Superlubricity ofBorophene: Tribological Propertiesin Comparison to hBN

Superlow friction and wear enabled by nanodiamond and hexagonal boron nitride on a-C:H films surfaces in dry nitrogen

An experimental study has been made of the friction properties of graphite, molybdenum disulphide, boron nitride and talc. The formation of surface layers of the lamellar solids on platinum, and the friction of these layers at elevated temperatures in air have been examined. There is evidence that the frictional behaviour of the solids is dominated by the forces acting between separate crystallites, cleavage of individual crystallites being of secondary importance. The structure of the lamellar solids gives rise to plate-like crystallites, and a large proportion of the surface area is composed of faces with relatively low surface energy, with a small proportion of high-energy edge surface. The edges can react with gases to give a surface of relatively low surface energy, and the adhesion between the crystallites is then small, giving the solids low friction properties. In general, the removal of adsorbed gases increases the adhesion between the crystallites (particularly at the edges) so that the friction increases. This is observed for the removal of physically adsorbed volatiles from the partly ionic boron nitride and talc, and for the removal of the chemically bound carbon oxides from graphite. Molybdenum disulphide behaves differently, for with this solid the presence of adsorbed water promotes hydrogen bonding between the crystallites and thereby increases the adhesion between them . Other workers have shown that the friction of outgassed graphite in vacuo decreases reversibly at high temperatures. A similar behaviour has been shown to occur for boron nitride in air at temperatures below that at which it oxidizes rapidly, and suggests its use as a high temperature lubricant. The decrease in friction is caused by a gradual weakening of the intercrystallite bonding as the temperature is increased. Small quantities of bulk impurities can have a large influence on the intercrystallite bonding. It is believed that the impurity responsible in the case of boron nitride is boric oxide, because it melts at a temperature close to that at which the reversible decrease in friction occurs. Thermogravimetric analysis has been used to show that when the materials undergo a chemical change such as rapid oxidation they no longer give a low friction even when present in excess on the surface. Electron diffraction studies show that on rubbing in air the lamellar solids tend to form oriented layers on them etal surface, so that the plate-like crystallites lie flat. The orientation does not cause the low friction, but the low adhesion between the crystallites allows them to become oriented in their most favourable position, and independently causes the friction to be low. Reflexion electron micrographs show that the lubricant is occluded within crevices in the surface. The micrographs of the tracks formed during lubricated sliding show that the metal is deformed plastically, but that failure occurs mainly or entirely within the lubricant. An important factor in the formation of satisfactory lubricant layers appears to be the hardness of the lubricant relative to that of the metal, as the lamellar solid may protect the metal by becoming embedded in the surface.

We demonstrate controlled chemical vapor deposition growth of lateral and vertical heterostructures of graphene and hexagonal boron nitride (h-BN) on Cu substrates by changing supply sequence of their precursors. When the graphene precursors are switched by the h-BN precursors, h-BN grows from the edges of the existing graphene islands, resulting in the lateral heterostrcutures. In the reversed supply sequence, on the other hand, graphene grows at the interface between the h-BN and Cu substrate, leading to the vertical heterostructures. This simple method of controlling vertical and lateral heterostructures allows to produce more complicated heterostructures by designing the precursor supply sequence.

The objective of this study was to find a trigger to make clear a mechanism of the ultra low friction by evaluating the friction property of DLC-DLC combination under lubrication with the simple fluid. The Pin-on-disc reciprocating and rotating sliding tests were conducted to evaluate the friction property. The super low friction property of pure sliding with the ta-C(T) pair coated by the filtered arc deposition process under oleic acid lubrication was found at the mixed lubrication condition. It was thought that the low share strength tribofilm composed of water and acid seemed to be formed on ta-C sliding interface. Additionally, the smooth sliding surface formed on ta-C(T) was seemed to be required to keep this tribofilm. Then, the super low friction was thought to be obtained by this superlubrication condition. Although the accurate and direct experimental data must be required to make clear this super low friction mechanism, the advanced effect obtained by the simple material combination is expected to be applied on the large industrial fields in near future.

Prandtl–Tomlinson (PT) model can be used to explain nanoscale friction in a variety of situations, except when a nanoscale object undergoes rolling. To alleviate this problem, we generalize the PT model as a collection of interacting point particles arranged on a ring of radius R. The center of mass of the ring is connected to a spring of stiffness k, whose other end is attached to a fictitious mass that moves with a constant velocity v. The entire assembly is driven in a composite force field, which is a product of (i) the familiar sinusoidal function used in the PT model and (ii) a parametrically controlled (λ) exponentially varying function that is dependent on the vertical coordinates of the particles. Our generalized model degenerates to the standard PT model if R≪1 and λ→0. With increasing k, for R≪1 and λ≠0, the ring undergoes a transition from sticky to smooth dynamics for both x and y directions. The dynamics, investigated numerically for the general case of R∼1 and λ≠0, reveals several interesting aspects of nanoscale tribology including the regimes where energy dissipation due to friction is minimum. Furthermore, the results from our proposed model are in agreement with those from molecular dynamics simulations as well. We believe that the simplicity of our model along with its similarity to the PT model may make it a popular tool for analyzing complicated nanotribological regimes.

Lateral two-dimensional (2D) heterostructures have opened up unprecedented opportunities in modern electronic device and material science. In this work, electronic properties of size-dependent MoTe2/WTe2 lateral heterostructures (LHSs) are investigated through the first-principles density functional calculations. The constructed periodic multi-interfaces patterns can also be defined as superlattice structures. Consequently, the direct band gap character remains in all considered LHSs without any external modulation, while the gap size changes within little difference range with the building blocks increasing due to the perfect lattice matching. The location of the conduction band minimum (CBM) and the valence band maximum (VBM) will change from P-point to Γ-point when m plus n is a multiple of 3 for A-mn LHSs as a result of Brillouin zone folding. The bandgap located at high symmetry Γ-point is favourable to electron transition, which might be useful to optoelectronic device and could be achieved by band engineering. Type-II band alignment occurs in the MoTe2/WTe2 LHSs, for electrons and holes are separated on the opposite domains, which would reduce the recombination rate of the charge carriers and facilitate the quantum efficiency. Moreover, external biaxial strain leads to efficient bandgap engineering. MoTe2/WTe2 LHSs could serve as potential candidate materials for next-generation electronic devices.

Graphene has received many attentions from the scientist due to its outstanding properties since it was mechanically exfoliated. The rise of graphene has stimulated immense research on analogous two-dimensional (2D) materials, such as the hexagonal boron nitride (h-BN), monolayer transition metal dichalcogenides, black phosphorene, silicene, borophene and germanene. However, as a semimetal, gapless graphene has been limited in the application like semiconducting electronics. Previous theoretical calculations predict that a small bandgap will arise when the graphene is placed onto an h-BN substrate. Then graphene/h-BN (G/h-BN) lateral heterostructures have been fabricated by many research groups on metal substrates. Comparing vertical vdW layer assembling, the atomic G/h-BN system combining both the high carrier mobility and tunable bandgap can build in-plane integrated circuit directly. However, to reduce the interface scattering, a sharp atomic interface between graphene and h-BN is required. To date, some experiments have built high quality G/h-BN heterostructures with sharp atomic interfaces, while many others did not. The underlying growth mechanism is crucial to control the size and shapes of the G/h-BN heterostructures of high quality. In this review, we first review the growth mechanism of graphene, mainly focusing on the nucleating process, which plays a vital role to reduce the grain boundary in the overall growth. The nucleation behavior of graphene is influenced by the growth temperature, substrate, carbon source partial pressure and hydrogen partial pressure. We review the structure and energy evaluation of clusters in the graphene nucleation stage on the Ir(111) and Ni(111) surface. Moreover, the nucleation barrier and size can also be obtained by combing the first-principles calculations and classical nucleation theory. Secondly, we present an overview on experimental fabrication of G/h-BN heterojunctions. Up to now, lateral G/h-BN heterojunctions has been produced on some metal substrates such as Cu foil, Ru(0001), Rh(111), Ni(111), Ir(111) and Pt(111). There are several methods to construct G/h-BN heterojunctions in experiments which can be classified into two kinds: Batch chemical vapor deposition and two/multi-step growth. In the earlier period, the large area randomly hybridized h-BN and graphene are fabricated by this method through supplying C precursors and NH3-BH3 at the same time. Another routine is two/multi-step growth technique in which G/h-BN is deposited on a metal surface beforehand as seeds of the second step of growth. And afterwards, by alternating the type of source supply, in plane heterojunctions with the predeposited grains will be formed. The growth sequence is proved to be a crucial factor that determines the quality of heterostructures. For multistep growth, lithographically technology and etching process are introduced to get spatially controlled heterostructures. Then, we introduce recent studies on the growth mechanism of G/h-BN lateral heterostructure both by atomistic first-principles calculations and experimental observations. The heteroepitaxial growth of graphene along the predeposited h-BN domains is systematically investigated by classical nucleation theory. At the initial stage, C atoms tend to attach linearly along the B/N-edges rather than perpendicular to the edges which resemble the graphene growth on the metal surface. When the carbon concentration is low, the nucleation barriers of graphene at B/N-edges are lower compared to the case of a Cu terrace. Nevertheless, under high carbon concentration, the nucleation barriers and nucleation rates in the two cases become comparable. Therefore, the chemical potential of the carbon source should be controlled within a suitable range to produce high quality G/h-BN lateral heterostructures with sharply and continuously atomic interface. Finally, we give a prospect on this rising field.

Fluorine-containing polymers have attracted a lot of interest due to their hydrophobic nature, which enables a number of functionalities such as low friction, self-cleaning, anti-sticking, etc. Fluorinated ethylene propylene (FEP) is a copolymer of hexafluoropropylene and tetrafluoroethylene. In contrast to polytetrafluoroethylene (PTFE), FEP is melt-processable using injection molding and it is highly transparent and resistant to sunlight. Here, we transform fluorinated ethylene propylene ( FEP) surfaces to superhydrophobic using plasma processing (i.e. plasma etching with simultaneous micro-nanotexturing followed by plasma deposition). Two different plasma reactors (an inductively coupled reactor and a reactive ion etcher) and different etching conditions are used in order to micro-nanotexture FEP surfaces. Superhydrophobicity is achieved after plasma deposition of a thin (30 nm) fluorocarbon coating. Subsequently, we probe their frictional properties against water drops using a tilting stage. In particular, we demonstrate wetting control of FEP surfaces with water static water contact angle ranging from 95 o to 168 o and hysteresis down to 1–2 o . Video analysis of drops moving on different FEP surfaces shows that water drops exhibit reduced friction as etching time increases so that a hierarchical morphology is created. This is achieved using both plasma reactors, whereas the optimum performance is observed for the 10 min etched surface in a inductively coupled plasma reactor in which friction force becomes 0.7–1 μN, which corresponds to an ultra-low dynamic coefficient of friction (at least one order of magnitude lower to that of untreated FEP). • Plasma Processing for wetting control of FEP surfaces. • Superhydrophobic FEP surfaces are demonstrated. • Water drops move with reduced friction on SH FEP surfaces. • Coefficient of friction is reduced to 0.007 for SH FEP surfaces.

The ultralow friction between atomic layers of hexagonal MoS2 , an important solid lubricant and additive of lubricating oil, is thought to be responsible for its excellent lubricating performances. However, the quantitative frictional properties between MoS2 atomic layers have not been directly tested in experiments due to the lack of conventional tools to characterize the frictional properties between 2D atomic layers. Herein, a versatile method for studying the frictional properties between atomic-layered materials is developed by combining the in situ scanning electron microscope technique with a Si nanowire force sensor, and the friction tests on the sliding between atomic-layered materials down to monolayers are reported. The friction tests on the sliding between incommensurate MoS2 monolayers give a friction coefficient of ≈10-4 in the regime of superlubricity. The results provide the first direct experimental evidence for superlubricity between MoS2 atomic layers and open a new route to investigate frictional properties of broad 2D materials.

Superlubricity of graphite and graphene has aroused increasing interest in recent years. Yet how to obtain a long-lasting superlubricity between graphene layers, under high applied normal load in ambient atmosphere still remains a challenge but is highly desirable. Here, we report a direct measurement of sliding friction between graphene and graphene, and graphene and hexagonal boron nitride (h-BN) under high contact pressures by employing graphene-coated microsphere (GMS) probe prepared by metal-catalyst-free chemical vapour deposition. The exceptionally low and robust friction coefficient of 0.003 is accomplished under local asperity contact pressure up to 1 GPa, at arbitrary relative surface rotation angles, which is insensitive to relative humidity up to 51% RH. This ultralow friction is attributed to the sustainable overall incommensurability due to the multi-asperity contact covered with randomly oriented graphene nanograins. This realization of microscale superlubricity can be extended to the sliding between a variety of two-dimensional (2D) layers.

Tribological study of cubic boron nitride film

Two-dimensional lateral heterostructures exhibit novel electronic and optical properties that are induced by their in-plane interface for which the mechanical properties of the interface are important for the stability of the lateral heterostructure. Therefore, we performed molecular dynamics simulations and developed a convolutional neural network-based machine learning model to study the fracture properties of the interface in a graphene/hexagonal boron nitride lateral heterostructure. The molecular dynamics (MD) simulations show that the shape of the interface can cause an 80% difference in the fracture stress and the fracture strain for the interface. By using 11 500 training samples obtained with help of high-cost MD simulation, the machine learning model is able to search out the strongest interfaces with the largest fracture strain and fracture stress in a large sample space with over 150 000 structures. By analyzing the atomic configuration of these strongest interfaces, we disclose two major factors dominating the interface strength, including the interface roughness and the strength of the chemical bond across the interface. We also explore the correlation between the fracture properties and the thermal conductivity for these lateral heterostructures by examining the bond type and the shape of the graphene/hexagonal boron nitride interface. We find that interfaces comprised of stronger bonds and smoother zigzag interfaces can relieve the abrupt change of the acoustic velocity, leading to the enhancement of the interface thermal conductivity. These findings will be valuable for the application of the two-dimensional lateral heterostructure in electronic devices.

Integration of graphene and hexagonal boron nitride (hBN) into lateral heterostructures has drawn focus due to the ability to broadly engineer the material properties. Hybrid monolayers with tuneable bandgaps have been reported, while the interface itself possesses unique electronic and magnetic qualities. Herein, we demonstrate lateral heteroepitaxial growth of graphene and hBN by sequential growth using high-temperature molecular beam epitaxy (MBE) on highly oriented pyrolytic graphite (HOPG). We find, using scanning probe microscopy, that graphene growth nucleates at hBN step edges and grows across the surface to form nanoribbons with a controlled width that is highly uniform across the surface. The graphene nanoribbons grow conformally from the armchair edges of hexagonal hBN islands forming multiply connected regions with the growth front alternating from armchair to zigzag in regions nucleated close to the vertices of hexagonal hBN islands. Images with lattice resolution confirm a lateral epitaxial alignment between the hBN and graphene nanoribbons, while the presence of a moiré pattern within the ribbons indicates that some strain relief occurs at the lateral heterojunction. These results demonstrate that high temperature MBE is a viable route towards integrating graphene and hBN in lateral heterostructures.

First-principles calculations combined with friction models to predict the moiré pattern effect on the interlayer friction of two-dimensional materials

Graphene and hexagonal boron nitride (h-BN), as typical two-dimensional (2D) materials, have long attracted substantial attention due to their unique properties and promise in a wide range of applications. Although they have a rather large difference in their intrinsic bandgaps, they share a very similar atomic lattice; thus, there is great potential in constructing heterostructures by lateral stitching. Herein, we present the in situ growth of graphene and h-BN lateral heterostructures with tunable morphologies that range from a regular hexagon to highly symmetrical star-like structure on the surface of liquid Cu. The chemical vapor deposition (CVD) method is used, where the growth of the h-BN is demonstrated to be highly templated by the graphene. Furthermore, large-area production of lateral G-h-BN heterostructures at the centimeter scale with uniform orientation is realized by precisely tuning the CVD conditions. We found that the growth of h-BN is determined by the initial graphene and symmetrical features are produced that demonstrate heteroepitaxy. Simulations based on the phase field and density functional theories are carried out to elucidate the growth processes of G-h-BN flakes with various morphologies, and they have a striking consistency with experimental observations. The growth of a lateral G-h-BN heterostructure and an understanding of the growth mechanism can accelerate the construction of various heterostructures based on 2D materials.