Piezoelectric response by PFM
WO3 thin films were deposited on sapphire with a resulting thickness of 225 nm and subsequently annealed at various temperatures, see experimental methods section. A piezoelectric response was found in the film annealed at 400°C, determined by characterization using piezo force microscopy technique in dual AC resonance tracking (DART) mode, as described extensively in [22, 26–28, 46, 47]. The surface topography of this film is shown in Fig. 1a, and Fig. 1b) and 1c) corresponds to the piezo force microscopy signal phase before and after measurements, revealing a local hysteresis loops; hysteresis loops corresponding to red circles, where piezo response domains appear as yellow, white and violet colored regions show polarization direction piezoelectric domains, as described by Kholkin et al. . The white regions are positive domains, i.e., polarization pointing towards the bottom electrode, which occurred by switching domains as observed in hysteresis loops shown in Fig. 1e. Furthermore, ferroelectric behavior was observed in the sample annealed at 400°C from hysteresis loops in phase and piezoelectric coefficient (d33) versus AC applied bias voltage as shown in Fig. 1f. The piezoelectric coefficient was determined by positioning the cantilever across a large grain of Al-WO3 − x (red circle). The amplitude (nm) versus AC bias voltage (V) exhibited a butterfly loop as presented in Fig. 1d and it is related to piezoelectric deformation under an applied AC bias voltage demonstrating a local polarization switching behavior . The latter indicates that a phase difference of 180° polarization switching under DC bias voltage related to the existence of piezoelectric domains and local d33 coefficient can be estimated by (V-V1)d33 = D-D1, where D is the measured piezoelectric deformation or amplitude, V is applied voltage, D1 is the piezoelectric deformation, and V1 is the applied voltage at the intersection as described by Roelofs et al. . The coercive voltage (2.7 V) was evaluated using the equation (Vc+-Vc−)/2 where Vc+ and Vc− are the forward and reverse coercive bias voltages, respectively. The piezoelectric coefficient (d33) of 35 ± 5 pmV− 1 was measured at the maximum voltage of 10 V for the film annealed at 400°C, which is four times higher than that reported by Kim et al.  for WO2.9 and the highest value found in the literature for this material, indicating a potential use in piezoelectric devices .
The measured d33 coefficient is assume to occur due to non-centrosymmetric phases in combination with potential oxygen vacancies and aluminum doping induced a different stoichiometric composition of WO3 − x films, mainly for those processed at 400°C in agreement with reports as found in the literature [38, 48–57]. And confirmed by XPS measurements (Supplemental material) which reveals a stoichiometry of WO2.7 in the surface of the film annealed at 400°C and in agreement with grazing incidence x-ray diffraction (GIXRD) and atom probe tomography (APT) as presented in this communication.
Crystallographic structure as determined by GIXRD
As presented in Fig. 2, an evolution of crystallinity occurred on the films from room temperature to annealing process at 400°C and 550 ºC corresponding to polycrystalline amorphous structure, reflections at 23.1° corresponds to (001), (021) and (121) and corresponds to monoclinic WO3 − x phase (γ-WO3 − x) , – with space group P21/c, a tetragonal WO3 − x phase (α-WO3 − x) with P4/nmm\(\)space group is formed for sample at 400°C in agreement with literature [61, 62]. The Raman spectra indicate a distorted octahedral tungsten vibration mode of monoclinic WO3 − x at 236.9 cm− 1, 691 cm− 1 and 803 cm− 1 attributed to bending O-W-O bonds and symmetric/antisymmetric stretching of W-O bonds (see supplemental material). However, as the annealing temperature increased and evolution of phases occurred with γ-WO3 − x or α-WO3 − x for 500°C and 550°C, respectively as observed. Furthermore, diffractions at ~ 37º which corresponds to (111) planes of metallic aluminum (FCC), only appear at 400°C and not at 500°C and 550°C, our believe is that aluminum clusters is formed by interdiffusion from sapphire substrates (Al2O3) during annealing process, in agreement with Li et al. oxygen vacancies can promote defects and dislocation; and potentially can induce diffusion of aluminum ions on lattice sites within WO3 ; because critical temperature of aluminum diffusion is reached at above 300°C in agreement with previous reports, and revealed by atom probe tomography, Fig. 3. Thus, segregated aluminum induces formation of mixed α-WO3 − x and γ-WO3 − x phases in agreement with previous reports , ; as it is described usually heavy metallic ions like induces recrystallization in thin films . Also, metallic species such as gold (Au) induces phase change from triclinic to monoclinic in WO3 . In here, we were able to determine that aluminum is diffused creating changes on the electronic states mainly on island form over film and mixed monoclinic and tetragonal (α-WO3 − x and γ-WO3 − x) specially when is processed at 400°C, in agreement with Ahart et al.  and Ibrahim et al.  who explained in detail that mixed phases can produce a morphotropic phase boundary (MPB).
Chemical distribution by atom probe tomography
In order to investigate chemical volume distribution a series of atom probe tomography characterizations were completed, which is an abrasive technique used to obtain time of flight mass spectrometry from events occurred due to laser pulse ionic evaporation at high-vacuum as described in the literature , for all APT measurements a well-defined interface between WO3 film and Al2O3 substrate was revealed. From mass spectrum it was possible to achieve chemical composition distribution mainly at the WO3 film thickness (0-400nm) and traces of aluminum, oxygen and tungsten was found as shown in Fig. 3. Tungsten concentration remains around 27% during annealing process and oxygen concentration is about 70% at 400 ºC with strong traces of aluminum (~ 3%) which forms clusters, as shown in Fig. 3a; it is our believe this clusters are formed during annealing process by ionic diffusion from sapphire substrate due to voids occurred by oxygen vacancies which allowed aluminum ions to undergo onto WO3 , , which is in agreement with mixed phases as encountered by grazing incidence x-ray diffraction. For sample processed at 500°C lower concentration (> 1%) of aluminum ions is found and corresponds mainly to α-WO3 − x as shown in Fig. 3b and no traces of grain boundaries was found for all samples.