Dissertation Defense: Joel Swanson
Candidate: Joel Swanson
Major: Chemistry
Advisor: Sarah Stoll, Ph.D.
Colloidal Preparation of Rare Earth Telluride Nanomaterials
The lanthanide telluride phases, LnTex (Ln = La-Lu; x = 1-3) host a wide variety of electronic, magnetic, and coupled magneto-electric properties. However, study of these properties as a function of material size have been limited by their challenging synthesis and air sensitivity. We sought a general colloidal approach to access these materials on the nanoscale, to allow for sufficient quantities of materials to screen for optical, electronic and magnetic changes observed on the nanoscale.
Due to the lack of a generalized colloidal synthetic scheme for the lanthanide tellurides, we began by designing a synthesis that yielded high quality single crystalline NdTe3. By small variations of reagents, two different thicknesses of this highly anisotropic material were grown. These nanosheets exhibited enhanced charge density wave ordering temperatures, and diminished antiferromagnetic character on the nanoscale.
We then studied how this synthesis could be adapted to access other lanthanide telluride phases. By decreasing the concentration of tellurium, dissolved as trioctylphosphine telluride, we found phase control between LnTe3 and LnTe2 (Ln = La-Nd, except Pm), and LnTe3 and Ln2Te3 (Ln = Sm-Tb, except Eu). This allowed for access to CeTe3 nanosheets, which exhibited a significant change from antiferromagnetism in the bulk, to ferro/ferri-magnetism on the nanoscale.
In an effort to achieve additional control over the thickness of the nanosheets, GdTe3 was investigated, where synthetic variables such as ligand identity, halide source and reaction time were studied. We found that the largest aspect ratio nanosheets were formed from GdI3, however, thickness control remains elusive.
Lastly, the divalent lanthanide tellurides, LnTe (Ln = Eu, Yb) and EuTe2, were formed on the nanoscale and their optical/magnetic properties were studied as a function of their particle size. A minor blue shift of the band gap with decreasing particle size was observed for EuTe nanoparticles, and EuTe2 showed a decreased critical field for its spin flop transition. This work is the first colloidal approach to multiple phases of the lanthanide telluride family of materials, allowing studying properties that appear to be highly sensitive to particle size changes.