zemin zheng

Nature, Published online: 22 December 2021; doi:10.1038/s41586-021-04151-5

Inelastic neutron scattering measurements show that superconductivity in UTe2 is associated with a resonance near antiferromagnetic order that suggests an unexpected spin-singlet component to the electron pairing.

Compared to multilayers, monolayer PtSe₂ exhibits a unique gap opening that enables the detailed characterization of defect induced electronic states. Pt-vacancies whose spin polarized nature has been linked to magnetic ordering are shown to be rare in the material. Instead, a metallization of zigzag edges is discovered with spin polarized edge states that may enable magnetic modifications of PtSe₂ nanomaterials.

Abstract

Edges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D-PtSe₂ are investigated where the 1.8 eV bandgap of monolayer PtSe₂ facilitates the detailed characterization of defect-induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe₂ are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se-vacancies, Pt-vacancies, and Se-antisites as the dominant defects in PtSe₂. In contrast to Se- or Pt-vacancies, the Se-antisites are almost devoid of gap states. Pt-vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe₂. The atomic-scale insights into defect-induced electronic states in monolayer PtSe₂ provide the fundamental underpinning for defect engineering of PtSe₂-monolayers and the newly identified spin-polarized edge states offer prospects for engineering magnetic properties in PtSe₂ nanoribbons.

The magnetic anisotropy of schreibersite (Fe,Ni)₃P with S ₄ symmetry is controlled by doping and its impact on the stability of antiskyrmions is investigated. It is demonstrated that the variation of the Ni content and slight doping with 4d transition metals considerably change the magnetic anisotropy, and subtle balance between uniaxial anisotropy and demagnetization energy is necessary to stabilize the antiskyrmions.

Abstract

Magnetic skyrmions, vortex-like topological spin textures, have attracted much interest in a wide range of research fields from fundamental physics to spintronics applications. Recently, growing attention is also paid to antiskyrmions emerging with opposite topological charge in non-centrosymmetric magnets with D _2d or S ₄ symmetry. In these magnets, complex interplay among anisotropic Dzyaloshinskii–Moriya interaction, uniaxial magnetic anisotropy, and magnetic dipolar interactions generates various magnetic textures. However, the precise role of these magnetic interactions in stabilizing antiskyrmions remains to be elucidated. In this work, the uniaxial magnetic anisotropy of schreibersite (Fe,Ni)₃P with S ₄ symmetry is controlled by doping and its impact on the stability of antiskyrmions is investigated. The authors’ magnetometry study, supported by ferromagnetic resonance spectroscopy, shows that the variation of the Ni content and slight doping with 4d transition metals considerably change the magnetic anisotropy. In particular, doping with Pd induces easy-axis anisotropy, giving rise to formation of antiskyrmions, while a temperature-induced spin reorientation is observed in an Rh-doped compound. In combination with Lorentz transmission electron microscopy and micromagnetic simulations, the stability of antiskyrmion as functions of uniaxial anisotropy and demagnetization energy is quantitatively analyzed, and demonstrated that subtle balance between them is necessary to stabilize the antiskyrmions.

Recent advances in the synthesis of 2D COF powders, single crystals, and thin films, as well as their advanced optical, electrical, and magnetic functionalities are systematically summarized. The challenging issues and potential opportunities are also proposed for further inspiring their development in structure, synthesis, and functionalities.

Abstract

Covalent organic frameworks (COFs), an emerging class of organic crystalline polymers with highly oriented structures and permanent porosity, can adopt 2D or 3D architectures depending on the different topological diagrams of the monomers. Notably, 2D COFs have particularly gained much attention due to the extraordinary merits of their extended in-plane π-conjugation and topologically ordered columnar π-arrays. These properties together with high crystallinity, large surface area, and tunable porosity distinguish 2D COFs as an ideal candidate for the fabrication of functional materials. Herein, this review surveys the recent research advances in 2D COFs with special emphasis on the preparation of 2D COF powders, single crystals, and thin films, as well as their advanced optical, electrical, and magnetic functionalities. Some challenging issues and potential research outlook for 2D COFs are also provided for promoting their development in terms of structure, synthesis, and functionalities.

2D Materials

In article number 2106674, Kai Xiao and co-workers develop a nonequilibrium chemical vapor deposition approach for selective formation of antisite defects in atomically thin 2D tungsten sulfide monolayer crystals by regulating the diffusion of tungsten into gold substrates. This work demonstrates a novel strategy for the selective formation of defects in 2D materials by tuning their formation energy during synthesis via choice of substrate and alloy design.

This study proposes a novel strategy for reducing the evaporation enthalpy by constructing micro-meniscuses and microdroplets. A high evaporation rate of 2.16 kg m⁻² h⁻¹ is obtained in pure water under 1 sun. Combining this strategy with the flowing salt-rejecting structure enables a stable evaporation rate in 10.0 wt% NaCl solution and simulated seawater.

Abstract

Interfacial solar water evaporation, a promising way to address water shortages and water pollution, has attracted increasing attention. However, low evaporation rates limit its practical applications. Reducing evaporation enthalpy is one of the most efficient ways to improve the evaporation rate. In this study, micro-meniscuses and microdroplets (MMDs) are found and observed on the surface of the polypyrrole nanoarrays on hydrophilic carbon cloth. The MMDs can reduce the evaporation enthalpy of the system, thus resulting in a high evaporation rate of 2.16 kg m⁻² h⁻¹ in pure water under 1 sun. Dynamic calculations imply that the evaporation rate of MMDs is approximately at least 1.7 times and 1.8 times that of a flat liquid film, respectively. Under 1 sun, the evaporators with MMDs enable stable evaporation in continuous 72 h in 10.0 wt% NaCl solution, simulated seawater, and actual wastewater, with an evaporation rate of 1.86, 1.99, and 1.82 kg m⁻² h⁻¹, respectively. As far as it is known, these evaporation rates are the highest reported values for the 2D interfacial solar evaporator in high-salinity brine or wastewater. It is believed that this work provided a novel pathway for designing an evaporator with low evaporation enthalpy and high evaporation performance.

A series of layered oxyselenides Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) are synthesized by a time-saving method. The origin of the excellent thermoelectric performance of Bi₂LnO₄Cu₂Se₂ is thoroughly investigated. A maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which proves to be a potential thermoelectric system for further investigation.

Abstract

Layered oxyselenides have been widely investigated as promising thermoelectric materials due to their unique merits such as super-lattice structural features and intrinsic complexity, which contributes to low thermal conductivity and easily controllable electrical properties. Newly developed Bi₂LnO₄Cu₂Se₂ (Ln stands for lanthanide) oxyselenides are found to be potential thermoelectric systems since they have excellent electrical conductivity over 10³ S cm⁻¹. In this work, unique energy and time-saving method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) is adopted to successfully prepare a highly pure Bi₂LnO₄Cu₂Se₂ instead of a traditional solid-state reaction. To explore the most suitable lanthanide for Bi₂LnO₄Cu₂Se₂, thermoelectric performance in a wide temperature range (300 to 923 K) of Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) is deeply evaluated and studied. Ultimately, with a relatively high electrical conductivity, moderate Seebeck coefficient, and extremely low thermal conductivity, a maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which is 4 times larger than that of the ever-reported Bi₂YO₄Cu₂Se₂ and proves a potential thermoelectric system for further investigation. This work may provide some enlightenment and broaden the horizon in finding new thermoelectric materials, especially for complex layered compounds.

Ni₃S₂ is co-doped by W heteroatoms and S vacancies to regulate the local spin state and band structure simultaneously for advanced activity of oxygen evolution reaction.

Abstract

The separate modulation of the adsorption of *O and *OOH is challenging in oxygen evolution reaction (OER), which results in a large overpotential and slow kinetics. To balance the adsorption of the two active species, here, a way to regulate the local spin state and band structure simultaneously in Ni₃S₂ nanosheets is reported. The adequate doping of W heteroatoms causes the electron depletion from the Ni active site, which modulates the spin state of e_g electrons, weakening the adsorption of *O. Additionally, the introduction of S vacancies contributes to the upshift of the d band center, which strengthens the adsorption of *OOH. In this manner, the adsorption of Ni₃S₂ for the active intermediates is optimized, resulting in a considerably improved overpotential of 246 mV at 100 mA cm⁻² and a Tafel slope of 66 mV dec⁻¹. This work provides insights into the exploration of OER catalysts through synergistic modulation of the spin state and the band structure.

The novel thermal properties of the Janus MoSSe/WSSe heterostructure are manipulated using thermo-mechanical coupling.

Abstract

2D Janus transition metal dichalcogenide (TMD) semiconductor materials have attracted great interest for their potential applications. Because of the increased requirement for thermal management in 2D devices with single-atom thickness, a fundamental understanding of interfacial thermal conduction (ITC) has emerging significance. In this work, the ITC of in-plane heterostructures constructed using MoSSe and WSSe is reported. In addition to the interface connected normally by MoSSe and WSSe with the same type of chalcogen atoms are on the same side of left and right sections, inversional interface by rotation of 180° of WSSe is also considered, in which S atoms are on the opposite side of the left and right sections. Interestingly, the ITC in the normally connected heterostructure is found to be almost twice as much as that in the inversely connected heterostructure. The unusually large change in ITC is attributed to the bending curvature and additional discontinuity in the inversely connected heterostructure. Euler–Bernoulli beam model gives further insight into the origin of such interface bending. The findings offer the very first insight into the phonon transport in Janus heterostructures, and benefit thermal management of 2D devices based on Janus monolayers.

The new discovery of the pH-dependent SCN⁻ poisoning kinetics on both married edge-adjacent Fe₂N₅ and graphitic N sites proves that the bi-active sites make contributions to excellent ORR activities in a wide pH range, which might be conducive to bridging the unresolved gaps between experimental and theoretical findings and constructing the guiding principles to design efficient carbon-based ORR catalysts.

Abstract

Transition metal-nitrogen-carbon-based catalysts (M-N-C) serve as promising alternatives for oxygen reduction reaction (ORR). However, their synthesis generally involves complex pyrolysis reactions, resulting in their high structure heterogeneity and consequently making it difficult to distinguish the catalytic active sites. Herein, atomically dispersed Fe₂ on the hollow carbon spheres are synthesized as the model for insight into the active sites at the atomic level. By virtue of the systematic SCN⁻ poisoning experiments and theoretical calculations, the authors find that both edge-adjacent Fe₂N₅ and graphitic N sites exhibit the pH-dependent poisoning kinetics, beyond a simple and traditional “SCN⁻ poisoning M-N _x sites” notion, helping us to discriminate the edge-adjacent Fe₂N₅ structure and graphitic N species as the bi-active ORR sites in a wide pH range. Moreover, this is the first work to synthesize the new married edge-adjacent Fe₂N₅ structure in an experimental aspect. The original work offers an important insight to pinpoint the active species in different pH media, which can broaden the fundamental understanding to design M-N-C and metal-free-carbon-based catalysts for ORR.

Despite the various techniques developed for the transfer of large area graphene grown by chemical vapor deposition (CVD), the conventional polymethylmethacrylate (PMMA) transferring technique has been widely applied in laboratories due to its convenience and economical cost. However, the complete removal of PMMA on graphene surface has become a troublesome, and the PMMA residue could degrade the properties of graphene significantly. We report here a facile water assisted technique to directly peel off the PMMA layer over centimeter-sized CVD graphene film for the first time. No organic solvents are involved in the whole transfer process. The transferred graphene film is clean and intact over large area because of the cooperative effect of the capillary force and the van der Waals force which facilitates the conformal contact between graphene film and the substrate. Various types of graphene samples (i.e. monolayer, multilayer, and incomplete domains) can be easily transferred t...

Nature Chemistry, Published online: 23 December 2021; doi:10.1038/s41557-021-00868-y

Bilayer borophene, predicted to be stabilized by interlayer linkages, has now been grown by molecular beam epitaxy on copper and silver surfaces in two independent studies. The growth substrate and temperature are found to influence the lattice structures formed.

New and in-depth insight into the fundamental mechanism of Fermi level pinning in 2D semiconductor devices is presented in this review. The related device characteristics and contact strategies utilizing both the Fermi level pinning and depinning are introduced.

Abstract

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Nature, Published online: 15 December 2021; doi:10.1038/s41586-021-04002-3

A study using local compressibility measurements reports fractional Chern insulator states at low magnetic field in magic-angle twisted bilayer graphene, and establishes the applied magnetic field as a means to tune the Berry curvature distribution.

Moiré superlattices (MSL) in twisted bilayer graphene (TBG) and its derived structures can host exotic correlated quantum phenomena because the narrow moiré flat minibands in those systems effectively enhance the electron-electron interaction. Correlated phenomena are also observed in 2H-transitional metal dichalcogenides MSL. However, the number of moiré systems that have been explored in experiments are still very limited. Here we theoretically investigate a series of two-dimensional (2D) twisted bilayer hexagonal materials beyond TBG at fixed angles of 7.34 ∘ and 67.34 ∘ with 22 2D van der Waals layered materials that are commonly studied in experiments. First-principles calculations are employed to systemically study the moiré minibands in these systems. We find that flat bands with narrow bandwidth generally exist in these systems. Some of the systems such as twisted bilayer In 2 Se 3 , InSe, GaSe, GaS and PtS 2 even host ultr...

Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe 2 /WS 2 vertical heterostructures as well as in pristine WS 2 and WSe 2 layers. For WS 2 , we find that photocurrents increase with increasing layer thickness, whereas for WSe 2 the layer dependence is more complex and depends on both the layer number and applied bias ( V b ). We further find that photocurrents in the WSe 2 /WS 2 heterostructur...

The development of scalable techniques to make two-dimensional (2D) material heterostructures is a major obstacle that needs to be overcome before these materials can be implemented in device technologies. Electrodeposition is an industrially compatible deposition technique that offers unique advantages in scaling 2D heterostructures. In this work, we demonstrate the electrodeposition of atomic layers of WS 2 over graphene electrodes using a single source precursor. Using conventional microfabrication techniques, graphene was patterned to create micro-electrodes where WS 2 was site-selectively deposited to form 2D heterostructures. We used various characterization techniques, including atomic force microscopy, transmission electron microscopy, Raman spectroscopy and x-ray photoelectron spectroscopy to show that our electrodeposited WS 2 layers are highly uniform and can be grown over graphene at a controllable deposition rate. This technique to sele...

MnBi 2 Te 4 /(Bi 2 Te 3 ) n materials system has recently generated strong interest as a natural platform for the realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic (FM) MnBi 2 Te 4 /(Bi 2 Te 3 ) n it is found that migration of Mn between MnBi 2 Te 4 septuple layers (SLs) and otherwise non-magnetic Bi 2 Te 3 quintuple layers (QLs) has systemic consequences—it induces FM coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (

We address the impact of crystal phase disorder on the generation of helicity-dependent photocurrents in layered MoTe 2 , which is one of the van der Waals materials to realize the topological type-II Weyl semimetal phase. Using scanning photocurrent microscopy, we spatially probe the phase transition and its hysteresis between the centrosymmetric, monoclinic 1Tʹ phase to the symmetry-broken, orthorhombic T ##IMG## [http://ej.iop.org/images/2053-1583/9/1/011002/tdmac3e03ieqn1.gif] {$_\textrm{d}$} phase as a function of temperature. We find a highly disordered photocurrent response in the intermediate temperature regime. Moreover, we demonstrate that helicity-dependent and ultrafast photocurrents in MoTe 2 arise most likely from a local breaking of the electronic symmetries. Our results highlight the prospects of local domain morphologies and ultrafast relaxation dynamics on the optoelectronic properties of low-dimensional van der Waals cir...

Nature Communications, Published online: 10 December 2021; doi:10.1038/s41467-021-27531-x

The reaction of a conductive ferroelectric matter to external electric field remains largely unknown. Here, the authors reveal the relationship between the electrically-driven crystalline domain transition along the multiple-polar directions and the resistance change.