On the thickness-driven metal–insulator transitions of buried LaNiO₃ films

Metal–insulator transitions (MITs) usually occur in metallic oxide films when scaling down the thickness below a critical value. It is difficult to pinpoint the intrinsic driving mechanism underlying the MIT because of the increasingly important impacts from the truncated surface at smaller film thickness. Herein, LaNiO₃ (LNO) films with precisely controlled layer thickness (N in unit cells) are encapsulated by LaMnO₃ (LMO) or LaFeO₃ (LFO) epilayers to maintain the bulk-like bonding environments for all the NiO₂ layers. Electron energy-loss spectroscopy reveals apparent electron transfer at the LNO/LMO interface, but not at the LNO/LFO interface. The resultant electron doping pushes the system toward a more insulating state for N≤2. Comparison between LNO plain films and buried layers reveals that the surface effects significantly degrade the conductivity of LNO only if N<4. Furthermore, epitaxial strain is found to have a notable difference on the resistivity within the intermediate thickness range (2<N<6) for buried LNO layers strained to SrTiO₃ and LaAlO₃ substrates. The electronic transports are dominated by the bulk electronic structure for N≥6, but governed by the dimensionality-induced gap opening for N≤2. Our results demonstrate controllable MITs in LNO through synergistic exploitation of multiple factors, offering design principles for low-dimensional oxide materials.