SPE-LEEM Studies of MoS2 and WSe2 Surface Structure
1. Studying
Surface Structure
of MoS2 and WSe2 Using
SPE-LEEM
SPEAKER: 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)
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. Chenet
Prof. R. M. Osgood
WenCan Jin
Jerry Dadap
Nader Zaki Peter Sutter
2APS March Meeting 2014 J31.00007
3. 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 bulk
ACS 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
3
Our aim:
Study sample quality: CVD vs. exfoliated MoS2
Find an ideal substrate for studying MoS2 EM and PE
Determine the surface corrugation and structure
Measure the electronic structure directly
4. 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
4
All in one!
5. SPE-LEEM: HOW DOES IT WORK?
Main Chamber
Objective
Illumination
Column
Imaging
Column
Sample
Detector
Energy
Analyzer
Electron Gun
UV beam
Synchrotron
Beam Splitter
5APS March Meeting 2014 J31.00007
6. 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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7. LEEM – EXFOLIATED MoS2 ON SiO2
2ML
1ML
10 µm
a. b.
10 µm
Charging hinders LEEM to extract information on:
• Depth profile
• Layer number
• Surface corrugation/defects
7APS March Meeting 2014 J31.00007
Charged up in 5min!
8. 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
Potassium
doping
LEEM imageOptical image
8
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
9. 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 process
for exfoliated MoS2
Rinsed in Acetone
for 24hr +
Anneal at 350C in UHV
chamber for 12hr +
9
In collaboration with the Herman group
APS March Meeting 2014 J31.00007
10. 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
10APS March Meeting 2014 J31.00007
Single
domain
Mirror-
Twin*
domains
*Ref: Nature Materials 12, 554–561 (2013)
12. A SNEAK PEEK ON MoS2 ARPES
Monolayer MoS2
12
Phys. Rev. Lett. 111, 106801 (2013)
Band structure EDCs MDCs
4:30 PM Session J31. 00009 by WenCan Jin.
13. ON GOING - WSe2 STUDIES
Optical image
5µm
20µm
1ML
2ML
3ML
2ML
LEEM image WSe2 transferred on Si substrate.
Contrast between layers.
Wrinkles and cracks.
LEED 1-3ML
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14. 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.
14
Preview – Part 2/2
ARPES 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)
15. 1-4ML + bulk, CVD + exfo IV analysis
Simplify this and bring up the multi-scattering theory
15APS March Meeting 2014 J31.00007
16. 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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17. LEED ANALYSIS
1ML 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)
17APS March Meeting 2014 J31.00007
18. ARPES BY SPE-LEEM
ARPES in SPE-LEEM
Bulk WSe2
Normal incident light;
without out-of-plane polarization
Photoelectron k-space
mapping
ARPES: Angle-resolved photoemission spectroscopy
Photons in, electrons out.
Direct measurement of band dispersion
ML WSe2
With DFT LDA bands
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19. 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
20. 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
20
Thickness Electron Mass Hole Mass Method Reference
Lattice
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-BSE
A. Ramasubramanim,
PRB 2012 3.32
ML 0.29a/0.26b 0.34a/0.33b DFT-GW-BSE
Hongliang 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
21. 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
21
Preview – Part 2/2
ARPES 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)
22. Figure 2
Angle‐integrated photoemission spectra of
exfoliated monolayer WSe2 extracted from high-
‐symmetry directions. (determine EF)
At 33eV, the cross section between W
5d and Se 4p has an order of
magnitude difference.
(a) (c)(b)
Γ
K
K’
M
High symmetry
points
27. • Talk about exfo MoS2 after transfer and the comparable quality to CVD?
APS March Meeting 2014 J31.00007 27
Notes de l'éditeur
Hi, everyone. My name is Po-Chun. I am from Prof. Osgood’s lab. Today, I will be talking about (title).Here, SPE-LEEM is stands for …
(Fast) First, I would like to thank our collaborators from Columbia and from Brookhaven National lab.
Why we want to study MoS2 and WSe2? We already know that graphene is the super star now and has drawn a lot of attention. Why we want to study this?
Because compare to graphene, MoS2 and WSe2 have a bandgap, which makes many application and devices possible such as Li-ion battery and transistors. Also, they have strong PL. In ML. Moreover, a strong spin-orbit coupling and splitting makes it valuable in Spintronics and Valleytronics applications.
Last-but-not-the-least, people have fabricated both n type and p type FET by using WSe2, which can be make into p-n junction devices.
In our study, we aim to …
Then, why choose SPE-LEEM? It has many merits, such as … Basically, it’s a combination of XX, XX, XX, and XX. For example, …(3 examples) To sum up, it’s an all-in-one system. [Cut](It is located in NSLS I in BNL, and will be upgraded and moved to NSLS II in the near future.)
*XPEEM. laterally resolved version of x-ray photoelectron spectroscopy (XPS)
Here’s a cartoon to show you how it works. Here is the electron beam. The electrons make a normal incidence on the sample and are collected for imaging. For ARPES, the UV comes from synchrotron and reaches the sample surface normally. Photoemission happens and we collect the electrons for mapping.
(Fast) Here’s a layout for SPELEEM.
In our sample preparation, part of it requires using wet transfer process.
CVD MoS2 was transferred via a PDS-based lift-off process.
Shape preserved. Surface is clean and flat. The transfer was successful.
The web-like wrinkles disappeared. *Workfunction of Si drops 2.1eV to the level of MoS2. The Fermi level shifts very little.
Annealing can swipe away the K ions. The workfunction restores to pre-doping level. This is a reversible process.
Maybe put the chart on the next slide and add some more nice graphs?
(Fast) Here’s a preview of ML MoS2 band dispersion.
(Fast) Ok, so now I will move to our current study on WSe2. Here’s the optical, LEEM, and LEED image of exfoliated WSe2. The transfer was successful.
(Stay on this page for Q&A)
*Only orbits from in-plane polarization survives. The interesting VBM is from W 5d orbits.
Here it’s a curvature analysis of the bands using the second derivative method.
These are the WSe2 band dispersion of 1-3ML and bulk; the red lines are from DFT calculation.
I just want to make one point here (click out the animation) The highest energy point of VB is at gamma in ML and shifts to K in 2ML and more. This VBM transition from K to G is a strong evidence for indirect to direct bandgap transition.
(Quick) This is our study of the effective mass at K point. The hole effective mass from our measurement agrees well with the theoretical calculations, in ML and bilayer cases.