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.
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.
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 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.
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.
Background subtraction for fluorescence EXAFS data of a very dilute dopant Z in Z+1 host, J. Synch. Rad. (2011) [PDF]
Degradation and local distortions in electroluminescent ZnS:Cu,Cl phosphors, IOP Conf. Ser.: Mater. Sci. Eng. (2010) [PDF]