Layered 2D Materials Research Articles

Layered 2D Nanomaterials to Tailor Friction and Wear in Machine Elements—A Review

AbstractRecent advances in 2D nanomaterials, such as graphene, transition metal dichalcogenides, boron nitride, MXenes, allow not only to discover several new nanoscale phenomena but also to address the scientific and industrial challenges associated with the design of systems with desired physical properties. One of the great challenges for mechanical systems is associated with addressing friction and wear problems in machine elements. In this review, the beneficial properties of layered 2D materials that enable the control of their tribological behavior and make them excellent candidates for efficient friction and wear reduction in dry‐running and boundary lubricated machine components are summarized. The recent studies highlighting the successful implementation of 2D structures when used as solid lubricant coatings or reinforcement phases in composites for various machine components including sliding and rolling bearings, gears, and seals are overviewed. The examples presented in the studies demonstrate the great potential for 2D materials to address the energy‐saving needs by friction and wear reduction.

Interlayer Quasi-Bonding Interactions in 2D Layered Materials: A Classification According to the Occupancy of Involved Energy Bands.

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

Lattice Orientation Heredity in the Transformation of 2D Epitaxial Films.

The ability to control lattice orientation is often an essential requirement in the growth of both 2D van der Waals (vdW) layered and nonlayered thin films. Here, a unique and universal phenomenon termed "lattice orientation heredity" (LOH) is reported. LOH enables product films (including 2D-layered materials) to inherit the lattice orientation from reactant films in a chemical conversion process, excluding the requirement on the substrate lattice order. The process universality is demonstrated by investigating the lattice transformations in the carbonization, nitridation, and sulfurization of epitaxial MoO2 , ZnO, and In2 O3 thin films. Their resultant compounds all inherit the mono-oriented crystal feature from their precursor oxides, including 2D vdW-layered semiconductors (e.g., MoS2 ), metallic films (e.g., MXene-like Mo2 C and MoN), wide-bandgap semiconductors (e.g., hexagonal ZnS), and ferroelectric semiconductors (e.g., In2 S3 ). Using LOH-grown MoN as a seeding layer, mono-oriented GaN is achieved on an amorphous quartz substrate. The LOH process presents a universal strategy capable of growing epitaxial thin films (including 2D vdW-layered materials) not only on single-crystalline but also on noncrystalline substrates.

Naturally occurring van der Waals heterostructure lengenbachite with strong in-plane structural and optical anisotropy

Lengenbachite is a naturally occurring layered mineral formed with alternating stacks of two constituent PbS-like and M2S3-like two-dimensional (2D) material layers due to the phase segregation process during the formation. Here, we demonstrate to achieve van der Waals (vdW) heterostructures of lengenbachite down to a few layer-pair thickness by mechanical exfoliation of bulk lengenbachite mineral. The incommensurability between the constituent isotropic 2D material layers makes the formed vdW heterostructure exhibit strong in-plane structural anisotropy, which leads to highly anisotropic optical responses in lengenbachite thin flakes, including anisotropic Raman scattering, linear dichroism, and anisotropic third-harmonic generation. Moreover, we exploit the nonlinear optical anisotropy for polarization-dependent intensity modulation of the converted third-harmonic optical vortices. Our study establishes lengenbachite as a new natural vdW heterostructure-based 2D material with unique optical properties for realizing anisotropic optical devices for photonic integrated circuits and optical information processing.

2D-2D MXene/ReS2 hybrid from Ti3C2Tx MXene conductive layers supporting ultrathin ReS2 nanosheets for superior sodium storage

Rhenium disulfide (ReS2) is a promising 2D-layered anode material for rechargeable batteries. However, this material suffers from severe agglomeration and volume expansion during ions insertion/extraction, leading to inferior battery performance. Building 3D hierarchical structure is an effective strategy to overcome these problems and simultaneously manifest synergistic effects. Herein, a one-pot hydrothermal method is introduced to construct N-doped carbon confined ReS2 ultrathin nanosheets anchored on few-layered Ti3C2Tx MXene substrate (MXene@ReS2@C). The MXene flakes can effectively reduce agglomeration of ReS2 nanosheets during charge/discharge process and generate chemical interaction with ReS2 nanosheets, thereby promoting charge transfer kinetic of hierarchical composite. Carbon incorporation not only stabilizes MXene from successive oxidation during hydrothermal procedure but also alleviates volume expansion of ReS2, ensuring cycling durability of electrode. When used as anode material of SIBs, this hierarchical MXene@ReS2@C hybrid with strong interfacial interactions demonstrates improved electrochemical performances with satisfying rate capability (138 mAh g−1 at 5.0 A g−1) and cycling stability (202 mAh g−1 at 2.0 A g−1 after 200 cycles). This work provides a new scope of synthesizing MXene-based transition metal sulfide composite for practical application in sodium ion batteries.

Hybrid van der Waals heterojunction based on two-dimensional materials

Since the discovery of graphene, two-dimensional (2D) layered materials have always been the focus of material research. The layers of 2D materials are covalent bonds, and the layers are weakly bonded to adjacent layers through van der Waals (vdW) interactions. Since any dangling-bond-free surface could be combined with another material through vdW forces, the concept can be extended. This can refer to the integration of 2D materials with any other non-2D materials through non-covalent interactions. The emerging mixed-dimensional (2D+nD, where n is 0, 1 or 3) heterostructure devices has been studied and represents a wider range of vdW heterostructures. New electronic devices and optoelectronic devices based on such heterojunctions have unique functions. Therefore, this article depicts the research progress of (2D+nD, where n is 0, 1 or 3) vdW heterojunctions based on 2D materials.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices.

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano‐electronics and nano‐optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed‐laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up‐to‐date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up‐to‐date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

Limitations and Benefits of MAX Phases in Electroanalysis

AbstractMAX phases are a group of layered 2D materials made of early transition metal, A‐group element (e.g., Al, Sn or Si), and C or N. These nanolaminated carbides and nitrides combine many attractive characteristics of metals and ceramics such as excellent electric and thermal conductivity and high chemical resistance. Although MAX phases have shown promising electrochemical results in the field of energy conversion, their use for electroanalytical approaches is nowadays an unexplored field. Herein, the potential use of MAX phases for electroanalytical approaches has been investigated. For this aim, seven different MAX phases (Cr2AlC, Mo2AlC, Ta2AlC, Ti2AlN, Ti2SnC, Ti3AlC2, Ti3SiC2, and V2AlC) have been drop‐casted upon a conventional glassy‐carbon electrode and tested at different pH media, also providing their potential towards the determination of different analytes. Overall, our findings elucidate the limitations and benefits of using MAX phases for electroanalysis, demonstrating that a proper combination of both MAX phases and electrolyte media is a must to direct efficient performances as electrode for electroanalysis. Accordingly, this work provides new knowledge about the electrochemical behaviour of MAX phases and their potential in the field of electronic devices.

Steering on Degrees of Freedom of 2D Van der Waals Heterostructures

2D heterostructures have garnered tremendous attention for potential applications in electronics and optoelectronics. Heterostructures can be constructed by assembling individual atomically thin layers of 2D materials into integrated devices, which involves three primary degrees of freedom (DOFs), i.e., Lego‐like basic building blocks, out‐of‐plane stacking order, and in‐plane twist‐angle alignment. By steering the DOFs of 2D materials, devices and structures such as artificial Shockley junction, quantum wells, and superlattices can be conveniently established based on well‐developed fabrication and/or assembly techniques, beneficial for next‐generation ultracompact information technologies. Herein, the recent progress on constructing the artificial atomic structures by taking advantage of three primary DOFs is overviewed. An outlook of the challenges and future developments is presented as well. Future advancements in the rational construction of complex devices and artificial heterostructures are also suggested.

Biogenic and Non-Biogenic Waste Utilization in the Synthesis of 2D Materials (Graphene, h-BN, g-C2N) and Their Applications

There is a significant amount of waste generated which creates a huge environmental issue for humanity/earth and a tremendous number of varieties of resources of a different kind are needed globally. In this context, nanoscience technology has shown its potential ability to solve the above issues and provides realistic applications and devices. The beauty of nanotechnology is its multidisciplinary approach, in which green nanotechnology has been translated to focus on waste materials. Waste materials are generally generated from biogenic (rice husk, dead leaves, waste food, etc.) and non-biogenic (several types of plastics waste, lard oil, etc.) materials produced from municipal or industrial waste. Currently, a large number of efforts have been made to utilize the waste materials for the synthesis of 2D materials in a greener way. This green synthetic approach has two advantages 1) it reduces the cost of synthesis and 2) includes minimal use of hazardous chemicals. Biogenic wastes (contains biomolecules) contain several significant constituents such as co-enzymes, enzymes, proteins, terpenoids, etc. These constituents or biomolecules are known to play an energetic role in the formation of a different variety of 2D materials and hence control the protocols of green synthesis of 2D materials. This review focuses on the exploration of the current understanding of 2D-layered material synthesis methods using waste material produce from biogenic and non-biogenic waste. It also investigates the applications of various 2D-layered materials in perspective with synthesis from waste and future challenges along with their limitations to industrial-scale synthesis.

Controllable Doping in 2D Layered Materials.

For each generation of semiconductors, the issue of doping techniques is always placed at the top of the priority list since it determines whether a material can be used in the electronic and optoelectronic industry or not. When it comes to 2D materials, significant challenges have been found in controllably doping 2D semiconductors into p- or n-type, let alone developing a continuous control of this process. Here, a unique self-modulated doping characteristic in 2D layered materials such as PtSSe, PtS0.8 Se1.2 , PdSe2 , and WSe2 is reported. The varying number of vertically stacked-monolayers is the critical factor for controllably tuning the same material from p-type to intrinsic, and to n-type doping. Importantly, it is found that the thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies to anion vacancies based on dynamic and thermodynamic analyses, which leads to p- and n-type conductance, respectively. By thickness-modulated doping, WSe2 diode exhibits a high rectification ratio of 4400 and a large open-circuit voltage of 0.38V. Meanwhile, the PtSSe detector overcomes the shortcoming of large dark-current in narrow-bandgap optoelectronic devices. All these findings provide a brand-new perspective for fundamental scientific studies and applications.

Gate-controlled quantum dots in monolayer WSe2

Quantum confinement and manipulation of charge carriers are critical for achieving devices practical for quantum technologies. The interplay between electron spin and valley, as well as the possibility to address their quantum states electrically and optically, makes two-dimensional (2D) transition metal dichalcogenides an emerging platform for the development of quantum devices. In this work, we fabricate devices based on heterostructures of layered 2D materials, in which we realize gate-controlled tungsten diselenide (WSe2) hole quantum dots. We discuss the observed mesoscopic transport features related to the emergence of quantum dots in the WSe2 device channel, and we compare them to a theoretical model.

Fabrication of permanent self-lubricating 2D material-reinforced nickel mould tools using electroforming

In the replication of polymeric micro/nano structures, adhesion and friction between mould tool and polymer cause significant feature failure. For the first time, this work developed a novel strategy for the fabrication of high-hardness and self-lubricating 2D material-reinforced nickel mould tools using electroforming to achieve high precision replication of polymeric micro structures. In this study, layered 2D material, including graphene oxide (GO), molybdenum disulfate (MoS2), and tungsten disulfide (WS2), for the fabrication of self-lubricating mould tools, were systematically studied. Our results demonstrated that nickel/WS2 mould tools, followed by nickel/GO and nickel/MoS2 mould tools, presented the most significant microhardness improvement. A maximum microhardness of ∼660 HV together with a minimum crystallite size of 12 nm was achieved from 0.5 g/L WS2, indicating a 3.67 times microhardness increase and 3 times crystallite size reduction relative to the pure nickel mould tool. The enhanced microhardness can be attributed to the 2D material-induced crystal refinement, and inherent hardness and incorporation content of 2D material. Additionally, friction and wear tests revealed that a low concentration of WS2 at 0.14 g/L achieved the lowest coefficient of friction (COF) and superior wear resistance. The COFs in the initial stage and steady state stage were 0.08 and 0.18, respectively, implying decreases of 42.8% and 72.3%, respectively, and a 27-fold increase in lifetime compared with those of the pure nickel mould tool. Such a significant improvement in tribological properties was due to the formation of self-lubricating transfer film by the interlayer shear effect of few-layered 2D material nanosheets. Finally, defect-free polymeric microfluidic chips were micro hot embossed using an optimal self-lubricating nickel/WS2 mould tool for validation. This work provides significant insight into the fabrication of potential self-lubricating micro/nano mould tools for microfluidics applications.

Two-dimensional coordination polymers containing permethylated motifs - promising candidates for 2D emerging materials. Structural, behavioral and functional particularities

Although three-dimensional metal-organic frameworks (3D MOFs) currently enjoy great attention in the scientific world due to their utility to address challenges concerning energy, environment cleaning and not only, very recently two-dimensional version of these, rather considered two-dimensional coordination polymers (2D CPs), have become of interest from the perspective of their approach as ultra-thin layered 2D materials. Different from the emerging 2D materials that are generally inorganic in nature (graphene or 2D allotropes of various elements, MXenes, etc.), 2D CPs contain both inorganic and organic blocks, offering wider possibilities for structural diversification and area of applicability. However, in the crystallization process, 2D CP layers often stack on the basis of intermolecular interactions, leading to higher dimensional materials, which limit the manifestation of the size-effect properties. Their isolation individually or in multilayers with thickness/surface aspect ratio as small as possible is a challenge that seeks solutions through top-down or bottom-up approaches, each of them having certain limits. An original pathway consists in the use of a permethylated ligand with lower surface energy which, through its orientation in the coordination process to the metal ion, favors the anisotropic growth of the polymer and ensures the shielding of the formed nanosheet structure preventing the establishment of noticeable interlayer interactions. Such unique two-dimensional coordination polymers recently reported by the authors and some new data about them are reviewed here from the perspective of their structural particularities, properties, reactivity, functionality, and reusability/recyclability potential.

Calcination effects of 2D molybdenum disulfides

Designing earth abundant, cost effective 2D-layered materials capable replace graphene and precious metal nanoparticulated arrays in optoelectronic and catalytically working devices are among global challenges. The key factor for these applications is thermostability of MoS2 layered nanostructures. Herein, calcination effects of hydrothermally synthesized MoS2 and commercial MoS2 crystal as well as 1.7 and 6.0 μm-sized MoS2 powders were studied in the synthetic air and argon gas environments by thermogravimetric and differential thermal analysis, mass spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. Markedly lower thermal stability of hydrothermally synthesized 2D MoS2 species both in the synthetic air and Ar environments compared to MoS2 crystal and commercial powders have been established and transformations up to 1000 °C estimated. It was determined that oxidation of hydrothermally synthesized MoS2 films in the synthetic air starts below 200 °C and results in a transformation of nanoplatelet-shaped films to MoO3 sheets and nanowires at higher temperatures. The annealing in argon atmosphere, however, resulted in the MoS2 crystallinity increase, S/Mo ratio reduction, and temperature-dependent transition of 3R-MoS2 phase to 2H-MoS2 no influencing markedly the morphology of nanoplatelets up to 520 °C.

Preparation and Research of Monolayer WS2 FETs Encapsulated by h-BN Material.

Functional devices that use vertical van der Waals (vdWs) heterostructure material can effectively combine the properties of single component materials, and the strong interlayer coupling effect can change their electronic and optical properties. According to our research, WS2/h-BN vertical vdWs heterostructure material can be synthesized by chemical vapor deposition (CVD) and wet transfer methods. Monolayer WS2 material and WS2/h-BN vertical vdWs heterostructure material can be tested and characterized using XPS, SEM, EDS, AFM and Raman spectroscopy, which can prove the existence of corresponding materials. When the thickness of the material decreases, the Coulomb scattering amongst two-dimensional (2D) layered materials increases. This is because both the shielding effect and the distance between the channel and the interface layer decrease. FET devices are then fabricated on WS2/h-BN vdWs heterostructure material by the electron beam lithography and evaporation processes. The effects of vdWs epitaxy on electrical transmission when WS2/h-BN vdWs heterostructure material is formed are explored. Finally, the related electrical performance of FET devices is tested and analyzed. Our experimental research provides guidance for the use of electronic devices with vdWs heterostructure material.

Robust type-II BP/AlN van der Waals heterostructure: A first-principles study

To achieve photoelectric appliance, effective carrier separation is a vital question. Here, we construct van der Waals (vdW) heterostructure by superimposing two different two-dimensional (2D) layer materials BP and AlN via vdW force. The results show that 2D BP/AlN heterostructure exhibits direct band gap characteristics and has a type-II band alignment, which accelerates the effective separation of electron-hole pairs. In addition, under the modulation of applied external electric field and biaxial strain, this heterostructure still keeps a type-II band alignment. This result reveals that 2D BP/AlN heterostructure shows a wide application prospect in the future photoelectric nanodevices.

Highly Dispersed Ru Nanoparticles on Boron‐Doped Ti3C2Tx (MXene) Nanosheets for Synergistic Enhancement of Electrocatalytic Hydrogen Evolution

2D-layered materials have attracted increasing attention as low-cost supports for developing active catalysts for the hydrogen evolution reaction (HER). In addition, atomically thin Ti3 C2 Tx (MXene) nanosheets have surface termination groups (Tx : F, O, and OH), which are active sites for effective functionalization. In this work, heteroatom (boron)-doped Ti3 C2 Tx (MXene) nanosheets are developed as an efficient solid support to host ultrasmall ruthenium (Ru) nanoparticles for electrocatalytic HER. The quantum-mechanical first-principles calculations and electrochemical tests reveal that the B-doping onto 2D MXene nanosheets can largely improve the intermediate H* adsorption kinetics and reduce the charge-transfer resistance toward the HER, leading to increased reactivity of active sites and favorable electrode kinetics. Importantly, the newly designed electrocatalyst based on Ru nanoparticles supported on B-doped MXene (Ru@B-Ti3 C2 Tx ) nanosheets shows a remarkable catalytic activity with low overpotentials of 62.9 and 276.9mV to drive 10 and 100mA cm-2 , respectively, for the HER, while exhibiting excellent cycling stabilities. Moreover, according to the theoretical calculations, Ru@B-Ti3 C2 Tx exhibits a near-zero value of Gibbs free energy (ΔGH* = 0.002eV) for the HER. This work introduces a facile strategy to functionalize MXene for use as a solid support for efficient electrocatalysts.

BaTiO3-assisted exfoliation of boron nitride nanosheets for high-temperature energy storage dielectrics and thermal management

Currently, massive preparation of boron nitride nanosheets (BNNSs) towards large-size and good structural integrity via ball milling remains a key challenge, limiting its extensive applications in thermal management and energy storage. Low-cost and recyclable BaTiO3 nanoparticles with good piezoelectric effect can concurrently act as collision buffers and strong reductants to suppress the in-plane crystal structure damage and surface oxidation triggered by high-energy mechanical impact. Herein, an environmentally friendly, simple to operate and easy to realize industrial production method, referred to as BaTiO3-assisted ball milling (BTABM), is demonstrated to prepare high quality few-layered BNNSs with an average lateral size of 0.56 μm and a high yield of 91.2%. This milling process involves the synergetic effects of mechanical shear and mechanochemical peeling provided by deformed BaTiO3. The incorporation of the prepared BNNSs into the P(VDF-CTFE) matrix by BTABM effectively improved simultaneously the dielectric properties and breakdown strength of the ternary P(VDF-CTFE)/BaTiO3/BNNS nanocomposites as well as their thermal management capability, due to the formation of 3D BNNS networks. This novel method provides a guidance for not only scalable exfoliation of other layered 2D materials but also massive fabrication of polymeric nanocomposites applied in high-temperature energy storage devices.

A simple fabrication strategy for orientationally accurate twisted heterostructures

Van der Waals (vdW) heterostructure is a type of metamaterial where multiple layers of 2D materials are vertically aligned at controlled misorientation. The relative rotation in between the adjacent layers, or the twist angle between them plays a crucial role in changing the electronic band structure of the superlattice. The assembly of multi-layers of precisely twisted two dimensional layered materials requires knowledge of the atomic structure at the edge of the flake. It may be artificially created by the ‘tear and stack’ process. Otherwise, the crystallographic orientation needs to be determined through invasive processes such as transmission electron microscopy or scanning tunneling microscopy, and via second-harmonic generation (SHG). Here, we demonstrate a simple and elegant transfer protocol using only an optical microscope as a edge identifier tool through which, controlled transfer of twisted homobilayer and heterobilayer transition metal dichalcogenides is performed with close to 100% yield. The fabricated twisted vdW heterostructures have been characterized by SHG, Raman spectroscopy and photoluminiscence spectroscopy, confirming the desired twist angle within ∼0.5° accuracy. The presented method is reliable, quick and prevents the use of invasive tools which is desirable for reproducible device functionalities.