Thermo Nicolet Nexus 670 Spectrometer Together, the observed COFs and MD simulation analysis provides insight into the physical and chemical origins of friction. Superior lubrication is achieved when the strain, terrain caused by terrain, and chemical bonding at the shear level are neglected. Significant friction during escalation over the edge of the 0.34 nm elevated graphene step can be attributed to the synergy of physical effects due to terrain and chemical effects due to bonding between faces. During the step-down movement, a negative topographic change results in a force that helps the sliding motion, while the chemical bonds between the moving surfaces inverted produce a resistance force. The balance of these two components will determine whether friction and COF are positive or negative.
The effect of topography can be explained in terms of the two forces acting on the AFM probe: the normal force applied from cantilevering and the strength from the edge of the graphene step. The horizontal component of the force from the edge of the step contributes to the measured lateral force in the experiment. It is resistant to upward movement and aiding downward movement, and its size increases with the increase of the normal applied load. The chemical effect arises from interactions with OH groups at the edge of the graphene step. This force always resists movement, regardless of the direction of the scan, and its size is related to the contact area between the tip and the surface (more precisely, the length at which the edge of the step exceeds the contact area). According to the contact model Derjaguin-Muller-Toporov (DMT) (25), the contact area is proportional to the cubic root of the normal force. Therefore, as the applied normal load increases, the auxiliary force increases due to the change of topographic height much faster than the resistance force due to chemical reactions during the stepping movement, which results in a negative COF (20). When the auxiliary force caused by the topography is greater than the resistance force caused by chemistry, negative friction can occur.
The newly created HOPG surface was analyzed by reflection absorption, polarization, and infrared absorption spectroscopy (PM-RAIRS). PM-RAIRS analysis was performed using a Thermo Nicolet Nexus 670 spectrometer equipped with a specially designed reflection absorption unit consisting of a ZnSe polarization modification crystal, an environmental control cell, and a mercury cadmium telluride (MCT) -A (32) detector. The PM operation was performed using a photoelectric modifier (HINDS Instruments PEM-90) and a demodulator (GWC tools). The angle of occurrence of the infrared beam was 81 ° from the normal surface. The PM-RAIRS spectra of the HOPG surface were normalized with the clean gold spectrum obtained in dry Ar to remove the Bessel function shape background from the phase removal process.