Engineering frictional characteristics of MoS2 structure by tuning thickness and morphology- An atomic, electronic structure, and exciton analysis

We performed atomic and electron dynamics analysis to study the impact of morphological and thickness changes of a MoS2 system on its tribological properties through a diamond tip. We had considered 4 cases: variable layers (1-4 layers) and number (2-8 indents), radius (12{\AA}, 16{\AA}, 20{\AA}, 24{\AA}), and pattern of indents (0{\deg}, 25{\deg}, 30{\deg}, 35{\deg}, 45{\deg}, 60{\deg}) resulting into 18 subcases. MD results showed changing the radius and number of indents were the most, and number of layers and indents' pattern were the least effective way to tune the frictional characteristics. Ground state ab-initio study demonstrated an increase in the number and radius of indents, raising the number of stretched bonds. Consequently, the volume covered by the HOMO iso-surface increases, and that of LUMO decreases. That makes higher area/volume available to lose/share electrons, resulting in stronger interlocking between layers and tip. And TD-DFT calculation proves the existence of interfacial excitons, resulting in stronger interlocking between the layer's surface and tip despite a contraction in the LUMO iso-surfaces' area/volume. We believe these interlayer excitons result in higher average Z-axis(hence frictional force) reaction forces for the indents number subcases and lower for indents radius subcases as the number and radius of indents increase.