Ballistic Dynamics of Flexural Thermal Movements in a Nano-membrane Revealed with Subatomic Resolution

TL;DR

This study reveals ballistic flexural motion in nano-membranes driven by thermal phonons, measured with sub-atomic resolution, providing insights into 2D material properties and potential applications in fast thermometry and sensing.

Contribution

First direct observation of ballistic flexural phonon motion in nano-membranes using a novel sub-atomic resolution technique.

Findings

Ballistic velocities around 300 μm/s observed within 10 μs intervals.

Verification of equipartition theorem and Maxwell-Boltzmann statistics for flexural modes.

Ballistic regime impacts understanding of 2D material properties.

Abstract

Flexural oscillations of free-standing films, nano-membranes and nano-wires are attracting growing attention for their importance to the thermal, electrical and mechanical properties of 2D materials. Here we report on the observation of short-timescale ballistic motion in the flexural mode of a nano-membrane cantilever, driven by thermal fluctuation of flexural phonons, including measurements of ballistic velocities and displacements performed with sub-atomic resolution, using a new free electron edge-scattering technique. Within intervals <10 {\mu}s, the membrane moves ballistically at a constant velocity, typically ~300 {\mu}m/s, while Brownian-like dynamics emerge for longer observation periods. Access to the ballistic regime provides verification of the equipartition theorem and Maxwell-Boltzmann statistics for flexural modes, and can be used in fast thermometry and mass sensing during atomic absorption/desorption processes on the membrane. We argue that the ballistic regime should be accounted for in understanding the electrical, optical, thermal and mechanical properties of 2D materials.