Hydrogen-associated electron-doping Mottronics for d-band correlated oxides (e.g., VO 2 ) opens up a new paradigm to regulate the electronic functionality via directly manipulating the orbital configuration and occupancy. Nevertheless, the role of hydrogen in the Mottronic transition of VO 2 is yet unclear because opposite orbital reconfigurations toward either the metallic or highly insulating states were both reported. Herein, we demonstrate the root cause for such hydrogen-induced multiple electronic phase transitions by 1 H quantification using nuclear reaction analysis. A low hydrogenation temperature is demonstrated to be vital in achieving a large hydrogen concentration (n H ≈ 10 22 cm −3 ) that further enhances the t 2g orbital occupancy to trigger electron localizations. In contrast, elevating the hydrogenation temperatures surprisingly reduces n H to ∼10 21 cm −3 but forms more stable metallic H 0.06 VO 2 . This leads to the recognition of a weaker hydrogen interaction that triggers electron localization within VO 2 via Mottronically enhancing the orbital occupancies.

Although vanadium dioxide (VO 2 ) exhibits the most abrupt metal-to-insulator transition (MIT) properties near room temperature, the present regulation of their MIT functionalities is insufficient owing to the high complexity and susceptibility associated with V 4+ . Herein, we demonstrate a spark plasma-assisted reactive sintering approach to simultaneously achieve in situ doping and sintering of VO 2 within a largely short period (∼10 min). This enables high convenience and flexibility in regulating the electronic structure of VO 2 via dopant elements covering Ti, W, Nb, Mo, Cr, and Fe, leading to a wide adjustment in their MIT temperature (T MIT ) and basic resistivity (ρ). Furthermore, the mechanical strength of the doped VO 2 is meanwhile largely improved via the compositing effect of the highmelting-point dopant oxide. The high adjustability in MIT properties and improved mechanical properties further pave the way toward practical applications of VO 2 in power electronics, thermochromism, and infrared camouflage.

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Although the hydrogen induced electronic transition within the perovskite family of rare-earth nickelate ( ReNiO3) beyond conventional semiconductors was recently discovered, the existing research stays at ReNiO3 with light rare-earth compositions. To further extend the cognition toward heavier rare-earth, herein we demonstrate hydrogen induced electronic transitions for quasi-single crystalline ReNiO3/LaAlO3 (001) heterostructures, covering a large variety of the rare-earth composition from Nd to Er. The hydrogen induced elevations in the resistivity of ReNiO3 ( RH/ R0) show an unexpected non-monotonic tendency with the atomic number of the rare-earth composition, e.g., first increases from Nd to Dy and afterwards decreases from Dy to Er. Although ReNiO3 with heavy rare-earth composition (e.g., DyNiO3) exhibits large RH/ R0 up to 107, their hydrogen induced electronic transition is not reversible. Further probing the electronic structures via near edge x-ray absorption fine structure analysis clearly demonstrates the respective transition in electronic structures of ReNiO3 from Ni3+ based electron itinerant orbital configurations toward the Ni2+ based electron localized state. Balancing the hydrogen induced transition reversibility with abruption in the variations of material resistivity, we emphasize that ReNiO3 with middle rare-earth compositions (e.g., Sm) are most suitable in catering to the potential applications in correlated electronic devices.

Although the discovery of the electrochemical protonation-induced electronic phase transition of rare-earth nickelates (ReNiO3) enables potential application in sensing the ocean electric field that simulates the working principle of the ampullary organ of marine animals, whether such a functionality is anisotropic is previously overlooked. Herein, we demonstrate the anisotropy in the protonation-induced electronic phase transition in ReNiO3 (Re = Sm, Nd, and Eu) thin films as electrochemically triggered in an ocean environment. A larger elevation in the material resistivity triggered by an electric field within an ocean environment is observed for ReNiO3/LaAlO3(110), compared to ReNiO3/LaAlO3(001) and ReNiO3/LaAlO3(111). This is attributed to the orientation-related in-plane oxygen atomic density that results in more effective in-plane proton diffusion along the adjacent oxygen position, as further confirmed by the electrochemical cyclic voltammetry characterization. In addition, the larger activation energy associated to the anisotropic in-plane electronic structures of ReNiO3/LaAlO3(110) is also expected to promote the formation of electron-localized orbital configurations upon hydrogenation. As demonstrated, anisotropy sheds light on another possibility that can be further introduced to regulate the protonation-induced electronic phase transition properties of ReNiO3 for its potential applications such as ocean electric field sensing or biosensing.