Mechanical, thermal, and electronic properties of transition metal dichalcogenides

Christopher Muratore

SBP MAT XIIIJoao Pessoa, BrazilSeptember 29, 2014

University of Dayton Chemical and Materials Engineering DepartmentAir 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

key co-workers (mechanical)

AFRL co-workers

External collaborators

Voevodin Zabinski Hu Bultman Safriet

Aouadi Southern Illinois U.

Rebelo de FigueiredoMittererU. of Leoben, Austria

WahlNaval Research Lab

SawyerU. of Florida

ClarkeHarvard 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)

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

key co-workers (thermal and electronic)

Current students, Randall Stevenson, Jessica Dagher, Phil Hagerty, Rachel Rai

tribology: study of contact interfaces in relative motion            (friction and wear of materials)

Wear of stainless steel (collaboration with Sawyer, University Florida)

interferometric analysis  of wear tracks during sliding tests

results from AFRL/UF collaboration found in Tribo. Lett. 32 (2008)92

c. wear track analysis (adaptive nanocomposite)

b. friction data (adaptive nanocomposite)

a. instrument

contact

interferometer objective

reciprocating stage

coating

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

Laboratory testing to accompany flight tests conducted in Areas A & C

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

Laboratory testing to accompany flight tests conducted in Areas A & C

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

Laboratory testing to accompany flight tests conducted in Areas A & C

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

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

Laboratory testing to accompany flight tests conducted in Areas A & C

nanocomposite materials with temperature adaptive properties

Existing and future aircraft are loaded with mission critical interfaces that must operate in extreme environments

Physical limits on ambient conditions required for materials characterization are often very different than operating environments

an overarching materials science dilemma:  linking performance to structure & composition

Structure and composition measured in a UHV environment

“materials science tetrahedron”

properties

performance

Performance measured in air at temperatures between -50 to >800 oC

processing

structure & composition

Drawing courtesy of Greg Sawyer

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

interferometric and spectroscopic analysis  of interfacial films through wear counterpart

slide courtesy of Sawyer and Wahl

transfer film after sliding

a. instrument b. transfer film thickness data (Pb-Mo-S coating)

as-deposited Pb-Mo-S coating

c. wear track analysis (Pb-Mo-S film)

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

FIB cutting

applied load

P

Ga+

Ga+

Ga+

Ga+

Sample

re-depos. mat.

1) re-deposition of incident Ga+ ions from cutting beam and sputtered carbon welds loaded contact in place 

10 mm

2) friction contact is now preserved on surface

3)liftout of cross-section

filmwear counterpart

Si substrate

preparation of contact pair cross-section for TEM analysis

Tribol. Lett. 32 (2008) 49

5nm

HRTEM of sliding contact interface

-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

wear counterpart

randomly oriented film

interfacial adaptation to loading

5 nm

each line represents one S-Mo-S layer 

Mo-W-S-Se composite film

Tribol. Lett. 32 (2008) 49

Interactive ISS experiments for in situ characterization of materials in space environments

NASA Image

Demonstration of multi-phase nanocomposites for terrestrial &

space applications (AFRL/AFOSR MURI/industry collaboration)

MoS2/graphite inclusions in ceramic matrix

250 mm 25 mm

FIB welding of loaded interface

Test apparatus

10 nm

5 cm

Atomic structure at contact interface

Environmental adaptation of mechanical properties

MISSE 7 test-bed

2 nm

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!

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

measurements during tests in diverse environments allow instantaneous identification of surface chemistry to reveal:

     -wear & failure mechanisms of coating materials

              -onset temperature for oxidation or sublimation

                 -evolution of compound formation

nitrogen cooling line

objective:  develop an understanding of TiCN run in process using in situ Raman analysis of WT

TiCN: interesting but difficult (low Raman intensities)

0 1000 2000 3000 4000 50000.0

0.2

0.4

0.6

0.8

1.0

fric

tion

co

effi

cie

nt

number of cycles

1200 1400 1600 1800

Raman shift (cm-1)

1200 1400 1600 1800

Raman shift (cm-1)

1200 1400 1600 1800

Raman shift (cm-1)

1200 1400 1600 1800

Raman shift (cm-1)

1200 1400 1600 1800

Raman shift (cm-1)

1035 cycles515 cycles0 cycles

2076 cycles

3638 cycles

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

0 1000 2000 3000 4000 50000.0

0.2

0.4

0.6

0.8

1.0

fric

tion

co

effi

cie

nt

number of cycles

2900 3000 3100 3200

Raman shift (cm-1)

2900 3000 3100 3200

Raman shift (cm-1)

2900 3000 3100 3200

Raman shift (cm-1)

2900 3000 3100 3200

Raman shift (cm-1)

3000 3200

Raman shift (cm-1)

a-C:H

a-C:H

3638 cycles

2076 cycles

a-C:Ha-C:H

1035 cycles515 cycles0 cycles

carbon hydrogenation induced by wear in humid air

Tribo. Lett. 40 (2010)

hydrogenated carbon signal increases  as 

test progresses

0 200 400 600 800 1000 1200 1400 1600

0.1

0.2

0.3

0.4

0

20

40

60

80

100

Coeffi

cien

t of

fric

tion

Number of cycles

Tra

nsf

er fi

lm t

hic

knes

s (n

m)

1000 2000 3000 4000 14000

0.2

0.4

0.6

0.8

1.0

3000 3200

Raman shift (cm-1)

13500 cycles

C-H

1400 1600

Raman shift (cm-1)

4680 cycles

3000 3200

Raman shift (cm-1)

1400 1600

Raman shift (cm-1)

3000 3200

Raman shift (cm-1)

C-H

3638 cycles

1400 1600

Raman shift (cm-1)

3000 3200

Raman shift (cm-1)

1400 1600

Raman shift (cm-1)

2076 cycles

3000 3200

Raman shift (cm-1)

1400 1600

Raman shift (cm-1)

515 cycles

3000 3200

Raman shift (cm-1)

1400 1600

Raman shift (cm-1)

3000 3200

Raman shift (cm-1)

1400 1600

Raman shift (cm-1)

1000 cycles

Coeffi

cient

of

fric

tion

Number of cycles

0 cycles

DG G

D

C-H

GD

C-H

Generation of wear debris

Lubricious C-H film sliding on 

TiCN

Transfer film accumulation

complimentary observations of transfer film during sliding on TiCN at 25% RH using NRL technique

Tribo. Lett. 40 (2010)

0 2500 5000 7500 10000 12500 150000.0

0.2

0.4

0.6

0.8

1.0

fric

tion c

oeffic

ient

number of cycles

200 400 600 800 10000

500

1000

inte

nsi

ty (

arb

. units

)

Raman shift (cm-1)

200 400 600 800 10000

250

500

750

1000

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

200 400 600 800 10000

250

500

750

1000

inte

nsi

ty (

arb

. units

)

Raman shift (cm-1)

200 400 600 800 10000

250

500

750

1000

inte

nsi

ty (

arb

. units

)

Raman shift (cm-1)

wear of MoS2  at 330-350 oC 

330 °C

MoS2

0-6500 cycles

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

MoO3

MoS2

MoS2

MoS2 Si

Wear 270 (2011)

(a)

(b)

(c)

Ann. Rev. Mat. Res. 39 (2010)

environmentally adaptive nanocomposites

catalytic tribo-oxidation at elevated temperatures

coatings relying on both lubrication mechanisms yield record low friction coefficients for the 25-700 °C temperature range 

200 400 600 800

5

10

MoS2

MoS2

MoS2

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

1000 cycles at 300 °C

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

1000 cycles at 700 °C

MoS2 transfer filmat moderate temperatures

S catalyzes Ag-Mo-O

200 400 600 800

5

10

MoS2

MoS2

MoS2

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

1000 cycles at 300 °C

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

1000 cycles at 700 °C

MoS2 transfer filmat moderate temperatures

-Mo-Oformation at high temperatures

YSZ-20%Ag-10%Mo-8%MoS2

-1

200 400 600 800

5

10

MoS2

MoS2

MoS2

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

1000 cycles at 300 °C

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

1000 cycles at 700 °C

MoS2 transfer filmat moderate temperatures

S catalyzes Ag-Mo-O

200 400 600 800

5

10

MoS2

MoS2

MoS2

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

1000 cycles at 300 °C

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

200 400 600 800 1000 12000

MoO 3

Ag 2MoO 4

MoO 3

MoO 3

Ag2MoO 4

Ag 2MoO 4Ag

2Mo

4O

7

5

10

Ag 2Mo 4O7

Ag 2MoO 4

Ag 2Mo 4O7

inte

nsity

(ar

b.

units

)

Raman shift (cm ) -1

1000 cycles at 700 °C

MoS2 transfer filmat moderate temperatures

-Mo-Oformation at high temperatures

YSZ-20%Ag-10%Mo-8%MoS2

-1

0 200 400 600 8000.0

0.2

0.4

0.6

YSZ-20%Ag-10%Mo-8%MoS2

YSZ-24%Ag-10%Mo

fric

tion

coef

ficen

t

temperature (°C)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

O-Ag-O layer

mixed MoO3 and AgO layers

analogous to MoS2?

AgMoO

200 400 600 800

5

10

MoS2 MoS

2MoS2

inte

nsi

ty (

arb

. u

nits

)

Raman shift (cm-1)

400 800 1200

MoO3

Ag2MoO

4

MoO3

Ag2Mo

4O

7

MoO3

Ag2MoO

4

Ag2MoO

4

Ag2Mo

4O

7

Ag2MoO

4

Ag2Mo

4O

7

Raman shift (cm-1)

Scripta Materialia 62 (2010) 735–738

Ann. Rev. Mat. Res. 39 (2010)

environmentally adaptive nanocomposites

MEMS heater device

Very steep thermal gradient means k is much lower than we expected

surprisingly low thermal conductivity for MoS2

Free-standing MoS2 ribbon

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

hot cold

Heat Flow

In plane phonons have high group velocity

Very small phonon group velocity across basal planes dxdTA

dtdQ

/

Step 1: Forces from bonded and non-bonded atomic interactions calculated and verified by simulating vibrational modesStep 2:Thermal conductivity calculated from Fourier Law analysis of steady temperature gradient in the crystal using this equation:

Predicted differences in thermal conductivity due to crystal anisotropy

i

iii lvCV3

1

Ci-spectral heat capacity

ni-group velocity

li-phonon mean free path

simulation results:  in-plane & out-of-plane

Comp.Mat.Sci. 48 (2010)

Mode-Locked Ti:Sapphire (140 fs) 775-830 nm

80 MHz

Electro – Optic Modulator @ 9.8MHz

Variable Delay

Sample Photodiode

RF Lock – in Amp.

Translation Stage

Lens

Iris

Ref.CCD Camera

OPO505-1600 nm

SpectrometerPulseCompressor

Lens Lensl Filter

Signal

time domain thermal reflectance (TDTR) measurement technique

TDTR schematic

Cahill, Rev. Sci. Instrum. 75 (2004) 5119Comp. Sci. Technol. 14 (2010), 2117

pumpprobe

reflective layer

material of interest

quantified interface for conductance

sample architecture for TDTR

substrate

reactive surface [2]

surface energy~25,000 mJ m-2

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

second 1 desorptiontsecond 1 11 / e

RTE

occdesorption

a

vkt

Desorption time is long on (001) planes allowing growth at low deposition rates

orientation control of layered atomic structures

5 nm

(002) oriented [higher rate & ion energy]

(100) oriented [lower rate & ion energy]

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

demonstration of orientation control of MoS2

Log-plot shows all orientations are accessible by selecting appropriate sputtering process

X-ray diffraction data

10 15 20 25 30 35 40

0.25

0.50

0.75

1.00

inte

nsi

ty (

arb

. u

nits

)

2 (degrees)

MoS2(002) 

MoS2(100) 

/intermittent sputtering

Intermittent sputtering for strong 002 orientation

deposit 5 atomic layers

example diffractogram of highly oriented sample

anneal

repeat until desired thickness is obtained

orientation and exposure history dependence on MoS2 thermal conductivity

MoS2

Depiction of Al cap

Al

MoS2 capped with Al in vacuo

10 15 20 25 30 35 400.00

0.25

0.50

0.75

1.00

inte

nsi

ty (

arb

. u

nits

)

2 (degrees)

(002)

(004)

Inconel substrate

Both orientations show k values ~4 x lower than predicted

0 50 100 150 2000.1

1

10

(002) pristine amorphous (002) 48 hour exposure (100) pristine

Th

erm

al C

on

du

cti

vity

(W

m-1K

-1)

Thickness (nm)

002 bulk crystal

5 nm

50 nm pristine MoS2

Phys. Chem. Chem. Phys. 16 (2014) 1008

10 15 20 25 30 35 400

500

1000

1500

2000

inte

nsi

ty (

arb

. u

nits

)

2 (degrees)

MoS2

WS2

TEM of WSe2 filmfilm surface

substrate5 nm

PVD processing of all MoX2  and WX2 TMDs

Identical microstructures under similar conditions (T, P, etc.)

XRD of MoS2 and WS2 films cross-sectional TEM shows basal plane alignment

Pulsed dc with TMD target

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)

Calculation of scattering length by summing scattering sources:

Simulated acoustic phonon dispersion for TMD materialsTEM of WSe2 film

film surface

substrate5 nm

Domain sizes ~ 3-10 nm

scattering at domain boundaries accounts for 10X reduction in thermal conductivity

2

3/13

T

NMB D

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

hot cold

Heat Flow

In plane phonons have high group velocity

Very small phonon group velocity across basal planes dxdTA

dtdQ

/

Step 1: Forces from bonded and non-bonded atomic interactions calculated and verified by simulating vibrational modesStep 2:Thermal conductivity calculated from Fourier Law analysis of steady temperature gradient in the crystal using this equation:

Predicted differences in thermal conductivity due to crystal anisotropy

i

iii lvCV3

1

Ci-spectral heat capacity

ni-group velocity

li-phonon mean free path

simulation results:  in-plane & out-of-plane

Comp.Mat.Sci. 48 (2010)

simulated defect scattering

20 interfaces

6 interfaces

2 interfaces

3 interfaces

4 interfaces

1 interface

Heat Flow

Heat Flow

Heat Flow

Heat Flow

Heat Flow

Heat Flow

Simulated value consistent with 50 W m-1K-1 value  reported by: Sahoo et al. J. Phys. Chem. C 117 (2013) 9042

Phys. Chem. Chem. Phys. 16 (2014) 1008

Tri-layer MoS2

Few-layer graphene 5 Å

Potential to build synthetic superlattices with no consideration of lattice constant

robust transistors

large strain accommodation for flexible electronics

2D molecular sensors with enhanced sensitivity/selectivity

Kis et al.

Naik and Muratore et al.Geim et al.

Yoon et al.

what are 2D TMDs good for?

Muratore       et al.

200 250 300 350 400 450

inte

nsi

ty (

arb

. u

nits

)

Raman shift (cm)-1

MoSe2

MoS230% lattice mismatch

accommodation at interface

Easy growth of multilayers

summary of 2D TMD processing SOTA

a

Solution-based or mechanical exfoliation Chemical vapor deposition

Interflake scattering inhibits application

Contamination and structural changes inhibit 

application

C. Lee et al. ACS Nano 4 2010

Najmaei  et al.Nat. Mater. 12 2013

Najmaei  et al.Nat. Mater. 12 2013

Zhan et al. small 8 2012

van der Zande Nat. Mater. 6 2013

K. Kaasbjerg, PRB 85 (2012)

UHV synthesis for pristine surfaces and interfaces

XPS analysis chamberSynthesis

chamber

Load lock

Composition measured in vacuuo after

growth

After 1 hour exposure to 22 oC air at

15% humidity

240 236 232 228 224 220

binding energy (eV)

240 236 232 228 224 220

binding energy (eV)

MoO3 S

Mo (+4)

Mo (+4)

S

edge oxidation

sputtering without energetic particle damage

0 2 4 6 8 10 12

10

100

1000

10000

100000

inte

nsity

(ar

b. u

nits

)

kinetic energy (eV)

It takes about 8 eV to create a vacancy via sputtering a sulfur atom from MoS21

1Komsa, et al. Phys. Rev. B 88 (2013) 035301

Incident ion energies can be modulated to stay below this threshold

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

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

Raman/PL characterization of PVD MoS2

softeningstiffening

Films demonstrate identical  shifts with thickness as exfoliated materials

FWHM analysis indicate small domain sizes (10s of nm)

524 525 526 550 600 650 700100

1000

10000

100000

inte

nsi

ty (

arb

. u

nits

)

wavelength (nm)

2 layers 4 layers

increasing PL intensity with reduced thickness

Raman PL

360 370 380 390 400 410 4200.00

0.25

0.50

0.75

1.00

inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

3 layers 4 layers 5 layers 6 layers

1 2 3 4 5 6 7

382

384

406

408

410

bulk A1g

A1g

E1

2g

Ram

an s

hift

(cm

-1)

MoS2 layers

bulk E1

2g

Raman frequency shift with thickness

are films really “large area”?

sample architecture

cross-sectional view

300 nm SiO2

450 mm Si                     (1-10 ohm-cm)

n-type/P doped

5 mm

physical isolation

Hall mobility measurements via Van der Pauw technique  over 1 cm

Strong T dependence above qD

Weak T dependence

thin MoS2

increasing domain size via increased thickness

Domain size increase with thickness reduces mobility (maybe)—increased phonon-electron coupling?

Promising development for TE applications

 Z = S2s/k

• 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

summary and conclusions