Mechanisms of metal-insulator transitions in ultrathin SrMoO3films.

As the thickness of a transition metal oxide thin film is reduced to several unit cells, dimensional and interfacial effects modulate its structure and properties, and initiate low-dimension quantum phase transitions different from its bulk counterparts. To check if a metal-insulator transition (MIT) occurs to a low-dimensional 4d2electron systems, we investigated SrMoO3thin films by characterizing and analyzing their lattice structures, electric transport properties and electronic states. Among various dimensional effects and interfacial effects, quantum confinement effect (QCE) was discerned as the dominating mechanism of the thickness-driven MIT. Surface/interface scattering contributes to the residual resistivity while the competition of several interactions modulated by QCE governs the temperature dependence of the resistivity of SrMoO3ultrathin films.

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.

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.

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.

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.

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.

Hydrogen Impurity Defects in Rutile TiO2.

Hydrogen-related defects play crucial roles in determining physical properties of their host oxides. In this work, we report our systematic experimental and theoretical (based on density functional theory) studies of the defect states formed in hydrogenated-rutile TiO2 in gaseous H2 and atomic H. In gas-hydrogenated TiO2, the incorporated hydrogen tends to occupy the oxygen vacancy site and negatively charged. The incorporated hydrogen takes the interstitial position in atom-hydrogenated TiO2, forming a weak O-H bond with the closest oxygen ion, and becomes positive. Both states of hydrogen affect the electronic structure of TiO2 mainly through changes of Ti 3d and O 2p states instead of the direct contributions of hydrogen. The resulted electronic structures of the hydrogenated TiO2 are manifested in modifications of the electrical and optical properties that will be useful for the design of new materials capable for green energy economy.

Structure and catalytic activities of ferrous centers confined on the interface between carbon nanotubes and humic acid.

Preparation of heterogeneous catalysts with active ferrous centers is of great significance for industrial and environmental catalytic processes. Nanostructured carbon materials (NCM), which possess free-flowing π electrons, can coordinate with transition metals, provide a confinement environment for catalysis, and act as potential supports or ligands to construct analogous complexes. However, designing such catalysts using NCM is still seldom studied to date. Herein, we synthesized a sandwich structured ternary complex via the coordination of Fe-loaded humic acid (HA) with C=C bonds in the aromatic rings of carbon nanotubes (CNTs), in which the O/N-Fe-C interface configuration provides the confinement environment for the ferrous sites. The experimental and theoretical results revealed octahedrally/tetrahedrally coordinated geometry at Fe centers, and the strong hybridization between CNT C π* and Fe 3d orbitals induces discretization of the atomic charges on aromatic rings of CNTs, which facilitates O2 adsorption and electron transfer from carbon to O2, which enhances O2 activation. The O2 activation by the novel HA/Fe-CNT complex can be applied in the oxidative degradation of phenol red (PR) and bisphenol A (BPA) in aqueous media.

Effects of oxygen vacancy on the electronic structure and multiferroics in sol-gel derived Pb(0.8)Co(0.2)TiO3 thin films.

The single phase Pb(0.8)Co(0.2)TiO3 thin films were synthesized on a Pt/Ti/SiO2/Si substrate by the sol-gel route. The present films exhibited homogeneous microstructure with low porosity. O 1s X-ray photoelectron spectroscopy (XPS) was used to detect the amount of oxygen vacancies. The ferroelectric measurements showed that the ferroelectricity deteriorates with the increase in the number of oxygen vacancies. X-ray absorption spectroscopy (XAS) and XPS were used to study the electronic structure. The results indicated that the decreased ferroelectricity might be ascribed to the weakened hybridization between O 2p and Pb 6s and Ti 3d orbitals. The ferromagnetic behaviors were also observed in the thin films and saturated magnetization raised monotonously with the oxygen vacancy rising due to the enhanced F-center exchange interaction. Magnetoelectric coupling of the films weakened with oxygen vacancy increase.

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy.

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.

Charge transfer dynamics of 3,4,9,10-perylene-tetracarboxylic-dianhydride molecules on Au(111) probed by resonant photoemission spectroscopy.

Charge transfer dynamics across the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) organic semiconductor molecules on Au(111) interface has been investigated using the core-hole clock implementation of resonant photoemission spectroscopy. It is found that the charge transfer time scale at the PTCDA∕Au(111) interface is much larger than the C 1s core-hole lifetime of 6 fs, indicating weak electronic coupling between PTCDA and the gold substrate due to the absence of chemical reaction and∕or bonding.

Electronic states of a C70 monolayer on the surface of Ag(111).

We have investigated the electronic states of a C(70) monolayer on the surface of Ag(111) (1 ML C(70)/Ag(111)) using synchrotron radiation photoelectron spectroscopy and soft x-ray absorption spectroscopy techniques. The experimental data exhibit metallic properties and at least 2.6 e(-) charge transfer per C(70) molecule. The screening effect of Ag(111) on the electronic structure of C(70) is remarkable; it greatly reduces or even eliminates the on-site Hubbard energy. The work functions of the C(70) multilayer and monolayer are determined as 4.53 eV and 4.52 eV respectively. The energy levels of C(70) align with the Fermi level of the Ag(111) substrate, and the shift of the vacuum level caused by C(70) adsorption is negligible. Potassium doping indicates that 1 ML C(70)/Ag(111) can still accommodate about nine electrons and that the sample remains metallic at any doping level.

Structural change of metallofullerene: an easier thermal decomposition.

We have studied for the first time the structural change of high-purity metallofullerene (Gd@C(82)) upon heat treatment in an ultra-high vacuum system (10(-10) Torr) and examined the decomposition product through successive analysis with MS, IR, Raman, TEM, EDS and XPS. It was found that metallofullerene (Gd@C(82)) had fully collapsed at 580 °C which was lower than that for the complete destruction of C(60). The easier decomposition should be ascribed to the encapsulated metal in the carbon cage which could induce the deformation of the C-C bond. The analysis indicated that the broken metallofullerene (Gd@C(82)) became a kind of graphite-like material with a lot of defects. The Gd atoms leaked out from the carbon cage and aggregated together to form a regular arrangement.

Supercritical synthesis and characterization of SWNT-based one dimensional nanomaterials.

The present study developed a novel, fast and efficient method to synthesize one dimensional nanotube-based materials via supercritical reactions and supercritical fluids. It was proved that supercritical organic fluids were good media to take materials into the nanocavity, not only as solvents but also as reaction agents. Different kinds of metals (Ni, Cu, Ag) and fullerenes (C(60), C(70), C(78), C(84), Gd@C(82), Er@C(82), Ho@C(82), Y@C(82)) were successfully inserted into nanotubes with small diameters by this technique, with various supercritical fluids such as C(2)H(5)OH, CH(3)OH or C(6)H(5)CH(3). The filling rates were proved to be more than 90%. The high filling efficiency and the properties of the as-generated materials were characterized by TEM, Raman, EDS and XPS. In principle, this technique can be applied to construct new types of nanomaterials, if we choose the appropriate supercritical reaction and fluid in the CNTs.

Self-construction of core-shell structure by metallofullerene-containing polymer.

Metallofullerene Gd@C82 offers the opportunity to produce novel and advanced polymer-based nanocomposite materials. In this work, we reported the synthesis of novel Gd@C82-containing copolymers with the optimum condition found by changing the temperature, initiator and fullerene contents of C60-PS. The developed materials, based on polystyrene, displayed unique nanostructures which were confirmed by many measurements (GPC, AFM, SEM, TGA/DSC and NEXAFS analysis). The mechanism, stability and structure of Gd@C82-containing copolymer were discussed. This approach offers a new possibility of optimizing the polymer performance with metallofullerene.