Center for Functional Nanomaterials Seminar
"Electron Diffraction and Its Application in Material Science"
Presented by Bin Jiang, FEI Company, Hillsboro, Oregon
Thursday, May 16, 2013, 11 am
Bldg. 735 - Conf Rm B
Hosted by: Eric Stach
Electron diffraction techniques are versatile method that can be employed to study many fundamental questions of crystal structure. In this presentation, I will introduce two important diffraction methods and their application in crystallography and material science. First topic is quantitative energy-filtered convergent beam electron diffraction (QCBED). It can be employed to measure the low order structure factor amplitudes and phases of microcrystals of known structure very accurately. Many difficulties of X-ray diffraction, such as extinction corrections, absorption corrections and scaling problems are avoided in electron diffraction. The accuracy is equivalent to that of the X-ray Pendullösung method, allowing the covalent and ionic contributions to be separately visualized. Accurate structure factors of Cu, GaN, AlN, SrTiO3, TiO2, Mg have been measured by QCBED. The measurement standard deviations are normally less than 0.2% for both amplitudes and phases of low order structure factors. Thus, accurate charge density maps have been obtained and the bonding character has been studied. Second topic is electron diffractive imaging. Diffractive imaging uses diffraction intensity and phase retrieval to form real-space images. Nanodiffraction patterns from carbon nanotubes and nanoparticles are recorded, and then a fienup algorithm is used to retrieve phases of these diffraction patterns to reconstruct atomic structure of these nanotubes and nano-particles. This lensless diffraction imaging method avoids the resolution limitation owning to objective lens aberration, and can enhance the resolution of TEM to sub-angstrom level using a conventional FEG gun TEM.