Air-Stable, Large-Area 2D Metals and Semiconductors.

Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.

Efficient Multiple Exciton Generation in Monolayer MoS2.

Utilization of the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEGs with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next-generation solar cells and photodetectors.

Remarkably Deep Moiré Potential for Intralayer Excitons in MoSe2/MoS2 Twisted Heterobilayers.

A moiré superlattice formed in twisted van der Waals bilayers has emerged as a new tuning knob for creating new electronic states in two-dimensional materials. Excitonic properties can also be altered drastically due to the presence of moiré potential. However, quantifying the moiré potential for excitons is nontrivial. By creating a large ensemble of MoSe2/MoS2 heterobilayers with a systematic variation of twist angles, we map out the minibands of interlayer and intralayer excitons as a function of twist angles, from which we determine the moiré potential for excitons. Surprisingly, the moiré potential depth for intralayer excitons is up to ∼130 meV, comparable to that for interlayer excitons. This result is markedly different from theoretical calculations based on density functional theory, which show an order of magnitude smaller moiré potential for intralayer excitons. The remarkably deep intralayer moiré potential is understood within the framework of structural reconstruction within the moiré unit cell.

Ultrafast laser ablation, intrinsic threshold, and nanopatterning of monolayer molybdenum disulfide.

Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the ultrafast ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on the femtosecond ablation of 2D materials is studied using MoS2 as an example. We show unambiguously that femtosecond ablation of MoS2 is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS2 with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power ultrafast oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes ultrafast laser ablation as a viable route to pattern 2D materials.

Using Exciton/Trion Dynamics to Spatially Monitor the Catalytic Activities of MoS2 during the Hydrogen Evolution Reaction.

The adsorption and desorption of electrolyte ions strongly modulates the carrier density or carrier type on the surface of monolayer-MoS2 catalyst during the hydrogen evolution reaction (HER). The buildup of electrolyte ions onto the surface of monolayer MoS2 during the HER may also result in the formation of excitons and trions, similar to those observed in gate-controlled field-effect transistor devices. Using the distinct carrier relaxation dynamics of excitons and trions of monolayer MoS2 as sensitive descriptors, an in situ microcell-based scanning time-resolved liquid cell microscope is set up to simultaneously measure the bias-dependent exciton/trion dynamics and spatially map the catalytic activity of monolayer MoS2 during the HER. This operando probing technique used to monitor the interplay between exciton/trion dynamics and electrocatalytic activity for two-dimensional transition metal dichalcogenides provides an excellent platform to investigate the local carrier behaviors at the atomic layer/liquid electrolyte interfaces during electrocatalytic reaction.

In Situ Atomic-Scale Observation of Monolayer MoS2 Devices under High-Voltage Biasing via Transmission Electron Microscopy.

2D materials have great potential for not only device scaling but also various applications. To prompt the development of 2D electronics and optoelectronics, a better understanding of the limitation of materials is essential. Material failure caused by bias can lead to variations in device behavior and even electrical breakdown. In this study, the structural evolution of monolayer MoS2 with high bias is revealed via in situ transmission electron microscopy at the atomic scale. The biasing process is recorded and studied with the aid of aberration-corrected scanning transmission electron microscopy. The effects of electron beam irradiation and biasing are also discussed through the combination of experiments and theory. It is found that the Mo nanoclusters result from disintegration of MoS2 and sulfur depletion, which are induced by Joule heating. The thermal stress can also damage the MoS2 layer and form long cracks in both in situ and ex situ biasing cases. Investigation of the results obtained with different applied voltages helps to further verify the mechanism of evolution and provide a comprehensive study of the function of biasing.

Time-resolved ARPES Determination of a Quasi-Particle Band Gap and Hot Electron Dynamics in Monolayer MoS2.

The electronic structure and dynamics of 2D transition metal dichalcogenide (TMD) monolayers provide important underpinnings both for understanding the many-body physics of electronic quasi-particles and for applications in advanced optoelectronic devices. However, extensive experimental investigations of semiconducting monolayer TMDs have yielded inconsistent results for a key parameter, the quasi-particle band gap (QBG), even for measurements carried out on the same layer and substrate combination. Here, we employ sensitive time- and angle-resolved photoelectron spectroscopy (trARPES) for a high-quality large-area MoS2 monolayer to capture its momentum-resolved equilibrium and excited-state electronic structure in the weak-excitation limit. For monolayer MoS2 on graphite, we obtain QBG values of ≈2.10 eV at 80 K and of ≈2.03 eV at 300 K, results well-corroborated by the scanning tunneling spectroscopy (STS) measurements on the same material.

Moiré potential impedes interlayer exciton diffusion in van der Waals heterostructures.

The properties of van der Waals heterostructures are drastically altered by a tunable moiré superlattice arising from periodically varying atomic alignment between the layers. Exciton diffusion represents an important channel of energy transport in transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers suggested that carriers and excitons exhibit long diffusion, a rich variety of scenarios can exist. In a moiré crystal with a large supercell and deep potential, interlayer excitons may be completely localized. As the moiré period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion should be the longest in commensurate heterostructures where the moiré superlattice is completely absent. Here, we experimentally demonstrate the rich phenomena of interlayer exciton diffusion in WSe2/MoSe2 heterostructures by comparing several samples prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles.

Layer-Dependent and In-Plane Anisotropic Properties of Low-Temperature Synthesized Few-Layer PdSe2 Single Crystals.

Palladium diselenide (PdSe2), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe2 promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe2 is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe2 flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe2 enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe2 in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe2 also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe2 for device applications.

Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment, and valley spin.

Excitons in monolayer semiconductors have a large optical transition dipole for strong coupling with light. Interlayer excitons in heterobilayers feature a large electric dipole that enables strong coupling with an electric field and exciton-exciton interaction at the cost of a small optical dipole. We demonstrate the ability to create a new class of excitons in hetero- and homobilayers that combines advantages of monolayer and interlayer excitons, i.e., featuring both large optical and electric dipoles. These excitons consist of an electron confined in an individual layer, and a hole extended in both layers, where the carrier-species-dependent layer hybridization can be controlled through rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of layer-hybridized valley excitons, which can be used for realizing strongly interacting polaritonic gases and optical quantum controls of bidirectional interlayer carrier transfer.

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.

Evidence of indirect gap in monolayer WSe2.

Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.Monolayer transition metal dichalcogenides have so far been thought to be direct bandgap semiconductors. Here, the authors revisit this assumption and find that unstrained monolayer WSe2 is an indirect-gap material, as evidenced by the observed photoluminescence intensity-energy correlation.

Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment.

Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.

NANOELECTRONICS. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface.

Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.