Browsing by Subject "Friction force microscopy"

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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
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.