Supporting Information
Fabrication of Stretchable MoS2 Thin-film Transistors Using Elastic
Ion-Gel Gate Dielectrics
J. Pu,1 Y. Zhang,2 Y. Wada,1 J. T.-W. Wang,3 L.-J. Li,3,a) Y. Iwasa,2,4,5 and T. Takenobu1,a)
1
Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan
2
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
3Institute
of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
4Correlated
Electron Research Group, RIKEN, Wako 351-0198, Japan
5Quantum-Phase
*E-mail:
Electronics Center, The University of Tokyo, Tokyo 113-8656, Japan
[email protected]
*E-mail address: [email protected]
1
S1 The strain dependence of the Au electrode conductance
Figure S1 (a) shows the schematic configuration of the stretch test of the Au electrode. These
electrodes were thermally deposited onto the PDMS substrates. DC measurements were performed
to investigate the variation in the Au electrode conductance under different applied tensile tensions.
Figure S1 (b) presents the strain dependence of the Au conductivity (red). The variation in the Au
conductivity is negligible within a strain of 20% because the Au conductivity is four magnitudes
larger than that of the MoS2 TFTs on the PDMS substrate (blue line shown in Fig. S1 (b)). The inset
shows the optical image of the relaxation between the two Au electrodes. The applied strains were
determined by directly observing the expansion of the distance between the two electrodes, which
corresponded to the channel region in the transistors. These results allow for the stretchability of the
electrical performance in semiconductors to be tested using an Au electrode.
2
S2 The strain dependence of the ion-gel capacitance
Figure S2 (a) presents the geometry of the ion-gel/Au interface capacitor on the PDMS substrate.
Impedance measurements were performed across the top and bottom electrodes with an applied
tensile tension. Figure S2 (b) shows the frequency profile of the specific capacitance under the
following three strain conditions: 0% (red), 11% (orange), and 22% (blue). The capacitance at 100
Hz is 9.05 (μF/cm2). The inset presents the phase angle as a function of the applied frequency, and
these results strongly suggest that the capacitance obtained at 100 Hz is reasonable because a phase
angle of -90º corresponds to a purely capacitive response. Figure S2 (c) demonstrates the strain
dependence of the ion-gel capacitance. This result demonstrates the stretchability of the ion-gel
capacitor under high strains because the capacitance was nearly constant during stretching at up to
20% strain.
3
S3 Optical and spectroscopic characterizations of the MoS2 thin films on PDMS substrates
Evidence of deformation in 2-D nanosheet materials is provided by the strain-induced peak
splitting in the Raman spectrum, which has already been observed for graphene.1-3 Figure S3(a)
presents the Raman spectra of the MoS2 films transferred onto the PDMS substrates under different
strains. The two characteristic Raman peaks correspond to the E12g mode at ~ 377 cm-1 and the A1g
mode at ~ 400 cm-1. The peak energy difference measured between the E12g and A1g modes (~ 23
cm-1) agrees reasonably well with the results for tri-layer MoS2 films produced by the mechanical
exfoliation method.4,5 Because the stretching of the MoS2 crystal structure should change the
S-Mo-S hexagonal geometry, changes in the Raman E12g and A1g peak energies are expected.6
However, we did not observe any clear change in the Raman spectra of the MoS2 films up to a strain
of 7%, suggesting negligible deformation of these films. These results are completely inconsistent
with the degradation of the transport properties under strain that is shown in Fig. 2.
Another possible experimental signature of uniaxial deformation in MoS2 is the closure of the
band gap. Very recently, several groups reported the effect of tensile strain on mono- and bi-layer
MoS2 films using first-principle calculations.6-9 These studies predicted that the band gap of MoS2
decreases as the strain increases and that a semiconductor-metal transition can be induced at a strain
of 10%.7-9 Following this theoretical prediction, to confirm the deformation of the MoS2 films, we
measured the absorption spectra under uniaxial strain. As shown in Fig. S3(b), we observed minor
variations below a strain of 15%, indicating negligible deformation of these films. Moreover, a
similar robustness up to a 7% strain was revealed in the photoluminescence spectra (Fig. S3(c)).
These optical and spectroscopic characterizations obviously indicate that the MoS2 lattice structure
was not strongly deformed by uniaxial strain, and they conflict with the results of the transistor
measurements.
References
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Funct. Mater. 22, 1385 (2012).
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(7) E. Scalise, M. Houssa, G. Pourtois, V. Afanas’ev, and A. Stesmans, Nano Res. 5, 43 (2012).
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FIG. S3. Optical and spectroscopic characterizations of the MoS2 thin films on PDMS substrates. (a)
Raman spectra of the MoS2 thin films under two strain conditions. The two characteristic Raman
peaks correspond to the E12g mode and the A1g mode. The energy difference between the two peaks
was ~ 23 cm-1. (b) The absorption and (c) Photoluminescence (PL) spectra are shown as a function
of energy. The Raman and PL spectra were measured with an excitation laser wavelength of 532 nm.
5
S4 Strain relaxation in the domain boundaries
Below 5% of strain, the applied deformation is relaxed by the ripples structure and transistors are
stable, as shown in Fig. S4. On the other hand, when the strain is larger than 5%, we observed the
degradation in transistor performance (Figs. 3). During the transistor experiments, we observed a
clear expansion of the channel length, which was measured directly as the distance between the
source and drain electrodes (Fig. 2). However, as shown in Figs. S3, optical and spectroscopic
characterizations deny the deformation of MoS2 films. These contradictory results lead to the
conclusion that the origin of the degradation of transport properties in CVD-grown MoS2 films is the
strain relaxation in the domain boundaries (Fig. S4). Although the harder single-crystalline part of
the CVD-grown MoS2 films is not seriously deformed by the stretching, the softer domain
boundaries change their bonding angles and distances, resulting in strain release. Because most of
the CVD-grown MoS2 films are single crystalline, the effective area of the domain boundaries is
very small. Therefore, optical and spectroscopic characterizations are not able to detect the
deformation of the boundaries. In contrast, because the undeformable single-crystalline part of the
films is inevitably surrounded by the deformable boundaries, the carriers must cross them, and the
transport properties are affected by strong stretching. This mechanism explains the strain dependence
of polycrystalline, CVD-grown nanosheet materials, such as graphene and MoS2 films.
FIG. S4. Schematic views of the stretching of a polycrystalline MoS2 thin film. Polycrystalline MoS2
thin films consist of single-crystalline MoS2 domains. In relatively weak strain (< 5%), the applied
strain is relaxed by the ripple structures. The domain boundaries play an important role in farther
stretching. Because the stiff MoS2 single crystals are undeformable, the domain boundaries deform
and relax the applied strain. This mechanism may explain the origin of the stretchability and
degradation of polycrystalline, CVD-grown, 2-D nanosheet materials.
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