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 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 Mo terminated 0% S terminated 50% S terminated7

8. MoS2 Edge Structures S terminated 100% S terminated 50% S terminated8

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
-2.1
-1.0
S Termination
-1.7
-2.1
-1.7
-1.8
-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),

12. 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

13. 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.

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

15. 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

16. 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: atoms
Temperature: ~100 K
Dynamic process

17. X Z Y

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