Abstract
We present the results of friction experiments performed by manipulation of oxidized platinum nanoislands on highly oriented pyrolytic graphite (HOPG) substrates through atomic force microscopy (AFM). The oxidation of the platinum nanoislands, performed via mild plasma exposure, is confirmed through X-ray photoelectron spectroscopy (XPS) and high-resolution energy-dispersive X-ray spectroscopy (EDX), the latter of which reveals partial oxidation on the sliding surfaces of the nanoislands. Oxidized platinum nanoislands are found to exhibit higher friction than non-oxidized islands, with a ~ 70% increase in mean shear stress over the investigated contact size regime. An increase in chemical interaction forces between the oxidized platinum and the graphite substrate is proposed to explain the increase in friction forces. Our results reveal that alteration of interfacial chemistry through oxidation leads to a noticeable modulation of friction forces, but not a total breakdown of the superlubric state (as evidenced by the signature observation of decreasing shear stress with increasing contact size), providing further feasibility for the design of superlubric mechanical systems to be operated under ambient conditions.Graphic
Highlights
A important grand challenge for humanity in the twenty-first century will be to drastically reduce our consumption of fossil fuel sources for energy
An important but so far unanswered question is the following: Can we tune friction forces in the structural superlubricity regime by modulating interfacial chemistry? importantly, as we know that surfaces of most metals chemically degrade under environmental conditions with time, do we expect a common phenomenon such as oxidation to affect, or perhaps lead to a complete breakdown of superlubric sliding? To take preliminary steps toward the answering of these important questions, we present here Atomic force microscopy (AFM)-based friction experiments performed on platinum nanoislands on highly oriented pyrolytic graphite (HOPG), whereby the surfaces of the islands have been oxidized through mild plasma exposure
We presented AFM-based friction experiments performed by manipulation of oxidized platinum nanoislands on HOPG, in order to answer the question of whether changes in interfacial chemistry would lead to a modulation of friction forces in the structural superlubricity regime, or alternatively, to its breakdown
Summary
A important grand challenge for humanity in the twenty-first century will be to drastically reduce our consumption of fossil fuel sources for energy. Atomic force microscopy (AFM) experiments in which nanoscale objects (e.g., nanotubes, nanoparticles, and pancake-like nanoislands) are manipulated (i.e., laterally moved) over certain substrates play an important role in fundamental friction research [4, 5] Such experiments, in contrast to conventional AFM-based friction measurements where contact geometries/chemistries at the tipsample junction remain largely uncharacterized [6], allow researchers to carefully study friction on the nanometer scale as a function of multiple parameters including but not limited to contact size, shape, and chemistry. This result underlined the important influence of interfacial chemistry on friction forces in the structural superlubricity regime
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