Transcript Slide 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
Experimental Context
MoS2 Structure
•Identify oxidation mechanisms
•Develop reactive bond-order (REBO) potential for MoS2
•MD Simulations of MoS2 tribology
A
B
A
B
A
B
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
0% S
terminated
50% S
terminated
Mo terminated
100% S
terminated
MoS2 Edge Structures
100% S
terminated
50% S
terminated
S terminated
0% S
terminated
Six MoS2 Edge Structures
0% coverage
Mo
Termination
S
Termination
50% coverage
100% coverage
Oxidation Energies of MoS2 Edge Structures
0% coverage
50% coverage
100% coverage
-1.7
-1.7
Mo
-1.4 -1.0
-1.7
-1.7
-1.7
-1.6
Termination
-1.0
-2.1
S
Termination
-1.7
-1.7
-2.1
-1.8
-1.7 -1.5
-1.1
-2.3
-1.3
Thermal Oxidation (AFM)
1000 nm
500 nm
MoO3 island on MoS2 (AFM)
a) Oxidation conditions:
480 °C in the furnace
with O2 flowing.
b) 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.
S..Mo..S …...S..Mo..S
MoS2 vs. Graphite
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

Eb   V R (rij )  bijV A (rij )
i
Repulsive Term:
Attractive Term:
Bond Order:
j i

rij
V (rij )  f (rij )(1  Q / rij ) Ae
R
c
Pair-wise parameters:
Q, A, α, B and β
 rij
V (rij )  f (rij ) Be
A
c
1
bij  (bij  b ji )
2
bij  [1   fik (rik )G(cos(ijk ))  Pij ( NiMo , NiS ...)]1/ 2
k i , j
Cut-off
function
• Each
Angular Term
Coordination Term
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
20
Mo
MoS2
Difference %
10
DFT
0
This Pot.
-10
-20
-30
a
B
c11
c12
a
c
B
c11
c12
Static Potential Energy Surface of MoS2
0.15
0.03
0.15
0.03
0.01
0.15
0.01
0.287
0.003
0.287
0.03
0.003
Path II 0.15
Path I
Y
0.15
0.03
0.01
0.03
0.03
0.15
0.01
X
DFT
0.003
Path II 0.287
Y
0.03
0.15
0.287
0.001
0.03
0.003
0.001
Path I
0.003
0.287
(nm)
0.001
0.003
0.003
0.287
0.003
0.287 0.001
X
REBO
(nm)
MD Simulation of MoS2 Tribology
DFT
Rigid moving
MD
Thermostat
Rigid moving
Active
Z
Y
X
Fixed
17.4 nm
Fixed
96 atoms
0K
Static process
System size: 12071 atoms
Temperature: ~100 K
Dynamic process
Z
Y
X
Dynamics of Frictional Sliding
(nm)
3
Y-disp
2
1
0
0.06
X-disp
Z-disp
0.04
0.02
0.00
-0.02
0
1
2
Sliding distance (nm)
3
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