Current Research

At Lawrence Berkeley National Lab, I collect and analyze X-ray absorption spectroscopy data, including XANES, EXAFS, and RIXS, of strongly correlated electron systems including lanthanide organometallic molecules and actinide intermetallic superconductors.

X-ray absorption spectroscopy in general and EXAFS in particular have a number of unique benefits for local structure determination. By tuning the incident X-ray energy, it is possible to examine individual elements within the sample and when interested in extremely dilute dopants, it is possible to collect data from fluorescence. Because the core-hole lifetime is so small, EXAFS has a time resolution of ~10-15 seconds.

Iron Organometallics

Iron organometallics are a class of materials with a spin-crossover in which the spin state can be switched on or off by changin external parameters such as temperature or pressure. I am studying the temperature-induced changes to the local structure behind the spin-crossover.

Actinide Supercondcutors

Actinides (and lanthanides) are very interesting elements due to the unique property of having partially filled f orbitals. In particular, uranium exhibts a unique mix of superconducting and magnetic properties, with the magnetic properties sometimes creating superconductivity, and sometimes destroying it. I am studying a variety of uranium compounds to better understand the physical basis for these phenomena.

Graduate Research

My research at UC Santa Cruz was primarily motivated by studying materials and phenomena with energy applications, including increasing efficiency and sustainable electricity generation. Some of the materials I've studied are electroluminescent ZnS:Cu, magnetic La1-xSrxCoO4, and thermoelectric clathrates. Most of the projects I worked on involved X-ray absorption spectroscopy to learn about the local structure of the particular material of interest.

I worked with Bud Bridges at UC Santa Cruz. We collected most of our data at the synchrotron at SLAC.

Electroluminescent ZnS Phosphors

ZnS:Cu contains CuS nanoprecipitates which amplify the local electric fields near the tips and enable AC Electroluminescence (EL) at voltages two orders of magnitude lower than the DC EL possible without Cu. After limited success in making smaller particles by grinding, I conducted EXAFS measurements to determine the cause of the damage and found evidence that many particles initially fracture through the CuS nanoprecipitates, leaving the CuS on the surface to be subjected to further damage and destroying field amplification.

ZnS:Cu AC EL devices degrade (exponentially) within a relatively short 100 hours of operation through a process that is still not understood. I took time-lapse microscopy images of several devices under degradation and found that the individual emission centers do not degrade evenly, but rather the light output of many of them drops suddenly. This helps to narrow the possible degradation mechanisms.

Thermoelectric Clathrates

Thermoelectric clathrates are a class of materials with a unit cell in the shape of a large cage in which a rattling atom sits. This rattler interferes with phonon conductivity but not electrical conductivity, resulting in useful thermoelectric properties. In order to better understand the cause of the thermal and electrical properties of thermoelectric materials, we compared the lattice structure as determined by EXAFS of various thermoelectric clathrates, such as Ba8Ga16Sn30 and Ba8Ga16Ge30.

Undergraduate Research

At Caltech, I worked with Nai-Chang Yeh on one project studying boiling liquid oxygen for enhanced effective gravity and another project fabricating electroluminescent spin-valves and characterizing their component layers.


X-ray Techniques

Background subtraction for fluorescence EXAFS data of a very dilute dopant Z in Z+1 host, J. Synch. Rad. (2011) [PDF]

Strongly Correlated Electron Systems

Self-irradiation damage to the local structure of plutonium and plutonium intermetallics, J. App. Phys. (2013) [PDF] [Journal]

Evolution of magnetic oxygen states in Sr-doped LaCoO3, Phys. Rev. Lett. (2012) [PDF] [Journal]


Local degradation of electroluminescent emission centers in ZnS:Cu,Cl phosphors, J. Luminescence (2013) [PDF] [Journal]

Probing the local structure of dilute Cu dopants in fluorescent ZnS nanocrystals using EXAFS, Nanoscale (2011) [PDF] [Journal]

Understanding and improving electroluminescence in mill-ground ZnS:Cu,Cl phosphors, J. Phys. D: Appl. Phys. (2011) [PDF] [Journal]

Enhanced Cu emission in ZnS:Cu,Cl/ZnS core-shell nanocrystals, Nanoscale (2010) [PDF] [Journal]

Degradation and local distortions in electroluminescent ZnS:Cu,Cl phosphors, IOP Conf. Ser.: Mater. Sci. Eng. (2010) [PDF]


Unusual distortions about Tl and Pb in PbTe:Tl, Phys. Status Solidi C (2012) [PDF] [Journal]


EXAFS evidence for a primary ZnLi dopant in LiNbO3, Phys. Rev. B (2012) [PDF] [Journal]

Pulsed chemical vapor deposition of Cu2S into a porous TiO2 matrix, J. Vac. Sci. & Tech. A (2011) [PDF] [Journal]