2.1 Formation mechanism of graphene directly synthesized on
arbitrary substrate by C-ion implantation
Figures 1a1-1a3 schematically illustrate our graphene growth
processes. A 100 nm-thickness Ni layer was evaporated on arbitrary
substrates and then implanted under room temperature with 70 keV C ions
with the pre-determined fluences, as presented in Figure 1a1 .
According to the Monte Carlo stopping and range of ions in matter (SRIM)
code, the projected area of the implanted C ions in Ni is about 80 nm
with a range of ~46 nm (standard deviation). The fluence
of implanted C ions and the lateral uniformity could be accurately
controlled by means of a 4-corner Faraday cup assembly, and the fluences
of 4×1015 and 8×1015atoms/cm2 are selected to achieve uniform single and
double graphene layer, respectively. Following the C ion implantation,
150 nm Cu film was placed on top of the Ni layer by e-beam evaporation
where the substrate was kept at room temperature (Figure 1a2 ).
The graphene with the expected thickness is obtained at the interface of
the Cu-Ni alloy and the objective substrate after the thermal treatment
(1000 ℃ for 10 min under 10-5 mbar) of the Cu/Ni dual
metal substrates (Figure 1a3 ). Afterward, the Cu-Ni alloys are
easily removed using thermal release tape, leaving the graphene on the
arbitrary substrate. The current strategy can directly synthesize
graphene on arbitrary substrates, bypassing the undesired post-synthesis
transfer process for further device fabrication. Besides, considering C
ions are pre-implanted into the Ni layer on the objective substrate
before the thermal treatment, the experimental conditions, especially
the gas sources operated for the annealing process, become less critical
as compared in other graphene synthesis processes where hydrogen capture
of oxygen from metal oxides is needed to facilitate graphene formation
at metal surfaces.
To investigate the forming process of graphene on an arbitrary substrate
(in this case, SiO2) through ion implantation, secondary
ion mass spectrometry (SIMS) is employed to evaluate the depth
distributions of C, Cu, and Ni in the Cu/Ni dual metal on
SiO2 substrates, as exhibited in Figures
1b1-1b3 . After the C ion implantation into the Ni layer, the
implanted C ions present a
Gaussian-like distribution in the Ni layer (Figure
1b1 ).[23,34-35] The implantation energy is
selected to keep the implanted C ions within the Ni layer based on their
projected ranges, as schematically illustrated in Figure 1c1 .
Note that the Si signal corresponding to SiO2 substrates
is also shown while the O signal is omitted to show better clarity for
the interested species. Figure 1b2 represents the SIMS result
of the deposition of 150 nm Cu film on top of the Ni layer, showing that
Cu, Ni, and C signals emerge at their respective locations in reference
to the surface (as schematically shown in Figure 1c2 ). As the
top Cu film is thicker compared to the bottom Ni layer of
~100 nm, when Cu/Ni inter-diffusion proceeds during the
thermal annealing, the thinner Ni is diluted by Cu and eventually
disappears entirely into the Cu-dominant Cu-Ni alloy. Development of
Cu-Ni alloy is verified through Ni showing up near the surface
(Figure 1b3 ) after 10 min under 1000 ℃. As is well-known that
Ni has a neutral C solubility of 0.4~2.7 at% at
600~1300 ℃,[12] while the
solubility of C in Cu is more than two orders of magnitudes smaller and
thus negligible (<0.003 at%) at 1000
℃.[13] Hence, the C solubility in the afresh
formed Cu-dominant Cu-Ni alloy is expected to decrease significantly
when inter-diffusion of Ni and Cu proceeds to form the Cu-Ni alloy.
Therefore, C atoms are driven towards the interface of the Cu-Ni alloy
and the objective substrate, and finally converted into graphene
structure promoted through the catalysis impact of the Cu-Ni
alloy,[23,32,34,36] as illustrated schematically
in Figure 1c3 .[7,23,31] The optimal
growth conditions of monolayer and bilayer graphene films synthesized by
C ion implantation into the Cu/Ni dual metal substrates were
investigated systematically and are detailed in SupplementaryFigures S1-S2 .