ANi5Bi5.6+δ (A = K, Rb, and Cs): Quasi-One-Dimensional Metals Featuring [Ni5Bi5.6+δ]- Double-Walled Column with Strong Diamagnetism.

A new series of compounds, ANi5Bi5.6+δ (where A = K, Rb, and Cs) are discovered with a quasi-one-dimensional (Q1D) [Ni5Bi5.6+δ]- double-walled column and a coaxial inner one-dimensional Bi atomic chain. The columns are linked to each other by intercolumn Bi-Bi bonds and separated by an A+ cation. Typical metallic behaviors with strong correlation of itinerant electrons and the Sommerfeld coefficient enhanced with the increasing cationic radius were experimentally observed and supported by first-principles calculations. Compared to AMn6Bi5 (where A = K, Rb, and Cs), the enhanced intercolumn distances and the substitution of Ni for Mn give rise to strong diamagnetic susceptibilities in ANi5Bi5.6+δ. First-principles calculations reveal possible uncharged Ni atoms with even number of electrons in ANi5Bi5.6+δ, which may explain the emergence of diamagnetism. ANi5Bi5.6+δ, as Q1D diamagnetic metals with strong electron correlation, provide a unique platform to understand exotic magnetism and explore novel quantum effects.

Single-Atom Fe Catalysts for Fenton-Like Reactions: Roles of Different N Species.

Recognizing and controlling the structure-activity relationships of single-atom catalysts (SACs) is vital for manipulating their catalytic properties for various practical applications. Herein, Fe SACs supported on nitrogen-doped carbon (SA-Fe/CN) are reported, which show high catalytic reactivity (97% degradation of bisphenol A in only 5 min), high stability (80% of reactivity maintained after five runs), and wide pH suitability (working pH range 3-11) toward Fenton-like reactions. The roles of different N species in these reactions are further explored, both experimentally and theoretically. It is discovered that graphitic N is an adsorptive site for the target molecule, pyrrolic N coordinates with Fe(III) and plays a dominant role in the reaction, and pyridinic N, coordinated with Fe(II), is only a minor contributor to the reactivity of SA-Fe/CN. Density functional theory (DFT) calculations reveal that a lower d-band center location of pyrrolic-type Fe sites leads to the easy generation of Fe-oxo intermediates, and thus, excellent catalytic properties.

Room-Temperature Ferromagnetism at an Oxide-Nitride Interface.

Heterointerfaces have led to the discovery of novel electronic and magnetic states because of their strongly entangled electronic degrees of freedom. Single-phase chromium compounds always exhibit antiferromagnetism following the prediction of the Goodenough-Kanamori rules. So far, exchange coupling between chromium ions via heteroanions has not been explored and the associated quantum states are unknown. Here, we report the successful epitaxial synthesis and characterization of chromium oxide (Cr_{2}O_{3})-chromium nitride (CrN) superlattices. Room-temperature ferromagnetic spin ordering is achieved at the interfaces between these two antiferromagnets, and the magnitude of the effect decays with increasing layer thickness. First-principles calculations indicate that robust ferromagnetic spin interaction between Cr^{3+} ions via anion-hybridization across the interface yields the lowest total energy. This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.

Electronic-structure evolution of SrFeO3-xduring topotactic phase transformation.

Oxygen-vacancy-induced topotactic phase transformation between the ABO2.5brownmillerite structure and the ABO3perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO3-xthin films during the topotactic phase transformation. The results indicate that the increase in oxygen content induces a new unoccupied state of O 2pcharacter near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to thedx2-y2orbital than to thed3z2-r2orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO3-xfilms during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 µB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Strain-Mediated High Conductivity in Ultrathin Antiferromagnetic Metallic Nitrides.

Strain engineering provides the ability to control the ground states and associated phase transition in epitaxial films. However, the systematic study of the intrinsic character and strain dependency in transition-metal nitrides remains challenging due to the difficulty in fabricating stoichiometric and high-quality films. Here the observation of an electronic state transition in highly crystalline antiferromagnetic CrN films with strain and reduced dimensionality is reported. By shrinking the film thickness to a critical value of ≈30 unit cells, a profound conductivity reduction accompanied by unexpected volume expansion is observed in CrN films. The electrical conductivity is observed surprisingly when the CrN layer is as thin as a single unit cell thick, which is far below the critical thickness of most metallic films. It is found that the metallicity of an ultrathin CrN film recovers from insulating behavior upon the removal of the as-grown strain by the fabrication of freestanding nitride films. Both first-principles calculations and linear dichroism measurements reveal that the strain-mediated orbital splitting effectively customizes the relatively small bandgap at the Fermi level, leading to an exotic phase transition in CrN. The ability to achieve highly conductive nitride ultrathin films by harnessing strain-control over competing phases can be used for utilizing their exceptional characteristics.

Electronic structure variations of polar and nonpolar ZnO lattices with nitrogen-ion bombardment using synchrotron-based in situ photoemission and X-ray absorption spectroscopy.

Surface polarity with different crystal orientations has been demonstrated as a crucial parameter in determining the physical properties and device applications in many transition metal oxide and semiconductor compound systems. The influences of surface polarity on electronic structures in nitrogen-incorporated ZnO lattices have been investigated in the present work. The successful doping of nitrogen atoms in ZnO lattices is suggested by the existence of N-related chemical bonds obtained from X-ray photoelectron spectroscopy analysis where a pronounced N-Zn peak intensity has been observed in the (000\bar 1)-terminated polar ZnO compound compared with the (10\bar 10)-terminated nonpolar ZnO compound. An energy shift of the valence band maximum towards the Fermi level has been resolved for both polar and nonpolar ZnO lattices, whereas a charge redistribution of the O 2p hybridized states is only resolved for o-plane ZnO with a polar surface. Angular-dependent X-ray absorption analyses at the O K-edge reveal enhanced surface-state contributions and asymmetric O 2p orbital occupations in the (000\bar 1)-terminated o-plane ZnO compound. The results shed light on the efficient nitrogen doping in ZnO lattices with polar surfaces. The comprehensive electronic structure investigations of correlations between impurity doping and surface polarity in ZnO lattices may also offer guidance for the material design in other transition metal oxide and semiconductor systems.

Evidence of Topological Edge States in Buckled Antimonene Monolayers.

Two-dimensional topological materials have attracted intense research efforts owing to their promise in applications for low-energy, high-efficiency quantum computations. Group-VA elemental thin films with strong spin-orbit coupling have been predicted to host topologically nontrivial states as excellent two-dimensional topological materials. Herein, we experimentally demonstrated for the first time that the epitaxially grown high-quality antimonene monolayer islands with buckled configurations exhibit significantly robust one-dimensional topological edge states above the Fermi level. We further demonstrated that these topologically nontrivial edge states arise from a single p-orbital manifold as a general consequence of atomic spin-orbit coupling. Thus, our findings establish monolayer antimonene as a new class of topological monolayer materials hosting the topological edge states for future low-power electronic nanodevices and quantum computations.

Interaction between the Non-Fullerene Acceptor ITIC and Potassium.

Using density functional theory calculations and photoemission measurements, we have studied the interaction between the non-fullerene small-molecule acceptor ITIC and K atoms (a representative of reactive metals). It is found that the acceptor-donor-acceptor-type geometric structure and the electronic structure of ITIC largely decide the interaction process. One ITIC molecule can combine with more than 20 K atoms. For stoichiometries K x≤6ITIC, the K atoms are attracted to the acceptor units of the molecule and donate their 4s electrons to the unoccupied molecular orbitals. K-ITIC organometallic complexes, characterized by the breaking of some S-C bonds in the donor unit of ITIC and the formation of K-S bonds, are formed for stoichiometries K x≥7ITIC. The complexes are still conjugated despite the breaking of some S-C bonds.

Synchrotron X-ray Absorption Spectroscopy Study of Local Structure in Al-Doped BiFeO3 Powders.

The Al-doped BiFeO3, i.e., BFAxO powder samples with x = 0, 0.025, 0.05, and 0.1, were prepared via the hydrothermal route. The effects of Al substitution on the structural, electrical, and optical properties of BFAxO samples were investigated. It is found that the substitution of Al ions at B-site of BiFeO3 did not cause structural change and it still retains the rhombohedral perovskite structure with R3c symmetry, which was confirmed by the X-ray diffraction (XRD) and Raman measurements. X-ray absorption fine structure (XAFS) above the Fe K-edge and Bi L3-edge in BFAxO powders was also measured and analyzed. Fe ions exhibit mixed valence states (Fe2+/Fe3+) while Bi ions keep the + 3 valence state in all the samples. Fe K-edge XAFS also indicated that there was a competition between hybridization of Fe 3d and Al 3d with O 2p orbitals and occurrence of the more 4p orbitals with Al doping. The Bi L3-edge XAFS revealed that transition from 2p3/2 to 6d state increased, so did the energy of 6d state. Besides, Al ion doping affected both the nearest-neighbor and next-nearest coordination shells of Fe atom and nearest-neighbor shells of Bi atom. Ultraviolet-visible (UV-Vis) spectroscopy results show the BFAxO prepared by hydrothermal method could be an appropriate visible-light photocatalytic material.

Synthesis of NiO Nanotubes via a Dynamic Thermal Oxidation Process.

Nickel oxide (NiO) nanotubes were synthesized via a thermal oxidation process from Ni nanowires. The effects of oxidation temperature on the morphology, microstructures, and composition of nanowires were investigated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results show that the Ni nanowires convert initially to Ni/NiO core-shell nanowires with increasing annealing temperatures, and then to the nanotubes at the critical transition temperature of about 425 °C. Our findings provide useful information for the preparation of NiO nanotubes to meet the required applications.

Proposal for a photoelectron spectroscopy and microscopy beamline (0.5-11†keV) at the High Energy Photon Source.

An optical design study of a beamline proposed for the new 6 GeV synchrotron, the High Energy Photon Source (HEPS), to be built in Beijing, China, is described. The beamline is designed to cover an energy range from 0.5 to 11 keV with two experimental stations, one for X-ray photoelectron spectroscopy (PES) experiments and the other for photoelectron emission microscopy (XPEEM) experiments. A 5 m APPLE II-type undulator with a relatively long magnetic period (55 mm) is used as the only radiation source. To optimize the optical efficiency for the full energy range, the beamline is split into a soft X-ray branch that is based on a variable-line-spacing plane-grating monochromator and a tender X-ray branch that uses a four-bounce monochromator with three Si channel-cut pairs. To allow both PES and XPEEM to be performed over the entire energy range, two toroidal mirrors and a bendable KB mirror pair are employed to deliver the soft and tender beams, respectively, to either of two experimental stations.

Strain-Enhanced Charge Transfer and Magnetism at a Manganite/Nickelate Interface.

The strain effect on charge transfer in correlated oxide La0.8Sr0.2MnO3/NdNiO3 (LSMO/NNO) heterostructures is investigated. This is achieved by carefully tailoring the strain on the two layers using various substrates. In contrast to bare LSMO films, the strain dependence of the enhanced magnetic moment of the LSMO/NNO bilayers strongly suggests that the charge transfer can be controlled via strain engineering in complex oxide heterostructures. Furthermore, our study also reveals that the coercive field, exchange bias, and conductivity are dramatically affected by the strain-modulated charge transfer in LSMO/NNO heterostructures. Our work thus points out a new path to control electronic states in oxide heterostructures to advance the use of interfaces in oxide-based electronics.

The formation of (NiFe)S2 pyrite mesocrystals as efficient pre-catalysts for water oxidation.

Designing intricate structures and searching for functional materials has attracted wide interest in nanoscience. Herein we have fabricated (NiFe)S2 pyrite mesocrystals in the form of nearly-single crystalline porous cubes, and studied their self-optimization to realize efficient activity toward water oxidation under electrochemical conditions. The growth mechanism of the mesocrystals was a non-classical mechanism, which was initiated by the formation of a large quantity of small nickel sulfide clusters, followed by the aggregation and transformation of these small clusters in an oriented manner. When these mesocrystals were tested for water oxidation under electrocatalytic conditions, the materials served as pre-catalysts and immediately self-optimized to form amorphous S-doped metal (oxy)hydroxides, which are the real catalytically active materials. As a result, the observed overpotential to reach a current density of 10 mA cm-2 on glassy carbon electrodes was less than 260 mV. The growth mechanism studied here may provide opportunities for constructing intricate sulfide structures, and the self-optimization process during water oxidation can inspire new thoughts on electrocatalysis.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain.

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO6 and tetrahedral CoO4 cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

Epitaxial Growth of Flat Antimonene Monolayer: A New Honeycomb Analogue of Graphene.

Group-V elemental monolayers were recently predicted to exhibit exotic physical properties such as nontrivial topological properties, or a quantum anomalous Hall effect, which would make them very suitable for applications in next-generation electronic devices. The free-standing group-V monolayer materials usually have a buckled honeycomb form, in contrast with the flat graphene monolayer. Here, we report epitaxial growth of atomically thin flat honeycomb monolayer of group-V element antimony on a Ag(111) substrate. Combined study of experiments and theoretical calculations verify the formation of a uniform and single-crystalline antimonene monolayer without atomic wrinkles, as a new honeycomb analogue of graphene monolayer. Directional bonding between adjacent Sb atoms and weak antimonene-substrate interaction are confirmed. The realization and investigation of flat antimonene honeycombs extends the scope of two-dimensional atomically-thick structures and provides a promising way to tune topological properties for future technological applications.

Epitaxially grown monolayer VSe2: an air-stable magnetic two-dimensional material with low work function at edges.

Recent experimental breakthroughs open up new opportunities for magnetism in few-atomic-layer two-dimensional (2D) materials, which makes fabrication of new magnetic 2D materials a fascinating issue. Here, we report the growth of monolayer VSe2 by molecular beam epitaxy (MBE) method. Electronic properties measurements by scanning tunneling spectroscopy (STS) method revealed that the as-grown monolayer VSe2 has magnetic characteristic peaks in its electronic density of states and a lower work-function at its edges. Moreover, air exposure experiments show air-stability of the monolayer VSe2. This high-quality monolayer VSe2, a very air-inert 2D material with magnetism and low edge work function, is promising for applications in developing next-generation low power-consumption, high efficiency spintronic devices and new electrocatalysts.

Epitaxial Growth and Air-Stability of Monolayer Antimonene on PdTe2.

Monolayer antimonene is fabricated on PdTe2 by an epitaxial method. Monolayer antimonene is theoretically predicted to have a large bandgap for nanoelectronic devices. Air-exposure experiments indicate amazing chemical stability, which is great for device fabrication. A method to fabricate high-quality monolayer antimonene with several great properties for novel electronic and optoelectronic applications is provided.

Electronic structure evolution of single bilayer Bi(1 1 1) film on 3D topological insulator Bi2Se x Te3-x surfaces.

The electronic state evolution of single bilayer (1BL) Bi(1 1 1) deposited on three-dimensional (3D) Bi2Se x Te3-x topological insulators at x = 0, 1.26, 2, 2.46, 3 is systematically investigated by angle-resolved photoemission spectroscopy (ARPES). Our results indicate that the electronic structures of epitaxial Bi films are strongly influenced by the substrate especially the topmost sublayer near the Bi films, manifesting in two main aspects. First, the Se atoms cause a stronger charge transfer effect, which induces a giant Rashba-spin splitting, while the low electronegativity of Te atoms induces a strong hybridization at the interface. Second, the lattice strain notably modifies the band dispersion of the surface bands. Furthermore, our experimental results are elucidated by first-principles band structure calculations.

Spontaneous Formation of a Superconductor-Topological Insulator-Normal Metal Layered Heterostructure.

2D materials with heterolayered structures beyond graphene are explored. A theoretically predicted superconductor-topological insulator-normal metal heterolayered structure is realized experimentally. The generated hybrid structure HfTe3 /HfTe5 /Hf has potential applications in both quantum-spin Hall effect-based and Majorana-based devices.