Date of Award

January 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics & Astrophysics

First Advisor

Deniz Cakır

Abstract

Next-generation spintronic nanoscale devices require two-dimensional (2D) materials with robust ferromagnetism. Among 2D materials, MXenes are favorable for spintronic applications due to their high electron conductivity, mobility, and chemical diversity. Since 2D materials have greater elastic strain limits than their bulk counterparts, their properties can be tuned effectively using strain engineering. In this dissertation, we discuss density functional theory (DFT) based first-principles studies on ferromagnetic MXenes for spintronics applications. In Chapter III, we investigated modifications in the structural, electronic, and magnetic properties produced by strain on 2D Hf2MnC2O2 and Hf2VC2O2 ferromagnetic semiconductors. The calculations in Chapter III reveal that the conduction bands near the Fermi-level are extremely sensitive to the biaxial and uniaxial strain. As a result, semiconductor-to-metal phase transitions occur at around 1-3 % biaxial compressive strain for both monolayers. At around 8-9 % biaxial tensile strain, those monolayers become half-metals. It could be shown that those results can be produced by applying uniaxial strain on Mn-based monolayer. In Chapter IV, surface defects were introduced to tune the electronic and magnetic properties of those two monolayers. Bare-Hf2MnC2O2 nanosheet exhibits easy-plane anisotropy, whereas bare-Hf2VC2O2 has easy-axis anisotropy. It could be found that defects change the anisotropy of Mn-based monolayer to easy- axis anisotropy. Moreover, the Curie temperature of Hf2MnC2O2H0.22 was predicted as 171 K by using the Monte Carlo simulations of the classical Heisenberg model .

Recently, extensive studies have been carried out to discover new techniques to improve the properties of the materials to meet the demand of high cycling stability, charge capacity, and energy density of rechargeable battery applications. The 2D materials have been highly investigated for energy storage due to their large surface areas, which facilitate enhanced ion adsorption. In this dissertation, several possible techniques were explored to modify the electrochemical properties of 2D battery electrodes. Chapter V studied the B-doped-graphene (B-Gr) based systems to intercalate highly abundant Na and Mg to lower the production cost. The Na and Mg intercalated bare graphene bilayers are energetically not favorable. Nevertheless, we could show that B-Gr bilayers provide a considerable capacity (238 mAh/g for Na and 320 mAh/g for Mg). Na intercalated T2CO2/B-Gr, and B-Gr/B-Gr systems provide energy barriers as low as 0.46 and 0.18 eV, respectively. So far, hydroxyl, oxygen, and fluorine-terminated MXenes have been widely studied for energy storage applications. In Chapter VI, sulfur functionalized MXene structures (i.e., M2CS2 with M= Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) have been proposed as candidate materials to enhance battery performance. It was found that all the M2CS2 monolayers provide the energy barriers less than 0.22 eV for single Li adsorbed systems. Among the considered MXenes, Ti2CS2 provides the highest gravimetric capacity (417.4 mAh/g). Chapter VII introduced pillared Ti3C2O2 bilayers for enhancing the ion storage capacity, minimizing the change in the interlayer distance between MXene layers and lowering the diffusion barrier of ions. Two different quinone molecules, namely 1,4-Benzoquinone (C6H4O2) and Tetrafluoro-1,4-benzoquinone (C6F4O2) were considered as the linkers between Ti3C2O2 layers. Even though only a single Li layer can be intercalated between Ti3C2O2 layers, quinone molecules provide enough space to store two layers of Li. Thus, high capacity can be expected. Moreover, pillared structures show a very lower diffusion barrier, which is around 0.3 eV, than that of Ti3C2O2 bilayers without quinone molecules (1.0 eV).

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