Anisotropic Charge Transport Research Articles

Positive and negative magnetoresistance and charge transport anisotropy in RB12 (R - Ho, Er, Tm) antiferromagnets with dynamic charge stripes

Magnetization M and magnetoresistance Δρρ(H,T) (MR) were studied and MR analyzed quantitatively in the complicated low temperature Néel phase of RB12 (R - Ho, Er and Tm) antiferromagnets with structural (originating from cooperative Jahn-Teller effect) and electronic (coming from dynamic charge stripes) instabilities. It is shown, that well below the Néel field (HN) the magnetoresistance is determined by the concurrence of carriers' scattering by spin-density waves (leading to a linear positive contribution Δρρ(H) ∼ H) from one side, and by ferromagnetic nanoscale clusters (leading to a linear negative MR) from the other. The development of different kind instabilities in the critical region just below HN results into the emergence of an additional negative quadratic MR term, attributed to the carriers’ scattering on 4f-5d local electron density fluctuations. The anisotropy of both two linear and one quadratic MR components is analyzed in detail and compared for RB12 compounds with different magnetic structure.

Highly Sensitive Direct X-Ray Detector Based on Copper(II) Coordination Polymer Single Crystal with Anisotropic Charge Transport.

Anisotropic charge transport plays a pivotal role in clarifying the conductivity mechanism in direct X-ray detection to improve the detection sensitivity. However, the anisotropic photoelectric effect of semiconductive single crystal responsive to X-ray is still lacking of theoretical and experimental proof. The semiconductive coordination polymers (CPs) with designable structures, adjustable functions, and high crystallinity provide a suitable platform for exploring the anisotropic conductive mechanism. Here,the study first reveals a 1D conductive transmission path for direct X-ray detection from the perspective of structural chemistry. The semiconductive copper(II)-based CP 1 single crystal detector exhibits unique anisotropic X-ray detection performance. Along the 1D π-π stacking direction, the single crystal device (1-SC-a) shows a superior sensitivity of 2697.15µCGyair -1 cm-2 and a low detection limit of 1.02µGyair s-1 among CPs-based X-ray detectors. This study provides beneficial guidance and deep insight for designing high-performance CP-based X-ray detectors.

Photocatalytic Cascade Reaction Driven by Directed Charge Transfer over VS -Zn0.5 Cd0.5 S/GO for Controllable Benzyl Oxidation.

Photocatalysis is an important technique for synthetic transformations. However, little attention has been paid to light-driven synergistic redox reactions for directed synthesis. Herein, the authors report tunable oxidation of benzyl to phenylcarbinol with the modest yield (47%) in 5h via singlet oxygen (1 O2 ) and proton-coupled electron transfer (PCET) over the photocatalyst Zn0.5 Cd0.5 S (ZCS)/graphene oxide (GO) under exceptionally mild conditions. Theoretical calculations indicate that the presence of S vacancies on the surface of ZCS/GO photocatalyst is crucial for the adsorption and activation of O2 , successively generating the superoxide radical (• O2 - ) and 1 O2 , attributing to the regulation of local electron density on the surface of ZCS/GO and photogenerated holes (h+ ). Meanwhile, accelerated transfer of photogenerated electrons (e- ) to GO caused by the π-π stacking effect is conducive to the subsequent aldehyde hydrogenation to benzyl alcohol rather than non-selective oxidation of aldehyde to carboxylic acid. Anisotropic charge transport driven by the built-in electric field can further promote the separation of e- and h+ for multistep reactions. Promisingly, one-pot photocatalytic conversion of p-xylene to 4-methylbenzyl alcohol is beneficial for reducing the harmful effects of aromatics on human health. Furthermore, this study provides novel insights into the design of photocatalysts for cascade reactions.

Highly Efficient Mixed Conduction in a Fused Oligomer n-Type Organic Semiconductor Enabled by 3D Transport Pathways.

Tailoring organic semiconductors to facilitate mixed conduction of ionic and electronic charges when interfaced with an aqueous media has spurred many recent advances in organic bioelectronics. The field is still restricted, however, by very few n-type (electron-transporting) organic semiconductors with adequate performance metrics. Here, a new electron-deficient, fused polycyclic aromatic system, TNR, is reported with excellent n-type mixed conduction properties including a µC* figure-of-merit value exceeding 30 F cm-1 V-1 s-1 for the best performing derivative. Comprising three naphthalene bis-isatin moieties, this new molecular design builds on successful small-molecule mixed conductors; by extending the molecular scaffold into the oligomer domain, good film-forming properties, strong π-π interactions, and consequently excellent charge-transport properties are obtained. Through judicious optimization of the side chains, the linear oligoether and branched alkyl chain derivative bgTNR is obtained which shows superior mixed conduction in an organic electrochemical transistor configuration including an electron mobility around 0.3 cm2 V-1 s-1 . By optimizing the side chains, the dominant molecular packing can be changed from a preferential edge-on orientation (with high charge-transport anisotropy) to an oblique orientation that can support 3D transport pathways which in turn ensure highly efficient mixed conduction properties across the bulk semiconductor film.

Angle-resolved magnetoresistance in the strongly anisotropic quantum magnet TmB4

Precise angle-resolved magnetoresistance (ARMR) measurements in various magnetic fields enabled us to create illustrative distributions of $\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}/\ensuremath{\rho}(\ensuremath{\varphi},\phantom{\rule{0.16em}{0ex}}H)$ in $\mathrm{Tm}{\mathrm{B}}_{4}$, where $\ensuremath{\varphi}$ is the angle between the sample $c$ axis and applied magnetic field $H$. These distributions reveal the charge transport anisotropy in this strongly Ising anisotropic quantum antiferromagnet with a geometrically frustrated Shastry-Sutherland lattice exhibiting fractional magnetization plateaus. While in the paramagnetic region $\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}/\ensuremath{\rho}(\ensuremath{\varphi},\phantom{\rule{0.16em}{0ex}}H)$ reaches its maxima for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}c$, below the N\'eel temperature ${T}_{N}=11.7$ K the situation is different. Here the main MR features appear for $H\ensuremath{\parallel}c$, i.e., along the easy axis of magnetic anisotropy, and correspond to magnetic phases and phase transitions between them. It is interesting that all the above features (maxima) related with the scattering of conduction electrons on spin magnetic structure are related with fractional magnetization plateaus. With increasing $\ensuremath{\varphi}$ MR anomalies shift to higher fields. Above the field of magnetic saturation, moreover, significant MR maxima have been observed at certain angles which correspond to specific directions in the crystal lattice, pointing to field directions in which the scattering of conduction electrons on the magnetic structure is the highest. Thus, ARMR appears to be a sensitive experimental tool reflecting the angular dependence of the interplay between charge carriers and magnetic structure as a function of temperature and applied magnetic field.

Maltese cross-type magnetic phase diagrams in Tm1-xYbxB12 antiferromagnets with Yb-valence instability and dynamic charge stripes

Precise X-ray diffraction, heat capacity, magnetoresistance and magnetization measurements have been carried out on high quality Tm1-xYbxB12 single crystals with the goal to reconstruct the H-T and angular H-φ magnetic phase diagrams in the (110) plane of these antiferromagnets (AF) with dynamic charge stripes and Yb-ion valence instability. The analysis developed here allowed us to conclude in favor of essential changes in the filamentary structure of fluctuating charges, and the emergence and variation of stripe-induced magnetic and charge transport anisotropy that are controlled by lowering the temperature and ytterbium doping. It was found that local charge and spin fluctuations on Yb-sites suppress strongly the complicated AF state. However, the magnetic anisotropy is conserved, and the only moderate modifications of the Maltese Cross –type magnetic phase diagrams are detected in the range x < 0.2. We argue that the AF ordering of Tm3+ magnetic moments is the main factor, which determines the anisotropy in the Néel phase of Tm1-xYbxB12 with carrier-mediated magnetic indirect RKKY exchange renormalized significantly by quantum fluctuations of the electron density along 〈110〉 directions.PACS: 73.22.-f, 75.47.-m, 71.27. + a.

Anisotropic Charge Transport in Cu(In,Ga)Se2 by Heavy Alkali Postdeposition Treatment for Reducing Cell‐to‐Module Efficiency Loss in Monolithically Integrated Photovoltaic Modules

The recent efficiency boosting of Cu(In,Ga)Se2 (CIGS) solar cells is undoubtedly triggered by heavy alkali postdeposition treatments (PDTs). However, the effects are not obvious under monolithically integrated CIGS modules where various current‐shunting sources can deteriorate the device performance. Herein, It is reported that KF PDT can effectively suppress the major shunting sources caused by P1 and P3 laser scribing for monolithic interconnection, reducing the cell‐to‐module (CTM) efficiency gap in CIGS photovoltaics. CIGS with NaF PDT exhibits nearly isotropic and high hole mobilities, causing a large CTM efficiency loss. CIGS with additional KF PDT, on the other hand, reveals much lower in‐plane hole mobility than the out‐of‐plane component, significantly increasing the P1 shunt resistance without exacerbating the photocarrier extraction in the active area. It is suggested that such anisotropic charge transport is due to carrier scattering by low‐conductivity phases at the CIGS grain boundaries. Furthermore, passivation of the front junction by KF PDT raises the tolerance to P3 scribing‐induced damage, increasing the P3 shunt resistance while preserving the junction property unlike the NaF PDT case. The work implies that the recent trend of employing heavy alkali PDTs for a high‐efficiency cell is also crucial for designing a high‐efficiency CIGS module.

Bulk Physical Properties of a Magnetic Weyl Semimetal Candidate NdAlGe Grown by a Laser Floating-Zone Method

In this study, we report the successful growth of single crystals of a magnetic Weyl semimetal candidate NdAlGe with the space group I41md. The crystals were grown using a floating-zone technique, which used five laser diodes, with a total power of 2 kW, as the heat source. To ensure that the molten zone was stably formed during the growth, we employed a bell-shaped distribution profile of the vertical irradiation intensity. After the nominal powder, crushed from an arc-melted ingot, was shaped under hydrostatic pressure, we sintered the feed and seed rods in an Ar atmosphere under ultra-low oxygen partial pressure (&lt;10−26 atm) generated by an oxygen pump made of yttria-stabilized zirconia heated at 873 K. Single crystals of NdAlGe were successfully grown to a length of 50 mm. The grown crystals showed magnetic order in bulk at 13.5 K. The fundamental physical properties were characterized by magnetic susceptibility, magnetization, specific heat, thermal expansion, and electrical resistivity measurements. This study demonstrates that the magnetic order induces anisotropic magnetoelasticity, magneto-entropy, and charge transport in NdAlGe.

A chiral SrSi2 (srs) superstructure constructed by a dual interaction system showing isotropic electrical conductivity

Most porous conductive frameworks are highly anisotropic in their structures thus leading to anisotropic charge transport. Here we report a supramolecular self-assembly which is constructed by intermolecular hydrogen bonding and π···π interactions. This material features a chiral, porous, cubic framework structure with π-stacked helical columns along all of the three Cartesian coordinates. As a result, isotropic charge transport with an electrical conductivity (σ) of 2.1 × 10–7 S/cm is achieved. By achieving isotropic charge transport in a π-stacked supramolecular assembly, these results provide a new type of isotropic conductive framework materials alternative to conductive metal-organic frameworks (MOFs).

Synergistic Effect of Sr-O Divacancy and Exposing Facets in SrTiO3 Micro/Nano Particle: Accelerating Exciton Formation and Splitting, Highly Efficient Co2+ Photooxidation.

As a typical perovskite-type crystal, polyhedral strontium titanate (SrTiO3 ) has shown anisotropic charge transport behavior in recent studies, however, the carrier transportation and transition of which has not been explained very clearly. This work presentthe existence of Sr and O divacancies in the novel rhombicuboctahedron SrTiO3 micro/nano particles (Sr1- x TiO3- x /TiO2- x ) with exposing (100), (110) and (111) facets and the diameter of 300-700nm synthesized via hydrothermal synthesis, and also summarizes the dissociation mechanism of self-trapped excitons (STEs) caused by the divacancy and facet effect. In addition, most importantly, the metastable STEs with ultra-low binding energy (Eb < 3 meV) under illumination are discovered. Combining the model of S-scheme heterojunction, a conversion mechanism of photoinduced carriers is proposed. The photocatalytic reaction of Co2+ is used as the probe reaction, and the unique Sr1- x TiO3- x /TiO2- x possesses a high photooxidation efficiency of Co2+ , by which 70.3% of Co2+ is oxidized to Co3+ (CoOOH) in 5min. This finding may provide a guideline for an optimal design of the photocatalytic materials for the recovery and extraction of metal ions based on SrTiO3 .

Nanostructural Analysis of SrTiO3:Al Photocatalyst Dispersed with Pt/Cr2O3/CoOOH Cocatalysts by Electron Microscopy

The distribution of cocatalysts on a water splitting photocatalyst was investigated using electron microscopy combined with energy dispersive X-ray spectroscopy. Pt, Cr, and Co cocatalyst components were dispersed on the surface of an Al-doped SrTiO3 photocatalyst by stepwise photodeposition. It was found that the Pt/Cr2O3 cocatalyst for hydrogen evolution and the CoOOH cocatalyst for oxygen evolution were selectively deposited on different crystal facets, indicating anisotropic charge transport.

Chain Alignment and Charge Transport Anisotropy in Blade-Coated P(NDI2OD-T2)/PS Blend Films

Chain Alignment and Charge Transport Anisotropy in Blade-Coated P(NDI2OD-T2)/PS Blend Films

Perfectness of the main-chain alignment in the conjugated polymer films prepared by the floating film transfer method

Polymer semiconductor/gate dielectric interface is well known to influence the charge transport behaviors in organic field-effect transistors (OFETs). In the unidirectional floating-film transfer method (UFTM), thin floating films of uniaxially aligned π-conjugated polymers are solidified at the air/liquid interface before transferring them onto device substrates. Therefore, packing structures in the films prepared by UFTM are not affected by the strong interactions with the device substrate as usually seen in other film growth methods. Here, one question arises: will the difference in interactions between the liquid substrate and air impact the orientational structure of the polymer chain near the two surfaces or not? To answer this question, we have studied the charge-transport anisotropy in top- and bottom-gate OFETs by changing the film thickness. As a result, the field-effect carrier mobility and its anisotropy did not significantly differ for both surfaces and were constant against the film thickness. These results indicate that the UFTM can provide unidirectional alignment of polymer chains throughout the film thickness and gives a similar superior performance either for a bottom- or a top-gate OFET.

Alignment of Organic Conjugated Molecules for High-Performance Device Applications.

High-performance organic semiconductor materials as the electroactive components of optoelectronic devices have attracted much attention and made them ideal candidates for solution-processable, large-area, and low-cost flexible electronics. Especially, organic field-effect transistors (OFETs) based on conjugated semiconductor materials have experienced stunning progress in device performance. To make these materials economically viable, comprehensive knowledge of charge transport mechanisms is required. The alignment of organic conjugated molecules in the active layer is vital to charge transport properties of devices. The present review highlights the recent progress of processing-structure-transport correlations that allow the precise and uniform alignment of organic conjugated molecules over large areas for multiple electronic applications, including OFETs, organic thermoelectric devices (OTEs), and organic phototransistors (OPTs). Different strategies for regulating crystallinity and macroscopic orientation of conjugated molecules are introduced to correlate the molecular packing, the device performance, and charge transport anisotropy in multiple organic electronic devices.

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.

Enhancing the photovoltaic properties of SnS-Based solar cells by crystallographic orientation engineering

Tin monosulfide (SnS) is a promising light-harvesting material for solar cell applications, owing to its potential for large-scale production, cost-effectiveness, eco-friendly source materials, and long-term stability. However, SnS crystallizes in an orthorhombic structure, which results in a highly anisotropic charge transport behavior. Tailoring the crystallographic orientation of the SnS absorber layer plays a critical role in the enhancement of the transfer of charge carriers and the power conversion efficiency (PCE). By controlling the substrate tilting angle and temperature ramp rate in vapor transport deposition, the crystal growth orientation was tuned to a preferred direction which significantly suppressed the unfavorable (040) crystallographic plane. Through the combination of these two approaches, the PCE could be increased from 0.11% to 2%. The effect of the tilting angle was numerically simulated to investigate its role in controlling the film uniformity and directing the film growth. In addition, the correlation between the texture coefficient of the (040) plane and the charge transport properties was determined by a combination of analytical methods such as device performance studies, electrochemical impedance spectroscopy, along with transient photovoltage, space-charge-limited current, and dark current measurements. These techniques were blended together to prove that the marked improvement in PCE can be ascribed to a reduced charge recombination (in both SnS bulk and interfaces) and an enhanced hole mobility.

Flat-Band-Induced Anomalous Anisotropic Charge Transport and Orbital Magnetism in Kagome Metal CoSn.

For solids, the dispersionless flat band has long been recognized as an ideal platform for achieving intriguing quantum phases. However, experimental progress in revealing flat-band physics has so far been achieved mainly in artificially engineered systems represented as magic-angle twisted bilayer graphene. Here, we demonstrate the emergence of flat-band-dominated anomalous transport and magnetic behaviors in CoSn, a paramagnetic kagome-lattice compound. By combination of angle-resolved photoemission spectroscopy measurements and first-principles calculations, we reveal the existence of a kagome-lattice-derived flat band right around the Fermi level. Strikingly, the resistivity within the kagome lattice plane is more than one order of magnitude larger than the interplane one, in sharp contrast with conventional (quasi-) two-dimensional layered materials. Moreover, the magnetic susceptibility under the out-of-plane magnetic field is found to be much smaller as compared with the in-plane case, which is revealed to be arising from the introduction of a unique orbital diamagnetism. Systematic analyses reveal that these anomalous and giant anisotropies can be reasonably attributed to the unique properties of flat-band electrons, including large effective mass and self-localization of wave functions. Our results broaden the already fascinating flat-band physics, and demonstrate the feasibility of exploring them in natural solid-state materials in addition to artificial ones.

Polymer assisted transferring of patterned electrodes for a weak anisotropic charge transport study in monolayer molecular crystals

Here, by taking the advantage of the varied adhesion forces between substrates/gold electrodes/polymer films, the prepatterned fan-shaped electrodes could be readily transferred to target MMCs for the anisotropy charge transport study.

Borazatruxenes as precursors for hybrid C-BN 2D molecular networks.

Synthesizing atomically thin, crystalline two-dimensional (2D) molecular materials which combine carbon with other elements is an emerging field requiring both custom-designed molecular precursors and their ability to organize into networks (hydrogen-bonded or covalent). Hybrid carbon-boron nitride (C-BN) networks face the additional challenge of needing hydrolytically-stable BN-containing molecular precursors. Here, we show that borazatruxenes (truxene-like molecules with a borazine core) and their halogenated derivatives are highly stable precursors suitable for on-surface assembly. Using scanning tunneling microscopy (STM) and density functional theory (DFT) simulations we demonstrate hierarchical H-bonded assembly based on chiral homodimers of tribromo-borazatruxenes (3Br-borazatruxenes) as building blocks for both 1D chains and 2D networks. A low-symmetry, H-bonded chiral 2D lattice forms on Au(111) from the C3-symmetric 3Br-borazatruxenes, leading to large enantiomorphic domains that are molecularly homochiral. Such homochiral segregation is a necessary condition if chiral C-BN covalent networks are to be obtained via subsequent on-surface reactions. We show via DFT that up to two Na atoms can be trapped within the small pores of this dense lattice, while further Na atoms can adsorb on preferred network sites; this leads to hybrid Na-molecular network electronic bands with anisotropic dispersion and significant (up to hundreds of meV) bandwidths, as well as significant doping, that can engender anisotropic transport through the network. Finally, electronic structure comparisons (combining both experiment and computation) between borazatruxene, its tri-brominated derivative and truxene show that the borazine core controls the band gap increase, while also inducing C-B pz-pz electron delocalization that facilitates a continuous electron path across the molecule. Furthermore, as shown by DFT, the borazine core drives inter-layer B-N polar interactions that promote adsorption of BN containing molecules in a staggered configuration, a mechanism to be exploited in layer-by-layer supra-molecular assembly of novel hybrid C-BN materials.

Anisotropic charge transport in 1D and 2D BeN4 and MgN4 nanomaterials: A first-principles study

Low-dimensional Dirac materials have a great potential for practical applications due to their exceptional electronic properties and large surface to volume ratio. Here, we use density functional theory calculations in combination with nonequilibrium Green’s functional formalism to study the ballistic transport properties of recently synthesized single layer BeN4 [Bykov et al., Phys. Rev. Lett. 126 (2021) 175501] and dynamically stable MgN4 [Mortazavi et al., Materials Today Nano 15 (2021) 100125]. We found that these new types of 2D Dirac material show strongly anisotropic electronic transport with current along the armchair direction being significantly larger than in the zig-zag direction. Such in-plane anisotropic electronic transport properties of the materials originate from lattice direction-dependent electron localizations and electrostatic potential variations. Anisotropic charge transport is also obtained for BeN4 nanotube structures. MgN4 sample shows larger current as compared to BeN4 which indicates the possibility of increasing the conductance of BeN4 by replacing Be atoms on the polymeric nitrogen chains with other metal atoms. These findings can be of practical interest in exploring the potential of this novel type of 2D materials for the development of anisotropic multifunctional electronic devices.