We fabricated transistor structures with channels of (100)-and (001)-oriented K2NiF4-type LaSrNiO4 (LSNO) epitaxial films and gate insulators of the proton-conducting electrolyte Nafion and investigated how the growth orientation influences their electrochemical responses. Gate-voltageinduced electrochemical reactions are found to modulate the resistance of 10-nm-thick (100) LSNO film channels by~ 103%, and the modulation ratio of the (100) LSNO channels decreases with increasing film thickness. On the other hand, the resistance of the (001) LSNO channels is hardly modulated electrochemically, and the modulation ratio is less than 1%, independent of the thickness. In addition, the electrochemical reactions result in no apparent changes in the LSNO lattices, regardless of the growth orientation and film thickness. Our results indicate that the electrochemical reactions produce oxygen vacancies in the limited region about 10 nm from the surface, and the oxygen ions diffuse preferentially within the NiO2 layer in LSNO. Electrochemical redox reactions enable one to control the structural and physical properties of transition metal oxides electrically. Recently, it has been shown that transistor structures with gate insulators of electrolytes and channels of oxides can be utilized for inducing electrochemical redox reactions and for electrically controlling oxides’ properties [1–12]. When electrolytes containing water molecules and protons are used as gate insulators, gate-voltage-induced redox reactions in transistor structures reversibly modulate oxygen contents and even introduce protons in channel materials, leading to reversible and nonvolatile changes in channels’ electrical and magnetic properties [13–17]. Given that oxygen vacancy formations and ionic diffusions during electrochemical reactions depend on arrangements of metal–oxygen bonds, electrochemical responses of materials would vary with their crystallographic orientations [18–25]. K2NiF4-type structured oxides have a crystal structure that can be regarded as a layered perovskite consisting of an alternating stacking of perovskite ABO3-and rock-salt AO-layers [26]. The presence of the rock-salt layer results in orientation-dependent differences in physical properties like electrical resistivity and magnetic properties, highlighting that K2NiF4-type structured oxides have a layered nature and provide a platform for exploring anisotropic physical properties [27–29]. However, obtaining single-crystal forms of K2NiF4-type structured oxides is rather difficult, and exploring and understanding their anisotropic properties have still been limited.

In this study, we focus on the K2NiF4-type structured nickelate LaSrNiO4 (LSNO)[30–32] and investigate the orientation dependence of its electrochemical response. We employed the epitaxial growth technique by pulsed laser deposition to make single-crystalline LSNO films with (100) and (001) orientations. We show that in transistor structures with gate insulators of proton-conducting electrolytes (Nafion), electrochemically induced changes in the electrical resistance of LSNO channels depend on their growth orientation. From the detailed characterization of electrochemical-reaction-induced changes in the channels’ resistance and structural properties, we discuss the origin of the orientation-dependent electrochemical responses of LSNO.