supporting information growth of 2h stacked wse2 …supporting information growth of 2h stacked wse2...
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Supporting Information
Growth of 2H Stacked WSe2 Bilayers on Sapphire
Ali Han1, Areej Aljarb1, Sheng Liu2, Peng Li1, Chun Ma1, Fei Xue1, Sergei Lopatin3, Chih-Wen
Yang1, Jing-Kai Huang,1,4 Yi Wan1, Xixiang Zhang1, Kuo-Wei Huang1, Qihua Xiong5, Vincent
Tung1,6*, Thomas D. Anthopoulos1*, Lain-Jong Li1,4*
1Physical Sciences and Engineering Division (PSE), King Abdullah University of Science and
Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
2Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
Nanyang Technological University, Singapore 637371
3King Abdullah University of Science and Technology (KAUST), Core Labs, Thuwal, 23955-
6900, Kingdom of Saudi Arabia
4School of Materials Science and Engineering, University of New South Wales, Sydney, NSW
2052, Australia.
5Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
Nanyang Technological University, Singapore 637371. MajuLab, CNRS-UNS-NUS-NTU
International Joint Research Unit, UMI 3654, Singapore 639798. NOVITAS, Nanoelectronics
Center of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological
University, Singapore 639798
6Molecular Foundry Division, Lawrence Berkeley National Lab, Berkeley 94720, USA
* E-mails: [email protected]; [email protected]; [email protected];
Electronic Supplementary Material (ESI) for Nanoscale Horizons.This journal is © The Royal Society of Chemistry 2019
Figure S1. Schematic illustration of the CVD setup and the relative position between WO3, Se and
substrate. The distance between Se powder and WO3 powder is ~25 cm.
Figure S2. Optical images for the WSe2 growth by CVD method. The mass amount of WO3 is
0.3g, while the amount of Se powder is increased. The high-purity of H2/Ar is as the carrier gas
with a fixed flow rate of 5/65 sccm/sccm. The TSe (temperature of Se) is maintained at 250 oC
while TWO3 (the temperature of WO3) is kept at 895 oC. The growth pressure of the furnace is 10
torr for the whole CVD growth. The growth time is 15 mins.
Figure S3. Optical images for the WSe2 growth by CVD method. The mass amount of WO3 is
0.3 g. The high-purity of H2/Ar is as the carrier gas with a fixed flow rate of 5/65 sccm/sccm. a-b,
The amount of Se powder is 5.5 g, TSe = 250 oC and TWO3 = 890 oC; c-d, The amount of Se powder
is 5.0 g, TSe = 260 oC and TWO3 = 895 oC; e-f, The amount of Se powder is 5.5 g, TSe = 250 oC and
TWO3 = 900 oC.
Figure S4. a, Left: optical micrograph of cloud bilayer WSe2 crystal with monolayer WSe2 as
reference; Right: The corresponding SHG mapping intensity obtained by pixel-to-pixel spatial
scanning on the crystals in Fig. S4a; b, The SH signal spectra of different layer number; c, Optical
micrograph of irregular bilayer WSe2 crystal with monolayer WSe2 as reference; d, The
corresponding SHG mapping intensity obtained by pixel-to-pixel spatial scanning on the crystals
in Fig. S4c; e, The SH signal spectra of different layer number.
Figure S5. Low-frequency Raman spectra of bilayer WSe2 crystals for 2H and 3R stacking
configurations with different morphologies.
a b c
Figure S6 a, Low-magnification HAADF-STEM image of top-view bilayer WSe2 sample; b-c, elemental mapping of the region (green frame) in Fig.a.
1.0 nm
-1.0nm
0.4 nm
-0.4 nm
a b
Area 1Bilayer growth
direction
Step direction
Area 1
0.76 nm
Monolayer
Figure S7. a, AFM topographic image of the monolayer WSe2 grain boundary crystal with bilayer
nuclei in the center area; b, The zoom-in AFM topographic image of area 1, indicative of initial
WSe2 bilayer nuclei aligned growth on the atomic steps. Scale bars: a, 2 µm; b, 100 nm.
3.0nm
2 µm 200 nm
2.0nmba
-3.0nm
-2.0nm
Area 1
Area 1
1 2
210.8 nm
0.8 nm
Bilayer growth direction
d
-2.0nm
Area 2
Bilayer growth direction
Step direction
2.0nmc
-2.0nm
Area 2
0.76 nm
2 µm 200 nm
Figure S8. a-b, AFM topographic images of monolayer WSe2 crystal with bilayer nuclei in different areas. The inset height profile is~0.8 nm, indicating a thickness of WSe2 monolayer. The zoom-in AFM image in Fig.R1b shows WSe2 bilayer nuclei initial growth on the atomic steps of sapphire; c-d, AFM topographic images of monolayer WSe2 crystal with bilayer nuclei in different areas. The inset height profile is ~0.76 nm, indicating a thickness of WSe2 monolayer The zoom-in AFM image in Fig. R1d shows WSe2 bilayer nuclei initial growth on the atomic steps of sapphire.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.4
-0.2
0.0
0.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-0.4
-0.2
0.0
0.2
0.4
Step width65 nm
Hei
ght (
nm)
Distance (m)
Anneal in H2/Ar
0.18 nm
Step height
0.21 nm62 pm
Anneal in air
Hei
ght (
nm)
Distance (m)-0.4 nm
-0.4 nm
0.4 nmc
a
1050 °C in H2/Ar 15 mins
1050 °C in air 60 mins
Step direction
Step direction
0.4 nm b
d
Figure S9. a, AFM topographic image of sapphire surface after high-temperature treatment (1050
°C in H2/Ar for 15 mins); b, the corresponding cross-section height profile of the atomic steps
along the vertical step direction in Fig. S7a; c, AFM topographic image of sapphire surface after
high-temperature treatment (1050 °C in air for 60 mins); d, the corresponding cross-section height
profile of the atomic steps along the vertical step direction in Fig. S7c. Scale bars: a, 100 nm; c,
100 nm.
1.0 nm
-1.0 nm
1.0 nm
-1.0 nm
0.76 nm
Sapphire Sapphire andMonolayer WSe2
a b
Step direction
Area 1
Area 1
1.0 nm
1.0 nm
-1.0 nm
-1.0 nm
Sapphire surface Roughness: 0.115 nm
Monolayer WSe2 surface Roughness: 0.081 nm
dc
Figure S10. a, AFM topographic image of one WSe2 crystal. The inset height profile was ~0.8
nm, indicating a monolayer thickness; b, The zoom-in AFM image in Fig. S8a. And the image
showed irregular atomic steps on bare sapphire surface without any pre-treatment. In contrast, the
apparently periodic atomic steps were shown after covering monolayer WSe2; c, The selected area
for the roughness calculation of bare sapphire surface (300 nm x 300 nm); d, The selected area for
the average roughness calculation of sapphire surface with monolayer WSe2 covering (300 nm x
300 nm). Scale bars: a, 2 µm; b, 200 nm; c, 2 µm; d, 200 nm.
Bilayer
Trilayer
200 250 300 350
B12g
2 LA (M)
Bilayer
Ram
an In
tens
ity (a
.u.)
Raman Shift (cm-1)
Trilayer
E12ga b
Figure S11. a, Optical micrograph of bilayer/trilayer WSe2 crystals as-grown on c-plane sapphire
substrate; b, The Raman spectra measurements for bilayer (red) and trilayer (orange) WSe2
crystals.
-3.0nm
-2.0nm
-3.0nm
-0.5 nm
a
d
c
b
3.0nm
3.0nm
0.5 nm
2.0nm
Area 1
Area 1
1 2
0.8 nm
0.75 nm2
1
Area 2
Bilayer growth
1.55 nm
Area 2
Bilayer
sapphire
2.0 nm
-2.0 nm
1.0 nm
-1.0 nm
e1.56 nm
Area 3
f Area 3
Monolayer
Bilayer growth
Monolayer sapphireStep direction
Step direction
Figure S12. AFM topographic images of three representative bilayer WSe2 crystals as-grown
on sapphire surface. a, The bilayer WSe2 crystal with irregular morphology. The inset height
profiles were both ~0.8 nm, indicating a bilayer thickness in the left part and a monolayer thickness
in the right part of the WSe2 crystal; b, The zoom-in AFM image showed the bilayer WSe2 nuclei
growth orientation following the atomic steps; c, the bilayer WSe2 crystal with truncated triangle
morphology. The inset height profile demonstrated a bilayer thickness of WSe2 crystal; d, The
zoom-in AFM image showed bilayer WSe2 nuclei growth orientation following the atomic steps;
e, The bilayer WSe2 crystal with grain boundary. The inset height profile demonstrated a bilayer
thickness of WSe2; f, The zoom-in AFM image showed bilayer WSe2 nuclei growth orientation
following the atomic steps. Scale bars: a, 2 µm; b, 200 nm; c, 1 µm; d, 200 nm; e, 1 µm; f, 100
nm.
Table S1. Roughness measurements on different surfaces
WSe2 crystals Sapphire Roughness
Monolayer Roughness
Bilayer Roughness
TrilayerRoughness
Sapphire Roughness (1050 ºC)1
Sapphire Roughness (1050 ºC)2
Sapphire (Fig. S7a) - - - - 0.056 nm -
Sapphire (Fig. S7c) - - - - - 0.060 nm
Crystal 1 (Fig. 2b) - 0.070 nm - - - -
Crystal 2 (Fig. 2d) 0.109 nm 0.073 nm 0.068 nm - - -
Crystal 3 (Fig. 3b) 0.129 nm 0.094 nm 0.097 nm 0.076 nm - -
Crystal 4 (Fig. S8b) 0.148 nm 0.056 nm - - - -
Crystal 5 (Fig. S10b) 0.110 nm 0.077 nm 0.082 nm - - -
Crystal 6 (Fig. S10d) 0.125 nm 0.081 nm 0.073 nm - - -
Crystal 7 (Fig. S10f) - 0.075 nm 0.086 nm - - -
1The as-supplied sapphire was annealed in the H2/Ar for 15 mins;2The as-supplied sapphire was annealed in the air for 60 mins.
200 250 300 350 400 600 650 700 750 800 850 900
WSe2 Monolayer
WSe2 Bilayer
WSe2 Monolayer (after device
fabracation)
WSe2 Bilayer (after device
fabracation)
WSe2 Monolayer
WSe2 Bilayer
WSe2 Monolayer
(after device fabracation)
WSe2 Bilayer
(after device fabracation)
Raman Shift (cm-1)
Inte
nsity
(a.u
.)
PL in
tens
ity (a
.u.)
Wavenumber (nm)
a b
Figure S13. a. The Raman spectra measurements for WSe2 monolayer and bilayer crystals before
and after device fabrication on c-plane sapphire substrate; b, The PL spectra measurements for
WSe2 monolayer and bilayer crystals before and after device fabrication on c-plane sapphire
substrate.
-2 -1 0 1 2
-0.8
-0.4
0.0
0.4
0.8
-2 -1 0 1 2
-15
-10
-5
0
5
10
15
I ds (
A)
Vg (V)
Bilayer WSe2
Vg (V)
I ds (n
A)
Monolayer WSe2
a b
-3 -2 -1 0 1 2 31E-6
1E-4
0.01
1
201 mV/dec
Vgs(V)
I ds (
A)
Monolayer WSe2
Bilayer WSe2
229 mV/dec
c
Figure S14. a, Ids as a function of Vg for monolayer device with two terminal. Ids as a function of
Vg for bilayer device with two terminal; b, Ids as a function of Vg for monolayer device. Ids as a
function of Vg for bilayer device. Cg = 5.0 µF/cm2; c, Ids as a function of Vgs for monolayer/bilayer
device. The subthreshold slopes of monolayer and bilayer crystals were measured to be 229 and
201 mV/dec, respectively.