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Supporting information Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe 2 Monolayers and Bilayers Jun Hong Park §,‡ , Suresh Vishwanath †,#,‡ , Xinyu Liu , Huawei Zhou , Sarah M. Eichfeld , Susan K. Fullerton-Shirey , Joshua A. Robinson , Randall M. Feenstra , Jacek Furdyna , Debdeep Jena †,#,∆ , Huili Grace Xing †,#,∆* , and Andrew C. Kummel §,±* *emails: [email protected], [email protected] § Materials Science & Engineering Program, ± Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093,USA. School of Electrical and Computer Engineering, School of Chemical and Biomolecular Engineering, Department of Materials Science and Engineering, Cornell University, Ithaca, NY, 14850, USA. # Electrical Engineering Department, Physics Department, University of Notre Dame, Notre Dame, IN, 46556, USA. Department Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA.
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Page 1: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Supporting information

Scanning Tunneling Microscopy and Spectroscopy

of Air Exposure Effects on Molecular Beam Epitaxy

Grown WSe2 Monolayers and Bilayers

Jun Hong Park§,‡

, Suresh Vishwanath†,#,‡

, Xinyu Liu∥

, Huawei Zhou‼ , Sarah M. Eichfeld

Ω ,

Susan K. Fullerton-Shirey⊥

, Joshua A. Robinson Ω

, Randall M. Feenstra€, Jacek Furdyna

∥,

Debdeep Jena†,#,∆

, Huili Grace Xing†,#,∆*

, and Andrew C. Kummel§,±*

*emails: [email protected], [email protected]

§Materials Science & Engineering Program, ±Department of Chemistry and Biochemistry, University of

California, San Diego, La Jolla, CA, 92093,USA.

†School of Electrical and Computer Engineering, ‼School of Chemical and Biomolecular Engineering,

∆Department of Materials Science and Engineering, Cornell University, Ithaca, NY, 14850, USA.

#Electrical Engineering Department, ∥Physics Department, University of Notre Dame, Notre Dame, IN,

46556, USA.

€Department Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA.

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ΩDepartment of Materials Science and Engineering, Pennsylvania State University, University Park, PA

16802, USA.

⊥Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, 15213,

USA.

1. RHEED evolution during the MBE growth of WSe2 and post-growth Raman

spectroscopy

Figure S1. RHEED evolution of MBE WSe2 during growth and Raman spectra taken post-growth.

RHEED patterns (a) before WSe2 growth (0 mins), (b) after ~0.25 ML (10 mins), (c) after ~0.75 ML (30

mins) and (d) after ~1.5ML (60 mins) growth of WSe2 are shown. The black arrows indicate the reflected

electron beam. The yellow arrows point to the RHEED from HOPG and the red arrows to the RHEED

from WSe2. (e) Raman spectra of bulk WSe2 and MBE WSe2 under a 488 nm excitation at an incident

power of ~ 760 µW.

Page 3: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Figures S1(a) to (d) show the gradual transition of the RHEED patterns from HOPG (yellow arrows B)

to WSe2 (red arrows C) through an intermediate state where both are simultaneously observed (Fig.

S1(c)). By taking the inverse ratio of the C-A separation over the B-A separation, the lattice constant ratio

of WSe2/HOPG is calculated as ~1.36, which closely matches the value reported for bulk materials: 1.34

at 400 °C1. The RHEED streaks from HOPG and sub monolayer (0.75 ML in Fig. S1 (c)) WSe2 are

observed at the same substrate rotation with each of them retaining their respective relaxed lattice

constants; this is consistent with the as grown film being closely oriented with the underlying substrate

and the growth proceeding by van der Waals (vdW) epitaxy respectively. This is also in good agreement

with previously reported CVD growth of transition metal dichalcodenides (TMDs) on graphene with

rotational alignment2, 3.

Raman spectra shown in Fig. S1 (e) confirm that the grown material is WSe2. All measurements were

performed using a low incident power, ~760 µW. In MBE-grown WSe2, two peaks are observed using

488 nm laser excitation: one at ~251 cm-1 and the other at ~261 cm-1. This is consistent with the Raman

spectra of exfoliated WSe2. The peak at ~251 cm-1 is due to the nearly degenerate A1g and E12g phonons4, 5.

The smaller peak at ~261 cm-1 is attributed to strain due to the growth process4 or a second order LA

phonons mode denoted as 2LA(M)6, 7. Because RHEED shows that the as-grown film is fully relaxed

within the measurement error, the smaller peak at ~261 cm-1 is likely intrinsic to the material arising from

second order processes, therefore denoted as the 2LA(M) peak. Similar Raman spectra have been

observed in CVD WSe2 grown on graphene2. In bulk WSe2 using a 488nm laser excitation, only one peak

is observed at ~252 cm-1 (Fig S1(e)), consistent with literature5. The slight blue shift of ~1 cm-1 of the A1g

and E12g peaks in MBE grown atomically thin layers with respect to bulk is also consistent with expected

shift in A1g peak position in ML WSe28. As expected for the backscattering configuration employed in this

experiment, the E1g peak (~176 cm-1) is absent because the incident beam is parallel to the c-axis9. In the

1.5 ML WSe2 sample, the bilayer (BL) peak around 310 cm-1 is not observed, which is probably due to

the small-size and disconnected BL domains. Similar to previous observations for MBE growth of MoSe2

Page 4: Scanning Tunneling Microscopy and Spectroscopy of Air ...

on substrates including CaF2, epi-graphene and HOPG, where no chemical interaction took place between

TMD and underlying substrates, no chemical intermixing between MBE WSe2 and HOPG is expected10.

2. The nucleation of WSe2 at mono atomic HOPG step edges

WSe2 growth via the step flowing mode can be observed on the mono atomic HOPG step edge, as

displayed in Fig. S2 (a), (b)and (c). In Fig. S2(a), one ML WSe2 flake crosses a monoatomic HOPG step

edge as a large continuous WSe2 layer, and the HOPG step edge is observed through the deposited WSe2

mono layer as depicted by the white arrows in Fig. S2(a) and the orange dashed lines in Fig. S2(b) and (c).

As shown by the line trace A in Fig. S2(b), the height different between ML WSe2 on the upper HOPG

step and ML WSe2 on the lower HOPG step is about 0.41 nm, close to the mono atomic thickness of

carbon, 0.3 nm. it is likely that the increased height of a monoatomic HOPG step edge slightly is

due to residual Se adatoms near the WSe2 domain edges. These Se adatoms can be removed by

annealing at higher temperatures, consistent with the relatively weak binding of excess Se on WSe2.

After the ML WSe2 is grown laterally from the step edge of HOPG to the basal plane of HOPG, the

nucleation of the BL WSe2 is initiated along the HOPG step edge as displayed by the line trace B in Fig.

S2(c). The single triangle shaped BL WSe2 is grown across the ML WSe2/HOPG step edge with a

thickness of about 0.7 nm. Based on the STM images and corresponding line traces, the HOPG step edge

provides preferential nucleation sites for both BL WSe2 and ML WSe2. It is noted that in Fig S2 (a),

although some of Se adlayers are deposited on top of WSe2 BL and ML, most of Se adlayers remain

under the WSe2 BL or ML, indicated with yellow arrows.

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Figure S2. As-decapped WSe2 layer at the monoatomic HOPG step edge. (a) STM image shown

WSe2 flake deposited across the monoatomic HOPG step edge. (b) Line trace corresponded to A in (a). (c)

Line trace corresponded to B in (a).

3. Growth of WSe2 layers on HOPG

Figure S3 illustrates a schematic model of a proposed growth model of WSe2 on HOPG in which two

growth modes are described. First, the broken bonds of the carbon network at step edges provide seeding

nuclei for growth, consistent with the step flow mode11-13. It is hypothesized that during the slow MBE

growth, the dangling bonds on the carbon atoms located at the step edges can trap the rapidly diffusing Se

and/or W atoms thereby seeding sites for the WSe2 growth. In previously reported DFT calculations,

formation of dangling bonds at edges of BL graphene result in localized electron clouds at the graphene

edge12, 14; for single layer graphene, the zip-zag edges have two dangling bonds in a sp3 type geometry;

these will bond to two hydrogen atoms, one above the plane and one below. Previous DFT calculations on

BL graphene shows that these zig-zap edge dangling bonds in adjacent layers overlap and become

chemically reactive14. Furthermore, for MBE growth conditions, there is no hydrogen present; therefore,

these edge states would be available for bonding to W or Se. It is hypothesized that the dangling bonds at

the edges of HOPG can nucleate the growth of WSe2 layers at both top and bottom HOPG planes nearly

simultaneously.

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The nucleation of WSe2 is simultaneously induced on the basal plane of HOPG; this is surprising

because for ideal HOPG, the carbon atoms have sp2 hybridization and no dangling bonds, resulting in an

inert surface. It can be hypothesized that if there are defect sites (e.g. C vacancies) or contaminants on the

basal plane of HOPG, these defects also can act as seeding sites for WSe2 growth. In addition, the flux of

W and Se onto HOPG may be sufficient that the W and Se surface concentrations exceed the critical

concentration to nucleate WSe2 or W2. Consequently, initial nucleation of the WSe2 layer is involved both

at the step edges and basal planes of HOPG; afterwards, the WSe2 layer propagates from nucleated sites to

un-deposited planes of HOPG.

Figure S3. Schematic diagram of growth process of WSe2. The initial nucleation of WSe2 occurs at the

HOPG step edges and then extends to the terraces.

Page 7: Scanning Tunneling Microscopy and Spectroscopy of Air ...

4. Rotation angle calculation from the STM Moiré pattern

The analytical expression for the temperature-dependent lattice constant in Ref.1 is used to extrapolate

the lattice constant of WSe2 at 100K (temperature where STM was performed),1 giving a value of 3.27 Å.

This value is close the value of ~3.2 Å measured from the lattice fringes observed in STM. Fig. S4 (b)

shows the height profile along the line 1 in Fig. S4 (a)

Figure S4. Lattice parameter of WSe2 from STM line scan. (a) STM image of the surface with a line

showing where the line scan was taken. (b) Line scan of the lattice fringes.

From the STM image, line scans were also taken to measure the Moiré lattice constant. Line scans

were taken along lines A and B in the figure below and the average of the difference between peaks (as

shown in Fig. S5 (b)) was measured to be ~10.46 Å. The angle θ between the Moire and the WSe2 lattice

was measured to be 7° (Shown in Fig. S5 (a)).

Page 8: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Figure S5. Measurements on Moiré pattern. (a) STM image showing the θ of 7° between the Moiré

and the WSe2 lattice fringes. Line A and B are shown the path of the line scan shown in (b). (b) Line

scans corresponding to lines A and B in Fig S5 (a) with vertical lines denoting the peak positions used to

calculate the Moiré pattern lattice constant.

Below is a similar analysis as that described in Ref 16 to model the Moiré pattern for graphene on h-

BN for the WSe2/graphene heterostructure15.

The lattice mismatch between graphene and WSe2,

1

where a and b are the in-plane lattice constants for graphene and WSe2 respectively.

The reciprocal lattice vectors,

2

1,0

Page 9: Scanning Tunneling Microscopy and Spectroscopy of Air ...

2

1 + cos,

where φ is the rotation misalignment between the graphene and WSe2 lattices.

The reciprocal lattice vector for the moiré pattern,

The wavelength of the moiré pattern,

2

||

The misoriented angle (between the moiré and the WSe2 lattice fringes),

! "#$

%& + 1

For the modeling, the lattice constant of graphene is taken to be 2.46 Å and a range of lattice constant

differences between graphene and WSe2 is simulated. For the extrapolated lattice constant of WSe2 of

3.27 Å the lattice mismatch δ is 0.3. With these parameters for a θ of 7° the λ is 10.56 Å and φ is 2.12°. λ

of 10.56 Å matches with in the range of measurement and fitting error with the measured λ of ~10.46 Å.

In reality, graphite expands when cooled and WSe2 could have a smaller lattice constant than the

extrapolated value of 3.27 Å at 100 K. Therefore, the simulation has been performed for a range of lattice

constant mismatches (dotted line). Given that WSe2 and MoSe2 have very close lattice constants and a

spread of ~4° is seen for rotational orientation of MoSe2 on graphene16, it is not surprising to see a

rotational misorientation of ~2° for WSe2 on HOPG.

Page 10: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Figure S6. Moiré pattern simulation: Moiré pattern wavelength and its rotation angle with respect to

WSe2 as a function of mis-orientation angle between graphene and WSe2 (several lattice mismatch values

are presented). The Solid blue drop down lines show the parameters consistent with the measured

parameters.

5. STM and STS on Bare HOPG.

Before deposition of the WSe2 on the HOPG, STM and STS measurements were taken on bare HOPG.

HOPG samples were cleaned by exfoliation in ambient conditions; afterwards, the samples were

transferred into the UHV chamber. To remove hydrocarbon, H2O or O2, The HOPG substrates were

gradually heated to 823 K over 30 mins, then held for 3 hrs in the UHV chamber (< 2 x 10-10 torr). All

STM and STS measurements were obtained at 100 K. Figure S7(a) displays the bare HOPG surface,

including the step edges and the terraces. As shown in the line trace in Fig. S7(b), the height of the step

edge is about 0.35 nm, consistent with mono carbon atomic thickness. Spatial STS measurements were

also taken along the white line to measure the varying band structure between the step edge and the

Page 11: Scanning Tunneling Microscopy and Spectroscopy of Air ...

terrace, as shown in Fig. S7(c). However, because the HOPG has zero band gap, it is difficult to detect

noticeable differences in STS.

Figure S7. STM image and spatial STS at the HOPG step edge. (a) STM image shown the HOPG step

edge. (b) line trace corresponded white line in (a). (c) Spatial STS measured along white line in (a),

including the HOPG terrace and the step edge.

The existence of increased localized density of states (DOS) at the step edge can be confirmed by bias-

dependent STM imaging, as shown in Fig. S8. It is noted that although all of Fig. S8(a) and (b) were

imaged at the same location on the HOPG step edge, the filled state STM images in Fig. S8(b) are

distorted by thermal drift. In the empty-state STM images obtained at + 0.5 V and + 1.0 V, the bright rims

are observed along the mono-atomic HOPG step edges, confirmed by corresponding line traces. This

observation of the bright rims is consistent with previous STM studies at the graphite edge17, 18. The

Page 12: Scanning Tunneling Microscopy and Spectroscopy of Air ...

brightness of the rims is enhanced in the STM image recorded at + 1.0 V, comparing with the STM image.

However, this brightness of rim is weaker or disappeared in the filled state STM images, as shown in Fig

S8(b). Comparing this observation of bright rims of the HOPG step edges with bright rims of the WSe2

edges, the bright rims of the HOPG step edge are shown in the empty state STM images, while the bright

rims of the WSe2 edges are displayed in the filled state STM images.

Figure S8. Bias-dependent STM images of the HOPG step edge. All of STM images were obtained

at the same HOPG step edge. (a) The empty state STM images obtained at + 0.5 and + 1.0 V. (b) The

Filled state STM images obtained at - 0.5 and - 1.0 V.

6. STM and STS of WSe2 surface exposed for 1 week, after annealing at 723 K for 30 mins.

WSe2 surface exposed to air for 1 week is stable and constant, even after annealing at 723 K for 30

mins. As shown in Fig. S9(a), after annealing at 723 K for 30 mins, no air-induced adsorbates are

observed on the WSe2 terraces. In figure S9(b), the STS of the WSe2 surface exposed to air for 1 week

Page 13: Scanning Tunneling Microscopy and Spectroscopy of Air ...

and annealed at 723 K for 30 mins shows a band gap of 2.09 ± 0.02 eV nearly identical to as decapped

WSe2 ML.

Figure S9. STM image and STS of air exposed WSe2 for 1 week and annealed at 723 K for 30 mins.

(a) STM image shown WSe2 surface exposed to air for 1 week and annealed at 723K for 30 mins. (b) STS

of ML WSe2 exposed to air for 1 week and annealed at 723K for 30 mins.

7. Large-area STM image shown air exposed WSe2 surface for 9 weeks.

Figure S10 displays a large-area, empty-state STM image of air-exposed WSe2/HOPG for 9 weeks.

This sample is annealed at 773 K for 1 hr. A large flake of WSe2 ML is deposited across the HOPG step

edge forming saw-tooth shape, while relatively smaller flakes are deposited on the terrace of HOPG.

Therefore, WSe2 layer coverage is about 0.5~0.6 ML on HOPG. The BL WSe2 are nucleated nearly at the

centers of WSe2 ML flakes as shown yellow arrows. Air exposure of this WSe2/HOPG sample induces the

oxidation of both WSe2 ML and BL edges, as shown by the bright protrusions.

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Figure S10. Large area STM image shown WSe2 layer exposed to air for 9 weeks.

8. Oxidation of WSe2 terrace by O3 exposure.

Figure S11. Oxidation of WSe2 terrace via O3 exposure for 1 min. (a) STM image of O3 exposed WSe2

after annealing at 723 K for 30 mins. (b) STS of the O3 induced oxidation state circled in Fig. S11(a).

Page 15: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Although ambient air does not induce oxidation of WSe2 terraces, WSe2 terraces can be oxidized by

O3 exposure. As shown in Fig S11(a), after exposure of O2 + O3 (0.6 Giga Langmuir ) on WSe2/HOPG at

300 K and subsequent annealing at 723 K for 30 min, the presence of the large states (white circles) are

induced on the WSe2 terraces, consistent with the high thermal stability of the O3 induced reaction

products. These new states have a larger band gaps (2.79 ± 0.04 eV) than air exposed WSe2 ML (2.15 ±

0.03 eV) and air-induced adsorbates (1.32 ± 0.02 eV), consistent with oxide formation. Further study of

O3 exposure on WSe2 will be performed to elucidate the oxidation process and the electronic effect on the

WSe2 terrace.

9. Growth on epitaxial graphene

To show that growth of WSe2 on graphene is identical to the growth on HOPG, growth was also

completed on epitaxial graphene. 2 samples were grown for different growth durations resulting in less

than 1 ML (Fig. S12 (a)) and greater than a ML, identical coverage to the sample used for STM (Fig.

S12(b)). AFM scans taken in an inert environment after transporting the sample shortly in air are shown

in Fig. S12.

Page 16: Scanning Tunneling Microscopy and Spectroscopy of Air ...

Figure S12. AFM of WSe2 growth on SiC. (a) <1ML (~0.5ML) equivalent film deposited (b) >1ML

(~1.5ML) equivalent film deposited.

Reference

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