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

Experimental Context

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

Vacuum-Air Cycling of MoS2 films

STM Characterization of MOS2 Surface

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

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

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

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

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 -2.1 -1.0 S Termination -1.7 -2.1 -1.7 -1.8 -1.7 -1.5 -1.1 -2.3 -1.3

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.

MoS2 vs. Graphite Directional bonding – angular terms S..Mo..S …...S..Mo..S Graphite MoS2 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

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

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

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

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

X Z Y

Dynamics of Frictional Sliding (nm)

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