The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO₂ and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO₂ as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., “wake-up”), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe’s confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

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.

The present research presents a portable bioelectronic platform for multiplex detection to read biosensor chips with several sensing sites for real-time analyte capture. The technique is based on Differential Impedance Sensing (DIS) of the target through functionalized nanoparticle amplification. Gold-interdigitated microelectrodes are the core of the biosensing system. They are designed in a differential configuration, reference and active sensor, to counteract all possible mismatches such as temperature fluctuations and variations in the ion content of the solution. The surface of the sensor is biochemically functionalized with a synthetic probe specifically developed for the selected target. The successful combination of all of these elements allowed the system to detect IgG antibodies spiked in buffer with a limit of detection of below 100 pg/mL. In a real case study for viral infection diagnosis, the system has been challenged with infected human serum samples for digital counts of anti-dengue virus antibodies, achieving the detection of clinically relevant target concentrations. Also, the bio-reconfigurability of the system has been successfully tested with oligonucleotide detection down to pM target concentration. To allow the portability of the entire measurement setup, the setup has been equipped with a custom electronic board based on an FPGA module allowing a multiplexing approach for the parallel reading of several electrodes. The final system provides simple and effective bio-reconfigurability, exploiting advances in bio-recognition through proper probe selection and boosting the possible use of multiplex sensing to a broad spectrum of needs.