Engineering Maxwell–Wagner relaxation and interface carrier confinement in Al2O3/TiO2 subnanometric laminates for high-density energy storage applications†

The Al₂O₃/TiO₂ nanolaminates (ATA NLs), with the dominant Maxwell–Wagner interfacial polarization, have been extensively explored in last decade due to their potential for new-generation energy storage applications. Here, we report the fabrication of device-grade sub-nanometric (<1 nm) ATA NLs using an optimized pulsed laser deposition technique, where the interface-confined carrier relaxation and sublayer conductivity contrast-induced Maxwell–Wagner interfacial polarization mechanism was engineered by precisely tailoring the individual Al₂O₃ and TiO₂ sublayer thickness along with the top-bottom capping layer thickness. The formation of oxygen vacancy-generated carriers in reduced titania sublayers across Al₂O₃/TiO₂ heterointerfaces and their relative response towards the applied field were responsible for both charge storage and leakage. An NL with a TiO₂ and Al₂O₃ sublayer thickness of ∼1 and 0.6 nm, respectively, sandwiched between ∼3 nm Al₂O₃ barrier layers, has demonstrated an improved capacitance density of ∼33.1 fF μm⁻² and a high cut-off frequency up to ∼0.5 MHz, along with a low dielectric loss of ∼0.032 and a reduced leakage current density of ∼3.08 × 10⁻⁷ A cm⁻² at 1 V. The calculated energy density value of ∼4.6 J cm⁻³ achieved with this optimized subnanometric Al₂O₃/TiO₂ laminate is comparable to those of state-of-the-art capacitive devices. These superior electrical properties and controllable dielectric relaxation make this laminate a promising high-k and low-loss dielectric material for next-generation nano-electronics and high-density energy storage capacitors.