Browsing by Subject "Nanotribology"

Now showing 1 - 7 of 7

Open Access
Atomic force microscopy: Methods and applications
(Elsevier, 2017) Baykara, Mehmet Z.; Schwarz, U. D.; Lindon, J.; Tranter, G. E.; Koppenaal, D.
This chapter provides an overview of atomic force microscopy, covering the fundamental aspects of the associated instrumentation and methodology as well as representative results from the literature highlighting a variety of application areas. In particular, atomic-resolution imaging and spectroscopy capabilities are emphasized, in addition to applications in biology, nanotribology and catalysis research. Finally, an outlook on emerging aspects and future prospects of atomic force microscopy is provided.
Open Access
Frictional properties of quasi-two-dimensional materials from the Prandtl-Tomlinson model
(2016-09) Adeel, Shaharyar
Tribology, the study of friction, is both an old theoretical problem in physics and an area of great practical importance. The invention of experimental instruments such as Atomic Force Microscope (AFM) has lead to the emergence of the eld of nanotribology, the exploration of friction phenomenon at the nanoscale. While more complete descriptions of friction make use of density functional theory (DFT) and molecular dynamics (MD) simulations, many essential features of frictional phenomena are accurately modeled by so called "reduced order models" such as the Prandtl-Tomlinson (PT) Model. We illustrate the PT model in both one-dimensional and two-dimensional forms via application to various crystal lattice surfaces (cubic, planar hexagonal) and reproduce important results from the literature by solving the resulting Langevin equation within the PT model. We also discuss the parameter dependence in this model via relevant simulations. We then generalize the PT model to a three-dimensional case and analyse quasi-two-dimensional systems. These systems thus exhibit a small amount of "buckling" - i.e. with out-of-plane basis atoms. The equations of motion of the Prandtl-Tomlinson model are solved numerically and the resulting friction force curves, tip path and lattice are analysed comparatively. The results agree with underlying theory and make testable predictions. We conclude that our generalized, three-dimensional PT model is a good approximation to the frictional dynamics at this scale for these systems and has the advantage of being computationally less intensive than full scale MD or DFT calculations.
Open Access
Influence of interface structure on the nanotribological properties of exfoliated graphene
(2016-07) Balkancı, Arda
On the nano- and micro-scale, conventional liquid-based lubrication cannot be utilized to minimize friction due to excessive surface tension and related effects. To overcome this limitation, solid lubricants suitable for use in nano- and microscale systems are needed. Being a two-dimensional material with outstanding mechanical properties, graphene emerges as a promising candidate for this purpose. Motivated as above, this M.S. thesis presents a comprehensive investigation of the nanotribological properties of mechanically-exfoliated graphene conducted via atomic force microscopy (AFM), whereby special emphasis is placed on the effect of interface structure. Graphene samples ranging from single- to few-layers were fabricated using the mechanical exfoliation method and transferred onto Si/SiO2 substrates. By utilizing optical microscopy and Raman spectroscopy, graphene akes exhibiting single- and bi-layer regions were located and identified. Furthermore, using topographical maps and associated profiles obtained via AFM, 3-, 4-layer and bulk graphite regions were found. Moreover, AFM probes were calibrated both for accurate normal force readings, and for obtaining quantitative friction force data from lateral force measurements conducted via contact-mode AFM under ambient conditions. Following sample preparation, identification and probe calibration, experiments aimed at measuring the effect of applied load on friction of single- and 2-, 3-, 4-layers of graphene were performed, confirming previous results reported in the literature as explained by the puckering phenomenon. Additionally, the effect of tip radius and thus, contact area, on the frictional behavior of graphene was quantitatively measured. In particular, thermal evaporation- and PECS (precision etching coating system)-based coating of gold onto AFM probes were utilized to modify tip radii. Results led to the determination of a new parameter affecting friction on graphene: interface roughness. In collaboration with scientists from UC Merced who performed molecular dynamics simulations complementing the experiments presented here, the effect of substrate roughness, which may be in addition to, or dominant over, the puckering phenomenon, was analyzed in terms of the frictional behavior of graphene. Presented experimental results provide a new perspective towards the layer-dependent frictional behavior of graphene, underlining the in uence of substrate roughness in addition to the phenomenon of puckering that is well-studied in the literature.
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
Nanotribological properties of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates
(2015) Demirbaş, Tuna
To extend the lifespan of mechanical systems, wear and friction must be minimized with the utilization of lubricants. On the other hand, traditional fluid-based lubrication schemes fail in nano- and micro-scale systems due to increasing surface-to-volume ratios and associated physical effects. As such, research efforts in recent years have been aimed at characterizing the structure and mechanical properties of various candidates for solid lubricants. Due to its outstanding electronic and mechanical properties, the two-dimensional “wonder material” graphene has been the focus of a large variety of experiments in the past decade. Based on its promise as a single-layer solid lubricant suitable for use in nano- and micro-scale systems, the nanotribological properties of graphene have been investigated in several studies in the literature. While frictional characteristics of mechanically exfoliated graphene samples as a function of layer number have been related to the effect of puckering, the nanotribological behavior of graphene samples grown by chemical vapor deposition (CVD) is still under investigation. Considering that high quality graphene of sufficient dimensions for practical applications is currently grown by CVD and requires transfer from metal foils onto various substrates, the need for an extensive understanding of the nanotribological properties of such graphene samples arises. Based on the discussion above, this M.S. thesis presents a comprehensive structural and nanotribological characterization of CVD-grown graphene transferred onto oxidized silicon substrates (SiO2/Si). In particular, the processes of sample preparation and post-preparation transfer onto SiO2/Si substrates are optimized via a series of experiments. Advanced microscopy techniques are utilized for the structural and morphological characterization of the obtained graphene films. In particular, optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) are used to inspect graphene coverage on the substrate and associated structural features. On the other hand, Raman spectroscopy is employed to confirm the single-layer character of CVD-grown samples. The nanotribological properties of CVD-grown graphene samples on SiO2/Si are studied by AFM in the friction force microscopy (FFM) mode under ambient conditions by measuring the evolution of friction force with increasing normal load. The effect of using different probe tips, growth conditions, and post-transfer cleaning procedures on frictional behavior is evaluated. A comparison of lubrication performance with mechanically-exfoliated graphene is also performed. Results indicate that CVD-grown graphene acts as a very good solid lubricant on SiO2/Si, reducing coefficients of friction by ~90% for all investigated samples. It is shown that as-transferred CVD-grown graphene exhibits the highest mean lubrication performance and that the associated values drop slightly with post-transfer cleaning. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values are quantitatively evaluated, with results revealing a substantial increase in friction on these structural defects.
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.
Open Access
Structure and nanotribology of thermally deposited gold nanoparticles on graphite
(Elsevier, 2015) Cihan, Ebru; Özoğul, Alper; Baykara, Mehmet Z.
We present experiments involving the structural and frictional characterization of gold nanoparticles (AuNP) thermally deposited on highly oriented pyrolytic graphite (HOPG). The effect of thermal deposition amount, as well as post-deposition annealing on the morphology and distribution of gold on HOPG is studied via scanning electron microscopy (SEM) measurements, while transmission electron microscopy (TEM) is utilized to confirm the crystalline character of the nanoparticles. Lateral force measurements conducted via atomic force microscopy (AFM) under ambient conditions are employed to investigate the nanotribological properties of the gold nanoparticles as a function of normal load. Finally, the increase in lateral force experienced at the edges of the nanoparticles is studied as a function of normal load, as well as nanoparticle height. As a whole, our results constitute a comprehensive structural and frictional characterization of the AuNP/HOPG material system, forming the basis for nanotribology experiments involving the lateral manipulation of thermally deposited AuNPs on HOPG via AFM under ambient conditions.