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Mechanical, thermal, and electronic properties of 
transition metal dichalcogenides 
Christopher Muratore 
University of D...
key co-workers (mechanical) 
AFRL co-workers 
Voevodin Zabinski Hu Bultman Safriet 
External collaborators 
Aouadi 
Southe...
key co-workers (thermal and electronic) 
Vikas Varshney-MD simulations 
Jamie Gengler—laser spectroscopy (TDTR measurement...
tribology: study of contact interfaces in relative motion 
(friction and wear of materials) 
Wear of stainless steel 
(col...
interferometric analysis of wear tracks during sliding tests 
c. wear track analysis (adaptive 
nanocomposite) 
b. frictio...
sensitivity of graphite to ambient atmosphere 
(Ramadanoff & Glass, Trans. AIEE, 1944) 
Laboratory testing to accompany fl...
sensitivity of graphite to ambient atmosphere 
(Ramadanoff & Glass, Trans. AIEE, 1944) 
Laboratory testing to accompany fl...
sensitivity of graphite to ambient atmosphere 
(Ramadanoff & Glass, Trans. AIEE, 1944) 
Laboratory testing to accompany fl...
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 fl...
nanocomposite materials with temperature adaptive properties
an overarching materials science dilemma: linking performance 
Existing and future 
aircraft are loaded 
with mission crit...
results available from prior in situ macroscopic tribology studies 
Low temperature 
lubricant (MoS2) 
-Raman studies 
-co...
interferometric and spectroscopic analysis of 
interfacial films through wear counterpart 
a. instrument b. transfer film ...
results available from prior in situ macroscopic tribology studies 
Low temperature 
lubricant (MoS2) 
-Raman studies 
-co...
preparation of contact pair cross-section for TEM analysis 
FIB cutting 
applied load 
P 
Ga+ 
Ga+ 
Ga+ 
Ga+ 
Sample 
1) r...
5nm 
HRTEM of sliding contact interface 
wear counterpart 
randomly 
oriented film 
5 nm 
-atomic scale reorientation and ...
Interactive ISS experiments for in situ characterization of 
materials in space environments 
Test 
apparatus 
NASA Image ...
knowledge gaps remaining with previously 
demonstrated in situ techniques 
-Raman studies 
-surface chemistry of coating l...
measurements during tests in diverse 
environments allow instantaneous 
identification of surface chemistry to reveal: 
sa...
TiCN: interesting but difficult 
(low Raman intensities) 
objective: develop an 
understanding of TiCN run 
in process usi...
0 cycles 515 cycles 1035 cycles 
1200 1400 1600 1800 
Raman shift (cm-1) 
1200 1400 1600 1800 
Raman shift (cm-1) 
0 1000 ...
carbon hydrogenation induced by wear in humid air 
2900 3000 3100 3200 
Raman shift (cm-1) 
2900 3000 3100 3200 
Raman shi...
complimentary observations of transfer film during sliding 
on TiCN at 25% RH using NRL technique 
100 
80 
60 
40 
20 
0 ...
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 ...
Ann. Rev. Mat. Res. 39 (2010) 
environmentally adaptive nanocomposites
catalytic tribo-oxidation at elevated temperatures 
YSZ-20%Ag-10%Mo-8%MoS2 
MoO 
Ag2MoO 4 
-1 
MoO 
Mo 
O 
Mo 
O 
MoO 
Mo ...
Ann. Rev. Mat. Res. 39 (2010) 
environmentally adaptive nanocomposites
surprisingly low thermal conductivity for MoS2 
MEMS heater device 
Free-standing 
MoS2 
ribbon 
Very steep thermal gradie...
simulation results: in-plane & out-of-plane 
dQ dt 
 
Very small phonon group velocity across basal planes dx 
hot cold 
...
Mode-Locked 
Ti:Sapphire (140 fs) 775-830 nm 
80 MHz 
Electro – Optic Modulator 
@ 9.8MHz 
Variable Delay 
RF Lock – in Am...
orientation control of layered atomic structures 
(100) oriented 
[lower rate & ion energy] 
(002) oriented 
[higher rate ...
demonstration of orientation control of MoS2 
X-ray diffraction data 
Log-plot shows all orientations 
are accessible by s...
orientation and exposure history dependence on 
MoS2 thermal conductivity 
MoS2 
Depiction of Al cap 
Al 
MoS2 capped with...
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 
5...
manipulating Slack parameters for k reduction: role 
of film structure and composition 
N = 6 for all compounds 
g = 2 
me...
scattering at domain boundaries accounts for 
10X reduction in thermal conductivity 
Simulated acoustic phonon dispersion ...
simulation results: in-plane & out-of-plane 
dQ dt 
 
Very small phonon group velocity across basal planes dx 
hot cold 
...
simulated defect scattering 
1 interface 
2 interfaces 
3 interfaces 
4 interfaces 
6 interfaces 
20 interfaces 
Heat Flow...
Naik and Muratore 
Geim et al. et al. 
Tri-layer 
MoS2 
Few-layer 
graphene 5 Å 
robust transistors 
Potential to build sy...
summary of 2D TMD processing SOTA 
Solution-based or mechanical exfoliation Chemical vapor deposition 
C. Lee et al. ACS N...
UHV synthesis for pristine surfaces and interfaces 
XPS analysis 
Synthesis chamber 
chamber 
Loa 
d 
lock 
Composition 
m...
sputtering without energetic particle damage 
0 2 4 6 8 10 12 
100000 
10000 
1000 
100 
10 
intensity (arb. units) 
kinet...
uniform application of TMD films over large areas 
Growth 
<250 oC 
Hybrid technique for evaluating uniformity over 
large...
Raman/PL characterization of PVD MoS2 
stiffening softening 
1.00 
0.75 
0.50 
0.25 
0.00 
Raman frequency shift with thic...
are films really “large area”? 
Hall mobility measurements via Van der Pauw technique over 1 cm 
sample 
architecture 
5 m...
increasing domain size via increased thickness 
Domain size increase with thickness reduces mobility 
(maybe)—increased ph...
summary and conclusions 
• Just like the best tribological coatings, scalable 2D TMD synthesis accessible by physical 
vap...
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Mechanical, thermal, and electronic properties of transition metal dichalcogenides.

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).

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Mechanical, thermal, and electronic properties of transition metal dichalcogenides.

  1. 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. 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. 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. 4. tribology: study of contact interfaces in relative motion (friction and wear of materials) Wear of stainless steel (collaboration with Sawyer, University Florida)
  5. 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. 6. sensitivity of graphite to ambient atmosphere (Ramadanoff & Glass, Trans. AIEE, 1944) Laboratory testing to accompany flight tests conducted in Areas A & C
  7. 7. sensitivity of graphite to ambient atmosphere (Ramadanoff & Glass, Trans. AIEE, 1944) Laboratory testing to accompany flight tests conducted in Areas A & C
  8. 8. sensitivity of graphite to ambient atmosphere (Ramadanoff & Glass, Trans. AIEE, 1944) Laboratory testing to accompany flight tests conducted in Areas A & C
  9. 9. sensitivity of graphite to ambient atmosphere (Ramadanoff & Glass, Trans. AIEE, 1944)
  10. 10. sensitivity of graphite to ambient atmosphere (Ramadanoff & Glass, Trans. AIEE, 1944) Laboratory testing to accompany flight tests conducted in Areas A & C
  11. 11. nanocomposite materials with temperature adaptive properties
  12. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 26. Ann. Rev. Mat. Res. 39 (2010) environmentally adaptive nanocomposites
  27. 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. 28. Ann. Rev. Mat. Res. 39 (2010) environmentally adaptive nanocomposites
  29. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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