Two-dimensional Transition Metal Dichalcogenides Heterostructures Research Articles

Automated mechanical exfoliation technique: a spin pumping study in YIG/TMD heterostructures.

Spintronics devices rely on the generation and manipulation of spin currents. Two-dimensional transition-metal dichalcogenides (TMDs) are among the most promising materials for a spin current generation due to a lack of inversion symmetry at the interface with the magnetic material. Here, we report on the fabrication of Yttrium Iron Garnet(YIG)/TMD heterostructures by means of a crude and fast method. While the magnetic insulator single-crystalline YIG thin films were grown by magnetron sputtering, the TMDs, namely MoS2 and MoSe2, were directly deposited onto YIG films using an automated mechanical abrasion method. Despite the brute force aspect of the method, it produces high-quality interfaces, which are suitable for spintronic device applications. The spin current density and the effective spin mixing conductance were measured by ferromagnetic resonance, whose values found are among the highest reported in the literature. Our method can be scaled to produce ferromagnetic materials/TMD heterostructures on a large scale, further advancing their potential for practical applications.

Efficient photoelectrodes based on two-dimensional transition metal dichalcogenides heterostructures: from design to construction

Efficient photoelectrodes based on two-dimensional transition metal dichalcogenides heterostructures: from design to construction

Plasmonic Photonic Crystal Mirror for Long-Lived Interlayer Exciton Generation

Interlayer excitons in van der Waals heterostructures of two-dimensional transition-metal dichalcogenides have recently emerged as a fascinating platform for quantum many-body effects, long-range interactions, and optovalleytronic applications. The practical implementation of such phenomena and applications requires further development of the long-lived character of interlayer excitons. Whereas material developments have successfully enhanced the nonradiative lifetime, the out-of-plane polarization nature of the interlayer excitons has made it challenging to improve the radiative lifetime with conventional photonic mirrors. Here, we propose and systematically analyze a plasmonic photonic crystal (PPhC) mirror that can increase the radiative lifetime of interlayer excitons by 2 orders of magnitude. Based on the vacuum field transition, the PPhC mirror supports spatially uniform radiative decay suppression over its territory, which is crucial for engineering the interlayer excitons not localized at a specific position. The PPhC mirror platform offers new possibilities for realizing long-lived interlayer exciton-based nanodevices.

Valley-Dependent Interlayer Excitons in Magnetic WSe2/CrI3.

Heterostructures of two-dimensional transition-metal dichalcogenides and ferromagnetic substrates are important candidates for the development of viable new spin- or valleytronic devices. For the prototypical bilayer of WSe2 on top of a ferromagnetic layer of CrI3, we find substantially different coupling of both WSe2 K-valleys to the sublayer. Besides an energy splitting of a few meV, the corresponding excitons have significantly different interlayer character with charge transfer allowed at the K̅- point but forbidden at K̅+. The different exciton wave functions result in a distinctly different response to magnetic fields with g factors of about -4.4 and -4.0, respectively. By means of ab initio GW/Bethe-Salpeter equation calculations, these findings establish g factors as tool for investigating the exciton character and shedding light on the detailed quantum-mechanical interplay of magnetic and optical properties which are essential for the targeted development of optoelectronic devices.

Interlayer Trions in 2D Transition-Metal Dichalcogenide Heterostructures

A dimensionally confined dielectric constant and reduced dielectric screening lead to two-dimensional transition-metal dichalcogenides (TMDs). The confined dielectric constant triggers a strong Coulomb interaction and high exciton binding energies between TMD heterostructures. Here we show a highly efficient interlayer charged exciton or trion formation and its generation sites are present in materials at room temperature. Two different transition metals (Mo, W) and chalcogenide (S, Se) elements were investigated in two different heterostructure combinations (MoS2—WS2 and MoSe2—WS2). Room temperature photoluminescence measurements demonstrate a highly efficient trion formation between and close to the heterostructure interfaces. This study highlights an effective band alignment, strong photoexciting Coulomb interaction, and formation of interlayer trions with different recombination energies. This investigation suggests the possibility of utilizing interlayer trions in promising optoelectronic devices in the future.

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMD) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMD monolayers and heterostructures in consideration of the material’s structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS2 flakes grown on a SiO2/ Si(001) substrate are presented.

Synthetic Engineering of Morphology and Electronic Band Gap in Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides.

Heterostructures of two-dimensional transition metal dichalcogenides (TMDs) can offer a plethora of opportunities in condensed matter physics, materials science, and device engineering. However, despite state-of-the-art demonstrations, most current methods lack enough degrees of freedom for the synthesis of heterostructures with engineerable properties. Here, we demonstrate that combining a postgrowth chalcogen-swapping procedure with the standard lithography enables the realization of lateral TMD heterostructures with controllable dimensions and spatial profiles in predefined locations on a substrate. Indeed, our protocol receives a monolithic TMD monolayer (e.g., MoSe2) as the input and delivers lateral heterostructures (e.g., MoSe2-MoS2) with fully engineerable morphologies. In addition, through establishing MoS2xSe2(1-x)-MoS2ySe2(1-y) lateral junctions, our synthesis protocol offers an extra degree of freedom for engineering the band gap energies up to ∼320 meV on each side of the heterostructure junction via changing x and y independently. Our electron microscopy analysis reveals that such continuous tuning stems from the random intermixing of sulfur and selenium atoms following the chalcogen swapping. We believe that, by adding an engineering flavor to the synthesis of TMD heterostructures, our study lowers the barrier for the integration of two-dimensional materials into practical optoelectronic platforms.

Epitaxial Synthesis of Monolayer PtSe2 Single Crystal on MoSe2 with Strong Interlayer Coupling.

PtSe2, a layered two-dimensional transition-metal dichalcogenide (TMD), has drawn intensive attention owing to its layer-dependent band structure, high air stability, and spin-layer locking effect which can be used in various applications for next-generation optoelectronic and electronic devices or catalysis applications. However, synthesis of PtSe2 is highly challenging due to the low chemical reactivity of Pt sources. Here, we report the chemical vapor deposition of monolayer PtSe2 single crystals on MoSe2. The periodic Moiré patterns from the vertically stacked heterostructure (PtSe2/MoSe2) are clearly identified via annular dark-field scanning transmission electron microscopy. First-principles calculations show a type II band alignment and reveal interface states originating from the strong-weak interlayer coupling (SWIC) between PtSe2 and MoSe2 monolayers, which is supported by the electrostatic force microscopy imaging. Ultrafast hole transfer between PtSe2 and MoSe2 monolayers is observed in the PtSe2/MoSe2 heterostructure, matching well with the theoretical results. Our study will shed light on the synthesis of Pt-based TMD heterostructures and boost the realization of SWIC-based optoelectronic devices.

(Invited) in-Vacuo Studies of MBE Grown Transition Metal Dichalcogenides

Here we present recent work on the synthesis and characterization of two-dimensional transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE). Bulk TMD crystals consist of two-dimensional layers bonded together by van der Waal interactions; the lack of primary bonds at the TMD interface allow for the growth of TMD heterostructures on a variety of substrates without the need to consider lattice constant mismatch or crystal structure of the substrate which we aim to demonstrate. TMDs are grown and characterized in an all in-vacuo ultra-high vacuum system which enables the simultaneous co-deposition of multiple transition metals and chalcogens allowing for the study of TMD ternary alloys and layered heterostructures followed by their compositional and intrinsic electronic characterization prior to atmospheric exposure. Presented will be a review of our work studying the interface chemistry between TMD and conventional semiconducting substrates, as well as our investigations of 2D-2D heterostructure grown by MBE. Figure 1 shows an example of in-vacuo x-ray photoelectron spectroscopy (XPS) carried out on GaAs(001) before an after the growth of monolayer MoSe2. The sample was initial passivated by a sulfur based treatment, and the presence of sulfur bonded to the surface can be observed in the As 2p and Ga 2p spectra. The asymmetry observed in the As 2p and Ga 2p core-levels suggest that some reactions with Se take place during the growth. The thermal stability of this interface will be investigated using in-situ UHV heating and XPS. In separate work we evaluated the valence band offsets using in-vacuo XPS and UPS (ultraviolet photoelectron spectroscopy). This is achieved by measuring core-level to valence band maximum separation for control samples as well as the core-level shifts in heterostructure. In-vacuo XPS analysis can also be used to reveal any interface reactors that may occur between the TMDs. It is found that for chalcogen deficient growths, metallic signatures can be detected. Of particular interest is the process structure relationships. Presented will be parametric studies showing impact of growth temperature and relative metal-chalcogen flux ratios on the electronic structure and interface chemistry in both the 2D-2D and 2D-3D systems. The all in-vacuo synthesis and analysis cluster tool allows these investigations to be carried out without air exposure which can induce substantial changes in the chemistry of these materials. Figure 1

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research, but also for the application of next generation electronic and optoelectronic devices. Herein, we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In2Se3/MoSe2 heterostructures. Transmission electron microscopy characterization reveals clearly that the In2Se3 has well-aligned lattice orientation with the substrate of monolayer MoSe2. Due to the interaction between the In2Se3 and MoSe2 layers, the heterostructure shows the quenching and red-shift of photoluminescence. Moreover, the current rectification behavior and photovoltaic effect can be observed from the heterostructure, which is attributed to the unique band structure alignment of the heterostructure, and is further confirmed by Kevin probe force microscopy measurement. The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.

Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures

Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.

Charge Separation and Recombination in Two-Dimensional MoS2/WS2: Time-Domain ab Initio Modeling

With applications in high performance electronics, photovoltaics and catalysis, two-dimensional transition metal dichalcogenides (TMDs) attract strong attention. Isolated TMDs, which are already remarkably complex, can stack in sequence to make even more complex heterostructures. Surprisingly, charge separation is ultrafast in layered TMD heterostructures, even though the interlayer interaction is weak. Also surprisingly, the charge separated state is long-lived, despite the close proximity of electron and hole. Using real-time time-dependent density functional theory combined with nonadiabatic (NA) molecular dynamics, we model hole and electron transfer, and electron–hole recombination at a MoS2/WS2 heterojunction. Hole transfer is ultrafast, in excellent agreement with the experiment, due to significant delocalization of the photoexcited state between the donor and acceptor materials. Electron transfer is 1 order of magnitude longer, due to weaker donor–acceptor and NA couplings, lower density of accept...

Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods.