Oxide heterostructures: Atoms on the move.

Complex oxide materials are a mixture of metal and oxygen ions that exhibit a wide variety of physical functionalities1. They are among the most abundant minerals on Earth, and are of interest in many commercial technologies, from displays to electronics and communication to sensors and actuators. The multifunctional nature of these materials has opened up many possibilities for fundamental research and created tremendous interest in the synthesis of novel materials. Among these, the An+1BnO3n+1 Ruddlesden–Popper phases, where A and B are cations, and O an oxygen anion, have attracted a lot of attention, as they offer functionalities such as di-, ferro-2 and piezoelectric, magnetic, superconducting3 as well as catalytic properties4. The crystal structure of these Ruddlesden–Popper phases can be described by the stacking of a finite number (n) of layers of perovskite ABO3 between rocksalt AO layers, as shown in Fig. 1. One of the most promising techniques to synthesize Ruddlesden–Popper thin films is molecular beam epitaxy, an ultrahighvacuum growth technique in which molecular or atomic beams react to an epitaxial thin-film on a heated crystalline substrate. In the most ideal case, this film is grown atomic layer-by-atomic layer. Molecules or atoms impinge on the surface and two-dimensional islands nucleate. These islands grow by attachment of further atoms, until an atomic layer is completed by coalescence of these islands. This process, of nucleation and growth, is repeated for every subsequent atomic layer. For the growth of Ruddlesden–Popper phases, a shuttered deposition process is normally used, allowing for the control of the composition of each atomic layer. Writing in Nature Materials and Nature Communications, two independent collaborations now show5,6 that a set of dynamic phenomena occur during the growth process of Ruddlesden–Popper films, thus explaining why they have been challenging to grow in the past, but also expanding the range of materials that can be prepared as epitaxial heterostructures in the future. The high degree of control over crystal growth provided by molecular beam epitaxy enables the precise control of surface composition and morphology, and therefore the reproducible synthesis of a large variety of heterostructures with extremely sharp interfaces. However, although many groups have successfully synthesized Ruddlesden– Popper phases using molecular beam epitaxy, fabricating these materials without defects, or indeed understanding the underlying mechanisms that govern the creation of such defects, has remained a challenge. At higher growth temperatures, both thermodynamic and kinetic effects are thought to disturb the atomic-layer control over the growth process, and therefore the resulting structure of the thin film. The work of Lee et al.5, however, now demonstrates that a dynamic rearrangement of atomic layers can occur during growth, leading to structures that are highly unexpected based on the intended layer sequence. Indeed, as Nie and colleagues6 independently demonstrate, the most atomically precise interfaces in Srn+1TinO3n+1 OXIDE HETEROSTRUCTURES