  Niobium pentoxide (Nb2O5) has been extensively reported for applications of
electrochemical energy storage, memristors, solar cells, light emitting diodes
(LEDs), and electrochromic devices. The thermal properties of Nb2O5 play a
critical role in device performance of these applications. However, very few
studies on the thermal properties of Nb2O5 have been reported and a fundamental
understanding of heat transport in Nb2O5 is still lacking. The present work
closes this gap and provides the first study of thermal conductivity of
amorphous Nb2O5 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. Density-function-theory (DFT) simulations
combined with a diffuson-mediated minimum-thermal-conductivity model confirms
this finding. Additionally, the measured thermal conductivity is lower than the
amorphous limit (Cahill model), which proves that the diffuson model works
better than the Cahill model to describe the thermal conduction mechanism in
the amorphous Nb2O5 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.
