## UNLV Theses, Dissertations, Professional Papers, and Capstones

12-15-2019

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

Ashkan Salamat

Qiang Zhu

Oliver Tschauner

Ravhi Kumar

Paul Forster

238

#### Abstract

The intrinsic link between long-range order, coordination geometry, and the electronic properties of a system must be understood in order to tailor function-specific materials. Although material properties are typically tailored using chemical dopants, such methods can cause irreversible changes to the structure, limiting the range of functionality. The application of high pressure may provide an alternative “clean” method to tune the electronic properties of semiconducting materials by tailoring their defect density and structure.

We have explored a number of optoelectronic relevant materials with promising characteristics, specifically Sn-(O,N) compounds which have been predicted to undergo pressure-mediated opening of their optical band gaps. Tin (IV) oxide (SnO$_2$) is a wide band gap semiconductor that belongs to a class of materials known as transparent conducting oxides (TCO). In SnO$_2$ we have discovered pressure-driven disorder in its rutile structure that may explain the origins of its conductivity. We predict this property to be a universal phenomenon across all rutile-structured materials, and could present a new route for strain engineering meta-stable states in rutile structures that are recoverable to ambient conditions. We have also developed a better understanding of the mechanisms that drive the pressure mediated band gap opening in tin (IV) nitride (Sn$_3$N$_4$).

X-ray absorption spectroscopy (XAS) is a multifaceted technique that can help elucidate how the behavior of lighter anion species affects the electronic band structure, structural properties, and vibrational dynamics of a material. In this thesis I will discuss how XAS can be combined together with x-ray diffraction (XRD) and Raman spectroscopy to construct a detailed picture regarding the different atomic species in Sn-(O,N) compounds. One difficulty with building a quantitative description based off of experimental data is that many of these materials are known to have highly kinetically hindered phase transitions. Because of this, they exhibit mixed phasing across a wide range of extreme conditions, leading to severe non-hydrostatic stresses within the system. By utilizing \textit{in situ} CO$_2$ laser annealing, I demonstrate that ground state structures can be accessed, overcoming many of the challenges that have thus far prevented a complete understanding of anion disordering and the role that it plays in a materials electronic properties.

#### Keywords

cumulant; extended x-ray absorption fine structure; SnO2; tin (IV) oxide; x-ray absorption spectroscopy; x-ray diffraction

#### Disciplines

Engineering Science and Materials | Materials Science and Engineering | Other Physics | Physics

pdf

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English