Moiré superlattices, the artificial quantum materials, have provided a wide range of possibilities for the exploration of completely new physics and device architectures. In this Review, we focus on the recent progress on emerging moiré photonics and optoelectronics, including but not limited to moiré excitons, trions, and polaritons; resonantly hybridized excitons; reconstructed collective excitations; strong mid- and far-infrared photoresponses; terahertz single-photon detection; and symmetry-breaking optoelectronics. We also discuss the future opportunities and research directions in this field, such as developing advanced techniques to probe the emergent photonics and optoelectronics in an individual moiré supercell; exploring new ferroelectric, magnetic, and multiferroic moiré systems; and using external degrees of freedom to engineer moiré properties for exciting physics and potential technological innovations. Description Patterning optoelectronics Moiré patterns develop when two or more periodic lattices are overlaid over each other. These long-range patterns can also be seen in two-dimensional materials and give rise to rich physics within these quantum materials systems. Du et al. reviewed recent developments in moiré-patterned two-dimensional materials with a focus on the photonic and optoelectronic aspects and how the properties can be controlled by engineering the stacking order and twist angle between the layers. The wealth of properties seen indicates that these systems may be a powerful platform to develop an exotic optoelectronic device technology. —ISO A review describes recent developments in moiré photonics and optoelectronics. BACKGROUND Moiré pattern refers to a new, long-length scale of structure arising from the interference between two or more periodic templates. The scientific and engineering value of moiré patterns was firstly appreciated by the physicist Lord Rayleigh in 1874. Thereafter, the study of moiré patterns enabled the birth of moiré techniques for a wide variety of practical applications, e.g., precision measurement and positioning, automatization, and strain measurements. In condensed-matter physics, moiré patterns (also referred to as moiré superlattices) can be formed by vertically stacking two or more two-dimensional (2D) layered materials with a tiny twist angle and/or a slight lattice mismatch. Over the past decade, moiré superlattices have made a spectacular appearance with a cornucopia of previously unachieved new phenomena and distinct functionalities, transforming the landscape of fundamental solid-state physics, materials science, and engineering. ADVANCES Moiré superlattices introduce a new length-scale potential much larger than the crystal periodicity of the constituent 2D layers, providing a powerful strategy for engineering electronic band structures and hence resulting in a plethora of emergent quantum phenomena. For example, the long-wavelength moiré periodic potential can fold the electronic band structure into a mini-Brillouin zone, giving rise to the formation of flat bands and a rich phase diagram of correlated states, such as unconventional superconductivity, orbital magnetism, Mott insulator states, and topological multiferroic orders. When moiré superlattices meet light, they provide a powerful platform for discovering emergent photonic and optoelectronic phenomena and device architectures. In early 2019, four research groups independently demonstrated the pronounced effects of moiré potential on excitons of transition-metal dichalcogenide heterostructures from the perspective of both real and momentum spaces. The new degree of freedom afforded by using moiré superlattices provides possibilities for engineering the exciton band structures and light-matter interactions for numerous applications, such as versatile quantum light sources, new quantum many-body physics, and long-sought boson crystals. These breakthroughs in the study of moiré excitons have excited tremendous interest in moiré photonics and optoelectronics. Indeed, a rich diversity of emergent moiré photonic and optoelectronic properties have been witnessed with unprecedented speed over the past few years—properties covering collective moiré excitations, moiré polaritons, photonics of moiré correlated electronic states, giant mid- and far-infrared photoresponses, tunable bulk photovoltaic effect, and so forth. Notably, the moiré bulk photovoltaic effects are ultrastrong and much larger than previous demonstrations, enabling the demonstration of intelligent infrared sensors with merely a subwavelength footprint potentially for next-generation nonlinear photonics and optoelectronics. OUTLOOK Currently, all measurements of moiré photonics and optoelectronics are performed in the far-field limit with a collection of signals from more than 10,000 moiré cells. Therefore, developing advanced techniques that can simultaneously probe the photonic and optoelectronic properties in an individual moiré supercell and evaluate the moiré potential landscape will be valuable for advancing our understanding. Meanwhile, current research efforts on moiré photonics and optoelectronics mainly focus on twisted graphene and twisted transition-metal dichalcogenides. Exploring new moiré systems, such as moiré superlattices composed of 2D ferroelectric, magnetic, or multiferroic crystals, and moiré devices involving two or multiple single moiré superlattices, will initiate research directions of moiré photonics and optoelectronics. Furthermore, engineering the moiré photonics and optoelectronics through external means (e.g., electric or magnetic field, strain, and ultrafast optical excitations) has the potential to spark the next “gold rush” for technical innovations (e.g., quantum nonlinear optics, ultracompact optical modulators, and terahertz single-photon devices). We are just starting down the path of exploring moiré photonics and optoelectronics, and there will be many more surprises to come. Moiré photonics and optoelectronics. Moiré superlattices introduce a paradigm to engineer the band structures of collective excitations and light-matter interaction for a plethora of emergent photonic and optoelectronic phenomena, such as moiré excitons, moiré polaritons, photonics of moiré correlated electronic states, intelligent infrared sensing, quantum light sources, single-photon detection, and symmetry-breaking optoelectronics.

Moiré photonics and optoelectronics

Borophene (B) is a promising two dimensional (2D) material because of the excellent carrier mobility, large specific surface area and very light mass. However, many problems (overactive borophene chemistry and difficulty) have limited it's further applications. To improve the catalytic hydrogen evolution reaction properties, here, we apply ab-initio calculations to study the structural stability of two defective borophene (χ3 and β12)/MoS2 heterojunction as a hydrogen evolution reaction (HER) catalyst. In particular, we further explore the influence noble metals (Pt and Pd) on the catalytic properties of borophene(χ3 and β12)/MoS2 heterojunction. The calculated results show that the involvement of MoS2 not only improves the structural stability of the defective borophene@MoS2 but also further improves the catalytic ability of the borophene@MoS2. In particular, the calculated hydrogen adsorption Gibbs free energy (ΔG) of the χ3/MoS2-III heterojunction is only −0.0004 eV, which is an ideal catalyst for HER. Furthermore, the calculated Gibbs free energy (ΔG) of borophene (β12)/Pd@MoS2-II is 0.0007 eV because of the strong localized hybridization between Pd and the borophene (β12)/MoS2-II. Therefore, we believe that the Pd-doping can effectively improve the catalytic properties of the borophene (β12)/Pd@MoS2 as a hydrogen evolution reaction (HER) catalyst. 

Enhancing the catalytic hydrogen evolution reaction (HER) of the defective borophene@Pt/Pd/MoS2 heterojunction

Friction behaviors of two-dimensional materials at the nanoscale

Macroscale superlubricity breakdown of lubricating materials caused by substrate surface roughening and mechanochemical modification poses great challenges for their practical tribological applications. Here, a facile way is reported to access robust macroscale superlubricity in a vacuum environment, via the operando formation of graphene/transition‐metal dichalcogenide (TMDC) heterostructures at wear‐induced rough surfaces. By trapping active amorphous carbon (a‐C) wear products between TMDC flakes, the sandwich structures readily transform into graphene/TMDC heterostructures during running‐in stage, based on shear‐induced confinement and load‐driven graphitization effects. Then they assemble into multipoint flake‐like tribofilms to achieve macroscale superlubricity at steady stage by reducing contact area, eliminating strong cross‐interface carbon–carbon interactions and polishing a‐C rough nascent surface. Atomistic simulations reveal the preferential formation of graphene/TMDC heterostructures during running‐in stage and demonstrate the superlubric sliding of TMDCs on the graphene. The findings are of importance to achieve robust superlubricity and provide a good strategy for the synthesis of other van der Waals heterostructures.

Operando Formation of Van der Waals Heterostructures for Achieving Macroscale Superlubricity on Engineering Rough and Worn Surfaces

Moiré superlattices and their interlayer interactions in van der Waals heterostructures have received surging attention for manipulating the properties of quantum materials. In this work, based on non-equilibrium molecular dynamics simulations, we find that the in-plane thermal conductivity of graphene/hexagonal boron nitride (h-BN) moiré superlattices decreases monotonically with the increase in the interlayer rotation angle within the small twisting range. The atomic stress amplitude exhibits the periodic distribution corresponding to a structural moiré pattern. Through the in-depth analysis at the atomic level, a competing mechanism between the magnitude and the directional change of the in-plane heat flow has been revealed, and the dominant role of directional change in determining the in-plane thermal conductivity of graphene/h-BN moiré superlattices at small rotation angle has also been confirmed. Finally, the monotonic decreasing trend of in-plane thermal conductivity at a small rotation angle is further explained by the reduced low-frequency phonon transmission and the blue shift of the transmission peak as the interlayer rotation angle increases. Our work provides the physical understanding of the moiré superlattice effect and a new approach for regulating the thermal conductivity of two-dimensional materials.

Impact of moiré superlattice on atomic stress and thermal transport in van der Waals heterostructures

Two-dimensional heterostructures are excellent platforms to realize twist-angle-independent ultra-low friction due to their weak interlayer van der Waals interactions and natural lattice mismatch. However, for finite-size interfaces, the effect of domain edges on the friction process remains unclear. Here we report the superlubricity phenomenon and the edge-pinning effect at MoS2/graphite and MoS2/hexagonal boron nitride van der Waals heterostructure interfaces. We found that the friction coefficients of these heterostructures are below 10−6. Molecular dynamics simulations corroborate the experiments, which highlights the contribution of edges and interface steps to friction forces. Our experiments and simulations provide more information on the sliding mechanism of finite low-dimensional structures, which is vital to understand the friction process of laminar solid lubricants. MoS2/graphite and MoS2/h-BN interfaces are shown to have ultra-low friction coefficients, whereas edges and interface steps mainly contribute to the friction force.

UItra-low friction and edge-pinning effect in large-lattice-mismatch van der Waals heterostructures.

Abstract The improvement of mechanical properties, electrical properties, and processing properties of ultra-high-filled thermal insulation polypropylene (PP) composites has been plagued by high filling amount of heat-conducting fillers such as alumina powder (Al2O3) and expanded graphite. In order to improve its properties, this article uses a high-temperature-resistant coupling agent (A153) to modify the PP composite. The results of this study show that when the content of A153 reaches 7.5 phr, the tensile strength of PP composite can reach 5 MPa, the elongation at break can reach 25%, and the volume resistivity can reach 12.8 × 1012 Ω·m, and the maximum thermal conductivity is 1.82 W·m−1·K−1. The processability also shows that the introduction of A153 can increase the melt flow rate and effectively improve the processability of the material. Graphical abstract

Effect of phenyltrimethoxysilane coupling agent (A153) on simultaneously improving mechanical, electrical, and processing properties of ultra-high-filled polypropylene composites

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UItra-low friction and edge-pinning effect in large-lattice-mismatch van der Waals heterostructures.

Effect of phenyltrimethoxysilane coupling agent (A153) on simultaneously improving mechanical, electrical, and processing properties of ultra-high-filled polypropylene composites