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

TL;DR

This study demonstrates the generation and detection of coherent longitudinal acoustic waves in ultrathin 1T'-MoTe2, revealing its mechanical properties and how they vary with temperature, using femtosecond optical techniques.

Contribution

It introduces a noninvasive optical method to measure mechanical properties of nanometer-thick 1T'-MoTe2 and reports the acoustic phonon modes and their temperature dependence.

Findings

Longitudinal sound speed of 2990 m/s in 1T'-MoTe2.

Normalized Young's modulus decreases linearly with temperature.

No significant change in mechanical properties across structural phase transition.

Abstract

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.