Strain-stabilized interfacial polarization tunes work function over 1 eV in RuO₂/TiO₂ heterostructures

Interfacial polarization - charge accumulation at the heterointerface - is a well-established tool in semiconductors, but its influence in metals remains unexplored. Here, we demonstrate that interfacial polarization can robustly modulate surface work function in metallic rutile RuO₂ layers in epitaxial RuO₂/TiO₂ heterostructures grown by hybrid molecular beam epitaxy. Using multislice electron ptychography, we directly visualize polar displacements of transition metal ions relative to oxygen octahedra near the interface, despite the conductive nature of RuO₂. This interfacial polarization enables over 1 eV modulation of the RuO₂ work function, controlled by small thickness variations (2-3 nm), as measured by Kelvin probe force microscopy, with a critical thickness of ~4 nm - corresponding to the transition from fully-strained to relaxed film. These results establish interfacial polarization as a powerful route to control electronic properties in metals and have implications for designing tunable electronic, catalytic, and quantum devices through interfacial control in polar metallic systems.