SUPPORTING INFORMATION FOR
Improvement of the electrical contact resistance at
rough interfaces using two dimensional materials
Jianchen Hu1,a, Chengbin Pan1,a,Heng Li2, Panpan Shen3, Hui Sun3, Huiling Duan3, Mario
Lanza1*
1
Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou
215123, China.2Department of Physics, State Key Laboratory for Mesoscopic Physics,
Peking University, Beijing 100871, China. 3 State Key Laboratory for Turbulence and
Complex System, Department of Mechanics and Engineering Science, CAPT, College of
Engineering, Peking University, Beijing 100871, China. a Equal contribution
EXPERIMENTAL DETAILS
Chemicals: The chemicals Zinc Acetate (Zn(AC)2), diethanol amine (DEA), Zinc nitrate
(Zn(NO3)2 • 6H2O), HexamethyleneTetramine (HMTA) and Poly-methyl methacrylate
(PMMA) were purchased from Sigma-Aldrich. Ethyl lactate, iron trichloride (FeCl3),
ammonia water and hydrochloride (HCl) were produced by Sinopharm Chemical Reagent
Beijing Co. Ltd. PMMA was dissolved in Ethyl lactate at 60 oC to form solution with the
concentration of 3%. FeCl3 was dissolved in diluted HCl to form the mixture solution with
the FeCl3 concentration of 0.6 M and HCl concentration of 0.72 M, respectively. All
chemicals were used as received.
Preparation of graphene: The graphene single layer was grown in low pressure chemical
vapor deposition (LPCVD) system on a 25 µm thick Cu foil (from Alfa Aesar). The Cu foil
was preprocessed at 1000 °C for 40 min under H2 flow (20 sccm) and then kept at 1000 ºC
for 10 min to remove Cu oxide from the surface. During the graphene growth process,
methane (10 sccm) was introduced into the system to grow graphene for 20-30 min.
Preparation of ZnO nanowire arrays: (1) Seed layer deposition: i) Mixture solution of
Zinc Acetate (0.1 M) and diethanol amine 0.1 M in ethanol was spin coated on p-type silicon
substrate (3000 rpm). The obtained sample was annealed at 385 °C for 20 minutes for
decomposition of zinc acetate into ZnO. (2) ZnO nanowire array growth: Ammonia water
was added in mixture solutuin of zinc nitrate (0.1 M) and HMTA (0.1 M) in DI water until
the solution getting clarifying. Immerse the substrate prepared in (1) in the solution prepared
in (2) which was contained in hydrothermal synthesis reactor, and heat the hydrothermal
synthesis reactor to 90 centigrade for 6 hours.
Transfer graphene single layer to ZnO nanowire array: The GSL/Cu stack sample was
spin-coated with a 150-200 nm thick PMMA film, after which the coated sample was etched
in the mixture solution of FeCl3 and HCl. Cu foil fully dissolving, the PMMA-graphene film
was lifted from the solution and washed in pure water and then transferred onto the ZnO
nanowire array. After the film was dried up, PMMA was removed by hot acetone.
Morphology characterization: SEM images were captured with a Hitachi S-4800 scanning
electron microscope. AFM characterization of graphene has been made using the Seiko 3000
atomic force microscope working in a high vacuum environment of 10−7torr and using Pt-Ir
coated Silicon AFM tips (k =0.2 N/m) and doped-Diamond coated Silicon AFM tips (k =2.8
N/m) from Bruker. The software used to analyze the images collected with the AFM was the
WSxM 5.0 Develop 6.1 from Nanotec. [1].
Characterization of residual PMMA at the graphene edges: The presence of rests of
polymer from the transfer process has been sporadically observed at the graphene edges.
After the graphene transfer, the PMMA layer is etched using acetone vapor. Despite most of
the graphene surface is PMMA-free after this step, we sporadically observe rests of PMMA
at the graphene edges.
Figure S1. SEM image of an area of the NWs sample partially covered with graphene. The
top left corner is free of graphene, while the bottom right is totally covered by graphene. The
dark area in the center is residual PMMA from the transfer process, which is more difficult
to be etched at the graphene edge.
We characterize the excess of PMMA at the edges by means of EDX surveys, and we
observe an increase in the Carbon signal. Moreover, in Figure S1 we show that the
remaining PMMA is much thicker than the graphene sheet, which impedes the correct
observation of the underlying nanowires.
Figure S2. EDX surveys captured at graphene-free NWs array (black curve, top-left of
Figure S1), graphene-covered NWs (green curve, bottom-right area of Figure S1) and
PMMA-covered graphene edge (red curve, central area of Figure S1). Panels (a), (b) and (c)
of this figure correspond to Carbon, Oxygen and Zinc signals. The signal of carbon at
graphene edge is much stronger than that of the other two locations. This demonstrates the
residual PMMA at the graphene edge is more difficult to be etched, leading to worse
observation of NWs beneath.
Figure S3: Preview of the FIG. 3a in the manuscript. The small gaps between the nanowires
can be clearly distinguished. (The area of the image is 5 μm × 5 μm, and the insert square is
1 μm × 1 μm)
Physical and electronic interaction between graphene and the nanowires: When a
piezoelectric ZnO nanowire bents (moved by the AFM tip), potentials of opposite polarities
are generated at the compressed and elongated surfaces (Figure S4a). If a metallic particle is
deposited on the top of a nanowire connecting all the points of its surface, the electronic
potential may be neutralized, as previously suggested by Wang et al. [Ref. 7 in the
manuscript] (Figure S4b). In our experiments, we observe that the use graphene as top
electrode not only doesn't compensate the piezoelectric potentials, but it also increases the
number of points showing large currents. Considering the exceptional conductivity of
graphene, and by looking at the current images recorded with the CAFM, the only possible
explanation for this behavior (at this point) is that graphene sheet may not be fully attached
to the top part of the nanowires, but only contacts the surface of the NW at the subjection
points (sides of the NW), leading to a small nanogap between them (Figure S4c, top). The
presence of nanogaps in graphene-covered materials has been previously observed [Ref. 24
in the manuscript]. Under this situation, it may be reasonable to believe that, when the NW
bends as due to the contact with the AFM tip, the graphene may detach from the compressed
(or elongated) area, avoiding potential neutralization (Figure S4c). Further studies should be
conducted to fully clarify this behavior, which are out of the scope of this letter. In any case,
the presence of larger currents in the graphene-coated NWs array is evident and repetitive
from CAFM maps.
Figure S4. Proposed model for GSL/NW contact. Free-standing (top) and bent (bottom)
nanowires without top electrode (a), with a metallic top electrode deposited on the nanowire
(b) and with graphene electrode (c). The graphene only contacts one side of the nanowire, so
that the potential is not neutralized, and currents can be collected, in agreement with Figure
4b of the manuscript.
Additional Reference
[1] Horcas, I.; Fernandez, R.;Gómez-Rodríguez, J.;Gómez-Herrero, J.;Baro, A. WSXM: A
Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci.
Instrum.2007,78, 013705.