Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Charge State Tuning of Spin Defects in Hexagonal Boron Nitride

Boron vacancies in hexagonal boron nitride (hBN) are among the most extensively studied optically active spin defects in van der Waals crystals, due to their promising potential to develop two-dimensional (2D) quantum sensors. In this letter, we demonstrate the tunability of the charge state of boron vacancies in ultrathin hBN layers, revealing a transition from the optically active singly negatively charged state to the optically inactive doubly negatively charged state when sandwiched between graphene electrodes. Notably, there is a photoluminescence quenching of a few percent upon the application of a bias voltage between the electrodes. Our findings emphasize the critical importance of considering the charge state of optically active defects in 2D materials, while also showing that the negatively charged boron vacancy remains robust against external perpendicular electric fields. This stability makes it a promising candidate for integration into various van der Waals heterostructures.

Effect of molecular adsorption on the conductivity of selectively grown, interconnected 2D-MoS2 atomically thin flake structures

The gas sensitivity of field-effect structures with 2D-MoS2 channels selectively grown between Mo electrodes using the Mo-CVD method was investigated by measuring the effect of molecular adsorption from air on the device source-drain current (I sd). The channels were composed of interconnected atomically thin MoS2 grains, with their density and average thickness varied by choosing two different distances (15 and 20 μm) between the Mo contacts. A high response to the tested stimuli, including molecule adsorption, illumination and gate voltage changes, was observed. A significant, persistent photoconduction was induced by positive charge accumulation on traps, most likely at grain boundaries and associated defects. I sd increased under high vacuum, both in the dark and under illumination. The relative dark current response to the transition from air to high vacuum reached up to 1000% at the turn-on voltage. When monitored during the gradual change in air pressure, I sd exhibited a non-monotonic function, sharply peaking at about 10-2 mbar, suggesting molecular adsorption on different defect sites and orientations of adsorbed H2O molecules, which were capable of inducing electron accumulation or depletion. Despite the screening of disorder by extra electrons, the #20 μm sample remained more sensitive to air molecules on its surface. The high vacuum state was also investigated by annealing devices at temperatures up to 340 K in high vacuum, followed by measurements down to 100 K. This revealed thermally stimulated currents and activation energies of trapping electronic states assigned to sulfur vacancies (230 meV) and other shallow levels (85-120 meV), possibly due to natural impurities, grain boundaries or disorder defects. The results demonstrate the high sensitivity of these devices to molecular adsorption, making the technology promising for the easy fabrication of chemical sensors.

High-performance boron nitride/graphene oxide composites modified with sodium thiosulfate for energy storage applications

Two-dimensional (2D) hybrid materials, particularly those based on boron nitride (BN) and graphene oxide (GO), have attracted significant attention for energy applications owing to their distinct structural and electronic properties. BN/GO composites uniquely combine the mechanical strength, thermal stability and electrical insulation of BN with the high conductivity and flexibility of GO, creating advanced materials ideal for the fabrication of batteries, supercapacitors and fuel cells. These hybrids offer synergistic effects, enhanced charge transport, increased surface area, and improved chemical stability, making them promising candidates for high-performance energy systems. Despite their potential, challenges, such as achieving scalable synthesis and uniform BN-GO dispersions and poor interface compatibility, have limited the widespread adoption of BN-GO hybrids. To address these limitations, this study is focused on the scalable synthesis of BN-GO composites via a liquid-phase exfoliation method with ultrasonication, followed by preparation of sodium thiosulfate (STS)-functionalized BN-GO composites (STBG), which exhibited high electrochemical properties suitable for energy storage. The structural identification was confirmed using FT-IR, Raman, XRD, and UV-vis spectroscopy. Thermal stability of the samples was assessed by TGA, while their morphological analysis was performed using HR-TEM, TEM, and SEM. Pristine BN showed negligible efficiency, whereas STS functionalization elevated the efficiency of STBN to 81.7%, while the incorporation of GO in STBG1 and STBG2 boosted their efficiency to 89.3% and 83.3%, respectively. STBG1 exhibited a nearly rectangular, symmetrical CV curve at various scan rates, demonstrating excellent capacitive behavior. Furthermore, it achieved the highest specific capacitance of 115.82 F g-1 at a current density of 1 A g-1, together with a coulombic efficiency of 89.3%, indicating its superior charge transfer and minimal energy loss. Additionally, STBG1 retained 87.3% of its capacity, while STBG2 retained 81.7% even after 3000 charge/discharge cycles. These findings highlight that STBG1 is a promising composite with high capacitance, strong rate capability, and exceptional coulombic efficiency, making it a viable candidate for next-generation energy storage systems.

Tuning the conducting types of VSi2N4 by van der Waals engineering

VSi2N4, as a notable member of the MA2Z4 material family, has consistently attracted great interest in the field of spintronics due to its distinct electronic properties. However, the nonexistence of states around the Fermi level makes it hardly directly applicable in many situations. This work presents an exhaustive exploration of the modulation of the electronic characteristics, and particularly, the conducting types of VSi2N4 by integrating VSi2N4 with MoS2, graphene, and NbS2, each noted as a representative of a semiconductor, a semimetal and a metal, respectively, to form van der Waals (vdW) heterostructures. First-principles calculations disclose the absence of charge transfer at the VSi2N4/MoS2 interface, thereby highlighting the ineffectiveness of MoS2 in electronic modulation of VSi2N4. In contrast, graphene was found to effectively induce charge transfer to VSi2N4 in both spin-up and spin-down channels. Importantly, NbS2, characterized by its large work function, emerges as a potent mediator of VSi2N4's electronic states, inducing an efficient transition from a half-semiconducting to a half-metallic phase. Interestingly, due to such a transition, fully spin polarized transport and giant tunnel magnetoresistance (with a tunnel magnetoresistance ratio of ∼1.15 × 104%) are achieved in a magnetic tunnel junction constructed with the VSi2N4/NbS2 vdW heterostructure. These findings provide a promising foundation for extending the application potential of VSi2N4 in the construction of nanoelectronic and spintronic devices.

Machine Learning Interatomic Potential for Modeling the Mechanical and Thermal Properties of Naphthyl-Based Nanotubes

Two-dimensional (2D) nanomaterials are at the forefront of potential technological advancements. Carbon-based materials have been extensively studied since synthesizing graphene, which revealed properties of great interest for novel applications across diverse scientific and technological domains. New carbon allotropes continue to be explored theoretically, with several successful synthesis processes for carbon-based materials recently achieved. In this context, this study investigates the mechanical and thermal properties of DHQ-based monolayers and nanotubes, a carbon allotrope characterized by 4-, 6-, and 10-membered carbon rings, with a potential synthesis route using naphthalene as a molecular precursor. A machine-learned interatomic potential (MLIP) was developed to explore this nanomaterial's mechanical and thermal behavior at larger scales than those accessible through the first-principles calculations. The MLIP was trained on data derived from the DFT/PBE (density functional theory/Perdew-Burke-Ernzerhof) level using ab initio molecular dynamics (AIMD). Classical molecular dynamics (CMD) simulations, employing the trained MLIP, revealed that Young's modulus of DHQ-based nanotubes ranges from 127 to 243 N/m, depending on chirality and diameter, with fracture occurring at strains between 13.6 and 17.4% of the initial length. Regarding thermal response, a critical temperature of 2200 K was identified, marking the onset of a transition to an amorphous phase at higher temperatures.

A brief review on the progress of MXene-based catalysts for electro- and photochemical water splitting for hydrogen generation

The development and generation of affordable and highly efficient energy, particularly hydrogen, are one of the best approaches to address the challenges posed by the depletion of non-renewable energy sources. Hydrogen energy, as a green and ecosystem-friendly source with zero carbon emission, can be generated through various methods, including water splitting (HER/OER) via either photo- or electrocatalytic reactions. To implement these reactions effectively in practical applications, it is highly desirable to develop extremely efficient and cost-effective catalytic materials that are comparable to contemporary catalysts. MXenes, a family of newly discovered 2D transition metal carbides, nitrides, or carbonitrides with surface termination groups, such as -OH, -O, and -F, have emerged as promising materials and substrates for photo- and electrocatalytic applications due to their unique characteristics. These include excellent conductivity provided by the transition metals, hydrophilic nature imparted by the surface termination groups, high mechanical stability, fast electronic transmission and extremely high surface area-to-volume ratios. In this review, we provide detailed insights into the synthesis, properties, and catalytic applications of MXenes. We systematically outline the photo- and electrocatalytic water splitting reactions carried out by various MXene-based heterostructures, supported by experimental data. A thorough deliberation on the structure-activity associations of reported catalysts and a basic understanding of the electrocatalytic applications of MXenes are also included. Furthermore, we offer an insight into the upcoming tasks, challenges, prospects and new research strategies for MXenes in water splitting applications. A noteworthy recognition of the design and optimization of extremely efficient MXene-based catalysts in water splitting applications is therefore offered in this review.

Exploring 1T/2H MoS2 quantum dots modified 2D CoPx nanosheets for efficient electrocatalytic hydrogen evolution reaction

The exploration of multiphases and 0D/2D heterojunction in transition metal phosphides (TMPs) and transition metal sulfides (TMDs) is of major interest for hydrogen evolution reaction (HER). Herein, a novel combination route where 0D mixed-phased 1T/2H molybdenum sulfide quantum dots (MoS2 QDs) are uniformly anchored on the 2D CoPx nanosheets is developed. MoS2 QDs and CoPx were prepared via hydrothermal method and mixed with different ratios (Mo:Co ratios of 2:1, 1:1, and 1:2) and annealed under different temperatures to modulate their application in acidic HER processes. Specifically, 2Mo/1Co exhibited advanced performance for HER in 0.5 M H2SO4 solution and required 14 mV to deliver 10 mA cm-2 and revealed a descended Tafel slope of 75.42 mV dec-1 with 240 h stability except obvious deactivation. The successful design and construction of 0D/2D mixed-dimensional materials would broaden the application of MoS2 and CoPx for electrocatalytic hydrogen evolution.

Eco-Friendly Alternatives to Toluene-Based 2D Inks for Inkjet and Electrohydrodynamic Jet Printing Processes: A Rheological Study

Green sustainable solvents have emerged as promising alternatives to petroleum-derived options, such as toluene. This study demonstrates the use of cyrene as an effective exfoliation medium for graphene nanoplatelets (GNPs) and hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2) particles. The incorporation of polyvinylpyrrolidone (PVP) attenuates the shear-thinning behavior of GNP and hBN suspensions, maintaining a constant shear viscosity over a wide range of shear rates regardless of PVP molecular weight. Despite the presence of polymer, elasticity is hindered by inertia effects, making it impossible to accurately measure the extensional relaxation time in the capillary breakup extensional rheometer (CaBER). Assuming the weak elasticity of the formulations has a negligible impact on the breakup mechanism, we estimated droplet sizes for drop-on-demand (DoD) inkjet printing and electrohydrodynamic (EHD) jet printing based on fluid properties, i.e., viscosity, surface tension and density, and nozzle inner diameter (Dnozzle). Results indicate that the droplet size ratio (Ddrop/Dnozzle) in DoD printing can be up to two orders of magnitude higher than the one predicted for EHD jet printing at the same flow rate. This work highlights the potential of cyrene-based 2D inks as eco-friendly alternatives for advanced printing technologies.

In Situ TEM Study of Electrical Property and Mechanical Deformation in MoS2/Graphene Heterostructures

We present a versatile method for synthesizing high-quality molybdenum disulfide (MoS2) crystals on graphite foil edges via chemical vapor deposition (CVD). This results in MoS2/graphene heterostructures with precise epitaxial layers and no rotational misalignment, eliminating the need for transfer processes and reducing contamination. Utilizing in situ transmission electron microscopy (TEM) equipped with a nano-manipulator and tungsten probe, we mechanically induce the folding, wrinkling, and tearing of freestanding MoS2 crystals, enabling the real-time observation of structural changes at high temporal and spatial resolutions. By applying a bias voltage through the probe, we measure the electrical properties under mechanical stress, revealing near-ohmic behavior due to compatible work functions. This approach facilitates the real-time study of mechanical and electrical properties of MoS2 crystals and can be extended to other two-dimensional materials, thereby advancing applications in flexible and bendable electronics.

Nanocrystalline transition metal tetraborides as efficient electrocatalysts for hydrogen evolution reaction at the large current density

While great efforts have been made to improve the electrocatalytic activity of existing materials toward hydrogen evolution reaction (HER), it is also importance for searching new type of nonprecious HER catalysts to realize the practical hydrogen evolution. Herein, we firstly report nanocrystalline transition metal tetraborides (TMB4, TM=W and Mo) as an efficient HER electrocatalyst has been synthesized by a single-step solid-state reaction. The optimized nanocrystalline WB4 exhibits an overpotential as low as 172 mV at 10 mA/cm2 and small Tafel slope of 63 mV/dec in 0.5 M H2SO4. Moreover, the nanocrystalline WB4 outperforms the commercial Pt/C at high current density region, confirming potential applications in industrially electrochemical water splitting. Theoretical study reveals that high intrinsic HER activity of WB4 is originated from its large work function that contributes to the weak hydrogen-adsorption energy. Therefore, this work provides new insights for development of robust nanocrystalline electrocatalysts for efficient HER.

Carbon-based Flame Retardants for Polymers: A Bottom-up Review

This state-of-the-art review is geared toward elucidating the molecular understanding of the carbon-based flame-retardant mechanisms for polymers via holistic characterization combining detailed analytical assessments and computational material science. The use of carbon-based flame retardants, which include graphite, graphene, carbon nanotubes (CNTs), carbon dots (CDs), and fullerenes, in their pure and functionalized forms are initially reviewed to evaluate their flame retardancy performance and to determine their elevation of the flammability resistance on various types of polymers. The early transition metal carbides such as MXenes, regarded as next-generation carbon-based flame retardants, are discussed with respect to their superior flame retardancy and multifunctional applications. At the core of this review is the utilization of cutting-edge molecular dynamics (MD) simulations which sets a precedence of an alternative bottom-up approach to fill the knowledge gap through insights into the thermal resisting process of the carbon-based flame retardants, such as the formation of carbonaceous char and intermediate chemical reactions offered by the unique carbon bonding arrangements and microscopic in-situ architectures. Combining MD simulations with detailed experimental assessments and characterization, a more targeted development as well as a systematic material synthesis framework can be realized for the future development of advanced flame-retardant polymers.

A review on the use of composites of a natural protein, silk fibroin with Mxene/carbonaceous materials in biomedical science

Silk fibroin (SF), a natural biodegradable and biocompatible protein, has garnered significant attention in biomedical applications due to its impressive properties, including excellent biocompatibility, biodegradability, and mechanical resilience. Nevertheless, its broader usage faces obstacles by its insufficient mechanical strength and electrical conductivity. In order to address these constraints, recent studies have concentrated on combining SF with cutting-edge nanomaterials like MXene and carbon-based materials. This review comprehensively examines the applications and potential of silk fibroin-MXene/carbon-based nanocomposites in biomedical fields. The unique properties of SF, MXene, and carbon-based materials are explored, emphasizing how their combination enhances mechanical strength, conductivity, and biocompatibility. These composites show substantial enhancements in performance for several biomedical applications by utilising the excellent conductivity and mechanical capabilities of MXene and carbonaceous elements. The innovative potential of these nanocomposites is highlighted by critically discussing key applications such as tissue engineering, drug delivery, and biosensing. In addition, the work discusses the latest research progress, difficulties, and future prospects in the sector, providing valuable insights into possible breakthroughs and uses. This review seeks to comprehensively analyse the existing information on silk fibroin-MXene/carbon based nanocomposites in healthcare.

Optical Active Meta-Surfaces, -Substrates, and Single Quantum Dots Based on Tuning Organic Composites with Graphene

In this communication, the design and fabrication of optical active metamaterials were developed by the incorporation of graphene and joining it to different substrates with variable spectroscopical properties. It focuses on how graphene and its derivatives could generate varied optical setups and materials considering modified and enhanced optics within substrates and surfaces. In this manner, it is discussed how light could be tuned and modified along its path from confined nano-patterned surfaces or through a modified micro-lens. In addition to these optical properties generated from the physical interaction of light, it should be added that the non-classical light pathways and quantum phenomena could participate. In this way, graphene and related carbon-based materials with particular properties, such as highly condensed electronics, pseudo-electromagnetic properties, and quantum and luminescent properties, could be incorporated. Therefore, the modified substrates could be switched by photo-stimulation with variable responses depending on the nature of the material constitution. Therefore, the optical properties of graphene and its derivatives are discussed in these types of metasurfaces with targeted optical active properties, such as within the UV, IR, and terahertz wavelength intervals, along with their further properties and respective potential applications.

Insoluble low-impedance organic battery cathode enabled by graphite grafting towards potassium storage

Organic electrode materials are extensively applied for potassium storage as their sustainability and low cost. However, the organic electrodes' (i) solubility (such as naphthalene-1,4,5,8-tetracarboxylic dianhydride, NTCDA; 2,6-diaminoanthanthraquinone, DAQ, which are easily soluble in organic solvents) and (ii) intrinsic poor conductivity often result in high impedance and inferior electrochemical performance. Herein, the monomers of NTCDA and DAQ were polymerized (PND) to obtain an insoluble organic cathode, and a 5 wt% graphite (G) was also used to graft the PND sheet and increase its conductivity. Consequently, the as-prepared organic cathode (PND-G) achieved a long-life cycling performance of over 1500 cycles at 100 mA g-1. This work may provide guidelines for designing and developing insoluble and high conductive organic electrode materials.

Recent advances in the role of MXene based hybrid architectures as electrocatalysts for water splitting

The development of non-noble metal based and cost-effective electrocatalysts for water splitting has attracted significant attention due to their potential in production of clean and green hydrogen fuel. Discovered in 2011, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, have demonstrated promising performance as electro catalysts in the water splitting process due to their high electrical conductivity, very large surface area and abundant catalytic active sites. However, their-long term stability and recyclability are limited due to restacking and agglomeration of MXene flakes. This problem can be solved by combining MXene with other materials to create their hybrid architectures which have demonstrated higher electrocatalytic performance than pristine MXenes. Electrolysis of water encompasses two half-cell reactions, hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. Firstly, this concise review explains the mechanism of water splitting. Then it provides an overview of the recent advances about applications of MXenes and their hybrid architectures as HER, OER and bifunctional electrocatalysts for overall water splitting. Finally, the recent challenges and potential outlook in the field have been presented. This concise review may provide further understanding about the role of MXene-based hybrid architectures to develop efficient electrocatalysts for water splitting.