Large-area high-quality 2D ultrathin Mo2C superconducting crystals

MXene: Evolutions in Chemical Synthesis and Recent Advances in Applications

Two-dimensional materials have secured a novel area of research in material science after the emergence of graphene. Now, a new family of 2D material-MXene is gradually growing and making itsmark in this field of study. MXenes since 2011 have been synthesized and experimented on in several ways.The HF treatment although successful poses some serious problems that gradually propelled the ideas of new synthesis methods. This review of the literature covers the major breakthroughs of MXene from the year of its discovery to recent endeavors, highlighting how the synthesis mechanisms have been developed over the years and also the importance of good characterization of data. Results and properties of this class of materials arealso briefly discussed alongwith recent advance in applications.

MXenes-A New Class of Two-Dimensional Materials: Structure, Properties and Potential Applications

A new class of two-dimensional nanomaterials, MXenes, which are carbides/nitrides/carbonitrides of transition and refractory metals, has been critically analyzed. Since the synthesis of the first family member in 2011 by Yury Gogotsi and colleagues, MXenes have quickly become attractive for a variety of research fields due to their exceptional properties. Despite the fact that this new family of 2D materials was discovered only about ten years ago, the number of scientific publications related to MXene almost doubles every year. Thus, in 2021 alone, more than 2000 papers are expected to be published, which indicates the relevance and prospects of MXenes. The current paper critically analyzes the structural features, properties, and methods of synthesis of MXenes based on recent available research data. We demonstrate the recent trends of MXene applications in various fields, such as environmental pollution removal and water desalination, energy storage and harvesting, quantum dots, sensors, electrodes, and optical devices. We focus on the most important medical applications: photo-thermal cancer therapy, diagnostics, and antibacterial treatment. The first results on obtaining and studying the structure of high-entropy MXenes are also presented.

MXene/Carbon Nanotube Hybrids: Synthesis, Structures, Properties, and Applications

Since the successful preparation of few-layer transition metal carbides from three-dimensional MAX phases in 2011, MXenes (known as a family of layered transition metal carbides, nitrides, and carbonitrides) have been intensively studied. Though MXenes have been adopted as active materials in many applications, issues including aggregation and restacking are likely to hamper their potential applications. In order to address these prevailing challenges, the concept of MXene/carbon nanotube (CNT) hybrids was proposed initially in 2015, where CNTs were incorporated as the spacers and conductive additives. Ever since, MXene/CNT hybrids with different architectures have been synthesized by a number of methods and applied in numerous fields. Herein, after the discussion about general synthesis approaches, architectures, and properties of the hybrids, this Review summarized the recent advances in the application of MXene/CNT hybrids in energy storage devices, sensors, electrocatalysis, electromagnetic interference shielding, and water treatment, in which the function of individual components was clarified. In the end, the current research trend in this field were discussed and several technical issues were highlighted along with some suggestions on future research directions.

Electrochemical and optical biosensors based on multifunctional MXene nanoplatforms: Progress and prospects

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (MXene) have emerged as a rising family of atomic layered nanomaterials which undergoes intensive investigations in interdisciplinary applications. The large surface-to-volume ratio, excellent mechanical strength, desirable biocompatibility, along with tunable electronic and optical properties, render 2D MXenes exceptional attractive as versatile nanoplatforms for biosensing. Herein, advanced progress and novel paradigms of MXene-based biosensors are reviewed, focusing on the combination of MXenes with various detection techniques that promotes target recognition and signal transducing. Regarding the nature of transducing signals, MXene-based biosensors are categorized into two groups where MXenes serve as electrical platforms or optical platforms, respectively. The merits of MXenes are critically compared with other 2D materials to illustrate the distinctive advantages of MXenes in biosensing, while challenges such as environmental vulnerability was discussed to guide the sensor design. Facing with the rapid development of wearable electronics and internet of medical things, as well as escalating demanding in precision medicine, perspectives are provided to elucidate the potential of MXenes in propelling advances in these trending biomedical applications.

Surface passivation induced a significant enhancement of superconductivity in layered two-dimensional MSi2N4 (M = Ta and Nb) materials

Two-dimensional (2D) transition metal di-nitrides (TMN₂) have been arousing great interest for their unique mechanic, electronic, optoelectronic, and magnetic properties. The recent successful growth of monolayer MSi₂N₄ (M = Mo and W) further motivates us to explore new physics and unusual properties behind this family. By using first-principles calculations and Bardeen-Cooper-Schrieffer theory, we predicted the existence of the superconductivity in single-layer (SL) 1T- and 1H-TaN₂ with superconducting transition temperatures (T_c) of ∼0.86 and 1.3 K. Specifically, the T_c could be greatly enhanced to ∼24.6 K by passivating the TaN₂ monolayer with Si-N bilayers. Furthermore, the superconductivity could be increased to ∼30.4 K via substituting lighter Nb for Ta. This enhancement of superconductivity mainly stems from the softer vibration modes consisting of in-plane Ta/Nb vibrations mixed with Si-xy vibrations. The superconductivity can be further tuned by applying external strains and carrier doping. This enhancement strategy of surface passivation and light atom substitution would suggest a new platform for 2D superconductors and provide an instructive pathway for next-generation nanoelectronics.

MXene-Based Materials for Solar Cell Applications

MXenes are a class of two-dimensional nanomaterials with exceptional tailor-made properties, making them promising candidates for a wide variety of critical applications from energy systems, optics, electromagnetic interference shielding to those advanced sensors, and medical devices. Owing to its mechano-ceramic nature, MXenes have superior thermal, mechanical, and electrical properties. Recently, MXene-based materials are being extensively explored for solar cell applications wherein materials with superior sustainability, performance, and efficiency have been developed in demand to reduce the manufacturing cost of the present solar cell materials as well as enhance the productivity, efficiency, and performance of the MXene-based materials for solar energy harvesting. It is aimed in this review to study those MXenes employed in solar technologies, and in terms of the layout of the current paper, those 2D materials candidates used in solar cell applications are briefly reviewed and discussed, and then the fabrication methods are introduced. The key synthesis methods of MXenes, as well as the electrical, optical, and thermoelectric properties, are explained before those research efforts studying MXenes in solar cell materials are comprehensively discussed. It is believed that the use of MXene in solar technologies is in its infancy stage and many research efforts are yet to be performed on the current pitfalls to fill the existing voids.

Superconducting 2D NbS2 Grown Epitaxially by Chemical Vapor Deposition

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS₂ crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS₂ crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS₂ is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS₂ a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS₂ below the transition temperature (T_c) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS₂, which is imperative for its practical applications in next-generation TMDC-based electrical devices.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

2D materials for bone therapy

Due to their prominent physicochemical properties, 2D materials are broadly applied in biomedicine. Currently, 2D materials have achieved great success in treating many diseases such as cancer and tissue engineering as well as bone therapy. Based on their different characteristics, 2D materials could function in various ways in different bone diseases. Herein, the application of 2D materials in bone tissue engineering, joint lubrication, infection of orthopedic implants, bone tumors, and osteoarthritis are firstly reviewed comprehensively together. Meanwhile, different mechanisms by which 2D materials function in each disease reviewed below are also reviewed in detail, which in turn reveals the versatile functions and application of 2D materials. At last, the outlook on how to further broaden applications of 2D materials in bone therapies based on their excellent properties is also discussed.

MXene-based designer nanomaterials and their exploitation to mitigate hazardous pollutants from environmental matrices

MXenes are a rapidly expanding and large family of two-dimensional (2D) materials that have recently garnered incredible research interests for diverse applications domains in various industrial sectors. Owing to unique inherent structural and physicochemical characteristics, such as high surface area, biological compatibility, robust electrochemistry, and high hydrophilicity, MXenes are appraised as a prospective avenue for environmental-clean-up technologies to detect and mitigate an array of recalcitrant hazardous contaminants from environmental matrices. MXene-based nanoarchitectures are thought to mitigate inorganic pollutants via interfacial chemical transformation and sorption, while three different mechanisms, including i) surface complexation and sorption (ii) catalytic activation and removal and (iii) radical's generation-based photocatalytic degradation, are involved in the removal of organic contaminants. Considering the application performance of MXenes on the incessant rise to expansion, in this review, we discuss the wide-spectrum applicability of diverse MXenes-based hybrid nanocomposites in environmental remediation. A brief description related to environmental pollutants, structural properties, chemical abilities, and synthesis route of MXenes is delineated at the start. Afterwards, the adsorption and degradative robustness of MXene-based designer nanomaterials for various contaminants including organic dyes, toxic heavy metals, pesticide residues, phenolics, antibiotics, radionuclides, and many others are thoroughly vetted to prove their potentiality in the arena of wastewater purification and remediation. Lastly, challenges and trends in assessing the wide-range applicability and scalability of MXenes are outlined. Seeing encouraging outcomes in plenty of reports, it can be concluded that MXenes-based nanostructures could be considered the next-generation candidates for water sustainability.

Adsorption and diffusion of magnesium on nitrogen-doped Mo2C monolayer

The Mg adsorption and diffusion behaviors on nitrogen-doped (N-doped) Mo₂C monolayer have been investigated by the first principles based on density functional theory (DFT). To investigate the effect of nitrogen concentration on adsorption energies, Mo₂C_1-xN_x (x=0.0625, 0.125, 0.1875, and 0.25) with four different nitrogen doping concentrations have been considered in the present work. The results show that N-doped Mo₂C is benefit for Mg adsorption. In particular, when the doping concentration reaches to 14.29%, the adsorption energies of Mg on Mo₂C_0.875N_0.125 are in the region between -1.639 and -1.517 eV, e.g., the adsorption energies of Mg on T_C1 and H₂ sites are -1.639 eV and -1.625 eV, which are decreased by 16.49% and 18.43% as compared with the pristine Mo₂C. The calculations on diffusion behaviors show that the Mg diffusing between two adjacent favored sites via a high-symmetry site along H₃-B-H₄ and H₁-B-H₁ paths possesses the barriers of 0.021 eV and 0.028 eV. Additionally, the partial density of states (PDOS) reveals the interaction between Mg and Mo₂C_0.875N_0.125, and indicates that nitrogen doping causes the PDOS peaks transfer to a lower energy level, which is benefit for the bonding between Mg and Mo₂C_0.875N_0.125. These results suggest that the adsorption and diffusion behaviors of Mg have been enhanced by nitrogen doping.

A minireview on chemical vapor deposition growth of wafer-scale monolayer h-BN single crystals

Hexagonal boron nitride (h-BN), with its excellent stability, flat surface, and large bandgap, plays a role in a variety of fundamental science and technology fields. The past few years have witnessed significant development in the scaled growth of h-BN single crystals. Currently, the size of h-BN crystal can be reached up to wafer-scale, paving the way towards industrial production and commercial applications. In this minireview, recent academic breakthroughs regarding the controlled growth of large-sized h-BN single crystals via chemical vapor deposition (CVD) are presented. The as-developed technique in terms of growth parameters, choice of catalysts, and the mechanism is fully emphasized, offering a guideline in enhancing the size and quality of h-BN. Several typical metal catalysts have been used in shaping scaled h-BN single crystals, of which the metal Cu substrate has drawn the most intensive attention. The significant advances in expanding the size of h-BN single crystals will largely push forward the way to h-BN industrialization and commercialization. The past few years have witnessed significant development in the scaled growth of h-BN single crystals. Currently, the size of h-BN crystal can be reached up to wafer-scale, paving the way towards industrial production and commercial applications. In this minireview, recent academic breakthroughs regarding controlled growth of large-sized h-BN single crystals via chemical vapor deposition (CVD) are present. The as-developed technique in terms of growth parameters, choice of catalysts and mechanism is fully emphasized, offering a guideline in enhancing size and quality of h-BN. Several typical metal catalysts are exhibited in shaping scaled h-BN single crystals, of which the metal Cu substrate has drawn the most intensive attentions.

Atomically Thin Materials for Next-Generation Rechargeable Batteries

Atomically thin materials (ATMs) with thicknesses in the atomic scale (typically <5 nm) offer inherent advantages of large specific surface areas, proper crystal lattice distortion, abundant surface dangling bonds, and strong in-plane chemical bonds, making them ideal 2D platforms to construct high-performance electrode materials for rechargeable metal-ion batteries, metal-sulfur batteries, and metal-air batteries. This work reviews the synthesis and electronic property tuning of state-of-the-art ATMs, including graphene and graphene derivatives (GE/GO/rGO), graphitic carbon nitride (g-C₃N₄), phosphorene, covalent organic frameworks (COFs), layered transition metal dichalcogenides (TMDs), transition metal carbides, carbonitrides, and nitrides (MXenes), transition metal oxides (TMOs), and metal-organic frameworks (MOFs) for constructing next-generation high-energy-density and high-power-density rechargeable batteries to meet the needs of the rapid developments in portable electronics, electric vehicles, and smart electricity grids. We also present our viewpoints on future challenges and opportunities of constructing efficient ATMs for next-generation rechargeable batteries.

Interfacial Engineering of 3D Hollow Mo-Based Carbide/Nitride Nanostructures

Molybdenum carbide and nitride nanocrystals have been widely recognized as ideal electrocatalyst materials for water splitting. Furthermore, the interfacial engineering strategy can effectively tune their physical and chemical properties to improve performance. Herein, we produced N-doped molybdenum carbide nanosheets on carbonized melamine (N-doped Mo₂C@CN) and 3D hollow Mo₂C-Mo₂N nanostructures (3D H-Mo₂C-Mo₂N) with tuneable interfacial properties via high-temperature treatment. X-ray photoelectron spectroscopy reveals that Mo₂C and Mo₂N nanocrystals in 3D hollow nanostructures are chemically bonded with each other and produce stable heterostructures. The 3D H-Mo₂C-Mo₂N nanostructures demonstrate lower onset potential and overpotential at a current density of 10 mV cm^-2 than the N-doped Mo₂C@CN nanostructure due to its higher active sites and improved interfacial charge transfer. The current work presents a strategy to tune metal carbide/nitride nanostructures and interfacial properties for the production of high-performance energy materials.

The structural, magnetic, Raman and electrical transport properties of Mn intercalated Ti3C2TX

The electronic and magnetic properties of the two-dimensional Ti₃C₂MXenes have attracted a lot of interests due to its potential applications. In this paper, Ti₃C₂T_xMXenes and Mn-doped Ti₃C₂T_xMXenes are synthesized and investigated. The experimental data shows that Mn²⁺ions are homogeneously and randomly intercalated between Ti₃C₂sheets as function terminals, which increase the interlayer distance between Ti₃C₂sheets and offer a mass of uncoupled magnetic moment. The temperature dependence of the electric resistivity of both samples show similar complex behavior although the resistivity increases dramatically as Mn doping. The inter-flake variable range hopping (VRH) dominates the low temperature electric transport behavior, while the inter-flake thermally activated hopping as well as the metallic intra-flake transport competing with the inter-flake VRH play important roles at high temperature. The increasing resistivity of the Mn-doped sample could be attributed to the increase of the interlayer distance and the enhancement of the localization of the transport electrons after Mn²⁺ions intercalation.

A graphene-Mo2C heterostructure for a highly responsive broadband photodetector

Photodetectors based on intrinsic graphene can operate over a broad wavelength range with ultrafast response, but their responsivity is much lower than commercial silicon photodiodes. The combination of graphene with two-dimensional (2D) semiconductors may enhance the light absorption, but there is still a cutoff wavelength originating from the bandgap of semiconductors. Here, we report a highly responsive broadband photodetector based on the heterostructure of graphene and transition metal carbides (TMCs, more specifically Mo₂C). The graphene-Mo₂C heterostructure enhanced light absorption over a broad wavelength range from ultraviolet to infrared. In addition, there is very small resistance for photoexcited carriers in both graphene and Mo₂C. Consequently, photodetectors based on the graphene-Mo₂C heterostructure deliver a very high responsivity from visible to infrared telecommunication wavelengths.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Wearable, Washable, and Highly Sensitive Piezoresistive Pressure Sensor Based on a 3D Sponge Network for Real-Time Monitoring Human Body Activities

Wearable pressure sensors are highly desirable for monitoring human health and realizing a nice human-machine interaction. Herein, a chitosan/MXene/polyurethane-sponge/polyvinyl alcohol (CS/MXene/PU sponge/PVA)-based 3D pressure sensor is developed to simultaneously achieve wearability, washability, and high sensitivity in a wide region. In the force-sensitive layer of the sensor, MXene and CS are fully attached to the PU sponge to ensure that the composite sponge has remarkable conductivity and washability. Benefiting from the highly resistive PVA-nanowire spacer, the initial current of the sensor is reduced significantly so that the sensor exhibits extremely high sensitivity (84.9 kPa^-1 for the less than 5 kPa region and 140.6 kPa^-1 for the 5-22 kPa region). Moreover, the sensor has an excellent fast response time of 200 ms and a short recovery time of 30 ms, as well as non-attenuating durability over 5000 cycles. With the high sensitivity in a wide range, the sensor is capable of detecting multiple human and animal activities in real time, ranging from the large pressure of joint activities to a subtle pressure of pulse. Furthermore, the sensor also demonstrates the potential application in measuring pressure distribution. Overall, such a multifunctional pressure sensor can supply a new platform for the design and development of wearable health-monitoring equipment and an efficient human-machine interface.

Efficient photocatalytic degradation of hazardous pollutants by homemade kitchen blender novel technique via 2D-material of few-layer MXene nanosheets

To attain elevated class MXene (Ti₃C₂Tx) through a homemade kitchen blender method, high shear mechanical exfoliation is highly required for the efficient delimitations of MXene nanosheets from bulk MAX (Ti₃AlC₂). We examine large-scale industrial productions of the MXene nanosheets, where combing the predicted 2D materials using a blender is a first-time novel approach with the delaminating solvent as a dimethyl sulfoxide (DMSO). And also manually created layered MXene systems (handmade) delaminating MXene sheets (MX-H) was furthermore employed for environmental dye-degradations applications. The materials characterizations was done for both the bulk MAX, MX-H and the MX-B. Additionally, the surface morphological studies like scanning electron microscopy (SEM) were investigated for both MX-H and MX-B as-prepared samples. SEM images indicated the high shear blander technique formations highly expanded/delaminated MXene (Ti₃C₂Tx) nanosheets compared to MX-H samples. FTIR technique is employed to identify -OH, C-H, C-O stretching vibrations for both materials. Raman spectroscopy analysis of MX-H and MX-B revealed 484.80 cm^-1 Raman shift assigned to E1g phonon mode of (Ti, C, O). The ultraviolet UV visible absorption spectra explored pure and catalyst added Methylene Blue (MB) dye stock solution using annular type photoreactor with visible light source of 300 W. The comparatives of MAX, MX-H and MX-B samples was investigated as photocatalytic activity, The blender made (MX-B) sample revealed 98% of efficiency.

Advances in the Synthesis of 2D MXenes

2D transition metal carbides, nitrides, and carbonitrides, also known as MXenes, are versatile materials due to their adjustable structure and rich surface chemistry. The physical and chemical diversity has recognized MXenes as a potential 2D material with a wide spectrum of application domains. Since the discovery of MXenes in 2011, a wide variety of synthetic routes has been proposed with advancement toward large-scale preparing methods for MXene nanosheets and derivative products. Herein, the critical synthesis aspects and the operating conditions that influence the physical and chemical characteristics of MXenes are discussed in detail. The emerging etching methods including HF etching methods, in situ HF-forming etching methods, electrochemical etching methods, alkali etching methods, and molten salt etching methods, as well as delamination strategies are discussed. Considering the future developments and practical applications, the large-scale synthesis routes and the antioxidation strategies of MXenes are also summarized. In summary, a generalized overview of MXenes synthesis protocols with an outlook for the current challenges and promising technologies for large-scale preparation and stable storage is provided.

CNSi/MXene/CNSi: Unique Structure with Specific Electronic Properties for Nanodevices

2D materials have been interesting for applications into nanodevices due to their intriguing physical properties. In this work, four types of unique structures are designed that are composed of MXenes and C/N-Si layers (CNSi), where MXene is sandwiched by the CNSi layers with different thicknesses, for their practical applications into integrated devices. The systematic calculations on their elastic constants, phonon dispersions, and thermodynamic properties show that these structures are stable, depending on the composition of MXene. It is found: 1) different from MXene or N-functionalized MXene (M₂ CN₂ ), SiN₂ /M₂ X/SiN₂ possess new electronic properties with free carriers only in the middle, leading to 2D free electron gas; 2) CNSi/MXene/CNSi shows an intrinsic Ohmic semiconductor-metal-semiconductor (S-M-S) contact, which is potential for applications into nanodevices; and 3) O/M₂ C/SiN₂ and N/M₂ C/OSiN are also stable and show different electronic properties, which can be semiconductor or metal as a whole depending on the interface. A method is further proposed to fabricate the 2D structures based on the industrial availability. The findings may provide a novel strategy to design and fabricate the 2D structures for their application into nanodevices and integrated circuits.

Ten Years of Progress in the Synthesis and Development of MXenes

Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes' unique properties, such as their metal-like electrical conductivity reaching ≈20 000 S cm^-1 , render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization.

Prospects and Challenges of MXenes as Emerging Sensing Materials for Flexible and Wearable Breath-Based Biomarker Diagnosis

A fully integrated, flexible, and functional sensing device for exhaled breath analysis drastically transforms conventional medical diagnosis to non-invasive, low-cost, real-time, and personalized health care. 2D materials based on MXenes offer multiple advantages for accurately detecting various breath biomarkers compared to conventional semiconducting oxides. High surface sensitivity, large surface-to-weight ratio, room temperature detection, and easy-to-assemble structures are vital parameters for such sensing devices in which MXenes have demonstrated all these properties both experimentally and theoretically. So far, MXenes-based flexible sensor is successfully fabricated at a lab-scale and is predicted to be translated into clinical practice within the next few years. This review presents a potential application of MXenes as emerging materials for flexible and wearable sensor devices. The biomarkers from exhaled breath are described first, with emphasis on metabolic processes and diseases indicated by abnormal biomarkers. Then, biomarkers sensing performances provided by MXenes families and the enhancement strategies are discussed. The method of fabrications toward MXenes integration into various flexible substrates is summarized. Finally, the fundamental challenges and prospects, including portable integration with Internet-of-Thing (IoT) and Artificial Intelligence (AI), are addressed to realize marketization.

Magnetic Doping Induced Superconductivity-to-Incommensurate Density Waves Transition in a 2D Ultrathin Cr-Doped Mo2C Crystal

In the vicinity of a competing electronic order, superconductivity emerges within a superconducting dome in the phase diagram, which has been demonstrated in unconventional superconductors and transition-metal dichalcogenides (TMDs), suggesting a scenario where fluctuations or a partial melting of a parent order are essential for inducing superconductivity. Here, we present a contrary example, the two-dimensional (2D) superconductivity in transition-metal carbide can be readily turned into charge density wave (CDW) phases via dilute magnetic doping. Low temperature scanning tunneling microscopy/spectroscopy (STM/STS), transport measurements, and density functional theory (DFT) calculations were employed to investigate Cr-doped superconducting Mo₂C crystals in the 2D limit. With ultralow Cr doping (2.7 atom %), the superconductivity of Mo₂C is heavily suppressed. Strikingly, an incommensurate density wave (IDW) and a related partially opened gap are observed at a temperature above the superconducting regime. The wave vector of IDW agrees well with the calculated Fermi surface nesting vectors. By further increasing the Cr doping level to 9.4 atom %, a stronger IDW with a smaller periodicity and a larger partial gap appear concurrently. The resistance anomaly implies the onset of the CDW phase. Spatial-resolved and temperature-dependent spectroscopy reveals that such CDW phases exist only in a nonsuperconducting regime and could form long-range orders uniformly. The results provide the understanding for the interplay between charge ordered states and superconductivity in 2D transition-metal carbide.

Two-dimensional molybdenum carbide 2D-Mo2C as a superior catalyst for CO2 hydrogenation

Early transitional metal carbides are promising catalysts for hydrogenation of CO₂. Here, a two-dimensional (2D) multilayered 2D-Mo₂C material is prepared from Mo₂CT_x of the MXene family. Surface termination groups T_x (O, OH, and F) are reductively de-functionalized in Mo₂CT_x (500 °C, pure H₂) avoiding the formation of a 3D carbide structure. CO₂ hydrogenation studies show that the activity and product selectivity (CO, CH₄, C₂–C₅ alkanes, methanol, and dimethyl ether) of Mo₂CT_x and 2D-Mo₂C are controlled by the surface coverage of T_x groups that are tunable by the H₂ pretreatment conditions. 2D-Mo₂C contains no T_x groups and outperforms Mo₂CT_x, β-Mo₂C, or the industrial Cu-ZnO-Al₂O₃ catalyst in CO₂ hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO₂ hydrogenation.

The development of robust and efficient catalysts for CO₂ hydrogenation to value-added chemicals is an urgent task. Here the authors report two-dimensional carbide catalyst based on earth-abundant molybdenum that hydrogenates CO₂ with high activity, stable performance and tunable selectivity.

Recent Progress in Emerging Two-Dimensional Transition Metal Carbides

As a new member in two-dimensional materials family, transition metal carbides (TMCs) have many excellent properties, such as chemical stability, in-plane anisotropy, high conductivity and flexibility, and remarkable energy conversation efficiency, which predispose them for promising applications as transparent electrode, flexible electronics, broadband photodetectors and battery electrodes. However, up to now, their device applications are in the early stage, especially because their controllable synthesis is still a great challenge. This review systematically summarized the state-of-the-art research in this rapidly developing field with particular focus on structure, property, synthesis and applicability of TMCs. Finally, the current challenges and future perspectives are outlined for the application of 2D TMCs.

Controlled Synthesis of MoxW1-xTe2 Atomic Layers with Emergent Quantum States

Recently, new states of matter like superconducting or topological quantum states were found in transition metal dichalcogenides (TMDs) and manifested themselves in a series of exotic physical behaviors. Such phenomena have been demonstrated to exist in a series of transition metal tellurides including MoTe₂, WTe₂, and alloyed Mo_xW_1-xTe_2. However, the behaviors in the alloy system have been rarely addressed due to their difficulty in obtaining atomic layers with controlled composition, albeit the alloy offers a great platform to tune the quantum states. Here, we report a facile CVD method to synthesize the Mo_xW_1-xTe₂ with controllable thickness and chemical composition ratios. The atomic structure of a monolayer Mo_xW_1-xTe₂ alloy was experimentally confirmed by scanning transmission electron microscopy. Importantly, two different transport behaviors including superconducting and Weyl semimetal states were observed in Mo-rich Mo_0.8W_0.2Te₂ and W-rich Mo_0.2W_0.8Te₂ samples, respectively. Our results show that the electrical properties of Mo_xW_1-xTe₂ can be tuned by controlling the chemical composition, demonstrating our controllable CVD growth method is an efficient strategy to manipulate the physical properties of TMDCs. Meanwhile, it provides a perspective on further comprehension and sheds light on the design of devices with topological multicomponent TMDC materials.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D β-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.

Fascinating MXene nanomaterials: emerging opportunities in the biomedical field

In recent years, there has been rapid progress in MXene research due to its distinctive two-dimensional structure and outstanding properties. Especially in biomedical applications, MXenes have attracted widespread favor with numerous studies on biosafety, bioimaging, therapy, and biosensing, although their development is still in the experimental stage. A comprehensive understanding of the current status of MXenes in biomedicine will promote their use in clinical applications. Here, we review advances in MXene research. First, we introduce the methods of synthesis, surface modification and functionalization of MXenes. Then, we summarize the biosafety and biocompatibility, paving the way for specific biomedical applications. On this basis, MXene nanostructures are described with respect to their use in antibacterial, bioimaging, cancer therapy, tissue regeneration and biosensor applications. Finally, we discuss MXene as a promising candidate material for further applications in biomedicine.

MXenes: synthesis, incorporation, and applications in ultrafast lasers

The rapid expansion of nanotechnology and material science prompts two-dimensional (2D) materials to be extensively used in biomedicine, optoelectronic devices, and ultrafast photonics. Owing to the broadband operation, ultrafast recovery time, and saturable absorption properties, 2D materials become the promising candidates for being saturable absorbers in ultrafast pulsed lasers. In recent years, the novel 2D MXene materials have occupied the forefront due to their superior optical and electronic, as well as mechanical and chemical properties. Herein, we introduce the fabrication methods of MXenes, incorporation methods of combining 2D materials with laser cavities, and applications of ultrafast pulsed lasers based on MXenes. Firstly, top-down and bottom-up approaches are two types of fabrication methods, where top-down way mainly contains acid etching and the chief way of bottom-up method is chemical vapor deposition. In addition to these two typical ones, other methods are also discussed. Then we summarize the advantages and drawbacks of these approaches. Besides, commonly used incorporation methods, such as sandwich structure, optical deposition, as well as coupling with D-shaped, tapered, and photonic crystal fibers are reviewed. We also discuss their merits, defects, and conditions of selecting different methods. Moreover, we introduce the state of the art of ultrafast pulsed lasers based on MXenes at different wavelengths and highlight some excellent output performance. Ultimately, the outlook for improving fabrication methods and applications of MXene-based ultrafast lasers is presented.

Recent Progress of Two-Dimensional Materials for Ultrafast Photonics

Owing to their extraordinary physical and chemical properties, two-dimensional (2D) materials have aroused extensive attention and have been widely used in photonic and optoelectronic devices, catalytic reactions, and biomedicine. In particular, 2D materials possess a unique bandgap structure and nonlinear optical properties, which can be used as saturable absorbers in ultrafast lasers. Here, we mainly review the top-down and bottom-up methods for preparing 2D materials, such as graphene, topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes. Then, we focus on the ultrafast applications of 2D materials at the typical operating wavelengths of 1, 1.5, 2, and 3 μm. The key parameters and output performance of ultrafast pulsed lasers based on 2D materials are discussed. Furthermore, an outlook regarding the fabrication methods and the development of 2D materials in ultrafast photonics is also presented.

MXene and black phosphorus based 2D nanomaterials in bioimaging and biosensing: progress and perspectives

Bioimaging and biosensing have garnered interest in early cancer diagnosis due to the ability of gaining in-depth insights into cellular functions and providing a wide range of diagnostic parameters. Emerging 2D materials of multielement MXenes and monoelement black phosphorous nanosheets (BPNSs) with unique intrinsic physicochemical properties such as a tunable bandgap and layer-dependent fluorescence, high carrier mobility and transport anisotropy, efficient fluorescence quenching capability, desirable light absorption and thermoelastic properties, and excellent biocompatibility and biosafety properties provide promising nano-platforms for bioimaging and biosensing applications. In view of the growing attention on the rising stars of the post-graphene age in the progress of bioimaging and biosensing, and their common feature characteristics as well as complementarity for constructing complexes, the main objective of this review is to reveal the recent advances in the design of MXene or BPNS based nanoplatforms in the field of bioimaging and biosensing. The preparation and surface functionalization methods, biosafety, and other important aspects of bioimaging and biosensing applications of MXenes and BPNSs have been assessed systematically, along with highlighting the main challenges in further biomedical application. The review not only focuses on the advancements in 2D materials for use in bioimaging and biosensing but also assesses the possibility of their future potential in bioapplications.

Real-Time Multiscale Monitoring and Tailoring of Graphene Growth on Liquid Copper

The synthesis of large, defect-free two-dimensional materials (2DMs) such as graphene is a major challenge toward industrial applications. Chemical vapor deposition (CVD) on liquid metal catalysts (LMCats) is a recently developed process for the fast synthesis of high-quality single crystals of 2DMs. However, up to now, the lack of in situ techniques enabling direct feedback on the growth has limited our understanding of the process dynamics and primarily led to empirical growth recipes. Thus, an in situ multiscale monitoring of the 2DMs structure, coupled with a real-time control of the growth parameters, is necessary for efficient synthesis. Here we report real-time monitoring of graphene growth on liquid copper (at 1370 K under atmospheric pressure CVD conditions) via four complementary in situ methods: synchrotron X-ray diffraction and reflectivity, Raman spectroscopy, and radiation-mode optical microscopy. This has allowed us to control graphene growth parameters such as shape, dispersion, and the hexagonal supra-organization with very high accuracy. Furthermore, the switch from continuous polycrystalline film to the growth of millimeter-sized defect-free single crystals could also be accomplished. The presented results have far-reaching consequences for studying and tailoring 2D material formation processes on LMCats under CVD growth conditions. Finally, the experimental observations are supported by multiscale modeling that has thrown light into the underlying mechanisms of graphene growth.

MXenes for Cancer Therapy and Diagnosis: Recent Advances and Current Challenges

MXenes endowed with several attractive physicochemical attributes, namely, specific large surface area, significant electrical conductivity, magnetism, low toxicity, luminescence, and high biocompatibility, have been considered as promising candidates for cancer therapy and theranostics. These two-dimensional (2D) nanostructures endowed with photothermal, chemotherapeutic synergistic, and photodynamic effects have shown promising potential for decidedly effectual and noninvasive anticancer treatments. They have been explored for photothermal/chemo-photothermal therapy (PTT) and for targeted anticancer drug delivery. Remarkably, MXenes with their unique optical properties have been employed for bioimaging and biosensing, and their excellent light-to-heat transition competence renders them an ideal biocompatible and decidedly proficient nanoscaled agent for PTT appliances. However, several important challenging issues still linger regarding their stability in physiological environments, sustained/controlled release of drugs, and biodegradability that need to be addressed. This Perspective emphasizes the latest advancements of MXenes and MXene-based materials in the domain of targeted cancer therapy/diagnosis, with a focus on the current trends, important challenges, and future perspectives.

The world of two-dimensional carbides and nitrides (MXenes)

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

Potential environmental applications of MXenes: A critical review

Various environmental pollutants (e.g., air, water and solid pollutants) are discharged into environments with the rapid development of industrializations, which is presently at the forefront of global attention. The high efficient removal of these environmental pollutants is of important concern due to their potential threat to human health and eco-diversity. Advanced nanomaterials may play an important role in the elimination of pollutants from environmental media. MXenes as the new intriguing class of graphene-like 2D transition metal carbides and/or carbonitrides have been widely used in energy storage, environmental remediation benefitting from exceptional structural properties such as highly active sites, high chemical stability, hydrophilicity, large interlayer spacing, huge specific surface area, superior sorption-reduction capacity. However, the comprehensive investigation concerning the removal of various environmental pollutants on MXenes is yet not available up to date. In this review, we summarized the synthesis and properties of MXenes to demonstrate the key roles in ameliorating their adsorption performance; then the recent advances and achievements in environmental application of MXenes on the removal of gases, organics, heavy metals and radionuclides were comprehensively reviewed in details; Finally, the formidable challenges and further perspectives regarding utilizing MXene in environmental remediation were proposed. Hopefully, this review can provide the useful information for environmental scientists and material engineers on designing versatile MXenes in actual environmental applications.

Recent Advances in 2D Superconductors

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (T_c ), continuous phase transition, and enhanced parallel critical magnetic field (B_c ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-T_c superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced B_c , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Molybdenum Carbide Buried in D-Shaped Fibers as a Novel Saturable Absorber Device for Ultrafast Photonics Applications

Study of nonlinear laser-matter interactions in 2D materials has promoted development of photonics applications. As a typical MXene material, molybdenum carbide (Mo₂C) has attracted much attention because of its graphene-like structure. Here, a type of D-shaped fiber (DF)-buried Mo₂C saturable absorber (SA) fabricated by magnetron-sputtering deposition (MSD) and sol-gel technique is reported. The Mo₂C material was buried between the bottom DF and the upper amorphous silica fabricated by sol-gel technology. Therefore, the DF-based SA effectively solves the problem of material shedding and aging, thus improving the stability and damage threshold of the fiber laser. Application of the SA in erbium-doped fiber laser and stable passive Q-switched operation with a maximum pulse energy of 430.47 nJ is realized. By adjusting the polarization state and pump power, high-power mode-locked pulses are generated with a pulse duration and output power of 199 fs and 54.13 mW, respectively. Further, bound-state soliton pulses are obtained with a pulse width of 312 fs and soliton interval of 1.26 ps for the first time based on MXene materials. Moreover, by application of the SA in ytterbium-doped fiber lasers, a stable dissipative soliton mode-locked pulse is obtained with a pulse width of 23 ps. These results indicate that the DF-based buried Mo₂C as a novel SA provides a reliable method for all-fiber and multifunctional high-power ultrafast laser.

MXene derivatives: synthesis and applications in energy convention and storage

Transition metal carbides or nitrides (MXene) have shown promising applications in energy convention and storage (ECS), owing to their high conductivity and adjustable surface functional groups. In the past several years, many MXene derivatives with different structures have been successfully prepared and their impressive performance demonstrated in ECS. This review summarizes the progress in the synthesis of MXene and typical Ti3C2T x MXene derivatives with different morphologies, including 0D quantum dots, 1D nanoribbons, 2D nanosheets and 3D nanoflowers. The mechanisms involved and their performance in photocatalysis, electrocatalysis and rechargeable batteries are also discussed. Furthermore, the challenges of MXene derivatives in ECS are also proposed.

Low-temperature synthesis and growth model of thin Mo2C crystals on indium

Chemical vapor deposition is a promising technique to produce Mo2C crystals with large area, controlled thickness, and reduced defect density. Typically, liquid Cu is used as a catalyst substrate; however, its high melting temperature (1085 °C) prompted research groups to search for alternatives. In this study, we report the synthesis of large-area thin Mo2C crystals at lower temperatures using liquid In, which is also advantageous with respect to the transfer process due to its facile etching. SEM, EDS, Raman spectroscopy, XPS, and XRD studies show that hexagonal Mo2C crystals, which are orthorhombic, grow along the [100] direction together with an amorphous carbon thin film on In. The growth mechanism is examined and discussed in detail, and a model is proposed. AFM studies agree well with the proposed model, showing that the vertical thickness of the Mo2C crystals decreases inversely with the thickness of In for a given reaction time.