Spectroscopic Evidence of Localized Small Polarons in Low-Dimensional Ionic Liquid Lead-Free Hybrid Perovskites.

Rational design is an important approach to consider in the development of low-dimensional hybrid organic-inorganic perovskites (HOIPs). In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical ionic liquid components in bismuth-based HOIPs. Element-sensitive X-ray absorption spectroscopy measurements of BMPBiBr4 and APIBiBr5 reveal distinct resonant excitation profiles across the N K-edges, where contrasting peak shifts are observed. These 1D-HOIPs exhibit a large Stokes shift due to the small polaron contribution, as probed by photoluminescence spectroscopy at room temperature. Interestingly, the incorporation of a small fraction of tin (Sn) into the APIBiBr5 (Sn/Bi mole ratio of 1:3) structure demonstrates a strong spectral weight transfer accompanied by a fast decay lifetime (2.6 ns). These phenomena are the direct result of Sn-substitution in APIBiBr5, decreasing the small polaron effect. By changing the active ionic liquid, the electronic interactions and optical responses can be moderately tuned by alteration of their intermolecular interaction between the semiconducting inorganic layers and organic moieties.

A Facile Two-Step Hydrothermal Synthesis of Co(OH)2@NiCo2O4 Nanosheet Nanocomposites for Supercapacitor Electrodes.

The composites of NiCo2O4 with unique structures were substantially investigated as promising electrodes. In this study, the unique structured nanosheets anchored on nickel foam (Ni foam) were prepared under the hydrothermal technique of NiCo2O4 and subsequent preparation of Co(OH)2. The Co(OH)2@NiCo2O4 nanosheet composite has demonstrated higher specific capacitances owing to its excellent specific surface region, enhanced rate properties, and outstanding electrical conductivities. Moreover, the electrochemical properties were analyzed in a three-electrode configuration to study the sample material. The as-designed Co(OH)2@NiCo2O4 nanosheet achieves higher specific capacitances of 1308 F·g-1 at 0.5 A·g-1 and notable long cycles with 92.83% capacity retention over 6000 cycles. The Co(OH)2@NiCo2O4 nanosheet electrode exhibits a long life span and high capacitances compared with the NiCo2O4 and Co(OH)2 electrodes, respectively. These outstanding electrochemical properties are mainly because of their porous construction and the synergistic effects between NiCo2O4 and Co(OH)2. Such unique Co(OH)2@NiCo2O4 nanosheets not only display promising applications in renewable storage but also reiterate to scientists of the unlimited potential of high-performance materials.

Unrevealing tunable resonant excitons and correlated plasmons and their coupling in new amorphous carbon-like for highly efficient photovoltaic devices.

An understanding on roles of excitons and plasmons is important in excitonic solar cells and photovoltaic (PV) technologies. Here, we produce new amorphous carbon (a-C) like films on Indium Tin Oxide (ITO) generating PV cells with efficiency three order of magnitude higher than the existing biomass-derived a-C. The amorphous carbon films are prepared from the bioproduct of palmyra sap with a simple, environmentally friendly, and highly reproducible method. Using spectroscopic ellipsometry, we measure simultaneously complex dielectric function, loss function as well as reflectivity and reveal coexistence of many-body resonant excitons and correlated-plasmons occurring due to strong electronic correlations. X-ray absorption and photoemission spectroscopies show the nature of electron and hole in defining the energy of the excitons and plasmons as a function of N or B doping. Our result shows new a-C like films and the importance of the coupling of resonant excitons and correlated plasmons in determining efficiency of photovoltaic devices.

Efficient Ternary Mn-Based Spinel Oxide with Multiple Active Sites for Oxygen Evolution Reaction Discovered via High-Throughput Screening Methods.

The discovery of more efficient and stable catalysts for oxygen evolution reaction (OER) is vital in improving the efficiency of renewable energy generation devices. Given the large numbers of possible binary and ternary metal oxide OER catalysts, high-throughput methods are necessary to accelerate the rate of discovery. Herein, Mn-based spinel oxide, Fe10 Co40 Mn50 O, is identified for the first time using high-throughput methods demonstrating remarkable catalytic activity (overpotential of 310 mV on fluorine-doped tin oxide (FTO) substrate and 237 mV on Ni foam at 10 mA cm-2 ). Using a combination of soft X-ray absorption spectroscopy and electrochemical measurements, the high catalytic activity is attributed to 1) the formation of multiple active sites in different geometric sites, tetrahedral and octahedral sites; and 2) the formation of active oxyhydroxide phase due to the strong interaction of Co2+ and Fe3+ . Structural and surface characterizations after OER show preservation of Fe10 Co40 Mn50 O surface structure highlighting its durability against irreversible redox damage on the catalytic surface. This work demonstrates the use of a high-throughput approach for the rapid identification of a new catalyst, provides a deeper understanding of catalyst design, and addresses the urgent need for a better and stable catalyst to target greener fuel.

Observation of resonant exciton and correlated plasmon yielding correlated plexciton in amorphous silicon with various hydrogen content.

Hydrogenated amorphous silicon (a-Si: H) has received great attention for rich fundamental physics and potentially inexpensive solar cells. Here, we observe new resonant excitons and correlated plasmons tunable via hydrogen content in a-Si: H films on Indium Tin Oxide (ITO) substrate. Spectroscopic ellipsometry supported with High Resolution-Transmission Electron Microscopy (HR-TEM) is used to probe optical properties and the density of electronic states in the various crystallinity from nano-size crystals to amorphous a-Si: H films. The observed optical and electronic structures are analyzed by the second derivative with analytic critical-point line shapes. The complex dielectric function shows good agreement with microscopic calculations for the energy shift and the broadening inter-band transitions based on the electron-hole interaction. Interestingly, we observe an unusual spectral weight transfer over a broad energy range revealing electronic correlations that cause a drastic change in the charge carrier density and determine the photovoltaic performance. Furthermore, the interplay of resonant excitons and correlated plasmons is discussed in term of a correlated plexciton. Our result shows the important role of hydrogen in determining the coupling of excitons and plasmons in a-Si: H film for photovoltaic devices.

A New Spin-Correlated Plasmon in Novel Highly Oriented Single-Crystalline Gold Quantum Dots.

A concept of spin plasmon, a collective mode of spin-density, in strongly correlated electron systems has been proposed since the 1930s. It is expected to bridge between spintronics and plasmonics by strongly confining the photon energy in the subwavelength scale within single magnetic-domain to enable further miniaturizing devices. However, spin plasmon in strongly correlated electron systems is yet to be realized. Herein, we present a new spin correlated-plasmon at room temperature in novel Mott-like insulating highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Interestingly, the spin correlated-plasmon is tunable from the infrared to visible, accompanied by spectral weight transfer yielding a large quantum absorption midgap state, disappearance of low-energy Drude response, and transparency. Supported with theoretical calculations, it occurs due to an interplay of surprisingly strong electron-electron correlations, s-p hybridization and quantum confinement in the s band. The first demonstration of the high sensitivity of spin correlated-plasmon in surface-enhanced Raman spectroscopy is also presented.

Coupled harmonic oscillator models for correlated plasmons in one-dimensional and quasi-one-dimensional systems.

A new phenomenon of correlated plasmons was first observed in the insulating phase of the Sr1-xNb1-yO3+δfamily (Asamaraet al2017Nat. Commun.815271). The correlated plasmons are tunable, have multiple plasmonic frequencies, and exhibit low loss-making them desirable in numerous plasmonic applications. However, their fundamental mechanism is yet to be explored. While conventional plasmons can be understood solely by considering long-range interactions, unconventional correlated plasmons arise in correlated electron systems and require consideration of the short-range interactions. Here, we report how the interplay of short-range and long-range interactions determines the correlated plasmon phenomena through a coupled harmonic oscillator model of both 1D and quasi-1D systems. In each system, the impact of various physical parameters like the number of oscillators, energy scale, free electron scattering parameter, quasi-particle concentration, charges, effective masses, and Coulomb interaction strengths are explored to gain an understanding of their impact on the complex dielectric function and loss function. We study both cases where the parameters are the same for all quasi-particles and where effective mass, Coulomb interaction strength, and charge are varied for individual quasi-particles. In an extended model of the quasi-1D system, we study both cases where the rung symmetry of all parameters is conserved and where it is broken. When rung symmetry is conserved, the overall trends in optical and plasmonic peaks are the same as the 1D model, though the peaks tend to shift to higher energies and amplitudes. When rung symmetry is broken, the quasi-1D behavior deviates significantly from the 1D model, including an increase in the maximum possible number of optical and plasmonic peaks. Overall, our results demonstrate the significance of the interplay of short-range and long-range interactions in determining the correlated plasmons and identifying how various parameters can be used to tune the resulting plasmons.

Teaching reaction kinetics through isomerization cases with the basis of density-functional calculations.

The fundamental mechanism of biochemistry lies on the reaction kinetics, which is determined by the reaction pathways. Interestingly, the reaction pathway is a challenging concept for undergraduate students. Experimentally, it is difficult to observe, and theoretically, it requires some degree of physics knowledge, namely statistical and quantum mechanics. However, students can utilize computational methods to study the reaction kinetics without paying too much attention but not wholly neglecting the comprehension of physics. We hereby provided an approach to study the reaction kinetics based on density-functional calculations. We particularized the study of the isomerization case involving five molecules at three different temperatures and emphasized the importance of the transition state in the study of reaction kinetics. The results we presented were in good agreement with the experiments and provided useful insights to assist students in the application of their knowledge into their research.

Emergent Topological Hall Effect at a Charge-Transfer Interface.

Exploring exotic interface magnetism due to charge transfer and strong spin-orbit coupling has profound application in the future development of spintronic memory. Here, the emergence and tuning of topological Hall effect (THE) from a CaMnO3 /CaIrO3 /CaMnO3 trilayer structure are studied in detail, which suggests the presence of magnetic Skyrmion-like bubbles. First, by tilting the magnetic field direction, the evolution of the Hall signal suggests a transformation of Skyrmions into topologically-trivial stripe domains, consistent with behaviors predicted by micromagnetic simulations. Second, by varying the thickness of CaMnO3 , the optimal thicknesses for the THE signal emergence are found, which allow identification of the source of Dzyaloshinskii-Moriya interaction (DMI) and its competition with antiferromagnetic superexchange. Employing high-resolution transmission electron microscopy, randomly distributed stacking faults are identified only at the bottom interface and may avoid mutual cancellation of DMI. Last, a spin-transfer torque experiment also reveals a low threshold current density of ≈109 A m-2 for initiating the bubbles' motion. This discovery sheds light on a possible strategy for integrating Skyrmions with antiferromagnetic spintronics.

Electron tunneling at the molecularly thin 2D perovskite and graphene van der Waals interface.

Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (~50 fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm2V-1s-1 between 1.7 to 200 K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite.

Interfacial Oxygen-Driven Charge Localization and Plasmon Excitation in Unconventional Superconductors.

Charge localization is critical to the control of charge dynamics in systems such as perovskite solar cells, organic-, and nanostructure-based photovoltaics. However, the precise control of charge localization via electronic transport or defect engineering is challenging due to the complexity in reaction pathways and environmental factors. Here, charge localization in optimal-doped La1.85 Sr0.15 CuO4 thin-film on SrTiO3 substrate (LSCO/STO) is investigated, and also a high-energy plasmon is observed. Charge localization manifests as a near-infrared mid-gap state in LSCO/STO. This is ascribed to the interfacial hybridization between the Ti3d-orbitals of the substrate and O2p-orbitals of the film. The interfacial effect leads to significant changes in the many-body correlations and local-field effect. The local-field effect results in an inhomogeneous charge distribution, and due to perturbation by an external field, the high polarizability of this nonuniform charge system eventually generates the high-energy plasmon. Transformation of the electronic correlations in LSCO/STO is further demonstrated via temperature-dependent spectral-weight transfer. This study of charge localization in cuprates and interfacial hybridization provides important clues to their electronic structures and superconductive properties.

Giant piezoelectricity in oxide thin films with nanopillar structure.

High-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient [Formula: see text] of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials.

Correction: The effect of crystallinity on the surface modification and optical properties of ZnO thin films.

Correction for 'The effect of crystallinity on the surface modification and optical properties of ZnO thin films' by Muhammad Abiyyu Kenichi Purbayanto et al., Phys. Chem. Chem. Phys., 2020, 22, 2010-2018, DOI: 10.1039/C9CP05464B.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides.

The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Optical constants and absorption properties of Te and TeO thin films in the 13-14†nm spectral range.

Undesired mask-induced effects caused by thick absorber layers in EUV photomasks reduce the quality of the projected patterns at the wafer stage in EUV photolithography scanners. New materials with better absorption properties than the state-of-the-art absorbers, TaN and TaBN, are required to mitigate these effects. In this work, we investigated the optical properties (δ and k) of Te and TeO films in the 13-14 nm range, and the absorption properties of these two materials at 13.5 nm. δ and k are obtained through fitting experimental values of reflectivity versus angle of incidence in the EUV range. We follow a methodology which combines different characterization techniques (X-ray reflectivity, EUV reflectivity, and X-ray photoemission spectroscopy) to reduce the number of free parameters in models and hence, increase the reliability of the optical constants obtained. At 13.5 nm, we obtain δ=0.03120, k = 0.07338 for Te, and δ=0.04099, k = 0.06555 for TeO. To experimentally verify the absorption properties of these materials, different thicknesses of Te and TeO films are cast on top of a state-of-the-art mask-quality EUV multilayer with 66.7% reflectivity at 13.5 nm. We found that a reflectivity of ∼0.7% can be attained with either 32.4 nm of Te, or 34.7 nm of TeO, greatly surpassing the absorption properties of TaN and TaBN. The morphology and surface roughness of the Te and TeO films deposited on the multilayer are also investigated.

Characteristic Lengths of Interlayer Charge Transfer in Correlated Oxide Heterostructures.

Using interlayer interaction to control functional heterostructures with atomic-scale designs has become one of the most effective interface-engineering strategies nowadays. Here, we demonstrate the effect of a crystalline LaFeO3 buffer layer on amorphous and crystalline LaAlO3/SrTiO3 heterostructures. The LaFeO3 buffer layer acts as an energetically favored electron acceptor in both LaAlO3/SrTiO3 systems, resulting in modulation of interfacial carrier density and hence metal-to-insulator transition. For amorphous and crystalline LaAlO3/SrTiO3 heterostructures, the metal-to-insulator transition is found when the LaFeO3 layer thickness crosses 3 and 6 unit cells, respectively. Such different critical LaFeO3 thicknesses are explained in terms of distinct characteristic lengths of the redox-reaction-mediated and polar-catastrophe-dominated charge transfer, controlled by the interfacial atomic contact and Thomas-Fermi screening effect, respectively. Our results not only shed light on the complex interlayer charge transfer across oxide heterostructures but also provide a new route to precisely tailor the charge-transfer process at a functional interface.

Direct Growth of Wafer-Scale, Transparent, p-Type Reduced-Graphene-Oxide-like Thin Films by Pulsed Laser Deposition.

Reduced graphene oxide (rGO) has attracted significant interest in an array of applications ranging from flexible optoelectronics, energy storage, sensing, and very recently as membranes for water purification. Many of these applications require a reproducible, scalable process for the growth of large-area films of high optical and electronic quality. In this work, we report a one-step scalable method for the growth of reduced-graphene-oxide-like (rGO-like) thin films via pulsed laser deposition (PLD) of sp2 carbon in an oxidizing environment. By deploying an appropriate laser beam scanning technique, we are able to deposit wafer-scale uniform rGO-like thin films with ultrasmooth surfaces (roughness <1 nm). Further, in situ control of the growth environment during the PLD process allows us to tailor its hybrid sp2-sp3 electronic structure. This enables us to control its intrinsic optoelectronic properties and helps us achieve some of the lowest extinction coefficients and refractive index values (0.358 and 1.715, respectively, at 2.236 eV) as compared to chemically grown rGO films. Additionally, the transparency and conductivity metrics of our PLD grown thin films are superior to other p-type rGO films and conducting oxides. Unlike chemical methods, our growth technique is devoid of catalysts and is carried out at lower process temperatures. This would enable the integration of these thin films with a wide range of material heterostructures via direct growth.

Magnetic Anisotropy of a Quasi Two-Dimensional Canted Antiferromagnet.

We report the control of the interplane magnetic exchange coupling in CaIrO3 perovskite thin films and superlattices with SrTiO3. By analyzing the anisotropic magneto-transport data, we demonstrate that a semimetallic paramagnetic CaIrO3 turns into a canted antiferromagnetic Mott insulator at reduced dimensions. The emergence of a biaxial magneto-crystalline anisotropy indicates the canted moment responding to the cubic symmetry. Extending to superlattices and probing oxygen octahedral rotation by half-integer X-ray Braggs diffraction, a more complete picture about the canted moment evolution with interplane coupling can be understood. Remarkably, a rotation of the canted moments' easy axes by 45° is also observed by a sign reversal of the in-plane strain. These results demonstrate the robustness of anisotropic magnetoresistance in revealing quasi two-dimensional canted antiferromagnets, as well as valuable insights about quadrupolar magnetoelastic coupling, relevant for designing future antiferromagnetic spintronic devices.

Unusual Hole and Electron Midgap States and Orbital Reconstructions Induced Huge Ferroelectric Tunneling Electroresistance in BaTiO3/SrTiO3.

Oxide heterostructures have attracted a lot of interest because of their rich exotic phenomena and potential applications. Recently, a greatly enhanced tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs) has been realized in such heterostructures. However, our understanding on the electronic structure of resistance response with polarization reversal and the origin of huge TER is still lacking. Here, we report on electronic structures, particularly at the interface and surface, and the control of the spontaneous polarization of BaTiO3 films by changing the termination of a SrTiO3 substrate. Interestingly, unusual electron and hole midgap states are concurrently formed and accompanied by orbital reconstructions, which determine the ferroelectric polarization orientation in the BaTiO3/SrTiO3. Such unusual midgap states, which yield a strong electronic screening effect, reduce the ferroelectric barrier width and height, and pin the ferroelectric polarization, lead to a dramatic enhancement of the TER effect. The midgap states are also observed in BaTiO3 films on electron-doped Nb/SrTiO3 revealing its universality. Our result provides new insight into the origin of the huge TER effect and opens a new route for designing ferroelectric tunnel junction-based devices with huge TER through interface engineering.