Transition metal oxide (TMO) thin films have shown intriguing physical phenomena as their structure and physical properties are strongly intertwined. Iontronics has emerged as a novel concept to tailor the lattice symmetry of TMOs and the associated physical phenomena (1,2). Here, I present the first realization of single crystalline T-Nb₂O₅ thin film growth critically with vertical ionic transport channels, leading to fast and colossal insulator-metal transitions and excellent reversibility via ionic liquid gating (ILG) (3). A change in lattice symmetry from orthorhombic to monoclinic coupled with electronic phase transitions are revealed by various in situ characterizations. Moreover, a new concept of synchronized local ionic gating of correlated oxides will be demonstrated, enabling local control of electronic, magnetic, and optical properties by gating through extended nano-windows (4). The gated V-antenna arrays in a continuous VO₂ layer exhibits optical metasurface responses with tunable anomalous reflection of light by electric-field and temperature control. Moreover, the synchronized ionic gating of ferromagnetic La_0.67Sr_0.33MnO₃ layer reveals the electrical formation of magnetic metamaterials. i.e., artificial spin ices that show collective magnetic phenomena via interaction between nano-islands. By employing a hydrogen spillover method, I present itinerant ferromagnetic SrRuO₃ (SRO) thin films whose resistivity and magnetism are extensively tuned (5). The hydrogenation induces a sign reversal of the anomalous Hall effect coupled with octahedral tilting, revealed by in situ crystal truncation rod measurements. Finally, I demonstrate exsolution of nanoparticles from perovskite thin films in a reducing environment. The roles of strain (6), facet (7), and anti-phase boundaries (8) in the exsolution phenomena will be presented. These studies pave a new way to tailor lattice symmetry and emergent physical phenomena and provide a novel approach to create and control complex electronic, magnetic, optical, and electrochemical devices.

1. Y. Guan†, H. Han† et al., Annual Review of Materials Research Under review.
2. H. Han et al., ACS Nano 16, 6206−6214 (2022).
3. H. Han* et al., Nature Materials Under review.
4. H. Han et al., Nature Materials Under review.
5. H. Han* et al., Advanced Materials doi.org/10.1002/adma.202207246 (2022).
6. H. Han et al., Nature Communications 10, 1471 (2019).
7. K. J. Kim†, H. Han† et al., Journal of the American Chemical Society 141, 7509−7517 (2019).
8. H. Han* et al., Nature Communications 13, 6682 (2022).