Single-exciton trapping in an electrostatically defined two-dimensional semiconductor quantum dot

Interlayer excitons have been extensively studied in monolayer transition metal dichalcogenide (TMD) heterobilayers mainly due to the long lifetime, which is beneficial for a wide range of optoelectronic applications. To date, the majority of investigations of interlayer excitons in TMD heterobilayers have been focusing on the geometric arrangement of structures, spin-valley lifetime, and interlayer valley excitons with interlayer hopping rules. Nevertheless, interlayer excitons in TMD heterobilayers strongly depend on the local atomic registry and coupling strength, which increase the complexity of the device fabrication. Here, we report pronounced interlayer exciton emission in two-dimensional (2D) perovskite/monolayer TMD heterostructures without the need of thermal annealing or specific geometric arrangements, and the interlayer exciton emission is rather general among 2D perovskites and monolayer TMDs. Such interlayer exciton emission completely dominates the emission spectrum at 78 K regardless of the stacking sequence, suggesting the robust interlayer coupling in 2D perovskite/monolayer TMD heterostructures. Furthermore, the interlayer exciton emission shows a large blue-shift with increasing laser intensity due to the repulsive dipole-dipole interaction and can persist above 220 K. Importantly, the interlayer exciton emission also possesses robust circular polarization in chiral 2D perovskite/monolayer WSe2 heterostructures, which can be applied to manipulate the valley degree of freedom for valleytronic devices. Our findings would provide a favorable platform to explore interlayer coupling and related physical processes in 2D perovskites and TMDs and further provoke more investigations into the understanding and controlling of excitonic effects and associated optoelectronic applications in van der Waals heterostructures over a broad-range spectral response.

Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles can potentially lead to low loss excitonic circuits. Here, we report unidirectional transport of IXs along nanoscale electrostatically defined channels in an MoSe2-WSe2 heterostructure. These results are enabled by a lithographically defined triangular etch in a graphene gate to create a potential energy "slide". By performing spatially and temporally resolved photoluminescence measurements, we measure smoothly varying IX energy along the structure and high speed exciton flow with a drift velocity up to 2 × 106 cm/s, an order of magnitude larger than previous experiments. Furthermore, exciton flow can be controlled by saturating exciton population in the channel using a second laser pulse, demonstrating an optically gated excitonic transistor. Our work paves the way toward low loss excitonic circuits, the study of bosonic transport in one-dimensional channels, and custom potential energy landscapes for excitons in van der Waals heterostructures.

Moiré superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes1-7, the confinement of excitons in artificial moiré lattices8-13 and the formation of exotic quantum phases14-18. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure19,20. To exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton-phonon scattering and subsequent charge transferat the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moiré modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moiré superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.

Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior complex scaling (ECS). We utilize an analytical bilayer Keldysh potential describing the interaction between the electron-hole pair, and validate its accuracy by comparing to the full multilayer Poisson equation. Based on this model, we obtain an analytical weak-field expression for the exciton dissociation rate. The heterostructures analysed are MoS2/MoSe2, MoS2/WS2, MoS2/WSe2, MoSe2/WSe2, WS2/MoSe2, and WS2/WSe2 in various dielectric environments. For weak electric fields, we find that WS2/WSe2 supports the fastest dissociation rates among the six structures. We, furthermore, observe that exciton dissociation rates in vdWHs are significantly larger than in their monolayer counterparts.

Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to their long lifetime, large exciton binding energy, and gate tunability. However, the charge-neutral nature of the excitons leads to weak response to the in-plane electric field and thus inhibits transport beyond the diffusion length. Here, we demonstrate the directional transport of interlayer excitons in bilayer WSe2 driven by the propagating potential traps induced by surface acoustic waves (SAW). We show that at 100 K, the SAW-driven excitonic transport is activated above a threshold acoustic power and reaches 20 μm, a distance at least ten times longer than the diffusion length and only limited by the device size. Temperature-dependent measurement reveals the transition from the diffusion-limited regime at low temperature to the acoustic field-driven regime at elevated temperature. Our work shows that acoustic waves are an effective, contact-free means to control exciton dynamics and transport, promising for realizing 2D materials-based excitonic devices such as exciton transistors, switches, and transducers up to room temperature.

Transition-metal dichalcogenide heterostructures exhibit moir\'e patterns that spatially modulate the electronic structure across the material's plane. For certain material pairs, this modulation acts as a potential landscape with deep, trigonally symmetric wells capable of localizing interlayer excitons, forming periodic arrays of quantum emitters. Here, we study these moir\'e localized exciton states and their optical properties. By numerically solving the two-body problem for an interacting electron-hole pair confined by a trigonal potential, we compute the localized exciton spectra for different pairs of materials. We derive optical selection rules for the different families of localized states, each belonging to one of the irreducible representations of the potential's symmetry group $C_{3v}$, and numerically estimate their polarization-resolved absorption spectra. We find that the optical response of localized moir\'e interlayer excitons is dominated by states belonging to the doubly-degenerate $E$ irreducible representation. Our results provide new insights into the optical properties of artificially confined excitons in two-dimensional semiconductors.

Artificially stacked van der Waals heterostructures (vdWH) of two-dimensional (2D) atomic layers have attracted considerable attention due to substantial interactions between different layers. In particular, the strongly bound interlayer exciton (IX) within vdWH offers a platform for exploring fundamental physics as well as innovative device applications. However, to date, it remains a critical challenge to modulate the IX emission energy, limiting the achievement of high-performance spin-valleytronics and excitonic devices. Here, we report a simple strain engineering approach to efficiently modulate the MoS2/WSe2 IX via uniaxial strain. By encapsulating the vdWH within a flexible substrate, the applied mechanical strain could be effectively transferred to the lattice of vdWH during the mechanical bending process, leading to an unprecedent IX modulation range of 144 meV with a linear fitted gauge factor of 121.8 meV per 1% strain. Furthermore, we found that the gauge factor of IX in vdWH is larger than that of individual MoS2 and WSe2 intralayer excitons, further confirming that the observed IX originates from the momentum-indirect exciton between the K point of the MoS2 conduction band and the Γ point of the WSe2 valence band. Our study not only achieves a high vdWH IX modulation value using efficient strain engineering but also provides a route to investigate the evolution of band energy for various two-dimensional (2D) materials as well as their vertical vdWH.

In van der Waals heterostructures consisting of stacked ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$ monolayers, optically bright interlayer excitons can be observed when the constituent layers are crystallographically aligned. The symmetry of the monolayers allows for two different types of alignment, in which the momentum-direct interlayer transitions are either valley conserving ($R$-type alignment) or change the valley index ($H$-type antialignment). Here, we study the valley polarization dynamics of interlayer excitons in magnetic fields up to 30 T by time-resolved photoluminescence. For all interlayer exciton types, we find a finite initial photoluminescence circular degree of polarization after unpolarized excitation in applied magnetic fields. For interlayer excitons in $H$-type heterostructures, we observe a systematic increase of the photoluminescence circular degree of polarization with time in applied magnetic fields, which saturates at values close to unity for the largest fields. By contrast, for interlayer excitons in $R$-type heterostructures, the photoluminescence circular degree of polarization shows a decrease and a zero crossing before saturating with opposite polarization. This unintuitive behavior can be explained by a model considering the different interlayer exciton states in $H$- and $R$-type heterostructures and their selection rules coupling photoluminescence helicity and valley polarization.

Two-dimensional semiconductors have emerged as a new class of materials for nanophotonics owing to their strong exciton-photon interaction1,2 and their ability to be engineered and integrated into devices3. Here we take advantage of these properties to engineer an efficient lasing medium based on direct-bandgap interlayer excitons in rotationally aligned atomically thin heterostructures4. Lasing is measured from a transition-metal dichalcogenide heterobilayer (WSe2-MoSe2) integrated in a silicon nitride grating resonator. An abrupt increase in the spatial coherence of the emission is observed across the lasing threshold. The work establishes interlayer excitons in two-dimensional heterostructures as a gain medium with spatially coherent lasing emission and potential for heterogeneous integration. With electrically tunable exciton-photon interaction strengths5 and long-range dipolar interactions, these interlayer excitons are promising for application as low-power, ultrafast lasers and modulators and for the study of many-body quantum phenomena6.

We perform a theoretical investigation about the valley dynamics of interlayer excitons (IXs) in $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{W}{\mathrm{S}}_{2}$ heterostructure with AB stacking configuration grown on ferromagnetic substrate. We find that the interlayer charge transfer process changes dramatically the IX emission as well as its valley polarization (VP). The magnetic proximity effect (MPE) exerted by ferromagnetic substrate, on the other hand, generates a zero-field valley Zeeman splitting, which suppresses the depolarization induced by electron-hole exchange interaction. Remarkably, unlike usual exciton-phonon scattering, which is harmful to the VP, the phonon-assisted intervalley scatterings between two split IX states in the different valleys foster an IX population imbalance in these states, giving rise to a giant VP. A combination of these experimentally tunable physical quantities (interlayer charge transfer rate, MPE intensity and exciton-phonon coupling strength) provides a promising tool for intriguing emerging magneto-optical emissions and their VPs. Considering a large family of layered materials, this study sheds light on the path for development of van der Waals heterostructures with detectable IX emissions and giant VP.

van der Waals (vdW) heterostructures provide a powerful method to control the alignment of energy bands of atomically thin 2D materials. Under light illumination, the optical responses are dominated by Coulomb-bound electron-hole quasiparticles, for example, excitons, trions, and biexcitons, whose contributions accordingly depend on the types of heterostructures. For type-II heterostructures, it has been well established that light excitation results in electrons and holes that are separated in different layers, and the radiative recombination is dominated by the interlayer excitons. On the contrary, little is known about the corresponding optical responses of type-I cases. Understanding the optical characteristics of type-I heterostructures is important to the full exploration of the quasiparticle physics of the 2D heterostacks. In this study, we performed optical spectroscopy on type-I vdW heterostacks composed of monolayer MoTe2 and WSe2. Photoluminescence and reflection contrast spectroscopy show that the light absorption and emission are dominated by the Coulomb-bound trions. Importantly, we observed that the MoTe2 trion emission gets stronger compared with the exciton emission under resonant light excitation to the WSe2 trion absorption state, especially in the WSe2/MoTe2/WSe2 heterotrilayer. A detailed study of photoluminescence excitation further reveals that the charge-transfer mechanism is likely responsible for our observation, which differs from the exciton-dominated dipole-dipole energy transfer in type-II structures. Our demonstration implies that the type-I vdW heterostack provides new opportunities to engineer the light-matter interactions through many-body Coulomb-bound states.

Interlayer excitons in transition metal dichalcogenide (TMD) heterostructures feature long lifetime and superior transport properties, yet their room‐temperature optical tracking remains challenging due to weak emission. Here, remote optical probing of interlayer exciton transport in a TMD heterostructure at room temperature is demonstrated. Optically generated interlayer excitons diffuse through the heterostructure and dissociate at boundaries, where liberated carriers further migrate into monolayers and recombine with native charges to give bright intralayer emission. With the intralayer emission as a remote optical probe, unique properties of interlayer exciton transport are observed including the characteristic decay length of ≈4.5 µm, the power‐dependent phase transition between exciton gas and electron‐hole plasma, the thermally activated transport enhancement, and the fast diffusivity of ≈2100 cm 2 s −1 . The research offers a new route for the study of interlayer exciton transport with enhanced visibility, which paves the way for room‐temperature exciton transistors and circuits.

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures have emerged as a platform for designing exciton superlattices. However, our understanding of the motion of excitons in moiré potentials is still limited. Here we investigated interlayer exciton dynamics and transport in WS2-WSe2 heterobilayers in time, space and momentum domains using transient absorption microscopy combined with first-principles calculations. We found that the exciton motion is modulated by twist-angle-dependent moiré potentials around 100 meV and deviates from normal diffusion due to the interplay between the moiré potentials and strong exciton-exciton interactions. Our experimental results verified the theoretical prediction of energetically favourable K-Q interlayer excitons and showed exciton-population dynamics that are controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. These results form a basis to investigate exciton and spin transport in van der Waals heterostructures, with implications for the design of quantum communication devices.

Controlling the valley polarization of interlayer excitons in van der Waals heterostructures is essential for developing valleytronic devices. Here, we demonstrate the manipulation of valley polarization of interlayer excitons in a monolayer WSe2/WS2 heterostructure using a dual-handedness metasurface. This metasurface, composed of left- and right-handed gold nanoantenna arrays, exhibits distinct chiral optical responses. Introducing the metasurface into heterostructures significantly enhances the degree of interlayer exciton valley polarization, which quantitatively corresponds to the circular dichroism (CD) of interlayer excitons, via chiral Purcell effects. A negative CD of -16% is yielded via the left-handed metasurface, and a positive CD of +25% is harvested via the right-handed metasurface under σ+ excitation of 532 nm at 77 K. Moreover, the CD is tunable with excitation wavelength, reaching a maximum of 38% at 620 nm under σ- excitation, the highest value from interlayer excitons reported so far without applying external fields. Further, we demonstrate that this platform enables direct and byte-invert arithmetic operations for information transmission. This capability highlights its potential for error detection in valleytronics-based optical communication systems.

Van der Waals (vdW) heterostructures (HSs) built on 2D materials provide an ideal platform for research of energy migration at the nanoscale. However, the underlying charge transfer mechanism in type II vdW HSs is still not well understood. Here, ultrafast exciton dynamics are investigated in trilayer‐WS2‐MoS2‐WSe2 and trilayer‐MoS2‐WSe2‐WS2 HSs by broadband pump‐probe spectroscopy. A two‐step process of exciton transfer in trilayer‐WS2‐MoS2‐WSe2 is directly observed when the band edge exciton of WSe2 is excited. The electrons in WSe2 are initially transferred to the high lying electronic state of MoS2‐WS2 on a time scale of tens of femtoseconds, and then electrons eventually relax into the conduction band minimum of MoS2 within 1 ps. Furthermore, the transfer of interlayer excitons is observed for the first time in trilayer‐MoS2‐WSe2‐WS2. Both transfer processes can be better understood by the Dexter charge exchange model. Due to the nature of Dexter type transfer that the exchange rate exponentially depends on the donor−acceptor distance, the interlayer exciton transfer rate is nearly a hundred times slower than that of exciton transition in bilayer HSs. The results deepen the understanding of charge transfer in 2D vdW HSs and also indicate that the exciton effect and orbital hybridization make HS a strong coupling system.