Xingxing Zhang

2D transition metal dichalcogenides have promising properties for future semiconductor technologies. Their integration into functional devices requires cost‐efficient and large‐scale passivation and protection against material deposition. This work introduces ultrathin Ga₂O₃ glass as a new, scalable capping material for monolayer WS₂. It exhibits a novel passivation mechanism and offers extraordinary protection against deposition of dielectric materials, for example, for top‐gating.

Abstract

Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all‐dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, it is shown that the novel, ultrathin Ga₂O₃ glass is an ideal centimeter‐scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga₂O₃ capping of monolayer WS₂ outperforms commercial‐grade hBN in both scalability and optical performance at room temperature. These properties make Ga₂O₃ highly suitable for large‐scale passivation and protection of monolayer TMDCs in functional heterostructures.

A new strategy of tuning the scattering mechanism to decouple electric conductivity with Seebeck coefficient for high‐performance thermoelectrics is realized by Jing Wu, Junpeng Lu, Zhenhua Ni, and co‐workers, as described in article number 2004786. By applying a gate voltage on a 2D Bi₂O₂Se‐based field‐effect transistor, a high thermoelectric power factor over a wide temperature range is achieved due to persistently high mobility arising from the highly gate‐tunable scattering mechanism.

Substantial coercivity reduction by the current, larger at least by two orders of magnitude than those in previous reports, is found in the van der Waals ferromagnet Fe₃GeTe₂. It is theoretically shown to arise from an unusual type of gigantic spin–orbit torque, which itself is directly related to its special symmetries, large Berry curvature, and band topology. A working model of a new robust nonvolatile magnetic memory based on Fe₃GeTe₂, controlled by a much smaller current, is also produced.

Abstract

Controlling magnetic states by a small current is essential for the next‐generation of energy‐efficient spintronic devices. However, it invariably requires considerable energy to change a magnetic ground state of intrinsically quantum nature governed by fundamental Hamiltonian, once stabilized below a phase‐transition temperature. Here, it is reported that, surprisingly, an in‐plane current can tune the magnetic state of the nanometer‐thin van der Waals ferromagnet Fe₃GeTe₂ from a hard magnetic state to a soft magnetic state. It is a direct demonstration of the current‐induced substantial reduction of the coercive field. This surprising finding is possible because the in‐plane current produces a highly unusual type of gigantic spin–orbit torque for Fe₃GeTe₂. In addition, a working model of a new nonvolatile magnetic memory based on the principle of the discovery in Fe₃GeTe₂, controlled by a tiny current, is further demonstrated. The findings open up a new window of exciting opportunities for magnetic van der Waals materials with potentially huge impact on the future development of spintronic and magnetic memory.

The distinct physical origin of 2D polarizations and modulation on degrees of freedom are comprehensively summarized based on the hysteresis behaviors and interface effects. The novel multifunctional polarized devices are discussed, from which perspectives and guidelines are proposed for developing “beyond Moore” integrated devices and circuits.

Abstract

The emergence of 2D polarized materials, including ferromagnetic, ferrovalley, and ferroelectric materials, has demonstrated unique quantum behaviors at atomic scales. These polarization behaviors are tightly bonded to the new degrees of freedom (DOFs) for next generation information storage and processing, which have been dramatically developed in the past few years. Here, the basic 2D polarized materials system and related devices’ application in spintronics, valleytronics, and electronics are reviewed. Specifically, the underlying physical mechanism accompanied with symmetry broken theory and the modulation process through heterostructure engineering are highlighted. These summarized works focusing on the 2D polarization would continue to enrich the cognition of 2D quantum system and promising practical applications.

A new ductile inorganic thermoelectric (TE) material, Ag₂₀S₇Te₃, with high carrier mobility, low lattice thermal conductivity, and a maximum zT of 0.80 at 600 K is reported. The prototype hetero‐shaped TE generator consisting of 10 Ag₂₀S₇Te₃ strips displays an open‐circuit voltage of 69.2 mV and a maximum power output of 17.1 μW under a temperature difference of 70 K.

Abstract

Hetero‐shaped thermoelectric (TE) generators (TEGs) can power the sensors used in safety monitoring systems of undersea oil pipelines, but their development is greatly limited by the lack of materials with both good shape‐conformable ability and high TE performance. In this work, a new ductile inorganic TE material, Ag₂₀S₇Te₃, with high TE performance is reported. At 300–600 K, Ag₂₀S₇Te₃ crystallizes in a body‐centered cubic structure, in which S and Te atoms randomly occupy the (0, 0, 1) site. Due to the smaller generalized stacking fault energy in the (101¯)[010] slip system, Ag₂₀S₇Te₃ shows better ductility than Ag₂S, yielding excellent shape‐conformability. The high carrier mobility and low lattice thermal conductivity observed in Ag₂₀S₇Te₃ result in a maximum dimensionless figure of merit (zT) of 0.80 at 600 K, which is comparable with the best commercial Bi₂Te₃‐based alloys. The prototype TEG consisting of 10 Ag₂₀S₇Te₃ strips displays an open‐circuit voltage of 69.2 mV and a maximum power output of 17.1 µW under the temperature difference of 70 K. This study creates a new route toward hetero‐shaped TEG.

Transition metal dichalcogenides (TMDs) are considered to be promising candidates over noble metal catalysts for electrochemical hydrogen production. The basic mechanism for the hydrogen evolution reaction (HER) is introduced, followed by a description of the different synthesis methods and modulation approaches to enhance the catalytic performance of TMD‐based catalysts toward the HER.

Abstract

Hydrogen has been deemed as an ideal substitute fuel to fossil energy because of its renewability and the highest energy density among all chemical fuels. One of the most economical, ecofriendly, and high‐performance ways of hydrogen production is electrochemical water splitting. Recently, 2D transition metal dichalcogenides (also known as 2D TMDs) showed their utilization potentiality as cost‐effective hydrogen evolution reaction (HER) catalysts in water electrolysis. Herein, recent representative research efforts and systematic progress made in 2D TMDs are reviewed, and future opportunities and challenges are discussed. Furthermore, general methods of synthesizing 2D TMDs materials are introduced in detail and the advantages and disadvantages for some specific methods are provided. This explanation includes several important regulation strategies of creating more active sites, heteroatoms doping, phase engineering, construction of heterostructures, and synergistic modulation which are capable of optimizing the electrical conductivity, exposure to the catalytic active sites, and reaction energy barrier of the electrode material to boost the HER kinetics. In the last section, the current obstacles and future chances for the development of 2D TMDs electrocatalysts are proposed to provide insight into and valuable guidelines for fabricating effective HER electrocatalysts.

In article number 1907818, Bo Song and co‐workers summarize the recent advances regarding the preparation and modulation strategies of electrocatalysts based on 2D transition metal dichalcogenides (TMDs). 2D TMDs are promising candidates for electrocatalytic water splitting, and the produced hydrogen (H₂) and oxygen (O₂) could be well utilized as continual power supply for space craft and life‐support system for astronauts, respectively. 2D‐TMDs‐based electrocatalysts with excellent catalytic performance could provide a promising solution for future interstellar exploration, such as to Mars.

The library of atomically thin 2D materials featuring non‐volatile resistive switching can provide a promising and broad platform for exploring the sub‐nanometer scaling limit, which is beneficial for emerging device concepts. A dissociation–diffusion–adsorption (DDA) model is proposed to describe the common conductive‐point mechanism behind 2D‐materials‐based universal resistive switching and supported by systematic density functional theory (DFT) calculations showing favorable adsorption of metal into native defects.

Abstract

Non‐volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few‐layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX₂, M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS₂/MoS₂), and an atomically thin insulator (h‐BN). These results indicate the universality of the phenomenon in 2D non‐conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming‐free characteristic. A dissociation–diffusion–adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive‐point mechanism supported by first‐principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

The WSe₂ monolayer in the 1T’ phase is a large‐gap quantum spin Hall insulator, but is thermodynamically metastable. Thanks to the enhanced interface interaction, a single‐phase 1T'‐WSe₂ monolayer is grown on SrTiO₃(100) substrate using the molecular beam epitaxy method, and the interlayer in‐plane strain also drives the 1T'‐WSe₂ into a semimetallic phase.

Abstract

The WSe₂ monolayer in 1T’ phase is reported to be a large‐gap quantum spin Hall insulator, but is thermodynamically metastable and so far the fabricated samples have always been in the mixed phase of 1T’ and 2H, which has become a bottleneck for further exploration and potential applications of the nontrivial topological properties. Based on first‐principle calculations in this work, it is found that the 1T’ phase could be more stable than 2H phase with enhanced interface interactions. Inspired by this discovery, SrTiO₃ (100) is chosen as substrate and WSe₂ monolayer is successfully grown in a 100% single 1T’ phase using the molecular beam epitaxial method. Combining in situ scanning tunneling microscopy and angle‐resolved photoemission spectroscopy measurements, it is found that the in‐plane compressive strain in the interface drives the 1T'‐WSe₂ into a semimetallic phase. Besides providing a new material platform for topological states, the results show that the interface interaction is a new approach to control both the structure phase stability and the topological band structures of transition metal dichalcogenides.

Inspired by the capillary phenomenon, a face‐to‐face confinement growth method is developed to grow high‐quality wafer‐sacle Cu₂(TCPP) (TCPP = meso‐tetra(4‐carboxyphenyl)porphine) metal–organic framework (MOF) film on dielectric substrates. The film exhibits p‐type semiconducting property and excellent photoelectric response. Meanwhile, this growth strategy can also be used to prepare other wafer‐sacle conductive MOF films.

Abstract

The preparation of large‐area 2D conductive metal–organic framework (MOF) films remains highly desirable but challenging. Here, inspired by the capillary phenomenon, a face‐to‐face confinement growth method to grow conductive 2D Cu₂(TCPP) (TCPP = meso‐tetra(4‐carboxyphenyl)porphine) MOF films on dielectric substrates is developed. Trace amounts of solutions containing low‐concentration Cu²⁺ and TCPP are pumped cyclically into a micropore interface to produce this growth. The crystal structures are confirmed with various characterization techniques, which include high‐resolution atomic force microscopy and cryogenic transmission electron microscopy (Cryo‐TEM). The Cu₂(TCPP) MOF film exhibit an electrical conductivity of ≈0.007 S cm⁻¹, which is approximately four orders of magnitude higher than other carboxylic‐acid‐based MOF materials (10⁻⁶ S cm⁻¹). Other wafer‐scale conductive MOF films such as M₃(HHTP)₂ (M = Cu, Co, and Ni; HHTP = 2,3,6,7,10,11‐triphenylenehexol) can be produced utilizing this strategy and suggests this method has widescale applicability potential.

A catalyst of iron single atoms on a nitrogen‐doped graphene (SAFe@NG) substrate is selected to catalyze reversible conversion in MoS₂ anodes. Importantly, the SAFe@NG catalyst with highly active Fe–N₄ sites significantly facilitates the conversion of Mo/Na₂S to MoS₂ in the charging processes, resulting in an efficient reversible conversion reaction of MoS₂↔NaMoS₂↔Mo/Na₂S in a complete cycle, which is demonstrated by spectroscopy, microscopy, and simulations.

Abstract

Sodium‐ion batteries (SIBs) based on conversion‐type metal sulfide (MS) anodes have attracted extraordinary attention due to relatively high capacity and intrinsic safety. The highly reversible conversion of M/Na₂S to pristine MS in charge plays a vital role with regard to the electrochemical performance. Here, taking conventional MoS₂ as an example, guided by theoretical simulations, a catalyst of iron single atoms on nitrogen‐doped graphene (SAFe@NG) is selected and first used as a substrate to facilitate the reaction kinetics of MoS₂ in the discharging process. In the following charging process, using a combination of spectroscopy and microscopy, it is demonstrated that the SAFe@NG catalyst enables an efficient reversible conversion reaction of Mo/Na₂S→NaMoS₂→MoS₂. Moreover, theoretical simulations reveal that the reversible conversion mechanism shows favorable formation energy barrier and reaction kinetics, in which SAFe@NG with the Fe–N₄ coordination center facilitates the uniform dispersion of Na₂S/Mo and the decomposition of Na₂S and NaMoS₂. Therefore, efficient reversible conversion reaction MoS₂↔NaMoS₂↔Mo/Na₂S is enabled by the SAFe@NG catalyst. This work contributes new avenues for designing conversion‐type materials with an efficient reversible mechanism.

A new strategy to dope transition metal dichalcogenides (TMDC) is demonstrated in which a high density (6.4–15%) of dopants is confined within 1D nanostripes embedded in the TMDC lattice. The growth of the nanostripe structures originates from the migration of dislocation cores and is also the key to introduce dopants into 2D materials.

Abstract

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface‐to‐volume ratio, such as transition metal dichalcogenides, various post‐synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post‐growth doping of single layers of WSe₂ is performed by adding transition metal (TM) atoms in a two‐step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4–15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe₂. Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Ab initio simulations and transport experiments elucidate why Sb₂Te₃ can form an insulating rocksalt structure, extending the exploration of Anderson insulators to binary chalcogenides. A systematic computational screening over binary and ternary chalcogenides, which leads to an in‐depth understanding of the critical factors that affect the stability of the rocksalt structure is also carried out.

Abstract

Tailoring the degree of disorder in chalcogenide phase‐change materials (PCMs) plays an essential role in nonvolatile memory devices and neuro‐inspired computing. Upon rapid crystallization from the amorphous phase, the flagship Ge–Sb–Te PCMs form metastable rocksalt‐like structures with an unconventionally high concentration of vacancies, which results in disordered crystals exhibiting Anderson‐insulating transport behavior. Here, ab initio simulations and transport experiments are combined to extend these concepts to the parent compound of Ge–Sb–Te alloys, viz., binary Sb₂Te₃, in the metastable rocksalt‐type modification. Then a systematic computational screening over a wide range of homologous, binary and ternary chalcogenides, elucidating the critical factors that affect the stability of the rocksalt structure is carried out. The findings vastly expand the family of disorder‐controlled main‐group chalcogenides toward many more compositions with a tunable bandgap size for demanding phase‐change applications, as well as a varying strength of spin–orbit interaction for the exploration of potential topological Anderson insulators.

Interfacial engineering is used to tune the bending stiffness of 2D material heterostructures composed of graphene and MoS₂ by over several hundred percent. The incorporation of twisted or heterointerfaces facilitates interlayer slip, which dramatically softens the 2D stacks. A bending model is developed to predict and design the deformability of 2D heterostructures as a function of composition, stacking order, and geometry of the structure.

Abstract

2D monolayers represent some of the most deformable inorganic materials, with bending stiffnesses approaching those of lipid bilayers. Achieving 2D heterostructures with similar properties would enable a new class of deformable devices orders of magnitude softer than conventional thin‐film electronics. Here, by systematically introducing low‐friction twisted or heterointerfaces, interfacial engineering is leveraged to tailor the bending stiffness of 2D heterostructures over several hundred percent. A bending model is developed and experimentally validated to predict and design the deformability of 2D heterostructures and how it evolves with the composition of the stack, the atomic arrangements at the interfaces, and the geometry of the structure. Notably, when each atomic layer is separated by heterointerfaces, the total bending stiffness reaches a theoretical minimum, equal to the sum of the constituent layers regardless of scale of deformation—lending the extreme deformability of 2D monolayers to device‐compatible multilayers.