Recent advances in graphene and other 2D materials

The isolation of the first two-dimensional material, graphene-a monolayer of carbon atoms arranged in a hexagonal lattice-opened new exciting opportunities in the field of condensed matter physics and materials. Its isolation and subsequent studies demonstrated that it was possible to obtain sheets of atomically thin crystals and that these were stable, and they also began to show its outstanding properties, thus opening the door to a whole new family of materials, known as two-dimensional materials or 2D materials. The great interest in different 2D materials is motivated by the variety of properties they show, being candidates for numerous applications.Additionally, the combination of 2D crystals allows the assembly of composite, on-demand materials, known as van der Waals heterostructures, which take advantage of the properties of those materials to create functionalities that otherwise would not be accessible. For example, the combination of 2D materials, which can be done with high precision, is opening up opportunities for the study of new challenges in fundamental physics and novel applications. Here we review the latest fundamental discoveries in the area of 2D materials and offer a perspective on the future of the field.

Towards a new avenue for rapid synthesis of electrocatalytic electrodes via laser-induced hydrothermal reaction for water splitting

Electrochemical production of hydrogen from water requires the development ofelectrocatalysts that are active,stable,and low-cost for water splitting.To address these challenges,researchers are increasingly exploring binder-free electrocatalytic integratedelectrodes (IEs) as an alternative to conventional powder-based electrode preparation methods,for the former is highly desirable to improve the catalytic activity and long-term stability for large-scale applications of electrocatalysts.Herein,we demonstrate a laser-inducedhydrothermal reaction (LIHR) technique to grow NiMoO4nanosheets on nickel foam,which is then calcined under H2/Ar mixed gases to prepare the IE IE-NiMo-LR.This electrode exhibits superior hydrogen evolution reaction performance,requiring overpotentials of 59,116 and143 mV to achieve current densities of 100,500 and 1000 mA·cm-2.During the 350 h chronopotentiometry test at current densities of 100 and 500 m A·cm-2,the overpotentialremains essentially unchanged.In addition,NiFe-layered double hydroxide grown on Ni foam is also fabricated with the same LIHR method and coupled with IE-NiMo-IR to achieve water splitting.This combination exhibits excellent durability under industrial current density.The energy consumption and production efficiency of the LIHR method are systematicallycompared with the conventional hydrothermal method.The LIHR method significantly improves the production rate by over 19 times,while consuming only 27.78%of the total energy required by conventional hydrothermal methods to achieve the same production.

A direct laser-synthesized magnetic metamaterial for low-frequency wideband passive microwave absorption

Microwave absorption in radar stealth technology is faced with challenges in terms of its effectiveness in low-frequency regions.Herein,we report a new laser-based method for producing an ultrawideband metamaterial-based microwave absorber with a highly uniform sheet resistance and negative magnetic permeability at resonant frequencies,which results in a wide bandwidth in the L-to S-band.Control of the electrical sheet resistance uniformity has been achieved with less than 5%deviation at 400Ωsq^(-1)and 6%deviation at 120Ωsq^(-1),resulting in a microwave absorption coefficient between 97.2%and 97.7%within a1.56–18.3 GHz bandwidth for incident angles of 0°–40°,and there is no need for providing energy or an electrical power source during the operation.Porous N-and S-doped turbostratic graphene 2D patterns with embedded magnetic nanoparticles were produced simultaneously on a polyethylene terephthalate substrate via laser direct writing.The proposed low-frequency,wideband,wide-incident-angle,and high-electromagnetic-absorption microwave absorber can potentially be used in aviation,electromagnetic interference(EMI)suppression,and 5G applications.

Freeze-assisted Tape Casting of Vertically Aligned MXene Films for High Rate Performance Supercapacitors

Conventional electrode preparation techniques of supercapacitors such as tape casting or vacuum filtration often lead to the restacking or agglomeration of twodimensional(2 D)materials.As a result,tortuous paths are created for the electrolyte ions and their adsorption onto the surfaces of the active materials can be prevented.Consequently,maintaining high rate performance while increasing the thickness of electrodes has been a challenge.Herein,a facile freeze-assisted tape-casting(Fa TC)method is reported for the scalable fabrication of flexible MXene(Ti3C2Tx)supercapacitor electrode films of up to 700μm thickness,exhibiting homogeneous ice-template microstructure composed of vertically aligned MXene walls within lamellar pores.The efficient ion transport created by the internal morphology allows for fast electrochemical charge–discharge cycles and near thickness-independent performance at up to 3000 m V s-1 for films of up to 300μm in thickness.By increasing the scan rate from 20 to 10,000 m V s-1,Ti3C2Tx films of 150μm in thickness sustain 50%of its specific capacitance(222.9 F g-1).When the film thickness is doubled to 300μm,its capacitance is still retained by 60%(at 213.3 F g-1)when the scan rate is increased from 20 to3000 m V s-1,with a capacitance retention above 97.7%for over 14,000 cycles at10 A g-1.They also showed a remarkably high gravimetric and areal power density of 150 k W kg-1 at 1000 A g-1 and 667 m W cm-2 at 4444 m A cm-2,respectively.Fa TC has the potential to provide industry with a viable way to fabricate electrodes formed from 2 D materials on a large scale,while providing promising performance for use in a wide range of applications,such as flexible electronics and wearable energy storage devices.

Graphene and other two-dimensional materials

Over the last few years,research on graphene [1-3]has progressed significantly,and as a result,a number of reallife applications of graphene have been realized [4,5].This carbon allotrope (for a review on other forms of carbon, see [6])resulted in a number of novel physical phenomena already discovered (ranging from new types of quantum Hall effect to universal optical conductivity)and dramatically expanded the range of possible applications for such materials (from transparent conductive coating to ultrafast photodetectors).

Self-sensing piezoresistive aerospace composites based on CNTs and 2D material coated fabric sensors

The ongoing fourth industrial revolution,also known as“Industry 4.0”is the driving force behind the digitalization of various manufacturing systems by incorporating smart autonomous systems,the Internet of Things(IoT),robotics,and artificial intelligence.In terms of aerospace composites,comprehensive research has been carried out in the past decade or so to manufacture smart and self-sensing fiber-reinforced polymer composites capable of monitoring their own health states.This review focuses on recent developments in smart,self-sensing fiberreinforced composites incorporating nanomaterial-coated piezoresistive fabric sensors such as carbon nanotubes(CNTs),graphene,and MXene.A comprehensive overview of process monitoring involving the complete resin infusion cycle,such as compaction response,resin flow monitoring,pressure variations within the mold,process-induced defects monitoring,and cure/post-cure monitoring,has been provided.The postmanufacturing structuring health monitoring(SHM)of composites has also been discussed in detail.An overview of the associated challenges of these sensors,such as manufacturability,robustness,conductivity/piezoresistivity calibration,and the effect on structural integrity,is presented.Finally,future insights into the application of these sensors in the physical and cyber domains for smart factories of the future have also been discussed.

Exploring van der Waals materials with high anisotropy:geometrical and optical approaches

The emergence of van der Waals(vdW)materials resulted in the discovery of their high optical,mechanical,and electronic anisotropic properties,immediately enabling countless novel phenomena and applications.Such success inspired an intensive search for the highest possible anisotropic properties among vdW materials.Furthermore,the identification of the most promising among the huge family of vdW materials is a challenging quest requiring innovative approaches.Here,we suggest an easy-to-use method for such a survey based on the crystallographic geometrical perspective of vdW materials followed by their optical characterization.Using our approach,we found As2S3 as a highly anisotropic vdW material.It demonstrates high in-plane optical anisotropy that is~20%larger than for rutile and over two times as large as calcite,high refractive index,and transparency in the visible range,overcoming the century-long record set by rutile.Given these benefits,As2S3 opens a pathway towards nextgeneration nanophotonics as demonstrated by an ultrathin true zero-order quarter-wave plate that combines classical and the Fabry–Pérot optical phase accumulations.Hence,our approach provides an effective and easy-to-use method to find vdW materials with the utmost anisotropic properties.

Convergent and divergent beam electron holography and reconstruction of adsorbates on free-standing two-dimensional crystals

Van der Waals heterostructures have been lately intensively studied because they offer a large variety of properties that can be controlled by selecting 2D materials and their sequence in the stack. The exact arrangement of the layers as well as the exact arrangement of the atoms within the layers, both are important for the properties of the resulting device. However, it is very difficult to control and characterize the exact position of the atoms and the layers in such heterostructures, in particular, along the vertical (z) dimension. Recently it has been demonstrated that convergent beam electron diffraction (CBED) allows quantitative three-dimensional mapping of atomic positions in three-dimensional materials from a single CBED pattern. In this study we investigate CBED in more detail by simulating and performing various CBED regimes, with convergent and divergent wavefronts, on a somewhat simplified system: a two-dimensional (2D) monolayer crystal. In CBED, each CBED spot is in fact an in-line hologram of the sample, where in-line holography is known to exhibit high intensity contrast in detection of weak phase objects that are not detectable in conventional in-focus imaging mode. Adsorbates exhibit strong intensity contrast in the zero and higher order CBED spots, whereas lattice deformation such as strain or rippling cause noticeable intensity contrast only in the first and higher order CBED spots. The individual CBED spots can thus be reconstructed as typical in-line holograms, and a resolution of 2.13 A can in principle be achieved in the reconstructions. We provide simulated and experimental examples of CBED of a 2D monolayer crystal. The simulations show that individual CBED spots can be treated as in-line holograms and sample distributions such as adsorbates, can be reconstructed. Individual atoms can be reconstructed from a single CBED pattern provided the later exhibits high-order CBED spots. The experimental results were obtained in a transmission electron microscope (TEM) at 80 keV on free-standing monolayer hBN containing adsorbates. Examples of reconstructions obtained from experimental CBED patterns at a resolution of 2.7 ? are shown. CBED technique can be potentially useful for imaging individual biological macromolecules, because it provides a relatively high resolution and does not require additional scanning procedure or multiple image acquisitions and therefore allows minimizing the radiation damage.

Stacking transition in rhombohedral graphite

Few-layer graphene (FLG) has recently been intensively investigated for its variable electronic properties, which are defined by a local atomic arrangement. While the most natural arrangement of layers in FLG is ABA (Bernal) stacking, a metastable ABC (rhombohedral) stacking, characterized by a relatively high-energy barrier, can also occur. When both types of stacking occur in one FLG device, the arrangement results in an in-plane heterostructure with a domain wall (DW). In this paper, we present two approaches to demonstrate that the ABC stacking in FLG can be controllably and locally turned into the ABA stacking. In the first approach, we introduced Joule heating, and the transition was characterized by 2D peak Raman spectra at a submicron spatial resolution. The transition was initiated in a small region, and then the DW was controllably shifted until the entire device became ABA stacked. In the second approach, the transition was achieved by illuminating the ABC region with a train of 790-nm-wavelength laser pulses, and the transition was visualized by transmission electron microscopy in both diffraction and dark-field imaging modes. Further, using this approach, the DW was visualized at a nanoscale spatial resolution in the dark-field imaging mode.

Laser solid-phase synthesis of single-atom catalysts

Single-atom catalysts(SACs)with atomically dispersed catalytic sites have shown outstanding catalytic performance in a variety of reactions.However,the development of facile and high-yield techniques for the fabrication of SACs remains challenging.In this paper,we report a laser-induced solid-phase strategy for the synthesis of Pt SACs on graphene support.Simply by rapid laser scanning/irradiation of a freeze-dried electrochemical graphene oxide(EGO)film loaded with chloroplatinic acid(H2PtCl6),we enabled simultaneous pyrolysis of H2PtCl6 into SACs and reduction/graphitization of EGO into graphene.The rapid freezing of EGO hydrogel film infused with H2PtCl6 solution in liquid nitrogen and the subsequent ice sublimation by freeze-drying were essential to achieve the atomically dispersed Pt.Nanosecond pulsed infrared(IR;1064 nm)and picosecond pulsed ultraviolet(UV;355 nm)lasers were used to investigate the effects of laser wavelength and pulse duration on the SACs formation mechanism.The atomically dispersed Pt on graphene support exhibited a small overpotential of−42.3 mV at−10 mA cm−2 for hydrogen evolution reaction and a mass activity tenfold higher than that of the commercial Pt/C catalyst.This method is simple,fast and potentially versatile,and scalable for the mass production of SACs.

1D/2D composite subnanometer channels for ion transport:The role of confined water

As a mass transport media,water is an alternative of organic solvent applied in rechargeable batteries,due to its unique properties,including fast ionic migration,easy-processibility,economic/environmental friendliness,and flame retardancy.However,due to the high activity of water molecules in aqueous electrolytes,the corrosion of metal anode,side reactions,and inferior metal electrodeposition behavior leads to unstable cycling performance,poor Coulombic efficiency(CE),and early-staged failure of batteries.Despite several attempts to regulate the activity of water,migration of ions is sacrificed,due to the limited methods to control the water states.Herein,we developed a subnanoscale confinement strategy based on a nacre-like structure to modulate the activity of water in the solid electrolytes.By tuning the ratio between the two-dimensional(2D)vermiculite and one-dimensional(1D)cellulose nanofibers(CNFs),the capillary size in the 1D/2D structure is altered to achieve a fast Zn^(2+)transport.Our dielectric relaxation and molecular dynamics studies indicate that the enhanced Zn^(2+)conductivity is attributed to the fast water relaxation in the precisely defined 1D/2D capillary.Taking advantage of the regulated activity of the confined water in 2D capillary,the composite vermiculite membrane can suppress the corrosion and side reactions between Zn electrode and water molecular,endowing a reversible Zn^(2+)stripping/plating behavior and a stable cycling performance for 900 h.Based on our confinement strategy to control the water states by 1D/2D structures,this work will open an avenue toward aqueous energy storage devices with excellent reversibility,high safety,and long-term stability.

A visibly transparent radiative cooling film with self-cleaning function produced by solution processing

Daylighting structures,including solar cells and building windows,utilize sunlight whilst suffering from undesired solar heat and outdoor dust contamination.A radiative cooling system that is transparent to sunlight and has a superhydrophobic surface would cool and clean the daylighting structures in a sustainable manner.However,the majority of the current daytime radiative cooling systems were designed to fully reflect the incident sunlight to maximize the cooling power.In this work,we optimized both the sunlight transmission and infrared thermal irradiation by modeling the size-dependent scattering and absorption of light by SiO_(2)spheres embedded in a polymer matrix,we found that the use of nanospheres(20 nm)enabled both high sunlight transmittance(>90%)and infrared emissivity(-0.85).This theoretical prediction was confirmed by experimental measurements of a solution-processed nanocomposite film.When coated on a solar cell,the as-prepared film not only preserved the power conversion efficiency of the cell(14.71%,uncoated cell has an efficiency of 14.79%)but also radiatively cooled the cell by up to 5℃under direct sunlight.This reduction of the operating temperature of the solar cell further enhanced its electrical power output,evidenced by an increase in the equilibrium temperature of the LED load by about 14℃.The nanoscale textured surface formed by the nanospheres further led to superhydrophobicity and thus excellent self-cleaning performance(efficient removal of dust by wind and/or water droplets).