Two-dimensional Layered Materials Research Articles

Gate-switchable rectification in isotype van der Waals heterostructure of multilayer MoTe2/SnS2 with large band offsets

Despite intensive studies on van der Waals heterostructures based on two-dimensional layered materials, isotype vdW heterojunctions, which consist of two different semiconductors with the same majority carrier, have received little attention. We demonstrate an n–n isotype field-effect heterojunction device composed of multilayer moly ditelluride (MoTe2) and tin disulfide (SnS2). The carrier transport flowing through the n-MoTe2/n-SnS2 heterojunction exhibits a clear rectifying behavior exceeding 103, even at a moderate source–drain voltage of 1 V in ambient environment. Owing to the large band offsets between the two materials, a potential barrier exceeding ~1 eV is formed, which is verified by comparing a numerical solution of Poisson’s equation and experimental data. In contrast to the conventional p–n heterostructure operating by diffusion of the minority carrier, we identify the carrier transport is governed by the majority carrier via the thermionic emission and tunneling-mediated process through the potential barrier. Furthermore, the gate voltage can completely turn off the device and even enhance the rectification. A ternary inverter based on the isotype MoTe2/SnS2 heterojunction and a SnS2 channel transistor is demonstrated for potential multivalued logic applications. Our results suggest that the isotype vdW heterojunction will become an able candidate for electronic or optoelectronic devices after suitable band engineering and design optimization.

Highly stable all-in-one photoelectrochemical electrodes based on carbon nanowalls

Photoelectrochemical (PEC) cells offer a promising approach for developing low-cost solar energy conversion systems. However, the lack of stable and cost-effective electrodes remains a bottleneck that hampers their practical applications. Here, we propose a kind of integrated all-in-one three-dimensional (3D) carbon nanowall (CNW) electrode without sensitized semiconductors for stable all-carbon PEC cells. The all-in-one CNW electrodes were fabricated by directly growing CNW on both sides of the SiO2/Si/SiO2 wafer employing the radio frequency plasmon enhanced chemical vapor deposition method. Benefitting from the interconnected 3D textured structure, the CNW can effectively absorb the incident light and provide a large electrochemical reaction interface at the CNW surface that promotes the separation of photogenerated charge carriers, which makes it a superior electrode material. Experimental results show that the all-in-one CNW electrodes possess excellent PEC performance with a photocurrent density of 830 μA cm−2. Moreover, the CNW electrodes exhibit excellent photoresponses over a wide waveband and superior stability with a maintained photocurrent response, even after 60 d, which outperforms the electrodes using the other two-dimensional layered materials or semiconductor sensitized electrodes. Such an all-in-one electrode with impressive photovoltaic properties provides a promising platform for PEC applications that is eco-friendly with high efficiency, excellent stability and low cost.

CoFe2O4 Nanoparticle-Decorated 2D MXene: A Novel Hybrid Material for Supercapacitor Applications

The development of highly efficient electrode materials for high power devices is one of the cutting-edge research areas in advanced energy applications. Recently, MXene has gained tremendous interest among the research community because of its extraordinary electrochemical properties as compared to other two-dimensional layered materials such as graphene/MoS₂. However, the supercapacitive performance of MXene as an electrode material is hindered by the restacking of its layers due to functional group interactions. To overcome this problem, here in this article, we explored MXene and its composites with cobalt ferrite [CoFe₂O₄] nanoparticles (CoF NPs) for battery-like hybrid supercapacitor applications. NPs were applied to use them as interlayer spacers between MXene layers. By the electrochemical studies, it is proved that the composite (CoF/MXene) can provide better electrochemical properties than individual ferrite or MXene. The maximum specific capacitance (Cₛₚ) of CoF NPs, MXene, and CoF/MXene composites was observed to be about 594, 1046.25, and 1268.75 Fg–¹ at 1 A g–¹, respectively. The calculated specific capacity (sp. capacity) of the CoF/MXene composite was about 440 Cg–¹ at 1 A g–¹ and proved to be an excellent hybrid electrode material by providing only 0.25 Ω charge transfer resistance. The as-synthesized material demonstrated the excellent capacitance retention, about 97%, up to 5000 cycles.

Atomic Structure and Electronic Properties of Intermetallic CaBi2 Thin Films.

Intermetallic bismuth-based compounds have attracted great interest as promising candidates for novel topological superconductivity. Among them, CaBi2 is a newly discovered member for which the atomic structure and electronic properties have never been systematically explored. Using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically characterized the atomic structure and electronic properties of CaBi2(010) thin films grown by molecular beam epitaxy (MBE) and found that their growth follows a Stranski-Krastanov mode. A nonreconstructed IBi layer and a (1 × 2) reconstructed IICa layer were found to be the most common surfaces. Nonreconstructed IIIBi and VCa layers were further exposed with reduced bismuth growth flux. All of these constituent layers exhibit unique features in the STS spectra, indicating that unique electronic properties exist in each specific constituent layer. Our findings provide for deeper understanding of the physical properties of this compound and suggest further studies of the two-dimensional (2D) layered materials family.

Two-dimensional Bi2O2CO3/δ-Bi2O3/Ag2O heterojunction for high performance of photocatalytic activity

Two-dimensional layered materials have attracted extensive interest recently due to their high efficiency solar energy conversion. However, 2D layered materials are usually stacked by individual monolayers through strong chemical bonds or van der Waal interactions. Therefore, it is still challenging to fabricate to 2D monolayered materials. Herein, monolayered Bi2O2CO3 composition dotted with δ-Bi2O3 assembling to a network structure is fabricated through a simple composite-salt mediated approach, which has strong adsorption of organic pollutant. After loaded Ag2O on the surface of it, Bi2O2CO3/δ-Bi2O3/Ag2O (BBA) composite exhibits a lower adsorption capability but higher photodegradation efficiency for methylene blue (MB). 97% MB is degraded by BBA-3 in 15 min under the irradiation of simulated sunlight of 100 mW/cm2, much better than that of commercial TiO2 (P25) and Bi2O2CO3/δ-Bi2O3. The short diffusion distance in the two-dimensional structure enables photoexcited carriers to diffuse to the surfaces almost without recombination and leads to efficient photodegradation of dyes on both sides of a monolayer or few layers. This novel 2D structure brings a new idea for developing highly efficient photocatalyst.

2D-SnSe2 Nanosheet Functionalized Piezo-resistive Flexible Sensor for Pressure and Human Breath Monitoring

Flexible sensors based on two-dimensional layered materials have gained huge attention in the field of intelligent electronics. The cellulose based platform provides a cost-effective way for fabric...

Evolutionary multi-objective optimization and Pareto-frontal uncertainty quantification of interatomic forcefields for thermal conductivity simulations

Predictive Molecular Dynamics simulations of thermal transport require forcefields that can simultaneously reproduce several structural, thermodynamic and vibrational properties of materials like lattice constants, phonon density of states, and specific heat. This requires a multi-objective optimization approach for forcefield parameterization. Existing methodologies for forcefield parameterization use ad-hoc and empirical weighting schemes to convert this into a single-objective optimization problem. Here, we provide and describe software to perform multi-objective optimization of Stillinger–Weber forcefields (SWFF) for two-dimensional layered materials using the recently developed 3rd generation non-dominated sorting genetic algorithm (NSGA-III). NSGA-III converges to the set of optimal forcefields lying on the Pareto front in the multi-dimensional objective space. This set of forcefields is used for uncertainty quantification of computed thermal conductivity due to variability in the forcefield parameters. We demonstrate this new optimization scheme by constructing a SWFF for a representative two-dimensional material, 2H-MoSe2 and quantifying the uncertainty in their computed thermal conductivity. Program summaryProgram Title: MOGA-NSGA3Program Files doi: http://dx.doi.org/10.17632/pbc6nb7hp9.1Licensing Provisions: GNU General Public License 3Programming Language: C++Nature of problem: Interatomic forcefields used for molecular dynamics simulations of thermal conductivity must be parameterized to accurately capture structural and vibrational properties of the material system being modeled. Therefore, these forcefields must be simultaneously optimized against several (n≥ 5) material properties. However, such parameterization is difficult using existing forcefield parameterization schemes, which are limited to optimization of a single or few (n< 3) objectives.Solution method: We present software to perform evolutionary optimization of forcefields for thermal conductivity simulations using the recently developed 3rd generation non-dominated sorting genetic algorithm (NSGA-III). The algorithm’s unique reference-point-based niching and non-dominated sorting schemes enable efficient exploration of higher-dimensional objective spaces while preserving diversity among forcefield populations. The best set of forcefields on the Pareto front are used for estimating uncertainty in computed thermal conductivity due to forcefield parameterization.

2D transition metal dichalcogenides, carbides, nitrides, and their applications in supercapacitors and electrocatalytic hydrogen evolution reaction

The development of renewable energy conversion and storage devices, aiming at high efficiency, stable operation, environmental friendliness, and low-cost goals, provides a promising approach to resolve the global energy crisis. Recently, two-dimensional (2D) layered materials have drawn enormous attention due to their unique layered structure and intriguing electrical characteristics, which brings the unprecedented board applications in the fields ranging from electronic, optical, optoelectronic, thermal, magnetic, quantum devices to energy storage and catalysis. Graphene-based 2D layered materials show promising applications in energy storage and conversion owing to their high specific surface area, which have been used for supercapacitor electrode materials based on the electrical double-layer capacitance model. However, graphene has a limited value of theoretical electrical double-layer capacitance when the whole surface area is fully utilized. Among several classes of 2D layered materials beyond graphene, transition metal dichalcogenides, transition metal carbides, and nitrides may exhibit excellent electrochemical properties due to the distinctive features of these 2D materials, such as large specific surface area, good hydrophilic nature, highly exposed active edge sites, and ease of intercalation and modification. Therefore, careful design and construction of these 2D compounds make them become potential candidates used for electrochemical supercapacitors and electrocatalytic hydrogen evolution. This review emphasizes the recent important advances of the 2D layered materials composed of transition metal dichalcogenides, transition metal carbides, and nitrides for supercapacitors and electrocatalysts. Furthermore, we discuss the challenges and perspectives in this energy field in terms of the classes of two-dimensional layered materials.

Facile preparation of black phosphorus-based rGO-BP-Pd composite hydrogels with enhanced catalytic reduction of 4-nitrophenol performances for wastewater treatment

The functionalization and self-assembly of two-dimensional layered materials (such as graphene, black phosphorus) play an important role in the wide application of nanomaterials. In present study, the graphene oxide (GO) was reduced by adding l-ascorbic acid to obtain a reduced graphene oxide (rGO) hydrogel. rGO hydrogel can be considered as an ideal support due to its wrinkled structure and residual oxygen atoms in GO surface, which can effectively reduce the aggregation of palladium nanoparticles (Pd NPs). The effect of adding black phosphorus nanosheets (BPNS) to the rGO-Pd hydrogel on the catalytic performance was also studied. The results show that the rGO-BP-Pd composite hydrogels exhibit enhanced catalytic performance compared to the rGO-Pd hydrogel due to the strong interaction between Pd and the P atom of the BPNS. The current research provides new clues for the preparation of functionalized composite hydrogel and BP nanomaterials.

Computational screening for enhanced hydrogen sensing by doped-2H and pristine-1T MoS2

Hydrogen is a kind of high-efficiency and clean energy, but easy to leak out. The detecting of hydrogen is important during storage and transport. Since the large surface area, two-dimensional layered materials have an advantage of sensing gas molecules. And monolayer MoS2 is a typical kind that has been widely studied recently. Here, the comparison between doped-2H MoS2 and pristine-1T MoS2 on sensing hydrogen (H2) molecules is performed using first principles for the first time. The results show that the four kinds of doped-2H and pristine-1T MoS2 all have stronger effects of orbital hybridization with hydrogen molecule than pristine-2H MoS2. The Si-doped one shows the best adsorption properties in multiple hydrogen situations, signifying a better sensing accuracy. Interestingly, pristine-1T MoS2 has much higher hydrogen adsorption energy than any kind of doped-2H MoS2, indicating a higher sensibility. Thus, trade-offs between doping processes and phase engineering should be considered for promoting the gas sensing performance of MoS2-based materials. With researches in this paper, a reference for further research on the application of highly integrated hydrogen sensing was provided.

A Novel Strategy for Liquid Exfoliation of Ultrathin Black Phosphorus Nanosheets.

As an emerging two-dimensional layered material, black phosphorus nanosheets show unparalleled optical and electronic properties. Although black phosphorus nanosheets have attracted much attention in the photoelectric field, their applications in biomedical field were still limited due to their poor biocompatibility of current synthesis strategies. Herein, we propose a novel synthetic strategy for black phosphorus nanosheets that rely on Tween 20-assisted liquid exfoliation and post-processing in deoxygenated water. Microscopic and spectroscopic analysis suggested that the produced black phosphorus nanosheets dispersions exhibited good stability and higher yield compared with other currently prepared methods. Because of their ultrahigh exfoliation efficiency, the black phosphorus flakes present few-layer and even monolayer, which are thinner than the most dispersions of black phosphorus. Thus, this method enables mass-production of high-quality few-layer black phosphorus with high biocompatibility, and has the potential to be directly used in the biological field.

Copper-doped induced ferromagnetic half-metal zirconium diselenide single crystals

Two-dimensional (2D) magnetic layered materials have attracted considerable attention in memory storage devices due to their exciting magnetic ordering. Herein, the electronic and magnetic properties of high-quality single crystals zirconium diselenide and copper (Cu)-doped zirconium diselenide as grown via chemical vapor transport technique combined with first principle density functional theory calculations were investigated. A semimetallic state is recognized for Cu0.052Zr0.93Se2 as measured through resistance versus temperature measurements and angle resolved photoemission spectroscopy (ARPES). The magnetic measurement shows diamagnetic semiconducting behaviour for ZrSe2, whereas Cu0.052Zr0.93Se2 exhibits ferromagnetic character via applying perpendicular magnetic field. Cu0.052Zr0.93Se2 reveals the room temperature magnetic moment ∼0.0125 emu g−1, while the Curie temperature is ∼363.49 K. Furthermore, first principle density functional theory (DFT) calculations show energetically long range ferromagnetic ordering in a half-metallic Cu-doped ZrSe2, while a diamagnetic state in case of ZrSe2 agrees well with experiment results. These results suggest that due to strong interaction elements at the octahedral site of zirconium atoms when replaced by copper atoms, which can change the spin ordering of electrons and make zirconium vacancy, while their magnetic moment is increased. Very importantly the half-metallic character of Cu0.052Zr0.93Se2 promotes much spin polarized electrons around the Fermi level, suggesting significant potential in future memory devices and spintronic applications.

Epitaxial nucleation and lateral growth of high-crystalline black phosphorus films on silicon

Black phosphorus (BP) is a promising two-dimensional layered semiconductor material for next-generation electronics and optoelectronics, with a thickness-dependent tunable direct bandgap and high carrier mobility. Though great research advantages have been achieved on BP, lateral synthesis of high quality BP films still remains a great challenge. Here, we report the direct growth of large-scale crystalline BP films on insulating silicon substrates by a gas-phase growth strategy with an epitaxial nucleation design and a further lateral growth control. The optimized lateral size of the achieved BP films can reach up to millimeters, with the ability to modulate thickness from a few to hundreds of nanometers. The as-grown BP films exhibit excellent electrical properties, with a field-effect and Hall mobility of over 1200 cm2V−1s−1 and 1400 cm2V−1s−1 at room temperature, respectively, comparable to those exfoliated from BP bulk crystals. Our work opens the door for broad applications with BP in scalable electronic and optoelectronic devices.

Vacuum-assisted assembly of iron cage intercalated layered double hydroxide composite membrane for water purification

Water purification is a significant approach to solve the serious global water shortage and pollution problems. Nanofiltration membranes assembled from two-dimensional layered materials have shown great potential in water purification. In this study, iron cage (Fe-cage) intercalated layered double hydroxide (LDH) composite membrane was formed on the surface of polyacrylonitrile ultrafiltration support through vacuum-assisted assembly strategy. Exfoliated LDH nanosheets obtained by solvothermal method were re-stacked with the intercalation of Fe-cage through the electrostatic interaction. The structure and chemical property of the nanochannels formed between the LDH nanosheets were tuned by Fe-cage. Therefore, the fabricated LDH/Fe-cage composite membranes possessed enhanced separation performance for the removal of dye and humic acid from water. In addition, the composite membrane exhibited a stable separation performance for a long-time running. These results indicated that this facile and effective method has an extensive potential in practical water purification process.

Nonlinear optical properties of anisotropic two-dimensional layered materials for ultrafast photonics

Abstract The discovery of graphene has intrigued the significant interest in exploring and developing the two-dimensional layered materials (2DLMs) for the photonics application in recent years. Unlike the isotropic graphene, a number of 2DLMs possess the in-plane anisotropic crystal structure with low symmetry, enabling a new degree of freedom for achieving the novel polarization-dependent and versatile ultrafast photonic devices. In this review article, we focus on the typical anisotropic 2DLMs including BP, ReS2, ReSe2, SnS, and SnSe and summarize the recent development of these anisotropic 2DLMs in the pulsed laser and the optical switch applications. First, we introduce the fabrication methods as well as the material characterization of the anisotropic 2DLMs by analyzing the polarized Raman configuration. Second, we discuss the anisotropic nonlinear optical properties of the anisotropic 2DLMs and concentrate on the anisotropic nonlinear absorption response. Next, we sum up state of the art of the anisotropic 2DLMs in the application of pulse lasers and optical switches. This review ends with perspectives on the challenge and outlook of the anisotropic 2DLMs for ultrafast photonics applications.

Recent Advances in the Rational Design and Synthesis of Two-Dimensional Materials for Multivalent Ion Batteries.

With the increase of device requirements, rechargeable lithium-ion batteries are facing tremendous challenges in large-scale applications due to the high price and gradual shortage of lithium sources. In contrast, multivalent ion batteries, such as aluminum, magnesium, and zinc, are promising candidates for the next-generation energy-storage systems because of their high volumetric energy density, safe operation, and abundant reserves. The strong intercalation between multivalent ions and the host materials, however, will cause lower ion-diffusion kinetics and a poor discharge capacity. One of the main challenges is to search for a suitable cathode material with a high capacity and good structural stability to overcome the abovementioned problems. Two-dimensional layered materials, with characteristic unique structural features, good conductivity, and high electrochemically active surface, have attracted attention from researchers during the past decade. In this review, the design approach and synthetic procedures for the preparation of two-dimensional materials as cathodes for multivalent ion batteries, including interlayer engineering, two-dimensional heterostructures, pore/hole engineering, and heteroatom doping, are summarized. Meanwhile, the relationship between the design configuration and optimized electrochemical performance is rationally and systematically presented. Additionally, perspectives for the sustainable synthesis of cathode materials are proposed for multivalent metal-ion chemistry.

Few-layer metal monochalcogenide saturable absorbers for high-energy Q-switched pulse generation

Two-dimensional layered materials have been widely utilized as nonlinear absorption materials to transfer continue-wave into pulse trains in fiber laser systems. Here, we prepare robust GaSe/GeSe composites with high power bearing capacity as saturable absorbers (SAs) and then investigate their nonlinear optical properties via broadband Z-scan measurement at 800 nm and 1550 nm, respectively. The modulation depths of GaSe/GeSe based SAs are measured to be 11.97% and 7.69% at 1550 nm. After incorporating the GaSe/GeSe SAs into an Erbium-doped fiber laser cavity, passively Q-switched pulse trains could be obtained with repetition rates changing from 83.58 to 136.78 kHz (70.41 to 161.65 kHz). The maximum output power and pulse energy are 52.1 mW/370.67 nJ (GaSe) and 21.6 mW/133.74 nJ (GeSe) under the maximum pump power of 600 mW. The results indicate that GaSe and GeSe possess outstanding thermal stability and could be employed as remarkable saturable absorption materials for high-energy pulses generation.

Neuromorphic nanoelectronic materials.

Memristive and nanoionic devices have recently emerged as leading candidates for neuromorphic computing architectures. While top-down fabrication based on conventional bulk materials has enabled many early neuromorphic devices and circuits, bottom-up approaches based on low-dimensional nanomaterials have shown novel device functionality that often better mimics a biological neuron. In addition, the chemical, structural and compositional tunability of low-dimensional nanomaterials coupled with the permutational flexibility enabled by van der Waals heterostructures offers significant opportunities for artificial neural networks. In this Review, we present a critical survey of emerging neuromorphic devices and architectures enabled by quantum dots, metal nanoparticles, polymers, nanotubes, nanowires, two-dimensional layered materials and van der Waals heterojunctions with a particular emphasis on bio-inspired device responses that are uniquely enabled by low-dimensional topology, quantum confinement and interfaces. We also provide a forward-looking perspective on the opportunities and challenges of neuromorphic nanoelectronic materials in comparison with more mature technologies based on traditional bulk electronic materials.

The influence of point defects on Na diffusion in black phosphorene: First principles study

Two-dimensional layered materials like graphene, phosphorene and silicene are promising materials for use as anodes in Li- and Na-ion batteries. However, during synthesis of these materials, point defects – single vacancy (SV), divacancy (DV) or Stone–Wales (SW) type – are quite likely to form and to change the performance as an anode material. In this study, the influences of these defects on Na adsorption performance in black phosphorene are investigated. We conclude that these defects could affect performance of phosphorene as an anode material: negatively in the case of DV-2, SV and SW-2 defects but positively for DV-1 and SW-1 defects. This impact on the performance is greatest in both paths (zigzag and armchair) for SW-2 defects with the diffusion coefficient almost zero. However, the SW-1 and DV-1 defects could improve Na diffusion in phosphorene making these desirable for phosphorene as anode material for Na-ion batteries.

Bubble-Free Transfer Technique for High-Quality Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Bubbles at the interface of two-dimensional layered materials in van der Waals heterostructures cause deterioration in the quality of materials, thereby limiting the size and design of devices. In this paper, we report a simple all-dry transfer technique, with which the bubble formation can be avoided. As a key factor in the technique, a contact angle between a picked-up flake on a viscoelastic polymer stamp and another flake on a substrate was introduced by protrusion at the stamp surface. Using this technique, we demonstrated the fabrication of high-quality devices on the basis of graphene/hexagonal boron nitride heterostructures with a large bubble-free region. Additionally, the technique can be used to remove unnecessary flakes on a substrate under an optical microscopic scale. Most importantly, it improves the yield and throughput for the fabrication process of high-quality van der Waals heterostructure-based devices.