Evidence for moiré excitons in van der Waals heterostructures

Anomalous Phonon Behavior and Tunable Exciton Emissions: Insights into Pressure-Driven Dynamics in Silicon Phosphide

IV-V two-dimensional materials have emerged as key contenders for polarization-sensitive and angle-resolved devices, given their inherent anisotropic physical properties. While these materials exhibit intriguing high-pressure quasi-particle behavior and phase transition, the evolution of quasi-particles and their interactions under external pressure remain elusive. Here, employing a diamond anvil cell and spectroscopic measurements coupled with first-principles calculations, we unveil rarely observed pressure-induced phonon-phonon coupling in layered SiP flakes. This coupling manifests as an anomalous phonon hardening behavior for the A1 mode within a broad wavenumber phonon softening region. Furthermore, we demonstrate the effective tuning of exciton emissions in SiP flakes under pressure, revealing a remarkable 63% enhancement in the degree of polarization (DOP) within the pressure range of 0-3.5 GPa. These findings contribute to our understanding of high-pressure phonon evolution in SiP materials and offer a strategic approach to manipulate the anisotropic performance of in-plane anisotropic 2D materials.

Crossover between rigid and reconstructed moiré lattice in h-BN-encapsulated twisted bilayer WSe2 with different twist angles

A moiré lattice in a twisted-bilayer transition metal dichalcogenide (tBL-TMD) exhibits a complex atomic reconstruction effect when its twist angle is less than a few degrees. The influence of the atomic reconstruction on material properties of the tBL-TMD has been of particular interest. In this study, we performed scanning transmission electron microscopy (STEM) imaging of a moiré lattice in h-BN-encapsulated twisted bilayer WSe2 with various twist angles. Atomic-resolution imaging of the moiré lattice revealed a reconstructed moiré lattice below a crossover twist angle of ∼4° and a rigid moiré lattice above this angle. Our findings indicate that h-BN encapsulation has a considerable influence on lattice reconstruction, as the crossover twist angle was larger in h-BN-encapsulated devices compared to non-encapsulated devices. We believe that this difference is due to the improved flatness and uniformity of the twisted bilayers with h-BN encapsulation. Our results provide a foundation for a deeper understanding of the lattice reconstruction in twisted TMD materials with h-BN encapsulation.

Boosting Monolayer Transition Metal Dichalcogenides Growth by Hydrogen-Free Ramping during Chemical Vapor Deposition

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H2) in their synthesis. Using CVD growth of MoSe2 as an example, this study illustrates the role of a H2-free environment during temperature ramping in suppressing the reduction of MoO3, which promotes effective vaporization and selenization of the Mo precursor to form MoSe2 monolayers with excellent crystal quality. As-synthesized MoSe2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm2/(V·s) with an on-off ratio of 7 × 107. This approach can be extended to other TMDs, such as WSe2, MoTe2, and MoSe2/WSe2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.

High Q Hybrid Mie-Plasmonic Resonances in van der Waals Nanoantennas on Gold Substrate

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices; however, they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain low losses, while achieving stronger mode confinement. Here, we use a high refractive index multilayer transition-metal dichalcogenide WS2 exfoliated on gold to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Perot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We measure the experimental quality factors of hybridized Mie-plasmonic (MP) modes to be up to 33 times that of standard Mie resonances in the nanoantennas on silica. We then tune the nanoantenna geometries to observe signatures of a supercavity mode with a further increased Q factor of over 260 in experiment. We show that this quasi-bound state in the continuum results from strong coupling between a Mie resonance and Fabry-Perot-plasmonic mode in the vicinity of the higher-order anapole condition. We further simulate WS2 nanoantennas on gold with a 5 nm thick hBN spacer in between. By placing a dipole within this spacer, we calculate the overall light extraction enhancement of over 107, resulting from the strong, subwavelength confinement of the incident light, a Purcell factor of over 700, and high directivity of the emitted light of up to 50%. We thus show that multilayer TMDs can be used to realize simple-to-fabricate, hybrid dielectric-on-metal nanophotonic devices granting access to high-Q, strongly confined, MP resonances, along with a large enhancement for emitters in the TMD-gold gap.

Moiré Engineering of Spin-Orbit Torque by Twisted WS2 Homobilayers

Artificial moiré superlattices created by stacking 2D crystals have emerged as a powerful platform with unprecedented material-engineering capabilities. While moiré superlattices are reported to host a number of novel quantum states, their potential for spintronic applications remains largely unexplored. Here, the effective manipulation of spin-orbit torque (SOT) is demonstrated using moiré superlattices in ferromagnetic devices comprised of twisted WS2/WS2 homobilayer (t-WS2) and CoFe/Pt thin films by altering twisting angle (θ) and gate voltage. Notably, a substantial enhancement of up to 44.5% is observed in SOT conductivity at θ ≈ 8.3°. Furthermore, compared to the WS2 monolayer and untwisted WS2/WS2 bilayers, the moiré superlattices in t-WS2 enable a greater gate-voltage tunability of SOT conductivity. These results are related to the generation of the interfacial moiré magnetic field by the real-space Berry phase in moiré superlattices, which modulates the absorption of the spin-Hall current arising from Pt through the magnetic proximity effect. This study highlights the moiré physics as a new building block for designing enhanced spintronic devices.

Twist piezoelectricity: giant electromechanical coupling in magic-angle twisted bilayer LiNbO3

Twisted a pair of stacked two-dimensional materials exhibit many exotic electronic and photonic properties, leading to the emergence of flat-band superconductivity, moiré engineering and topological polaritons. These remarkable discoveries make twistronics the focus point of tremendous interest, but mostly limited to the concept of electrons, phonons or photons. Here, we present twist piezoelectricity as a fascinating paradigm to modulate polarization and electromechanical coupling by twisting precisely the stacked lithium niobate slabs due to the interlayer coupling effect. Particularly, the inversed and twisted bilayer lithium niobate is constructed to overcome the intrinsic mutual limitation of single crystals and giant effective electromechanical coupling coefficientk t 2 is unveiled at magic angle of 11 1 ∘ , reaching 85.5%. Theoretical analysis based on mutual energy integrals shows well agreements with numerical and experimental results. Our work opens new venues to flexibly control multi-physics with magic angle, stimulating progress in wideband acoustic-electric, and acoustic-optic components, which has great potential in wireless communication, timing, sensing, and hybrid integrated photonics.

Low-frequency Raman active modes of twisted bilayer MoS2

We study the low-frequency Raman active modes of twisted bilayer MoS2for several twist angles using a force-field approach and a parametrized bond polarizability model. We show that twist angles near high-symmetry stacking configurations exhibit stacking frustration that leads to significant buckling of the moiré superlattice. We find that atomic relaxation due to the twist is of prime importance. The periodic displacement of the Mo atoms shows the realization of a soliton network, and in turn, leads to the emergence of a number of frequency modes not seen in the high-symmetry stacking systems. Some of the modes are only seen in theXZRaman polarization setup while others are seen in theXYsetup. The symmetry of the normal modes, and how this affects the Raman tensors is examined in detail.

Chirality-Dependent Valley Polarization in Magnetic van der Waals Heterostructures via Spin-Selective Charge Transfer

Magnetic proximity interaction provides a promising route to manipulate the spin and valley degrees of freedom in van der Waals heterostructures. Here, we report a control of valley pseudospin in the WS2/MoSe2 heterostructure by utilizing the magnetic proximity effect of few-layered CrBr3 and, for the first time, observe a substantial difference in valley polarization of intra/interlayer excitons under different circularly polarized laser excitations, referred to as chirality-dependent valley polarization. Theoretical and experimental results reveal that the spin-selective charge transfer between MoSe2 and CrBr3, as well as between MoSe2 and WS2, is mostly responsible for the chiral feature of valley polarization in comparison with the proximity exchange field. This means that a long-distance manipulation of exciton behaviors in multilayer heterostructures can be achieved through spin-selective charge transfer. This work marks a significant advancement in the control of spin and valley pseudospin in multilayer structures.

Exciton Localization Modulated by Ultradeep Moiré Potential in Twisted Bilayer γ-Graphdiyne

Twisted moiré superlattice is featured with its moiré potential energy, the depth of which renders an effective approach to strengthening the exciton-exciton interaction and exciton localization toward high-performance quantum photonic devices. However, it remains as a long-standing challenge to further push the limit of moiré potential depth. Herein, owing to the pz orbital induced band edge states enabled by the unique sp-C in bilayer γ-graphdiyne (GDY), an ultradeep moiré potential of ∼289 meV is yielded. After being twisted into the hole-to-hole layer stacking configuration, the interlayer coupling is substantially intensified to augment the lattice potential of bilayer GDY up to 475%. The presence of lateral constrained moiré potential shifts the spatial distribution of electrons and holes in excitons from the regular alternating mode to their respective separated and localized mode. According to the well-established wave function distribution of electrons contained in excitons, the AA-stacked site is identified to serve for exciton localization. This work extends the materials systems available for moiré superlattice design further to serial carbon allotropes featured with benzene ring-alkyne chain coupling, unlocking tremendous potential for twistronic-based quantum device applications.

Research Progress of Single-Photon Emitters Based on Two-Dimensional Materials

From quantum communications to quantum computing, single-photon emitters (SPEs) are essential components of numerous quantum technologies. Two-dimensional (2D) materials have especially been found to be highly attractive for the research into nanoscale light-matter interactions. In particular, localized photonic states at their surfaces have attracted great attention due to their enormous potential applications in quantum optics. Recently, SPEs have been achieved in various 2D materials, while the challenges still remain. This paper reviews the recent research progress on these SPEs based on various 2D materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and twisted-angle 2D materials. Additionally, we summarized the strategies to create, position, enhance, and tune the emission wavelength of these emitters by introducing external fields into these 2D system. For example, pronounced enhancement of the SPEs' properties can be achieved by coupling with external fields, such as the plasmonic field, and by locating in optical microcavities. Finally, this paper also discusses current challenges and offers perspectives that could further stimulate scientific research in this field. These emitters, due to their unique physical properties and integration potential, are highly appealing for applications in quantum information and communication, as well as other physical and technological fields.

Twist angle-dependent valley polarization switching in heterostructures

The twist engineering of moiré superlattice in van der Waals heterostructures of transition metal dichalcogenides can manipulate valley physics of interlayer excitons (IXs), paving the way for next-generation valleytronic devices. However, the twist angle-dependent control of excitonic potential on valley polarization is not investigated so far in electrically controlled heterostructures and the physical mechanism underneath needs to be explored. Here, we demonstrate the dependence of both polarization switching and degree of valley polarization on the moiré period. We also find the mechanisms to reveal the modulation of twist angle on the exciton potential and the electron-hole exchange interaction, which elucidate the experimentally observed twist angle-dependent valley polarization of IXs. Furthermore, we realize the valley-addressable devices based on polarization switch. Our work demonstrates the manipulation of the valley polarization of IXs by tunning twist angle in electrically controlled heterostructures, which opens an avenue for electrically controlling the valley degrees of freedom in twistronic devices.

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Nonbosonic Moiré Excitons

Optical excitations in moiré transition metal dichalcogenide bilayers lead to the creation of excitons, as electron-hole bound states, that are generically considered within a Bose-Hubbard framework. Here, we demonstrate that these composite particles obey an angular momentum commutation relation that is generally nonbosonic. This emergent spin description of excitons indicates a limitation to their occupancy on each site, which is substantial in the weak electron-hole binding regime. The effective exciton theory is accordingly a spin Hamiltonian, which further becomes a Hubbard model of emergent bosons subject to an occupancy constraint after a Holstein-Primakoff transformation. We apply our theory to three commonly studied bilayers (MoSe_{2}/WSe_{2}, WSe_{2}/WS_{2}, and WSe_{2}/MoS_{2}) and show that in the relevant parameter regimes their allowed occupancies never exceed three excitons. Our systematic theory provides guidelines for future research on the many-body physics of moiré excitons.

Preparation of core-shell structure Ag@TiO2 plasma photocatalysts and reduction of Cr(VI): Size dependent and LSPR effect

The poor light absorption and low carrier separation efficiency of Titanium dioxide (TiO2) limit its further application. The introduction of plasma metal Ag have the potential to solve these drawbacks owing to its plasma resonance effect. Thus core-shell structure Ag@TiO2 plasma photocatalysts was prepared by using facile reduction method in this work. More specifically, Ag@TiO2 composite catalysts with different Ag loading amounts were prepared in the presence of surfactant PVP. Ag@TiO2 demonstrates excellent light absorption performance and photoelectric separation efficiency compared with pure TiO2. As a result, it displays excellent performance of Cr(VI) reduction under visible light. The optimal composite catalysts Ag@TiO2-5P achieves exceptional visible-light-driven photocatalytic Cr(VI) reduction efficiency of 0.01416 min-1 that is 2.29 times greater than pure TiO2. To investigate the role of PVP, we also synthesized Ag@TiO2-5 without PVP. The experimental results show that although Ag@TiO2-5 Cr(VI) reduction performance is superior to pure TiO2, it significantly decreases compared with Ag@TiO2-5P. The results of TEM and optoelectronic testing show that agglomeration of Ag particles leads to a decrease in the photoelectric separation efficiency of Ag@TiO2-5. The smaller Ag particles provide more active sites and demonstrating a stronger overall local surface plasmon resonance (LSPR) effect. DMPO spin-trapping ESR spectra testing indicates that ∙O2- and ∙OH are the main reactive species. This research provides a potential strategy to prepare Ag-based plasma photocatalysts for environment protection.

Resonant Band Hybridization in Alloyed Transition Metal Dichalcogenide Heterobilayers

Bandstructure engineering using alloying is widely utilized for achieving optimized performance in modern semiconductor devices. While alloying has been studied in monolayer transition metal dichalcogenides, its application in van der Waals heterostructures built from atomically thin layers is largely unexplored. Here, heterobilayers made from monolayers of WSe2 (or MoSe2) and MoxW1 - xSe2 alloy are fabricated and nontrivial tuning of the resultant bandstructure is observed as a function of concentration x. This evolution is monitored by measuring the energy of photoluminescence (PL) of the interlayer exciton (IX) composed of an electron and hole residing in different monolayers. In MoxW1 - xSe2/WSe2, a strong IX energy shift of ≈100 meV is observed for x varied from 1 to 0.6. However, for x < 0.6 this shift saturates and the IX PL energy asymptotically approaches that of the indirect bandgap in bilayer WSe2. This observation is theoretically interpreted as the strong variation of the conduction band K valley for x > 0.6, with IX PL arising from the K - K transition, while for x < 0.6, the bandstructure hybridization becomes prevalent leading to the dominating momentum-indirect K - Q transition. This bandstructure hybridization is accompanied with strong modification of IX PL dynamics and nonlinear exciton properties. This work provides foundation for bandstructure engineering in van der Waals heterostructures highlighting the importance of hybridization effects and opening a way to devices with accurately tailored electronic properties.

Imaging moiré excited states with photocurrent tunnelling microscopy

Moiré superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moiré excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moiré correlated ground states at the atomic scale. However, imaging of quantum excited states in moiré heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moiré exciton in twisted bilayer WS2. The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the in-plane charge transfer moiré exciton in twisted bilayer WS2, predicted by our GW-Bethe-Salpeter equation calculations, that emerges from the competition between the electron-hole Coulomb interaction and the moiré potential landscape. Our technique enables the exploration of photoexcited non-equilibrium moiré phenomena at the atomic scale.

Low-Temperature Vapor-Phase Growth of 2D Metal Chalcogenides

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

Conduction Band Replicas in a 2D Moiré Semiconductor Heterobilayer

Stacking monolayer semiconductors creates moiré patterns, leading to correlated and topological electronic phenomena, but measurements of the electronic structure underpinning these phenomena are scarce. Here, we investigate the properties of the conduction band in moiré heterobilayers of WS2/WSe2 using submicrometer angle-resolved photoemission spectroscopy with electrostatic gating. We find that at all twist angles the conduction band edge is the K-point valley of the WS2, with a band gap of 1.58 ± 0.03 eV. From the resolved conduction band dispersion, we deduce an effective mass of 0.15 ± 0.02 me. Additionally, we observe replicas of the conduction band displaced by reciprocal lattice vectors of the moiré superlattice. We argue that the replicas result from the moiré potential modifying the conduction band states rather than final-state diffraction. Interestingly, the replicas display an intensity pattern with reduced 3-fold symmetry, which we show implicates the pseudo vector potential associated with in-plane strain in moiré band formation.

Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Excitonic systems, facilitated by optical pumping, electrostatic gating or magnetic field, sustain composite particles with fascinating physics. Although various intriguing excitonic phases have been revealed via global measurements, the atomic-scale accessibility towards excitons has yet to be established. Here, we realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl3/graphite heterostructure. Combining scanning tunneling microscope and theoretical calculations, the excitonic in-gap states are directly profiled. The out-of-plane excitonic charge clouds exhibit oscillating Rydberg nodal structure, while their in-plane arrangements are determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy. Our results demonstrate the feasibility of mapping out the charge clouds of excitons microscopically and pave a brand-new way to directly investigate the nanoscale order of exotic correlated phases.

Excitonic Thermalization Bottleneck in Twisted TMD Heterostructures

Twisted van der Waals heterostructures show intriguing interface exciton physics, including hybridization effects and emergence of moiré potentials. Recent experiments have revealed that moiré-trapped excitons exhibit remarkable dynamics, where excited states show lifetimes that are several orders of magnitude longer than in monolayers. The origin of this behavior is still under debate. Based on a microscopic many-particle approach, we investigate the phonon-driven relaxation cascade of nonequilibrium moiré excitons in the exemplary MoSe2-WSe2 heterostructure. We track exciton relaxation pathways across different moiré mini-bands and identify the phonon-scattering channels assisting the spatial redistribution of excitons into low-energy pockets of the moiré potential. We unravel a phonon bottleneck in the flat band structure at low twist angles preventing excitons from fully thermalizing into the lowest state, explaining the measured enhanced emission intensity and lifetime of excited moiré excitons. Overall, our work provides important insights into exciton relaxation dynamics in flat-band exciton materials.

Contrast-enhanced phase-resolved second harmonic generation microscopy

The characterization of inverted structures (crystallographic, ferroelectric, or magnetic domains) is crucial in the development and application of novel multi-state devices. However, determining these inverted structures needs a sensitive probe capable of revealing their phase correlation. Here a contrast-enhanced phase-resolved second harmonic generation (SHG) microscopy is presented, which utilizes a phase-tunable Soleil-Babinet compensator and the interference between the SHG fields from the inverted structures and a homogeneous reference. By this means, such inverted structures are correlated through the π-phase difference of SHG, and the phase difference is ultimately converted into the intensity contrast. As a demonstration, we have applied this microscopy in two scenarios to determine the inverted crystallographic domains in two-dimensional van der Waals material MoS2. Our method is particularly suitable for applying in vacuum and cryogenic environments while providing optical diffraction-limited resolution and arbitrarily adjustable contrast. Without loss of generality, this contrast-enhanced phase-resolved SHG microscopy can also be used to resolve other non-centrosymmetric inverted structures, e.g. ferroelectric, magnetic, or multiferroic phases.

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.

Gold-Template-Assisted Mechanical Exfoliation of Large-Area 2D Layers Enables Efficient and Precise Construction of Moiré Superlattices

Moiré superlattices, consisting of rotationally aligned 2D atomically thin layers, provide a highly novel platform for the study of correlated quantum phenomena. However, reliable and efficient construction of moiré superlattices is challenging because of difficulties to accurately angle-align small exfoliated 2D layers and the need to shun wet-transfer processes. Here, efficient and precise construction of various moiré superlattices is demonstrated by picking up and stacking large-area 2D mono- or few-layer crystals with predetermined crystal axes, made possible by a gold-template-assisted mechanical exfoliation method. The exfoliated 2D layers are semiconductors, superconductors, or magnets and their high quality is confirmed by photoluminescence and Raman spectra and by electrical transport measurements of fabricated field-effect transistors and Hall devices. Twisted homobilayers with angle-twisting accuracy of ≈0.3°, twisted heterobilayers with sub-degree angle-alignment accuracy, and multilayer superlattices are precisely constructed and characterized by their moiré patterns, interlayer excitons, and second harmonic generation. The present study paves the way for exploring emergent phenomena in moiré superlattices.

Spatiotemporal evolution of ultrafast photocarrier dynamics across WS2-ReS2 lateral interface

2D lateral heterostructures possess atomically sharp lateral interfaces, while understanding of their ultrafast photocarrier dynamics from a spatiotemporal viewpoint is rather elusive. In this study, we have investigated the spatiotemporal evolution of photocarrier dynamics across the 1D lateral interface of a WS2-ReS2 2D lateral heterostructure utilizing femtosecond laser pump-probe. The nontrivial band offset across the 1D lateral interface markedly mediates the spatiotemporal photocarrier transfer and transport processes. Subsequently, a hole accumulation region on the WS2 side and an electron accumulation region (1DEG) on the ReS2 side have been spatially identified by correlating ultrafast photocarrier signals. The measured width of the unilateral depletion region is 1360 ± 160 nm. Our work has provided substantial insights into mediated photocarrier dynamics in the 2D lateral heterostructure, which will benefit explorations in 2D interfacial physics and 2D lateral optoelectronic devices.

Topological minibands and interaction driven quantum anomalous Hall state in topological insulator based moiré heterostructures

The presence of topological flat minibands in moiré materials provides an opportunity to explore the interplay between topology and correlation. In this work, we study moiré minibands in topological insulator films with two hybridized surface states under a moiré superlattice potential created by two-dimensional insulating materials. We show the lowest conduction (highest valence) Kramers' pair of minibands can beZ 2 non-trivial when the minima (maxima) of moiré potential approximately form a hexagonal lattice with six-fold rotation symmetry. Coulomb interaction can drive the non-trivial Kramers' minibands into the quantum anomalous Hall state when they are half-filled, which is further stabilized by applying external gate voltages to break inversion. We propose the monolayer Sb2 on top of Sb2Te3 films as a candidate based on first principles calculations. Our work demonstrates the topological insulator based moiré heterostructure as a potential platform for studying interacting topological phases.

Imaging Valley Excitons in a 2D Semiconductor with Scanning Tunneling Microscope-Induced Luminescence

Valley excitons dominate the optoelectronic response of transition-metal dichalcogenides and are drastically affected by structural and environmental inhomogeneities localized in these materials. Critical to understanding and controlling these nanoscale excitonic changes is the ability to correlate the imaging of excitonic states with crystalline structures on the atomic scale. Here, we apply scanning tunneling microscope-induced luminescence microscopy to image valley excitons in a semiconducting transition-metal dichalcogenide monolayer decoupled by a 10 nanometer-thick hexagonal-boron-nitride flake incorporated in a lateral homojunction on an Au electrode surface. This design enables the observation of chiral excitonic emission arising from neutral and charged valley excitons of the monolayer semiconductor at ambipolar voltages with a quantum efficiency up to ∼10-5 photon/electron. The measured light helicity demonstrates considerable circular polarization dependent on the sample voltage, reaching as much as 40%. The real-space luminescence imaging maps─at subnanometer resolution─of the valley excitons reveal striking spatial variations associated with localized inhomogeneities, including surface impurities and possibly nanoscale dielectric and/or potential disorders in the monolayer. Our study introduces a promising format for 2D materials to explore and tailor their optoelectronic processes at the atomic scale.

Ultralow Auger-Assisted Interlayer Exciton Annihilation in WS2/WSe2 Moiré Heterobilayers

Transition metal dichalcogenide (TMD) heterobilayers have emerged as a promising platform for exploring solid-state quantum simulators and many-body quantum phenomena. Their type II band alignment, combined with the moiré superlattice, inevitably leads to nontrivial exciton interactions and dynamics. Here, we unveil the distinct Auger annihilation processes for delocalized interlayer excitons in WS2/WSe2 moiré heterobilayers. By fitting the characteristic efficiency droop and bimolecular recombination rate, we quantitatively determine an ultralow Auger coefficient of 1.3 × 10-5 cm2 s-1, which is >100-fold smaller than that of excitons in TMD monolayers. In addition, we reveal selective exciton upconversion into the WSe2 layer, which highlights the significance of intralayer electron Coulomb interactions in dictating the microscopic scattering pathways. The distinct Auger processes arising from spatial electron-hole separation have important implications for TMD heterobilayers while endowing interlayer excitons and their strongly correlated states with unique layer degrees of freedom.

Van der Waals epitaxy of tunable moirés enabled by alloying

The unique physics in moiré superlattices of twisted or lattice-mismatched atomic layers holds great promise for future quantum technologies. However, twisted configurations are thermodynamically unfavourable, making accurate twist angle control during growth implausible. While rotationally aligned, lattice-mismatched moirés such as WSe2/WS2 can be synthesized, they lack the critical moiré period tunability, and their formation mechanisms are not well understood. Here, we report the thermodynamically driven van der Waals epitaxy of moirés with a tunable period from 10 to 45 nanometres, using lattice mismatch engineering in two WSSe layers with adjustable chalcogen ratios. Contrary to conventional epitaxy, where lattice-mismatch-induced stress hinders high-quality growth, we reveal the key role of bulk stress in moiré formation and its unique interplay with edge stress in shaping the moiré growth modes. Moreover, the superlattices display tunable interlayer excitons and moiré intralayer excitons. Our studies unveil the epitaxial science of moiré synthesis and lay the foundations for moiré-based technologies.

Twisted van der Waals Quantum Materials: Fundamentals, Tunability, and Applications

Twisted van der Waals (vdW) quantum materials have emerged as a rapidly developing field of two-dimensional (2D) semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single photon emission, nonlinear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This paper offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes, lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

Mixed Stochastic-Deterministic Approach for Many-Body Perturbation Theory Calculations

We present an approach for GW calculations of quasiparticle energies with quasiquadratic scaling by approximating high-energy contributions to the Green's function in its Lehmann representation with effective stochastic vectors. The method is easy to implement without altering the GW code, converges rapidly with stochastic parameters, and treats systems of various dimensionality and screening response. Our calculations on a 5.75° twisted MoS_{2} bilayer show how large-scale GW methods include geometry relaxations and electronic correlations on an equal basis in structurally nontrivial materials.

Field-Induced Hybridization of Moiré Excitons in MoSe_{2}/WS_{2} Heterobilayers

We study experimentally and theoretically the hybridization among intralayer and interlayer moiré excitons in a MoSe_{2}/WS_{2} heterostructure with antiparallel alignment. Using a dual-gate device and cryogenic white light reflectance and narrow-band laser modulation spectroscopy, we subject the moiré excitons in the MoSe_{2}/WS_{2} heterostack to a perpendicular electric field, monitor the field-induced dispersion and hybridization of intralayer and interlayer moiré exciton states, and induce a crossover from type I to type II band alignment. Moreover, we employ perpendicular magnetic fields to map out the dependence of the corresponding exciton Landé g factors on the electric field. Finally, we develop an effective theoretical model combining resonant and nonresonant contributions to moiré potentials to explain the observed phenomenology, and highlight the relevance of interlayer coupling for structures with close energetic band alignment as in MoSe_{2}/WS_{2}.

Simultaneously Regulated Highly Polarized and Long-Lived Valley Excitons in WSe2/GaN Heterostructures

Interlayer excitons, with prolonged lifetimes and tunability, hold potential for advanced optoelectronics. Previous research on the interlayer excitons has been dominated by two-dimensional heterostructures. Here, we construct WSe2/GaN composite heterostructures, in which the doping concentration of GaN and the twist angle of bilayer WSe2 are employed as two ingredients for the manipulation of exciton behaviors and polarizations. The exciton energies in monolayer WSe2/GaN can be regulated continuously by the doping levels of the GaN substrate, and a remarkable increase in the valley polarizations is achieved. Especially in a heterostructure with 4°-twisted bilayer WSe2, a maximum polarization of 38.9% with a long lifetime is achieved for the interlayer exciton. Theoretical calculations reveal that the large polarization and long lifetime are attributed to the high exciton binding energy and large spin flipping energy during depolarization in bilayer WSe2/GaN. This work introduces a distinctive member of the interlayer exciton with a high degree of polarization and a long lifetime.

Competition of Moiré Network Sites to Form Electronic Quantum Dots in Reconstructed MoX2/WX2 Heterostructures

Twisted bilayers of two-dimensional semiconductors offer a versatile platform for engineering quantum states for charge carriers using moiré superlattice effects. Among the systems of recent interest are twistronic MoX2/WX2 heterostructures (X = Se or S), which undergo reconstruction into preferential stacking domains and highly strained domain wall networks, determining the electron/hole localization across moiré superlattices. Here, we present a catalogue of options for the formation of self-organized quantum dots and wires in lattice-reconstructed marginally twisted MoX2/WX2 bilayers with a relative lattice mismatch δ ≪ 1 for twist angles ranging from perfect alignment to θ ∼ 1°. On the basis of multiscale modeling taking into account twirling of domain wall networks, we analyze bilayers with both parallel and antiparallel orientations of their unit cells and describe crossovers between different positioning of band edges for electrons and holes across moiré superlattices when θ < δ and θ > δ.

Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure

In two-dimensional semiconductors, cooperative and correlated interactions determine the material's excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton's hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.

Second-harmonic generation in 2D moiré superlattices composed of bilayer transition metal dichalcogenides

Moiré superlattices (MSLs) in twisted two-dimensional van der Waals materials feature twist-angle-dependent crystal symmetry and strong optical nonlinearities. By adjusting the twist angle in bilayer van der Waals materials, the second-harmonic generation (SHG) can be controlled. Here, we focus on exploring the electronic and SHG properties of MSLs in 2D bilayer transition metal dichalcogenides (TMDs) with different twist angles through first-principles calculations. We constructed MSL structures of five TMD materials, including three single-phase materials (MoS2, WS2, and MoSe2) and two heterojunctions (MoS2/MoSe2 and MoS2/WS2) with twist angles of 9.4°, 13.2°, 21.8°, 32.2°, and 42.1° without lattice mismatch. Our findings demonstrate a consistent variation in the SHG susceptibility among different TMD MSLs as a response to twist-angle changes. The underlying reason for the twist-angle dependence of SHG is that the twist angle regulates the interlayer coupling strength, affecting the optical band gap of MSLs and subsequently tuning the SHG susceptibility. Through a comparison of the static SHG susceptibility values, we identified the twist angle of 9.4° as the configuration that yields the highest SHG susceptibility (e.g. 358.5 pm V-1 for the 9.4° MoSe2 MSL). This value is even twice that of the monolayer (173.3 pm V-1 for monolayer MoSe2) and AA'-stacked bilayer structures (139.8 pm V-1 for AA' MoSe2). This high SHG susceptibility is attributed to the strong interlayer coupling in the 9.4° MSL, which enhances the valence band energy (contributed by the antibonding orbitals of chalcogen-pz and transition metal-dz2) and consequently leads to a small optical band gap, thus improving the optical transitions. The findings of this study provide a straightforward way to improve the SHG performance of bilayer TMDs and also throw light on the sensitive relationship between the twist angle, band structure and SHG properties of TMD MSLs.

Localisation-to-delocalisation transition of moiré excitons in WSe2/MoSe2 heterostructures

Moiré excitons (MXs) are electron-hole pairs localised by the periodic (moiré) potential forming in two-dimensional heterostructures (HSs). MXs can be exploited, e.g., for creating nanoscale-ordered quantum emitters and achieving or probing strongly correlated electronic phases at relatively high temperatures. Here, we studied the exciton properties of WSe2/MoSe2 HSs from T = 6 K to room temperature using time-resolved and continuous-wave micro-photoluminescence also under a magnetic field. The exciton dynamics and emission lineshape evolution with temperature show clear signatures that MXs de-trap from the moiré potential and turn into free interlayer excitons (IXs) for temperatures above 100 K. The MX-to-IX transition is also apparent from the exciton magnetic moment reversing its sign when the moiré potential is not capable of localising excitons at elevated temperatures. Concomitantly, the exciton formation and decay times reduce drastically. Thus, our findings establish the conditions for a truly confined nature of the exciton states in a moiré superlattice with increasing temperature and photo-generated carrier density.

Dynamically tunable moiré exciton Rydberg states in a monolayer semiconductor on twisted bilayer graphene

Moiré excitons are emergent optical excitations in two-dimensional semiconductors with moiré superlattice potentials. Although these excitations have been observed on several platforms, a system with dynamically tunable moiré potential to tailor their properties is yet to be realized. Here we present a continuously tunable moiré potential in monolayer WSe2, enabled by its proximity to twisted bilayer graphene (TBG) near the magic angle. By tuning local charge density via gating, TBG provides a spatially varying and dynamically tunable dielectric superlattice for modulation of monolayer WSe2 exciton wave functions. We observed emergent moiré exciton Rydberg branches with increased energy splitting following doping of TBG due to exciton wave function hybridization between bright and dark Rydberg states. In addition, emergent Rydberg states can probe strongly correlated states in TBG at the magic angle. Our study provides a new platform for engineering moiré excitons and optical accessibility to electronic states with small correlation gaps in TBG.

Enhancing Metallicity and Basal Plane Reactivity of 2D Materials via Self-Intercalation

Intercalation (ic) of metal atoms into the van der Waals (vdW) gap of layered materials constitutes a facile strategy to create materials whose properties can be tuned via the concentration of the intercalated atoms. Here we perform systematic density functional theory calculations to explore various properties of an emergent class of crystalline 2D materials (ic-2D materials) comprising vdW homobilayers with native metal atoms on a sublattice of intercalation sites. From an initial set of 1348 ic-2D materials, generated from 77 vdW homobilayers, we find 95 structures with good thermodynamic stability (formation energy within 200 meV/atom of the convex hull). A significant fraction of the semiconducting host materials are found to undergo an insulator to metal transition upon self-intercalation, with only PdS2, PdSe2, and GeS2 maintaining a finite electronic gap. In five cases, self-intercalation introduces magnetism. In general, self-intercalation is found to promote metallicity and enhance the chemical reactivity on the basal plane. Based on the calculated H binding energy, we find that self-intercalated SnS2 and Hf3Te2 are promising candidates for hydrogen evolution catalysis. All the stable ic-2D structures and their calculated properties can be explored in the open C2DB database.

High-Q optical resonances with robustness based on the quasi-guided modes in waveguide moiré gratings

High-Q resonances, especially those with high spectral tunability and large robustness of the Q factors, are always sought in photonic research for enhanced light-matter interactions. In this work, by rotating the 1D ridge grating on a slab waveguide in both the clockwise and counterclockwise directions by a certain angle θ, we show that the original subwavelength lattice can be converted into waveguide moiré gratings (WMGs), with the period increased to a larger value determined by the value of θ. These period-increasing perturbations will cause the First Brillouin Zone (FBZ) of the 1D grating to shrink, and thus convert the non-radiating guided modes with the dispersion band below the light line into quasi-guided modes (QGMs) above the light line, which can be accessed by free space radiations. We present the numerically calculated dispersion band and the Q-values for the QGMs supported by the WMGs with θ = 60°, and demonstrate that high-Q resonances can be achieved in a wide region of the energy-momentum space with the Q-values exhibiting large robustness over wavevectors. As an example of application, we show that the QGMs in the WMGs can be exploited to produce quite high optical gradient forces at different wavenumbers or wavelengths. Our results show that the QGMs supported by the WMGs work as a new type of high-Q resonances and may find prospective applications in various photonic systems.

Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures

Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations.

Modulating Thermoelectric Properties of the MoSe2/WSe2 Superlattice Heterostructure by Twist Angles

In the current era of limited resources, the matter of energy conversion holds significant importance. Thermoelectric materials possess the ability to transform thermal energy into electric power. Achieving an impressive thermoelectric figure of merit (ZT) necessitates the presence of a high power factor alongside low thermal conductivity. Stimulated by recent experimental reports on the in-plane lateral MoSe2/WSe2 heterostructure in the application thermoelectric device [Zhang Y. et al., Simultaneous electrical and thermal rectification in a monolayer lateral heterojunction. Science 2022, 378, 169], in this study, the method of twisting angle is used to modulate the energy bands of van der Waals MoSe2/WSe2 superlattice heterostructures to optimize the carrier concentration, band gap, electric conductance, thermal conductivity, and ZT of the heterostructure. The 21.79° twisted heterostructures among different twisting-angle heterostructures benefit from both the high power factor and low thermal conductivity, ultimately leading to significantly improved ZT compared to the untwisted counterpart.

Single Quasi-1D Chains of Sb2Se3 Encapsulated within Carbon Nanotubes

The realization of stable monolayers from 2D van der Waals (vdW) solids has fueled the search for exfoliable crystals with even lower dimensionalities. To this end, 1D and quasi-1D (q-1D) vdW crystals comprising weakly bound subnanometer-thick chains have been discovered and demonstrated to exhibit nascent physics in the bulk. Although established micromechanical and liquid-phase exfoliation methods have been applied to access single isolated chains from bulk crystals, interchain vdW interactions with nonequivalent strengths have greatly hindered the ability to achieve uniform single isolated chains. Here, we report that encapsulation of the model q-1D vdW crystal, Sb2Se3, within single-walled carbon nanotubes (CNTs) circumvents the relatively stronger c-axis vdW interactions between the chains and allows for the isolation of single chains with structural integrity. High-resolution transmission electron microscopy and selected area electron diffraction studies of the Sb2Se3@CNT heterostructure revealed that the structure of the [Sb4Se6]n chain is preserved, enabling us to systematically probe the size-dependent properties of Sb2Se3 from the bulk down to a single chain. We show that ensembles of the [Sb4Se6]n chains within CNTs display Raman confinement effects and an emergent band-like absorption onset around 600 nm, suggesting a strong blue shift of the near-infrared band gap of Sb2Se3 into the visible range upon encapsulation. First-principles density functional theory calculations further provided qualitative insight into the structures and interactions that could manifest in the Sb2Se3@CNT heterostructure. Spatial visualization of the calculated electron density difference map of the heterostructure indicated a minimal degree of electron donation from the host CNT to the guest [Sb4Se6]n chain. Altogether, this model system demonstrates that 1D and q-1D vdW crystals with strongly anisotropic vdW interactions can be precisely studied by encapsulation within CNTs with suitable diameters, thereby opening opportunities in understanding dimension-dependent properties of a plethora of emergent vdW solids at or approaching the subnanometer regime.

Terahertz linear/non-linear anomalous Hall conductivity of moiré TMD hetero-nanoribbons as topological valleytronics materials

Twisted moiré van der Waals heterostructures hold promise to provide a robust quantum simulation platform for strongly correlated materials and realize elusive states of matter such as topological states in the laboratory. We demonstrated that the moiré bands of twisted transition metal dichalcogenide (TMD) hetero-nanoribbons exhibit non-trivial topological order due to the tendency of valence and conduction band states in K valleys to form giant band gaps when spin-orbit coupling (SOC) is taken into account. Among the features of twisted WS[Formula: see text]/MoS[Formula: see text] and WSe[Formula: see text]/MoSe[Formula: see text], we found that the heavy fermions associated with the topological flat bands and the presence of strongly correlated states, enhance anomalous Hall conductivity (AHC) away from the magic angle. By band analysis, we showed that the topmost conduction bands from the ± K-valleys are perfectly flat and carry a spin/valley Chern number. Moreover, we showed that the non-linear anomalous Hall effect in moiré TMD hetero-nanoribbons can be used to manipulate terahertz (THz) radiation. Our findings establish twisted heterostructures of group-VI TMD nanoribbons as a tunable platform for engineering topological valley quantum phases and THz non-linear Hall conductivity.

Twistronics in two-dimensional transition metal dichalcogenide (TMD)-based van der Waals interface

Transition metal dichalcogenides (TMD) based heterostructures have gained significant attention lately because of their distinct physical properties and potential uses in electronics and optoelectronics. In the present work, the effects of twist on the structural, electronic, and optical properties (such as the static dielectric constant, refractive index, extinction coefficient, and absorption coefficient) of vertically stacked TMD heterostructures, namely MoSe2/WSe2, WS2/WSe2, MoSe2/WS2 and MoS2/WSe2, have been systematically studied and a thorough comparison is done among these heterostructures. In addition, the absence of negative frequency in the phonon dispersion curve and a low formation energy confirm the structural and thermodynamical stability of all the proposed TMD heterostructures. The calculations are performed using first-principles-based density functional theory (DFT) method. Beautiful Moiré patterns are formed due to the relative rotation of the layers as a consequence of the superposition of the periodic structures of the TMDs on each other. Twist engineering allows the modulation of bandgaps and a phase change from direct to indirect band gap semiconductors as well. The high optical absorption in the visible range of spectrum makes these twisted heterostructures very promising candidates in photovoltaic applications.

Low-Frequency Raman Study of Large-Area Twisted Bilayers of WS2 Stacked by an Etchant-Free Transfer Method

Monolayer transition metal dichalcogenides have strong intracovalent bonding. When stacked in multilayers, however, weak van der Waals interactions dominate interlayer mechanical coupling and, thus, influence their lattice vibrations. This study presents the frequency evolution of interlayer phonons in twisted WS2 bilayers, highly subject to the twist angle. The twist angle between the layers is controlled to modulate the spacing between the layers, which, in turn, affects the interlayer coupling that is probed by Raman spectroscopy. The shifts of high-frequency E2g1 (Γ) and A1g (Γ) phonon modes and their frequency separations are dependent on the twist angle, reflecting the correlation between the interlayer mechanical coupling and twist angle. In this work, we fabricated large-area, twisted bilayer WS2 with a clean interface with controlled twist angles. Polarized Raman spectroscopy identified new interlayer modes, which were not previously reported, depending on the twist angle. The appearance of breathing modes in Raman phonon spectra provides evidence of strong interlayer coupling in bilayer structures. We confirm that the twist angle can alter the exciton and trion dynamics of bilayers as indicated by the photoluminescence peak shift. These large-area controlled twist angle samples have practical applications in optoelectronic device fabrication and twistronics.

Interlayer Phonon Coupling from Heavy and Light Sublayers in a Natural Van der Waals Superlattice

Layered compounds characterized by van der Waals gaps are often associated with relatively weak interlayer particle interactions. However, in specific scenarios, these seemingly feeble forces can exert an impact on interlayer interactions through subtle energy fluctuations, which can give rise to a diverse range of physical and chemical properties, particularly intriguing in the context of thermal transport. In this study, taking a natural superlattice composed of alternately stacked PbS and SnS2 sublayers as a model, we proposed that in a superlattice, there is strong hybridization between acoustic phonons of heavy sublayers and optical phonons of light sublayers. We identified newly generated vibration modes in the superlattice, such as interlayer shear and breathing, which exhibit lower sound velocity and contribute less to heat transport compared to their parent materials, which significantly alters the thermal behaviors of the superlattice compared to its bulk counterparts. Our findings on the behavior of interlayer phonons in superlattices not only can shed light on developing functional materials with enhanced thermal dissipation capabilities but also contribute to the broader field of condensed matter physics, offering insights into various fields, including thermoelectrics and phononic devices, and may pave the way for technological advancements in these areas.

Direct Visualization of Dark Interlayer Exciton Transport in Moiré Superlattices

Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide heterostructures are expected to achieve high-temperature exciton condensation. However, the exciton degeneracy state is usually optically inactive due to the finite momentum of interlayer excitons. Experimental observation of dark interlayer excitons in moiré potentials remains challenging. Here we directly visualize the dark interlayer exciton transport in WS2/h-BN/WSe2 heterostructures using femtosecond transient absorption microscopy. We observe a transition from classical free exciton gas to quantum degeneracy by imaging temperature-dependent exciton transport. Below a critical degeneracy temperature, exciton diffusion rates exhibit an accelerating downward trend, which can be explained well by a nonlinear quantum diffusion model. These results open the door to quantum information processing and high-precision metrology in moiré superlattices.

Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2

The intriguing photophysical properties of monolayer stacks of different transition-metal dichalcogenides (TMDs), revealing rich exciton physics including interfacial and moiré excitons, have recently prompted an extension of similar investigations to hybrid systems of TMDs and organic films, as the latter combine large photoabsorption cross sections with the ability to tailor energy levels by targeted synthesis. To go beyond single-molecule photoexcitations and exploit the excitonic signatures of organic solids, crystalline molecular films are required. Moreover, a defined registry on the substrate, ideally an epitaxy, is desirable to also achieve an excitonic coupling in momentum space. This poses a certain challenge as excitonic dipole moments of organic films are closely related to the molecular orientation and film structure, which critically depend on the support roughness. Using X-ray diffraction, optical polarization, and atomic force microscopy, we analyzed the structure of pentacene (PEN) multilayer films grown on WSe2(001) and WS2(001) and identified an epitaxial alignment. While (022)-oriented PEN films are formed on both substrates, their azimuthal orientations are quite different, showing an alignment of the molecular L-axis along the ⟨ 110 ⟩ WSe 2 and ⟨ 100 ⟩ WS 2 directions. This intrinsic epitaxial PEN growth depends, however, sensitively on the substrates surface quality. While it occurs on exfoliated TMD single crystals and multilayer flakes, it is hardly found on exfoliated monolayers, which often exhibit bubbles and wrinkles. This enhances the surface roughness and results in (001)-oriented PEN films with upright molecular orientation but without any azimuthal alignment. However, monolayer flakes can be smoothed by AFM operated in contact mode or by transferring to ultrasmooth substrates such as hBN, which again yields epitaxial PEN films. As different PEN orientations result in different characteristic film morphologies (elongated mesa islands vs pyramidal dendrites), which can be easily distinguished by AFM or optical microscopy, this provides a simple means to judge the roughness of the used TMD surface.

Nonlinear and Negative Effective Diffusivity of Interlayer Excitons in Moiré-Free Heterobilayers

Interlayer exciton diffusion is studied in atomically reconstructed MoSe_{2}/WSe_{2} heterobilayers with suppressed disorder. Local atomic registry is confirmed by characteristic optical absorption, circularly polarized photoluminescence, and g-factor measurements. Using transient microscopy we observe propagation properties of interlayer excitons that are independent from trapping at moiré- or disorder-induced local potentials. Confirmed by characteristic temperature dependence for free particles, linear diffusion coefficients of interlayer excitons at liquid helium temperature and low excitation densities are almost 1000 times higher than in previous observations. We further show that exciton-exciton repulsion and annihilation contribute nearly equally to nonlinear propagation by disentangling the two processes in the experiment and simulations. Finally, we demonstrate effective shrinking of the light emission area over time across several hundreds of picoseconds at the transition from exciton- to the plasma-dominated regimes. Supported by microscopic calculations for band gap renormalization to identify the Mott threshold, this indicates transient crossing between rapidly expanding, short-lived electron-hole plasma and slower, long-lived exciton populations.

Tuning commensurability in twisted van der Waals bilayers

Moiré superlattices based on van der Waals bilayers1-4 created at small twist angles lead to a long wavelength pattern with approximate translational symmetry. At large twist angles (θt), moiré patterns are, in general, incommensurate except for a few discrete angles. Here we show that large-angle twisted bilayers offer distinctly different platforms. More specifically, by using twisted tungsten diselenide bilayers, we create the incommensurate dodecagon quasicrystals at θt = 30° and the commensurate moiré crystals at θt = 21.8° and 38.2°. Valley-resolved scanning tunnelling spectroscopy shows disparate behaviours between moiré crystals (with translational symmetry) and quasicrystals (with broken translational symmetry). In particular, the K valley shows rich electronic structures exemplified by the formation of mini-gaps near the valence band maximum. These discoveries demonstrate that bilayers with large twist angles offer a design platform to explore moiré physics beyond those formed with small twist angles.