  Understanding the thermoelastic response of a nanostructure is crucial for
the choice of materials and interfaces in electronic devices with improved and
tailored transport properties, at the length scales of the present technology.
Here we show how the deposition of a $MoS_2$ monolayer can strongly modify the
nanoscale thermoelastic dynamics of silicon substrates close to their
interface. We achieve this result by creating a transient grating with extreme
ultraviolet light, using ultrashort free-electron laser pulses, whose
$\approx$84 nm period is comparable to the size of elements typically used in
nanodevices, such as electric contacts and nanowires. The thermoelastic
response, featured by coherent acoustic waves and an incoherent relaxation, is
tangibly modified by the presence of monolayer $MoS_2$. Namely, we observed a
major reduction of the amplitude of the surface mode, which is almost
suppressed, while the longitudinal mode is basically unperturbed, aside from a
faster decay of the acoustic modulations. We interpret this behavior as a
selective modification of the surface elasticity and we discuss the conditions
to observe such effect, which might be of immediate relevance for the design of
Si-based nanoscale devices.
