band structure and surface properties of 1-4 layers of mos2
TRANSCRIPT
Studying
Surface Structure
of MoS2 and WSe2 Using
SPE-LEEMSPEAKER: PO-CHUN YEH (FIGO)
ADVISOR: PROF. R. M. OSGOOD
“SPE-LEEM” = Spectroscopic Photo-Emission and Low
Energy Electron Microscopy
1APS March Meeting 2014 J31.00007
J31. 00007: SPE-LEEM Studies on the Surface and Electronic Structure of 2-D TMDC (Part 1/2)
MANY THANKS!
Also, credits and thanks to:Jonathan Liou, XiaoXiao Zhang, and YuMeng You
Jurek Sadowski
DaTong Zhang
Arend van der Zande
Abdullah Al-Mahboob
Prof. James Hone
Prof. Irving Herman
Daniel A. ChenetProf. R. M. Osgood
WenCan Jin
Jerry Dadap
Nader Zaki Peter Sutter
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WHY WE WANT TO STUDY THIS?
• Spin-orbit coupling
• It has a bandgap! • Photoluminescence
• WSe2: both p- and n- type FET are fabricated
Strong PL in monolayer MoS2 Nano. Lett. 10, 1271-1275 (2010)
High quantum efficiency 1000 times stronger PL in ML
WS2, WSe2 than in bulkACS Nano 7 (1), 791–797 (2013)
Direct bandgap in ML Thin, flexible devices E.g. Li-ion battery and transistors
Nano Lett., 11 (9), pp 3768–3773 (2011)
Chem. Commun. , 47, 4252-4254 (2011)
Enhanced spin lifetimes Large spin Hall angles VBM S-O splitting up to 456meV in WSe2
PRB 84, 153402 (2011)
Nano Lett. 13 (7), pp 3106–3110 (2013)
Nano Lett. 13(5), pp 1983–1990 (2013)
http://meetings.aps.org/Meeting/MAR14/Session/D51.1
Nano Lett., 2012, 12 (7), pp 3788–3792
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Our aim: Study sample quality: CVD vs. exfoliated MoS2
Find an ideal substrate for studying MoS2 EM and PEDetermine the surface corrugation and structureMeasure the electronic structure directly
WHY SPE-LEEM?Micron-size spot, Direct band structure, fast real time imaging, large area mapping, UHV, clean, surface doping, depth profile.
NSLS I NSLS II
1. LEED – Crystal orientation.2. LEEM – Surface corrugation, quality probe.3. ARPES – Energy resolved k space mapping.4. XPEEM – study ionization, core level orbitals.
Now Future
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All in one!
SPE-LEEM: HOW DOES IT WORK?
Main Chamber
Objective
IlluminationColumn
Imaging Column
Sample
Detector
EnergyAnalyzer
Electron Gun
UV beamSynchrotron
Beam Splitter
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SPE-LEEM - PERSPECTIVES
ELMITEC SPLEEM
Energy Analyzer
Manipulator.Grounded.(High voltage @ 2kV)
Preparation chambers
Photon energy: 15-150eV Good energy resolution: 100meV Good spatial resolution: 8nm Large mapping area: d = 100µm
Thermal coupler
Sample holder
d ~ 10mm
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LEEM – EXFOLIATED MoS2 ON SiO2
2ML1ML
10 µm
a. b.
10 µm
Charging hinders LEEM to extract information on:
• Depth profile
• Layer number
• Surface corrugation/defects
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Charged up in 5min!
5 150
10 µm
Photoluminescence(Courtesy of the
Hone group)
20 µm
Grain Boundaries
LEEM – CVD MoS2 ON SiO2
Similar charging effect hinders imaging
Precise surface doping: SEAS Getter dispenser, 730C, Y = 0.2 mg/cm, adhesion rate: 0.7
Surface corrugation Periodicity: sub-micron
Potassiumdoping
LEEM imageOptical image
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In collaboration with the Hone group
APS March Meeting 2014 J31.00007
Nature Materials 12, 554–561 (2013)
Need a conducting substrate! Use native oxide silicon
SAMPLE PREPARATION - TRANSFER
Si
Flakes on PMMA layer
Water soluble PVA layer
DI water
Scoop
PMMA
Sample on PMMA film suspended on scoop
Si, patterned
Wet transfer processfor exfoliated MoS2
Rinsed in Acetone for 24hr +
Anneal at 350C in UHV chamber for 12hr +
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In collaboration with the Herman group
APS March Meeting 2014 J31.00007
TRANSFER MoS2 onto Silicon SUBSTRATE
10 µm
15 µm
10 µm
15 µm
10 µm
15 µm
With LEEM, flakes are visible on native oxide silicon substrate Transfer preserves the sample morphology Wrinkles (growth-induced, strain) removed Doping changes work function a lot, but a little to Fermi level
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Single domain
Mirror-Twin*
domains
*Ref: Nature Materials 12, 554–561 (2013)
LEED ANALYSIS1ML 2ML 3ML 4ML
LEED (00) spot Correlation length 𝜉 (in) Roughness 𝛼 (in) Substrate scattering (ex)
Bulk
Compare to Graphene
LEED 1st order spots Correlation length 𝜉 (in) Roughness 𝛼 (in) Substrate scattering (ex)
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00 spot42eV
𝐹𝑊𝐻𝑀 ∝ 𝑘⊥𝜔1/𝛼 /𝜉
Exfoliated on SiO2, at 42eV FWHM in Å-1 1st order
Graphene 1ML 1.20
Graphene 2ML 0.51
Graphene 3ML 0.37
MoS2 1ML, exfoliated 0.34 0.30
MoS2 1ML, CVD 0.52 0.34
MoS2 1ML, transferred on Si 0.67 0.50
A SNEAK PEEK ON MoS2 ARPES
Monolayer MoS2
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Phys. Rev. Lett. 111, 106801 (2013)
Band structure EDCs MDCs
4:30 PM Session J31. 00009 by WenCan Jin.
ON GOING - WSe2 STUDIES
Optical image
5µm
20µm
1ML2ML
3ML2ML
LEEM image WSe2 transferred on Si substrate. Contrast between layers. Wrinkles and cracks.
LEED 1-3ML
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CONCLUSION
SPE-LEEM is a strong and versatile tool for layered material.
LEEM gives good depth profile, layer number, and surface images if there is no charging effect.
LEED reflects sample quality via careful analysis on spot width; it’s layer dependent.
CVD MoS2 has a comparable quality to exfoliated MoS2.
Transfer onto native oxide Si substrate made doing ARPES possible. It also removes the strain-induced wrinkles.
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Preview – Part 2/2ARPES result of MoS2 1-4ML and bulk.
Substrate-induced compression and effective mass. Session J31. 00009 by WenCan Jin.
APS March Meeting 2014 J31.00007
J31. 00007: SPE-LEEM Studies on the Surface and Electronic Structure of 2-D TMDC (Part 1/2)
1-4ML + bulk, CVD + exfo IV analysis
Simplify this and bring up the multi-scattering theory
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LEEM IV ANALYSIS – DIFFERENT ORIGINS AND LAYER DEPENDENCE
Among 1ML CVD samples(magenta curve is exfo)
1-4ML + bulk exfoliated samples
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LEED ANALYSIS1ML 2ML 3ML 4ML
LEED (00) spot Surface corrugation (in) Number of defects (in) Substrate (ex)
Bulk
Compare to Graphene
Exfoliated on SiO2, at 42eV FWHM in Å-1 1st order
Graphene 1ML 1.20
Graphene 2ML 0.51
Graphene 3ML 0.37
MoS2 1ML 0.34 0.30
MoS2 1ML, CVD 0.52 0.34
MoS2 1ML, transferred 0.67 0.50
LEED 1st order spots Surface corrugation (in) Number of defects (in) Substrate (ex)
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ARPES BY SPE-LEEM
ARPES in SPE-LEEMBulk WSe2
Normal incident light;without out-of-plane polarization
Photoelectron k-spacemapping
ARPES: Angle-resolved photoemission spectroscopy Photons in, electrons out. Direct measurement of band dispersion
ML WSe2With DFT LDA bands
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ARPES AND CURVATURE ANALYSIS Uses 2nd order derivatives of the ARPES intensity mapping. Separates band dispersion from linear background and detector artifacts.
VBM transits from K to Γ.A strong evidence for indirect to direct bandgap transition.
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Low
2ML 3ML Bulk1ML
High
EFFECTIVE MASS AT K POINT
a: experimental lattices, ref Phys. Rev. B 85 (2012). b: optimized lattices from calculation
Hole effective mass agrees well with the calculations, for both 1ML and 2ML
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Thickness Electron Mass Hole Mass Method ReferenceLattice
Constant
ML N/A 0.435 LDA Our results. 3.28
ML N/A 0.564c Experiment Our results. 3.28
ML 0.53 0.52 DFT-GW-BSEA. Ramasubramanim,
PRB 2012 3.32
ML 0.29a/0.26b 0.34a/0.33b DFT-GW-BSEHongliang Shi, PRB
2013 3.286
ML 0.23 0.41 LDA A. Kumar, EPJB 2012 3.282
ML 0.19 0.4 FLAPW-GGA W. S. Yun. PRB 2012 3.286
2ML N/A 0.545 LDA Our results. 3.28
2ML N/A 0.432 Experiment Our results. 3.28
2ML 0.3 0.49 LDA A. Kumar, EPJB 2012 3.282
2ML 0.3 0.3 FLAPW-GGA W. S. Yun. PRB 2012 3.286
CONCLUSION
SPE-LEEM is a strong and versatile tool for layered material.
LEEM gives good depth profile, layer number, and surface images.
LEED reflects sample quality via careful analysis on spot width; it’s layer dependent.
Transfer onto native oxide Si substrate made doing ARPES possible. It also removes the strain-induced wrinkles.
CVD is as good as exfoliated MoS2.
Si serves as a
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Preview – Part 2/2ARPES result of MoS2 1-4ML and bulk. Substrate-induced compression and effective mass. Session J31. 00009 by WenCan Jin.
APS March Meeting 2014 J31.00007
J31. 00007: SPE-LEEM Studies on the Surface and Electronic Structure of 2-D TMDC (Part 1/2)
Figure 2
Angle‐integrated photoemission spectra ofexfoliated monolayer WSe2 extracted from high-‐symmetry directions. (determine EF)
At 33eV, the cross section between W5d and Se 4p has an order ofmagnitude difference.
(a) (c)(b)
Γ
K
K’M
High symmetry
points
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CVD MOS2 – A STAR
MoS2 on SiO2 with K doping24APS March Meeting 2014 J31.00007
diffractioncontrast
sample
contrastaperture
objective
[0,0]
[h,j]
SURFACE STRUCTURE
Au+O/Rh(110)
quantum sizecontrast
d
FILM THICKNESS
Co/W(110)
geometricphase contrast
MORPHOLOGY
Mo(110)
WHAT CAN BE MEASURED WITH LEEM?
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• Talk about exfo MoS2 after transfer and the comparable quality to CVD?
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