From Surface Atomic Exchange Diffusion to Layer-by-Layer Oxygen Vacancy Exchange Diffusion
——The Nature in Highly Epitaxial Multifunctional Complex Oxide Thin Films
University of Texas San Antonio, Texas 78249, USA
Surface atomic exchange diffusion is one of the basic surface diffusion mechanisms, which become the key role in governing thin film growth, nanostructure nucleation, and surface alloy formation for various modern nanoelectronic device fabrication technique. The oxygen vacancy exchange diffusion with oxygen and hydrogen were discovered in the single crystalline LnBaCo2O5.5 (LnBCO, Ln = Er, Pr; 0.0 < δ < 0.5) epitaxial films, which paves a new avenue in the development of ultrasensitive chemical nanosensors, energy harvest devices, and many others.By measuring the resistance changes under a switching flow of oxidizing gas (O2) and reducing gas (H2) in the temperature range of 250~800ºC, the highly epitaxial LnBCO films were found to exhibit ultrafast oxygen/hydrogen diffusivity and high sensitivity to redox environments. The oscillation chemical dynamics in the resistance changes vs. time plots with time period during the oxidation cycle under O2 suggests that the oxygen vacancy diffuses in the films with oxygen and hydrogen by taking the atomic layer by layer exchange diffusion mechanism. This unprecedented oscillation phenomenon provides the first direct experimental evidence that oxygen/hydrogen atoms diffuse through the LnBCO thin films layer by layer via the oxygen-vacancy-exchange mechanism. Furthermore, the superfast chemical surface exchange dynamic behavior were discovered on the surfaces of the highly epitaxial thin films of single-crystalline cobalt double-perovskite LnBCO thin films. The excellent chemical dynamics and ultrafast layer-by-layer oxygen vacancy exchange diffusion suggest that the as-grown LnBCO can be an excellent candidate for energy harvest and chemical sensor developments.