Oxide thin film properties

Chapter 4 focuses on the various physico-chemical properties of thin films, and in particular on their thickness dependence. It aims at bridging the gap between the well established knowledge acquired on oxide surfaces and truely two-dimensional oxides. It reviews the consequences of the reduced local environment of their atoms and of the confinement effects due to their finite thickness. It analyzes the various manifestations of structural, electronic, vibrational, ferroelectric and magnetic modifications. A special focus is put on the interaction of thin films with metal or oxide substrates, possibly leading to interfacial dislocations, electron transfer or mixing, with consequences on their chemical reactivity.

Introduction

In the introductory chapter, the stage is set for the title materials, oxide thin films and nanostructures, and their widespread occurrence and importance in everyday life, science and technology are pointed out. The “nanostructure” term is defined and the specific characteristics of nanomaterials, distinguishing them from macroscopic bulk materials, are lined out. A preview of the following eight chapters gives an impression of the content of the book.

Growth of Oxide thin Films and Nanoparticles: Methods of Fabrication

This chapter outlines the fabrication methods of oxide thin films, from the oxidation of the outer layers of bulk elemental solids to thin film deposition methodologies. The classical theories treating the thermal oxidation of metals and silicon are reviewed. A particular focus is put on the oxidation of alloy single crystal surfaces to generate ultrathin oxide films and the formation of surface oxides, the latter as precursor layers for thicker bulk-type oxide phases. The diverse deposition techniques to grow epitaxial thin oxide films are introduced, with a classification into physical and chemical methods for the ease of presentation; the benefits and disadvantages of the different methods are pointed out. The synthesis of oxide nanoparticles is discussed in the gas phase and in liquid phase environments. The fundamental concepts of nucleation and growth of thin films and nanoparticles are introduced, including the classical capillary approach and atomistic descriptions.

Methods of study

The experimental and theoretical characterization of oxide nanostructures is addressed. The experimental techniques are classified according to their information content, revealing atomic geometry, chemical composition, electronic structure as well as magnetic, vibrational and chemical properties. Due to the nanometer scale dimensions of oxide nanosystems, many experimental techniques are derived fom the field of surface science and involve ultrahigh vacuum technology. The quantum-theoretical simulations for the description of oxide materials are presented by progressing from simple to increasingly sophisticated methods; the latter become necessary to accurately treat electron correlation effects, which are significant in many oxide materials, in particular at low dimension. Electronic structure methods, total energy methods and atomic structure simulation methods are introduced and discussed.

Clay mineral layers and nanoparticles

This chapter provides an introduction to the properties of naturally occurring oxide ultra-thin films and nanoparticles of complex composition, namely clay minerals. Clays represent a wide family of nanomaterials, formed from the weathering of primary rocks at the Earth surface. Their layered structure is responsible for their extreme anisotropy and their extremely high specific surface area which is at the origin of their unique properties and of their strong interaction with their surrounding. The focus of this chapter is on their structural and composition characteristics, the conditions of their formation, their interaction with water and their chemical properties. A mention is given of their uses in human cultural activities, in dwelling constructions and statuary, in porcelain fabrication, in retention of metal contaminants in the environment, in the pharmaceutical and cosmetic industries, and as lubricants, catalysts, therapeuthic agents, among others.

Synopsis and outlook

The synopsis part of this last chapter gives a brief summary of the book content. The outlook attempts to identify future areas of scientific activity, in which according to the authors´ visions nano-oxide materials may promote new developments. Among them are the controlled synthesis of oxide nanosheets and the experimental realization of oxide nanoribbons. The preparation of well-defined oxide heterostructures may reveal novel emergent states and new topological phases of matter. Mixed nano-oxides will be of interest for band structure engineering and to adjust band edges for photochemical reactivity. Programmable defect chemistry may open up new selective pathways for catalytic reactions. In parallel with experimental progress, advanced theoretical and simulation methods will take advantage of the ever-increasing computer power to tackle highly correlated materials and allow highthroughput computing. The interaction of nano-oxides with biological systems has great potential for opening up new avenues in the biotechnological area.

Oxide nanoparticles

Oxide micro- and nanoparticles are ubiquitous in the natural environment. They have long been used as catalysts, but have found novel applications with the advent of nanotechnologies. From a fundamental point of view, they bridge the gap between the properties of isolated atoms or molecules and those of bulk condensed phases. They present a large spectrum of atomic and electronic structures, due to their finite size in the three dimensions of space and the fact that their atoms are nearly all surface atoms. They represent the first stages of nucleation and growth of larger size oxides, and their stability and properties which depend upon the thermodynamic conditions under which they are formed, largely impact the final product. This chapter reviews their specific properties and analyses their physical origin, both when they are produced in the gas phase, in an aqueous environment, or when cation mixing takes place.

Two-dimensional oxides

The novel physical and chemical properties and functionalities of two-dimensional (2-D) oxide materials are assessed. The synthesis of one unit-cell thick 2-D oxides poses particular challenges, since in contrast to other 2-D materials, which can be fabricated by exfoliation of layered bulk compounds, the majority of oxides do not occur in layered bulk structures. Most 2-D oxides are therefore prepared by thin-film deposition methods on substrates. However the fabrication of free-standing quasi-2-D oxide nanosheets, with less restrictive several monolayer thickness, has been successfully achieved by wet chemical procedures. New geometry concepts and electronic properties are observed in 2-D oxides, due to quantum confinement and interface proximity effects. Atomic geometries, electronic structure, ferroic properties and catalytic behaviour of 2-D oxides are discussed, together with promising prototypical proof-of-concept experiments for prospective applications. The edge states in oxide nanoribbons, 2-D objects of limited width, and their polarity aspects are discussed.

Surface chemistry, energy conversion, and related applications.

Oxide nanomaterials have an impact in many interdisciplinary fields of the emerging nanotechnologies as reported here. They are components of heterogeneous catalyst systems with specific features that are outlined. In photocatalysis, chemical reactions are catalytically enabled by photon-energy conversion: oxide photocatalysts are prominent and discussed in relation to the photochemical water splitting reaction. Solid oxide fuel cells are promising energy sources, in which oxide nanomaterials are expected to boost further progress. Solar energy materials are elements of the “green chemistry” revolution for energy saving: the chromogenic functionality of oxide nanolayers with use in advanced fenestration is introduced. The formation and structure of corrosion protective nanolayer oxides, which is vital for the everyday use of metallic components, are examined. Biotechnology applications of oxide nanostructures comprise their biocompatibility, antibacterial properties, theranostic systems as well as biosensor platforms. An interesting bioapplication of ferroelectric oxide thin films is reported.