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.