Mirror twin grain boundaries in molybdenum dichalcogenides

Mirror twin grain boundaries (MTBs) exist at the interface between two grains of 60° rotated hexagonal transition metal dichalcogenides (TMDC). These grain boundaries form a regular atomic structure that extends in one dimension and thus may be described as a one-dimensional (1D) lattice embedded in the 2D TMDC. In this review, the different atomic structures and compositions of these MTBs are discussed. The obvious formation of MTBs is by coalescence of two twinned grains. In addition, however, in MoSe₂ and MoTe₂ a different formation mechanism has been revealed for the formation of Mo-rich MTBs. It has been shown that excess Mo can be incorporated into the TMDC lattices. These excess Mo atoms can then reorganize into closed, triangular MTB-loops that can grow in size by adding more Mo atoms to them. This mechanism allows the formation of dense MTB networks in MoSe₂ and MoTe₂. Such MTB networks have been observed in samples grown by molecular beam epitaxy (MBE) and consequently their presence needs to be considered in understanding the properties of MBE grown MoSe₂ and MoTe₂. Density functional theory as well as photoemission spectroscopy of MTB networks have shown that MTBs exhibit dispersing 1D-bands that intersect the Fermi-level, thus suggesting that these are 1D electron systems. Consequently, experimental data have been interpreted to reveal a charge density wave (or Peierls) instability, as well as a Tomonaga-Luttinger liquid behavior for electrons confined in 1D. We discuss these observations and the controversies that remain in the interpretation of some data. The metallic properties of the MTBs and their formation in dense networks also sparked the potential use of such crystal modifications for making metallic contacts to MoTe₂ or MoSe₂. Moreover, these crystal modifications may also boost the catalytic properties of these materials.

Moiré Intralayer Excitons in a MoSe2/MoS2 Heterostructure

Spatially periodic structures with a long-range period, referred to as a moiré pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra- and interlayer excitons of transition metal dichalcogenide heterostructures. Here, we report on the impact of the moiré pattern formed in a MoSe₂/MoS₂ heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe₂ exciton and trion in emission and (for the exciton) absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.

Ultrafast formation and dynamics of interlayer exciton in a large-area CVD-grown WS2/WSe2 heterostructure

A WS₂/WSe₂ heterostructure is constructed by stacking a WS₂ monolayer on the top of WSe₂ monolayer fabricated with chemical vapor deposition (CVD) method. Ultrafast transient spectroscopy is used to demonstrate the ultrafast charge transfer and interlayer exciton dynamics in the heterostructure. When the WS₂/WSe₂ heterostructure was photoexcitated at 617 nm (2.01 eV) to excite the A-exciton transition of WS₂, an ultrafast photobleaching was observed around the WSe₂ A-exciton transition at 749 nm. The bleaching signal lasts several nanoseconds, which is much longer than the A-exciton lifetime in both the WS₂ and WSe₂ monolayer film. Moreover, by selectively photoexciting the A-exciton of WSe₂ at 749 nm in the heterostructure, an ultrafast photobleaching occurs around the WS₂ A-exciton transition, the recovery of the bleaching shows a single exponential relaxation with typical time constant of ~1.8 ps. The very fast relaxation in the heterostructure probing around 620 nm is indicative that rich defect states exist below the conduction band in WS₂, which can efficiently trap these electrons transferred from the WSe₂ upon photoexcitation. Our spectroscopic results reveal that our CVD-grown WS₂/WSe₂ bilayer film has a type II heterostructure in nature at room temperature. With photoexcitation, electrons and holes can be separately confined in the WS₂ and WSe₂ layer, respectively; as a result, interlayer excitons are formed.

Controllable one-step growth of bilayer MoS2-WS2/WS2 heterostructures by chemical vapor deposition

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) offer attractive prospects for practical applications by combining unique physical properties that are distinct from those of traditional structures. In this paper, we demonstrate a three-stage chemical vapor deposition method for the growth of bilayer MoS₂-WS₂/WS₂ heterostructures with the bottom layers being the lateral MoS₂-center/WS₂-edge monolayer heterostructures and the top layers being the WS₂ monolayers. The alternative growth of lateral and vertical heterostructures can be realized by adjusting both the temperature and the carrier gas flow direction. The combined effect of both reverse gas flow and higher growing temperature can promote the epitaxial growth of second layer on the activated nucleation centers of the first monolayer heterostructures. By using customized temperature profiles, single heterostructures including monolayer lateral MoS₂-WS₂ heterostructures and bilayer lateral WS₂(2L)-MoS₂(2L) heterostructures could also be obtained. Atomic force microscopy, photoluminescence and Raman mapping studies clearly reveal that these different heterostructure samples are highly uniform. These results thus provide a promising and efficient method for the synthesis of complex heterostructures based on different TMDs materials, which would greatly expand the heterostructure family and broaden their applications.

Interlayer valley excitons in heterobilayers of transition metal dichalcogenides

Stacking different two-dimensional crystals into van der Waals heterostructures provides an exciting approach to designing quantum materials that can harness and extend the already fascinating properties of the constituents. Heterobilayers of transition metal dichalcogenides are particularly attractive for low-dimensional semiconductor optics because they host interlayer excitons-with electrons and holes localized in different layers-which inherit valley-contrasting physics from the monolayers and thereby possess various novel and appealing properties compared to other solid-state nanostructures. This Review presents the contemporary experimental and theoretical understanding of these interlayer excitons. We discuss their unique optical properties arising from the underlying valley physics, the strong many-body interactions and electrical control resulting from the electric dipole moment, and the unique effects of a moiré superlattice on the interlayer exciton potential landscape and optical properties.

Ultrafast dynamics in van der Waals heterostructures

Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-to-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this Review, we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of long-lived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from timescales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic, valleytronic and spintronic devices.

Electroluminescent Devices Based on 2D Semiconducting Transition Metal Dichalcogenides

Ultrathin layers of van der Waals inorganic semiconductors represent a new class of excitonic materials with attractive light-emitting properties. Recent observation of valley polarization, optically pumped lasing, exciton-polaritons, and single-photon emission highlights the exciting prospects for two-dimensional (2D) semiconductors for applications in novel photonic devices. Development of efficient and reliable light sources based on excitonic electroluminescence in 2D semiconductors is of fundamental importance toward the practical implementation of photonic devices. Achieving electroluminescence in these atomically thin layers requires unconventional device designs and in-depth understanding of the carrier injection and transport mechanisms. Herein, various strategies for electrically generating excitons in 2D semiconducting transition metal dichalcogenides such as monolayer MoS₂ are reviewed and challenges and opportunities are outlined. Furthermore, novel device concepts such as tunable chiral emission, electrically driven quantum emission, and high-frequency modulation are highlighted.

Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2

Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by nonradiative energy dissipation, has greatly limited practical applications. To reveal the details for nonradiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS₂. With this method, the many-body effects, occupation effects, and phonon dynamics are clearly identified. In particular, thermalization of the MoS₂ lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within tens to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (∼9 ps) is mainly dissipated from the MoS₂ surfaces to external phonons.

Interlayer excitons in transition-metal dichalcogenide heterostructures with type-II band alignment

Combining ab initio density functional theory with the Dirac-Bloch and gap equations, excitonic properties of transition-metal dichalcogenide hetero-bilayers with type-II band alignment are computed. The existence of interlayer excitons is predicted, whose binding energies are as large as 350 meV, only roughly 100 meV less than those of the coexisting intralayer excitons. The oscillator strength of the interlayer excitons reaches a few percent of the intralayer exciton resonances and their radiative lifetime is two orders of magnitude larger than that of the intralayer excitons.

Interfacially Bound Exciton State in a Hybrid Structure of Monolayer WS2 and InGaN Quantum Dots

van der Waals heterostructures that are usually formed using atomically thin transition-metal dichalcogenides (TMDCs) with a direct band gap in the near-infrared to the visible range are promising candidates for low-dimension optoelectronic applications. The interlayer interaction or coupling between two-dimensional (2D) layer and the substrate or between adjacent 2D layers plays an important role in modifying the properties of the individual 2D material or device performances through Coulomb interaction or forming interlayer excitons. Here, we report the realization of quasi-zero-dimensional (0D) photon emission of WS₂ in a coupled hybrid structure of monolayer WS₂ and InGaN quantum dots (QDs). An interfacially bound exciton, i.e., the coupling between the excitons in WS₂ and the electrons in QDs, has been identified. The emission of this interfacially bound exciton inherits the 0D confinement of QDs as well as the spin-valley physics of excitons in monolayer WS₂. The effective coupling between 2D materials and conventional semiconductors observed in this work provides an effective way to realize the 0D emission of 2D materials and opens the potential of compact on-chip integration of valleytronics and conventional electronics and optoelectronics.

New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures

Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS₂/WSe₂ interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

Tunable photoluminescence in a van der Waals heterojunction built from a MoS2 monolayer and a PTCDA organic semiconductor

We report the photoluminescence (PL) characteristics of a van der Waals (vdW) heterojunction constructed by simply depositing an organic semiconductor of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) onto a two-dimensional MoS2 monolayer. The crystallinity of PTCDA on MoS2 is significantly improved due to the vdW epitaxial growth. We observe an enhanced PL intensity and PL peak shift of the MoS2/PTCDA heterojunction compared with the solo MoS2 and PTCDA layer. The synergistic PL characteristics are believed to originate from the hybridization interaction between the MoS2 and the PTCDA as evidenced by density functional theory calculations and Raman measurements. The hybridization interfacial interaction is found to be greatly influenced by the crystalline ordering of the PTCDA film on the 2D MoS2. Our study opens up a new avenue to tune the PL of vdW heterojunctions consisting of TMDs and organic semiconductors for optoelectronic applications.

Valley-contrasting optics of interlayer excitons in Mo- and W-based bulk transition metal dichalcogenides

Recently, spatially indirect ("interlayer") excitons have been discovered in bulk 2H-MoTe2. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (XIL) in bulk 2H-MoSe2 and 2H-WSe2 using valley-resolved magneto-reflectance spectroscopy under high magnetic fields of up to 29 T combined with ab initio GW-BSE calculations. In the experiments, we observe interlayer excitons in MoSe2, while their signature is surprisingly absent in WSe2. In the calculations, we find that interlayer excitons exist in both Mo- and W-based TMDCs. However, their energetic positions and their oscillator strengths are remarkably different. In Mo-based compounds, the interlayer exciton resonance XIL is clearly separated from the intralayer exciton X1sA and has a high amplitude. In contrast, in W-based compounds, XIL is close in energy to the intralayer A exciton X1sA and possesses a small oscillator strength, which explains its absence in the experimental data of WSe2. Our combined experimental and theoretical observations demonstrate that interlayer excitons can gain substantial oscillator strength by mixing with intralayer states and hence pave the way for exploring interlayer exciton physics in Mo-based bulk transition metal dichalcogenides.

Twistable electronics with dynamically rotatable heterostructures

In heterostructures of two-dimensional materials, electronic properties can vary dramatically with relative interlayer angle. This effect makes it theoretically possible to realize a new class of twistable electronics in which properties can be manipulated on demand by means of rotation. We demonstrate a device architecture in which a layered heterostructure can be dynamically twisted in situ. We study graphene encapsulated by boron nitride, where, at small rotation angles, the device characteristics are dominated by coupling to a long-wavelength moiré superlattice. The ability to investigate arbitrary rotation angle in a single device reveals features of the optical, mechanical, and electronic response in this system not captured in static rotation studies. Our results establish the capability to fabricate twistable electronic devices with dynamically tunable properties.

Semiconducting van der Waals Interfaces as Artificial Semiconductors

Recent technical progress demonstrates the possibility of stacking together virtually any combination of atomically thin crystals of van der Waals bonded compounds to form new types of heterostructures and interfaces. As a result, there is the need to understand at a quantitative level how the interfacial properties are determined by the properties of the constituent 2D materials. We address this problem by studying the transport and optoelectronic response of two different interfaces based on transition-metal dichalcogenide monolayers, namely WSe₂-MoSe₂ and WSe₂-MoS₂. By exploiting the spectroscopic capabilities of ionic liquid gated transistors, we show how the conduction and valence bands of the individual monolayers determine the bands of the interface, and we establish quantitatively (directly from the measurements) the energetic alignment of the bands in the different materials as well as the magnitude of the interfacial band gap. Photoluminescence and photocurrent measurements allow us to conclude that the band gap of the WSe₂-MoSe₂ interface is direct in k space, whereas the gap of WSe₂/MoS₂ is indirect. For WSe₂/MoSe₂, we detect the light emitted from the decay of interlayer excitons and determine experimentally their binding energy using the values of the interfacial band gap extracted from transport measurements. The technique that we employed to reach this conclusion demonstrates a rather-general strategy for characterizing quantitatively the interfacial properties in terms of the properties of the constituent atomic layers. The results presented here further illustrate how van der Waals interfaces of two distinct 2D semiconducting materials are composite systems that truly behave as artificial semiconductors, the properties of which can be deterministically defined by the selection of the appropriate constituent semiconducting monolayers.

Microsecond Valley Lifetime of Defect-Bound Excitons in Monolayer WSe_{2}

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer WSe_{2} reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than 1  μs. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

Valley-Selective Response of Nanostructures Coupled to 2D Transition-Metal Dichalcogenides

Monolayer (1L) transition-metal dichalcogenides (TMDCs) are attractive materials for several optoelectronic applications because of their strong excitonic resonances and valley-selective response. Valley excitons in 1L-TMDCs are formed at opposite points of the Brillouin zone boundary, giving rise to a valley degree of freedom that can be treated as a pseudospin, and may be used as a platform for information transport and processing. However, short valley depolarization times and relatively short exciton lifetimes at room temperature prevent using valley pseudospins in on-chip integrated valley devices. Recently, it was demonstrated how coupling these materials to optical nanoantennas and metasurfaces can overcome this obstacle. Here, we review the state-of-the-art advances in valley-selective directional emission and exciton sorting in 1L-TMDC mediated by nanostructures and nanoantennas. We briefly discuss the optical properties of 1L-TMDCs paying special attention to their photoluminescence/absorption spectra, dynamics of valley depolarization, and the valley Hall effect. Then, we review recent works on nanostructures for valley-selective directional emission from 1L-TMDCs.

Two-dimensional semiconductors in the regime of strong light-matter coupling

The optical properties of transition metal dichalcogenide monolayers are widely dominated by excitons, Coulomb-bound electron-hole pairs. These quasi-particles exhibit giant oscillator strength and give rise to narrow-band, well-pronounced optical transitions, which can be brought into resonance with electromagnetic fields in microcavities and plasmonic nanostructures. Due to the atomic thinness and robustness of the monolayers, their integration in van der Waals heterostructures provides unique opportunities for engineering strong light-matter coupling. We review first results in this emerging field and outline future opportunities and challenges.

Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors

Monolayers of transition metal dichalcogenides (TMDC) have recently emerged as excellent platforms for exploiting new physics and applications relying on electronic valley degrees of freedom in two-dimensional (2D) systems. Here, we demonstrate that Coulomb screening by 2D carriers plays a critical role in excitonic valley pseudospin relaxation processes in naturally carrier-doped WSe₂ monolayers (1L-WSe₂). The exciton valley relaxation times were examined using polarization- and time-resolved photoluminescence spectroscopy at temperatures ranging from 10 to 160 K. We show that the temperature-dependent exciton valley relaxation times in 1L-WSe₂ under various exciton and carrier densities can be understood using a unified framework of intervalley exciton scattering via momentum-dependent long-range electron-hole exchange interactions screened by 2D carriers that depend on the carrier density and the exciton linewidth. Moreover, the developed framework was successfully applied to engineer the valley polarization of excitons in 1L-WSe₂. These findings may facilitate the development of TMDC-based opto-valleytronic devices.

Monolayer Transition Metal Dichalcogenides as Light Sources

Reducing the dimensions of materials is one of the key approaches to discovering novel optical phenomena. The recent emergence of 2D transition metal dichalcogenides (TMDCs) has provided a promising platform for exploring new optoelectronic device applications, with their tunable electronic properties, structural controllability, and unique spin valley-coupled systems. This progress report provides an overview of recent advances in TMDC-based light-emitting devices discussed from several aspects in terms of device concepts, material designs, device fabrication, and their diverse functionalities. First, the advantages of TMDCs used in light-emitting devices and their possible functionalities are presented. Second, conventional approaches for fabricating TMDC light-emitting devices are emphasized, followed by introducing a newly established, versatile method for generating light emission in TMDCs. Third, current growing technologies for heterostructure fabrication, in which distinct TMDCs are vertically stacked or laterally stitched, are explained as a possible means for designing high-performance light-emitting devices. Finally, utilizing the topological features of TMDCs, the challenges for controlling circularly polarized light emission and its device applications are discussed from both theoretical and experimental points of view.

Intervalley Scattering of Interlayer Excitons in a MoS2/MoSe2/MoS2 Heterostructure in High Magnetic Field

Degenerate extrema in the energy dispersion of charge carriers in solids, also referred to as valleys, can be regarded as a binary quantum degree of freedom, which can potentially be used to implement valleytronic concepts in van der Waals heterostructures based on transition metal dichalcogenides. Using magneto-photoluminescence spectroscopy, we achieve a deeper insight into the valley polarization and depolarization mechanisms of interlayer excitons formed across a MoS₂/MoSe₂/MoS₂ heterostructure. We account for the nontrivial behavior of the valley polarization as a function of the magnetic field by considering the interplay between exchange interaction and phonon-mediated intervalley scattering in a system consisting of Zeeman-split energy levels. Our results represent a crucial step toward the understanding of the properties of interlayer excitons with strong implications for the implementation of atomically thin valleytronic devices.

Imaging of pure spin-valley diffusion current in WS2-WSe2 heterostructures

Transition metal dichalcogenide (TMDC) materials are promising for spintronic and valleytronic applications because valley-polarized excitations can be generated and manipulated with circularly polarized photons and the valley and spin degrees of freedom are locked by strong spin-orbital interactions. In this study we demonstrate efficient generation of a pure and locked spin-valley diffusion current in tungsten disulfide (WS₂)-tungsten diselenide (WSe₂) heterostructures without any driving electric field. We imaged the propagation of valley current in real time and space by pump-probe spectroscopy. The valley current in the heterostructures can live for more than 20 microseconds and propagate over 20 micrometers; both the lifetime and the diffusion length can be controlled through electrostatic gating. The high-efficiency and electric-field-free generation of a locked spin-valley current in TMDC heterostructures holds promise for applications in spin and valley devices.

Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure

An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe₂/WSe₂ heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations: they show that both emission features originate from excitonic transitions that are indirect in momentum space and are split by spin-orbit coupling. Also, the electron is strongly hybridized between both the MoSe₂ and WSe₂ layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. This work represents a significant advance in our understanding of the static and dynamic properties of TMD heterostructures.

Exciton Diffusion and Halo Effects in Monolayer Semiconductors

We directly monitor exciton propagation in freestanding and SiO_{2}-supported WS_{2} monolayers through spatially and time-resolved microphotoluminescence under ambient conditions. We find a highly nonlinear behavior with characteristic, qualitative changes in the spatial profiles of the exciton emission and an effective diffusion coefficient increasing from 0.3 to more than 30  cm^{2}/s, depending on the injected exciton density. Solving the diffusion equation while accounting for Auger recombination allows us to identify and quantitatively understand the main origin of the increase in the observed diffusion coefficient. At elevated excitation densities, the initial Gaussian distribution of the excitons evolves into long-lived halo shapes with μm-scale diameter, indicating additional memory effects in the exciton dynamics.

Indirect excitons in van der Waals heterostructures at room temperature

Indirect excitons (IXs) are explored both for studying quantum Bose gases in semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III-V and II-VI semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS₂ separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.

Interlayer Excitons with Large Optical Amplitudes in Layered van der Waals Materials

Vertically stacked two-dimensional materials form an ideal platform for controlling and exploiting light-matter interactions at the nanoscale. As a unique feature, these materials host electronic excitations of both intra- and interlayer type with distinctly different properties. In this Letter, using first-principles many-body calculations, we provide a detailed picture of the most prominent excitons in bilayer MoS₂, a prototypical van der Waals material. By applying an electric field perpendicular to the bilayer, we explore the evolution of the excitonic states as the band alignment is varied from perfect line-up to staggered (Type II) alignment. For moderate field strengths, the lowest exciton has intralayer character and is almost independent of the electric field. However, we find higher lying excitons that have interlayer character. They can be described as linear combinations of the intralayer B exciton and optically dark charge transfer excitons, and interestingly, these mixed interlayer excitons have strong optical amplitude and can be easily tuned by the electric field. The first-principles results can be accurately reproduced by a simple excitonic model Hamiltonian that can be straightforwardly generalized to more complex van der Waals materials.

Negative circular polarization emissions from WSe2/MoSe2 commensurate heterobilayers

Van der Waals heterobilayers of transition metal dichalcogenides with spin–valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe₂/MoSe₂ heterobilayers with well-defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.

The interlayer coupling in van der Waals heterostructures is sensitive to the interlayer atomic registry. Here, the authors investigate the polarisation properties of epitaxially grown, commensurate WSe₂/MoSe₂ heterobilayers with well-defined atomic registry, and observe negative, circularly polarized photoluminescence from interlayer excitons.

Two-Dimensionally Layered p-Black Phosphorus/n-MoS2/p-Black Phosphorus Heterojunctions

Layered heterojunctions are widely applied as fundamental building blocks for semiconductor devices. For the construction of nanoelectronic and nanophotonic devices, the implementation of two-dimensional materials (2DMs) is essential. However, studies of junction devices composed of 2DMs are still largely focused on single p-n junction devices. In this study, we demonstrate a novel pnp double heterojunction fabricated by the vertical stacking of 2DMs (black phosphorus (BP) and MoS₂) using dry-transfer techniques and the formation of high-quality p-n heterojunctions between the BP and MoS₂ in the vertically stacked BP/MoS₂/BP structure. The pnp double heterojunctions allowed us to modulate the output currents by controlling the input current. These results can be applied for the fabrication of advanced heterojunction devices composed of 2DMs for nano(opto)electronics.

Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBN

A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS₂/MoS₂, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS₂ or MoS₂ monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G₀ W₀, we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.

Quantum-Confined Electronic States Arising from the Moiré Pattern of MoS2-WSe2 Heterobilayers

A two-dimensional (2D) heterobilayer system consisting of MoS₂ on WSe₂, deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS₂ and WSe₂. Significant energy shifts are observed in tunneling spectra observed at the maxima of the moiré corrugation, as compared with spectra obtained at corrugation minima, consistent with prior work. Furthermore, at the minima of the moiré corrugation, sharp peaks in the spectra at energies near the band edges are observed for spectra acquired at 5 K. The peaks correspond to discrete states that are confined within the moiré unit cells. Conductance mapping is employed to reveal the detailed structure of the wave functions of the states. For measurements at 80 K, the sharp peaks in the spectra are absent, and conductance maps of the band edges reveal little structure.

Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2)

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe_{2} layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs.

Microsecond dark-exciton valley polarization memory in two-dimensional heterostructures

Transition metal dichalcogenides have valley degree of freedom, which features optical selection rule and spin-valley locking, making them promising for valleytronics devices and quantum computation. For either application, a long valley polarization lifetime is crucial. Previous results showed that it is around picosecond in monolayer excitons, nanosecond for local excitons and tens of nanosecond for interlayer excitons. Here we show that the dark excitons in two-dimensional heterostructures provide a microsecond valley polarization memory thanks to the magnetic field induced suppression of valley mixing. The lifetime of the dark excitons shows magnetic field and temperature dependence. The long lifetime and valley polarization lifetime of the dark exciton in two-dimensional heterostructures make them promising for long-distance exciton transport and macroscopic quantum state generations.

Substrate modified thermal stability of mono- and few-layer MoS2

Two-dimensional semiconducting transition metal dichalcogenides have been employed as key components in various electronic devices. The thermal stability of these ultrathin materials must be carefully considered in device applications because the heating caused by current flow, light absorption, or other harsh environmental conditions is usually unavoidable. In this work, we found that the substrate plays a role in modifying the thermal stability of mono- and few-layer MoS₂. Triangular etching holes, which are considered to initiate from defect sites, form on MoS₂ when the temperature exceeds a threshold. On Al₂O₃ and SiO₂, monolayer MoS₂ is found to be more stable in thermal annealing than few-layer MoS₂ either in atmospheric-pressure air or under vacuum; while on mica, the absolute opposite behavior exists. However, this difference due to substrates appears to vanish when using defective, chemical-vapor-deposited MoS₂ samples. The substrate modification of the thermal stability of MoS₂ with various thicknesses is attributed to the competition between MoS₂-substrate interface interaction and MoS₂-MoS₂ interlayer interaction. Our findings provide important design rules for MoS₂-based devices, and also potentially point to a route of controlled patterning of MoS₂ with substrate engineering.

Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure

Heterostructures of van der Waals bonded layered materials offer unique means to tailor dielectric screening with atomic-layer precision, opening a fertile field of fundamental research. The optical analyses used so far have relied on interband spectroscopy. Here we demonstrate how a capping layer of hexagonal boron nitride (hBN) renormalizes the internal structure of excitons in a WSe₂ monolayer using intraband transitions. Ultrabroadband terahertz probes sensitively map out the full complex-valued mid-infrared conductivity of the heterostructure after optical injection of 1s A excitons. This approach allows us to trace the energies and line widths of the atom-like 1s-2p transition of optically bright and dark excitons as well as the densities of these quasiparticles. The excitonic resonance red shifts and narrows in the WSe₂/hBN heterostructure compared to the bare monolayer. Furthermore, the ultrafast temporal evolution of the mid-infrared response function evidences the formation of optically dark excitons from an initial bright population. Our results provide key insight into the effect of nonlocal screening on electron-hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures.

Interlayer Trions in the MoS2/WS2 van der Waals Heterostructure

Electronic excitations in van der Waals heterostructures can have interlayer or intralayer character depending on the spatial localization of the involved charges (electrons and holes). In the case of neutral electron-hole pairs (excitons), both types of excitations have been explored theoretically and experimentally. In contrast, studies of charged trions have so far been limited to the intralayer type. Here we investigate the complete set of interlayer excitations in a MoS₂/WS₂ heterostructure using a novel ab initio method, which allows for a consistent treatment of both excitons and trions at the same theoretical footing. Our calculations predict the existence of bound interlayer trions below the neutral interlayer excitons. We obtain binding energies of 18/28 meV for the positive/negative interlayer trions with both electrons/holes located on the same layer. In contrast, a negligible binding energy is found for trions which have the two equally charged particles on different layers. Our results advance the understanding of electronic excitations in doped van der Waals heterostructures and their effect on the optical properties.

Interlayer resistance of misoriented MoS2

Interlayer misorientation in transition metal dichalcogenides alters their interlayer distance, total energy, electronic band structure, and vibrational modes, but its effect on the interlayer resistance is not known. This study analyzes the interlayer resistance of misoriented bilayer MoS₂ as a function of the misorientation angle, and it shows that interlayer misorientation exponentially increases the electron resistivity while leaving the hole resistivity almost unchanged. The physics, determined by the wave functions at the high symmetry points, are generic among the popular semiconducting transition metal dichalcogenides (TMDs). The asymmetrical effect of misorientation on the electron and hole transport may be exploited in the design and optimization of vertical transport devices such as a bipolar transistor. Density functional theory provides the interlayer coupling elements used for the resistivity calculations.

Synthesis of In-Plane Artificial Lattices of Monolayer Multijunctions

Recently, monolayers of van der Waals materials, including transition metal dichalcogenides (TMDs), are considered ideal building blocks for constructing 2D artificial lattices and heterostructures. Heterostructures with multijunctions of more than two monolayer TMDs are intriguing for exploring new physics and materials properties. Obtaining in-plane heterojunctions of monolayer TMDs with atomically sharp interfaces is very significant for fundamental research and applications. Currently, multistep synthesis for more than two monolayer TMDs remains a challenge because decomposition or compositional alloying is thermodynamically favored at the high growth temperature. Here, a multistep chemical vapor deposition (CVD) synthesis of the in-plane multijunctions of monolayer TMDs is presented. A low growth temperature synthesis is developed to avoid compositional fluctuations of as-grown TMDs, defects formations, and interfacial alloying for high heterointerface quality and thermal stability of monolayer TMDs. With optimized parameters, atomically sharp interfaces are successfully achieved in the synthesis of in-plane artificial lattices of the WS₂ /WSe₂ /MoS₂ at reduced growth temperatures. Growth behaviors as well as the heterointerface quality are carefully studied in varying growth parameters. Highly oriented strain patterns are found in the second harmonic generation imaging of the TMD multijunctions, suggesting that the in-plane heteroepitaxial growth may induce distortion for unique material symmetry.

Electrical control of charged carriers and excitons in atomically thin materials

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices^1-3. The unique band structure^4-7 of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley-spin qubits ⁸ . 2D TMDs also provide a platform for novel quantum optoelectronic devices^9-11 due to their large exciton binding energy^12,13. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings^14-16. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin-valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons.

Resonant Raman and Exciton Coupling in High-Quality Single Crystals of Atomically Thin Molybdenum Diselenide Grown by Vapor-Phase Chalcogenization

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ E_ex ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm^-1. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe₂ crystals. We further present an in-depth investigation of the energy-dependent variation of the position of this and other peaks and provide evidence of C-exciton-phonon coupling in monolayer MoSe₂. Finally, we show how the signature peak positions and intensities vary as a function of layer thickness in these samples.

Electrical Tuning of Interlayer Exciton Gases in WSe2 Bilayers

van der Waals heterostructures formed by stacking two-dimensional atomic crystals are a unique platform for exploring new phenomena and functionalities. Interlayer excitons, bound states of spatially separated electron-hole pairs in van der Waals heterostructures, have demonstrated potential for rich valley physics and optoelectronics applications and been proposed to facilitate high-temperature superfluidity. Here, we demonstrate highly tunable interlayer excitons by an out-of-plane electric field in homobilayers of transition metal dichalcogenides. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved, which is not possible in heterobilayers due to the presence of large built-in interfacial electric fields. A large linear field-induced redshift up to ∼100 meV has been observed in the exciton resonance energy. The Stark effect is accompanied by an enhancement of the exciton recombination lifetime by more than two orders of magnitude to >20 ns. The long recombination lifetime has allowed the creation of an interlayer exciton gas with density as large as 1.2 × 10¹¹ cm^-2 by moderate continuous-wave optical pumping. Our results have paved the way for the realization of degenerate exciton gases in atomically thin semiconductors.

Two-dimensional light-emitting materials: preparation, properties and applications

We review the recent development in two-dimensional (2D) light-emitting materials and describe their preparation methods, optical/optoelectronic properties and applications.

Step-like band alignment and stacking-dependent band splitting in trilayer TMD heterostructures

We propose a kind of trilayer TMD heterostructure with step-like band alignment, and the effects of interlayer coupling, strain and SOC are also discussed.

Highly mobile charge-transfer excitons in two-dimensional WS2/tetracene heterostructures

Charge-transfer (CT) excitons at heterointerfaces play a critical role in light to electricity conversion using organic and nanostructured materials. However, how CT excitons migrate at these interfaces is poorly understood. We investigate the formation and transport of CT excitons in two-dimensional WS₂/tetracene van der Waals heterostructures. Electron and hole transfer occurs on the time scale of a few picoseconds, and emission of interlayer CT excitons with a binding energy of ~0.3 eV has been observed. Transport of the CT excitons is directly measured by transient absorption microscopy, revealing coexistence of delocalized and localized states. Trapping-detrapping dynamics between the delocalized and localized states leads to stretched-exponential photoluminescence decay with an average lifetime of ~2 ns. The delocalized CT excitons are remarkably mobile with a diffusion constant of ~1 cm² s^-1. These highly mobile CT excitons could have important implications in achieving efficient charge separation.

Emerging photonic architectures in two-dimensional opto-electronics

This review summarizes recent developments in opto-electronic device architectures comprising van der Waals two-dimensional materials for enhanced light–matter interactions.

Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (X_D) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of X_D states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the X_D emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe₂ on a gold substrate, we demonstrate ~6 × 10⁵-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 10³ of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS2 and Yttrium Iron Garnet

Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin-up (-down) electrons are distributed in a +K (-K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many-body effects in a heterostructure geometry comprising a MoS₂ monolayer and YIG. High-order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ∼10¹³ cm^-2, resulting in a large splitting of ∼40 meV between trions and excitons. The trions exhibit a high circular polarization of ∼80% under optical pumping by circularly polarized light at ∼1.96 eV; it is confirmed experimentally that both phonon scattering and electron-hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS₂ monolayer was observed, and a giant MR ratio of ∼30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS₂/CoFe₂O₄ heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.

Robust Stacking-Independent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS₂/WS₂ bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.