Tailoring Valley Polarization of Interlayer Excitons in van der Waals Heterostructure toward Optical Communication

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.

Electrically Pumped Valley Emitter in Transition Metal Dichalcogenides with Magnetic Manipulation

Optical pumping has been extensively employed as a straightforward and efficient method for the investigation of excitonic effects in 2D transition metal dichalcogenides (TMDCs). However, the challenge of achieving well-matched resonant excitation makes it difficult to conduct a comprehensive and rigorous comparative study across different TMDCs systems. In this work, electrical pumping is utilized on quantum well structures of TMDCs, enabling equivalent carrier injection with similar kinetic energy while effectively mitigating the effects of non-resonant excitation. Valley-polarized electroluminescence (VP-EL) is systematically investigated under varying magnetic fields, demonstrating that without magnetic electrodes or substrates, reversing the magnetic field direction induces a corresponding reversal in the EL valley polarization. A comparative analysis of VP-EL from monolayer WS2, its homobilayer (WS2/WS2), and heterobilayer (WS2/WSe2) reveals that large spin-orbit coupling (SOC) and dark exciton ground state of WS2 enable the polarization reversal tunable by interlayer charge transfer and spin-matched interlayer hopping. This work elucidates the roles of SOC and the excitonic states for magneto-electroluminescence and demonstrates electrical pumping as a vital technique for the exploration of optical properties of 2D semiconductors.

Friction Tuning of Interlayer Exciton Recombination in Van der Waals Heterostructures

Van der Waals heterostructures combine low friction with excellent optoelectronic properties, making them suitable for opto-nano-electromechanical systems. While the long lifetime of interlayer excitons in these materials helps reduce energy loss, friction in mechanical systems is unavoidable and can shorten the exciton recombination lifetime, undermining the low-friction benefits. Despite its importance, the fundamental mechanism underlying friction-induced changes in exciton recombination remains unexplored, mainly due to the difficulty of probing long-lifetime exciton recombination at friction interfaces. Here, time-resolved photoluminescence combined with an atomic force microscope is used to detect exciton recombination at the friction interface of MoS2/WS2 heterostructures. The findings show that friction generates defects, which trap electrons and create additional recombination pathways, shortening exciton recombination lifetimes. This, in turn, increases friction by altering charge density evolution and raising the friction sliding barrier. Density functional theory calculations confirm this mechanism. These results reveal how friction influences exciton recombination, paving the way for advancements in low-friction nano-opto-electromechanical devices.

On-chip manipulation of trion drift in suspended WS2 monolayer at room temperature

Excitons, which are bound states of electrons and holes, in transition metal dichalcogenides (TMDCs) have been studied as an information carrier for realizing new types of optoelectronic devices. However, the charge neutrality of excitons inhibits the electric control of their motion, as seen in conventional electronic devices, except when utilizing a heterostructure. Here, we investigated the drift motion of trions, quasiparticles composed of an exciton bound to an excess charge, at room temperature in a suspended WS2 monolayer by applying a gate-tunable electric field. Using a simple bottom-gate device, we can tune the electric field intensity and exciton-to-trion conversion ratio by increasing the charge density in the monolayer. Consequently, we experimentally observed that locally excited trions drift toward the center of the suspended monolayer. To understand the underlying mechanisms, we numerically simulated the trion drift using the drift-diffusion equation, accounting for the contributions from both the electric field and strain. The results confirmed that the electric field plays the dominant role in the drift phenomena. Our work offers a useful platform for realizing trion-based optoelectronic devices that are capable of operating even at room temperature.

Emergent 2D van der Waals materials photonic sources

Over the past two decades, two-dimensional (2D) van der Waals (vdW) semiconductors have garnered significant attention in the field of light sources due to their unique optoelectronic properties, such as high excitonic binding energy, tunable bandgaps, and strong optical anisotropy. These properties make 2D vdW semiconductors highly promising for next-generation light sources, offering advantages like enhanced efficiency, wavelength tunability, and polarization control. In this review, we summarize the development of various 2D vdW material-based light sources and their modulation mechanisms. We first provide an overview of excitonic properties and light-emission principles that aim to develop light sources with low-power, high-efficiency. Next, we discuss advances in 2D semiconductor lasers, including intralayer and interlayer exciton lasers, cavity-free systems, and exciton-polariton sources. We then look into single-photon emission and their integration into on-chip systems, followed by studies on nonlinear optical properties like high-order harmonic generation and P-band emission. Additionally, we cover advancements in electrically pumped light sources. The review concludes with an outlook on future developments of 2D vdW semiconductor light sources.

Origin of Interlayer Exciton-Phonon Coupling in 2D Heterostructures

The coupling between excitons and phonons across adjacent layers has been experimentally observed in various heterostructures of layered materials. However, the precise mechanism underlying this phenomenon remains elusive. Using the WSe2@hBN heterostructure as an example, we study the origin of the interlayer exciton-phonon coupling and its signature in resonant Raman scattering through first-principles calculations. Our study emphasizes the central role of crystal symmetries in the interlayer exciton-phonon scattering processes, which are responsible for the anomalous resonant Raman intensities of the in-plane and the out-of-plane hBN phonon modes. We find that the deformation potential induced by the hBN phonon interacts with the hybridized hole density of WSe2 excitons near the hBN interface, leading to interlayer exciton-phonon coupling.

Spectra-Orthogonal Optical Anisotropy in Wafer-Scale Molecular Crystal Monolayers

Controlling the spectral and polarization response of two-dimensional (2D) crystals is vital for developing ultrathin platforms for compact optoelectronic devices. However, independently tuning optical anisotropy and spectral response remains challenging in conventional semiconductors due to the intertwined nature of their lattice and electronic structures. Here, we report spectra-orthogonal optical anisotropy─where polarization anisotropy is tuned independently of spectral response─in wafer-scale, one-atom-thick 2D molecular crystal (2DMC) monolayers synthesized on monolayer transition-metal dichalcogenide (TMD) crystals. Utilizing the concomitant spectral consistency and structural tunability of perylene derivatives, we demonstrate tunable optical polarization anisotropy in 2DMCs with similar spectral profiles, as confirmed by room-temperature scanning tunneling microscopy and cross-polarized reflectance microscopy. Additional angle-dependent analysis of the single-crystal and polycrystalline molecular domains reveals an epitaxial relationship between the 2DMC and TMD. Our results establish a scalable, molecule-based 2D crystalline platform for unique and tunable functionalities unattainable in covalent 2D solids.

Twist Angle-Dependent Exciton Mobility in WS2 Bilayers

Bilayer WS2 shows exceptional promise for excitonic devices due to its defect tolerance, high carrier density, and angle-tunable electronic properties. However, fundamental understanding of twist angle-dependent exciton transport remains limited due to challenges in sample preparation and interplays between interlayer coupling and moiré potential. Using transient reflection microscopy (TRM), we systematically studied exciton mobility in chemical vapor deposition-grown (CVD-grown) bilayer WS2 with different twist angles. At 0°, strong interlayer coupling without moiré potential effects yielded the highest exciton mobility (87.3 cm2/V s)- 10-fold greater than monolayer WS2-with a 1.06 μm diffusion length, while the 25° sample showed reduced mobility (44.5 cm2/(V s)) and shorter diffusion length (0.88 μm) due to weakened coupling and moiré potential effects, and the 60° case exhibited intermediate characteristics. This work demonstrates that interlayer coupling and moiré potential modulation critically determine exciton transport dynamics in layered two-dimensional semiconductors, providing essential guidelines for device engineering.

Frozen non-equilibrium dynamics of exciton Mott insulators in moiré superlattices

Moiré superlattices, such as those formed from transition metal dichalcogenide heterostructures, have emerged as an exciting platform for exploring quantum many-body physics. They have the potential to serve as solid-state analogues to ultracold gases for quantum simulations. A key open question is the coherence and dynamics of the quantum phases arising from photoexcited moiré excitons, particularly amid dissipation. Here we use transient photoluminescence and ultrafast reflectance microscopy to image non-equilibrium exciton phase transitions. Counterintuitively, experimental results and theoretical simulations indicate that strong long-range dipolar repulsion freezes the motion of the Mott insulator phase for over 70 ns. In mixed electron-exciton lattices, reduced dipolar interactions lead to diminished freezing dynamics. These findings challenge the prevailing notion that repulsion disperses particles, whereas attraction binds them. The observed phenomenon of frozen dynamics due to strong repulsive interactions is characteristic of highly coherent systems, a feature previously realized exclusively in ultracold gases.

Designable excitonic effects in van der Waals artificial crystals with exponentially growing thickness

When disassembled into monolayers from their bulk crystals, two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit exotic optical properties dominated by strong excitonic effects. Reassembling 2D TMDC layers to build bulk excitonic crystals can significantly boost their optical performance and introduce emerging functionalities toward optoelectronic and valleytronic applications. However, maintaining or manipulating 2D excitonic properties in bulk structures or superlattices is challenging. Herein, we developed a method to precisely construct m∙2N-layer artificial excitonic crystals with only a number N of stacking operations (m denotes the layer number of the initial material unit), referred to as the "2^N method". We successfully fabricated a millimeter-scale weakly coupled 16-layer MoS2 single crystal with zero interlayer twist angle, which retains monolayer-like exciton properties and exhibits remarkable enhancements up to 643% and 646% in their absorption and photoluminescence (PL) features, respectively. Moreover, we created a WSe2/(MoS2/WSe2)3/MoS2 superlattice starting from monolayer WSe2 and MoS2, which demonstrated an intensity increase of up to 400% in quadrupolar interlayer exciton (IX) emission as compared to dipolar IXs in its bilayer counterpart. Our work shows a promising approach for the design and bottom-up fabrication of excitonic crystals, promoting the exploration of excitonic physics in complex van der Waals (vdW) structures and their applications in optoelectronic devices.

Controlling Coulomb correlations and fine structure of quasi-one-dimensional excitons by magnetic order

Many surprising properties of quantum materials result from Coulomb correlations defining electronic quasiparticles and their interaction chains. In van der Waals layered crystals, enhanced correlations have been tailored in reduced dimensions, enabling excitons with giant binding energies and emergent phases including ferroelectric, ferromagnetic and multiferroic orders. Yet, correlation design has primarily relied on structural engineering. Here we present quantitative experiment-theory proof that excitonic correlations can be switched through magnetic order. By probing internal Rydberg-like transitions of excitons in the magnetic semiconductor CrSBr, we reveal their binding energy and a dramatic anisotropy of their quasi-one-dimensional orbitals manifesting in strong fine-structure splitting. We switch the internal structure from strongly bound, monolayer-localized states to weakly bound, interlayer-delocalized states by pushing the system from antiferromagnetic to paramagnetic phases. Our analysis connects this transition to the exciton's spin-controlled effective quantum confinement, supported by the exciton's dynamics. In future applications, excitons or even condensates may be interfaced with spintronics; extrinsically switchable Coulomb correlations could shape phase transitions on demand.

Ultrafast chirality-dependent dynamics from helicity-resolved transient absorption spectroscopy

Chirality, a pervasive phenomenon in nature, is widely studied across diverse fields including the origins of life, chemical catalysis, drug discovery, and physical optoelectronics. The investigations of natural chiral materials have been constrained by their intrinsically weak chiral effects. Recently, significant progress has been made in the fabrication and assembly of low-dimensional micro and nanoscale chiral materials and their architectures, leading to the discovery of novel optoelectronic phenomena such as circularly polarized light emission, spin and charge flip, advocating great potential for applications in quantum information, quantum computing, and biosensing. Despite these advancements, the fundamental mechanisms underlying the generation, propagation, and amplification of chirality in low-dimensional chiral materials and architectures remain largely unexplored. To tackle these challenges, we focus on employing ultrafast spectroscopy to investigate the dynamics of chirality evolution, with the aim of attaining a more profound understanding of the microscopic mechanisms governing chirality generation and amplification. This review thus provides a comprehensive overview of the chiral micro-/nano-materials, including two-dimensional transition metal dichalcogenides (TMDs), chiral halide perovskites, and chiral metasurfaces, with a particular emphasis on the physical mechanism. This review further explores the advancements made by ultrafast chiral spectroscopy research, thereby paving the way for innovative devices in chiral photonics and optoelectronics.

Resonantly Enhanced Hybrid Wannier-Mott-Frenkel Excitons in Organic-Inorganic Van Der Waals Heterostructures

Hybrid excitons formed via resonant hybridization in 2D material heterostructures feature both large optical and electrical dipoles, providing a promising platform for many-body exciton physics and correlated electronic states. However, hybrid excitons at organic-inorganic interface combining the advantages of both Wannier-Mott and Frenkel excitons remain elusive. Here, hybrid excitons are reported in the copper phthalocyanine/molybdenum diselenide (CuPc/MoSe2) heterostructure (HS) featuring strong molecular orientation dependence by low-temperature photoluminescence and absorption spectroscopy. The hybrid Wannier-Mott-Frenkel excitons exhibit a large oscillator strength and display signatures of the Frenkel excitons in CuPc and the Wannier-Mott excitons in MoSe2 simultaneously through the delocalized electrons. The density functional theory (DFT) calculations further confirm the strong hybridization between the lowest unoccupied molecular orbital (LUMO) of CuPc and the conduction band minimum (CBM) of MoSe2. The out-of-plane molecular orientation is further employed to tune the hybridization strength and tailor the hybrid exciton states. The results reveal the hybrid excitons at the CuPc/MoSe2 interface with tunability by molecular orientation, suggesting that the organic-inorganic HS constitutes a promising platform for many-body exciton physics such as exciton condensation and optoelectrical applications.

Room-Temperature, Strong Emission of Momentum-Forbidden Interlayer Excitons in Nanocavity-Coupled Twisted van der Waals Heterostructures

The emission efficiency of interlayer excitons (IEs) in twisted 2D heterostructures has long suffered from momentum mismatch, limiting their applications in ultracompact excitonic devices. Here, we report strong room-temperature emission of the momentum-forbidden IEs in a 30°-twisted MoS2/WS2 heterobilayer. Utilizing the Purcell effect of a compact plasmonic nanocavity boosts the IE emission intensity in the cavity by over 2 orders of magnitude. We further study the interplay of this Purcell enhancement and phonon assistance in 30°- and 0°-twisted heterostructures. Temperature-dependent and time-resolved spectroscopic measurements reveal that the IE enhancement in the 30°-twisted case involves competition between IE and intralayer-exciton emissions, which is remarkably distinct from the 0°-twisted case. We propose an exciton decay model capturing the features of phonon-assisted momentum compensation and Purcell enhancement in the IE emission, showing consistency with the experimental measurements. Our results enrich the understanding of the nanocavity-assisted light-matter interaction for momentum-indirect excitonic transitions.

Long-Range Charge Transport Facilitated by Electron Delocalization in MoS2 and Carbon Nanotube Heterostructures

Controlling charge transport at the interfaces of nanostructures is crucial for their successful use in optoelectronic and solar energy applications. Mixed-dimensional heterostructures based on single-walled carbon nanotubes (SWCNTs) and transition metal dichalcogenides (TMDCs) have demonstrated exceptionally long-lived charge-separated states. However, the factors that control the charge transport at these interfaces remain unclear. In this study, we directly image charge transport at the interfaces of single- and multilayered MoS2 and (6,5) SWCNT heterostructures using transient absorption microscopy. We find that charge recombination becomes slower as the layer thickness of MoS2 increases. This behavior can be explained by electron delocalization in multilayers and reduced orbital overlap with the SWCNTs, as suggested by nonadiabatic (NA) molecular dynamics (MD) simulations. Dipolar repulsion of interfacial excitons results in rapid density-dependent transport within the first 100 ps. Stronger repulsion and longer-range charge transport are observed in heterostructures with thicker MoS2 layers, driven by electron delocalization and larger interfacial dipole moments. These findings are consistent with the results from NAMD simulations. Our results suggest that heterostructures with multilayer MoS2 can facilitate long-lived charge separation and transport, which is promising for applications in photovoltaics and photocatalysis.

Regulation of the valleytronic properties in single-layer NbSeCl

The regulation of the valleytronic properties of two-dimensional materials can contribute to the in-depth study of valley physics and improve its potential for applications in valleytronic devices. Herein, we systematically investigate the electronic properties and the modulation of the valleytronic properties in single-layer NbSeCl. Our results reveal that NbSeCl is a semiconductor with a 105.2 meV valley splitting at K and K' valleys in the valence band. Magnetic doping and constructing heterostructures can significantly manipulate the valleytronic properties. Specifically, a large valley polarization of 79.5 meV can be induced by Cr atom doping, and the construction of heterostructures with HfN2 can dramatically increase the valley splitting to 147.5 meV. Intriguingly, the type-II band alignment of the NbSeCl/HfN2 heterostructure may extend the lifetime of valley excitons and realize the valley Hall effect in different layers by doping with electrons or holes.

Circular Dichroism and Interlayer Exciton Hall Effect in Transition Metal Dichalcogenides Homobilayers

In van der Waals (vdW) architectures of transition metal dichalcogenides (TMDCs), the coupling between interlayer exciton and quantum degrees of freedom opens unprecedented opportunities for excitonic physics. Taking the MoSe2 homobilayer as representative, we identify that the interlayer registry defines the nature and dynamics of the lowest-energy interlayer exciton. The large layer polarization (Pn) is proved, which ensures the formation of layer-resolved interlayer excitons. In particular, sliding ferroelectric polarization couples to the dipole orientation of the interlayer exciton, thus achieving the long-sought electric control of excitonic states. In line with the phase winding of the Bloch states under C3 rotational symmetry, we clarify the valley optical circular dichroism, enriching the exciton valleytronics. We also elucidate the Hall effect of the layer- and valley-polarized interlayer excitons, which advances our understanding of the spatial transport properties of the composite particles and provides new insights into the exciton-based applications.

Identification of Two-Dimensional Interlayer Excitons and Their Valley Polarization in MoSe2/WSe2 Heterostructure with h-BN Spacer Layer

Interlayer excitons (IXs) in the heterostructure of monolayer transition metal dichalcogenides (TMDs) are considered as a promising platform to study fundamental exciton physics and for potential applications of next generation optoelectronic devices. The IXs trapped in the moiré potential in a twisted monolayer TMD heterostructure such as MoSe2/WSe2 form zero-dimensional (0D) moiré excitons. Introducing an atomically thin insulating layer between TMD monolayers in a twisted heterostructure would modulate the moiré potential landscape, thereby tuning 0D IXs into 2D IXs. However, the optical characteristics of IXs have not been elucidated. Here, we have experimentally investigated the significant optical characteristics arising from IXs in a MoSe2/h-BN/WSe2 heterostructure by optical spectroscopy. The experimental results of time-resolved photoluminescence spectroscopy combined with phenomenological rate equation analysis reveal that the radiative decay rate of IXs in the MoSe2/h-BN/WSe2 heterostructure changes as a function of temperature, which strongly suggests the emergence of 2D IXs by the modulation of potential. Moreover, we demonstrate the valley polarization arising from the prolonged valley relaxation lifetime of 2D IXs reaching 100 ns at low temperature, which is dominated by electron-hole exchange interactions. These findings provide us with an effective strategy to tailor the dimensionality of IXs and elucidate the desired optoelectronic response of IXs in monolayer semiconductor heterostructures.

Imaging interlayer exciton superfluidity in a 2D semiconductor heterostructure

Excitons, which are Coulomb bound electron-hole pairs, are composite bosons and thus at low temperature can form a superfluid state with a single well-defined amplitude and phase. We directly image this macroscopic exciton superfluid state in an hBN-separated MoSe2-WSe2 heterostructure. At high density, we identify quasi-long-range order over the entire active area of our sample, through spatially resolved coherence measurements. By varying the exciton density and sample temperature, we map out the phase diagram of the superfluid. We observe the superfluid phase persisting to a temperature of 15 K, which is in excellent agreement with theoretical predictions. This works paves the way to realizing on chip superfluid structures capable of studying fundamental physical behaviors and quantum devices that use superfluidity.

Momentum-Direct Infrared Interlayer Exciton and Photodetection in Multilayer van der Waals Heterostructures

Interlayer excitons (IX), spatially separated electron-hole pair quasiparticles, can form in type-II van der Waals heterostructures (vdWH). To date, the most widely studied IX in hetero- and homobilayer transition metal dichalcogenides feature momentum-indirect and visible interlayer recombinations. However, momentum-direct IX emissions and interlayer absorptions, especially in the infrared, are crucial for excitonic devices but remain underexplored. In this work, we propose and construct a multilayer WSe2/InSe vdWH that hosts momentum-direct IX and manifests near-infrared interlayer absorptions at room temperature, verified by first-principles density functional theory calculations. We conduct power- and temperature-dependent photoluminescence spectroscopies and extract the IX binding energy to be 43 ± 5 meV. Furthermore, we manipulate the IX emission electrically via the Stark effect and tune its energy by 180 meV. Taking advantage of the direct interlayer absorption, we fabricate a near-infrared vdWH photodetector modulated by strong photogating effect, and achieve the optimal photoresponsivity, specific detectivity, and response time of 33 A W-1, 1.8 × 1010 Jones, and 3.7 μs at 1150 nm. In addition, we test the imaging capability of the photodetector by integrating it into a single-pixel imaging system. Our work showcases the possibility for constructing infrared-responsive vdWH that hosts momentum-direct IX for future excitonic devices of optoelectronic applications.

Theoretical insights into Z-scheme BAs/GeC van der Waals heterostructure for high-efficiency solar cell

The urgent need for solar electricity production is critical for ensuring energy security and mitigating climate change. Achieving the optimal optical bandgap and effective carrier separation, essential for high-efficiency solar cells, remains a significant challenge when utilizing a single material. In this study, we design a BAs/GeC heterostructure using density functional theory. Our findings indicate that the BAs/GeC heterostructure exhibits direct bandgap semiconductor characteristics. Notably, the BAs/GeC heterostructure demonstrates excellent optical absorption within the infrared and visible light spectrum. Moreover, significant carrier spatial separation was suggested, facilitated by a Z-scheme pathway. Furthermore, applying biaxial strains revealed that the BAs/GeC heterostructure is unstable under compressive strain. However, the electronic and optical properties can be tuned using tensile biaxial strains. The calculated power conversion efficiency (PCE) of the BAs/GeC heterostructure is approximately 31%, as determined by the Scharber method. Hence, the combination of an appropriate bandgap, substantial carrier separation, and superior photoelectric conversion efficiency positions the BAs/GeC heterostructure as a promising candidate for high-efficiency solar cells.

Tailored Growth of Transition Metal Dichalcogenides' Monolayers by Chemical Vapor Deposition

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the "standard" TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD1-TMD2 ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.

Long-distance decay-less spin transport in indirect excitons in a van der Waals heterostructure

In addition to its fundamental interest, the long-distance spin transport is essential for spintronic devices. However, the spin relaxation caused by scattering of the particles carrying the spin limits spin transport. We explored spatially indirect excitons (IXs) in van der Waals heterostructures composed of atomically thin layers of transition-metal dichalcogenides as spin carries. We observed the long-distance spin transport: the spin polarized excitons travel over the entire sample,  ~10 micron away from the excitation spot, with no spin density decay. This transport is characterized by the 1/e decay distances reaching  ~100 micron. The 1/e decay distances are extracted from fits over the  ~10 micron sample size. The emergence of long-distance spin transport is observed at the densities and temperatures where the IX transport decay distances and, in turn, scattering times are strongly enhanced. The suppression of IX scattering suppresses the spin relaxation and enables the long-distance spin transport.

Graphene-Templated Achiral Hybrid Perovskite for Circularly Polarized Light Sensing

This study points out the importance of the templating effect in hybrid organic-inorganic perovskite semiconductors grown on graphene. By combining two achiral materials, we report the formation of a chiral composite heterostructure with electronic band splitting. The effect is observed through circularly polarized light emission and detection in a graphene/α-CH(NH2)2PbI3 perovskite composite, at ambient temperature and without a magnetic field. We exploit the spin-charge conversion by introducing an unbalanced spin population through polarized light that gives rise to a spin photoconductive effect rationalized by Rashba-type coupling. The prepared composite heterostructure exhibits a circularly polarized photoluminescence anisotropy gCPL of ∼0.35 at ∼2.54 × 103 W cm-2 confocal power density of 532 nm excitation. A carefully engineered interface between the graphene and the perovskite thin film enhances the Rashba field and generates the built-in electric field responsible for photocurrent, yielding a photoresponsivity of ∼105 A W-1 under ∼0.08 μW cm-2 fluence of visible light photons. The maximum photocurrent anisotropy factor gph is ∼0.51 under ∼0.16 μW cm-2 irradiance. The work sheds light on the photophysical properties of graphene/perovskite composite heterostructures, finding them to be a promising candidate for developing miniaturized spin-photonic devices.

Deterministic fabrication of graphene hexagonal boron nitride moiré superlattices

The electronic properties of moiré heterostructures depend sensitively on the relative orientation between layers of the stack. For example, near-magic-angle twisted bilayer graphene (TBG) commonly shows superconductivity, yet a TBG sample with one of the graphene layers rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided experimental observation of orbital ferromagnetism. To create samples with aligned graphene/hBN, researchers often align edges of exfoliated flakes that appear straight in optical micrographs. However, graphene or hBN can cleave along either zig-zag or armchair lattice directions, introducing a [Formula: see text] ambiguity in the relative orientation of two flakes. By characterizing the crystal lattice orientation of exfoliated flakes prior to stacking using Raman and second-harmonic generation for graphene and hBN, respectively, we unambiguously align monolayer graphene to hBN at a near-[Formula: see text], not [Formula: see text], relative twist angle. We confirm this alignment by torsional force microscopy of the graphene/hBN moiré on an open-face stack, and then by cryogenic transport measurements, after full encapsulation with a second, nonaligned hBN layer. This work demonstrates a key step toward systematically exploring the effects of the relative twist angle between dissimilar materials within moiré heterostructures.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Nature of Long-Lived Moiré Interlayer Excitons in Electrically Tunable MoS2/MoSe2 Heterobilayers

Interlayer excitons in transition-metal dichalcogenide heterobilayers combine high binding energy and valley-contrasting physics with a long optical lifetime and strong dipolar character. Their permanent electric dipole enables electric-field control of the emission energy, lifetime, and location. Device material and geometry impact the nature of the interlayer excitons via their real- and momentum-space configurations. Here, we show that interlayer excitons in MoS2/MoSe2 heterobilayers are formed by charge carriers residing at the Brillouin zone edges, with negligible interlayer hybridization. We find that the moiré superlattice leads to the reversal of the valley-dependent optical selection rules, yielding a positively valued g-factor and cross-polarized photoluminescence. Time-resolved photoluminescence measurements reveal that the interlayer exciton population retains the optically induced valley polarization throughout its microsecond-long lifetime. The combination of a long optical lifetime and valley polarization retention makes MoS2/MoSe2 heterobilayers a promising platform for studying fundamental bosonic interactions and developing excitonic circuits for optical information processing.

Heterostrain-Induced Zeeman-like Splitting in h-BN-Encapsulated Bilayer WSe2

Heterostrain is predicted to induce exceptionally rich physics in atomically thin two-dimensional structures by modifying the symmetry and optical selection rules. In this work, we introduce heterostrain into WSe2 bilayers by combining h-BN encapsulation and high-temperature vacuum annealing. Nonvolatile heterostrain gives rise to a Zeeman-like splitting associated with the elliptically polarized optical emission of interlayer K-K excitons. Further manipulation of the interlayer exciton emission in an external magnetic field reveals that the Zeeman-like splitting cannot be eliminated even in a magnetic field of up to ±6 T. We propose a microscopic picture with respect to the layer and valley pseudospin to interpret the results. Our findings imply an intriguing way to encode binary information with the layer pseudospin enabled by the heterostrain and open a venue for manipulating the layer pseudospin with heterostrain engineering, optical pseudospin injection, and an external magnetic field.

Interlayer and Moiré excitons in atomically thin double layers: From individual quantum emitters to degenerate ensembles

Abstract

Interlayer excitons (IXs), composed of electron and hole states localized in different layers, excel in bilayers composed of atomically thin van der Waals materials such as semiconducting transition-metal dichalcogenides (TMDs) due to drastically enlarged exciton binding energies, exciting spin-valley properties, elongated lifetimes, and large permanent dipoles. The latter allows modification by electric fields and the study of thermalized bosonic quasiparticles, from the single particle level to interacting degenerate dense ensembles. Additionally, the freedom to combine bilayers of different van der Waals materials without lattice or relative twist-angle constraints leads to layer-hybridized and Moiré excitons, which can be widely engineered. This article covers fundamental aspects of IXs, including correlation phenomena as well as the consequence of Moiré superlattices with a strong focus on TMD homo- and heterobilayers.

Graphical abstract

Infrared Interlayer Excitons in Twist-Free MoTe2/MoS2 Heterobilayers

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8 eV (1550 nm) in a chemically vapor-deposited, strictly aligned MoTe2/MoS2 heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum-space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin-orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150 meV (270 nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

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.

Ferrovalley and Quantum Anomalous Hall Effect in Janus TiTeCl Monolayer

Ferrovalley materials are garnering significant interest for their potential roles in advancing information processing and enhancing data storage capabilities. This study utilizes first-principles calculations to determine that the Janus monolayer TiTeCl exhibits the properties of a ferrovalley semiconductor. This material demonstrates valley polarization with a notable valley splitting of 80 meV. Additionally, the Berry curvature has been computed across the first Brillouin zone of the monolayer TiTeCl. The research also highlights that topological phase transitions ranging from ferrovalley and half-valley metals to quantum anomalous Hall effect states can occur in monolayer TiTeCl under compressive strains ranging from -1% to 0%. Throughout these strain changes, monolayer TiTeCl maintains its ferromagnetic coupling. These characteristics make monolayer TiTeCl a promising candidate for the development of new valleytronic and topological devices.

Tunable Localized Charge Transfer Excitons in Nanoplatelet-2D Chalcogenide van der Waals Heterostructures

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D-2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate-2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe2 and WSe2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

Integration of conventional surface science techniques with surface-sensitive azimuthal and polarization dependent femtosecond-resolved sum frequency generation spectroscopy

Experimental insight into the elementary processes underlying charge transfer across interfaces has blossomed with the wide-spread availability of ultra-high vacuum (UHV) setups that allow the preparation and characterization of solid surfaces with well-defined molecular adsorbates over a wide range of temperatures. Within the last 15 years, such insights have extended to charge transfer heterostructures containing solids overlain by one or more atomically thin two dimensional materials. Such systems are of wide potential interest both because they appear to offer a path to separate surface reactivity from bulk chemical properties and because some offer completely novel physics, unrealizable in bulk three dimensional solids. Thick layers of molecular adsorbates or heterostructures of 2D materials generally preclude the use of electrons or atoms as probes. However, with linear photon-in/photon-out techniques, it is often challenging to assign the observed optical response to a particular portion of the interface. We and prior workers have demonstrated that by full characterization of the symmetry of the second order nonlinear optical susceptibility, i.e., the χ(2), in sum frequency generation (SFG) spectroscopy, this problem can be overcome. Here, we describe an UHV system built to allow conventional UHV sample preparation and characterization, femtosecond and polarization resolved SFG spectroscopy, the azimuthal sample rotation necessary to fully describe χ(2) symmetry, and sufficient stability to allow scanning SFG microscopy. We demonstrate these capabilities in proof-of-principle measurements on CO adsorbed on Pt(111) and on the clean Ag(111) surface. Because this setup allows both full characterization of the nonlinear susceptibility and the temperature control and sample preparation/characterization of conventional UHV setups, we expect it to be of great utility in the investigation of both the basic physics and applications of solid, 2D material heterostructures.

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

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

Deterministic Areal Enhancement of Interlayer Exciton Emission by a Plasmonic Lattice on Mirror

The emergence of interlayer excitons (IX) in atomically thin heterostructures of transition metal dichalcogenides (TMDCs) has drawn great attention due to their unique and exotic optical and optoelectronic properties. Because of the spatially indirect nature of IX, its oscillator strength is 2 orders of magnitude smaller than that of the intralayer excitons, resulting in a relatively low photoluminescence (PL) efficiency. Here, we achieve the PL enhancement of IX by more than 2 orders of magnitude across the entire heterostructure area with a plasmonic lattice on mirror (PLoM) structure. The significant PL enhancement mainly arises from resonant coupling between the amplified electric field strength within the PLoM gap and the out-of-plane dipole moment of IX excitons, increasing the emission efficiency by a factor of around 47.5 through the Purcell effect. This mechanism is further verified by detuning the PLoM resonance frequency with respect to the IX emission energy, which is consistent with our theoretical model. Moreover, our simulation results reveal that the PLoM structure greatly alters the far-field radiation of the IX excitons preferentially to the surface normal direction, which increases the collection efficiency by a factor of around 10. Our work provides a reliable and universal method to enhance and manipulate the emission properties of the out-of-plane excitons in a deterministic way and holds great promise for boosting the development of photoelectronic devices based on the IX excitons.

Electrical Control and Transport of Tightly Bound Interlayer Excitons in a MoSe_{2}/hBN/MoSe_{2} Heterostructure

Controlling interlayer excitons in Van der Waals heterostructures holds promise for exploring Bose-Einstein condensates and developing novel optoelectronic applications, such as excitonic integrated circuits. Despite intensive studies, several key fundamental properties of interlayer excitons, such as their binding energies and interactions with charges, remain not well understood. Here we report the formation of momentum-direct interlayer excitons in a high-quality MoSe_{2}/hBN/MoSe_{2} heterostructure under an electric field, characterized by bright photoluminescence (PL) emission with high quantum yield and a narrow linewidth of less than 4 meV. These interlayer excitons show electrically tunable emission energy spanning ∼180  meV through the Stark effect, and exhibit a sizable binding energy of ∼81  meV in the intrinsic regime, along with trion binding energies of a few millielectronvolts. Remarkably, we demonstrate the long-range transport of interlayer excitons with a characteristic diffusion length exceeding 10  μm, which can be attributed, in part, to their dipolar repulsive interactions. Spatially and polarization-resolved spectroscopic studies reveal rich exciton physics in the system, such as valley polarization, local trapping, and the possible existence of dark interlayer excitons. The formation and transport of tightly bound interlayer excitons with narrow linewidth, coupled with the ability to electrically manipulate their properties, open exciting new avenues for exploring quantum many-body physics, including excitonic condensate and superfluidity, and for developing novel optoelectronic devices, such as exciton and photon routers.

Highly tunable ground and excited state excitonic dipoles in multilayer 2H-MoSe2

The fundamental properties of an exciton are determined by the spin, valley, energy, and spatial wavefunctions of the Coulomb-bound electron and hole. In van der Waals materials, these attributes can be widely engineered through layer stacking configuration to create highly tunable interlayer excitons with static out-of-plane electric dipoles, at the expense of the strength of the oscillating in-plane dipole responsible for light-matter coupling. Here we show that interlayer excitons in bi- and tri-layer 2H-MoSe2 crystals exhibit electric-field-driven coupling with the ground (1s) and excited states (2s) of the intralayer A excitons. We demonstrate that the hybrid states of these distinct exciton species provide strong oscillator strength, large permanent dipoles (up to 0.73 ± 0.01 enm), high energy tunability (up to ~200 meV), and full control of the spin and valley characteristics such that the exciton g-factor can be manipulated over a large range (from -4 to +14). Further, we observe the bi- and tri-layer excited state (2s) interlayer excitons and their coupling with the intralayer excitons states (1s and 2s). Our results, in good agreement with a coupled oscillator model with spin (layer)-selectivity and beyond standard density functional theory calculations, promote multilayer 2H-MoSe2 as a highly tunable platform to explore exciton-exciton interactions with strong light-matter interactions.

Twist angle-dependent valley polarization switching in heterostructures

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

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

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

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

Spin-Polarized Charge Separation at Two-Dimensional Semiconductor/Molecule Interfaces

Spin-polarized electrons can improve the efficiency and selectivity of photo- and electro-catalytic reactions, as demonstrated in the past with magnetic or magnetized catalysts. Here, we present a scheme in which spin-polarized charge separation occurs at the interfaces of nonmagnetic semiconductors and molecular films in the absence of a magnetic field. We take advantage of the spin-valley-locked band structure and valley-dependent optical selection rule in group VI transition metal dichalcogenide (TMDC) monolayers to generate spin-polarized electron-hole pairs. Photoinduced electron transfer from WS2 to fullerene (C60) and hole transfer from MoSe2 to phthalocyanine (H2Pc) are found to result in spin polarization lifetimes that are 1 order of magnitude longer than those in the TMDC monolayers alone. Our findings connect valleytronic properties of TMDC monolayers to spin-polarized interfacial charge transfer and suggest a viable route toward spin-selective photocatalysis.

Valleytronics in bulk MoS2 with a topologic optical field

The valley degree of freedom1-4 of electrons in materials promises routes towards energy-efficient information storage with enticing prospects for quantum information processing5-7. Current challenges in utilizing valley polarization are symmetry conditions that require monolayer structures8,9 or specific material engineering10-13, non-resonant optical control to avoid energy dissipation and the ability to switch valley polarization at optical speed. We demonstrate all-optical and non-resonant control over valley polarization using bulk MoS2, a centrosymmetric material without Berry curvature at the valleys. Our universal method utilizes spin angular momentum-shaped trefoil optical control pulses14,15 to switch the material's electronic topology and induce valley polarization by transiently breaking time and space inversion symmetry16 through a simple phase rotation. We confirm valley polarization through the transient generation of the second harmonic of a non-collinear optical probe pulse, depending on the trefoil phase rotation. The investigation shows that direct optical control over the valley degree of freedom is not limited to monolayer structures. Indeed, such control is possible for systems with an arbitrary number of layers and for bulk materials. Non-resonant valley control is universal and, at optical speeds, unlocks the possibility of engineering efficient multimaterial valleytronic devices operating on quantum coherent timescales.

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.

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

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

Unraveling the strain tuning mechanism of interlayer excitons in WSe2/MoSe2heterostructure

Atomically thin transition metal dichalcogenides (TMDs) exhibit rich excitonic physics, due to reduced dielectric screening and strong Coulomb interactions. Especially, some attractive topics in modern condensed matter physics, such as correlated insulator, superconductivity, topological excitons bands, are recently reported in stacking two monolayer (ML) TMDs. Here, we clearly reveal the tuning mechanism of tensile strain on interlayer excitons (IEXs) and intralayer excitons (IAXs) in WSe2/MoSe2heterostructure (HS) at low temperature. We utilize the cryogenic tensile strain platform to stretch the HS, and measure by micro-photoluminescence (μ-PL). The PL peaks redshifts of IEXs and IAXs in WSe2/MoSe2HS under tensile strain are well observed. The first-principles calculations by using density functional theory reveals the PL peaks redshifts of IEXs and IAXs origin from bandgap shrinkage. The calculation results also show the Mo-4d states dominating conduction band minimum shifts of the ML MoSe2plays a dominant role in the redshifts of IEXs. This work provides new insights into understanding the tuning mechanism of tensile strain on IEXs and IAXs in two-dimensional (2D) HS, and paves a way to the development of flexible optoelectronic devices based on 2D materials.

Coexisting Phases in Transition Metal Dichalcogenides: Overview, Synthesis, Applications, and Prospects

Over the past decade, significant advancements have been made in phase engineering of two-dimensional transition metal dichalcogenides (TMDCs), thereby allowing controlled synthesis of various phases of TMDCs and facile conversion between them. Recently, there has been emerging interest in TMDC coexisting phases, which contain multiple phases within one nanostructured TMDC. By taking advantage of the merits from the component phases, the coexisting phases offer enhanced performance in many aspects compared with single-phase TMDCs. Herein, this review article thoroughly expounds the latest progress and ongoing efforts on the syntheses, properties, and applications of TMDC coexisting phases. The introduction section overviews the main phases of TMDCs (2H, 3R, 1T, 1T', 1Td), along with the advantages of phase coexistence. The subsequent section focuses on the synthesis methods for coexisting phases of TMDCs, with particular attention to local patterning and random formations. Furthermore, on the basis of the versatile properties of TMDC coexisting phases, their applications in magnetism, valleytronics, field-effect transistors, memristors, and catalysis are discussed. Lastly, a perspective is presented on the future development, challenges, and potential opportunities of TMDC coexisting phases. This review aims to provide insights into the phase engineering of 2D materials for both scientific and engineering communities and contribute to further advancements in this emerging field.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

Direct Visualization of Dark Interlayer Exciton Transport in Moiré Superlattices

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

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

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