Band structure and surface properties of 1-4 layers of MoS2

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


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


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


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


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


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


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)


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


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.



1-4ML + bulk, CVD + exfo IV analysis

Simplify this and bring up the multi-scattering theory


LEEM IV ANALYSIS – DIFFERENT ORIGINS AND LAYER DEPENDENCE

Among 1ML CVD samples(magenta curve is exfo)

1-4ML + bulk exfoliated samples


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)


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


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.


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.



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


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?



• Talk about exfo MoS2 after transfer and the comparable quality to CVD?

APS March Meeting 2014 J31.00007 27

Science

Band structure and surface properties of 1-4 layers of MoS2