Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy

Controlled growth of 3R phase niobium diselenide and its properties

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6-7 μs, 0.5 ms - 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

Electrically tunable non-radiative lifetime in WS2/WSe2 heterostructures

Van der Waals heterostructures based on transition metal dichalcogenides (TMDs) have emerged as excellent candidates for next-generation optoelectronics and valleytronics, due to their fascinating physical properties. The understanding and active control of the relaxation dynamics of heterostructures play a crucial role in device design and optimization. Here, we investigate the back-gate modulation of exciton dynamics in a WS2/WSe2 heterostructure by combining time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) at cryogenic temperatures. We find that the non-radiative relaxation lifetimes of photocarriers in heterostructures can be electrically controlled for samples with different twist-angles, whereas such lifetime tuning is not present in standalone monolayers. We attribute such an observation to doping-controlled competition between interlayer and intralayer recombination pathways in high-quality WS2/WSe2 samples. The simultaneous measurement of TRPL and TAS lifetimes within the same sample provides additional insight into the influence of coexisting excitons and background carriers on the photo-response, and points to the potential of tailoring light-matter interactions in TMD heterostructures.

Quantification of Two-Dimensional Interfaces: Quality of Heterostructures and What Is Inside a Nanobubble

Trapped materials at the interfaces of two-dimensional heterostructures (HS) lead to reduced coupling between the layers, resulting in degraded optoelectronic performance and device variability. Further, nanobubbles can form at the interface during transfer or after annealing. The question of what is inside a nanobubble, i.e., the trapped material, remains unanswered, limiting the studies and applications of these nanobubble systems. In this work, we report two key advances. First, we quantify the interface quality using RAW format optical imaging (unprocessed image data) and distinguish between ideal and non-ideal interfaces. The HS/substrate ratio value is calculated using a transfer matrix model and is able to detect the presence of trapped layers. The second key advance is the identification of water as the trapped material inside a nanobubble. To the best of our knowledge, this is the first study to show that optical imaging alone can quantify interface quality and find the type of trapped material inside spontaneously formed nanobubbles. We also define a quality index parameter to quantify the interface quality of HS. Quantitative measurement of the interface will help answer the question whether annealing is necessary during HS preparation and will enable creation of complex HS with small twist angles. Identification of the trapped materials will pave the way toward using nanobubbles for optical and engineering applications.

Enhancing Layer-Engineered Interlayer Exciton Emission and Valley Polarization in van der Waals Heterostructures via Strain

Layer-engineered interlayer excitons from heterostructures of transition-metal dichalcogenides (TMDCs) exhibit a rich variety of emissive states and intriguing valley spin-selection rules, the effective modulation of which is crucial for excitonic physics and related device applications. Strain or high pressure provides the possibility to tune the energy of the interlayer excitons; however, the reported emission intensity is substantially quenched, which greatly limits their practical application in optoelectronic devices. Here, via applying uniaxial strain based on polyvinyl alcohol (PVA) encapsulation technique, we report enhanced layer-engineered interlayer exciton emission intensity with largely modulated emission energy in WSe2/WS2 heterobilayer and heterotrilayer. Both momentum-direct and momentum-indirect interlayer excitons were observed, and their emission energies show an opposite shift tendency upon applied strain, which agrees with our DFT calculations. We further demonstrate that intralayer and interlayer exciton states with low phonon interactions can be modulated through the mechanical strain applied to the PVA substrate at low temperatures. Due to strain-induced breaking of the 3-fold rotational symmetry, we observe the enhanced valley polarization of interlayer excitons. Our study contributes to the understanding and modulation of the optical properties of interlayer excitons, which could be exploited for optoelectronic device applications.

A new charge transfer pathway in the MoSe2-WSe2 heterostructure under the conditions of B-excitons being resonantly pumped

Most transition metal dichalcogenide (TMD) heterostructures (HSs) exhibit a type II band alignment, leading to a charge transfer process accompanied by the transfer of spin-valley polarization and spontaneous formation of interlayer excitons. This unique band structure facilitates achieving a longer exciton lifetime and extended spin-valley polarization lifetime. However, the mechanism of charge transfer in type II TMD HSs is not fully comprehended. Here, the ultrafast charge transfer process is studied in MoSe2-WSe2 HS via valley-solved broadband pump-probe spectroscopy. Under the conditions of B-excitons of WSe2 and MoSe2 being resonantly pumped, a new charge transfer pathway through the higher energy state associated with the B-exciton is found. Meanwhile, the holes (electrons) in the WSe2 (MoSe2) layer of MoSe2-WSe2 HS produce obvious spin-valley polarization even under the condition of B-exciton of WSe2 (MoSe2) being resonantly pumped, and the lifetime can reach tens of ps, which is in stark contrast to the absence of A-exciton spin-valley polarization in monolayer WSe2 (MoSe2) under the same pumping condition. The results deepen the insight into the charge transfer process in type II TMD HSs and show the great potential of TMD HSs in the application of spin-valley electronics devices.

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.

Atomically thin MXene/WSe2Schottky heterojunction towards enhanced photogenerated charge carrier

Two-dimensional materials garner increasing interest in next-generation electronics and optoelectronic devices due to their atomic-thin nature and distinctive physical properties. Building on these advances, we present the successful synthesis of a heterostructure composed of the semi-metallic Ti3C2-MXene and the semiconducting WSe2, in which the atomic layers are vertically aligned. The wet impregnation method effectively synthesizes an atomically thin Ti3C2-MXene/WSe2heterostructure characterized by atomic force microscopy, Raman and time-resolved photoluminescence (TRPL) analysis. In addition, the current-voltage characteristics at the heterostructure reveal the Schottky junction probed by the scanning tunnelling microscopy and the conductive atomic force microscopy tip. The Schottky heterojunction also exhibits enhanced photocatalytic properties by improving the photogenerated charge carriers and inhibiting recombination. This work demonstrates the unique 2D-2D Ti3C2-MXene/WSe2vertical heterojunction possesses superior photon trapping ability and can efficiently transport photogenerated charge carriers to the reaction sites to enhance photocatalysis performance.

Annealing temperature effects on monolayer WS2-veiled Ag nanoparticle array for surface catalytic reaction

Plasmonic-WS2 hybrids have attracted widespread interest for plasmon driven catalytic reactions. In this work, a Ag nanoparticles (NPs)/WS2 hybrid was fabricated by utilizing a one-step anodized Al template-assisted vacuum thermal evaporation technique and wet transfer method. To optimize the catalytic performance, the morphological evolution and corresponding changes in the catalytic properties of the Ag NPs/WS2 hybrid at different thermal annealing temperatures were investigated. It was found that the surface-enhanced Raman scattering (SERS) and catalytic activity of the hybrid were optimized by tuning the annealing temperature, with the optimal SERS and catalytic properties observed at 290 °C. These results may open new avenues for improving the efficiency and expanding the research field of plasmon-driven reactions.

Recent progress of exciton transport in two-dimensional semiconductors

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Probing the interlayer excitation dynamics in WS2/WSe2 heterostructures with broadly tunable pump and probe energies

van der Waals heterostructures based on transition metal dichalcogenides (TMDs) provide a fascinating platform for exploring new physical phenomena and novel optoelectronic functionalities. Revealing the energy-dependence of photocarrier population dynamics in heterostructures is key for developing optoelectronic or valleytronic devices. Here, the broadband transient dynamics of interlayer excitation of a nearly-aligned WS2/WSe2 heterostructure is investigated by using energy-dependent pump-probe spectroscopy at cryogenic temperatures. Interestingly, WS2/WSe2 interlayer excitation, herein comprising a mixture of intra- and inter-layer excitons, exhibits largely constant lifetimes of a few hundred picoseconds across a broad energy range, in stark contrast to the salient energy-dependent dynamics of intralayer excitons in monolayer WSe2. While the PL emission of the WS2/WSe2 heterostructure is found to be strongly affected by electrostatic doping, the lifetimes of interlayer excitation show negligible changes. Our work elaborates the signatures of ultrafast dynamics introduced by intra- and interlayer co-existing excitonic species and enriches the understanding of interlayer couplings in van der Waals heterostructures.

Tuning and exploiting interlayer coupling in two-dimensional van der Waals heterostructures

Two-dimensional (2D) layered materials can stack into new material systems, with van der Waals (vdW) interaction between the adjacent constituent layers. This stacking process of 2D atomic layers creates a new degree of freedom-interlayer interface between two adjacent layers-that can be independently studied and tuned from the intralayer degree of freedom. In such heterostructures (HSs), the physical properties are largely determined by the vdW interaction between the individual layers,i.e.interlayer coupling, which can be effectively tuned by a number of means. In this review, we summarize and discuss a number of such approaches, including stacking order, electric field, intercalation, and pressure, with both their experimental demonstrations and theoretical predictions. A comprehensive overview of the modulation on structural, optical, electrical, and magnetic properties by these four approaches are also presented. We conclude this review by discussing several prospective research directions in 2D HSs field, including fundamental physics study, property tuning techniques, and future applications.

Probing the Twist-Controlled Interlayer Coupling in Artificially Stacked Transition Metal Dichalcogenide Bilayers by Second-Harmonic Generation

Interlayer coupling plays a critical role in the electronic band structures and optoelectronic properties of van der Waals (vdW) materials and heterostructures. Here, we utilize optical second-harmonic generation (SHG) measurements to probe the twist-controlled interlayer coupling in artificially stacked WSe2/WSe2 homobilayers and WSe2/WS2 and WSe2/MoS2 heterobilayers with a postannealing procedure. In the large angle twisted WSe2/WSe2 and WSe2/WS2, the angular dependence of the SHG intensity follows the interference relations up to angles above 10°. For lower angles, the SHG is significantly suppressed. Furthermore, for the twisted WSe2/MoS2 the SHG intensity largely deviates from the coherent superposition model and shows consistent quenching for all the stacking angles. The suppressed SHG in twisted transition metal dichalcogenide (TMDC) bilayers is predominantly attributed to the interlayer coupling between the two adjacent monolayers. The evolution of the interlayer Raman mode in WSe2 demonstrates that the interlayer coupling in the twisted WSe2/WSe2 and WSe2/WS2 is highly angle-dependent. Alternatively, the interlayer coupling generally exists in the twisted WSe2/MoS2, regardless of the different angles. The interlayer coupling is further confirmed by the quenching and red-shift of the photoluminescence of WSe2 in the twisted TMDC bilayers. Combined with density functional theory calculations, we reveal that the stacking-angle-modulated interlayer coupling originates from the variation of the interlayer spacing and the binding energy in the twisted TMDC bilayers.

Unraveling the Effect of Stacking Configurations on Charge Transfer in WS2 and Organic Semiconductor Heterojunctions

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals (vdW) heterostructures constructed of inorganic nanostructures and organic materials. However, the effect of molecular stacking configurations on charge transfer dynamics is less understood. In this study, we demonstrated the tunability of interfacial charge separation in a type-II heterojunction between monolayer (ML) WS2 and an organic semiconducting molecule [2-(3″',4'-dimethyl-[2,2':5',2':5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium halide (4Tm)] by rational design of relative stacking configurations. The assembly between ML-WS2 and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a self-aggregation state. In contrast, a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice. The face-to-face stacking was proved to be more favorable for hole transfer from WS2 to 4Tm and led to interlayer excitons (IEs) emission. Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs. On the other hand, the face-to-edge stacking resulted in much slower hole transfer without formation of IEs. This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS2, leading to the formation of negative trions. These investigations offer important fundamental insights into the charge transfer processes at organic-inorganic interfaces.

High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenides

van der Waals (vdW) layered materials have attracted much attention because their physical properties can be controlled by varying the twist angle and layer composition. However, such twisted vdW assemblies are often prepared using mechanically exfoliated monolayer flakes with unintended shapes through a time-consuming search for such materials. Here, we report the rapid and dry fabrication of twisted multilayers using chemical vapor deposition (CVD) grown transition metal chalcogenide (TMDC) monolayers. By improving the adhesion of an acrylic resin stamp to the monolayers, the single crystals of various TMDC monolayers with desired grain size and density on a SiO2/Si substrate can be efficiently picked up. The present dry transfer process demonstrates the one-step fabrication of more than 100 twisted bilayers and the sequential stacking of a twisted 10-layer MoS2 single crystal. Furthermore, we also fabricated hBN-encapsulated TMDC monolayers and various twisted bilayers including MoSe2/MoS2, MoSe2/WSe2, and MoSe2/WS2. The interlayer interaction and quality of dry-transferred, CVD-grown TMDCs were characterized by using photoluminescence (PL), cathodoluminescence (CL) spectroscopy, and cross-sectional electron microscopy. The prominent PL peaks of interlayer excitons can be observed for MoSe2/MoS2 and MoSe2/WSe2 with small twist angles at room temperature. We also found that the optical spectra were locally modulated due to nanosized bubbles, which are formed by the presence of interface carbon impurities. The present findings indicate the widely applicable potential of the present method and enable an efficient search of the emergent optical and electrical properties of TMDC-based vdW heterostructures.

Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.

Twist-angle-dependent momentum-space direct and indirect interlayer excitons in WSe2/WS2 heterostructure

Interlayer excitons (ILEs) in the van der Waals (vdW) heterostructures of type-II band alignment transition metal dichalcogenides (TMDCs) have attracted significant interest owing to their unique exciton properties and potential in quantum information applications. However, the new dimension that emerges with the stacking of structures with a twist angle leads to a more complex fine structure of ILEs, presenting both an opportunity and a challenge for the regulation of the interlayer excitons. In this study, we report the evolution of interlayer excitons with the twist angle in the WSe₂/WS₂ heterostructure and identify the direct (indirect) interlayer excitons by combining photoluminescence (PL) and density functional theory (DFT) calculations. Two interlayer excitons with opposite circular polarization assigned to the different transition paths of K-K and Q-K were observed. The nature of the direct (indirect) interlayer exciton was confirmed by circular polarization PL measurement, excitation power-dependent PL measurement and DFT calculations. Furthermore, by applying an external electric field to regulate the band structure of the WSe₂/WS₂ heterostructure and control the transition path of the interlayer excitons, we could successfully realize the regulation of interlayer exciton emission. This study provides more evidence for the twist-angle-based control of heterostructure properties.

Controllable Synthesis and Charge Density Wave Phase Transitions of Two-Dimensional 1T-TaS2 Crystals

1T-TaS₂ has attracted much attention recently due to its abundant charge density wave phases. In this work, high-quality two-dimensional 1T-TaS₂ crystals were successfully synthesized by a chemical vapor deposition method with controllable layer numbers, confirmed by the structural characterization. Based on the as-grown samples, their thickness-dependency nearly commensurate charge density wave/commensurate charge density wave phase transitions was revealed by the combination of the temperature-dependent resistance measurements and Raman spectra. The phase transition temperature increased with increasing thickness, but no apparent phase transition was found on the 2~3 nm thick crystals from temperature-dependent Raman spectra. The transition hysteresis loops due to temperature-dependent resistance changes of 1T-TaS₂ can be used for memory devices and oscillators, making 1T-TaS₂ a promising material for various electronic applications.

The effects of intercalated environmental gas molecules on carrier dynamics in WSe2/WS2 heterostructures

Effective tuning of carrier dynamics in two-dimensional (2D) materials is significant for multi-scene device applications. Using first-principles and ab initio nonadiabatic molecular dynamics calculations, the kinetics of O₂, H₂O, and N₂ intercalation into 2D WSe₂/WS₂ van der Waals heterostructures and its effect on carrier dynamics have been comprehensively explored. It is found that the O₂ molecule prefers to dissociate into atomic O atoms spontaneously after intercalation of WSe₂/WS₂ heterostructures, whereas H₂O and N₂ molecules remain intact. O₂ intercalation significantly speeds up the electron separation process, while H₂O intercalation largely speeds up the hole separation process. The lifetime of excited carriers can be prolonged by O₂ or H₂O or N₂ intercalations. These intriguing phenomena can be attributed to the effect of interlayer coupling, and the underlying physical mechanism for tuning the carrier dynamics is fully discussed. Our results provide useful guidance for the experimental design of 2D heterostructures for optoelectronic applications in photocatalysts and solar energy cells.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer

Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe₂/WS₂ monolayers. Following the selective excitation of WSe₂, we measure the concurrent heating of both WSe₂ and WS₂ on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.

Visualizing Large Facet-Dependent Electronic Tuning in Monolayer WSe2 on Au Surfaces

Two-dimensional transition metal dichalcogenides (TMDs) have shown great importance in the development of novel ultrathin optoelectronic devices owing to their exceptional electronic and photonic properties. Effectively tuning their electronic band structures is not only desired in electronics applications but also can facilitate more novel properties. In this work, we demonstrate that large electronic tuning on a WSe₂ monolayer can be realized by different facets of a Au-foil substrate, forming in-plane p-n junctions with remarkable built-in electric fields. This facet-dependent tuning effect is directly visualized by using scanning tunneling microscopy and differential conductance (dI/dV) spectroscopy. First-principles calculations reveal that the atomic arrangement of the Au facet effectively changes the interfacial coupling and charge transfer, leading to different magnitudes of charge doping in WSe₂. Our study would be beneficial for future customized fabrication of TMD-junction devices via facet-specific construction on the substrate.

Nano-optical Visualization of Interlayer Interactions in WSe2/WS2 Heterostructures

The interplay between excitons and phonons governs the optical and electronic properties of transition metal dichalcogenides (TMDs). Even though a number of linear and nonlinear optical-, electron-, and photoelectron-based approaches have been developed and/or adopted to characterize excitons and phonons in single/few-layer TMDs and their heterostructures, no existing method is capable of directly probing ultralow-frequency and interlayer phonons on the nanoscale. To this end, we developed ultralow-frequency tip-enhanced Raman spectroscopy, which allows spectrally and spatially resolved chemical and structural nanoimaging of WSe₂/WS₂ heterostructures. In this work, we apply this method to analyze phonons in nanobubbles that are sustained in these heterobilayers. Our method is capable of directly probing interlayer (de)coupling using our novel structurally sensitive nano-optical probe and the interplay between excitons and interlayer/intralayer phonons through correlation analysis of the recorded spectral images.

Observation of Ultrafast Interfacial Exciton Formation and Relaxation in Graphene/MoS2 Heterostructure

Heterostructures constructed from graphene and transition metal dichalcogenides (TMDs) have established a new platform for optoelectronic applications. After a large number of studies, one intriguing debate is the existence of the interfacial exciton in graphene/TMDs. Hereby, by combined optical pump-terahertz probe spectroscopy and transient absorption spectroscopy, we report the observation of the interfacial exciton in graphene/MoS2 heterostructure. With the photon energy well below the band gap of monolayer MoS2, the hot electrons of graphene are transferred to MoS2 within 0.5 ps; subsequently, the relaxation of the holes in graphene and electrons in MoS2 shows an identical time scale of 15-18 ps, which manifests the formation and relaxation of the interfacial exciton in the heterostructure following photoexcitation. Moreover, a model of the carrier heating and photogating effect in graphene is proposed to estimate the amount of transferred charge, which agrees well with the experimental results. Our study provides insights into the dynamics of graphene-based heterostructure interfacial non-equilibrium carriers.

Visualization of Band Shifting and Interlayer Coupling in WxMo1-xS2 Alloys Using Near-Field Broadband Absorption Microscopy

Beyond-diffraction-limit optical absorption spectroscopy provides in-depth information on the graded band structures of composition-spread and stacked two-dimensional materials, in which direct/indirect bandgap, interlayer coupling, and defects significantly modify their optoelectronic functionalities such as photoluminescence efficiency. We here visualize the spatially varying band structure of monolayer and bilayer transition metal dichalcogenide alloys by using near-field broadband absorption microscopy. The near-field spectral and spatial information manifests the excitonic band shift that results from the interplay of composition spreading and interlayer coupling. These results enable us to identify, notably, the top layer of the bilayer alloy as pure WS₂. We also use the aberration-free near-field transmission images to demarcate the exact boundaries of alloyed and pure transition metal dichalcogenides. This technology can offer valuable insights on various layered structures in the era of "stacking science" in the quest of quantum optoelectronic devices.

Radiative pattern of intralayer and interlayer excitons in two-dimensional WS2/WSe2 heterostructure

Two-dimensional (2D) heterostructures (HS) formed by transition-metal dichalcogenide (TMDC) monolayers offer a unique platform for the study of intralayer and interlayer excitons as well as moiré-pattern-induced features. Particularly, the dipolar charge-transfer exciton comprising an electron and a hole, which are confined to separate layers of 2D semiconductors and Coulomb-bound across the heterojunction interface, has drawn considerable attention in the research community. On the one hand, it bears significance for optoelectronic devices, e.g. in terms of charge carrier extraction from photovoltaic devices. On the other hand, its spatially indirect nature and correspondingly high longevity among excitons as well as its out-of-plane dipole orientation render it attractive for excitonic Bose–Einstein condensation studies, which address collective coherence effects, and for photonic integration schemes with TMDCs. Here, we demonstrate the interlayer excitons’ out-of-plane dipole orientation through angle-resolved spectroscopy of the HS photoluminescence at cryogenic temperatures, employing a tungsten-based TMDC HS. Within the measurable light cone, the directly-obtained radiation profile of this species clearly resembles that of an in-plane emitter which deviates from that of the intralayer bright excitons as well as the other excitonic HS features recently attributed to artificial superlattices formed by moiré patterns.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Janus Monolayers for Ultrafast and Directional Charge Transfer in Transition Metal Dichalcogenide Heterostructures

Charge transfer properties of van der Waals heterostructures formed by Janus and regular transition metal dichalcogenide monolayers are studied by time-resolved pump-probe measurements and photoluminescence spectroscopy. Measurements of electron and hole transfer in three heterostructures with atomic layer sequences of S-W-Se/S-W-S, Se-W-S/S-W-S, and S-W-Se/Se-W-Se reveal that charge transfer from regular to Janus monolayers is ultrafast regardless of the direction of the built-in electric field of the Janus monolayer (Janus field). However, the charge transfer from Janus to regular layers is directional and controlled by the Janus field. When the current direction is along the field, the charge transfer is ultrafast and efficient, while the field blocks the charge transfer with an opposite charge current direction. The transferred carriers form interlayer excitons with extended lifetimes compared to the intralayer excitons. The demonstrated ultrafast and directional charge transfer between Janus and regular TMD layers shows that the Janus structures can be used to make 2D heterostructures with efficient and directional charge transfer properties.

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe₂ and ReS₂, nonradiative energy transfer (ET) from higher to lower work function material (ReS₂ to MoSe₂) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe₂ photoluminescence (PL) enhancement as compared to the MoSe₂ area alone. In a completely encapsulated sample, the HS PL emission further increases by a factor of 6.4. After completely blocking the CT process, more than 1 order of magnitude higher MoSe₂ PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS₂ and WSe₂, CT dominates over ET, resulting in a severely quenched WSe₂ PL. This study not only provides significant insight into the competing interlayer processes but also shows an innovative way to increase the PL emission intensity of the desired TMD material using the ET process by carefully choosing the right material combination for HS.

Scalable Moiré Lattice with Oriented TMD Monolayers

Moiré lattice in artificially stacked monolayers of two-dimensional (2D) materials effectively modulates the electronic structures of materials, which is widely highlighted. Formation of the electronic Moiré superlattice promises the prospect of uniformity among different moiré cells across the lattice, enabling a new platform for novel properties, such as unconventional superconductivity, and scalable quantum emitters. Recently, epitaxial growth of the monolayer transition metal dichalcogenide (TMD) is achieved on the sapphire substrate by chemical vapor deposition (CVD) to realize scalable growth of highly-oriented monolayers. However, fabrication of the scalable Moiré lattice remains challenging due to the lack of essential manipulation of the well-aligned monolayers for clean interface quality and precise twisting angle control. Here, scalable and highly-oriented monolayers of TMD are realized on the sapphire substrates by using the customized CVD process. Controlled growth of the epitaxial monolayers is achieved by promoting the rotation of the nuclei-like domains in the initial growth stage, enabling aligned domains for further grain growth in the steady-state stage. A full coverage and distribution of the highly-oriented domains are verified by second-harmonic generation (SHG) microscopy. By developing the method for clean monolayer manipulation, hetero-stacked bilayer (epi-WS₂/epi-MoS₂) is fabricated with the specific angular alignment of the two major oriented monolayers at the edge direction of 0°/ ± 60°. On account of the optimization for scalable Moiré lattice with a high-quality interface, the observation of interlayer exciton at low temperature illustrates the feasibility of scalable Moiré superlattice based on the oriented monolayers.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂ . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Strain regulated interlayer coupling in WSe2/WS2heterobilayer

Strain engineering can effectively modify the materials lattice parameters at atomic scale, hence it has become an efficient method for tuning the physical properties of two-dimensional (2D) materials. The study of the strain regulated interlayer coupling is deserved for different kinds of heterostructures. Here, we systematically studied the strain engineering of WSe₂/WS₂heterostructures as well as their constituent monolayers. The measured Raman and photoluminescence spectra demonstrate that the strain can evidently modulate the phonon energy and exciton emission of monolayer WSe₂and WS₂as well as the WSe₂/WS₂heterostructures. The tensile strain can tune the electronic band structure of WSe₂/WS₂heterostructure, as well as enhance the interlayer coupling. It is further revealed that the photoluminescence intensity ratio of WS₂to WSe₂in our WSe₂/WS₂heterobilayer increases monotonically with tensile strain. These findings can broaden the understanding and practical application of strain engineering in 2D materials with nanometer-scale resolution.

First-principles study of borophene/phosphorene heterojunction as anode material for lithium-ion batteries

It is urgent to explore high-capacity and efficient anode materials for rechargeable lithium-ion batteries. For borophene and phosphorene, two configurations are considered to form a heterojunction: twist angles of 0° (I) and 90° (II). There is a less degree of mismatch and larger formation energy in the formation of a B/P heterojunction, implying that borophene and phosphorene form the stable heterojunction. The heterojunctions of these two configurations demonstrate good conductivity, and the electrons near the Fermi level are mainly provided by borophene. Very importantly, the low energy barrier for interlayer migration of Li is observed in configuration I (0.14eV) and II (0.06 eV), and the migration of Li on the borophene and phosphorene side of the heterojunction still maintains its original energy barrier in bare monolayer. Moreover, the two configurations show the theoretical capacity as high as 738.69 and 721.86 mA h g^-1, respectively, which is comparable to bare phosphorene. Furthermore, compared with bare phosphorene, the average voltage is greatly reduced after the formation of heterojunction. Hence, the overall electrochemical properties of the B/P heterojunction have been enhanced by combining the advantages of the individual phosphorene and borophene monolayers, which guarantees the B/P heterojunction as a good candidate for the anode material used in Li-ion batteries.

Anomalously polarised emission from a MoS2/WS2 heterostructure

We report circularly polarised emission, with helicity opposite to the optical excitation, from a van der Waals heterostructure (HS) consisting of a monolayer MoS₂ and three-layer WS₂. Selective excitation of the MoS₂ layer confirms that this cross-polarized emission is due to the charge transfer from the WS₂ layers to the MoS₂ layer. We propose that the high levels of n-doping in the constituent layers due to sulphur vacancies and defects give rise to an enhanced transition rate of electrons from the k valley of WS₂ to the k' valley of MoS₂, which leads to the emission, counter polarized to the excitation. Simulations based on the rate equation model support this conclusion.

Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump-Probe Photoluminescence Spectroscopy

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump-probe photoluminescence intensity of monolayer WS₂ in the WS₂/WSe₂ heterostructures. During thermal annealing of WS₂/WSe₂, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10^-3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at approximately 1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental PL spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation (GW-BSE) approach indicates that both the electron and hole of the exciton are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the usually called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insights into understanding the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2)

The emergence of rapidly expanding infectious diseases such as coronavirus (COVID-19) demands effective biosensors that can promptly detect and recognize the pathogens. Field-effect transistors based on semiconducting two-dimensional (2D) materials (2D-FETs) have been identified as potential candidates for rapid and label-free sensing applications. This is because any perturbation of such atomically thin 2D channels can significantly impact their electronic transport properties. Here, we report the use of FET based on semiconducting transition metal dichalcogenide (TMDC) WSe2 as a promising biosensor for the rapid and sensitive detection of SARS-CoV-2 in vitro. The sensor is created by functionalizing the WSe2 monolayers with a monoclonal antibody against the SARS-CoV-2 spike protein and exhibits a detection limit of down to 25 fg/μL in 0.01X phosphate-buffered saline (PBS). Comprehensive theoretical and experimental studies, including density functional theory, atomic force microscopy, Raman and photoluminescence spectroscopies, and electronic transport properties, were performed to characterize and explain the device performance. The results demonstrate that TMDC-based 2D-FETs can potentially serve as sensitive and selective biosensors for the rapid detection of infectious diseases.

Dissecting Interlayer Hole and Electron Transfer in Transition Metal Dichalcogenide Heterostructures via Two-Dimensional Electronic Spectroscopy

Monolayer transition metal dichalcogenides (ML-TMDs) are two-dimensional semiconductors that stack to form heterostructures (HSs) with tailored electronic and optical properties. TMD/TMD-HSs like WS2/MoS2 have type II band alignment and form long-lived (nanosecond) interlayer excitons following sub-100 fs interlayer charge transfer (ICT) from the photoexcited intralayer exciton. While many studies have demonstrated the ultrafast nature of ICT processes, we still lack a clear physical understanding of ICT due to the trade-off between temporal and frequency resolution in conventional transient absorption spectroscopy. Here, we perform two-dimensional electronic spectroscopy (2DES), a method with both high frequency and temporal resolution, on a large-area WS2/MoS2 HS where we unambiguously time resolve both interlayer hole and electron transfer with 34 ± 14 and 69 ± 9 fs time constants, respectively. We simultaneously resolve additional optoelectronic processes including band gap renormalization and intralayer exciton coupling. This study demonstrates the advantages of 2DES in comprehensively resolving ultrafast processes in TMD-HS, including ICT.

Tuning Valleys and Wave Functions of van der Waals Heterostructures by Varying the Number of Layers: A First-Principles Study

In van der Waals heterostructures of 2D transition-metal dichalcogenides (2D TMDCs) electron and hole states are spatially localized in different layers forming long-lived interlayer excitons. Here, the influence of additional electron or hole layers on the electronic properties of a MoS₂ /WSe₂ heterobilayer (HBL), which is a direct bandgap material, is investigated from first principles. Additional layers modify the interlayer hybridization, mostly affecting the quasiparticle energy and real-space extend of hole states at the Γ and electron states at the Q valleys. For a sufficient number of additional layers, the band edges move from K to Q or Γ, respectively. Adding electron layers to the HBL leads to more delocalized K and Q states, while Γ states do not extend much beyond the HBL, even when more hole layers are added. These results suggest a simple and yet powerful way to tune band edges and the real-space extent of the electron and hole wave functions in TMDC heterostructures, potentially affecting strongly the lifetime and dynamics of interlayer excitons.

Understanding Substrate-Guided Assembly in van der Waals Epitaxy by in Situ Laser Crystallization within a Transmission Electron Microscope

Understanding the bottom-up synthesis of atomically thin two-dimensional (2D) crystals and heterostructures is important for the development of new processing strategies to assemble 2D heterostructures with desired functional properties. Here, we utilize in situ laser-heating within a transmission electron microscope (TEM) to understand the stages of crystallization and coalescence of amorphous precursors deposited by pulsed laser deposition (PLD) as they are guided by 2D crystalline substrates into van der Waals (vdW) epitaxial heterostructures. Amorphous clusters of tungsten selenide were deposited by PLD at room temperature onto graphene or MoSe₂ monolayer crystals that were suspended on TEM grids. The precursors were then stepwise evolved into 2D heterostructures with pulsed laser heating treatments within the TEM. The lattice-matching provided by the MoSe₂ substrate is shown to guide the formation of large-domain, heteroepitaxial vdW WSe₂/MoSe₂ bilayers both during the crystallization process via direct templating and after crystallization by assisting the coalescence of nanosized domains through nonclassical particle attachment processes including domain rotation and grain boundary migration. The favorable energetics for domain rotation induced by lattice matching with the substrate were understood from first-principles calculations. These in situ TEM studies of pulsed laser-driven nonequilibrium crystallization phenomena represent a transformational tool for the rapid exploration of synthesis and processing pathways that may occur on extremely different length and time scales and lend insight into the growth of 2D crystals by PLD and laser crystallization.

Controllable Epitaxial Growth of Large-Area MoS2 /WS2 Vertical Heterostructures by Confined-Space Chemical Vapor Deposition

The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS₂ /WS₂ heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H₂ flow on and off: MoS₂ /WS₂ heterostructures with multiple WS₂ domains can be achieved without introducing the H₂ flow due to the numerous nucleation centers on the bottom MoS₂ monolayer during the transition stage between the MoS₂ and WS₂ monolayer growth. In contrast, isolated MoS₂ /WS₂ heterostructures with single WS₂ domain can be obtained with introducing the H₂ flow due to the reduced nucleation centers on the bottom MoS₂ monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS₂ /WS₂ heterostructures feature high quality. The photodetectors based on the isolated MoS₂ /WS₂ heterostructures exhibit a high responsivity of 68 mA W^-1 and a short response time of 35 ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS₂ /WS₂ heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.

Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures

Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) generally possess a type-II band alignment that facilitates the formation of interlayer excitons between constituent monolayers. Manipulation of the interlayer excitons in TMD vdW heterostructures holds great promise for the development of excitonic integrated circuits that serve as the counterpart of electronic integrated circuits, which allows the photons and excitons to transform into each other and thus bridges optical communication and signal processing at the integrated circuit. As a consequence, numerous studies have been carried out to obtain deep insight into the physical properties of interlayer excitons, including revealing their ultrafast formation, long population recombination lifetimes, and intriguing spin-valley dynamics. These outstanding properties ensure interlayer excitons with good transport characteristics, and may pave the way for their potential applications in efficient excitonic devices based on TMD vdW heterostructures. At present, a systematic and comprehensive overview of interlayer exciton formation, relaxation, transport, and potential applications is still lacking. In this review, we give a comprehensive description and discussion of these frontier topics for interlayer excitons in TMD vdW heterostructures to provide valuable guidance for researchers in this field.

Stacking-Engineered Heterostructures in Transition Metal Dichalcogenides

The layer-by-layer assembly of 2D transition metal dichalcogenide monolayer blocks to form a 3D stack, with a precisely chosen sequence/angle, is the newest development for these materials. In this way, one can create "van der Waals heterostructures (HSs)," opening up a new realm of materials engineering and novel devices with designed functionalities. Herein, a detailed systematic review of transition metal dichalcogenide stacking-engineered heterostructures, from controllable fabrication to typical characterization, and stacking-correlated physical behaviors is presented. Furthermore, recent advances in stacking design, such as stacking sequence, twist angles, and moiré superlattice heterojunctions, are also comprehensively summarized. Finally, the remaining challenges and possible strategies for using stacking engineering to tune the properties of 2D materials are also outlined.

Thickness-Dependent Interlayer Charge Transfer in MoSe2/MoS2 Heterostructures Studied by Femtosecond Transient Absorption Measurements

We report observations of a strong thickness dependence for charge transfer (CT) from MoSe₂ to MoS₂, as evidenced by transient absorption measurements. By time-resolving CT from MoSe₂ monolayers (1Ls) to MoS₂ flakes of varying thicknesses, including 1L, bilayer (2L), and trilayer (3L), we find that the CT time is several picoseconds in the 1L-MoSe₂/3L-MoS₂ heterostructure, which is much longer than that of 1L-MoSe₂/1L-MoS₂ and 1L-MoSe₂/2L-MoS₂ heterostructures. In addition, the recombination lifetime of the interlayer excitons in the 1L/3L heterostructure is several times longer than that of 1L/1L and 1L/2L heterostructures, reaching 800 ps. Furthermore, we show that a prepulse can reduce the CT time and enhance the interlayer exciton recombination in the 1L/3L heterostructure. These findings illustrate that layer thickness can be an important parameter to control the CT property of van der Waals heterostructures. These experimental results also provide important information for further refining the understanding of the physical mechanisms of CT in van der Waals heterostructures.

A two-step chemical vapor deposition process for the growth of continuous vertical heterostructure WSe2/h-BN and its optical properties

Direct growth of WSe₂ on hexagonal boron nitride via a two step CVD process.

Mechanisms and Applications of Steady-State Photoluminescence Spectroscopy in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors exhibit many important structural and optoelectronic properties, such as strong light-matter interactions, direct bandgaps tunable from visible to near-infrared regions, flexibility and atomic thickness, quantum-confinement effects, valley polarization possibilities, and so on. Therefore, they are regarded as a very promising class of materials for next-generation state-of-the-art nano/micro optoelectronic devices. To explore different applications and device structures based on 2D TMDs, intrinsic material properties, their relationships, and evolutions with fabrication parameters need to be deeply understood, very often through a combination of various characterization techniques. Among them, steady-state photoluminescence (PL) spectroscopy has been extensively employed. This class of techniques is fast, contactless, and nondestructive and can provide very high spatial resolution. Therefore, it can be used to obtain optoelectronic properties from samples of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. In this article, the mechanism and applications of steady-state PL spectroscopy in 2D TMDs are reviewed. The first part of this review details the physics of PL phenomena in semiconductors and common techniques to acquire and analyze PL spectra. The second part introduces various applications of PL spectroscopy in 2D TMDs. Finally, a broader perspective is discussed to highlight some limitations and untapped opportunities of PL spectroscopy in characterizing 2D TMDs.

Manipulation of Valley Pseudospin by Selective Spin Injection in Chiral Two-Dimensional Perovskite/Monolayer Transition Metal Dichalcogenide Heterostructures

Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted great interest in spintronics and valleytronics due to the spin-valley locking effect. To efficiently control and manipulate the valley pseudospin is of paramount importance for valley-based electronics and optoelectronics. A variety of strategies have been developed to address the valley pseudospin including optical, electrical, and magnetic methods; nonetheless, they involve either below liquid-nitrogen temperature or an external magnetic field, which increases the cost and complexity of the devices. Here, we report a straightforward way to manipulate valley polarization in monolayer TMDs via selective spin injection in chiral 2D perovskite/monolayer TMD (e.g., MoS₂ and WSe₂) van der Waals heterostructures without requiring an external magnetic field or specially designed device structures. We show the dangling-bond-free vdW interface can allow an impressive average spin injection efficiency of 78% to produce persistent valley polarization in monolayer MoS₂ (WSe₂) over 10% from liquid-nitrogen temperature to above 200 K. We attribute the valley polarization of monolayer MoS₂ (WSe₂) to selective spin injection from chiral 2D perovskites, which can effectively introduce population imbalance between valleys in monolayer MoS₂ (WSe₂). Our findings provide an alternative strategy to manipulate the valley polarization in TMDs without requiring circularly polarized light excitation, below liquid-nitrogen temperature, or external magnetic field, and thus would promote the development of perovskite-based spintronic and valleytronic devices.

Efficient hot-electron extraction in two-dimensional semiconductor heterostructures by ultrafast resonant transfer

Energy loss from hot-carrier cooling sets the thermodynamic limit for the photon-to-power conversion efficiency in optoelectronic applications. Efficient hot-electron extraction before cooling could reduce the energy loss and leads to efficient next generation devices, which, unfortunately, is challenging to achieve in conventional semiconductors. In this work, we explore hot-electron transfer in two-dimensional (2D) layered semiconductor heterostructures, which have shown great potential for exploring new physics and optoelectronic applications. Using broadband micro-area ultrafast spectroscopy, we firmly established a type I band alignment in the WS₂-MoTe₂ heterostructure and ultrafast (∼60 fs) hot-electron transfer from photoexcited MoTe₂ to WS₂. The hot-electron transfer efficiency increases with excitation energy or excess energy as a result of a more favorable continuous competition between resonant electron transfer and cooling, reaching 90% for hot electrons with 0.3 eV excess energy. This study reveals exciting opportunities of designing extremely thin absorber and hot-carrier devices using 2D semiconductors and also sheds important light on the photoinduced interfacial process including charge transfer and generation in 2D heterostructures and optoelectronic devices.

Selective Synthesis of Bi2Te3/WS2 Heterostructures with Strong Interlayer Coupling

The vertical integration of atomically thin-layered materials to create van der Waals heterostructures (vdWHs) has been proposed as a method to design nanostructures with emergent properties. In this work, epitaxial Bi2Te3/WS2 vdWHs are synthesized via a two-step vapor deposition process. It is calculated that the vdWH has an indirect band gap with a valence band edge that bridges the vdW gap, resulting in a quenched photoluminescence (PL) from the WS2 monolayer, reduced intensity of its resonance Raman vibrational peaks, improved vertical charge transport, and a decrease in the intensity of second harmonic generation (SHG). Furthermore, it is observed that induced defects strongly influence the nucleation and growth of vdWHs. By creating point defects in WS2 monolayers, it is shown that the growth of Bi2Te3 platelets can be patterned. This work offers important insights into the synthesis, defect engineering, and moiré engineering of an emerging class of two-dimensional (2D) heterostructures.