Electron Materials Research Articles

Multipolar superconductivity in Luttinger semimetals

Topological superconductivity in multiband systems has received much attention due to a variety of possible exotic superconducting order parameters as well as non-trivial bulk and surface states. While the impact of coexisting magnetic order on superconductivity has been studied for many years, such as ferromagnetic superconductors, the implication of coexisting multipolar order has not been explored much despite the possibility of multipolar hidden order in a number of $f$-electron materials. In this work, we investigate topological properties of multipolar superconductors that may arise when quadrupolar local moments are coupled to conduction electrons in the multiband Luttinger semimetal. We show that the multipolar ordering of local moments leads to various multipolar superconductors with distinct topological properties. We apply these results to the quadrupolar Kondo semimetal system, PrBi, by deriving the microscopic multipolar Kondo model and examining the possible superconducting order parameters. We also discuss how to experimentally probe the topological nature of the Bogoliubov quasiparticles in distinct multipolar superconductors via doping and external pressure, especially in the context of PrBi.

Rational design of naphthalimide based small molecules non-fullerene acceptors for organic solar cells

To explore optical electronic and charge transfer properties, a series of D-π-A type of molecules with central core of 9,9-dimethyl-9H-fluorene as donor linked by 6-fluoro-4-(prop-1-yn-1-yl)benzo[1,2,5]thiadiazole as π-bridge to variable end group acceptor materials have been designed for organic solar cells (OSCs). Optoelectronic properties of designed molecules M1-M4 with similar central core and π-bridge but with different end groups were compared with R as representative of our system with 1,8-naphthalimide as end group. These optoelectronic properties are influenced by different end groups. Lower band gaps and longer wavelength of absorption have been observed for molecules by analyzing their frontier molecular orbitals. Furthermore, the computed reorganization energies for designed molecules are also comparable to the R so these molecules can be used as electron and hole transport materials. Among designed molecules M3 has higher wavelength of absorption along with minimum band gap and suitable distribution pattern of HOMO and LUMO during transition. The results presented display that varying the end groups is a highly promising approach in order to develop a series of D-π-A type of materials for organic photovoltaics. So, our computed results display that these designed molecules can be used as an excellent candidates for OSCs.

Single walled carbon nanotube incorporated Titanium dioxide and Poly(3-hexylthiophene) as electron and hole transport materials for perovskite solar cells

This study focuses on incorporating single wall carbon nanotubes (CNTs) in both electron conducting porous Titanium dioxide (TiO2) and the hole conducting Poly(3-hexylthiophene) (P3HT) layers in order to enhance the performance of perovskite solar cells. The CNT was incorporated at TiO2/CH3NH3PbIxCl3-x interface by dip coating TiO2 electrodes with CNT solution prior to deposition of CH3NH3PbIxCl3-x and by blending with P3HT, respectively. Raman spectra and Atomic Force Microscopic (AFM) images confirmed the presence of CNT in TiO2 electrodes and in P3HT, respectively. Optimized devices with the CNT show overall power conversion efficiencies (PCE) over 14%, under 100 mW/cm2 illumination with Air Mass 1.5 filter, which is 50% higher than that of the control device which is not having CNT. The enhancement is predominantly due to increase in short circuit current density (JSC) and fill factor, which resulted from surface passivation at the interface by CNT.

Non-Fullerene Small Molecule Electron-Transporting Materials for Efficient p-i-n Perovskite Solar Cells.

PC61BM is commonly used in perovskite solar cells (PSC) as the electron transport material (ETM). However, PC61BM film has various disadvantages, such as its low coverage or the many pinholes that appear due to its aggregation behavior. These faults may lead to undesirable direct contact between the metal cathode and perovskite film, which could result in charge recombination at the perovskite/metal interface. In order to overcome this problem, three alternative non-fullerene electron materials were applied to inverted PSCs; they were evaluated on suitability as electron transport layers. The roles and effects of these non-fullerene ETMs on device performance were studied using photoluminescence (PL) measurements, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), internal resistance in PSC measurements, and conductive atomic force microscopy (C-AFM). It was found that one of the tested materials, IT-4f, showed excellent electron extraction ability and was associated with reduced recombination. The PSC with IT-4f as the ETM produced better cell-performance; it had an average PCE of 11.21%, which makes it better than the ITIC and COi8DFIC-based devices. Finally, IT-4f was compared with PC61BM; it was found that the two materials have quite comparable efficiency and stability levels.

Using graphdiyne (GDY) as a catalyst support for enhanced performance in organic pollutant degradation and hydrogen production: A review

The development of carbon materials brings a new two-dimensional catalyst support, graphdiyne (GDY), which is attracting increasing interest in the field of catalysis. This article presents a systematical review of recent studies about the characteristics, design strategies, and applications of GDY-supported catalysts. The sp- and sp2-hybridized carbon, high electrical conductivity, direct band gap, and high intrinsic carrier mobility are key characteristics for GDY to serve as a competitive catalyst support. Hydrothermal method (or solvothermal method), GDY in-situ growth, and electrochemical deposition are commonly used to load catalysts on GDY support. In the applications of GDY-supported photocatalysts, GDY mainly serves as an electron or hole transfer material. For the electrocatalytic hydrogen production, the unique electronic structure and high electrical conductivity of GDY can promote the electron transfer and water splitting kinetics. This review is expected to provide meaningful insight and guidance for the design of GDY-supported catalysts and their applications.

Simple and fast fabrication of single crystal VO2 microtube arrays

Single crystal VO2 is a strongly correlated electron material that has shown great potential for a wide range of high-performance modern device applications, such as microbolometers, lithium ion batteries, microactuators and strain sensors. However, the present fabrication methods for single crystal VO2 almost always require complicated procedures, strict conditions and long reaction times of up to one week. Here, we report a simple, fast, low-cost and green method for fabricating single crystal VO2 using a thermal oxidation route based on resistive heating of a vanadium foil in air. Our method not only reduces the complete fabrication time from hours to tens of seconds but also naturally forms single crystal VO2 microtube arrays that are nearly vertically aligned on the surface of a V2O5 substrate. Microstructure characteristics and the reversible phase transition between the monoclinic VO2 and rutile VO2 phases demonstrate that the obtained single crystal VO2 is the same as that achieved by other fabrication methods.

Thermoelectric probe of defect state induced by ionic liquid gating in vanadium dioxide

Thermoelectric measurements detect the asymmetry between the density of states above and below the chemical potential in a material. They provide insights into small variations in the density of states near the chemical potential, complementing electron transport measurements. Here, we report combined resistance and thermoelectric power measurements of vanadium dioxide (VO2), a prototypical correlated electron material, under ionic-liquid (IL) gating. We show that under our gating conditions, the charge transport below the metal-to-insulator-transition (MIT) temperature remains in the thermally activated regime, while the Seebeck coefficient exhibits an apparent transition from semiconducting to metallic behavior. The contrasting behavior indicates changes in the electronic structure upon IL gating, due to the formation of oxygen defect states. The experimental results are corroborated by numerical simulations based on a model density of states incorporating a gating-induced defect band. Our study reveals thermoelectric measurements to be a convenient and sensitive probe for the role of defect states induced by IL gating in suppressing the MIT in VO2, which remains benign in charge transport measurements, and possibly for studying defect states in other materials.

Aggregate-State Effects in the Atomistic Modeling of Organic Materials for Electrochemical Energy Conversion and Storage Devices: A Perspective.

Development of new functional materials for novel energy conversion and storage technologies is often assisted by ab initio modeling. Specifically, for organic materials, such as electron and hole transport materials for perovskite solar cells, LED (light emitting diodes) emitters for organic LEDs (OLEDs), and active electrode materials for organic batteries, such modeling is often done at the molecular level. Modeling of aggregate-state effects is onerous, as packing may not be known or large simulation cells may be required for amorphous materials. Yet aggregate-state effects are essential to estimate charge transport rates, and they may also have substantial effects on redox potentials (voltages) and optical properties. This paper summarizes recent studies by the author’s group of aggregation effects on the electronic properties of organic materials used in optoelectronic devices and in organic batteries. We show that in some cases it is possible to understand the mechanism and predict specific performance characteristics based on simple molecular models, while in other cases the inclusion of effects of aggregation is essential. For example, it is possible to understand the mechanism and predict the overall shape of the voltage-capacity curve for insertion-type organic battery materials, but not the absolute voltage. On the other hand, oligomeric models of p-type organic electrode materials can allow for relatively reliable estimates of voltages. Inclusion of aggregate state modeling is critically important for estimating charge transport rates in materials and interfaces used in optoelectronic devices or when intermolecular charge transfer bands are important. We highlight the use of the semi-empirical DFTB (density functional tight binding) method to simplify such calculations.

Coincidence angle-resolved photoemission spectroscopy: Proposal for detection of two-particle correlations

The angle-resolved photoemission spectroscopy (ARPES) is one powerful experimental technique to study the electronic structure of materials. As many electron materials show unusual many-body correlations, the technique to detect directly these many-body correlations will play important roles in the study of their many-body physics. In this article, we propose a technique to detect directly the two-particle correlations, a coincidence ARPES (cARPES) where two incident photons excite two respective photoelectrons which are detected in coincidence. While the one-photon-absorption and one-photoelectron-emission ARPES provides the single-particle spectrum function, the proposed cARPES with two-photon absorption and two-photoelectron emission is relevant to a two-particle Bethe-Salpeter wave function. Examples of the coincidence detection probability of the cARPES for a free Fermi gas and a Bardeen-Cooper-Schrieffer (BCS) superconducting state are studied in detail. We also propose another two experimental techniques, a coincidence angle-resolved photoemission and inverse-photoemission spectroscopy (cARP/IPES) and a coincidence angle-resolved inverse-photoemission spectroscopy (cARIPES). As all of these proposed coincidence techniques can provide the two-particle frequency Bethe-Salpeter wave functions, they can show the momentum and energy dependent two-particle dynamical physics of the material electrons in the particle-particle or particle-hole channel. Thus, they can be introduced to study the Cooper-pair physics in the superconductor, the itinerant magnetism in the metallic ferromagnet/antiferromagnet, and the particle-hole pair physics in the metallic nematic state. Moreover, as the two-particle Bethe-Salpeter wave functions also involve the inner-pair dynamical physics, these proposed coincidence techniques can be used to study the inner-pair time-retarded physics.

Electronic structure of bulk manganese oxide and nickel oxide from coupled cluster theory

We describe the ground- and excited-state electronic structure of bulk MnO and NiO, two prototypical correlated electron materials, using coupled cluster theory with single and double excitations (CCSD). As a corollary, this work also reports an implementation of unrestricted periodic ab initio equation-of-motion CCSD. Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 and 4.83 eV for MnO and NiO, respectively, for the 16-unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated in the thermodynamic limit. From the character of the correlated electronic bands we find both MnO and NiO to lie in the intermediate Mott/charge-transfer insulator regime, although NiO appears as a charge transfer insulator when only the fundamental gap is considered. While the lowest quasiparticle excitations are of metal $3d$ and O $2p$ character in most of the Brillouin zone, near the $\mathrm{\ensuremath{\Gamma}}$ point, the lowest conduction band quasiparticles are of $s$ character. Our study supports the potential of coupled cluster theory to provide high-level many-body insights into correlated solids.

Improved Photovoltaic Effect of p-i-n Structured BiFeO₃ Film Deposited by Radio-Frequency Magnetron Sputtering.

Pure phase polycrystalline BiFeO₃ film was deposited onto FTO substrate by RF magnetron sputtering method. SEM result shows that BiFeO₃ film has the obvious porosity and large clusters which lead to the poor ferroelectric and photovoltaic properties in FTO/BiFeO₃/Ag device. However, these properties are improved in p-i-n structured FTO/TiO₂/BiFeO₃/HTM/Ag device by incorporating the electron and hole transport materials. The hysteresis loop measurement demonstrates the excellent ferroelectric property with large remnant polarization (2Pr = 180 μC/cm²) and low leakage current. The J-V curve shows the short-circuit current density is dozens of times larger than that of FTO/BiFeO₃/Ag device. Moreover, the photovoltaic output depends on the poling field where the positive poling improves the short-circuit current density to -85 μA/cm₂ and the negative poling reduces both the photocurrent and photovoltage. It is believed that the ferroelectric polarization plays a dominant role in the photovoltaic effect.

Relationship between exceptional points and the Kondo effect in f -electron materials

We study the impact of nonhermiticity due to strong correlations in f-electron materials. One of the most remarkable phenomena occurring in nonhermitian systems is the emergence of exceptional points at which the effective nonhermitian Hamiltonian becomes non-diagonalizable. We here demonstrate that Kondo temperature is related to the temperature at which exceptional points appear around the Fermi level. For this purpose, we study the periodic Anderson model with local and nonlocal hybridization in the insulating and metallic regimes. By analyzing the effective nonhermitian Hamiltonian, which describes the single-particle spectral function, and the temperature dependence of the screening of the magnetic moment, from which the Kondo temperature can be found, we show that exceptional points appear at the temperature at which the magnetic moment is screened. These results suggest that the well-known crossover between localized and itinerant f electrons in $f$-electron materials is related to the emergent exceptional points in the single-particle spectral function.

Robust Hot Electron and Multiple Topological Insulator States in PtBi2.

A two-dimensional topological insulator features (only) one bulk gap with nontrivial topology, which protects one-dimensional boundary states at the Fermi level. We find a quantum phase of matter beyond this category: a multiple topological insulator. It possesses a ladder of topological gaps; each gap protects a robust edge state. We prove a monolayer of van der Waals material PtBi2 as a two-dimensional multiple topological insulator. By means of scanning tunneling spectroscopy, we directly visualize the one-dimensional hot electron (and hole) channels with nanometer size on the samples. Furthermore, we confirm the topological protection of these channels by directly demonstrating their robustness to variations of crystal orientation, edge geometry, and sample temperature. The discovered topological hot electron materials may be applied as efficient photocatalysts in the future.

Optically Induced Phase Change for Magnetoresistance Modulation

AbstractOptical methods for magnetism manipulation have been considered as a promising strategy for ultralow‐power and ultrahigh‐speed data storage and processing, which have become an emerging field of spintronics. However, a widely applicable and efficient method has rarely been demonstrated. Here, the strongly correlated electron material vanadium dioxide (VO2) is used to realize the optically induced phase change for control of the magnetism in NiFe. The NiFe/VO2 bilayer heterostructure features appreciable modulations of electrical conductivity (32%), coercivity (37.5%), and magnetic anisotropy (25%). Further analyses indicate that interfacial strain coupling plays a crucial role in the magnetic modulation. Utilizing this heterostructure, which can respond to both optical and magnetic stimuli, a phase change controlled anisotropic magnetoresistance (AMR) device is fabricated, and reconfigurable Boolean logics are implemented. As a demonstration of phase change spintronics, this work may pave the way for next‐generation opto‐electronics in the post‐Moore era.

Exploring itinerant states in divalent hexaborides using rare-earth L edge resonant inelastic x-ray scattering

We present a study of resonant inelastic x-ray scattering (RIXS) spectra collected at the rare-earth L edges of divalent hexaborides YbB6 and EuB6. In both systems, RIXS-active features are observed at two distinct resonances separated by eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak resemble the unoccupied 5d density of states calculated using density functional theory. We discuss possible origins of this correspondence including a scenario which changes the 4f valence. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results demonstrate the potential for L-edge RIXS to assess the itinerant-state properties of f -electron materials.

Surface modification of patterned electrospun nanofibrous films via the adhesion of DOPA-bFGF and DOPA-ponericin G1 for skin wound healing

Combining biodegradable materials with bioactive factors for skin wound healing has been receiving significant attention. This study was inspired by the adhesion mechanism of a marine organism, the mussel. 3,4-Dihydroxyphenylalanine (DOPA) was introduced to basic fibroblast growth factor (bFGF) and ponericin G1 (PonG1) using tyrosine hydroxylation, which are expected to have strong binding affinities to the surfaces of materials. DOPA-bFGF and DOPA-PonG1 were applied for the surface modification of patterned poly(lactic-co-glycolic acid) (PLGA) electrospun nanofibrous films for skin wound healing. The DOPA-bFGF- and DOPA-PonG1-modified patterned PLGA nanofibrous films were analysed using scanning electron microscopy (SEM), contact angle, and materials testing machine. Then, the immobilization efficiencies and antibacterial abilities of the different films were examined. The results show that the DOPA-bFGF- and DOPA-PonG1-modified PLGA nanofibrous films exhibited bionic performance, improved tensile strength, and hydrophilicity. DOPA-bFGF and DOPA-PonG1 exhibited stronger binding abilities to films compared with bFGF and PonG1. DOPA-bFGF and DOPA-PonG1 films can significantly promote BALB/c 3T3 cell attachment, proliferation and tissue repair-related gene expression. In vivo experiments indicated that DOPA-PonG1/DOPA-bFGF@PLGA nanofibrous films shortened the wound healing time, accelerated epithelialization and promoted skin remodelling.

Crucial role of charge transporting layers on ion migration in perovskite solar cells

The device preconditioning dependent hysteresis and the consequential performance degradation hinder the actual performance and stability of the perovskite solar cells. Ion migration and charge trapping in the perovskite with large contribution from grain boundaries are the most common interpretations for the hysteresis. Yet, the high performing devices often include intermediate hole and electron transporting layers, which can further complicate the dynamical process in the device. Here, by using Kelvin Probe Force Microscopy and Confocal Photoluminescence Microscopy, we elucidate the impact of charge-transporting layers and excess MAI on the spatial and temporal variations of the photovoltage on the MAPbI3-based solar cells. By studying the devices layer by layer, we found that the light-induced ion migration occurs predominantly in the presence of an imbalanced charge extraction in the solar cells, and the charge transporting layers play crucial role in suppressing it. Careful selection and processing of the electron and hole-transporting materials are thus essential for making perovskite solar cells free from the ion migration effect.

Quantum engineered Kondo lattices

Atomic manipulation techniques have provided a bottom-up approach to investigating the unconventional properties and complex phases of strongly correlated electron materials. By engineering artificial systems containing tens to thousands of atoms with tailored electronic or magnetic properties, it has become possible to explore how quantum many-body effects emerge as the size of a system is increased from the nanoscale to the mesoscale. Here we investigate both theoretically and experimentally the quantum engineering of nanoscale Kondo lattices – Kondo droplets – exemplifying nanoscopic replicas of heavy-fermion materials. We demonstrate that by changing a droplet’s real-space geometry, we can not only create coherently coupled Kondo droplets whose properties asymptotically approach those of a quantum-coherent Kondo lattice, but also markedly increase or decrease the droplet’s Kondo temperature. Furthermore we report on the discovery of a new quantum phenomenon – the Kondo echo – a signature of droplets containing Kondo holes functioning as direct probes of spatially extended, quantum-coherent Kondo cloud correlations.

Investigation on microstructure, mechanical properties and work hardening behavior of as-extruded Mg-4.5Zn-0.75Y-xZr alloys

The effects of Zr content on microstructures, mechanical properties and work hardening behavior of the as-extruded Mg-4.5Zn-0.75Y-xZr (x = 0.14, 0.28, 0.42, in at%) alloys were investigated with the assistance of optical microscope (OM), x-ray diffraction (XRD), scanning electron microscope (SEM) and material testing machine. The results show that the matrix microstructures of the as-extruded Mg-4.5Zn-0.75Y-xZr alloys present as typical bimodal structures, which are characterized by coarse α-Mg grains surrounded by many relatively fine dynamically recrystallized grains. The average grain sizes (GSs) of the extruded alloys show a trend of first decreasing and then slightly increasing with increasing Zr content, but the I-phase (Mg3Zn6Y) volume fractions keep almost constant. Among the three alloys, Mg-4.5Zn-0.75Y-0.28Zr alloy exhibits the optimum comprehensive mechanical properties (the ultimate tensile strength, yield strength and elongation to failure are 353 MPa, 296 MPa and 26.5%, respectively), which is corresponded to its smallest average GS. Meanwhile, with the reduction of the average GS, the work hardening (WH) rate, hardening capacity and WH exponent of the extruded alloys decrease. The WH rate decreases linearly with increasing deformation at the third stage of WH. Moreover, all the studied alloys show good ductility, and their ductile fracture features can be observed from the morphologies of tensile fracture surfaces at room temperature.

Characteristics of matrix-related pores associated with various lithofacies of marine shales inside of Guizhong Basin, South China

Matrix-related pore characteristics of between Lower Carboniferous shale and Middle Devonian shale in Guizhong Basin were investigated to explore the reservoir capacity. Via the combination of organic geochemistry analysis, X-ray diffraction, gas adsorption, mercury intrusion porosimetry, helium porosimetry and focused ion beam-scanning electron microscopy, material basics and physical properties were studied. Five lithofacies were divided as comparable units in two formations according to thermal maturity, OM abundance and mineral components, namely organic-moderate argillaceous shale with high thermal maturity (OMAS-H) and organic-lean argillaceous shale with high thermal maturity (OLAS-H) in Lower Carboniferous Formation, and organic-lean mixed shale with over thermal maturity (OLMS-O), organic-rich mixed shale with over thermal maturity (ORMS-O) as well as organic-rich calcareous shale with over thermal maturity (ORCS-O) in Middle Devonian Formation. Results showed that (1) OMAS-H and OLAS-H performed better porous features than OLMS-O, ORMS-O and ORCS-O, in terms of PV, PSA and porosity, (2) pores developed better in OMAS-H and OLAS-H than those in OLMS-O, ORMS-O and ORCS-O, in terms of pores of rigid grains, pores associated with clay flakes and OM-hosted pores, (3) specific PVs of OM were dramatically larger than those of mineral components, while specific PVs of OM in both OMAS-H (0.348 ml/g × 10−6) and OLAS-H (0.248 ml/g × 10−6) were about an order of magnitude higher than those in OLMS-O (0.055 ml/g × 10−6), ORMS-O (0.025 ml/g × 10−6) and ORCS-O (0.011 ml/g × 10−6). The ability of pore contribution largely depends on whether corresponding components are suited in the appropriate evolution stage with suitable thermal and pressure circumstance. Then sufficient degree of pore carriers (matrixes) need to be thought about, especially OM and clay as the secondary pore providers. Also, the demand for pore preservation highlights the importance of brittle minerals which can establish rigid frameworks. Therefore, in the case of controlling factors on pore characteristics, the importance ranking should be taken into consideration.