Nano-mechanical and structural study of WO3 thin films

• Three depth-regions of interest were identified from the nanoindentation tests.
• The annealed sample exhibited higher values for hardness and elastic modulus.
• Values of nano-mechanical properties in region I are substrate-independent.
• The monoclinic P21/c and triclinic P-1 structures were evidenced in annealed sample.
• A polyimide film is suggested as substrate for a future flexible gas sensor.

We present results of nano-mechanical and spectroscopic measurements of tungsten oxide (WO3) thin films fabricated by high vacuum DC magnetron sputtering as a step in the search for flexible gas sensors. In order to understand the mechanical properties at the nano-scale, the thin film samples were subjected to nanoindentation measurements in continuous stiffness measurement (CSM) mode, obtaining higher values for hardness (H) and elastic modulus (E) in samples annealed at 500 °C as compared to as-deposited samples (room temperature), within three depth regions of interest. Scanning electron microscopy (SEM) was used for morphological characterization. To determine their crystalline structure, grazing incidence X-ray diffraction (GIXD) analysis was performed on the films. An amorphous structure was determined for the as-deposited sample. To reach a final conclusion about the structural phases present in the material, spectroscopic measurements using confocal Raman spectroscopy and mapping were carried out, yielding two regions with different spectral characteristics within the annealed sample and enabling the visualization of those spatial domains. These measurements confirmed the polycrystalline nature of the annealed film, evidencing a triclinic (space group, P1̅) and a non-conventional monoclinic (space group, P21/c) structures.