Transition Metal Dichalcogenide Heterostructures Research Articles

Cathodoluminescence emission and electron energy loss absorption from a 2D transition metal dichalcogenide in van der Waals heterostructures

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2 flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2 make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

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 600nm. 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 60ps 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.

Spatial Filtering of Interlayer Exciton Ground State in WSe2/MoS2 Heterobilayer.

Long-life interlayer excitons (IXs) in transition metal dichalcogenide (TMD) heterostructure are promising for realizing excitonic condensates at high temperatures. Critical to this objective is to separate the IX ground state (the lowest energy of IX state) emission from other states' emissions. Filtering the IX ground state is also essential in uncovering the dynamics of correlated excitonic states, such as the excitonic Mott insulator. Here, we show that the IX ground state in the WSe2/MoS2 heterobilayer can be separated from other states by its spatial profile. The emissions from different moiré IX modes are identified by their different energies and spatial distributions, which fits well with the rate-diffusion model for cascading emission. Our results show spatial filtering of the ground state mode and enrich the toolbox to realize correlated states at elevated temperatures.

Spatial Filtering of Interlayer Exciton Ground State in WSe2/MoS2 Heterobilayer.

Long-life interlayer excitons (IXs) in transition metal dichalcogenide (TMD) heterostructure are promising for realizing excitonic condensates at high temperatures. Critical to this objective is to separate the IX ground state (the lowest energy of IX state) emission from other states' emissions. Filtering the IX ground state is also essential in uncovering the dynamics of correlated excitonic states, such as the excitonic Mott insulator. Here, we show that the IX ground state in the WSe2/MoS2 heterobilayer can be separated from other states by its spatial profile. The emissions from different moiré IX modes are identified by their different energies and spatial distributions, which fits well with the rate-diffusion model for cascading emission. Our results show spatial filtering of the ground state mode and enrich the toolbox to realize correlated states at elevated temperatures.

Excitonic response in transition metal dichalcogenide heterostructures from first principles: Impact of stacking, twisting, and interlayer distance

Excitonic response in transition metal dichalcogenide heterostructures from first principles: Impact of stacking, twisting, and interlayer distance

Excitonic response in transition metal dichalcogenide heterostructures from first principles: Impact of stacking, twisting, and interlayer distance

Excitonic response in transition metal dichalcogenide heterostructures from first principles: Impact of stacking, twisting, and interlayer distance

Electrically tunable non-radiative lifetime in WS2/WSe2 heterostructures.

Van der Waals heterostructures based on transition metal dichalcogenides (TMDs) have emerged as excellent candidates for next-generation optoelectronics and valleytronics, due to their fascinating physical properties. The understanding and active control of the relaxation dynamics of heterostructures play a crucial role in device design and optimization. Here, we investigate the back-gate modulation of exciton dynamics in a WS2/WSe2 heterostructure by combining time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) at cryogenic temperatures. We find that the non-radiative relaxation lifetimes of photocarriers in heterostructures can be electrically controlled for samples with different twist-angles, whereas such lifetime tuning is not present in standalone monolayers. We attribute such an observation to doping-controlled competition between interlayer and intralayer recombination pathways in high-quality WS2/WSe2 samples. The simultaneous measurement of TRPL and TAS lifetimes within the same sample provides additional insight into the influence of coexisting excitons and background carriers on the photo-response, and points to the potential of tailoring light-matter interactions in TMD heterostructures.

Control of Hybrid Exciton Lifetime in MoSe2/WS2 Moiré Heterostructures.

Hybrid excitons, characterized by their strong oscillation strength and long lifetimes, hold great potential as information carriers in semiconductors. They offer promising applications in exciton-based devices and circuits. MoSe2/WS2 heterostructures represent an ideal platform for studying hybrid excitons, but how to regulate the exciton lifetime has not yet been explored. In this study, layer hybridization is modulated by applying electric fields parallel or antiparallel to the dipole moment, enabling us to regulate the exciton lifetime from 1.36 to 4.60ns. Furthermore, the time-resolved photoluminescence decay traces are measured at different excitation power. A hybrid exciton annihilation rate of 8.9 × 10-4cm2s-1 is obtained by fitting. This work reveals the effects of electric fields and excitation power on the lifetime of hybrid excitons in MoSe2/WS2 1.5° moiré heterostructures, which play important roles in high photoluminescence quantum yield optoelectronic devices based on transition-metal dichalcogenides heterostructures.

Twist angle-dependent valley polarization switching in heterostructures.

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

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

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

Exploring Electrostatic Confinement Transport in MoS2/WSe2 Heterostructure via Triple‐Gated Point Contact Device

AbstractThe exponential development in quantum phenomena is directly correlated with the decreasing size of nano‐semiconductor transistors. Consequently, the use of a quantum structure that deviates from traditional transistor types becomes imperative. Electrostatically defined nanoscale devices within 2D semiconductor heterostructures serve as foundational elements for diverse quantum electrical circuits. Van der Waals heterostructures, distinguished by atomically flat interfaces and inherent 2D characteristics, offer advantages such as large‐scale uniformity, flexibility, and portability over conventional bulk semiconductor heterostructures. Herein, the intricate electronic behavior of a MoS2/WSe2 encapsulated heterostructure governed by split‐gate and middle‐gate configurations is investigated, revealing a distinctive step‐like current profile at a low temperature of 77 K. The observed four regimes in the current highlight the impact of quantum confinement induced by reduced lateral dimensions, coupled with precise electrostatic confinement controlled by gate voltages. The temperature dependence of the phenomena emphasizes the role of thermal effects on carrier scattering mechanisms. In addition, the pinch‐off characteristics with different temperatures, middle‐gate voltages, and drain biases are explored. This study contributes to a deeper understanding of electrostatic effects in 2D transition metal dichalcogenide heterostructures and holds promise for the development of advanced electronic devices with tailored confinement for enhanced functionalities.

High-Performance Broadband Image Sensing Photodetector Based on MnTe/WS2 van der Waals Epitaxial Heterostructures.

Two-dimensional transition metal dichalcogenide (TMDC) heterostructure is receiving considerable attention due to its novel electronic, optoelectronic, and spintronic devices with design-oriented and functional features. However, direct design and synthesis of high-quality TMDC/MnTe heterostructures remain difficult, which severely impede further investigations of semiconductor/magnetic semiconductor devices. Herein, the synthesis of high-quality vertically stacked WS2/MnTe heterostructures is realized via a two-step chemical vapor deposition method. Raman, photoluminescence, and scanning transmission electron microscopy characterizations reveal the high-quality and atomically sharp interfaces of the WS2/MnTe heterostructure. WS2/MnTe-based van der Waals field effect transistors demonstrate high rectification behavior with rectification ratio up to 106, as well as a typical p-n electrical transport characteristic. Notably, the fabricated WS2/MnTe photodetector exhibits sensitive and broadband photoresponse ranging from UV to NIR with a maximum responsivity of 1.2 × 103 A/W, a high external quantum efficiency of 2.7 × 105%, and fast photoresponse time of ∼50 ms. Moreover, WS2/MnTe heterostructure photodetectors possess a broadband image sensing capability at room temperature, suggesting potential applications in next-generation high-performance and broadband image sensing photodetectors.

Anomalous redshift in interlayer exciton emission with increasing twist angle in WSe2/MoSe2 heterostructures

Van der Waals heterostructures utilizing semiconducting transition metal dichalcogenide (TMDC) monolayers have surfaced as compelling candidates due to their intriguing optical characteristics, which can be effectively controlled by the manipulation of the stacking twist angle. This study investigates the intricate correlation between twist angle, band offset, and interlayer exciton emission within twisted WSe2/MoSe2 heterostructures. Our findings suggest a crucial influence of monolayer stacking order on the band offset and the dipole orientation in twisted heterostructures that leads to either blueshift or redshift in emission energy. Herein, we fabricate heterobilayers with twist angles varying from 1 ∘ to 56 ∘ and observe an anomalous redshift energy of 100 meV in the interlayer exciton emission. Additionally, photoluminescence excitation spectroscopy measurements highlight the systematic twist angle dependence of intralayer exciton resonances, indicating significant angle dependent effects on individual monolayer bandgaps and on the interlayer coupling strength. Our fundamental study of exciton resonances provides comprehensive insights into the nuanced interplay between twist angle, dipole orientation, and dielectric asymmetry, providing a deeper understanding of the factors governing the optical properties of layered TMDC heterostructures.

Electrochemically deposited transition metal dichalcogenide heterostructures as electrocatalysts: Accelerated kinetics for the hydrogen evolution reaction

Transition metal dichalcogenide (TMD) heterostructures have been discovered to have improved catalytic activity towards the hydrogen evolution reaction (HER). This study explores the stability and HER catalytic activity including reaction kinetics of heterolayers of different TMDs (MoS2, MoSe2 and WS2). The stability of the heterolayers varied with those having an overlayer of electrodeposited MoS2 being more stable as compared to those with MoSe2 overlayer which degraded with each scan in acidic media. Investigation into the HER kinetics of the heterolayers involved Tafel analysis and electrochemical rate constant calculation. There was an improvement in Tafel values calculated in comparison to reported values for these heterolayers. WS2/MoS2 and MoSe2/MoS2 heterolayers registered rate constants of (3.20 ± 0.10) × 10−4 cm s−1 and (1.73 ± 0.03) × 10−4 cm s−1 respectively, which was an improvement of up to an order of magnitude compared to the reported rate constant of electrodeposited MoS2 of (3.17 ± 0.30) × 10−5 cm s−1. All this highlights the improved HER catalytic activity of the heterolayers.

Synthesis of two-dimensional MoO2 nanoplatelets and its multistep sulfurization into MoS2

To control the growth of layered two-dimensional structures, such as transition metal dichalcogenide materials or heterostructures, understanding the growth mechanism is crucial. Here, we report the synthesis of ultra-thin MoO2 nanoplatelets through the sublimation of MoO3. Rhombus MoO2 nanoplatelets with the P21/c space group were characterized using various microscopic and spectroscopic techniques. Introducing sulfur sources into the chemical vapor deposition system also leads to the formation of monoclinic MoO2 nanoflakes due to the incomplete sulfurization of MoO3. With a gradual increase in the vapor concentration of sulfur, MoO3 undergoes stepwise reduction into MoS2/MoO2 and eventually into MoS2. Additionally, utilizing MoO2 as a precursor for Mo sources enables the formation of monolayer MoS2 single crystals. This work provides an effective approach for growing MoO2 nanoplatelets and elucidates the mechanism behind the stepwise sulfurization of MoO3.

Spotlight: Visualization of Moiré Quantum Phenomena in Transition Metal Dichalcogenide with Scanning Tunneling Microscopy.

Transition metal dichalcogenide (TMD) moiré superlattices have emerged as a significant area of study in condensed matter physics. Thanks to their superior optical properties, tunable electronic band structure, strong Coulomb interactions, and quenched electron kinetic energy, they offer exciting avenues to explore correlated quantum phenomena, topological properties, and light-matter interactions. In recent years, scanning tunneling microscopy (STM) has made significant impacts on the study of these fields by enabling intrinsic surface visualization and spectroscopic measurements with unprecedented atomic scale detail. Here, we spotlight the key findings and innovative developments in imaging and characterization of TMD heterostructures via STM, from its initial implementation on the in situ grown sample to the latest photocurrent tunneling microscopy. The evolution in sample design, progressing from a conductive to an insulating substrate, has not only expanded our control over TMD moiré superlattices but also promoted an understanding of their structures and strongly correlated properties, such as the structural reconstruction and formation of generalized two-dimensional Wigner crystal states. In addition to highlighting recent advancements, we outline upcoming challenges, suggest the direction of future research, and advocate for the versatile use of STM to further comprehend and manipulate the quantum dynamics in TMD moiré superlattices.

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 equationmodel 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 of 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.

Twistronics in two-dimensional transition metal dichalcogenide (TMD)-based van der Waals interface.

Transition metal dichalcogenides (TMD) based heterostructures have gained significant attention lately because of their distinct physical properties and potential uses in electronics and optoelectronics. In the present work, the effects of twist on the structural, electronic, and optical properties (such as the static dielectric constant, refractive index, extinction coefficient, and absorption coefficient) of vertically stacked TMD heterostructures, namely MoSe2/WSe2, WS2/WSe2, MoSe2/WS2 and MoS2/WSe2, have been systematically studied and a thorough comparison is done among these heterostructures. In addition, the absence of negative frequency in the phonon dispersion curve and a low formation energy confirm the structural and thermodynamical stability of all the proposed TMD heterostructures. The calculations are performed using first-principles-based density functional theory (DFT) method. Beautiful Moiré patterns are formed due to the relative rotation of the layers as a consequence of the superposition of the periodic structures of the TMDs on each other. Twist engineering allows the modulation of bandgaps and a phase change from direct to indirect band gap semiconductors as well. The high optical absorption in the visible range of spectrum makes these twisted heterostructures very promising candidates in photovoltaic applications.

Changes of phonon modes and electron transfer induced by interface interactions of Pd/MoS2 heterostructures.

As functional materials and nano-catalysts, Pd nanoparticles (NPs) are often used to modify two-dimensional (2D) materials. In the heterostructures of metal NPs and 2D transition metal dichalcogenides, the interface atomic configuration and interface effect greatly affect material properties and stability. Therefore, the rational design of interface structures and in-depth analysis of interface interactions are of vital importance for the preparation of specific functional devices. In this work, Pd NPs were deposited on mechanically exfoliated MoS2 flakes and the epitaxial relationship between Pd and MoS2 was observed, accompanied by distinct moiré patterns. Raman spectra of the Pd NPs/MoS2 heterostructure showed an E12g' vibration mode indicative of the local strain in MoS2. A new vibration mode A'1g appeared in the higher-frequency direction compared with the pristine A1g peak. Combined with X-ray photoelectron spectra and density functional theory calculations, the new vibration mode can be attributed to the bonding between Pd and MoS2. Besides, graphene was inserted between Pd NPs and MoS2, and the decoupling of the interfacial effect by graphene was investigated. This study will help deepen our understanding on the interaction mechanism between metals and MoS2, thereby enabling the modulation of optoelectronic properties and the performance of these hybrid materials.

A new charge transfer pathway in the MoSe2-WSe2 heterostructure under the conditions of B-excitons being resonantly pumped.

Most transition metal dichalcogenide (TMD) heterostructures (HSs) exhibit a type II band alignment, leading to a charge transfer process accompanied by the transfer of spin-valley polarization and spontaneous formation of interlayer excitons. This unique band structure facilitates achieving a longer exciton lifetime and extended spin-valley polarization lifetime. However, the mechanism of charge transfer in type II TMD HSs is not fully comprehended. Here, the ultrafast charge transfer process is studied in MoSe2-WSe2 HS via valley-solved broadband pump-probe spectroscopy. Under the conditions of B-excitons of WSe2 and MoSe2 being resonantly pumped, a new charge transfer pathway through the higher energy state associated with the B-exciton is found. Meanwhile, the holes (electrons) in the WSe2 (MoSe2) layer of MoSe2-WSe2 HS produce obvious spin-valley polarization even under the condition of B-exciton of WSe2 (MoSe2) being resonantly pumped, and the lifetime can reach tens of ps, which is in stark contrast to the absence of A-exciton spin-valley polarization in monolayer WSe2 (MoSe2) under the same pumping condition. The results deepen the insight into the charge transfer process in type II TMD HSs and show the great potential of TMD HSs in the application of spin-valley electronics devices.