Investigation of structural lubricity on platinum nanoparticles under ambient conditions

Abstract

Structural lubricity describes a state of ultra-low friction involving relative motion between atomically flat and molecularly clean surfaces with incommensurate structures. While the occurrence of structural lubricity was quantitatively confirmed under ultrahigh vacuum (UHV) conditions first, recently reported experiments have demonstrated that structurally lubric sliding is achievable under ambient conditions as well, specifically at mesoscopic interfaces formed between thermally deposited gold nanoparticles and graphite. The question that is covered in this thesis is whether the observation of structural lubricity under ambient conditions is limited solely to gold nanoparticles. To answer this question, an investigation of the frictional behavior of platinum nanoparticles laterally manipulated on graphite has been conducted. In particular, platinum nanoparticles have been prepared by e-beam evaporation of a thin film of platinum on graphite, followed by post-deposition annealing. Morphological characterization of the particles was performed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), revealing a crystalline structure. X-Ray photoelectron spectroscopy (XPS) revealed no significant change in the electronic structure of platinum upon exposure to air, ruling out the oxidation of platinum nanoparticles under ambient conditions. This finding was supported by cross-sectional TEM measurements demonstrating the absence of an oxide layer on the particle surfaces. Lateral manipulation experiments have been performed on platinum nanoparticles of mesoscopic dimensions (4000 − 75,000 nm2) under ambient conditions via atomic force microscopy (AFM), whereby results indicated the occurrence of structural lubricity, although with a higher magnitude of friction forces when compared with gold nanoparticles. Thus, it is confirmed that the occurrence of structural lubricity between incommensurate, atomically flat surfaces should be independent of material choice, as stated in the theory. Lastly, an attempt has been made to alter the structure and chemistry of the sliding surface of platinum nanoparticles. For this purpose, platinum nanoparticles have been oxidised in a reactive oxygen plasma atmosphere. XPS results confirmed the existence of oxidised platinum, and structural characterization performed by SEM showed that there was no significant change in morphology. Lateral manipulation experiments performed on oxidised platinum nanoparticles showed that these nanoparticles experience approximately two times as much friction as platinum nanoparticles. The potential reasons behind this observation are discussed.