Switchable moiré potentials in ferroelectric WTe2/WSe2 superlattices

Mechanical force-induced interlayer sliding in interfacial ferroelectrics

Moiré superlattices in two-dimensional stacks have attracted worldwide interest due to their unique electronic properties. A typical example is the moiré ferroelectricity, where adjacent moirés exhibit opposite spontaneous polarization that can be switched through interlayer sliding. However, in contrast to ideal regular ferroelectric moiré domains (equilateral triangles) built in most theoretical models, the unavoidable irregular moiré supercells (non-equilateral triangles) induced by external strain fields during the transfer process have been given less attention. Manipulation of controllable polarization evolutions is also a big challenge due to an interlinked network of polarized domains. In this study, we employ a sliding-disturb measurement to examine and modulate these irregular moirés via mechanical force. By introducing a curved substrate, the irregular moirés are fabricated, and three distinct types of moiré domains with different patterns are identified and modulated by external mechanical force disturbing. They exhibit reduced pinning forces when the shear direction is not aligned with the strain direction. The shift of the moirés is observed to be orthogonal to the shear direction. This work offers an effective pathway for the controlled switch of the polarization in interfacial ferroelectricity.

Continuous Strain Modulation of Moiré Superlattice Symmetry From Triangle to Rectangle

Moiré superlattices formed in van der Waals (vdW) bilayers of 2D materials provide an ideal platform for studying previously undescribed physics, including correlated electronic states and moiré excitons, owing to the wide-range tunability of their lattice constants. However, their crystal symmetry is fixed by the monolayer structure, and the lack of a straightforward technique for modulating the symmetry of moiré superlattices has impeded progress in this field. Herein, a simple, room-temperature, ambient method for controlling superlattice symmetry is reported. The method uses vdW heterostructures on a flexible substrate; by bending the substrate, a uniaxial strain is introduced. Based on numerical calculations, a strain condition is designed to deform the moiré superlattice from triangular to rectangular, and visualized the continuous deformation of real-space moiré superlattices using piezoresponse force microscopy. The band calculations show that nearly flat moiré minibands remain in the rectangular lattice; therefore, this method provides an additional tuning knob for the Hamiltonian of moiré quantum matter.

Ferroelectricity with concomitant Coulomb screening in van der Waals heterostructures

Interfacial ferroelectricity emerges in non-centrosymmetric heterostructures consisting of non-polar van der Waals (vdW) layers. Ferroelectricity with concomitant Coulomb screening can switch topological currents or superconductivity and simulate synaptic response. So far, it has only been realized in bilayer graphene moiré superlattices, posing stringent requirements to constituent materials and twist angles. Here we report ferroelectricity with concomitant Coulomb screening in different vdW heterostructures free of moiré interfaces containing monolayer graphene, boron nitride (BN) and transition metal chalcogenide layers. We observe a ferroelectric hysteretic response in a BN/monolayer graphene/BN, as well as in BN/WSe2/monolayer graphene/WSe2/BN heterostructure, but also when replacing the stacking fault-containing BN with multilayer twisted MoS2, a prototypical sliding ferroelectric. Our control experiments exclude alternative mechanisms, such that we conclude that ferroelectricity originates from stacking faults in the BN flakes. Hysteretic switching occurs when a conductive ferroelectric screens the gating field electrically and controls the monolayer graphene through its polarization field. Our results relax some of the material and design constraints for device applications based on sliding ferroelectricity and should enable memory device or the combination with diverse vdW materials with superconducting, topological or magnetic properties.

Symmetry Breaking in Twisted Mixed-Dimensional Heterostructure Interfaces for Multifunctional Polarization-Sensitive Photodetection

Moiré superlattices, created by stacking different van der Waals materials at twist angles, have emerged as a versatile platform for exploring intriguing phenomena such as topological properties, superconductivity, the quantum anomalous Hall effect, and the unconventional Stark effect. Additionally, the formation of moiré superlattice potential can generate spontaneous symmetry breaking, leading to an anisotropic optical response and electronic transport behavior. Herein, we propose a two-step chemical vapor deposition (CVD) strategy for synthesizing WS2/Sb2S3 moiré superlattices. Density functional theory calculations show that the moiré potential and interlayer distance at the WS2/Sb2S3 interface can generate anisotropic electronic states. The atomic-resolution HAADF-STEM image clearly reveals angle-dependent complicated moiré periodicity. The polarization-dependent second harmonic generation, Raman, photoluminescence, and absorption spectroscopy of the WS2/Sb2S3 heterostructure confirm optical anisotropic behavior due to symmetry breaking by the moiré superlattice formation. The WS2/Sb2S3 device exhibits high on/off ratios up to 106, a relatively low leakage current of 10-13 A, and a broadband optoelectronic response range from 360 to 914 nm. Notably, the broken symmetry by C2-symmetric Sb2S3 nanowires grown on a C3-symmetric WS2 nanosheet endows the WS2/Sb2S3 photodetector with strong polarization-dependent photocurrent intensity and high-resolution polarization imaging capability. Our study demonstrates the potential for constructing multifunctional moiré materials by incorporating symmetry-breaking engineering.

Stacking Engineering toward Giant Second Harmonic Generation in Twisted Graphene Superstructures

The nonlinear optical response in graphene is finding increasing applications in nanophotonic devices. The activation and enhancement of second harmonic generation (SHG) in graphene, which is generally forbidden in monolayer and AB-stacked bilayer graphene due to their centrosymmetry, is of urgent need for nanophotonic applications. Here, we present a comprehensive study of SHG performance of twisted multilayer graphene structures based on stacking engineering. It is found that the modulation of in-plane and out-of-plane SHG susceptibility components by stacking few-layer graphene is essential in producing giant SHG response in twisted multilayer graphene. Giant SHG intensity in twisted multilayer graphene is observed, reaching nearly 10 times that of monolayer MoS2 under 1064 nm excitation, which significantly outperformed graphene structures reported to date. Our findings present a facile and effective approach to enhance SHG in graphene structures, showing promise for future application of graphene in second harmonic nanophotonic devices as well as prospects for the study of SHG among two-dimensional (2D) structures in general.

Nonlinear physics of moiré superlattices

Nonlinear physics is one of the most important research fields in modern physics and materials science. It offers an unprecedented paradigm for exploring many fascinating physical phenomena and realizing diverse cutting-edge applications inconceivable in the framework of linear processes. Here we review the recent theoretical and experimental progress concerning the nonlinear physics of synthetic quantum moiré superlattices. We focus on the emerging nonlinear electronic, optical and optoelectronic properties of moiré superlattices, including but not limited to the nonlinear anomalous Hall effect, dynamically twistable harmonic generation, nonlinear optical chirality, ultralow-power-threshold optical solitons and spontaneous photogalvanic effect. We also present our perspectives on the future opportunities and challenges in this rapidly progressing field, and highlight the implications for advances in both fundamental physics and technological innovations.

Conductance Quantization in 2D Semi-Metallic Transition Metal Dichalcogenides

Conductance quantization of 2D materials is significant for understanding the charge transport at the atomic scale, which provides a platform to manipulate the quantum states, showing promising applications for nanoelectronics and memristors. However, the conventional methods for investigating conductance quantization are only applicable to materials consisting of one element, such as metal and graphene. The experimental observation of conductance quantization in transition metal dichalcogenides (TMDCs) with complex compositions and structures remains a challenge. To address this issue, an approach is proposed to characterize the charge transport across a single atom in TMDCs by integrating in situ synthesized 1T'-WTe2 electrodes with scanning tunneling microscope break junction (STM-BJ) technique. The quantized conductance of 1T'-WTe2 is measured for the first time, and the quantum states can be modulated by stretching speed and solvent. Combined with theoretical calculations, the evolution of quantized and corresponding configurations during the break junction process is demonstrated. This work provides a facile and reliable avenue to characterize and modulate conductance quantization of 2D materials, intensively expanding the research scope of quantum effects in diverse materials.

WSe2/chitosan-based wearable multi-functional platform for monitoring electrophysiological signals, pulse rate, respiratory rate, and body movements

A cleanroom free optimized fabrication of a low-cost facile tungsten diselenide (WSe2) combined with chitosan-based hydrogel device is reported for multifunctional applications including tactile sensing, pulse rate monitoring, respiratory rate monitoring, human body movements detection, and human electrophysiological signal detection. Chitosan being a natural biodegradable, non-toxic compound serves as a substrate to the semiconducting WSe2 electrode which is synthesized using a single step hydrothermal technique. Elaborate characterization studies are performed to confirm the morphological, structural, and electrical properties of the fabricated chitosan/WSe2 device. Chitosan/WSe2 sensor with copper contacts on each side is put directly on skin to capture human body motions. The resistivity of the sample was calculated as 26 kΩ m-1. The device behaves as an ultrasensitive pressure sensor for tactile and arterial pulse sensing with response time of 0.9 s and sensitivity of around 0.02 kPa-1. It is also capable for strain sensing with a gauge factor of 54 which is significantly higher than similar other reported electrodes. The human body movements sensing can be attributed to the piezoresistive character of WSe2 that originates from its non-centrosymmetric structure. Further, the sensor is employed for monitoring respiratory rate which measures to 13 counts/min for healthy individual and electrophysiological signals like ECG and EOG which can be used later for detecting numerous pathological conditions in humans. Electrophysiological signal sensing is carried out using a bio-signal amplifier (Bio-Amp EXG Pill) connected to Arduino. The skin-friendly, low toxic WSe2/chitosan dry electrodes pave the way for replacing wet electrodes and find numerous applications in personalized healthcare.

Manipulation of Moiré Superlattice in Twisted Monolayer-multilayer Graphene by Moving Nanobubbles

In the heterostructure of two-dimensional (2D) materials, many novel physics phenomena are strongly dependent on the Moiré superlattice. How to achieve the continuous manipulation of the Moiré superlattice in the same sample is very important to study the evolution of various physical properties. Here, in minimally twisted monolayer-multilayer graphene, we found that bubble-induced strain has a huge impact on the Moiré superlattice. By employing the AFM tip to dynamically and continuously move the nanobubble, we realized the modulation of the Moiré superlattice, like the evolution of regular triangular domains into long strip domain structures with single or double domain walls. We also achieved controllable modulation of the Moiré superlattice by moving multiple nanobubbles and establishing the coupling of nanobubbles. Our work presents a flexible method for continuous and controllable manipulation of Moiré superlattices, which will be widely used to study novel physical properties in 2D heterostructures.

Odd Nonlinear Conductivity under Spatial Inversion in Chiral Tellurium

Electrical transport in noncentrosymmetric materials departs from the well-established phenomenological Ohm's law. Instead of a linear relation between current and electric field, a nonlinear conductivity emerges along specific crystallographic directions. This nonlinear transport is fundamentally related to the lack of spatial inversion symmetry. However, the experimental implications of an inversion symmetry operation on the nonlinear conductivity remain to be explored. Here, we report on a large, nonlinear conductivity in chiral tellurium. By measuring samples with opposite handedness, we demonstrate that the nonlinear transport is odd under spatial inversion. Furthermore, by applying an electrostatic gate, we modulate the nonlinear output by a factor of 300, reaching the highest reported value excluding engineered heterostructures. Our results establish chiral tellurium as an ideal compound not just to study the fundamental interplay between crystal structure, symmetry operations and nonlinear transport; but also to develop wireless rectifiers and energy-harvesting chiral devices.

High-Performance Sliding Ferroelectric Transistor Based on Schottky Barrier Tuning

Sliding ferroelectricity associated with interlayer translation is an excellent candidate for ferroelectric device miniaturization. However, the weak polarization gives rise to the poor performance of sliding ferroelectric transistors with a low on/off ratio and a narrow memory window, which restricts its practical application. To address the issue, we propose a facile strategy by regulating the Schottky barrier in sliding ferroelectric semiconductor transistors based on γ-InSe, in which a high performance with a large on/off ratio (10⁶) and a wide memory window (4.5 V) was ultimately acquired. Additionally, the memory window of the device can be further modulated by electrostatic doping or light excitation. These results open up new ways for designing novel ferroelectric devices based on emerging sliding ferroelectricity.