Structure, friction, and Raman spectroscopy of Re-doped bulk MoS$_2$ from first principles

Abstract

Doping MoS 2 with Re is known to alter the electronic, structural, and tribological properties.Re-doped MoS 2 has been mainly studied in monolayer or few-layer form, but can also be relevant for applications in many-layer or bulk form. In this work, we use density functional theory to explore the phase stability, Raman spectra, and atomic force microscopy (AFM) sliding interactions of bulk Re-doped MoS 2 . We consider the possibility of the Re dopant existing at different locations and provide experimentally distinguishable characteristics of the most likely sites-Mo-substitution and tetrahedral (t-) intercalation. We demonstrate and benchmark an approach to calculate Raman spectra of doped materials with metallic densities of states by using atomic Raman tensors from the pristine material. Applying this method to the metallic Re-doped structures, we find consistent shifts in the Raman-active peaks depending on Re dopant position: redshifts in both A 1g and E 1 2g peaks in the t-intercalated case versus a redshift for A 1g and blueshift for E 1 2g peaks in the Mo-substituted case. AFM friction forces, if the MoS 2 surface remains flat and unpuckered, are increased by Re dopants, similar for Mo-substitution and t-intercalation, and would decrease with the number of layers.

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

“…Therefore modifications in bending stiffness cannot explain the observed trends. This stiffening effect on C 33 is significantly more pronounced for t-intercalation than Mo-substitution, due to the formation of interlayer covalent bonds by Re [27], similar to the general increase in interlayer coupling for Ni-intercalated MoS 2 [37]. More importantly for the present discussion, the stiffening effect is proportional to the dopant concentration, suggesting a model of rigid layers connected in series by springs, in which the dopant contributes a stronger spring constant (figure 5(a), inset).…”

“…For transitionmetal dichalcogenides, particularly in many-layer form, determination of doping site is a significant experimental challenge, only definitively resolved in a few cases [25,26]. Intercalation between the layers, and substitution for the chemically similar Mo, are the most typical sites for a transition-metal dopant such as Re in MoS 2 , and are the most thermodynamically favorable for Re [27]. We performed plane-wave DFT calculations with Quantum ESPRESSO [28,29] of Raman spectra [30] using the approximation we developed for metallic doped systems [27].…”

“…Intercalation between the layers, and substitution for the chemically similar Mo, are the most typical sites for a transition-metal dopant such as Re in MoS 2 , and are the most thermodynamically favorable for Re [27]. We performed plane-wave DFT calculations with Quantum ESPRESSO [28,29] of Raman spectra [30] using the approximation we developed for metallic doped systems [27]. We studied bulk (multi-layer) MoS 2 , undoped and with Re in tetrahedral (t-) intercalation and Mo substitution sites, to identify frequency shifts in the two prominent peaks (figure 4).…”