Two-dimensional Layered Materials Research Articles

Generalized method of image dyons for quasi-two dimensional slabs with ordinary - topological insulator interfaces

Electrostatic charges near the interface between topological (TI) and ordinary (OI) insulators induce magnetic fields in the medium that can be described through the so-called method of image dyons (electric charge - magnetic monopole pairs), the magnetoelectric extension of the method of image charges in classical electrostatics. Here, we provide the expressions for the image dyons and ensuing magnetoelectric potentials in a system comprised by two planar-parallel OI-TI interfaces conforming a finite-width slab. The obtained formulae extend earlier work in that they account for all different combinations of materials forming the slab and its surroundings, including asymmetric systems, as well as all possible combinations of external magnetization orientations on the interfaces. The equations are susceptible of implementation in simple computational codes, to be solved recurrently, in order to model magnetoelectric fields in topological quantum wells, thin films, or layers of two-dimensional materials. We exemplify this by calculating the magnetic fields induced by a point charge in nanometer-thick quantum wells, by means of a Mathematica code made available in repositories.

Controllable growth α-In2Se3 flakes by chemical vapor deposition

Boosting large-scale application of two-dimensional ferroelectric materials as memory transistors has stimulated intensive research interest. Layered α-In2Se3 features strikingly ferroelectric properties, which allows unique opportunities for the engineering of functional ferroelectric devices. However, the large-scale and controllable growth of the α-In2Se3 has remained a challenge due to the complexity of indium-selenium growth phase diagram. Here, we report the controllable growth of α-In2Se3 flakes using the developed confined-space modulated chemical vapor deposition (CVD) method. The α-In2Se3 flakes are preferred to be deposited on the inner surfaces of the confined space while the β-In2Se3 flakes are mainly grown on the outer surfaces of the confined space. In addition, the grown α-In2Se3 flake possesses a large size of ∼ 60 μm, uniform thickness of ∼ 2.8 nm and distinct ferroelectric properties. Our findings offer an effective and easily accessible method for controllable growth of two-dimensional layered materials and hold promise for further insight into the fascinating physical properties and potential practical devices applications of van der Waal crystals.

Synthesis of MnFe2O4/MXene/NF nanosized composite for supercapacitor application

One of the most promising topics in the study of advanced energy applications is the creation of extraordinarily effective electrodes substances for higher-power devices. Scientific community has recently become interested in MXene because of its exceptional electrochemical characteristics, especially in comparison with two-dimensional (layered materials) like graphene/MY (M = Metal and YS, O, Se, N, P etc). Layer restacking of MXene, however, reduces its supercapacitive performance as an electrode material because of functional group interactions. This work examines MXene and its composites with manganese ferrite MnFe2O4 nanostructure (MnF) for use as a battery-like hybrid supercapacitor in an effort to resolve this problem. In this work, MnF were used as interlayer spacers between layers of MXene nanosheet. When compared to either MnF or MXene alone, the electrochemical investigations demonstrate that the composite (MnF/MXene) has improved electrochemical characteristics. For MnF, MXene, and MnF/MXene composites, maximum (specific capacitance, Cs) around 1A g−1 was calculated to be around of 594, 1046.25, and 1268.75 F g−1, respectively. The MnF/MXene composites exhibited the high surface area of 975.3 cm2 resultant decreased the lesser charge transfer resistance (Rct) around 0.25 Ω, and shows a stable behavior for 50 h over 5000 cycles. In this study, 2D MXene was found to be the most effective capacitive electrode, outperforming carbon-based electrodes in an asymmetric assembly. As a result, the hierarchical MnF structure produced on different substrates in conjunction with MXene is a promising material for storage of energy.

Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS[formula omitted] by X-ray absorption spectroscopy and ab initio molecular dynamics simulations

Understanding interlayer and intralayer coupling in two-dimensional layered materials (2DLMs) has fundamental and technological importance for their large-scale production, engineering heterostructures, and development of flexible and transparent electronics. At the same time, the quantification of weak interlayer interactions in 2DMLs is a challenging task, especially, from the experimental point of view. Herein, we demonstrate that the use of X-ray absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations can provide useful information on both interlayer and intralayer coupling in 2DLM 2Hc-MoS2. The analysis of the low-temperature (10–300 K) Mo K-edge extended X-ray absorption fine structure (EXAFS) using RMC simulations allows for obtaining information on the means-squared relative displacements σ2 for nearest and distant Mo–S and Mo–Mo atom pairs. This information allowed us further to determine the strength of the interlayer and intralayer interactions in terms of the characteristic Einstein frequencies ωE and the effective force constants κ for the nearest ten coordination shells around molybdenum. The studied temperature range was extended up to 1200 K employing AIMD simulations which were validated at 300 K using the EXAFS data. Both RMC and AIMD results provide evidence of the reduction of correlation in thermal motion between distant atoms and suggest strong anisotropy of atom thermal vibrations within the plane of the layers and in the orthogonal direction.

GeAs as an emerging p-type van der Waals semiconductor and its application in p–n photodiodes

van der Waals (vdW) layered materials have shown great potential for future optoelectronic applications owing to their unique and variable properties. In particular, two-dimensional layered materials enable the creation of various circuital building blocks via vertical stacking, e.g. the vertical p–n junction as a key one. While numerous stable n-type layered materials have been discovered, p-type materials remain relatively scarce. Here, we report on the study of multilayer germanium arsenide (GeAs), another emerging p-type vdW layered material. We first verify the efficient hole transport in a multilayer GeAs field-effect transistor with Pt electrodes, which establish low contact potential barriers. Subsequently, we demonstrate a p–n photodiode featuring a vertical heterojunction of a multilayer GeAs and n-type MoS2 monolayer, exhibiting a photovoltaic response. This study promotes that 2D GeAs is a promising candidate for p-type material in vdW optoelectronic devices.

Robust Threshold-Switching Behavior Assisted by Cu Migration in a Ferroionic CuInP2S6 Heterostructure.

The two-dimensional layered material CuInP2S6 (CIPS) has attracted significant research attention due to its nontrivial physical properties, including room-temperature ferroelectricity at the ultrathin limit and substantial ionic conductivity. Despite many efforts to control its ionic conductance and develop electronic devices, such as memristors, improving the stability of these devices remains a challenge. This work presents a highly stable threshold-switching device based on the Cu/CIPS/graphene heterostructure, achieved after a comprehensive investigation of the activation of Cu's ionic conductivity. The device exhibits exceptional threshold-switching performance, including good cycling endurance, a high on/off ratio of up to 104, low operation voltages, and an ultrasmall subthreshold swing of less than 1.8 mV/decade for the resistance-switching process. Through temperature-dependent electrical and Raman spectroscopy measurements, the stable resistive-switching mechanism is interpreted with a drifting and diffusion model of Cu ions under the electric field, rather than the conventional conducting filament mechanism. These results make the layered ferroionic CIPS material a promising candidate for information storage devices, demonstrating a compelling approach to achieving high-performance threshold-switching memristor devices.

Ultrasensitive Phototransistor Based on Laser-Induced P-Type Doped WSe2/MoS2 Van der Waals Heterojunction

Out-of-plane p-n heterojunctions based on two-dimensional layered materials (2DLMs) with unusual physical characteristics are attracting extensive research attention for their application as photodetectors. However, the present fabrication method based on 2DLMs produces out-of-plane p-n homojunction devices with low photoresponsivity and detectivity. This work reports an ultrasensitive phototransistor based on a laser-induced p-doped WSe2/MoS2 van der Waals heterojunction. The laser treatment is used for p-doping WSe2 nanoflakes using high work function WOx. Then, an n-type MoS2 nanoflake is transferred onto the resulting p-doped WSe2 nanoflake. The built-in electric field of p-doped WSe2/MoS2 is stronger than that of pristine WSe2/MoS2. The p-n junction between p-doped WSe2 and MoS2 can separate more photogenerated electron–hole pairs and inject more electrons into MoS2 under laser illumination than pristine WSe2/MoS2. Thus, a high photoresponsivity (R) of ~1.28 × 105 A·W−1 and high specific detectivity (D*) of ~7.17 × 1013 Jones are achieved under the illumination of a 633 nm laser, which is approximately two orders higher than the best phototransistor based on a WSe2/MoS2 heterojunction. Our work provides an effective and simple method to enhance photoresponsivity and detectivity in two-dimensional (2D) heterojunction phototransistors, indicating the potential applications in fabricating high-performance photodetectors based on 2DLMs.

Two-Dimensional Layered Materials Meet Perovskite Oxides: A Combination for High-Performance Electronic Devices.

As the Si-based transistors scale down to atomic dimensions, the basic principle of current electronics, which heavily relies on the tunable charge degree of freedom, faces increasing challenges to meet the future requirements of speed, switching energy, heat dissipation, and packing density as well as functionalities. Heterogeneous integration, where dissimilar layers of materials and functionalities are unrestrictedly stacked at an atomic scale, is appealing for next-generation electronics, such as multifunctional, neuromorphic, spintronic, and ultralow-power devices, because it unlocks technologically useful interfaces of distinct functionalities. Recently, the combination of functional perovskite oxides and two-dimensional layered materials (2DLMs) led to unexpected functionalities and enhanced device performance. In this paper, we review the recent progress of the heterogeneous integration of perovskite oxides and 2DLMs from the perspectives of fabrication and interfacial properties, electronic applications, and challenges as well as outlooks. In particular, we focus on three types of attractive applications, namely field-effect transistors, memory, and neuromorphic electronics. The van der Waals integration approach is extendible to other oxides and 2DLMs, leading to almost unlimited combinations of oxides and 2DLMs and contributing to future high-performance electronic and spintronic devices.

Electrochemical aptasensor based on gold nanoparticle decorated Ti3C2Tx nanocomposites for chloramphenicol detection.

Using gold (Au) nanoparticle decorated Ti3C2Tx (Ti3C2Tx-Au) nanocomposites, a highly sensitive electrochemical aptasensor for the effective detection of chloramphenicol has been developed. As a two-dimensional layered material, the prepared composite not only provides high surface area, good conductivity, and thermal stability but also substantial binding sites for aptamers with high sensitivity and selectivity for the accurate determination of chloramphenicol. Interestingly, the conductivity and active sites were enhanced by freeze-drying Ti3C2Tx and in situ formation of Ti3C2Tx-Au nanocomposite. The fabricated aptasensor exhibited a very low detection limit (S/N ≥ 3) of 13.18fgmL-1 with a linear range of 1 ~ 700pgmL-1 and correlation coefficient of 0.9992. The fabricated aptasensor demonstrated an excellent reproducibility, repeatability, long-term stability, and high selectivity toward chloramphenicol. Further, the aptasensor was applied to real milk samples, and the recoveries were ranged from 98.93 to 101.93%.

Ion irradiation of monolayer graphene-Nd:YAG hybrid waveguides: fabrication and laser.

Hybrid waveguides consisting of two-dimensional layered materials pad on the surface of optical waveguides suffer from a nonuniform and loose contact between the two-dimensional material and the waveguide, which can reduce the efficiency of the pulsed laser. Here, we present high-performance passively Q-switched pulsed lasers in three distinct structures of monolayer graphene-Nd:YAG hybrid waveguides irradiated by energetic ions. The ion irradiation enables the monolayer graphene a tight contact and strong coupling with the waveguide. As a result, Q-switched pulsed lasers with narrow pulse width and high repetition rate are obtained in three designed hybrid waveguides. The narrowest pulse width is 43.6 ns, provided by the ion-irradiated Y-branch hybrid waveguide. This study paves the way toward developing on-chip laser sources based on hybrid waveguides by using ion irradiation.

Evaluating In-Plane Thermal Expansion of Two-Dimensional Layered Materials via Effective Descriptors

Two-dimensional layered materials show promising applications in miniaturized devices, such as transistors, spintronics, and field emitters. However, substantial thermal management issues, including thermal mismatch and thermal stress, may degrade device performance. To address such challenges, the thermal expansion (TE) anisotropy determined by the structure feature of layered material needs to be well understood. Here, we propose two new descriptors to evaluate the TE behavior of layered materials, namely the axial elastic deviation factor σi and the axial net thermal stress fi along the ith direction. The former, defined as the normalized elastic element difference of material elastic tensor C and compliance tensor S, can distinguish whether the thermal expansion of a material is driven by phonons (with small σi) or elastic property (with large σi) with few computational costs. The latter, axial stress (in GPa/K) induced by temperature, shows an accurate determination of the positive or negative thermal expansion along different in-plane directions of layered materials. Based on the analysis of descriptors, we found that PtS2 and PtSe2 are featured with a larger axial elastic deviation factor (>23%). Considering the elastic property, we for the first time report the in-plane negative thermal expansion in PtS2 (−1.2 ppm/K) and PtSe2 (−0.8 ppm/K). Our work provides a unified understanding of TE causes of layered materials via effective descriptors, which can serve as a guideline for high-throughput screening of thermal expansion materials and subsequent device design.

MoS2-functionalized conductive carbon heterostructure embedded with ferroelectric polymers for bipolar memristive applications

Heterostructures of two-dimensional layered materials, integrating two or more building blocks with complementing counterparts, can regulate the confinement and transportation of charge carriers via vacancy-induced defect and interfacial states. Herein, reduced graphene oxide-molybdenum disulfide (rGO-MoS2) nanohybrid were fabricated and reinforced with various polymers [poly methyl methacrylate (PMMA), poly (vinylidene fluoride) (PVDF), and PMMA-PVDF (20:80) blend] to study the resistive memory properties in a metal–insulator-metal configuration. The scanning electron microscopy analysis presents a hierarchical 3D flower-like MoS2 intercalated with rGO nanosheets. Transmission electron microscopy image exhibits MoS2 nanoflakes well interspersed and grafted on layered rGO sheets, forming sandwich heterostructures. Raman analysis shows a higher I D/I G ratio for rGO-MoS2 than rGO, demonstrating numerous defect states in rGO. The x-ray diffraction analysis of the polymer blend containing rGO-MoS2 exhibits β-crystal phases with a polarity-dependent internal electric field (E-field). The J-V characteristics of pure MoS2-polymer films display a write-once-read-many behavior with a current I ON/I OFF ratio of ∼102–103, in contrast to pristine polymer films exhibiting repeatable electrical hysteresis. Instead, the rGO-MoS2-based devices display bipolar characteristics (I ON/I OFF ratio of ∼103–104) due to charge transfer interaction with the conductive carbon substrates. The ferroelectric polarization-induced E-field coupled with the external bias is responsible for the improved memristive performances. A plausible conduction mechanism is proposed to discuss the carrier transport through the devices.

First-Principles Investigations on the Semiconductor-to-Metal Phase Transition of 2D Si2Te3 for Reversible Resistive Switching.

Si2Te3 is attracting attention due to its compatibility with Si technology while still showing advantages as a two-dimensional layered material. Although recent experimental studies have observed the resistive switching process in Si2Te3-based memristors, the mechanism has not been clearly identified. In this study, first-principles density functional theory calculations are employed to understand the relationship between the phase transition of Si2Te3 and the reversible resistive switching of the Si2Te3-based memristor. Our calculation results show that although semiconducting Si2Te3 is energetically more stable than two metallic Si2Te3 phases (α and β), two metallic Si2Te3 can be energetically stabilized by excess holes. The enhanced energetic preference of two metallic Si2Te3 by excess holes is explained by the reduced occupation of antibonding states between Si and Te. Our study finds that the energy barrier for the phase transition between semiconducting Si2Te3 and α-metallic Si2Te3 varies significantly by excess charge carriers so the phase transition can be directly connected to the reversible resistive switching of the Si2Te3-based memristor under external bias. Our finding will serve as a cornerstone for optimizing the resistive switching process of the Si2Te3-based memristor.

Field electron emission and field ion microscope images of layered material MoS2

Field emission microscopy and field ion microscopy (FEM and FIM) of two-dimensional layered material, MoS2, were carried out. MoS2 field emitters were prepared by mechanical peeling of a single crystal and by deposition of MoS2 suspension. FEM exhibited an array of elongated broad spots, and the corresponding FIM images presented patterns consisting of streaked spots and split spots (double spots), depending on electric voltages applied to the emitter. Since the probability of field ionization of imaging gas atoms (Ne gas was used in the present study) is governed by both field strength near surface atomic sites and accessible vacant electronic states, these patterns reflect the atomic site and/or spatial distribution of unoccupied electronic orbitals at the edges of MoS2 layers.

Elimination of iron in molybdenum concentrate via vacuum chlorination

Due to the great affinity and the similarity of the physical performance of iron and molybdenum disulfide in the concentrate, it is difficult to affect the removal and separation of iron during the procession of molybdenum concentrate. The application of molybdenum disulfide two-dimensional layered materials is seriously restricted by the relatively high content of iron. Considering the direct purification of MoS2, vacuum chlorination roasting for decreasing iron of molybdenum concentrate is proposed. Thermodynamics of iron removal by vacuum chlorination roasting is analyzed. FeS can be chloridized into FeCl2, which volatilizes easily by solid chlorination agent in vacuum. The experimental results showed that the iron content decreased to 0.43% at 1223 K and P ∼1 Pa for 60 min, and the Fe content was reduced to 0.082% after acid washing, with a removal rate of 94.2%. which offers a novel way to replace the conventional method for elimination of iron sulfide in molybdenum ore. Possible separation mechanism of iron sulfide is also discussed in this work.

Construction of Cu2O-g-C3N4/MoS2 composite material through the decoration of Cu2O nanoparticles onto the surface of two-dimensional g-C3N4/MoS2 heterostructure for their application in electrochemical hydrogen evolution

This work focused on developing an efficient electrocatalyst comprising a cost-effective metal oxide and two-dimensional layered materials. Tricomponent Cu2O-g-C3N4/MoS2 composite material was synthesized using a facile two-step procedure for their catalytic application in electrochemical hydrogen evolution. In addition to the more electrochemically active surface area (ECSA), the presence of synergistic properties among the components in the Cu2O-g-C3N4/MoS2 composite material contributed to the excellent efficiency of the catalyst in the evolution of hydrogen from an aqueous acidic solution. The composite demonstrated a low overpotential of −0.14 V vs RHE at 10 mA/cm2 and a high total current density of 237.6 mA/cm2 at −0.8 V. The improvement in the performance of the composite material for hydrogen evolution (HER) was supported by PEIS results in addition to CV and LSV. The reaction kinetics of the HER catalyzed by Cu2O-g-C3N4/MoS2 composite material was explained with the help of the Tafel slope. Moreover, the high durability of the catalyst with no drop in the current density for many hours was established through CA measurement.

Application of Layered Double Hydroxides as a Slow-Release Phosphate Source: A Comparison of Hydroponic and Soil Systems.

The utilization of slow-release fertilizer materials capable of responding to their environment and releasing nutrient ions efficiently over a prolonged period is an emerging research area in agricultural materials sciences. In this study, two-dimensional layered materials were prepared to release phosphor ions (P) slowly into the soil as well as in the hydroponic system. Various P-intercalated layered double hydroxides (LDHs) (Mg/Al, Zn/Al, and Mg-Zn/Al-LDHs) with a molar ratio of 2:1 were synthesized using an ion-exchange method from corresponding LDHs containing NO3 - ions within the layers. Sodium alginate (SA) was used to encapsulate P-intercalated Mg/Al-LDH to produce bionanocomposite beads (LB) to check the effect of the biopolymer matrix on the release characteristics. The prepared materials were characterized by XRD and FTIR to confirm the incorporation of P in LDHs. TGA, SEM, and elemental analysis were also performed to study the thermal decomposition pattern, surface morphology, and chemical composition of synthesized materials. The P-release experiments were conducted in a soil solution. The performance of the prepared materials was investigated in soil as well as in a hydroponic system for tomato plants under a controlled atmosphere of humidity, temperature, and light. The fertilization ability of the prepared materials was compared with that of a soluble P source (KH2PO4), commercial hydroponic fertilizer (Nutrifeed), and a commercial soil slow-release fertilizer (Wonder plant starter). The prepared materials demonstrated a slow release of P in the soil solution. P-intercalated LDHs were not very effective under hydroponic conditions; however, the LDHs were more effective in the soil system in terms of dry matter production and P content in dry matter. Furthermore, LDHs were able to increase the soil pH value over time.

Thermal Conductivity of a Two-Dimensional Diamondene Sheet: A Molecular Study

Diamondene, as a novel two-dimensional layered material, exhibits a high carrier mobility resembling those of diamond thin films and holds great promise in electronic devices, while its thermal properties are yet to be investigated. In this paper, the thermal behaviors of diamondene are investigated using molecular dynamic simulation, and the effects of orientation, temperature, hydrogenation, and defect on the thermal conductivity are considered. At simulated temperatures of 300 K, for diamondene, the thermal conductivities of the armchair direction consist of with that of the zigzag direction. The thermal conductivity of quantum correction is reduced to 89% of the classical MD results. The thermal conductivity is sensitive to strain engineering but almost independent of surface chemical functionalization. Thermal conductivity decreases with defect concentration and then enhances (stable stage at a defect concentration range of 20 and 80%). The phenomena are explained by phonon-density-state and micro-heat-flux scatting. It is attributed to the interplay between two cooperating/competing mechanisms of phonon and the compressive effect of the scatted phonon at different chirality. Our study offers a fundamental understanding of thermal transport in diamondene, which will enrich the studies of 2D materials in the field of thermal transfer.

Liquid-Phase Friction of Two-Dimensional Molybdenum Disulfide at the Atomic Scale.

Tribological properties depend strongly on environmental conditions such as temperature, humidity, and operation liquid. However, the origin of the liquid effect on friction remains largely unexplored. Herein, taking molybdenum disulfide (MoS2) as a model system, we explored the nanoscale friction of MoS2 in polar (water) and nonpolar (dodecane) liquids through friction force microscopy. The friction force exhibits a similar layer-dependent behavior in liquids as in air; i.e., thinner samples have a larger friction force. Interestingly, friction is significantly influenced by the polarity of the liquid, and it is larger in polar water than in nonpolar dodecane. Atomically resolved friction images together with atomistic simulations reveal that the polarity of the liquid has a substantial effect on friction behavior, where liquid molecule arrangement and hydrogen-bond formation lead to a higher resistance in polar water in comparison to that in nonpolar dodecane. This work provides insights into the friction on two-dimensional layered materials in liquids and holds great promise for future low-friction technologies.

Order-disorder phase transition driven by interlayer sliding in lead iodides

A variety of phase transitions have been found in two-dimensional layered materials, but some of their atomic-scale mechanisms are hard to clearly understand. Here, we report the discovery of a phase transition whose mechanism is identified as interlayer sliding in lead iodides, a layered material widely used to synthesize lead halide perovskites. The low-temperature crystal structure of lead iodides is found not 2H polytype as known before, but non-centrosymmetric 4H polytype. This undergoes the order-disorder phase transition characterized by the abrupt spectral broadening of valence bands, taken by angle-resolved photoemission, at the critical temperature of 120 K. It is accompanied by drastic changes in simultaneously taken photocurrent and photoluminescence. The transmission electron microscopy is used to reveal that lead iodide layers stacked in the form of 4H polytype at low temperatures irregularly slide over each other above 120 K, which can be explained by the low energy barrier of only 10.6 meV/atom estimated by first principles calculations. Our findings suggest that interlayer sliding is a key mechanism of the phase transitions in layered materials, which can significantly affect optoelectronic and optical characteristics.