Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain

Phase Modulation of 2D Semiconducting GaTe from Hexagonal to Monoclinic through Layer Thickness Control and Strain Engineering

Phase engineering offers a novel approach to modulate the properties of materials for versatile applications. Two-dimensional (2D) GaTe, an emerging III-VI semiconductor, can exist in hexagonal (h) or monoclinic (m) phases with fascinating phase-dependent properties (e.g., isotropic or anisotropic electrical transport). However, the key factors governing GaTe phases remain obscure. Herein, we achieve phase modulation of GaTe by tuning two previously overlooked factors: layer thickness and strain. The precise layer-controlled synthesis of GaTe from a monolayer (1L) to >10L is achieved via molecular beam epitaxy. A layer-dependent phase transition from h-GaTe (1-5L) to m-GaTe (>10L) is unambiguously unveiled by scanning tunneling microscopy/spectroscopy, driven by system energy minimization according to density functional theory calculations. Local phase transitions from ultrathin h-GaTe to m-GaTe are also obtained via introduced tensile strain. This work clarifies the factors influencing GaTe phases, providing valuable guidance for the phase engineering of other 2D materials toward the desired properties and applications.

Strain-Assisted Large-Scale 1T-MoS2 Synthesis and its Optical Synaptic Flash Memory Application

2D transition-metal dichalcogenides (TMDCs) have attracted attention as promising materials for next-generation devices owing to their versatile electronic and optical properties. The phase variety of TMDCs provides strategic opportunities for performance enhancement. Herein, a novel method is proposed to synthesize wafer-scale 1T phase MoS₂ and, simultaneously, induce a phase transition via a plasma-assisted metal-sulfidation process and spontaneous internal strain. With thicker MoS2 layers, the strong internal strain during synthesis suppresses the undesirable phase transition from the metastable 1T phase to the 2H phase, ensuring stabilization of the 1T phase. Furthermore, as-synthesized 1T-MoS₂ shows remarkable electrical properties owing to the narrow bandgap (0.4 eV) of its semi-metallic state. As a result, the 1T-phase MoS₂ floating gate (1T-FG) flash memory demonstrates a wider memory window, a higher on/off ratio, and improved stability compared to the 2H-phase MoS₂ floating gate (2H-FG) flash memory. A 5 × 5 array structure is constructed to validate large-scale integration. Notably, under light irradiation, a single 1T-FG memory enables carrier trapping in the floating gate, even in the off state. This study introduces a facile phase control strategy and provides insights into advanced nonvolatile memory and optoelectronic synaptic functionalities.

Physics of 2D Materials for Developing Smart Devices

Rapid industrialization advancements have grabbed worldwide attention to integrate a very large number of electronic components into a smaller space for performing multifunctional operations. To fulfill the growing computing demand state-of-the-art materials are required for substituting traditional silicon and metal oxide semiconductors frameworks. Two-dimensional (2D) materials have shown their tremendous potential surpassing the limitations of conventional materials for developing smart devices. Despite their ground-breaking progress over the last two decades, systematic studies providing in-depth insights into the exciting physics of 2D materials are still lacking. Therefore, in this review, we discuss the importance of 2D materials in bridging the gap between conventional and advanced technologies due to their distinct statistical and quantum physics. Moreover, the inherent properties of these materials could easily be tailored to meet the specific requirements of smart devices. Hence, we discuss the physics of various 2D materials enabling them to fabricate smart devices. We also shed light on promising opportunities in developing smart devices and identified the formidable challenges that need to be addressed.

Strain-Engineered 2D Materials: Challenges, Opportunities, and Future Perspectives

Strain engineering is a powerful strategy that can strongly influence and tune the intrinsic characteristics of materials by incorporating lattice deformations. Due to atomically thin thickness, 2D materials are excellent candidates for strain engineering as they possess inherent mechanical flexibility and stretchability, which allow them to withstand large strains. The application of strain affects the atomic arrangement in the lattice of 2D material, which modify the electronic band structure. It subsequently tunes the electrical and optical characteristics, thereby enhances the performance and functionalities of the fabricated devices. Recent advances in strain engineering strategies for large-area flexible devices fabricated with 2D materials enable dynamic modulation of device performance. This perspective provides an overview of the strain engineering approaches employed so far for straining 2D materials, reviewing their advantages and disadvantages. The effect of various strains (uniaxial, biaxial, hydrostatic) on the characteristics of 2D material is also discussed, with a particular emphasis on electronic and optical properties. The strain-inducing methods employed for large-area device applications based on 2D materials are summarized. In addition, the future perspectives of strain engineering in functional devices, along with the associated challenges and potential solutions, are also outlined.

Interface Engineering of 2D Materials toward High-Temperature Electronic Devices

High-temperature electronic materials and devices are highly sought after for advanced applications in aerospace, high-speed automobiles, and deep-well drilling, where active or passive cooling mechanisms are either insufficient or impractical. 2D materials (2DMs) represent promising alternatives to traditional silicon and wide-bandgap semiconductors (WBG) for nanoscale electronic devices operating under high-temperature conditions. The development of robust interfaces is essential for ensuring that 2DMs and their devices achieve high performance and maintain stability when subjected to elevated temperatures. This review summarizes recent advancements in the interface engineering of 2DMs for high-temperature electronic devices. Initially, the limitations of conventional silicon-based materials and WBG semiconductors, alongside the advantages offered by 2DMs, are examined. Subsequently, strategies for interface engineering to enhance the stability of 2DMs and the performance of their devices are detailed. Furthermore, various interface-engineered 2D high-temperature devices, including transistors, optoelectronic devices, sensors, memristors, and neuromorphic devices, are reviewed. Finally, a forward-looking perspective on future 2D high-temperature electronics is presented. This review offers valuable insights into emerging 2DMs and their applications in high-temperature environments from both fundamental and practical perspectives.

Growth Pathway and Phase Transition From Quasi-layered Pt5Se4 to Layered PtSe2 Nanocrystals

The discovery and control of phases is a significant endeavor for materials science. PtSe2, as a stable Pt─Se phase, is controllably synthesized for electronic, optoelectronic, and electrocatalytic applications, while research on another stable Pt─Se phase, Pt5Se4, is limited to calculations. Moreover, the growth mechanisms and phase transition of Pt─Se compounds remain unclear. Here, we report a two-step growth pathway of PtSe2 with a stable Pt5Se4 phase as intermediate and reveal the Pt-Pt5Se4-PtSe2 transition process. The synthesis of PtSe2 and Pt5Se4 nanocrystals is achieved by controlling the degree of selenization. Theoretical calculations prove that Pt5Se4 phase is thermodynamically favorable under Se-deficient conditions and PtSe2 nanocrystals are formed along with the diffusion of Pt and Se atoms. This work greatly enriches the knowledge on the growth mechanism and phase transition of Pt─Se compounds, and offers insights into the controlled synthesis of Pt5Se4 for future electrocatalysis and electronic devices.

Control of the Phase Distribution in TMDs by Strain Engineering and Kirigami Techniques

Materials exhibiting both metallic and semiconducting states, including two-dimensional transition metal dichalcogenides (TMDs), have numerous applications. We therefore investigate the effects of axial and shear strains on the phase energetics of pristine and striped TMDs using density functional theory and classical molecular dynamics simulations. We demonstrate that control of the phase distribution can be achieved by the integration of strain engineering and Kirigami techniques. Our results extend the understanding of the phase energetics in TMDs and reveal an effective strategy for creating virtually defect-free metal-semiconductor-metal junctions.

Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electonic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

Ultra-broadband all-optical nonlinear activation function enabled by MoTe2/optical waveguide integrated devices

All-optical nonlinear activation functions (NAFs) are crucial for enabling rapid optical neural networks (ONNs). As linear matrix computation advances in integrated ONNs, on-chip all-optical NAFs face challenges such as limited integration, high latency, substantial power consumption, and a high activation threshold. In this work, we develop an integrated nonlinear optical activator based on the butt-coupling integration of two-dimensional (2D) MoTe2 and optical waveguides (OWGs). The activator exhibits an ultra-broadband response from visible to near-infrared wavelength, a low activation threshold of 0.94 μW, a small device size (~50 µm2), an ultra-fast response rate (2.08 THz), and high-density integration. The excellent nonlinear effects and broadband response of 2D materials have been utilized to create all-optical NAFs. These activators were applied to simulate MNIST handwritten digit recognition, achieving an accuracy of 97.6%. The results underscore the potential application of this approach in ONNs. Moreover, the classification of more intricate CIFAR-10 images demonstrated a generalizable accuracy of 94.6%. The present nonlinear activator promises a general platform for three-dimensional (3D) ultra-broadband ONNs with dense integration and low activation thresholds by integrating a variety of strong nonlinear optical (NLO) materials (e.g., 2D materials) and OWGs in glass.

Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors

Transition-metal dichalcogenides (TMDs) are ideal systems for two-dimensional (2D) optoelectronic applications owing to their strong light-matter interaction and various band gap energies. New techniques to modify the crystallographic phase of TMDs have recently been discovered, allowing the creation of lateral heterostructures and the design of all-2D circuitry. Thus, far, the potential benefits of phase-engineered TMD devices for optoelectronic applications are still largely unexplored. The dominant mechanisms involved in photocurrent generation in these systems remain unclear, hindering further development of new all-2D optoelectronic devices. Here, we fabricate locally phase-engineered MoTe2 optoelectronic devices, creating a metal (1T') semiconductor (2H) lateral junction and unveil the main mechanisms at play for photocurrent generation. We find that the photocurrent originates from the 1T'-2H junction, with a maximum at the 2H MoTe2 side of the junction. This observation, together with the nonlinear IV-curve, indicates that the photovoltaic effect plays a major role in the photon-to-charge current conversion in these systems. Additionally, the 1T'-2H MoTe2 heterojunction device exhibits a fast optoelectronic response over a wavelength range of 700-1100 nm, with a rise and fall times of 113 and 110 μs, respectively, 2 orders of magnitude faster when compared to a directly contacted 2H MoTe2 device. These results show the potential of local phase-engineering for all-2D optoelectronic circuitry.

Nanoscale Operando Imaging of Electrically Driven Charge-Density Wave Phase Transitions

Structural transformations in strongly correlated materials promise efficient and fast control of materials' properties via electrical or optical stimulation. The desired functionality of devices operating based on phase transitions, however, will also be influenced by nanoscale heterogeneity. Experimentally characterizing the relationship between microstructure and phase switching remains challenging, as nanometer resolution and high sensitivity to subtle structural modifications are required. Here, we demonstrate nanoimaging of a current-induced phase transformation in the charge-density wave (CDW) material 1T-TaS2. Combining electrical characterizations with tailored contrast enhancement, we correlate macroscopic resistance changes with the nanoscale nucleation and growth of CDW phase domains. In particular, we locally determine the transformation barrier in the presence of dislocations and strain, underlining their non-negligible impact on future functional devices. Thereby, our results demonstrate the merit of tailored contrast enhancement and beam shaping for advanced operando microscopy of quantum materials and devices.

Concentration-Dependent Layer-Stacking and the Influence on Phase-Conversion in Colloidally Synthesized WSe2 Nanocrystals

We report a synthesis of WSe2 nanocrystals in which the number of layers is controlled by varying the precursor concentration. By altering the ratios and concentrations of W(CO)6 and Ph2Se2 in trioctylphosphine oxide, we show that high [Se] and large Se/W ratios lead to an increased number of layers per nanocrystal. As the number of layers per nanocrystal is increased, the nanocrystal ensembles show less phase-conversion from the metastable 2M phase to the thermodynamically favored 2H phase. Density functional theory calculations indicate that the interlayer binding energy increases with the number of layers, indicating that the stronger interlayer interactions in multilayered nanocrystals may increase the energy barrier to phase-conversion. The results presented herein provide insights for directing phase-conversion in solution-phase syntheses of transition metal dichalcogenides.

Emergent Optical Resonances in Atomically Phase-Patterned Semiconducting Monolayers of WS2

Atomic-scale control of light-matter interactions represents the ultimate frontier for many applications in photonics and quantum technology. Two-dimensional semiconductors, including transition-metal dichalcogenides, are a promising platform to achieve such control due to the combination of an atomically thin geometry and convenient photophysical properties. Here, we demonstrate that a variety of durable polymorphic structures can be combined to generate additional optical resonances beyond the standard excitons. We theoretically predict and experimentally show that atomic-sized patches of the 1T phase within the 1H matrix form unique electronic bands that lead to the emergence of robust optical resonances with strong absorption, circularly polarized emission, and long radiative lifetimes. The atomic manipulation of two-dimensional semiconductors opens unexplored scenarios for light harvesting devices and exciton-based photonics.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Manipulating 2D Materials through Strain Engineering

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

Synthesis and Polymorph Manipulation of FeSe2 Monolayers

Polymorph engineering involves the manipulation of material properties through controlled structural modification and is a candidate technique for creating unique two-dimensional transition metal dichalcogenide (TMDC) nanodevices. Despite its promise, polymorph engineering of magnetic TMDC monolayers has not yet been demonstrated. Here we grow FeSe2 monolayers via molecular beam epitaxy and find that they have great promise for magnetic polymorph engineering. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we find that FeSe2 monolayers predominantly display a 1T' structural polymorph at 5 K. Application of voltage pulses from an STM tip causes a local, reversible transition from the 1T' phase to the 1T phase. Density functional theory calculations suggest that this single-layer structural phase transition is accompanied by a magnetic transition from an antiferromagnetic to a ferromagnetic configuration. These results open new possibilities for creating functional magnetic devices with TMDC monolayers via polymorph engineering.

Lithiation Induced Phases in 1T'-MoTe2 Nanoflakes

Multiple polytypes of MoTe2 with distinct structures and intriguing electronic properties can be accessed by various physical and chemical approaches. Here, we report electrochemical lithium (Li) intercalation into 1T'-MoTe2 nanoflakes, leading to the discovery of two previously unreported lithiated phases. Distinguished by their structural differences from the pristine 1T' phase, these distinct phases were characterized using in situ polarization Raman spectroscopy and in situ single-crystal X-ray diffraction. The lithiated phases exhibit increasing resistivity with decreasing temperature, and their carrier densities are two to 4 orders of magnitude smaller than the metallic 1T' phase, as probed through in situ Hall measurements. The discovery of these gapped phases in initially metallic 1T'-MoTe2 underscores electrochemical intercalation as a potent tool for tuning the phase stability and electron density in two-dimensional (2D) materials.

A Mini Review: Phase Regulation for Molybdenum Dichalcogenide Nanomaterials

Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) have been regarded as ideal and promising nanomaterials that bring broad application prospects in extensive fields due to their ultrathin layered structure, unique electronic band structure, and multiple spatial phase configurations. TMDCs with different phase structures exhibit great diversities in physical and chemical properties. By regulating the phase structure, their properties would be modified to broaden the application fields. In this mini review, focusing on the most widely concerned molybdenum dichalcogenides (MoX2: X = S, Se, Te), we summarized their phase structures and corresponding electronic properties. Particularly, the mechanisms of phase transformation are explained, and the common methods of phase regulation or phase stabilization strategies are systematically reviewed and discussed. We hope the review could provide guidance for the phase regulation of molybdenum dichalcogenides nanomaterials, and further promote their real industrial applications.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

In-plane anisotropic two-dimensional materials for twistronics

In-plane anisotropic two-dimensional (2D) materials exhibit in-plane orientation-dependent properties. The anisotropic unit cell causes these materials to show lower symmetry but more diverse physical properties than in-plane isotropic 2D materials. In addition, the artificial stacking of in-plane anisotropic 2D materials can generate new phenomena that cannot be achieved in in-plane isotropic 2D materials. In this perspective we provide an overview of representative in-plane anisotropic 2D materials and their properties, such as black phosphorus, group IV monochalcogenides, group VI transition metal dichalcogenides with 1T' and Tdphases, and rhenium dichalcogenides. In addition, we discuss recent theoretical and experimental investigations of twistronics using in-plane anisotropic 2D materials. Both in-plane anisotropic 2D materials and their twistronics hold considerable potential for advancing the field of 2D materials, particularly in the context of orientation-dependent optoelectronic devices.

Phase Engineering of Nanomaterials: Transition Metal Dichalcogenides

Crystal phase, a critical structural characteristic beyond the morphology, size, dimension, facet, etc., determines the physicochemical properties of nanomaterials. As a group of layered nanomaterials with polymorphs, transition metal dichalcogenides (TMDs) have attracted intensive research attention due to their phase-dependent properties. Therefore, great efforts have been devoted to the phase engineering of TMDs to synthesize TMDs with controlled phases, especially unconventional/metastable phases, for various applications in electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectrics. Considering the significant progress in the synthesis and applications of TMDs, we believe that a comprehensive review on the phase engineering of TMDs is critical to promote their fundamental studies and practical applications. This Review aims to provide a comprehensive introduction and discussion on the crystal structures, synthetic strategies, and phase-dependent properties and applications of TMDs. Finally, our perspectives on the challenges and opportunities in phase engineering of TMDs will also be discussed.

Transforming friction: unveiling sliding-induced phase transitions in CVD-grown WS2 monolayers under single-asperity sliding nanocontacts

Transition metal dichalcogenides (TMDs) exhibit diverse properties across different phases, making them promising materials for various engineering applications. In the present work, we employed a comprehensive approach, combining experimental investigations and computational simulations to elucidate the remarkable tunable frictional characteristics of chemical vapor deposition (CVD) grown WS2 monolayers through the sliding-induced transitions between the 1H and 1T' phases. Our atomic force microscopy (AFM) measurements reveal a significant contrast in friction between the two phases, with the 1H phase displaying higher friction (∼52%) than the 1T' phase. Surprisingly, under repeated scanning at constant stress, the friction of the 1H phase decreases, eventually matching the lower friction values of the 1T' phase. It was observed that the phase transformation is irreversible and is strongly dependent on contact stresses and is accelerated as the contact stress is increased by increasing the applied normal load. Molecular dynamics (MD) simulations provide further insights into the phase transition mechanism, highlighting the role of localized lateral stress and strain induced by sliding an AFM tip on the 1H phase. The simulations confirm that sliding induced localized lateral strain plays a crucial role in the phase transition, ultimately resulting in a decrease in friction. Moreover, our simulations unveil an intriguing connection between friction, potential energy surfaces, and the localized lateral strain during the phase transformation process. Our findings not only offer insights into the tribological properties of TMD materials but also open new possibilities for tailoring their performance in various applications where reducing friction and wear is crucial.

Mechanism of Fermi Level Pinning for Metal Contacts on Molybdenum Dichalcogenide

The high contact resistance of transition metal dichalcogenide (TMD)-based devices is receiving considerable attention due to its limitation on electronic performance. The mechanism of Fermi level (EF) pinning, which causes the high contact resistance, is not thoroughly understood to date. In this study, the metal (Ni and Ag)/Mo-TMD surfaces and interfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunneling microscopy and spectroscopy, and density functional theory systematically. Ni and Ag form covalent and van der Waals (vdW) interfaces on Mo-TMDs, respectively. Imperfections are detected on Mo-TMDs, which lead to electronic and spatial variations. Gap states appear after the adsorption of single and two metal atoms on Mo-TMDs. The combination of the interface reaction type (covalent or vdW), the imperfection variability of the TMD materials, and the gap states induced by contact metals with different weights are concluded to be the origins of EF pinning.

Phase transition in WSe2-xTex monolayers driven by charge injection and pressure: a first-principles study

Alloying strategies permit new probes for governing structural stability and semiconductor-semimetal phase transition of transition metal dichalcogenides (TMDs). However, the possible structure and phase transition mechanism of the alloy TMDs, and the effect of an external field, have been still unclear. Here, the enrichment of the Te content in WSe2-xTex monolayers allows for coherent structural transition from the H phase to the T' phase. The crystal orbital Hamiltonian population (COHP) uncovers that the bonding state of the H phase approaches the high-energy domain near the Fermi level as the Te concentration increases, posing a source of structural instability followed by a weakened energy barrier for the phase transition. In addition, the structural phase transition driven by charge injection opens up new possibilities for the development of phase-change devices based on atomic thin films. For WSe2-xTex monolayers with the H phase as the stable phase, the critical value of electron concentration triggering the phase transition decreases with an increase in the x value. Furthermore, the energy barrier from the H phase to the T' phase can be effectively reduced by applying tensile strain, which could favor the phase switching in electronic devices. This work provides a critical reference for controllable modulation of phase-sensitive devices from alloy materials with rich phase characteristics.

Temperature-dependent failure of atomically thin MoTe2

CONTEXT

In this study, we investigated the mechanical responses of molybdenum ditelluride (MoTe2) using molecular dynamics (MD) simulations. Our key focus was on the tensile behavior of MoTe2 with trigonal prismatic phase (2H-MoTe2) which was investigated under uniaxial tensile stress for both armchair and zigzag directions. Crack formation and propagation were examined to understand the fracture behavior of such material for varying temperatures. Additionally, the study also assesses the impact of temperature on Young's modulus and fracture stress-strain of a monolayer of 2H-MoTe2.

METHOD

The investigation was done using molecular dynamics (MD) simulations using Stillinger-Weber (SW) potentials. The tensile behavior was simulated for temperature for 10 K and then from 100 to 600 K with a 100-K interval. The crack propagation and formation of 10 K and 300 K 2H-MoTe2 for both directions at different strain rates was analyzed using Ovito visualizer. All the simulations were conducted using a strain rate of 10-4 ps-1. The results show that the fracture strength of 2H-MoTe2 in the armchair and zigzag direction at 10 K is 16.33 GPa (11.43 N/m) and 13.71429 GPa (9.46 N/m) under a 24% and 18% fracture strain, respectively. The fracture strength of 2H-MoTe2 in the armchair and zigzag direction at 600 K is 10.81 GPa (7.56 N/m) and 10.13 GPa (7.09 N/m) under a 12.5% and 12.47% fracture strain, respectively.

Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.

High intrinsic phase stability of ultrathin 2M WS2

Metallic 2M or 1T'-phase transition metal dichalcogenides (TMDs) attract increasing interests owing to their fascinating physicochemical properties, such as superconductivity, optical nonlinearity, and enhanced electrochemical activity. However, these TMDs are metastable and tend to transform to the thermodynamically stable 2H phase. In this study, through systematic investigation and theoretical simulation of phase change of 2M WS2, we demonstrate that ultrathin 2M WS2 has significantly higher intrinsic thermal stabilities than the bulk counterparts. The 2M-to-2H phase transition temperature increases from 120 °C to 210 °C in the air as thickness of WS2 is reduced from bulk to bilayer. Monolayered 1T' WS2 can withstand temperatures up to 350 °C in the air before being oxidized, and up to 450 °C in argon atmosphere before transforming to 1H phase. The higher stability of thinner 2M WS2 is attributed to stiffened intralayer bonds, enhanced thermal conductivity and higher average barrier per layer during the layer(s)-by-layer(s) phase transition process. The observed high intrinsic phase stability can expand the practical applications of ultrathin 2M TMDs.

Patternable Process-Induced Strain in 2D Monolayers and Heterobilayers

Strain engineering in two-dimensional (2D) materials is a powerful but difficult to control approach to tailor material properties. Across applications, there is a need for device-compatible techniques to design strain within 2D materials. This work explores how process-induced strain engineering, commonly used by the semiconductor industry to enhance transistor performance, can be used to pattern complex strain profiles in monolayer MoS2 and 2D heterostructures. A traction-separation model is identified to predict strain profiles and extract the interfacial traction coefficient of 1.3 ± 0.7 MPa/μm and the damage initiation threshold of 16 ± 5 nm. This work demonstrates the utility to (1) spatially pattern the optical band gap with a tuning rate of 91 ± 1 meV/% strain and (2) induce interlayer heterostrain in MoS2-WSe2 heterobilayers. These results provide a CMOS-compatible approach to design complex strain patterns in 2D materials with important applications in 2D heterogeneous integration into CMOS technologies, moiré engineering, and confining quantum systems.

Coexisting Phases in Transition Metal Dichalcogenides: Overview, Synthesis, Applications, and Prospects

Over the past decade, significant advancements have been made in phase engineering of two-dimensional transition metal dichalcogenides (TMDCs), thereby allowing controlled synthesis of various phases of TMDCs and facile conversion between them. Recently, there has been emerging interest in TMDC coexisting phases, which contain multiple phases within one nanostructured TMDC. By taking advantage of the merits from the component phases, the coexisting phases offer enhanced performance in many aspects compared with single-phase TMDCs. Herein, this review article thoroughly expounds the latest progress and ongoing efforts on the syntheses, properties, and applications of TMDC coexisting phases. The introduction section overviews the main phases of TMDCs (2H, 3R, 1T, 1T', 1Td), along with the advantages of phase coexistence. The subsequent section focuses on the synthesis methods for coexisting phases of TMDCs, with particular attention to local patterning and random formations. Furthermore, on the basis of the versatile properties of TMDC coexisting phases, their applications in magnetism, valleytronics, field-effect transistors, memristors, and catalysis are discussed. Lastly, a perspective is presented on the future development, challenges, and potential opportunities of TMDC coexisting phases. This review aims to provide insights into the phase engineering of 2D materials for both scientific and engineering communities and contribute to further advancements in this emerging field.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

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

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

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS2

The distorted phases of monolayer 1T-MoS2 have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases and find not only the ones usually reported but also two further phases (3 × 3 and 4 × 4), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after removal of dopants, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in 3 × 3 which─compared to the recently measured 1T″─has 100 times greater ferroelectric polarization, a lower energy, and a larger band gap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics.

Phase transformations in single-layer MoTe2stimulated by electron irradiation and annealing

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), MoTe2is predestined for phase-engineering applications due to the small difference in free energy between the semiconducting H-phase and metallic 1T'-phase. At the same time, the complete picture of the phase evolution originating from point defects in single-layer of semiconducting H-MoTe2via Mo6Te6nanowires to cubic molybdenum has not yet been reported so far, and it is the topic of the present study. The occurring phase transformations in single-layer H-MoTe2were initiated by 40-80 kV electrons in the spherical and chromatic aberration-corrected high-resolution transmission electron microscope and/or when subjected to high temperatures. We analyse the damage cross-section at voltages between 40 kV and 80 kV and relate the results to previously published values for other TMDs. Then we demonstrate that electron beam irradiation offers a route to locally transform freestanding single-layer H-MoTe2into one-dimensional (1D) Mo6Te6nanowires. Combining the experimental data with the results of first-principles calculations, we explain the transformations in MoTe2single-layers and Mo6Te6nanowires by an interplay of electron-beam-induced energy transfer, atom ejection, and oxygen absorption. Further, the effects emerging from electron irradiation are compared with those produced byin situannealing in a vacuum until pure molybdenum crystals are obtained at temperatures of about 1000 °C. A detailed understanding of high-temperature solid-to-solid phase transformation in the 2D limit can provide insights into the applicability of this material for future device fabrication.

Exploring the Surface Oxidation and Environmental Instability of 2H-/1T'-MoTe2 Using Field Emission-Based Scanning Probe Lithography

An unconventional approach for the resistless nanopatterning 2H- and 1T'-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3 /MoOx as well as TeO2 /TeOx . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T'-/(Td )-MoTe2 to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Directionally-Resolved Phononic Properties of Monolayer 2D Molybdenum Ditelluride (MoTe2) under Uniaxial Elastic Strain

Understanding the phonon characteristics of two-dimensional (2D) molybdenum ditelluride (MoTe2) under strain is critical to manipulating its multiphysical properties. Although there have been numerous computational efforts to elucidate the strain-coupled phonon properties of monolayer MoTe2, empirical validation is still lacking. In this work, monolayer 1H-MoTe2 under uniaxial strain is studied via in situ micro-Raman spectroscopy. Directionally dependent monotonic softening of the doubly degenerate in-plane E2g1 phonon mode is observed with increasing uniaxial strain, where the E2g1 peak red-shifts -1.66 ± 0.04 cm-1/% along the armchair direction and -0.80 ± 0.07 cm-1/% along the zigzag direction. The corresponding Grüneisen parameters are calculated to be 1.09 and 0.52 along the armchair and zigzag directions, respectively. This work provides the first empirical quantification and validation of the orientation-dependent strain-coupled phonon response in monolayer 1H-MoTe2 and serves as a benchmark for other prototypical 2D transition-metal tellurides.

Strain-Induced 2H to 1T' Phase Transition in Suspended MoTe2 Using Electric Double Layer Gating

MoTe2 can be converted from the semiconducting (2H) phase to the semimetallic (1T') phase by several stimuli including heat, electrochemical doping, and strain. This type of phase transition, if reversible and gate-controlled, could be useful for low-power memory and logic. In this work, a gate-controlled and fully reversible 2H to 1T' phase transition is demonstrated via strain in few-layer suspended MoTe2 field effect transistors. Strain is applied by the electric double layer gating of a suspended channel using a single ion conducting solid polymer electrolyte. The phase transition is confirmed by simultaneous electrical transport and Raman spectroscopy. The out-of-plane vibration peak (A1g)─a signature of the 1T' phase─is observed when VSG ≥ 2.5 V. Further, a redshift in the in-plane vibration mode (E2g) is detected, which is a characteristic of a strain-induced phonon shift. Based on the magnitude of the shift, strain is estimated to be 0.2-0.3% by density functional theory. Electrically, the temperature coefficient of resistance transitions from negative to positive at VSG ≥ 2 V, confirming the transition from semiconducting to metallic. The approach to gate-controlled, reversible straining presented here can be extended to strain other two-dimensional materials, explore fundamental material properties, and introduce electronic device functionalities.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Phase-dependent Friction on Exfoliated Transition Metal Dichalcogenides Atomic Layers

The fundamental aspects of energy dissipation on 2-dimensional (2D) atomic layers are extensively studied. Among various atomic layers, transition metal dichalcogenides (TMDs) exists in several phases based on their lattice structure, which give rise to the different phononic and electronic contributions in energy dissipation. 2H and 1T' (distorted 1T) phase MoS2 and MoTe2 atomic layers exfoliated on mica substrate are obtained and investigated their nanotribological properties with atomic force microscopy (AFM)/ friction force microscopy (FFM). Surprisingly, 1T' phase of both MoS2 and MoTe2 exhibits ≈10 times higher friction compared to 2H phase. With density functional theory analyses, the friction increase is attributed to enhanced electronic excitation, efficient phonon dissipation, and increased potential energy surface barrier at the tip-sample interface. This study suggests the intriguing possibility of tuning the friction of TMDs through phase transition, which can lead to potential application in tunable tribological devices.

Local Nanostrain Engineering of Monolayer MoS2 Using Atomic Force Microscopy-Based Thermomechanical Nanoindentation

Strain engineering in two-dimensional materials (2DMs) has important application potential for electronic and optoelectronic devices. However, achieving precise spatial control, adjustable sizing, and permanent strain with nanoscale resolution remains challenging. Herein, a thermomechanical nanoindentation method is introduced, inspired by skin edema caused by mosquito bites, which can induce localized nanostrain and bandgap modulation in monolayer molybdenum disulfide (MoS2) transferred onto a poly(methyl methacrylate) film utilizing a heated atomic force microscopy nanotip. Via adjustment of the machining parameters, the strains of MoS2 are manipulated, achieving an average strain of ≤2.6% on the ring-shaped expansion structure. The local bandgap of MoS2 is spatially modulated using three types of nanostructures. Among them, the nanopit has the largest range of bandgap regulation, with a substantial change of 56 meV. These findings demonstrate the capability of the proposed method to create controllable and reproducible nanostrains in 2DMs.

Classification and Simulation of Structural Phase Transformation-Induced Interfacial Defects in Group VI Transition-Metal Dichalcogenide Monolayers

Polymorphic 2D materials have recently emerged as promising candidates for use in nanoelectronic devices by way of their ability to undergo structural phase transformations induced by external fields. Under cyclic transformations, however, induced interfacial defects may proliferate and compromise the system properties. Herein, we first employ geometric analysis to classify such defects generated during the 2H ↔ 1T and 2H ↔ 1T' transformations in group VI transition-metal dichalcogenide monolayers. Then, simulations of a mesoscale model with atomistic spatial resolution are conducted to assess the proliferation of such defects during cyclic 2H ↔ 1T transformations. It is shown that defect densities reach a steady state, with the 2H phase remaining more pristine than the 1T and 1T' states. We expect that the effects of these defects on the device performance are application-dependent and will require further inquiry.

Phase Engineering of 2D Materials

Polymorphic 2D materials allow structural and electronic phase engineering, which can be used to realize energy-efficient, cost-effective, and scalable device applications. The phase engineering covers not only conventional structural and metal-insulator transitions but also magnetic states, strongly correlated band structures, and topological phases in rich 2D materials. The methods used for the local phase engineering of 2D materials include various optical, geometrical, and chemical processes as well as traditional thermodynamic approaches. In this Review, we survey the precise manipulation of local phases and phase patterning of 2D materials, particularly with ideal and versatile phase interfaces for electronic and energy device applications. Polymorphic 2D materials and diverse quantum materials with their layered, vertical, and lateral geometries are discussed with an emphasis on the role and use of their phase interfaces. Various phase interfaces have demonstrated superior and unique performance in electronic and energy devices. The phase patterning leads to novel homo- and heterojunction structures of 2D materials with low-dimensional phase boundaries, which highlights their potential for technological breakthroughs in future electronic, quantum, and energy devices. Accordingly, we encourage researchers to investigate and exploit phase patterning in emerging 2D materials.

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Charge-Transfer-Driven Phase Transition of Two-Dimensional MoTe2 in Donor-Acceptor Heterostructures

In this work, based on first-principles calculations, we propose that electrene can be considered as an electron-donating substrate to drive the phase transition of MoTe2 from the H to T' phase, which is a topic of long-standing interest and importance. In particular, new electrenes Ca2XN2 (X = Zr, Hf) are predicted with the existence of a nearly free two-dimensional (2D) electron gas and ultralow work functions. In MoTe2/Ca2XN2 donor-acceptor heterostructures, we find significantly large charge transfer (∼0.4e per MoTe2 unit cell) from Ca2XN2 to MoTe2, which stabilizes the T' phase and decreases the phase transition barrier (from ∼0.9 to ∼0.5 eV per unit cell). In addition, the phase transition of MoTe2 on Ca2XN2 remains effective as the interlayer distance varies. It therefore can be confirmed conclusively that our results open a new avenue for phase transition study and provide new insights for the large-scale synthesis of metastable high-quality T'-phase MoTe2.

Imaging Field-Driven Melting of a Molecular Solid at the Atomic Scale

Solid-liquid phase transitions are basic physical processes, but atomically resolved microscopy has yet to capture their full dynamics. A new technique is developed for controlling the melting and freezing of self-assembled molecular structures on a graphene field-effect transistor (FET) that allows phase-transition behavior to be imaged using atomically resolved scanning tunneling microscopy. This is achieved by applying electric fields to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-decorated FETs to induce reversible transitions between molecular solid and liquid phases at the FET surface. Nonequilibrium melting dynamics are visualized by rapidly heating the graphene substrate with an electrical current and imaging the resulting evolution toward new 2D equilibrium states. An analytical model is developed that explains observed mixed-state phases based on spectroscopic measurement of solid and liquid molecular energy levels. The observed nonequilibrium melting dynamics are consistent with Monte Carlo simulations.

Nonmetal-Mediated Atomic Spalling of Large-Area Monolayer Transition Metal Dichalcogenide

Transition metal dichalcogenides (TMDCs) have attracted intense interest; however, despite the considerable effort of researchers, a universal manufacturing method that can guarantee both high material quality and throughput has not been realized to date. Herein, a universal approach to producing high-quality monolayer TMDCs on a large scale via germanium (Ge)-mediated atomic spalling is presented. Through the modified analytic model, the study verifies that the thin Ge film could be a suitable stressor that effectively reduces the crack propagation depth at the sub-nanometer range. In particular, an acid-etching process is not required in the overall atomic spalling process due to the water-soluble nature of the Ge, enabling it widely applicable to various TMDCs. Under the optimized spalling conditions, a millimeter-sized monolayer of stable MoS2, as well as unstable MoTe2, is successfully achieved. Through detailed spectroscopic and electrical characterizations, it is confirmed that the proposed methodology for obtaining large-area atomic layers does not introduce any significant structural defects or chemical contaminations.

Tunable electrical properties and multiple-phases of ferromagnetic GdS2, GdSe2 and Janus GdSSe monolayers

With the continuous miniaturization and integration of spintronic devices, the two-dimensional (2D) ferromagnet coupling of ferromagnetic and diverse electrical properties has become increasingly important. Herein, we report three ferromagnetic monolayers: GdS2, GdSe2 and Janus GdSSe. They are bipolar magnetic semiconductors and demonstrate ferroelasticity with a large reversible strain of 73.2%. Three monolayers all hold large magnetic moments of about 8μB f.u.-1 and large spin-flip energy gaps in both the conduction and valence bands, which are highly desirable for applications in bipolar field effect spin filters and spin valves. Our calculations have testified to the feasibility of the experimental achievement of the three monolayers and their stability. Additionally, intrinsic valley polarization occurs in the three monolayers owing to the cooperative interplay between spin-orbit coupling and magnetic exchange interaction. Moreover, we identified square lattices for GdS2 and GdSe2 monolayers. The new and stable square lattices of GdS2 and GdSe2 monolayers show robust ferromagnetism with high Curie temperatures of 648 and 312 K, respectively, and the characteristics of spin-gapless semiconductors. Overall, these findings render GdS2, GdSe2 and Janus GdSSe monolayers promising candidate materials for multifunctional spintronic devices at the nanoscale.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Structural Transformation of Unconventional-Phase Materials

The structural transformation of materials, which involves the evolution of different structural features, including phase, composition, morphology, etc., under external conditions, represents an important fundamental phenomenon and has drawn substantial research interest. Recently, materials with unconventional phases that are different from their thermodynamically stable ones have been demonstrated to possess distinct properties and compelling functions and can further serve as starting materials for structural transformation studies. The identification and mechanism study of the structural transformation process of unconventional-phase starting materials can not only provide deep insights into their thermodynamic stability in potential applications but also offer effective approaches for the synthesis of other unconventional structures. Here, we briefly summarize the recent research progress on the structural transformation of some typical starting materials with various unconventional phases, including the metastable crystalline phase, amorphous phase, and heterophase, induced by different approaches. The importance of unconventional-phase starting materials in the structural modulation of resultant intermediates and products will be highlighted. The employment of diverse in situ/operando characterization techniques and theoretical simulations in studying the mechanism of the structural transformation process will also be introduced. Finally, we discuss the existing challenges in this emerging research field and provide some future research directions.