Excited State Dynamics of First-Row Transition Metal Oxide Clusters
Description
Transition metal oxides are used for numerous applications, includingsemiconductors, batteries, solar cells, catalysis, magnetic devices, and are commonly
observed in interstellar media. However, the atomic-scale properties which dictate the
overall bulk material activity is still lacking fundamental details. Most importantly, how
the electron shells of metals and O atoms mix is inherently significant to reactivity. This
thesis compares the binding and excited state properties of highly correlated first-row
transition metal oxides using four separate transition metal systems of Ti, Cr, Fe and Ni.
Laser ablation coupled with femtosecond pump-probe spectroscopy is utilized to resolve
the time-dependent excited state relaxation dynamics of atomically precise neutral
clusters following 400 nm (3.1 eV) photoexcitation. All transition metal oxides form
unique stable stoichiometries with excited state dynamics that evolve due to oxidation,
size, or geometry. Theoretical calculations assist in experimental analysis, showing
correlations between charge transfer characteristics, electron and hole localization, and
magnetic properties to the experimentally determined excited state lifetimes.
This thesis finds that neutral Ti and Cr form stable stoichiometries of MO2 (M =
Ti, Cr) which easily lose up to two O atoms, while neutral Fe and Ni primarily form MO
(M = Fe, Ni) geometries with suboxides also produced. TiO2 clusters possess excited
state lifetimes that increase with additional cluster units to ~600 fs, owing to a larger
delocalization of excited charge carriers with cluster size. CrO2 clusters show a unique
inversed metallic behavior with O content, where the fast (~30 fs) metallic relaxation
component associated with electron scattering increases with higher O content, connected
to the percent of ligand-to-metal charge transfer (LMCT) character and higher density of
states. FeO clusters show a decreased lifetime with size, reaching a plateau of ~150 fs at
the size of (FeO)5 related to the density of states as clusters form 3D geometries. Finally,
neutral (NiO)n clusters all have similar fast lifetimes (~110 fs), with suboxides possessing
unexpected electronic transitions involving s-orbitals, increasing excited state lifetimes
up to 80% and causing long-lived states lasting over 2.5 ps. Similarities are drawn
between each cluster system, providing valuable information about each metal oxide
species and the evolution of excited state dynamics as a result of the occupied d-shell.
The work presented within this thesis will lead to novel materials of increased reactivity
while facilitating a deeper fundamental understanding on the effect of electron
interactions on chemical properties.
Date Created
The date the item was original created (prior to any relationship with the ASU Digital Repositories.)
2021
Agent
- Author (aut): Garcia, Jacob M.
- Thesis advisor (ths): Sayres, Scott G
- Committee member: Yarger, Jeffery
- Committee member: Steimle, Timothy
- Publisher (pbl): Arizona State University