Strong Interlayer Coupling Research Articles

Direct formation of interlayer exciton in two-dimensional van der Waals heterostructures.

In atomically thin two-dimensional van der Waals (2D vdW) heterostructures, spatially separated interlayer excitons play an important role in the optoelectronic performance and show great potential for the exploration of many-body quantum phenomena. A commonly accepted formation mode for interlayer excitons is via a two-step intralayer exciton transfer mechanism, namely, photo-excited intralayer excitons are initially generated in individual sublayers, and photogenerated electrons and holes are then separated into opposite sublayers based on the type-II band alignment. Herein, we expand the concept of interlayer exciton formation and reveal that bright interlayer excitons can be generated in one step by direct interlayer photoexcitation in 2D vdW heterostructures that have strong interlayer coupling and a short photoexcitation channel. First-principles and many-body perturbation theory calculations demonstrate that indium selenide/antimonene and indium selenide/black phosphorus heterostructures are two promising systems that show an exceptionally large interlayer transition probability (>500 Debye2). This study enriches the understanding of interlayer exciton formation and provides a new avenue to acquiring strong interlayer excitons in artificial 2D vdW heterostructures.

Optical conductivity of twisted bilayer graphene under heterostrain

<sec>Twisted bilayer graphene (TBG) is a two-dimensional material composed of two layers stacked at a certain angle. When the twisted angle decreases, the lattice mismatch between two layers produces moiré pattern at a long wavelength which significantly modifies the low-energy band structure. In particular, when the twisted angle is close to the so-called “magic angle”, two moiré flat bands are formed near a charge neutral point due to the strong interlayer coupling. These flat bands with high density of states are essential in realizing superconductivity and correlated insulating states. More recently, the magic angle TBG combining an hBN system has exhibited spin-valley polarization when 3/4 of flat bands are filled, thereby providing an ideal platform to achieve quantum anomalous Hall states. Whether it is TBG system or TBG-hBN system, the flat band becomes a crucial condition for discovering so rich physical connotations. Besides the twisted angle, the strain gives an alternative way to modulate flat bands. It has been reported that applying heterostrain in magic angle TBG can makes flat moiré band tunable; strain can also generate flat bands in non-magic angle TBG. Moreover, the reconstruction of TBG due to the strain gives rise to a serial of novel physical phenomena such as topological protected soliton and photonic crystal. Another reason for studying strain effect is that the strain is ubiquitous in the fabrication progress. The strain can also be controlled via piezoelectric substrate which makes possible the in situ modulation of correlated states, topology and quantum effect. </sec><sec>Our work is to study the heterostrain effect in TBG band structure and optical conductivity by using a continuum model. Although the resulting conduction band and valence bands keep connected through Dirac points protected by the <i>C</i><sub>2</sub> symmetry, their separation increases significantly when heterostrain is applied while the Dirac point is also shifted. The optical conductivity is characterized by a series of peaks associated with van Hove singularities, and the peak energies are systematically shifted with the strain amplitude. These changes show that the heterostrain exerts a great influence on electron property of TBG.</sec>

Anti-phase relay synchronization of wave structures in a heterogeneous multiplex network of 2D lattices

We explore numerically relay synchronization of wave structures in a heterogeneous three-layer network of coupled two-dimensional (2D) lattices of continuous-time systems. Remote layers, which are not directly connected but interact via a relay layer, consist of coupled van der Pol oscillators, while the middle layer is described by a lattice of interacting FitzHugh-Nagumo neurons. We show for the first time that already for weak inter-layer coupling, anti-phase relay synchronization of target wave patterns occurs in the considered network. This is a novel effect which can be observed in multiplex networks of interacting 2D lattices of oscillatory systems. Our numerical studies indicate that strong inter-layer coupling leads to in-phase synchronization of spatio-coherent structures in the network layers. We also analyze the impact of inter-layer coupling ranges in the relay and the remote layers on the relay synchronization effect.

Surface-Redox Features of Molecular-Level Controlled 2D Oxidized Black Phosphorus

Two-dimensional (2D) nanomaterials have gained much attention for energy applications over other dimensional materials due to their unique atomic orientation and physiochemistry, which allows fully utilization of exposed surface. Here, we highlight the importance of surface-redox features on 2D oxidized black phosphorus (oBP) surface chemically coupled with percolated graphene. The redox-active oxidized P sites and strong covalent bonds between oBP and graphene play the pivotal role for highly efficient energy storage. The interplay of oxidized P sites on the surface and strong interlayer coupling at heterointerface on the unique charge storage mechanism were investigated by several in-situ spectroscopic techniques and computational method along with electrochemical performance of hybrid cell.

Self‐Driven WSe2/Bi2O2Se Van der Waals Heterostructure Photodetectors with High Light On/Off Ratio and Fast Response

AbstractBenefiting from the superior electron mobility and good air‐stability, the emerging layered bismuth oxyselenide (Bi2O2Se) nanosheet has received considerable attention with the promising prospects for electronics and optoelectronics applications. However, the high charge carrier concentration and bolometric effect of Bi2O2Se give rise to the high dark current and relatively slow photoresponse, which severely impede further improvement of the performance of Bi2O2Se based photodetectors. Here, a WSe2/Bi2O2Se Van der Waals p‐n heterostructure is reported with a pronounced rectification ratio of 105 and a low reverse dark current of 10−11 A, as well as an enhanced light on/off ratio up to 618 under 532 nm light illumination. The device also exhibits a fast response speed of 2.6 µs and a broadband detection capability from 365 to 2000 nm due to the efficient charge separation and strong interlayer coupling at the interface of the two flakes. Importantly, the built‐in potential in the WSe2/Bi2O2Se heterostructure offers a competitive self‐powered photodetector with the light on/off ratio above 105 and a photovoltaic responsivity of 284 mA W−1. The WSe2/Bi2O2Se heterostructure shows promising potentials for high‐performance self‐driven photodetector applications.

Strong coupling and pressure engineering in WSe2–MoSe2 heterobilayers

Two-dimensional materials offer an exciting platform that enables the creation of van der Waals heterostructures with rich functions and intriguing physical properties that stem from different band alignments and diverse interlayer interactions. However, further exploration of two-dimensional van der Waals heterostructures is hindered by the limited coupling strength and lack of efficient methods for tuning the interlayer interactions. Here, by using a two-step chemical vapour deposition method, we realize high-quality 2H-stacked WSe2–MoSe2 heterostructures with strong interlayer coupling, and effective tuning of their interlayer interaction by hydrostatic pressure. We unambiguously establish the strong coupling nature in these WSe2–MoSe2 heterostructures through the existence of exclusive interlayer excitons instead of the typical intralayer excitons. We further demonstrate efficient tuning of the interlayer coupling by using pressure engineering, and observe a clear evolution and transition of interlayer excitons in WSe2–MoSe2 heterostructures with a pressure-induced band changeover, which is further confirmed by first-principles calculations. Our findings provide new opportunities for producing, exploring and tuning van der Waals heterostructures with strong interlayer coupling that can lead towards the realization of future excitonic devices based on tailor-made, atomically thin, two-dimensional stacks. High-quality WSe2–MoSe2 heterostructures support strong coupling between the two layers, which is associated with tight hybridization and effective charge separation. In these structures, the bands of the interlayer excitons can be pressure-engineered.

Synchronization effects for dissipative and inertial coupling between multiplex lattices

We analyse the synchronization of spatiotemporal patterns based on spiral and target waves, which are realized in a multiplex network consisting of two nonidentical 2D lattices of the nonlocally coupled van der Pol oscillators. We study the syncronization and its features for two types of the inter-layer unidirectional coupling, namely dissipative and inertial. We show that the synchronization features for these cases are significantly different. When the inter-layer coupling is dissipative the synchronization between the layers is observed only for the strong inter-layer coupling or is even impossible. For the case of inertial interaction the layers are synchronized at sufficiently weak coupling strength. At the same time, there are “desynchronization windows”, in which the layer regimes are no longer synchronized. We explain these phenomena by a noticeably different influence of the inter-layer coupling on the dynamics of individual oscillators in the cases of dissipative and inertial interaction of the layers.

Gate-Tuned Interlayer Coupling in van der Waals Ferromagnet Fe_{3}GeTe_{2} Nanoflakes.

The weak interlayer coupling in van der Waals (vdW) magnets has confined their application to two dimensional (2D) spintronic devices. Here, we demonstrate that the interlayer coupling in a vdW magnet Fe_{3}GeTe_{2} (FGT) can be largely modulated by a protonic gate. With the increase of the protons intercalated among vdW layers, interlayer magnetic coupling increases. Because of the existence of antiferromagnetic layers in FGT nanoflakes, the increasing interlayer magnetic coupling induces exchange bias in protonated FGT nanoflakes. Most strikingly, a rarely seen zero-field cooled (ZFC) exchange bias with very large values (maximally up to 1.2kOe) has been observed when higher positive voltages (V_{g}≥4.36 V) are applied to the protonic gate, which clearly demonstrates that a strong interlayer coupling is realized by proton intercalation. Such strong interlayer coupling will enable a wider range of applications for vdW magnets.

Current-induced CrI3 surface spin-flop transition probed by proximity magnetoresistance in Pt

By exploiting proximity coupling, we probe the spin state of the surface layers of CrI3, a van der Waals magnetic semiconductor, by measuring the induced magnetoresistance (MR) of Pt in Pt/CrI3 nano-devices. We fabricate the devices with clean and stable interfaces by placing freshly exfoliated CrI3 flake atop pre-patterned thin Pt strip and encapsulating the Pt/CrI3 heterostructure with hexagonal boron nitride (hBN) in a protected environment. In devices consisting of a wide range of CrI3 thicknesses (30–150 nm), we observe that an abrupt upward jump in Pt MR emerge at a 2 T magnetic field applied perpendicularly to the layers when the current density exceeds 2.5 × 1010 A m−2, followed by a gradual decrease over a range of 5 T. These distinct MR features suggest a spin-flop transition which reveals strong antiferromagnetic interlayer coupling in the surface layers of CrI3. We study the current dependence by holding the Pt/CrI3 sample at approximately the same temperature to exclude the joule heating effect, and find that the MR jump increases with the current density, indicating a spin current origin. This spin current effect provides a new route to control spin configurations in insulating antiferromagnets, which is potentially useful for spintronic applications.

Two ultra-stable novel allotropes of tellurium few-layers**Project supported by the Science Fund from the Ministry of Science and Technology (MOST) of China (Grant No. 2018YFE0202700), the National Natural Science Foundation of China (Grant Nos. 11274380, 91433103, 11622437, 61674171, 11974422, and 61761166009), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB30000000), the Fundamental Research Funds for

At least four two- or quasi-one-dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te, namely the α, β, γ, δ, and chiral-α + δ phases. Among them the γ and α phases were found to be the most stable phases for monolayer and thicker layers, respectively. Here, we found two novel low-dimensional phases, namely the ε and ζ phases. The ζ phase is over 29 meV/Te more stable than the most stable monolayer γ phase, and the ε phase shows comparable stability with the most stable monolayer γ phase. The energetic difference between the ζ and α phases reduces with respect to the increased layer thickness and vanishes at the four-layer (12-sublayer) thickness, while this thickness increases under change doping. Both ε and ζ phases are metallic chains and layers, respectively. The ζ phase, with very strong interlayer coupling, shows quantum well states in its layer-dependent bandstructures. These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases.

Selective Synthesis of Bi2Te3/WS2 Heterostructures with Strong Interlayer Coupling.

The vertical integration of atomically thin-layered materials to create van der Waals heterostructures (vdWHs) has been proposed as a method to design nanostructures with emergent properties. In this work, epitaxial Bi2Te3/WS2 vdWHs are synthesized via a two-step vapor deposition process. It is calculated that the vdWH has an indirect band gap with a valence band edge that bridges the vdW gap, resulting in a quenched photoluminescence (PL) from the WS2 monolayer, reduced intensity of its resonance Raman vibrational peaks, improved vertical charge transport, and a decrease in the intensity of second harmonic generation (SHG). Furthermore, it is observed that induced defects strongly influence the nucleation and growth of vdWHs. By creating point defects in WS2 monolayers, it is shown that the growth of Bi2Te3 platelets can be patterned. This work offers important insights into the synthesis, defect engineering, and moiré engineering of an emerging class of two-dimensional (2D) heterostructures.

Twisted bilayer graphene fabricated by direct bonding in a high vacuum

Twisted bilayer graphene (TBG), in which two monolayer graphene are stacked with an in-plane rotation angle, has recently become a hot topic due to unique electronic structures. TBG is normally produced in air by the tear-and-stack method of mechanical exfoliation and transferring graphene flakes, by which a sizable, millimeter-order area, and importantly clean interface between layers are hard to obtain. In this study, we resolved these problems by directly transferring the easy-to-exfoliate CVD-grown graphene on SiC substrate to graphene in a high vacuum without using any transfer assisting medium and observed electronic band modulations due to the strong interlayer coupling.

Observation of excitonic series in monolayer and few-layer black phosphorus

We report the full sequence of excitonic states in monolayer and few-layer black phosphorus (BP). High-quality BP samples are acquired by treating the silicon oxide substrate with a self-assembled monolayer hydrophobic molecule, which facilitates the access to their ground and higher-lying states of intrinsic excitons through the reflection and photoluminescence excitation spectra. Combined with first-principles and model calculations, we further determine that the exciton binding energy shrinks by \ensuremath{\sim}6-fold from 940 meV in monolayer to 160 meV in bilayer BP, yet it only decreases slightly to 108 meV in trilayer. This could be attributed to the strong interlayer coupling and lattice shrinking in multilayer BP samples. Our results shed light on the unique excitonic optical properties of two-dimensional BP.

Modulating Charge Separation with Hexagonal Boron Nitride Mediation in Vertical Van der Waals Heterostructures.

Tuning the optical and electrical properties by stacking different layers of two-dimensional (2D) materials enables us to create unusual physical phenomena. Here, we demonstrate an alternative approach to enhance charge separation and alter physical properties in van der Waals heterojunctions with type-II band alignment by using thin dielectric spacers. To illustrate our working principle, we implement a hexagonal boron nitride (h-BN) sieve layer in between an InSe/GeS heterojunction. The optical transitions at the junctions studied by photoluminescence and the ultrafast pump-probe technique show quenching of emission without h-BN layers exhibiting an indirect recombination process. This quenching effect due to strong interlayer coupling was confirmed with Raman spectroscopic studies. In contrast, h-BN layers in between InSe and GeS show strong enhancement in emission, giving another degree of freedom to tune the heterojunction property. The two-terminal photoresponse study supports the argument by showing a large photocurrent density for an InSe/h-BN/GeS device by avoiding interlayer charge recombination. The enhanced charge separation with h-BN mediation manifests a photoresponsivity and detectivity of 9 × 102 A W-1 and 3.4 × 1014 Jones, respectively. Moreover, a photogain of 1.7 × 103 shows a high detection of electrons for the incident photons. Interestingly, the photovoltaic short-circuit current is switched from positive to negative, whereas the open-circuit voltage changes from negative to positive. Our proposed enhancement of charge separation with 2D-insulator mediation, therefore, provides a useful route to manipulate the physical properties of heterostructures and for the future development of high-performance optoelectronic devices.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers.

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS2) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS2/hBN heterostructures, which is absent in bare MoS2 layers (or homoepitaxial MoS2/MoS2 dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS2 in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS2 as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS2 layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

The Large-Scale Preparation and Optical Properties of MoS2/WS2 Vertical Hetero-Junction

A variety of hetero-junctions can be constructed to form the basic structural units in the different optoelectronic devices, such as the photo-detectors, solar cells, sensors and light-emitting diodes. In our research, the large-area high-quality MoS2/WS2 vertical hetero-junction are prepared by the two-step atmospheric pressure chemical vapor deposition (APCVD) methods and the dry transfer method, and the corresponding optimal reaction conditions of MoS2/WS2 vertical hetero-junction are obtained. The morphology, composition and optical properties of MoS2/WS2 vertical hetero-junction are systematically characterized by the optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and the field emission scanning electron microscopy. Compared to the mechanical transfer method, the MoS2/WS2 vertical hetero-junction sample obtained by the APCVD and dry transfer methods have lower impurity content, cleaner interfaces and tighter interlayer coupling. Besides, the strong interlayer coupling and effective interlayer charge transfer of MoS2/WS2 vertical hetero-junction are also further studied. The photoluminescence intensity of MoS2/WS2 vertical hetero-junction is significantly reduced compared to the single MoS2 or WS2 material. In general, this research can help to achieve the large-scale preparation of various Van der Waals hetero-junctions, which can lay the foundation for the new application of optoelectronic devices.

Two-dimensional van der Waals heterostructure of indium selenide/hexagonal boron nitride with strong interlayer coupling

Indium selenide (InSe) is a promising two-dimensional photodetector material. However, band-gap and work-function of low-dimension InSe are too large to act as photodetector devices. Here, we design InSe-based vdWs heterostructure, InSe/hBN (hexagonal boron nitride), using first-principles. InSe/hBN possesses rather low work-function (~3.0 eV) compared with that of isolated InSe monolayer due to strong interlayer coupling. Besides, strong interlayer coupling induces extra emerging states, markedly reducing band-gap of InSe/hBN. Meanwhile, these emerging states ensure red-shift of optical absorption, from infrared to ultraviolet light-harvesting. Moreover, different heterostructure stacking mode doesn’t affect optical absorption and electron emission, convenient to the application of optoelectronics.

Graphene/WS2 heterostructure saturable absorbers for ultrashort pulse generation in L-band passively mode-locked fiber lasers.

Graphene/WS2 (G/WS2) van der Waals (vdW) heterostructures are utilized as saturable absorbers (SAs) in compact mode-locked fiber lasers operating in the telecommunication L-band for the first time. The interlayer coupling is confirmed by Raman and photoluminescence spectra. In comparison with pure WS2, the heterostructure exhibits excellent nonlinear optical properties in terms of larger modulation depth and lower saturation intensity due to the strong interlayer coupling. By incorporating the G/WS2-based SA into an all-anomalous-dispersion fiber laser, stable conventional-soliton pulses with a pulse duration down to 660 fs can be realized at 1601.9 nm, manifesting better output performance compared to pure WS2. In addition, through shifting the cavity dispersion to the net-normal dispersion, the G/WS2 SA can also be applied for dissipative-soliton generation. Resultant output pulses feature the central wavelength of 1593.5 nm and the pulse duration of 55.6 ps. Our results indicate that the G/WS2 vdW heterostructure is a promising candidate as SA for pulsed laser applications, which pave the way for the development of novel ultrafast photonic devices with desirable performance.

Strain-tunable III-nitride/ZnO heterostructures for photocatalytic water-splitting: A hybrid functional calculation

Solar to fuel energy conversion is one of the momentous topics nowadays considering the urgent demand for clean energy supplies. In this work, the tunable electronic and optical properties of III-nitride/ZnO 2D/2D heterostructures (including GaN/ZnO, AlN/ZnO, and GaN/AlN) by strain engineering were investigated by first-principles calculations. The studied heterostructures feature a small interlayer distance, with the cation of one layer directly above the anion of the other layer, and vice versa. This leads to a strong binding energy and interlayer coupling across the heterostructure. The built-in field induced by the charge redistribution facilitates the photoexcited carrier migration, which is beneficial to the photocatalytic water splitting application. The stable III-nitride/ZnO heterostructures exhibit decent band edge positions with biaxial strain engineering and feature an enhancement of optical absorption under tensile strain. Our results indicate that the III-nitride/ZnO 2D/2D heterostructures are promising photocatalysts for solar to hydrogen generation by water splitting.

BaHgSn: A Dirac semimetal with surface hourglass fermions

We proposed that BaHgSn is a Dirac semimetal (DSM) which can host hourglass-like surface states (HSSs) as protected by nonsymmorphic glide symmetry. Compared to KHgSb, an isostructural topological crystalline insulator with the same HSSs, BaHgSn has an additional band inversion at $\Gamma$ point. This band inversion is induced by the stronger interlayer coupling among Hg-Sn honeycomb layers than that among Hg-Sb-layers in KHgSb, which leads to bulk Dirac nodes in BaHgSn along the layer stacking direction $\Gamma$-$A$. In addition, the mirror Chern number $C_{i}$ protected by the mirror plane $\overline{M}_{z}$ ($k_z$=0) changes from 2 in KHgSb to 3 in BaHgSn. Therefore, when a compressive uniaxial strain is applied along the $y$ axis to break the rotation symmetry protecting the DSM state, BaHgSn becomes a strong topological insulator with $Z_{2}$ indices of $(1;000)$ and the topological surface Dirac cone co-exists with HSSs on the (010) surface. The Wilson-loop spectra have been calculated to verify these topological features. The calculated surface states, the Fermi surfaces and their quasiparticle interference patterns are ready to be compared with experimental measurements.