Phase transitions of a water overlayer on charged graphene: from electromelting to electrofreezing

Cheh J., Gao Y., Wang C., Zhao H., Fang H.

Qiu H., Guo W.

In sharp contrast to the prevailing view that electric fields promote water freezing, here we show by molecular dynamics simulations that monolayer ice confined between two parallel plates can melt into liquid water under a perpendicularly applied electric field. The melting temperature of the monolayer ice decreases with the increasing strength of the external field due to the field-induced disruption of the water-wall interaction induced well-ordered network of the hydrogen bond. This electromelting process should add an important new ingredient to the physics of water.

Zhu C., Li H., Huang Y., Zeng X.C., Meng S.

We report simulation evidence that the structure of the first water layer next to the surface can strongly affect the contact angle of water droplets. Molecular dynamics simulations show that a small uniform strain (± 3%) applied to the lattice constant of a multilayer hydrophilic surface can introduce a marked change in the wetting tendency. In particular, when the lattice constant of a hydrophilic surface matches the projected oxygen-oxygen distance of bulk water to the surface, a contact-angle minimum is resulted. In stark contrast, such a lattice strain has little effect on the wetting properties of hydrophobic surfaces. The structure of the first water layer next to the hydrophilic surface gradually loses characteristics of liquid water when moving away from the contact-angle minimum. Our results demonstrate a close correlation among the length of lattice constant, contact angle of the water droplet, and the structure and dynamics of vicinal water.

Xuan Lim C.H., Sorkin A., Bao Q., Li A., Zhang K., Nesladek M., Loh K.P.

The hardness and virtual incompressibility of diamond allow it to be used in high-pressure anvil cell. Here we report a new way to generate static pressure by encapsulating single-crystal diamond with graphene membrane, the latter is well known for its superior nano-indentation strength and in-plane rigidity. Heating the diamond–graphene interface to the reconstruction temperature of diamond (~1,275 K) produces a high density of graphene nanobubbles that can trap water. At high temperature, chemical bonding between graphene and diamond is robust enough to allow the hybrid interface to act as a hydrothermal anvil cell due to the impermeability of graphene. Superheated water trapped within the pressurized graphene nanobubbles is observed to etch the diamond surface to produce a high density of square-shaped voids. The molecular structure of superheated water trapped in the bubble is probed using vibrational spectroscopy and dynamic changes in the hydrogen-bonding environment are observed. The hardness and incompressibility of diamond makes it an ideal material for high-pressure anvil cells. Here, a method for generating static pressure is described in which graphene-coated diamond is heated, forming graphene nanobubbles that trap water at pressures sufficient to etch the surface of diamond.

Lv W., Wu R.

A computational investigation was carried out to understand the aggregation of nanoscale graphene with two typical assembly pathways of stacking assembly and sliding assembly in water. The interfacial-organized monolayer water film (MWF) hindering the aggregation of nanographene in both stacking and sliding assembly pathways was reported for the first time. By means of potential mean forces (PMFs) calculation, no energy barrier was observed during the sliding assembly of two graphene nanosheets, while the PMF profiles could be impacted by the contact forms of nanographene and the MWF within the interplate of two graphene nanosheets. To explore the potential physical basis of the hindering-role of self-organized interfacial water, the dynamical and structural properties as well as the status of hydrogen bonds (H-bonds) for interfacial water were investigated. We found that the compact, ordered structure and abundant H-bonds of the MWF could be taken as the fundamental aspects of the hindering-role of interfacial water for the hydrophobic assembly of nanographene. These findings are displaying a potential to further understand the hydrophobic assembly which mostly dominate the behaviors of nanomaterials, proteins etc. in aqueous solutions.

Zhu X., Yuan Q., Zhao Y.

Molecular dynamics simulations were carried out to explore the capillary wave propagation induced by the competition between one upper precursor film (PF) on the graphene and one lower PF on the substrate in electro-elasto-capillarity (EEC). During the wave propagation, the graphene was gradually delaminated from the substrate by the lower PF. The physics of the capillary wave was explored by the molecular kinetic theory. Besides, the dispersion relation of the wave was obtained theoretically. The theory showed that the wave was controlled by the driving work difference of the two PFs. Simulating the EEC process under different electric field intensities (E), the wave velocity was found insensitive to E. We hope this research could expand our knowledge on the wetting, electrowetting and EEC. As a potential application, the electrowetting of the PF between the graphene and the substrate is a promising candidate for delaminating graphene from substrate.

Feng X., Maier S., Salmeron M.

The adsorption and reactions of small molecules, such as water and oxygen, with graphene films is an area of active research, as graphene may hold the key to unique applications in electronics, batteries, and other technologies. Since the graphene films produced so far are typically polycrystalline, with point and line defects that can strongly affect gas adsorption, there is a need to understand their reactivity with environmentally abundant molecules that can adsorb and alter their properties. Here we report a study of the adsorption and reactions of water, oxygen, hydrogen, and ammonia on epitaxial graphene grown on Ru and Cu substrates using scanning tunneling microscopy (STM). We found that on Ru(0001) graphene line defects are extremely fragile toward chemical attack by water, which splits the graphene film into numerous fragments at temperatures as low as 90 K, followed by water intercalation under the graphene. On Cu(111) water can also split graphene but far less effectively, indicating that the chemical nature of the substrate strongly affects the reactivity of the C–C bonds in epitaxial graphene. Interestingly, no such effects were observed with other molecules, including oxygen, hydrogen, and ammonia also studied here.

Li H., Zeng X.C.

Born-Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron-nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net positive charges on the contacting surface of the graphene and monolayer BN sheets, and such charge induction may affect electronic structure of the contacting graphene in view that graphene is a semimetal. Contact angles of nanodroplets of water in a supercooled state on the graphene are also computed. It is found that under the supercooled condition, water nanodroplets exhibit an appreciably larger contact angle than under the ambient condition.

Nair R.R., Wu H.A., Jayaram P.N., Grigorieva I.V., Geim A.K.

Porous Membranes Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by Paul ). Karan et al. (p. 444 ) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. Nair et al. (p. 442 ) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water.

Teschke O.

In this work, ice was viewed at the nanoscale by scanning an atomic force microscopy tip over a highly oriented pyrolytic graphite (HOPG) surface in air. At low scan velocities, the tip exhibited stick−slip motion with a period of 0.13 nm corresponding to the scanner step; at higher velocities, the HOPG lattice and the periodicity of the ice were visible. A hexagonal structure with a 0.45 ± 0.04 nm periodicity was observed in which the distance between the second neighbors of the HOPG coincided with the distance of the first neighbors for the ice-like arrangement. Small water clusters were also nucleated with an ice-Ic structure (0.34 ± 0.03 nm), and thus, the ice layers consisted of extensive sets composed of arrangements of hexamers and tetramers.

Han S., Choi M.Y., Kumar P., Stanley H.E.

Phase transitions in water are normally classified as first or second order. But in confined quasi-one-dimensional films of water, simulations show that the solid–liquid transition can take place by means of a first-order transition or a continuous one without a distinction between solid and liquid. Bulk water has three phases: solid, liquid and vapour. In addition to undergoing a phase transition (of the first order) between them, liquid and vapour can deform continuously into each other without crossing a transition line—in other words, there is no intrinsic distinction between the two phases. Hence, the first-order line of the liquid–vapour phase transition should terminate at a critical point. In contrast, the first-order transition line between solid and liquid is believed to persist indefinitely without terminating at a critical point1. In recent years, however, it was reported that inside carbon nanotubes, freezing of water may occur continuously as well as discontinuously through a first-order phase transition2. Here we present simulation results for water in a quasi-two-dimensional hydrophobic nanopore slit, which are consistent with the idea that water may freeze by means of both first-order and continuous phase transitions. Our results lead us to hypothesize the existence of a connection point at which first-order and continuous transition lines meet3,4.

Miller D.L., Kubista K.D., Rutter G.M., Ruan M., de Heer W.A., First P.N., Stroscio J.A.

Rotational misalignment of two stacked honeycomb lattices produces a moir\'e pattern that is observable in scanning tunneling microscopy as a small modulation of the apparent surface height. This is known from experiments on highly-oriented pyrolytic graphite. Here, we observe the combined effect of three-layer moir\'e patterns in multilayer graphene grown on SiC ($000\bar{1}$). Small-angle rotations between the first and third layer are shown to produce a "double-moir\'e" pattern, resulting from the interference of moir\'e patterns from the first three layers. These patterns are strongly affected by relative lattice strain between the layers. We model the moir\'e patterns as a beat-period of the mismatched reciprocal lattice vectors and show how these patterns can be used to determine the relative strain between lattices, in analogy to strain measurement by optical moir\'e interferometry.

Balog R., Jørgensen B., Nilsson L., Andersen M., Rienks E., Bianchi M., Fanetti M., Lægsgaard E., Baraldi A., Lizzit S., Sljivancanin Z., Besenbacher F., Hammer B., Pedersen T.G., Hofmann P., et. al.

Several routes designed to induce a bandgap opening in graphene have been proposed. It is now demonstrated that hydrogen adsorption on the Moiré pattern induced by an iridium substrate can induce a bandgap of 450 meV. Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many emerging areas such as graphene-based electronic devices1. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication2,3. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice4,5,6, but experimental evidence for this is so far lacking. Here, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the Moiré superlattice positions of graphene grown on an Ir(111) substrate.

González M.A., Abascal J.L.

In this work, the shear viscosity at ambient conditions of several water models (SPC/E, TIP4P, TIP5P, and TIP4P/2005) is evaluated using the Green–Kubo formalism. The performance of TIP4P/2005 is excellent, that of SPC/E and TIP5P is more or less acceptable, whereas TIP4P and especially TIP3P give a poor agreement with experiment. Further calculations have been carried out for TIP4P/2005 to provide a wider assessment of its performance. In accordance with experimental data, TIP4P/2005 predicts a minimum in the shear viscosity for the 273 K isotherm, a shift in the minimum toward lower pressures at 298 K, and its disappearance at 373 K.

Patra N., Wang B., Král P.

We demonstrate by molecular dynamics simulations that water nanodroplets can activate and guide the folding of planar graphene nanostructures. Once the nanodroplets are deposited at selected spots on the planar nanostructure, they can act as catalytic elements that initiate conformational changes and help to overcome deformation barriers associated with them. Nanodroplets can induce rapid bending, folding, sliding, rolling, and zipping of the planar nanostructures, which can lead to the assembly of nanoscale sandwiches, capsules, knots, and rings.

Zhao C., Lin Y., Wu X., Ma L., Chu F.

Smirnov A., Anisimkin V., Ageykin N., Datsuk E., Kuznetsova I.

An important technical task is to develop methods for recording the phase transitions of water to ice. At present, many sensors based on various types of acoustic waves are suggested for solving this challenge. This paper focuses on the theoretical and experimental study of the effect of water-to-ice phase transition on the properties of Lamb and quasi shear horizontal (QSH) acoustic waves of a higher order propagating in different directions in piezoelectric plates with strong anisotropy. Y-cut LiNbO3, 128Y-cut LiNbO3, and 36Y-cut LiTaO3 plates with a thickness of 500 μm and 350 μm were used as piezoelectric substrates. It was shown that the amplitude of the waves under study can decrease, increase, or remain relatively stable due to the water-to-ice phase transition, depending on the propagation direction and mode order. The greatest decrease in amplitude (42.1 dB) due to glaciation occurred for Lamb waves with a frequency of 40.53 MHz and propagating in the YX+30° LiNbO3 plate. The smallest change in the amplitude (0.9 dB) due to glaciation was observed for QSH waves at 56.5 MHz propagating in the YX+60° LiNbO3 plate. Additionally, it was also found that, in the YX+30° LiNbO3 plate, the water-to-ice transition results in the complete absorption of all acoustic waves within the specified frequency range (10–60 MHz), with the exception of one. The phase velocities, electromechanical coupling coefficients, elastic polarizations, and attenuation of the waves under study were calculated. The structures “air–piezoelectric plate–air”, “air–piezoelectric plate–liquid”, and “air–piezoelectric plate–ice” were considered. The results obtained can be used to develop methods for detecting ice formation and measuring its parameters.

Smirnov A., Anisimkin V., Voronova N., Kashin V., Kuznetsova I.

The detection of the liquid-to-ice transition is an important challenge for many applications. In this paper, a method for multi-parameter characterization of the liquid-to-ice phase transition is proposed and tested. The method is based on the fundamental properties of bulk acoustic waves (BAWs). BAWs with shear vertical (SV) or shear horizontal (SH) polarization cannot propagate in liquids, only in solids such as ice. BAWs with longitudinal (L) polarization, however, can propagate in both liquids and solids, but with different velocities and attenuations. Velocities and attenuations for L-BAWs and SV-BAWs are measured in ice using parameters such as time delay and wave amplitude at a frequency range of 1–37 MHz. Based on these measurements, relevant parameters for Rayleigh surface acoustic waves and Poisson’s modulus for ice are determined. The homogeneity of the ice sample is also detected along its length. A dual sensor has been developed and tested to analyze two-phase transitions in two liquids simultaneously. Distilled water and a 0.9% solution of NaCl in water were used as examples.

Xu Q., Shen Y., Zhang C., Xu R., Gu Q., Guo H., Meng S.

Jin Y., Wu C., Sun P., Wang M., Cui M., Zhang C., Wang Z.

Lang H., Zou K., Chen R., Huang Y., Peng Y.

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.

Alosious S., Kannam S.K., Sathian S.P., Todd B.D.

Electrostatic interactions in nanoscale systems can influence the heat transfer mechanism and interfacial properties. This study uses molecular dynamics simulations to investigate the impact of various electrostatic interactions on the Kapitza resistance (Rk) on a hexagonal boron nitride-water system. The Kapitza resistance at hexagonal boron nitride nanotube (hBNNT)-water interface reduces with an increase in diameter of the nanotube due to more aggregation of water molecules per unit surface area. An increase in the partial charges on boron and nitride caused the reduction in Rk. With the increase in partial charge, a better hydrogen bonding between hBNNT and water was observed, whereas the structure and order of the water molecules remain the same. Nevertheless, the addition of NaCl salt into water does not have any influence on interfacial thermal transport. Rk remains unchanged with electrolyte concentration because the cumulative Coulombic interaction between the ions and the hBNNT is significantly less when compared with water molecules. Furthermore, the effect of electric field strength on interfacial heat transfer is also investigated by providing uniform positive and negative surface charges on the outermost hBN layers. Rk is nearly independent of the practical range of applied electric fields and decreases with an increasing electric field for extreme field strengths until the electrofreezing phenomenon occurs. The ordering of water molecules toward the charged surface leads to an increase in the layering effect, causing the reduction in Rk in the presence of an electric field.

Yenigun O., Barisik M.

We introduced an active heat transfer control between graphene and water using interdigitated electrodes (IDEs). Oppositely charged co-planer electrodes embedded on a graphene surface created a non-uniform electric field. Resulted interface localized liquid dielectrophoresis (LDEP) perpendicular to surface enhanced the water/graphene coupling and decreased interfacial thermal resistance (ITR) substantially. We correlated the theoretical calculations of average electric field strength near surface with Kapitza values measured at corresponding electrode configurations. We obtained a unified linear variation of Kapitza as a function of average electric strength independent of electrode size and charge. By increasing the electric field strength, we measured up to 96% decrease of Kapitza near electrodes. Since the IDEs generated electric field was only interface localized, it required lower electrode charges than any parallel-plate capacitor systems. We showed that ITR remains effective in heat transfer behavior for systems as big as 100nm such that interface localized electric field can at least increase the heat removal 50% by eliminating the ITR from both graphene/water interfaces of a channel system. By converting hydrophobic few-layer graphene to super-hydrophilic condition with ultra-low Kapitza, current results are important for graphene-based materials considered for the solution of the thermal management problem of current and next generation micro/nano-electronics.

Chin H., Klimes J., Hu I., Chen D., Nguyen H., Chen T., Ma S., Hofmann M., Liang C., Hsieh Y.

We here report on the direct observation of ferroelectric properties of water ice in its 2D phase. Upon nanoelectromechanical confinement between two graphene layers, water forms a 2D ice phase at room temperature that exhibits a strong and permanent dipole which depends on the previously applied field, representing clear evidence for ferroelectric ordering. Characterization of this permanent polarization with respect to varying water partial pressure and temperature reveals the importance of forming a monolayer of 2D ice for ferroelectric ordering which agrees with ab-initio and molecular dynamics simulations conducted. The observed robust ferroelectric properties of 2D ice enable novel nanoelectromechanical devices that exhibit memristive properties. A unique bipolar mechanical switching behavior is observed where previous charging history controls the transition voltage between low-resistance and high-resistance state. This advance enables the realization of rugged, non-volatile, mechanical memory exhibiting switching ratios of 106, 4 bit storage capabilities and no degradation after 10,000 switching cycles. Ferroelectric ordering of water has been at the heart of intense debates due to its importance in enhancing our understanding of the condensed matter. Here, the authors observe ferroelectric properties of water ice in a two dimensional phase under confinement between two graphene layers.

Lu H., Xu Q., Wu J., Hong R., Zhang Z.

In this letter, we performed molecular dynamics simulations of ice nucleation on a rigid surface model of cubic zinc blende structure with different surface dipole strength and orientation. It follows that, despite the excellent lattice match between cubic ice and substrates, the ice nucleation happened only when the interfacial water molecules (IWs) have the same or similar orientations as that of the water molecules in cubic ice. The free energy landscapes revealed that, for substrates with improper dipole strength/orientation, large free energy barriers arose to prevent the dipole of IWs rotating to the right orientation to trigger ice formation. Our results suggest that the traditional concept of lattice match, the similarity of lattice length between a substrate and the new-formed crystalline, should be extended to a broader match include the similarity between the molecular orientations of the interfacial component and the component in the specific new-formed crystalline face.

Bespaly A., Dey I., Papeer J., Shaham A., Komm P., Hadad I., Marcus G., Zigler A.

Abstract Microstructured targets demonstrate an enhanced coupling of high-intensity laser pulse to a target and play an important role in laser-induced ion acceleration. Here we demonstrate an approach that enables us to control the morphology of amorphous solid water (ASW) microstructured targets, by deposition of water vapor on a charged substrate, cooled down to 100 K. The morphology of the deposited ASW structures is controlled by varying the surface charge on the substrate and the pressure of water vapor. The obtained target is structured as multiple, dense spikes, confined by the charged area on the substrate, with increased aspect ratio of up to 5:1 and having a diameter comparable with the typical spot size of the laser focused onto the target.

Li J., Zhu Y., Xia J., Fan J., Wu H., Wang F.

In this work, we explored how the structure of monolayer water confined between two graphene sheets is coupled to its dynamic behavior. Our molecular dynamics simulations show that there is a remarkable interrelation between the friction of confined water with two walls and its structure under extreme confinement. When the water molecules formed a regular quadrilateral structure, the friction coefficient is dramatically reduced. Such a low-friction coefficient can be attributed to the formation of long-range ordered hydrogen bond network, which not only decreases the structure corrugation in the direction perpendicular to the walls but also promotes the collective motion of the confined water. The regular quadrilateral structure can be formed only if the number density of confined water falls within a certain range. Higher number density results in larger structure corrugations, which increases the friction, while smaller number density leads to an irregular hydrogen bond network in which the collective motion cannot play the role. We demonstrated that there are four distinct stages in the diagram of the friction coefficient vs the number density of confined water. This research clearly established the connection between the dynamic characteristics of confined monolayer water and its structure, which is beneficial to further understand the mechanism of the high-speed water flow through graphene nanocapillaries observed in recent experiments.

Yenigun O., Barisik M.

Graphene-based materials are considered for the solution of the thermal management problem of current and next generation micro/nano-electronics with high heat generation densities. However, the hydrophobic nature of few-layer graphene makes passing heat to a fluid very challenging. We introduced an active and local manipulation of heat transfer between graphene and water using an applied, non-uniform electric field. When water undergoes electric field induced orientation polarization and liquid dielectrophoresis, a substantial increase in heat transfer develops due to a decrease in interfacial thermal resistance and increase in thermal conductivity. By using two locally embedded pin and plate electrodes of different sizes, we demonstrated a two-dimensional heat transfer control between two parallel few-layer graphene slabs. We obtained local heat transfer increase up to nine times at pin electrode region with an ultra-low Kapitza resistance through the studied non-uniform electric field strength range creating highly-ordered compressed water in the experimentally measured density limits. With this technique, heat can be (i) distributed from a smaller location to a larger section and/or (ii) collected to a smaller section from a larger region. Current results are important for hot spot cooling and/or heat focusing applications.

Anisimkin V., Kolesov V., Kuznetsova A., Shamsutdinova E., Kuznetsova I.

It is shown that, in spite of the wave radiation into the adjacent liquid, a large group of Lamb waves are able to propagate along piezoelectric plates (quartz, LiNbO3, LiTaO3) coated with a liquid layer (distilled water H2O). When the layer freezes, most of the group’s waves increase their losses, essentially forming an acoustic response towards water-to-ice transformation. Partial contributions to the responses originating from wave propagation, electro-mechanical transduction, and wave scattering were estimated and compared with the coupling constants, and the vertical displacements of the waves were calculated numerically at the water–plate and ice–plate interfaces. The maximum values of the responses (20–30 dB at 10–100 MHz) are attributed to the total water-to-ice transformation. Time variations in the responses at intermediate temperatures were interpreted in terms of a two-phase system containing both water and ice simultaneously. The results of the paper may turn out to be useful for some applications where the control of ice formation is an important problem (aircraft wings, ship bodies, car roads, etc.).

Xie Z., Li Z., Li J., Kou J., Yao J., Fan J.

Gas dissolution or accumulation regulating in an aqueous environment is important but difficult in various fields. Here, we performed all-atom molecular dynamics simulations to study the dissolution/accumulation of gas molecules in aqueous solutions. It was found that the distribution of gas molecules at the solid–water interface is regulated by the direction of the external electric field. Gas molecules attach and accumulate to the interface with an electric field parallel to the interface, while the gas molecules depart and dissolve into the aqueous solutions with a vertical electric field. The above phenomena can be attributed to the redistribution of water molecules as a result of the change of hydrogen bonds of water molecules at the interface as affected by the electric field. This finding reveals a new mechanism of regulating gas accumulation and dissolution in aqueous solutions and can have tremendous applications in the synthesis of drugs, the design of microfluidic device, and the extraction of natural gas.