Bilayer Material Research Articles

Influence of Atomic Relaxations on the Moiré Flat Band Wave Functions in Antiparallel Twisted Bilayer WS2.

Twisting bilayers of transition metal dichalcogenides gives rise to a moiré potential resulting in flat bands with localized wave functions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS2 bilayer twisted approximately 3° off the antiparallel alignment. Scanning tunneling spectroscopy reveals localized states in the vicinity of the valence band onset, which is observed to occur first in regions with S-on-S Bernal stacking. In contrast, density functional theory calculations on twisted bilayers that have been relaxed in vacuum predict the highest-lying flat valence band to be localized in regions of AA' stacking. However, agreement with experiment is recovered when the calculations are performed on bilayers in which the atomic displacements from the unrelaxed positions have been reduced, reflecting the influence of the substrate and finite temperature. This demonstrates the delicate interplay of atomic relaxations and the electronic structure of twisted bilayer materials.

Tunable properties of two-dimensional bilayer C3N as anode material: Bandgap, binding energy, and diffusion barrier

Numerous studies have demonstrated that adjusting the stacking pattern of bilayer materials can modulate their electrical properties. However, whether such adjustments can also influence other properties of the material still requires further examination. In this work, the potential of bilayer C3N with various stacking structures as anode materials for lithium-ion batteries was systematically investigated through first-principles calculations. The calculated results show that the bilayer C3N possesses good regulable electronic properties (band gap range from 0.35 to 0.90 eV), Li binding strength (the adsorption energy of Li range from −1.60 to −0.73 eV) and Li migration capability (the interlayer migration barrier of Li range from 0.082 to 0.580 eV). Notably, the AA′ stacked structure exhibits an ultrafast interlayer migration channel for Li (the lowest interlayer migration barrier of 0.082 eV) and lowest bandgap of 0.35 eV. Further research revealed that the formation of ultrafast Li migration channels in the AA′ stacking structure results from the combined effect of charge transfer and layer distance. These results provide a new strategy for modulating the electronic properties and the diffusion properties of ion in 2D van der Waals materials.

New Approach for Manufacturing Ti–6Al–4V+40%TiC Metal-Matrix Composites by 3D Printing Using Conic Electron Beam and Cored Wire. Pt. 2: Layered MMC/Alloy Materials, Their Main Characteristics, and Possible Application as Ballistic Resistant Materials

Bilayer samples comprised of hard metal-matrix composite top layer and ductile 10 mm Ti–6Al–4V plate are produced with 3D printing by conical electron-beam method using specially prepared core (powder) wire that allows forming hard top layer of metal-matrix (Ti–6Al–4V) composite (MMC) reinforced by means of fine TiC particles with thickness up to 4 mm. Ballistic tests performed with 7.62×51 AP ammunition show a good ballistic resistance of this protective structure, i.e., it is not perforated. Only minor penetration and partial fracture are occurred exclusively in the surface MMC layer. Either no traces of plastic deformation are found at the boundary with the base layer or inside it that indicates that the MMC layer absorbs the entire impact energy of the projectile. Based on studies of the fine structure and texture of the interface between the layers, a reasonable assumption is made that wavy geometry of MMC layer provides additional deflection and scattering of stress waves generated during impact. Comparing the results of ballistic tests of various metallic materials, it is concluded that the 3D-printed bilayer material consisting of the upper Ti–6Al–4V + 40% TiC layer and the base Ti–6Al–4V layer has an undeniable advantage in ballistic performance when it is tested with cartridges of this type.

Li-decorated bilayer borophene as a potential hydrogen storage material: A DFT study

Two-dimensional (2D) materials have recently attracted much attention in the field of hydrogen storage, owing to their beneficial properties such as high surface-to-volume ratio and light weight. Motivated by recent experimental developments of bilayer borophene (BL) sheets, we investigate the suitability of hydrogen storage on Li-decorated BL via density functional theory (DFT) calculations. The H2 molecules are adsorbed on BL sheet via weak physisorption. By decorating BL with Li atoms, its hydrogen storage performance has been improved. We find that Li atoms can firmly adsorb on the substrate without forming clusters. Meanwhile, the Li-decorated structure exhibits good stability, verified by molecular dynamics calculations. Remarkably, in the fully loaded case, the compound obtains a 6.08 wt% H2 content and 0.13 eV/H2 average adsorption energy, which is promising for onboard hydrogen storage. Furthermore, electronic structure analysis reveals the electrostatic nature of interactions between the H2 molecules and the protruding Li atoms on BL sheets, which originates from polarization and orbital hybridization. Overall, our results suggest that Li-decorated BL is a promising candidate for potential hydrogen storage applications, and contribute to a better understanding of bilayer materials for hydrogen storage.

Beeswax: A review on the recent progress in the development of superhydrophobic films/coatings and their applications in fruits preservation.

Recently, the design and fabrication of bio-inspired superhydrophobic materials using natural lipid additives such as beeswax (BW) have aroused great attention in food packaging as they can minimize the transfer rate of water molecules and have effective moisture barriers. This review discusses the recent progress in the design and fabrication of BW-containing edible films/coatings (e.g., emulsion and blend films, bilayer materials, bionanocomposites, and antimicrobial materials) and their potential applications on the postharvest life and quality attributes of various fruits. Incorporation of BW into polysaccharides- and proteins-based emulsion films effectively improved their hydrophobicity, water vapor, and UV/visible light barrier properties, as well as the film tensile properties. The addition of nanoparticles to BW-based polymeric matrices often results in improved physico-mechanical properties. BW coatings have been also applied to prolong the shelf-life of various climacteric fruits, however, optimization of the wax concentration can be further investigated to develop targeted food storage systems.

Modulating intrinsic anomalous Hall effect in Fe3GeTe2 monolayer via strain engineering

The Fe3GeTe2 monolayer, a ferromagnetic topological candidate with a high Curie temperature of 130 K, has recently garnered considerable interest. We investigated the impact of strain on the electronic, magnetic, and topological properties of the Fe3GeTe2 monolayer using density functional theory calculations. Our results showed that the Fe3GeTe2 monolayer was an itinerant ferromagnet with a high spin polarization of 69.93% and out-of-plane easy magnetization. Tensile strain had no profound impact on the anomalous Hall conductivity (AHC). However, when applying a compressive biaxial strain of −3%, the AHC at the Fermi level was dramatically enhanced to 1.62 e2/h, which is much larger than that of the bulk (0.7 e2/h) and bilayer (1.5 e2/h) material. The large AHC nearly coincided with the singularity of the energy bands near the M point. Our results highlight the potential of using strain engineering to control and optimize the properties of two-dimensional topological materials.

Characterizing high-frequency wave interaction in rigid porous bilayer materials: Insights from acoustic parameter sensitivity analysis

This study aims to investigate the sensitivity of high-frequency acoustic parameters in rigid air-saturated porous bilayer materials using the equivalent fluid theory. The interaction between the fluid and solid phases of the porous medium incorporates visco-inertial and thermal exchange, characterized by two functions: the dynamic tortuosity α(ω) proposed by Johnson etal. and the dynamic compressibility β(ω) proposed by Allard, refined by Sadouki for the low-frequency domain of ultrasound. Various parameters, including porosity, tortuosity, viscous characteristic length, thermal characteristic length, as well as viscous and thermal shape factors, are examined in this study. A 10% variation in these parameters is considered to assess their impact on the amplitudes of transmitted waves. The findings of this study contribute to a deeper understanding of high-frequency wave behavior in porous bilayer materials. The investigation sheds light on the sensitivity of the parameters and provides valuable insights for the design and optimization of such materials in engineering, technology, and applied sciences.

Resolidified Chalcogen-Assisted Growth of Bilayer Semiconductors with Controlled Stacking Orders.

Bilayer semiconductors have attracted much attention due to their stacking-order-dependent properties. However, as both 3R- and 2H-stacking are energetically stable at high temperatures, most of the high-temperature grown bilayer materials have random 3R- or 2H-stacking orders, leading to non-uniformity in optical and electrical properties. Here, a chemical vapor deposition method is developed to grow bilayer semiconductors with controlled stacking order by modulating the resolidified chalcogen precursors supply kinetics. Taking tungsten disulfide (WS2 ) as an example, pure 3R-stacking (100%) and 2H-stacking dominated (87.6%) bilayer WS2 are grown by using this method and both show high structural and optical quality and good uniformity. Importantly, the bilayer 3R-stacking WS2 shows higher field effect mobility than 2H-stacking samples, due to the difference in stacking order-dependent surface potentials. This method is universal for growing other bilayer semiconductors with controlled stacking orders including molybdenum disulfide and tungsten diselenide, paving the way to exploit stacking-order-dependent properties of these family of emerging bilayer materials.

(Invited) wearable Elastography Via Surface Mechano-Acoustic Sensing for Continuous Monitoring of Tissue Stiffness

Recent progress in wearable devices provides new opportunities for precise, non-invasive, long-term recording of body mechanics. The soft device incorporating a single MEMS accelerometer captures subtle vibration of the skin with a resolution of 1×10-4 g/√Hz in the frequency range from 0 to 800 Hz. In this study, we introduce an on-body mechano-acoustic sensing technology based on a skin-mounted accelerometer array to assess the mechanical profiles of subdermal tissues in-vivo, similar to seismology. A system-level wearable device construction, optimized for a comfortable skin interface and high precision, incorporates a broadband dual-accelerometer sensor, an audio actuator, and a Bluetooth System-on-Chip and enables a wireless, automated operation. An automated algorithm, leveraging the spectral analysis of surface waves (SASW) methods, computes the depth-sensitive, elastic modulus information of the propagation media from the mechanical dispersion relationship. Comprehensive theoretical and experimental investigation on bi-layer phantom materials and biological tissues, with a storage moduli range of 19–1439 kPa, demonstrates the capability of rapid and robust evaluation of modulus in a depth range of 2–46 mm. The device identifies the softening of porcine tissues with increasing injected water content and the changes of modulus of muscle under different levels of tension. The results are in agreement with the in-parallel Ultrasound Elastography measurements. A quantitative assessment of the stiffness profile of the bicep brachii and rectus femoris muscle during gym exercises demonstrate the device operation in an ambulant environment for a non-invasive and continuous assessment of deep tissue stiffness with a temporal resolution of 0.5 s.

Revealing the Hidden Spin-Polarized Bands in a Superconducting Tl Bilayer Crystal.

The interplay of spin-orbit coupling and crystal symmetry can generate spin-polarized bands in materials only a few atomic layers thick, potentially leading to unprecedented physical properties. In the case of bilayer materials with global inversion symmetry, locally broken inversion symmetry can generate degenerate spin-polarized bands, in which the spins in each layer are oppositely polarized. Here, we demonstrate that the hidden spins in a Tl bilayer crystal are revealed by growing it on Ag(111) of sizable lattice mismatch, together with the appearance of a remarkable phenomenon unique to centrosymmetric hidden-spin bilayer crystals: a novel band splitting in both spin and space. The key to success in observing this novel splitting is that the interaction at the interface has just the right strength: it does not destroy the original wave functions of the Tl bilayer but is strong enough to induce an energy separation.

A novel perovskite solar cell comprising CsPbI2Br/Cs2SnI6 (CsGeI3) cascade absorbing bilayer and Cr2O3 electron transport material: A theoretical comparison study

In order to prevent the limitation on absorbing energies by a single-layer perovskite solar cells (PSCs), a cell comprising bilayer heterostructure in the form of FTO/Cr2O3/CsPbI2Br +Cs2SnI6 (CsGeI3)/spiro-OMeTAD/Au is simulated and its photovoltaic performance including incident photon-to-current conversion efficiency, fill factor, open circuit voltage and short circuit current is evaluated using SCAPS 1D software package. It is shown by introducing these structures, not only a wider range of the sunlight spectrum is exploited, but also by achieving a favorable band-edge bending as graded band gap, more charge carriers can be collected. Moreover, further improvement in the performance of devices is obtained by optimizing the thickness of absorbing layers and work function of the back electrode. The thermal stability of PSCs is investigated by varying the temperature in the range of 300–480 K and the effect of various electron transport materials (ETMs) on the performance of the cell is evaluated. Under optimized condition, it is found that the efficiencies of the cells with CsPbI2Br+Cs2SnI6 and CsPbI2Br+CsGeI3 bilayer heterostructures increase from 13.38% for non-heterojunction structure to 15.14% and 16.34%, respectively. Furthermore, the simulation results reveal that the photovoltaic cell made of CsPbI2Br+CsGeI3 absorbing layer having higher efficiency and greater thermal stability (compared to PSC made of CsPbI2Br+Cs2SnI6 absorbing layer) is an appropriate and promising candidate as efficient perovskite solar cell.

First-principles study on the electronic structure of siligraphene on a ZnO monolayer

Density functional theory was employed to investigate the electronic structures of atomic bilayer materials that form between graphene (g-C) or graphitic silicon carbide (also known as siligraphene: g-SiC and g-SiC2) and graphitic zinc oxide (g-ZnO). The results indicate that g-C/g-ZnO bilayers have semimetallic properties with an energy band gap of zero like in graphene. For a g-SiC/g-ZnO bilayer, an ensemble of three sp 2-hybridized carbon atoms periodically separated by three silicon atoms on g-ZnO has indirect and direct band gaps of 3.32 and 3.78 eV, respectively, which is suitable for use in light-emitting diode applications. For a g-SiC2/g-ZnO bilayer, an ensemble of four sp 2-hybridized carbon atoms periodically separated by two silicon atoms on g-ZnO has a direct band gap of 1.15 eV, which approaches the optimal value of the band gap (E opt ≃ 1.3 eV) for solar cell applications. The results show that increasing Si content in siligraphene can help to open the band gap of graphene and enhance the band gap of graphitic silicon carbide. The band gaps of siligraphene/g-ZnO bilayers depend on a smaller band gap from the monolayer component. Therefore, adjusting the Si content in siligraphene permits tuning of the band gap, and constructing a bilayer in the presence of a g-ZnO monolayer can slightly decrease the band gap. These results could lead to a new design of heterostructures with tunable band gaps for various applications.

Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy.

Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

An extended plane wave framework for the electronic structure calculations of twisted bilayer material systems

In this study, we propose an extended plane wave framework to ensure that the electronic structure calculations of twisted bilayer 2D material systems are practically feasible. Based on the foundations presented in Zhou et al. (2019), we extend the PW framework in the following aspects: (1) a tensor-product basis set, which adopts plane waves (PWs) in the incommensurate dimensions and a localized basis in the interlayer dimension; (2) a practical application of a novel cutoff technique that we recently developed; and (3) a quasi-band structure picture under small twist angles and weak interlayer coupling limits. Based on (1) and (2), the dimensions of the Hamiltonian matrix are reduced by approximately two orders of magnitude compared with the original framework. Moreover, (3) enables us to better organize the calculations and understand the results. As a numerical example, we study the electronic structure of a linear bilayer graphene lattice system with a magic twist angle (∼1.05°). The well-known flat bands are reproduced ensuring that their features quantitatively agreed with those obtained experimentally and via other theoretical calculations. Moreover, the extended framework has a much lower computational cost than the commensurate cell approximations and is more extendable than the traditional model Hamiltonians and tight binding models. Finally, this framework readily accommodates nonlinear models, thereby laying the foundation for more effective and accurate density functional theory (DFT) calculations.

High throughput calculations for a dataset of bilayer materials

Bilayer materials made of 2D monolayers are emerging as new systems creating diverse opportunities for basic research and applications in optoelectronics, thermoelectrics, and topological science among others. Herein, we present a computational bilayer materials dataset containing 760 structures with their structural, electronic, and transport properties. Different stacking patterns of each bilayer have been framed by analyzing their monolayer symmetries. Density functional theory calculations including van der Waals interactions are carried out for each stacking pattern to evaluate the corresponding ground states, which are correctly identified for experimentally synthesized transition metal dichalcogenides, graphene, boron nitride, and silicene. Binding energies and interlayer charge transfer are evaluated to analyze the interlayer coupling strength. Our dataset can be used for materials screening and data-assisted modeling for desired thermoelectric or optoelectronic applications.

Preparation and performance of lauric acid phase change material with the double-layer structure for solar energy storage

The use of phase change materials (PCM) to absorb and store solar energy is one of the ways to solve the problem of increasing energy consumption in buildings. However, most of the PCM has low thermal conductivity and poor photothermal conversion performance, which limits their application in the building. In this study, it was proposed that a lauric acid-based bilayer material with good thermal conductivity and photothermal conversion performance. Specifically, sodium dodecyl sulfate (SDS) was used to combine carbon black (CB) and lauric acid (LA) in the upper layer to enhance photothermal conversion, and expanded graphite (EG) was added in lauric acid in the lower layer to enhance heat transfer. The synthesis ratio, thermal conductivity, latent heat, and heating rate of the LA/CB-LA/EG were tested. This study concluded that the preferred ratio of the photothermal conversion material in the upper layer is 100:5:1 (LA: CB: SDS), and the preferred ratio of the lower layer material is 100:1.4 (LA: EG). In addition, the addition of carbon material has only a small effect on the latent heat properties of the whole material. To verify the physical properties of the bilayer material, we tested the surface heating rate of the material and the inner material during the photothermal conversion of the material. The experimental results show that the surface temperature rise rate of LA/CB-LA/EG is 217 % of that of LA and 109 % of that of LA/EG in the time range of 30 min. The internal temperature rise rate of LA/CB-LA/EG is 226 % of LA and 136 % of LA/EG, respectively. Finally, we build the model in the numerical study and test it after inputting the physical parameters. The results show that the heating hours can be reduced by more than 50 % in winter using the above scheme, so this work achieves the purpose of energy saving and emission reduction.

Covalent bonded bilayers from germanene and stanene with topological giant capacitance effects

The discovery of twisted bilayer graphene with tunable superconductivity has diverted great focus at the world of twisted van der Waals heterostructures. Here we propose a paradigm for bilayer materials, where covalent bonding replaces the van der Waals interaction between the layers. On the example of germanene-stanene bilayer, we show that such systems demonstrate fascinating topological properties and manifest giant capacitance effects of the order of C = 102μ F as well as dipole-like charge densities of q = 1 − 2 × 10−4μ C cm−2, showing promise for 2D ferroelectricity. The observed unique behaviour is closely linked to transverse strain-induced buckling deformations at the bilayer/substrate interface. In alternative GeSn bilayer structures with low twist angles the strain distortions trigger rich topological defect physics. We propose that the GeSn bilayer topology may be switched locally by a substrate-strain-induced electric fields. We demonstrate an approach to fabricate covalent bilayer materials, holding vast possibilities to transform applications technologies across solar, energy and optoelectronic sectors.

Pd/ZnS/ZnO sensitive selective detection photoelectrochemical sensor for the detection of Cd2+

A sensitive photoelectrochemical method for the detection of Cd2+ ions was realized using Pd/ZnS/ZnO nanoparticle modified electrodes. The ZnS/ZnO bilayer material was prepared by hydrothermal method using nickel foam as the substrate then the three-layer nanomaterial photoelectric sensor was prepared using pulse plating method. Then, a series of electrochemical and photoelectrochemical tests were conducted to measure the electrochemical and photoelectrochemical properties. The test results show that the composite electrode has good electronic conductivity, improved photosensitivity, and a detection response range of 0.1 μM–100 μM for Cd2+ with good sensitivity and selectivity and lower detection limit compared with the conventional Cd2+ sensor.

Enhancing the parameters of ZnO/CdZnS thin film photodetector by thermal annealing

This work aims to manufacture photodetectors from semiconducting bilayer materials of zinc oxide nanorods and coated with a thin layer of CdZnS by chemical bath deposition method. The effect of annealing time on the optical and electrical properties of the prepared devices and on the parameters of the devices was also studied. The prepared films were thermally treated by annealing at a temperature of 200 °C for several times for the purpose of comparison. The annealing times were 15, 30, 35, 60 min, respectively, by means of a vacuum furnace. The effect of the annealing time was noticeable and obvious on the optical and electrical properties of the manufactured photodetectors.

Controlling I-V Hysteresis in Al/Pt Bilayer Symmetric SQUIDs at Millikelvin Temperatures

We study operation of a superconducting quantum interference devices (SQUIDs) based on a new bilayer material. They can be used for the ultra-sensitive detection of magnetic momentum at temperatures down to milliKelvin range. Typically, thermal origin hysteresis of the symmetric SQUID current-voltage curves limits operating temperatures to T>0.6Tc. We used a new bilayer material for SQUID fabrication, namely proximity-coupled superconductor/normal-metal (S/N) bilayers (aluminum 25 nm/platinum 5 nm). Because of the 5 nm Pt-layer, Al/Pt devices show nonhysteretic behavior in a broad temperature range from 20 mK to 0.8 K. Furthermore, the Al/Pt bilayer devices demonstrate an order of magnitude lower critical current compared to the Al devices, which decreases the screening parameter (βL) and improves the modulation depth of the critical current by magnetic flux. Operation at lower temperatures reduces thermal noise and increases the SQUID magnetic field resolution. Moreover, we expect strong decrease of two-level fluctuators on the surface of aluminum due to Pt-layer oxidation protection and hence significant reduction of the 1/f noise. Optimized geometry of Al/Pt symmetric SQUIDs is promising for the detection of single-electron spin flip.