Defense Date

7-10-2014

Graduation Date

2014

Availability

Immediate Access

Submission Type

dissertation

Degree Name

PhD

Department

Chemistry and Biochemistry

Committee Chair

Jennifer A Aitken

Committee Member

Tomislav Pintauer

Committee Member

Stephanie J Wetzel

Committee Member

Charles H Lake

Keywords

Characterization, Diamond-like, Nonlinear, Second harmonic generation, Semiconductor, Solid-state synthesis

Abstract

Diamond-like semiconductors (DLSs) have structures that are derived from cubic or hexagonal diamond. The compositions of DLSs are predictable and flexible allowing for properties to be tuned to target technological applications. The fields of photovoltaics, thermoelectrics, and nonlinear optics (NLO) are in need of materials with improved efficiencies for practical devices. Toward the goal of discovering advanced materials, systematic studies of DLSs allowing for the elucidation of structure-property and composition-property relationships were carried out. Here, in-depth structural, physicochemical, and computational studies have been performed on several DLSs to correlate the crystal and electronic structure with the physicochemical properties.

The DLSs, α-Cu2ZnSiS4, β-Cu2ZnSiS4, Cu2CdSiS4, Cu2CdSnS4 and AgInSe2:Mn were synthesized via high-temperature, solid-state synthesis. Synchrotron X-ray powder diffraction (XRPD) of Cu2ZnSiS4 revealed the presence of two polymorphs, α-Cu2ZnSiS4 and β-Cu2ZnSiS4. Rietveld refinement of synchrotron XRPD and single crystal X-ray diffraction data showed that α-Cu2ZnSiS4 crystallizes with the wurtz-stannite structure, in the noncentrosymmetric space group Pmn21. The new polymorph, b-Cu2ZnSiS4, crystallizes with the wurtz-kesterite structure, in the noncentrosymmetric space group Pn. X-ray photoelectron spectroscopy of the α/β-Cu2ZnSiS4 sample and Cu2CdSiS4 verified the expected oxidation states of the ions. The calculated electronic structures (ES) of all of the studied materials indicate that the top of the valence band is dominated by the hybridization of the Cu/Ag-d and chalcogen-p states. The second-order nonlinear optical susceptibility (χ(2)), 62±3 pm/V, the third-order nonlinear optical susceptibility, (χ(3)), [(8.0±2.0)´104 pm2/V2], and the laser-damage threshold (LDT), 0.2 GW/cm2, for Cu2CdSnS4 are similar to AgGaSe2, a benchmark infrared (IR) NLO material. While χ(2) and χ(3) of α/β-Cu2ZnSiS4 are lower, the material outshines benchmark IR NLO materials in LDT (2.0 GW/cm2). These results align with the ideas that a higher degree of covalency leads to larger NLO susceptibility, while a wider bandgap leads to better LDT. Rietveld refinements using laboratory XRPD data indicated that Ag1-2xMnxInSe2 (x=0-0.07) and Ag0.95Mn0.05InSe2 are single-phase materials, where manganese was found to occupy the Ag (4a) site. The incorporation of manganese provides a simple means for physical property tuning, and likely represents a general mechanism for the substitution of silver ions with higher valence dopants in this system.

Format

PDF

Language

English

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