Wet Chemical Synthesis of Patterned Bismuth Ferrite Thin Films by Direct Writing (printing) and Characterization Using Printed Electrodes

BiFeO₃ (BFO) is the only naturally occurring room-temperature multiferroic with high Neel (643 K) and Curie (1103 K) temperatures enabling effective magnetic-ferroelectric coupling, thus opening avenues of non-volatile FRAM (ferroelectric random access memory), spintronics, MEMS and one of three realistic candidates to replace lead zirconate titanate (PZT) in ferroelectric and piezoelectric applications. Owing to lead's toxicity, safety, and disposal hazards, PZT is being phased out, and BFO leads over potassium sodium niobate (KNN) and NBT possess severe volatility and non-stoichiometric defects as well as a narrower operating temperature range due to lower Curie temperatures even after significant doping.<br/>Wet chemical routes proffer the distinct advantage of large-scale production with lower temperatures compared to conventional solid-state reaction and lack of high vacuum requirements as in physical vapor deposition techniques. Of the routes available, sol-gel was chosen due to the flexibility in the choice of nanostructures that could be obtained- from thin films to nanoparticles.<br/>In this work, BFO coatings were deposited on fluorine-doped tin oxide (FTO), indium-doped tin oxide (ITO), and quartz substrates via direct writing using a customized with optimized printing and extrusion parameters. A precursor-based ink was developed that remains stable in dispersion form for up to 9 months without aggregation or any phase separation. Choice of solvent, precursor, solvent-precursor ratio, chelating agent, and temperature were critical, which aided in achieving phase pure BFO at a low sintering temperature of 500 °C for 1 hour. Phase purity was ascertained by structural characterization- XRD, SEM-EDS, Raman spectroscopy, and UV-vis spectroscopy, while the BFO ink characteristics were optimized for printability using contact angle (wettability) and viscosity tests. Optical profilometry confirmed the thickness of films deposited, while UV-vis spectroscopy also indicated optical bandgap in the range of 2.6 to 2.9 eV using Tauc plots.<br/>Traditionally, top and bottom electrodes were deposited by evaporation or sputtering, which necessitated high vacuum and expensive pure gold targets. This work utilized direct writing to print silver nanoparticles as electrodes to get Ag/BFO/ITO, Ag/BFO/FTO, and Ag/BFO/quartz heterostructures to frugally obtain contacts required for electrical characterization. By optimizing extrusion and nozzle parameters during printing, the significantly expensive material loss during sputtering/evaporation was bypassed, while flexibility with the printer allowed for choice of electrode shape (square/circular) and dimension without the need for a mask.<br/>References:<br/>[1] H Liu, J Liu, H Zhu, Y Wang, J Ouyang, Materials Letters, 320, 132387 (2022)<br/>[2] N Spaldin, R Ramesh, Nature Materials, 18, 203 (2019)