Inhomogeneous and three-dimensional strain engineering in two dimensional materials opens up new avenues to straintronic devices for control strain sensitive photonic properties. Here we present a method to tune strain by wrinkling monolayer WSe2 attached to a 15 nm thick ALD support layer and compressing the heterostructure on a soft substrate. The ALD film stiffens the 2D material, enabling optically resolvable micron scale wrinkling rather than nanometer scale crumpling and folding. Using photoluminescence spectroscopy, we show the wrinkling introduces periodic modulation of the bandgap by 47 meV, corresponding with strain modulation from +0.67% tensile strain at the wrinkle crest to -0.31% compressive strain at the trough. Moreover, we show that cycling the substrate strain mechanically reconfigures the magnitude and direction of wrinkling and resulting band tuning. These results pave the way towards stretchable multifuctional devices based on strained 2D materials.
We demonstrate THz imaging and time-domain spectroscopy of a single-layer graphene film. The large-area graphene
was grown by chemical vapor deposition on Cu-foil and subsequently transferred to a Si substrate. We took a
transmission image of the graphene/Si sample measured by a Si:bolometer (pixel size is 0.4-mm). The graphene film
(transmission: 36 - 41%) is clearly resolved against the background of the Si substrate (average transmission: 56.6%).
The strong THz absorption by the graphene layer indicates that THz carrier dynamics are dominated by intraband
transitions. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet
resistance varies across the sample from 420 to 590 Ohm, consistent with electron mobility ~ 3,000 cm2V-1s-1. We also
measured time-resolved THz waveforms through the Si substrate and the graphene/Si sample. The waveforms consist of
a series of single-cycle THz pulses: a directly transmitted pulse, then subsequent "echos" corresponding to multiple
reflections from the substrate. The amplitude difference between graphene/Si pulses and Si pulses becomes more
pronounced as the pulses undergo more reflections. From these measurements, we obtained spectrally flat transmission
spectra of the transmitted pulses and the average sheet resistance 480 Ohm, consistent with the results of the power
transmission measurement. The flat spectral responses indicate that the carrier scattering time in our graphene sample is
much shorter than the THz pulse duration.
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