Title

Crystal Structure, Electronic Structure, and Physicochemical Characterization of I2-II-IV-VI4 and I4-II-IV2-VI7 Diamond-Like Semiconductors Prepared by High-Temperature Solid-State Synthesis

Defense Date

7-8-2015

Graduation Date

Summer 1-1-2015

Availability

Immediate Access

Submission Type

thesis

Degree Name

MS

Department

Chemistry and Biochemistry

School

Bayer School of Natural and Environmental Sciences

Committee Chair

Jennifer Aitken

Committee Member

Stephanie Wetzel

Committee Member

Tomislav Pintauer

Committee Member

Charles Lake

Keywords

Diamond-like semiconductors, crystal structure, semiconductors, high-temperature solid-state synthesis

Abstract

Diamond-like semiconductors are compounds that possess bandgaps in the visible to near-infrared and crystallize in structures derived from that of cubic or hexagonal diamond. The compositions of quaternary DLSs are predictable and flexible, which allows for the tuning of physicochemical properties for technological applications. Chapter 1 provides an overview of DLSs and potential applications for which they are useful. In this work, diamond-like semiconductors (DLSs) of the family I2-II-IV-VI4, namely, Cu2ZnGeSe4 and Cu2MnGeSe4, were prepared via high-temperature solid-state synthesis Investigations to prepare Cu2ZnGeSe4, led to the serendipitous discovery of a new compound, Cu4ZnGe2Se7. To better understand the relationships between these compounds and their potential for applications, crystal structure, electronic structure, and physicochemical characterization were performed. Chapter 2 details the synthesis and characterization techniques implemented for these compounds.

Chapter 3 describes the relationships between these I2 II IV-VI4 and I4-II-IV2-VI7 compounds, specifically Cu2ZnGeSe4 and the Cu4ZnGe2Se7, along with the differences in the resulting physicochemical properties. Single crystal X-ray diffraction was used to solve and refine the structure of Cu4ZnGe2Se7, which was found to crystallize in the monoclinic C2 space group. Rietveld refinement of synchrotron X-ray powder diffraction (XRPD) data for Cu4ZnGe2Se7 confirmed the structure and indicated ~ 95 wt. % phase purity with ~ 5 wt. % of Cu2ZnGeSe4 present as a secondary phase. Rietveld refinement of synchrotron XRPD data for Cu2ZnGeSe4 confirmed the previously reported stannite structure in space group I 42m. Cu2ZnGeSe4 and Cu4ZnGe2Se7 both were found to be direct-gap semiconductors by optical diffuse reflectance spectroscopy, with bandgap values of 1.38 eV and 0.91 eV, respectively.

The discovery of the new compound, Cu4ZnGe2Se7, prompted further research into the family of materials having the formula I4-II-IV2-VI7, specifically Cu4-II-Ge2-Se7 where II = Mn, Fe, Co, and Ni. This investigation was successful by leading into the discovery of more new I4-II-IV2-VI7 phases as described in the Appendix. Due to relatively lower bandgaps, a more distorted diamond-like structure, and a chemical formula naturally containing more copper, these compounds may be even better candidates for thermoelectric applications.

Format

PDF

Language

English

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