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Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions, and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to identify the biexciton state in BN-encapsulated single-layer WSe 2 . The biexciton state only exists in charge-neutral WSe 2 , which is realized through the control of efficient electrostatic gating. In the lightly electron-doped WSe 2 , one free electron binds to a biexciton and forms the trion–exciton complex. Improved understanding of the biexciton and trion–exciton complexes paves the way for exploiting the many-body physics in TMDs for novel optoelectronics applications. Owing to strong Coulomb interactions, atomically thin transition metal dichalcogenides host strongly bound excitonic complexes. Here, the authors report charge-neutral biexciton and negatively charged trion-exciton complexes in hBN encapsulated monolayer WSe 2 by employing low-temperature photoluminescence spectroscopy.

Heterobilayers of transition metal dichalcogenides (TMDCs) can form a moiré superlattice with flat minibands, which enables strong electron interaction and leads to various fascinating correlated states. These heterobilayers also host interlayer excitons in a type-II band alignment, in which optically excited electrons and holes reside on different layers but remain bound by the Coulomb interaction. Here we explore the unique setting of interlayer excitons interacting with strongly correlated electrons, and we show that the photoluminescence (PL) of interlayer excitons sensitively signals the onset of various correlated insulating states as the band filling is varied. When the system is in one of such states, the PL of interlayer excitons is relatively amplified at increased optical excitation power due to reduced mobility, and the valley polarization of interlayer excitons is enhanced. The moiré superlattice of the TMDC heterobilayer presents an exciting platform to engineer interlayer excitons through the periodic correlated electron states. Heterobilayers of transition metal dichalcogenides host moiré superlattices that give rise to strong electron interactions. Here, the authors study the photoluminescence from interlayer excitons in a WS 2 /WSe 2 heterobilayer to reveal the onset of various correlated insulating states.

Daily travel is an important means for everyone to obtain the right to development. With the development of the economy and the progress of the times, the equalization of public infrastructure has become an important concern. The accessibility and fairness of transportation have become a hot topic of research in various fields. To promote transport equity and formulate more reasonable transport planning and policies, this paper takes the Kunshan city as the research object, based on the mobile phone signaling data and the travel time consumption data from the application programming interface (API) of Gaode Map, using weighted average accessibility and the Theil index to investigate the accessibility and equity of public transport and car traffic in the Kunshan city. The study found that the accessibility of public transport is lower than that of car transport in the same research unit, but the equity of public transport is better than that of car transport, that is, the public transport is fair and the efficiency is neglected. In the same mode of transportation, equity presents a high four-week low distribution in the central urban area, and the spatial equity difference is mainly caused by the difference in accessibility levels between cell units. According to the research conclusions, it is recommended that Kunshan further optimize the spatial layout of public transportation infrastructure and adopt measures such as bus speed increase to achieve equity and efficiency and improve the competitiveness of public transportation.

The strong electron interactions in the minibands formed in moiré superlattices of van der Waals materials, such as twisted graphene and transition metal dichalcogenides, make such systems a fascinating platform with which to study strongly correlated states1–19. In most systems, the correlated states appear when the moiré lattice is filled by an integer number of electrons per moiré unit cell. Recently, correlated states at fractional fillings of 1/3 and 2/3 holes per moiré unit cell have been reported in the WS2/WSe2 hetero-bilayer, hinting at the long-range nature of the electron interaction16. Here we observe a series of correlated insulating states at fractional fillings of the moiré minibands on both electron- and hole-doped sides in angle-aligned WS2/WSe2 hetero-bilayers, with certain states persisting at temperatures up to 120 K. Simulations reveal that these insulating states correspond to ordering of electrons in the moiré lattice with a periodicity much larger than the moiré unit cell, indicating a surprisingly strong and long-range interaction beyond the nearest neighbours.Twisted bilayers of WS2 and WSe2 have correlated states that correspond to real-space ordering of the electrons on a length scale much longer than the moiré pattern.

Tungsten-based monolayer transition metal dichalcogenides host a long-lived “dark” exciton, an electron-hole pair in a spin-triplet configuration. The long lifetime and unique spin properties of the dark exciton provide exciting opportunities to explore light-matter interactions beyond electric dipole transitions. Here we demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe 2 . Gate and magnetic-field dependent PL measurements unveil a circularly-polarized replica peak located below the dark exciton by 21.6 meV, equal to E″ phonon energy from Se vibrations. First-principles calculations show that the exciton-phonon interaction selectively couples the spin-forbidden dark exciton to the intravalley spin-allowed bright exciton, permitting the simultaneous emission of a chiral phonon and a circularly-polarized photon. Our discovery and understanding of the phonon replica reveals a chirality dictated emission channel of the phonons and photons, unveiling a new route of manipulating valley-spin. The long lifetime and spin properties of dark excitons in atomically thin transition metal dichalcogenides offer opportunities to explore light-matter interactions beyond electric dipole transitions. Here, the authors demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe 2

Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g −1 @200 mA g −1 current density) and rate capability (~70 mAh g −1 @1000 mA g −1 ), while achieving capacity retention close to 100% after 400 charge−discharge steps. VS 2 is a promising cathode material for lithium-ion batteries, but is susceptible to Peierls distortion during (de)lithiation. Here the authors show that VS 2 cathodes can be stabilized by conformally coating them with a nanoscale TiS 2 protective layer, leading to impressive electrochemical performance.

The heterostructure of monolayer transition metal dichalcogenides (TMDCs) provides a unique platform to manipulate exciton dynamics. The ultrafast carrier transfer across the van der Waals interface of the TMDC hetero-bilayer can efficiently separate electrons and holes in the intralayer excitons with a type II alignment, but it will funnel excitons into one layer with a type I alignment. In this work, we demonstrate the reversible switch from exciton dissociation to exciton funneling in a MoSe 2 /WS 2 heterostructure, which manifests itself as the photoluminescence (PL) quenching to PL enhancement transition. This transition was realized through effectively controlling the quantum capacitance of both MoSe 2 and WS 2 layers with gating. PL excitation spectroscopy study unveils that PL enhancement arises from the blockage of the optically excited electron transfer from MoSe 2 to WS 2 . Our work demonstrates electrical control of photoexcited carrier transfer across the van der Waals interface, the understanding of which promises applications in quantum optoelectronics. The ultrafast carrier dynamics across the van der Waals interface of transition metal dichalcogenide heterostructures govern the formation and funnelling of excitons. Here, the authors demonstrate a reversible switch from exciton dissociation to exciton funnelling in a MoSe 2 /WS 2 heterostructure, which manifests itself as a photoluminescence quenching-to-enhancement transition.

The charge transfer between two layers of different two-dimensional materials occurs at a much faster speed than expected, holding promise for efficient optoelectronic devices. Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX 2 ), give rise to fascinating new phenomena 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . MX 2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because two-dimensional MX 2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light–matter interactions 2 , 3 , 11 . Theory predicts that many stacked MX 2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron–hole separation for light detection and harvesting 12 , 13 , 14 , 15 , 16 . Here, we report the first experimental observation of ultrafast charge transfer in photoexcited MoS 2 /WS 2 heterostructures using both photoluminescence mapping and femtosecond pump–probe spectroscopy. We show that hole transfer from the MoS 2 layer to the WS 2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled two-dimensional layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel two-dimensional devices for optoelectronics and light harvesting.

The enhanced Coulomb interaction in two dimensions leads to not only tightly bound excitons but also many-particle excitonic complexes: excitons interacting with other quasiparticles, which results in improved and even new exciton properties with better controls. Here, we summarize studies of excitonic complexes in monolayer transition metal dichalcogenides and their moiré heterojunctions, envisioning how to utilize them for exploring quantum many-body physics.

Two-dimensional moiré superlattices provide a highly tunable platform to study strongly correlated physics. In particular, the moiré superlattices of two-dimensional semiconductor heterojunctions have been shown to host tunable correlated electronic states such as a Mott insulator and generalized Wigner crystals 1 – 4 . Here we report the observation of an excitonic insulator 5 – 7 , a correlated state with strongly bound electrons and holes, in an angle-aligned monolayer WS 2 /bilayer WSe 2 moiré superlattice. The moiré coupling induces a flat miniband on the valence-band side only in the first WSe 2 layer interfacing WS 2 . The electrostatically introduced holes first fill this miniband and form a Mott insulator when the carrier density corresponds to one hole per moiré supercell. By applying a vertical electric field, we tune the valence band in the second WSe 2 layer to overlap with the moiré miniband in the first WSe 2 layer, realizing the coexistence of electrons and holes at equilibrium, which are bound as excitons due to a strong Coulomb interaction. We show that this new bound state is an excitonic insulator with a transition temperature as high as 90 K. Our study demonstrates a moiré system for the study of correlated many-body physics in two dimensions. Stacking monolayer WS 2 on top of bilayer WSe 2 creates conditions where electrons and holes can coexist in the structure. Their Coulomb interaction allows them to form bound pairs and hence an excitonic insulator state.