Bilayer 2D Research Articles

Photoredox phase engineering of transition metal dichalcogenides

Crystallographic phase engineering plays an important part in the precise control of the physical and electronic properties of materials. In two-dimensional transition metal dichalcogenides (2D TMDs), phase engineering using chemical lithiation with the organometallization agent n-butyllithium (n-BuLi), to convert the semiconducting 2H (trigonal) to the metallic 1T (octahedral) phase, has been widely explored for applications in areas such as transistors, catalysis and batteries1–15. Although this chemical phase engineering can be performed at ambient temperatures and pressures, the underlying mechanisms are poorly understood, and the use of n-BuLi raises notable safety concerns. Here we optically visualize the archetypical phase transition from the 2H to the 1T phase in mono- and bilayer 2D TMDs and discover that this reaction can be accelerated by up to six orders of magnitude using low-power illumination at 455 nm. We identify that the above-gap illumination improves the rate-limiting charge-transfer kinetics through a photoredox process. We use this method to achieve rapid and high-quality phase engineering of TMDs and demonstrate that this methodology can be harnessed to inscribe arbitrary phase patterns with diffraction-limited edge resolution into few-layer TMDs. Finally, we replace pyrophoric n-BuLi with safer polycyclic aromatic organolithiation agents and show that their performance exceeds that of n-BuLi as a phase transition agent. Our work opens opportunities for exploring the in situ characterization of electrochemical processes and paves the way for sustainably scaling up materials and devices by photoredox phase engineering.

Direct synthesis of controllable ultrathin heteroatoms-intercalated 2D layered materials

Two-dimensional (2D) layered materials have been studied in depth during the past two decades due to their unique structure and properties. Transition metal (TM) intercalation of layered materials have been proven as an effective way to introduce new physical properties, such as tunable 2D magnetism, but the direct growth of atomically thin heteroatoms-intercalated layered materials remains untapped. Herein, we directly synthesize various ultrathin heteroatoms-intercalated 2D layered materials (UHI-2DMs) through flux-assisted growth (FAG) approach. Eight UHI-2DMs (V1/3NbS2, Cr1/3NbS2, Mn1/3NbS2, Fe1/3NbS2, Co1/3NbS2, Co1/3NbSe2, Fe1/3TaS2, Fe1/4TaS2) were successfully synthesized. Their thickness can be reduced to the thinnest limit (bilayer 2D material with monolayer intercalated TM), and magnetic ordering can be induced in the synthesized structures. Interestingly, due to the possible anisotropy-stabilized long-range ferromagnetism in Fe1/3TaS2 with weak interlayer coupling, the layer-independent magnetic ordering temperature of Fe1/3TaS2 was revealed by magneto-transport properties. This work establishes a general method for direct synthesis of heteroatom-intercalated ultrathin 2D materials with tunable chemical and physical properties.

Quantum simulations with bilayer 2D Bose gases in multiple-RF-dressed potentials

Quantum simulations with bilayer 2D Bose gases in multiple-RF-dressed potentials

Novel bilayer 2D V2O5 as a potential catalyst for fast photodegradation of organic dyes

Two-dimensional (2D) materials have recently drawn interest in various applications due to their superior electronic properties, high specific surface area, and surface activity. However, studies on the catalytic properties of the 2D counterpart of V2O5 are scarce. In the present study, the catalytic properties of 2D V2O5 vis-à-vis bulk V2O5 for the degradation of methylene blue dye are discussed for the first time. The 2D V2O5 catalyst was synthesized using a modified chemical exfoliation technique. A massive increase in the electrochemically active surface area of 2D V2O5 by one order of magnitude greater than that of bulk V2O5 was observed in this study. Simultaneously, ~ 7 times increase in the optical absorption coefficient of 2D V2O5 significantly increases the number of photogenerated electrons involved in the catalytic performance. In addition, the surface activity of the 2D V2O5 catalyst is enhanced by generating surface oxygen vacancy defects. In the current study, we have achieved ~ 99% degradation of 16 ppm dye using the 2D V2O5 nanosheet catalysts under UV light exposure with a remarkable degradation rate constant of 2.31 min−1, which is an increase of the order of 102 from previous studies using V2O5 nanostructures and nanocomposites as catalysts. Since the enhanced photocatalytic activity emerged from the surface and optical properties of the catalyst, the current study shows great promise for the future application of 2D V2O5 in photo- and electrocatalysis.

Two-dimensional crystallization of precise side-chain giant molecules with constant building blocks ratio

This paper describes the two-dimensional (2D) crystallization of side-chain giant molecules (SCGMs) having the fixed number ratio of two types of functionalized building blocks with 1:1. SCGMs are regarded to a certain degree as size-amplified versions of universal synthetic polymers that are prepared by precisely connecting molecular nanoparticles (MNPs) to polymer chains. Our previous experimental results have shown that individually changing the number ratio of one of the building blocks would significantly affect the structural parameters of molecular crystals, mainly the longitudinal thickness. Herein, we systematically discussed the 2D crystallization of two categories of POSS-based SCGMs with constant building blocks ratio. Solution self-assembly of such SCGMs resulted in well-defined 2D nanosheets with similar structural configurations, showing great tolerance in the number of building blocks. Subsequently, we proposed a “sandwiched-type” mode to illustrate the bilayer 2D supramolecular framework, in which crystalline blocks pack in head-to-head manner. In addition, sequence isomers provide an ideal platform to further verify that the thickness of 2D nanocrystal is linked with the tethering density determined by the number ratio of building blocks. We believe that this study could be a better complement to our previous work and then make us to be more in-depth understand the crystallization mechanism of SCGMs.

Modulating electronic structure by interlayer spacing and twist on bilayer bismuthene.

Modulation of the electronic structure has played a crucial role in advancing the field of two-dimensional materials, but there are still many unexplored directions, such as the twist angle for a novel degree of freedom, for modulating the properties of heterostructures. We observed a distinct pattern in the energy bands of bilayer bismuthene, demonstrating that modulating the twist angle and interlayer spacing significantly influences interlayer interactions. Our study of various interlayer spacings and twist angles revealed a close relationship between bandgap size and interlayer spacing, while the twist angle notably affects the shape of the energy bands. Furthermore, we observed a synergistic effect between these two factors. As the twist angle decreases, the energy bands become flat, and flat bands can be generated without requiring a specific angle on bilayer bismuthene. Our results suggest a promising way to tailor the energy band structure of bilayer 2D materials by varying the interlayer spacing and twist angle.

High-efficiency fabrication of uniform photonic crystals by a vertical deposition method from multiple glass slides of simple moulds and their application in isomer recognitions

Vertical deposition method is a simple, cheap and common method for fabricating photonic crystals (PCs). However, a majority of PCs are obtained based on the traditional vertical deposition method from one glass slide, the method is low efficiency. Herein, it is proposed to efficiently fabricate uniform PCs by an optimized vertical deposition method from double or multiple glass slides of simple moulds. Amounts of SiO2 spheres, distances of two glass slides of moulds, preparation temperatures and different diameters of SiO2 spheres are investigated for obtaining uniform PCs. And different types of PCs (the bottom-middle-top 3D PCs, bilayer 2D PCs and amorphous PCs) are also fabricated based on the optimized vertical deposition method. The above method is a simple, cheap and high-efficiency way for mass production of uniform PCs. Meanwhile, the chemicals with similar refractive indices (isomers and cis-trans isomers) can be easily distinguished under high humidity based on dynamic reflection spectra of the PCs. For example, the difference in the refractive indices of m-xylene and p-xylene is only 0.002, the difference of desorption balance time of PCs is 263 s. The PCs have potential applications in visual detections, isomer recognitions and monitoring of volatilization rates of trace liquids.

A Cu3BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling for Highly Efficient Photoinduced Charge Separation.

Two-dimensional van der Waals heterostructures (2D vdWhs) are of significant interest due to their intriguing physical properties critically defined by the constituent monolayers and their interlayer coupling. Synthetic access to 2D vdWhs based on chemically tunable monolayer organic 2D materials remains challenging. Herein, the fabrication of a novel organic-inorganic bilayer vdWh by combining π-conjugated 2D coordination polymer (2DCP, i.e., Cu3BHT, BHT = benzenehexathiol) with graphene is reported. Monolayer Cu3BHT with detectable µm2-scale uniformity and atomic flatness is synthesized using on-water surface chemistry. A combination of diffraction and imaging techniques enables the determination of the crystal structure of monolayer Cu3BHT with atomic precision. Leveraging the strong interlayer coupling, Cu3BHT-graphene vdWh exhibits highly efficient photoinduced interlayer charge separation with a net electron transfer efficiency of up to 34% from Cu3BHT to graphene, superior to those of reported bilayer 2D vdWhs and molecular-graphene vdWhs. This study unveils the potential for developing novel 2DCP-based vdWhs with intriguing physical properties.

Accurate construction of monolayer, bilayer, sandwich bilayer, four-layer, multi-layer and chiral bilayer 2D pillararene-type supramolecular networks.

The accurate construction of mono-, bi- and multi-layer networks has been an important challenge, especially for bi- and multi-layer networks. Monolayer, bilayer, sandwich bilayer, four-layer, and multi-layer two-dimensional pillararene-type metal-organic coordination networks have been constructed from functionalized pillar[5]arene and pillar[6]arene by utilizing the coordination interaction of cobalt and copper ions and combining with temperature control and guest induction. These two-dimensional coordination networks exhibit the excellent plasticity of pillararenes and structural variety, which are characterized by X-ray single crystal diffraction and PXRD, confirming that pillararenes units can function as excellent tunable scaffolds for structural regulation. Two-dimensional chiral double-layer structure products are also constructed from R- and S-pillar[6]arene, which are obtained by high-performance liquid chromatography. Atomic force microscopic imaging confirms the thicknesses of these networks. Moreover, these networks also exhibit high iodine adsorption capacity in aqueous environments at ambient temperature. The monolayer, bilayer, sandwich bilayer, four-layer and multi-layer structures of the pillararene-type networks represent a new facile supramolecular self-assembly strategy and platform for designing more mono-, bi- and multi-layer two-dimensional nanomaterials and chiral two-dimensional double-layer structures provide a new method for the construction of more two-dimensional chiral polymers.

Resistive Switching in Bigraphene/Diamane Nanostructures Formed on a La3Ga5SiO14 Substrate Using Electron Beam Irradiation.

Memristors, resistive switching memory devices, play a crucial role in the energy-efficient implementation of artificial intelligence. This study investigates resistive switching behavior in a lateral 2D composite structure composed of bilayer graphene and 2D diamond (diamane) nanostructures formed using electron beam irradiation. The resulting bigraphene/diamane structure exhibits nonlinear charge carrier transport behavior and a significant increase in resistance. It is shown that the resistive switching of the nanostructure is well controlled using bias voltage. The impact of an electrical field on the bonding of diamane-stabilizing functional groups is investigated. By subjecting the lateral bigraphene/diamane/bigraphene nanostructure to a sufficiently strong electric field, the migration of hydrogen ions and/or oxygen-related groups located on one or both sides of the nanostructure can occur. This process leads to the disruption of sp3 carbon bonds, restoring the high conductivity of bigraphene.

Comprehensive excited state carrier dynamics of 2D selenium: One-photon and multi-photon absorption regimes

Semiconductors play a critical role in optoelectronic applications, and recent research has identified group-VI 2D semiconductors as promising materials for this purpose. Here, we report the comprehensive excited state carrier dynamics of bilayer, two-dimensional (2D) selenium (Se) in one-photon and multi-photon absorption regimes using transient reflection (TR) spectroscopy. Carrier lifetime obtained from TR measurement is used to theoretically predict the photo-responsivity for 2D Se photo-detectors operating in the one-photon-absorption regime. We also calculate a giant two-photon absorption cross section of 2.9×105 GM at 750 nm hinting possible application of 2D Se as a sub-bandgap photo-detector. The carrier recombination process is dominated by surface and sub-surface defect states in one- and multi-photon absorption regimes, respectively, resulting nearly one order increased carrier lifetime in a three-photon-absorption regime (1700 ps) compared to a one-photon-absorption regime (103 ps). Femtosecond Z-scan measurement shows saturation behavior for above bandgap excitation, further indicating the possibility of 2D Se as a saturable absorber material for passive Q-switching. Our study provides comprehensive insight into the excited state carrier dynamics of bilayer 2D Se and highlights its potential as a versatile material for various linear and non-linear optoelectronic applications.

High-Contrast Bidirectional Optoelectronic Synapses based on 2D Molecular Crystal Heterojunctions for Motion Detection.

Light-stimulated optoelectronic synaptic devices are fundamental compositions of the neuromorphic vision system. However, there are still huge challenges to achieving both bidirectional synaptic behaviors under light stimuli and high performance. Herein, a bilayer 2D molecular crystal (2DMC) p-n heterojunction is developed to achieve high-performance bidirectional synaptic behaviors. The 2DMC heterojunction-based field effect transistor (FET) devices exhibit typical ambipolar properties and remarkable responsivity (R) of 3.58×104 A W-1 under weak light as low as 0.008mW cm-2 . Excitatory and inhibitory synaptic behaviors are successfully realized by the same light stimuli under different gate voltages. Moreover, a superior contrast ratio (CR) of 1.53×103 is demonstrated by the ultrathin and high-quality 2DMC heterojunction, which transcends previous optoelectronic synapses and enables application for the motion detection of the pendulum. Furthermore, a motion detection network based on the device is developed to detect and recognize classic motion vehicles in road traffic with an accuracy exceeding 90%. This work provides an effective strategy for developing high-contrast bidirectional optoelectronic synapses and shows great potential in the intelligent bionic device and future artificial vision.

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.

Strong Interlayer Coupling in Twisted Transition Metal Dichalcogenide Moiré Superlattices.

Moiré superlattices in twisted van der Waals materials offer a powerful platform for exploring light-matter interactions. The periodic moiré potentials in moiré superlattices can induce strongly correlated quantum phenomena that depend on the moiré potential associated with interlayer coupling at the interface. However, moiré superlattices are primarily prepared by mechanical exfoliation and manual stacking, where the transfer methods easily cause interfacial contamination, and the preparation of high-quality bilayer 2D materials with small twist angles by growth methods remains a significant challenge. In this work, WSe2 /WSe2 homobilayers with different twist angles by chemical vapor deposition (CVD),using a heteroatom-assisted growth technique, are synthesized. Using low-frequency Raman scattering, the uniformity of the moiré superlattices is mapped to demonstrate the strong interfacial coupling of the CVD-fabricated twist-angle homobilayers. The moiré potential depths of the CVD-grown and artificially stacked homostructures with twist angles of 1.5° are 115 and 45 meV (an increase of 155%), indicating that the depth of moiré potential can be modulated by the interfacial coupling. These results open a new avenue to study the modulation of moiré potential by strong interlayer coupling and provide a foundation for the development of twistronics.

Unveiling the moiré pattern evolution and superlubricity in twisted bilayer 2D phosphorene at atomistic scale

Moiré patterns are commonly evidenced on two-dimensional (2D) van der Waals (vdW) bilayers stacked with rotational misalignment or slight lattice mismatch, which can dramatically influence the interface mechanical properties. Understanding the basic characteristics of moiré patterns are essential to tune the interfacial mechanical properties. Herein, molecular dynamics (MD) simulations are performed to capture the atomic evolution in the interface of the bilayer phosphorene during rotation and sliding. The hexagonal moiré patterns are observed in bilayer blue phosphorene, while the transition from quadrate to one dimensional stripe-like moiré pattern is evidenced in bilayer black phosphorene as the twist angle increases. However, the unique one-dimensional stripe-like moiré pattern always exists in black phosphorene/blue phosphorene heterogeneous interface during rotation. The reconstruction and deformation of Moiré pattern are determined by the competition between intralayer strain and interlayer coupling. The energy barrier for the rotation of heterogeneous bilayers is almost an order of magnitude smaller than that in homogeneous bilayers. As a result, the friction forces of heterogeneous bilayer during sliding can fall in the scope of superlubricity more possibly. However, the surface roughness, contact size and twist angle also alter the friction force. The results provide a comprehensive understanding on the Moiré superlattices.

Band Flattening and Landau Level Merging in Strongly-Correlated Two-Dimensional Electron Systems

We review recent experimental results indicating the band flattening and Landau level merging at the chemical potential in strongly-correlated two-dimensional (2D) electron systems. In ultra-clean, strongly interacting 2D electron system in SiGe/Si/SiGe quantum wells, the effective electron mass at the Fermi level increases monotonically in the entire range of electron densities, while the energy-averaged mass saturates at low densities. The qualitatively different behavior of the two masses reveals a precursor to the interaction-induced single-particle spectrum flattening at the chemical potential in this electron system, in which case the fermion “condensation” at the Fermi level occurs in a range of momenta, unlike the condensation of bosons. In strong magnetic fields, perpendicular to the 2D electron layer, a similar effect of different fillings of quantum levels at the chemical potential—the merging of the spin- and valley-split Landau levels at the chemical potential—is observed in Si inversion layers and bilayer 2D electron system in GaAs. Indication of merging of the quantum levels of composite fermions with different valley indices is also reported in ultra-clean SiGe/Si/SiGe quantum wells.

Theoretical study on CO2 reduction to methanol catalyzed by highly stable single-atom transition metal riveted bilayer 2D material

The main factor limiting the high activity of single-atom catalysts (SACs) is the structural instability, which is caused by atomic aggregation or attack of reaction intermediates. In this paper, the single-atom transition metal copper and palladium was riveted between two layers of the monolayer materials (carbon nitride and graphene) respectively to enhance the structural stability, and used C3N-TM-C3N and GR-TM-C3N (TM= Cu, Pd) in the theoretical study of CO2 reduction to methanol (CO2RR). It is found that a single TM atom can be firmly anchored between two C3N or GR-C3N layers, which can prevent the aggregation of TM atoms (TM= Cu, Pd) in SACs and inhibit the attack of reaction intermediates. The TM atom enhances the catalytic activity of the material system by providing electrons to the protective layer GR/C3N. And C3N-Cu-C3N material has the best performance and the lowest reaction energy barrier in CO2RR, which is only 2.094 eV. It provides a feasible method to improve the catalytic activity and maintain the stability for the practical application of SACs systems.

Utilizing trapped charge at bilayer 2D MoS2/SiO2 interface for memory applications

In this work we use conductive atomic force microscopy (cAFM) to study the charge injection process from a nanoscale tip to a single isolated bilayer 2D MoS2 flake. The MoS2 is exfoliated and bonded to ultra-thin SiO2/Si substrate. Local current–voltage (IV) measurements conducted by cAFM provides insight in charge trapping/de-trapping mechanisms at the MoS2/SiO2 interface. The MoS2 nano-flake provides an adjustable potential barrier for embedded trap sites where the charge is injected from AFM tip is confined at the interface. A window of (ΔV ∼ 1.8 V) is obtain at a reading current of 2 nA between two consecutive IV sweeps. This is a sufficient window to differentiate between the two states indicating memory behavior. Furthermore, the physics behind the charge entrapment and its contribution to the tunneling mechanisms is discussed.

Tailoring Layer Number of 2D Porphyrin-Based MOFs Towards Photocoupled Electroreduction of CO2.

Inspired by the success of graphene, a series of single- or few-layer 2D materials have been developed and applied in the past decade. Here, the successful preparation of monolayer and bilayer 2D porphyrin-based metal-organic frameworks (MOFs) by a facile solvothermal method is reported. The structure transition from monolayer to bilayer drives distinct electronic properties and restructuring behaviors, which finally results in distinct catalytic pathways towards CO2 electrocatalysis. The monolayer favors CO2 -to-C2 pathway due to the restructuring of CuO4 sites, while CO and HCOO- are the major products over the bilayer. In photocoupled electrocatalysis, the Faradaic efficiency (FE) of the C2 compounds shows a nearly fourfold increase on the monolayer than that under dark conditions (FEC2 increases from 11.9% to 41.1% at -1.4V). For comparison, the light field plays a negligible effect on the bilayer. The light-induced selectivity optimization is investigated by experimental characterization and density functional theory (DFT) calculations. This work opens up a novel possibility to tune the selectivity of carbon products just by tailoring the layer number of the 2D material.

Recent advances in the controlled chemical vapor deposition growth of bilayer 2D single crystals

Recent advances in the controllable growth of bilayer 2D single crystals via the chemical vapor deposition method are comprehensively presented.