Article
Methods
Growth of Cu (111) thin films
The as-received sapphire substrates were first etched in a mixed H 2 SO 4 /
H 3 PO 4 aqueous solution at 300 °C for 20 min. The etched sapphire
substrates were cleaned by immersion in ultra-pure water for 5 min.
After cleaning, the sapphire substrates were loaded into a sputtering
chamber for Cu deposition. The chamber was maintained at room
temperature under an argon pressure of 0.3 mtorr, giving rise to a Cu
deposition rate of 2 nm s−1 (ref.^10 ).
Chemical vapour deposition of hBN
The monolayer hBN films were grown in a three-inch furnace tube with
three heating zones using low-pressure chemical vapour deposition
(LPCVD). The two-inch Cu (111)/sapphire substrate was placed in the cen-
tral heating zone of the main chamber. Ammonia borane (97%, roughly
60 mg) was used as the precursor and loaded into a subchamber at the
upstream side of the main chamber. The furnace was first pumped down
to a base pressure of 5.0 torr. Before growth, the substrate was annealed
at 1,050 °C for 60 min under a hydrogen gas flow of 300 standard cubic
centimetres per minute (sccm). The subchamber (with precursors) was
heated to 85 °C using a heating belt and maintained there for 30 min.
The precursor was then introduced into the main chamber for 30 min
in order to grow hBN on the Cu (111)/sapphire substrates, with the
substrates facing downwards. After hBN growth, the subchamber was
closed, and the main chamber was naturally cooled to room temperature
under a hydrogen gas flow of 30 sccm.
Transfer of hBN films onto arbitrary substrates
The as-grown monolayer hBN film was detached from the Cu (111)/
sapphire substrate by electrochemical delamination. A poly(methyl
methacrylate) (PMMA) film was first spin-coated on the as-grown hBN/
Cu (111)/sapphire as a protection layer. Then a thermal release tape
(TRT; catalogue number 3195M from Nitto) was applied to the PMMA/
hBN/Cu (111)/sapphire in order to avoid possible folding during the
transfer process. Electrochemical delamination was performed using
an aqueous solution of NaOH (1 M) as the electrolyte, with the Cu layer
in the TRT/PMMA/hBN/Cu (111)/sapphire stack as the cathode and a
platinum foil as the anode, under an applied DC voltage of 4 V. During
this process, the TRT/PMMA/hBN stacked film was detached from the
Cu (111)/sapphire through the generation of hydrogen bubbles at the
hBN/Cu interface. After detachment, the TRT/PMMA/hBN stacked film
can be placed on the target substrate. The TRT can be released by bak-
ing the TRT/PMMA/hBN/substrate on a hot-plate at 180 °C. The PMMA
film was finally removed by immersing the sample in hot acetone for
40 min, leaving behind a monolayer hBN film on the target substrate.
Chemical vapour deposition of MoS 2
Highly oriented monolayer MoS 2 was grown on sapphire substrates by
CVD in a horizontal hot-wall three-inch furnace tube with two heating
zones. The maximum substrate size can be a wafer of up to two inches.
High-purity sulfur (99.5%, Alfa) and MoO 3 (99%, Aldrich) powders were
used as the reaction precursors. The sulfur powder was placed in the front
heating zone at the upstream side of the furnace, and the temperature was
maintained at 140 °C during the reaction. The MoO 3 powder was put into
a quartz boat in the central heating zone of the furnace. The temperature
of this central heating zone was gradually ramped to 740 °C and held for
5 min. During this process, MoS 2 was grown on the sapphire substrates
placed at the downstream side of the MoO 3 quartz boat. All growth was
performed in argon flowing gas (90 sccm) at a base pressure of 30 torr.
Finally, the furnace was naturally cooled to room temperature^17.
Transfer of monolayer MoS 2
After CVD growth, monolayer MoS 2 films on sapphire were
transferred onto a target substrate using PMMA. PMMA was first
spin-coated onto the as-grown MoS 2 /sapphire as a protection
layer. Then a TRT was applied to the PMMA/MoS 2 /sapphire in order
to avoid possible folding during the transfer process. The TRT/
PMMA/MoS 2 /sapphire stacked film was immersed in an NH 4 OH
solution (NH 4 OH/deionized water, 17/100) at 100 °C for 20 min.
Next, the detached TRT/PMMA/MoS 2 stacked films were immersed
in deionized water in order to dilute the etchant and residues. The
TRT/PMMA/MoS 2 stacked film is placed on the target substrate;
the TRT is released by baking on a hot-plate at 180 °C; and the
PMMA film is finally removed by immersing the sample in hot
acetone for 40 min, leaving behind a monolayer MoS 2 film on the
target substrate^17.LEEM/PEEM/μ-LEED characterizations
We carried out structural analysis of hBN using low-energy electron
microscopy (LEEM) with a field-emission gun, photoemission elec-
tron microscopy (PEEM) excited by a mercury lamp, and microspot
low-energy electron diffraction (μ-LEED, with an area of about
3 μm in diameter). The system (Elmitec LEEM-III) is composed of
a preparation chamber, a main chamber for imaging, and a deep
ultraviolet laser^18. The sample was first annealed in the prepara-
tion chamber in ultrahigh vacuum (1 × 10−9 torr) at 600 °C for 6 h
and then transferred to the main chamber for LEEM/PEEM/μ-LEED
analysis.STM characterizations
We used an Omicron ultrahigh vacuum (UHV) variable-temperature
(VT)-STM system for atomic-scale structural characterization, with a
base pressure better than 10−10 mbar. The hBN/Cu sample was annealed
at 900 K for 5.5 h before STM measurements. All of the the STM images
were captured under a constant-current mode at room temperature.
The atomic-scale morphology of hBN/Cu was analysed directly by high-
resolution STM.EBSD characterization
EBSD was performed on a JEOL JSM-7800F PrimeSEM with collection
accessory (from EDAX) at an accelerating voltage of 20 kV and the sam-
ple stage tilting at 70°.TEM characterization
We obtained TEM images from transferred hBN/SiO 2 /Si with an accelera-
tion voltage of 200 keV using a FEI Tecnai Osiris transmission electron
microscope.AFM characterization
AFM data were acquired in tapping mode over the scan area, using a
silicon tip.XPS measurements
XPS spectra were obtained using a Perkin Elmer PHI 5400 system
equipped with hemispherical analyser with an overall resolution of
0.05 eV. The energy span and energy scale were calibrated by setting
the 4f7/2 line of gold and the 2p3/2 line of copper to 84 eV and 932.67 eV,
respectively. XPS spectra were measured from the as-grown monolayer
hBN film on Cu (111)/sapphire. The B 1s and N 1s emission peaks con-
firm the formation of hBN. The binding energies of B 1s and N 1s are
located at 190.4 eV and 398 eV, respectively. The B/N atomic ratio is
calculated from the integrated intensities of these peaks, yielding a
ratio of 1/1.03, indicative of a good stoichiometry for the CVD-grown
monolayer hBN films.Raman measurements
The Raman measurements were carried out on transferred hBN
film using a 532-nm solid-state laser as the excitation source. Excita-
tion light with a power of 2.5 mW was focused onto the sample with