The Piezoresponse in WO 3 Thin Films Due to N 2 -Filled Nanovoids Enrichment by Atom Probe Tomography.

Tungsten trioxide (WO 3 ) is a versatile n -type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d 33 = 35 pmV -1 on WO 3 thin films ~200 nm deposited using RF-sputtering onto alumina (Al 2 O 3 ) substrate with post-deposit annealing treatment of 400 °C in a 3% H 2 /N 2 -forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO 3 with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N 2 -filled nanovoids that acts as strain centers producing a local deformation of the WO 3 lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.