Tunable Formation of Ferromagnetic Nanoparticle Rings: Experiments and Monte Carlo Simulations

Anisotropic interactions in self-assembly of nanostructures always result in novel patterns. We demonstrate that, by the aid of high-power sonication, 16 nm ε-cobalt ferromagnetic nanoparticles (FMNPs) dispersed in dilute suspensions at room temperature would self-assemble into rings as small as ∼50 nm in diameter. The well-defined size and shape and the uniform surfactant coating layer of the cobalt nanoparticles enable quantitative calculations of particle–particle and particle–interface interactions. The experiments, in conjunction with cluster-moving Monte Carlo simulations mimicking the self-assembly in solution and dynamics during solvent evaporation, have revealed three key factors that influence the ring formation most, i.e., FMNP density, dipolar strength, and surfactant layers. Two very different mechanisms of the FMNP-ring formation are found by changing these factors. The results provide a guide to the fabrication of nanorings as well as diverse patterns assembled by FMNPs

Controllable Photovoltaic Effect of Microarray Derived from Epitaxial Tetragonal BiFeO₃ Films

Recently, the ferroelectric photovoltaic (FePV) effect has attracted great interest due to its potential in developing optoelectronic devices such as solar cell and electric–optical sensors. It is important for actual applications to realize a controllable photovoltaic process in ferroelectric-based materials. In this work, we prepared well-ordered microarrays based on epitaxially tetragonal BiFeO₃ (T-BFO) films by the pulsed laser deposition technique. The polarization-dependent photocurrent image was directly observed by a conductive atomic force microscope under ultraviolet illumination. By choosing a suitable buffer electrode layer and controlling the ferroelectric polarization in the T-BFO layer, we realized the manipulation of the photovoltaic process. Moreover, based on the analysis of the band structure, we revealed the mechanism of manipulating the photovoltaic process and attributed it to the competition between two key factors, i.e., the internal electric field caused by energy band alignments at interfaces and the depolarization field induced by the ferroelectric polarization in T-BFO. This work is very meaningful for deeply understanding the photovoltaic process of BiFeO₃-based devices at the microscale and provides us a feasible avenue for developing data storage or logic switching microdevices based on the FePV effect