Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides

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.

Distinct Valley Polarization in Vertical Heterobilayers: Difference between Edge- and Center-Nucleated WS2/MoS2

Structural imperfections can cause both beneficial and detrimental consequences on the excitonic characteristics of transition metal dichalcogenides (TMDs). Regarding valley selection, structural defects typically promote valley depolarization in monolayer TMDs, but defect healing via an additional growth process can restore valley polarization in vertical heterobilayers (VHs). In this study, we analyzed the valley polarization of center-nucleated and edge-nucleated VHs (WS2/MoS2) grown using a controlled growth process and discovered that defect-related photoluminescence (PL) is strongly suppressed in the center-nucleated VHs due to defect healing. Additionally, we demonstrated that the valley polarization of lower-lying intralayer excitons is more sensitive to the defect density of the sample than to higher-lying intralayer excitons. Despite defect healing in the center-nucleated VHs, the temperature-dependent PL study indicated that valley depolarization of the lower-lying intralayer excitons becomes significant below 100 K because of stronger hybridization of defect states. Also, we conducted a comprehensive study on the excitation intensity dependence to investigate the electron-doping-induced Auger recombination mechanism, which also contributes to valley depolarization of intralayer excitons via regeneration of intervalley trions. Our findings provide valuable insight into the development of VH-based valleytronic devices.

Revealing the Optical Transition Properties of Interlayer Excitons in Defective WS2/WSe2 Heterobilayers

Manipulation of physical properties in multidimensional tunable moiré superlattice systems is a key focus in nanophotonics, especially for interlayer excitons (IXs) in two-dimensional materials. However, the impact of defects on IXs remains unclear. Here, we thoroughly study the optical properties of WS2/WSe2 heterobilayers with varying defect densities. Low-temperature photoluminescence (PL) characterizations reveal that the low-energy IXs are more susceptible to defects compared to the high-energy IXs. The low-energy IXs also show much faster PL quenching rate with temperature, faster peak width broadening rate with laser power, shorter lifetime, and lower circular polarization compared to the low-energy IXs in the region with fewer defects. These effects are attributed to the combined effects of increased electron scattering, exciton-phonon interactions, and nonradiative channels introduced by the defects. Our findings aid in optimizing moiré superlattice structures.

Enhancement of Interlayer Exciton Emission in a TMDC Heterostructure via a Multi-Resonant Chirped Microresonator Upto Room Temperature

We report on multi-resonance chirped distributed Bragg reflector (DBR) microcavities. These systems are employed to investigate the light-mater interaction with both intra- and inter-layer excitons of transition metal dichalcogenide (TMDC) bilayer heterostructures. The chirped DBRs consisting of SiO2 and Si3N4 layers of gradually varying thickness exhibit a broad stopband with a width exceeding 600 nm. Importantly, the structures provide multiple resonances across a broad spectral range, which can be matched to resonances of the embedded TMDC heterostructures. Studying cavity-coupled emission of both intra- and inter-layer excitons from an integrated WSe2/MoSe2 heterostructure in a chirped microcavity system, an enhanced interlayer exciton emission with a Purcell factor of 6.67 ± 1.02 at 4 K is observed. The cavity-enhanced emission of the interlayer exciton is used to investigate its temperature-dependent luminescence lifetime of 60 ps at room temperature. The cavity system modestly suppresses intralayer exciton emission by intentional detuning, thereby promoting a higher IX population and enhancing cavity-coupled interlayer exciton emission. This approach provides an intriguing platform for future studies of energetically distant and confined excitons in different semiconducting materials, which paves the way for various applications such as microlasers and single-photon sources by enabling precise emission control and utilizing multimode resonance light-matter interaction.

Temperature-driven journey of dark excitons to efficient photocatalytic water splitting in β-AsP

Limited availability of photogenerated charge carriers in two-dimensional (2D) materials, due to high exciton binding energies, is a major bottleneck in achieving efficient photocatalytic water splitting (PWS). Strong excitonic effects in 2D materials demand precise attention to electron-electron correlation, electron-hole interaction and electron-phonon coupling simultaneously. In this work, we explore the temperature-dependent electronic and optical responses of an efficient photocatalyst, blue-AsP (β-AsP), by integrating electron-phonon coupling into state-of-the-art GW + BSE calculations. Interestingly, strong electron-lattice interaction at high temperature promotes photocatalytic water splitting with an increasing supply of long-lived dark excitons. This work presents an atypical observation contrary to the general assumption that only bright excitons enhance the PWS due to prominent absorption. Dark excitons, due to the low recombination rate, exhibit long-lived photogenerated electron-hole pairs with high exciton lifetime increasing with temperature up to ∼0.25 μs.

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equation model to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time ofτ ET ≈ ( 0 . 35 - 0.15 + 0.65 ) $\tau _{\rm ET} \approx (0.35^{+0.65}_{-0.15})$  ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Lasing from a Large-Area 2D Material Enabled by a Dual-Resonance Metasurface

Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD lasers and the limited size due to mechanical exfoliation presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present room temperature lasing from a large-area tungsten disulfide (WS2) monolayer, grown by a wafer-scale chemical vapor deposition (CVD) technique. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission, resulting in an ultralow threshold operation (threshold well below 1 W/cm2). We provide a thorough study of the laser performance, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 μm, which is several times greater than what has been reported for 2D material lasers so far. Our realization of a single-mode laser from a CVD-grown monolayer presents exciting opportunities for integration and the development of real-world applications.

Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal Dichalcogenides

The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton-exciton annihilation processes in monolayer and bilayer MoS2 are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS2 nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.

Optical spectra and exciton radiative lifetimes in bulk transition metal dichalcogenides

The optical response of layered transition metal dichalcogenides (TMDCs) exhibits remarkable excitonic properties which are important from both fundamental and device application viewpoints. One of these phenomena is the observation of intralayer/interlayer excitons. While much effort has been done to characterize excitons in monolayer TMDCs and their heterostructures, a quite limited number of works have addressed the exciton spectra of their bulk counterparts. In this work, we employ ab initio many-body perturbation calculations to investigate the exciton dynamics and spectra of bulk 2H-MX2 (M = Mo, W, and X = S, Se). For molybdenum-based systems, we find the presence of interlayer excitons at energies higher than the first bright exciton (XA), with non-negligible strength intensity. Our results also show that interlayer excitons in tungsten-based systems are almost degenerate in energy with XA and possess very small oscillator strengths when compared with molybdenum-based systems. At room temperature, and considering the thermal exciton fine-structure population for the XA-exciton, we estimate effective radiative lifetimes in the range of ∼4-14 ns. For higher energy excitons we predict longer effective lifetimes of tens of nanoseconds.

Ultra-Confined Phonon Polaritons and Strongly Coupled Microcavity Exciton Polaritons in Monolayer MoSi2N4 and WSi2N4

The 2D semiconductors are an ideal platform for exploration of bosonic fluids composed of coupled photons and collective excitations of atoms or excitons, primarily due to large excitonic binding energies and strong light-matter interaction. Based on first-principles calculations, it is demonstrated that the phonon polaritons formed by two infrared-active phonon modes in monolayer MoSi2N4 and WSi2N4 possess ultra-high confinement factors of around ≈105 and 103, surpassing those of conventional polaritonic thin-film materials by two orders of magnitude. It is observed that the first bright exciton possesses a substantial binding energies of 750 and 740 meV in these two monolayers, with the radiative recombination lifetimes as long as 25 and 188 ns, and the Rabi splitting of the formed cavity-exciton polaritons reaching 373 and 321 meV, respectively. The effective masses of the cavity exciton polaritons are approximately 10-5me, providing the potential for high-temperature quantum condensation. The ultra-confined and ultra-low-loss phonon polaritons, as well as strongly-coupled cavity exciton polaritons with ultra-small polaritonic effective masses in these two monolayers, offering the flexible control of light at the nanoscale, probably leading to practical applications in nanophotonics, meta-optics, and quantum materials.

Core/Shell-Like Localized Emission at Atomically Thin Semiconductor-Au Interface

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Excitons, optical spectra, and electronic properties of semiconducting Hf-based MXenes

Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely, Hf3C2O2 and Hf2CO2. Using the first-principles calculation and excited state corrections, we proved their dynamical stability, reconciled their semiconducting behavior, and obtained fundamental gaps by using the many-body GW method (indirect 1.1 and 2.2 eV; direct 1.4 and 3.5 eV). Using the Bethe-Salpeter equation, we subsequently provided optical gaps (0.9 and 2.7 eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed for a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the Γ-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron-hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf3C2O2 MXene lead to a 4% and 13% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28% absorptance peak in the visible region appears in Hf2CO2 MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Four-phonon scattering of so-As and improvement of the thermoelectric properties by increasing the buckling height

In recent years, more and more thermoelectric (TE) materials have been discovered as the research boom of TE materials advances. However, due to the low conversion efficiency, most of the current TE materials cannot meet the commercial demand. The low-dimensional nanomaterials are promising to break the current status quo of low conversion efficiency of TE materials. Here, we predicted a stable two-dimensional TE material, namely so-As, based on density functional theory. The so-As has an ultra-low lattice thermal conductivity,κl= 1.829 W m-1K-1at 300 K, and when the temperature rises to 700 K theκlis only 0.788 W m-1K-1. This might be caused by the strong anharmonic interaction among the so-As phonon and the out-of-plane vibration of the low-frequency acoustic modes. Moreover, the maximumZTvalue of thep-type so-As is 0.18 at room temperature (0.45 at 700 K), while that of then-type can even reach 0.75 at 700 K. In addition, we have also studied the difference between the four- and three-phonon scattering rates. The increase of scattering channels leads to the ultra-lowκl, which is only 3.33 × 10-4W m-1K-1at room temperature, showing an almost adiabatic property. Finally, we adjust the TE properties of so-As by changing the buckling height. With the buckling height is increased by 2%, the scattering rate of so-As is extremely high. WhenTis 700 K, the maximumZTof then-type is 0.94 (p-type can also reach 0.7), which is 25% higher than the pristine one. Our work reveals the impact of buckling height on the TE figure of merit, which provides a direction for future search and regulation of the highZTTE materials.

Room-temperature high-speed electrical modulation of excitonic distribution in a monolayer semiconductor

Excitons in monolayer semiconductors, benefitting from their large binding energies, hold great potential towards excitonic circuits bridging nano-electronics and photonics. However, achieving room-temperature ultrafast on-chip electrical modulation of excitonic distribution and flow in monolayer semiconductors is nontrivial. Here, utilizing lateral bias, we report high-speed electrical modulation of the excitonic distribution in a monolayer semiconductor junction at room temperature. The alternating charge trapping/detrapping at the two monolayer/electrode interfaces induces a non-uniform carrier distribution, leading to controlled in-plane spatial variations of excitonic populations, and mimicking a bias-driven excitonic flow. This modulation increases with the bias amplitude and eventually saturates, relating to the energetic distribution of trap density of states. The switching time of the modulation is down to 5 ns, enabling high-speed excitonic devices. Our findings reveal the trap-assisted exciton engineering in monolayer semiconductors and offer great opportunities for future two-dimensional excitonic devices and circuits.

Control of the Bright-Dark Exciton Splitting Using the Lamb Shift in a Two-Dimensional Semiconductor

We investigate the exciton fine structure in atomically thin WSe_{2}-based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting Δ between the bright and dark exciton is measured by photoluminescence spectroscopy. We demonstrate that Δ can be tuned by a few meV as a result of a significant Lamb shift of the optically active exciton that arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measure strong variations of the bright exciton radiative linewidth as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field fluctuations. We find a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control we demonstrate here in the weak light-matter coupling regime should apply to any semiconductor structures.

Dilute Rhenium Doping and its Impact on Defects in MoS2

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS₂ sandwiched between top and bottom graphene, here we demonstrate fully valley-coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

Intrinsic Control of Interlayer Exciton Generation in Van der Waals Materials via Janus Layers

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. We investigate different MoS₂@Janus layer combinations using first-principles methods including excitons and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between dark interlayer exciton states and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS₂@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient generation of charge-separated electron-hole pairs.

Rydberg Excitons and Trions in Monolayer MoTe2

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS₂, MoSe₂, WS₂, and WSe₂), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe₂). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe₂ to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton-phonon coupling. Furthermore, we observe a strongly gate-tunable exciton-trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe-Salpeter equation approach calculations. Our results help bring monolayer MoTe₂ closer to its potential applications in near-infrared optoelectronics and photonic devices.

All-optical control of high-purity trions in nanoscale waveguide

The generation of high-purity localized trions, dynamic exciton-trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS₂ monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal-insulator-metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion.

Ultrafast Dynamics of Valley-Polarized Excitons in WSe2 Monolayer Studied by Few-Cycle Laser Pulses

We report on the experimental investigation of the ultrafast dynamics of valley-polarized excitons in monolayer WSe2 using transient reflection spectroscopy with few-cycle laser pulses with 7 fs duration. We observe that at room temperature, the anisotropic valley population of excitons decays on two different timescales. The shorter decay time of approximately 120 fs is related to the initial hot exciton relaxation related to the fast direct recombination of excitons from the radiative zone, while the slower picosecond dynamics corresponds to valley depolarization induced by Coloumb exchange-driven transitions of excitons between two inequivalent valleys.

Impact of synthesis temperature and precursor ratio on the crystal quality of MOCVD WSe2monolayers

Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe₂) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe₂ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1000 up to 100 000 leads to less defect-related PL at low temperatures.

Quantum theory of electronic excitation and sputtering by transmission electron microscopy

Many computational models have been developed to predict the rates of atomic displacements in two-dimensional (2D) materials under electron beam irradiation. However, these models often drastically underestimate the displacement rates in 2D insulators, in which beam-induced electronic excitations can reduce the binding energies of the irradiated atoms. This bond softening leads to a qualitative disagreement between theory and experiment, in that substantial sputtering is experimentally observed at beam energies deemed far too small to drive atomic dislocation by many current models. To address these theoretical shortcomings, this paper develops a first-principles method to calculate the probability of beam-induced electronic excitations by coupling quantum electrodynamics (QED) scattering amplitudes to density functional theory (DFT) single-particle orbitals. The presented theory then explicitly considers the effect of these electronic excitations on the sputtering cross section. Applying this method to 2D hexagonal BN and MoS₂ significantly increases their calculated sputtering cross sections and correctly yields appreciable sputtering rates at beam energies previously predicted to leave the crystals intact. The proposed QED-DFT approach can be easily extended to describe a rich variety of beam-driven phenomena in any crystalline material.

Interlayer and Intralayer Excitons in AlN/WS2 Heterostructure

The study of intra and interlayer excitons in 2D semiconducting vdW heterostructures is a very hot topic not only from a fundamental but also an applicative point of view. Due to their strong light-matter interaction, Transition Metal Dichalcogenides (TMD) and group-III nitrides are particularly attractive in the field of opto-electronic applications such as photo-catalytic and photo-voltaic ultra-thin and flexible devices. Using first-principles ground and excited-state simulations, we investigate here the electronic and excitonic properties of a representative nitride/TMD heterobilayer, the AlN/WS2. We demonstrate that the band alignment is of type I, and low energy intralayer excitons are similar to those of a pristine WS2 monolayer. Further, we disentangle the role of strain and AlN dielectric screening on the electronic and optical gaps. These results, although they do not favor the possible use of AlN/WS2 in photo-catalysis, as envisaged in the previous literature, can boost the recently started experimental studies of 2D hexagonal aluminum nitride as a good low screening substrate for TMD-based electronic and opto-electronic devices. Importantly, our work shows how the inclusion of both spin-orbit and many-body interactions is compulsory for the correct prediction of the electronic and optical properties of TMD/nitride heterobilayers.

Optical Control of High-Harmonic Generation at the Atomic Thickness

High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light-matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.

Exciton Dynamics in MoS2-Pentacene and WSe2-Pentacene Heterojunctions

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS₂-pentacene and WSe₂-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS₂ and WSe₂ monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS₂ to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS₂-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS₂ on SiO₂. The B-exciton decay in WSe₂ shows faster hole transfer rates for WSe₂-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe₂ also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions.

Tuning moiré excitons in Janus heterobilayers for high-temperature Bose-Einstein condensation

Using first-principles calculations, we predict that moiré excitons in twisted Janus heterobilayers could realize tunable and high-temperature Bose-Einstein condensation (BEC). The electric dipole in the Janus heterobilayers leads to charge-transfer interlayer and intralayer moiré excitons with exceptionally long lifetimes, in the absence of spacer layers. The electric dipole is also expected to enhance exciton-exciton repulsions at high exciton densities and can modulate moiré potentials that trap excitons for their condensation. The key parameters for exciton condensation, including exciton Bohr radius, binding energy, effective mass, and critical Mott density, are examined as a function of the twist angle. Last, exciton phase diagrams for the Janus heterobilayers are constructed from which one can estimate the BEC (>100 K) and superfluid (~30 K) transition temperatures. In addition to indirect interlayer excitons, we find that direct intralayer excitons can also condense at high temperatures, consistent with experiments.

Strain Effects on the Two-Dimensional Cr2N MXene: An Ab Initio Study

Structural, electronic, and magnetic properties of two-dimensional Cr₂N MXene under strain were studied. The uniaxial and biaxial strain was considered from -5 to 5%. Phonon dispersion was calculated; imaginary frequency was not found for both kinds of strain. Phonon density of states displays an interesting relation between strain and optical phonon gaps (OPGs), that it implies tunable thermal conductivity. When we apply biaxial tensile strain, the OPG increases; however, this is not appreciable under uniaxial strain. The electronic properties of the dynamically stable systems were investigated by calculating the band structure and electron localization function (ELF) along the (110) plane. The band structure showed a metallic behavior under compressive strain; nevertheless, under tensile strain, the system has a little indirect band gap of 0.16 eV. By analyzing, the ELF interactions between Cr-N are determined to be a weaker covalent bonding Cr₂N under tensile strain. On the other hand, if the Cr atoms reduce or increase their self-distance, the magnetization alignment changes, also the magnetic anisotropy energy displays out-of-plane spin alignment. These properties extend the potential applications of Cr₂N in the spintronic area as long as they can be grown on substrates with high lattice mismatch, conserving their magnetic properties.

Optical Introduction and Manipulation of Plasmon-Exciton-Trion Coupling in a Si/WS2/Au Nanocavity

Strong plasmon-exciton coupling, which has potential applications in nanophotonics, plasmonics, and quantum electrodynamics, has been successfully demonstrated by using metallic nanocavities and two-dimensional materials. Dynamical control of plasmon-exciton coupling strength, especially by using optical methods, remains a big challenge although it is highly desirable. Here, we report the optical introduction and manipulation of plasmon-exciton-trion coupling realized in a dielectric-metal hybrid nanocavity, which is composed of a silicon (Si) nanoparticle and a thin gold (Au) film, with an embedded tungsten disulfide (WS₂) monolayer. We employ scattering and photoluminescence spectra to characterize the coupling strength between plasmons and excitons in Si/WS₂/Au nanocavities constructed by using Si nanoparticles with different diameters. We enhance the plasmon-exciton and plasmon-trion coupling strength by injecting excitons and trions into the WS₂ monolayer with a 488 nm laser beam. It is revealed that the emission intensities of excitons and trions with respect to the reference WS₂ monolayer can be modified through the change in the coupling strength induced by the laser light. Interestingly, the coupling strength between the plasmons and the excitons/trions can be manipulated from weak to strong coupling regime by simply increasing the laser power, which is clearly resolved in the scattering spectra of Si/WS₂/Au nanocavities. More importantly, the plasmon-exciton-trion coupling induced by the laser light is confirmed by the energy exchange between excitons and trions. Our findings indicate the possibility for optically manipulating plasmon-exciton interaction and suggest the practical applications of dielectric-metal hybrid nanocavities in nanoscale plasmonic devices.

Engineering the Dynamics and Transport of Excitons, Trions, and Biexcitons in Monolayer WS2

The study of transport and diffusion dynamics of quasi-particles such as excitons, trions, and biexcitons in two-dimensional (2D) semiconductors has opened avenues for their application in high-speed excitonic and optoelectronic devices. However, long-range transport and fast diffusion of these quasi-particles have not been reported for 2D systems such as transition metal dichalcogenides (TMDCs). The reported diffusion coefficients from TMDCs are low, limiting their use in high-speed excitonic devices and other optoelectronic applications. Here, we report the highest exciton diffusion coefficient value in monolayer WS₂ achieved via engineering the radiative lifetime and diffusion lengths using static back-gate voltage and substrate engineering. Electrostatic doping is observed to modulate the radiative lifetime and in turn the diffusion coefficient of excitons by ∼three times at room temperature. By combining electrostatic doping and substrate engineering, we push the diffusion coefficient to an extremely high value of 86.5 cm²/s, which has not been reported before in TMDCs and is even higher than the values in some 1D systems. At low temperatures, we further report the control of dynamic and spatial diffusion of excitons, trions, and biexcitons from WS₂. The electrostatic control of dynamics and transport of these quasi-particles in monolayers establishes monolayer TMDCs as ideal candidates for high-speed excitonic circuits, optoelectronic, and photonic device applications.

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS2

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Visualizing Ultrafast Defect-Controlled Interlayer Electron-Phonon Coupling in Van der Waals Heterostructures

Engineering ultrafast interlayer coupling provides access to new quantum phenomena and novel device functionalities in atomically thin van der Waals heterostructures. However, due to all the atoms of a monolayer material being exposed at the interfaces, the interlayer coupling is extremely susceptible to defects, resulting in high energy dissipation through heat and low device performance. The study of how defects affect the interlayer coupling at ultrafast and atomic scales remains a challenge. Here, using femtosecond transient absorption microscopy, a new defect-induced ultrafast interlayer electron-phonon coupling pathway is identified in a WS₂ /graphene heterostructure, involving a three-body collision between electrons in WS₂ and both acoustic phonons and defects in graphene. This interaction manifests as the reduced defect-related Raman resonant activity and the accelerated electron-phonon scattering time from 7.1 to 2.4 ps. Furthermore, the ultrafast interlayer coupling process is directly imaged. These insights will advance the fundamental knowledge of heat dissipation in nanoscale devices, and enable new ways to dynamically manipulate electrons and phonons via defects in van der Waals heterostructures.

First-Principles Study of the Optical Dipole Trap for Two-Dimensional Excitons in Graphane

Recent studies on excitons in two-dimensional materials have been widely conducted for their potential usages for novel electronic and optical devices. Especially, sophisticated manipulation techniques of quantum degrees of freedom of excitons are in demand. In this Letter we propose a technique of forming an optical dipole trap for excitons in graphane, a two-dimensional wide gap semiconductor, based on first-principles calculations. We develop a first-principles method to evaluate the transition dipole matrix between excitonic states and combine it with the density functional theory and GW+BSE calculations. We reveal that in graphane the huge exciton binding energy and the large dipole moments of Wannier-like excitons enable us to induce the dipole trap of the order of meV depth and μm width. This Letter opens a new way to control light-exciton interacting systems based on newly developed numerically robust ab initio calculations.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (R_C), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe₂/Au and 1 L MoSe₂/graphene junctions were estimated to be 147.8 Ω μm and 54.9 Ω μm, respectively. Density functional theory (DFT) simulations revealed that the larger R_C of the former was possibly due to the existence of intrinsic effective potential difference (Φ_barrier) between mTMD and metal. Both junctions exhibit an increasing tendency of R_C as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of R_C was observed in 1 L MoSe₂/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

Giant excitonic upconverted emission from two-dimensional semiconductor in doubly resonant plasmonic nanocavity

Phonon-assisted upconverted emission is the heart of energy harvesting, bioimaging, optical cryptography, and optical refrigeration. It has been demonstrated that emerging two-dimensional (2D) semiconductors can provide an excellent platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, there is little research on the further enhancement of excitonic upconverted emission in 2D semiconductors. Here, we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavities. Owing to the enhanced light collection, enhanced excitation rate, and quantum efficiency enhancement arising from the Purcell effect, an upconverted emission amplification of >1000-fold and a decrease of 2~3 orders of magnitude in the saturated excitation power are achieved. These findings pave the way for the development of excitonic upconversion lasing, nanoscopic thermometry, and sensing, revealing the possibility of optical refrigeration in future 2D electronic or excitonic devices.

Chemiresistive NH3 detection at sub-zero temperatures by polypyrrole- loaded Sn1-xSbxO2 nanocubes

Chemiresistive gas sensors operate mainly at high temperatures, primarily due to the need of energy for surface adsorption-desorption of analytes. As a result, the operating temperature of the chemiresistive sensors could be reduced only to room temperature. Hence, a plethora of sensing requirements at temperatures below ambient have remained outside the scope of chemiresistive materials. In this work, we have developed an antimony-doped SnO₂ nanocube-supported expanded polypyrrole network that could detect low ppm ammonia gas (≤20 ppm) at sub-zero temperatures with high response (∼4), selectivity, and short response and recovery times. The low temperature chemiresistive sensing has been explained in terms of the interplay of an extended conducting network of an in situ deposited polymer, effective transport properties of majority charge carriers and a loosely bound exciton-like electron-hole pair formation and breakage mechanism.

Optimizing cathodoluminescence microscopy of buried interfaces through nanoscale heterostructure design

Mapping the optical response of buried interfaces with nanoscale spatial resolution is crucial in several systems where an active component is embedded within a buffer layer for structural or functional reasons. Here, we demonstrate that cathodoluminescence microscopy is not only an ideal tool for visualizing buried interfaces, but can be optimized through heterostructure design. We focus on the prototypical system of monolayers of semiconducting transition metal dichalcogenide sandwiched between hexagonal boron nitride layers. We leverage the encapsulating layers to tune the nanoscale spatial resolution achievable in cathodoluminescence mapping while also controlling the brightness of the emission. Thicker encapsulation layers result in a brighter emission while thinner ones enhance the spatial resolution at the expense of the signal intensity. We find that a favorable trade-off between brightness and resolution is achievable up to about ∼100 nm of total encapsulation. Beyond this value, the brightness gain is marginal, while the spatial resolution enters a regime that is achievable by diffraction-limited optical microscopy. By preparing samples of varying encapsulation thickness, we are able to determine a surprisingly isotropic exciton diffusion length of >200 nm within the hexagonal boron nitride which is the dominant factor that determines spatial resolution. We further demonstrate that we can overcome the exciton diffusion-limited spatial resolution by using spectrally distinct signals, which is the case for nanoscale inhomogeneities within monolayer transition metal dichalcogenides.

Resonance Photoluminescence Enhancement of Monolayer MoS2 via a Plasmonic Nanowire Dimer Optical Antenna

Two-dimensional transition-metal dichalcogenides (TMDs) such as monolayer MoS2 exhibit remarkable optical properties. However, the intrinsic absorption and emission rates of MoS2 are very low, thus severely hindering its application in electronics and photonics. Combining MoS2 with a plasmonic optical antenna is an alternative solution to enhance the emission rates of the 2D semiconductor, and this can drastically increase the photoresponsivity of the corresponding photodetector. Herein, we have constructed a plasmonic gap cavity of a nanowire dimer (NWD) system as an optical antenna to brighten the emission of MoS2 off the hot spot. Different from the conventional enhancement concept which occurred in the plasmonic hot spot, the light emission off the nanogap hot spot was thoroughly investigated. We demonstrate that this new plasmonic optical nanostructure leads to a strong enhancement due to the Purcell effect. The NWD optical antenna can trap light to the near field through a high-efficiency plasmonic gap mode (PGM); then the PL emission was enhanced drastically up to 14.5-fold due to the resonance of the plasmonic gap mode (PGM) in the NWD with the excitonic band of monolayer MoS2. Theoretical simulations reveal that this NWD can alter the efficiency of convergence and excitation, which was consistent with our experimental results. This study can provide a pathway toward enhancing and controlling PGM-enhanced light emission of TMD materials beyond the plasmonic hot spot.

Theory of Excitons in Atomically Thin Semiconductors: Tight-Binding Approach

Atomically thin semiconductors from the transition metal dichalcogenide family are materials in which the optical response is dominated by strongly bound excitonic complexes. Here, we present a theory of excitons in two-dimensional semiconductors using a tight-binding model of the electronic structure. In the first part, we review extensive literature on 2D van der Waals materials, with particular focus on their optical response from both experimental and theoretical points of view. In the second part, we discuss our ab initio calculations of the electronic structure of MoS2, representative of a wide class of materials, and review our minimal tight-binding model, which reproduces low-energy physics around the Fermi level and, at the same time, allows for the understanding of their electronic structure. Next, we describe how electron-hole pair excitations from the mean-field-level ground state are constructed. The electron-electron interactions mix the electron-hole pair excitations, resulting in excitonic wave functions and energies obtained by solving the Bethe-Salpeter equation. This is enabled by the efficient computation of the Coulomb matrix elements optimized for two-dimensional crystals. Next, we discuss non-local screening in various geometries usually used in experiments. We conclude with a discussion of the fine structure and excited excitonic spectra. In particular, we discuss the effect of band nesting on the exciton fine structure; Coulomb interactions; and the topology of the wave functions, screening and dielectric environment. Finally, we follow by adding another layer and discuss excitons in heterostructures built from two-dimensional semiconductors.

Self-Trapped Interlayer Excitons in van der Waals Heterostructures

The self-trapped state (STS) of the interlayer exciton (IX) has aroused enormous interest owing to its significant impact on the fundamental properties of the van der Waals heterostructures (vdWHs). Nevertheless, the microscopic mechanisms of STS are still controversial. Herein, we study the corrections of the binding energies of the IXs stemming from the exciton-interface optical phonon coupling in four kinds of vdWHs and find that these IXs are in the STS for the appropriate ratio of the electron and hole effective masses. We show that these self-trapped IXs could be classified into type I with the increasing binding energy in the tens of millielectronvolts range, which are very agreement with the red-shift of the IX spectra in experiments, and type II with the decreasing binding energy, which provides a possible explanation for the blue-shift and broad line width of the IX's spectra at low temperatures. Moreover, these two types of exciton states could be transformed into each other by adjusting the structural parameters of vdWHs. These results not only provide an in-depth understanding for the self-trapped mechanism but also shed light on the modulations of IXs in vdWHs.

Dark-Exciton Driven Energy Funneling into Dielectric Inhomogeneities in Two-Dimensional Semiconductors

The optoelectronic and transport properties of two-dimensional transition metal dichalcogenide semiconductors (2D TMDs) are highly susceptible to external perturbation, enabling precise tailoring of material function through postsynthetic modifications. Here, we show that nanoscale inhomogeneities known as nanobubbles can be used for both strain and, less invasively, dielectric tuning of exciton transport in bilayer tungsten diselenide (WSe₂). We use ultrasensitive spatiotemporally resolved optical scattering microscopy to directly image exciton transport, revealing that dielectric nanobubbles are surprisingly efficient at funneling and trapping excitons at room temperature, even though the energies of the bright excitons are negligibly affected. Our observations suggest that exciton funneling in dielectric inhomogeneities is driven by momentum-indirect (dark) excitons whose energies are more sensitive to dielectric perturbations than bright excitons. These results reveal a new pathway to control exciton transport in 2D semiconductors with exceptional spatial and energetic precision using dielectric engineering of dark state energetic landscapes.

Picosecond energy transfer in a transition metal dichalcogenide-graphene heterostructure revealed by transient Raman spectroscopy

Hot carrier–based energy harvesting is critically implied in the performances of optoelectronic devices based on van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD). The way electron–hole couples initially photogenerated in the TMD are converted into an electric current in Gr is a controversial issue. In this work we identify the interlayer interaction occurring during the first picoseconds following photoexcitation as an energy transfer process that is much faster than (other) photogating phenomena implied in optoelectronic applications.

Intense light–matter interactions and unique structural and electrical properties make van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunneling transistors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics, and quantum light emitting devices (QLEDs), bright and narrow-line emitters using minimal amounts of active absorber material. The performance of such devices is critically ruled by interlayer interactions which are still poorly understood in many respects. Specifically, two classes of coupling mechanisms have been proposed, charge transfer (CT) and energy transfer (ET), but their relative efficiency and the underlying physics are open questions. Here, building on a time-resolved Raman scattering experiment, we determine the electronic temperature profile of Gr in response to TMD photoexcitation, tracking the picosecond dynamics of the G and 2D Raman bands. Compelling evidence for a dominant role of the ET process accomplished within a characteristic time of ∼4 ps is provided. Our results suggest the existence of an intermediate process between the observed picosecond ET and the generation of a net charge underlying the slower electric signals detected in optoelectronic applications.

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

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

Dipole moment and pressure dependent interlayer excitons in MoSSe/WSSe heterostructures

The broken mirror symmetry of two-dimensional (2D) Janus materials brings novel quantum properties and various application prospects. Particularly, when stacking into heterostructures, their intrinsic dipole moments and large band offsets are very favorable to the photoexcited properties concerning electron-hole pairs, i.e., excitons. However, the effect of the intrinsic dipole moments on the interlayer excitons in the heterostructures composed of 2D Janus materials is still unclear. Here we use the GW/BSE methods to explore the effect of the intrinsic dipole moments on the interlayer excitons via varying the stacking configuration of MoSSe/WSSe heterostructures. Surprisingly, our results reveal that the parallel-arranged intrinsic dipole moments enhance the interlayer coupling in the heterostructures, and hence make the lowest interlayer exciton have an intensity comparable to the bright excitons while accompanied by a large binding energy and a radiative lifetime as long as 10^-7 s at 300 K, though it is almost a spin-forbidden process, and with the out-of-plane light polarization, long lifetime interlayer excitons are observed under the effect of selection rules. More intriguingly, we found that the photoexcited properties of the interlayer excitons considering the momentum in the stacking configuration with parallel-arranged intrinsic dipole moments are greatly tunable through hydrostatic pressure. These explorations provide a basic perspective for optoelectronic applications by means of engineering the intrinsic dipole moments in Janus heterostructures.

Bright and Dark Exciton Coherent Coupling and Hybridization Enabled by External Magnetic Fields

Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer WSe₂ combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe₂ plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe₂ plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K' valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically thin semiconductors.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Drift-dominant exciton funneling and trion conversion in 2D semiconductors on the nanogap

Understanding and controlling the nanoscale transport of excitonic quasiparticles in atomically thin two-dimensional (2D) semiconductors are crucial to produce highly efficient nano-excitonic devices. Here, we present a nanogap device to selectively confine excitons or trions of 2D transition metal dichalcogenides at the nanoscale, facilitated by the drift-dominant exciton funneling into the strain-induced local spot. We investigate the spatiospectral characteristics of the funneled excitons in a WSe₂ monolayer (ML) and converted trions in a MoS₂ ML using hyperspectral tip-enhanced photoluminescence imaging with <15-nm spatial resolution. In addition, we dynamically control the exciton funneling and trion conversion rate by the gigapascal-scale tip pressure engineering. Through a drift-diffusion model, we confirm an exciton funneling efficiency of ∼25% with a significantly low strain threshold (∼0.1%), which sufficiently exceeds the efficiency of ∼3% in previous studies. This work provides a previously unexplored strategy to facilitate efficient exciton transport and trion conversion of 2D semiconductor devices.

Effects of the Structure and Temperature on the Nature of Excitons in the Mo0.6W0.4S2 Alloy

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo_0.6W_0.4S₂ compared to pure MoS₂ and WS₂ grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS₂ and WS₂. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS₂ than those for WS₂. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.