Effect Of Interlayer Coupling Research Articles

Self-Healing Originated van der Waals Homojunctions with Strong Interlayer Coupling for High-Performance Photodiodes.

The dangling-bond-free surfaces of van der Waals (vdW) materials make it possible to build ultrathin junctions. Fundamentally, the interfacial phenomena and related optoelectronic properties of vdW junctions are modulated by the interlayer coupling effect. However, the weak interlayer coupling of vdW heterostructures limits the interlayer charge transfer efficiency, resulting in low photoresponsivity. Here, a bilayer MoS2 homogeneous junction is constructed by stacking the as-grown onto the self-healed monolayer MoS2. The homojunction barrier of ∼165 meV is obtained by the electronic structure modulation of defect self-healing. This homojunction reveals the stronger interlayer coupling effect in comparison with vdW heterostructures. This ultrastrong interlayer coupling effect is experimentally verified by Raman spectra and angle-resolved photoemission spectroscopy. The ultrafast interlayer charge transfer takes place within ∼447 fs, which is faster than those of most vdW heterostructures. Furthermore, the homojunction photodiode manifests outstanding rectifying behavior with an ideal factor of ∼1.6, perfect air stability over 12 months, and high responsivity of ∼54.6 mA/W. Moreover, the interlayer exciton peak of ∼1.66 eV is found in vdW homojunctions. This work offers an uncommon vdW junction with strong interlayer coupling and perfects the relevance of interlayer coupling and interlayer charge transfer.

Probing Interfacial Surface State Excitons in Nanoscale Synthesized CuxS/MoS2 Heterostructure

AbstractTwo dimensional (2D) layer integration of different materials facilitates new functionalities in material engineering to produce novel structures with specialized properties. Fabrication of large‐scale 2D heterostructure with atomically clean interface, and unveiling the distinct interlayer transition, is a challenging process. In this paper, the vertical heterostructure of copper sulfide (CuxS) and molybdenum disulfide (MoS2) nanosheets is synthesized by sulfurization of pre‐deposited metal films. Optical bandgap transition and phonon vibration frequency are observed in the heterojunction area with respect to individual nanosheets. Raman shift in phonon vibration modes of MoS2 shows the interlayer coupling effect. Photoluminescence (PL) intensity of monolayer MoS2 is quenched due to bandgap offset interlayer transition. Bandgap alignment of CuxS (1 < x < 2) with MoS2 is a tunable staggered symmetry that yields charge transfer at the heterostructure interface. Photoexcited electron–hole pairs are moving across the interface much faster than recombining into the intralayer excitons. The synthesized CuxS/MoS2 heterostructure is well defined over a large area, with controllable thickness and a scalable position. Heterostructure has great potential for future device applications in optoelectronics based on optical excitonic response and charge carrier dynamics.

Carrier dynamics and spin–valley–layer effects in bilayer transition metal dichalcogenides

Transition metal dichalcogenides are an interesting class of low dimensional materials in mono- and few-layer form with diverse applications in valleytronic, optoelectronic and quantum devices. Therefore, the general nature of the band-edges and the interplay with valley dynamics is important from a fundamental and technological standpoint. Bilayers introduce interlayer coupling effects which can have a significant impact on the valley polarization. The combined effect of spin-orbit and interlayer coupling can strongly modify the band structure, phonon interactions and overall carrier dynamics in the material. Here we use first-principles calculations of electron-electron and electron-phonon interactions to investigate bilayer MoS2 and WSe2 in both the AA' and AB stacking configurations. We find that in addition to spin-orbit coupling, interlayer interactions present in the two configurations significantly alter the near-band-edge dynamics. Scattering lifetimes and dynamic behavior are highly material-dependent, despite the similarities and typical trends in TMDCs. Additionally, we capture significant differences in dynamics for the AA' and AB stacking configurations, with lifetime values differing by up to an order of magnitude between them for MoS2. Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe2 in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and 'magic angle' physics in TMDCs.

Computational design of GeSe/graphene heterojunction based on density functional theory

Based on density functional theory, we computational designed the hetero junction composed by GeSe monolayer and graphene. The effects of interlayer coupling, strains and electric fields on the electronic structures of the designed GeSe/graphene (G/g) hetero structure are explored. We demonstrated that both the intrinsic electric properties of the GeSe monolayer and graphene are well preserved in G/g hetero structure. It is found that an energy gap of 0.17 eV in graphene is opened by decreasing the interlayer distance in G/g hetero structure. The height of Schottky barrier can be effectively tuned by the interlayer distance between GeSe monolayer and graphene. Moreover, we found that applying in-plane strains and the electric fields perpendicular to the G/g hetero structure can control the Schottky barriers at the G/g interface. Our results predict that the ultra-thin G/g hetero structure can be used as two-dimensional semiconductor-based optoelectronic devices.

Tuning the band alignment of p-type graphene-AsSb Schottky contact by electric field

By combining the electronic structures of graphene and monolayer AsSb via van der Waals force interaction, the intrinsic p-type Schottky contact can be obtained. Here, a series of theoretic calculations are performed to survey the effects of interlayer coupling and the band realignment of graphene-AsSb heterointerface. It reveals that intrinsic p-type Schottky barriers of 0.184 and 0.381 eV are formed for the two types of configurations. Besides, the intrinsic electronic properties of graphene and AsSb are roughly preserved. When the external electric field is applied, the Schottky barrier can be effectively tuned up by changing the external electric field intensity and further convert the p-type contact into the n-type contact. A variation of the Schottky barriers indicates a partial Fermi level pinning at the interfaces of AsSb. It results from the low density of interfacial states between graphene and AsSb. The barrier height of AsSb and the corresponding contact type can be flexibly tuned, which is of great importance in the design of novel transistors based two-dimensional materials and they provide meaningful guidelines.

Quasiparticle electronic structure of honeycomb C3N: from monolayer to bulk

Layer-dependent quasiparticle band structures of the newly emerged honeycomb C3N are systematically studied using both density functional theory and GW methods. The calculated GW band gap for monolayer C3N is about 1.5 eV. This moderate band gap may be ideal for future electronics applications. Our result is in marked contrast with a recent experimental report of 0.39 eV and calls for future experimental verifications. Interlayer chemical coupling effects on the electronic structure of C3N are investigated using several bilayer models. The electronic structure of bilayer C3N depends sensitively on the layer stacking pattern with the calculated quasiparticle band gap ranging from 0.87 to 1.35 eV. Finally, we illustrate the effects of interlayer chemical interaction and bulk dielectric screening on the electronic properties of C3N. Depending on the specific bulk stacking, C3N may be metallic or semiconducting with a narrow gap of about 0.6 eV, even though different bulk phases are essentially degenerate energetically. As a result, it may be challenging to prepare single-phase semiconducting bulk C3N unless synthetic kinetics can somehow prefer or prohibit certain stacking patterns. This issue deserves further investigations.

Two-dimensional phosphorene/C3N p-n heterostructure: Effect of contact type on electronic and optical properties

p-n heterostructure (HTS) is a fundamental component for high-performance electronic and optoelectronic device. Vertical stacking through van der Waals (vdW) force is emerging as a feasible technique to construct p-n HTS. Herein, we designed a novel kind of direct-bandgap C3N monolayer, via adjusting the arrangement of C and N atoms in C3N hexagonal cell. On the basis of the density functional theory combined with the non-equilibrium Green’s function method, we built two-dimensional vdW-contact phosphorene (BP)/C3N p-n HTS, and analyzed its electronic and optical properties in comparison with the inplane-jointed ones. The strong charge transfer between BP and C3N segments results in a wide bandgap of 0.48 eV for joint-contact type BP/C3N HTS, whereas the effective interlayer coupling in vdW-contact type leads to an improved light adsorption as compared to the isolated C3N monolayer. By fabricating dual-gated BP/C3N HTS field-effect transistors (FETs), the dynamic transport behaviors demonstrated that the band bending under a lower threshold voltage makes band-to-band tunneling possible for vdW-contact type. Our work suggests that vdW-contact type is superior to joint-contact type in constructing p-n HTS for high-performance electronic and optoelectronic devices.

Strain and interlayer coupling tailored magnetic properties and valley splitting in layered ferrovalley 2H-VSe2

Geometry structure, band structure, magnetic moment and magnetocrystalline anisotropy energy of layered 2H-VSe2 are investigated by first-principles calculations. It is found that the VSe2 is intrinsic ferromagnetic with an in-plane magnetocrystalline anisotropy no matter how the change of the strain and layer thickness is. Additionally, the valley splitting in monolayer VSe2 can be modulated by strain due to the modulated intrinsic exchange interaction of V 3d electrons. Interestingly, the valley splitting decreases and the V magnetic moment monotonously increases as a strain increases, showing that the valley splitting is related to the magnetic moment. Furthermore, the slight effect of interlayer coupling on V magnetic moment appears. The tunable magnetism and valley splitting may provide novel applications in multifunctional nanoelectronics.

Transition from saturable absorption to reverse saturable absorption in MoTe2 nano-films with thickness and pump intensity

Nonlinear optical effect such as saturable absorption (SA) and reverse saturable absorption (RSA) play a key role for all-optical logic gates, in which transition between them is a challenging issue for the optoelectronic applications. Herein, we find MoTe2 nano-films fabricated with liquid-phase exfoliation and a vacuum filtration method which demonstrate both SA and RSA with different thicknesses and pump intensity. The SA nonlinear optical susceptibility for thick film (100 nm) is Imχ(3) ∼ 10−11 esu. However, the thin film (30 nm) exhibited RSA with the Imχ(3) as high as ∼9.96 × 10−11 esu. The different nonlinear optical response with the thickness is due to the interlayer coupling effect. Under high pump intensity, the thick MoTe2 film also demonstrates transition from SA to RSA. The four energy-level model is employed to analyze the competition between the ground-state and excited-state absorption. We found that the conversion of SA to RSA depends not only on pump intensity but also on the absorption cross-section and transition probability between energy level. The transition properties of SA and RSA for MoTe2 films can be used in all-optical logic gates, fast optical switch, optical limiter, mode storage and so on.

Anomalous interlayer vibrations in strongly coupled layered PdSe2**Notice: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this

In this work, we show unusual effects of strong interlayer coupling on low-frequency (LF) Raman scattering in exfoliated PdSe2 crystals with different number of layers. Unlike many other layered materials, it is found that the measured frequencies of the breathing modes cannot be simply described by a conventional linear chain model (LCM) that treats each layer as a single rigid object. By using first-principles calculations, we show that strong deviations from layer rigidity can occur for the LF breathing vibrations of PdSe2, which accounts for the observed disagreement with the conventional LCM. The layer non-rigidity and strong interlayer coupling could also explain the unusual strong intensities of the LF breathing modes that are comparable with those of the high-frequency Raman modes. These strong intensities allowed us to use a set of the measured LF Raman lines as unique fingerprints for a precise assignment of the layer numbers. The assignment of the layer numbers was further confirmed using second harmonic generation that appeared only in the noncentrosymmetric even-layer PdSe2 crystals. This work thus demonstrates a simple and fast approach for the determination of the number of layers in 2D materials with strong interlayer coupling and non-rigid interlayer vibrations.

Interlayer coupling and the phase transition mechanism of stacked MoS2/TaS2 heterostructures discovered using temperature dependent Raman and photoluminescence spectroscopy.

Ultrathin 1T (tetragonal)-TaS2 and monolayer MoS2 heterostructures were prepared to study their phase transition (PT) mechanisms and band structure modulation. The temperature dependency of photoluminescence (PL) and Raman spectra was utilized to study interlayer coupling and band structure. The PL results indicate that the band structure of MoS2/TaS2 heterostructures undergoes a sharp change at 214 K. This is attributed to the PT of 1T-TaS2 from a Mott insulator state to a metastable state. In addition, the temperature dependency of the MoS2/TaS2 Raman spectra illustrates that the phonon vibration of the heterojunction is softened due to the effect of interlayer coupling. The present work could provide an avenue to create material systems with abundant functionalities and physical effects.

Inter-Layer Coupling Effects on Vertical Electron Transport in Multilayer Graphene Nanoribbons

We have performed nonequilibrium Green function simulations on the vertical transport characteristics in multilayer graphene nanoribbons sandwiched between two graphene contact layers with varying the inter-layer coupling strength β. We find that the integrated transmission function thorough the top (or bottom) layer is hardly affected by β when the incident energy is close to the Dirac point. The β-insensitive energy window becomes narrower as the channel length increases.

Structural Vibration of A Flexible Complex System Under A Harmonic Oscillation Moving Force

This paper focuses on the free and forced transverse vibration of a double-string complex system with elastic interlayer under a harmonic oscillation moving force. The paper includes the study of a dynamic behaviour of a finite, simply supported double-string flexible complex system subject to harmonic force moving with a constant velocity on the top string. The strings are identical, parallel one upon the other. The elastic interlayer is described by the Winkler’s model consists of a Hookean resilient spring distributed in parallel. The classical solution of the response of complex systems subjected to harmonic oscillation force moving with a constant velocity has a form of an infinite series. But also, it is possible to show that in the considered case part of the solution can be presented in a closed, analytical form instead of an infinite series. The presented method to search for a solution in a closed-form is based on the observation that the solution of the system of partial differential equations in the form of an infinite series is also a solution of an appropriate system of ordinary differential equations. The double string connected in parallel by linear elastic elements can be studied as a theoretical model of composite structure in which impact of layer interaction, interlayer coupling effects and transverse wave effects is taken into account.

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Atomic Interface Engineering and Electric-Field Effect in Ultrathin Bi2 MoO6 Nanosheets for Superior Lithium Ion Storage.

Ultrathin 2D materials can offer promising opportunities for exploring advanced energy storage systems, with satisfactory electrochemical performance. Engineering atomic interfaces by stacking 2D crystals holds huge potential for tuning material properties at the atomic level, owing to the strong layer-layer interactions, enabling unprecedented physical properties. In this work, atomically thin Bi2 MoO6 sheets are acquired that exhibit remarkable high-rate cycling performance in Li-ion batteries, which can be ascribed to the interlayer coupling effect, as well as the 2D configuration and intrinsic structural stability. The unbalanced charge distribution occurs within the crystal and induces built-in electric fields, significantly boosting lithium ion transfer dynamics, while the extra charge transport channels generated on the open surfaces further promote charge transport. The in situ synchrotron X-ray powder diffraction results confirm the material's excellent structural stability. This work provides some insights for designing high-performance electrode materials for energy storage by manipulating the interface interaction and electronic structure.

Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T'-ReSe2.

Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe2 regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

Interlayer coupling effect on the performance of monolithic three-dimensional inverters and its dependence on the interlayer dielectric thickness

We study the interlayer coupling in monolithic three-dimensional (3D) inverters and investigate its effect on the performance of 3D inverters using technology computer-aided design simulation. The interlayer coupling in 3D inverters consisting of silicon-on-insulator metal–oxide–semiconductor field-effect transistors (FETs) improves the current driving capability of the top FETs. Owing to this improvement, 3D inverters have a smaller intrinsic delay than the corresponding two-dimensional (2D) inverters although the switching energies of 3D and 2D inverters are comparable to each other. We analyze the relationship of such interlayer coupling effects with the interlayer dielectric (ILD) thickness and find that there exists an appropriate ILD thickness in terms of various aspects of the performance such as speed, energy efficiency, and both. It is also found that decreasing the power supply voltage enhances the interlayer coupling effects. In addition, we reveal that the interlayer coupling in 3D inverters is strongly dependent on the original structure of the constituent FETs.

Layer-number dependent high-frequency vibration modes in few-layer transition metal dichalcogenides induced by interlayer couplings**Project supported by the National Basic Research Program of China (No. 2016YFA0301200), the National Natural Science Foundation of China (Nos. 11225421, 11474277, 11434010, 61474067, 11604326, 11574305 and 51527901), and the National Young 1000 Talent Plan of China.

Two-dimensional transition metal dichalcogenides (TMDs) have attracted extensive attention due to their many novel properties. The atoms within each layer in two-dimensional TMDs are joined together by covalent bonds, while van der Waals interactions combine the layers together. This makes its lattice dynamics layer-number dependent. The evolutions of ultralow frequency (< 50 cm−1) modes, such as shear and layer-breathing modes have been well-established. Here, we review the layer-number dependent high-frequency (> 50 cm−1) vibration modes in few-layer TMDs and demonstrate how the interlayer coupling leads to the splitting of high-frequency vibration modes, known as Davydov splitting. Such Davydov splitting can be well described by a van der Waals model, which directly links the splitting with the interlayer coupling. Our review expands the understanding on the effect of interlayer coupling on the high-frequency vibration modes in TMDs and other two-dimensional materials.

Spin heat capacity of monolayer and AB-stacked bilayer MoS2 in the presence of exchange magnetic field

Dirac theory and Green's function technique are carried out to compute the spin dependent band structures and corresponding electronic heat capacity (EHC) of monolayer (ML) and AB-stacked bilayer (BL) molybdenum disulfide (MoS2) two-dimensional (2D) crystals. We report the influence of induced exchange magnetic field (EMF) by magnetic insulator substrates on these quantities for both structures. The spin-up (down) subband gaps are shifted with EMF from conduction (valence) band to valence (conduction) band at both Dirac points in the ML because of the spin-orbit coupling (SOC) which leads to a critical EMF in the K point and EHC returns to its initial states for both spins. In the BL case, EMF results split states and the decrease (increase) behavior of spin-up (down) subband gaps has been observed at both K and K′ valleys which is due to the combined effect of SOC and interlayer coupling. For low and high EMFs, EHC of BL MoS2 does not change for spin-up subbands while increases for spin-down subbands.

Experimental Investigation of Electronic Structure of La(O,F)BiSe2**Supported by the National Basic Research Program of China under Grant Nos 2015CB921300, 2013CB921700 and 2016YFA0300404, the National Natural Science Foundation of China under Grant Nos 11474340, 11234014, U1532267 and 11674327, and the Chinese Academy of Sciences under Grant No XDB07000000.

La(O,F)BiSe2 is a layered superconductor and has the same crystal structure with La(O,F)BiS2. We investigate the electronic structure of La(O,F)BiSe2 using the angle-resolved photoemission spectroscopy. Two electron-like Fermi surfaces around X(π, 0) are observed, corresponding to the electron doping of 0.23 per Bi site. We clearly resolve anisotropic band splitting along both Γ—X and M—X due to the cooperative effects of large spin-orbit coupling and interlayer coupling. Moreover, we observe an almost non-dispersive electronic state around −0.2eV between the electron-like bands. This state vanishes after in-situ K evaporation, indicating that it could be the localized surface state caused by defects on the cleaved surface.