Oxide Thin Films, Multilayers, and Nanocomposites

The high upper critical field and critical current density (J _c) of the cuprate materials superconductor REBa₂Cu₃O _y (REBCO, RE: rare earth) coated conductors make it promising for its use in the construction of superconducting magnets for medical and electrical power components. For the applications, its J _c in high magnetic fields needs to be improved. In this chapter, we describe how to improve it for REBCO-coated conductors derived from trifluoroacetate metal organic deposition, such as artificially controlled crystallinity, composition, nanosize defects, and in-field superconducting properties.

For the fabrication of new devices taking advantage of interface phenomena, growth of thin films with different orientations is necessary. Epitaxial thin films of c-axis type (001) Bi-2201, (001) Bi-2212, and (001) Bi-2223 and of non-c-axis type (115) Bi-2201, (117) Bi-2212, and (119) Bi-2223 were grown by spin coating and subsequent annealing on single-crystal substrates of SrTiO₃, MgO, or LaAlO₃ with (001) and (110) orientations, respectively. We show that lattice matching relationship between the substrate and the film is the key condition for the orientation control in Bi–Sr–Ca–Cu–O thin films obtained by this route. This is similar to Bi–Sr–Ca–Cu–O films grown by vapor deposition methods. Therefore, it is concluded that the growth method is not essential for the orientation control of the film. However, for the epitaxial growth it is necessary to ensure a high mobility of the atomic species at the substrate-film interface and for the spin-coated films this criterion suggests the presence of a liquid at the interface during annealing. Our films have shown specific structural features and occurrence of impurity phases or orientations. Further research is required.

Our work suggests that the growth by spin coating or by related chemical methods of high-quality epitaxial oxide thin films is a promising research direction.

Given the layered structure of cuprate high-transition-temperature superconductors (HTS), essentially constituted by infinite layer (IL) blocks, containing the CuO₂ planes, separated by charge reservoir (CR) blocks, the use of heterostructures and superlattices, made by stacking HTS with different magnetic and conductivity properties (metallic, superconducting, insulating), such as YBa₂Cu₃O_7−x, PrBa₂Cu₃O_7−x, and Bi₂Sr₂Cu_nCa_n−1O_x, have been extensively exploited to investigate many fundamental issues in HTS physics. Successively, superconducting heterostructures made by a metallic (but non-superconducting) cuprate and an insulating cuprate have been successfully synthesized, such as CaCuO₂/BaCuO_2+x and La₂CuO₄/La_2−xSr_xCuO₄, with the idea to mimic the intrinsic IL/CR structure of HTS, revealing the circumstance that interface effects are important for the occurrence of high T _c superconductivity. The discovery of 2D electron gas at the interface between two insulating perovskites induced us to develop a new kind of heterostructures based on an insulating cuprate with IL structure, CaCuO₂ (CCO), and an insulating perovskite, SrTiO₃ (STO), with the idea that the 2D electron gas eventually formed at the interface could dope the CuO₂ planes in CaCuO₂ block, giving rise to high T _c superconductivity. Indeed, CCO/STO superlattices are superconducting at T _c = 50 K. The doping of CCO occurs thanks to extra oxygen ions, which are incorporated at the CCO/STO interface during the growth in strongly oxidizing conditions. The CR role is thus here played by the interface layer, and the superconductivity is confined at the interface within few CCO unit cells. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here.

Three-dimensional nano/micro-machining (3D machining) is a versatile technique to fabricate devices in scale to quantum level. Three-dimensional focused ion beam (3D-FIB) etching technique is one of the most promising mask-less 3D-machining techniques which give freedom of milling through whisker or thin films and help to fabricate tunnel junction devices. In this chapter, the base materials of these devices are high-T _c superconductor whiskers and thin films. Particularly, this chapter explains the fabrication process of Josephson junction stack devices of high-T _c superconductor Bi₂Sr₂CaCu₂O_8+δ single-crystal whiskers and multilayered YBa₂Cu₃O₇/PrBa₂Cu₃O₇ thin films. With 3D-FIB etching technique, it is possible to mill down to nano- and submicron scale. The smallest in-plane area of about 0.16 μm² is discussed in this chapter.

Nanocomposite oxide high-temperature bulk superconductors can be used as “quasi-magnets.” Thanks to the recent progress of material processing, “quasi-magnet” with 26 mm diameter can generate a large field of 17.6 T at 26 K. These results are highly attractive for applications, involving levitation of permanent magnets on the bulk superconductors. Indeed, several other applications such as motors and magnetic resonance microscope using bulk superconductors have been proposed and demonstrated. In this chapter, we describe several techniques to improve the magnetic properties for bulk superconductors together with some basics such as phase diagrams and solidifications.

The goal of this science fair project was to construct prototype high-T _c superconducting train model. The train would be propelled and would be levitated by a melt-processed GdBa₂Cu₃O _y “Gd-123” superconducting material over a magnetic rail (track). The oval-shaped track was constructed using 189 Nd-Fe-B permanent magnets which were arranged on the iron plate. In addition, the train bodies were constructed with FRP sheets to maintain the temperature of liquid nitrogen (77 K). Finally, the train bodies were attached to a small train toy. The stability, speed, and safety of the superconducting train were tested for various gaps (1–15 mm) between the train and the track. The experimental results indicated that trains with 1–2 mm gaps could not run properly due to the friction applied to the track. The trains with 10 or 15 mm gaps did not run stable on the track. Our results confirmed that a gap of 5 mm is the optimum to run the train which had stability to run fast on the track. The present results clearly demonstrate that the stability of the superconducting trains depends on the gap between the rail and train, which will be very useful for Maglev trains feasible.

Rapid progress in thermoelectric performance of oxide materials has been conducted virtually exclusively in Japan, resulting in more than ten times increase in the ZT values of oxides within the last two decades. This has caused a revolutionary change in the guiding principles of thermoelectric materials research, in which oxide materials had been disregarded as a potential candidate until early 1990s. Promising oxide thermoelectric materials having been discovered include CaMnO₃-based perovskites, Al-doped ZnO, layered cobalt oxides represented by NaCo₂O₄ and Ca₃Co₄O₉, and SrTiO₃-related phases. This chapter reviews the current aspects of bulk oxide thermoelectric materials, and some strategies for selective reduction of the lattice thermal conductivity (selective phonon scattering) in bulk oxides are also discussed.

Thermoelectrics can be used for direct conversion of heat into electricity without moving parts, which is promising for sustainable energy harvesting. In this chapter, thermoelectric properties of oxide thin films: n-type ZnO and p-type Ca₃Co₄O₉ are described in relation to preparation techniques, experimental conditions, substrates used, structure, and morphology. Nanostructuration and artificial nanodefects engineering as ultimate approaches to enhance the conversion efficiency of oxide thin films are also discussed.

The combination of atomic and molecular layer deposition enables the fabrication of layered hybrid inorganic–organic structures with a high degree of controllability over the thickness and composition of individual layers. In addition to the increased potential for obtaining novel combinations of properties from the interplay of the inorganic and organic phases, layered hybrid structures offer opportunities for nanostructuring through the creation of superlattice structures where the organic layer thicknesses have been reduced to a single molecule. Even with very low organic content, this superlattice approach can lead to significant improvements in the inorganic host material’s performance by influencing properties such as thermal conductivity or the electronic band structure.

The conducting quasi-two-dimensional electron system (q2DES) formed at the interface between LaAlO₃ and SrTiO₃ band insulators is confronting the condensed matter physics community with new paradigms. While the mechanism for the formation of the q2DES is debated, new conducting interfaces have been discovered paving the way to possible applications in electronics, spintronics and optoelectronics. This chapter is an overview of the research on the LAO/STO system, presenting some of the most important results obtained in the last decade to clarify the mechanism of formation of the q2DES at the oxide interfaces and its peculiar electronic properties as compared to semiconducting 2D-electron gas.

A review of investigations on CeO₂ films with nanostructured surfaces modified by high-temperature annealing is presented. The influence of film deposition method employed in the preparation of the films, and effect of various annealing conditions on CeO₂ microstructure, crystallinity, and development of porosity is highlighted in this chapter. The application of nanostructured CeO₂ films as buffer layers in multilayered structures is discussed.

Zinc oxide, as one of the promising semiconductor materials, has attracted considerable attention in optoelectronic applications due to its promising properties, including a wide band gap of 3.37 eV at room temperature, a large excitation binding energy of 60 meV, high chemical stability, and transparency. Recently, one-dimensional ZnO nanostructures have been extensive investigated due to their potential application for nanodevices such as solar cells, lighting, chemical sensors, and electrical devices and a variety of display units.

In this chapter, we introduce the most convenient method to deposit reproducible and homogeneous ZnO thin films over a large-area substrate at a lower deposition temperature. The critical parameters which will influence on the property of ZnO thin film are investigated. It is found that the gas ratio (Ar/O₂) and deposition pressure significantly influence the structural and optical properties of ZnO film. In order to apply the ZnO nanostructures to the photovoltaic device, we also developed a novel method to fabricate well-aligned ZnO nanostructures from as-deposited ZnO thin film by thermal annealing method. The substrate dependence and growth mechanism are analyzed. Moreover, a novel mist chemical vapor deposition method is introduced for the first time to modify the obtained ZnO nanostructures. The morphology of ZnO nanostructures could be controlled well by adjusting the deposition time and carrier gas.

This work provides a much useful technique to fabricate high quality ZnO thin film and nanostructures which can be expected to apply in thin film transistor, sensor, dye-sensitized solar cell industries in the near future.

The photocatalytic process and various approaches to enhance the efficiency of photocatalysis in metal oxides nanostructures are described in this chapter. The concept of coupling nanostructured metal oxide to another metal oxide or noble metal nanoparticles (NPs) to form hybrid material with novel properties is an attractive and efficient approach for enhancing the photocatalytic efficiency. The hybrid photocatalysts have shown to improve the carrier separation and visible light absorption and thus the photocatalytic efficiency. The effect of various noble metal NPs (Pt, Au, Ag) decoration/coupling on the photocatalytic activity of the metal oxide (TiO₂, ZnO) semiconductors is discussed with the possible mechanisms. Various hybrid oxide nanostructures such as TiO₂–ZnO, TiO₂–SnO₂–TiO₂–WO₃, TiO₂–SiO₂, ZnO–Fe₂O₃, ZnO–Fe₃O₄, ZnO–SnO₂, ZnO–Cu₂O are discussed for their photocatalytic activities. The promising photocatalytic properties of metal oxides hybrid with carbon based materials like carbon nanotubes and graphene is also described.

Solar energy or visible light, as a renewable free sources using diverse types of nanomaterials as photocatalysts for air remediation or solar cell applications, can give solutions to environmental problems by controlling the nanomaterial’s morphology shape or doping condition, as well as the adjustment of required bandgap according to specific compositions.