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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Article
  • Peer Reviewed
UC Merced Previously Published Works (2020)
We perform atomic force microscopy (AFM) experiments on mechanically exfoliated, single-layer and bulk molybdenum disulfide (MoS2) in order to probe friction forces as a function of sliding speed. The results of the experiments demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe, manifesting in the form of varying contact stiffness and tip-sample interaction potential parameters in the thermally activated Prandtl-Tomlinson model. Our study contributes to the formation of a mechanistic understanding of the speed dependence of nanoscale friction on two-dimensional materials.
    Cover page: Speed dependence of friction on single-layer and bulk MoS2 measured by atomic force microscopy
    • Thesis
    • Peer Reviewed
    UC Merced Electronic Theses and Dissertations (2020)

    The potential use of two-dimensional (2D) materials as solid lubricants in micro-and nano-scale mechanical systems draws significant attention, mainly due to the fact that liquid based lubrication schemes fail at such small length scales. Within this context, the lamellar material molybdenum disulfide (MoS2), in the form of a single or few layers, emerges as a promising candidate for the solid lubrication of small-scale mechanical systems.

    Motivated as above, this thesis focuses on the nanotribological properties of mechanically exfoliated MoS2, explored via state-of-the-art atomic force microscopy (AFM) experiments. First, the dependence of friction force on sliding speed is investigated for single-layer and bulk MoS2 samples. The results demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe. The direction dependence (i.e. anisotropy) of friction on MoS2 is studied next. In particular, high-resolution AFM measurements conducted by our collaborators at McGill University lead to the direct imaging of atomic-scale ripples on few-layer MoS2 samples, allowing to explain the various symmetries for friction anisotropy that are observed in our experiments as a function of scan size. Finally, the nanotribological properties of Re-doped MoS2 are studied, revealing a surprising, inverse dependence of friction force on number of layers, in contradiction with the seemingly universal trend of decreasing friction with increasing number of layers on 2D materials. Attempts are made to uncover the physical mechanisms behind this striking observation by way of roughness and adhesion measurements.

    In summary, the results reported in this thesis contribute to the formation of a comprehensive, mechanistic understanding of the nanotribological properties of MoS2 in particular, and 2D materials, in general. While the speed dependence and anisotropy results are relatively self-contained, further work needs to be conducted in order to explain the inverse layer-dependence of friction observed on Re-doped MoS2.

      Cover page: Nanotribology of MoS2 Investigated via Atomic Force Microscopy
      • Article
      • Peer Reviewed
      UC Merced Previously Published Works (2020)
      Abstract: Theory predicts that two-dimensional (2D) materials may only exist in the presence of out-of-plane deformations on atomic length scales, frequently referred to as ripples. While such ripples can be detected via electron microscopy, their direct observation via surface-based techniques and characterization in terms of interaction forces and energies remain limited, preventing an unambiguous study of their effect on mechanical characteristics, including but not limited to friction anisotropy. Here, we employ high-resolution atomic force microscopy to demonstrate the presence of atomic-scale ripples on supported samples of few-layer molybdenum disulfide (MoS2). Three-dimensional force/energy spectroscopy is utilized to study the effect of ripples on the interaction landscape. Friction force microscopy reveals multiple symmetries for friction anisotropy, explained by studying rippled sample areas as a function of scan size. Our experiments contribute to the continuing development of a rigorous understanding of the nanoscale mechanics of 2D materials.
        Cover page: Direct imaging, three-dimensional interaction spectroscopy, and friction anisotropy of atomic-scale ripples on MoS2
        • Article
        • Peer Reviewed
        UC Merced Previously Published Works (2020)
        We present a comparative study of the tribological properties of Pd-, Pt-, and Zr-based bulk metallic glasses (BMG-Pd, BMG-Pt, and BMG-Zr, respectively) under unlubricated conditions. In particular, micro-tribometry is utilized with a 52,100 steel ball, showing that BMG-Pt exhibits a significantly higher coefficient of friction (COF) (0.58 ± 0.08) when compared with BMG-Pd (0.30 ± 0.02) and BMG-Zr (0.20 ± 0.03). Topographical roughness on and off wear scars is characterized via atomic force microscopy (AFM), with results that do not correlate with the observed frictional behavior. On the other hand, scanning electron microscopy (SEM) is utilized to reveal contrasting wear mechanisms for the three samples: while BMG-Pd and BMG-Zr exhibit predominantly abrasive wear, there is evidence of adhesive wear on BMG-Pt. Consequently, the occurrence of adhesive wear emerges as a potential mechanism behind the observation of relatively high coefficients of friction on BMG-Pt, suggesting stronger interactions with steel when compared with the other BMG samples.
          • Article
          • Peer Reviewed
          UC Merced Previously Published Works (2020)
          We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e., inverse dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.
            Cover page: Intercalation Leads to Inverse Layer Dependence of Friction on Chemically Doped MoS_{2}
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2022)
            We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined withab initiocalculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.
              Cover page: Intercalation leads to inverse layer dependence of friction on chemically doped MoS<sub>2</sub>.
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