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

TL;DR

This study investigates how the thickness and morphology of MoS2 influence its frictional properties, revealing that indents' radius and number significantly affect interlayer interactions and exciton behavior, which in turn modulate friction.

Contribution

It provides a comprehensive atomic, electronic, and excitonic analysis of how morphological and thickness modifications tune MoS2's tribological characteristics, combining MD, ab-initio, and TD-DFT methods.

Findings

Increasing indent radius and number enhances interlayer bonding and exciton effects.

Number and radius of indents are the most effective parameters for tuning friction.

Interlayer excitons contribute to variations in frictional forces despite geometric contractions.

Abstract

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.