Enhancement of Friction by Water Intercalated between Graphene and Mica

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.

Anomalous friction of supercooled glycerol on mica

Although friction of liquids on solid surfaces is traditionally linked to wettability, recent works have unveiled the role of the solid's internal excitations on interfacial dissipation. In order to directly evidence such couplings, we take advantage of the considerable variation of the molecular timescales of supercooled glycerol under mild change of temperature to explore how friction depends on the liquid's molecular dynamics. Using a dedicated tuning-fork AFM, we measure the slippage of glycerol on mica. We report a 100 fold increase of slip length upon cooling, while liquid-solid friction exhibits a linear scaling with molecular relaxation rate at high temperature. This scaling can be explained by a contribution of mica's phonons which resonate with density fluctuations in the liquid, allowing efficient momentum transfer to mica. These results suggest that engineering phononic spectra of materials could enhance flow performance in nanofluidic channels and industrially relevant membranes.

Observation of the early stages of environmental contamination in graphene by friction force

Exposure to ambient air contaminates the surface of graphene sheets. Contamination may arise from different sources, and its nature alters the frictional behavior of the material. These changes in friction enable the observation of the early stages of contaminants' adsorption in graphene. Using a friction force microscope, we show that molecular adsorption initiates at the edges and mechanical defects in the monolayer. Once the monolayer is covered, the contaminants spread over the additional graphene layers. With this method, we estimate the contamination kinetics. In monolayer graphene, the surface area covered with adsorbed molecules increases with time of air exposure at a rate of 10-14 m2/s, while in bilayer graphene, it is one order of magnitude smaller. Finally, as the contaminants cover the additional graphene layers, friction no longer has a difference concerning the number of graphene layers.

Phase-dependent Friction on Exfoliated Transition Metal Dichalcogenides Atomic Layers

The fundamental aspects of energy dissipation on 2-dimensional (2D) atomic layers are extensively studied. Among various atomic layers, transition metal dichalcogenides (TMDs) exists in several phases based on their lattice structure, which give rise to the different phononic and electronic contributions in energy dissipation. 2H and 1T' (distorted 1T) phase MoS2 and MoTe2 atomic layers exfoliated on mica substrate are obtained and investigated their nanotribological properties with atomic force microscopy (AFM)/ friction force microscopy (FFM). Surprisingly, 1T' phase of both MoS2 and MoTe2 exhibits ≈10 times higher friction compared to 2H phase. With density functional theory analyses, the friction increase is attributed to enhanced electronic excitation, efficient phonon dissipation, and increased potential energy surface barrier at the tip-sample interface. This study suggests the intriguing possibility of tuning the friction of TMDs through phase transition, which can lead to potential application in tunable tribological devices.

Dynamically tuning friction at the graphene interface using the field effect

Dynamically controlling friction in micro- and nanoscale devices is possible using applied electrical bias between contacting surfaces, but this can also induce unwanted reactions which can affect device performance. External electric fields provide a way around this limitation by removing the need to apply bias directly between the contacting surfaces. 2D materials are promising candidates for this approach as their properties can be easily tuned by electric fields and they can be straightforwardly used as surface coatings. This work investigates the friction between single layer graphene and an atomic force microscope tip under the influence of external electric fields. While the primary effect in most systems is electrostatically controllable adhesion, graphene in contact with semiconducting tips exhibits a regime of unexpectedly enhanced and highly tunable friction. The origins of this phenomenon are discussed in the context of fundamental frictional dissipation mechanisms considering stick slip behavior, electron-phonon coupling and viscous electronic flow.

Does twist angle affect the properties of water confined inside twisted bilayer graphene?

Graphene nanoslit pores are used for nanofluidic devices, such as, in water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices, because a small misalignment leads to a significant alteration in various properties of the devices. Here, we focus on the rotational misalignment between two parallel graphene sheets. Using molecular dynamics simulation, we probe the structure and dynamics of monolayer water confined inside graphene nanochannels for a range of commensurate twist angles. With SPC/E and TIP4P/2005 water models, our simulations reveal the independence of the equilibrium number density- n ∼ 13 nm^-2 for SPC/E and n ∼ 11.5 nm^-2 for TIP4P/2005- across twists. Based on the respective densities of the water models, the structure and dielectric constant are invariant of twist angles. The confined water structure at this density shows square ice ordering for SPC/E water only. TIP4P/2005 shows ordering at the vicinity of a critical density (n ∼ 12.5 nm^-2). The average perpendicular dielectric constant of the confined water remains anomalously low (∼2 for SPC/E and ∼6 for TIP4P/2005) for the studied twist angles. We find that the friction coefficient of confined water molecules varies for small twist angles, while becoming independent for twists greater than 5.1°. Our results indicate that a small, angular misalignment will not impair the dielectric properties of monolayer water within a graphene slit-pore, but can significantly influence its dynamics.

Water adsorption and dynamics on graphene and other 2D materials: Computational and experimental advances

The interaction of water and surfaces, at molecular level, is of critical importance for understanding processes such as corrosion, friction, catalysis and mass transport. The significant literature on interactions with single crystal metal surfaces should not obscure unknowns in the unique behaviour of ice and the complex relationships between adsorption, diffusion and long-range inter-molecular interactions. Even less is known about the atomic-scale behaviour of water on novel, non-metallic interfaces, in particular on graphene and other 2D materials. In this manuscript, we review recent progress in the characterisation of water adsorption on 2D materials, with a focus on the nano-material graphene and graphitic nanostructures; materials which are of paramount importance for separation technologies, electrochemistry and catalysis, to name a few. The adsorption of water on graphene has also become one of the benchmark systems for modern computational methods, in particular dispersion-corrected density functional theory (DFT). We then review recent experimental and theoretical advances in studying the single-molecular motion of water at surfaces, with a special emphasis on scattering approaches as they allow an unparalleled window of observation to water surface motion, including diffusion, vibration and self-assembly.

Role of Interfacial Water in the Tribological Behavior of Graphene in an Electric Field

Friction properties in the electric field are important for the application of graphene as a solid lubricant in graphene-based micro/nanoelectromechanical systems. The studies based on conductive atomic force microscopy show that interfacial water between graphene and the SiO₂/Si substrate affects the friction of graphene in the electric field. Friction without applying voltage remains low because the interfacial water retains a stable ice-like network. However, friction after applying voltage increases because the polar water molecules are attracted by the electric field and gather around the tip. The gathered interfacial water not only increases the deformation of graphene but is also pushed by the tip during frictional sliding, which results in the increased friction. These studies provide beneficial guidelines for the applications of graphene as a solid lubricant in the electric field.

Direct Observation of Atomic-Scale Gliding on Hydrophilic Surfaces

Nanoscale friction behavior on hydrophilic surfaces (HS), influenced by a probe gliding on a confined water layer, has been investigated with friction force microscopy under various relative humidity (RH) conditions. The topographical and frictional responses of the mechanically exfoliated single-layer graphene (SLG) on native-oxide-covered silicon (SiO₂/Si) and mica were both influenced by RH conditions. The ordinary phenomena at ambient conditions (i.e., higher friction on a HS than on a SLG due to different hydrophilicity), nondistinguishable height, friction of SLG with SiO₂/Si at high RH (>98%), and the superlubricating behavior of friction on a HS were observed. Furthermore, the subdomain within SLG, consisting of an ice-like water layer intercalated between SLG and SiO₂/Si, showed friction enhancement. These results suggest that the abundant water molecules at the interface of the probe and a HS can make a slippery surface that overcomes capillary and viscosity effects through the gliding motion of the probe.

Exploring Nanoscale Lubrication Mechanisms of Multilayer MoS2 During Sliding: The Effect of Humidity

Solid lubricants have received substantial attention due to their excellent frictional properties. Among others, molybdenum disulfide (MoS₂) is one of the most studied lubricants. Humidity results in a deterioration of the frictional properties of MoS₂. The actual mechanism at the nanoscale is still under debate, although there are indications that chemical reactions are not likely to occur in defect-free structures. In this study, we performed nonequilibrium molecular dynamics simulations to study the frictional properties of multilayer MoS₂ during sliding in the presence of water. Moreover, we also investigated the effect of sliding speed and normal load. We confirmed earlier results that a thin layer of water organizes as a solidified, ice-like network of hydrogen bonds as a result of being confined in a two-dimensional fashion between MoS₂. Moreover, we found that there exists an energy-driven, rotational dependence of the water network atop/beneath MoS₂. This orientational anisotropy is directly related to the dissipative character of MoS₂ during sliding. Finally, three distinct frictional regimes were identified, two for a thin layer of water and one for bulk water. In the case of a thin layer and low coverage, water represents a solid-like contaminant, causing high energy dissipation. For a thin layer and high coverage, water starts to act as a solid-like lubricant, reducing dissipation during sliding. Finally, a regime where water acts as a liquid lubricant, characterized by a clear velocity dependence was found.

Tribology of 2D Nanomaterials: A Review

The exfoliation of graphene has opened a new frontier in material science with a focus on 2D materials. The unique thermal, physical and chemical properties of these materials have made them one of the choicest candidates in novel mechanical and nano-electronic devices. Notably, 2D materials such as graphene, MoS2, WS2, h-BN and black phosphorus have shown outstanding lowest frictional coefficients and wear rates, making them attractive materials for high-performance nano-lubricants and lubricating applications. The objective of this work is to provide a comprehensive overview of the most recent developments in the tribological potentials of 2D materials. At first, the essential physical, wear and frictional characteristics of the 2D materials including their production techniques are discussed. Subsequently, the experimental explorations and theoretical simulations of the most common 2D materials are reviewed in regards to their tribological applications such as their use as solid lubricants and surface lubricant nano-additives. The effects of micro/nano textures on friction behavior are also reviewed. Finally, the current challenges in tribological applications of 2D materials and their prospects are discussed.

Atomic structure causing an obvious difference in thermal conductance at the Pd-H2O interface: a molecular dynamics simulation

Thermal transfer across solid-liquid interfaces is influenced by multiple factors such as surface wettability, interfacial water layer density, molecular structure, and mass density depletion length. However, the dominant factors in interfacial heat transport are yet to be investigated. In this work, we explore the contributions from these factors by employing the Pd-water model for water molecules forming ordered, partially ordered, and disordered structures on Pd (100), (110) and (111) surfaces, respectively. The results revealed that the ordered water layer on the (100) surface can introduce a "phonon bridge" at the solid-liquid interface to improve thermal transfer, while the partially ordered water layer on the (110) surface can further promote thermal transfer due to the enhanced interfacial friction. On the other hand, the decreased density depletion length also makes dominant contributions to the enhancement of interfacial thermal transfer. The results are explained by the interfacial friction coefficient, surface potential energy distribution and density depletion length. We also introduce an efficient technique by tuning the vacancy defects on the solid surface to tune the atomic structure as well as the thermal transfer. Our study reveals the complex relationship between the atomic structure of the crystal face, the water layer structure and the thermal boundary conductance, which will inspire more experimental and theoretical studies toward the improvement of interfacial thermal transport by tuning the structure of the water layer.

A hillock-like phenomenon with low friction and adhesion on a graphene surface induced by relative sliding at the interface of graphene and the SiO2 substrate using an AFM tip

Graphene demonstrates high potential as an atomically thin solid lubricant for sliding interfaces in industry. However, graphene as a coating material does not always exhibit strong adhesion to any substrates. When the adhesion of graphene to its substrate weakens, it remains unknown whether relative sliding at the interface exists and how the tribological properties of the graphene coating changes. In this work, we first designed a method to weaken the adhesion between graphene and its SiO₂ substrate. Then the graphene with weakened adhesion to its substrate was rubbed using an AFM tip, where we found a novel phenomenon: the monolayer graphene not only no longer protected the SiO₂ substrate from deformation and damage, but also prompted the formation of hillock-like structures with heights of approximately tens of nanometers. Moreover, the surface of the hillock-like structure exhibited very low adhesion and a continuously decreasing friction force versus sliding time. Comparing the hillock-like structure on the bare SiO₂ surface and the proposed force model, we demonstrated that the emergence of the hillock-like structure (with very low adhesion and continuously decreasing friction) was ascribed to the relative sliding at the graphene/substrate interface caused by the mechanical shear of the AFM tip. Our findings reveal a potential failure of the graphene coating when the adhesion strength between graphene and its substrate is damaged or weakened and provide a possibility for in situ fabrication of a low friction and adhesion micro/nanostructure on a SiO₂/graphene surface.

Formation of Linear Water Chains on Ni(110)

Materials that bind strongly to water structure the contact layer, modifying its chemical and physical properties in a manner that depends on the symmetry and reactivity of the surface. Although detailed models have been developed for several inert surfaces, much less is known about reactive surfaces, particularly those with a symmetry different from that of ice. Here we investigate water adsorption on a rectangular surface, Ni(110), an active re-forming catalyst that interacts strongly with water. Instead of forming a network of H-bonded cyclic rings, water forms flat 1D water chains, leaving half the Ni atoms exposed. Second layer water also follows the surface symmetry, forming chains of alternating pentamer and heptamer rings in preference to an extended 2D structure. This behavior is different from that found on other surfaces studied previously and is driven by the short lattice spacing of the solid and the strength of the Ni-water bond.

Super-Slippery Degraded Black Phosphorus/Silicon Dioxide Interface

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO₂)/silicon (Si) substrate, generally considered as a solid-solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO₂/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO₂ interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO₂ interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Hydrogen bonding sewing interface

Hydrogen bonding and van der Waals (vdW) forces have been precisely measured and distinguished by an in-house nanomechanical testing system.

Fluorinated Graphene: A Promising Macroscale Solid Lubricant under Various Environments

Graphene-related materials are promising solid lubricants owing to their easy shear between lattice layers. However, the coefficient of friction (COF) of graphene is not sufficiently low at the macroscale, and the lubrication performance is largely restricted by the external environment. In this study, we fabricated a fluorinated graphene (FG) coating on a stainless-steel substrate by a simple electrophoretic deposition in ethanol. The FG coating exhibited an excellent lubrication performance, which reduced the COF by 54.0 and 66.2% compared to those of pristine graphene and graphene oxide coatings, respectively. The lubrication enhancement of FG coating is attributed to its extremely low surface energy and interlaminar shear strength. The formation of ionic metal-fluorine chemical bonds provided a robust solid tribofilm and transfer layer on the friction pairs, which further increased the lubrication performance of the FG coating. The limited influence of the humidity on the lubrication performance of the FG coating is attributed to the hydrophobicity of the FG nanoflakes, which could prevent the influence of water molecules on the sliding interface. The excellent lubrication performance and better environmental adaptability of the FG make it a promising solid lubricant for mechanical engineering applications.

Thickness and Structure of Adsorbed Water Layer and Effects on Adhesion and Friction at Nanoasperity Contact

Most inorganic material surfaces exposed to ambient air can adsorb water, and hydrogen bonding interactions among adsorbed water molecules vary depending on, not only intrinsic properties of material surfaces, but also extrinsic working conditions. When dimensions of solid objects shrink to micro- and nano-scales, the ratio of surface area to volume increases greatly and the contribution of water condensation on interfacial forces, such as adhesion (Fa) and friction (Ft), becomes significant. This paper reviews the structural evolution of the adsorbed water layer on solid surfaces and its effect on Fa and Ft at nanoasperity contact for sphere-on-flat geometry. The details of the underlying mechanisms governing water adsorption behaviors vary depending on the atomic structure of the substrate, surface hydrophilicity and atmospheric conditions. The solid surfaces reviewed in this paper include metal/metallic oxides, silicon/silicon oxides, fluorides, and two-dimensional materials. The mechanism by which water condensation influences Fa is discussed based on the competition among capillary force, van der Waals force and the rupture force of solid-like water bridge. The condensed meniscus and the molecular configuration of the water bridge are influenced by surface roughness, surface hydrophilicity, temperature, sliding velocity, which in turn affect the kinetics of water condensation and interfacial Ft. Taking the effects of the thickness and structure of adsorbed water into account is important to obtain a full understanding of the interfacial forces at nanoasperity contact under ambient conditions.

A Hybrid Two-Axis Force Sensor for the Mesoscopic Structural Superlubricity Studies

Structural superlubricity (SSL) is a state of nearly zero friction and zero wear between two directly contacted solid surfaces. Recently, SSL was achieved in mesoscale and thus opened the SSL technology which promises great applications in Micro-electromechanical Systems (MEMS), sensors, storage technologies, etc. However, load issues in current mesoscale SSL studies are still not clear. The great challenge is to simultaneously measure both the ultralow shear forces and the much larger normal forces, although the widely used frictional force microscopes (FFM) and micro tribometers can satisfy the shear forces and normal forces requirements, respectively. Here we propose a hybrid two-axis force sensor that can well fill the blank between the capabilities of FFM and micro tribometers for the mesoscopic SSL studies. The proposed sensor can afford 1mN normal load with 10 nN lateral resolution. Moreover, the probe of the sensor is designed at the edge of the structure for the convenience of real-time optical observation. Calibrations and preliminary experiments are conducted to validate the performance of the design.

Thick Two-Dimensional Water Film Confined between the Atomically Thin Mica Nanosheet and Hydrophilic Substrate

The interesting properties of water molecules confined in a two-dimensional (2D) environment have aroused great attention. However, the study of 2D-confined water at the hydrophilic-hydrophilic interface is largely unexplored due to the lack of appropriate system. In this work, the behavior of water molecules confined between an atomically thin mica nanosheet and a hydrophilic SiO₂/Si substrate was investigated using an atomic force microscope in detail at ambient conditions. The confined water molecules aggregated as droplets when the relative humidity (RH) of the environment was 11%. A large-area 2D water film with a uniform thickness of ∼2 nm was observed when the mica flake was incubated at 33% RH for 1 h before being mechanically exfoliated on a SiO₂/Si substrate. Interestingly, the water film showed ordered edges with a predominant angle of 120°, which was the same with the lattice orientation of the mica nanosheet on top of it. The water film showed a fluidic behavior at the early stage and reached a stable state after 48 h under ambient conditions. The surface properties of the upper mica nanosheet and the underlying substrate played a crucial role in manipulating the behavior of confined water molecules. When the surface of the upper mica nanosheet was modified by Na⁺, Ni²⁺_, and aminopropyltriethoxysilane (APS), only some small water droplets were observed instead of a water film. The surface of the underlying SiO₂/Si substrate was functionalized by hydrophilic APS and hydrophobic octadecyltrimethoxysiliane (OTS). The small water droplets were imaged on a hydrophobic OTS-SiO₂/Si substrate, while the water film with regular edges was maintained on a hydrophilic APS-SiO₂/Si substrate. Our results might provide an alternative molecular view for investigating structures and properties of confined water molecules in 2D environments.

Tuning the Friction of Graphene on Mica by Alcohol Intercalation

The friction of graphene on mica was studied using lateral force microscopy. We observed that intercalation of alcohol molecules significantly increases the friction of graphene, as compared to water. An increase of 1.8, 2.4, and 5.9 times in friction between the atomic force microscopy tip and  single-layer graphene was observed for methanol, ethanol, and 2-propanol, respectively. Moreover, the friction of graphene is found to be higher for single-layer graphene than for multilayer graphene. We attribute the increase in friction to the additional vibrational modes of alcohol molecules. The significant variation of the frictional characteristics of graphene at the nanoscale by altering the intercalant could open up applications for the next-generation nanolubricants and nanodevices.

Influence of interface hydration on sliding of graphene and molybdenum-disulfide single-layers

Humidity influences friction in layered materials in peculiar ways. For example, while water improves the lubricating properties of graphite, it deteriorates those of molybdenum disulfide (MoS₂). The reasons remain debated, not the least due to the difficulty in experimentally comparing dry and hydrated interface frictions. Here we show that the hydration of interfaces between a mica substrate and single-layers of graphene and MoS₂ with a molecularly thin water layer affects strain transfer from the substrate to the 2D materials. For this, we strain the substrate and detect strain in graphene and MoS₂ by changes in Raman and photoluminescence spectra, respectively. Strain relaxation in graphene changes from stick-slip in dry contact, to viscous when hydrated. In contrast, there is no viscous relaxation in MoS₂ regardless of hydration. Our work provides a novel approach for better understanding the impact of hydration on friction in layered materials.

Slippery and Sticky Graphene in Water

Understanding modulation of water molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials. Here, we examine normal and shear forces on supported single-layer graphene using atomic force microscopy and find that the electrolyte composition affects the molecular slippage of nanometer thick films of aqueous electrolytes along the graphene surface. In light of the shear-assisted thermally activated theory, water molecules along the graphene plane are very mobile when subjected to shear. However, upon addition of an electrolyte, the cations can make water stick to graphene, while ion-specific and concentration effects are present. Recognizing the tribological and tribochemical utility of graphene, we also evaluate the impact of this behavior on its frictional response in the presence of water. It appears that the addition of an electrolyte to pure water causes a reduction of the thermal activation energy and of the shear-activation length at several concentrations, both results conversely affecting the friction force. Further, this work can inspire innovation in research areas where changes of the molecular slippage through the modulation of the doping characteristics of graphene in liquid environment can be of use, including molecular sensing, lubrication, and energy storage.

Frictional characteristics of nano-confined water mediated hole-doped single-layer graphene on silica surface

We have investigated the frictional properties of single-layer graphene (SLG) coated rough silica substrate under the influence of nano-confined hydration layer underneath SLG. Through the friction and surface potential measurements by atomic force microscopy (AFM), we found polygonal features in AFM images of SLG-protected silica surface that exhibit simultaneously larger friction and higher surface potential as compared to their surrounding areas due to water layers confined under SLG. Nano-confined water layers at the SLG-silica interface can induce the hole-doping effect in SLG, resulting in a more positively-charged and hydrophilic surface that favors adsorption of ambient water molecules. Therefore, during friction measurements, nanoscale capillary bridges can form within the interstices of AFM probe-SLG contact, leading to larger adhesion and friction. The friction forces were found to respectively have negative and positive dependence on the sliding velocity inside and outside the polygonal regions due to different surface wettability. Hence, it is possible to manipulate the frictional properties of SLG-coated silica by the amount of hydration layer confined underneath SLG. Our results may find applications in friction control for future nano-devices.

Operational and environmental conditions regulate the frictional behavior of two-dimensional materials

The friction characteristics of single-layer h-BN, MoS₂, and graphene were systematically investigated via friction force microscopy measurements at various operational (e.g., normal force and sliding speed) and environmental (e.g., relative humidity and thermal annealing) conditions. The low friction characteristics of these single-layer materials were clearly observed from the normal force-dependent friction results, and their interfacial shear strengths were further estimated using a Hertz-plus-offset model. In addition, speed-dependent friction characteristics clearly demonstrated two regimes of friction as a function of sliding speed - the first is the logarithmic increase in friction with sliding speed regime at sliding speeds smaller than the critical speed and the second is the friction plateau regime at sliding speeds greater than the critical speed. Fundamental parameters such as effective shape of the interaction potential and its corrugation amplitude for these single-layer materials were characterized using the thermally-activated Prandtl-Tomlinson model. Moreover, friction of single-layer h-BN, MoS₂, and graphene was found to increase with relative humidity and decrease with thermal annealing; these trends were attributed to the diffusion of water molecules to the interface between the single-layer materials and their substrates, which leads to an increase in the puckering effect at the tip-material interface and interaction potential corrugation. The enhanced puckering effect was verified via molecular dynamics simulations. Overall, the findings enable a comprehensive understanding of friction characteristics for several classes of two-dimensional materials, which is important to elucidate the feasibility of using these materials as protective and solid-lubricant coating layers for nanoscale devices.

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.

Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide

In this work, the effect of humidity and water intercalation on the friction and wear behavior of few-layers of graphene and graphene oxide (GO) was studied using friction force microscopy. Thickness measurements demonstrated significant water intercalation within GO affecting its surface topography (roughness and protrusions), whereas negligible water intercalation of graphene was observed. It was found that water intercalation in GO contributed to wearing of layers at a relative humidity as low as ∼30%. The influence of surface wettability and water adsorption was also studied by comparing the sliding behavior of SiO₂/GO, SiO₂/Graphene, and SiO₂/SiO₂ interfaces. Friction for the SiO₂/GO interface increased with relative humidity due to water intercalation and condensation of water. In contrast, it was observed that adsorption of water molecules lubricated the SiO_2/SiO₂ interface due to easy shearing of water on the hydrophobic surface, particularly once the adsorbed water layers had transitioned from "ice-like water" to "liquid-like water" structures. Lastly, an opposite friction trend was observed for the graphene/SiO₂ interface with water molecules failing to lubricate the interface as compared to the dry graphene/SiO₂ contact.

Anomalous Nanoscale Optoacoustic Phonon Mixing in Nematic Mesogens

Recent inelastic X-ray scattering (IXS) experiments on mesogens have revealed entirely new capabilities with regards to their nanoscale phonon-assisted heat management. Mesogens such as nematic liquid crystals (LCs) are appealing systems for study because their structure and morphology can easily be tuned. We report on Q-resolved ultra-high-resolution IXS, X-ray diffraction, and THz time-domain spectroscopy experiments combined with large-scale all-atom molecular dynamics simulations on the dynamic properties of 5CB LCs. For the first time, we observe a strong mixing of phonon excitations originating from independent in-phase and out-of-phase van-der-Waals-mediated displacement patterns. The coexistence of transverse acoustic and optical modes of 5CB LCs at near room temperature is revealed through the emergent transverse phonon gap and THz light-phonon coupling taking place within the same energy range. Furthermore, our experimental observations are supported by analysis showing correlations of spontaneous fluctuations of LCs on picosecond time scales. These findings are significant for the design of a new generation of soft molecular vibration-sensitive nanoacoustic and optomechanical applications.

Rippling of graphitic surfaces: a comparison between few-layer graphene and HOPG

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bio-organic molecules (l-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 ± 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity "relaxes" to 6.2 ± 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

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.

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.

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.