Visualizing the motion of graphene nanodrums

TL;DR

This study visualizes the vibrational modes of graphene nanodrums with high spatial resolution, revealing mode splitting, imperfections, and complex dynamics crucial for their reliable use in nanomechanical applications.

Contribution

It introduces a phase-sensitive interferometric method to directly visualize and analyze the vibrational modes of graphene nanodrums, clarifying mode splitting and deformation effects.

Findings

Resonance frequencies up to the eighth mode match theoretical predictions.

Mode splitting explains previously unexplained spectral features.

Imperfections deform nodal lines more in higher vibrational modes.

Abstract

Membranes of suspended two-dimensional materials show a large variability in mechanical properties, in part due to static and dynamic wrinkles. As a consequence, experiments typically show a multitude of nanomechanical resonance peaks, which makes an unambiguous identification of the vibrational modes difficult. Here, we probe the motion of graphene nanodrum resonators with spatial resolution using a phase-sensitive interferometer. By simultaneously visualizing the local phase and amplitude of the driven motion, we show that unexplained spectral features represent split degenerate modes. When taking these into account, the resonance frequencies up to the eighth vibrational mode agree with theory. The corresponding displacement profiles however, are remarkably different from theory, as small imperfections increasingly deform the nodal lines for the higher modes. The Brownian motion, which is used to calibrate the local displacement, exhibits a similar mode pattern. The experiments clarify the complicated dynamic behaviour of suspended two-dimensional materials, which is crucial for reproducible fabrication and applications.