2024-03-28

Graphene nanoribbons grown in hBN stacks for high-performance electronics

Van der Waals encapsulation of two-dimensional materials in hexagonal boron nitride (hBN) stacks is a promising way to create ultrahigh-performance electronic devices. However, contemporary approaches for achieving van der Waals encapsulation, which involve artifcial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination and unscalable. Here we report the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) in hBN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow (<5nm) and homochiral with zigzag edges. Our atomistic simulations show that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA′-stacked hBN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR feld-efect devices that exhibit excellent performance at room temperature with mobilities of up to 4,600cm²V⁻¹s⁻¹and on–off ratios of up to 10⁶ . This paves the way for the bottom-up fabrication of high-performance electronic devices based on embedded layered materials.

2020-09-08

Controllable Thermal Conductivity in Twisted Homogeneous Interfaces of Graphene and Hexagonal Boron Nitride

Thermal conductivity of homogeneous twisted stacks of graphite is found to strongly depend on the misfit angle. The underlying mechanism relies on the angle dependence of phonon–phonon couplings across the twisted interface. Excellent agreement between the calculated thermal conductivity of narrow graphitic stacks and corresponding experimental results indicates the validity of the predictions. This is attributed to the accuracy of interlayer interaction descriptions obtained by the dedicated registry-dependent interlayer potential used. Similar results for h-BN stacks indicate overall higher conductivity and reduced misfit angle variation. This opens the way for the design of tunable heterogeneous junctions with controllable heat-transport properties ranging from substrate-isolation to efficient heat evacuation.

2019-12-09

Mechanical and Tribological Properties of Layered Materials under High Pressure: Assessing the Importance of Many-Body Dispersion Effects

The importance of many-body dispersion effects in layered materials subjected to high external loads is evaluated. State-of-the-art many-body dispersion density functional theory calculations performed for graphite, hexagonal boron nitride, and their heterostructures were used to fit the parameters of a classical registry-dependent interlayer potential. Using the latter, we performed extensive equilibrium molecular dynamics simulations and studied the mechanical response of homogeneous and heterogeneous bulk models under hydrostatic pressures up to 30 GPa. Comparison with experimental data demonstrates that the reliability of the many-body dispersion model extends deep into the subequilibrium regime. Friction simulations demonstrate the importance of many-body dispersion effects for the accurate description of the tribological properties of layered material interfaces under high pressure.

2016-02-01

Frictional Properties of Nanojunctions Including Atomically Thin Sheets

Using nonequilibrium molecular dynamics simulations and a coarse-grained description of a system, we have investigated frictional properties of nanojunctions including atomically thin sheets embedded between metal surfaces. We found that the frictional properties of the junctions are determined by the interplay between the lattice mismatch of the contacting surfaces and out-of-plane displacements of the sheet. The simulations provide insight into how and why the frictional characteristics of the nanojunctions are affected by the commensurate–incommensurate transition. We demonstrated that in order to achieve a superlow friction, the graphene sheet should be grown on or transferred to the surface with morphology, which is close to that of the graphene (for instance, Cu), while the second confining surface should be incommensurate with the graphene (e.g., Au). Our results suggest an avenue for controlling nanoscale friction in layered materials and provide insights in the design of heterojunctions for nanomechanical applications.

2021-05-27

Parity-Dependent Moiré Superlattices in Graphene/h−BN Heterostructures: A Route to Mechanomutable Metamaterials

The superlattice of alternating graphene/hBN few-layered heterostructures is found to exhibit strong dependence on the parity of the number of layers within the stack. Odd-parity systems show a unique flamingolike pattern, whereas their even-parity counterparts exhibit regular hexagonal or rectangular superlattices. When the alternating stack consists of 7 layers or more, the flamingo pattern becomes favorable, regardless of parity. Notably, the out-of-plane corrugation of the system strongly depends on the shape of the superstructure resulting in significant parity dependence of its mechanical properties. The predicted phenomenon originates in an intricate competition between moiré patterns developing at the interface of consecutive layers. This mechanism is of general nature and is expected to occur in other alternating stacks of closely matched rigid layered materials as demonstrated for homogeneous alternating junctions of twisted graphene and hBN. Our findings thus allow for the rational design of mechanomutable metamaterials based on van der Waals heterostructures.

2022-10-17

Origin of frictional scaling law in circular twist layered interfaces: Simulations and theory

Structural superlubricity based on twisted layered materials has stimulated great research interests. Recent MD simulations show that the circular twisted bilayer graphene (tBLG) presenting a size scaling of friction with strong Moiré-level oscillations. To reveal the physical origin of observed abnormal scaling, we proposed a theoretical formula and derived the analytic expression of frictional size scaling law of tBLG. The predicted twist angle dependent scaling law agrees well with MD simulations and provides a rationalizing explanation for the scattered power scaling law measured in various experiments. Finally, we show clear evidence that the origin of the scaling law comes from the Moiré boundary, that is, the remaining part of the twisted layered interfaces after subtracting the internal complete Moiré supercells. Our work provides new physical insights into the friction origin of layered materials and highlights the importance of Moiré boundary in the thermodynamic models of layered materials.

2024-04-01

Twist-Dependent Anisotropic Thermal Conductivity in Homogeneous MoS2 Stacks

Thermal transport property of homogeneous twisted molybdenum disulfide (MoS2) is investigated using non-equilibrium molecular dynamics simulations with the state-of-art force fields. The simulation results demonstrate that the cross-plane thermal conductivity strongly depends on the interfacial twist angle, while it has only a minor effect on the in-plane thermal conductivity, exhibiting a highly anisotropic nature. A frequency-decomposed phonon analysis showed that the cross-plane and in-plane thermal conductivity of MoS2 are dominated by the phonons with frequencies below 12.5 THz and 7.5 THz, respectively. As the interfacial twist angle increases, these low-frequency phonons significantly attenuate the phonon transport across the interface, leading to impeded cross-plane thermal transport. However, the in-plane phonon transport is almost unaffected, which allows for maintaining high in-plane thermal conductivity. Furthermore, our study revealed a strong size dependence for both cross-plane and in-plane thermal conductivities due to the influence of low-frequency phonons in MoS2. The maximum thermal conductivity anisotropy ratio is estimated as ∼400 for twisted MoS2 from our simulation, which is in the same order of magnitude as recent experimental results (∼900). Our study highlights the potential of twist engineering as a tool for tailoring the thermal transport properties of layered materials.

2024-04-01

Shape-dependent friction scaling laws in twisted layered material interfaces

Static friction induced by moiré superstructures in twisted incommensurate finite layered material interfaces reveals unique double periodicity and lack of scaling with contact size. The underlying mechanism involves compensation of incomplete moiré tiles at the rim of rigid polygonal graphene flakes sliding atop fixed graphene or h-BN substrates. The scaling of friction (or lack thereof) with contact size is found to strongly depend on the shape of the slider and the relative orientation between its edges and the emerging superstructure, partially rationalizing scattered experimental data. A phenomenological analytical model is developed, which agrees well with detailed atomistic calculations. By carefully considering the edge orientation, twist angle, and sliding direction of the flake relative to the substrate, one should therefore be able to achieve large-scale superlubricity via shape tailoring.

2022-09-01

Microscopic mechanisms of frictional aging

Frictional aging is observed at a wide range of length- and time-scales, and plays a crucial role in functioning of micro- and nanomachines, as well as in the nucleation and recurrence of earthquakes. Here, we developed an analytical model for description of frictional aging mediated by dynamical formation and rupture of microscopic interfacial contacts. The model accounts for the presence of various types of contacts at the frictional interface and exhibits three different aging regimes: (i) linear aging at short hold times, (ii) logarithmic (or logarithmic-like) aging for intermediate time scales and (iii) levelling off in the static friction for long hold times. It is demonstrated that the linear aging regime is a universal feature of frictional aging for the interfaces including various types of contacts, and the slope of variation of the static friction with the hold time depends on a distribution of energy barriers for contact formation. The conditions for the existence of a pronounced logarithmic aging regime, covering a long-time interval, have been established. Frictional aging has been found to manifest itself not only in slide-hold-slide measurements, but also in sliding experiments exhibiting stick-slip mode of motion, and a relationship has been established between these two regimes of aging. The predicted dependencies of frictional aging on the normal load and temperature are in good agreement with the experimental observations. Our work shows that experimental studies of load and temperature dependencies of aging, carried out over a wide range of time scales, offer promising opportunities for understanding the microscopic mechanisms of frictional aging and revealing the physical meaning of state variables that determine temporal evolution of friction described by phenomenological rate and state laws.

2023-08-01

Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity

The stacking state of atomic layers critically determines the physical properties of twisted van der Waals materials. Unfortunately, precise characterization of the stacked interfaces remains a great challenge as they are buried internally. With conductive atomic force microscopy, we show that the moiré superlattice structure formed at the embedded interfaces of small-angle twisted multilayer graphene (tMLG) can noticeably regulate surface conductivity even when the twisted interfaces are 10 atomic layers beneath the surface. Assisted by molecular dynamics (MD) simulations, a theoretical model is proposed to correlate surface conductivity with the sequential stacking state of the graphene layers of tMLG. The theoretical model is then employed to extract the complexstructure of a tMLG sample with crystalline defects. Probing and visualizing the internal stacking structures of twisted layered materials is essential for understanding their unique physical properties, and our work offers a powerful tool for this via simple surface conductivity mapping.

2023-05-21

The Origin of Moiré-Level Stick-Slip Behavior on Graphene/h-BN Heterostructures

Frictional behavior of a nanoscale tip sliding on superlattice of aligned graphene/(hexagonal boron nitride) h-BN heterostructure is found to be strongly regulated by the moiré superlattices, resulting in long-range stick-slip modulation in experimental measurements. Through molecular dynamics simulations, it is shown that the origin of moiré-level stick-slip comes from the strong coupling between in-plane deformation and out-of-plane distortion of the moiré superlattice. The periodicity of long-range modulation decreases as the interfacial twist angle increases, once the periodicity of moiré becomes smaller than the contact region between the tip and graphene, the long-range modulation becomes smooth and the stick-slip behavior disappears. It is found that the contact trajectory of the tip during sliding is the key to reveal the underlying mechanism, based on which a modified Prandtl-Tomlinson model is proposed considering deformation coupled effect to reproduce the frictional properties observed in molecular dynamics simulation. These findings emphasize the critical role of the moiré superlattice on frictional properties of van der Waals (vdW) heterostructures and open an avenue for the rational design of vdW devices with controllable tribological properties.

2022-11-04

Nanoserpents Graphene Nanoribbon Motion on Two-Dimensional Hexagonal Materials

We demonstrate snake-like motion of graphene nanoribbons atop graphene and hexagonal boron nitride (h-BN) substrates using fully atomistic nonequilibrium molecular dynamics simulations. The sliding dynamics of the edge-pulled nanoribbons is found to be determined by the interplay between in-plane ribbon elasticity and interfacial lattice mismatch. This results in an unusual dependence of the friction-force on the ribbon’s length, exhibiting an initial linear rise that levels-off above a junction-dependent threshold value dictated by the pre-slip stress distribution within the slider. As part of this letter, we present the LAMMPS implementation of the registry-dependent interlayer potentials for graphene, h-BN, and their heterojunctions that were used herein, which provides enhanced performance and accuracy.

2022-11-02

Load-velocity-temperature relationship in frictional response of microscopic contacts

Frictional properties of interfaces with dynamic chemical bonds have been the subject of intensive experimental investigation and modeling, as it provides important insights into the molecular origin of the empirical rate and state laws, which have been highly successful in describing friction from nano to geophysical scales. Using previously developed theoretical approaches requires time-consuming simulations that are impractical for many realistic tribological systems. To solve this problem and set a framework for understanding microscopic mechanisms of friction at interfaces including multiple microscopic contacts, we developed an analytical approach for description of friction mediated by dynamical formation and rupture of microscopic interfacial contacts, which allows to calculate frictional properties on the time and length scales that are relevant to tribological experimental conditions. The model accounts for the presence of various types of contacts at the frictional interface and predicts novel dependencies of friction on sliding velocity, temperature and normal load, which are amenable to experimental observations. Our model predicts the velocity-temperature scaling, which relies on the interplay between the effects of shear and temperature on the rupture of interfacial contacts. The proposed scaling can be used to extrapolate the simulation results to a range of very low sliding velocities used in nanoscale friction experiments, which is still unreachable by simulations. For interfaces including two types of interfacial contacts with distinct properties, our model predicts novel double-peaked dependencies of friction on temperature and velocity. Considering friction force microscopy experiments (FFM), we found that the non-uniform distribution of normal load across the interface leads to a distribution of barrier heights for contact formation. The results obtained in this case allowed to reveal a mechanism of nonlinear dependence of friction on normal load observed in recent FFM experiments and predict the effect of normal load on velocity and temperature dependencies of friction. Our work provides a promising avenue for the interpretation of the experimental data on friction at interfaces including microscopic contacts and opens new pathways for the rational control of the frictional response.

2022-10-21

Load and Velocity Dependence of Friction Mediated by Dynamics of Ilnterfacial Contacts

Studying the frictional properties of interfaces with dynamic chemical bonds advances understanding of the mechanism underlying rate and state laws, and offers new pathways for the rational control of frictional response. In this work, we revisit the load dependence of interfacial chemical-bond-induced (ICBI) friction experimentally and find that the velocity dependence of friction can be reversed by changing the normal load. We propose a theoretical model, whose analytical solution allows us to interpret the experimental data on timescales and length scales that are relevant to experimental conditions. Our work provides a promising avenue for exploring the dynamics of ICBI friction.