A Delicate Balance Between Spin-Wave Mediated Weak Localization and Electron-Phonon Scattering in the Design of Zero Temperature Coefficient of Resistivity.

Zero thermal coefficients of resistivity (ZTCR) materials exhibit minimal changes in resistance with temperature variations, making them essential in modern advanced technologies. The current ZTCR materials, which are based on the resistivity saturation effect of heavy metals, tend to function at elevated temperatures because the mean free path approaches the lower limit of the semiclassical Boltzmann theory when the temperature is sufficiently high. ZTCR materials working at low-temperatures are difficult to achieve due to electron-phonon scattering, which results in increased resistivity according to Bloch's theory. In this work, the ZTCR behavior at low-temperatures is realized in pre-microstrained Mn3NiN. The delicate balance between the resistivity contribution from electron-phonon scattering and spin-wave mediated weak localization is well revealed. A remarkable temperature coefficient of resistivity (TCR) value as low as 1.9 ppm K-1 (50 K ≤ T ≤ 200 K) is obtained, which is significantly superior to the threshold value of ZTCR behavior and the application standard of commercial ZTCR materials. The demonstration provides a unique paradigm in the design of ZTCR materials through the contraction effects of two opposite conductance mechanisms with positive and negative thermal coefficients of resistivity.

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure.

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of Tc up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

Electric-Field-Driven Negative Differential Conductance in 2D van der Waals Ferromagnet Fe3GeTe2.

Understanding quantum tunneling principles over two-dimensional (2D) van der Waals (vdW) ferromagnets at the atomic level is essential and complementary to the fundamental study of low-dimensional strong correlated systems and is critical for the development of magnetic tunneling devices. Here, we demonstrate a local electric-field controlled negative differential conductance (NDC) in 2D vdW ferromagnet Fe3GeTe2 (FGT) by using scanning tunneling microscopy (STM). The STM reveals that NDC shows an atomic position dependence and can be precisely modulated by altering the tunneling junction. The band shift together with electric-field-driven 3d-orbital occupancy modulates the sensitive magnetic anisotropic energy (MAE) in 2D FGT and consequently leads to electric-field-tunable NDC, which is also verified by theoretical simulation. This work realizes the electric-field-driven NDC in 2D ferromagnet FGT, which paves a way to design and develop applications based on 2D vdW magnets.

Kondo Holes in the Two-Dimensional Itinerant Ising Ferromagnet Fe3GeTe2.

Heavy Fermion (HF) states emerge in correlated quantum materials due to the intriguing interplay between localized magnetic moments and itinerant electrons but rarely appear in 3d-electron systems due to high itinerancy of d-electrons. Here, an anomalous enhancement of Kondo screening is observed at the Kondo hole of local Fe vacancies in Fe3GeTe2 which is a recently discovered 3d-HF system featuring Kondo lattice and two-dimensional itinerant ferromagnetism. An itinerant Kondo-Ising model is established to reproduce the experimental results and provides insight into the competition between Ising ferromagnetism and Kondo screening. Our work explains the microscopic origin of the d-electron HF states in Fe3GeTe2 and inspires future studies of the enriched quantum many-body effects with Kondo holes.

Moiré-Potential-Induced Band Structure Engineering in Graphene and Silicene.

A moiré pattern results from the projection of one periodic pattern to another with relative lattice constant or misalignment and provides great periodic potential to modify the electronic properties of pristine materials. In this Review, recent research on the effect of the moiré superlattice on the electronic structures of graphene and silicene, both of which possess a honeycomb lattice, is focused on. The moiré periodic potential is introduced by the interlayer interaction to realize abundant phenomena, including new generation of Dirac cones, emergence of Van Hove singularities (vHs) at the cross point of two sets of Dirac cones, Mott-like insulating behavior at half-filling state, unconventional superconductivity, and electronic Kagome lattice and flat band with nontrivial edge state. The role of interlayer coupling strength, which is determined by twist angle and buckling degree, in these exotic properties is discussed in terms of both the theoretical prediction and experimental measurement, and finally, the challenges and outlook for this field are discussed.

Atomic Structural Evolution of Single-Layer Pt Clusters as Efficient Electrocatalysts.

The rational synthesis of single-layer noble metal directly anchored on support materials is an elusive target to accomplish for a long time. This paper reports well-defined single-layer Pt (Pt-SL) clusters anchored on ultrathin TiO2 nanosheets-as a new frontier in electrocatalysis. The structural evolution of Pt-SL/TiO2 via self-assembly of single Pt atoms (Pt-SA) is systematically recorded. Significantly, the Pt atoms of Pt-SL/TiO2 possess a unique electronic configuration with PtPt covalent bonds surrounded by abundant unpaired electrons. This Pt-SL/TiO2 catalyst presents enhanced electrochemical performance toward diverse electrocatalytic reactions (such as the hydrogen evolution reaction and the oxygen reduction reaction) compared with Pt-SA, multilayer Pt nanoclusters, and Pt nanoparticles, suggesting an efficient new type of catalyst that can be achieved by constructing single-layer atomic clusters on supports.

Efficient Ammonia Electrosynthesis from Nitrate on Strained Ruthenium Nanoclusters.

The limitations of the Haber-Bosch reaction, particularly high-temperature operation, have ignited new interests in low-temperature ammonia-synthesis scenarios. Ambient N2 electroreduction is a compelling alternative but is impeded by a low ammonia production rate (mostly <10 mmol gcat-1 h-1), a small partial current density (<1 mA cm-2), and a high-selectivity hydrogen-evolving side reaction. Herein, we report that room-temperature nitrate electroreduction catalyzed by strained ruthenium nanoclusters generates ammonia at a higher rate (5.56 mol gcat-1 h-1) than the Haber-Bosch process. The primary contributor to such performance is hydrogen radicals, which are generated by suppressing hydrogen-hydrogen dimerization during water splitting enabled by the tensile lattice strains. The radicals expedite nitrate-to-ammonia conversion by hydrogenating intermediates of the rate-limiting steps at lower kinetic barriers. The strained nanostructures can maintain nearly 100% ammonia-evolving selectivity at >120 mA cm-2 current densities for 100 h due to the robust subsurface Ru-O coordination. These findings highlight the potential of nitrate electroreduction in real-world, low-temperature ammonia synthesis.

Hydrogen Terminated Germanene for a Robust Self-Powered Flexible Photoelectrochemical Photodetector.

As a rising star in the family of graphene analogues, germanene shows great potential for electronic and optical device applications due to its unique structure and electronic properties. It is revealed that the hydrogen terminated germanene not only maintains a high carrier mobility similar to that of germanene, but also exhibits strong light-matter interaction with a direct band gap, exhibiting great potential for photoelectronics. In this work, few-layer germanane (GeH) nanosheets with controllable thickness are successfully synthesized by a solution-based exfoliation-centrifugation route. Instead of complicated microfabrication techniques, a robust photoelectrochemical (PEC)-type photodetector, which can be extended to flexible device, is developed by simply using the GeH nanosheet film as an active electrode. The device exhibits an outstanding photocurrent density of 2.9 µA cm-2 with zero bias potential, excellent responsivity at around 22 µA W-1 under illumination with intensity ranging from 60 to 140 mW cm-2 , as well as short response time (with rise and decay times, tr = 0.24 s and td = 0.74 s). This efficient strategy for a constructing GeH-based PEC-type photodetector suggests a path to promising high-performance, self-powered, flexible photodetectors, and it also paves the way to a practical application of germanene.

Hierarchical Nanoassembly of MoS2/Co9S8/Ni3S2/Ni as a Highly Efficient Electrocatalyst for Overall Water Splitting in a Wide pH Range.

The design of low-cost yet high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) over a wide pH range is highly challenging. We now report a hierarchical co-assembly of interacting MoS2 and Co9S8 nanosheets attached on Ni3S2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance toward HER and OER over a very broad pH range. By adjusting the molar ratio of the Co:Mo precursors, we have created CoMoNiS-NF- xy composites ( x: y means Co:Mo molar ratios ranging from 5:1 to 1:3) with controllable morphology and composition. The three-dimensional composites have an abundance of active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103, and 117 mV for HER and 166, 228, and 405 mV for OER) to achieve a current density of 10 mA cm-2 in alkaline, acidic, and neutral electrolytes, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on the distinguished catalytic performance of CoMoNiS-NF-31 toward HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45, and 1.80 V at 10 mA cm-2 in alkaline, acidic, and neutral media, respectively. First-principles calculations suggest that the high OER activity arises from electron transfer from Co9S8 to MoS2 at the interface, which alters the binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH = 0-14) electrocatalysts for overall water splitting.

Direct Observation of Oxygen Vacancy Self-Healing on TiO2 Photocatalysts for Solar Water Splitting.

Oxygen vacancy (Vo) on transition metal oxides plays a crucial role in determining their chemical/physical properties. Conversely, the capability to directly detect the changing process of oxygen vacancies (Vos) will be important to realize their full potentials in the related fields. Herein, with a novel synchronous illumination X-ray photoelectron spectroscopy (SI-XPS) technique, we found that the surface Vos (surf-Vos) exhibit a strong selectivity for binding with the water molecules, and sequentially capture an oxygen atom to achieve the anisotropic self-healing of surface lattice oxygen. After this self-healing process, the survived subsurface Vos (sub-Vos) promote the charge excitation from Ti to O atoms due to the enriched electron located on low-coordinated Ti sites. However, the excessive sub-Vos would block the charge separation and transfer to TiO2 surfaces resulted from the destroyed atomic structures. These findings open a new pathway to explore the dynamic changes of Vos and their roles on catalytic properties, not only in metal oxides, but in crystalline materials more generally.

Solid Base Bi24 O31 Br10 (OH)δ with Active Lattice Oxygen for the Efficient Photo-Oxidation of Primary Alcohols to Aldehydes.

The selective oxidation of primary alcohols to aldehydes by O2 instead of stoichiometric oxidants (for example, MnVII , CrVI , and OsIV ) is an important but challenging process. Most heterogeneous catalytic systems (thermal and photocatalysis) require noble metals or harsh reaction conditions. Here we show that the Bi24 O31 Br10 (OH)δ photocatalyst is very efficient in the selective oxidation of a series of aliphatic (carbon chain from C1 to C10 ) and aromatic alcohols to their corresponding aldehydes/ketones under visible-light irradiation in air at room temperature, which would be challenging for conventional thermal and light-driven processes. High quantum efficiencies (71 % and 55 % under 410 and 450 nm irradiation) are reached in a representative reaction, the oxidation of isopropanol. We propose that the outstanding performance of the Bi24 O31 Br10 (OH)δ photocatalyst is associated with basic surface sites and active lattice oxygen that boost the dehydrogenation step in the photo-oxidation of alcohols.

Three-Dimensional Hierarchical Architectures Derived from Surface-Mounted Metal-Organic Framework Membranes for Enhanced Electrocatalysis.

Inspired by the rapid development of metal-organic-framework-derived materials in various applications, a facile synthetic strategy was developed for fabrication of 3D hierarchical nanoarchitectures. A surface-mounted metal-organic framework membrane was pyrolyzed at a range of temperatures to produce catalysts with excellent trifunctional electrocatalytic efficiencies for the oxygen reduction, hydrogen evolution, and oxygen evolution reactions.

Magnetic field actuated manipulation and transfer of oil droplets on a stable underwater superoleophobic surface.

The transport of fluids at functional interfaces, driven by the external stimuli, is well established. The lossless transport of oil-based fluids under water remains a challenge, however, due to their high stickiness towards the surface. Here, a superhydrophilic and underwater superoleophobic tri-phase water/oil/solid nanoarray surface has been designed and prepared. The unique tri-phase surface exhibits underwater superoleophobic properties with an extremely low stickiness towards oil-based fluids. The magnetic-field-driven manipulation and transport of oil-based magnetic fluids are demonstrated under water, which opens up a new pathway to design flexible and smart devices for the control and transfer of liquid droplets by using tri-phase systems.

Variation of the coordination environment and its effect on the white light emission properties in a Mn-doped ZnO-ZnS complex structure.

Mn-doped ZnO-ZnS complex nanocrystals were fabricated through coating of dodecanethiol on Mn-doped ZnO nanocrystals. The relationship between the component of white light emission and the coordination environments of Mn-dopants were experimentally investigated. It was shown that Mn ions mainly formed Mn(3+)O6 octahedra in as prepared Mn-doped ZnO, while the Mn(3+) ions on the surface of ZnO transferred into Mn(2+) ions at the interface between ZnO and ZnS after dodecanethiol coating. The Mn(2+)S4 tetrahedron density and the orange emission intensity increased upon enhancing the dodecanethiol content. These results provide an alternative way to optimize the white emission spectrum from nanocrystals of Mn-doped ZnS-ZnO complex structures through modulation of the coordination environment of Mn ions.

Design of higher Chern number two-band structures from topological defect perspective.

In this article, we propose two methods for designing higher Chern number models from the topological defect perspective. Based on the fact that the Chern number is equal to a summation of the charges of meron defects, we show that the higher Chern number structures can be realized by either moving the positions of merons or increasing the amount of them. The combination of the two methods is also verified to be a viable approach. We shall construct several models and investigate their energy spectrum. More than one gapless state can be observed on the edges of these models. Expectedly, our theory promises to provide not only a simple approach to obtain the Chern number without computing any integrals, but also a practical technique for new material design.

Epitaxial Growth of Monolayer SnP3 with High Mobility and Chiral Boundary Junctions.

Two-dimensional materials with layered structures, appropriate band gaps, and high carrier mobility have attracted tremendous interest for their potential applications. Here we report the growth of monolayer SnP3 on Au(111) surfaces by molecular beam epitaxy. The kinetic processes for the growth and the crystalline properties are studied by scanning tunneling microscopy. The weak interaction between SnP3 and its Au(111) substrate is signified by the random crystal orientation distributions of SnP3 nanosheets. The electronic structures exhibit a band gap of ∼0.25 eV and high charge carrier mobility comparable to that of black phosphorus engineered by compressive strain. Additionally, domain boundary junctions with opposite chirality are observed, resulting from the strained film in the epitaxial growth process. Our work provides a method to fabricate high-quality monolayer SnP3 and suggests that the monolayer SnP3 is a promising candidate for applications in nanoelectronics and optoelectronics.

Controllable Modulation of the Electronic Properties of a Two-Dimensional Ambipolar Semiconductor by Interface Ferroelectric Polarization.

Precise control of charge carrier type and density of two-dimensional (2D) ambipolar semiconductors is the prerequisite for their applications in next-generation integrated circuits and electronic devices. Here, by fabricating a heterointerface between a 2D ambipolar semiconductor (hydrogenated germanene, GeH) and a ferroelectric substrate (PbMg1/3Nb2/3O3-PbTiO3, PMN-PT), fine-tuning of charge carrier type and density of GeH is achieved. Due to ambipolar properties, proper band gap, and high carrier mobility of GeH, by applying the opposite local bias (±8 V), a lateral polarization in GeH is constructed with a change of work function by 0.6 eV. Besides, the built-in polarization in GeH nanoflake could promote the separation of photoexcited electron-hole pairs, which lead to 4 times enhancement of the photoconductivity after poling by 200 V. In addition, a gradient regulation of the work function of GeH from 4.94 to 5.21 eV by adjusting the local substrate polarization is demonstrated, which could be used for data storage at the micrometer size by forming p-n homojunctions. This work of constructing such heterointerfaces provides a pathway for applying 2D ambipolar semiconductors in nonvolatile memory devices, photoelectronic devices, and next-generation integrated circuit.

Observation of Anomalous Planar Hall Effect Induced by One-Dimensional Weak Antilocalization.

The confinement of electrons in one-dimensional (1D) space highlights the prominence of the role of electron interactions or correlations, leading to a variety of fascinating physical phenomena. The quasi-1D electron states can exhibit a unique spin texture under spin-orbit interaction (SOI) and thus could generate a robust spin current by forbidden electron backscattering. Direct detection of such 1D spin or SOI information, however, is challenging due to complicated techniques. Here, we identify an anomalous planar Hall effect (APHE) in the magnetotransport of quasi-1D van der Waals (vdW) topological materials as exemplified by Bi4Br4, which arises from the quantum interference correction of 1D weak antilocalization (WAL) to the ordinary planar Hall effect and demonstrates a deviation from the usual sine and cosine curves. The occurrence of 1D WAL is correlated to the line-shape Fermi surface and persistent spin texture of (100) topological surface states of Bi4Br4, as revealed by both our angle-resolved photoemission spectroscopy and first-principles calculations. By generalizing the observation of APHE to other non-vdW bulk materials, this work provides a possible characteristic of magnetotransport for identifying the spin/SOI information and quantum interference behavior of 1D states in 3D topological material.

Manipulation of domain wall mobility by oxygen vacancy ordering in multiferroic YMnO3.

The mobility of the ferroelectric domain phases and the local conductivity of ferroelectric domain walls in multiferroic YMnO3 crystals grown in air and reduced atmosphere were studied by piezoresponse force microscopy (PFM), tip-enhanced Raman spectroscopy (TERS) and conductive atomic force microscopy (CAFM). Oxygen vacancies were found to reduce the strength of 4d-2p (Y(3+)-O(2-)) hybridization and structural trimerization distortion, leading to the disappearance of the six wedge-shaped ferroelectric domain phases in oxygen deficient YMnO3-δ crystals. We observed anisotropic domain wall motion such that the wedge-shaped domain configuration joined at one point could be changed to the stripe domain configuration by applying high electric fields in oxygen deficient YMnO3-δ single crystals. The local conductivity of the domain walls increased significantly in poled YMnO3-δ single crystals. The straight conductive domain walls in YMnO3-δ, instead of the twisted insulating ones in the stoichiometric crystal, are induced by the ordered oxygen vacancies which are verified by TERS measurements.

Design of ultrathin CoAl-LDHs/ZnIn2S4 with strong interfacial bonding and rich oxygen vacancies for highly efficient hydrogen evolution activity.

Designing a semiconductor-based heterostructure photocatalyst is very important way to enhance the hydrogen production activity. Here, a novel 2D/2D CoAl-LDHs/ZnIn2S4 S-scheme heterostructure with an ultrathin structure was synthesized by electrostatic attraction between CoAl-LDHs and ZnIn2S4 nanosheets. The presence of oxygen vacancies in the monolayer CoAl-LDHs nanosheet promotes the formation of Co-SX bonds, which serve as charge transfer channels at the interface of the CoAl-LDHs/ZnIn2S4 heterostructure. The ultrathin CoAl-LDHs/ZnIn2S4 exhibits broadened light absorption in the near-infrared range due to the occurrence of Co-SX chemical bonds. The CoAl-LDHs/ZnIn2S4 with a mass ratio of 1:2 demonstrated the highest photocatalytic hydrogen evolution activity (1563.64 µmol g-1 h-1) under the simulated sunlight, which is 4.6 and 9.7 times than that of the ZnIn2S4 and CoAl-LDHs/ZnIn2S4(bulk), respectively. The enhanced photocatalytic activity of ultrathin 2D/2D CoAl-LDHs/ZnIn2S4 should attributed to the shorter carriers path that benefit from the ultrathin structure and the quicker photogenerated charge transfer and the S-scheme migration pathway accelerated by the charge channel of Co-SX bonds. These new ideas should be inspiring for the design and construction of heterostructures for higher photocatalytic hydrogen evolution activity.