Browsing by Subject "Superlubricity"

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Open Access
Investigation of structural lubricity on platinum nanoparticles under ambient conditions
(2017-05) Özoğul, Alper
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.
Open Access
Structure and nanotribology of thermally deposited gold nanoparticles on graphite
(2015) Cihan, Ebru
Forming a complete understanding of the physical mechanisms that govern friction on the nanometer and atomic scales is an ongoing endeavor for scientists from various disciplines. While atomic force microscopy (AFM) has proven to be invaluable for the detailed study of nano-scale frictional properties associated with various surfaces, issues related to the precise characterization of the contact formed by the probe tip and the sample surface remain largely unsolved. In recent years, an alternative approach to nanotribology experiments has involved the lateral manipulation of well-characterized nanoparticles on sample surfaces via AFM and the measurement of associated frictional forces. In line with this idea, ambient-condition structural/nanotribological characterization and nano-manipulation experiments involving gold nanoparticles (AuNP) thermally deposited on highly oriented pyrolytic graphite (HOPG) are presented in this thesis. The effect of deposition amount on thin film morphology is discussed and post- deposition annealing procedure in terms of different annealing temperatures and times are tackled in order to characterize AuNP formation on HOPG. The morphology and distribution of AuNPs on HOPG are studied via scanning electron microscopy (SEM) while the confirmation of AuNP crystallinity via transmission electron microscopy (TEM) is also described. Topographical characterization of the resulting AuNP/HOPG material system performed via contact-mode AFM is demonstrated. Lateral force measurements are also presented, in terms of the dependence of friction force on normal load as well as the dependence of friction force increase at AuNP edges on normal load and particle height. Subsequent to comprehensive structural and frictional characterization, the results of nano-manipulation experiments performed on AuNPs on the HOPG substrate are reported and it is observed that AuNPs experience remarkably low frictional forces during sliding. A detailed study of friction with respect to contact area firmly confirms the occurrence of structurally lubric sliding under ambient conditions for this material system. This result constitutes the first observation of structurally lubric sliding under ambient conditions between different materials in the scientific literature.