Trilayer Heterostructures Research Articles

Diverse Excitonic Phenomena in Asymmetric Trilayer Transition Metal Dichalcogenide Heterostructures.

Interlayer excitons formed in two-dimensional transition metal dichalcogenide (TMD) heterostructures can be easily tuned due to the large spatial separation. In this work, we discuss the electronic and excitonic optical properties of trilayer heterostructures MoS2/MoSSe/WSe2 and MoS2/MoSSe/MoSe2 using state-of-the-art GW+BSE calculations. In both trilayer geometries, we discover a variety of exciton states, including interlayer excitons, every-other-layer excitons, and their hybridized states, h-IX. Importantly, the h-IXs are optically bright through hybridizing with the intralayer excitons, and the radiative lifetimes of h-IXs range from subnanoseconds to tens of microseconds at 77 K, depending on their compositions. We also reveal that the diversity of the low-lying IXs in MoS2/MoSSe/MoSe2 is higher than that of MoS2/MoSSe/WSe2, because more energy levels participate in transitions in MoS2/MoSSe/MoSe2. Our findings demonstrate that the appropriate energy alignment via manipulating the Janus layer is crucial for realizing rich excitonic states in trilayer TMD heterostructures.

Quadrupolar and dipolar excitons in symmetric trilayer heterostructures: insights from first principles theory

Excitons in van der Waals heterostructures come in many different forms. In bilayer structures, the electron and hole may be localized on the same layer or they may be separated forming an interlayer (IL) exciton with a finite out-of-plane dipole moment. Using first principles calculations, we investigate the excitons in a symmetric WS2/MoS2/WS2 heterostructure in the presence of a vertical electric field. The excitons exhibit a quadratic Stark shift for low field strengths and a linear Stark shift for stronger fields. This behavior is traced to the coupling of IL excitons with opposite dipole moments, which lead to the formation of quadrupolar excitons at small fields. The formation of quadrupolar excitons is determined by the relative size of the electric field-induced splitting of the dipolar excitons and the coupling between them given by the hole tunneling across the MoS2 layer. For the inverted structure, MoS2/WS2/MoS2, the dipolar excitons are coupled by electron tunneling across the WS2 layer. Because this effect is much weaker, the resulting quadrupolar excitons are more fragile and break at a weaker electric field.

DFT Calculations of Trilayer Heterostructures from MoSe2, PtS2 Monolayers in Different Orders with Promising Optoelectronic Properties

vVan der Waals (vdW) heterostructures have taken the dominant place in commercialization of the optoelectronic devices. MoSe2 and PtS2 are two-dimensional semiconductors, Using first-principles computations, the optical and electronic characteristics of trilayer van der Waals (vdW) heterostructures with four distinct orders were investigated. We demonstrate that all innovative heterostructures investigated are semiconductors. In addition, it should be emphasized that the indirect band gaps of the ABA, BAA, ABB, and BAB orders (where A is MoSe2 and B is PtS2) are approximately 0.875, 0.68, 0.595, and 0.594 eV, respectively. Positively, the optical characteristics reveal that the trilayer heterostructures strongly absorb light with energies ranging from infrared to ultraviolet. Therefore, these heterostructures can be utilized in optoelectronic devices in these regions.

Epitaxial growth and characterization of (110)-oriented YBCO/PBCGO bilayer and YBCO/PBCGO/YBCO trilayer heterostructures

We have grown and characterized (110)-oriented YBa2Cu3O7−x (YBCO)/PrBa2(Cu0.8Ga0.2)3O7−x (PBCGO) bilayer and YBCO/PBCGO/YBCO trilayer heterostructures, which were deposited by pulsed laser deposition technique for the nanofabrication of (110)-oriented YBCO-based superconductor (S)/insulator (I)/superconductor (S) tunneling vertical geometry Josephson junction and other superconductor electronic devices. The structural properties of these heterostructures, investigated through various x-ray diffraction techniques (profile, x-ray reflectivity, pole figure, and reciprocal mapping), showed (110)-oriented epitaxial growth with a preferred c-axis-in-plane direction for all layers of the heterostructures. The atomic force microscopy measurement on the top surface of the heterostructures showed crack-free and pinhole-free, compact surface morphology with about a few nanometer root mean square roughness over the 5 × 5 μm2 region. The electrical resistivity measurements on the (110)-direction of the heterostructures showed superconducting critical temperature (Tc) values above 77 K and a very small proximity effect due to the interfacial contact of the superconducting YBCO layers with the PBCGO insulating layer. Raman spectroscopy measurements on the heterostructures showed the softening of the Ag-type Raman modes associated with the apical oxygen O(4) and O(2)-O(3)-in-phase vibrations compared to the stand-alone (110)-oriented PBCGO due to the residual stress and additional two Raman modes at ∼600 and ∼285 cm−1 frequencies due to the disorder at the Cu–O chain site of the PBCGO. The growth process and structural, electrical transport, and Raman spectroscopy characterization of (110)-oriented YBCO/PBCGO bilayer and YBCO/PBCGO/YBCO trilayer heterostructures are discussed in detail.

Two-dimensional trilayer heterostructures with cascade dual Z-schemes to achieve efficient hydrogen evolution reaction

Electronic properties and diabatic molecular dynamics simulations reveal that the maximum solar-to-hydrogen efficiency of photocatalytic cascade dual Z-schemes with Bi(InAs3)/HfSeTe/ZrSe2 heterostructures can reach 41.04%.

Electrical transport mechanism of manganite-based trilayer heterostructures in the Cusp and insulating regions

In the present study, efforts are made to study the temperature dependent resistivity (ρ-T) variations of manganites (La0.7Sr0.3MnO3 (LSMO) single layer) and manganites-based systems ((LSMO/γ-Fe2O3/LSMO) trilayer heterostructures) in two distinct regions (Cusp region and Insulating region). Cusp region is an intermediate region between Metallic region and Insulating region. The resistivity-temperature curves are studied and analysed by various resistive models and fitting equations. The co-existence of ferromagnetic clusters and polarons manifests in the Cusp region. The Insulating region is explored and fitted under two regimes: Low Temperature Insulating region and High Temperature Insulating region. Various mathematical models viz. Thermal Activation model, Small Polaron Hopping model and Variable Range Hopping model were employed to study the ρ-T variations under various temperature ranges for LSMO (60 nm) single layer system and LSMO/γ-Fe2O3/LSMO (150 nm) trilayer heterostructures. In Insulating region, the deviations in the fitted curves from the experimental data is due to intrinsic disorder at the R/A site in case of LSMO single layer system while in case of trilayer system disorder exists at both R/A and B sites. The study is crucial to understand the microscopic picture of electrical transport behavior in both the systems near and above Metal-Insulator Transition.

Bilayer and Trilayer C3N/Blue-Phosphorene Heterostructures as Potential Anode Materials for Potassium-Ion Batteries.

Two-dimensional (2D) van der Waals heterostructures outperform conventional anode materials for postlithium-ion batteries in terms of mechanical, thermal, and electrochemical properties. This study systemically investigates the performance of bilayer and trilayer C3N/blue phosphorene (C3N/BlueP) heterostructures as anode materials for potassium-ion batteries (KIBs) using first-principles density functional theory calculations. This study reveals that the adsorption and diffusion of K ions on bilayer and trilayer C3N/BlueP heterostructures are markedly superior to those of their monolayer counterparts. A bilayer heterostructure (C3N/BlueP) effectively reduces the bandgap of the BlueP monolayer (1.98 eV) to 0.02 eV, whereas trilayer heterostructures (bilayer-C3N/BlueP and C3N/bilayer-BlueP) exhibit metallic behavior with no bandgap. Additionally, the theoretical capacity of the bilayer and trilayer heterostructures ranges from 636.7 to 755.5 mA h g-1, considerably higher than the theoretical capacity of other prospective 2D heterostructures for KIBs investigated in the literature. This study also shows that the heterostructures exhibit K-ion diffusion barriers as low as 0.042 eV, ensuring the relatively fast diffusion of K ions.

Effect of strain on electronic properties of tri-layer MoS2/h-BN/graphene van der Waals heterostructures

The two-dimensional materials have grabbed the attention of researchers because of their high functionality and widely controllable properties. Due to the dimensionality reduction, the external factors have a significant influence on their properties. The density functional theory calculations have been performed on the heterostructures of single-layer graphene (G), boron nitride (BN) and molybdenum disulfide (MoS2) to investigate the alteration in their electronic properties. Our calculations predict that electronic properties such as bandgap, work function, Schottky barrier height and tunnelling barrier probability can be modulated by applying mechanical strain. The application of strain resulted in a band gap opening of about 1 eV in trilayer heterostructures. At low values of strain, heterostructures maintained a low resistance ohmic contact. At −9% strain, the p-to-n-type transition of Schottky contact was observed indicating the flow of electrons from semiconductor to metal. The MoS2 monolayer was found to be more electron deficient and exhibited a greater charge accumulation. MoS2/BN/G heterostructure can be used for the fabrication of capacitive devices. Whereas BN/G/MoS2 and BN/MoS2/G heterostructures can act as potential candidates for trilayer Schottky devices due to the presence of low-barrier metal-semiconductor junction.

Achieving near-perfect light absorption in atomically thin transition metal dichalcogenides through band nesting

Near-perfect light absorbers (NPLAs), with absorbance, {{{{{{{mathcal{A}}}}}}}}, of at least 99%, have a wide range of applications ranging from energy and sensing devices to stealth technologies and secure communications. Previous work on NPLAs has mainly relied upon plasmonic structures or patterned metasurfaces, which require complex nanolithography, limiting their practical applications, particularly for large-area platforms. Here, we use the exceptional band nesting effect in TMDs, combined with a Salisbury screen geometry, to demonstrate NPLAs using only two or three uniform atomic layers of transition metal dichalcogenides (TMDs). The key innovation in our design, verified using theoretical calculations, is to stack monolayer TMDs in such a way as to minimize their interlayer coupling, thus preserving their strong band nesting properties. We experimentally demonstrate two feasible routes to controlling the interlayer coupling: twisted TMD bi-layers and TMD/buffer layer/TMD tri-layer heterostructures. Using these approaches, we demonstrate room-temperature values of {{{{{{{mathcal{A}}}}}}}}=95% at λ=2.8 eV with theoretically predicted values as high as 99%. Moreover, the chemical variety of TMDs allows us to design NPLAs covering the entire visible range, paving the way for efficient atomically-thin optoelectronics.

Stacking order effects on the energetics and electronic properties of [formula omitted]-doped graphene/h-BN van der Waals heterostructures on SiC(0001)

Heterostructures based on the stacking of two-dimensional materials with different electronic properties have been the subject of several studies addressing the development of novel devices with multiple functionalities. Graphene (G)/hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively investigated, including studies on the large-scale synthesis of h-BN on graphene/SiC(0001) substrate. This work is a theoretical study using first-principles calculations of bilayer (G/h-BN) and trilayer (G/h-BN/G) heterostructures on SiC(0001) covered by a carbon buffer layer (SiC). The results show an energetic preference for h-BN encapsulation below a single layer of graphene on SiC, viz.: G/h-BN/SiC in the case of bilayer systems and G/h-BN/G/SiC in the case of trilayer systems. Indeed, the electronic structure calculations reveal the preservation of graphene linear energy band dispersion in the bilayer systems. The trilayer systems’ electronic structure depends on the stacking order, with the emergence of parabolic bands in energetically less stable systems. Structural characterizations of these bilayers and trilayers on SiC were carried out based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near edge spectroscopy (XANES) with the goal of assisting future experimental spectroscopy in these graphene/h-BN vdW systems.

Designing strong and tunable magnetoelectric coupling in 2D trilayer heterostructures

The quest for electric-field control of nanoscale magnetic states such as skyrmions, which would impact the field of spintronics, has led to a challenging search for multiferroic materials or structures with strong magnetoelectric coupling and efficient electric-field control. Here we report a theoretical prediction that such phenomena can be realized in two-dimensional (2D) bilayer FE/PMM and trilayer FE/PMM/FE heterostructures (two-terminal and three-terminal devices), where FE is a 2D ferroelectric and PMM is a polar magnetic metal with strong spin–orbit coupling. Such a PMM has strong Dzyaloshinskii-Moriya interactions (DMI) that can generate skyrmions, while the FE can generate strong magnetoelectric coupling through polarization-polarization interactions. In trilayer heterostructures, contact to the metallic PMM layer enables multiple polarization configurations for electric-field control of skyrmions. We report density-functional-theory calculations for particular material choices that demonstrate the effectiveness of these arrangements, with the key driver being the polarization-polarization interactions between the PMM and FE layers. The present findings provide a method to achieve strong magnetoelectric coupling in the 2D limit and a new perspective for the design of related spintronics.

Hard/soft magnetic trilayer: Monte Carlo simulation

We present computer models of three-layer (trilayer) hard/soft magnetic heterostructures for the study of magnetic reversal processes under the external magnetic field using the Monte Carlo method. The comparisons of data for the magnetic SmCo/Fe trilayer models with the results for the magnetic SmCo/Fe bilayer model are presented. For the SmCo/Fe trilayer heterostructures, the exchange-bias field Hex and the irreversibility field Hirr increase by 150% and 30%, respectively, than for the SmCo/Fe bilayer model. Phase diagrams of dependencies of the Hex and Hirr values on the thickness of the hard magnetic layer of the trilayer magnetic heterostructures are plotted.

Efficient interlayer electron transfer in a MoTe2/WS2/MoS2 trilayer heterostructure

Electron transfer and carrier dynamics in MoTe2/WS2/MoS2 trilayer heterostructures are investigated by transient absorption and photoluminescence measurements. Monolayer flakes of MoTe2, WS2, and MoS2 are obtained by mechanical exfoliation from their bulk crystals and are used to fabricate the heterostructures by a dry-transfer technique. Photoluminescence spectroscopic measurements indicate that the recombination of the MoS2 and WS2 intralayer excitons is significantly suppressed in the heterostructure, illustrating the efficient interlayer charge transfer processes. Layer-selective time-resolved differential reflectance measurements show that the electrons excited in MoTe2 can transfer to MoS2 within 0.3 ps. The transferred electrons show a long lifetime of several hundred picoseconds due to their slow recombination with the spatially separated holes that reside in MoTe2. Furthermore, the charge transfer and recombination processes are weakly dependent on the injected carrier density. These results demonstrate the feasibility of constructing van der Waals multilayer heterostructures involving the infrared-sensitive MoTe2 with emergent properties and provide important information to quantify the performance of MoTe2-based devices.

Moir\xe9 excitons in MoSe2-WSe2 heterobilayers and heterotrilayers

Layered two-dimensional materials exhibit rich transport and optical phenomena in twisted or lattice-incommensurate heterostructures with spatial variations of interlayer hybridization arising from moiré interference effects. Here, we report experimental and theoretical studies of excitons in twisted heterobilayers and heterotrilayers of transition metal dichalcogenides. Using MoSe2-WSe2 stacks as representative realizations of twisted van der Waals bilayer and trilayer heterostructures, we observe contrasting optical signatures and interpret them in the theoretical framework of interlayer moiré excitons in different spin and valley configurations. We conclude that the photoluminescence of MoSe2-WSe2 heterobilayer is consistent with joint contributions from radiatively decaying valley-direct interlayer excitons and phonon-assisted emission from momentum-indirect reservoirs that reside in spatially distinct regions of moiré supercells, whereas the heterotrilayer emission is entirely due to momentum-dark interlayer excitons of hybrid-layer valleys. Our results highlight the profound role of interlayer hybridization for transition metal dichalcogenide heterostacks and other realizations of multi-layered semiconductor van der Waals heterostructures.

3D Hybrid Trilayer Heterostructure: Tunable Au Nanorods and Optical Properties.

Engineering plasmonic nanostructures from three dimensions (3D) is very attractive toward controllable and tunable nanophotonic components and devices. Herein, Au-based trilayer heterostructures composed of a dielectric spacer sandwiched by hybrid Au-TiN vertically aligned nanocomposite (VAN) nanoplasmonic claddings are demonstrated with a broad range of geometries and property tuning. Two types of spacer layers, that is, a pure dielectric BaTiO3 layer and a hybrid plasmonic Au-BaTiO3 VAN layer, contribute to the tuning of the Au nanorod dimension. Such geometrical variations of Au nanostructures originate from the surface energy and lattice strain tuned by the spacer layers. Optical measurements and numerical simulations suggest the change of the localized surface plasmon resonance which is strongly affected by the tailored Au nanorods as either separated or channeled. The uniaxial dielectric tensors suggest a tunable hyperbolic property affected by such a metal-insulator-metal trilayer stack. The complex 3D heterostructures offer additional tuning parameters and design flexibilities in hybrid plasmonic metamaterials toward potential applications in light harvesting, sensing, and nanophotonic devices.

Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure

AbstractWhen the spatial dimensions of metallic heterostructures shrink below the mean free path of its conduction electrons, the transport of electrons and hence the transport of thermal energy by electrons continuously changes from diffusive to ballistic. Electron–phonon coupling sets the mean free path to the nanoscale and the time for equilibration of electron and lattice temperatures to the picosecond range. A particularly intriguing situation occurs in trilayer heterostructures combining metals with very different electron–phonon coupling strength: Heat energy deposited in few atomic layers of Pt is transported into a nanometric Ni film, which is heated more than the Cu film through which the heat is released. Femtosecond pump‐probe experiments with hard X‐ray pulses provide a layer‐specific probe of the heat energy. A purely diffusive two‐temperature model with increased thermal conductivity of hot electrons excellently reproduces the observed signals from all three layers. At the time when the Ni lattice is maximally heated, no significant heat has entered the Cu lattice. This phenomenon would be enhanced in thinner layers where ballistic transport dominates. In this context it is shown that purely diffusive transport can lead to a linear time‐to‐length dependence that must not be misinterpreted as ballistic transport.

Strong spin-dephasing in a topological insulator-paramagnet heterostructure

The interface between magnetic materials and topological insulators can drive the formation of exotic phases of matter and enable functionality through the manipulation of the strong spin polarized transport. Here, we report that the transport processes that rely on strong spin-momentum locking in the topological insulator Bi2Se3 are completely suppressed by scattering at a heterointerface with the kagome-lattice paramagnet, Co7Se8. Bi2Se3–Co7Se8–Bi2Se3 trilayer heterostructures were grown using molecular beam epitaxy, where magnetotransport measurements revealed a substantial suppression of the weak antilocalization effect for Co7Se8 at thicknesses as thin as a monolayer, indicating a strong dephasing mechanism. Bi2−xCoxSe3 films, in which Co is in a non-magnetic 3+ state, show weak antilocalization that survives to higher than x = 0.4, which, in comparison with the heterostructures, suggests that the unordered moments of Co2+ act as a far stronger dephasing element. This work highlights several important points regarding coherent transport processes involving spin-momentum locking in topological insulator interfaces and how magnetic materials can be integrated with topological materials to realize both exotic phases and novel device functionality.

Current-in-plane spin-valve magnetoresistance in ferromagnetic semiconductor (Ga,Fe)Sb heterostructures with high Curie temperature

We demonstrate spin-valve magnetoresistance (MR) with a current-in-plane (CIP) configuration in (Ga,Fe)Sb/InAs (thickness tInAs nm)/(Ga,Fe)Sb trilayer heterostructures, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC). An MR curve with an open minor loop is clearly observed at 3.7 K in a sample with tInAs = 3 nm, which originates from the parallel-antiparallel magnetization switching of the (Ga,Fe)Sb layers and spin-dependent scattering at the (Ga,Fe)Sb/InAs interfaces. The MR ratio increases (from 0.03 to 1.6%) with decreasing tInAs (from 9 to 3 nm) due to the enhancement of the interface scattering. This demonstration of the spin-valve effect in Fe-doped FMS heterostructures paves the way for device applications of the high-TC FMS.

Stacking-dependent electronic properties of aluminene based multilayer van der Waals heterostructures

Aluminene, one of the group III elemental monolayers, is predicted to be stable in the honeycomb configuration with metallic characteristics. In this paper, we consider aluminene-based heterostructures, investigating their stability, structural and electronic properties. The results based on density functional theory find that the interaction between aluminene and BN or graphene monolayers is weak, and is dominated by the van der Waals forces. The electronic structures calculated using the HSE06 functional show aluminene/graphene and aluminene/BN bilayers to be metallic. Likewise, graphene/aluminene/graphene trilayers are predicted to be metallic. However, BN/aluminene/BN trilayers are predicted to be semiconducting in nature with finite gaps. The results suggest that the interaction of Al atoms with N atoms facilitates the opening up of the gap in trilayer heterostructures, unlike the graphene/BN heterostructures, where inequivalence of the carbon lattice leads to the bandgap. The stacking-dependence of the electronic structure is affirmed by the electron tunneling characteristics calculated for the trilayer heterostructures. The predicted relationship between topology and electronic structure can, therefore, be exploited to tailor the electronic properties of aluminene-based heterostructures.

Interplay of charge transfer and disorder in optoelectronic response in Graphene/hBN/MoS2 van der Waals heterostructures

Strong optoelectronic response in the binary van der Waals heterostructures of graphene and transition metal dichalcogenides (TMDCs) is an emerging route towards high-sensitivity light sensing. While the high sensitivity is an effect of photogating of graphene due to inter-layer transfer of photo-excited carriers, the impact of intrinisic defects, such as traps and mid-gap states in the chalcogen layer remain largely unexplored. Here we employ graphene/hBN (hexagonal boron nitride)/MoS2 (molybdenum disulphide) trilayer heterostructures to explore the photogating mechanism, where the hBN layer acts as interfacial barrier to tune the charge transfer timescale. We find two new features in the photoresponse: First, an unexpected positive component in photoconductance upon illumination at short times that preceeds the conventional negative photoconductance due to charge transfer, and second, a strong negative photoresponse at infrared wavelengths (up to 1720 nm) well-below the band gap of single layer MoS2. Detailed time and gate voltage-dependence of the photoconductance indicates optically-driven charging of trap states as possible origin of these observations. The responsivity of the trilayer structure in the infrared regime was found to be extremely large (> 108 A/W at 1550 nm using 20 mV source drain bias at 180 K temperature and ≈ − 30 V back gate voltage). Our experiment demonstrates that interface engineering in the optically sensitive van der Waals heterostructures may cast crucial insight onto both inter- and intra-layer charge reorganization processes in graphene/TMDC heterostructures.