2D Materials Research Articles

Multifunctional UV photodetect-memristors based on area selective fabricated Ga2S3/graphene/GaN van der Waals heterojunctions.

Multifunctional devices based on van der Waals heterojunctions have drawn significant attention owing to their portable size, low power consumption and various application scenarios. However, high fabrication equipment requirements, complex device structures and limited operating conditions hinder their potential value. Herein, multifunctional UV photodetect-memristors based on Ga2S3/graphene/GaN van der Waals heterojunctions via area selective deposition have been proposed for the first time. The Ga2S3/graphene/GaN heterojunctions are firstly grown via area selective deposition (ASD) without a mask plate or lithography process. And the corresponding molecular dynamics (MD) and density functional theory (DFT) simulation further confirmed its feasibility and physical properties. Subsequently, multifunctional devices based on Ga2S3/graphene/GaN heterojunctions are fabricated accordingly, and exhibit ultrafast (<80 μs) response at 0 V and stable, highly sensitive (1150.4 A W-1) memory features at 3 V. Here, the huge hole barriers formed on the two edges of graphene set the foundation of trapping and detecting light-induced carriers. Afterwards, handwriting numeral recognition tasks are carried out based on the performance extracted from the device and a simplified noise filtering and improved recognition accuracy system is proposed, confirming its application potential in the artificial intelligence area. This study proposes a practical way to grow large-size 2D materials selectively, shows the valuable application potential of p-g-n heterojunctions in various application fields, and expands an innovative path of device development in the post-Moorish era.

Two-dimensional BiTeX crystals with persistent luminescence induced by photochemical reactions.

Two-dimensional (2D) materials are highly valued for their unique properties and potential applications, as they can display exotic behaviors differing from those of their bulk forms. Research on elementary and binary solids has been making great progress recently, while synthesizing multi-component 2D materials experimentally remains a challenge, despite the possibility of greatly extending the number of members of the 2D realm. In this study, we synthesized ternary BiTeX (X = Cl, Br, I) nanosheets with high crystallinity through an electrochemical exfoliation method. Structural analysis confirmed the retention of bulk composition in these nanosheets. Interestingly, the BiTeX nanosheets display a persistent luminescence effect, where the photogenerated carriers exhibit lifetimes of over 100 seconds. Furthermore, the recovery time increased at lower temperatures. The persistent luminescence in 2D BiTeX, which can be ascribed to reversible bond cleavage and recovery under photoexcitation, exhibits potential for applications in fluorescence and photo-responsive systems.

Machine Learning Interatomic Potential for Modeling the Mechanical and Thermal Properties of Naphthyl-Based Nanotubes.

Two-dimensional (2D) nanomaterials are at the forefront of potential technological advancements. Carbon-based materials have been extensively studied since synthesizing graphene, which revealed properties of great interest for novel applications across diverse scientific and technological domains. New carbon allotropes continue to be explored theoretically, with several successful synthesis processes for carbon-based materials recently achieved. In this context, this study investigates the mechanical and thermal properties of DHQ-based monolayers and nanotubes, a carbon allotrope characterized by 4-, 6-, and 10-membered carbon rings, with a potential synthesis route using naphthalene as a molecular precursor. A machine-learned interatomic potential (MLIP) was developed to explore this nanomaterial's mechanical and thermal behavior at larger scales than those accessible through the first-principles calculations. The MLIP was trained on data derived from the DFT/PBE (density functional theory/Perdew-Burke-Ernzerhof) level using ab initio molecular dynamics (AIMD). Classical molecular dynamics (CMD) simulations, employing the trained MLIP, revealed that Young's modulus of DHQ-based nanotubes ranges from 127 to 243 N/m, depending on chirality and diameter, with fracture occurring at strains between 13.6 and 17.4% of the initial length. Regarding thermal response, a critical temperature of 2200 K was identified, marking the onset of a transition to an amorphous phase at higher temperatures.

Structural and electronic transition of layered PtSe2 into non-layered PtSe.

Research into novel two-dimensional (2D) materials has boomed over the past decade, with a bewildering diversity of distinct properties being discovered. In this work, layered PtSe2, grown by chemical vapor deposition and thermally converted to non-layered tetragonal PtSe, is experimentally and theoretically investigated. Notably, the resultant PtSe is distinctly metallic, which highlights the significance of sub-stoichiometric phases within transition metal dichalcogenide films. This shows that thermal conversion can drastically affect the atomic and electronic properties of these phases.

A First-Principles Calculation Study of the Catalytic Properties of Two-Dimensional Bismuthene Materials for Carbon Dioxide Reduction

A First-Principles Calculation Study of the Catalytic Properties of Two-Dimensional Bismuthene Materials for Carbon Dioxide Reduction

Advancements in 2D Titanium Carbide (MXene) for Electromagnetic Wave Absorption: Mechanisms, Methods, Enhancements, and Applications.

With the advent of the 5G era, there has been a marked increase in research interest concerning electromagnetic wave-absorbing materials. A critical challenge remains in improving the wave-absorbing properties of these materials while satisfying diverse application demands. MXenes, identified as prominent "emerging" 2D materials for wave absorption, offer unique advantages that are expected to drive advancements and innovations in this field. This review emphasizes the synthesis benefits provided by the unique structural characteristics of MXenes and the performance enhancements achieved through their combination with other absorbing materials. Material requirements, synthesis approaches, and conceptual frameworks are integrated to underscore these advantages. The study provides a thorough analysis of MXene-absorbing composites, going beyond basic classification to address preparation and modification processes affecting the absorption properties of MXenes and their composites. Attention is directed to synthesis techniques, structural design principles, and their influence on composite performance. Additionally, the potential applications of MXenes in electromagnetic wave absorbing devices are summarized. The review concludes by addressing the challenges currently confronting MXene materials and outlining expected developmental trends, aiming to offer guidance for subsequent research in this domain.

2D Materials Coated Flexible Origami for Low-Frequency Energy Harvesting

2D Materials Coated Flexible Origami for Low-Frequency Energy Harvesting

Deep-Learning-Enabled Fast Raman Identification of the Twist Angle of Bi-Layer Graphene.

Twisted bilayer graphene (TBG) has drawn considerable attention due to its angle-dependent electrical, optical, and mechanical properties, yet preparing and identifying samples at specific angles on a large scale remains challenging and labor-intensive. Here, a data-driven strategy that leverages Raman spectroscopy is proposed in combination with deep learning to rapidly and non-destructively decode and predict the twist angle of TBG across the full angular range. By processing high-dimensional Raman data, the deep learning model extracts hidden information to achieve precise twist angle identification. This approach is further extended to a 2D plane, enabling accurate orientational mapping within individual samples. Through interpretability analysis, the model is validated in conjunction with first-principles theoretical calculations, ensuring robust and explainable results. This data-driven methodology not only facilitates efficient TBG characterization but also introduces a broadly applicable framework for studying other angle-dependent 2D materials, thereby advancing the field of material spectroscopy and analysis.

Molecular dynamics simulations of functionalized hBN nanopores in water: Ab initio force field and implications for water desalination.

Heteropolar two-dimensional materials, including hexagonal boron nitride (hBN), are promising candidates for seawater desalination and osmotic power harvesting, but previous simulation studies have considered bare, unterminated nanopores in molecular dynamics (MD) simulations. There is presently a lack of force fields to describe functionalized nanoporous hBN in aqueous media. To address this gap, we conduct density functional theory (DFT)-based abinitio MD simulations of hBN nanopores surrounded by water molecules. The results reveal a high propensity for hydrogen (H) and hydroxyl (OH) functionalization at boron edges, while nitrogen edges are functionalized with H and occasionally with oxygen (O), highlighting a route to tune membranes. We demonstrate the role of the Grotthuss mechanism during the functionalization of hBN edges in water. We develop high-fidelity force fields for H- and OH-functionalized hBN nanopores using potential energy surface fitting based on DFT calculations. The nonbonded parameters for H functionalization are obtained by training a force field for borazine (B3N3H6). We find that the proposed force field enables stable MD simulations of water/ion transport through B- and N-terminated hBN nanopores. Our results also indicate that previous studies that considered bare nanopores without functional groups overestimated the water flux and underestimated the ionic rejection of nanoporous hBN. Overall, our work is expected to enable the realistic modeling of edge-functionalized hBN in aqueous media for various application areas.

Octupolar Vortex Crystal and Toroidal Moment in Twisted Bilayer MnPSe3.

Experimental detection of antiferromagnetic order in two-dimensional materials is a challenging task. Identifying multidomain antiferromagnetic textures via the current techniques is even more difficult. Therefore, we investigate the higher-order multipole moments in twisted bilayer MnPSe3. While the monolayers of MnPSe3 exhibit antiferromagnetic order, the moiré superlattices display a two-domain phase. We show that the octupolar moments M33+ and M33- are significant in this multidomain phase. In addition, when [M33+,M33-] are represented by the x and y components of a vector, the resultant pattern of these octupole moments forms vortex crystals which leads to octupolar toroidal moments, Txyz and Tzβ. Txyz and Tzβ can give rise to a magnetoelectric effect and gyrotropic birefringence that may provide indirect ways of detecting multidomain antiferromagnetic order. Our results highlight the importance of higher-order multipole moments for identification of complex spin textures in moiré magnets.

Monolayer MoS2 with S vacancy defects doped with Group V non-metallic elements (N, P, As): a first-principles study.

This study systematically investigated the effects of single S-atom vacancy defects and composite defects (vacancy combined with doping) on the properties of MoS2 using density functional theory. The results revealed that N-doped S-vacancy MoS2 has the smallest composite defect formation energy, indicating its highest stability. Doping maintained the direct band gap characteristic, with shifts in the valence band top. The Fermi level slightly shifted down in N- and P-doped systems, with N-doped MoS2 showing a larger increase in valence band top energy. Doping also significantly altered the density of states at the Fermi level and weakened the dielectric properties of MoS2. The maximum dielectric peaks of doped systems appeared near 2.7 eV with reduced intensities and red-shifted energies. Optical properties were significantly changed, with decreased reflectance, narrower reflectance spectra, and blue-shifted absorption spectra. These findings suggest that introducing composite defects can effectively reduce the forbidden bandwidth of MoS2, enhancing electrical conductivity. This research provides theoretical guidance for novel material design and offers insights into composite defect behavior in other two-dimensional materials. The Materials-Studio CASTEP module was used to calculate density functional theory (DFT). A plane wave ultrasoft pseudopotential is used to optimize the crystal structure, and the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) is used to characterize the exchange correlation energy. After the convergence test, the truncation energy and dot settings were finally selected to be 450 eV and 3 × 3 × 1, respectively, the convergence accuracy was set to 1.0e-5eV/atom, and the convergence criterion for the interatomic interaction force was 0.02 eV/Å. The parameters were all at or better than the accuracy settings. The vacuum layer between the layers wasset to 18 Å to avoid interactions caused by the periodic calculation method.

Principles and Applications of Two-Dimensional Semiconductor Material Devices for Reconfigurable Electronics

With the advances in edge computing and artificial intelligence, the demands of multifunctional electronics with large area efficiency are increased. As the scaling down of the conventional transistor is restricted by physical limits, reconfigurable electronics are developed to promote the functional integration of integrated circuits. Reconfigurable electronics refer to electronics with switchable functionalities, including reconfigurable logic operation functionalities and reconfigurable responses to electrical or optical signals. Reconfigurable electronics integrate data-processing capabilities with reduced size. Two-dimensional (2D) semiconductor materials exhibit excellent modulation capabilities through electrical and optical signals, and structural designs of 2D material devices achieve versatile and switchable functionalities. 2D semiconductors have great potential to develop advanced reconfigurable electronics. Recent years witnessed the rapid development of 2D material devices for reconfigurable electronics. This work focuses on the working principles of 2D material devices used for reconfigurable electronics, discusses applications of 2D-material-based reconfigurable electronics in logic operation and artificial intelligence, and further provides a future outlook for the development of reconfigurable electronics based on 2D material devices.

Optimization of Thermal Insulation Parameters for Vertical Perimeters in Buildings with Single-Layer Walls

This article presents the results of a study on the impact of selected parameters of a building’s ground-floor zone (the junction of the external and foundation walls, the ground floor slab, and the ground itself) on the temperature field within the external envelope. This study aims to analyze the influence and optimize the parameters of vertical perimeter insulation in the ground-floor zone of the building. Mathematical modeling was selected as the research method. This paper analyzes the relationships between the temperature ϑimg on the inner surface of the wall at the analyzed node and the linear thermal transmittance coefficient ψim of the thermal bridge occurring in this location, as influenced by the following parameters: dp—thickness of the insulation layer in the ground floor slab; df—thickness of the vertical perimeter insulation layer in the foundation wall; t—location of the insulation layer in the ground floor slab relative to the “external wall-foundation wall” contact surface; r—location of the vertical perimeter insulation layer in the foundation wall; and λs—thermal conductivity of the single-layer external wall material. The analysis was conducted under the climatic conditions of Białystok, Poland. Using THERM 7.6 software for computational experiments, data were obtained to develop deterministic mathematical models of these relationships. The models enabled the assessment of the degree and nature of the influence of the studied factors on ϑimg and ψim, optimization of selected parameters, and mathematical description of safe operating conditions for external walls in the ground-floor zone of heated buildings. It was found that the parameters dp, df, and r have favorable effects, increasing ϑimg by 2.57%, 2.51%, and 4.17%, respectively, when varying from their minimum to maximum levels. Conversely, the parameters t and λs negatively impact ϑimg, contributing reductions of −12.01% and −4.66%, respectively, with a cumulative effect of −16.67%. Optimal parameter values were determined based on energy efficiency criteria. This information may be useful for researchers, designers, engineers, and decision-makers when making informed decisions during the design phase of heated buildings.

Computational prediction of novel two-dimensional tungsten nitride superconductors

Transition metal nitrides are well-known 3D superconductor materials. However, there is a lack of knowledge related to their two-dimensional (2D) counterparts, which have several potential technological applications. In this work, we predict, using an evolutionary algorithm coupled with a first-principles approach, a set of novel 2D superconductive structures based on tungsten nitride. Through a systematic process including energetic and dynamical analysis, three thermodynamically stable compositions along with metastable compounds were studied in the following stoichiometries: W4N2, W2N2, and W2N3. Their superconductive temperature (Tc) values, estimated by means of the Eliashberg superconductive theory and the McMillan equation, range from 2.3 to 21.6 K, where the highestTcvalue corresponds to a W2N3metastable hexagonal system. A systematic analysis of the structural, electronic, vibrational and electron-phonon (e-ph) properties, allowed us to recognize the variables that modulate theTcin theses systems. The superconductive behavior is strongly affected by changes in the nitrogen density of states at Fermi level, the e-ph coupling constant and the lattice symmetry. The present results aim to encourage further theoretical and experimental efforts over non fully explored superconductors in two dimensions.

Strain free thin film growth of vanadium dioxide deposited on 2D atomic layered material of hexagonal boron nitride investigated by their thickness dependence of insulator-metal transition behavior

Abstract We report preparation of Vanadium dioxide (VO2) ultrathin films on hexagonal boron nitride (hBN), which is a typical two-dimensional material, to show clear Metal -Insulator transition owing to weak van der Waals interaction at their surface. It is confirmed that VO2 films on hBN with the thickness ranging from 10 to 40 nm exhibit bulk like Metal -Insulator transition without degradation by using the Raman scattering spectroscopy and electric transport measurements. These results are demonstrating the importance of 2D material nature of hBN for producing the strain-free oxide thin films.

2.5-dimensional topological superconductivity in twisted superconducting flakes

Multilayer flakes of two-dimensional materials were recently shown to be tunable by twisting monolayers on their surface. This raises the question whether qualitatively new phenomena can occur in such finite-thickness moiré systems. Here we demonstrate the emergence of distinct topological phases and transitions in N-layered flakes of nodal superconductors with a single monolayer twisted on top of it. We show that a c-axis current transforms the whole system into a chiral topological superconductor. Increasing the current drives a sequence of topological transitions between states characterized by a Chern number increasing from ~O(N) up to ~O(N2), well beyond the additive effect of stacking N layers. We predict thickness-independent signatures of these states in the thermal Hall and tunneling microscopy measurements. Twisted superconductor flakes thus provide an example of a “2.5-dimensional” material where the synergy of two-dimensional layers extended in a third dimension realize states inaccessible in either monolayer or bulk materials.

Type-I and type-II interfaces in a MoSe2/WS2 van der Waals heterostructure

We report experimental evidence that MoSe2 and WS2 allow the formation of type-I and type-II interfaces, according to the thickness of the former. Heterostructure samples are obtained by stacking a monolayer WS2 flake on top of a MoSe2 flake that contains regions of thickness from one to four layers. Photoluminescence spectroscopy and transient absorption measurements reveal a type-II interface in the regions of monolayer MoSe2 in contact with monolayer WS2. In other regions of the heterostructure formed by multilayer MoSe2 and monolayer WS2, features of type-I interface are observed, including the absence of charge transfer and dominance of intralayer excitons in MoSe2. The coexistence of type-I and type-II interfaces in a single heterostructure offers opportunities to design sophisticated two-dimensional materials with finely controlled photocarrier behaviors.

Effects of Unit Cell Size on Thermal Conductivity in Two Different Polymorphs of Niobium Diselenide.

2D materials possess weak inter-layer van der Waals bonding, allowing them to exist as different polymorphs depending on the stacking sequence of the layers. Herein, the thermal conductivities of the 2H-NbSe2 and 2H-3R-NbSe2 polymorphs by conducting experimental measurements and theoretical analysis are comparatively studied. Owing to its 1.8 times larger unit cell, 2H-3R-NbSe2 has a considerably greater number of optical phonon branches than does 2H-NbSe2, suggesting that 2H-3R-NbSe2 absorbs thermal energy rather than transporting it. In addition, scattering is more likely to occur in 2H-3R-NbSe2 because a far greater number of states satisfy the selection rule. As a result of these, the 2H-3R-NbSe2 has considerably lower thermal conductivity than that of the 2H-NbSe2. The results highlight how the size of the unit cell affects the thermal conductivities of polymorphs.

Sensitive Mechanism and Instability Modeling Methods of Flexible Sensing Films Based on 2D Materials

This paper examines sensing mechanisms and stability issues of flexible sensors used in morphing wing applications. A key challenge is the lack of theoretical frameworks that accurately predict sensor behavior during complex deformation. Current models struggle to fully capture the relationships between mechanical strain, electrical response, and material properties. We first analyze the microscopic mechanisms and macroscopic sensing characteristics of 2D material-based sensitive films, developing strain-sensitive models based on crack effects and pressure-sensitive models based on slip effects. Through power spectrum analysis, we establish a quantitative model linking microscopic cracks to macroscopic electrical properties. Using this model, we study the factors affecting flexible sensing stability and propose a quantitative description model using dual-layer multi-channel flexible sensors. After simulation validation, our model successfully guides the structural design of flexible sensing films, offering a clear approach to improve flexible sensor stability.

Coupled pyroelectric-photovoltaic effect in 2D ferroelectric α-In2Se3

Pyroelectric and photovoltaic effects are vital in cutting-edge broadband sensors and solar energy harvesting. Recent advances revealed great potential of the bulk photovoltaic effect in two-dimensional (2D) materials to surpass the Shockley-Queiseer limit. Moreover, the atomic thickness, high thermal conductivity and room-temperature ferroelectricity endow 2D ferroelectrics with a superior pyroelectric response. Herein, we combined direct pyroelectric-photovoltaic measurements in 2D α-In2Se3. The results reveal a gigantic pyroelectric coefficient of ∼30.7 mC/m2K and a figure of merit of ∼135.9 m2/C. Moreover, a coupled pyroelectric-photovoltaic effect was demonstrated, where the pyroelectric current follows the temperature derivative, while the short-circuit current follows temperature. Finally, we utilized the intercoupled ferroelectricity of In2Se3 to realize a non-volatile, self-powered photovoltaic memory operation, demonstrating stable short-circuit current switching with 103 ON-OFF ratio. The coupled pyroelectric-photovoltaic effect, along with reconfigurable photocurrent, pave the way for a novel integrated thermal and optical response, in-memory logic and energy harvesting.