| Dawn Bonnell
|
Ph.D., Materials Science & Engineering, University of Michigan, 1986
M.S., Engineering Materials, University of Michigan, 1984
B.S.E., Materials Science & Engineering, University of Michigan, 1983
Research Interests
Issues involving properties of oxides at nm size scales are limiting in several emerging technologies, including ferroelectric DRAM, chemical micro sensors, and microwave filters for wireless communications. While several accurate tools exist for structure determination in solids, local property variations have been much less accessible. The application of Scanning Probe analysis has yielded considerable insight as to size dependent properties and surface or interface mediated behavior of ceramics. We have used atomic resolution STM of transition metal oxide surfaces to show that compositional variations are accommodated through the stabilization of surface phases, as well as by surface reconstruction. Recent studies in our lab focus on perturbation of local properties near interfaces. Electric field variation at field emitter arrays, size dependent contact potential at metal cluster/oxide interfaces, the atomic basis of potential at oxide grain boundaries, limitation to current flow in superconductors, and ferroelectric phase transformations are examples where nanometer scale property variations dictate behavior. Comparison of in situ measurements to continuum or quantum mechanical models are used to elucidate fundamental behavior.
Current Research Projects
The atomic basis of potential at oxide interfaces is determined in model studies using SrTiO3 bicrystal grain boundaries. Grain boundary structure, majority/minority carrier, and carrier concentrations are systematically varied. Theoretical simulations and in situ measurements of electric fields are combined to elucidate the effects of grain boundary structure and chemistry on electrical potential barriers. Local potential variations and electrostatic force interactions are compared to atomic structure determined by high resolution and z contrast electron microscopy. (collaboration with Oak Ridge National Lab)
Localized surface charge results from atomic polarization in ferroelectric oxides. Dynamics of ferroelectric domains through phase transitions are used to determine charge compensation mechanisms. The effect of surface reactivity on domains and vice versa are used to stabilize nanometer scale charge patterns.
Novel probes of nanometer scale property variations are developed based on Scanning Probe Microscopies. Particular attention is paid to quantification of local properties which requires theoretical analyses of electromagnetic interactions between the probe tip and complex surfaces. Examples include piezoelectric response imaging, magnetic force imaging with surface potential correction, electrostatic phase shift imaging of defects. Spatial resolution is on the characteristic length scale of the phenomena.
Group Members: