Delamination-assisted ultrafast wrinkle formation in a freestanding film

TL;DR

This study uses ultrafast electron diffraction to observe the rapid formation of wrinkles in freestanding 2D films, revealing that delamination at the boundary enables ultrafast wrinkle development, which is crucial for materials engineering.

Contribution

It provides a new experimental protocol to quantify wrinkle dynamics in freestanding films and uncovers the role of delamination in ultrafast wrinkle formation.

Findings

Wrinkle formation occurs within picoseconds.

Delamination at the boundary facilitates wrinkle development.

Film-substrate interactions are key to freestanding film properties.

Abstract

The ability to synthesize and control two-dimensional (2D) crystals creates numerous opportunities for studying emergent states of matter and their novel functionalities. Freestanding films provide a particularly versatile platform for materials engineering in 2D thanks to additional structural motifs not found in films adhered to a flat substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time at the nanoscopic scale. Here, we use ultrafast electron diffraction to directly observe light-activated wrinkling formation in a freestanding La$_{2/3}$Ca$_{1/3}$MnO$_3$ film. Via a ``lock-in'' analysis of a single mode of oscillation in the diffraction peak position, intensity, and width, we are able to quantitatively reconstruct how wrinkles develop at the picosecond timescale. Contrary to the common assumption of a fixed boundary condition for freestanding films, we found that wrinkle development is associated with ultrafast delamination at the contact point between the film boundary and the underlying substrate, avoiding a strain build-up in the film. Not only does our work provide a generic protocol to quantify wrinkling dynamics in freestanding films, it also highlights the importance of film-substrate interaction for determining the properties of freestanding structures.