. The maximum ZT value reaches 2.85 (0.40) along the x-axis (y-axis) at 700 K, revealing that the TE properties of the Ir2Cl2O2 monolayer are highly anisotropic. This work reveals that the anisotropic layer Ir2Cl2O2 exhibits high TE performance, which confirms that it is feasible to screen excellent TE materials via high-throughput calculations.

A multi scale multi domain model for large format lithium-ion batteries

A multi scale multi domain (MSMD) model for large format lithium-ion battery (LIB) cells is presented. In our approach the homogenization is performed on two scales (i) from the particulate electrodes to homogenized electrode materials using an extended Newman model and (ii) from individual cell layer materials to a homogenized battery material with anisotropic electrical and thermal transport properties. Both intertwined homogenizations are necessary for considering electrochemical-thermal details related to microstructural and material features of electrode and electrolyte layers at affordable computational costs. Simulation results validate the MSMD model compared to the homogenized Newman model for isothermal cases. The strength of the MSMD model is demonstrated for non-isothermal conditions, namely for a 120 Ah cell discharged with four different cooling concepts: (i) without cooling (ii) with a base plate cooling (iii) with a tab cooling and (iv) with a side cooling. As one result, temperature gradients cause a local peak discharge up to 2.8 C for a global 2 C discharge rate.

Smectic vortex glass

We show that in type-II superconductors a magnetic field applied transversely to correlated columnar disorder, drives a phase transition to a distinct "smectic" vortex glass (SmVG) state. SmVG is characterized by an infinitely anisotropic electrical transport, resistive (dissipationless) for current perpendicular to (along) columnar defects. Its positional order is also quite unusual, long-ranged with true Bragg peaks along columnar defects and logarithmically rough vortex lattice distortions with quasi-Bragg peaks transverse to columnar defects. For low temperatures and sufficiently weak columnar-only disorder, SmVG is a true topologically-ordered "Bragg glass", characterized by a vanishing dislocation density. At sufficiently long scales the residual ever-present point disorder converts this state to a more standard, but highly anisotropic vortex glass.

In-plane anisotropic electronic properties in layered α′-In2Se3

In2Se3 polymorphs have been extensively studied because of their diverse physical properties such as piezoelectricity, photoelectricity, and ferroelectricity, thereby showing plentiful promising applications in integrated electronic devices. These diverse properties are strongly dependent on or affected by their atomic bonding arrangement in the crystal phases. Combining lattice symmetry and local atomic perturbation, we demonstrate a novel layered α′-In2Se3 phase by using the first-principles calculations, which is reconstructed from the inverted tetrahedral bonding configuration by the in-plane displacive middle layer Se atom. The optimized structure of monolayer α′-In2Se3 has triple degenerated atomic configurations with different Se atom orientations. We noted that these degenerated atomic configurations exhibit a moderate switching barrier (about 61 meV/f.u.) between them. To further explore this atom-oriented anisotropic property in α′-In2Se3, the electronic properties were studied with an orthorhombic unit cell. The comparative results for the orthogonal Se atom orientations suggest that the nonbonding orbital coupling of the displacive Se atoms induces large in-plane anisotropic optical absorption and electrical transport properties. This study of the layered α′-In2Se3 phase can extend the realm of switchable anisotropic optoelectronic applications in future electronic devices.

Role of grain alignment and oxide impurity in thermoelectric properties of textured n-type Bi–Te–Se alloy

A nanostructured n-type Bi2Te2.7Se0.3 (BTS) alloy with a unique microstructure was prepared using a facile melting-rotation-quenching process followed by ball-milling and uniaxial hot-press sintering at 623 K. Anisotropy in the resulting microstructure showed anisotropic electrical and thermal transport properties in two directions normal to the pressing axis. The texture of the nanostructured BTS alloy was analyzed by x-ray diffraction and scanning electron microscopy. Based on the geometric phase analysis of a high resolution transmission electron microscopy images, abundant dislocations, high grain boundary density, and oxide impurity were identified, which act as phonon scattering centers. Higher anisotropy in thermal conductivity combined with oxide impurity resulted in an ultra-low phonon thermal conductivity of ∼0.305 W mK−1 at 423 K in the nanostructured n-type BTS in the direction parallel to the pressing axis. Laser power- and temperature-dependent Raman spectra analyses provided a deeper insight into the anisotropy in thermal transport properties. Optimum power factor and low thermal conductivity, due to the combination of grain alignment and oxide impurity, resulted in a dimensionless figure of merit (zT ) value of ∼0.75 at 423 K. In comparison, the high and opposite temperature dependences of electrical conductivity and thermal conductivity led to a better average zT value of ∼0.68 and a thermoelectric energy conversion efficiency percentage of ∼4.4% in the operating temperature range (300–423 K) in the direction parallel to the pressing axis.

Highly In‐Plane Anisotropic 2D PdSe2 for Polarized Photodetection with Orientation Selectivity

AbstractPolarized photodetection based on anisotropic two‐dimensional materials display promising prospects for practical application in optical communication and optoelectronic fields. However, most of the reported polarized photodetection are limited by the lack of valid tunable strategy and low linear dichroism ratio. A peculiar noble metal dichalcogenide—PdSe2 with a puckered pentagonal structure and abnormal linear dichroism conversion—potentially removes these restrictions and is demonstrated in this study. Herein, azimuth‐dependent reflectance difference microscopy combined with anisotropic electrical transport measurements indicate strong in‐plane anisotropic optical and electrical properties of two‐dimensional PdSe2. Remarkably, the typical polarization‐resolved photodetection exhibits anisotropic photodetection characteristics with a dichroic ratio up to ≈1.8 at 532 nm and ≈2.2 at 369 nm, and their dominant polarization orientation differs by 90° corresponding to the a‐axis and b‐axis, respectively. The unique orientation selection behavior in polarization‐dependent photodetection can be attributed to the intrinsic linear dichroism conversion. The results make 2D PdSe2 a promising platform for investigating anisotropic structure–property correlations and integrated optical applications for novel polarization‐sensitive photodetection.

Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.

Geometrical Structure Optimization Design of High-Performance Bi2Te3-Based Artificially Tilted Multilayer Thermoelectric Devices

Artificially tilted multilayer thermoelectric devices (ATMTDs) are a kind of thermoelectric device that can directly convert heat into electricity based on transverse thermoelectric effect. Although the devices have a simplified multilayer structure, their geometrical optimization is a complicated task. In this work, n-type Bi2Te2.7Se0.3 and p-type Bi0.1Sb1.9Te3 materials, which have the most important commercial applications in conventional thermoelectric devices, were selected as the component materials to assemble a promising high-performance Bi2Te3-based ATMTD. A numerical analysis method was employed to optimize the device length, device width, and thickness of the component materials. The results revealed that a large device width/length ratio, a small device aspect ratio, and a small thickness of component materials are favorable for achieving a high conversion efficiency. The temperature and charge distributions inside the ATMTD are studied based on finite element simulation. The nonuniform distribution of temperature field inside the device strongly depends on the thermal conductivity of component materials. The accumulation of a transverse electric field, accompanied with the cancellation of longitudinal electric field, is a consequence of different electric field distributions in the two component materials. This work provides a better understanding on the anisotropic electrical and thermal transport behaviors in transverse thermoelectric devices.

Recent progress of two‐dimensional lead halide perovskite single crystals: Crystal growth, physical properties, and device applications

AbstractIn recent years, research on organic‐inorganic lead halide two‐dimensional (2D) perovskites has been blossoming. 2D perovskites have completely different layered structures from three‐dimensional perovskites, with interleaved organic and inorganic layers, leading to 2D perovskite materials being more stable and possessing anisotropic electrical transport. Meanwhile, 2D organic‐inorganic lead halide perovskite single crystals are receiving increasing attention because of their remarkable properties, such as long charge carrier lifetime, low defect density, and high photoluminescence quantum yield, increasing their potential for optoelectronic applications. Previously, a series of material systems based 2D perovskites single crystals has been synthesized and applied in various device applications. In this review, the preparation methods of organic‐inorganic lead halide 2D perovskite single crystals, the growth principles, their photoelectric properties and several device applications are discussed.image