Weronika Walkosz

Electron energy-loss spectroscopy study of metallic Nb and Nb oxides

We present a series of electron energy-loss spectroscopy (EELS) studies on niobium (Nb) and its oxides (NbO, NbO2, and Nb2O5) to develop a reliable method for quantifying the oxidation state in mixed niobium oxide thin films. Our approach utilizes a combination of transmission electron microscopy and EELS experiments with density functional theory calculations to distinguish between metallic niobium and the different niobium oxides. More specifically, the differences in the near-edge fine-structure of the Nb M-edge and O K-edge provide sufficient information to determine the valence state of niobium. Based on these observed changes in the core-loss edges, we propose a linear relationship that correlates the peak positions in the Nb M- and O K-edges with the Nb valence state. The methods developed in this paper are also applied to ultrathin niobium oxide films to examine the effects of low-temperature baking on the films’ oxidation states.

Overview of Experimental Tools

This chapter reviews a range of transmission electron microscopy (TEM) techniques, including conventional electron microscopy, High-Resolution Electron Microscopy (HREM), Z-contrast imaging, Electron Energy-Loss Spectroscopy (EELS), and spectrum imaging (SI). The latter three will be discussed in relation to Scanning TEM (STEM). The key aspects of these techniques will be described in detail, emphasizing their strengths and drawbacks. Moreover, various simulation techniques will be discussed in the chapter, and a short summary of advances in electron optics will be provided.

Theoretical Methods and Approximations

This chapter describes the theoretical approaches and approximations used in standard first principles calculations. First, the Born Oppenheimer approximation is stated. Next, an overview of the basic formulation of density functional theory (DFT) is given, underlying its merits in describing various materials and their properties, but also pointing out the aspects which need further improvements. The Kohn–Sham ansatz, which provides a means of calculating properties of many-body system using independent-particle methods, is presented along with practical schemes for solving the resulting Kohn–Sham equations. Finally, the DFT+U method and its implementation within a pseudopotential-plane wave framework are introduced.

Silicon Nitride Ceramics

The widespread interest in silicon nitride ceramics stems from their desirable physical and mechanical properties in many high temperature and pressure applications [1, 2, 3, 4, 5]. Good resistance to oxidation and corrosive environments, low coefficient of friction and thermal expansion, negligible creep, and high decomposition temperature are some of these important properties. Because of these, silicon nitride (especially its polymorph) is widely used in gas turbines, engine parts, bearings, dental drills and gauges, and cutting tools. In addition, thin films and coatings have been studied in relation to high-speed memory devices [6, 7, 8, 9, 10] and optical waveguide applications [11].

Imaging Bulk α-Si3N4

In this chapter I describe results on bulk α-Si3N4. In particular, the problem regarding the stability of this polymorph is addressed in detail with aberration-corrected Scanning Transmission Electron Microscopy (STEM) and ab initio methods. I start this chapter with an introduction to α-Si3N4 followed by the discussion of the obtained Z-contrast images and the Bloch-wave Z-contrast image simulations. Next, I present my Density Functional Theory (DFT) calculations along with the Electron Energy-Loss Spectroscopy (EELS) results confirming the presence of interstitial O in bulk α-Si3N4.

Atomic-Resolution Study of β-Si3N4/SiO2 Interfaces

In this chapter I describe results of the experimental study of (hbox{Si}_3hbox{N}_4/hbox{SiO}_2) interface. This work was carried out in an effort to examine the morphology of (hbox{Si}_3hbox{N}_4,(10overline{1}0)) surface in the absence of rare-earth elements to better understand their importance in stabilizing the ceramics open-ring termination discussed in Chap. 4. The chapter commences with an introduction to (hbox{Si}_3hbox{N}_4/hbox{SiO}_2) interfaces, followed by a short summary of the experimental methods used in this study. Next, the results of the investigation are presented in detail, focusing in particular on the composition and bonding characteristics across the interface.