zemin zheng

Nature, Published online: 23 February 2022; doi:10.1038/s41586-021-04337-x

Multiple complementary optical signatures confirm the persistence of ferroelectricity and inversion-symmetry-breaking magnetic order down to monolayer NiI2, introducing the physics of type-II multiferroics into the area of van der Waals materials.

A low-voltage memristor array based on ultrathin PdSeO _x /PdSe₂ heterostructure is demonstrated using controllable ultraviolet–ozone treatment. By confining the formation of conductive filaments in the heterostructure, the memristor achieves a remarkable uniform switching with low set and reset voltage variability of 4.8% and −3.6%, respectively. The crossbar array further enables multiple convolutional image processing with a high image-recognition accuracy of ≈93.4%.

Abstract

In-memory computing based on memristor arrays holds promise to address the speed and energy issues of the classical von Neumann computing system. However, the stochasticity of ions’ transport in conventional oxide-based memristors imposes severe intrinsic variability, which compromises learning accuracy and hinders the implementation of neural network hardware accelerators. Here, these challenges are addressed using a low-voltage memristor array based on an ultrathin PdSeO _x /PdSe₂ heterostructure switching medium realized by a controllable ultraviolet (UV)–ozone treatment. A distinctively different ions’ transport mechanism is revealed in the heterostructure that can confine the formation of conductive filaments, leading to a remarkable uniform switching with low set and reset voltage variability values of 4.8% and −3.6%, respectively. Moreover, convolutional image processing is further implemented using various crossbar kernels that achieve a high recognition accuracy of ≈93.4% due to the highly linear and symmetric analog weight update as well as multiple conductance states, manifesting its potential beyond von Neumann computing.

Nature Communications, Published online: 29 March 2022; doi:10.1038/s41467-022-29259-8

Exotic states emerge from the interplay between band topology and ferromagnetism, but it remains less known in canted-antiferromagnetic phase. Here, the authors realize a canted-antiferromagnetic Chern insulator in atomically-thin MnBi2Te4 with electrical control of chiral-edge state transport.

Nature Communications, Published online: 07 April 2022; doi:10.1038/s41467-022-29495-y

The recent thrust toward flexible nanoscale devices creates a need for two-dimensional piezoelectric materials. Here, the authors find large piezoelectric response in NbOI2 flakes ranging from 4 nm to the bulk.

Nature Communications, Published online: 08 April 2022; doi:10.1038/s41467-022-29545-5

Tunable coupling between magnetism and the lattice is important for on-demand manipulation of magnetic phases. Here, the authors demonstrate that lattice vibrations can coherently modulate the interlayer magnetic exchange coupling in the magnetic topological insulator MnBi2Te4.

A creation of clean van der Waals contacts is reported. Clean contacts made from 2D metal Cl–SnSe₂ can completely eliminate the Femi-level pinning, permitting ideal Schottky barrier heights and polarity-controllable transistors. With the ability to control the carrier polarity, various functional logic gates and circuits (inverter, NAND, and NOR) are demonstrated by integrating WSe₂ transistors with Cl–SnSe₂ contacts.

Abstract

Precise control over the polarity of transistors is a key necessity for the construction of complementary metal–oxide–semiconductor circuits. However, the polarity control of 2D transistors remains a challenge because of the lack of a high-work-function electrode that completely eliminates Fermi-level pinning at metal–semiconductor interfaces. Here, a creation of clean van der Waals contacts is demonstrated, wherein a metallic 2D material, chlorine-doped SnSe₂ (Cl–SnSe₂), is used as the high-work-function contact, providing an interface that is free of defects and Fermi-level pinning. Such clean contacts made from Cl–SnSe₂ can pose nearly ideal Schottky barrier heights, following the Schottky–Mott limit and thus permitting polarity-controllable transistors. With the integration of Cl–SnSe₂ as contacts, WSe₂ transistors exhibit pronounced p-type characteristics, which are distinctly different from those of the devices with evaporated metal contacts, where n-type transport is observed. Finally, this ability to control the polarity enables the fabrication of functional logic gates and circuits, including inverter, NAND, and NOR.

Atomically resolved multiple 2D phase transformations (MoS₂ → Mo₄S₆, MoSe₂ → L-, Z-Mo₆Se₆) is observed in monolayer transition metal dichalcogenides under in situ heating with stoichiometry control by electron beam irradiation. Through chalcogen sliding and reconstruction mechanisms, phase transformations are well manipulated to fabricate diphase heterostructures with atomically sharp interfaces, which will pave the way to phase engineered optoelectronics.

Abstract

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS₂ and MoSe₂. The multiphase transformations: MoS₂ → Mo₄S₆ and MoSe₂ → Mo₆Se₆ which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor–metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.

Controllable and reversible photoconductance polarity switching can enhance broadband photodetection performance besides enabling multi-level optoelectronic logic and memory applications. In this article, wavelength-controlled negative (NPC) and positive photoconductance (PPC), along with reversible switching between the two, have been demonstrated in a BP-MoS₂ heterostructure phototransistor. The NPC-to-PPC crossover wavelength can be tuned by varying MoS₂ thickness.

Abstract

Layered 2D van der Waals semiconductors and their heterostructures have been shown to exhibit positive photoconductance (PPC) in many studies. A few recent reports have demonstrated negative photoconductance (NPC) as well that can enable broadband photodetection besides multi-level optoelectronic logic and memory. Controllable and reversible switching between PPC and NPC is a key requirement for these applications. This report demonstrates visible-to-near infrared wavelength-driven NPC and PPC, along with reversible switching between the two, in an air stable, high mobility, broadband black phosphorus field effect transistor covered with a few layer MoS₂ flake. The crossover switching wavelength can be tuned by varying the MoS₂ bandgap through its flake thickness and the NPC and PPC photoresponsivities can be modulated using electrostatic gating as well as laser power. Recombination-driven NPC and PPC, as supported by density functional theory calculations, allows for reversible switching. Further, gate voltage-dependent negative persistent photoconductance is well-suited for optosynaptic applications.

Nature Communications, Published online: 18 March 2022; doi:10.1038/s41467-022-29182-y

The industrial application of two-dimensional (2D) materials strongly depends on the large-scale manufacturing of high-quality 2D films and powders. Here, the authors analyze three state-of-the art mass production techniques, discussing the recent progress and remaining challenges for future improvements.

Ferroelastic–ferroelectric multiferroic materials can switch orientations of crystal structure and polarization with external strain. 2D ferroelastic–ferroelectric multiferroicity in single-crystalline rhenium dichalcogenides is discovered. Reorientation of the physical properties based on reversible bond switching between the rhenium atoms provides insights for 2D multiferroic phase transitions and opens up new opportunities for applications such as multilevel memory.

Abstract

2D multiferroics with combined ferroic orders have gained attention owing to their novel functionality and underlying science. Intrinsic ferroelastic–ferroelectric multiferroicity in single-crystalline van der Waals rhenium dichalcogenides, whose symmetries are broken by the Peierls distortion and layer-stacking order, is demonstrated. Ferroelastic switching of the domain orientation and accompanying anisotropic properties is achieved with 1% uniaxial strain using the polymer encapsulation method. Based on the electron localization function and bond dissociation energy of the Re–Re bonds, the change in bond configuration during the evolution of the domain wall and the preferred switching between the two specific orientation states are explained. Furthermore, the ferroelastic switching of ferroelectric polarization is confirmed using the photovoltaic effect. The study provides insights into the reversible bond-switching process and potential applications based on 2D multiferroicity.

The increasing demand for wearable electronics has boosted the development of energy-saving and self-powered personal thermal management systems. This review highlights the unique advantages of thermoelectric technology comparing with other technologies, summarizes corresponding key parameters, fundamental functions, material and device advancements of thermoelectric personal thermal management, and further points out corresponding future research directions.

Abstract

With the ever-increasing demand for wearable electronics and energy-saving technologies, self-powered thermoelectric personal thermal management (PTM) has attracted extensive research interest. In this review, the unique characteristics of thermoelectric PTM comparing with other technologies are first highlighted, and the key parameters and fundamental functions of thermoelectric PTM are systematically summarized. Then, the advances in thermoelectric PTM are overviewed from the material design to the wearable device design viewpoints. Finally, the key challenges and future research directions of thermoelectric PTM, where both high-performance flexible materials and proper device designs are in urgent need, are pointed out. This review will deliver a systematic understanding and guideline for thermoelectric PTM.

Nature Communications, Published online: 16 March 2022; doi:10.1038/s41467-022-29001-4

Graphene and related two-dimensional (2D) materials have remained an active field of research in science and engineering for over fifteen years. Here, the authors investigate why the transition from laboratories to fabrication plants appears to lag behind expectations, and summarize the main challenges and opportunities that have thus far prevented the commercialisation of these materials.

On epitaxial substrates with moderate surface interactions, density functional theory calculations show that a step with appropriate height can not only promote simultaneous nucleation of thickness-matched MoS₂ domains with aligned edges but also suppresses the nucleation of thinner MoS₂ domains, by which uniform multilayer MoS₂ with targeted thickness can be grown.

Abstract

Multilayer MoS₂ shows superior performance over the monolayer MoS₂ for electronic devices while the growth of multilayer MoS₂ with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS₂ domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS₂ domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS₂ domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS₂ on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS₂ with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS₂ films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Spin–phonon coupling in monolayer chromium tribromide is investigated via Raman spectroscopy in combination with first principle calculations. The experimental Curie temperature and phonon shifts are in good agreement with numerical simulations. This work demonstrates how magnetic exchange interactions affect the phonon vibrations and establishes the missing guidelines for the design of the 2D magnetic materials and related devices.

Abstract

Novel 2D magnets exhibit intrinsic electrically tunable magnetism down to the monolayer limit, which has significant value for nonvolatile memory and emerging computing device applications. In these compounds, spin–phonon coupling (SPC) typically plays a crucial role in magnetic fluctuations, magnon dissipation, and ultimately establishing long-range ferromagnetic order. However, a systematic understanding of SPC in 2D magnets that combines theory and experiment is still lacking. In this work, monolayer chromium tribromide is studied to investigate SPC in 2D magnets via Raman spectroscopy and first principle calculations. The experimental Curie temperature and phonon shifts are found to be in good agreement with the numerical simulations. Specifically, it is demonstrated how magnetic exchange interactions affect phonon vibrations, which helps establish design fundamentals for 2D magnetic materials and other related devices.

Nature Nanotechnology, Published online: 14 March 2022; doi:10.1038/s41565-021-01068-y

This Review discusses the recent progress in the emerging field of exciton phenomena in semiconductor moiré superlattices.

The two-dimensional (2D) metal nitrides (MNs), including group IIA nitrides, group IIIA nitrides, nitride MXene and other transition metal nitrides (TMNs), exhibit unique electronic and magnetic characteristics. The 2D MNs have been widely studied by experimental and computational approaches and some of them have been synthesized. Herein we systematically reviewed the structural, electronic, thermal, mechanical, magnetic and optical properties of the 2D MNs that have been reported in recent years. Based on their unique properties, the related applications of 2D MNs on fields like electronics, spintronics, sensing, catalysis, and energy storage were discussed. Additionally, the lattice structures and synthetic routes were also summarized as supplements of the research progress of 2D MNs family. Furthermore, we provided insights into the research prospects and future efforts that need to be made on 2D MNs.

The unique optical and electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) make them promising materials for applications in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have broad potential applications owing to their composition-controlled properties. Several important challenges remain regarding controllable and scalable fabrication of these alloys, such as achieving control over their atomic ordering (i.e. clustering or random mixing of the transition metal atoms within the 2D layers). In this work, atomic layer deposition is used to synthesize the TMD alloy Mo 1− x W x S 2 with excellent composition control along the complete composition range 0 ⩽ x ⩽ 1. Importantly, this composition control allows us to control the atomic ordering of the alloy from well-mixed to clustered while keeping the alloy composition fixed, as is confirmed directly through atomic-resolution ...

This work presents an organic/inorganic hybrid route that small molecular copper(II) phthalocyanine nanoparticles can be exploited to effectively improve the thermoelectric performance of Bi_0.5Sb_1.5Te₃ compounds, yielding a high zT _avg of 1.1 between 300 and 523 K and a high conversion efficiency of 6.8% for single TE leg at ΔT = 223 K.

Abstract

Bismuth telluride alloys favor the applications of low-grade waste heat recovery if their figure-of-merits are improved within the larger temperature range from 300 to 523 K. Herein, this work reports a synergistic optimization for Bi_0.5Sb_1.5Te₃ (BST) by incorporating the copper(II) phthalocyanine (CuPc), which is preferentially distributed at the grain boundary of BST after the spark plasma sintering process and suppresses the grain growth of BST. The lattice thermal conductivity of composites is then extensively reduced by the multiscale scattering induced by the CuPc. In addition, the Cu atoms diffuse into the lattice of BST and increase the whole concentration, thus suppressing the bipolar effect. As a result, the average zT value is effectively enhanced from 0.7 to 1.1 in the temperature range between 300 and 523 K. A high conversion efficiency of 6.8% is achieved in a single BST/CuPc₅ leg, which is 41.7% higher than that of BST at temperature different ΔT = 223 K. This result proves that the composition optimization of the BST/CuPc is a promising strategy to improve the application of BST-based TE modules.

Nature Nanotechnology, Published online: 10 March 2022; doi:10.1038/s41565-022-01076-6

Detectivities of WS₂/HfS₂ heterojunctions

Electronic structure of XY haeckelite compounds (X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) is studied from the first principles. The calculations reveal these compounds possess fascinating electronic properties with nontrivial band topologies relevant to future optoelectronic and quantum information technologies.

Abstract

The family of III–V element compounds (i.e., XY compounds; X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) have been intensively investigated for several decades because of their enormous applications for many optoelectronic devices. Here, by employing first-principles calculations, the electronic structures of bulk XY haeckelite compounds are examined. It is identified that InSb (TlN and TlP) is Dirac semimetal (are strong topological insulators). The other fifteen XY compounds are semiconducting. The effect of biaxial and uniaxial tensile and compressive strains on the electronic structures are studied. These materials offer diverse topological orders. The semiconducting band gaps are mainly found between the bonding and antibonding states of the mixed X(p)–Y(p) orbitals at the top of the valence band and the bottom of the conduction bands, respectively. The topological insulating nature of the XY compounds is explained based on the degenerate p _x  + p _y orbitals and their orbital energies relative to the p _z orbitals near the Fermi energy. The nontrivial band topologies of TlN and TlP are confirmed by calculating the Z ₂ (1;000) index, surface states, and Wilson loop calculations. The bands split into two branches by including spin-orbit interaction. The results demonstrate that haeckelite compounds are fascinating materials with broad potential applications in optoelectronics and possessing the possibility of hosting emergent physical phenomena.