New ultrathin oxide films on metal substrates – NOUS

Ternary oxides with perovskite structures ABO3 have two cationic species that can be selected among transition metals, lanthanides, simple metals, rare earths or even alkalis. So many possible associations result in a large class of materials exhibiting various properties, from superconductivity to giant magnetoresistance, from metal-insulator transitions to 2D electron gases. When synthesized in the form of thin layers, new phenomena may appear related to the 2D nature of the layers. In particular, when these ultrathin layers are supported on a metal, it becomes possible to more precisely control the stoichiometry of the films and their oxygen vacancy concentration compared to an all-oxide system, because interdiffusion phenomena at the interfaces are largely suppressed. The metallic substrate can also be used to modify the formation, structure and properties of the films, modifying the stress magnitude and the charge transfers at the interface. This is a rapidly growing field that has revealed the existence of new 2D oxide phases whose structures have no equivalent in bulk form. We will call them UTOx hereafter (UltraThin reduced Oxide layers).

From a structural point of view, these UTOx phases can often be described from a few elementary tiles, such as MeO4 squares (Me: a metal) or MeO3 triangles. These tiles can be assembled together in different ways to pave the plane and form structures of various complexities, ranging from a simple hexagonal honeycomb structure to quasiperiodic structures with dodecagonal symmetry and their approximant. Such complex phases seem to appear in several perovskites thin films grown on hexagonal surfaces of transition or noble metals. This is a new class of 2D ternary oxides whose physicochemical properties are poorly understood. In addition, the number of systems investigated so far is still very limited compared to the vast landscape offered by the numerous possibility of ABO3/Me(111) couples.

These ultrathin oxides are generally formed on single crystal metal substrates. The NOUS project proposes an original “all-thin film” approach that is more versatile and more easily transferable for applications. We will use conventional and inexpensive substrates such as sapphire on which heterostructures will be fabricated by MBE (metal) and PLD (oxides). The general objective of the project is to be able to synthesize a whole range of new ultrathin oxides with flexible physicochemical properties from a detailed understanding of the growth phenomena and of the chemical bonds and frustration at interfaces. The work consists in understanding the interfacial interactions favoring the emergence of these new 2D phases and characterize their structural and electronic properties as well as their chemical reactivity and wetting properties in view of potential applications, for example in catalysis or electronics. The experimental work will be guided by a theoretical approach using numerical simulations based on the density functional theory (DFT). The goal will be to study the geometric, electronic, magnetic and thermodynamic properties of ultrathin oxides derived from ABO3/Me(111) interfaces.

NOUS brings together three teams from two different departments of the Jean Lamour Institute as well as a competence center in charge of the unique TUBE facility. This consortium gathers complementary expertise in the fields of i / epitaxial growth of thin metallic films and oxides by MBE and PLD, ii / full chemical and physical characterization at the atomic scale of surfaces and interfaces and iii / surface science and modeling of complex quasicrystalline systems and their approximant.

The NOUS project proposes a novel “all thin film” approach involving a synergic contribution of advanced experimental and computational tools which we believe is necessary to discover new UTOx and shed light on the properties of this novel class of materials, as demonstrated by promising preliminary results.