  Layered transition metal dichalcogenides have attracted substantial attention
owing to their versatile functionalities and compatibility with current
nanofabrication technologies. Thus, noninvasive means to determine the
mechanical properties of nanometer (nm) thick specimens are of increasing
importance. Here, we report on the detection of coherent longitudinal acoustic
phonon modes generated by impulsive femtosecond (fs) optical excitation.
Broadband fs-transient absorption experiments in 1T'-MoTe2 flakes as a function
of thickness (7 nm - 30 nm) yield a longitudinal sound speed of 2990 m/s. In
addition, temperature dependent measurements unveil a linear decrease of the
normalized Young's modulus with a slope of -0.002 per K and no noticeable
change caused by the Td - 1T' structural phase transition or variations in film
thickness.
