Anisotropic Electrical Transport Research Articles

Enhancing the Thermoelectric Properties of Misfit Layered Sulfides (MS)1.2+q (NbS2) n (M = Gd and Dy) through Structural Evolution and Compositional Tuning.

The misfit monolayered sulfides, (GdS)1.20NbS2, (DyS)1.22NbS2, (Gd0.1Dy0.9S)1.21NbS2, (Gd0.2Dy0.8S)1.21NbS2, and (Gd0.5Dy0.5S)1.21NbS2 and the misfit bilayered sulfide (GdS)0.60NbS2 were synthesized via sulfurization under flowing CS2/H2S gas and consolidated by pressure-assisted sintering. The thermoelectric properties of the monolayered and bilayered sulfides perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction were investigated over a temperature range of 300–873 K. The crystal grains in all the sintered samples were preferentially oriented perpendicular to the pressing direction, which resulted in highly anisotropic electrical and thermal transport properties. All the sintered samples exhibited degenerate n-type semiconductor-like behavior, leading to a large thermoelectric power factor. The misfit layered structure yielded low lattice thermal conductivity. The evolution of the monolayered structures into bilayered structures affected their thermoelectric properties. The thermoelectric figure of merit (ZT) of monolayered (GdS)1.20NbS2 was higher than that of bilayered (GdS)0.60NbS2 due to the larger power factor and lower lattice thermal conductivity of (GdS)1.20NbS2. The lattice thermal conductivity of the monolayered sulfide was lower in (GdxDy1-xS)1.2+qNbS2 solid solutions. The large power factor and low lattice thermal conductivity allowed a ZT value of 0.13 at 873 K in (Gd0.5Dy0.5S)1.21NbS2 perpendicular to the pressing direction.

Complementary doping of van der Waals materials through controlled intercalation for monolithically integrated electronics

Doping control has been a key challenge for electronic applications of van der Waals materials. Here, we demonstrate complementary doping of black phosphorus using controlled ionic intercalation to achieve monolithic building elements. We characterize the anisotropic electrical transport as a function of ion concentrations and report a widely tunable resistivity up to three orders of magnitude with characteristic concentration dependence corresponding to phase transitions during intercalation. As a further step, we develop both p-type and n-type field effect transistors as well as electrical diodes with high device stability and performance. In addition, enhanced charge mobility from 380 to 820 cm2/(V·s) with the intercalation process is observed and explained as the suppressed neutral impurity scattering based on our ab initio calculations. Our study provides a unique approach to atomically control the electrical properties of van der Waals materials, and may open up new opportunities in developing advanced electronics and physics platforms.

Anisotropic Electrical, Magnetic, and Thermal Properties of In3Ni2 Intermetallic Catalyst

We present the anisotropic electrical and thermal transport coefficients (electrical resistivity, magnetoresistance, thermoelectric power, thermal conductivity), the magnetic properties, the specific heat and the electronic density of states of a monocrystalline In3Ni2 intermetallic compound, representing a precious‐metal‐free (and noble‐metal‐free) intermetallic catalyst for the selective hydrogenation of α,β‐unsaturated aldehydes. The investigated physical parameters were determined along three orthogonal crystal‐symmetry directions of the trigonal structure, the twofold axis, the 3 axis and within the mirror plane. All the investigated tensorial and vectorial quantities show the same anisotropy, with the quantities being isotropic for the twofold direction and in the mirror plane, whereas there is small, though still significant anisotropy to the 3 direction. The In3Ni2 crystal conducts the electricity and heat somewhat less efficiently along the 3 direction than along the twofold direction and in the mirror plane, but the differences are not large, of about 20 %. In3Ni2 is a diamagnetic intermetallic compound, with a presumably simple Fermi surface and electrons as the majority charge carriers.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects.

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Large anomalous Hall effect in ferromagnetic Weyl semimetal candidate PrAlGe

We present the structure and anisotropic magnetic and electrical transport properties on the flux-grown PrAlGe single crystal, a recently proposed magnetic Weyl semimetal candidate. From the powder and single-crystal x-ray diffraction analysis, the grown crystal is revealed to crystallize in tetragonal LaPtSi-type structure with space group of I41md. The PrAlGe exhibits strong Ising-type magnetic anisotropy with ferromagnetic moments ∼2.32μB/Pr along easy c-axis below transition temperature Tc ∼ 15 K. Accordingly, anomalous Hall effect (AHE) is observed for field (H) along c axis in contrast to a axis, and large anomalous Hall conductivity value reaches up to ∼680 Ω−1 cm−1 close to the theoretical expected value based on intrinsic Berry-curvature mechanism. Moreover, the linear scaling behaviors between the anomalous Hall resistivity and longitudinal resistivity also support the intrinsic Karplus-Luttinger mechanism as a dominant role on the observed AHE rather than extrinsic scattering mechanism.

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi2Te3, we prepared Bi2Te3 nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi2Te3 samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3 μW cm−1 K−2 at 300 K) than that along the out-of-plane direction (8.1 μW cm−1 K−2 at 300 K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440 K for the nano-bulk n-type Bi2Te3 sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi2Te3 nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.

Microstructure and Electrical Transport Properties of Bi3TiNbO9 High-Temperature Piezoceramics

High-density ceramic samples of the Bi3TiNbO9 ferroelectric phase with a layered perovskite-like structure have been produced by tape casting slurry technology and hot pressing processes. The samples have been characterized by X-ray diffraction and dielectric, piezoelectric, and pyroelectric measurements. The results demonstrate that the samples have a well-defined texture due to the preferential orientation of the basal planes of their platelike grains across the pressing axis or along the film casting direction. The Curie temperature of the samples has been determined to be TC = 1180 K. The samples have anisotropic electrical transport characteristics. Their piezoelectric constants d33 in directions perpendicular and parallel to the texture plane is 22 and 5 pC/N, respectively. The conclusion has been drawn that the synthesized materials are potentially attractive for producing high-temperature piezoelectric elements.

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

AbstractAnisotropic 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.

Anisotropic electrical, thermal and magnetic properties of Al13Ru4 decagonal quasicrystalline approximant

Abstract We present measurements of the anisotropic electrical and thermal transport coefficients (the electrical resistivity, the thermoelectric power, the thermal conductivity), the magnetization and the specific heat of the Al13Ru4 monoclinic approximant to the decagonal quasicrystal, in comparison to the isostructural Al13Fe4. The electrical and thermal transport parameters of Al13Ru4 were found to exhibit significant anisotropy, qualitatively similar to that found previously in the Al13Fe4 (P. Popčević, et al., Phys. Rev. B 2010, 81, 184203). The crystallographic b direction, corresponding to the stacking direction of the (a,c) atomic planes, is the most conducting direction for the electricity and heat. The thermopower is strongly anisotropic with a complicated temperature dependence, exhibiting maxima, minima, crossovers and sign change. The electronic density of states (DOS) at the Fermi energy is reduced to 35% of the DOS of Al metal. The magnetic susceptibility is diamagnetic and the diamagnetism is by a factor of 2 stronger for the magnetic field along the stacking b direction.

Quasi-two-dimensional massless Dirac fermions in CaMnSb2

We report a study of the magnetic and magnetotransport properties of layered transition-metal pnictide ${\mathrm{CaMnSb}}_{2}$. ${\mathrm{CaMnSb}}_{2}$ is a quasi-two-dimensional system with an orthorhombic crystal structure, containing two distinct layers of a distorted Sb square sheet and an edge-sharing ${\mathrm{MnSb}}_{4}$ tetrahedron. It was found that ${\mathrm{CaMnSb}}_{2}$ orders antiferromagnetically below ${T}_{\mathrm{N}}$ = 302 K and exhibits the expected highly anisotropic magnetic and electrical transport properties. At low temperatures and high fields, we observed quantum oscillations in both magnetization and resistivity, which reveal the multiband Fermi surfaces with very small cross-sectional areas. Nonzero Berry phase, very small cyclotron mass, and high carrier mobility indicate the existence of nearly massless Dirac fermions in ${\mathrm{CaMnSb}}_{2}$.

Thermal and electronic transport characteristics of highly stretchable graphene kirigami.

For centuries, cutting and folding papers with special patterns have been used to build beautiful, flexible and complex three-dimensional structures. Inspired by the old idea of kirigami (paper cutting), and the outstanding properties of graphene, recently graphene kirigami structures were fabricated to enhance the stretchability of graphene. However, the possibility of further tuning the electronic and thermal transport along the 2D kirigami structures has remained original to investigate. We therefore performed extensive atomistic simulations to explore the electronic, heat and load transfer along various graphene kirigami structures. The mechanical response and thermal transport were explored using classical molecular dynamics simulations. We then used a real-space Kubo-Greenwood formalism to investigate the charge transport characteristics in graphene kirigami. Our results reveal that graphene kirigami structures present highly anisotropic thermal and electrical transport. Interestingly, we show the possibility of tuning the thermal conductivity of graphene by four orders of magnitude. Moreover, we discuss the engineering of kirigami patterns to further enhance their stretchability by more than 10 times as compared with pristine graphene. Our study not only provides a general understanding concerning the engineering of electronic, thermal and mechanical response of graphene, but more importantly can also be useful to guide future studies with respect to the synthesis of other 2D material kirigami structures, to reach highly flexible and stretchable nanostructures with finely tunable electronic and thermal properties.

Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However, the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows a significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculations. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with a reduced thickness. A low-temperature Hall experiment sheds light on more intrinsic anisotropic electrical transport, with a hole mobility of 3000 and 1500 cm2/V·s along the a-axis and c-axis, respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with the highest electron mobility up to 44 000 cm2/V·s.

Interplay between nanolaminated structure and electron-phonon coupling in Ti-based MAX phases

A linear-response method to the density functional theory is used to derive lattice dynamics, the transport spectral function, and the electron-phonon coupling (EPC) constant of $\mathrm{T}{\mathrm{i}}_{2}\mathrm{AlC}$, a member of the very large class of nanolaminated conducting ceramics named MAX phases (where $M$ is a transition metal, $A$ is an element from groups IIIA to VIA, and $X$ is carbon and/or nitrogen). By coupling ab initio calculations with the semiclassical Boltzmann transport theory for electron-phonon scattering, the experimentally observed anisotropic electrical transport properties of $\mathrm{T}{\mathrm{i}}_{2}\mathrm{AlC}$ are rationalized. Our results indicate that in $\mathrm{T}{\mathrm{i}}_{2}\mathrm{AlC}$, because of the weak dependence of the EPC constant $\phantom{\rule{4pt}{0ex}}{\ensuremath{\lambda}}_{\mathrm{tr},\ensuremath{\alpha}}$ $(\ensuremath{\alpha}=xx$ and $zz)$ on the crystallographic direction, the anisotropy of $\ensuremath{\rho}(T)$ results from the anisotropy of the Fermi surface. These conclusions, in contrast with those obtained on $\mathrm{T}{\mathrm{i}}_{3}\mathrm{Si}{\mathrm{C}}_{2}$ (another member of the MAX phases family) using a similar approach, establish a correlation between the nanolaminated structure of the MAX phases and the origin of the anisotropy of their transport properties.

Anisotropic electrical and lattice transport properties of ordered quaternary phases Cr2TiAlC2 and Mo2TiAlC2: A first principles study

Electrical conductivities of Cr2TiAlC2 and Mo2TiAlC2 in a and c directions are calculated from semi-classic Boltzmann transport theory. The values are found to be σa=5.68×105 S/m (6.56×105 S/m) and σc=2.15×105 S/m (2.69×105 S/m) for Cr2TiAlC2 (Mo2TiAlC2) at 300 K. Using the phonon-mode Debye temperature and Slack-model, the lattice thermal conductivities in the two directions are also evaluated, and the values are κa=18.71 W/m K (16.11 W/m K) and κc=0.48 W/m K(0.25 W/m K) for Cr2TiAlC2 (Mo2TiAlC2) at room temperature. The anisotropy in lattice thermal conductivity is found to be stronger than that of electrical conductivity. The predicted Seebeck coefficients and thermoelectric figure of merit (ZT) indicate that they are poor thermoelectric materials. Due to the relatively high conductivities, they might be used to fabricate high temperature conductive components in aerospace industry. In addition, our results in a direction have the direct implications for the relevant properties of MXenes (Cr2TiC2 and Mo2TiC2), produced from their bulk phases.

Self-Assembled Magnetic Metallic Nanopillars in Ceramic Matrix with Anisotropic Magnetic and Electrical Transport Properties.

Ordered arrays of metallic nanopillars embedded in a ceramic matrix have recently attracted considerable interest for their multifunctionality in advanced devices. A number of hurdles need to be overcome for achieving practical devices, including selections of metal-ceramic combination, creation of tunable and ordered structure, and control of strain state. In this article, we demonstrate major advances to create such a fine nanoscale structure, i.e., epitaxial self-assembled vertically aligned metal-ceramic composite, in one-step growth using pulsed laser deposition. Tunable diameter and spacing of the nanopillars can be achieved by controlling the growth parameters such as deposition temperature. The magnetic metal-ceramic composite thin films demonstrate uniaxial anisotropic magnetic properties and enhanced coercivity compared to that of bulk metal. The system also presents unique anisotropic electrical transport properties under in-plane and out-of-plane directions. This work paves a new avenue to fabricate epitaxial metal-ceramic nanocomposites, which can simulate broader future explorations in nanocomposites with novel magnetic, optical, electrical, and catalytical properties.

Electronic and Structural Properties of N-Type Microcrystalline Silicon Oxide (Mc-Siox:H) Films for Applications in Thin Film Silicon Solar Cells

The growth of n-type μc-SiOx:H layers in the thickness range of up to 700nm is investigated. We show that the electrical and structural properties of μc-SiOx:H are thickness dependent but to a weaker extend in comparison with n-type μc-Si:H. μc-SiOx:H layers with low dark conductivity in the order of 10-11 S/cm, measured in the planar direction, can be successfully used as n-layers in amorphous silicon solar cells, resulting in an improved cell performance due to reduced parasitic absorption in the n-type layer. The results suggest anisotropic electrical transport in μc-SiOx:H materials, supported by an observation of elongated crystalline regions in high resolution TEM microscopy.

Structural Complexity Meets Transport and Magnetic Anisotropy in Single Crystalline Ln30Ru4Sn31 (Ln = Gd, Dy)

We present the structure of Ln(30)Ru(4+x)Sn(31-y) (Ln = Gd, Dy) and the anisotropic resistivity, magnetization, thermopower, and thermal conductivity of single crystal Ln(30)Ru(4+x)Sn(31-y) (Ln = Gd, Tb). Gd(30)Ru(4.92)Sn(30.54) crystallizes in a new structure-type with space group Pnnm and dimensions of a = 11.784(1) Å, b = 24.717(1) Å, and c = 11.651(2) Å, and V = 3394(1) Å(3). Magnetic anisotropy and highly anisotropic electrical transport behavior were observed in the single crystals of Gd(30)Ru(4.92)Sn(30.54) and Tb(30)Ru(6)Sn(29.5). Additionally, the lattice thermal conductivity of Tb(30)Ru(6)Sn(29.5) is quite low, and a comparison is made to other Sn-containing compounds.

Super-long barnesite Na2V6O16·3H2O nanobelts for aligned film electrodes with enhanced anisotropic electrical transport

Developing a novel, aligned super-long nanobelt film electrode is essential for achieving outstanding electrical transport properties. However, pursuing a convenient way to fabricate an aligned super-long nanobelt film electrode has been a challenge so far. Herein, the aligned super-long barnesite Na2V6O16·3H2O nanobelt film electrode is realized as a dense and thick film for the first time by a convenient solution route, providing a new strategy to synthesize aligned nanobelt film electrodes. The Na2V6O16·3H2O nanobelts with lengths of 100–200 μm growing along [010] are fabricated successfully, and the super-long single nanobelt enables the first investigation of electrical transport property. Encouragingly, the individual nanobelt with aspect ratios exceeding 1000 has a high conductivity of ∼100 S m−1. Furthermore, the synthetic aligned super-long barnesite Na2V6O16·3H2O nanobelt film electrode displays an enhanced anisotropic electrical transport property and good electrical conductivity, presenting an ideal candidate as a nanoelectrode.

Anisotropic Electrical Transport Properties of Graphene Nanoplatelets/Pyrene Composites by Electric-Field-Assisted Thermal Annealing

Dispersion of graphene nanoplatelets (GNPs) with a pyrene derivative compound (1,3,6,8-tetrakis(4-t-butylphenyl)pyrene (TBPP)) in tetrahydrofuran as solvent medium is investigated. Field-emission electron microscopy and atomic force microscopy demonstrated the π–π stacking interaction between the TBPP compound and the GNPs resulting in the deposition of the TBPP compound onto the exfoliated graphene sheets. We found that the electrical conductivity of the films obtained by the prepared dispersions can be activated in temperature and oriented by the application of an external electric field. We found an increase of the composite conductivity measured along the substrate with respect to the composite conductivity measured perpendicular to the substrate. This result provides an initial understanding of how electric fields can be used to control the bulk physical properties of such composites.

Anisotropic electrical transport properties of a two-dimensional electron gas at SrTiO3–LaAlO3 interfaces

Experimental evidence of strong in-plane anisotropy in electrical properties of the confined electron gas at the SrTiO3–LaAlO3 interface on top of (LaAlO3)0.3(Sr2AlTaO3)0.7 substrates is provided by detailed transport measurements. Structured measurement geometries in multiple directions are used to show dependence of the sheet resistance with the in-plane angle θ, which is fitted with a sine function with a period of 180°. The carrier density remains constant and a directional dependence of the carrier mobility of more than one order of magnitude is determined with respect to the orientation of the unit cell height steps present at the SrTiO3–LaAlO3 interface.