Dissertation Defense: Nicholas Rosa
Candidate Name: Nicholas Rosa
Advisor: Sarah Stoll, Ph.D.
Title: SYNTHESIS AND PHYSICAL PROPERTIES OF EUROPIUM CHALCOGENIDE SOLID-SOLUTION NANOMATERIALS
The europium chalcogenides (oxide, sulfide, selenide, and telluride) are intrinsic condensed magnetic semiconductors. This class of materials exhibits strong coupling of the electronic, optical, and magnetic properties that gives rise to phenomena that are of particular interest in device applications. These materials display colossal magnetoresistance, strong Faraday and magneto-optical Kerr effects, and the ability to create a spin-polarized current by spin filtering. These effects depend on the magnetic behavior of the material, which changes from ferromagnetic in EuO and EuS to antiferromagnetic in EuTe. Europium selenide (EuSe), however, is a special case which displays metamagnetism. This is attributed to the close balance of the ferromagnetic and antiferromagnetic interactions. This balance can be tipped to favor one
or the other using factors such as pressure, applied field, or small changes in the composition.
As the physical properties of nanoparticles change with size and shape, we have developed methods for size and shape control in EuSe nanoparticles. This has been achieved by changing the identity of the capping ligand and europium source used. We have observed size and crystallinity (single vs polycrystalline) differences in the optical bandgap.
In order to examine compositional effects to the electronic, optical, and magnetic properties, we have synthesized nanocrystals of EuS, EuSe, and solid-solutions of the formula EuS1-xSex, where 0≤x≤1, using solution phase thermolysis of Eu, S, and Se precursors. Bulk solids of the same formula were also synthesized using traditional solid-state techniques to serve as a comparison for examining nanoscale properties. We have carefully determined the structure and composition of these materials to ensure that the elemental distribution is homogenous throughout large numbers of nanocrystals and no clustering or phase separation is present.
The optical properties of these materials have been determined in the bulk and nanoscale. A linear trend between the bandgap and composition is observed in the bulk solid but significant optical bowing is observed for nanocrystal samples. The MCD spectrum has been obtained for nanocrystal samples that shows a shift of the location of the signal to higher energy with increasing selenium.
Wednesday, August 23 at 12:00pm to 2:00pm
Reiss Science Building, 238
37th and O St., N.W., Washington