High-Entropy Strategy to Achieve Electronic Band Convergence for High-Performance Thermoelectrics.

Multiband convergence has attracted significant interest due to its positive effects on further improving thermoelectric performance. However, the current research mainly focuses on two- or three-band convergence in lead chalcogenides through doping and alloying. Therefore, exploring a new strategy to facilitate more-band convergence has instructive significance and practical value in thermoelectric research. Herein, we first propose a high-entropy strategy to achieve four-band convergence for optimizing thermoelectric performance. Taking high-entropy AgSbPbSnGeTe5 as an example, we found that the emergence of more-band convergence occurs as the configuration entropy increases; in particular, the four-band convergence occurs in high-entropy AgSbPbSnGeTe5. The overlap of multiatom orbitals in the high-entropy sample contributes to the convergence of four valence bands, promoting the improvement of electrical performance. Meanwhile, due to large lattice distortion and disordered atoms, the phonon mean free path is effectively compressed, resulting in low lattice thermal conductivity of high-entropy AgSbPbSnGeTe5. Consequently, AgSbPbSnGeTe5 achieved an intrinsically high ZT value of 1.22 at 673 K, providing a cornerstone for further optimizing thermoelectric performance. For example, by generally optimizing the carrier concentration, a peak ZT value of ∼1.75 at 723 K is achieved. These insights offer a comprehensive understanding of the band structure affected by unique structures of high-entropy materials and also shed useful light on innovation mechanisms and functionalities for future improvement of thermoelectric performance.

Even-Odd-Layer-Dependent Ferromagnetism in 2D Non-van-der-Waals CrCuSe2.

Van der Waals (vdW) layered materials with strong magnetocrystalline anisotropy have attracted significant interest as the long-range magnetic order in these systems can survive even when their thicknesses is reduced to the 2D limit. Even though the interlayer coupling between the neighboring magnetic layers is very weak, it has a determining effect on the magnetism of these atomic-thickness materials. Herein, a new 2D ferromagnetic material, namely, non-vdW CuCrSe2 nanosheets with even-odd-layer-dependent ferromagnetism when laminated from an antiferromagnetic bulk is reported. Monolayer and even-layer CuCrSe2 exhibit the anomalous Hall effect and a significantly enhanced magnetic ordering temperature of more than 125 K. In contrast, the linear Hall effect exists in the odd-layer samples. Theoretical calculations indicate that the layer-dependent magnetic coupling is attributable to the orbital shift of the Cr atoms in the CrSe2 layers owing to the Cu-induced breaking of the centrosymmetry. Thus, this work sheds light on the exotic magnetic properties of layered materials that exhibit phenomena beyond weak interlayer interactions.

Phonon Symphony of Stacked Multilayers and Weak Bonds Lowers Lattice Thermal Conductivity.

Controlling lattice vibrations to obtain intrinsic low thermal conductivity play a critical role in thermal management of electronic and photonic devices, energy converters, and thermal insulation, which necessitates exploring new compounds and a thorough understanding of their chemical structure, bonding, and lattice dynamics. Herein, a new chalcogenide, Ga6 Cr5 Se16 , shows intrinsic low lattice thermal conductivity κlat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe4 layers (g'), close-packed Cr3+ Se6 layers (c), GaSe4 layers (g) and loosely-stacked Cr2+ Se6 layers (c') along the c-axis. In this structure, a wide variety of chemical bonding is arranged in each layer, such as covalent Ga-Se, covalent Cr3+ -Se, and weaker Cr2+ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga6 Cr5 Se16 realizes an intrinsic low κlat of 0.79 W m- 1  K- 1 at 323 K, which is almost four times, or twice lower than that of Cr3 Se4 (2.95 W m- 1  K- 1 ), or Cr2 Se3 (1.56 W m- 1  K- 1 ), Ga2 Se3 (1.36 W m- 1 K- 1 ) at 323 K, respectively. These insights will offer comprehensive understanding of the phonon propagation in complex layered chalcogenides, and also shed useful light on future design of low-κlat solids.

Parasitic Ferromagnetism in Few-Layered Transition-Metal Chalcogenophosphate.

Since the rise of two-dimensional (2D) semiconductors, it seems that electronic devices will soon be upgraded with spintronics, in which the manipulation of spin degree of freedom endows it obvious advantages over conventional charge-based electronics. However, as the most crucial prerequisite for the above-mentioned expectation, 2D semiconductors with adjustable magnetic interaction are still rare, which has greatly hampered the promotion of spintronics. Recently, transition metal phosphates have attracted tremendous interest due to their intrinsic antiferromagnetism and potential applications in spintronics. In the work described herein, parasitic ferromagnetism is achieved for the first time by exfoliating an antiferromagnetic chalcogenophosphate to a few layers. Taking the transition metal chalcogenophosphate Mn2P2S6 as an example, the antiferromagnetic transition at the Néel temperature is completely suppressed, and the magnetic behaviors of the as-obtained few-layered Mn2P2S6 are dominated by parasitic ferromagnetism. We experimentally verify an electron redistribution by which part of the Mn 3d electrons migrate and redistribute on P atoms in few-layered Mn2P2S6 due to the introduced Mn vacancies. The results demonstrated here broaden the tunability of the material's magnetic properties and open up a new strategy to rationally design the magnetic behaviors of 2D semiconductors, which could accelerate the applications of spintronics.

Freestanding Cubic ZrN Single-Crystalline Films with Two-Dimensional Superconductivity.

The successful fabrication of freestanding two-dimensional (2D) crystals that exhibit unprecedented high crystal quality and macroscopic continuity renovates the conventional cognition that 2D long-range crystalline order cannot stably exist at finite temperatures. Current progresses are primarily limited to van der Waals (vdW) layered materials, while studies on how to obtain 2D materials from nonlayered bulk crystals remain sparse. Herein, we report the experimental realization of vdW-like cubic ZrN single crystal and emphasize the significant role of confined electrons in stabilizing the atomic structure at the 2D limit. Furthermore, the exfoliated ZrN single-crystal films with a few nanometers thick exhibit dimensional crossover effect of emerging 2D superconductivity with the unconventional upper critical field beyond Pauli paramagnetic limit, which suggests a dimensional effect in the pairing mechanism of dimensionally confined superconductors.

High Phase Purity of Large-Sized 1T'-MoS2 Monolayers with 2D Superconductivity.

The development of transition metal dichalcogenides has greatly accelerated research in the 2D realm, especially for layered MoS2 . Crucially, the metallic MoS2 monolayer is an ideal platform in which novel topological electronic states can emerge and also exhibits excellent energy conversion and storage properties. However, as its intrinsic metallic phase, little is known about the nature of 2D 1T'-MoS2 , probably because of limited phase uniformity (<80%) and lateral size (usually <1 µm) in produced materials. Herein, solution processing to realize high phase-purity 1T'-MoS2 monolayers with large lateral size is demonstrated. Direct chemical exfoliation of millimeter-sized 1T' crystal is introduced to successfully produce a high-yield of 1T'-MoS2 monolayers with over 97% phase purity and unprecedentedly large size up to tens of micrometers. Furthermore, the large-sized and high-quality 1T'-MoS2 nanosheets exhibit clear intrinsic superconductivity among all thicknesses down to monolayer, accompanied by a slow drop of transition temperature from 6.1 to 3.0 K. Prominently, unconventional superconducting behavior with upper critical field far beyond the Pauli limit is observed in the centrosymmetric 1T'-MoS2 structure. The results open up an ideal approach to explore the properties of 2D metastable polymorphic materials.

The role of oxygen vacancies in water oxidation for perovskite cobalt oxide electrocatalysts: are more better?

We explore the role of oxygen vacancies in the oxygen evolution reaction (OER) for double perovskite PrBaCo2O6-δ. Interestingly, we find that largely increasing oxygen vacancies leads to a significant reduction in the intrinsic OER activity. Structural studies reveal that oxygen vacancies tend to orderly align in PrO1-δ. This ordered structure not only lowers the cobalt oxidation states but also triggers a spin-state transition from high-spin to low-spin states for cobalt ions, both greatly slowing the OER kinetics.

Disorder Enhanced Superconductivity toward TaS2 Monolayer.

Appearance of disorder is commonly known as detrimental to two-dimensional (2D) superconductivity, and typically results in decrement of the critical transition temperature ( Tc). Herein, an anomalous enhancement of superconductivity was observed in TaS2 monolayer with function of disorder induced by structural defect. Owing to controlled pore density by acid concentration during chemical exfoliation, the disorder level in TaS2 framework can be effectively regulated. Dome-shaped behavior was uncovered in disorder dependence of superconductivity toward the chemically functionalized TaS2 monolayers, with Tc enhanced from 2.89 to 3.61 K when below critical disorder level. The disorder-engineered Tc enhancement, which distinctly differs from monotonic decrement in deposited 2D superconductors, can be ascribed to the increment of carrier density induced by Ta atom absence. The exotic superconducting enhancement would give help to deeply understand the correlation between superconductivity and disorder in 2D system.

Manipulating the quantum interference effect and magnetotransport of ZnO nanowires through interfacial doping.

We carefully prepared interfacial Al-doped (IAD) and interfacial natively-doped (IND) ZnO nanowires (NWs) by introducing atomic-layer interfacial Δ-doping between the two steps of CVD growth. Variable-temperature electron transport as well as magnetotransport behaviours of these NWs were systematically investigated. By virtue of the unique architecture and the quality-guaranteed growth technique, a series of quantum interference effects were clearly observed in the IAD ZnO NWs, including weak localization, universal conductance fluctuation and Altshuler-Aronov-Spivak oscillations. The phase-coherence length (Lφ) of electrons exceeds 100 nm in the IAD ZnO NWs, much longer than those in the IND ones and most conventionally doped ZnO NWs. This ability to efficiently manipulate a variety of quantum interference effects in ZnO NWs is very desirable for applications in nano-optoelectronics, nano- & quantum-electronics and solid-state quantum computing.

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering.

2D transition-metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS2 ), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS2 . An overall control of the electronic states of VS2 is also demonstrated: bond-enlarging triggering a metal-to-semiconductor electronic transition and bond-compression inducing metallization in 2D VS2 . The pristine VS2 lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.

Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials.

Electronic state transitions, especially metal-insulator transitions (MIT), offer physical properties that are useful in intriguing energy applications and smart devices. But to-date, very few simple metal oxides have been shown to undergo electronic state transitions near room temperature. Herein, we demonstrate experimentally that chemical induction of double-exchange in two-dimensional (2D) nanomaterials brings about a MIT near room temperature. In this case, valence-state regulation of a 2D MnO2 nanosheet induces a Mn(III)-O-Mn(IV) structure with the double-exchange effect, successfully triggering a near-room-temperature electronic transition with an ultrahigh negative magneto-resistance (MR). Double-exchange in 2D MnO2 nanomaterials exhibits an ultrahigh MR value of up to -11.3% (0.1 T) at 287 K, representing the highest reported negative MR values in 2D nanomaterials approaching room temperature. Also, the MnO2 nanosheet displays an infrared response of 7.1% transmittance change on going from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure promises novel magneto-transport properties and sensitive responses for smart devices.

Metallic Nickel Hydroxide Nanosheets Give Superior Electrocatalytic Oxidation of Urea for Fuel Cells.

The direct urea fuel cell (DUFC) is an important but challenging renewable energy production technology, it offers great promise for energy-sustainable developments and mitigating water contamination. However, DUFCs still suffer from the sluggish kinetics of the urea oxidation reaction (UOR) owing to a 6 e(-) transfer process, which poses a severe hindrance to their practical use. Herein, taking ß-Ni(OH)2 nanosheets as the proof-of-concept study, we demonstrated a surface-chemistry strategy to achieve metallic Ni(OH)2 nanosheets by engineering their electronic structure, representing a first metallic configuration of transition-metal hydroxides. Surface sulfur incorporation successfully brings synergetic effects of more exposed active sites, good wetting behavior, and effective electron transport, giving rise to greatly enhanced performance for UOR. Metallic nanosheets exhibited a much higher current density, smaller onset potential and stronger durability.

Hydrogen Treatment for Superparamagnetic VO2 Nanowires with Large Room-Temperature Magnetoresistance.

One-dimensional (1D) transition metal oxide (TMO) nanostructures are actively pursued in spintronic devices owing to their nontrivial d electron magnetism and confined electron transport pathways. However, for TMOs, the realization of 1D structures with long-range magnetic order to achieve a sensitive magnetoelectric response near room temperature has been a longstanding challenge. Herein, we exploit a chemical hydric effect to regulate the spin structure of 1D V-V atomic chains in monoclinic VO2 nanowires. Hydrogen treatment introduced V(3+) (3d(2) ) ions into the 1D zigzag V-V chains, triggering the formation of ferromagnetically coupled V(3+) -V(4+) dimers to produce 1D superparamagnetic chains and achieve large room-temperature negative magnetoresistance (-23.9 %, 300 K, 0.5 T). This approach offers new opportunities to regulate the spin structure of 1D nanostructures to control the intrinsic magnetoelectric properties of spintronic materials.

Engineering electrocatalytic activity in nanosized perovskite cobaltite through surface spin-state transition.

The activity of electrocatalysts exhibits a strongly dependence on their electronic structures. Specifically, for perovskite oxides, Shao-Horn and co-workers have reported a correlation between the oxygen evolution reaction activity and the eg orbital occupation of transition-metal ions, which provides guidelines for the design of highly active catalysts. Here we demonstrate a facile method to engineer the eg filling of perovskite cobaltite LaCoO3 for improving the oxygen evolution reaction activity. By reducing the particle size to ∼80 nm, the eg filling of cobalt ions is successfully increased from unity to near the optimal configuration of 1.2 expected by Shao-Horn's principle. Consequently, the activity is significantly enhanced, comparable to those of recently reported cobalt oxides with eg(∼1.2) configurations. This enhancement is ascribed to the emergence of spin-state transition from low-spin to high-spin states for cobalt ions at the surface of the nanoparticles, leading to more active sites with increased reactivity.

Signature of coexistence of superconductivity and ferromagnetism in two-dimensional NbSe2 triggered by surface molecular adsorption.

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb-Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.

Superparamagnetic Reduced Graphene Oxide with Large Magnetoresistance: A Surface Modulation Strategy.

The graphene system is actively pursued in spintronics for its nontrivial sp electron magnetism and its potential for the flexible surface chemical tuning of magnetoelectronic functionality. The magnetoresistance (MR) of graphene can be effectively tuned under high magnetic fields at cryogenic temperatures, but it remains a challenge to achieve sensitive magnetoelectric response under ambient conditions. We report the use of surface modulation to realize superparamagnetism in reduced graphene oxide (rGO) with sensitive magnetic field response. The superparamagnetic rGO was obtained by a mild oxidation process to partially remove the thiol groups covalently bound to the carbon framework, which brings about large low-field negative MR at room temperature (-8.6 %, 500 Oe, 300 K). This strategy provides a new approach for optimizing the intrinsic magnetoelectric properties of two-dimensional materials.

Engineering the electronic state of a perovskite electrocatalyst for synergistically enhanced oxygen evolution reaction.

A surface hydrogen effect to modulate the pure electronic-state transition in perovskite Ca0.9 Yb0.1 MnO3 synergistically generates a more suitable eg electron filling status and better conductivity. This achieves 100 times higher catalytic activity compared to that of a pristine sample.

Surface chemical-modification for engineering the intrinsic physical properties of inorganic two-dimensional nanomaterials.

Two-dimensional (2D) nanomaterials, especially the inorganic ultrathin nanosheets with single or few-atomic layers, have been extensively studied due to their special structures and rich physical properties coming from the quantum confinement of electrons. With atomic-scale thickness, 2D nanomaterials have an extremely high specific surface area enabling their surface phase to be as important as bulk counterparts, and therefore provide an alternative way of modifying the surface phase for engineering the intrinsic physical properties of inorganic 2D nanomaterials. In this review, we focus on recent research concerning surface chemical modification strategies to effectively engineer the intrinsic physical properties of inorganic 2D nanomaterials. We highlight the newly developed regulation strategies of surface incorporation, defect engineering, and structure modulation of inorganic 2D nanomaterials, which respectively influence the intrinsic conductivity, band structure, and magnetism while maintaining the primary 2D freestanding structures that are vital for 2D based ultrasensitive electronic response, enhanced catalytic and magnetocaloric capabilities.

Hydrogen dangling bonds induce ferromagnetism in two-dimensional metal-free graphitic-C3N4 nanosheets.

Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin-orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C3N4 ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C3N4 ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.

Large negative magnetoresistance induced by anionic solid solutions in two-dimensional spin-frustrated transition metal chalcogenides.

We report an anionic solid solution process that induces frustrated magnetic structures within two-dimensional transition metal chalcogenides, which leads to huge negative magnetoresistance effects. Ultrathin nanosheets of TiTe(2-x)I(x) solid solutions, which are a new class of inorganic two-dimensional magnetic material, exhibit negative magnetoresistance with a value of up to -85%, due to the spin-dependent scattering effects of local Ti(3+) 3d(1) moments that are antiferromagnetically coupled. Moreover, TiTe(2-x)I(x) serials show unique transport behaviors with continuous evolution from metallic to semiconducting states. We anticipate that anionic doping will be a powerful tool for optimizing the intrinsic physical properties of two-dimensional transition metal chalcogenide system.