Zhijiang Ye

Role of wrinkle height in friction variation with number of graphene layers

Molecular dynamics simulations are performed to study the frictional behavior of graphene. It is found that the friction between a diamond tip and graphene decreases with increasing number of graphene layers. This behavior is also affected by the graphene sheet size; specifically, the effect of the number of layers on friction becomes significant only when the modeled graphene sheets exceed a critical length. We further show that the frictional behavior can be directly correlated to the height of near-contact wrinkles that resist sliding. These observations are rationalized in terms of the ability of multiple sheets to act as a single material as they resist wrinkle formation.

Thermal Conductivity in Nanostructured Films: From Single Cellulose Nanocrystals to Bulk Films

We achieved a multiscale description of the thermal conductivity of cellulose nanocrystals (CNCs) from single CNCs (∼0.72-5.7 W m(-1) K(-1)) to their organized nanostructured films (∼0.22-0.53 W m(-1) K(-1)) using experimental evidence and molecular dynamics (MD) simulation. The ratio of the approximate phonon mean free path (∼1.7-5.3 nm) to the lateral dimension of a single CNC (∼5-20 nm) suggested a contribution of crystal-crystal interfaces to polydisperse CNC films heat transport. Based on this, we modeled the thermal conductivity of CNC films using MD-predicted single crystal and interface properties along with the degree of CNC alignment in the bulk films using Hermans order parameter. Film thermal conductivities were strongly correlated to the degree of CNC alignment and the direction of heat flow relative to the CNC chain axis. The low interfacial barrier to heat transport found for CNCs (∼9.4 to 12.6 m(2) K GW(-1)), and their versatile alignment capabilities offer unique opportunities in thermal conductivity control.

Correlation Between Probe Shape and Atomic Friction Peaks at Graphite Step Edges

Molecular dynamics simulation and atomic force microscopy are used to study the nature of friction between nanoscale tips and graphite step edges. Both techniques show that the width of the lateral force peak as the probe moves up a step is directly correlated with the size and shape of the tip. The origin of that relationship is explored and the similarities and differences between the measurements and simulations are discussed. The observations suggest that the relationship between lateral force peak width and tip geometry can be used as a real-time monitor for tip wear during atomic scale friction measurements.

Load-Dependent Friction Hysteresis on Graphene

Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip-graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene-water interaction strength. The size of the water-graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading.

Effect of tip shape on atomic-friction at graphite step edges

Materials such as graphite exhibit step edges that affect their frictional behavior. Recent experimental studies found that an atomic force microscope tip can experience either an assisting force that facilitates sliding or a resistive force that impedes motion as it scans down a step. Here, an atomistic model is used to show that tip shape affects its trajectory on a graphite step edge, which determines the potential energy, and thus the frictional behavior. The relationship between trajectory and potential energy is confirmed using density-functional theory, which provides insight into the origin of the energy barrier at a step edge.

Atomic friction at exposed and buried graphite step edges: Experiments and simulations

The surfaces of layered materials such as graphite exhibit step edges that affect friction. Step edges can be exposed, where the step occurs at the outmost layer, or buried, where the step is underneath another layer of material. Here, we study friction at exposed and buried step edges on graphite using an atomic force microscope (AFM) and complementary molecular dynamics simulations of the AFM tip apex. Exposed and buried steps exhibit distinct friction behavior, and the friction on either step is affected by the direction of sliding, i.e., moving up or down the step, and the bluntness of the tip. These trends are analyzing in terms of the trajectory of the AFM tip as it moves over the step, which is a convolution of the topography of the surface and the tip shape.

Structural and Chemical Evolution of the Near-Apex Region of an Atomic Force Microscope Tip Subject to Sliding

Atomic force microscopy and molecular dynamics simulation are used to study the nanoscale wear of a silicon dioxide tip sliding on a copper substrate. Wear is characterized in terms of structural and chemical evolution of the system where the latter is possible experimentally using atom probe tomography of the slid tips. Comparison of the experimentally observed and simulation-predicted wear reveals that adhesive wear is dominant in the short sliding distances of the simulation at any applied load, while the sliding distances in the experiments are long enough to observe load-induced transitions between adhesive-dominated and abrasive-dominated wear.

Atomistic Simulation of the Load Dependence of Nanoscale Friction on Suspended and Supported Graphene

Suspended graphene exhibits distinct behavior in which nanoscale friction first increases and then decreases with load; this is in contrast to the monotonic increase of friction with load exhibited by most materials, including graphene supported by a substrate. In this work, these friction trends are reproduced for the first time using molecular dynamics simulations of a nanoscale probe scanning on suspended and supported graphene. The atomic-scale detail available in the simulations is used to correlate friction trends to the presence and size of a wrinkle on the graphene surface in front of the probe. The simulations also provide information about how frictional load dependence is affected by the size of the graphene, the size of the probe, and the strength of the interaction between graphene and probe.

Oscillatory motion in layered materials: graphene, boron nitride, and molybdenum disulfide

Offset-driven self-retraction and oscillatory motion of bilayer graphene has been observed experimentally and is potentially relevant for nanoscale technological applications. In a previous article, we showed that friction between laterally offset graphene layers is controlled by roughness and proposed a simple reduced-order model based on density-functional theory (DFT) and molecular dynamics (MD) data, with which predictions on the experimental size-scale could be made. In this article, we extend our study to other layered materials, with emphasis on boron nitride (BN) and molybdenum disulfide (MoS2). Using MD and DFT simulations of these systems and a generalized version of the reduced-order model, we predict that BN will exhibit behavior similar to graphene (heavily-damped oscillation with a decay rate that increases with roughness) and that MoS2 shows no oscillatory behavior even in the absence of roughness. This is attributed to the higher energy barrier for sliding in MoS2 as well as the surface structure. Our generalized reduced-order model provides a guide to predicting and tuning experimental oscillation behavior using a few parameters that can be derived from simulation data.

The role of roughness-induced damping in the oscillatory motion of bilayer graphene

A multi-scale theoretical model is presented that is the first to offer quantitative agreement with experimental measurements of self-retraction and oscillation of bilayer graphene. The model integrates density-functional theory calculations of the energetics driving flake retraction and molecular-dynamics simulations capturing the dynamic response of laterally-offset rough surfaces. We demonstrate that nanoscale roughness explains self-retraction motion and propose a recipe for tuning that motion by controlling friction.