Resumen de Nanostructured nickel oxide thin films grown by reactive RF Magnetron Sputtering
Transition metal oxides have attained much attention in the last years due to their outstanding properties that make them attractive not only for fundamental studies, but also for a wide range of applications in nanotechnology. Among the transition metal oxides, Nickel Oxide (NiO) is being used nowadays in several technological applications, such as electrochromic display devices, gas and fluid sensors, resistive-switching memories, catalysis, electrochemical capacitors, or electrodes in lithium-ion batteries. In many of these applications, such as in catalysis or gas sensing, the interaction with the surrounding environment plays a very important role. The use of nanostructured materials increases the effective surface, and, consequently, enhances the yield of the surface reactions of interest. The physical properties of any nano-material, and thus the actual feasibility of its application, strongly depend on the stoichiometry, density of defects, and crystallinity, which are very sensitive to growth conditions.
Even though NiO has been profoundly studied during decades and has been pointed as a prototypic solid state system due to its basic structure but unexpected properties, it still has obscure fundamental attributes which make it attractive as an object of study [1, 2]. In particular, the electronic structure of Nickel Oxide together with its magnetic properties are subject of controversy and many studies and interpretations have appeared [3, 4]. Different electrical and optical models proposed do not agree with the experimental data, increasing the interest on this material. Whether the electrical conduction in NiO is driven by free polarons or electron hopping depends on the crystal structure, the environment conditions and the chemical composition. Nickel is said to have only one valence, though several works assign to it more oxidation states mainly due to the flexibility of its orbital bonding.
The aim of this work is to study the properties of Nickel Oxide when deposited in the form of thin films with dimensions in the nano-scale by means of reactive RF magnetron sputtering. Are those properties still unclear in this nanostructured form? Which of them are modified when reducing the thickness, creating pores or increasing the impurities concentration? Answers to these questions will be given following a method of working which starts from understanding the morphology and structure of the deposited films, continues with the electro-optical properties and ends with the understanding of the electronic structure, providing models and correlated results that support the work.
Nickel Oxide properties change dramatically when the system is arranged in nanostructured thin films specially due to the lack of stoichiometry. Dealing with such systems need a comprehensive study of its different physical properties. We will show that it is possible to tune some of the properties of the thin films, especially the electrical, by choosing the suitable growth conditions, considering the degrees of freedom of our experimental set-up. The first approach is to explore and control the deposition method, reactive RF magnetron sputtering, a reproducible, scalable and middle-range energy PVD method, as it is shown in chapter 4.
A model for the growth of the porous nanostructured films will be given in chapter 4, and correlated with the internal structure and morphology, obtained by means of scanning electron microscopy (SEM), profilometry, X-ray diffraction (XRD) and X-ray absorption (EXAFS). By controlling the morphology it will be possible to control the effective surface. Upon certain conditions, out-of-equilibrium growth leads to disorder and impurities, which in turn lead to new properties far from the stoichiometric NiO.
The electrical and optical aspects of a coating are not only useful to complete the comprehension of fundamental properties but are also one of the most valuable properties for industrial applications. A perfect stoichiometric Nickel Oxide is very complicated to obtain due to its tendency to have impurities in many forms. In chapter 5 we will show, regarding the structural information obtained previously, the consequences of the disorder and non-stoichiometry on the properties of the thin films. The controlled presence of Ni vacancies in the films will make it possible to tune their resistivity down to the range of some ohm-cm, which, together with their partial transparency, makes these films very interesting for electro-optical applications. Inspired by the native oxidation layer of metals, we propose a model to explain coherently the electrical resistivity evolution of the films, and the definite influence of thermal treatment. As expected, the optical absorption is also modified at will, placing these films at the center of potential applications arising from these oxides [5].
Finally, in chapter 6 the electronic structure of the NiO oxide films will be correlated with the previous results on the geometric structure and the functional properties. Although many works studied the NiO electronic structure, only some approaches explain the non-local screening effects and surface contribution to the photoemission spectra. However, the possibility of existence Ni3+ ions due to unstable non-stoichiometric NiO remains as an open question. We show by means of X-ray absorption (XANES) with cluster model calculations and X-ray photoemission (XPS) measurements and interpretations of Ni vacancies as such ions. This strongly supports the previous results conforming the impurities influence in the conduction band and hence the decrease in resistivity. Thermal treatment is shown to be highly efficient by healing the surface crystal structure, reducing the amount of Ni vacancies.
[1] Fujimori A. Siratori K. Tsuda N., Nasu K. Electronic conduction in oxides. Springer, 2000.
[2] Bärbel Fromme. d-d Excitations in Transition-Metal Oxides. Springer, 2001.
[3] G. van der Laan, J. Zaanen, G. A. Sawatzky, R. Karnatak, and J.M. Esteva. Comparison of X-ray absorption with X-ray photoemission of nickel dihalides and NiO. Phys. Rev. B, 33:4253-4263, 1986.
[4] Atsushi Fujimori and Fujio Minami. Valence-band photoemission and optical absorption in nickel compounds. Phys. Rev. B, 30:957{971, 1984.
[5] H. Sato, T. Minami, S. Takata, and T. Yamada. Transparent conducting p-type NiO thin films prepared by magnetron sputtering. Thin Solid Films, 236(1-2):27-31, 1993.
