Transition Metal Dichalcogenide Heterostructures Research Articles

Microwave Facilitated Covalent Organic Framework/Transition Metal Dichalcogenide Heterostructures.

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS2, MoSe2, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe2, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures

In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and hBN layers, and show that it introduces avoided crossings between the TMD and hBN bands, with umklapp processes opening ‘ghost’ avoided crossings in individual bands. Comparison between density functional theory (DFT) and ARPES spectra for the MoSe2/hBN heterostructure shows that the valence bands of MoSe2 and hBN are significantly further separated in energy in experiment as compared to DFT. We then show that a novel scissor operator can be applied to the hBN valence states in the DFT calculations, to correct the band alignment and enable quantitative comparison to ARPES, explaining avoided crossings and other features of band visibility in the ARPES spectra.

Exciton optics, dynamics, and transport in atomically thin semiconductors

Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this Research Update, we review the recent progress in the understanding of exciton optics, dynamics, and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hexagonal boron nitride-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved, angle-resolved photoemission spectroscopy studies. We also show the impact of exciton density, strain, and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.

Proximity spin-orbit coupling in graphene on alloyed transition metal dichalcogenides

The negligible intrinsic spin-orbit coupling (SOC) in graphene can be enhanced by proximity effects in stacked heterostructures of graphene and transition metal dichalcogenides (TMDCs). The composition of the TMDC layer plays a key role in determining the nature and strength of the resultant SOC induced in the graphene layer. Here, we study the evolution of the proximity--induced SOC as the TMDC layer is deliberately defected. Alloyed $\mathrm{G}/{\mathrm{W}}_{\ensuremath{\chi}}{\mathrm{Mo}}_{1\ensuremath{-}\ensuremath{\chi}}{\mathrm{Se}}_{2}$ heterostructures with diverse compositions ($\ensuremath{\chi}$) and defect distributions are simulated using density functional theory. Comparison with continuum and tight-binding models allows both local and global signatures of the metal-atom alloying to be clarified. Our findings show that, despite some dramatic perturbation of local parameters for individual defects, the low-energy spin and electronic behavior follow a simple effective medium model which depends only on the composition ratio of the metallic species in the TMDC layer. Furthermore, we demonstrate that the topological state of such alloyed systems can be feasibly tuned by controlling this ratio.

Stacking-order-dependent interlayer coupling in Janus WSSe/WS2 heterostructures

In transition metal dichalcogenide (TMDC) heterostructures, interlayer coupling plays a crucial role in the design and emergence of diverse properties. Here, we show the control of the interlayer coupling in Janus WSSe/WS2 heterostructures by changing the stacking order. Low-frequency Raman measurements revealed that heterostructures with the S/S interface show strong interlayer coupling compared to those with the S/Se interface. In the case of the S/S interface, photoluminescence (PL) from WS2 was quenched due to charge transfer in the type-II band alignment, while, interestingly, in the case of the S/Se interface, it was enhanced. The stacking-order dependence of the interlayer coupling and photoluminescence are attributed to the direction of interfacial electric field caused by broken out-of-plane mirror symmetry in a WSSe layer. In the case of the S/S interface, the interfacial electric field promotes the charge transfer as evidenced by strong PL quenching. On the contrary, in heterostructures with the S/Se interface, the charge transfer is blocked as the electric field prevents carrier transfer and modulates the band offset. Our results clearly show that Janus TMDCs with intrinsic fields can be effectively used to manipulate the interlayer interactions and optical properties in heterostructures.

Localized interlayer excitons in MoSe2–WSe2 heterostructures without a moiré potential

Interlayer excitons (IXs) in MoSe2–WSe2 heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers. We compare the doping, electric field, magnetic field, and temperature dependence of IXs in a directly contacted MoSe2–WSe2 region to those in a region separated by bilayer hBN. The doping, electric field, and temperature dependence of the narrow IX lines are similar for both regions, but their excitonic g-factors have opposite signs, indicating that the origin of narrow IX PL is not the moiré potential.

Layer-Number Engineered Momentum-Indirect Interlayer Excitons with Large Spectral Tunability.

Interlayer excitons (IXs) in type II van der Waals (vdW) heterostructures are equipped with an oriented permanent dipole moment and long lifetime and thus would allow promising applications in excitonic and optoelectronic devices. However, based on the widely studied heterostructures of transition-metal dichalcogenides (TMDs), IX emission is greatly influenced by the lattice mismatch and geometric misalignment between the constituent layers, increasing the complexity of the device fabrication. Here, we report on the robust momentum-indirect IX emission in TMD/two-dimensional (2D) perovskite vdW heterostructures, which were fabricated without considering the orientation arrangement or momentum mismatch. The IXs show a large diffusion coefficient of ∼10 cm2 s-1, and importantly the IX emission energy can be widely tuned from 1.3 to 1.6 eV via changing the layer number of the 2D perovskite or the thickness of TMD flakes, shedding light on the applications of vdW interface engineering to broad-spectrum optoelectronics.

Strain and external electric field modulation of the electronic and optical properties of GaN/WSe2 vdWHs

Recently, the GaN/WSe2 van der Waals heterostructures (vdWHs) were successfully synthesized in experiments, providing a pathway for designing the nitride semiconductor materials with transition metal dichalcogenides (TMDs) heterostructures. To insight into the physical mechanism of the GaN/WSe2 vdWHs, the geometrical, electronic and optical properties of the systems are examined based on the first-principles calculations. Our results indicate that the GaN/WSe2 heterostructures exhibit typical type-I band alignment with the bandgap values around 2.20 eV at HSE06 level. Biaxial and external electric field not only can cause the evolution of the band alignment to transfer from type-I to type-II, but also can realize semiconductor-metal transition. In addition, compressive strain enhances the light absorption intensity in the ultraviolet region and tensile strain enables the GaN/WSe2 vdWHs to appear a distinct peak in infrared (IR) region. These flexible and tunable characteristics of type-I GaN/WSe2 vdWHs are more convenient for the design of IR detectors, photovoltaic and light emission devices.

Ultrafast phonon‐driven charge transfer in van der Waals heterostructures

AbstractVan der Waals heterostructures built by vertically stacked transition metal dichalcogenides (TMDs) exhibit a rich energy landscape, including interlayer and intervalley excitons. Recent experiments demonstrated an ultrafast charge transfer in TMD heterostructures. However, the nature of the charge transfer process has remained elusive. Based on a microscopic and material‐realistic exciton theory, we reveal that phonon‐mediated scattering via strongly hybridized intervalley excitons governs the charge transfer process that occurs on a sub‐100fs timescale. We track the time‐, momentum‐, and energy‐resolved relaxation dynamics of optically excited excitons and determine the temperature‐ and stacking‐dependent charge transfer time for different TMD bilayers. The provided insights present a major step in microscopic understanding of the technologically important charge transfer process in van der Waals heterostructures.Key Points Microscopic and fully quantum‐mechanic model is developed to calculate exciton dynamics in van der Waals heterostructures Charge transfer occurs on a femtosecond timescale and is a phonon‐mediated two‐step process Strongly hybridized dark exciton states play a crucial role for the charge transfer

(Digital Presentation) Growth of Vdw Heterostructures with Tunable Moiré Patterns

Overlaying two two-dimensional (2D) lattices with different periodicities or a relative twist angle gives rise to a larger (quasi-) periodic lattice called a moiré pattern. In van der Waals (vdW) heterostructures, the moiré pattern has recently been shown to display interesting novel physical phenomena. For instance, twisted bilayer graphene near the “magic angle” of 1.1° hosts flat electronic bands and strongly correlated states such as unconventional superconductivity [1, 2]. On the other hand, the moiré pattern has been found to both localize and tune interlayer excitons in transition metal dichalcogenides (TMDCs) heterostructures with small twist angle [3-5]. Such moiré heterostructures are generally produced by mechanical exfoliation and stacking to precisely control the twist angle and moiré periodicity. However, this laborious method leads to interfacial contamination and twist angle inhomogeneities and is inherently not scalable. In principle, direct growth of van der Waals heterostructures can overcome these limitations. However, due to energetic considerations, control over the twist angle, and therefore the moiré lattice, has not yet been achieved. Herein, using TMDCs as a model system, we demonstrate the scalable growth of moiré heterostructures with continuously tunable periodicity. By controlling the substitutional alloying of larger atoms within the same group of the periodic table (S, Se), we precisely engineer the lattice parameters of the 2D layers and the moiré period. The grown moiré heterostructures have lateral size up to 200 μm and are shown to host moiré excitons. These results lay the groundwork for the integration of moiré heterostructures in emerging quantum technologies.[1] Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Nature 2018, 556 (7699), 43-50.[2] Cao, Y., Fatemi, V., Demir, A., Fang, S., Tomarken, S. L., Luo, J. Y., ... & Jarillo-Herrero, P. 2018. Nature, 556 (7699), 80-84.[3] Yu, H.; Liu, G.-B.; Tang, J.; Xu, X.; Yao, W. (2017). Science Advances 2017. 3 (11), e1701696.[4] Seyler, K. L.; Rivera, P.; Yu, H.; Wilson, N. P.; Ray, E. L.; Mandrus, D. G.; Yan, J.; Yao, W.; Xu, X. Nature 2019, 567 (7746), 66.[5] Jin, C.; Regan, E. C.; Yan, A.; Utama, M. I. B.; Wang, D.; Zhao, S.; Qin, Y.; Yang, S.; Zheng, Z.; Shi, S.; et al. Nature 2019, 567 (7746), 76.

Hybrid heterostructure of transition metal dichalcogenides as potential photocatalyst for hydrogen evolution

Transition metal dichalcogenides (TMDCs) heterostructure with property diversity is of scientific and technological interest for nanoelectronics and photoelectronic devices. Herein, we investigate electronic properties of TMDCs hybrid heterostructures, i.e., (WS2-MoS2)/WS2, (WS2-MoS2)/MoS2 and WS2/(WS2-MoS2)/WS2 heterostructures. From analyses of energetics, charge transfer and interaction strength, we find there is a synergistic effect, i.e., strong intralayer and moderate interlayer interaction, in these hybrid heterostructures. Electrostatic potential analysis implies there is a build-in electric field at interfaces of these hybrid heterostructures, which would influence the migration of photoinduced carriers. Combination of build-in electric field with band offset calculations, separation mechanism of photoinduced carriers is discussed. Our calculated results show that (WS2-MoS2)/WS2 and WS2/(WS2-MoS2)/WS2 hybrid heterostructures are potential type-II photocatalyst for hydrogen evolution. (WS2-MoS2)/MoS2 heterostructure is a candidate material for infrared laser light emitting devices due to its type-I band alignment. Finally, the influence of strain on band gap of (WS2-MoS2)/WS2 hybrid heterostructure is also investigated. We believe our study suggests potential direction for fabrication of novel electronic and photoelectric devices based on TMDCs heterostructures.

Imaging dynamic exciton interactions and coupling in transition metal dichalcogenides.

Transition metal dichalcogenides (TMDs) are regarded as a possible material platform for quantum information science and related device applications. In TMD monolayers, the dephasing time and inhomogeneity are crucial parameters for any quantum information application. In TMD heterostructures, coupling strength and interlayer exciton lifetimes are also parameters of interest. However, many demonstrations in TMDs can only be realized at specific spots on the sample, presenting a challenge to the scalability of these applications. Here, using multi-dimensional coherent imaging spectroscopy, we shed light on the underlying physics-including dephasing, inhomogeneity, and strain-for a MoSe2 monolayer and identify both promising and unfavorable areas for quantum information applications. We, furthermore, apply the same technique to a MoSe2/WSe2 heterostructure. Despite the notable presence of strain and dielectric environment changes, coherent and incoherent coupling and interlayer exciton lifetimes are mostly robust across the sample. This uniformity is despite a significantly inhomogeneous interlayer exciton photoluminescence distribution that suggests a bad sample for device applications. This robustness strengthens the case for TMDs as a next-generation material platform in quantum information science and beyond.

Non-asymptotic quantum scattering theory to design high-mobility lateral transition-metal dichalcogenide heterostructures

Atomistic determination of carrier scattering properties is essential for designing nano-electronic devices in two-dimensional (2D) materials. Traditional quantum scattering theory is developed in an asymptotic limit, thus making it inapplicable for 2D materials and heterostructures. Here, we introduce a new paradigm of non-asymptotic quantum scattering theory to obtain the carrier scattering properties at finite distances from active scattering centers. We develop an atomistic multiscale formalism built on the k⋅p Hamiltonian, supplemented with parameters from first-principles electronic structure calculations. We apply this framework to investigate electron transport in lateral transition-metal dichalcogenide heterostructures and demonstrate enhanced high mobility of the order of 103cm2V−1s−1 at room temperature. The non-asymptotic quantum scattering formalism provides a new frontier to design high-performance mesoscopic devices in 2D materials.

Phonon-assisted transport in van der Waals heterostructure tunnel devices

In this work, we present a first-principles study of quantum transport in tunnel FETs based on van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDs). We focus on 1T-HfSe2 and 1T-SnS2 monolayers to construct a vertical heterostructure with a type-II band alignment. By including dissipative effects due to the electron–phonon interaction, we show that vdW tunnel FETs are highly sensible to the phonon coupling due to polar optical phonons present in TMDs which results in an increased sub-threshold swing (SS) and reduced ON-current. However, vdW TFETs are still able to provide high ON-current values due to the inversion of CB and VB at high VGS and high inter-valley tunneling.

Ultrafast Pump–Probe Microscopy on 2D Transition Metal Dichalcogenides

Although microscopic techniques have been used to characterize transition metal dichalcogenides (TMDs), direct observation of charge carrier dynamics distribution in TMDs with diverse shapes remains unexplored. Herein, ultrafast pump–probe microscopy (UPPM) is employed to reveal the carrier dynamics distribution in molybdenum disulfide (MoS2) and tungsten disulfide (WS2) monolayer of four shapes: triangular (t‐MoS2), curved triangular (c‐MoS2), triangular (t‐WS2), and hexagonal (h‐WS2). Monitoring the photon transmission T at 1.55 eV after pumping with a photon energy of 3.1 eV, a negative ΔT/T occurs in t‐MoS2 and c‐MoS2, while a positive ΔT/T is detected in t‐WS2 and h‐WS2 after 3–7 ps time evolution. This distinctive behavior is attributed to deep/shallow defects below the conduction band minimum (CBM) in MoS2 and WS2. Spatial‐independent ΔT/T is observed in t‐MoS2 and t‐WS2, while the ΔT/T in c‐MoS2 has a rapid decay of photoexcited carriers at the vertices and curved edges. Additionally, a threefold symmetry of ΔT/T is revealed in h‐WS2, attributed to the dissimilar occupation of defect states near the h‐WS2 CBM. This work paves the way for examining charge carrier dynamics of various shapes of TMDs and provides a unique microscopic method for studying the charge carrier dynamics in emerging TMDs heterostructures.

Resonance Couplings in Si@MoS2 Core-Shell Architectures.

Heterostructures of transition metal dichalcogenides and optical cavities that can couple to each other are rising candidates for advanced quantum optics and electronics. This is due to their enhanced light-matter interactions in the visible to near-infrared range. Core-shell structures are particularly valuable for their maximized interfacial area. Here, the chemical vapor deposition synthesis of Si@MoS2 core-shells and extensive structural characterization are presented. Compared with traditional plasmonic cores, the silicon dielectric Mie resonator core offers low Ohmic losses and a wider spectrum of optical modes. The magnetic dipole (MD) mode of the silicon core efficiently couples with MoS2 through its large tangential component at the core surface. Using transmission electron microscopy and correlative single-particle scattering spectroscopy, MD mode splitting is experimentally demonstrated in this unique Si@MoS2 core-shell structure. This is evidence for resonance coupling, which is limited to theoretical proposals in this particular system. A coupling constant of 39 meV is achieved, which is ≈1.5-fold higher than previous reports of particle-on-film geometries with a smaller interfacial area. Finally, higher-order systems with the potential to tune properties are demonstrated through a dimer system of Si@MoS2 , forming the basis for emerging architectures for optoelectronic and nanophotonic applications.

Modification of monolayer 1T-VSe2 by selective deposition of vanadium and tellurium

Hetero-structures of transition metal dichalcogenide (TMD) layers offer vast possibilities of new materials properties. The growth behaviors of such TMD hetero-structures can be complex. In this study, we report the structure modification of a VSe2 monolayer by the molecular beam deposition of Te, V, or both. With a typical growth temperature of 300 °C and a nominal flux rate, we found that Te deposition leaves the VSe2 intact. Vanadium deposition, in contrast, leads to small clusters ordered in stripes along well-defined directions. The charge density wave symmetry of monolayer VSe2 surprisingly changes in small regions enclosed by such vanadium stripes. With V and Te co-deposition, a dramatic change of the monolayer surface structure to a (2 × 1) ordered phase is observed. This study illustrates the unexpected complexities involved in preparing even a simple bilayer TMD hetero-structure, such as VTe2/VSe2.

Direct STM measurements of R-type and H-type twisted MoSe2/WSe2

When semiconducting transition metal dichalcogenide heterostructures are stacked, the twist angle and lattice mismatch lead to a periodic moiré potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0° or 60°, interesting characteristics and properties, such as modulations in the band edges, flat bands, and confinement, are predicted to occur. Here, we report scanning tunneling microscopy and spectroscopy measurements on the bandgaps and band modulations in MoSe2/WSe2 heterostructures with near 0° rotation (R-type) and near 60° rotation (H-type). We find a modulation of the bandgap for both stacking configurations with a larger modulation for R-type than for H-type as predicted by theory. Furthermore, local density of states images show that electrons are localized differently at the valence band and conduction band edges.

Two-Dimensional Perovskite/HfS2 van der Waals Heterostructure as an Absorber Material for Photovoltaic Applications

Van der Waals (vdW) heterostructures of perovskites and transition metal dichalcogenides (TMDCs) have attracted increased interest owing to their extraordinary optoelectronic properties and encouraging applications. Two-dimensional (2D) TMDCs, i.e., hafnium disulfide (HfS2), are also interesting because of their unique optoelectronic properties. Therefore, the combination of these different types of materials is very smart in terms of the fundamental science of interface interaction, as well as for the understanding of ultrathin optoelectronic devices with superior performance. Here, we have systematically modeled the 2D CH3NH3PbI3/HfS2 vdW heterostructure by using first-principles calculations. The substituted interface has enhanced visible-light sensitivity and photoelectrocatalytic activity by reducing the transition energies. The interfacial interaction of both materials effectively tunes the band gap of the interface; therefore, it would significantly improve the photoreactivity for solar cell applications. Due to the presence of small effective masses of electrons–holes, high optical absorption on the order of 105 and high spectroscopic limited maximum efficiency of 28.45% in the CH3NH3PbI3/HfS2 vdW heterostructure will be better candidates in the field of absorber materials. The considered systems are expected to be more efficient in separating the photogenerated electrons–holes and active in the visible spectrum. These theoretical results suggest that the CH3NH3PbI3/HfS2 vdW heterostructure may lead to many novel applications in efficient light-absorbing materials for photovoltaic applications.

Enhancing excitons by oleic acid treatment in WS2, MoS2, and WS2/MoS2 heterostructure

The neutral and interlayer exciton originates from intralayer and interlayer coupling, respectively. Unlike neutral exciton, the interlayer excitons at room temperature are hard to observe and manipulate due to instability. In this work, we show the photoluminescence of WS2 and MoS2 neutral exciton can be improved by oleic acid passivation, allowing trion peaks to be observed at room temperature. More importantly, a 3-fold increase in peak intensity of interlayer excitons is achieved, and the energy peak is blue-shifted 107 meV. Our work paves the way to investigate excitons in two-dimensional transition metal dichalcogenides monolayers and heterostructures at room temperature.