Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis.

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Ultrafast Energy Transfer Process in Confined Gold Nanospheres Revealed by Femtosecond X-ray Imaging and Diffraction.

Femtosecond laser pulses drive nonequilibrium phase transitions via reaction paths hidden in thermal equilibrium. This stimulates interest to understand photoinduced ultrafast melting processes, which remains incomplete due to challenges in resolving accompanied kinetics at the relevant space-time resolution. Here, by newly establishing a multiplexing femtosecond X-ray probe, we have successfully revealed ultrafast energy transfer processes in confined Au nanospheres. Real-time images of electron density distributions with the corresponding lattice structures elucidate that the energy transfer begins with subpicosecond melting at the specimen boundary earlier than the lattice thermalization, and proceeds by forming voids. Two temperature molecular dynamics simulations uncovered the presence of both heterogeneous melting with the melting front propagation from surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Supported by experimental and theoretical results, we provide a comprehensive atomic-scale picture that accounts for the ultrafast laser-induced melting and evaporation kinetics.

Tunable spin injection and detection across a van der Waals interface.

Van der Waals heterostructures with two-dimensional magnets offer a magnetic junction with an atomically sharp and clean interface. This attribute ensures that the magnetic layers maintain their intrinsic spin-polarized electronic states and spin-flipping scattering processes at a minimum level, a trait that can expand spintronic device functionalities. Here, using a van der Waals assembly of ferromagnetic Fe3GeTe2 with non-magnetic hexagonal boron nitride and WSe2 layers, we demonstrate electrically tunable, highly transparent spin injection and detection across the van der Waals interfaces. By varying an electrical bias, the net spin polarization of the injected carriers can be modulated and reversed in polarity, which leads to sign changes of the tunnelling magnetoresistance. We attribute the spin polarization reversals to sizable contributions from high-energy localized spin states in the metallic ferromagnet, so far inaccessible in conventional magnetic junctions. Such tunability of the spin-valve operations opens a promising route for the electronic control of next-generation low-dimensional spintronic device applications.

Skin-Deep Aspect of Thermopower in Bi2Q3, PbQ, and BiCuQO (Q = Se, Te): Hidden One-Dimensional Character of Their Band Edges Leading to High Thermopower.

ConspectusThermoelectric (TE) materials have received much attention because of their ability to convert heat energy to electrical energy. At a given temperature T, the efficiency of a TE material for this energy conversion is measured by the figure of merit zT, which is related to the thermopower (or Seebeck coefficient) S, the thermal conductivity κ, and the electrical conductivity σ of the TE material as zT = (S2σT)/κ. Bi2Q3 and PbQ (Q = Se, Te) are efficient TE materials with high zT, although they are not ecofriendly and their stability is poor at high temperature. In principle, a TE material can have a high zT if it has a low thermal conductivity and a high electrical conductivity, but the latter condition is hardly met in a real material because the parameters S, σ and κ have a conflicting dependence on material properties. The difficulty in searching for TE materials of high zT is even more exasperated because the relationship between the thermopower S and the carrier density n (hereafter, the S-vs-n relationship) for the well-known hole-doped samples of BiCuSeO showed that the hole carriers responsible for their thermopower are associated largely with the electronic states lying within ∼0.5 eV of its valence band maximum (VBM). Thus, the states governing the TE properties lie in the "skin-deep" region from the VBM. For electron-doped TE systems, the electron carriers responsible for their thermopower should also be associated with the electronic states lying within ∼0.5 eV of the conduction band minimum (CBM). This makes it difficult to predict TE materials of high zT. One faces a similar skin-deep phenomenon in searching for superconductors of high transition temperature because the transition from a normal metallic to a superconducting state involves the normal metallic states in the vicinity of the Fermi level EF. Other skin-deep phenomena in metallic compounds include the formation of charge density wave (CDW), which involves the electronic states in the vicinity of their Fermi levels. For magnetic materials of transition-metal ions, the preferred orientation of their spin moments is a skin-deep phenomenon because it is governed by the interaction between the highest-occupied and the lowest unoccupied d-states of these ions. In the present work we probe the issues concerning how to find the possible range of thermopower expected for a given TE material and hence how to recognize what experimental values of thermopower are expected or unusual. For these purposes, we analyze the accumulated S and n data on the three well-studied TE materials, Bi2Q3, PbQ, and BiCuQO (Q = Se, Te), as representative examples, in terms of the ideal theoretical S-vs-n relationships, which we determine for their defect-free Bi2Q3, PbQ, and BiCuQO structures using density functional theory (DFT) calculations under the rigid band approximation. We find that the general trends in the experimental S-vs-n relationships are reasonably well explained by the calculated S-vs-n relationships, and the carrier densities covering these relationships are associated with the states lying within ∼0.5 eV from their band edges confirming the skin-deep nature of their thermoelectric properties. Despite the fact that these TE materials are not one-dimensional (1D) in structure, they mostly possess sharp density-of-state peaks around their band edges because their band dispersions have a hidden 1D character so their thermopower is generally high in magnitude.

Optical Transitions of a Single Nodal Ring in SrAs_{3}: Radially and Axially Resolved Characterization.

SrAs_{3} is a unique nodal-line semimetal that contains only a single nodal ring in the Brillouin zone, uninterrupted by any trivial bands near the Fermi energy. We performed axis-resolved optical reflection measurements on SrAs_{3} and observed that the optical conductivity exhibits flat absorption up to 129 meV in both the radial and axial directions, confirming the robustness of the universal power-law behavior of the nodal ring. The axis-resolved optical conductivity, in combination with theoretical calculations, further reveals fundamental properties beyond the flat absorption, including the overlap energy of the topological bands, the spin-orbit coupling gap along the nodal ring, and the geometric properties of the nodal ring such as the average ring radius, ring ellipticity, and velocity anisotropy. In addition, our temperature-dependent measurements revealed a spectral weight transfer between intraband and interband transitions, indicating a possible violation of the optical sum rule within the measured energy range.

Evolution of the Kondo lattice electronic structure above the transport coherence temperature.

The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the basic f-c hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. Here, the temperature dependence of f-c hybridized band dispersions and Fermi-energy f spectral weight in the Kondo lattice system CeCoIn5 is investigated using f-resonant angle-resolved photoemission spectroscopy (ARPES) with sufficient detail to allow direct comparison to first-principles dynamical mean-field theory (DMFT) calculations containing full realism of crystalline electric-field states. The ARPES results, for two orthogonal (001) and (100) cleaved surfaces and three different f-c hybridization configurations, with additional microscopic insight provided by DMFT, reveal f participation in the Fermi surface at temperatures much higher than the lattice coherence temperature, [Formula: see text] K, commonly believed to be the onset for such behavior. The DMFT results show the role of crystalline electric-field (CEF) splittings in this behavior and a T-dependent CEF degeneracy crossover below [Formula: see text] is specifically highlighted. A recent ARPES report of low T Luttinger theorem failure for CeCoIn5 is shown to be unjustified by current ARPES data and is not found in the theory.

Ghost-Template-Faceted Synthesis of Tunable Amorphous Hollow Silica Nanostructures and Their Ordered Mesoscale Assembly.

Despite the enormous applications of and fundamental scientific interest in amorphous hollow-silica nanostructures (h-SiNSs), their synthesis in crystal-like nonspherical polygonal architectures is challenging. Herein, we present a facile one-shot synthetic procedure for various unconventional h-SiNSs with controllable surface curvatures (concave, convex, or angular), symmetries (spherical, polygonal, or Janus), and interior architectures (open or closed walls) by the addition of a metal salt and implementing kinetic handles of silica precursor (silanes/ammonia) concentrations and reverse-micellar volume. During the silica growth, we identified the key role of transiently in situ crystallized metal coordination complexes as a nanopolyhedral "ghost template", which provides facet-selective interactions with amino-silica monomers and guides the differential silica growth that produces different h-SiNSs. Additionally, crystal-like well-defined polygonal h-SiNSs with flat surfaces, assembled as highly ordered close-packed octahedral mesoscale materials (ca. 3 µm) where h-SiNSs with different nanoarchitectures act as building units (ca. 150 nm) to construct customizable cavities and nanospaces, differ from conventionally assembled materials.

Orbital Order Melting at Reduced Dimensions.

The orbital degree of freedom, strongly coupled with the lattice and spin, is an important factor when designing correlated functions. Whether the long-range orbital order is stable at reduced dimensions and, if not, what the critical thickness is remains a tantalizing question. Here, we report the melting of orbital ordering, observed by controlling the dimensionality of the canonical eg1 orbital system LaMnO3. Epitaxial films are synthesized with vertically aligned orbital ordering planes on an orthorhombic substrate, so that reducing film thickness changes the two-dimensional planes into quasi-one-dimensional nanostrips. The orbital order appears to be suppressed below the critical thickness of about six unit cells by changing the characteristic phonon modes and making the Mn d orbital more isotropic. Density functional calculations reveal that the electronic energy instability induced by bandwidth narrowing via the dimensional crossover and the interfacial effect causes the absence of orbital order in the ultrathin thickness.

Chemical Vapor Deposition of Edge-on Oriented 2D Conductive Metal-Organic Framework Thin Films.

We demonstrated the synthesis of a conductive two-dimensional metal-organic framework (MOF) thin film by single-step all-vapor-phase chemical vapor deposition (CVD). The synthesized large-area thin film of Cu3(C6O6)2 has an edge-on-orientation with high crystallinity. Cu3(C6O6)2 thin film-based microdevices were fabricated by e-beam lithography and had an electrical conductivity of 92.95 S/cm. Synthesis of conductive MOF thin films by the all-vapor-phase CVD will enable fundamental studies of physical properties and may help to accomplish practical applications of conductive MOFs.

Spin-Peierls Distortion of TiPO4 Causing a Transition from a Magnetic to a Nonmagnetic Insulating State and Its Effect on the Thermoelectric Properties: Density Functional Theory Analysis.

Density functional theory calculations were carried out to probe the nature of the electronic structure change in TiPO4 before and after its spin-Peierls distortion at 74.5 K, which is characterized by the dimerization in the chains of Ti3+ (d1) ions present in TiPO4. These calculations suggest strongly that the electronic state of TiPO4 is magnetic insulating before the distortion, but becomes nonmagnetic insulating after the distortion. Consistent with this suggestion, the phonon dispersion relations calculated for TiPO4 show that the undistorted TiPO4 is stable, while the distorted TiPO4 is not, if each Ti3+ ion has a spin moment, and that the opposite is true if each Ti3+ ion has no spin moment. These observations suggest that the driving force for the spin-Peierls distortion is the formation of direct metal-metal bonds leading to the dimerized chains of Ti3+ ions. The abrupt change in the electronic structures from a magnetic insulating state to a nonmagnetic insulating state explains why the spin-Peierls distortion of TiPO4 exhibits a first-order character. Although the two electronic states of TiPO4 before and after the distortion have a band gap, the substantial spin-Peierls distortion is found to enhance the thermoelectric properties of TiPO4.

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals.

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

Observation of Spin-Dependent Dual Ferromagnetism in Perovskite Ruthenates.

We performed in situ angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) experiments to investigate the relationship between electronic band structures and ferromagnetism in SrRuO_{3} (SRO) thin films. Our high quality ARPES and SARPES results show clear spin-lifted band structures. The spin polarization is strongly dependent on momentum around the Fermi level, whereas it becomes less dependent at high-binding energies. This experimental observation matches our dynamical mean-field theory results very well. As temperature increases from low to the Curie temperature, spin-splitting gap decreases and band dispersions become incoherent. Based on the ARPES study and theoretical calculation results, we found that SRO possesses spin-dependent electron correlations in which majority and minority spins are localized and itinerant, respectively. Our finding explains how ferromagnetism and electronic structure are connected, which has been under debate for decades in SRO.

Second-Harmonic Young's Interference in Atom-Thin Heterocrystals.

Second-harmonic generation (SHG) is a nonlinear optical process that converts two identical photons into a new one with doubled frequency. Two-dimensional semiconductors represented by transition-metal dichalcogenides are highly efficient SHG media because of their excitonic resonances. Using spectral phase interferometry, here we directly show that SHG in heterobilayers of MoS2 and WS2 is governed by optical interference between two coherent SH fields that are phase-delayed differently in each material. We also quantified the frequency-dependent phase difference between the two, which agreed with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young's double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.

Ultrafast Infrared Photoresponse from Heavily Hydrogen-Doped VO2 Single Crystalline Nanoparticles.

Infrared photodetectors are sought for diverse applications and their performance relies on photoactive materials and photocurrent generation mechanisms. Here, we fabricate IR photodetectors with heavily hydrogen-doped VO2 (i.e., HVO2) single-crystalline nanoparticles which show two orders greater resistivities than pure VO2. The I-V plots obtained under IR light irradiation are expressed by space charge limited current mechanism and the increase in photocurrent occurs due to the increase in the number of photoinduced trap sites. This phenomenon remarkably improves the key parameters at λ = 780 nm of high responsivity of 35280 A/W, high detectivity of 1.12 × 1013 Jones, and strikingly fast response times of 0.6-2.5 ns, that is, 3 orders of magnitude faster than the best records of two-dimensional structures and heterostructures. Density functional theory calculations illustrate that the generation of photoinduced trap sites is attributed to the movement of hydrogen atoms to less stable interstitial sites in VO2 under light exposure.

Reversible Polymorphic Transition and Hysteresis-Driven Phase Selectivity in Single-Crystalline C8-BTBT Rods.

Organic semiconductors (OSCs) are highly susceptible to the formation of metastable polymorphs that are often transformed by external stimuli. However, thermally reversible transformations in OSCs with stability have not been achieved due to weak van der Waals forces, and poor phase homogeneity and crystallinity. Here, a polymorph of a single crystalline 2,7-dioctyl[1] benzothieno[3,2-b][1]benzothio-phene rod on a low molecular weight poly(methyl methacrylate) (≈120k) that limits crystal coarsening during solvent vapor annealing is fabricated. Molecules in the polymorph lie down slightly toward the substrate compared to the equilibrium state, inducing an order of greater resistivity. During thermal cycling, the polymorph exhibits a reversible change in resistivity by 5.5 orders with hysteresis; this transition is stable toward bias and thermal cycling. Remarkably, varying cycling temperatures leads to diverse resistivities near room temperature, important for nonvolatile multivalue memories. These trends persist in the carrier mobility and on/off ratio of the polymorph field-effect transistor. A combination of in situ grazing incident wide angle X-ray scattering analyses, visualization for electronic and structural analysis simulations, and density functional theory calculations reveals that molecular tilt governs the charge transport characteristics; the polymorph transforms as molecules tilt, and thereby, only a homogeneous single-crystalline phase appears at each temperature.

Metal@SiO2 Core-Shells with Self-Arrested Migrating Core.

Developing easy and customizable strategies for the directional structure modulation of multicomponent nanosystems to influence and optimize their properties are a paramount but challenging task in nanoscience. Here, we demonstrate highly controlled eccentric off-center positioning of metal-core in metal@silica core-shells by utilizing an in situ generated biphasic silica-based intraparticle solid-solid interface. In the synthetic strategy, by including Ca2+-ions in silica-shell and successive oxidative and reductive annealing at high temperature, a unique hairline-biphasic interface is evolved via the heat-induced concentric radial segregation of calcium silicate phase at the interior and normal silica phase at the exterior of core-shell, which can effectively arrest the outwardly migrating metal-core within rubbery calcium silicate phase, affording various eccentric core-shells, where core-positions are flexibly controlled by the annealing time and amounts of initially added Ca2+-ions. In the structure-property correlation study, the strategy allows fine-tuning of dipolar interaction-based blocking temperatures and magnetic anisotropies of different eccentric core-shells as the function of variable off-center distance of magnetic core without changing the overall size of nanoparticles. This work demonstrates the discovery and potential application of biphasic solid-solid media interface in controlling the heat-induced migration of metal nanocrystals and opens the avenues for exploiting the rarely studied high-temperature solid-state nanocrystal conversion chemistry and migratory behavior for directional nanostructure engineering.

Electrodynamic Force Derived in-Channel Separation and Detection of Au Nanoparticles Using an Electrochemical AC Microfluidic Channel.

In this study, we have established the separation of Au nanoparticles (AuNPs) using a symmetrical AC electric field applied-electrochemical microfluidic device composed of carbon channel and detection electrodes. The lateral movement of AuNPs in the channel under the AC field was analyzed by simulation using the mathematically derived equations, which were formulated from Newtonian fluid mechanics. It shows that the nanoparticles are precisely separated according to their respective mass or size difference in a short time. The experimental parameters affecting the separation and detection of AuNPs were optimized in terms of applied frequency, amplitude, flow rate, buffer concentration, pH dependency, and temperature. The final separation was performed at 1.0 V amplitude with 8.0 MHz frequency at 0.4 µL/min flow rate for the separation, and the potential of 1.0 V was applied for the amperometric detection of AuNPs in a 0.1 M PBS. Before and after the separation, AuNPs (diameter range: 3-60 nm) were confirmed by UV-visible spectroscopy and transmission electron microscopy. In this case, the separation resolution was 3 nm with an enhanced separation efficiency of up to 597,503 plates/m for the AuNPs. In addition, the amperometric current response of the detection electrode under the AC field application was also enhanced by the sensitivity 5-fold compared with the absence of the AC field.

Large anomalous Hall current induced by topological nodal lines in a ferromagnetic van der Waals semimetal.

Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose Fe3GeTe2, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin-orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.

Persistent Charge-Density-Wave Order in Single-Layer TaSe2.

We present the electronic characterization of single-layer 1H-TaSe2 grown by molecular beam epitaxy using a combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory calculations. We demonstrate that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of Fermi surface topology. Enhanced spin-orbit coupling and lattice distortion in the single-layer play a crucial role in the formation of CDW order. Our findings provide a deeper understanding of the nature of CDW order in the two-dimensional limit.

Hybrid density functional study on the electronic structures and properties of P3HT-PbS and P3HT-CdS hybrid interface for photovoltaic applications.

The efficiency of charge transport mainly depends on the interfacial energy level alignment between the conjugated polymer and the inorganic substrate. It provides an accurate understanding, predicting as well as controlling the optimal power conversion efficiency of various type of hybrid photovoltaic systems. In this article, we use hybrid functional (HSE06) to study the electronic structures and properties at the interface of poly(3-hexylthiophene)(P3HT)/CdS and P3HT/PbS for solar cell applications. We found that the dangling bonds at the inorganic surface introduce in-gap states and greatly reduce the device performance. We used pseudo-hydrogen atoms as the passivation agent to remove the dangling bonds and eliminate the in-gap states to construct the energy alignment at the hybrid interface. The calculated interfacial density of states reveal a better performance in P3HT/CdS, compared to P3HT/PbS. P3HT/CdS possesses a LUMOP3HT /CBMCdS and HOMOP3HT /VBMCdS energy offset large enough for sufficient exciton separation across the interface and prevents charge recombination. In contrast, the reason for low power conversion efficiency in P3HT/PbS lies on its HOMOP3HT /VBMPbS offset which is too small to break the exciton binding energy for charge separation. Moreover, we reported the dependency of the energy level alignment and open circuit voltage on the interfacial molecular orientations. Our DFT calculation can be used to predict candidate materials for the development of efficiency optoelectronic devices. © 2018 Wiley Periodicals, Inc.