Materials Science and Engineering PhD Thesis Defense by Mohammadreza Khodabakhsh

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MATERIALS SCIENCE AND ENGINEERING

PhD THESIS DEFENSE BY MOHAMMADREZA KHODABAKHSH

Title: Synthesis and Optical Characterization of Aurivillius Layered Perovskites

Speaker : Mohammadreza Khodabakhsh

Time: August 01, 2018 at 9:00 AM

Place: ENG B29

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Dr. Uğur Ünal (Advisor, Koç University)

Assoc. Prof. Dr. Alper Uzun (Koç University)

Asst. Prof. Dr. Sarp Kaya (Koç University)

Assoc. Prof. Dr. Burç Mısırlıoğlu (Sabanci University)

Assoc. Prof. Dr. Nuri Solak (Istanbul Technical University)

Abstract:

The main objective of this study is to synthesis layered perovskite with Aurivillius structure and explore their photocatalytic and photovoltaic properties. The results of the experiments as well as their possible applications are described in detail within each chapter. Chapter two focuses on the preparation of Aurivillius double perovskites with layered structure and their exfoliation by chemical processing (Liquid Exfoliation route). There is no report on the upconversion luminescence from the nanosheets of exfoliated perovskite type layered structures doped with lanthanides. The upconversion (UC) behaviour of Aurivillius phase Bi₂SrTa₂O₉ layered oxide doped with Er(III), Ho(III), Tm(III) and Yb(III) is demonstrated. This ferroelectric Aurivillius phase showed tunable upconversion emission at room temperature. The doped Aurivillius phase was synthesized with solid state method with desired amount of lanthanides substituting Sr(II) sites in the perovskite layer. The upconversion emission from lanthanides was observed also for the proton exchanged and exfoliated Aurivillius phase, which proves that lanthanides are doped into the perovskite [SrTa₂O₇]²- layer. The effect of concentration of lanthanides and sensitizer to activator ratio on the upconversion spectra are investigated and also possible energy transfer and upconversion mechanisms are suggested based on the number of photons participating in UC process. Herein, we report the 2D single nanosheets of doped Aurivillius phase layered perovskite capable of upconversion emission. Chapter three describes research performed on the thermal sensing behavior in Er(III) doped Aurivillius phase oxides with general formula of Bi₂Sr_0.95Er_0.05B₂O₉ and Bi₂Sr_0.75Yb_0.20Er_0.05B₂O₉ [B: Ta⁵⁺ and Nb⁵⁺] based on fluorescence intensity ratio (FIR) technique.  Pure oxides are synthesized through solid state reaction and their structural and luminescence properties are studied at different temperatures. While singly doped samples have shown sensitivity comparable to previous reports of layered perovskites, it is interesting to see that addition of sensitizer (Yb³⁺) to the structure can significantly enhance sensitivity by modifying crystalline size and structure of energy levels. The maximum sensitivity of 0.0073K^-1 and 0.0098K^-1 was obtained for BST and BSN samples respectively when temperature approaches 400°C.  Owning to high curie temperature, good thermal stability and superior sensitivity at high temperature, these oxides can be promising temperature probes for wide range and high temperature applications. Chapter four describes the study of dual luminescence behaviour in Er³⁺ doped BST nanoparticles. The superiority of coexistence of stoke and anti-stoke emission in a single host lattice with a single activator ion, besides to tunability of UC luminescence only by controlling sensitizer/activator ratio are very interesting features which can be used to produce dual mode multicolor luminescent ink with high security level against forgery.