Percolation-Limited Threshold Switching in Strain-Graded Mott Devices

The threshold switch operated by a field-driven insulator-to-metal transition in VO₂ has attracted considerable interest for emerging devices due to its nonlinear and sensitive response to external voltage. However, the nucleation barrier intrinsic to the first-order phase transition causes a finite time delay before the abrupt rise in currents under voltage pulses, thereby hindering energy-efficient device operation. Here, we demonstrate that the strain-graded VO₂ epilayer on Pt nanoislands (NIs) enables percolation-limited threshold switching by promoting the nucleation process of metallic phases during voltage-triggered phase transitions. Unlike constantly strained VO₂, the Pt NIs locally disrupt lattice coherency at the VO₂/TiO₂ interface, facilitating gradual relaxation of misfit strain energy; tailoring the spatial strain distribution in the strain-graded VO₂ films effectively lowers the activation barrier for the nucleation events of metallic domains, achieving one-twentieth lower incubation time (τ_inc) compared to constantly strained VO₂ films. Moreover, this percolation-limited phase evolution stabilizes an intermediate metastable phase (i.e., negative differential resistance), enabling robust self-oscillatory behavior across a wide current range with enhanced tunability and dynamic controllability. These findings tailor the phase transition dynamics for ultrafast and energy-efficient switching applications.