Generation and detection of coherent longitudinal acoustic waves in ultrathin 1T’-MoTe2

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 (4–30 nm) yield a longitudinal sound speed of v L = (2990 ± 90) m s−1. In addition, temperature-dependent measurements unveil a linear decrease in the normalized Young's modulus E L / E L , 295 K with a slope of δ ( E L / E L , 295 K ) / δ T = (–2.0 ± 0.1) 10−3 K−1 and no noticeable change caused by the Td – 1T’ structural phase transition or variations in film thickness.