Van Der Waals Heterobilayers Research Articles

Exciton-Exciton Interactions in Van der Waals Heterobilayers

Exciton-exciton interactions are key to understanding nonlinear optical and transport phenomena in van der Waals heterobilayers, which emerged as versatile platforms to study correlated electronic states. We present a combined theory-experiment study of excitonic many-body effects based on first-principle band structures and Coulomb interaction matrix elements. Key to our approach is the explicit treatment of the fermionic substructure of excitons and dynamical screening effects for density-induced energy renormalization and dissipation. We demonstrate that dipolar blueshifts are almost perfectly compensated by many-body effects, mainly by screening-induced self-energy corrections. Moreover, we identify a crossover between attractive and repulsive behavior at elevated exciton densities. Theoretical findings are supported by experimental studies of spectrally narrow, mobile interlayer excitons in atomically reconstructed, h-BN-encapsulated MoSe2/WSe2 heterobilayers. Both theory and experiment show energy renormalization on a scale of a few meV even for high injection densities in the vicinity of the Mott transition. Our results revise the established picture of dipolar repulsion dominating exciton-exciton interactions in van der Waals heterostructures and open up opportunities for their external design. Published by the American Physical Society 2024

Infrared Interlayer Excitons in Twist-Free MoTe2/MoS2 Heterobilayers.

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8eV (1550nm) in a chemically vapor-deposited, strictly aligned MoTe2/MoS2 heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum-space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin-orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150meV (270nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

A study of 2D GeI2/InTe van der Waals hetero bilayer as a photocatalyst material

The computational study of the van der Waals hetero (vdW) bilayer GeI2/InTe has been carried out in present study. The isolated monolayer GeI2 and InTe have been studied first and the results were compared to the previous studies. The possible stackings are considered after the vdW interaction correction is applied in the structure relaxation. The vdW hetero bilayer stability has been checked from the phonon dispersion and ab initio Molecular Dynamics calculations. The charge transfer from InTe to GeI2 monolayer. Type-II indirect band gap (1.98, 2.01 eV) is verified by the projected band structure and band alignment calculations. The vdW hetero bilayer is a superior photocatalyst for the pH value up to pH = 0 to 11. The optical properties are calculated from the complex dielectric constant. The absorption coefficient shows the enhance absorption of light in the visible and ultraviolet regions. The vdW hetero bilayer has shown low reflectivity (37%) and a high refractive index (2.80) in the visible region. The enhanced optical properties have shown its possible applications in optoelectronic devices.

Excitonic Interactions and Mechanism for Ultrafast Interlayer Photoexcited Response in van der Waals Heterostructures.

Optical dynamics in van der Waals heterobilayers is of fundamental scientific and practical interest. Based on a time-dependent adiabatic GW approach, we discover a new many-electron (excitonic) channel for converting photoexcited intralayer to interlayer excitations and the associated ultrafast optical responses in heterobilayers, which is conceptually different from the conventional single-particle picture. We find strong electron-hole interactions drive the dynamics and enhance the pump-probe optical responses by an order of magnitude with a rise time of ∼300 fs in MoSe_{2}/WSe_{2} heterobilayers, in agreement with experiment.

Tuning Interlayer Exciton Emission with TMD Alloys in van der Waals Heterobilayers of Mo0.5W0.5Se2 and Its Binary Counterparts.

Semiconductor heterostructures have been the backbone of developments in electronic and optoelectronic devices. One class of structures of interest is the so-called type II band alignment, in which optically excited electrons and holes relax into different material layers. The unique properties observed in two-dimensional transition metal dichalcogenides and the possibility to engineer van der Waals heterostructures make them candidates for future high-tech devices. In these structures, electronic, optical, and magnetic properties can be tuned through the interlayer coupling, thereby opening avenues for developing new functional materials. We report the possibility of explicitly tuning the emission of interlayer exciton energies in the binary-ternary heterobilayer of Mo0.5W0.5Se2 with MoSe2 and WSe2. The respective interlayer energies of 1.516 eV and 1.490 eV were observed from low-temperature photoluminescence measurements for the MoSe2- and WSe2- based heterostructures, respectively. These interlayer emission energies are above those reported for MoSe2/WSe2 (≃1.30-1.45 eV). Consequently, binary-ternary heterostructure systems offer an extended energy range and tailored emission energies not accessible with the binary counterparts. Moreover, even though Mo0.5W0.5Se2 and MoSe2 have almost similar optical gaps, their band offsets are different, resulting in charge transfer between the monolayers following the optical excitation. Thus, confirming TMDs alloys can be used to tune the band-offsets, which adds another design parameter for application-specific optoelectronic devices.

Nonlinear Optical Responses of Janus MoSSe/MoS2 Heterobilayers Optimized by Stacking Order and Strain.

Nonlinear optical responses in second harmonic generation (SHG) of van der Waals heterobilayers, Janus MoSSe/MoS2, are theoretically optimized as a function of strain and stacking order by adopting an exchange-correlation hybrid functional and a real-time approach in first-principles calculation. We find that the calculated nonlinear susceptibility, χ(2), in AA stacking (550 pm/V) becomes three times as large as AB stacking (170 pm/V) due to the broken inversion symmetry in the AA stacking. The present theoretical prediction is compared with the observed SHG spectra of Janus MoSSe/MoS2 heterobilayers, in which the peak SHG intensity of AA stacking becomes four times as large as AB stacking. Furthermore, a relatively large, two-dimensional strain (4%) that breaks the C3v point group symmetry of the MoSSe/MoS2, enhances calculated χ(2) values for both AA (900 pm/V) and AB (300 pm/V) stackings 1.6 times as large as that without strain.

Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide-Graphene Heterostructures.

Associating atomic vacancies to excited-state transport phenomena in two-dimensional semiconductors demands a detailed understanding of the exciton transitions involved. We study the effect of such defects on the electronic and optical properties of WS2-graphene and MoS2-graphene van der Waals heterobilayers, employing many-body perturbation theory. We find that chalcogen defects and the graphene interface radically alter the optical properties of the transition-metal dichalcogenide in the heterobilayer, due to a combination of dielectric screening and the many-body nature of defect-induced intralayer and interlayer optical transitions. By analyzing the intrinsic radiative rates of the subgap excitonic features, we show that while defects introduce low-lying optical transitions, resulting in excitons with non-negligible oscillator strength, they decrease the optical response of the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.

Strain engineering of type-II C2N/WS2 van der Waals heterojunction for highly enhanced photocatalytic hydrogen evolution

The first-principles calculations demonstrate that the photocatalytic performance of type-II C2N/WS2 van der Waals heterobilayer can be improved to a new and high level by applying weak biaxial strain, with desired band structure, tunable interface charge transfer, superior visible-light absorption, and ideal hydrogen evolution reaction (HER) activity. Tensile strains can largely reduce the energy gap, strengthen the interfacial interactions, and promote the visible-light absorption of C2N/WS2 heterostructure, and even induce a direct band gap of 1.70 eV for overall water splitting at 2% strain. Compressive strains like −2% can upshift the valence band and conduction band positions of the C2N/WS2 bilayer to acquire outstanding redox abilities, and enhance the interface charge transfer and separation by increasing the electrostatic potential drop. More importantly, the cycle between spontaneous H∗ adsorption and energetically favorable H2 release may be realized by dynamic strains between ±2%, maximizing the HER activities of C2N/WS2 heterostructure.

Charge quenching at defect states in transition metal dichalcogenide–graphene van der Waals heterobilayers

The electronic and spin properties of van der Waals materials are greatly influenced by the atomic features of the interface. Combining $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ methods, model Hamiltonian, and quantum master equation, the authors calculate the electronic transition times and the dynamics of electron population and depopulation of chalcogen vacancies at the interface between graphene and transition metal dichalcogenides. Their findings give insight into the semiclassical dynamics, reveal the interplay between lattice symmetry and spin through the spin-orbit interaction, and examine its impact on the charge quenching.

Negative piezoelectricity and enhanced electrical conductivity at the interfaces of two-dimensional dialkali oxide and chalcogenide monolayers

The negative piezoelectric coefficient in ferroelectric polymers has been widely studied. However, the same in the two-dimensional (2D) materials is scarcely reported, which stresses the need for its thorough investigation. Here, the 2D van der Waals (vdW) heterobilayers (vdWHs) of the dialkali metal monochalcogenide ${M}_{2}X$ ($M=\mathrm{Na}$, K, Rb, or Cs; and $X=\mathrm{O}$, S, Se, or Te) monolayers show negative out-of-plane piezoelectricity. Out of all the explored heterobilayers of the family, the maximum piezoelectric strain coefficient in the out-of-plane direction ${d}_{33}=\ensuremath{-}39\phantom{\rule{0.28em}{0ex}}\mathrm{pm}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}$ is found in the ${\mathrm{Na}}_{2}\mathrm{Te}$/${\mathrm{Cs}}_{2}\mathrm{S}$ system. This strong piezoelectric coefficient arises from the notable electrostatic potential energy difference and band offset between the constituent monolayers. The small out-of-plane Young's modulus ($Y\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$), originating from the weak vdW interlayer bonding relative to the stronger intralayer bonding, accounts for the anomalous negative piezoelectricity in the heterostructure. The combination of minute out-of-plane ($Y=\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) and in-plane ($Y=\ensuremath{\sim}23\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) Young's moduli with small bending modulus (8.83 eV) and negative piezoelectric coefficient underlines the suitability of the proposed vdWH for stretchable and flexible piezotronic devices. Moreover, external perturbations are found to modify the properties of the system. The vertical compressive strain widens the band gap by elevating the conduction band minimum in the heterostructure. An externally applied electric field of low strength ($\ensuremath{-}0.33$ and $+0.31\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\AA{}$) is found to change the semiconducting heterostructure to metallic upon the closure of the band gap, which is attributed to the dropping of nearly free electron gas (NFEG) states to the Fermi level. A significant enhancement in electrical conductivity has been brought up by the NFEG states upon reaching the Fermi level, which inspires the use of the heterostructure in a nonresistive charge carrier transport application in electronics.

Negative Valley Polarization of the Intralayer Exciton via One-Step Growth of H-Type Heterobilayer WS2/MoS2

Vertical type II van der Waals heterobilayers of transition metal dichalcogenides (TMDs) have attracted wide attention due to their distinctive features mostly arising from the emergence of intriguing electronic structures that include moiré-related phenomena. Owing to strong spin-orbit coupling under a noncentrosymmetric environment, TMD heterobilayers host nonequivalent +K and -K valleys of contrasting Berry curvatures, which can be optically controlled by the helicity of optical excitation. The corresponding valley selection rules are well established by not only intralayer excitons but also interlayer excitons. Quite intriguingly, here, we experimentally demonstrate that unusual valley switching can be achieved using the lowest-lying intralayer excitons in H-type heterobilayer WS2/MoS2 prepared by one-step growth. This TMD combination provides an ideal case for interlayer coupling with an almost perfect lattice match, thereby also in the momentum space between +K and -K valleys in the H-type heterostructure. The underlying valley-switching mechanism can be understood by bright-to-dark conversion of initially created electrons in the valley of WS2, followed by interlayer charge transfer to the opposite valley in MoS2. Our suggested model is also confirmed by the absence of valley switching when the lowest-lying excitons in MoS2 are directly generated in the heterobilayer. In contrast to the H-type case, we show that no valley switching is observed from R-type heterobilayers prepared by the same method, where interlayer charge transfer does not occur between the opposite valleys. We compare the case with the series of valley polarization data from other heterobilayer combinations obtained under different excitation energies and temperatures. Our valley switching mechanism can be utilized for valley manipulation by controlling the excitation photon energy together with the photon helicity in valleytronic devices derived from H-type TMD heterobilayers.

Flattening conduction and valence bands for interlayer excitons in a moiré MoS2/WSe2 heterobilayer.

We explore the flatness of conduction and valence bands of interlayer excitons in MoS2/WSe2 van der Waals heterobilayers, tuned by interlayer twist angle, pressure, and external electric field. We employ an efficient continuum model where the moiré pattern from lattice mismatch and/or twisting is represented by an equivalent mesoscopic periodic potential. We demonstrate that the mismatch moiré potential is too weak to produce significant flattening. Moreover, we draw attention to the fact that the quasi-particle effective masses around the Γ-point and the band flattening are reduced with twisting. As an alternative approach, we show (i) that reducing the interlayer distance by uniform vertical pressure can significantly increase the effective mass of the moiré hole, and (ii) that the moiré depth and its band flattening effects are strongly enhanced by accessible electric gating fields perpendicular to the heterobilayer, with resulting electron and hole effective masses increased by more than an order of magnitude - leading to record-flat bands. These findings impose boundaries on the commonly generalized benefits of moiré twistronics, while also revealing alternative feasible routes to achieve truly flat electron and hole bands to carry us to strongly correlated excitonic phenomena on demand.

Excellent photocatalytic properties in 2D ZnO/SiC van der Waals hetero-bilayers: water-splitting H2-fuel production.

This research unravels the photocatalytic properties of a 2D ZnO/SiC van der Waals hetero-bilayer for potential water-splitting applications by first-principles calculations. Four unique stacking patterns are considered in studying the electronic and optical properties in the presence and absence of biaxial external strain. For pattern-I and II, large negative binding energy and positive phonon frequencies are observed, denoting chemical and mechanical stabilities. Under the HSE-06 pseudo potential, the calculated bandgap value for pattern-I and II reaches 2.86 eV and 2.74 eV, respectively. 2D ZnO/SiC shows a high absorption coefficient (∼105 cm-1). The absorption peak under biaxial strain could reach ∼3.5 times the peak observed under unstrained conditions. Under strain, a shift from compressive to tensile biaxial strain (-6% to 6%) results in a bandgap decrease from 3.18 eV to 2.52 eV and 3.09 eV to 2.43 eV, for pattern-I and II, respectively. The observed strain-driven kinetic overpotential for 2D ZnO/SiC pattern-I and II easily engenders photocatalytic redox reactions. The excellent mechanical durability and strain-driven large kinetic overpotential suggest 2D ZnO/SiC heterobilayers as a prospective material for water-splitting H2-fuel production.

Two-dimensional SiC/AlN based type-II van der Waals heterobilayer as a promising photocatalyst for overall water disassociation

Two-dimensional (2D) van der Waals (vdW) heterostructures made by vertical assembling of two different layers have drawn immense attention in the photocatalytic water disassociation process. Herein, we suggest a novel 2D/2D vdW heterobilayer consisting of silicon carbide (SiC) and aluminum nitride (AlN) as an exciting photocatalyst for solar-to-hydrogen conversion reactions using first-principles calculations. Notably, the heterostructure presents an inherent type-II band orientation wherein the photogenic holes and electrons are spatially separated in the SiC layer and the AlN layer, respectively. Our results indicate that the SiC/AlN heterostructure occupies a suitable band-gap of 2.97 eV which straddles the kinetic overpotentials of the hydrogen production reaction and oxygen production reaction. Importantly, the built-in electric field at the interface created by substantial charge transfer prohibits carrier recombination and further improves the photocatalytic performance. The heterostructure has an ample absorption profile ranging from the ultraviolet to the near-infrared regime, while the intensity of the absorption reaches up to 2.16 × 105 cm−1. In addition, external strain modulates the optical absorption of the heterostructure effectively. This work provides an intriguing insight into the important features of the SiC/AlN heterostructure and renders useful information on the experimental design of a novel vdW heterostructure for solar energy-driven water disassociation with superior efficiency.

Spectroscopic imaging ellipsometry of two-dimensional TMDC heterostructures

Semiconducting two-dimensional materials and their heterostructures gained a lot of interest for applications as well as fundamental studies due to their rich optical properties. Assembly in van der Waals heterostacks can significantly alter the intrinsic optical properties as well as the wavelength-dependent absorption and emission efficiencies, making a direct comparison of, e.g., photoluminescence intensities difficult. Here, we determine the dielectric function for the prototypical MoSe2/WSe2 heterobilayer and their individual layers. Apart from a redshift of 18–44 meV of the energetically lowest interband transitions, we find that for larger energies, the dielectric function can only be described by treating the van der Waals heterobilayer as a new artificial homobilayer crystal rather than a stack of individual layers. The determined dielectric functions are applied to calculate the Michelson contrast of the individual layers and the bilayer in dependence of the oxide thickness of often used Si/SiO2 substrates. Our results highlight the need to consider the altered dielectric functions impacting the Michelson interference in the interpretation of intensities in optical measurements such as Raman scattering or photoluminescence.

Prediction of Effective Photoelectron and Hole Separation in Type-I MoS2/PtSe2 van der Waals Junction.

Understanding the dynamics of charge transfer at vertical heterostructures of transition metal dichalcogenide monolayers is fundamentally important for future technological applications, given the unique feature of van der Waals interactions at the interface. Here, we employ time-dependent density functional theory formalism combined with molecular dynamics to investigate photoexcited electrons and holes in the type-I MoS2/PtSe2 van der Waals heterobilayer. While type-I junctions have been traditionally viewed as being ineffective in photocarrier separation, we show that here a different mechanism from type-II is at play, which effectively separates photoelectrons from photoholes. The key is the phonon bottleneck, arising from the characteristically different dynamic band alignments in the valence and conduction bands, respectively, which only affects the transfer of holes but not electrons. The disparity between electron and hole transfer rates offers a new direction for effective control of charge separation at interfaces.

Transition metal substituted MoS2/WS2 van der Waals heterostructure for realization of dilute magnetic semiconductors

Atomically thin doped two dimensional (2D) layered materials manifest excellent magnetic features beneficial to the potential applications in spintronics. With the implementation of extensive first principles calculations, we demonstrate the MS2 (M = Mo, W) monolayers, as well as their van der Waals (vdW) hetero-bilayers as promising candidates for the successful realization of 2D dilute magnetic semiconductors with the incorporation of Mn and Co dopants. Under various pairwise doping configurations at different host atom sites, we report the electronic properties modifications induced change in magnetic exchange interactions. The magnetic coupling among the dopant pairs can be tuned between FM and AFM orderings via suitable doping adjustments. The developed interlayer exchange coupling between the vdW layers leads to strong and long-ranged ferromagnetic interactions which unleash robust magnetic moments with stable doping configurations. Our findings address the novel magnetic behavior of the layered vdW heterostructures and may further guide the future experimental efforts for the possible applications in modern electronics and nanoscale magnetic storage devices.

Tailoring high-frequency magnonics in monolayer chromium trihalides

Monolayer chromium-trihalides, the archetypal two-dimensional (2D) magnetic materials, are readily suggested as a promising platform for high-frequency magnonics. Here we detail the spin-wave properties of monolayer CrBr3 and CrI3, using spin-dynamics simulations parametrized from the first principles. We reveal that spin-wave dispersion can be tuned in a broad range of frequencies by strain, paving the way towards flexo-magnonic applications. We further show that ever-present halide vacancies in these monolayers host sufficiently strong Dzyaloshinskii-Moriya interaction to scatter spin-waves, which promotes design of spin-wave guides by defect engineering. Finally we discuss the spectra of spin-waves propagating across a moiré-periodic modulation of magnetic parameters in a van der Waals heterobilayer, and show that the nanoscale moiré periodicities in such samples are ideal for realization of a magnonic crystal in the terahertz frequency range. Recalling the additional tunability of magnetic 2D materials by electronic gating, our results situate these systems among the front-runners for prospective high-frequency magnonic applications.

Interface-dependent phononic and optical properties of GeO/MoSO heterostructures.

The interface-dependent electronic, vibrational, piezoelectric, and optical properties of van der Waals heterobilayers, formed by buckled GeO (b-GeO) and Janus MoSO structures, are investigated by means of first-principles calculations. The electronic band dispersions show that O/Ge and S/O interface formations result in a type-II band alignment with direct and indirect band gaps, respectively. In contrast, O/O and S/Ge interfaces give rise to the formation of a type-I band alignment with an indirect band gap. By considering the Bethe-Salpeter equation (BSE) on top of G0W0 approximation, it is shown that different interfaces can be distinguished from each other by means of the optical absorption spectra as a consequence of the band alignments. Additionally, the low- and high-frequency regimes of the Raman spectra are also different for each interface type. The alignment of the individual dipoles, which is interface-dependent, either weakens or strengthens the net dipole of the heterobilayers and results in tunable piezoelectric coefficients. The results indicate that the possible heterobilayers of b-GeO/MoSO asymmetric structures possess various electronic, optical, and piezoelectric properties arising from the different interface formations and can be distinguished by means of various spectroscopic techniques.

Insights into CrS2 monolayer and [formula omitted]-CrS2/[formula omitted]-HfN2 interface for low-power digital and analog nanoelectronics

CrS2 monolayers exhibit excellent mechanical, electronic, piezoelectric and thermal properties as compared to other well-known transition metal dichalcogenides. CrS2/HfN2 van der Waals heterobilayer (vdWH) forms a pn-heterojunction where CrS2 and HfN2 act as an n-type and p-type semiconductor respectively, thereby suggesting its utility in nano-electronics and photodetectors. The perpendicular electric field is effective in manipulating band alignments and band gap in CrS2/HfN2 vdWH. Weaker electric field is able to close the band gap in the vdWH relative to CrS2 ML. Therefore, the vdWH may be a promising candidate in low-power analog and digital nano-electronics. Vertical electric field is found to drive the nearly free electron gas (NFEG) states to near the Fermi level in CrS2 ML. These surface states, reported in a few low-dimensional materials, will enhance charge transport and conductivity in electronics. Furthermore, the combined effect of spin–orbit coupling and a small magnitude of vertical electric field helps to reach a type-II band alignment in the CrS2/HfN2 vdWH. It unusually shows a large spin splitting at both conduction and valence band edges, which is very crucial for ambipolar spin transport. This work demonstrates possibilities for futuristic nano-electronic devices based on CrS2 ML & CrS2 based vdWH.