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The 2D materials exhibit excellent tribological properties due to their weak inter-plane interactions, such as the ultra-low friction, which can be further tuned by number of layers, application of electric bias, stacking of different materials into a van der Waals heterostructure, and change of substrate. In this work, the tribological properties of 2D materials were investigated experimentally by means of atomic force microscopy techniques in ultra-high vacuum and theoretically with atomistic simulations. Friction measurements on epitaxial graphene on SiC(0001) show that the ultra-low friction is limited by a normal load threshold, above which friction increases by one order of magnitude. Simulations suggest that, at contact pressures above 10 GPa, the high-friction regime is a result of an intermittent sp3 rehybridization of graphene and the formation of covalent bonds. Friction on the MoS2/graphene heterostructure is dominated by adhesion due to the out-of-plane deformation of the MoS2 layers. Increasing the number of MoS2 layers decreases friction as the flexural compliance decreases. Higher friction was recorded on MoSe2/hBN compared to graphene/hBN heterostructure or pristine hBN. Work on exfoliated materials was facilitated by the application of navigational microstructures.
Epitaxial graphene on SiC(0001) exhibits superlow friction due to its weak out-of-plane interactions. Friction-force microscopy with silicon tips shows an abrupt increase of friction by one order of magnitude above a threshold normal force. Density-functional tight-binding simulations suggest that this wearless high-friction regime involves an intermittent sp3 rehybridization of graphene at contact pressure exceeding 10 GPa. The simultaneous formation of covalent bonds with the tip's silica surface and the underlying SiC interface layer establishes a third mechanism limiting the superlow friction on epitaxial graphene, in addition to dissipation in elastic instabilities and in wear processes.
By differential thermal analysis, a concentration field suitable for the growth of Zr, Mg co-doped strontium hexagallate crystals was observed that corresponds well with known experimental results. It was shown that the melting point of doped crystal is ca. 60 K higher than that of undoped crystals. This higher melting points indicate hexagallate phase stabilization by Zr, Mg co-doping and increase the growth window of (Mg,Zr):SrGa12O19, compared to undoped SrGa12O19 that grows from SrO–Ga2O3 melts.
Stacked hetero-structures of two-dimensional materials allow for a design of interactions with corresponding electronic and mechanical properties. We report structure, work function, and frictional properties of 1 to 4 layers of MoS2 grown by chemical vapor deposition on epitaxial graphene on SiC(0001). Experiments were performed by atomic force microscopy in ultra-high vacuum. Friction is dominated by adhesion which is mediated by a deformation of the layers to adapt the shape of the tip apex. Friction decreases with increasing number of MoS2 layers as the bending rigidity leads to less deformation. The dependence of friction on applied load and bias voltage can be attributed to variations in the atomic potential corrugation of the interface, which is enhanced by both load and applied bias. Minimal friction is obtained when work function differences are compensated.
