Anomalous Phonon Evolution via Out-of-Plane Strain Engineering in van der Waals Epitaxial VO₂ Film

Vanadium oxide (VO₂) exhibits a unique phase transition, making it promising for ultrafast switches, neuromorphic computing, and memory devices. Interfacial strain engineering is a universal strategy for manipulating the phase transition behavior of VO₂ films, but most studies have focused only on in-plane strain (epitaxial) due to the challenges of applying out-of-plane strain. Here, we address this gap by employing a diamond anvil cell-based high-pressure method to investigate the phase transition properties of VO₂ films under strong out-of-plane strain. We elucidate the role of substrate clamping in epitaxial VO₂ films under out-of-plane compression by designing van der Waals epitaxial VO₂ films on flexible mica substrates (as-grown VO₂) and freestanding membranes (f-VO₂) through a lift-off process. The phase transition threshold of the as-grown VO₂ film is 62% higher than that of the f-VO₂ membrane under a high pressure. Strikingly, an anomalous phonon mode in the as-grown VO₂ film was observed, but not in the f-VO₂ membrane, which is attributed to the coupling of out-of-plane phonon and stronger substrate clamping (interlayer interaction) under high pressure. Our findings open up the possibility of probing complex phonon behavior in oxide films by manipulating the interlayer interaction by, for example, heterostructural engineering.