Supporting Information
Electronic properties of graphene under large local strain
Xin He,1 Li Gao,2 Ning Tang, 1 ,a) Junxi Duan,1 Fuhong Mei,1 Hu Meng,1 Fangchao Lu,1
Fujun Xu,1 Xinqiang Wang,1 ,3 Xuelin Yang,1 Weikun Ge,4 ,a) and Bo Shen1 ,3 ,a)
1
State Key Laboratory for artificial Microstructure and Mesoscopic Physics, Peking
University, Beijing 100871, PR China
2
Institute of Life Sciences, Jiangsu University, Zhenjiang 212013, Jiangsu Province, PR
China
3
Collaborative Innovation Center of Quantum Matter, Beijing 100871, PR China
4
Department of physics, Tsinghua University, Beijing 100084, PR China
ntang@pku.edu.cn, geweikun@mail.tsinghua.edu.cn, bshen@pku.edu.cn
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S1 The process of fabricating the strain device
The process of fabricating the strain device is illustrated in Fig. S1. An
oxidized 300 m thick Si substrate whose oxidized layer being 300 nm
thick was glued onto a supporting frame which was made up of a
piezocrystal whose two ends were fixed to two separated L-shaped
brackets as shown in Fig. 1(b) in the manuscript. After that, the Si substrate
was cleaved into two parts to form a gap parallel to the cleavage plane, see
step A1 to A2 in Fig. S1 (the width of the gap at this experiment was 12.2
m). Subsequently, a drop of polydimethylsiloxane (PDMS) was spun on
a patterned bare copper foil at a high speed of rotation (8000 rounds per
minute) to get an about 5 m thick membrane. After baking at 75 ℃ for
1 hour to make the PDMS solidify, the PDMS/Cu layer was transferred into
a saturated FeCl3 solution to etch the copper foil, and then the FeCl3 residue was
washed away in deionized water for several times (step B1 to B3). After
that, the PDMS membrane was transferred onto the silicon crack (step D1)
and baked in an oven at 120℃ for 10 minutes to make them tightly bond
(the PDMS layer here was used to support the graphene layer and deliver
strain from the underlying piezocrystal). At the same time, two circular
shaped electrodes (Ni/Au: 5 nm/ 50 nm) with diameter of 1000 m and
distanced of 2000 m in between were deposited on a graphene flake
grown on a copper foil by using shadow mask and magnetron sputtering.
After spinning polymethylmethacrylate (PMMA) on the flake, the copper
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foil was dissolved by a saturated FeCl3 solution once more (step C1 to C4).
Finally, the PMMA-graphene layer was transferred onto the PDMS layer
to cover the gap below (step D2) and then baked in an oven at 120℃ to
make the graphene bond tightly with the PDMS layer. When the PMMA
layer was removed by acetone, a local strain can then be loaded in the
suspended part of the PDMS layer by applying piezo voltage ( V p ) to the
piezocrystal and subsequently transmitted to the graphene on top.
Figure S1. Schematic diagrams of the process of fabricating the strain
device.
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S2 The strain transmission ratio of the PDMS substrate
The extension ratio of the piezocrystal is approximately 97.2 nm/V .
However, the mean effective deformation of the graphene cannot reach
such a value, because the PDMS layer was about 5 m thick and only the
bottom surface was stuck onto the underlying frame. When the
deformation was transmitted to the graphene layer, it would be
counteracted by the PDMS layer to some extent. The thicker the PDMS
layer was, the lower the transmission efficiency would be. That was why
the rotational speed of spinning PDMS should be high enough to make it
thin. We define the ratio between the extension values of the graphene layer
and the piezocrystal as the strain transmission ratio  
x1  x2
( x1  x2
X 1  X 2
and X1  X 2 are the extension values of the bottom and upper surfaces of
the PDMS layer respectively, as depicted in Fig. S2). To measure this ratio,
we made a mark by ink on the upper surface of the PDMS layer and
measured its real extension value when strain was loaded by the
piezocrystal. The ratio was measured to be ~ 0.7.
Figure S2. Schematic diagrams of transmitting strain to sample via PDMS.
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S3 The transfer characteristics of the four-probe measurement
A four-electrode graphene device was fabricated to find why the
asymmetry of electron-hole conduction appeared. The ohmic contacts were
prepared using the same process as described in our manuscript. Three
different constant currents were applied and then the transfer
characteristics were plotted in Fig. S3.
Figure S3. Transfer characteristics from a four-probe measurement.
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S4 The transfer characteristics of graphene on different
substrates
To find why the mobility of graphene was low on PDMS, we acquired
the transfer characteristics by transferring graphene onto different
substrates using the process described in our manuscript and then making
electronic measurements.
Figure S4. The transfer characteristics of graphene on PDMS and SiO2.
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