Fig. 1. (Color online) Features of lattice and band structures of TMD monolayers and heterostructures. (a) 2D hexagonal lattice representing monolayer TMDs.R_i are the vectors connecting nearest metal atoms. (b) Schematic drawing of valley-contrasting splitting at the K and K′ valley of the band structure at the band edges. (c) Spin and valley coupled optical selection rules. K (K′) valley couple toσ⁺ (σ⁻) circularly polarized light^[15]. Copyright 2012, American Physical Society. (d) Band structures calculated with exchange-correlation energy functions, and the corresponding Brillouin zone (the top right)^[31]. Copyright 2013, American Physical Society. (e) Calculated band-edge energies for various TMDs based on ab initio density functional theory calculation using the Perdew-Burke-Ernzerhof functional (blue) and G0W0 (pink), respectively^[32]. Copyright 2016, Institute of Physics.

Fig. 2. (Color online) Interlayer gap dependence of energy and charge transfer at the 2D interface. (a) The energies of the band-edge states as a function of the interlayer gap in MoS₂/WS₂ heterostructures^[41]. Copyright 2013, American Physical Society. (b, c) Schemes of Fӧrster and Dexter energy transfer mechanism (left), and schemes (right) of representative TMD heterobilayers showing the direction of Fӧrster and Dexter energy transfer^[45,46]. Copyright 2016, American Chemical Society (b) and 2019, American Chemical Society (c). (d) Scheme of WSe₂/WS₂ heterostructures with different BN intermediate layers. (e) Charge transfer kinetics for heterostructures with different BN layer thicknesses^[47]. Copyright 2020, American Chemical Society.

Fig. 3. (Color online) Twist-angle-dependent features of the 2D interface. (a) Twist-angle dependence of the average layer distance of the WS₂/WSe₂ heterobilayer based on AA-stacking (blue) and AB-stacking (red) configurations, calculated by dispersion-corrected DFT. (b) Calculated K–K (blue) and Γ–K (red) transition energies for AB-stacked heterobilayers with different twist angles^[56]. Copyright 2021, Oxford University Press. (c–e) Transient kinetics in the MoS₂ layer for WSe₂/MoS₂ heterobilayers with three different twist angles, including a single exponential rise (CT: charge transfer process) and a single exponential decay (CR: charge recombination process). (f) Interfacial charge transfer lifetime and charge recombination lifetime as a function of Δϕ (bottom axis) and momentum change (top axis) in WSe₂/MoS₂ heterojunctions. Inset shows scheme illustrating the twist angle in momentum space^[42]. Copyright 2017, American Chemical Society.

Fig. 4. (Color online) Enhanced emission by interfacial interaction in TMD heterostructures. (a, b) Förster energy transfer in MoSe₂/WS₂ heterobilayers, including photoluminescence excitation intensity map at 78 K where the color scale represents emission intensity (a) and photoluminescence spectra for MoSe₂ emission from different heterostructures excited in resonance with A exciton of WS₂ (W-A) at 2.00 eV at room temperature (b)^[45]. Copyright 2016, American Chemical Society. (c, d) Trion-mediated Förster energy transfer and optical gating effect in WS₂/hBN/MoSe₂ heterostructures, including the scheme of trion-mediated energy transfer and optical gating effect (c), and photoluminescence spectra at different positions of heterostructures (d)^[48]. Copyright 2020, American Chemical Society. (e, f) Dexter energy transfer in WSe₂/MoTe₂ heterostructures, including photoluminescence spectra of different domains (e) and schematic depiction of near-unity energy transfer of both bright and dark excitons from WSe₂ to MoTe₂ (f)^[46]. Copyright 2019, American Chemical Society. (g, h) Enhanced emission of MoSe₂/MoS₂ heterostructures by interlayer charge transfer, including a schematic of the band alignment describing the transfer of electrons and holes (g) and photoluminescence intensity maps of the MoSe₂/MoS₂ with one or two layers h-BNs at the interface. Scale bars are 5μm^[72]. Copyright 2016, American Chemical Society.

Fig. 5. (Color online) Fluorescence blinking of 2D emitters. (a) Optical image (left) and fluorescence images (three panels on the right) of a WS₂/MoSe₂ heterobilayer showing bright, neutral, and dark emission state of WS₂ at the heterobilayer region. (b) Spectra of WS₂ and MoSe₂, and optical image of WS₂/MoSe₂ heterobilayer (inset). (c) Time-dependent intensity of WS₂ (red curve) and MoSe₂ (blue curve) emission from the spectra shown in (b)^[36]. Copyright 2017, The Authors. (d) Capture of interfacial photocurrent in a blinking WSe₂/WS₂ circuit at zero bias. The photocurrent and fluorescence intensities are recorded synchronously^[39]. Copyright 2021, American Chemical Society.

Fig. 6. (Color online) Intralayer moiré excitons in TMD heterostructures. (a) Left: schematic illustration of the moiré superlattice with a twist angleθ and moiré potential perioda_M. Right: the moiré Brillouin zone corresponding to the moiré unit cell. (b) Schematic diagram of excitons trapped by the periodic moiré potential. (c) Observation of moiré excitons in twisted WS₂/WS₂ homobilayer superlattices. (d) Linear energy distribution of different peaks fitted by the Lorentzian function^[87]. Copyright 2022, American Physical Society. (e, f) Excitons in a reconstructed moiré potential. (e) Scanning electron microscopy image of twisted WSe₂ bilayer showing a reconstructed moiré pattern^[90]. Copyright 2021, Wiley-VCH GmbH. (f) Photoluminescence spectra at five positions in a WSe₂ homobilayer^[91]. Copyright 2022, Royal Society of Chemistry.

Fig. 7. (Color online) Emission features of interlayer excitons in TMD heterostructures. (a) Photoluminescence of WSe₂ and MoSe₂ monolayers and the heterobilayers^[95]. Copyright 2022, Royal Society of Chemistry. (b) Interlayer exciton energies and calculated transition energies for heterobilayers with different twist angles. (c) Left: band alignment diagram. Right: the misaligned Brillouin zones of MoS₂ (blue) and WSe₂ (green) where both K–K and Γ–K transitions arek-space indirect^[96]. Copyright 2020, Royal Society of Chemistry. (d) Intensity and (e) energy of the interlayer exciton emission versus the twist angle. The inset in (d) shows the schema of MoSe₂/WSe₂ heterobilayers^[97]. Copyright 2017, American Chemical Society. (f, g) Spin-valley polarization of the interlayer exciton in MoSe₂/WSe₂ heterostructures. (f) Circular polarization-resolved photoluminescence spectra of the interlayer exciton showing the generation of strong valley polarization. (g) Spatial maps ofσ⁺ (left) andσ⁻ (right) interlayer exciton photoluminescence^[98]. Copyright 2016, Science.

Fig. 8. (Color online) Modulated emissions of interlayer moiré excitons in TMD heterostructures. (a) A moiré superlattice formed by a MoSe₂/WSe₂ vertical heterostructure, showing three highlighted regions with three-fold rotational symmetry. (b) Optical selection rules of different atomic configurations in K valley. (c) Left: the moiré potential of the interlayer exciton transition, showing a local minimum and maximum at different sites. Right: spatial map of the degree of circular polarization for K-valley excitons^[111]. Copyright 2021, Springer Nature. (d, e) Helicity-resolved photoluminescence spectra of trapped interlayer excitons in MoSe₂/WSe₂ heterobilayers with twist angles of (d) 57° and (e) 2°^[112]. Copyright 2021, Royal Society of Chemistry. (f-h), Modulation of interlayer moiré excitons by strain, which changes the moiré patterns. (f, g) Piezoresponse force microscopy (PFM) images at two locations of the heterostructure subjected to stress^[113]. Copyright 2021, Wiley‐VCH GmbH. (h) Excitation power-dependent emission spectra of 1D moiré exciton^[114]. Copyright 2021, American Chemical Society. (i) Trapped interlayer trions in a moiré superlattice^[115]. Copyright 2021, Nature.

Fig. 9. (Color online) Quantum emission from moiré excitons. (a) Nano-patterned quantum emitter arrays. Left: distribution of oscillator strength of the interlayer exciton. Right: nanodot confinements atA points, realizing a periodic array of excitonic quantum emitters. Three high-symmetry points corresponding to the moiré superlattices are labeled asA,B, andC, respectively^[109]. Copyright 2017, Science. (b) Normalized emission spectra excited by photon energy of 1.55 eV with different excitation power densities^[118]. Copyright 2021, American Chemical Society. (c–f) Quantum features of moiré interlayer excitons. (c) Photoluminescence spectrum of MoSe₂/WSe₂ moiré superlattices at 4 K. (d) Time-resolved normalized PL intensity of the single emitter fitted by single exponential decay fit (red line), which reveals a lifetime of 12.1 ± 0.3 ns. (e) Integrated PL intensity of the same single emission peak (1.401 eV) at different excitation powers. (f) Photon antibunching revealed by second-order photon correlation statistics (g⁽²⁾(τ)) of emission with the experimental data (red solid line) and the Poissonian interval error (red shadowed area)^[119]. Copyright 2020, Science.

Table 1. Primary progresses on novel van der Waals emitters based on interface engineering.