Highly Efficient Uniaxial In-Plane Stretching of a 2D Material via Ion Insertion

Electrochemical Control of the Ultrafast Lattice Response of a Layered Semimetal

The unique layer-stacking in two-dimensional (2D) van der Waals materials facilitates the formation of nearly degenerate phases of matter and opens novel routes for the design of low-power, reconfigurable functional materials. Electrochemical ion intercalation between stacked layers offers a promising approach to stabilize bulk metastable phases and to explore the effects of extreme carrier doping and strain. However, in situ characterization methods to study the structural evolution and dynamical functional properties of these intercalated materials remains limited. Here a novel experimental platform is presented capable of simultaneously performing electrochemical lithium-ion intercalation and multimodal ultrafast characterization of the lattice using both electron diffraction and nonlinear optical techniques. Using the layered semimetal WTe2 as a model system, the interlayer shear phonon mode that modulates stacking between 2Dlayers is probed, showing that small amounts of lithiation enhance the amplitude and lifetime of the phonon, contrary to expectations. This results from the dynamically fluctuating and anharmonic structure between nearly degenerate phases at room temperature, which can be stabilized by electronic carriers accompanying the inserted lithium ions. At high lithiation, the Td' structure emerges and quenches the phonon response. This work defines new approaches for using electrochemistry to engineer the dynamic structure of 2D materials.

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.

Electrochemically-driven actuators: from materials to mechanisms and from performance to applications

Soft actuators, pivotal for converting external energy into mechanical motion, have become increasingly vital in a wide range of applications, from the subtle engineering of soft robotics to the demanding environments of aerospace exploration. Among these, electrochemically-driven actuators (EC actuators), are particularly distinguished by their operation through ion diffusion or intercalation-induced volume changes. These actuators feature notable advantages, including precise deformation control under electrical stimuli, freedom from Carnot efficiency limitations, and the ability to maintain their actuated state with minimal energy use, akin to the latching state in skeletal muscles. This review extensively examines EC actuators, emphasizing their classification based on diverse material types, driving mechanisms, actuator configurations, and potential applications. It aims to illuminate the complicated driving mechanisms of different categories, uncover their underlying connections, and reveal the interdependencies among materials, mechanisms, and performances. We conduct an in-depth analysis of both conventional and emerging EC actuator materials, casting a forward-looking lens on their trajectories and pinpointing areas ready for innovation and performance enhancement strategies. We also navigate through the challenges and opportunities within the field, including optimizing current materials, exploring new materials, and scaling up production processes. Overall, this review aims to provide a scientifically robust narrative that captures the current state of EC actuators and sets a trajectory for future innovation in this rapidly advancing field.

Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal

Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

Strain-induced excellent photocatalytic performance in Z-scheme BlueP/γ-SnS heterostructures for water splitting

Constructing Z-scheme heterojunction photocatalysts with high solar-to-hydrogen (STH) efficiency is a practical alternative to produce clean and recyclable hydrogen energy on a large scale. This paper presents the design of stable Z-scheme blue phosphorene (BlueP)/γ-SnS heterostructures with excellent photocatalytic activities by applying strains. The first-principles calculations show that the BlueP/γ-SnS heterobilayer is a type-I heterojunction with an indirect bandgap of 1.41 eV and strong visible-light absorption up to 105 cm-1. Interestingly, biaxial strains (ε) can effectively regulate its bandgap width (semiconductor-metal) and induce the band alignment transition (type-I-type-II). Compressive and tensile strains can significantly enhance the interfacial interaction and visible-light absorption, respectively. More intriguingly, compressive strains can not only modulate the heterojunction types but also make the band edges meet the requirements for overall water splitting. In particular, the Z-scheme (type-I) BlueP/γ-SnS bilayer at -8% (-2%) strain exhibits a relatively high STH efficiency of 18% (17%), and the strained Z-scheme system (-8% ≤ ε ≤ -6%) also exhibits high and anisotropic carrier mobilities (158-2327 cm2 V-1 s-1). These strain-induced outstanding properties make BlueP/γ-SnS heterostructures promising candidates for constructing economically feasible photocatalysts and flexible nanodevices.

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

Stacking orders in 2D van der Waals (vdW) materials dictate the relative sliding (lateral displacement) and twisting (rotation) between atomically thin layers. By altering the stacking order, many new ferroic, strongly correlated and topological orderings emerge with exotic electrical, optical and magnetic properties. Thanks to the weak vdW interlayer bonding, such highly flexible and energy-efficient stacking order engineering has transformed the design of quantum properties in 2D vdW materials, unleashing the potential for miniaturized high-performance device applications in electronics, spintronics, photonics, and surface chemistry. This Review provides a comprehensive overview of stacking order engineering in 2D vdW materials and their device applications, ranging from the typical fabrication and characterization methods to the novel physical properties and the emergent slidetronics and twistronics device prototyping. The main emphasis is on the critical role of stacking orders affecting the interlayer charge transfer, orbital coupling and flat band formation for the design of innovative materials with on-demand quantum properties and surface potentials. By demonstrating a correlation between the stacking configurations and device functionality, we highlight their implications for next-generation electronic, photonic and chemical energy conversion devices. We conclude with our perspective of this exciting field including challenges and opportunities for future stacking order engineering research.

Infrared Nanoimaging of Hydrogenated Perovskite Nickelate Memristive Devices

Solid-state devices made from correlated oxides, such as perovskite nickelates, are promising for neuromorphic computing by mimicking biological synaptic function. However, comprehending dopant action at the nanoscale poses a formidable challenge to understanding the elementary mechanisms involved. Here, we perform operando infrared nanoimaging of hydrogen-doped correlated perovskite, neodymium nickel oxide (H-NdNiO3, H-NNO), devices and reveal how an applied field perturbs dopant distribution at the nanoscale. This perturbation leads to stripe phases of varying conductivity perpendicular to the applied field, which define the macroscale electrical characteristics of the devices. Hyperspectral nano-FTIR imaging in conjunction with density functional theory calculations unveils a real-space map of multiple vibrational states of H-NNO associated with OH stretching modes and their dependence on the dopant concentration. Moreover, the localization of excess charges induces an out-of-plane lattice expansion in NNO which was confirmed by in situ X-ray diffraction and creates a strain that acts as a barrier against further diffusion. Our results and the techniques presented here hold great potential for the rapidly growing field of memristors and neuromorphic devices wherein nanoscale ion motion is fundamentally responsible for function.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

High-frequency and rapid response tungsten sulfide nano onion-based electrochemical actuators

Poor rate capability, the biggest barrier to potential applications of electrochemical actuators (ECAs), is primarily resulted from symmetric electrochemical reactions. This makes it extremely difficult for ECAs to actuate above 1 Hz while maintaining sufficient displacement retainability compared with their actuations at relatively low frequencies, particularly when working in liquids. Here, tungsten trisulfide (WS₃) assisted tungsten disulfide nano onions are synthesized through a one-step laser-assisted strategy. Using the irreversibility of WS₃ in adsorbing hydrogen in an acidic solution, the electrochemical reaction of tungsten sulfide nano onions is tailored to realize an asymmetric redox reaction for breaking the symmetry of the electrical double layer and battery-like process. Experiments demonstrate that the ECA's response rate (0.24 mm^-1 s^-1) is at least 10 times faster than that of the previously reported ECAs. Moreover, this ECA can actuate at 30 Hz and reaches top performance in liquids at 4 Hz with long-term durability (>90% after 23 000 cycles), which is comparable to that of electromagnetic and electrothermal actuators. To understand the electrochemical actuation of tungsten sulfide from the atomic scale to the macroscopic scale, density functional theory calculations are conducted and an electrochemomechanical coupling model is proposed. A new generation of subvolt electric-driven actuators used in underwater robotics can be developed by modulating the electrochemical response and chemomechanical coupling effect.

A Gapped Phase in Semimetallic Td -WTe2 Induced by Lithium Intercalation

The Weyl semimetal WTe₂ has shown several correlated electronic behaviors, such as the quantum spin Hall effect, superconductivity, ferroelectricity, and a possible exciton insulator state, all of which can be tuned by various physical and chemical approaches. Here, a new electronic phase in WTe₂ induced by lithium intercalation is discovered. The new phase exhibits an increasing resistivity with decreasing temperature and its carrier density is almost two orders of magnitude lower than the carrier density of the semimetallic T_d phase, probed by in situ Hall measurements as a function of lithium intercalation. The theoretical calculations predict the new lithiated phase to be a potential charge density wave (CDW) phase with a bandgap of ≈0.14 eV, in good agreement with the in situ transport data. The new phase is structurally distinct from the initial T_d phase, characterized by polarization-angle-dependent Raman spectroscopy, and large lattice distortions close to 6% are predicted in the new phase. This finding of a new gapped phase in a 2D semimetal demonstrates electrochemical intercalation as a powerful tuning knob for modulating electron density and phase stability in 2D materials.