Investigating Defects and Magnetism in Nanocrystals by Magnetic Spectroscopies

Abstract

Magnetically-active nanocrystals have long attracted significant attention for their prospective applications in spintronics and other emergent technologies. Additionally, nanocrystals doped with magnetic impurities are also of interest for various phosphor, and other opto-electronic technologies. In order to improve and facilitate the use of these materials for devices, a thorough understanding of their electronic and magnetic properties is needed. Here, we explore the incorporation and speciation of transition metal-dopants in luminescent nanocrystals by taking advantage of their magnetic properties, as well as the effects of size confinement on the magnetic properties of nanocrystals. Chapter 1 introduces nanocrystals with magnetically-active dopants, explores the spectroscopic techniques used to investigate said nanocrystals, and expands upon the evolution of magnetism upon size confinement. Chapter 2 investigates through a combination of electron paramagnetic resonance (EPR) and photoluminescence spectroscopies the speciation of Mn2+ dopants in halide perovskite nanocrystals during typical post-synthetic anion exchange manipulations. This study reveals that during the anion exchange process, Mn2+ dopants cluster into regions of high lighter-halide (i.e., Cl ions) concentrations forming antiferromagnetic domains. Chapter 3 discusses using variabletemperature magnetic circular dichroism (MCD) spectroscopy to study the magnetic band gap splittings in colloidal EuS and EuSe nanocrystals as well as EPR spectroscopy to investigate the magnetization of EuS nanocrystals and the role trivalent impurity ions have on this magnetism. These studies document the giant band-gap splittings in europium(II) monochalcogenide nanocrystals as well as reveal how common approaches for mediating the magnetic ordering temperature (Curie temperature) of bulk EuS may not be suited for accomplishing the same in nanocrystals. This latter study also addresses what role of trivalent impurity ions play in the magnetization of EuS nanocrystals. In Chapter 4, we introduce the first preparation of nanocrystals of the chromium trihalides (Br-, I-). These materials have been shown to have layer-dependent magnetism as exfoliated bulk materials. We show that these effects also apply to nanocrystals of the materials though magnetic circular dichroism spectroscopy measurements. We additionally investigate the effects of lateral size miniaturization. In short, this work focuses on using unique magnetic spectroscopic tools to investigate the properties opto-electronic and magnetic properties of materials at the nanoscale.