Metal–Insulator Transition Induced by Oxygen Vacancies from Electrochemical Reaction in Ionic Liquid‐Gated Manganite Films

Abstract

It was proposed that ionic liquids (IL) used as the gated dielectric in EDLT can form a strong electric fi eld at the interface between IL and oxides to electrostatically manipulate the high carrier density in a range of ≈10 14 -10 15 cm −2 , leading to the change of the electronic phase of transitional metal oxides, while it is not achievable via fi eld-effect transistor gated with conventional dielectric. [4][5][6][7][8][9][10][11][12][13][14][15][16] However, some recent reports argued that the mechanism of electronic phase change is the creation of oxygen vacancies caused by the electric fi eld in oxides, [17][18][19][20] instead of electrostatic carrier doping. On the other hand, some other results claimed that the change of the electronic phase in correlated oxides is the result of chemical doping via an electrochemical reaction. [21][22][23][24] Despite these great efforts that have been made, the underlying mechanism of IL gating driving MIT has been a matter of debate so far. Therefore, it is of vital importance to reveal the origin of the electrical control of MIT in transitional metal oxides using IL as gate mediums.The perovskite manganite materials are of signifi cant interest for those who wish to control and understand MIT in correlated oxides, as well as an excellent platform for exploring how the oxygen vacancies occur and what role they play in the electronic phase transition by electrolyte gating, because of the high correlation with the valence state of manganese determined by the oxygen stoichiometry. [ 2,25 ] In this work, we focus on a typical hole-doped perovskite oxide material, La 0.8 Sr 0.2 MnO 3 (LSMO), featured with colossal magnetoresistance and phase separation. [ 25 ] First, we will present the manipulation of the electronic phase transition by using IL gating, and the realization of a resistance increase of more than four orders of magnitude in the LSMO fi lms. Then it will be shown that the oxygen vacancies are responsible for MIT of LSMO fi lms. Finally from a series of comparative experiments presented in this work, we conclude that these oxygen vacancies are originated by the electrochemical reaction at the interface gated by IL electrolytes, and the amount of the trace water contained in the IL plays an indispensable role in the generation of oxygen vacancies in LSMO fi lms.The thin fi lms of LSMO were grown on single crystal (001) LaAlO 3 (LAO) substrates by pulsed laser deposition. The crosssectional high-angle annular dark-fi eld (HAADF) scanning transmission electron microscopy (STEM) image exhibits the LSMO/LAO sharp interface, indicating a high quality of the epitaxial heterostructure ( Figure S1, Supporting Information). It was patterned into a Hall-bar structure with coplanar In correlated transitional metal oxides, the coupling of charge, spin, orbital, and lattice degree of freedoms gives rise to the rich physics, including metal-insulator transition (MIT) [ 1 ] and interface related multifunctional properties. [ 2 ] The electric fi eld control of these intriguing p...

Hybrid supercapacitors for reversible control of magnetism

“…Here ionic intercalation into LSMO is unlikely as the DEME41 and TFSI37 ions are nearly twice the size of the LSMO unit cell30. A minor contribution due to oxygen ions migration into LSMO may be attributed to the decomposition of residual traces of water42 present in the ionic liquid (see Methods section). This is compatible with the appearance of small irreversible redox peaks on expansion of the potential window up to 3.7 V (see arrows in Fig.…”

Insulating phase at low temperature in ultrathin La0.8Sr0.2MnO3 films

“…film, both the Mn valence and the saturation magnetization exhibits bulk-like values, while the lowered Mn valence in the 8 u.c. film indicates the decrease of the effective doping density12 and the reduction of the Mn 4+ content in the film, leading to the suppression of the double-exchange mechanism. Since both the magnetic and electric properties are dominated by the completion of the double-exchange and super-exchange mechanism35, this decrease of Mn valence results in the decrease of the Curie temperature Tc and the suppression of the magnetic properties of the ultrathin films.…”

“…Epitaxial strain in the manganite thin films, which can induce structural distortions and produce spin, orbital and charge reconstructions at the interface, has been proposed to play an important role in such behaviors910. However, understanding for these deviations from the bulk material still remains elusive, and many other factors have been found to account for such behaviors, such as the oxygen vacancies1112 and charge transfer at the interfaces13. In addition, Liao et al have demonstrated that the nonmetallic behavior in LSMO thin films is mainly due to the disorder–induced localization effects which can be amplified by the reduction in dimensionality14.…”

Emergent electric field control of phase transformation in oxide superlattices

“…To fully exploit this mechanism, materials that can be electrically switched from one crystalline phase to another, with distinct physical properties, are highly desirable. Although ion transfer has been reported in many singlephase TMOs [8][9][10][11][12][13][14][15][16][17][18] , reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature (RT) has been limited to a few systems, including binary oxides VO2 8 and WO3 12,13 , and perovskite oxides SrCoO3-δ 14 and SrFeO3-δ 17 that exhibit topotactic transformations. Among these systems, coupled electronic, magnetic and optical phase transitions have only been demonstrated in SrCoO3-δ thus far 14 .…”