Manipulate The Properties Of Transition Metal Oxides Through Electric-Field Induced Protonation

Transition metal oxides have been attracting increasing attentions due to their fundamental physics and functional applications in the last decades,with a wealth of novel phenomena emergent,such as high temperature superconductivity in cuprate,metal-insulator transition in nickelate,and colossal magnetoresistance in manganite,etc.During these studies,one of the most important things is to effectivly manipulate the charge and spin degrees of freedom and their correlation with the crystalline lattice,and a series of methodologies have been developed,such as pressure/strain,chemical doping,magnetic-field and electric-field.Since the invention of the first transistor in1947,the electric-field-controlled electronic state has been realized in various material systems through the electrostatic charge modulation across the interface between the material and the gate layer.However,because of the limitation of charge screening effect,the modulated charge density is only about<10¹⁴/cm²,which however is not enough the trigger the electronic phase transition in oxides due to their dramtically higher carrier cenctration as compared with the semicondutors.Recently the transistor with ionic liquid?IL?electrolyte as the gate layer was developed as a more powerful method to realize higher charge modulation(<10¹⁵/cm²),which was well employed in many oxide systems.It is worth noting that the conventional ionic liquid gating?ILG?method employes mainly the pure electrostatic effect with charge accumulation manipulated at the liquid/solid interface,while the associated electrochemical reaction was carefully avoided.However,recent experiments have demonstrated that the residual water would exist ubiquitously within the IL,which may induce electrochemical reaction through electrolysis into H⁺and O^2-ions during the ILG.Especially,the H⁺ion?proton?as the lightest and smallest ion,can be intercalated into many oxide lattices,and hence induce ultra-large electron doping(>10¹⁶/cm²).With this,the electric-field controlled protonation through ILG emerges as a novel strategy to manipulate the electronic and magnetic properties of oxides.Clearly,the study of ILG induced protonation is still in its early stage.To obtain a further understanding on its underlying mechanism and expand its manipulation capability,we chose WO₃ and Ni Co₂O₄ as the model systems in this thesis.In WO₃,we realize an insulator to metal transition through ILG induced proton evolution,and solve the controversy of phase transition mechanism.In the study of Ni Co₂O₄,we further invesitage the mechanism of proton migration from liquid/solid interface to the whole film,and then develop a high-temperature ILG strategy to realize effieciency-improved protonation process.Furthremore we demo the protonation can induce a phase transition from Ni Co₂O₄ to H₂Ni Co₂O₄ with novel switching of the magnetic and electronic states from ferimagnetic metallic to antiferromagnetic insulator.In summary,through the studies in WO₃ and inverse spinel Ni Co₂O₄,we obtain a further understanding of the underlying mechanism of electric-field controlled protonation and demonstrate that this new method forms a generic tuning knob to realize rich phenomena and functionalities in oxides.