  Strain engineering has arisen as a powerful technique to tune the electronic
and optical properties of two-dimensional semiconductors like molybdenum
disulfide (MoS2). Although several theoretical works predicted that biaxial
strain would be more effective than uniaxial strain to tune the band structure
of MoS2, a direct experimental verification is still missing in the literature.
Here we implemented a simple experimental setup that allows to apply biaxial
strain through the bending of a cruciform polymer substrate. We used the setup
to study the effect of biaxial strain on the differential reflectance spectra
of 12 single-layer MoS2 flakes finding a redshift of the excitonic features at
a rate between -40 meV/% and -110 meV/% of biaxial tension. We also directly
compare the effect of biaxial and uniaxial strain on the same single-layer MoS2
finding that the biaxial strain gauge factor is 2.3 times larger than the
uniaxial strain one.
