Lead-free piezoceramics aiming at replacing the market-dominant Pb(Zr_xTi_1−x)O₃ have been extensively researched for more than a decade worldwide due to the toxicity of lead. In this context, (K_0.5Na_0.5)NbO₃ (KNN) triggered the attention of the scientific community thanks to its bulk record piezoelectric coefficient combined with high critical temperature, which make it a good candidate for applications. In this paper, (001)-oriented KNN thin films grown by pulsed laser deposition on Pt(111)/TiO₂/SiO₂/Si substrates are investigated. We highlight the relevance of the template substrate in determining the structure of the film. Developing a suitable treatment for the Pt substrate is of great importance to film morphologic and topographic quality and to electric, ferroelectric, and piezoelectric properties. From local characterization of piezoelectric properties, we find a piezoelectric coefficient d₃₃ of about 80 pm/V, comparable to the highest values reported for state-of-the-art undoped KNN thin films. Finally, electrical characterization of fabricated micro-capacitors allows the investigation of dielectric performance and shows remanent ferroelectric polarization over microscopic areas, thus paving the way to the integration of these KNN films in microfabricated actuator devices.

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.

Integrated optical biosensors are gaining increasing attention for their exploitation in lab-on-chip platforms. The standard detection method is based on the measurement of the shift of some optical quantity induced by the immobilization of target molecules at the surface of an integrated optical element upon biomolecular recognition. However, this requires the acquisition of said quantity over the whole hybridization process, which can take hours, during which any external perturbation (e.g., temperature and mechanical instability) can seriously affect the measurement and contribute to a sizeable percentage of invalid tests. Here, we present a different assay concept, named Opto-Magnetic biosensing, allowing us to optically measure off-line (i.e., post hybridization) tiny variations of the effective refractive index seen by microring resonators upon immobilization of magnetic nanoparticles labelling target molecules. Bound magnetic nanoparticles are driven in oscillation by an external AC magnetic field and the corresponding modulation of the microring transfer function, due to the effective refractive index dependence on the position of the particles above the ring, is recorded using a lock-in technique. For a model system of DNA biomolecular recognition we reached a lowest detected concentration on the order of 10 $\mathrm{p}$m, and data analysis shows an expected effective refractive index variation limit of detection of $7.5\times {10}^{-9}$ RIU, in a measurement time of just a few seconds.