Liquid to Solid State Dynamic TEM/ Coherent Electron Imaging and Source Development
The research in this program is aimed at in situ observations of structure and dynamics of solution phase reaction dynamics. Most chemistry and biology occurs within the liquid state, which would seem to be out of the reach of electrons as structural probes. As discussed above samples need to be on the order of 100 nm thin to allow sufficient electron transmission for either real space or diffraction imaging. How can one obtain stable liquid sample this thin? To solve this problem , the Miller group has made a significant advance in the development of nanofluidics with flow. To overcome the requirement of high vacuum and ultrathin liquid pathlengths, nanocells with “electron transparent” SiN windows have been used. The SiN windows must be on the order of 30 nm thin to allow sufficient e transmission and acceptable scatter. The major problem is that such thin windows, and the requirement for 100 micron viewing windows, leads to high instability. The windows have extremely low compliance (think of wildly flapping flags in a hurricane). It was discovered that the pathlengths could be stabilized to a few Angstroms by using a concept of dynamic stabilization (e.g. spinning a plate on stick) using a pathlength sensor in a negative feedback mode to a computer controlled syringe system. Real space imaging of single nanoparticles undergoing Brownian motion and even single biological macromolecules have been resolved with better than 1 nm resolution in real space imaging. This advance has opened a new window on in situ probing structural dynamics with electron probes. Stable liquid pathlengths as short as 45 nm have been achieved in a robust nanofluidic cell design (see Mueller et al, JPC Lett 2013). The next technical challenge is the electron source. The electron energy must be on the order of 200-300 KeV to have sufficient penetration depth and minimal scatter to enable electron diffraction to capture atomic motions in liquid phase. New methods in scaling electron energies while maintaining high temporal-spatial coherence are being developed, along with REGAE above, to provide the electron probes.
Specific research topics within this subgroup include the development of coherent electron sources for coherent imaging, solution phase reaction dynamics of classic photochemical reactions, in situ studies of biological functions over all relevant space-time domains. We now have the ability to go from simultaneous 10 femtosecond time resolution and sub-Angstrom spatial resolution in reciprocal space imaging of atomic motions. For real space imaging, new methods of phasing are being explored that promise to resolve correlated motions with sub-nm resolution and time resolution from nanosecond to seconds. This latter feature enables connection of far-from-equilibrium fluctuations captured with femtosecond electron diffraction to equilibrium fluctuations to make the link to thermally driven processes and biological functions.