Interface-Governed Deformation of Nanobubbles and Nanotents Formed by Two-Dimensional Materials

Nanoblisters such as nanobubbles and nanotents formed by two-dimensional (2D) materials have been extensively exploited for strain engineering purposes as they can produce self-sustained, nonuniform in-plane strains through out-of-plane deformation. However, deterministic measure and control of strain fields in these systems are challenging because of the atomic thinness and unconventional interface behaviors of 2D materials. Here, we experimentally characterize a simple and unified power law for the profiles of a variety of nanobubbles and nanotents formed by 2D materials such as graphene and MoS_{2} layers. Using membrane theory, we analytically unveil what sets the in-plane strains of these blisters regarding their shape and interface characteristics. Our analytical solutions are validated by Raman spectroscopy measured strain distributions in bulged graphene bubbles supported by strong and weak shear interfaces. We advocate that both the strain magnitudes and distributions can be tuned by 2D material-substrate interface adhesion and friction properties.

Non-monotonous Wetting of Graphene-Mica and MoS2-Mica Interfaces with a Molecular Layer of Water

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale, the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS₂ deposited on dry mica are used to investigate wetting of the two-dimensional (2D) material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17% for graphenes and 8-9% for MoS₂ and concludes with a homogeneous layer at 25-30 and 15-20%, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25% RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter causing at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at submonolayer concentrations may also be a quite general phenomenon.

Graphene-Encapsulated DNA Nanostructure: Preservation of Topographic Features at High Temperature and Site-Specific Oxidation of Graphene

This paper reports the effect of graphene encapsulation on the thermal stability of DNA nanostructures and the thermal oxidation of graphene in the presence of DNA nanostructures. Triangular-shaped DNA nanostructures were deposited onto a Si/SiO₂ substrate and covered with single-layer graphene. The apparent height of the DNA nanostructure significantly decreased upon thermal annealing at 250 °C and higher temperatures. The topographical features of the DNA nanostructure, as measured by atomic force microscopy (AFM), disappeared after annealing at 300 °C for 5 h but reappeared after 23 h. In contrast, in the absence of a graphene coating, the topographical features of DNA nanostructure disappeared after heating at 300 °C for 45 min. After heating at 300 °C for 29 h, oxidation produced nanometer-sized holes on graphene, some of which were triangular and spatially overlapped with DNA nanostructures. These results suggest that the inorganic residues produced by the decomposition of DNA nanostructures enhance the oxidation of graphene in a site-specific manner.

Structuring of Organic Solvents at Solid Interfaces and Ramifications for Antimalarial Adsorption on β-Hematin Crystals

A critical aspect of material synthesis is solvent structuring at solid-liquid interfaces, which can impact the adsorption of solute and growth modifiers on an underlying substrate. In general, the impact of solvent structuring on molecular sorbate interactions with solid sorbents is poorly understood. This is particularly true for processes that occur in organic media, such as hematin crystallization, which is crucial to the survival of malaria parasites. Here, we use chemical force microscopy and molecular modeling to analyze the interactions between functional moieties of known antimalarials and the interface between β-hematin crystals and a mixed organic (octanol)-aqueous solvent. We show that the β-hematin surface, patterned in parallel hydrophobic and hydrophilic stripes, engenders the assembly of up to five layers of octanol molecules aligned parallel to the crystal surface. In contrast, studies of solvent structuring on a disordered glass surface reveal that octanol molecules align perpendicular to the interface. The distinct octanol arrays direct molecule adsorption at the respective interfaces. At both substrates, we also find stabilized pockets of aqueous nanophase lining the surfaces. A combination of experimental analyses and modeling of solvent structuring provides crucial insights into the association of hematin molecules with growing crystals as well as the adsorption and mobility of antimalarial drugs. Moreover, our findings offer a general perspective on the collective behaviors of complex organic solvents that may apply to a broad range of interactions at solid-liquid interfaces.

Water clusters contributed to molecular interactions of ionizable organic pollutants with aromatized biochar via π-PAHB: Sorption experiments and DFT calculations

Molecular interactions between biochars and ionizable organic pollutants (IOPs) are of great concern in natural environments, however the role of water clusters on the biochar surface remain unclear. The pH-dependent adsorption of aniline, phenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 4-methylphenol and 4-nitrophenol onto bamboo wood derived biochar (BW700) as a model was conducted to identify conventional and novel interaction mechanisms between aromatized surface and IOPs. The dissociation constant (pK_a,surface) of surface functional groups of BW700 was characterized by acid-base titration and Zeta potential measurements. The pH-dependent adsorption behavior depended on the pK_a,IOP of IOPs and also related to the pK_a,surface of biochar surface. An obvious peak of adsorption coefficients (K_d) in the range of solution pH was shaped at pH_peak = (pK_a,IOP + pK_a,surface)/2, which cannot be well explained by the conventional mechanisms such as hydrophobic effects, π-π interaction, electrostatic attractions, and hydrogen-binding. The contribution of ice-like adlayer (water clusters) on aromatic surface as H-acceptors is proposed for the first time to the adsorption peak of IOP as H-donors at pH_peak. Density functional theory (DFT) calculations provided a possible structure of the complex combined with ice-like adlayer and aromatic substrate of BW700, and indicated that the adsorbing peak resulted from the multiple π-bond and polarization assisted H-bond (π-PAHB) interactions. Three distinct properties of π-PAHB were given, based on multiple π-bond, hydrophobicity-dependence and pH sensitivity. This novel mechanism extends the definition of H-bonds for better understanding the molecular interactions of IOP with carbonaceous materials and their environmental fate.

Ti2C3Tx nanosheets as photothermal agents for near-infrared responsive hydrogels

Poly(N-isopropylacrylamide) (PNIPAM) is broadly applicable in many fields due to its temperature-induced phase transition property. Herein, a facile method to incorporate exfoliated Ti2C3Tx nanosheets in the PNIPAM network is reported. Due to compatibility, stability and photothermal properties of the incorporated Ti2C3Tx nanosheets, the obtained MXene/PNIPAM composite hydrogel shows excellent photothermal properties, expanding the pure thermal-responsive property of the PNIPAM hydrogel. Based on the smart composite hydrogel, remote light-control of the microfluidic pipeline is also demonstrated.

Transforming ground mica into high-performance biomimetic polymeric mica film

Biomimetic assembly of high-quality nanosheets into nacre-like structures can produce macroscopic films with favorable mechanical and optical performances due to the intrinsic properties and high level of ordering of the nanoscale building blocks. Natural ground mica is abundant and exhibits great application potential. However, large size and low aspect ratio greatly limit its biomimetic assembly. Moreover, exfoliation of ground mica into high-quality nanosheets remains a significant challenge. Here, we report that large-scale exfoliation of ground mica into mono- or few-layered mica nanosheets with a production rate of ~1.0 g h-1 can be successfully achieved. The mica nanosheets are then assembled into strong biomimetic polymeric mica film that inherits the high electric insulation, excellent visible transmittance, and unique ultraviolet-shielding properties of natural mica. Its overall performance is superior to that of natural sheet mica and other biomimetic films, making the polymeric mica film a suitable substrate for flexible and transparent devices.

Mechanics of spontaneously formed nanoblisters trapped by transferred 2D crystals

Layered systems of 2D crystals and heterostructures are widely explored for new physics and devices. In many cases, monolayer or few-layer 2D crystals are transferred to a target substrate including other 2D crystals, and nanometer-scale blisters form spontaneously between the 2D crystal and its substrate. Such nanoblisters are often recognized as an indicator of good adhesion, but there is no consensus on the contents inside the blisters. While gas-filled blisters have been modeled and measured by bulge tests, applying such models to spontaneously formed nanoblisters yielded unrealistically low adhesion energy values between the 2D crystal and its substrate. Typically, gas-filled blisters are fully deflated within hours or days. In contrast, we found that the height of the spontaneously formed nanoblisters dropped only by 20-30% after 3 mo, indicating that probably liquid instead of gas is trapped in them. We therefore developed a simple scaling law and a rigorous theoretical model for liquid-filled nanoblisters, which predicts that the interfacial work of adhesion is related to the fourth power of the aspect ratio of the nanoblister and depends on the surface tension of the liquid. Our model was verified by molecular dynamics simulations, and the adhesion energy values obtained for the measured nanoblisters are in good agreement with those reported in the literature. This model can be applied to estimate the pressure inside the nanoblisters and the work of adhesion for a variety of 2D interfaces, which provides important implications for the fabrication and deformability of 2D heterostructures and devices.

Optical characterization of surface adlayers and their compositional demixing at the nanoscale

Under ambient conditions, the behavior of a solid surface is often dominated by a molecularly thin adsorbed layer (adlayer) of small molecules. Here we develop an optical approach to unveil the nanoscale structure and composition of small-molecule adlayers on glass surfaces through spectrally resolved super-resolution microscopy. By recording the images and emission spectra of millions of individual solvatochromic molecules that turn fluorescent in the adlayer phase, we obtain ~30 nm spatial resolution and achieve concurrent measurement of local polarity. This allows us to establish that the adlayer dimensionality gradually increases through a sequence of 0D (nanodroplets), 1D (nano-lines), and 2D (films) for liquids of increasing polarity. Moreover, we find that in adlayers, a solution of two miscible liquids spontaneously demixes into nanodroplets of different compositions that correlate strongly with droplet size and location. We thus reveal unexpectedly rich structural and compositional behaviors of surface adlayers at the nanoscale.

Characterization of adsorbed molecular layers on surfaces is the key to wide-ranging applications, but elucidating the structure and composition of such adlayers remains challenging. Here the authors develop an approach to unveil the nanoscale structure and composition of adlayers through spectrally resolved super-resolution microscopy.

Versatile technique for assessing thickness of 2D layered materials by XPS

X-ray photoelectron spectroscopy (XPS) has been utilized as a versatile method for thickness characterization of various two-dimensional (2D) films. Accurate thickness can be measured simultaneously while acquiring XPS data for chemical characterization of 2D films having thickness up to approximately 10 nm. For validating the developed technique, thicknesses of few-layer graphene (FLG), MoS₂ and amorphous boron nitride (a-BN) layer, produced by microwave plasma chemical vapor deposition (MPCVD), plasma enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD) respectively, were accurately measured. The intensity ratio between photoemission peaks recorded for the films (C 1s, Mo 3d, B 1s) and the substrates (Cu 2p, Al 2p, Si 2p) is the primary input parameter for thickness calculation, in addition to the atomic densities of the substrate and the film, and the corresponding electron attenuation length (EAL). The XPS data was used with a proposed model for thickness calculations, which was verified by cross-sectional transmission electron microscope (TEM) measurement of thickness for all the films. The XPS method determines thickness values averaged over an analysis area which is orders of magnitude larger than the typical area in cross-sectional TEM imaging, hence provides an advanced approach for thickness measurement over large areas of 2D materials. The study confirms that the versatile XPS method allows rapid and reliable assessment of the 2D material thickness and this method can facilitate in tailoring growth conditions for producing very thin 2D materials effectively over a large area. Furthermore, the XPS measurement for a typical 2D material is non-destructive and does not require special sample preparation. Therefore, after XPS analysis, exactly the same sample can undergo further processing or utilization.

Capillary Condensation in 8 nm Deep Channels

Condensation on the nanoscale is essential to understand many natural and synthetic systems relevant to water, air, and energy. Despite its importance, the underlying physics of condensation initiation and propagation remain largely unknown at sub-10 nm, mainly due to the challenges of controlling and probing such small systems. Here we study the condensation of n-propane down to 8 nm confinement in a nanofluidic system, distinct from previous studies at ∼100 nm. The condensation initiates significantly earlier in the 8 nm channels, and it initiates from the entrance, in contrast to channels just 10 times larger. The condensate propagation is observed to be governed by two liquid-vapor interfaces with an interplay between film and bridging effects. We model the experimental results using classical theories and find good agreement, demonstrating that this 8 nm nonpolar fluid system can be treated as a continuum from a thermodynamic perspective, despite having only 10-20 molecular layers.

Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy

Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields but the disturbance of the probes on the hydrogen-bonding structure of water has remained an intractable problem. Here, we report submolecular-resolution imaging of the water clusters on a NaCl(001) surface within the nearly noninvasive region by a qPlus-based noncontact atomic force microscopy. Comparison with theoretical simulations reveals that the key lies in probing the weak high-order electrostatic force between the quadrupole-like CO-terminated tip and the polar water molecules at large tip–water distances. This interaction allows the imaging and structural determination of the weakly bonded water clusters and even of their metastable states with negligible disturbance. This work may open an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces, ion hydration, and biological water with atomic precision.

Scanning probe microscopy has been extensively applied to probe interfacial water but the probes tend to disturb the structure of water easily. Here, the authors report submolecular-resolution imaging of water clusters within the nearly non-invasive region by qPlus noncontact atomic force microscopy.

Thermodynamic, structural, and dynamical properties of nano-confined water using SPC/E and TIP4P models by molecular dynamics simulations

Different morphologies of water molecules are confined between two parallel graphene surfaces.

Friction induced structural transformations of water monolayers at graphene/Cu interfaces

The strong association of friction characteristics with structural transformations of water monolayers at graphene/Cu interfaces.

The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP

A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the individual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.

The evolution in graphitic surface wettability with first-principles quantum simulations: the counterintuitive role of water

The surface wettability of graphite has gained a lot of interest in nanotechnology and fundamental studies alike, but the types of adsorptions that dominate its time resolved surface property variations in ambient environment are still elusive.

Pressure-Induced Melting of Confined Ice

The classic regelation experiment of Thomson in the 1850s deals with cutting an ice cube, followed by refreezing. The cutting was attributed to pressure-induced melting but has been challenged continuously, and only lately consensus emerged by understanding that compression shortens the O:H nonbond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point. The hot debate survived well over 150 years, mainly due to a poorly defined heat exchange with the environment in the experiment. In our current experiment, we achieved thermal isolation from the environment and studied the fully reversible ice-liquid water transition for water confined between graphene and muscovite mica. We observe a transition from two-dimensional (2D) ice into a quasi-liquid phase by applying a pressure exerted by an atomic force microscopy tip. At room temperature, the critical pressure amounts to about 6 GPa. The transition is completely reversible: refreezing occurs when the applied pressure is lifted. The critical pressure to melt the 2D ice decreases with temperature, and we measured the phase coexistence line between 293 and 333 K. From a Clausius-Clapeyron analysis, we determine the latent heat of fusion of two-dimensional ice at 0.15 eV/molecule, being twice as large as that of bulk ice.

Reduction in Step Height Variation and Correcting Contrast Inversion in Dynamic AFM of WS2 Monolayers

A model has been developed to account for and prevent the anomalies encountered in topographic images of transition metal dichalcogenide monolayers using dynamic atomic force microscopy (dAFM). The height of WS2 monolayers measured using dAFM appeared to be increased or decreased, resulting from the interactions between the tip and the surface. The hydrophilic SiO2 substrate appeared higher than the weakly hydrophilic WS2 when the tip amplitude was low or at a high set point (high force). Large amplitudes and low set points corrected the step height inversion, but did not recover the true step height. Removing water from the sample resulted in an order of magnitude reduced variation in step height, but the WS2 appeared inverted except at low amplitudes and high set points. Our model explains the varying step heights in dAFM of TMDs as a result of varying tip-sample interactions between the sample and substrate, in the presence or absence of capillaries. To eliminate contrast inversion, high amplitudes can be used to reduce the effect of capillary forces. However, when capillaries are not present, low amplitudes and high set points produce images with proper contrast due to tool operation in the repulsive regime on both materials.

Phase imaging and nanoscale energy dissipation of supported graphene using amplitude modulation atomic force microscopy

We investigate the phase imaging of supported graphene using amplitude modulation atomic force microscopy (AFM), the so-called tapping mode. The phase contrast between graphene and the neighboring substrate grows in hard tapping conditions and the contrast is enhanced compared to the topographic one. Therefore, phase measurements could enable the high-contrast imaging of graphene and related two-dimensional materials and heterostructures, which is not achievable with conventional AFM based topographic measurements. Obtained phase maps are then transformed into energy dissipation maps, which are important for graphene applications in various nano-mechanical systems. From a fundamental point of view, energy dissipation gives further insight into mechanical properties. Reliable measurements, obtained in the repulsive regime, show that the energy dissipation on a graphene-covered substrate is lower than that on a bare one, so graphene provides certain shielding in tip-substrate interaction. Based on the obtained phase curves and their derivatives, as well as on correlation measurements based on AFM nanoindentation and force modulation microscopy, we conclude that the main dissipation channels in graphene-substrate systems are short-range hysteresis and long-range interfacial forces.

Charge Induced Dynamics of Water in a Graphene-Mica Slit Pore

We use atomic force microscopy to in situ investigate the dynamic behavior of confined water at the interface between graphene and mica. The graphene is either uncharged, negatively charged, or positively charged. At high humidity, a third water layer will intercalate between graphene and mica. When graphene is negatively charged, the interface fills faster with a complete three layer water film, compared to uncharged graphene. As charged positively, the third water layer dewets the interface, either by evaporation into the ambient or by the formation of three-dimensional droplets under the graphene, on top of the bilayer. Our experimental findings reveal novel phenomena of water at the nanoscale, which are interesting from a fundamental point of view and demonstrate the direct control over the wetting properties of the graphene/water interface.

Controlling Water Intercalation Is Key to a Direct Graphene Transfer

The key steps of a transfer of two-dimensional (2D) materials are the delamination of the as-grown material from a growth substrate and the lamination of the 2D material on a target substrate. In state-of-the-art transfer experiments, these steps remain very challenging, and transfer variations often result in unreliable 2D material properties. Here, it is demonstrated that interfacial water can insert between graphene and its growth substrate despite the hydrophobic behavior of graphene. It is understood that interfacial water is essential for an electrochemistry-based graphene delamination from a Pt surface. Additionally, the lamination of graphene to a target wafer is hindered by intercalation effects, which can even result in graphene delamination from the target wafer. For circumvention of these issues, a direct, support-free graphene transfer process is demonstrated, which relies on the formation of interfacial water between graphene and its growth surface, while avoiding water intercalation between graphene and the target wafer by using hydrophobic silane layers on the target wafer. The proposed direct graphene transfer also avoids polymer contamination (no temporary support layer) and eliminates the need for etching of the catalyst metal. Therefore, recycling of the growth template becomes feasible. The proposed transfer process might even open the door for the suggested atomic-scale interlocking-toy-brick-based stacking of different 2D materials, which will enable a more reliable fabrication of van der Waals heterostructure-based devices and applications.

Preventing Thin Film Dewetting via Graphene Capping

A monolayer 2D capping layer with high Young's modulus is shown to be able to effectively suppress the dewetting of underlying thin films of small organic semiconductor molecule, polymer, and polycrystalline metal, respectively. To verify the universality of this capping layer approach, the dewetting experiments are performed for single-layer graphene transferred onto polystyrene (PS), semiconducting thienoazacoronene (EH-TAC), gold, and also MoS₂ on PS. Thermodynamic modeling indicates that the exceptionally high Young's modulus and surface conformity of 2D capping layers such as graphene and MoS₂ substantially suppress surface fluctuations and thus dewetting. As long as the uncovered area is smaller than the fluctuation wavelength of the thin film in a dewetting process via spinodal decomposition, the dewetting should be suppressed. The 2D monolayer-capping approach opens up exciting new possibilities to enhance the thermal stability and expands the processing parameters for thin film materials without significantly altering their physical properties.

Nanoscale Insight into the Mechanism of a Highly Oriented Pyrolytic Graphite Edge Surface Wetting by "Interferencing" Water

The new molecular dynamics simulation results showing the influence of the edge carbon surface atoms on the wettability of a highly oriented pyrolytic graphite (HOPG) surface with water nanodroplets are reported. The conditions for the occurrence of the Wenzel effect are discussed, and the Cassie-to-Wenzel transition (CTWT) mechanism in the nanoscale is explored. This transition is detected by the application of a new procedure showing that the CTWT point shifts toward larger values of carbon-oxygen potential well depth with the decrease in the HOPG side angle. It is concluded that the Wenzel effect significantly contributes to the contact angles (CAs) measured for the HOPG surfaces. The Wenzel effect is also very important for the "HOPG" structures possessing the disturbed C-C interlayer distance, and its influence on the water nanodroplet CAs is strongly pronounced. The structure of water confined inside slits and on a HOPG surface is studied using the analysis of the density profiles, the number of hydrogen bonds, and, modified for the purpose of this study, structure factor. The detailed analysis of all parameters describing confined water leads to the conclusion about the presence of characteristic interference patterns revealed as a result of long-term simulation. A simple model describing this effect is proposed as the starting point for further considerations.

Molecular Dynamics Study on the Effect of Surface Hydroxyl Groups on Three-Phase Wettability in Oil-Water-Graphite Systems

In this paper, a hydroxylated graphite surface is generated as a hydrophilic oleophobic material for the application of oil-water separation, and the effects of hydroxyl density on the three-phase wettability are studied in oil-water-graphite systems. We analyze the adsorption of water molecules on the hydroxylated surfaces and obtain the relationship between water-oil-solid interfacial properties and the hydroxyl density, which results from the synthetic effects of the orientation of molecules and hydrogen bonds. With the increase of hydroxyl density, the water-solid contact angle first decreases rapidly, and then remains constant. The density of the hydrogen bond formed between hydroxyls and water molecules in the adsorption layer can explain the regularity of the three-phase wettability. The orientation of the water molecules in the adsorption layer shows insignificant variation, owing to the hydrogen bond network formed between the water molecules; thus, little change is observed in the hydrogen bond density in the adsorption layer.

Enhancement of Friction by Water Intercalated between Graphene and Mica

Common experience shows that friction converts mechanical energy into heat. The first part of this process is vibrational excitation of atoms at the interface between rubbing bodies. The second part is the removal of the vibration energy by transferring it from the interface to the substrate. However, it is difficult to disentangle the excitation and energy transfer processes. We solved this by using a system consisting of a SiO₂-terminated tip sliding over graphene deposited on mica with intercalated water between them. The intercalated water was found to increase friction by a factor of ∼3 relative to dry mica. Density functional theory calculations show that water broadens the spectral range of graphene vibrations-particularly the low-frequency flexural modes-thus providing new excitation channels and also by increasing the overlap with the atomic vibrations of the mica substrate, which facilitates coupling and energy transfer.

Quasi-stabilized hydration layers on muscovite mica under a thin water film grown from humid air

The interfaces between solids and water films in air play fundamental roles in physicochemical phenomena, biological functions, and nano-fabrication. Though the properties of the interfaces have been considered to be irrelevant to the water film thickness, we found distinctive mechanical features of the interface between a cleaved muscovite mica surface and a thin water film grown in humid air, dissimilar to those in bulk water, using frequency-modulation atomic force microscopy. The thin water film grew with quasi-stabilized hydration networks of water molecules, tightly bound each other at the interface, to a thickness of ~2 nm at near-saturating humidity. Consequently, defective structures of the hydration networks persisted vertically through the hydration layers at the interface, and K⁺ ions on the cleaved surface remained without dissolution into the water film. The results provide atomistic insights into thin water films in regard to epitaxial-like growth from vapour and the motion of water molecules and ions therein.

Dependence of the shape of graphene nanobubbles on trapped substance

Van der Waals (vdW) interaction between two-dimensional crystals (2D) can trap substances in high pressurized (of order 1 GPa) on nanobubbles. Increasing the adhesion between the 2D crystals further enhances the pressure and can lead to a phase transition of the trapped material. We found that the shape of the nanobubble can depend critically on the properties of the trapped substance. In the absence of any residual strain in the top 2D crystal, flat nanobubbles can be formed by trapped long hydrocarbons (that is, hexadecane). For large nanobubbles with radius 130 nm, our atomic force microscopy measurements show nanobubbles filled with hydrocarbons (water) have a cylindrical symmetry (asymmetric) shape which is in good agreement with our molecular dynamics simulations. This study provides insights into the effects of the specific material and the vdW pressure on the microscopic details of graphene bubbles.

Graphene nanobubbles can act as enclosures for holding small volumes of substances. Here the authors find a correlation between bubble shape and the encapsulated material providing a potential method for determining the graphene bubble content by its deformation.

From Flatland to Spaceland: Higher Dimensional Patterning with Two-Dimensional Materials

The creation of three-dimensional (3D) structures from two-dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out-of-plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.

Formation of Water Layers on Graphene Surfaces

Although graphitic materials were thought to be hydrophobic, recent experimental results based on contact angle measurements show that the hydrophobicity of graphitic surfaces stems from airborne contamination of hydrocarbons. This leads us to question whether a pristine graphitic surface is indeed hydrophobic. To investigate the water wettability of graphitic surfaces, we use molecular dynamics simulations of water molecules on the surface of a single graphene layer at room temperature. The results indicate that a water droplet spreads over the entire surface and that a double-layer structure of water molecules forms on the surface, which means that wetting of graphitic surfaces is possible, but only by two layers of water molecules. No further water layers can cohere to the double-layer structure, but the formation of three-dimensional clusters of liquid water is confirmed. The surface of the double-layer structure acts as a hydrophobic surface. Such peculiar behavior of water molecules can be reasonably explained by the formation of hydrogen bonds: The hydrogen bonds of the interfacial water molecules form between the first two layers and also within each layer. This hydrogen-bond network is confined within the double layer, which means that no "dangling hydrogen bonds" appear on the surface of the double-layer structure. This formation of hydrogen bonds stabilizes the double-layer structure and makes its surface hydrophobic. Thus, the numerical simulations indicate that a graphene surface is perfectly wettable on the atomic scale and becomes hydrophobic once it is covered by this double layer of water molecules.

Interaction between Iron and Graphene Nanocavity: Formation of Iron Membranes, Iron Clusters, or Iron Carbides

Motivated from a recent experimental study on filling of a graphene nanocavity by iron membrane at room temperature (Science 2014, 343, 1228), we perform a comprehensive study of morphology changes of two-dimensional Fe membranes and iron carbides embedded in graphene nanocavities with specific sizes and shapes using the first-principles calculations and ab initio molecular dynamics simulations. Our simulations show that Fe atoms tend to gradually seal the graphene nanocavity via growing a metastable Fe membrane until the nanocavity is completely covered. Notably, a densely packed Fe membrane in the graphene nanocavity shows higher structural stability than a loosely packed one as long as more triangular lattices can form to release high tensile strain. The Fe membrane under high tensile strain tends to collapse and turns into a three-dimensional Fe cluster upon detaching from the edge. The structural transformation of Fe nanostructures follows the melting recrystallization mechanism at ambient temperatures in high vacuum. Moreover, the iron carbide can also exist in the graphene nanocavity and once formed can be highly stable even at 1200 K.

Interfacial Mechanical Properties of Graphene on Self-Assembled Monolayers: Experiments and Simulations

Self-assembled monolayers (SAMs) have been widely used to engineer the electronic properties of substrate-supported graphene devices. However, little is known about how the surface chemistry of SAMs affects the interfacial mechanical properties of graphene supported on SAMs. Fluctuations and changes in these properties affect the stress transfer between substrate and the supported graphene and thus the performance of graphene-based devices. The changes in interfacial mechanical properties can be characterized by measuring the out-of-plane elastic properties. Combining contact resonance atomic force microcopy experiments with molecular dynamics simulations, we show that the head group chemistry of a SAM, which affects the interfacial interactions, can have a significant effect on the out-of-plane elastic modulus of the graphene-SAM heterostructure. Graphene supported on hydrophobic SAMs leads to heterostructures stiffer than those of graphene supported on hydrophilic SAMs, which is largely due to fewer water molecules present at the graphene-SAM interface. Our results provide an important, and often overlooked, insight into the mechanical properties of substrate-supported graphene electronics.

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

Graphene Visualizes the Ion Distribution on Air-Cleaved Mica

The distribution of potassium (K⁺) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer. The K⁺ ions prefer to minimize the number of nearest neighbour K⁺ ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K⁺ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.

Perspective: Structure and dynamics of water at surfaces probed by scanning tunneling microscopy and spectroscopy

The detailed and precise understanding of water-solid interaction largely relies on the development of atomic-scale experimental techniques, among which scanning tunneling microscopy (STM) has proven to be a noteworthy example. In this perspective, we review the recent advances of STM techniques in imaging, spectroscopy, and manipulation of water molecules. We discuss how those newly developed techniques are applied to probe the structure and dynamics of water at solid surfaces with single-molecule and even submolecular resolution, paying particular attention to the ability of accessing the degree of freedom of hydrogen. In the end, we present an outlook on the directions of future STM studies of water-solid interfaces as well as the challenges faced by this field. Some new scanning probe techniques beyond STM are also envisaged.

Surface chemistry and catalysis confined under two-dimensional materials

Interfaces between 2D material overlayers and solid surfaces provide confined spaces for chemical processes, which have stimulated new chemistry under a 2D cover.

Influence of O2, H2O and airborne hydrocarbons on the properties of selected 2D materials

Adsorption of molecules from the ambient environment significantly changes the optical, electrical, electrochemical, and tribological properties of 2D materials.

The physics and chemistry of graphene-on-surfaces

This review describes the major “graphene-on-surface” structures and examines the roles of their properties in governing the overall performance for specific applications.

Humidity-accelerated spreading of ionic liquids on a mica surface

The role of relative humidity (RH) on the wetting behavior of droplets of two [Rmim][NTf₂] ionic liquids (ILs) on a mica surface was investigated and water vapor adsorption was found to enhance the ILs precursor film formation and droplet spreading.

Probing the shape of a graphene nanobubble

Universal shape behavior and ∼1 GPa vdW pressure in a small ∼10 nm graphene nanobubble.

CO Separation from H2 via Hydrate Formation in Single-Walled Carbon Nanotubes

Hydrogen is an alternative fuel without generating greenhouse gas or other harmful emissions. Industrial hydrogen production, however, always contains a small fraction of carbon monoxide (CO) (∼0.5-2%) that must be removed for use in fuel cells. Here, we present molecular dynamics simulation evidence on facile separation of CO from H₂ at ambient pressure via the formation of quasi-one-dimensional (Q1D) clathrate hydrates within single-walled carbon nanotubes (SW-CNTs). At ambient pressure, Q1D CO (or H₂) clathrates in SW-CNTs are formed spontaneously when the SW-CNTs are immersed in CO (or H₂) aqueous solution. More interestingly, for the CO/H₂ aqueous solution, highly preferential adsorption of CO over H₂ occurs within the octagonal or nonagonal ice nanotubes inside of SW-CNTs. These results suggest that the formation of Q1D hydrates within SW-CNTs can be a viable and safe method for the separation of CO from H₂, which can be exploited for hydrogen purification in fuel cells.

High-Resolution Characterization of Preferential Gas Adsorption at the Graphene-Water Interface

The contact of water with graphene is of fundamental importance and of great interest for numerous promising applications, but how graphene interacts with water remains unclear. Here we used atomic force microscopy (AFM) to investigate hydrophilic mica substrates with some regions covered by mechanically exfoliated graphene layers in water. In water containing air gas close to the saturation concentration (within ∼40%), cap-shaped nanostructures (or interfacial nanobubbles) and ordered-stripe domains were observed on graphene-covered regions but not on pure mica regions. These structures did not appear on graphene when samples were immersed in highly degassed water, indicating that their formation was caused by the adsorption of gas dissolved in water. Thus, atomically thin graphene, even at a narrow width of 20 nm, changes the local surface chemistry of a highly hydrophilic substrate. Furthermore, surface hydrophobicity significantly affects gas adsorption, which has broad implications for diverse phenomena in water.

Universal shape and pressure inside bubbles appearing in van der Waals heterostructures

Trapped substances between a two-dimensional (2D) crystal and an atomically flat substrate lead to the formation of bubbles. Their size, shape and internal pressure are determined by the competition between van der Waals attraction of the crystal to the substrate and the elastic energy needed to deform it, allowing to use bubbles to study elastic properties of 2D crystals and conditions of confinement. Using atomic force microscopy, we analysed a variety of bubbles formed by monolayers of graphene, boron nitride and MoS₂. Their shapes are found to exhibit universal scaling, in agreement with our analysis based on the theory of elasticity of membranes. We also measured the hydrostatic pressure induced by the confinement, which was found to reach tens of MPa inside submicron bubbles. This agrees with our theory estimates and suggests that for even smaller, sub-10 nm bubbles the pressure can be close to 1 GPa and may modify properties of a trapped material.

The interface between vertically stacked 2D materials can host contaminants trapped within bubbles. Here, the authors show that such nano-bubbles can be used as a platform to explore the van der Waals pressure and elasticity in atomically thin films, in a previously inaccessible confined environment.

Nanoscopic characterization of the water vapor-salt interfacial layer reveals a unique biphasic adsorption process

Our quantitative understanding of water adsorption onto salt surfaces under ambient conditions is presently quite poor owing to the difficulties in directly characterizing this interfacial layer under these conditions. Here we determine the thickness of the interfacial layer on NaCl at different relative humidities (RH) based on a novel application of atomic force spectroscopy and capillary condensation theory. In particular, we take advantage of the microsecond-timescale of the capillary condensation process to directly resolve the magnitude of its contribution in the tip-sample interaction, from which the interfacial water thickness is determined. Further, to correlate this thickness with salt dissolution, we also measure surface conductance under similar conditions. We find that below 30% RH, there is essentially only the deposition of water molecules onto this surface, typical of conventional adsorption onto solid surfaces. However, above 30% RH, adsorption is simultaneous with the dissolution of ions, unlike conventional adsorption, leading to a rapid increase of surface conductance. Thus, water adsorption on NaCl is an unconventional biphasic process in which the interfacial layer not only exhibits quantitative differences in thickness but also qualitative differences in composition.