Ionic Liquid Gate-Induced Modifications of Step Edges at SrCoO2.5 Surfaces.

Intense electric fields developed during gating at the interface between an ionic liquid and an oxide layer have been shown to lead to significant structural and electronic phase transitions in the entire oxide layer. An archetypical example is the reversible transformation between the brownmillerite SrCoO2.5 and the perovskite SrCoO3 engendered by ionic liquid gating. Here we show using in situ atomic force microscopy studies with photothermal excitation detection, that allows for high quality measurements in the viscous environment of the ionic liquid that the edges of atomically smooth terraces at the surface of SrCoO2.5 films are significantly modified by ionic liquid gating but that the terraces themselves remain smooth. The edges develop ridges that we show, using complementary X-ray photoemission spectroscopy studies, result from the adsorption of hydroxyl groups. Our findings exhibit a way of electrically controlled surface modifications in emergent ionitronic applications.

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures.

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO2.5 and the oxygen ordered phase perovskite SrCoO3. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.