Bilayer Heterostructures Research Articles

Magnetic proximity-induced anomalous Hall effect in 2D CrOCl/Pt heterostructure

Two-dimensional (2D) van der Waals antiferromagnetic (AFM) materials boast exceptional properties for spintronics, including high spin-wave speeds and negligible stray fields. Their layer-by-layer assembly into heterostructures enables the exploration of next-generation spintronic devices. However, most 2D AFM materials are semiconductors or insulators. Thus, magneto-transport, a key segment of spintronics, is difficult to obtain especially at low temperatures. Herein, we report the observation of anomalous Hall effect (AHE) in 2D CrOCl/Pt bilayer heterostructure. Magneto-transport measurements supported by density functional theory calculations reveal that the appearance of AHE is generated by spin polarization in Pt due to the magnetic proximity effect. In addition, it is demonstrated that the magnetic easy-axis changes from the z-axis to the xy-plane at the interface of the heterostructure. Our work sheds light on the magneto-transport properties of 2D CrOCl and its potential in emerging spintronic devices.

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

Solid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s‐1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

The effect of interface strains induced by various ferroelectric top layers on the magnetoelectric behavior for bilayer heterostructure

The effect of interface strains induced by various ferroelectric top layers on the magnetoelectric behavior for bilayer heterostructure

Resistive switching behaviors and conduction mechanisms of IGZO/ZnO bilayer heterostructure memristors

Resistive switching behaviors and conduction mechanisms of IGZO/ZnO bilayer heterostructure memristors

Bilayer Heterostructure Electrolytes Were Prepared by a UV-Curing Process for High Temperature Lithium-Ion Batteries.

Solid-state electrolytes are widely anticipated to revitalize high-energy-density and high-safety lithium-ion batteries. However, low ionic conductivity and high interfacial resistance at room temperature pose challenges for their practical application. In this work, the dual-matrix concept is applied to the design of a bilayer heterogeneous structure. The electrolyte in contact with the cathode blends PVDF-HFP and oxidation-resistant PAN. In contrast, the electrolyte in contact with the anode blends PVDF-HFP and reduction-resistant PEO. A UV-curing process was used to fabricate the bilayer heterostructure electrolyte. The heterostructure electrolyte exhibits an ionic conductivity of 4.27 × 10-4 S/cm and a Li+ transference number of 0.68 at room temperature. Additionally, when assembled into LiFePO4/CPEs/Li batteries, it shows a high initial discharge capacity at room temperature (168 mAh g-1 at 0.1 C and 60 mAh g-1 at 2 C), with a capacity retention of 93.3% after 100 cycles at a current density of 0.2 C. Notably, at 60 °C, the battery maintains a discharge capacity of 90 mAh g-1 at 2 C, with a capacity retention of 97.4% after 100 cycles at 0.2 C. Therefore, solid-state batteries using this bilayer heterogeneous structure electrolyte demonstrate promising performance, including effective capacity output and cycling stability.

Designing Bilayer Heterostructure Functional Polymer Electrolytes with Interfacial Engineering Strategy for High‐Performance Lithium Metal Batteries

AbstractThe uncontrollable lithium (Li) dendrites growth and complex electrode/electrolyte interface (EEI) problems are hindering the further application of high energy density lithium metal batteries (LMBs) in practice. Herein, a bilayer heterostructure gel polymer electrolyte (BGPE) is designed by directly curing functional boron‐containing monomers on the electrode surface to ensure excellent conductivity while solving the interface problems, achieving durable high voltage resistance and Li dendrites suppression. The unoccupied p‐orbital boron moiety of the 3D crosslinked network in BGPE not only improves the Li+ transference number (0.78), but also enhances the interfacial stability of the Li metal and inhibits the dendrites growth by anchoring PF6− anions and regulating the uniform Li deposition, thus ensuring a long cycle for Li/BGPE/Li cells. In addition, the functional additives tris(trimethylsilyl) phosphite and tris(pentafluorophenyl)borane can preferentially oxidize and decompose to form stable B, F, and Si‐rich EEIs, and effectively regulate the uniform growth of EEI. Thus, the LiNi0.5Co0.2Mn0.3O2/BGPE/Li and LiFePO4/BGPE/Li full cells exhibit stable cycling and excellent rate performance. This work provides a guiding design direction to address the EEI problems for high energy density LMBs.

Electronic and Optical Properties of 2D Heterostructure Bilayers of Graphene, Borophene and 2D Boron Carbides from First Principles.

In the present work the atomic, electronic and optical properties of two-dimensional graphene, borophene, and boron carbide heterojunction bilayer systems (Graphene-BC3, Graphene-Borophene and Graphene-B4C3) as well as their constituent monolayers are investigated on the basis of first-principles calculations using the HSE06 hybrid functional. Our calculations show that while borophene is metallic, both monolayer BC3 and B4C3 are indirect semiconductors, with band-gaps of 1.822 eV and 2.381 eV as obtained using HSE06. The Graphene-BC3 and Graphene-B4C3 bilayer heterojunction systems maintain the Dirac point-like character of graphene at the K-point with the opening of a very small gap (20-50 meV) and are essentially semi-metals, while Graphene-Borophene is metallic. All bilayer heterostructure systems possess absorbance in the visible region where the resonance frequency and resonance absorption peak intensity vary between structures. Remarkably, all heterojunctions support plasmons within the range 16.5-18.5 eV, while Graphene-B4C3 and Graphene-Borophene exhibit a π-type plasmon within the region 4-6 eV, with the latter possessing an additional plasmon at the lower energy of 1.5-3 eV. The dielectric tensor for Graphene-B4C3 exhibits complex off-diagonal elements due to the lower P3 space group symmetry indicating it has anisotropic dielectric properties and could exhibit optically active (chiral) effects. Our study shows that the two-dimensional heterostructures have desirable optical properties broadening the potential applications of the constituent monolayers.

Bilayer graphene, bilayer GeC and graphene/GeC bilayer heterostructure as anode materials for lithium-ion batteries: First-principles calculations

There is a great effort to develop the high-efficient anode materials for lithium-ion batteries with high stability and mobility. In this paper, we adopt the first-principles calculations to study the electrochemical properties of Li intercalation within bilayer graphene (BLG), bilayer GeC (BLGeC) and graphene/GeC bilayer heterostructure (BLGGeC) as anode materials. Our calculations disclose the following findings: (1) The interlayer is modulated by the stacking patterns and bilayer species. (2) The most energetically favorite intercalation of the Li atom is achieved in BLG with AA stacking because of the lowest adsorption energy. (3) The descending order of energy barriers is BLGGeC > BLGeC > BLG. The low diffusion barriers of BLG (0.025 eV) and BLGeC (0.09 eV) imply their high charging/discharging rates. Finally, the findings underline that the bilayer is a promising anode material and its electrochemical characteristics can be changed by adjusting the stacking configurations and its species.

Highly tunable skyrmion-like polar nanodomains for high-density ferroelectric hard disks

Emerging topological polar domains have a wide range of potential applications in electronic devices. It is critical to accurately manipulate these topological domains by electrical fields and explore their exotic properties for making more energy-efficient high-density non-volatile memories. Herein, we demonstrate that skyrmion-like polar nanodomains appear at room temperature in SrTiO3/PbTiO3 bilayer heterostructures by balancing the elastic and electrostatic energies via varying the SrTiO3 capping layer thickness. These polar nanodomains, stable at room temperature, can be electrically written, erased, and rewritten into the bilayer by applying an appropriate bias on the conductive tip of an atomic force microscope. The lateral size and location of these polar nanodomains can be precisely controlled. Moreover, ring-shaped conductive domain walls are observed around these polar nanodomains, with on/off ratios of more than two orders of magnitude with respect to the ferroelectric background. Based on these characteristics, the polar nanodomains can be created, erased, and probed electrically, suggesting applications for high-density ferroelectric hard disks.

Distinguishing carrier transport and interfacial recombination at perovskite/transport-layer interfaces using ultrafast spectroscopy and numerical simulation

In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast terahertz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest-performing CTLs, namely C60, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite’s surface recombination rate, likely contributing to its success in solar cell devices. Published by the American Physical Society 2024

Negative photo conductivity triggered with visible light in wide bandgap oxide-based optoelectronic crossbar memristive array for photograph sensing and neuromorphic computing applications

Photoresponsivity studies of wide-bandgap oxide-based devices have emerged as a vibrant and popular research area. Researchers have explored various material systems in their quest to develop devices capable of responding to illumination. In this study, we engineered a mature wide-bandgap oxide-based bilayer heterostructure synaptic memristor to emulate the human brain for applications in neuromorphic computing and photograph sensing. The device exhibits advanced electric and electrophotonic synaptic functions, such as long-term potentiation (LTP), long-term depression (LTD), and paired-pulse facilitation (PPF), by applying successive electric and photonic pulses. Moreover, the device exhibits exceptional electrical SET and photonic RESET endurance, maintaining its stability for a minimum of 1200 cycles without any degradation. Density functional theory calculations of the band structures provide insights into the conduction mechanism of the device. Based on this memristor array, we developed an autoencoder and convolutional neural network for noise reduction and image recognition tasks, which achieves a peak signal-to-noise ratio of 562 and high accuracy of 84.23%, while consuming lower energy by four orders of magnitude compared with the Tesla P40 GPU. This groundbreaking research not only opens doors for the integration of our device into image processing but also represents a significant advancement in the realm of in-memory computing and photograph-sensing features in a single cell.

Enhancement of Interlayer Exciton Emission in a TMDC Heterostructure via a Multi-Resonant Chirped Microresonator Upto Room Temperature.

We report on multi-resonance chirped distributed Bragg reflector (DBR) microcavities. These systems are employed to investigate the light-mater interaction with both intra- and inter-layer excitons of transition metal dichalcogenide (TMDC) bilayer heterostructures. The chirped DBRs consisting of SiO2 and Si3N4 layers of gradually varying thickness exhibit a broad stopband with a width exceeding 600nm. Importantly, the structures provide multiple resonances across a broad spectral range, which can be matched to resonances of the embedded TMDC heterostructures. Studying cavity-coupled emission of both intra- and inter-layer excitons from an integrated WSe2/MoSe2 heterostructure in a chirped microcavity system, an enhanced interlayer exciton emission with a Purcell factor of 6.67 ± 1.02 at 4 K is observed. The cavity-enhanced emission of the interlayer exciton is used to investigate its temperature-dependent luminescence lifetime of 60ps at room temperature. The cavity system modestly suppresses intralayer exciton emission by intentional detuning, thereby promoting a higher IX population and enhancing cavity-coupled interlayer exciton emission. This approach provides an intriguing platform for future studies of energetically distant and confined excitons in different semiconducting materials, which paves the way for various applications such as microlasers and single-photon sources by enabling precise emission control and utilizing multimode resonance light-matter interaction.

Relaxation effects in transition metal dichalcogenide bilayer heterostructures

While moiré structures in twisted bilayer transition metal dichalcogenides (TMDCs) have been studied for over a decade, the importance of lattice relaxation effects was pointed out only in 2021 by DiAngelo and MacDonald1, who reported the emergence of a Dirac cone upon relaxation. TMDCs of group 6 transition metals MX2 (M = Mo, W, X = S, Se) share layered structures with pronounced interlayer interactions, exhibiting a direct band gap when exfoliated to a two-dimensional (2D) monolayer. As their heterolayers are incommensurable, moiré structures are present in the bilayers even if stacked without a twist angle. This study addresses the challenge of accurately modeling and understanding the structural relaxation in twisted TMDC heterobilayers. We show that the typical experimental situation of finite-size flakes stacked upon larger flakes can reliably be modeled by fully periodic commensurate models. Our findings reveal significant lattice reconstruction in TMDC heterobilayers, which strongly depend on the twist angle. We can categorize the results in two principal cases: at or near the untwisted configurations of 0° and 60°, domains with matching lattice constants form and the two constituting layers exhibit significant in-phase corrugation—their out-of-plane displacements are oriented towards the same direction in all local stackings—while at large twist angles—deviating from the 0° and 60°—the two layers show an out-of-phase corrugation. In particular, we reveal that the lattice reconstruction results from the competition between the strain energy cost and the van der Waals energy gain. Additionally, our systematical study highlights structural disparities between heterostructures composed of different or identical chalcogen atoms. Our research not only confirms the reliability of using periodic commensurate models to predict heterostructure behavior but also enriches the understanding of TMDC bilayer heterostructures.

Giant asymmetric proximity-induced spin–orbit coupling in twisted graphene/SnTe heterostructure

We analyze the spin–orbit coupling effects in a 3∘-degree twisted bilayer heterostructure made of graphene and an in-plane ferroelectric SnTe, with the goal of transferring the spin–orbit coupling from SnTe to graphene, via the proximity effect. Our results indicate that the point-symmetry breaking due to the incompatible mutual symmetry of the twisted monolayers and a strong hybridization has a massive impact on the spin splitting in graphene close to the Dirac point, with the spin splitting values greater than 20 meV. The band structure and spin expectation values of graphene close to the Dirac point can be described using a symmetry-free model, triggering different types of interaction with respect to the threefold symmetric graphene/transition-metal dichalcogenide heterostructure. We show that the strong hybridization of the Dirac cone’s right movers with the SnTe band gives rise to a large asymmetric spin splitting in the momentum space. Furthermore, we discover that the ferroelectricity-induced Rashba spin–orbit coupling in graphene is the dominant contribution to the overall Rashba field, with the effective in-plane electric field that is almost aligned with the (in-plane) ferroelectricity direction of the SnTe monolayer. We also predict an anisotropy of the in-plane spin relaxation rates. Our results demonstrate that the group-IV monochalcogenides MX (M = Sn, Ge; X = S, Se, Te) are a viable alternative to transition-metal dichalcogenides for inducing strong spin–orbit coupling in graphene.

Quantitative Complex Refractive Index Changes in Thin Films: A Pump-Probe Spectroscopy Analysis Approach.

Photoexcitation induces intricate changes in both the real and imaginary components of the complex refractive index of thin film materials, which is essential for interpreting transient spectral features. Here, we employ a Kramers-Kronig-based analytical approach to elucidate light-induced changes in the complex refractive index from transient transmission spectra of thin films. Using gold-perovskite films as model systems, we conduct experimental measurements of transient transmission spectra for both individual gold and perovskite films, as well as for the bilayer heterostructure. Our analysis reveals significant changes in the refractive index and absorption for these systems. Notably, we observe negligible photocarrier transfer between the gold and perovskite layers based on transient spectroscopic analysis. These findings have implications for the design and optimization of bilayer heterostructures in optoelectronic applications. This work highlights the importance of spectroscopic techniques in studying the photophysical properties of heterostructure films.

Topologically influenced terahertz emission in Co2MnGa with a large anomalous Hall effect

The terahertz (THz) spectral zone is one of the most exciting but least explored domains of the electromagnetic spectrum. To extend the applicability of THz waves, the present objective is to develop an efficient, compact, durable, and low-cost THz emitter source. A spintronic THz emitter consisting of a ferromagnetic/nonmagnetic bilayer heterostructure is a promising innovation that can provide an alternative solution/replacement for conventional THz emitters. To further develop these spin-based THz emitters, we demonstrate an efficient and strong THz emission from a single layer of Co2MnGa with a large anomalous Hall effect (AHE) influenced by its Weyl semimetallic nature. Strong correlations among the THz emission, AHE, and chemical ordering of the full Heusler crystal structures for Co2MnGa are shown. Based on proper structural and chemical design, the topological nature of this material facilitates systematic optimization. Our initial findings provide a new design concept for the topological influences on spin-based THz emitters, and these emitters are expected to facilitate the further development of the intriguing Weyl physics.

A resilient type-III broken gap Ga2O3/SiC van der Waals heterogeneous bilayer with band-to-band tunneling effect and tunable electronic property

The potential of van der Waals (vdW) heterostructure to incorporate the outstanding features of stacked materials to meet a variety of application requirements has drawn considerable attention. Due to the unique quantum tunneling mechanisms, a type-III broken-gap obtained from vdW heterostructure is a promising design strategy for tunneling field-effect transistors. Herein, a unique Ga2O3/SiC vdW bilayer heterostructure with inherent type-III broken gap band alignment has been revealed through first-principles calculation. The underlying physical mechanism to form the broken gap band alignment is thoroughly studied. Due to the overlapping band structures, a tunneling window of 0.609 eV has been created, which enables the charges to tunnel from the VBM of the SiC layer to the CBM of the Ga2O3 layer and fulfills the required condition for band-to-band tunneling. External electric field and strain can be applied to tailor the electronic behavior of the bilayer heterostructure. Positive external electric field and compressive vertical strain enlarge the tunneling window and enhance the band-to-band tunneling (BTBT) scheme while negative electric field and tensile vertical strain shorten the BTBT window. Under external electric field as well as vertical and biaxial strain, the Ga2O3/SiC vdW hetero-bilayer maintains the type-III band alignment, revealing its capability to tolerate the external electric field and strain with resilience. All these results provide a compelling platform of the Ga2O3/SiC vdW bilayer to design high performance tunneling field effect transistor.

Universal Machine Learning Kohn–Sham Hamiltonian for Materials

Abstract While density functional theory (DFT) serves as a prevalent computational approach in electronic structure calculations, its computational demands and scalability limitations persist. Recently, leveraging neural networks to parameterize the Kohn–Sham DFT Hamiltonian has emerged as a promising avenue for accelerating electronic structure computations. Despite advancements, challenges such as the necessity for computing extensive DFT training data to explore each new system and the complexity of establishing accurate machine learning models for multi-elemental materials still exist. Addressing these hurdles, this study introduces a universal electronic Hamiltonian model trained on Hamiltonian matrices obtained from first-principles DFT calculations of nearly all crystal structures on the Materials Project. We demonstrate its generality in predicting electronic structures across the whole periodic table, including complex multi-elemental systems, solid-state electrolytes, Moiré twisted bilayer heterostructure, and metal-organic frameworks. Moreover, we utilize the universal model to conduct high-throughput calculations of electronic structures for crystals in GNoME datasets, identifying 3940 crystals with direct band gaps and 5109 crystals with flat bands. By offering a reliable efficient framework for computing electronic properties, this universal Hamiltonian model lays the groundwork for advancements in diverse fields, such as easily providing a huge data set of electronic structures and also making the materials design across the whole periodic table possible.

A broad-band investigation of ferromagnetic resonance in exchange coupled permalloy/phthalocyanine bilayer heterostructures at room temperature

The phenomenon of ferromagnet/molecule interfacial electronic hybridization enables the possibility of modifying magnetic characteristics of downscaled systems for spintronic applications. Ferromagnetic resonance (FMR) at the nanoscale level through a coplanar waveguide (CPW) over a wide frequency range is a remarkably reliable technique for investigation of the magnetic heterostructure’s interfacial properties and their feasibility in applications as spintronic devices. Here, magnetron sputtering and organic molecular beam epitaxy methods are used to fabricate the following nanoscale heterostructures: permalloy (Py)/Fe phthalocyanine (Pc), Py/ZnPc, and Py/Cr. Magnetization dynamics of the bilayer heterostructures is investigated in a CPW by studying frequency dependence (5–50 GHz) of the resonance field and linewidth with parallel and perpendicular geometries of the static applied magnetic field, thereby providing insights into possible intrinsic and extrinsic sources of FMR damping. Planar Pc molecules at the Py layer interface are found to have the potential to contribute to magnetic stabilization based on calculated magnetic parameters. This is evidenced by the larger results of surface anisotropy of the Py/MPc bilayer heterostructures induced by exchange coupling interaction at room temperature as well as the lower values of damping constant.