Tracing the ionic evolution during ILG induced phase transformation in strontium cobaltite thin films

Strain-Induced Uphill Hydrogen Distribution in Perovskite Oxide Films

Incorporating hydrogen into transition-metal oxides (TMOs) provides a facile and powerful way to manipulate the performances of TMOs, and thus numerous efforts have been invested in developing hydrogenation methods and exploring the property modulation via hydrogen doping. However, the distribution of hydrogen ions, which is a key factor in determining the physicochemical properties on a microscopic scale, has not been clearly illustrated. Here, focusing on prototypical perovskite oxide (NdNiO3 and La0.67Sr0.33MnO3) epitaxial films, we find that hydrogen distribution exhibits an anomalous "uphill" feature (against the concentration gradient) under tensile strain, namely, the proton concentration enhances upon getting farther from the hydrogen source. Distinctly, under a compressive strain state, hydrogen shows a normal distribution without uphill features. The epitaxial strain significantly influences the chemical lattice coupling and the energy profile as a function of the hydrogen doping position, thus dominating the hydrogen distribution. Furthermore, the strain-(H+) distribution relationship is maintained in different hydrogenation methods (metal-alkali treatment) which is first applied to perovskite oxides. The discovery of strain-dependent hydrogen distribution in oxides provides insights into tailoring the magnetoelectric and energy-conversion functionalities of TMOs via strain engineering.

Realizing Metastable Cobaltite Perovskite via Proton-Induced Filling of Oxygen Vacancy Channels

The interaction between transition-metal oxides (TMOs) and protons has become a key issue in magneto-ionics and proton-conducting fuel cells. Until now, most investigations on oxide-proton reactions rely on electrochemical tools, while the direct interplay between protons and oxides remains basically at simple dissolution of metal oxides by an acidic solution. In this work, we find classical TMO brownmillerite SrCoO_2.5 (B-SCO) films with ordered oxygen vacancy channels experiencing an interesting transition to a metastable perovskite phase (M-SCO) in a weak acidic solution. M-SCO exhibits a strong ferromagnetism (1.01 μ_B/Co, T_c > 200 K) and a greatly elevated electrical conductivity (∼10⁴ of pristine SrCoO_2.5), which is similar to the prototypical perovskite SrCoO₃. Besides, such M-SCO tends to transform back to B-SCO in a vacuum environment or heating at a relatively low temperature. Two possible mechanisms (H₂O addition/active oxygen filling) have been proposed to explain the phenomenon, and the control experiments demonstrate that the latter mechanism is the dominant process. Our work finds a new way to realize cobaltite perovskite with enhanced magnetoelectric properties and may deepen the understanding of oxide-proton interaction in an aqueous solution.

Nonvolatile n-Type Doping and Metallic State in Multilayer-MoS2 Induced by Hydrogenation Using Ionic-Liquid Gating

Manipulation of the carrier density of layered transition-metal dichalcogenides (TMDs) is of fundamental significance for a wide range of electronic and optoelectronic applications. Herein, we applied the ionic-liquid-gating (ILG) method to inject the smallest ions, H⁺, into layered MoS₂ to manipulate its carrier concentration. The measurements demonstrate that the injection of H⁺ realizes a nonvolatile n-type doping and metallic state in multilayer-MoS₂ with a concentration of injection electron of ∼1.08 × 10¹³ cm^-2 but has no effect on monolayer-MoS₂, which clearly reveals that the H⁺ is injected into the interlayer of MoS₂, not in the crystal lattice. The H⁺-injected multilayer-MoS₂ was then used as the contact electrodes of a monolayer-MoS₂ field effect transistor to improve the contact quality, and its performance has been enhanced. Our work deepens the understanding of the ILG technology and extends its application in TMDs.

High-Conductive Protonated Layered Oxides from H2 O Vapor-Annealed Brownmillerites

Protonated 3d transition-metal oxides often display low electronic conduction, which hampers their application in electric, magnetic, thermoelectric, and catalytic fields. Electronic conduction can be enhanced by co-inserting oxygen acceptors simultaneously. However, the currently used redox approaches hinder protons and oxygen ions co-insertion due to the selective switching issues. Here, a thermal hydration strategy for systematically exploring the synthesis of conductive protonated oxides from 3d transition-metal oxides is introduced. This strategy is illustrated by synthesizing a novel layered-oxide SrCoO₃ H from the brownmillerite SrCoO_2.5 . Compared to the insulating SrCoO_2.5 , SrCoO₃ H exhibits an unprecedented high electronic conductivity above room temperature, water uptake at 250 °C, and a thermoelectric power factor of up to 1.2 mW K^-2 m^-1 at 300 K. These findings open up opportunities for creating high-conductive protonated layered oxides by protons and oxygen ions co-doping.