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Infinite-layer Nd_{1-x}Sr_{x}NiO_{2} thin films with Sr doping level x from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome x between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x≥0.18, the normal state Hall coefficient (R_{H}) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing x from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.

A controllable electronic manipulation in a frustrated magnetic system such as solution-based two-dimensional (2D) all-inorganic perovskites offers a possible route for their integrations with electronic and magnetic devices for their advanced applications. Here, we perform element-specific investigations of an emergent class of quasi-2D all-inorganic perovskites Cs

High quality rutile Ti 1- x Sn x O 2 nanofilms were successfully grown in a layer-by-layer mode at a moderately low temperature of 400 °C using pulsed laser deposition (PLD). High solid solubility of up to x = 0.216 has been achieved in the Ti 1- x Sn x O 2 films despite theoretical prediction by Density functional theory (DFT) of large formation energy (∼5.64 eV) required for the substitutional alloy to exist at such high Sn concentration. The resultant films have smooth interfaces and step-terraced surfaces with well controlled stoichiometry and are optically transparent. Sn L 3 -edge Extended X-ray absorption fine structure (EXAFS) reveals the substitution of Sn 4+ in the Ti 4+ lattice sites of TiO 2 . The lattice spacing along [110] increases linearly with increment in x due to substitution of Sn 4+ ions in the Ti lattice sites of the Ti 1- x Sn x O 2 films. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering (RBS) show that Sn is uniformly distributed on the surface and in the bulk of the films. These results are crucial when considering Ti 1- x Sn x O 2 with suitable composition for making TiO 2 based quantum structures in advanced optoelectronic devices and solar energy materials, where high-quality crystalline thin film-substrates are important.

Control of magnetic states by external factors has garnered a mainstream status in spintronic research for designing low power consumption and fast-response information storage and processing devices. Previously, magnetic-cation substitution is the conventional means to induce ferromagnetism in an intrinsic antiferromagnet. Theoretically, the anion-doping is proposed to be another effect means to change magnetic ground states. Here we demonstrate the synthesis of high-quality single-phase chromium oxynitride thin films using in-situ nitrogen doping. Unlike antiferromagnetic monoanionic chromium oxide and nitride phases, chromium oxynitride exhibits a robust ferromagnetic and insulating state, as demonstrated by the combination of multiple magnetization probes and theoretical calculations. With increasing the nitrogen content, the crystal structure of chromium oxynitride transits from trigonal (R3c) to tetragonal (4mm) phase and its saturation magnetization reduces significantly. Furthermore, we achieve a large and controllable exchange bias field in the chromia heterostructures by synthetic anion engineering. This work reflects the anion engineering in functional oxides towards the potential applications in giant magnetoresistance and tunnelling junctions of modern magnetic sensors and read heads.

Dielectric polymer composites with high dielectric constant and low dielectric loss are highly demanded for various types of devices. In this work, the core–shell structured polydopamine functionalized silicon carbide whiskers (SiC@PDA) were successfully synthesized. The presence of PDA effectively suppressed the dielectric loss of PVDF/SiC@PDA composites because it could inhibit the connection between fillers and reduce the interfacial polarity. Moreover, the thicker PDA layer contributed to improving dielectric breakdown strength and reducing AC conductivity of PVDF composites. The insulating PVDF/SiC@PDA-a-30 composite containing 30 wt% of SiC@PDA-a had a dielectric constant of 35.1 while the dielectric loss still maintained as low as 0.037 at 1 KHz. Meanwhile, PVDF/SiC@PDA-a-30 achieved the maximum stress of 58.8 MPa with elongation at break of 17.3% due to PDA layer improved the compatibility between SiC@PDA and PVDF. This simple coated PDA shell strategy could be extended to inorganic fillers to develop high performance dielectric polymer composites.

To reach Paris Agreement's target of 1.5 degrees C global temperature increase by 2100, low emission strategies are crucial. In synthetic chemistry, the phototransformation of light alcohols (methanol, ethanol and iso-propanol) into hydrogen fuel and chemicals promises high potential utility value for industry. However, this process is often practiced using promoters (i.e. water, acid or base), with and several issues remained unsolved, e.g. water promoter induce CO2 emission; acid/base promoters restrict catalyst selection, and lead to complicated product purification and cost increase. In this work, we report solar-driven promoter-free multivalorization of light alcohols into self-separable hydrogen gas/fuel and liquid chemicals without COx emission based on the twodimensional (2D) SnS/g-C3N4 heterojunction. The hydrogen production efficiency reaches 1.27 mmol.g(-1). h(-1) with similar to 95 % recoverability. The process reduction occurs via a sequence of redox reactions, combination and disproportionation, as confirmed from experimental and product analysis. This work demonstrates the effectiveness of 2D heterojunctions for promoter-free, zero-emission alcohol phototransformations.

The titanomagnetites (Fe2-xTixO4,x⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.

Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO2, TiO2 and SrTiO3 originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-Tc cuprates, NdBa2Cu3O7-{\delta} (NBCO) and Pr2-xCexCuO4 (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific., Comment: 4 figures; Supporting Information included at the end of the main text

Polymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS2. Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge–lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoS2 on gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T’ phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H–1T’ phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge–lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS2, thereby unlocking new possibilities for 2D-TMD-based device fabrication., MoS2 exhibits multiple electronic properties associated with different crystal structures. Here, the authors observe inverted and fundamental gaps through a designed annealing-based strategy, to induce a semiconductor-to-metal phase transition in monolayer-MoS2 on Au, facilitated by interfacial strain and electron transfer from Au to MoS2.

The molecular orientation of organic semiconductors on a solid surface could be an indispensable factor to determine the electrical performance of organic-based devices. Despite its fundamental prominence, a clear description of the emergent two-dimensional layered material–organic interface is not fully understood yet. In this study, we reveal the molecular alignment and electronic structure of thermally deposited N,N′-dibutyl-3,4,9,10-perylene-dicarboximide (PTCDI-C4) molecules on natural molybdenum disulfide (MoS2) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The average tilt angle determination reveals that the anisotropy in the π* symmetry transition of the carbon K-edge (284–288 eV range) is present at the sub-monolayer regime. Supported by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and resonant photoemission spectroscopy (RPES) measurements, we find that our spectroscopic measurements indicate a weak charge transfer established at...

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

We present a combined experimental and theoretical study of monolayer vanadium ditelluride, VTe

We present a combined experimental and theoretical study of monolayer vanadium ditelluride, VTe2, grown on highly oriented pyrolytic graphite by molecular-beam epitaxy. Using various in situ microscopic and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, together with theoretical analysis by density functional theory calculations, we demonstrate direct evidence of the metallic 1T phase and 3d1 electronic configuration in monolayer VTe2 that also features a (4 × 4) charge density wave order at low temperatures. In contrast to previous theoretical predictions, our element-specific characterization by X-ray magnetic circular dichroism rules out a ferromagnetic order intrinsic to the monolayer. Our findings provide essential knowledge necessary for understanding this interesting yet less explored metallic monolayer in the emerging family of van der Waals magnets.

Transition metal oxides exhibit strong structure-property correlations, which has been extensively investigated and utilized for achieving efficient oxygen electrocatalysts. However, high-performance oxide-based electrocatalysts for hydrogen evolution are quite limited, and the mechanism still remains elusive. Here we demonstrate the strong correlations between the electronic structure and hydrogen electrocatalytic activity within a single oxide system Ti2O3. Taking advantage of the epitaxial stabilization, the polymorphism of Ti2O3 is extended by stabilizing bulk-absent polymorphs in the film-form. Electronic reconstructions are realized in the bulk-absent Ti2O3 polymorphs, which are further correlated to their electrocatalytic activity. We identify that smaller charge-transfer energy leads to a substantial enhancement in the electrocatalytic efficiency with stronger hybridization of Ti 3d and O 2p orbitals. Our study highlights the importance of the electronic structures on the hydrogen evolution activity of oxide electrocatalysts, and also provides a strategy to achieve efficient oxide-based hydrogen electrocatalysts by epitaxial stabilization of bulk-absent polymorphs., Converting solar energy to hydrogen fuel requires light-absorbers that well-match the wavelengths of incoming sunlight. Here, authors prepare a broadband visible-light-absorbing molecular complex that efficiently produces hydrogen from water.

Monolayer VSe2, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim R-143-000-652-112, MOE2016-T2-2-110 National Research Foundation Singapore, NRF Agency for Science, Technology and Research, A*STAR: 1527000016 R-723-000-001-281 Ministry of Science and Higher Education of the Russian Federation: 3.7372.2017/8.9 P.K.J.W. and W.Z. contributed equally to this work. The authors acknowledge financial support from the Singapore Ministry of Education Tier 2 Grants (MOE2016-T2-2-110 and R-143-000-652-112), the National Research Foundation Medium Sized Centre Programme (R-723-000-001-281), and the A*STAR 2D PHAROS Grant (1527000016). D.W.B. acknowledges support from the Ministry of Science and Higher Education of the Russian Federation (Project No. 3.7372.2017/8.9). This work used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231, and was partially performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR Italy Progetti Internazionali) project. The authors would also like to acknowledge the Singapore Synchrotron Light Source (SSLS), a National Research Infrastructure under the National Research Foundation Singapore, for providing the facility necessary for conducting the research. The authors appreciate valuable discussions with Yuanping Feng and Jun Zhou, and experimental assistance from Xiaojiang Yu at the SINS beam line of SSLS.

In article number 1900446, Shi Jie Wang, Wenjing Zhang, Andrivo Rusydi, Andrew T. S. Wee, and co‐workers observe high‐energy excitons that are generated by a new mechanism in monolayer‐MoS2 on SrTiO3, which are attributed to the change in many‐body interactions that couples with interfacial orbital‐hybridization. The interfacial interactions lead to a fermi‐surface feature at the interface. The results provide an understanding of 2D‐transition metal dichalcogenides heterointerfaces and show the crucial role that many‐body interactions play at the atomic level.

The interplay among spin, lattice, charge and orbit is of central importance for several rich and fascinating properties of oxides, and is the subject of intense research at present. Here, we present an approach to manipulate this interplay by Sn doping to effectively apply uniaxial strain on the TiO2 lattice. The evolution of this interplay in pseudo-homoepitaxial Ti1-xSnxO2 films is measured using a combination of X-ray absorption near edge spectroscopy at the O K and Ti L3,2-edges. Supported by various theoretical calculations, we find that the multiplet-type electronic correlations, long-range bonding and hybridization in the system can be controlled by independently modifying uniaxial strain, thereby allowing us to establish the correlations among these effects, doping concentration, and strain. This significantly widens the phase space for experimental exploration of predictive models and leads to new possibilities for manipulation over materials’ functional properties. The methodology presented here can be applied in general to study the nature of the multiplet-type electronic correlations and bonding properties in octahedral-coordinated 3dN transition metal oxides. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

The quasi-metallic 1T'-phase is one of the notable polymorphic two-dimensional transition metal dichalcogenide phases and possesses a unique structure, comprising a one-dimensional zigzag transition metal chain along a single axis. This structure gives rise to unprecedented anisotropic electronic properties, including new anisotropic mid-infrared plasmon excitations. The emergence of highly correlated electronic modes in such anisotropic systems is relevant to the intrinsic electronic and optical properties of two-dimensional transition metal dichalcogenides. In this review, we highlight the unique features of these materials in their anisotropic quasi-metallic 1T'-phase, not found in the other structural phases. We discuss how this anisotropic structure brings about the onset of unique physical phenomena such as the electronic band inversion, anomalously large magnetoresistance, and intrinsic or induced superconductivity in multiple transition metal dichalcogenide systems. The unique structural properties of these systems have led to spatially resolved transport and optical properties that can be further manipulated for use in a wide variety of applications ranging from nano-electronics to ultra-sensitive photonic systems but also their incorporation in high performance field-electric transistors, photodetectors, hydrogen evolution reactions, and piezoelectric devices cater to technologies of the future.

Dielectric polymer nanocomposites with high dielectric permittivity and low dielectric loss have great applications in energy storage field. In this study, a series of PVDF/MXene nanocomposites was prepared by solution method. The surface of MXene was functionalized with cetyltrimethylammonium bromide (MXene@CTAB) to achieve better dispersion in poly(vinylidene fluoride) (PVDF) matrix. The results revealed that the MXene@CTAB dispersed uniformly and had a good compatibility with PVDF matrix. In addition, the dynamical rheological test revealed that the viscosity and storage modulus of PVDF nanocomposites increased with the increasing of MXene@CTAB loading. The PVDF/MXene@CTAB nanocomposite containing 7 wt% MXene@CTAB exhibited a dielectric permittivity of 82.1 and a loss factor as low as 0.2 at 1 kHz. The presence of CTAB could serve as an insulating layer to suppress dielectric loss and AC conductivity of PVDF nanocomposites simultaneously. The PVDF/MXene@CTAB nanocomposite with high dielectric permittivity may find potential applications in energy storage capacitors for electronics. Moreover, the addition of MXene@CTAB contributed to the formation of Beta phase of PVDF.

The magnetic properties of a series of IrMn(Ir35Mn65)/permalloy (Ni80Fe20) exchange-biased films are investigated using X-ray magnetic circular dichroism (XMCD). The ferromagnetic permalloy films in different samples are of the same thickness (5 nm) while the antiferromagnetic IrMn films have various thickness between 15 and 40 nm. The exchange-field and coercivity is observed to increase with IrMn thickness. XMCD spectra are used to obtain the magnetic moments. It is shown that when the thickness of IrMn layer increases from 20 to 30 nm, a significant increase of the Fe spin-moment occurs. However, the spin-moment of Ni shows no obvious change. We argue that this effect is attributed to the strong coupling between Fe and Mn, while the coupling between Ni and Mn is relatively weaker. The results show the importance of understanding magnetic coupling effects at the exchange-biased FM/AFM interfaces. The study of these interfacial effects can potentially pave the way to the future development of spin-valve devices.

In article number https://doi.org/10.1002/advs.201902726, Xinmao Yin, Andrew T. S. Wee, and co‐workers observe 1D mid‐IR range anisotropic plasmons in quasi‐metallic WSe2 and MoS2 monolayers. In‐depth analysis reveals that the long‐range inter‐chain coupling is responsible for the 1D‐charge dynamics and the plasmon energy is dictated by the effects of spin–orbit coupling. Detailed investigation into these collective modes in 2D‐chained systems opens new opportunities for novel device applications and holds significant implications to the underlying mechanism that governs superconductivity in 2D‐transition‐metal dichalcogenide systems.

We present an optical and photoelectron spectroscopic study to elucidate the interfacial electronic properties of organic-inorganic semiconductor heterojunctions formed in a kinetically blocked heptazethrene triisopropylsilyl ethynylene (HZ-TIPS) and its homologue, octazethrene (OZ-TIPS) on an all-inorganic perovskite cesium lead bromide (CsPbBr3) surface. The photoluminescence behavior of the underlying perovskites upon differing molecular doping conditions was examined. It turns out that the charge transfer dynamics of thermally-evaporated OZ-TIPS molecule exhibited a faster average lifetime than that of the HZ-TIPS case suggesting the importance of the biradical state in the former molecule. An interfacial dipole was formed at the interface due to the competing interaction between the dispersion force of the bulky TIPS-substituent group and the attractive van der Waals interaction at the first few layers. Photoemission spectroscopy of the physisorbed HZ-TIPS shows chemical shifts, which indicates electron transfer from HZ-TIPS molecules to the CsPbBr3 perovskite single crystal. In contrast, the adsorbed OZ-TIPS molecular layer on CsPbBr3 demonstrates the opposite trend indicating a hole transfer process. The average molecular orientation as determined by near edge X-ray absorption fine structure (NEXAFS) suggests that the HZ-TIPS molecular plane is generally lifted with respect to the perovskite surface. We suggest that the nature of the closed-shell electronic ground state of HZ-TIPS could contribute to the formation of interfacial dipole at the molecule/perovskite interface. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Perovskite-structured (ABO3) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare-earth nickelates (RNiO3) thin films is investigated with controlled A-site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three-electrode system in O2-saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A-site element ionic radius which lowers the conductivity of RNiO3 (R = La, La0.5Nd0.5, La0.2Nd0.8, Nd, Nd0.5Sm0.5, Sm, and Gd) films, with LaNiO3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La0.2Nd0.8NiO3. Moreover, the OER activity remains comparable within error through Sm-doped NdNiO3. Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni3+ to Ni2+ as a result of oxygen vacancies, which increases the average occupancy of the eg antibonding orbital to more than one. The work highlights the importance of tuning A-site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts. MOE (Min. of Education, S’pore)

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic, and magnetic properties induced by the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic, and magnetic properties of La0.7Sr0.3MnO3 thin films on SrTiO3 (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2 nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin film nanostructure LSMO/STO at ∼4 eV, which could be attributed to the formation of anomalous charged excitonic complexes. On the basis of the spectral weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic correlation in LSMO films....

Magnetite Fe3O4 (ferrite) has attracted considerable interest for its exceptional physical properties: It is predicted to be a semimetallic ferromagnetic with a high Curie temperature, it displays a metal-insulator transition, and has potential oxide-electronics applications. Here, we fabricate a high-magnetization (> 1 Tesla) high-resistance (~0.1 Ω·cm) sub-nanostructured (grain size < 3 nm) Fe3O4 film via grain-size control and nano-engineering. We report a new phenomenon of spin-flipping of the valence-spin tetrahedral Fe3+ in the sub-nanostructured Fe3O4 film, which produces the high magnetization. Using soft X-ray magnetic circular dichroism and soft X-ray absorption, both at the Fe L3,2- and O K-edges, and supported by first-principles and charge-transfer multiple calculations, we observe an anomalous enhancement of double exchange, accompanied by a suppression of the superexchange interactions because of the spin-flipping mechanism via oxygen at the grain boundaries. Our result may open avenues for developing spin-manipulated giant magnetic Fe3O4-based compounds via nano-grain size control.

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO3 (STO) crystals and LaAlO3 (LAO) overlayer through the measurements of electrical transport, X-ray absorption spectroscopy (XAS) and photoluminescence (PL) spectra. We found that oxygen vacancies (Ovac) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of Ovac, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces., 5 figures; Supplementary materials included at the end of the main text

Physical review / B covering condensed matter and materials physics 95(16), 165108(2017). doi:10.1103/PhysRevB.95.165108, Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order tounderstand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity(up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarizationof the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holesin lightly hole-doped $La_{1.95}Sr_{0.05}Cu_{0.95}Zn_{0.05}O_4$. The changes of the local spin polarization manifest stronglyin the temperature-dependent optical conductivity at ∼7.2 eV, with an anomaly at the magnetic stripe phase(∼ 25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoreticalcalculations, we also assign high-energy optical transitions and their corresponding temperature dependence,particularly at ∼2.5, ∼8.7, ∼9.7, ∼11.3, and ∼21.8 eV. Our result shows the importance of a strong mixture ofspin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magneticcorrelations using high-energy optical conductivity in correlated electron systems., Published by Inst., Woodbury, NY

The molecular orientation of organic semiconductors on a solid surface could be an indispensable factor to determine the electrical performance of organic-based devices. Despite its fundamental prominence, a clear description of the emergent two-dimensional layered material-organic interface is not fully understood yet. In this study, we reveal the molecular alignment and electronic structure of thermally deposited N,N'-dibutyl-3,4,9,10-perylene-dicarboximide (PTCDI-C4) molecules on natural molybdenum disulfide (MoS

As an iron oxyhydroxide, nanosized ferrihydrite (Fh) is important in Earth science, biology, and industrial applications. However, its basic structure and origin of its magnetism have long been debated. We integrate synchrotron-based techniques to explore the chemical structures of 2-line ferrihydrite and to determine the origin of its magnetism during hydrothermal aging in air. Our results demonstrate that both the magnetism and X-ray magnetic circular dichroism (XMCD) signal of 2-line ferrihydrite are enhanced with aging time, and that XMCD spectral patterns resemble that of maghemite (γ-Fe2O3) rather than magnetite (Fe3O4). Fe L-edge and K-edge X-ray absorption spectroscopy (XAS) further indicate formation of both maghemite and hematite (α-Fe2O3) with increasing concentrations with longer hydrothermal aging time. Thus, magnetic enhancement with longer hydrothermal aging time is attributed to increasing maghemite concentration instead of a magnetically ordered ferrihydrite as previously reported. Moreov...

The properties of perovskite nickelates are very sensitive to their oxygen content, which allows us to tune their electronic structures by varying the oxygen partial pressure during film deposition. Under the optimized condition, we have obtained GdNiO3 films that are sensitive to a wide spectrum of light. By combining the GdNiO3 film with Nb-doped SrTiO3 to form a heterojunction, we design a self-powered photodetector with high sensitivity toward light with a wavelength between 650 nm and 365 nm. Under 365 nm illumination (50 μW/cm2), the device shows a responsivity of 0.23 A/W at 0 V bias, comparable to or even better than the ultraviolet photodetectors made of semiconductor materials such as GaN or ZnO. The photo-dark ratio can be close to 103 when the power light density reaches 0.6 mW/cm2. Moreover, the device performance is very stable without any decay after 6 months. MOE (Min. of Education, S’pore) Published version

Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS$_2$ isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A ($1.92$ eV) and B ($2.02$ eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the bandstructure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This work unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS$_2$., Comment: Accepted in Physical Review Letters; Main Article: 6 pages, 4 figures; Supplemental Material: 6 pages, 7 figures, 10 tables

The perovskite oxide LaNiO3 is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO3 films (La1− xSrxNiO3) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at x = 0.5 comparable to the state‐of‐the‐art catalyst Ba0.5Sr0.5Co0.8Fe0.2O3− δ. By combining X‐ray photoemission and X‐ray absorption spectroscopies with density functional theory, it is shown that an upward energy shift of the O 2p band relative to the Fermi level occurs with increasing x in La1− xSrxNiO3. This alloying step strengthens Ni 3d–O 2p hybridization and decreases the charge transfer energy, which in turn accounts for the enhanced OER activity.

2D transition metal dichalcogenides (2D‐TMDs) and their unique polymorphic features such as the semiconducting 1H and quasi‐metallic 1T′ phases exhibit intriguing optical and electronic properties, which can be used in novel electronic and photonic device applications. With the favorable quasi‐metallic nature of 1T′‐phase 2D‐TMDs, the 1H‐to‐1T′ phase engineering processes are an immensely vital discipline exploited for novel device applications. Here, a high‐yield 1H‐to‐1T′ phase transition of monolayer‐MoS2 on Cu and monolayer‐WSe2 on Au via an annealing‐based process is reported. A comprehensive experimental and first‐principles study is performed to unravel the underlying mechanism and derive the general trends for the high‐yield phase transition process of 2D‐TMDs on metallic substrates. While each 2D‐TMD possesses different intrinsic 1H‐1T′ energy barriers, the option of metallic substrates with higher chemical reactivity plays a significantly pivotal role in enhancing the 1H‐1T′ phase transition yield. The yield increase is achieved via the enhancement of the interfacial hybridizations by the means of increased interfacial binding energy, larger charge transfer, shorter interfacial spacing, and weaker bond strength. Fundamentally, this study opens up the field of 2D‐TMD/metal‐like systems to further scientific investigation and research, thereby creating new possibilities for 2D‐TMDs‐based device applications.

Advances in thin film growth technologies make it possible to obtain ultra-thin perovskite oxide films and open the window for controlling novel electronic phases for use in functional nanoscale electronics, such as switches and sensors. Here, we study the thickness-dependent transport characteristics of high-quality ultrathin Nd

We report the first observation of the coexistence of a distinct midgap state and a Mott state in undoped and their evolution in electron and hole-doped ambipolar Y_{0.38}La_{0.62}(Ba_{0.82}La_{0.18})_{2}Cu_{3}O_{y} films using spectroscopic ellipsometry and x-ray absorption spectroscopies at the O K and Cu L_{3,2} edges. Supported by theoretical calculations, the midgap state is shown to originate from antiferromagnetic correlation. Surprisingly, while the magnetic state collapses and its correlation strength weakens with dopings, the Mott state in contrast moves toward a higher energy and its correlation strength increases. Our result provides important clues to the mechanism of electronic correlation strengths and superconductivity in cuprates.

The interactions between delocalized and localized charges play important roles in correlated electron systems. Here, using a combination of transport measurements, spectroscopic ellipsometry (SE), and X-ray absorption spectroscopy (XAS) supported by theoretical calculations, we reveal the important role of interfacial localized charges and their screening effects in determining the mobility of (La0.3 Sr0.7 )(Al0.65 Ta0.35 )O3 /SrTiO3 (LSAT/SrTiO3 ) interfaces. When the LSAT layer thickness reaches the critical value of 5 uc, the insulating interface abruptly becomes conducting, accompanied by the appearance of a new midgap state. This midgap state emerges at ≈1 eV below the Ti t2g band and shows a strong character of Ti 3dxy - O 2p hybridization. Increasing the LSAT layer from 5 to 18 uc, the number of localized charges increases, resulting in an enhanced screening effect and higher mobile electron mobility. This observation contradicts the traditional semiconductor interface where the localized charges always suppress the carrier mobility. These results demonstrate a new strategy to probe localized charges and mobile electrons in correlated electronic systems and highlight the important role of screening effects from localized charges in improving the mobile electron mobility at complex oxide interfaces.

This work was supported by the King Abdullah University of Science and Technology (KAUST). Y.K.W. and S.D. were supported by National Natural Science Foundation of China (Grant No. 11674055). J.S.C. was supported by the Singapore National Research Foundation under CRP Award No. NRF-CRP10-2012-02 and IIP award No. NRF-IIP001-001.

Perovskite manganites exhibit fascinating transport and magnetic properties, essential for fundamental research and applications. With the development of thin film technologies, more exotic properties have been observed in doped-manganites over a wide range of temperature. Unraveling the interplay of spin, charge and orbital degrees of freedom that drives exotic, macroscopic properties is therefore crucial for the understanding of strongly correlated electron systems. Here, using a combination of transport, spectroscopic ellipsometry, X-ray absorption spectroscopy and X-ray magnetic circular dichroism, we observe two concomitant electronic and magnetic phases (insulating paramagnetic phase for T>195 K and insulating canted-ferromagnetic for T

Transition metal oxides have a range of unique properties due to coupling of charge, spin, orbital and lattice degrees of freedom and nearly degenerate multiple ground states. These properties make them interesting for applications and for fundamental investigations. Here we report a new phase with abnormal transport anisotropy in La0.7Sr0.3MnO3 ultra-thin films under large tensile strain. This anisotropy is absent in films under smaller tensile strain or compressive strain. Furthermore, thickness and magnetic-field-dependent experiments suggest that the tensile-strain-induced two-dimensional character is crucial for the observed phenomena. X-ray absorption spectroscopy results indicate that this anisotropy is likely driven by O 2p orbital, which hybridizes with Mn 3d. Ab initio calculations confirm this result. Our results may help to understand the anisotropic transport behaviour observed in other systems., Transition metal oxides can have exotic electronic phases because of the interplay of charge, spin and orbital degrees of freedom. Here, the authors investigate thin films of La0.7Sr0.3MnO3 and show a new phase with transport anisotropy under large tensile strain.

Advances in thin film growth technologies make it possible to obtain ultra-thin perovskite oxide films and open the window for controlling novel electronic phases for use in functional nanoscale electronics, such as switches and sensors. Here, we study the thickness-dependent transport characteristics of high-quality ultrathin Nd0.9Sr0.1NiO3 (Sr-NNO) films, which were grown on LaAlO3 (0 0 1) single-crystal substrates by using pulsed laser deposition method. Thick Sr-NNO films (25 unit cells) exhibit metallic behavior with the electrical resistivity following the T n (n

We report the optical and electronic properties of multilayer graphene films grown epitaxially on the carbon face (C-face) of 4H-SiC probed using spectroscopic ellipsometry (SE) and angle-resolved photoemission spectroscopy (ARPES). The optical conductivity in the energy range from 1.0 to 5.3 eV extracted from SE reveals two important features: the presence of universal conductivity at the near-infrared region and asymmetric resonant excitons at 4.5 eV. Furthermore, ARPES shows the presence of independent linear electronic dispersion. These features resemble quasi-freestanding properties of multilayer graphene grown on the C-face of SiC.