Effect of Surface Termination on the Electronic Properties of ${\mathrm{LaNiO}}_3$ Films

The electronic and structural properties of thin ${\mathrm{LaNiO}}_3$ films grown by using molecular beam epitaxy are studied as a function of the net ionic charge of the surface terminating layer. We demonstrate that electronic transport in nickelate heterostructures can be manipulated through changes in the surface termination due to a strong coupling of the surface electrostatic properties to the structural properties of the Ni—O bonds that govern electronic conduction. We observe experimentally and from first-principles theory an asymmetric response of the structural properties of the films to the sign of the surface charge, which results from a strong interplay between electrostatic and mechanical boundary conditions governing the system. The structural response results in ionic buckling in the near-surface ${\mathrm{NiO}}_2$ planes for films terminated with negatively charged ${\mathrm{NiO}}_2$ and bulklike ${\mathrm{NiO}}_2$ planes for films terminated with positively charged LaO planes. The ability to modify transport properties by the deposition of a single atomic layer can be used as a guiding principle for nanoscale device fabrication.

Figure 1

Temperature-dependent transport measurements for ${\mathrm{LaNiO}}_3$ films as a function of surface termination and film thickness (a perovskite u.c. corresponds to a ${\mathrm{NiO}}_2$-LaO bilayer). Films terminated with LaO (solid curves) are metallic, and films terminated with ${\mathrm{NiO}}_2$ (squares) are insulating.

Figure 2

Vertical cuts through experimentally determined COBRA-derived electron density maps along (a) [110] and (b) [100] directions for a 3.5-u.c.-thick ${\mathrm{LaNiO}}_3$ film terminated with LaO. The horizontal lines indicate the centers of mass in the vertical direction of the cation and anion positions.

Figure 3

Theoretically predicted structures for (a) 3-u.c. ${\mathrm{NiO}}_2$- and (b) 3.5-u.c. LaO-terminated ${\mathrm{LaNiO}}_3$ films epitaxially strained to ${\mathrm{LaAlO}}_3$ substrates. The blue arrows indicate the direction of the surface fields due to the nominal charge of the termination layer. The red arrow indicates the direction of the mechanical force exerted by the apical oxygen on the Ni ion below it. Layer-resolved cation-anion displacements are shown for (c) ${\mathrm{NiO}}_2$- and (d) LaO-terminated films. The film-substrate interface is at $z=0$. Positive displacement indicates cation movement towards the vacuum compared to oxygen in the same layer. The solid lines are shown as a guide to the eye. Experimentally measured displacements are shown as open symbols.

Figure 4

Oxygen $K$ edge absorption spectra for 3.5-u.c. (LaO-terminated) and 3-u.c. (${\mathrm{NiO}}_2$-terminated) -thick ${\mathrm{LaNiO}}_3$ films grown on (001)-oriented ${\mathrm{LaAlO}}_3$ substrates. The inset shows a comparison of the prepeak intensities for 3–4-u.c. LNO films. The surface terminations are indicated in parentheses.