Microsoft Word - 2074130_File000003_34718897.docxof Air Exposure Effects on Molecular Beam Epitaxy
Grown WSe2 Monolayers and Bilayers
Jun Hong Park §,‡
,
,
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.
16802, USA.
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.
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 E1 2g 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 E1 2g peaks in MBE grown atomically thin layers with respect to bulk is also consistent with expected
shift in A1g peak position in ML WSe2 8. 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
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.
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.
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.
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)).
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,
1 + cos,
where φ is the rotation misalignment between the graphene and WSe2 lattices.
The reciprocal lattice vector for the moiré pattern,

2
||
! "#$
%& + 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.
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
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
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
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.
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).
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.
Figure S12. AFM of WSe2 growth on SiC. (a) <1ML (~0.5ML) equivalent film deposited (b) >1ML
(~1.5ML) equivalent film deposited.
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