Xingxing Zhang

Metallic/semimetallic transition metal dichalcogenides (m-TMDs) have grabbed widespread attention in recent years due to their exotic physical properties and potential applications in various fields. The state-of-the-art progress in m-TMDs is reviewed, including electronic and crystal structures, synthetic methods, physical properties, and practical applications. Moreover, views on development, challenges, and future prospects of m-TMDs are put forward.

Abstract

2D materials and the associated heterostructures define an ideal material platform for investigating physical and chemical properties, and exhibiting new functional applications in (opto)electronic devices, electrocatalysis, and energy storage. 2D transition metal dichalcogenides (2D TMDs), as a member of the 2D materials family including 2D semiconducting TMDs (s-TMDs) and 2D metallic/semimetallic TMDs (m-TMDs) have attracted considerable attention in the scientific community. Over the past decade, the 2D s-TMDs have been extensively researched and reviewed elsewhere. Because of their distinctive physical properties including intrinsic magnetism, charge-density-wave order and superconductivity, and potential applications, such as high-performance electronic devices, catalysis, and as metal electrode contacts, 2D m-TMDs have grabbed widespread attention in recent years. However, reviews demonstrating the m-TMDs systematically and comprehensively have been rarely reported. Here, the recent advances in 2D m-TMDs in the aspects of their unique structures, synthetic approaches, distinctive physical properties, and functional applications are highlighted. Finally, the current challenges and perspectives are discussed.

Some of the recent research progress of rationally activating the basal plane of transition metal dichalcogenides (TMDCs) for electrochemical energy conversion is summarized. The activated TMDC basal plane, with the integrated advantages of structure and composition, shows much enhanced electrochemical activity.

Abstract

Transition metal dichalcogenides (TMDCs) hold great promise for electrochemical energy conversion technologies in view of their unique structural features associated with the layered structure and ultrathin thickness. Because the inert basal plane accounts for the majority of a TMDC's bulk, activation of the basal plane sites is necessary to fully exploit the intrinsic potential of TMDCs. Here, recent advances on TMDCs-based hybrids/composites with greatly enhanced electrochemical activity are reviewed. After a summary of the synthesis of TMDCs with different sizes and morphologies, comprehensive in-plane activation strategies are described in detail, mainly including in-plane-modification-induced phase transformation, surface-layer modulation, and interlayer modification/coupling. Simultaneously, the underlying mechanisms for improved electrochemical activities are highlighted. Finally, the strategic evaluation on further research directions of TMDCs in-plane activation is featured. This work would shed some light on future design trends of TMDCs-based functional materials for electrochemical energy-related applications.

Combining interface assembly of 2D organic semiconductor crystal (2DOSCs) with a poly(dimethylsiloxane)-mold-assisted selective contact evaporation printing technique, large-area, high-resolution (1271 dpi), and layer-controlled 2DOSC arrays are fabricated. OFETs based on the 2DOSC arrays show high electrical performance and a small relative variation of 12.5% in mobility. This method will promote the research and application of 2DOSCs.

Abstract

2D organic semiconductor crystals (2DOSCs) have extraordinary charge transport capability, adjustable photoelectric properties, and superior flexibility, and have stimulated continuous research interest for next-generation electronic and optoelectronic applications. The prerequisite for achieving large-area and high-throughput optoelectronic device integration is to fabricate high-resolution 2DOSC arrays. Patterned substrate- and template-assisted self-assembly is an effective strategy to fabricate OSC arrays. However, the film thickness is difficult to control due to the complicated crystallization process during solvent evaporation. Therefore, the manufacturing of 2DOSC arrays with high-resolution and controllable molecular-layer numbers through solution-based patterning methods remains a challenge. Herein, a two-step strategy to produce high-resolution layer-controlled 2DOSC arrays is reported. First, large-scale 2DOSCs with well-defined layer numbers are obtained by a solution-processed organic semiconductor crystal engineering method. Next, the high-resolution layer-controlled 2DOSC arrays are fabricated by a polydimethylsiloxane mold-assisted selective contact evaporation printing technique. The organic field-effect transistors (OFETs) based on 2DOSC arrays have high electrical performance and excellent uniformity. The 2,6-bis(4-hexylphenyl)anthracene 2DOSC arrays-based OFETs have a small variation of 12.5% in mobility. This strategy can be applied to various organic semiconductors and pattern arrays. These demonstrations will offer more opportunities for 2DOSCs for integrated optoelectronic devices.

Saturated molecules are not preferred in the research of organic thermoelectrics because of intrinsically wide bandgaps and poor thermopower. Graphene electrodes are demonstrated to be able to enhance thermopower of monolayers of saturated compounds. A noncovalent amine anchor induces the creation of in-gap states and n-type doping of graphene, thereby leading to an increased Seebeck coefficient relative to analogous covalent gold–thiolate monolayers.

Abstract

Enhancing thermopower is a key goal in organic and molecular thermoelectrics. Herein, it is shown that introducing noncovalent contact with a single-layer graphene (SLG) electrode improves the thermopower of saturated molecules as compared to the traditional gold–thiolate covalent contact. Thermoelectric junction measurements with a liquid-metal technique reveal that the value of Seebeck coefficient in large-area junctions based on n-alkylamine self-assembled monolayers (SAMs) on SLG is increased up to fivefold compared to the analogous junction based on n-alkanethiolate SAMs on gold. Experiments with Raman spectroscopy and field-effect transistor analysis indicate that such enhancements benefit from the creation of new in-gap states and electron doping through noncovalent interaction between the amine anchor and the SLG electrode, which leads to a reduced energy offset between the Fermi level and the transport channel. This work demonstrates that control of interfacial bonding nature in molecular junctions improves the Seebeck effect in saturated molecules.

Ion insertion in between the 2D layers of WTe₂ is shown to induce an exotic crystallographic phase. This novel phase is linked to uniquely large uniaxial in-plane strain. On-chip electrochemical control over the lithium content in single flakes of WTe₂ is demonstrated as an efficient, fast, and reversible way to control the strain, making Li _x WTe₂ a promising microscale actuator.

Abstract

On-chip dynamic strain engineering requires efficient micro-actuators that can generate large in-plane strains. Inorganic electrochemical actuators are unique in that they are driven by low voltages (≈1 V) and produce considerable strains (≈1%). However, actuation speed and efficiency are limited by mass transport of ions. Minimizing the number of ions required to actuate is thus key to enabling useful “straintronic” devices. Here, it is shown that the electrochemical intercalation of exceptionally few lithium ions into WTe₂ causes large anisotropic in-plane strain: 5% in one in-plane direction and 0.1% in the other. This efficient stretching of the 2D WTe₂ layers contrasts to intercalation-induced strains in related materials which are predominantly in the out-of-plane direction. The unusual actuation of Li _x WTe₂ is linked to the formation of a newly discovered crystallographic phase, referred to as Td', with an exotic atomic arrangement. On-chip low-voltage (<0.2 V) control is demonstrated over the transition to the novel phase and its composition. Within the Td'-Li_{0.5− δ} WTe₂ phase, a uniaxial in-plane strain of 1.4% is achieved with a change of δ of only 0.075. This makes the in-plane chemical expansion coefficient of Td'-Li_0.5−δWTe₂ far greater than of any other single-phase material, enabling fast and efficient planar electrochemical actuation.

High-entropy atomic layers of transition-metal carbide (HE-MXene) are produced via selectively etching a high-entropy MAX phase, in which five transition-metal species are homogeneously dispersed into one MX slab, giving rise to stable transition-metal carbide in the atomic layers. The resultant HE-MXene has distinct lattice distortions and strains, efficiently guiding the nucleation and uniform growth of lithium.

Abstract

High-entropy materials (HEMs) have great potential for energy storage and conversion due to their diverse compositions, and unexpected physical and chemical features. However, high-entropy atomic layers with fully exposed active sites are difficult to synthesize since their phases are easily segregated. Here, it is demonstrated that high-entropy atomic layers of transition-metal carbide (HE-MXene) can be produced via the selective etching of novel high-entropy MAX (also termed M _{n +1}AX _n (n = 1, 2, 3), where M represents an early transition-metal element, A is an element mainly from groups 13–16, and X stands for C and/or N) phase (HE-MAX) (Ti_1/5V_1/5Zr_1/5Nb_1/5Ta_1/5)₂AlC, in which the five transition-metal species are homogeneously dispersed into one MX slab due to their solid-solution feature, giving rise to a stable transition-metal carbide in the atomic layers owing to the high molar configurational entropy and correspondingly low Gibbs free energy. Additionally, the resultant high-entropy MXene with distinct lattice distortions leads to high mechanical strain into the atomic layers. Moreover, the mechanical strain can efficiently guide the nucleation and uniform growth of dendrite-free lithium on HE-MXene, achieving a long cycling stability of up to 1200 h and good deep stripping–plating levels of up to 20 mAh cm⁻².

A low-cost post-processing method to etch the defects in phase-transition-assisted CVD-grown 2H-MoTe₂ by using the triiodide ion solution is reported. The etching mechanism is discussed based on the high Te-vacancy densities in defects and 1T′ phase. The etching results are confirmed by electrical measurements and chemical analysis.

Abstract

2D molybdenum ditelluride (MoTe₂) with polymorphism is a promising candidate to developing phase-change memory, high-performance transistors and spintronic devices. The phase-transition-assisted chemical vapor deposition (CVD) process has been used to prepare large-scale 2H-MoTe₂ with large grain size and low density of grain boundary. However, because of the lack of precise control of the growth condition, some defects including the amorphous regions and grain boundaries in 2H-MoTe₂ are hardly avoidable. Here, a facile method of selectively etching defects in large-scale CVD-grown 2H-MoTe₂ by triiodide ion (I₃ ⁻) solution is reported. The defect etching is attributed to the reduced lattice symmetry, high chemisorption activity and high conductivity of the defects due to the high density of Te vacancies. The treated 2H-MoTe₂ shows the suppressed hysteresis in the electrical transfer curve, enhances hole mobility and the higher effective barrier height on the metal contact, suggesting the decreased density of defects. Further chemical analysis indicates that the 2H-MoTe₂ is not damaged or doped by I₃ ⁻ solution during the etching process. This simple and low-cost post-processing method is effective for etching the defects in large-area 2H-MoTe₂ for high-performance device applications.

With lithography-free, direct-laser-writing-controlled MoS₂/MoS_{2− x} O _δ grain boundaries, synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously, as well as low energy consumption that is over 40 times lower than that of conventional complementary metal–oxide–semiconductor (CMOS) devices.

Abstract

Synaptic devices based on 2D-layered materials have emerged as high-efficiency electronic synapses and neurons for neuromorphic computing. Lateral 2D synaptic devices have the advantages of multiple functionalities by responding to diverse stimuli, but they consume large amounts of energy, far more than the human brain. Moreover, current lateral devices employ several mechanisms based on conductive filaments and grain boundaries (GBs), but their formation is random and difficult to control, also hindering their practical applications. Here, four-terminal, lateral synaptic devices with artificially engineered GBs are reported, which are made from monolayer MoS₂. With lithography-free, direct-laser-writing-controlled MoS₂/MoS_{2− x} O _δ GBs, such synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously. This enables detailed simulations of biological learning and cognitive processes as well as image perception and processing. In particular, the device exhibits low energy consumption, similar to that of the human brain and much lower than those of other lateral 2D synaptic devices. This work provides an effective way to fabricate lateral synaptic devices for practical application development and sheds light on controllable electrical state switching for neuromorphic computing.

Moiré superlattices produced through artificial stacking are often quite inhomogeneous and are challenging to characterize noninvasively. In this work, interlayer-breathing mode and moiré phonons are proven to be highly susceptible to the moiré period, and enable large-scale mapping of moiré superlattices by hyperspectral Raman imaging.

Abstract

Moiré superlattices can induce correlated-electronic phases in twisted van der Waals materials: strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here, it is demonstrated in twisted-bilayer transition-metal dichalcogenides that low-frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer-breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. Hyperspectral Raman imaging visualizes microscopic domains of a 5° twisted-bilayer sample with an effective twist-angle resolution of about 0.1°. This ambient methodology can be conveniently implemented to characterize and preselect high-quality areas of samples for subsequent device fabrication, and for transport and optical experiments.

Wafer-scale growth of single-grain 2D metal chalcogenides remains a challenge in the commercialization of various electronic devices based on these materials. A generalized surface-diffusion-induced epitaxial self-planarization method to produce wafer-scale single-grain metal chalcogenide thin films is described. This process also enables the practical applications of single-grain metal chalcogenides without involving a transfer process.

Abstract

Although wafer-scale single-grain thin films of 2D metal chalcogenides (MCs) have been extensively sought after during the last decade, the grain size of the MC thin films is still limited in the sub-millimeter scale. A general strategy of synthesizing wafer-scale single-grain MC thin films by using commercial wafers (Si, Ge, GaAs) both as metal source and epitaxial collimator is presented. A new mechanism of single-grain thin-film formation, surface diffusion, and epitaxial self-planarization is proposed, where chalcogen elements migrate preferentially along substrate surface and the epitaxial crystal domains flow to form an atomically smooth thin film. Through synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy, the formation of single-grain Si₂Te₃, GeTe, GeSe, and GaTe thin films on (111) Si, Ge, and (100) GaAs is verified. The Si₂Te₃ thin film is used to achieve transfer-free fabrication of a high-performance bipolar memristive electrical-switching device.

Phase-transition-induced growth is effective for synthesizing layer-controlled and electronic-state-modulated MoS₂ films. The energetically unstable amorphous MoS _x O _y is efficiently converted to a thermodynamically stable crystalline MoS₂ film on a wafer scale with excellent uniformity. The number of layers is precisely controlled layer-by-layer from one to eleven layers, which is applicable in the growth of both intrinsic and heteroatom-inserted MoS₂.

Abstract

Wafer-scale growth of transition metal dichalcogenides with precise control over the number of layers, and hence the electronic state is an essential technology for expanding the practical application of 2D materials. Herein, a new growth method, phase-transition-induced growth (PTG), is proposed for the precisely controlled growth of molybdenum disulfide (MoS₂) films consisting of one to eleven layers with spatial uniformity on a 2 in. wafer. In this method, an energetically unstable amorphous MoS _x O _y (a-MoS _x O _y ) phase is effectively converted to a thermodynamically stable crystalline MoS₂ film. The number of MoS₂ layers is readily controlled layer-by-layer by controlling the amount of Mo atoms in a-MoS _x O _y , which is also applicable for the growth of heteroatom-inserted MoS₂. The electronic states of intrinsic and Nb-inserted MoS₂ with one and four layers grown by PTGare are analyzed based on their work functions. The work function of monolayer MoS₂ effectively increases with the substitution of Nb for Mo. As the number of layers increases to four, charge screening becomes weaker, dopant ionization becomes easier, and ultimately the work function increases further. Thus, better electronic state modulation is achieved in a thicker layer, and in this respect, PTG has the advantage of enabling precise control over the film thickness.

Atomically thin NbSe₂ shows near-infrared plasmonic resonance that is thickness-dependant and can be in situ modulated by an electrostatic voltage applied through an ion gel. These results demonstrate a new strategy to access electrostatically tunable near-infrared plasmonic sources.

Abstract

In the broad spectral range, near-infrared (NIR) plasmonics find applications in telecommunication, energy harvesting, sensing, and more, all of which would benefit from an electrostatically controllable NIR plasmon source. However, it is difficult to control bulk NIR plasmonics directly with electrostatics because of the strong electric-field screening effect and high carrier concentration required to support NIR plasmons. Here, this constraint is overcome and the observation of NIR plasmonic resonances that can be modulated electrostatically over a range of ≈360 cm^–1 in few-layer NbSe₂ gratings is reported, thanks to the enhanced electrostatics of atomically thin 2D materials and the high-quality film produced by a solution method. NbSe₂ plasmons also render strong field confinement due to their atomic thickness and provide an extra degree of resonance frequency modulation from the layered structure. This study identifies metallic 2D materials as promising (easily produced and well-performing) candidates to extend electrostatically tunable plasmonics to the technologically important NIR range.

Nanodots derived from layered materials not only exhibit the intriguing properties of traditional nanodots due to the size confinement effect, but also inherit some unique properties of layered materials, making them promising candidates in various applications. The state-of-the-art progress on the preparation and applications of nanodots derived from layered materials is reviewed.

Abstract

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10 nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

SnTe/PbTe monolayer lateral heterostructures are experimentally created through a two-step molecular beam epitaxial growth process. A vortex-oriented quadrant ferroelectric domain configuration is observed in the van der Waals 2D material heterostructure in this work.

Abstract

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

18-fold-enhanced electron mobility is achieved for n-channel organic field-effect transistors with co-crystalized C₆₀:C₇₀ blend films formed on nanodot-structured dipolar poly(2-ethyl-2-oxazoline) (PEOz) interlayers that also reduce the work function of the electrodes. The PEOz nanodot layers further deliver high spatial uniformity in electron mobility and enhance device stability for transistor array fabrication.

Abstract

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C₆₀-I_h)[5,6]fullerene (C₆₀) and (C₇₀-D_5h(6))[5,6]fullerene (C₇₀) blend films, with an increase from 0.1 to 2.1 cm² V⁻¹ s⁻¹. The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C₆₀:C₇₀ co-crystal structure within the blend films.

In field-effect transistors with solid ionic conductors as the gate dielectric, the easy-axis of the ferromagnetism of Cr₂Ge₂Te₆ thin flakes can be uniformly tuned from the out-of-plane direction to the in-plane direction by an electric field, coinciding with a significant increase of the Curie temperature. The surface of the sample is fully exposed in this type of devices, making further heterostructure-engineering possible.

Abstract

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr₂Ge₂Te₆. The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr₂Ge₂Te₆. Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

A novel 2D-heterostructure mixing cell allows sub-nanometer resolution characterization of calcium carbonate precipitation via nonclassical nucleation using scanning transmission electron microscopy. A trigger-based mixing mechanism using controlled damage to the MoS₂ membrane enables visualization of the entire reaction timeline with high spatial resolution, including liquid–liquid phase separation, formation of amorphous calcium carbonate, and crystallization.

Abstract

Liquid-phase transmission electron microscopy is used to study a wide range of chemical processes, where its unique combination of spatial and temporal resolution provides countless insights into nanoscale reaction dynamics. However, achieving sub-nanometer resolution has proved difficult due to limitations in the current liquid cell designs. Here, a novel experimental platform for in situ mixing using a specially developed 2D heterostructure-based liquid cell is presented. The technique facilitates in situ atomic resolution imaging and elemental analysis, with mixing achieved within the immediate viewing area via controllable nanofracture of an atomically thin separation membrane. This novel technique is used to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment. The observations provide the first direct visual confirmation of the recently developed liquid-liquid phase separation theory, while the technological advancements open an avenue for many other studies of early stage solution-phase reactions of great interest for both the exploration of fundamental science and developing applications.

2D materials exhibit unique physical phenomena and features that are absent in conventional bulk semiconductors. The theoretical and ab initio methods that yield the optically excited states in 2D materials are reviewed. Several analytical and numerical approaches are introduced and their results are compared with experiments.

Abstract

Recent studies of the optical properties of 2D materials have reported unique phenomena and features that are absent in conventional bulk semiconductors. Many of these interesting properties, such as enhanced light-matter coupling, gate-tunable photoluminescence, and unusual excitonic optical selection rules arise from the nature of the two- and multi-particle excited states such as strongly bound Wannier excitons and charged excitons. The theory, modeling, and ab initio calculations of these optically excited states in 2D materials are reviewed. Several analytical and ab initio approaches are introduced. These methods are compared with each other, revealing their relative strength and limitations. Recent works that apply these methods to a variety of 2D materials and material-defect systems are then highlighted. Understanding of the optically excited states in these systems is relevant not only for fundamental scientific research of electronic excitations and correlations, but also plays an important role in the future development of quantum information science and nano-photonics.

A scalable approach is developed to achieve ferroelectric polymer P(VDF-TrFE) thin films composed of close-packed crystalline nanowires with out-of-plane polar axis via interface-epitaxy with 1T′-ReS₂, which leads to over six orders of magnitude higher ferroelectric field-effect modulation in ReS₂ transistors and coercive voltages as low as 0.1 V, making them viable for large-scale, low-power, high-performance device applications.

Abstract

The flexible, transparent, and low-weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization-enabled device functionalities, large-scale fabrication of polymer thin films with well-controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir–Blodgett, spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) thin films composed of close-packed crystalline nanowires via interface-epitaxy with 1T′-ReS₂. Upon controlled thermal treatment, uniform P(VDF-TrFE) films restructure into about 10 and 35 nm-wide (010)-oriented nanowires that are crystallographically aligned with the underlying ReS₂, as revealed by high-resolution transmission electron microscopy. Piezoresponse force microscopy studies confirm the out-of-plane polar axis of the nanowire films and reveal coercive voltages as low as 0.1 V. Reversing the polarization can induce a conductance switching ratio of >10⁸ in bilayer ReS₂, over six orders of magnitude higher than that achieved by an untreated polymer gate. This study points to a cost-effective route to large-scale processing of high-performance ferroelectric polymer thin films for flexible energy-efficient nanoelectronics.

NbOI₂ has a strong in-plane structural anisotropy, where the [NbO₂I₄] octahedra are connected through I–I edges along the c-axis and cornered O atoms along the b-axis. The I anions with weaker electronegativity contribute to the electrons near the Fermi surface, leading to the highly anisotropic dispersion of band structures along the c-axis. Hence, NbOI₂ exhibits large anisotropic factor of 1.75 and 1.7 for optical absorbance and photoresponsivity, which makes it a promising platform for novel polarization-sensitive photodetection applications.

Abstract

Exploring in-plane anisotropic 2D materials is of great significance to the fundamental studies and further development of polarizationsensitive optoelectronics. Herein, chiral niobium oxide diiodide (NbOI₂) is introduced into the intriguing anisotropic 2D family with the experimental demonstration of anisotropic optical and electrical properties. 2D NbOI₂ crystals exhibit highly anisotropic dispersed band structures around the Fermi surface and strong in-plane anisotropy of phonon vibrations owing to the different bonding modes of Nb atoms along the b- and c-axes. Consequently, the anisotropic factors of optical absorbance and photoresponsivity in 2D NbOI₂ crystals reach up to 1.75 and 1.7, respectively. These anisotropic properties make 2D NbOI₂ an interesting platform for novel polarization-sensitive optoelectronic applications.

Monolayer graphene is stronger than bulk graphite: the elastic properties of graphene are such that it could hypothetically withstand an elephant balancing on a pencil. A contactless optical technique, reported by Bartlomiej Graczykowski and co-workers in article number 2008614 now shows that this is not the case for MoSe₂, an attractive semiconducting 2D material. This material becomes significantly softer by reducing its thickness from bulk to a few layers.

In-memory computing that performs computations in situ within a nonvolatile memory unit is considered as one of the mainstream hardware implementations for future data-intensive computing applications. The recent progress from 2D memory devices to their applications for in-memory computing is summarized, and the current challenges and potential strategies in this exciting field are proposed.

Abstract

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Besides the 2D structure, the unique feature of black phosphorus (BP) is the lone-pair electrons on each P atom. The interaction between the lone-pair electrons of BP and the acceptor component can be utilized for passivation of BP and contributes toward BP-based applications, either indirectly or directly.

Abstract

2D materials have experienced rapid and explosive development in the past decades. Among them, black phosphorus (BP) is one of the most promising materials on account of its thickness-dependent bandgap, high charge-carrier mobility, in-plane anisotropic structure, and excellent biocompatibility, as well as the broad applications brought by the properties. In view of the electron configuration, the most unique feature of BP is the lone-pair electrons on each P atom. The lone-pair electrons inevitably cause high reactivity of BP, particularly toward water/oxygen, which greatly limits the practical application of BP under ambient conditions. The other side of the coin is that BP can serve as an electron donor to promote the construction of BP-based hybrid materials and/or to boost the performance of BP or BP-based hybrid materials in applications. Here, recent advances in passivation and application of BP by addressing the interaction between the lone-pair electrons of BP and the other materials are discussed, and prospects for future research on BP are also proposed.

The graphene-like 2D III-nitride semiconductor materials have unique physical properties such as high stability, wide and tunable bandgap, and magnetism, etc. Those advantages prove them to be useful in optoelectronic, electronic, and spin-based devices and so on. The pro perties, growth methods, and applications of graphene-like 2D III-nitride materials are introduced and reviewed.

Abstract

2D III-nitride materials have been receiving considerable attention recently due to their excellent physicochemical properties, such as high stability, wide and tunable bandgap, and magnetism. Therefore, 2D III-nitride materials can be applied in various fields, such as electronic and photoelectric devices, spin-based devices, and gas detectors. Although the developments of 2D h-BN materials have been successful, the fabrication of other 2D III-nitride materials, such as 2D h-AlN, h-GaN, and h-InN, are still far from satisfactory, which limits the practical applications of these materials. In this review, recent advances in the properties, growth methods, and potential applications of 2D III-nitride materials are summarized. The properties of the 2D III-nitride materials are mainly obtained by first-principles calculations because of the difficulties in the growth and characterizations of these materials. The discussion on the growth of 2D III-nitride materials is focused on 2D h-BN and h-AlN, as the developments of 2D h-GaN and h-InN are yet to be realized. Therefore, applications have been realized mostly based on the 2D h-BN materials; however, many potential applications are cited for the entire range of 2D III-nitride materials. Finally, future research directions and prospects in this field are also discussed.

The patterned synthesis of high-quality 2D PdTe₂ is demonstrated. The as-grown PdTe₂ exhibits high electrical and thermal conductivities, showing potential as an ideal contact material in nanoelectronics. Based on the nondestructive synthesis of PdTe₂ patterns directly on various low-dimensional semiconductors, high-performance field-effect transistors (FETs) with reduced contact barriers are chemically constructed.

Abstract

Low-dimensional semiconductors provide promising ultrathin channels for constructing more-than-Moore devices. However, the prominent contact barriers at the semiconductor–metal electrodes interfaces greatly limit the performance of the obtained devices. Here, a chemical approach is developed for the construction of p-type field-effect transistors (FETs) with low contact barriers by achieving the simultaneous synthesis and integration of 2D PdTe₂ with various low-dimensional semiconductors. The 2D PdTe₂ synthesized through a quasi-liquid process exhibits high electrical conductivity (≈4.3 × 10⁶ S m⁻¹) and thermal conductivity (≈130 W m⁻¹ K⁻¹), superior to other transition metal dichalcogenides (TMDCs) and even higher than some metals. In addition, PdTe₂ electrodes with desired geometry can be synthesized directly on 2D MoTe₂ and other semiconductors to form high-performance p-type FETs without any further treatment. The chemically derived atomically ordered PdTe₂–MoTe₂ interface results in significantly reduced contact barrier (65 vs 240 meV) and thus increases the performance of the obtained devices. This work demonstrates the great potential of 2D PdTe₂ as contact materials and also opens up a new avenue for the future device fabrication through the chemical construction and integration of 2D components.

A double-gate MoS₂ field-effect transistor is designed for potential applications in multifunctional reconfigurable logic circuits. Featuring the feasible tuning of threshold voltage and optimizing of subthreshold swing of the transistors, the system can both function well as full logic swing binary logic with high noise margin and be dynamically reconfigured between binary and ternary logic.

Abstract

Multifunctional reconfigurable devices, with higher information capacity, smaller size, and more functions, are urgently needed and draw most attention in frontiers in information technology. 2D semiconductors, ascribing to ultrathin body and easy electrostatic control, show great potential in developing reconfigurable functional units. This work proposes a novel double-gate field-effect transistor architecture with equal top and bottom gate (TG and BG) and realizes flexible optimization of the subthreshold swing (SS) and threshold voltage (V _TH). While the TG and BG are used simultaneously, as a single gate to drive the transistor, ultralow average SS value of 65.5 mV dec⁻¹ can be obtained in a large current range over 10⁴, enabling the application in high gain inverter. While one gate is used to initialize the channel doping, full logic swing inverter circuit with high noise margin (over 90%) is demonstrated. Such device prototype is further extended for designing reconfigurable logic applications and can be dynamically switched and well maintained between binary and ternary logics. This study provides important concept and device prototype for future multifunctional logic applications.