Nanotribology of MoS 2 : Microscopic Simulations of Oxidation and Friction

1. Nanotribology of MoS2: Microscopic Simulations of Oxidation and Friction Tao Liang, W. Gregory Sawyer*, Scott S. Perry, Susan B. Sinnott and Simon R. Phillpot University of Florida Materials Science and Engineering *Mechanical and Aerospace Engineering

2. Experimental Context

3. MoS2 Structure A B A B A B • Identify oxidation mechanisms • Develop reactive bond-order (REBO) potential for MoS2 • MD Simulations of MoS2 tribology

4. Vacuum-Air Cycling of MoS2 films

5. STM Characterization of MOS2 Surface

6. Substitution O for S of Bulk Structure • Atomic oxygen prevalent in low earth-orbit conditions • On space station, each sulfur is hit by 1 atom oxygen per second Oxygen DFT-LDA calculations show: DE ~ -1.7 eV (-39 kcal/mol)  Substitution of S for O strongly energetically favored

7. MoS2 Edge Structures 0% S terminated 50% S terminated 100% S terminated Mo terminated

8. MoS2 Edge Structures 100% S terminated 50% S terminated 0% S terminated S terminated

9. Six MoS2 Edge Structures 50% coverage 100% coverage 0% coverage Mo Termination S Termination

10. Oxidation Energies of MoS2 Edge Structures 50% coverage 100% coverage 0% coverage -1.7 -1.7 -1.4 -1.0 -1.7 Mo Termination -1.7 -1.7 -1.6 -1.0 -2.1 S Termination -1.8 -1.7 -2.1 -1.7 -1.7 -1.5 -1.1 -2.3 -1.3

11. Thermal Oxidation (AFM) 1000 nm 500 nm MoO3 island on MoS2 (AFM) • Oxidation conditions: 480 °C in the furnace with O2 flowing. • The MoO3 island surface is not flat. 5 nm 5 nm MoS2 MoO3 Sheehan, Paul E.; Lieber, Charles M. Nanotribology and nanofabrication of MoO3 structures by atomic force microscopy. Science (1996), 272(5265), 1158-1161.

12. MoS2 vs. Graphite • Directional bonding – angular terms • Layered structures with vdW interactions • Captured for graphite in Adapted Intermolecular Reactive Empirical Bond Order (AIREBO) potential • Adapt REBO for MoS2 S..Mo..S …...S..Mo..S Graphite MoS2

13. Repulsive Term: Pair-wise parameters: Q, A, α, B and β Attractive Term: Cut-off function Angular Term Coordination Term REBO Potential for Mo-S Systems Bond Order: • Each bond has one set of pair-wise parameters. • Each element has one set of many body parameters, G and P.

14. Validation of Mo-S potential Mo MoS2 a B c11 c12 a c B c11 c12 Exp. 3.16 Å 76 GPa 12.3 Å 52 GPa 238 GPa 3.15 Å 450 GPa 230 GPa 173 GPa

15. 0.003 0.287 0.003 0.001 0.287 0.287 0.001 0.003 Path II Path I 0.287 0.003 Y 0.003 0.287 0.001 0.287 0.003 0.003 0.287 0.001 0.003 X Static Potential Energy Surface of MoS2 0.03 0.15 0.03 0.01 0.15 0.15 0.01 0.03 Path II Path I 0.15 0.03 Y 0.03 0.15 (nm) (nm) 0.01 0.15 0.03 0.03 0.15 0.01 0.03 X DFT REBO

16. DFT Rigid moving Fixed 96 atoms 0 K Static process MD Simulation of MoS2 Tribology MD Thermostat 18.9 nm Rigid moving V 6.2 nm Active Z Y Fixed 17.4 nm X • System size: 12071 atoms • Temperature: ~100 K • Dynamic process

17. Z Y X

18. Dynamics of Frictional Sliding (nm)

19. Accomplishments • Thermodynamics for oxidation is strongly favorable • Flexible REBO potential for MoS2 • MD simulation of sliding friction of MoS2 • Thermal-transport properties of MoS2 (with Andrey Voevodin, AFRL) Opportunities • Oxidation kinetics • Elucidating nature of experimentally observed electronic defects • Role of step edges and oxidation on tribological performance