Double-layer Heterostructure Research Articles

Plasmons and loss function in a double-layer silicene-graphene heterostructure at zero-temperature

We compute theoretically the plasmon frequencies and the loss function in a double layer silicene-graphene heterostructure within random phase approximation (RPA) at zero temperature. In the long wavelength approximation we obtain analytical expressions for the acoustic and optical plasmons as well as for the loss function restricted to the acoustic and optical plamon branches. In the q→0 limit the loss function restricted to the acoustic plasmon behaves as −ImTr[ΠR(q,ωac(q→0))]∼q while its restriction to the optical plasmon displays a behavior −ImTr[ΠR(q,ωop(q→0))]∼q32. Numerical simulations show that the acoustic plasmon crosses the ω=vFgq line tending towards the ω=12vFgq. To the best of our knowledge, this behavior is not exhibited in other double layer systems. Another novel feature of this system is that as the acoustic plasmon passes the ω=vFgq, the loss function restricted to the acoustic plasmon curve displays a highly concentrated peak with almost no broadening on a small interval until it disperses and shows the usual broadened peak for damped plasmons.

Double-layer heterostructure in situ grown from stainless steel substrate for overall water splitting

Developing free-standing electrode based on cost-effective industrial substrate materials is a promising way for efficient water splitting applications. The common stainless steel (S S) consisting of catalytic active transition metal components (e.g. Ni, Fe, etc.) has the potential to be an ideal substrate to prepare free-standing electrodes. In this work, a facile hydrothermal oxidization treatment was proposed to oxidize three types of SS substrates (i.e. 304, 316L and 310S) to prepare efficient free-standing electrodes for water electrolysis. Compared with the 316L and 310S, the relatively low content of Mo and Cr in 304 SS make it easier to be oxidized by alkaline hydrogen peroxide to produce a well-confined double layer heterostructured catalytic film with Fe-rich microcrystals at the top and Ni-rich nanocrystals at the bottom. Consequently, the as obtained 304-SSO27 sample exhibits low overpotentials of 136 mV and 285 mV at the current density of 10 mA·cm−2 towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Moreover, the bifunctional 304-SS-O27 electrode displays a low cell voltage of 1.67 V to realize overall water splitting at 10 mA·cm−2. These results provide a convincing demonstration of fabricating cost-effective and frees-standing electrodes via a facile one step hydrothermal oxidization for water splitting applications.

Study on the mechanism of buffer absorbing energy of double-layer heterostructure based on viscoelastic materials for MEMS devices

Under the high overload impact environment, MEMS devices are prone to failure due to their complex mechanical structure. Therefore, improving the sensor's resistance to high overload is crucial to ensure its normal service. The dynamic response model of the double-layer heterostructure is established based on high-impact accelerometers and second-order oscillation systems. The peak attenuation ratio of the inertial load was obtained to characterize the buffering and energy absorption properties of the structure. Through Ls-Dyna numerical simulation and overload impact tests, it was verified that the error was less than 6.67%. The double-layer heterostructure with an outer epoxy resin and inner polyurethane exhibited the best buffering performance among the tested structures, with a maximum improvement of 35.6% compared to the single-layer structure. The peak attenuation ratio of inertial load can reach 44%. These findings provide a theoretical foundation and technical support for the research and application of protective structures for MEMS sensors against high overload.

Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures.

A supersolid, a counterintuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realized. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic density modulation or clustering of the superfluid. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid-solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

Plasmon Damping Rates in Coulomb-Coupled 2D Layers in a Heterostructure.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

Correlated interlayer exciton insulator in heterostructures of monolayer WSe2 and moiré WS2/WSe2

Moiré superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moiré bilayer that are separated by ultrathin hexagonal boron nitride. The moiré WS2/WSe2 bilayer features a Mott insulator state when the density of holes is one per moiré lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moiré bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The interlayer exciton insulator is stable up to a critical hole density in the WSe2 monolayer, beyond which the interlayer exciton dissociates. Our study highlights the opportunities for realizing quantum phases in double-layer moiré systems due to the interplay between the moiré flat band and strong interlayer electron interactions.

Giant Extrinsic Spin Hall Effect in Platinum-Titanium Oxide Nanocomposite Films.

Although the spin Hall effect provides a pathway for efficient and fast current‐induced manipulation of magnetization, application of spin–orbit torque magnetic random access memory with low power dissipation is still limited to spin Hall materials with low spin Hall angles or very high resistivities. This work reports a group of spin Hall materials, Pt1 −x (TiO2) x nanocomposites, that combines a giant spin Hall effect with a low resistivity. The spin Hall angle of Pt1 −x (TiO2) x in an yttrium iron garnet/Pt1 −x (TiO2) x double‐layer heterostructure is estimated from a combination of ferromagnetic resonance, spin pumping, and inverse spin Hall experiments. A giant spin Hall angle 1.607 ± 0.04 is obtained in a Pt0.94(TiO2)0.06 nanocomposite film, which is an increase by an order of magnitude compared with 0.051 ± 0.002 in pure Pt thin film under the same conditions. The great enhancement of spin Hall angle is attributed to strong side‐jump induced by TiO2 impurities. These findings provide a new nanocomposite spin Hall material combining a giant spin Hall angle, low resistivity and excellent process compatibility with semiconductors for developing highly efficiency current‐induced magnetization switching memory devices and logic devices.

Single-layer Ga2O3/graphene heterogeneous structure with optical switching effect

Both single layer Ga2O3 (SLGO) and graphene are attractive due to their respective electronic and mechanical properties such as wide bandgap and high electrical conductivity. Bringing them together by using van der Waals force to form a heterogeneous structure is new and worth to investigate. In this work, density functional theory (DFT) study of SLGO/graphene has been conducted through varying the interlayer distance. Standard procedures of calculating double-layer heterostructures using DFT has been followed, and several interesting phenomena have been unveiled, for example, band opening in conduction bands of the SLGO and graphene and switching effect of the in-plane optical absorption.

Bridge role of weak chemical bonding in photocatalytic performance of asymmetric 2H-MoS2/BiOCl Janus heterostructure

The geometric and electronic structure, partial (band decomposed) charge density, charge transfer, electron localization function and photocatalytic mechanism of the asymmetric 2H-MoS2/BiOCl Janus heterostructure were systematically studied with first-principles density functional theory. Our calculations showed that there exist several newly formed weak Bi-S bonds with shorter bond lengths between BiOCl and 2H-MoS2 which act as an electron transport bridge along the direction perpendicular to the heterojunction interface. This newly weak bonds lead to the formation of occupied shallow defect levels approximately 0.0–0.9 eV below the bottom of the conduction band. Electrons located at these defect levels can easily jump into the conduction band as a donor energy level under thermal fluctuations and simultaneously further promote the effective separation of photo-generated electron-hole pairs in the BiOCl. The photogenerated electrons located around Bi-atom layer in the conduction band of BiOCl transfer to the valence band of 2H-MoS2 around the S-atom layer through the interface of the asymmetric 2H-MoS2/BiOCl Janus heterostructure, which significantly reduce photo-generated holes in the 2H-MoS2 and electrons in the BiOCl. The large numbers of photogenerated electrons from the 2H-MoS2 cannot recombine with holes owing to lack of sufficient holes. They will move to the surface and greatly improve the hydrogen production activity in the 2H-MoS2. While the photogenerated holes from the BiOCl will significantly improve the ability of BiOCl to oxidize pollutant in the water owing to the absence of sufficient electrons. Our studies provide new way for the design of asymmetric Janus double-layer heterostructures with newly formed weak chemical bonding.

Growth and characterization of ZnO/MgZnO thin film hetero structures on p-Si for visible light detectors

ZnO- and MgZnO-based single- and double-layer heterostructures have been grown using an electron-beam evaporation system. Structural, morphological, optical and electrical characteristics were elaborated for all the configurations. Using x-ray diffraction, it was inferred that a hexagonal wurtzite structure is maintained for both ZnO and MgZnO with fairly good crystallinity. Field emission scanning electron microscopy (FESEM) images showed the homogeneous distribution of particles in ZnO and MgZnO throughout the films. Both atomic force microscopy and FESEM images exhibit a larger size for ZnO particles. The UV emission for ZnO at ∼371 nm and MgZnO at ∼359 nm was anticipated from photoluminescence spectra. The visible photoconductive properties of all the different configurations were studied in the dark and under the illumination of a white light source. The highest responsivities measured for the ZnO/MgZnO/Si structure were 0.242 A W−1 and 0.164 A W−1 for as deposited and annealed at 400 °C, respectively. These results show the suitability of bilayer photo detectors for visible light detection.

ZnO/CoO@NiCoS nanohybrids with double heterogeneous interface for high-performance hybrid supercapacitors

Hybrid supercapacitors (HSC) with high power density and long cycle life are received more attention in the field of energy storage devices, however, the wide application of HSC is limited by the low energy density owing to the insufficient active sites and poor electronic conductivity of electrode materials. Here, we successfully synthesize the ZnO/CoO@NiCoS nanohybrids with double-layer heterojunction by a three-step method. The conductivity of the as-prepared electrode can be improved by the built-in electric field formed between ZnO and CoO. The coating of amorphous NiCoS can provide sufficient active sites and strengthen the physical/chemical stability of ZnO/CoO nanowires. The obtained ZnO/CoO@NiCoS delivers a high specific capacity of 934 C g−1 (1868 F g−1), which is 5.5 times the pristine ZnO electrode. Furthermore, a HSC with ZnO/CoO@NiCoS shows an improved power density (3987.7 W kg−1), a high energy density of 39.2 Wh kg−1, and an excellent cycling stability (initial capacity retention rate of 81.7% after 6,000 cycles). This study not only proposes an effective strategy for the preparation of low-cost, deep-discharge electrodes for supercapacitors, but also provides a novel method to construct nanomaterials with double-layer heterostructure and built-in electric field.

Effects of parallel electric and magnetic fields on Rydberg excitons in buckled two-dimensional materials

We study direct and indirect magnetoexcitons in Rydberg states in monolayers and double-layer heterostructures of Xenes (silicene, germanene, and stanene) in external parallel electric and magnetic fields, applied perpendicular to the monolayer and heterostructure. We calculate binding energies of magnetoexcitons for the Rydberg states 1$s$, 2$s$, 3$s$, and 4$s$, by numerical integration of the Schr\"{o}dinger equation using the Rytova-Keldysh potential for direct magnetoexciton and both the Rytova-Keldysh and Coulomb potentials for indirect excitons. Latter allows understanding a role of screening in Xenes. In the external perpendicular electric field, the buckled structure of the Xene monolayers leads to appearance of potential difference between sublattices allowing to tune electron and hole masses and, therefore, the binding energies and diamagnetic coefficients (DMCs) of magnetoexcitons. We report the energy contribution from electric and magnetic fields to the binding energies and DMCs. The tunability of the energy contribution of direct and indirect magnetoexcitons by electric and magnetic fields is demonstrated. It is also shown that DMCs of direct excitons can be tuned by the electric field, and the DMCs of indirect magnetoexcitons can be tuned by the electric field and manipulated by the number of h-BN layers. Therefore, these allowing the possibility of electronic devices design that can be controlled by external electric and magnetic fields and the number of h-BN layers. The calculations of the binding energies and DMCs of magnetoexcitons in Xenes monolayers and heterostructures are novel and can be compared with the experimental results when they will be available.

Effect of Mismatched Electron-Hole Effective Masses on Superfluidity in Double Layer Solid-State Systems

Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We systematically investigate the sensitivity to unequal masses of the superfluid properties and the self-consistent screening of the electron-hole pairing interaction. We find that the superfluid properties are insensitive to mass imbalance in the low density BEC regime of strongly-coupled boson-like electron-hole pairs. At higher densities, in the BEC-BCS crossover regime of fermionic pairs, we find that mass imbalance between electrons and holes weakens the superfluidity and expands the density range for the BEC-BCS crossover regime. This permits screening to kill the superfluid at a lower density than for equal masses.

Compact SQUID Realized in a Double-Layer Graphene Heterostructure.

2D systems that host 1D helical states are advantageous from the perspective of scalable topological quantum computation when coupled to a superconductor. Graphene is particularly promising for its high electronic quality, its versatility in van der Waals heterostructures, and its electron- and hole-like degenerate 0th Landau level. Here we study a compact double-layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by the independent gate control of the chemical potentials in both layers. Furthermore, both Josephson junctions show a skewed current-phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime, we measure a well-defined conductance plateau of 2e2/h indicative of counter-propagating edge channels in the two layers.

Point interactions with bound states: A zero-thickness limit of a double-layer heterostructure

A heterostructure composed of two parallel homogeneous layers is studied in the limit as their width and the distance between them shrinks to zero simultaneously. The problem is considered in one dimension and the squeezing potential in the Schrödinger equation is chosen in the form of a piecewise constant function. As a result, two families of point interactions with bound state energy are realized from this structure. The specific feature of these interactions is the resonant-tunneling transmission of electrons through one-point singular potentials under certain conditions described by transcendental equations. The solutions to these equations define so-called resonance sets of Lebesgue’s measure zero. A particular example is the potential in the form of the derivative of Dirac’s delta function. For a whole family of point interactions including this example, the existence of a bound state is proven, contrary to the widespread opinion on the non-existence of bound states in δ'-like systems.

Terahertz plasma wave localization and velocity control in tapered double-layer graphene heterostructure

Terahertz plasma wave (plasmon) localization and velocity control in tapered heterostructure (taper) with double-layer graphene were numerically studied. We employ a simple rigorous theoretical model to find out the plasmon localization length and energy velocity values along the tapered structure. It is shown that the plasmon decelerates while moving from the taper apex and the deceleration process is accompanied with increase of the plasmon wave localization. While moving along the structure from the taper apex, the plasmon energy velocity as well as the plasmon localization length can become nearly an order of magnitude smaller as compared to the values near the taper apex.

Transition Metal Dichalcogenides as Strategy for High Temperature Electron-Hole Superfluidity

Condensation of spatially indirect excitons, with the electrons and holes confined in two separate layers, has recently been observed in two different double layer heterostructures. High transition temperatures were reported in a double Transition Metal Dichalcogenide (TMD) monolayer system. We briefly review electron-hole double layer systems that have been proposed as candidates for this interesting phenomenon. We investigate the double TMD system WSe 2 /hBN/MoSe 2 , using a mean-field approach that includes multiband effects due to the spin-orbit coupling and self-consistent screening of the electron-hole Coulomb interaction. We demonstrate that the transition temperature observed in the double TMD monolayers, which is remarkably high relative to the other systems, is the result of (i) the large electron and hole effective masses in TMDs, (ii) the large TMD band gaps, and (iii) the presence of multiple superfluid condensates in the TMD system. The net effect is that the superfluidity is strong across a wide range of densities, which leads to high transition temperatures that extend as high as T B K T = 150 K.

Electrical plasmon injection in double-layer graphene heterostructures

It is by now well established that high-quality graphene enables a gate-tunable low-loss plasmonic platform for the efficient confinement, enhancement, and manipulation of optical fields spanning a broad range of frequencies, from the mid infrared to the Terahertz domain. While all-electrical detection of graphene plasmons has been demonstrated, electrical plasmon injection (EPI), which is crucial to operate nanoplasmonic devices without the encumbrance of a far-field optical apparatus, remains elusive. In this work, we present a theory of EPI in double-layer graphene, where a vertical tunnel current excites acoustic and optical plasmon modes. We first calculate the power delivered by the applied inter-layer voltage bias into these collective modes. We then show that this system works also as a spectrally-resolved molecular sensor.

First-principles quantum corrections for carrier correlations in double-layer two-dimensional heterostructures

We present systematic ab initio calculations of the charge carrier correlations between adjacent layers of two-dimensional materials in the presence of both charged impurity and strain disorder potentials using the examples of monolayer and bilayer graphene. For the first time, our analysis yields unambiguous first-principles quantum corrections to the Thomas--Fermi densities for interacting two-dimensional systems described by orbital-free density functional theory. Specifically, using density-potential functional theory, we find that quantum corrections to the quasi-classical Thomas-Fermi approximation have to be taken into account even for heterostructures of mesoscopic size. In order for the disorder-induced puddles of electrons and holes to be anti-correlated at zero average carrier density for both layers, the strength of the strain potential has to exceed that of the impurity potential by at least a factor of ten, with this number increasing for smaller impurity densities. Furthermore, our results show that quantum corrections have a larger impact on puddle correlations than exchange does, and they are necessary for properly predicting the experimentally observed Gaussian energy distribution at charge neutrality.

A hierarchical sandwich-structured MoS2/SnO2/CC heterostructure for high photocatalysis performance

A sandwich-structured MoS2/SnO2/Carbon cloth (MoS2/SnO2/CC) photocatalyst was successfully synthesized through simple two-step hydrothermal method. The MoS2/SnO2/CC composite exhibited an excellent photocatalysis performance for degradation of RhB dye under visible light. A hierarchical double-layer heterostructure is formed via introducing carbon cloth substrates, which not only enhances the photocatalysis activity by accelerating electrons transfer rate and further intensifying electron-hole separation, but also is convenient for recycle. The specific photocatalysis, optical properties and photogenerated electron-hole transmission mechanism are discussed in detail. The improved photocatalysis performance of the MoS2/SnO2/CC suggests the possibilities of developing the multi-layer heterostructure for further photocatalysis applications.