First-principles studies of perovskite thin films and heterostructures

The growth of oxides on semiconductors is of great interest for electronics applications; however, the effects of film growth, atomic adsorption, and strain can have fundamental effects on the properties of the oxides in question. In this dissertation, we use density functional theory to calculate the properties of SrTiO₃ and BaTiO₃, and discover the effects of the environment on the electronic and atomic properties of these systems. We examine the effects of H adsorption on the SrTiO₃ and BaTiO₃(001) surfaces, and discover the coverage-dependent onset and retreat of metallic surface states. We calculate the effect of Pt film growth on BaTiO₃, and study the effects on the polarization of BaTiO₃ for different Pt/BaTiO₃ interfaces. We study how strain and interfacial chemistry affect the ferroelectricity of BaTiO₃/Ge and BaTiO₃/SrTiO₃/Ge heterostructures. We also discuss the development of two-dimensional conducting states created in BaTiO₃/SrTiO₃ heterostructures.Physic

Strain enhancement of the electro-optical response in BaTiO_3 films integrated on Si(001)

ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-379

Tuning the Basal Plane Functionalization of Two-Dimensional Metal Carbides (MXenes) To Control Hydrogen Evolution Activity

Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T_<i>x</i>) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti₃C₂T_<i>x</i> produced by different fluorine-containing etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo₂CT_<i>x</i> with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm^–2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo₂CT_<i>x</i> are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS₂, in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed

Two-Dimensional Molybdenum Carbide (MXene) as an Efficient Electrocatalyst for Hydrogen Evolution

The hydrogen evolution reaction (HER) is an important energy conversion process that underpins many clean energy technologies including water splitting. Herein, we report for the first time the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Our computational screening study of 2D layered M₂XT_<i>x</i> (M = metal; X = (C, N); and T_<i>x</i> = surface functional groups) predicts Mo₂CT_<i>x</i> to be an active catalyst candidate for the HER. We synthesized both Mo₂CT_<i>x</i> and Ti₂CT_<i>x</i> MXenes, and in agreement with our theoretical predictions, Mo₂CT_<i>x</i> was found to exhibit far higher HER activity than Ti₂CT_<i>x</i>. Theory suggests that the basal planes of Mo₂CT_<i>x</i> are catalytically active toward the HER, unlike in the case of widely studied MoS₂, in which only the edge sites of the 2H phase are active. This work paves the way for the development of novel 2D layered materials that can be applied in a multitude of other clean energy reactions for a sustainable energy future