Intercalation leads to inverse layer dependence of friction on chemically doped MoS₂

Abstract

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.

Structure, Thermodynamics, and Raman Spectroscopy of Rhenium-Doped Bulk MoS2 from First Principles

“…This result is consistent with experimental observation of Mo-substituted monolayers, which have a comparable geometry. Although Re adatoms on monolayers are much less stable, bulk intercalation of Re has been inferred in experimental samples, which can be due to out-of-equilibrium or higher-temperature synthesis processes. The picture is quite different from that for Ni-doping of MoS 2 , in which only t-intercalation was compatible with pristine MoS 2 stability (except at the highest doping levels) …”

“…The favored site in bulk is unclear and has not been established by experiments, which are not necessarily able to distinguish between the sites in multi-layer structures. In particular, there is some experimental evidence of Re intercalation in 2H-MoS 2 from Raman spectroscopy interpreted by our density functional theory (DFT) calculations 17 and in 3R-MoS 2 from energy-dispersive X-ray analysis on samples from chemical vapor transport. 21 Moreover, DFT studies indicate the stability of other transition metals in tetrahedral (t-) intercalation, 22,23 sites beyond Mo substitution are possibilities worth investigating.…”

“…High-frequency vibrations of the Re atom exist above the A 1g peak (Figure S4), but the (out-ofplane) A 1g -like vibrations are lower in frequency than the pristine�despite the increase in out-of-plane stiffness found in our calculations. 17 Upon comparing Mo−S interatomic force constants, it is seen that those near Re are lower in magnitude by about 25% than those farther from Re in the z-direction. This is consistent with the observation that Re doping lowers the A 1g frequency in both cases.…”

“…Most of the recent interest in Re-doped MoS 2 systems has been in single-layer structures, , with few multilayer or bulk studies. , Studying Re-doping in bulk is nevertheless also interesting since bulk MoS 2 shares some of the opto-electronic properties of single layers and as an infinite-layer limit helps understand trends with the increasing number of layers. Bulk MoS 2 is also important in macroscale applications such as solid lubrication. , …”

“…Bulk MoS 2 is also important in macroscale applications such as solid lubrication. 2,17 MoS 2 is an effective solid lubricant owing to the ease of shearing along the basal planes. 2 Doping MoS 2 alters material growth patterns 18 and enhances tribological properties.…”