Diffuson-driven ultralow thermal conductivity in amorphous N b2 O5 thin films

Niobium pentoxide (Nb ₂ O ₅ ) has been investigated extensively for applications such as electrochemical energy storage, memristors, solar cells, light emitting diodes, and electrochromic devices. The thermal properties of Nb ₂ O ₅ play a critical role in device performance of these applications. However, very few studies on the thermal properties of Nb ₂ O ₅ have been reported and a fundamental understanding of heat transport in Nb ₂ O ₅ is still lacking. The present paper closes this gap and provides a study of thermal conductivity of amorphous Nb ₂ O ₅ thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. By using the vibrational density of states of the single-crystal phase obtained from density functional theory simulations as an approximation, a diffuson-mediated minimum thermal conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit, which proves that the diffuson model works better than the amorphous limit model to describe the thermal conduction mechanism in the amorphous Nb ₂ O ₅ thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.