Two-dimensional Layered Materials Research Articles

Two-dimensional layered material photodetectors: what could be the upcoming downstream applications beyond prototype devices?

With distinctive advantages spanning excellent flexibility, rich physical properties, strong electrostatic tunability, dangling-bond-free surface, and ease of integration, 2D layered materials (2DLMs) have demonstrated tremendous potential for photodetection. However, to date, most of the research enthusiasm has been merely focused on developing novel prototype devices. In the past few years, researchers have also been devoted to developing various downstream applications based on 2DLM photodetectors to contribute to promoting them from fundamental research to practical commercialization, and extensive accomplishments have been realized. In spite of the remarkable advancements, these fascinating research findings are relatively scattered. To date, there is still a lack of a systematic and profound summarization regarding this fast-evolving domain. This is not beneficial to researchers, especially researchers just entering this research field, who want to have a quick, timely, and comprehensive inspection of this fascinating domain. To address this issue, in this review, the emerging downstream applications of 2DLM photodetectors in extensive fields, including imaging, health monitoring, target tracking, optoelectronic logic operation, ultraviolet monitoring, optical communications, automatic driving, and acoustic signal detection, have been systematically summarized, with the focus on the underlying working mechanisms. At the end, the ongoing challenges of this rapidly progressing domain are identified, and the potential schemes to address them are envisioned, which aim at navigating the future exploration as well as fully exerting the pivotal roles of 2DLMs towards the practical optoelectronic industry.

Tunable structured AlGaN-based nanoporous distributed Bragg reflectors for light-coupling enhancement in monolayer MoS2

In this work, we demonstrated tunable structured AlGaN-based nanoporous (NP) distributed Bragg reflectors (DBRs) with high-reflectivity through one-step vertical-lateral electrochemical etching (ECE). The aperture of nanopores in the doping layer and the size and density of V-pits on the surface of NP-DBRs can be tuned by controlling the etching bias. Furthermore, by further tuning the thickness of alternating layers, we achieved high reflectivity of NP-DBRs at multiple center wavelengths within the wavelength range from 280 to 800 nm. To explore its potential applications, MoS2 with a photoluminescence (PL) emission in the red light region was grown for the first time on the NP-DBRs. Due to the light localized effect of surface V-pits and the high reflectivity of the bottom DBRs, the PL intensity of MoS2 was significantly enhanced by a factor of 37.3 compared to the region without NP-DBRs. The PL enhancement of MoS2 on porous AlGaN and different periodic NP-DBRs further validated the PL enhancement mechanism. The corresponding theoretical simulations also revealed that the PL enhancement should be attributed to the combined effect of enhanced light absorption and NP-DBRs induced emission interference. The fabricated AlGaN-based NP-DBRs provide a new approach for improving the quantum efficiency of two-dimensional layered materials, which holds significant importance for advancing the development of miniaturized and integrated 2D/3D heterogeneous structure systems in the field of optoelectronics.

The enhanced adsorption mechanism of ethanol on (Fe-Zn) bimetallic-doped Ti2CO2 monolayers: a DFT study

Ti2C as a new two-dimensional layered MXene material has a certain potential in the sensor field. The adsorption properties of gases (NH3, NO, ethanol, CO) on intrinsic Ti2CO2 and transition metal atoms (Fe, Zn) doped Ti2CO2 were studied by density functional theory systematically. The geometric structure, adsorption energy, molecular dynamics, energy band structure, density of states, charge transfer, and differential charge density of these molecules at five sites of intrinsic Ti2CO2, Fe-doped Ti2CO2, Zn-doped Ti2CO2, FeZn-adjacent co-doped Ti2CO2 and FeZn-pair co-doped Ti2CO2 were analyzed. The results show that the activity of the substrate can be improved by the introduction of Fe-Zn transition metals. The high adsorption energy of ethanol gas at (A) FeZn site is −1.98eV with the short adsorption distance. The high charge transfer between the metal co-doped Ti2CO2 substrate and ethanol gas was also found. The results suggest that the Fe-Zn bimetallic-doped Ti2CO2 monolayer may be a potential sensing material for the detection of ethanol gas.

Room-Temperature, Current-Induced Magnetization Self-Switching in A Van Der Waals Ferromagnet.

2D layered materials with broken inversion symmetry are being extensively pursued as spinsource layers to realize high-efficiency magnetic switching. Such low-symmetry layered systems are, however, scarce. In addition, most layered magnets with perpendicular magnetic anisotropy show a low Curie temperature. Here, the experimental observation of spin-orbit torque magnetization self-switching at room temperature in a layered polar ferromagnetic metal, Fe2.5 Co2.5 GeTe2 is reported. The spin-orbit torque is generated from the broken inversion symmetry along the c-axis of the crystal. These results provide a direct pathway toward applicable 2D spintronic devices.

Highly selective and ultra-sensitive gas sensor based on Fe2O3/Ti3C2Tx MXene heterostructure for ppb-level n-butanol detection

The sensitive and rapid detection of toxic gases, particularly at concentrations as low as parts per billion (ppb), is crucial in preventing explosion and gas leakage accidents. Herein, we have developed a ppb-level n-butanol sensor by constructing a heterojunction structure through in situ growth of n-type oxide semiconductor Fe2O3 nanoparticles on two-dimensional layered carbide materials Ti3C2Tx MXene. The obtained results from that the Fe2O3/MXene-1% sensors display ultra-low detection limit, exhibiting a 1.31 response towards 70 ppb n-butanol. In addition, the Fe2O3/MXene-1% sensors show a remarkable response of 83.7 towards 100 ppm n-butanol at 150 °C, which represents a 22-fold improvement when comparison with the pure Fe2O3 sensors. The sensors also exhibit brilliant selectivity, reproducibility and excellent long-term stability. The remarkable enhancement in gas sensitive performance depends on Schottky heterojunctions formed between metal oxides and Ti3C2Tx MXene and the synergistic effect. The incorporation of MXene decreases materials’ resistance and enhances the carrier transfer rate. The design of this study provides a novel approach for achieving ppb-level n-butanol detection.

Exploring charge transfer mechanisms and optical properties in vdW heterostructures of MoS2 and Bi2Se3 at nanoscale regime

We report here the two-dimensional layered material of MoS2/Bi2Se3 vdW heterostructures formed by ultrasonication of different concentrations of exfoliated MoS2 and Bi2Se3 nanosheets. The vdW heterostructures show unique optical characteristics compared to individual exfoliated nanosheets. The enhanced lifetime of heterostructures explained the charge transfer with increasing efficacy for producing optoelectronic devices. Raman spectroscopy confirms the presence of intrinsic Raman active modes of Bi2Se3, MoS2, and respective modes of vdW heterostructures. TEM images show characteristic lattice planes in both materials and clear overlapping regimes, confirming successful heterojunction formation. Further, Zeta potential confirms the enhanced stability of the heterostructures formed while the significant PL quenching, strongly suggests the presence of charge transfer processes within the heterostructures. Ultrafast pump-probe spectroscopy further confirms the non-linear charge transfer and relaxation mechanism of the heterostructures. We employed Surface Xplorer and Glotaran software for model fitting and Glotaran's global fitting revealed unresolved charge carrier dynamics.

Type-II CoMoO4/Graphdiyne heterojunction promotes visible-light-driven photocatalytic hydrogen production activity

Photocatalytic hydrogen production technology has great challenges in designing and synthesizing stable photocatalyst and inhibiting high recombination rate of carriers. In this paper, CoMoO4/GDY type-II heterojunctions were successfully constructed by coupling a new two-dimensional layered carbon material graphdiyne (GDY) with molybdate (CoMoO4), and the positive effect of GDY on photocatalytic hydrogen production system was studied. The introduction of GDY provides a large number of reaction sites for the reaction. Moreover, due to the strong light absorption and excellent electrical conductivity of GDY, the photoelectrochemical properties of molybdate (CoMoO4) are enhanced. The construction of type-II heterojunctions effectively promotes the spatial separation of photogenerated electrons and holes. The electron transfer mechanism of the photocatalyst was verified based on DFT and in-situ irradiation X-ray photoelectron spectroscopy. The experimental results show that the photocatalytic performance of CoMoO4/GDY photocatalyst is the best (121 μmol), which is 2 times and 45 times that of CoMoO4 and GDY, respectively. This study provides a unique idea for the application of GDY in the field of photocatalysis.

Studying the Adsorption of Gas Molecules and Defects on Modulating the Electronic Transport Characteristics of Monolayer Penta-BN2-Based Devices.

Two-dimensional atomic layer materials, as an important part of the post-Moore era, have recently become an ideal choice for the preparation of high-efficiency, low-power, and miniaturized gas sensors. In this work, our study utilized density functional theory and the nonequilibrium Green's function method to investigate the electronic properties of the pentagonal BN2 (P-BN2) monolayer, as well as its gas-sensing properties for organic and inorganic gases. We also investigated how defects affect the quantum transport properties of the P-BN2-based device. Our findings demonstrate that the CO, H2S, NH3, SO2, C2H5OH, C3H6OH, CH3OH, and CH4 undergo physisorption on the P-BN2 monolayer, while NO, NO2, C2H2, C2H4, and HCHO undergo chemisorption. Then, we analyzed the impact of gas molecules chemisorbed on the P-BN2 monolayer on the electronic transport properties of the P-BN2-based gas sensor. When these five gas molecules are adsorbed, the current of the P-BN2-based gas sensor is greatly reduced. In addition, the effect of defects on the quantum transport properties of the P-BN2-based device is investigated. The results indicate that defects of N, B, and BN atoms lead to a decrease in the current of P-BN2-based nanodevices. Moreover, both the adsorption of gas molecules and the formation of vacancy defects leading to a decrease in device current can be revealed by the local device density of states near the zero-bias Fermi level, elucidating their microscopic mechanisms. Finally, gas molecules can also cause a decrease in the current of defect systems. These theoretical studies are of great significance for exploring two-dimensional atomic layer materials as high-efficiency gas sensors.

Two-dimensional layered materials for modifying solid-state electrolytes in lithium batteries via interface engineering

Lithium-ion batteries have been widely used in mobile electronic devices and electric vehicles, but the safety issues of lithium-ion batteries have become increasingly prominent owing to the risk of leak out and explosion with liquid electrolytes. Solid-state electrolytes (SSEs) with high ionic conductivity and low cost are considered as one of the most attractive alternatives to replacing liquid electrolytes. However, the poor interfacial compatibilities of SSE in lithium batteries lead to the failure, which severely hinders their development. Recent studies have shown that two-dimensional (2D) layered materials can improve the interface stability of SSE to lithium metal. In this Review, we will mainly introduce the fundamentals of SSEs and interface issues, and then summarize 2D materials-based modification strategy for SSEs. Recent advances have been highlighted in the aspects of using 2D materials for improving the interface of SSEs, which are very promising for energy applications. The availability of the 2D materials offers a rich playground, which not only improves the electrochemical performance of solid-state lithium batteries, but also conceives deep understanding the mechanism of interface modulation. Lastly, the perspective is provided to address the current challenges and issues of SSEs in lithium batteries.

Macroscale ultradurable superlubricity on passivated transition-metal diborides

Superlubricity is a sliding regime with almost vanishing friction sustained under stringent material conditions. This remarkable phenomenon was first found between surfaces with incommensurate lattice structures, and later extended to materials with weak shear, as in many two-dimensional layered materials. Such materials, however, face great challenges that limit their applications, including the difficulty to scale up to meet the needs of many practical implementations and low durability stemming from the intrinsic structural weakness. Overcoming these limitations hinges on new design and fabrication of advanced superlubricating materials. Here, we report on the realization of macroscale ultradurable superlubricity on passivated transition-metal diborides (TMB2) following an unconventional design principle that first construct materials with strong anti-wear ability then create superlubricity via effective surface passivation. TMB2 possess superior mechanical strength and durability, along with the ability to activate in-situ load-induced passivation layers via tribochemical actions at the surface. The resultant passivated TMB2 exhibits characteristic superlubricating and ultradurable friction coefficient (0.002) and wear rate (10−19 m3/N·m). These findings introduce a fresh approach toward rational design and implementation of superlubricating materials, opening a new avenue for versatile applications at large (up to industrial) scales.

Nitrogen-doped carbon quantum dots assembled on layered Ti3C2Tx exhibit excellent tribological properties

Nitrogen-doped carbon quantum dots (NCQDs) as a lubricant additive, is limited by its single tribological mechanisms and difficult to surface functionalization. Therefore, it will be anticipated to combine NCQDs with two-dimensional layered materials, so as to improve lubrication and interface bonding performance. Due to the active groups existing on the surface of Ti3C2Tx, the NCQDs is assembled on the two-dimensional material Ti3C2Tx, thus the NCQDs@Ti3C2Tx composite can be used as a favorable lubricant additive. The NCQDs@Ti3C2Tx composite can exert synergistic lubrication and chemical reactions during the friction process in comparison to NCQDs as lubricant additives, resulting in forming a friction transfer film between the surface. Therefore, this study confirms that NCQDs@Ti3C2Tx composite exhibits enormous potential as a lubricant additive.

Rotation angle dependent Li intercalation and the induced phase transition in bilayer MoTe2

In this work, taking twisted bilayer MoTe2 (tb-MoTe2) as an example, density functional theory (DFT) calculations are done to investigate the interlayer rotation angle dependent lithium intercalation as well as the induced 2H → 1T phase transition in two-dimensional layered materials. Both static equipotential energy diagrams and dynamic ab initio molecular dynamics simulations (AIMD) are utilized to obtain the reliable lithium diffusion pathways and barriers. The results reveal that the interlayer rotation leads to increased interlayer separation in tb-MoTe2, and provides a stable environment for Li insertion. Moreover, the diffusion barrier is substantially reduced with increasing twist angle, promoting the fast Li diffusion. At room temperature, the mobility of Li in tb-MoTe2 with the twist angle of θ = 21.79˚ and θ = 60˚ is increased by a factor of 10 and 105, respectively. The energy barrier for the 2H → 1T phase transition is lowered from 1.23 eV in the non-twisted MoTe2 to 0.88 eV in the tb-MoTe2 with a twist angle θ = 21.79˚ for a given concentration of inserted Li. Above all, the enhanced diffusion and lower critical electron concentration facilitate the 2H-1T phase transition in tb-MoTe2. The results provide valuable insights into the non-uniform intercalation and present an opportunity to modulate the phase transition of twisted 2D systems.

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Advances in the two-dimensional layer materials for cancer diagnosis and treatment: unique advantages beyond the microsphere.

In recent years, two-dimensional (2D) layer materials have shown great potential in the field of cancer diagnosis and treatment due to their unique structural, electronic, and chemical properties. These non-spherical materials have attracted increasing attention around the world because of its widely used biological characteristics. The application of 2D layer materials like lamellar graphene, transition metal dichalcogenides (TMDs), and black phosphorus (BPs) and so on have been developed for CT/MRI imaging, serum biosensing, drug targeting delivery, photothermal therapy, and photodynamic therapy. These unique applications for tumor are due to the multi-variable synthesis of 2D materials and the structural characteristics of good ductility different from microsphere. Based on the above considerations, the application of 2D materials in cancer is mainly carried out in the following three aspects: 1) In terms of accurate and rapid screening of tumor patients, we will focus on the enrichment of serum markers and sensitive signal transformation of 2D materials; 2) The progress of 2D nanomaterials in tumor MRI and CT imaging was described by comparing the performance of traditional contrast agents; 3) In the most important aspect, we will focus on the progress of 2D materials in the field of precision drug delivery and collaborative therapy, such as photothermal ablation, sonodynamic therapy, chemokinetic therapy, etc. In summary, this review provides a comprehensive overview of the advances in the application of 2D layer materials for tumor diagnosis and treatment, and emphasizes the performance difference between 2D materials and other types of nanoparticles (mainly spherical). With further research and development, these multifunctional layer materials hold great promise in the prospects, and challenges of 2D materials development are discussed.

Enhanced in-plane thermoelectric properties achieved through the vertical van der Waals stacking of black phosphorus and Ti2C

Vertical van der Waals heterostructures (vdWHs) exhibit considerable freedom in integrating two-dimensional layered materials for achieving excellent performance. In this study, we construct a double-layer Ti2C/BP vdWH by stacking pristine monolayer black phosphorus (BP) and Ti2C vertically. The in-plane thermoelectric properties of BP, Ti2C and Ti2C/BP vdWHs at different temperatures are investigated using density functional theory combined with the non-equilibrium Green's function method. Results showed that the thermoelectric figure of merit (ZT) of Ti2C/BP vdWHs at room temperature (300 K) was approximately 103 and 104 times that of Ti2C and BP, respectively. Because of the mutual compensation of the electronic energy bands and interfacial phonon scattering, Ti2C/BP vdWHs have higher electron conductance and lower phonon conductance than both BP and Ti2C, which significantly enhances the ZT value of these structures. More importantly, the prominent thermoelectric properties of Ti2C/BP vdWHs maintain greater stability as the temperature rises (300–900 K), which leads to a high thermoelectric power factor and ZT value without relying on high temperatures. These results provide a new idea for designing two-dimensional functionalised nanoscale devices with excellent thermoelectric performance.

Observation of spatial self-phase modulation excited by off-axis integer and fractional vortex beams

Spatial self-phase modulation (SSPM) is widely used to characterize the nonlinear refractive index of two-dimensional layered materials and design passive nonlinear photonic devices. In this work, we investigate the SSPM phenomena of black phosphorus dispersed in agarose solid under the excitation of on-axis and off-axis integer and half-integer vortex beams. It is found that with the increase of the off-axis distance of the Gaussian vortex beam, the pattern of the far-field self-diffraction intensity gradually breaks from the initial ring and finally evolves into a tail outside the self-diffraction ring. Moreover, for half-integer vortex beams, the size of the innermost tail of the self-diffraction ring is nearly half smaller than that of the second outer tail. The underlying mechanism of the observed SSPM phenomenon is also analyzed. Interestingly, the trailing number and rotation direction of the self-diffraction ring pattern quantitatively reflect the magnitude and sign of the topological charge (TC) of the off-axis vortex beam, respectively. This work provides a novel method to measure the TC of off-axis vortex beam in the nonlinear optics regime.

A potentially efficient ORR catalyst: Transition metal-doped 2D Haeckelite MoS2

Enhancing the efficiency of the proton exchange membrane fuel cell (PEMFC) relies significantly on the development of low-cost but highly efficient catalysts for oxygen reduction reaction (ORR). In the present work, a specially structured two-dimensional layered material called Haeckelite (HL) MoS2 is investigated as an electrocatalyst for ORR by using density functional theory (DFT). In order to further improve the ORR catalytic performance, transition metals (TMs = Mn, Ru, Rh and Os) are independently doped on the surface of HL-MoS2. The detailed calculation includes the TMs doping, O2 molecule adsorption, electronic structure analysis, and Gibbs free energy comparison. The final results demonstrate that all TM/HL-MoS2 systems exhibit metallic properties, especially Ru/HL-MoS2 has the best catalytic performance for ORR in view of its moderate adsorption energy of O2 molecule (≈ 0.57 eV) and the lowest overpotential (≈ 0.503 V). Therefore, the combination of defect structure and TMs doping in MoS2 can be regarded as an effective approach to achieve both enhancement of catalytic performance and reduction of consumption of precious metals in the oxygen reduction reaction.

Advanced Two-Dimensional Layered Compound-Based Materials for High Performance Lithium/Sodium-Ion Capacitors

Advanced Two-Dimensional Layered Compound-Based Materials for High Performance Lithium/Sodium-Ion Capacitors

First-principles study on the electronic structure and catalytic properties of two-dimensional MX2N4 systems (M = Ti, Zr; X = Si, Ge)

Recently, the two-dimensional MA2Z4 family with a seven-atom layer structure (experimentally synthesized a MoSi2N4 monolayer), which has good stability and exhibits semiconductivity, providing a platform for photocatalytic hydrolysis studies of two-dimensional polyatomic layer materials. In this work, we systematically investigated the thermal stability, dynamical stability, mechanical stability, electronic properties, and HER performance of the MX2N4 systems (M = Ti, Zr; X = Si, Ge). All monolayers exhibit excellent hydrogen evolution reaction (HER) performance (The Gibbs free energy (ΔGH*) of TiSi2N4; ZrSi2N4; ZrGe2N4 are 0.11 eV; 0.05 eV; −0.03 eV, respectively) and meet the thermodynamic requirements of OER for photocatalytic. Furthermore, the effects of biaxial strain and pH value on ZrGe2N4 monolayer photocatalytic hydrolysis performance were studied. ZrGe2N4 monolayer with 1% compression strain has well HER performance (ΔGH∗=0.21eV) under light conditions (Ue = 0.04 eV, pH = 0), and the photocatalytic oxidation process of water can take place spontaneously under light conditions and pH ≥ 7. In addition, the ZrGe2N4 monolayer also has excellent absorption in the visible range (high absorption coefficients over 105 cm−1). Undoubtedly, the potential applications and properties of MX2N4 monolayers in the fields of electrocatalysis and photocatalysis are poised to expand significantly. These findings hold great promise and are expected to offer valuable insights for experimental research in these areas.

Synthesis of 2D type A molecular sieves by alkaline removal of natural kaolin

Type A molecular sieves have been extensively employed in various fields. It is noteworthy that the direct synthesis of type A molecular sieves from natural kaolin is a common practice among researchers. Traditional type A molecular sieves are characterized by a three-dimensional cubic lattice structure. In this study, our objective is to facilitate the transformation of type A molecular sieves into a two-dimensional layered configuration by employing a two-dimensional layered material as a templating agent. In this research, natural kaolin serves as the primary source material. To eliminate impurities including quartz, illite, and dolomite, an alkali-based solvent extraction method is employed, yielding amorphous silicon and aluminum compounds. Subsequently, a graphene-based templating agent is introduced, and a hydrothermal synthesis process is employed to fabricate two-dimensional type A molecular sieves. The method described herein yields two-dimensional layered type A molecular sieves with a crystallinity exceeding 90%, thereby resulting in a specific surface area that is approximately 11-fold greater compared to their three-dimensional type A counterparts. The applicability of this methodology can be extended to the valorization of low-grade natural mineral resources, optimizing their utility. Furthermore, the approach presented herein for the synthesis of two-dimensional molecular sieves is of a universal nature, offering valuable insights that can serve as a reference for the synthesis of various other categories of two-dimensional molecular sieves.