Wetting transparency of graphene

Directional Water Transport in Flexible Nanochannels

Directional liquid transport in nanochannels composed of two-dimensional (2D) materials shows great potential in applications such as liquid diodes and biochemical nanosensors. However, experimental research on water transport at the nanoscale is challenging, and most previous theoretical studies have focused on rigid nanochannels while overlooking the coupling effect of channel flexibility on liquid transport. Considering the deformation of the 2D material, molecular dynamics simulations and theoretical analysis are employed to assess the transport of water droplets in a flexible 2D material nanochannel. The applicability of the Laplace equation at the nanoscale is verified first. And the deformation of the nanochannel confined with water droplets is analyzed using membrane theory, considering a significant interfacial effect at the nanoscale. Regardless of the wettability, thickness, and droplet size of the flexible graphene nanochannel, the droplets consistently move spontaneously toward the center of the channel, except in the case of a critical wettability state, where the water contact angle (WCA) is close to 90°. The passive transport mechanism is further explained from the perspective of the driving force on the droplet and the evolution of the system potential energy, both deduced from the asymmetric deformation of the channel. Relevant results reveal that for both hydrophilic and hydrophobic channels, as the droplet moves toward the channel center, the driving force and system potential energy decrease, reaching zero and a minimum at the center, respectively. However, in the cases of a critical wettability state, the driving force and potential energy remain zero and constant, respectively.

Untangling the Diffusion Mechanism of Water in a Heterogeneous Nanochannel

Heterogeneous confinement systems attract increasing attention owing to their widespread applications in diverse areas. However, it is still lacking an in-depth understanding of the diffusion mechanism and physical properties of water in the heterogeneous nanochannel through molecular simulations. Here, high-precision TIP4P-BGWT water molecules confined in molybdenum disulfide (MoS2) and graphene walls are utilized to investigate the influences of variables, i.e., channel height, wettability of walls, charge of MoS2, and temperature, on the diffusion mechanism and physical properties. The simulation results indicate that the diffusion mechanism is significantly affected by the channel height and temperature but weakly influenced by the wettability of walls. Observable impacts on the physical properties can be observed with the channel height and temperature, but slight impacts are observed with the wettability of walls. Considered variables, excluding charge of MoS2, remarkably influence density distribution, while limiting mean square displacement at the channel height depends solely upon the effective diffusion distance. It is worth noting that, compared to the homostructure, significant discrepancy in the density distribution can be obtained from the heterogeneous nanochannel due to different solid-liquid interactions. The present study offers a solid foundation for the design of nanodevices, such as nanomembrane, nanosensor, microfluidic chip, etc.

Hybrid structure combining essential oil derivatives and polydopamine for anti-bacterial coatings

The development of effective antimicrobial surfaces is crucial for reducing the risk of medical device-associated infections. This study investigates the antibacterial potential of carvacrol (CAR), a natural essential oil, after their surface immobilization onto gold (Au) substrates through a polydopamine (pDA) layer. The successful deposition and properties of each layer were characterized using ellipsometry, water contact angle (WCA) measurements, Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS), X-ray Photoelectron Spectroscopy (XPS), and Atomic Force Microscopy (AFM). The resulting coatings displayed a thin, uniform film with smooth topography and with enhanced hydrophilicity. Antibacterial efficacy was assessed against Staphylococcus epidermidis, a relevant etiological agent in this context. The results revealed that the polydopamine-carvacrol coated surfaces (Au-pDA-CAR) exhibited a significant reduction in bacterial viability, achieving a 96 % decrease compared to unmodified gold surfaces. This was contrasted with minimal antibacterial activity from surfaces with either only polydopamine (Au-pDA) or carvacrol (Au-CAR). Live/Dead bacterial viability assays confirmed the bactericidal effect of the Au-pDA-CAR surface, demonstrating its effectiveness in killing bacteria rather than merely preventing adhesion. Our findings indicate that the pDA-CAR coating presents a promising approach for developing antimicrobial surfaces with enhanced performance against biofilm-forming pathogens. The development of this coating is an important step towards the establishment of a new technological platform capable of preventing medical device associated infections.

Water as Solvent for the Dispersion of 2D Nanostructured Materials

The development of large number of two-dimensional (2D) nanostructured materials that followed the success of graphene and the need for their handling and manipulation e. g., in inks, brought to the fore the study of solvents and substances that contribute to the stabilization of 2D nanomaterials in the liquid phase. The successful dispersion of 2D materials in solvents is combined with one of the most widespread preparation methods, that of liquid phase exfoliation. In this article, a review for the role of water in the preparation of different 2D nanostructures and their stable dispersions in the liquid phase is discussed. The use of water as a solvent or dispersant is instrumental in promoting materials with an ecological footprint, low cost, and sustainability.

Nanoscopic Supercapacitance Elucidations of the Graphene-Ionic Interface with Suspended/Supported Graphene in Different Ionic Solutions

Graphene-based supercapacitors have gained significant attention due to their exceptional energy storage capabilities. Despite numerous research efforts trying to improve the performance, the challenge of experimentally elucidating the nanoscale-interface molecular characteristics still needs to be tackled for device optimizations in commercial applications. To address this, we have conducted a series of experiments using substrate-free graphene field-effect transistors (SF-GFETs) and oxide-supported graphene field-effect transistors (OS-GFETs) to elucidate the graphene-electrolyte interfacial arrangement and corresponding capacitance under different surface potential states and ionic concentration environments. For SF-GFET, we observed that the hysteresis of the Dirac point changes from 0.32 to -0.06 V as the ionic concentration increases. Moreover, it results in the interfacial capacitance changing from 4 to 2 F/g. For OS-GFET, the hysteresis of the Dirac point remains negative (-0.15 to -0.07 V). Furthermore, the corresponding capacitance of OS-GFET decreases (53-16 F/g) as the ionic concentration increases. These suggest that the orderly oriented water structure at the graphene-water interface is gradually replaced by ionic hydration clusters and results in the difference of capacitance. The relationship between Dirac-point hysteresis value and ionic concentration can be modeled by using the first-order Hill equation to obtain the half occupation value (K = 1.0131 × 10-4 for KCl solution and K = 6.6237 × 10-5 for MgCl2 solution). This also agrees with the variances of two minerals in ion hydration within the inner water layer at the interface. This work illustrates the influence of interfacial nanoscale arrangement on interface capacitance formation and layout implications for the development of supercapacitors.

Remote epitaxy and exfoliation of vanadium dioxide via sub-nanometer thick amorphous interlayer

The recently emerged remote epitaxy technique, utilizing 2D materials (mostly graphene) as interlayers between the epilayer and the substrate, enables the exfoliation of crystalline nanomembranes from the substrate, expanding the range of potential device applications. However, remote epitaxy has been so far applied to a limited range of material systems, owing to the need of stringent growth conditions to avoid graphene damaging, and has therefore remained challenging for the synthesis of oxide nanomembranes. Here, we demonstrate the remote epitaxial growth of an oxide nanomembrane (vanadium dioxide, VO2) with a sub-nanometer thick amorphous interlayer, which can withstand potential sputtering-induced damage and oxidation. By removing the amorphous interlayer, a 4-inch wafer-scale freestanding VO2 nanomembrane can be obtained, exhibiting intact crystalline structure and physical properties. In addition, multi-shaped freestanding infrared bolometers are fabricated based on the epitaxial VO2 nanomembranes, showing high detectivity and low current noise. Our strategy provides a promising way to explore various freestanding heteroepitaxial oxide materials for future large-scale integrated circuits and functional devices.

Anisotropic Wetting and Diffusion Behavior of Water Droplets on Biphenylene Compared to Graphene

We comparatively studied the wetting behavior of water droplets on graphene and biphenylene using molecular dynamics simulations. The research showed that pristine biphenylene (BPN), unlike graphene, exhibits greater hydrophobicity and anisotropic wettability. This specific anisotropy can be tuned by the layer number and vacancy concentration. Particularly, there was a decrease in the water contact angle with increasing BPN layer number, highlighting the importance of water-BPN interactions. As the concentration of vacancies increases, the contact angle increases both along the zigzag direction and the armchair direction, while the anisotropy decreases. In planar defective biphenylene heterojunctions, water droplets spontaneously move from the defective area to the pristine area, where the zigzag direction exhibits a larger energy gradient compared to the armchair direction, leading to a faster movement of water droplets in the zigzag direction. The energy factor plays an important role in the directional movement of the water droplets. Our study explores new 2D materials with strong hydrophobicity and highlights the direction-dependent wetting behavior of biphenylene.

Capillary filling dynamics in closed-end carbon nanotubes-Defying the classical Lucas-Washburn paradigm

The emergence of two-dimensional (2D) materials such as carbon nanotubes (CNTs) offers the possibility of exploring new regimes of capillarity and wetting that remained inaccessible with traditional microfluidic and nanofluidic substrates. Here, we bring out the non-intuitive capillary filling regimes in closed-end CNTs using molecular-level investigations. Contrary to the existing understanding of the advancing liquid meniscus getting retarded by the viscous resistance offered by an entrapped vapor phase in a three-dimensional capillary, here the liquid meniscus is shown to accelerate toward the later stages of the dynamic wetting, which is attributed to the modified surface friction due to a 2D interface. This apparently counterintuitive observation is qualitatively linked to the local pressure fluctuations across the meniscus caused by the spontaneous bombardment of the entrapped vapor molecules, which may ramify into hitherto unexplored phenomena of a shape-reversed meniscus advancing in the 2-D pore. We further develop a simple analytical model to represent the essential physics of the resulting capillary filling dynamics, featuring significant deviations from the classical Lucas-Washburn paradigm. These results may turn out to be imperative in realizing new regimes of capillarity in 2D materials in multifarious applications, ranging from energy storage and water filtration to thin film flows in integrated electronics and photonic devices.

2D Embedded Ultrawide Bandgap Devices for Extreme Environment Applications

Ultrawide bandgap semiconductors such as AlGaN, AlN, diamond, and β-Ga2O3 have significantly enhanced the functionality of electronic and optoelectronic devices, particularly in harsh environment conditions. However, some of these materials face challenges such as low thermal conductivity, limited P-type conductivity, and scalability issues, which can hinder device performance under extreme conditions like high temperature and irradiation. In this review paper, we explore the integration of various two-dimensional materials (2DMs) to address these challenges. These materials offer excellent properties such as high thermal conductivity, mechanical strength, and electrical properties. Notably, graphene, hexagonal boron nitride, transition metal dichalcogenides, 2D and quasi-2D Ga2O3, TeO2, and others are investigated for their potential in improving ultrawide bandgap semiconductor-based devices. We highlight the significant improvement observed in the device performance after the incorporation of 2D materials. By leveraging the properties of these materials, ultrawide bandgap semiconductor devices demonstrate enhanced functionality and resilience in harsh environmental conditions. This review provides valuable insights into the role of 2D materials in advancing the field of ultrawide bandgap semiconductors and highlights opportunities for further research and development in this area.

Atomic-Scale Friction and Adhesion at Ambient Pressure

In this Perspective, we present the recent advancement and the prospects of atomic-scale friction and adhesion measurements across the pressure gap between ultrahigh vacuum and ambient pressure environments using variable-pressure atomic force microscopy (VP-AFM). We introduce the VP-AFM that enables nanotribological studies under various gas conditions with partial pressure ranging from UHV (1.0 × 10-10 mbar) to 1 bar. We highlight the frictional behaviors of ultrananocrystalline diamond surface in oxygen and water gas environments, as well as the chemical states probed with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The atomic scale degradation processes of MA(CH3NH3)PbBr3, which is an organic-inorganic hybrid perovskite (OHP) investigated with VP-AFM are introduced. Finally, we discuss the potential works on catalytic model systems including bimetallic Pt3Ni(111) and TiO2(110) and the future perspective of nanotribology under ambient conditions.

Hybrid fibrous architectures-mediated gene transfer by pDNA nanoparticles into macrophages

Gene therapy is one of the most potential therapeutic approaches in direct and specific regulation of biological functions of macrophages at the gene level for efficient cell therapy. However, the delivery of genetic material to macrophages is extremely challenging, because of low stability, specificity and inability of therapeutic genes to efficiently enter the cells. Here, we present a method that uses the hybrid electrospun architectures based on gelatin-alginate decorated with carboxylated graphene oxide (HAG/G) as efficient substrate for loading and in vitro local and controlled delivery of plasmid DNA (pDNA) to macrophages as an alternative to systemic gene delivery carriers. Polyethyleneimine (PEI) is employed to assemble PEI/pDNA nanoparticles (Np) - used as model of carrier. The dispersion of GO-COOH sheets shifts the surface zeta potential of HAG/G to high negative value (SZP = -16.8 ± 2.21 mV) and further increases the encapsulation efficiency of PEI/pDNA Np onto hybrid HAG/G electrospun architectures to ∼ 69 % (HAG/G-Np). The in vitro biological investigations show a good metabolic activity of macrophages seeded onto HAG/G-Np (MTT assay), while gene expression experiments (fluorescent microscopy) show a 30 % increase in transient gene transfection of cells cultured in the presence of HAG/G-Np as compared to those incubated with free PEI/pDNA Np.

Role of Underlying Substrates on the Interfacial Thermal Transport in Supported Graphene Nanochannels: Implications of Thermal Translucency

We study the role of underlying substrates on interfacial heat transfer within supported graphene nanochannels. Due to graphene's translucency, the underlying substrate, apart from its known hydrodynamic impact on fluid flow, also influences heat transport. We introduce the term "thermal translucency" to describe this phenomenon in the context of interfacial heat transfer. Our findings reveal that the Kapitza resistance, RK, is dependent on the specific underlying substrate. The specific underlying substrate alters the water-graphene interface potential landscape due to graphene's translucency, leading to a breakdown in the inverse relationship between interfacial water density peaks and RK values, typically observed at water-graphene and water-graphite interfaces. Remarkably, higher interfacial water density peaks correlate with more ordered energy patterns, not necessarily tied to more hydrophilic substrates as the literature commonly suggests for lower RK values. The insights provided have implications for controlling and tuning thermal transport and heat storage in nanofluidic devices.

Slip Opacity and Fast Osmotic Transport of Hydrophobes at Aqueous Interfaces with Two-Dimensional Materials

We investigate the interfacial transport of water and hydrophobic solutes on van der Waals bilayers and heterostructures formed by stacking graphene, hBN, and MoS2 using extensive ab initio molecular dynamics simulations. We compute water slippage and the diffusio-osmotic transport coefficient of hydrophobic particles at the interface by combining hydrodynamics and the theory of the hydrophobic effect. We find that slippage is dominated by the layer that is in direct contact with water and only marginally altered by the second layer, leading to a so-called "slip opacity". The screening of the lateral forces, where the liquid does not feel the forces coming from the second nearest layer, is one of the factors leading to the "slip opacity" in our systems. The diffusio-osmotic transport of small hydrophobes (with a radius below 2.5 Å) is also affected by the slip opacity, being dramatically enhanced by slippage. Furthermore, the direction of diffusio-osmotic flow is controlled by the solute size, with the flow in the opposite direction of the concentration gradient for smaller hydrophobes, and vice versa for larger ones. We connect our findings to the wetting properties of two-dimensional materials, and we propose that slippage and wetting can be controlled separately: whereas the slippage is mostly determined by the layer in closer proximity to water, wetting can be finely tuned by stacking different two-dimensional materials. Our study advances the computational design of two-dimensional materials and van der Waals heterostructures, enabling precise control over wetting and slippage properties for applications in coatings and water purification membranes.

Accurate Simulation for 2D Lubricating Materials in Realistic Environments: From Classical to Quantum Mechanical Methods

2D materials such as graphene, MoS2, and hexagonal BN are the most advanced solid lubricating materials with superior friction and anti-wear performance. However, as a typical surface phenomenon, the lubricating properties of 2D materials are largely dependent on the surrounding environment, such as temperature, stress, humidity, oxygen, and other environmental substances. Given the technical challenges in experiment for real-time and in situ detection of microscopic environment-material interaction, recent years have witnessed the acceleration of computational research on the lubrication behavior of 2D materials in realistic environments. This study reviews the up-to-date computational studies for the effect of environmental factors on the lubrication performance of 2D materials, summarizes the theoretical methods in lubrication from classical to quantum-mechanics ones, and emphasizes the importance of quantum method in revealing the lubrication mechanism at atomic and electronic level. An effective simulation method based on ab initio molecular dynamics is also proposed to try to provide more ways to accurately reveal the friction mechanisms and reliably guide the lubricating material design. On the basis of current development, future prospects, and challenges for the simulation and modeling in lubrication with realistic environment are outlined.

Ice-Enabled Transfer of Graphene on Copper Substrates Enhanced by Electric Field and Cu2O

Graphene films grown by the chemical vapor deposition (CVD) method suffer from contamination and damage during transfer. Herein, an innovative ice-enabled transfer method under an applied electric field and in the presence of Cu2O (or Cu2O-Electric-field Ice Transfer, abbreviated as CEIT) is developed. Ice serves as a pollution-free transfer medium while water molecules under the electric field fully wet the graphene surface for a bolstered adhesion force between the ice and graphene. Cu2O is used to reduce the adhesion force between graphene and copper. The combined methodology in CEIT ensures complete separation and clean transfer of graphene, resulting in successfully transferred graphene to various substrates, including polydimethylsiloxane (PDMS), Teflon, and C4F8 without pollution. The graphene obtained via CEIT is utilized to fabricate field-effect transistors with electrical performances comparable to that of intrinsic graphene characterized by small Dirac points and high carrier mobility. The carrier mobility of the transferred graphene reaches 9090 cm2 V-1 s-1, demonstrating a superior carrier mobility over that from other dry transfer methods. In a nutshell, the proposed clean and efficient transfer method holds great potential for future applications of graphene.

Metal-water interface formation: Thermodynamics from ab initio molecular dynamics simulations

Metal-water interfaces are central to many electrochemical, (electro)catalytic, and materials science processes and systems. However, our current understanding of their thermodynamic properties is limited by the scarcity of accurate experimental and computational data and procedures. In this work, thermodynamic quantities for metal-water interface formation are computed for a range of FCC(111) surfaces (Pd, Pt, Au, Ag, Rh, and PdAu) through extensive density functional theory based molecular dynamics and the two-phase entropy model. We show that metal-water interface formation is thermodynamically favorable and that most metal surfaces studied in this work are completely wettable, i.e., have contact angles of zero. Interfacial water has higher entropy than bulk water due to the increased population of low-frequency translational modes. The entropic contributions also correlate with the orientational water density, and the highest solvation entropies are observed for interfaces with a moderately ordered first water layer; the entropic contributions account for up to ∼25% of the formation free energy. Water adsorption energy correlates with the water orientation and structure and is found to be a good descriptor of the internal energy part of the interface formation free energy, but it alone cannot satisfactorily explain the interfacial thermodynamics; the interface formation is driven by the competition between energetic and entropic contributions. The obtained results and insight can be used to develop, parameterize, and benchmark theoretical and computational methods for studying metal-water interfaces. Overall, our study yields benchmark-quality data and fundamental insight into the thermodynamic forces driving metal-water interface formation.

Ultraclean Suspended Graphene by Radiolysis of Adsorbed Water

Access to intrinsic properties of a 2D material is challenging due to the absence of a bulk that would dominate over surface contamination, and this lack of bulk also precludes effective conventional cleaning methods that are almost always sacrificial. Suspended graphene and carbon contaminants represent the most salient challenge. This work has achieved ultraclean graphene, attested by electron energy loss (EEL) spectra unprecedentedly exhibiting fine-structure features expected from bonding and band structure. In the cleaning process in a transmission electron microscope, radicals generated by radiolysis of intentionally adsorbed water remove organic contaminants, which would otherwise be feedstock of the notorious electron irradiation induced carbon deposition. This method can be readily adapted to other experimental settings and other materials to enable previously inhibited undertakings that rely on the intrinsic properties or ultimate thinness of 2D materials. Importantly, the method is surprisingly simple and robust, easily implementable with common lab equipment.

Enhancement in Heat Transfer Performance of Water Vapor Condensation on Graphene-Coated Copper Surfaces: A Molecular Dynamics Study

The condensation of water vapor plays a crucial role in various applications, including combating water scarcity. In this study, by employing molecular dynamics simulations, we delved into the impact of graphene coatings on water vapor condensation on copper surfaces. Unique to this work was the exploration of various levels of graphene coverage and distribution, a facet largely unexplored in prior investigations. The findings demonstrated a notable increase in the rate of water vapor condensation and heat transfer performance as the graphene coverage was reduced. Using graphene coverages of 84%, 68%, and 52%, the numbers of condensed water molecules were 664, 735, and 880 molecules/ns, respectively. One of the most important findings was that when using the same graphene coverage of 68%, the rate of water vapor condensation and heat transfer performance increased as the graphene coating became more distributed. The overall performance of the water condensation correlated well with the energy and vibrational interaction between the graphene and the copper. This phenomenon suggests how a hybrid surface can enhance the nucleation and growth of a droplet, which might be beneficial for tailoring graphene-coated copper surfaces for applications demanding efficient water vapor condensation.

Wetting Transparency of Single-Layer Graphene on Liquid Substrates

Graphene's wetting transparency offers promising avenues for creating multifunctional devices by allowing real-time wettability control on liquid substrates via the flow of different liquids beneath graphene. Despite its potential, direct measurement of floating graphene's wettability remains a challenge, hindering the exploration of these applications. The current study develops an experimental methodology to assess the wetting transparency of single-layer graphene (SLG) on liquid substrates. By employing contact angle measurements and Neumann's Triangle model, the challenge of evaluating the wettability of floating free-suspended single-layer graphene is addressed. The research reveals that for successful contact angle measurements, the testing and substrate liquids must be immiscible. Using diiodomethane as the testing liquid and ammonium persulfate solution as liquid substrate, the study demonstrates the near-complete wetting transparency of graphene. Furthermore, it successfully showcases the feasibility of real-time wettability control using graphene on liquid substrates. This work not only advances the understanding of graphene's interaction with liquid interfaces but also suggests a new avenue for the development of multifunctional materials and devices by exploiting the unique wetting transparency of graphene.

Modeling Water Interactions with Graphene and Graphite via Force Fields Consistent with Experimental Contact Angles

Accurate simulation models for water interactions with graphene and graphite are important for nanofluidic applications, but existing force fields produce widely varying contact angles. Our extensive review of the experimental literature reveals extreme variation among reported values of graphene-water contact angles and a clustering of graphite-water contact angles into groups of freshly exfoliated (60° ± 13°) and not-freshly exfoliated graphite surfaces. The carbon-oxygen dispersion energy for a classical force field is optimized with respect to this 60° graphite-water contact angle in the infinite-force-cutoff limit, which in turn yields a contact angle for unsupported graphene of 80°, in agreement with the mean of the experimental results. Interaction force fields for finite cutoffs are also derived. A method for calculating contact angles from pressure tensors of planar equilibrium simulations that is ideally suited to graphite and graphene surfaces is introduced. Our methodology is widely applicable to any liquid-surface combination.

Study on Confined Water in Flexible Graphene/GO Nanochannels

The structural evolution of flexible nanochannels within a 2D material membrane, influenced by the ingress of water molecules, plays a crucial role in the membrane's filtration and structural stability. However, the experimental observation of nanoscale water is challenging, and current studies mostly focus on rigid nanochannels. Further investigation on the nanoconfined water is urgently needed, considering the flexibility and deformation of the channel. In this work, MD simulations and theoretical analyses are conducted to investigate the water structure and thermodynamic properties when confined within both rigid and flexible graphene/graphene oxide (GO) nanochannels. In free rigid graphene nanochannels, the interlayer distance exhibits a quantized increase with the number of water molecules, along with sudden changes in entropy, potential energy, and free energy of the water molecules. Meanwhile, in flexible graphene nanochannels, the average interlayer space increases linearly with the number of water molecules. In free rigid GO nanochannels, with the increase of oxidation concentration, the quantized increase in the interlayer space gradually diminishes, accompanied by a decrease in both potential energy and free energy. This work provides insights into the configurational evolution of flexible nanochannels within water, offering guidance in fields such as desalination and mass transport of 2D material membranes.

Experimental and Theoretical Insights into Interfacial Properties of 2D Materials for Selective Water Transport Membranes: A Critical Review

Interfacial properties, such as wettability and friction, play critical roles in nanofluidics and desalination. Understanding the interfacial properties of two-dimensional (2D) materials is crucial in these applications due to the close interaction between liquids and the solid surface. The most important interfacial properties of a solid surface include the water contact angle, which quantifies the extent of interactions between the surface and water, and the water slip length, which determines how much faster water can flow on the surface beyond the predictions of continuum fluid mechanics. This Review seeks to elucidate the mechanism that governs the interfacial properties of diverse 2D materials, including transition metal dichalcogenides (e.g., MoS2), graphene, and hexagonal boron nitride (hBN). Our work consolidates existing experimental and computational insights into 2D material synthesis and modeling and explores their interfacial properties for desalination. We investigated the capabilities of density functional theory and molecular dynamics simulations in analyzing the interfacial properties of 2D materials. Specifically, we highlight how MD simulations have revolutionized our understanding of these properties, paving the way for their effective application in desalination. This Review of the synthesis and interfacial properties of 2D materials unlocks opportunities for further advancement and optimization in desalination.

Competition between Water-Water Hydrogen Bonds and Water-π Bonds in Pyrene-Water Cluster Anions

We present infrared spectra and density functional theory calculations of hydrated pyrene anion clusters with up to four water molecules. The experimental spectra were acquired by using infrared Ar messenger photodissociation spectroscopy. Water molecules form clusters on the surface of the pyrene, forming hydrogen bonds with the π-system. The structures of the water clusters and their interaction with the π-system are encoded in OH stretching vibrational modes. We find that the interactions between water molecules are stronger than the interactions between water molecules and the π-system. While all clusters show multiple conformers, three- and four-membered rings are the lowest energy structures in the larger hydrates.

Role of Carbon Nanotube Wetting Transparency in Rapid Water Transport for a Nanopore Membrane

A carbon nanotube (CNT) may facilitate near-frictionless water transport within it. In this work, we elucidate the slip flow characteristics for a CNT embedded in a silicon nitride matrix using the molecular dynamics (MD) method. We reveal that the wetting transparency of a CNT, the transmission of the membrane matrix wetting property over a CNT, cannot be ignored. Due to the effect of CNT wetting transparency, the orientation flip behavior of water molecules should be the primary cause of the entrance and exit losses, which is a dominant factor influencing the interfacial friction coefficient for the thin CNT membrane. The relationship between the friction coefficient and pore size follows a logarithmic function, which agrees well with the reported experimental data. Our findings bridge the gap between the MD prediction and experimental observation for water transport in a CNT membrane and provide a clear understanding of the mechanism behind its ultrafast flow performance.

Environmentally Friendly and All-Dry Hydrophobic Patterning of Graphene Oxide for Fog Harvesting

This study examines the fog-harvesting ability of graphene oxide surfaces patterned by hydrophobic domains. The samples were prepared from graphene deposited using low pressure chemical vapor deposition, which was later plasma oxidized to obtain hydrophilic graphene oxide (GO) surfaces. Hydrophobic domains on GO surfaces were formed by initiated CVD (iCVD) of a low-surface-energy poly(perfluorodecyl alkylate) (PPFDA) polymer. Hence, patterned surfaces with hydrophobic/hydrophilic contrast were produced with ease in an all-dry manner. The structures of the as-deposited graphene and PPFDA films were characterized using Raman and Fourier transform infrared spectrophotometer analyses, respectively. The fog harvesting performance of the samples was measured using the fog generated by a nebulizer, in which the average diameter of the fog droplets is comparable to meteorological fog. According to the fog harvesting experiment results, 100 cm2 of the as-patterned surface can collect fog up to 2.5 L in 10 h in a foggy environment. Hence, hydrophilic/hydrophobic patterned surfaces in this study can be considered as promising fog harvesting materials. Both CVD techniques used in the production of hydrophilic/hydrophobic patterned surfaces can be easily applied to the production of large-scale materials.

The potential of graphene coatings as neural interfaces

Recent advances in nanotechnology design and fabrication have shaped the landscape for the development of ideal cell interfaces based on biomaterials. A holistic evaluation of the requirements for a cell interface is a highly complex task. Biocompatibility is a crucial requirement which is affected by the interface's properties, including elemental composition, morphology, and surface chemistry. This review explores the current state-of-the-art on graphene coatings produced by chemical vapor deposition (CVD) and applied as neural interfaces, detailing the key properties required to design an interface capable of physiologically interacting with neural cells. The interfaces are classified into substrates and scaffolds to differentiate the planar and three-dimensional environments where the cells can adhere and proliferate. The role of specific features such as mechanical properties, porosity and wettability are investigated. We further report on the specific brain-interface applications where CVD graphene paved the way to revolutionary advances in biomedicine. Future studies on the long-term effects of graphene-based materials in vivo will unlock even more potentially disruptive neuro-applications.

The effect of surface hydrophobicity and hydrophilicity on ion-ion interactions at water-solid interfaces

Condensation and arrangement of ions at water-solid interfaces are of great importance in the formation of electrical double layers (EDL) and the transport of ions under a confined geometry. So far, the microscopic understanding of interfacial ion configurations is still far from complete, especially when the local ion concentration is high and ion-ion interactions become prominent. In this study, we directly visualized alkali metal cations within the hydrogen-bonding network of water on graphite and Cu(111)-supported graphene surfaces, using qPlus-based noncontact atomic force microscopy (NC-AFM). We found that the codeposition of the alkali cations and water molecules on the hydrophobic graphite surface leads to the formation of an ion-doped bilayer hexagonal ice (BHI) structure, where the ions are repelled from each other and scattered in a disordered distribution. In contrast, the hydrated alkali cations aggregate in one dimension on the more hydrophilic graphene/Cu(111) surface, forming a nematic state with a long-range order. Such a nematic state arises from the delicate interplay between water-ion and water-water interactions under surface confinement. These results reveal the high sensitivity of ion-ion interactions and ionic ordering to the surface hydrophobicity and hydrophilicity.

Ambient-mediated wetting on smooth surfaces

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research has demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity, of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the transition from wetting (hydrophilicity) to non-wetting (hydrophobicity) on somooth surfaces due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure as a function of the surface carbon content has also been developed. In addition, we analyse recent works that address these wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding this surface contamination phenomenon within the engineering, interfacial science, and physical chemistry domains.

Challenges for Field-Effect-Transistor-Based Graphene Biosensors

Owing to its outstanding physical properties, graphene has attracted attention as a promising biosensor material. Field-effect-transistor (FET)-based biosensors are particularly promising because of their high sensitivity that is achieved through the high carrier mobility of graphene. However, graphene-FET biosensors have not yet reached widespread practical applications owing to several problems. In this review, the authors focus on graphene-FET biosensors and discuss their advantages, the challenges to their development, and the solutions to the challenges. The problem of Debye screening, in which the surface charges of the detection target are shielded and undetectable, can be solved by using small-molecule receptors and their deformations and by using enzyme reaction products. To address the complexity of sample components and the detection mechanisms of graphene-FET biosensors, the authors outline measures against nonspecific adsorption and the remaining problems related to the detection mechanism itself. The authors also introduce a solution with which the molecular species that can reach the sensor surfaces are limited. Finally, the authors present multifaceted approaches to the sensor surfaces that provide much information to corroborate the results of electrical measurements. The measures and solutions introduced bring us closer to the practical realization of stable biosensors utilizing the superior characteristics of graphene.

Solvent-Assisted Graphene Exfoliation from Graphite Using Umbrella Sampling Simulations

We employed molecular dynamics (MD) simulations coupled with umbrella sampling to explore the thermodynamics governing the exfoliation of a single graphene layer from a graphitic substrate in five different solvents such as dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), cyclohexane (CHX), and water. The substrate was modeled as a stack of three identical graphene layers with the graphene sheet undergoing exfoliation positioned on top of this stack. The initial configurations for each umbrella simulation were generated through steered MD simulations carried out along two distinct coordinates: one parallel and the other perpendicular to the graphene layers. Our analyses revealed a uniform wetting behavior for both the nanosheet and the graphitic substrate in all of the tested solvents. Consistent with experimental observations, the steered simulations confirmed that exfoliation is more favorable along the parallel direction than along the perpendicular one. All non-water solvents exhibit comparable effectiveness in the exfoliation of graphene. The calculated free energies of these solvents in parallel exfoliation consistently fell within the range of 90-100 kJ/mol/nm2. In perpendicular exfoliation, however, the corresponding energies converge to lower values. This difference is attributed to the nonequilibrium nature of the perpendicular exfoliation, primarily caused by the great steering velocity of the graphene sheet immediately after detachment from the substrate. This rapid motion of the nanosheet along the perpendicular coordinate results in an elevated system energy and heating.

Long-Distance Continuous Self-Transport of a Droplet by Merging Droplets on a Graphene-Covered Multibranch Gradient Groove Surface

Although the self-transport of liquid droplets by a gradient-textured substrate can break away from the energy input, the long distance and even continuous spontaneous motion of droplets will be limited by the length in the surface-gradient direction. This article introduces a novel design with a monolayer graphene-covered multibranch gradient groove surface (GMGGS). The design aims to achieve long-distance, continuous self-transport of a mercury (Hg) droplet by merging with other mercury droplets, and the process is carried out using molecular dynamics (MD) simulation. This method achieves the merging of mercury droplets through the structure of multibranch gradient grooves, and we have observed that the merged mercury droplet can be reaccelerated in the gradient groove. The results demonstrate that droplet merging allows for control over the surface morphology variations of mercury droplets within the gradient groove. This creates a forward pressure difference, which leads to reacceleration of the mercury droplets. In light of this mechanism, the trunk droplet can achieve long-distance continuous self-transport on the GMGGS by continuously merging with branch droplets. These findings will broaden our comprehension of droplet merging and self-transport behavior, offering corresponding theoretical support for the long-distance continuous self-transport of droplets.

Size Matters: Free-Energy Calculations of Amino Acid Adsorption over Pristine Graphene

There is still growing interest in graphene interactions with proteins, both for its possible biological applications and due to concerns over detrimental effects at the cellular level. As with any process involving proteins, an understanding of amino acid composition is desirable. In this work, we systematically studied the adsorption process of amino acids onto pristine graphene via rigorous free-energy calculations. We characterized the free energy, potential energy, and entropy of the adsorption of all proteinogenic amino acids. The energetic components were further separated into pair interaction contributions. A linear correlation was found between the free energy and the solvent accessible surface area change during adsorption (ΔSASAads) over pristine graphene and uncharged amino acids. Free energies over pristine graphene were compared with adsorption onto graphene oxide, finding an almost complete loss of the favorability of amino acid adsorption onto graphene. Finally, the correlation with ΔSASAads was used to successfully predict the free energy of adsorption of several penta-l-peptides in different structural states and sequences. Due to the relative ease of calculating the ΔSASAads compared to free-energy calculations, it could prove to be a cost-effective predictor of the free energy of adsorption for proteins onto nonpolar surfaces.

Dielectric-free electrowetting on graphene

Electrowetting is a simple way to induce the spreading and retraction of electrolyte droplets. This method is widely used in "device" applications, where a dielectric layer is applied between the electrolyte and the conducting substrate. Recent work, including contributions from our own laboratory, have shown that reversible electrowetting can be achieved directly on conductors. We have shown that graphite surfaces, in particular when combined with highly concentrated electrolyte solutions, show a strong wetting effect. The process is driven by the interactions between the electrolyte ions and the surface, hence models of double-layer capacitance are able to explain changes in the equilibrium contact angles. Herein, we extend the approach to the investigation of electrowetting on graphene samples of varying thickness, prepared by chemical vapor deposition. We show that the use of highly concentrated aqueous electrolytes induces a clear yet subtle electrowetting response due to the adsorption of ions and the suppression of the negative effect introduced by the surface impurities accumulating during the transfer process. The latter have been previously reported to fully hinder electrowetting at lower electrolyte concentrations. An amplified wetting response is recorded in the presence of strongly adsorbed/intercalated anions in both aqueous and non-aqueous electrolytes. The phenomenon is interpreted based on the anion-graphene interactions and their influence on the energetics of the interface. By monitoring the dynamics of wetting, an irreversible behaviour is identified in all cases as a consequence of the irreversibility of anion adsorption and/or intercalation. Finally, the effect of the underlying reactions on the timescales of wetting is also examined.

3He spin-echo scattering indicates hindered diffusion of isolated water molecules on graphene-covered Ir(111)

The dynamics of water diffusion on carbon surfaces are of interest in fields as diverse as furthering the use of graphene as an industrial-coating technology and understanding the catalytic role of carbon-based dust grains in the interstellar medium. The early stages of water-ice growth and the mobility of water adsorbates are inherently dependent on the microscopic mechanisms that facilitate water diffusion. Here, we use 3He spin-echo quasi-inelastic scattering to probe the microscopic mechanisms responsible for the diffusion of isolated water molecules on graphene-covered and bare Ir(111). The scattering of He atoms provides a non-invasive and highly surface-sensitive means to measure the rate at which absorbates move around on a substrate at very low coverage. Our results provide an approximate upper limit on the diffusion coefficient for water molecules on GrIr(111) of < 10 - 12  m2/s, an order of magnitude lower than the coefficient that describes the diffusion of water molecules on the bare Ir(111) surface. We attribute the hindered diffusion of water molecules on the GrIr(111) surface to water trapping at specific areas of the corrugated moiré superstructure. Lower mobility of water molecules on a surface is expected to lead to a lower ice nucleation rate and may enhance the macroscopic anti-icing properties of a surface.

Nanoscale Elasto-Capillarity in the Graphene-Water System under Tension: Revisiting the Assumption of a Constant Wetting Angle

Wetting highly compliant surfaces can cause them to deform. Atomically thin materials, such as graphene, can have exceptionally small bending rigidities, leading to elasto-capillary lengths of a few nanometers. Using large-scale molecular dynamics (MD), we have studied the wetting and deformation of graphene due to nanometer-sized water droplets, focusing on the wetting angle near the vesicle transition. Recent continuum theories for wetting of flexible membranes reproduce our MD results qualitatively well. However, we find that when the curvature is large at the triple-phase contact line, the wetting angle increases with decreasing tension. This is in contrast to existing macroscopic theories but can be amended by allowing for a variable wetting angle.

Application of a Fluctuating Charge Polarization Model to Large Polyaromatic Hydrocarbons and Graphene Nanoflakes

We present a polarization model incorporating coupled fluctuating charges and point inducible dipoles that is able to accurately describe the dipole polarizabilities of small hydrocarbons and, for sufficiently large graphene nanoflakes, reproduce the classical image potential of an infinite conducting sheet. When our fluctuating charge model is applied to the hexagonal carbon nanoflake C60000 we attain excellent agreement with the image potential and induced charge distribution of a conducting sheet. With the inclusion of inducible dipole terms, the model predicts an image plane of zim = 1.3334 a0, which falls in line with prior estimates for graphene. We consider the case of two charges placed on opposite sides of C60000 and find that the fluctuating charge model reproduces classical electrostatics once again. By testing opposing and similar signs of the external charges, we conclude that an atomically thin molecule or extended system does not fully screen their interaction.

Structure evolution at the gate-tunable suspended graphene-water interface

Graphitic electrode is commonly used in electrochemical reactions owing to its excellent in-plane conductivity, structural robustness and cost efficiency1,2. It serves as prime electrocatalyst support as well as a layered intercalation matrix2,3, with wide applications in energy conversion and storage1,4. Being the two-dimensional building block of graphite, graphene shares similar chemical properties with graphite1,2, and its unique physical and chemical properties offer more varieties and tunability for developing state-of-the-art graphitic devices5-7. Hence it serves as an ideal platform to investigate the microscopic structure and reaction kinetics at the graphitic-electrode interfaces. Unfortunately, graphene is susceptible to various extrinsic factors, such as substrate effect8-10, causing much confusion and controversy7,8,10,11. Hereby we have obtained centimetre-sized substrate-free monolayer graphene suspended on aqueous electrolyte surface with gate tunability. Using sum-frequency spectroscopy, here we show the structural evolution versus the gate voltage at the graphene-water interface. The hydrogen-bond network of water in the Stern layer is barely changed within the water-electrolysis window but undergoes notable change when switching on the electrochemical reactions. The dangling O-H bond protruding at the graphene-water interface disappears at the onset of the hydrogen evolution reaction, signifying a marked structural change on the topmost layer owing to excess intermediate species next to the electrode. The large-size suspended pristine graphene offers a new platform to unravel the microscopic processes at the graphitic-electrode interfaces.

Unveiling the mechanism of remote epitaxy of crystalline semiconductors on 2D materials-coated substrates

Remote epitaxy has opened novel opportunities for advanced manufacturing and heterogeneous integration of two-dimensional (2D) materials and conventional (3D) materials. The lattice transparency as the fundamental principle of remote epitaxy has been studied and challenged by recent observations defying the concept. Understanding remote epitaxy requires an integrated approach of theoretical modeling and experimental validation at multi-scales because the phenomenon includes remote interactions of atoms across an atomically thin material and a few van der Waals gaps. The roles of atomically thin 2D material for the nucleation and growth of a 3D material have not been integrated into a framework of remote epitaxy research. Here, we summarize studies of remote epitaxy mechanisms with a comparison to other epitaxy techniques. In the end, we suggest the crucial topics of remote epitaxy research for basic science and applications.

The Influence of Adventitious Carbon Groups on the Wetting of Copper: A Study on the Effect of Microstructure on the Static Contact Angle

Tuning of the wetting behavior of metallic surfaces by chemical and topographical modification has become popular in recent years. Still, there is a lack in the understanding of fundamental relations between intrinsic properties of the material and its resulting water contact angle. It is widely accepted in the literature that transitions from a hydrophilic to increasingly hydrophobic behavior upon exposure to ambient conditions happen due to the adsorption of adventitious hydrocarbons. In order to investigate the role of metallic bulk microstructure in the wetting behavior and its transition properties, we created three different grain sizes and deformation states on copper by preparation combined with heat treatment. We found that for freshly prepared surfaces, differences in the wetting behavior show a higher static contact angle for mechanically prepared surfaces with a fine-crystalline deformation layer compared to the electropolished cold-rolled copper sheet and the annealed defect-free coarse-grained surface. Already after five days of storage time, most of this difference vanishes, and all surfaces show a wetting behavior with a contact angle in the range of 97-100° after 30 days. Though long-term wetting behavior seems largely independent of microstructure, correlated XPS measurements showed an increased adsorption of organic contaminants of the mechanically polished surface. Preparation-induced near-surface defects seem to accelerate adsorption, while varying grain size and slight bulk deformation from rolling processes did not show significant effects. Complex relations between the amount of adsorbed carbon and the polarity of the adsorption film were found to depend on the sample age and influence the contact angle.

Moderate binding of villin headpiece protein to C3N3 nanosheet reveals the suitable biocompatibility of this nanomaterial

Since its recent successful synthesis and due to its promising physical and chemical properties, the carbon nitrite nanomaterial, C3N3, has attracted considerable attention in various scientific areas. However, thus far, little effort has been devoted to investigating the structural influence of the direct interaction of this 2D nanomaterial and biomolecules, including proteins and biomembranes so as to understand the physical origin of its bio-effect, particularly from the molecular landscape. Such information is fundamental to correlate to the potential nanotoxicology of the C3N3 nanomaterial. In this work, we explored the potential structural influence of a C3N3 nanosheet on the prototypical globular protein, villin headpiece (HP35) using all-atom molecular dynamics (MD) simulations. We found that HP35 could maintain its native conformations upon adsorption onto the C3N3 nanosheet regardless of the diversity in the binding sites, implying the potential advantage of C3N3 in protecting the biomolecular structure. The adsorption was mediated primarily by vdW interactions. Moreover, once adsorbed on the C3N3 surface, HP35 remains relatively fixed on the nanostructure without a distinct lateral translation, which may aid in keeping the structural integrity of the protein. In addition, the porous topological structure of C3N3 and the special water layer present on the C3N3 holes conjointly contributed to the restricted motion of HP35 via the formation of a high free energy barrier and a steric hindrance to prevent the surface displacement. This work revealed for the first time the potential influence of the 2D C3N3 nanomaterial in the protein structure and provided the corresponding in-depth molecular-level mechanism, which is valuable for future applications of C3N3 in bionanomedicine.

Pure Graphene Acts as an "Entropic Surfactant" at the Octanol-Water Interface

Graphene oxide (GO) has proved itself as a nanomaterial capable of acting as a surfactant by lowering the interfacial tension of the oil-water interface due to its polar oxygen groups. However, the surfactant behavior of the pure graphene sheet─since prevention of edge oxidation in experimental setups is nontrivial─is still an unresolved issue in graphene research despite significant progress in the field in recent years. Here, we conduct both atomistic and coarse-grained simulations to demonstrate that─surprisingly─even pristine graphene, which only consists of hydrophobic carbon atoms, is attracted to the octanol-water interface and consequently reduces its surface tension by 2.3 kBT/nm2 or about 10 mN/m. Interestingly, the location of the free energy minimum is not precisely at the oil-water interface itself but is rather buried about two octanol layers into the octanol phase, being about 0.9 nm from the water phase. We demonstrate that the observed surfactant behavior is purely entropically driven and can be attributed to the unfavorable lipid-like structuring of octanol molecules at the free octanol-water interface. In essence, graphene enhances the inherent lipid-like behavior of octanol at the water interface rather than directly acting as a surfactant. Importantly, graphene does not display surfactant-like behavior in corresponding Martini coarse-grained simulations of the octanol-water system since the free liquid-liquid interface loses essential structure at the lower coarse-grained resolution. However, a similar surfactant behavior is recovered in coarse-grained simulations of longer alcohols such as dodecan-1-ol and hexadecan-1-ol. The observed discrepancies at different model resolutions enable us to construct a comprehensive model of the surfactant behavior of graphene at the octanol-water interface. The here-gained insights may facilitate the broader utilization of graphene in numerous domains of nanotechnology. Furthermore, since a drug's octanol-water partition coefficient is a crucial physicochemical parameter in rational drug discovery, we also believe that the universality of the here-illustrated entropic surfactant behavior of planar molecules deserves special attention in the drug design and development field.

Plasma-Induced Surface Modification of Sapphire and Its Influence on Graphene Grown by Plasma-Enhanced Chemical Vapour Deposition

In this work, we study the influence of the different surface terminations of c-plane sapphire substrates on the synthesis of graphene via plasma-enhanced chemical vapor deposition. The different terminations of the sapphire surface are controlled by a plasma process. A design of experiments procedure was carried out to evaluate the major effects governing the plasma process of four different parameters: i.e., discharge power, time, pressure and gas employed. In the characterization of the substrate, two sapphire surface terminations were identified and characterized by means of contact angle measurements, being a hydrophilic (hydrophobic) surface and the fingerprint of an Al- (OH-) terminated surface, respectively. The defects within the synthesized graphene were analyzed by Raman spectroscopy. Notably, we found that the I_D/I_G ratio decreases for graphene grown on OH-terminated surfaces. Furthermore, two different regimes related to the nature of graphene defects were identified and, depending on the sapphire terminated surface, are bound either to vacancy or boundary-like defects. Finally, studying the density of defects and the crystallite area, as well as their relationship with the sapphire surface termination, paves the way for increasing the crystallinity of the synthesized graphene.

Wetting of MXenes and Beyond

MXenes are a class of 2D nanomaterials with exceptional tailor-made properties such as mechano-ceramic nature, rich chemistry, and hydrophilicity, to name a few. However, one of the most challenging issues in any composite/hybrid system is the interfacial wetting. Having a superior integrity of a given composite system is a direct consequence of the proper wettability. While wetting is a fundamental feature, dictating many physical and chemical attributes, most of the common nanomaterials possesses poor affinity due to hydrophobic nature, making them hard to be easily dispersed in a given composite. Thanks to low contact angle, MXenes can offer themselves as an ideal candidate for manufacturing different nano-hybrid structures. Herein this review, it is aimed to particularly study the wettability of MXenes. In terms of the layout of the present study, MXenes are first briefly introduced, and then, the wettability phenomenon is discussed in detail. Upon reviewing the sporadic research efforts conducted to date, a particular attention is paid on the current challenges and research pitfalls to light up the future perspectives. It is strongly believed that taking the advantage of MXene's rich hydrophilic surface may have a revolutionizing role in the fabrication of advanced materials with exceptional features.

The First-Water-Layer Evolution at the Graphene/Water Interface under Different Electro-Modulated Hydrophilic Conditions Observed by Suspended/Supported Field-Effect-Device Architectures

Interfacial water molecules affect carrier transportation within graphene and related applications. Without proper tools, however, most of the previous works focus on simulation modeling rather than experimental validation. To overcome this obstacle, a series of graphene field-effect transistors (GFETs) with suspended (substrate-free, SF) and supported (oxide-supported, OS) configurations are developed to investigate the graphene-water interface under different hydrophilic conditions. With deionized water environments, in our experiments, the electrical transportation behaviors of the graphene mainly originate from the evolution of the interfacial water-molecule arrangement. Also, these current-voltage behaviors can be used to elucidate the first-water layer at the graphene-water interface. For SF-GFET, our experimental results show positive hysteresis in electrical transportation. These imply highly ordered interfacial water molecules with a separated-ionic distributed structure. For OS-GFET, on the contrary, the negative hysteresis shows the formation of the hydrogen-bond interaction between the interfacial water layer and the SiO₂ substrate under the graphene. This interaction further promotes current conduction through the graphene/water interface. In addition, the net current-voltage relationship also indicates the energy required to change the orientation of the first-layer water molecules during electro-potential change. Therefore, our work gives an insight into graphene-water interfacial evolution with field-effect modulation. Furthermore, this experimental architecture also paves the way for investigating 2D solid-liquid interfacial features.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Modulation of Remote Epitaxial Heterointerface by Graphene-Assisted Attenuative Charge Transfer

Remote epitaxy (RE), substrate polarity can "penetrate" two-dimensional materials (2DMs) and act on the epi-layer, showing a prospective universal growth strategy. However, essentially, the role that 2DMs plays in RE has not been deeply investigated so far. Here, the RE of single-crystal films on the weakest polarity/iconicity substrate is realized to reveal its essence physical properties. Graphene facilitates attenuative charge transfer (ACT) from a substrate to epi-layer to construct remote interactions. Interfacial atoms are assembled into "incommensurate" epitaxial relationships through graphene to reduce misfit dislocations in the epi-layer. Moreover, graphene reduces the atomic migration barrier, leading to a tendency toward a "layer-by-layer" growth mode. Such film growth mode is different with the conventional epitaxy (CE), and it is beneficial for the fast growth of epi-layers and the reduction of dislocations at coalescence boundaries. The insightful revelation of the role of graphene reveals the interface physics of RE and provides a more valuable guide to using 2DMs to expand three-dimensional materials (3DMs) for application in devices.

Growing Nanocrystalline Graphene on Aggregates for Conductive and Strong Smart Cement Composites

Highly conductive concrete/mortar has been long pursued to realize structural health monitoring in the development of smart-cement-based facilities. However, it remains challenging to significantly increase the electrical conductivity of concrete/mortar without lowering the compressive strength and flowability. Here, nanocrystalline-graphene-coated aggregates (termed Gr@AGs) are synthesized to break this conductivity-strength tradeoff. Admixing Gr@AGs with cement enables the construction of a conductive network of graphene that simultaneously strengthens the interfacial transition zone between aggregates and paste. As a result, high conductivity and improved mechanical properties have been simultaneously realized in Gr@AGs-based smart mortars. The significant positive effects of Gr@AGs are further enhanced by combining them with a low percentage of carbon fiber. Typically, the 28-day compressive/flexural strength of the optimized mortar increases by 12.2%/19.4%, with the electrical resistivity reduced by over 3 orders of magnitude from ∼4.6 × 10⁵ to 182 Ω cm. On this basis, we demonstrate high-sensitivity cement-based piezoresistive sensors with a fractional change in resistivity as high as ∼25%, which is more than 1 order of magnitude higher than those reported in comparable systems. This study provides a solution to the critical issues in developing smart cementitious composites by taking full advantage of graphene's properties.

Review on Hybrid Reinforced Polymer Matrix Composites with Nanocellulose, Nanomaterials, and Other Fibers

The use of composite materials has seen many new innovations for a large variety of applications. The area of reinforcement in composites is also rapidly evolving with many new discoveries, including the use of hybrid fibers, sustainable materials, and nanocellulose. In this review, studies on hybrid fiber reinforcement, the use of nanocellulose, the use of nanocellulose in hybrid forms, the use of nanocellulose with other nanomaterials, the applications of these materials, and finally, the challenges and opportunities (including safety issues) of their use are thoroughly discussed. This review will point out new prospects for the composite materials world, enabling the use of nano- and micron-sized materials together and creating value-added products at the industrial scale. Furthermore, the use of hybrid structures consisting of two different nano-materials creates many novel solutions for applications in electronics and sensors.

Water flow in graphene nanochannels driven by imposed thermal gradients: the role of flexural phonons

Accurate control of fluid transport in nanoscale structures is key to enable the design of foreseeable nanofluidic devices with applications in many fields such as chip cooling, energy conversion, drug delivery and medical diagnosis. Here, inspired by the experimental observation of intrinsic thermal ripples in graphene and by recent advances in the manipulation of 2D nanomaterials, we introduce a graphene-based thermal nanopump which produces controlled and continuous liquid flow in nanoslit channels. We investigate the performance of this thermal nanopump employing large scale molecular dynamics simulations. Upon systematically imposing thermal gradients, a net water flow towards the low-temperature zone is observed, achieving flow velocities up to 4 m s^-1. We observe that water flow rates increase monotonically due to larger ripple fluctuations on the graphene layers as higher thermal gradients are applied. Moreover, we find that the out-of-plane flexural phonons in graphene are responsible for flow generation wherein lower frequency phonon branches are activated with higher imposed thermal gradients. Furthermore, by modifying the wettability of the channel walls, an increase of 50% in the water flow rates is observed, showing that the efficiency of the proposed thermal pump can be enhanced by tuning the channel wall hydrophobicity. Our results indicate that thermal gradients can be employed to drive continuous water flow in graphene nanoslit channels with potential applications in nanofluidic devices.