Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets.

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Hierarchical palladium catalyst for highly active and stable water oxidation in acidic media.

Acidic water electrolysis is of great importance for boosting the development of renewable energy. However, it severely suffers from the trade-off between high activity and long lifespan for oxygen evolution catalysts on the anode side. This is because the sluggish kinetics of oxygen evolution reaction necessitates the application of a high overpotential to achieve considerable current, which inevitably drives the catalysts far away from their thermodynamic equilibrium states. Here we demonstrate a new oxygen evolution model catalyst-hierarchical palladium (Pd) whose performance even surpasses the benchmark Ir- and Ru-based materials. The Pd catalyst displays an ultralow overpotential (196 mV), excellent durability and mitigated degradation (66 µV h-1) at 10 mA cm-2 in 1 M HClO4. Tensile strain on Pd (111) facets weakens the binding of oxygen species on electrochemical etching-derived hierarchical Pd and thereby leads to two orders of magnitudes of enhancement of mass activity in comparison to the parent Pd bulk materials. Furthermore, the Pd catalyst displays the bifunctional catalytic properties for both oxygen and hydrogen evolutions and can deliver a current density of 2 A cm-2 at a low cell voltage of 1.771 V when fabricated into polymer electrolyte membrane electrolyser.

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.

Host-Guest Intercalation Chemistry for the Synthesis and Modification of Two-Dimensional Transition Metal Dichalcogenides.

Intercalation chemistry is of great importance in solid-state physics and chemistry for the ability to modulate electronic structures for constructing new materials with exotic properties. This ancient and versatile discipline has recently become prevailing in the synthesis and regulation of 2D transition metal dichalcogenides (TMDs) with atomic thickness due to diverse host-guest configurations and their impact on layered frameworks, which bring in extensive applications in electronics, optoelectronics, and other energy-based devices. In order to prepare 2D TMD materials with desired structure and properties, it is essential to gain in-depth understanding of the key role the intercalation chemistry plays in the preparation process. A focused review on recent advances regarding 2D TMD materials through intercalation exfoliation from the view of host, guest, and solvent interactions is provided. The effect of intercalation chemistry on TMD nanosheets synthesis and modification is comprehensively reviewed. The interactions between host and guest from the aspects of lattice strain, interlayer distance, and carrier density are considered. Finally, a prospectus of the future research opportunities for the intercalation chemistry of 2D materials is provided.

Stoichiometric two-dimensional non-van der Waals AgCrS2 with superionic behaviour at room temperature.

Layered materials have attracted tremendous interest for accessing two-dimensional structures. Materials such as graphite or transition metal dichalcogenides, in which the layers are held together by van der Waals interactions, can be exfoliated through a variety of processes in a manner that retains the structure and composition of the monolayers, but this has proven difficult for solids with stronger interlayer interactions. Here, we demonstrate the exfoliation of AgCrS2, a member of the AMX2 family (where A is a monovalent metal, M is a trivalent metal and X is a chalcogen), through intercalation with tetraalkylammonium cations, chosen for their suitable redox potential. The as-exfoliated nanosheets consist of Ag layers sandwiched between two CrS2 layers, similar to their structure in the bulk. They show superionic behaviour at room temperature, with an ionic conductivity of 33.2 mS cm-1 at 298 K that originates from Ag+ ions rapidly hopping between neighbouring tetrahedral interstices; in the bulk, this behaviour is only observed above 673 K.

Spin-Dependent Transport at 2D Solids: From Nonmagnetic Layers to Ferromagnetic van der Waals Structures.

Spintronics is a promising alternative to the conventional silicon transistor-based electronics that are gradually approaching their physical limitations. Ultrathin two-dimensional van der Waals (vdW) materials (2D materials) with controllable spin degrees of freedom are recognized as extremely promising spintronic materials in architectures for the post-Moore era. In this Perspective, we review recent progress on spin-dependent transport behaviors (SDTBs) confined at the 2D scale, which are the mainstream paradigms for spintronic devices. We first present the mechanism and the key factors of SDTBs in 2D nonmagnetic materials-based hybrid devices. Then, some chemical modulation strategies for inducing short-range magnetic order and magneto-electric performance into 2D nonmagnetic materials are discussed. Furthermore, we concentrate on introducing intriguing SDTBs in 2D long-range ferromagnetic materials-based vdW devices. Finally, we highlight the current challenges in the study of spin-dependent transport of 2D modified materials and 2D material-based spintronic devices, in the hope of accelerating their applications.

Fast Lithium Ion Conductivity in Layered (Li-Ag)CrS2.

Fast ionic conductors are of great importance for novel technologies in high-performance and rechargeable energy storage components with reliable safety and thermal stability. Here, we demonstrate a new concept of the pillar effect to construct two-dimensional (2D) fast Li+ conductors. Our developed layered LixAg1-xCrS2 (0 < x < 0.4) structure, with larger-radius Ag+ served as "pillars" to effectively rigidify the interlayer ionic channel, leads to multi-ion concerted migration behavior and thus contributes to low activation energy and fast Li+ diffusion. Consequently, the room-temperature ionic conductivity in (Li-Ag)CrS2 system reaches up to 19.6 mS·cm-1 for x is 0.31, which is comparable to that of currently best Li-ion conductors. Furthermore, the pillared structure exhibits unique ionic transport that the conductivity decreases as temperature elevated, which can be ascribed to the competition between Li+ and Ag+ migration through tetrahedral viods in 2D channel. We anticipated that pillar effect would pave a new way to explore new catalogue of Li superionic conductors.

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.

Electron Transport in Low Dimensional Solids: A Surface Chemistry Perspective.

Electron transport is a fundamental process that controls the intrinsic chemical and physical properties of solid materials. The surface phase becomes dominant when downsized dimensionality into cluster scale in nanomaterials, and surface chemistry plays more and more important role in regulating electron transport. During past decades, varieties of chemical approaches have been developed to modify the surface of low dimensional solids, substantially providing versatile perspectives on engineering electron transport. In this Perspective, we focus on recent researches concerning surface chemical modification strategies, such as surface molecular adsorption, atomic incorporation, defect engineering and spin scattering to engineer electron transport of typical one-/two-dimensional systems. Under the framework of Drude's transport model, we highlight the core role of micro degrees of freedom, i.e., charge, lattice, and spin, in molecular-level understanding and optimizing the regulation effect of surface chemistry. Finally, based on the discussion and current achievements of surface chemistry effect on electron transport of low dimensional solids, some personal perspectives on the future development are also presented.

Solution Processing for Lateral Transition-Metal Dichalcogenides Homojunction from Polymorphic Crystal.

Homojunctions comprised of transition-metal dichalcogenides (TMD) polymorphs are attractive building blocks for next-generation two-dimensional (2D) electronic circuitry. However, the synthesis of such homojunctions, which usually involves elaborate manipulation at the nanoscale, still remains a great challenge. Herein, we demonstrated a solution-processing strategy to successfully harvest lateral semiconductor-metal homojunctions with high yield. Specially, through precisely controlled lithiation process, precursors of polymorphic crystal arranged with 1T-2H domains were successfully achieved. A programmed exfoliation procedure was further employed to orderly laminate each phase in the polymorphic crystal, thus leading to 1T-2H TMD homojunction monolayers with sizes up to tens of micrometers. Moreover, the atomically sharp boundaries and superior band alignment improved the device on the basis of the semiconductor-metal homojunction with 50% decrease of electric field strength required in the derivation of state transition. We anticipate that solution processing based on programmed exfoliation would be a powerful tool to produce new configurations of 2D nanomaterials.

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.

Two-Dimensional Tellurium Nanosheets Exhibiting an Anomalous Switchable Photoresponse with Thickness Dependence.

Two-dimensional (2D) tellurium (Te) was recently predicted to be promising for diverse electronic and optoelectronic applications. However, the synthesis of high-quality 2D Te structures remains challenging, which greatly hinders the exploration of its full properties. Herein, an anomalous photoresponse from negative to positive as a function of thickness in Te nanosheets is reported. Ultrathin Te layers with large size and clean interface were obtained through a topotactic transformation, in which the 2D Te structure was derived from a layered MTe2 (M=Ti, Mo, W) matrix by excessive lithiation. Prominently, the photoresponse in Te nanosheets exhibits negative behavior when the thickness is less than 5 nm, which turns positive as the thickness increases. This unusual photoresponse will shed light on the full exploration of 2D non-layered materials with exotic properties.

Acid-Assisted Exfoliation toward Metallic Sub-nanopore TaS2 Monolayer with High Volumetric Capacitance.

Conductive porous structures are favorable as active electrode materials for energy storage by boosting the active sites and specific surface area but have been rarely achieved in transition metal dichalcogenides. Here, we developed acid-assisted exfoliation for the first time, to successfully exfoliate TaS2 into very large-sized conductive monolayers with controllable in-plane sub-nanopores. By inducing both interlayer lattice expansion and basal in-plane etching, hydrogen ion, previously regarded disastrous in charged system, was creatively utilized as an efficient and easily accessible assistant in simultaneous exfoliation and controllable structural modification. Benefiting from pore size (∼0.95 nm) matching well with electrolyte ion size, coexistence of ultrahigh conductivity and fast ion transport was achieved in metallic large-sized monolayers. Notably, the as-produced TaS2-based electrode delivers large volumetric capacitance (508 F/cm3 at scan rate of 10 mV/s) and high energy density (58.5 Wh/L) when fabricated into a micro-supercapacitor. We anticipate acid-assisted exfoliation to be a promising strategy in constructing 2D nanomaterials with novel structure for wide energy applications.

Molecule-Confined Engineering toward Superconductivity and Ferromagnetism in Two-Dimensional Superlattice.

Superconductivity is mutually exclusive with ferromagnetism, because the ferromagnetic exchange field is often destructive to superconducting pairing correlation. Well-designed chemical and physical methods have been devoted to realize their coexistence only by structural integrity of inherent superconducting and ferromagnetic ingredients. However, such coexistence in freestanding structure with nonsuperconducting and nonferromagnetic components still remains a great challenge up to now. Here, we demonstrate a molecule-confined engineering in two-dimensional organic-inorganic superlattice using a chemical building-block approach, successfully realizing first freestanding coexistence of superconductivity and ferromagnetism originated from electronic interactions of nonsuperconducting and nonferromagnetic building blocks. We unravel totally different electronic behavior of molecules depending on spatial confinement: flatly lying Co(Cp)2 molecules in strongly confined SnSe2 interlayers weaken the coordination field, leading to spin transition to form ferromagnetism; meanwhile, electron transfer from cyclopentadienyls to the Se-Sn-Se lattice induces superconducting state. This entirely new class of coexisting superconductivity and ferromagnetism generates a unique correlated state of Kondo effect between the molecular ferromagnetic layers and inorganic superconducting layers. We anticipate that confined molecular chemistry provides a newly powerful tool to trigger exotic chemical and physical properties in two-dimensional matrixes.

Half-Metallic Behavior in 2D Transition Metal Dichalcogenides Nanosheets by Dual-Native-Defects Engineering.

Two-dimensional transition metal dichalcogenides (TMDs) have been regarded as one of the best nonartificial low-dimensional building blocks for developing spintronic nanodevices. However, the lack of spin polarization in the vicinity of the Fermi surface and local magnetic moment in pristine TMDs has greatly hampered the exploitation of magnetotransport properties. Herein, a half-metallic structure of TMDs is successfully developed by a simple chemical defect-engineering strategy. Dual native defects decorate titanium diselenides with the coexistence of metal-Ti-atom incorporation and Se-anion defects, resulting in a high-spin-polarized current and local magnetic moment of 2D Ti-based TMDs toward half-metallic room-temperature ferromagnetism character. Arising from spin-polarization transport, the as-obtained T-TiSe1.8 nanosheets exhibit a large negative magnetoresistance phenomenon with a value of -40% (5T, 10 K), representing one of the highest negative magnetoresistance effects among TMDs. It is anticipated that this dual regulation strategy will be a powerful tool for optimizing the intrinsic physical properties of TMD systems.

Very Large-Sized Transition Metal Dichalcogenides Monolayers from Fast Exfoliation by Manual Shaking.

For two-dimensional transition metal dichalcogenides (TMD) materials, achieving large size with high quality to provide a basis for the next generation of electronic device geometries has been a long-term need. Here, we demonstrate that, by only manual shaking within several seconds, very large-sized TMD monolayers that cover a wide range of group IVB-VIB transition metal sulfides and selenides can be efficiently harvested from intercalated single-crystal counterparts. Taking TaS2 as examples, monolayers up to unprecedented size (>100 µm) are obtained while maintaining high crystalline quality and the phase structure of the starting materials. Furthermore, benefiting from the gentle manual shaking, we unraveled the atomic-level correlation between the intercalated lattice-strain effects and exfoliated nanosheets, and that strong tensile strain usually led to very large sizes. This work helps to deepen the understanding of exfoliation mechanism and provides a powerful tool for producing large-sized and high-quality TMD nanosheets appealing for further applications.

Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam.

In correlated systems, intermediate states usually appear transiently across phase transitions even at the femtosecond scale. It therefore remains an open question how to determine these intermediate states-a critical issue for understanding the origin of their correlated behaviour. Here we report a surface coordination route to successfully stabilize and directly image an intermediate state in the metal-insulator transition of vanadium dioxide. As a prototype metal-insulator transition material, we capture an unusual metal-like monoclinic phase at room temperature that has long been predicted. Coordinate bonding of L-ascorbic acid molecules with vanadium dioxide nanobeams induces charge-carrier density reorganization and stabilizes metallic monoclinic vanadium dioxide, unravelling orbital-selective Mott correlation for gap opening of the vanadium dioxide metal-insulator transition. Our study contributes to completing phase-evolution pathways in the metal-insulator transition process, and we anticipate that coordination chemistry may be a powerful tool for engineering properties of low-dimensional correlated solids.

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.