42 research outputs found
Structural lubricity under ambient conditions.
Despite its fundamental importance, physical mechanisms that govern friction are poorly understood. While a state of ultra-low friction, termed structural lubricity, is expected for any clean, atomically flat interface consisting of two different materials with incommensurate structures, some associated predictions could only be quantitatively confirmed under ultra-high vacuum (UHV) conditions so far. Here, we report structurally lubric sliding under ambient conditions at mesoscopic (∼4,000-130,000 nm(2)) interfaces formed by gold islands on graphite. Ab initio calculations reveal that the gold-graphite interface is expected to remain largely free from contaminant molecules, leading to structurally lubric sliding. The experiments reported here demonstrate the potential for practical lubrication schemes for micro- and nano-electromechanical systems, which would mainly rely on an atomic-scale structural mismatch between the slider and substrate components, via the utilization of material systems featuring clean, atomically flat interfaces under ambient conditions
A Computational Study of Cluster Dynamics in Structural Lubricity: Role of Cluster Rotation
We present a computational study of sliding between gold clusters and a
highly oriented pyrolytic graphite substrate, a material system that exhibits
ultra-low friction due to structural lubricity. By means of molecular dynamics,
it is found that clusters may undergo spontaneous rotations during manipulation
as a result of elastic instability, leading to attenuated friction due to
enhanced interfacial incommensurability. In the case of a free cluster, shear
stresses exhibit a non-monotonic dependency on the strength of the tip-cluster
interaction, whereby rigid clusters experience nearly constant shear stresses.
Finally, it is shown that the suppression of the translational degrees of
freedom of a cluster's outermost-layer can partially annihilate out-of-plane
phonon vibrations, which leads to a reduction of energy dissipation that is in
compliance with Stokesian damping. It is projected that the physical insight
attained by the study presented here will result in enhanced control and
interpretation of manipulation experiments at structurally lubric contacts
Direct Imaging, Three-dimensional Interaction Spectroscopy, and Friction Anisotropy of Atomic-scale Ripples on MoS
Theory predicts that two-dimensional (2D) materials may only exist in the
presence of out-ofplane 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
(MoS). 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.Comment: 22 pages including 4 figures in the main text, 2 figures in the
supplemental informatio
Inverse Layer Dependence of Friction on Chemically Doped MoS_{2}
We present the results of atomic-force-microscopy-based friction measurements
on Re-doped molybdenum disulfide (MoS2). In stark contrast to the seemingly
universal 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 dependency on the number of layers. Raman spectroscopy
measurements revealed signatures of Re intercalation, leading to a decoupling
between neighboring MoS2 layers and enhanced electron-phonon interactions, thus
resulting in increasing friction with increasing number of layers: a new
paradigm in the mechanics of 2D materials.Comment: 15 pages incl. Supplemental Material, 5 figure