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 FloridaMaterials Science and Engineering

*Mechanical and Aerospace Engineering

Experimental Context

MoS2 Structure

•Identify oxidation mechanisms

•Develop reactive bond-order (REBO) potential for MoS2

•MD Simulations of MoS2 tribology

ABA

BAB

Vacuum-Air Cycling of MoS2 films

STM Characterization of MOS2 Surface

Substitution O for S of Bulk Structure

DFT-LDA calculations show:

E ~ -1.7 eV (-39 kcal/mol)

Substitution of S for O strongly energetically favored

• Atomic oxygen prevalent in low earth-orbit conditions• On space station, each sulfur is hit by 1 atom oxygen per second

Oxygen

MoS2 Edge Structures

0% S terminated

50% S terminated

100% S terminatedMo terminated

MoS2 Edge Structures

100% S terminated

50% S terminated

0% S terminated

S terminated

Six MoS2 Edge Structures

Mo Termination

S Termination

0% coverage 50% coverage 100% coverage

Oxidation Energies of MoS2 Edge Structures

Mo

Termination

S Termination

0% coverage 50% coverage 100% coverage

-1.7 -1.5

-1.1

-1.7 -2.1

-2.3

-1.7

-1.3

-1.8

-1.7

-1.6

-1.7

-1.7

-1.0

-1.4 -1.0

-1.7

-2.1

-1.7

5 nm 5 nm

1000 nm 500 nm

MoS2 MoO3

MoO3 island on MoS2 (AFM)

a)Oxidation conditions: 480 °C in the furnace with O2 flowing.

b)The MoO3 island surface is not flat.

Sheehan, Paul E.; Lieber, Charles M. Nanotribology and nanofabrication of MoO3 structures by

atomic force microscopy. Science (1996), 272(5265), 1158-1161.

Thermal Oxidation (AFM)

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

Graphite

MoS2

S..

Mo.

.S …

...S

..M

o..S

REBO Potential for Mo-S Systems

i ij

ijA

ijijR

b rVbrVE )()(

)(2

1jiijij bbb Bond Order:

2/1,

,

...)]())(cos()(1[

Si

Moiijijk

jikikikij NNPGrfb

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.

ijrijij

cij

R AerQrfrV )/1)(()(

ijrij

cij

A BerfrV )()(

Repulsive Term:

Attractive Term:Pair-wise parameters: Q, A, α, B and β

Validation of Mo-S potentialE

xp.

a B c11 c12 a c B c11 c12

3.15

Å

173

GP

a

3.16

Å

12.3

Å

76 G

Pa

238

GP

a

52 G

Pa

450

GP

a

230

GP

a

Mo MoS2

Static Potential Energy Surface of MoS2

(nm)

Path I

0.0030.003

0.003

0.003

0.003

0.0030.003

0.003

0.001

0.001

0.001

0.001

0.287

0.287

0.287

0.287

0.287

0.287

0.287

X

YPath II

(nm)

Path I

0.030.03

0.03

0.03

0.03

0.030.03

0.03

0.01

0.01

0.01

0.01

0.15

0.15

0.15

0.15

0.15

0.15

0.15

X

YPath II

DFT REBO

MD Simulation of MoS2 Tribology

System size: 12071 atomsTemperature: ~100 KDynamic process

Fixed

Rig

id m

ovin

g

DFT

96 atoms0 KStatic process

MD

X

Z

Y Fixed

Rigid moving

Thermostat

Active

17.4 nm

18.9 nm

6.2

nm

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