Gigantic Maximum of Nanoscale Noncontact Friction

Abstract

We report measurements of noncontact friction between surfaces of NbSe2 and SrTiO3, and a sharp Pt-Ir tip that is oscillated laterally by a quartz tuning fork cantilever. At 4.2 K, the friction coefficients on both the metallic and insulating materials show a giant maximum at the tip-surface distance of several nanometers. The maximum is strongly correlated with an increase in the spring constant of the cantilever. These features can be understood phenomenologically by a distancedependent relaxation mechanism with distributed time scales.PACS numbers: 68.35. Af, 68.35.Ja, 68.37.Ps Friction has been studied for a long time as one of the fundamental subjects in physics. However, the microscopic mechanism of friction is still in dispute [1]. Nanotribology, namely study of friction at nanoscale, is the most important subject not only for understanding friction but also for the development of micro-and nanoelectromechanical devices, which need control of friction at the nanoscale. A significant amount of research effort has been devoted to revealing the mechanism of friction at the nanoscale [2].Interestingly, there is a novel type of friction, so-called noncontact friction, at the nanoscale. In contrast to the ordinary contact friction, noncontact friction occurs when two bodies are not in direct contact. It has been observed in scanning probe microscopy experiments, in which a sharp metal tip oscillates laterally near a flat surface [3][4][5]. Stipe et al. observed the noncontact friction between a Au(111) surface and a Au-coated probe tip attached to a very soft cantilever (spring constant k 0 ∼ 10 −4 N/m) [4]. At temperatures 4< T <300 K, the friction coefficient Γ was approximately 10 −12 kg/s at a tip-sample distance d <10 nm. As a possible mechanism of the noncontact friction, Ohmic losses caused by fluctuating electromagnetic fields were proposed [6,7]. However, the observed friction coefficient is 7 -8 orders of magnitude larger than the values derived by the theories. Some additional mechanisms that could explain the large noncontact friction have been proposed [8,9], but the discrepancy has not been solved.The above-mentioned theories predict that noncontact friction is proportional to the electrical resistivity of samples. It is therefore expected to be higher on insulating materials than on metals [8,9]. It was found experimentally that the noncontact friction coefficients of insulating silica and polymer films were an order of magnitude larger than the value on the Au(111) surface [4,5]. This tendency is qualitatively consistent with the theories, but quantitative contradiction still remains. On the other hand, Karrai and Tiemann (KT) observed a huge Γ, which is estimated to be ∼ 10 −4 kg/s, between a conductive graphite surface and a gold probe tip attached to a hard (k 0 ∼ 10 4 N/m) quartz tuning fork (QTF) at d <10 nm at room temperature [10]. They attributed the origin of the friction to the viscous damping caused by residual adsorbates such as carbon oxide. The friction observed by KT was theref...

Noncontact Friction and Relaxational Dynamics of Surface Defects

“…10 Specifically, we propose that the observed behavior of NCF is due to the relaxation dynamics of the surface defects.…”

“…Charge-charge interaction: For completeness, we also consider the possibility that friction arises from interactions in the charge channel, though this mechanism may not apply for 10 . We notice that, as was shown in 24 , coupling between charge on the cantilever tip and ion vibrations on the sample surface can produce strong enhancement of NCF.…”

“…Analysis of the experimental data indeed indicates a distribution of relaxation times. 10 Alternatively defects can be local charged sites. In that case the charging and uncharging dynamics of the defects leads to the random electric field probed by a cantilever.…”

“…Experiments also indicate a distribution of relaxation times. 10 We thus assume C(ω) to have the usual phenomenological form typical for a glass system C(ω) = C 0 /(1 − iωτ d ) a , with the exponent 0 < a ≤ 1, and the relaxation time τ d . 16-18 γ(ω) now takes the form…”

Friction Modification by Shifting of Phonon Energy Dissipation in Solid Atoms

“…Nevertheless, the details of the nonequilibrium atomic process are still vague because the friction interface is difficult to access experimentally. Recent sophisticated nanoscale experiments by atomic force microscopy (AFM) have shed light on the energy dissipation process [9][10][11][12][13][14][15][16]. For example, Saitoh et al measured the noncontact friction between solids and an AFM probe under high vacuum and observed a significant maximum value at a distance of a few nanometers between them [10].…”