Structure Modulation in Confined Nanoparticles: The Role of the Strain Gradient

Microstructure modulation is an integral part of materials engineering, fundamentally manipulating its properties. Here, we present a combined computational and experimental investigation on the role of the strain gradient on structure modulation in Co-CoO nanoparticles confined in Al2O3 matrix. From the series of finite-element simulations, by tuning the gradient of the strain, the confined nanoparticle could be modulated from the initial Co solid structure to transitional Co/CoO yolk/shell structures and then to a final CoO hollow structure. Experimentally, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy are employed to monitor the microstructure and chemical component of the confined nanoparticles prepared via a pulsed laser deposition system and rapid thermal annealing technique, confirming the successful synthesis of these three distinct structures as predicted by simulations. In contrast to Co ferromagnetic nanoparticles and CoO antiferromagnetic hollow nanoparticles, the Co/CoO yolk/shell nanoparticles display enhanced magnetic properties and optical nonlinearities that are attributed to the novel ferromagnetic/antiferromagnetic yolk/shell structure with decentring core and void. Demonstration of structure modulation through altering the strain gradient presents opportunities to tailor the properties of confined nanoparticles for diverse applications, particularly in the fields of spintronics and nonlinear optics.