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Since their discovery single-layer semiconducting transition metaldichalcogenides have attracted much attention thanks to their outstandingoptical and mechanical properties. Strain engineering in these two-dimensionalmaterials aims to tune their bandgap energy and to modify their optoelectronicproperties by the application of external strain. In this paper we demonstratethat biaxial strain, both tensile and compressive, can be applied and releasedin a timescale of a few seconds in a reproducible way on transition metaldichalcogenides monolayers deposited on polymeric substrates. We can controlthe amount of biaxial strain applied by letting the substrate expand orcompress. To do this we change the substrate temperature and choose materialswith a large thermal expansion coefficient. After the investigation of thesubstrate-dependent strain transfer, we performed micro-differentialspectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2,WS2, WSe2) under the application of biaxial strain and measured their opticalproperties. For tensile strain we observe a redshift of the bandgap thatreaches a value as large as 95 meV/% in the case of single-layer WS2 depositedon polypropylene. The observed bandgap shifts as a function of substrateextension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoreticalcalculations of these four materials under biaxial strain predict the sametrend for the material-dependent rates of the shift and reproduce well thefeatures observed in the measured reflectance spectra.

Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides