Low-temperature solution-processed sol-gel K-rich KNN thin films for flexible electronics

ABSTRACTSputtered lead-free piezoelectric materials like potassium sodium niobate (K1-xNaxNbO3 or KNN) have received significant technological interest in recent years in light of several reports of piezoelectric constants comparable to lead zirconium titanate (PZT). Potential applications include self-powered sensors, actuators, and low acoustic impedance transducers. For large area printed applications, it is vital to develop low-temperature solution processed deposition methods. In this work, sol-gel synthesis of K-rich (70:30) KNN was carried out under an argon atmosphere, using acetate precursors, followed by precipitation of white KNN powder upon careful drying. Powder X-ray diffraction (XRD) scans of the product with a Cu Kα source after calcination revealed a dominant (110) peak, accompanied by smaller (100) and (010) peaks, in agreement with published standard KNN data. The composition of K-rich phase was confirmed using energy dispersive X-ray spectroscopy (EDX). To produce thin films, the sol was spin coated on a surface-treated Au-coated Si substrate, followed by slow annealing to obtain low surface roughness films (RMS roughness ﹤∼10 nm) of thickness ∼200 nm after solvent removal. Atomic force microscopy (AFM) scans revealed an unremarkable amorphous film. However, deposition of the sol on the Au-coated backside of Si wafer under similar processing conditions revealed limited polycrystalline film formation observed using optical profilometry. Thin film XRD measurements of the deposited film reveal orthorhombic phase growth of KNN, though the unannealed film was more amorphous than the calcined KNN film. Preliminary piezoresponse force microscopy (PFM) scans were used to estimate a piezoelectric constant (d33) ∼ 2.7 pC/N, consistent with the general expectation of lower piezoelectric constants for thin sol-gel films. The highest processing temperature used at any step during the deposition process was 90°C, consistent with the applications involving flexible polyimide substrates. This low-temperature thin-film growth suggests a potential route towards integration of large area piezoelectric generators for environmentally-friendly autonomous flexible sensor applications, with better control of phase and composition during the solution-phase deposition of KNN.