Atomic-scale friction of rotating N2/CO2 adsorbed on black phosphorene surface: Ultra-low friction and “superlubricity” induced by the critical loads

Abstract

N2 and CO2 in the atmospheric environment may be adsorbed on the black phosphorene (BP) surface and maybe affect its friction property. Based on first-principles, the adsorption energy (Ea) of N2/BP and CO2/BP is smaller than that of Si2(v)/BP, suggesting that the friction property of BP surface can be enhanced. Under the zero load, the static lateral force (fstatic) of N2(p1)/BP and CO2(p2)/BP is reduced by 7.5 and 2.6 times than that of Si2(v)/BP. Under the non-zero load, a clear boundary of the ultra-low/high frictions is shown in N2(p1/p2)/BP and CO2(p1/p2)/BP. On the one hand, adsorbed gas on BP surface reduces greatly its surface friction under the low load. On the other hand, the decomposition of adsorbed gas (as pollutants) on BP surface leads to failure of the ultra-low friction under the high load. Excitingly, the intersection of adsorption energy (Ea) of N2(p1/p2)/BP and CO2(p1/p2)/BP predicts the theoretical “superlubricity” behavior. Normally, “superlubricity” behavior results mainly from the flattening PES under the critical load, which is controlled by electrostatic action. It is worth noting that there is competition between adsorption states (p1 and p2) or types (N2 and CO2) caused by the load. The friction property of BP surface is dominated by N2(p1)/BP under the mixed atmosphere (N2 and CO2), while it is dominated by CO2(p1/p2)/BP under a pure CO2 environment. In addition, the orientation of adsorbed gas on BP surface can be changed by the external potential fields (electricity, magnetic field, etc.) to obtain appropriate and acceptable energy dissipation and friction, which expands their application area and flexibility.