Invited lecture of the Simposium N "Surface Engineering - functional coatings and modified surfaces" at the XIII SBPMat (Brazilian MRS) meeting, in João Pessoa (Brazil). The lecture took place on September 29th, 2014. The speaker was Christopher Muratore, "Wright Brothers Institute Endowed Chair Professor" at the Department of Chemical and Materials Engineering from University of Dayton (USA).

1. Mechanical, thermal, and electronic properties of
transition metal dichalcogenides
Christopher Muratore
University of Dayton Chemical and Materials Engineering Department
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH USA
Research funded by Air Force Office of Scientific Research, Air Force Research
Laboratory, and Dayton Area Graduate Studies Institute
SBP MAT XIII
Joao Pessoa, Brazil
September 29, 2014

2. key co-workers (mechanical)
AFRL co-workers
Voevodin Zabinski Hu Bultman Safriet
External collaborators
Aouadi
Southern Illinois U.
Rebelo de Figueiredo
Mitterer
U. of Leoben, Austria
Wahl
Naval Research Lab
Sawyer
U. of Florida
Clarke
Harvard University
(ex) students and post docs, including: Matt Hamilton (UF), Tim Smith (OSU),
Rich Chromik, Colin Baker (NCSU), Jason Steffens (UF) and D’Arcy Stone (SIU)

3. key co-workers (thermal and electronic)
Vikas Varshney-MD simulations
Jamie Gengler—laser spectroscopy (TDTR measurements)
Mike Jespersen—XPS analysis
John Bultman—thin film growth, XPS
Aman Haque (PSU)—device nanofabricaton and characterization
Jianjun Hu—Transmission electron microscopy
Andrey Voevodin—XPS analysis
Ajit Roy—MD simulations
Current students, Randall Stevenson, Jessica Dagher, Phil Hagerty, Rachel Rai

4. tribology: study of contact interfaces in relative motion
(friction and wear of materials)
Wear of stainless steel
(collaboration with Sawyer, University Florida)

5. interferometric analysis of wear tracks during sliding tests
c. wear track analysis (adaptive
nanocomposite)
b. friction data (adaptive nanocomposite)
results from AFRL/UF collaboration found in Tribo. Lett. 32 (2008)92
a. instrument
contact
interferometer
objective
reciprocating
stage
coating

6. sensitivity of graphite to ambient atmosphere
(Ramadanoff & Glass, Trans. AIEE, 1944)
Laboratory testing to accompany flight tests conducted in Areas A & C

7. sensitivity of graphite to ambient atmosphere
(Ramadanoff & Glass, Trans. AIEE, 1944)
Laboratory testing to accompany flight tests conducted in Areas A & C

8. sensitivity of graphite to ambient atmosphere
(Ramadanoff & Glass, Trans. AIEE, 1944)
Laboratory testing to accompany flight tests conducted in Areas A & C

9. sensitivity of graphite to ambient atmosphere
(Ramadanoff & Glass, Trans. AIEE, 1944)

10. sensitivity of graphite to ambient atmosphere
(Ramadanoff & Glass, Trans. AIEE, 1944)
Laboratory testing to accompany flight tests conducted in Areas A & C

11. nanocomposite materials with temperature adaptive properties

12. an overarching materials science dilemma: linking performance
Existing and future
aircraft are loaded
with mission critical
interfaces that must
operate in extreme
environments
to structure & composition
Performance measured in air at
temperatures between -50 to >800 oC
Physical limits on ambient
conditions required for
materials characterization
are often very different
than operating
environments
Structure and composition
measured in a UHV environment
Drawing courtesy of Greg Sawyer
properties
performance
processing
structure & composition
“materials science tetrahedron”

13. results available from prior in situ macroscopic tribology studies
Low temperature
lubricant (MoS2)
-Raman studies
-composition and thickness of transfer film
-relationship between friction coefficient
and transfer film thickness
-interferometry studies
-steady state wear rates
-correlation of friction and coating wear
-electron microscopy studies
-atomic scale view of contact pair
Optical
image of
contact
interface

14. interferometric and spectroscopic analysis of
interfacial films through wear counterpart
a. instrument b. transfer film thickness data (Pb-Mo-S coating)
slide courtesy of Sawyer and Wahl
c. wear track analysis (Pb-Mo-S film)
transfer film after sliding
as-deposited Pb-Mo-S coating

15. results available from prior in situ macroscopic tribology studies
Low temperature
lubricant (MoS2)
-Raman studies
-composition and thickness of transfer film
-relationship between friction coefficient
and transfer film thickness
-interferometry studies
-steady state wear rates
-correlation of friction and coating wear
-electron microscopy studies
-atomic scale view of contact pair
Optical
image of
contact
interface

16. preparation of contact pair cross-section for TEM analysis
FIB cutting
applied load
P
Ga+
Ga+
Ga+
Ga+
Sample
1) re-deposition
of incident Ga+
ions from cutting
beam and
sputtered carbon
welds loaded
contact in place
re-depos. mat.
2) friction
contact is now
preserved on
surface
10 mm
3)liftout of
cross-section
wear counterpart
film
Si substrate
Tribol. Lett. 32 (2008) 49

17. 5nm
HRTEM of sliding contact interface
wear counterpart
randomly
oriented film
5 nm
-atomic scale reorientation and recrystallization of
TMD surface at contact interface
-in situ technique holds promise for identifying where
sliding takes place and how friction is reduced at
solid-solid interfaces
each line represents
one S-Mo-S layer
Mo-W-S-Se composite film
Tribol. Lett. 32 (2008) 49

18. Interactive ISS experiments for in situ characterization of
materials in space environments
Test
apparatus
NASA Image
FIB welding of loaded
interface
Demonstration of multi-phase
nanocomposites for terrestrial &
space applications (AFRL/AFOSR
MURI/industry collaboration)
MoS2/graphite
inclusions in ceramic
matrix
250 mm 25 mm
10 nm
5 cm
Atomic structure at contact interface
Environmental adaptation of mechanical properties
MISSE 7 test-bed
2 nm

19. knowledge gaps remaining with previously
demonstrated in situ techniques
-Raman studies
-surface chemistry of coating leading to
changes in friction coefficient?
-coating failure mechanisms?
-interferometry studies
-surface chemistry leading to friction events?
-high temperature friction events?
-electron microscopy studies
-high temperature friction events?
-low throughput!

20. measurements during tests in diverse
environments allow instantaneous
identification of surface chemistry to reveal:
sample rotation
Raman tribospectrometer for in situ measurements
cut-away of heater
assembly
high temperature
Raman probe
V-block
mount
test
sample
Raman
spectrometer
scattered light
Ar laser
ball
holder
laser sampling
area
objective
lens
friction contact
-wear & failure mechanisms of coating materials
-onset temperature for oxidation or sublimation
-evolution of compound formation
nitrogen cooling
line

21. TiCN: interesting but difficult
(low Raman intensities)
objective: develop an
understanding of TiCN run
in process using in situ
Raman analysis of WT

22. 0 cycles 515 cycles 1035 cycles
1200 1400 1600 1800
Raman shift (cm-1)
1200 1400 1600 1800
Raman shift (cm-1)
0 1000 2000 3000 4000 5000
1.0
0.8
0.6
0.4
0.2
0.0
friction coefficient
number of cycles
1200 1400 1600 1800
Raman shift (cm-1)
2076 cycles
1200 1400 1600 1800
Raman shift (cm-1)
3638 cycles
1200 1400 1600 1800
Raman shift (cm-1)
in situ detection of amorphous carbon decay
during “run in” of TiCN
a-C a-C a-C
Tribo. Lett. 40 (2010)
amorphous
carbon peak
is absent after
peak friction
coefficient is
reached

23. carbon hydrogenation induced by wear in humid air
2900 3000 3100 3200
Raman shift (cm-1)
2900 3000 3100 3200
Raman shift (cm-1)
0 1000 2000 3000 4000 5000
1.0
0.8
0.6
0.4
0.2
0.0
friction coefficient
number of cycles
2900 3000 3100 3200
Raman shift (cm-1)
a-C:H
2076 cycles
2900 3000 3100 3200
Raman shift (cm-1)
a-C:H
3638 cycles
3000 3200
Raman shift (cm-1)
a-C:H a-C:H
0 cycles 515 cycles 1035 cycles
Tribo. Lett. 40 (2010)
hydrogenated
carbon signal
increases as
test
progresses

24. complimentary observations of transfer film during sliding
on TiCN at 25% RH using NRL technique
100
80
60
40
20
0 200 400 600 800 1000 1200 1400 1600
0.4
0.3
0.2
0.1
0
Coefficient of friction
Number of cycles
Transfer film thickness (nm)
1400 1600
Raman shift (cm-1)
C-H
1400 1600
Raman shift (cm-1)
3000 3200
Raman shift (cm-1)
1400 1600
Raman shift (cm-1)
C-H
1400 1600
Raman shift (cm-1)
G G
1400 1600
Raman shift (cm-1)
1400 1600
Raman shift (cm-1)
D
Raman shift (cm-1)
1000 2000 3000 4000 14000
1.0
0.8
0.6
0.4
0.2
3000 3200
Raman shift (cm-1)
13500 cycles
4680 cycles
3000 3200
Raman shift (cm-1)
3638 cycles
3000 3200
Raman shift (cm-1)
2076 cycles
3000 3200
Raman shift (cm-1)
515 cycles
3000 3200
Raman shift (cm-1)
1400 1600
3000 3200
Raman shift (cm-1)
1000 cycles
Coefficient of friction
Number of cycles
0 cycles
D
C-H
G
D
C-H
Generation
of wear
debris
Lubricious C-H
film sliding on
TiCN
Transfer film
accumulation
Tribo. Lett. 40 (2010)

25. 1000
500
1000
750
500
250
wear of MoS2 at 330-350 oC
1000
750
500
250
1000
750
500
250
0-6500 cycles
MoO3
0 2500 5000 7500 10000 12500 15000
1.0
0.8
0.6
0.4
0.2
0.0
friction coefficient
number of cycles
200 400 600 800 1000
0
intensity (arb. units)
Raman shift (cm-1)
200 400 600 800 1000
0
intensity (arb. units)
Raman shift (cm-1)
200 400 600 800 1000
0
intensity (arb. units)
Raman shift (cm-1)
200 400 600 800 1000
0
intensity (arb. units)
Raman shift (cm-1)
330 °C
MoS2
7100 cycles
From the data we can
see :
(a) the evolution of the
wear track
composition from
MoS2 (at 330 °C)
(b) to a mixture of
MoS2/MoO3 (7000
cycles)
(c) the failed coating
where the
substrate peak is
just as prominent
as the coating
8100 cycles
8700 cycles
Increase temperature
to 350 °C
MoO3
MoO3
MoS2
MoS2
MoS2 Si
Wear 270 (2011)
(a)
(b)
(c)

26. Ann. Rev. Mat. Res. 39 (2010)
environmentally adaptive nanocomposites

27. catalytic tribo-oxidation at elevated temperatures
YSZ-20%Ag-10%Mo-8%MoS2
MoO
Ag2MoO 4
-1
MoO
Mo
O
Mo
O
MoO
Mo
O
coatings relying on both
lubrication mechanisms yield
record low friction
coefficients for the 25-700 °C
temperature range
10
10
5
MoS2
MoS2
MoS2
1000 cycles
at 300 °C
5
MoS
2 MoS
2 MoS
2
200 400 600 800
intensity (arb. units)
Raman shift (cm-1)
200 400 600 800 1000 1200
10
0
MoO 3
MoO 3
MoO 3
Ag2MoO 4
Ag 2MoO 4
Ag2Mo4O7
5
Ag2Mo4O7
Ag2MoO 4
Ag2Mo4O7
intensity (arb. units)
Raman shift (cm ) -1
1000 cycles
at 700 °C
MoS2 transfer film
at moderate temperatures
S catalyzes Ag--Mo-O
formation at high temperatures
YSZ-24%Ag-10%Mo
0 200 400 600 800
0.6
0.4
0.2
0.0
YSZ-20%Ag-10%Mo-8%MoS2
friction coefficent
temperature (°C)
O-Ag-O
layer
Surf. Coat. Technol. 201 (2006) 4125
Ag2MoO4
Ag-O bond (220 kJ mol-1)
Mo-O bond (560 kJ mol-1)
O-Ag-O
layer
O-Ag-O
layer
mixed
MoO3 and
AgO layers
analogous
to MoS2?
Ag
Mo
O
200 400 600 800
intensity (arb. units)
Raman shift (cm-1)
400 800 1200
MoO
3
Ag
2
4
MoO
3
Ag
2
4
7
MoO
3
Ag
2
4
Ag
2
MoO
4
Ag
2
4
7
Ag
2
4
Ag
2
4
7
Raman shift (cm-1)
Scripta Materialia 62 (2010) 735–738

28. Ann. Rev. Mat. Res. 39 (2010)
environmentally adaptive nanocomposites

29. surprisingly low thermal conductivity for MoS2
MEMS heater device
Free-standing
MoS2
ribbon
Very steep thermal gradient means
k is much lower than we expected

30. simulation results: in-plane & out-of-plane
dQ dt

Very small phonon group velocity across basal planes dx
hot cold
In plane phonons have high group velocity
2.26 nm
Tilted view of
simulated MoS2
crystal
k across basal planes: 4.2 W m -1K-1
k along basal planes:: 18.0 W m-1 K-1
Heat Flow
Heat Flow
A dT

/
k
Step 1: Forces from bonded and non-bonded atomic interactions calculated
and verified by simulating vibrational modes
Step 2:Thermal conductivity calculated from Fourier Law analysis of steady
temperature gradient in the crystal using this equation:
1
 
i
i i i C v l
3V
-group velocity
li-phonon mean free path
Predicted differences in thermal conductivity due to
crystal anisotropy
k
Ci-spectral heat capacity
ni
Comp.Mat.Sci. 48 (2010)

31. Mode-Locked
Ti:Sapphire (140 fs) 775-830 nm
80 MHz
Electro – Optic Modulator
@ 9.8MHz
Variable Delay
RF Lock – in Amp.
Sample Photodiode
Translation Stage
Lens
Iris
Ref.
CCD
Camera
OPO
505-1600 nm
Pulse Spectrometer
Compressor
Lens Lens
l Filter
Signal
time domain thermal reflectance (TDTR) measurement technique
TDTR schematic
Cahill, Rev. Sci. Instrum. 75 (2004) 5119
Comp. Sci. Technol. 14 (2010), 2117
probe pump
reflective layer
material of
interest
quantified
interface for
conductance
sample architecture for TDTR

32. orientation control of layered atomic structures
(100) oriented
[lower rate & ion energy]
(002) oriented
[higher rate & ion energy]
substrate
reactive surface [2]
surface energy~25,000 mJ m-2
substrate
MoS2 (100) edge planes
Deposited atoms
are more likely to
desorb from
(002) surface if
burial is slower
than 1 second
 1 second desorption 1 second t
1 1 /
E RT
  e 
c oc
desorption
a
k v
t
Desorption time is
long on (001)
planes allowing
growth at low
deposition rates
5 nm
Thin Solid Films 517 (2009)
Crystal orientation dependence on
growth rate and ion energy magnetron
sputtering
Control of MoS2
orientation via
plasma power
modulation
Processing development enables studies of anisotropic
crystal properties
MoS2 (002) basal planes

33. demonstration of orientation control of MoS2
X-ray diffraction data
Log-plot shows all orientations
are accessible by selecting
appropriate sputtering process
1.00
0.75
0.50
0.25
anneal
repeat until desired
thickness is obtained
10 15 20 25 30 35 40
intensity (arb. units)
2 (degrees)
MoS2
(002)
MoS2
(100)
/intermittent sputtering
Intermittent sputtering for strong
002 orientation
deposit 5
atomic layers
example diffractogram of
highly oriented sample

34. orientation and exposure history dependence on
MoS2 thermal conductivity
MoS2
Depiction of Al cap
Al
MoS2 capped with Al in vacuo
1.00
0.75
0.50
0.25
0.00
50 nm pristine MoS2
10 15 20 25 30 35 40
intensity (arb. units)
2 (degrees)
(002)
(004)
Inconel
substrate
Both orientations show k values ~4 x
lower than predicted
002 bulk crystal
0 50 100 150 200
10
1
0.1
(002) pristine
amorphous
(002) 48 hour exposure
(100) pristine
Thermal Conductivity (W m-1K-1)
Thickness (nm)
5 nm
Phys. Chem. Chem. Phys. 16 (2014) 1008

35. Pulsed dc with
TMD target
XRD of MoS2 and WS2 films cross-sectional TEM shows
10 15 20 25 30 35 40
2000
1500
1000
500
0
intensity (arb. units)
2 (degrees)
MoS
2
WS
2
TEM of WSe2 film
film surface
substrate
5 nm
PVD processing of all MoX2 and WX2 TMDs
Identical
microstructures
under similar
conditions (T, P, etc.)
basal plane alignment

36. manipulating Slack parameters for k reduction: role
of film structure and composition
N = 6 for all compounds
g = 2
measured and predicted thermal conductivities for 20 nm
(002) oriented transition metal dichalcogenide films
10 x reduction of k for thin films
with identical microstructures
Appl. Phys. Lett. 102 (2013)

37. scattering at domain boundaries accounts for
10X reduction in thermal conductivity
Simulated acoustic phonon dispersion for TMD materials
Calculation of scattering length by
summing scattering sources:
TEM of WSe2 film
film surface
substrate
5 nm
Domain sizes ~ 3-10 nm
3 1/3
BM N D 
2

g
k
T

38. simulation results: in-plane & out-of-plane
dQ dt

Very small phonon group velocity across basal planes dx
hot cold
In plane phonons have high group velocity
2.26 nm
Tilted view of
simulated MoS2
crystal
k across basal planes: 4.2 W m -1K-1
k along basal planes:: 18.0 W m-1 K-1
Heat Flow
Heat Flow
A dT

/
k
Step 1: Forces from bonded and non-bonded atomic interactions calculated
and verified by simulating vibrational modes
Step 2:Thermal conductivity calculated from Fourier Law analysis of steady
temperature gradient in the crystal using this equation:
1
 
i
i i i C v l
3V
-group velocity
li-phonon mean free path
Predicted differences in thermal conductivity due to
crystal anisotropy
k
Ci-spectral heat capacity
ni
Comp.Mat.Sci. 48 (2010)

39. simulated defect scattering
1 interface
2 interfaces
3 interfaces
4 interfaces
6 interfaces
20 interfaces
Heat Flow
Heat Flow
Heat Flow
Heat Flow
Heat Flow
Heat Flow
Phys. Chem. Chem. Phys.
16 (2014) 1008
Simulated value consistent with
50 W m-1K-1 value reported by:
Sahoo et al. J. Phys. Chem. C 117 (2013) 9042

40. Naik and Muratore
Geim et al. et al.
Tri-layer
MoS2
Few-layer
graphene 5 Å
robust transistors
Potential to build synthetic superlattices with no
consideration of lattice constant
large strain
accommodation for
flexible electronics
Kis
et al.
2D molecular sensors with
enhanced sensitivity/selectivity
Yoon
et al.
what are 2D TMDs good for?
Muratore
et al.
MoSe2
MoS2
200 250 300 350 400 450
intensity (arb. units)
Raman shift (cm)-1
30% lattice mismatch
accommodation at interface
Easy growth of
multilayers

41. summary of 2D TMD processing SOTA
Solution-based or mechanical exfoliation Chemical vapor deposition
C. Lee et al. ACS Nano 4 2010
a
Zhan et al. small 8 2012
Interflake scattering
inhibits application
Contamination and
structural changes inhibit
application
Najmaei et al.Nat.
Mater. 12 2013
Najmaei et al.Nat.
Mater. 12 2013
van der Zande Nat.
Mater. 6 2013
K. Kaasbjerg, PRB 85
(2012)

42. UHV synthesis for pristine surfaces and interfaces
XPS analysis
Synthesis chamber
chamber
Loa
d
lock
Composition
measured in
vacuuo after
growth
After 1 hour
exposure to
22 oC air at
15%
humidity
Mo (+4)
S
240 236 232 228 224 220
binding energy (eV)
Mo (+4)
edge oxidation
MoO3 S
240 236 232 228 224 220
binding energy (eV)

43. sputtering without energetic particle damage
0 2 4 6 8 10 12
100000
10000
1000
100
10
intensity (arb. units)
kinetic energy (eV)
It takes about 8 eV to create a
vacancy via sputtering a sulfur
atom from MoS2
1
Incident ion energies can be
modulated to stay below this
threshold
1Komsa, et al. Phys. Rev. B 88 (2013) 035301
A narrow window of growth
rates, energetic particle fluxes
and energies results in high
quality, ultra-thin TMD films
at low temperatures
Kinetic energy of incident flux
defect
generation
threshold

44. uniform application of TMD films over large areas
Growth
<250 oC
Hybrid technique for evaluating uniformity over
large areas
5 layer MoS2 on thermally grown
SiO2 ,R = 5 nm

45. Raman/PL characterization of PVD MoS2
stiffening softening
1.00
0.75
0.50
0.25
0.00
Raman frequency shift with thickness
410
408
406
384
Films demonstrate identical shifts with thickness as exfoliated materials
FWHM analysis indicate small domain sizes (10s of nm)
100000
10000
1000
100
524 525 526 550 600 650 700
intensity (arb. units)
wavelength (nm)
2 layers
4 layers
increasing PL intensity with
reduced thickness
Raman
PL
360 370 380 390 400 410 420
intensity (arb. units)
Raman shift (cm-1)
3 layers
4 layers
5 layers
6 layers
1 2 3 4 5 6 7
382
bulk A1g
A1g
E1
2g
Raman shift (cm-1)
MoS2 layers
bulk E1
2g

46. are films really “large area”?
Hall mobility measurements via Van der Pauw technique over 1 cm
sample
architecture
5 mm
450 mm Si
(1-10 ohm-cm)
n-type/P doped
300 nm SiO2
cross-sectional
view
physical
isolation
Strong T
dependence
above D
Weak T
dependence
thin MoS2

47. increasing domain size via increased thickness
Domain size increase with thickness reduces mobility
(maybe)—increased phonon-electron coupling?
Z = S2s/k
Promising development
for TE applications

48. summary and conclusions
• Just like the best tribological coatings, scalable 2D TMD synthesis accessible by physical
vapor deposition techniques such as sputtering, etc.
• comparison of different dichalcogenides with similar microstructure demonstrate
strong composition dependence of k, consistent with Slack law prediction
– uniform translation of measured k values suggests effect of TMD micro- or atomic structure
• very low thermal conductivity (0.07<k<0.25 W m-1 K-1) measured for thin film members
of TMD family of compounds
• Suggested mechanism for massive mobility in PVD TMDs related to coherent nanoscale
domains
• Just like MoS2 coatings revolutionized aerospace tribology, they will impact hot
technological areas including ultra-efficient thermoelectrics and selective biosensors