Self‐Organization of Heteroepitaxial CeO2 Nanodots Grown from Chemical Solutions

Abstract

The appearance of new functionalities and devices arising from sizeand shape-dependent properties has triggered the interest in creating well-defined structures at the nanometricscale. While conventional lithography is used to fabricate structures in the range of hundreds of nanometers, self-organizing and self-assembling processes following a bottom-up approach can easily decrease this size limit and also cover cost-effectively much larger surfaces. Semiconductors, metals, oxides and molecular materials are different areas where such principles are intensively pursued to generate nanostructures. In particular, self-assembling based on the stress associated to heteroepitaxial growth is an attractive fabrication route in which the generation of semiconductor nanostructures has been extensively investigated and, recently, it has spread to other emerging fields such as oxide-based nanotechnology. Complex oxides attract a great interest for a wealth of different physical and chemical properties and applications such as ferromagnetism, ferroelectricity, colossal magnetoresistance, high dielectric constants, catalysis, optical properties, high temperature superconductivity, solar cells, etc. Based on these properties, many device concepts are under investigation requiring lateral confinement at the nanometric scale, therefore, it constitutes a real scientific challenge to understand the formation mechanisms of nanometric structures. So far the mechanisms that drive self-organized nanodot growth have been studied in certain detail in epitaxial materials prepared from vapor deposition techniques, including oxides, and either Stranski–Krastanov or Volmer–Weber mechanisms were found to apply. On the other hand, vicinal substrates have been widely considered as templates for growing low dimensional nanostructures. The role of lattice steps on the growth mechanism of oxide films has been analyzed by several authors, though the formation of self-organized nanostructures using the terraces as templates has been very scarce. Much less attention has been devoted to investigate the capabilities of Chemical Solution Deposition (CSD), a preparation methodology bearing a high interest for many functionalities and practical applications requiring the use of large areas or long lengths. Particularly, CeO2 is a key material which is being extensively investigated because of its very high intrinsic interest in many areas such as catalysis, ionic conductivity, optical and dielectric properties, buffer layer for oxide superconductors, or nanotemplates to manipulate the vortex properties in superconducting materials. In the last case, the use of self-organized templates in combination with superconducting films could lead to a wealth of new superconducting phenomena. It appears then as highly demanding, scientifically and technologically, the study of the mechanisms generating nanostructured networks of CeO2. Very thin films grown by CSD have been found to be unstable under high temperature annealings and so they can become the source of nanostructure generation, though under these conditions the size and morphology control may become problematic. In this work, a new methodology of general validity has been devised for the formation of quasi one-dimensional arrays of nanodots based on the control of the interfacial energy of heteroepitaxial structures grown from ultradiluted chemical solutions. The terraces of vicinal single crystals of LaAlO3 (LAO) or SrTiO3 (STO) perovskite substrates have been used as templates where CeO2 and Ce1–xGdxO2–y (CGO) nanostructures have been grown by CSD. We have been able to confine within the terraces rows of nanodots with heights ∼ 20 times higher than the unit cell steps of the vicinal substrate. We will show that the underlying mechanism leading to nanodot self-organization is grounded on an enhanced energy barrier at the surface steps due to the structural dissimilarity of the heterostructure. Figure 1a displays a typical image of a heat treated LAO vicinal substrate where the straightness of the lattice steps is clearly appreciated and Figure 1b shows the corresponding line scan where single perovskite unit cell steps are distinguished, thus demonstrating that only one type of crystal termination has been achieved. From the width of the terraces we have determined that the miscut angles of the substrates used in this work were in the range h = 0.2 ± 0.1°. A reproC O M M U N IC A IO N