Yunfeng Shi

Wetting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates.

The recent interest and excitement in graphene has also opened up a pandoras box of other two-dimensional (2D) materials and material combinations which are now beginning to come to the fore. One family of these emerging 2D materials is transition metal dichalcogenides (TMDs). So far there is very limited understanding on the wetting behavior of monolayer TMD materials. In this study, we synthesized large-area, continuous monolayer tungsten disulfide (WS2) and molybdenum disulfide (MoS2) films on SiO2/Si substrates by the thermal reduction and sulfurization of WO3 and MO3 thin films. The monolayer TMD films displayed an advancing water contact angle of ∼83° as compared to ∼90° for the bulk material. We also prepared bilayer and trilayer WS2 films and studied the transition of the water contact angle with increasing number of layers. The advancing water contact angle increased to ∼85° for the bilayer and then to ∼90° for the trilayer film. Beyond three layers, there was no significant change in the measured water contact angle. This type of wetting transition indicates that water interacts to some extent with the underlying silica substrate through the monolayer TMD sheet. The experimentally observed wetting transition with numbers of TMD layers lies in-between the predictions of one continuum model that considers only van der Waals attractions and another model that considers only dipole-dipole interactions. We also explored wetting as a function of aging. A clean single-layer WS2 film (without airborne contaminants) was shown to be strongly hydrophilic with an advancing water contact angle of ∼70°. However, over time, the sample ages as hydrocarbons and water present in air adsorb onto the clean WS2 sheet. After ∼7 days, the aging process is completed and the advancing water contact angle of the aged single-layer WS2 film stabilizes at ∼83°. These results suggest that clean (i.e., nonaged) monolayer TMDs are hydrophilic materials. We further show that substitution of sulfur atoms by oxygen in the lattice of aged monolayer WS2 and MoS2 films can be used to generate well-defined hydrophobic-hydrophilic patterns that preferentially accumulate and create microdrop arrays on the surface during water condensation and evaporation experiments.

Intrinsic ductility of glassy solids

Glasses are usually brittle, seriously limiting their practical usage. Recently, the intrinsic ductility of glass was found to increase with the Poissons ratio (v), with a sharp brittle-to-ductile (BTD) transition at vBTDu2009=u20090.31-0.32. Such a correlation between far-from-equilibrium fracture and near-equilibrium elasticity is unexpected and not understood. Molecular dynamics simulations, on three families of glasses (metallic glasses, amorphous silicon, and silica) with controlled bonding, processing, and testing conditions, show that glasses with low covalency and high structural disorder have high v and ductility, and vice versa. The BTD transitions triggered by the aforementioned causes in each system correspond to a unified vBTD value, which increases with its average coordination number (CN). The vBTD-CN relation can be comprehended by recognizing v as a measure of covalency and disorder, and the BTD transition as a competition between shear and cleavage. Our results provide guidelines for developing...

Graphene Drape Minimizes the Pinning and Hysteresis of Water Drops on Nanotextured Rough Surfaces

Previous studies of the interaction of water with graphene-coated surfaces have been limited to flat (smooth) surfaces. Here we created a rough surface by nanopatterning and then draped the surface with a single-layer graphene sheet. We found that the ultrasheer graphene drape prevents the penetration of water into the textured surface thereby drastically reducing the contact angle hysteresis (which is a measure of frictional energy dissipation) and preventing the liquid contact line from getting pinned to the substrate. This has important technological implications since the main obstacle to the motion of liquid drops on rough surfaces is contact angle hysteresis and contact line pinning. Graphene drapes could therefore enable enhanced droplet mobility which is required in a wide range of applications in micro and nanofluidics. Compared to polymer coatings that could fill the cavities between the nano/micropores or significantly alter the roughness profile of the substrate, graphene provides the thinnest (i.e., most sheer) and most conformal drape that is imaginable. Despite its extreme thinness, the graphene drape is mechanically robust, chemically stable, and offers high flexibility and resilience which can enable it to reliably drape arbitrarily complex surface topologies. Graphene drapes may therefore provide a hitherto unavailable ability to tailor the dynamic wettability of surfaces for a variety of applications.

Protecting Silicon Film Anodes in Lithium-Ion Batteries Using an Atomically Thin Graphene Drape

Silicon (Si) shows promise as an anode material in lithium-ion batteries due to its very high specific capacity. However, Si is highly brittle, and in an effort to prevent Si from fracturing, the research community has migrated from the use of Si films to Si nanoparticle based electrodes. However, such a strategy significantly reduces volumetric energy density due to the porosity of Si nanoparticle electrodes. Here we show that contrary to conventional wisdom, Si films can be stabilized by two strategies: (a) anchoring the Si films to a carbon nanotube macrofilm (CNM) current collector and (b) draping the films with a graphene monolayer. After electrochemical cycling, the graphene-coated Si films on CNM resembled a tough mud-cracked surface in which the graphene capping layer suppresses delamination and stabilizes the solid electrolyte interface. The graphene-draped Si films on CNM exhibit long cycle life (>1000 charge/discharge steps) with an average specific capacity of ∼806 mAh g-1. The volumetric capacity averaged over 1000 cycles of charge/discharge is ∼2821 mAh cm-3, which is 2 to 5 times higher than what is reported in the literature for Si nanoparticle based electrodes. The graphene-draped Si anode could also be successfully cycled against commercial cathodes in a full-cell configuration.

Self-heating–induced healing of lithium dendrites

Healing away the dendrites The formation of lithium dendrites during charge-discharge cycles limits the development of lithium metal batteries, because the dendrites can cause electrical shorting of the cells. A number of tricks have been used to try to prevent dendrite formation. Li et al. took the opposite approach (see the Perspective by Mukhopadhyay and Jangid). They operated their cells at higher current densities, under which one would expect dendrites to form owing to the higher nucleation rates. However, under these conditions, the dendrites that started to form heated up and annealed, leading to their disappearance. Science, this issue p. 1513; see also p. 1463 Lithium metal dendrites can be healed in situ by Joule self-heating of the dendritic particles. Lithium (Li) metal electrodes are not deployable in rechargeable batteries because electrochemical plating and stripping invariably leads to growth of dendrites that reduce coulombic efficiency and eventually short the battery. It is generally accepted that the dendrite problem is exacerbated at high current densities. Here, we report a regime for dendrite evolution in which the reverse is true. In our experiments, we found that when the plating and stripping current density is raised above ~9 milliamperes per square centimeter, there is substantial self-heating of the dendrites, which triggers extensive surface migration of Li. This surface diffusion heals the dendrites and smoothens the Li metal surface. We show that repeated doses of high-current-density healing treatment enables the safe cycling of Li-sulfur batteries with high coulombic efficiency.

Commonalities in frequency-dependent viscoelastic damping in glasses in the MHz to THz regime

We use non-equilibrium molecular dynamics oscillatory shear simulations to study frequency-dependent viscoelastic damping spanning nearly six decades in frequency range (MHz to THz), in a wide range of model glasses including binary glasses such as Cu-Zr metallic glass (MG), Wahnstrom glass and amorphous silica, and unary glasses, namely, Dzugutov glass and amorphous silicon. First, for the Cu-Zr MG, we elucidate the role of quench rate, number of shear cycles, shear amplitude, and shear temperature on the damping characteristics. We observe striking commonalities in damping characteristics for all glasses studied—(i) a peak in the loss modulus in the high-frequency regime (∼THz) and (ii) persistent damping in the low-frequency regime (extending down to 10u2009s of MHz). The high-frequency peak is seen to overlap with the range of natural vibrational frequencies for each glass, and arises from coupling between the excited harmonic vibrational modes. On the other hand, persistent damping at intermediate and lo...

On measuring the fracture energy of model metallic glasses

We report a heuristic approach to measure the fracture energy of model metallic glasses using molecular dynamics simulation. Specifically, we adopted the Rivlin-Thomas method, simplified by Suo et al., which is applicable even with the presence of plastic flow. We further modified the testing condition with semi-rigid holders in our molecular simulations, to avoid unintended fracture near the holders. This method was first applied in measuring the fracture energy of a brittle model glass, which agrees well with direct K I C and J I C measurements (both measurements are independent of the crack size). Furthermore, the fracture energy values of a family of model metallic glasses, ranging from brittle to ductile (BTD), were measured. The Poissons ratio-fracture energy (v-G, or v-G/2γ, normalized by the surface energy) relation obtained here exhibits a BTD transition at a critical Poissons ratio of 0.31–0.32, consistent with experimental results.We report a heuristic approach to measure the fracture energy of model metallic glasses using molecular dynamics simulation. Specifically, we adopted the Rivlin-Thomas method, simplified by Suo et al., which is applicable even with the presence of plastic flow. We further modified the testing condition with semi-rigid holders in our molecular simulations, to avoid unintended fracture near the holders. This method was first applied in measuring the fracture energy of a brittle model glass, which agrees well with direct K I C and J I C measurements (both measurements are independent of the crack size). Furthermore, the fracture energy values of a family of model metallic glasses, ranging from brittle to ductile (BTD), were measured. The Poissons ratio-fracture energy (v-G, or v-G/2γ, normalized by the surface energy) relation obtained here exhibits a BTD transition at a critical Poissons ratio of 0.31–0.32, consistent with experimental results.

Utilizing van der Waals slippery interfaces to enhance the electrochemical stability of Silicon film anodes in Lithium-ion batteries

High specific capacity anode materials such as silicon (Si) are increasingly being explored for next-generation, high performance lithium (Li)-ion batteries. In this context, Si films are advantageous compared to Si nanoparticle based anodes since in films the free volume between nanoparticles is eliminated, resulting in very high volumetric energy density. However, Si undergoes volume expansion (contraction) under lithiation (delithiation) of up to 300%. This large volume expansion leads to stress build-up at the interface between the Si film and the current collector, leading to delamination of Si from the surface of the current collector. To prevent this, adhesion promotors (such as chromium interlayers) are often used to strengthen the interface between the Si and the current collector. Here, we show that such approaches are in fact counter-productive and that far better electrochemical stability can be obtained by engineering a van der Waals slippery interface between the Si film and the current collector. This can be accomplished by simply coating the current collector surface with graphene sheets. For such an interface, the Si film slips with respect to the current collector under lithiation/delithiation, while retaining electrical contact with the current collector. Molecular dynamics simulations indicate (i) less stress build-up and (ii) less stress cycling on a van der Waals slippery substrate as opposed to a fixed interface. Electrochemical testing confirms more stable performance and much higher Coulombic efficiency for Si films deposited on graphene-coated nickel (i.e., slippery interface) as compared to conventional nickel current collectors.

A minimalist's reactive potential for efficient molecular modelling of chemistry

We review a minimalists reactive force field, reactive state summation (RSS) potential. The essence of RSS potential scheme is to model each reactive state by individual non-reactive force fields, then modulate each term by a reaction-coordinate-dependent weight function, finally sum together to obtain the reactive potential. Compared with existing reactive potentials, RSS potential is easier to formulate and parameterise and is computationally efficient, at the expense of lesser accuracy. Thus, RSS potential can be regarded as a ‘reactive Lennard-Jones’ potential. Three exemplary RSS potentials are described in the context of their respective chemical systems: RSS-nitrogen for modelling detonation, RSS-carbon for modelling pyrolysis of activated carbon and RSS-fuel-catalyst for modelling catalytic chemical reaction.