The ability to control a variety of functionalities with external stimuli is one of the main issues in correlated oxides and their heterostructures. Due to the extreme sensitivity of those material systems to external stimuli, the control of the versatile functionalities can achieve unique phenomena, such as metal-insulator phase transition, which can be applicable for various future electronic devices that require high sensitivity and steepness. Among the various stimuli, (intrinsic or extrinsic) atomic defects have a strong influence on the d-band filling, which is the core concept of correlated electronic systems.
Hydrogen, the smallest and the lightest one among atomic elements, is reversibly incorporated into the interstitial site of vanadium dioxide (VO2), a 3d1 correlated metal oxide undergoing metal-insulator transition at ~ 68 °C, and then induces dramatic electronic phase modulation. Here, we present that hydrogenation can be achieved up to two hydrogen atoms per VO2 unit cell, and hydrogen is reversibly absorbed into and released out of VO2 without destroying the lattice framework due to the low temperature annealing process . More importantly, this massive hydrogenation process allows to elucidate phase modulation of vanadium oxyhydride (HxVO2), remarkably demonstrating two-step insulator (VO2) – metal (HxVO2) – insulator (HVO2) phase transition during inter-integer d band filling. Furthermore, this massive hydrogenation was also universally observed in facet-dependent  and E-field-controlled  phase modulation using precisely controlled experiments. Our finding not only shows the possibility of reversible and dynamic control of topotactic phase transition in VO2, but it also opens up the potential application of functional defects in correlated oxides for various electronic applications.
This work was performed in collaboration with Hyojin Yoon, Minguk Jo, Jaeseoung Park, Hyunah Kwon, Prof. Ji-Young Jo, Prof. Minseok Choi, Prof. Si-Young Choi, Dr. Kyuwook Ihm, Prof. Donghwa Lee and Prof. Jong-Kyu Kim.
 H. Yoon et al., Nature Mater. 15, 1113 (2016)
 H. Yoon et al., Adv. Electron. Mater. 1800128 (2018)
 M. Jo et al., Adv. Funct. Mater. (Accepted)