Laboratory for Ultrafast Nanoscale Imaging and Spectroscopy
Department of Physics and Astronomy
My research emphasizes on developing new tools for real-time molecular imaging applied to the studies of complex molecules and nanometer scale materials. Atomic scale resolution in structures of complex materials has been achieved in the late 20th century through modern diffraction and microscopy. The question remains on whether we can obtain temporal resolution required to characterize the molecular motions. This is critical for the understanding of mechanisms and functions on the mesoscopic scales, particularly for those associated with complex materials and macromolecules. The electron diffractions are very useful tools in the studies of molecules, surfaces and nano-meter scale materials because of the large cross-section of electron scattering with matters (5-6 orders larger than that of X-ray). Taking advantage of this high sensitivity, my earlier work with Professor Zewail at Caltech involved combining the spatial resolution of electron diffraction with the temporal resolutions of femtosecond laser to probe the real-time dynamics of complex molecules. This so-called ultrafast electron diffraction (UED) technique employs the “pump-probe” scheme to make movies for molecular reactions. Photo-chemical and photo-physical processes such as the breaking and reforming of chemical bonds and the internal energy redistributes in complex potential energy landscape were captured by electron diffraction in ultrashort time window. The ability to determine the short-lived transition state structure on an excited energy landscape is an important step towards quantum control of reactions.
The recent progress of ultrafast electron crystallography (UEC) takes advantages of the rapidly developing atomic scale preparations of functionalized nanocrystals and assemblies on surfaces, in line with the developments for molecular scale electronics and materials for sensing and catalysis. By interfacing the UED with ultrahigh vacuum and precision sample manipulations and preparations, it is now possible to isolate the structures and dynamics of the surfaces and adsorbates from those of the lattices. This ability allows one to visualize the patterns of energy flow from lattices to the surfaces and adsorbates or vice versa. It also enables the atomic scale studies of the hydrophobic and hydrophilic interactions of interfacial water on chemically modified surfaces, as well as the phase transitions on the nanometer scale.
The new development made at MSU includes an ultrafast electron nanocrystallography system for studying interfaces and nano-materials, and more recently an rf-enabled high-brightness electron microscope for studying complex materials and nano-film solution phase chemical and biological processes. With a proximity-coupled electron optical system, dynamical pulse compression, femtosecond laser pulse shaping, and nanoscaled sample manipulation and preparation, enhanced versatility and resolutions are being implemented to examine complex dynamical patterns of atoms and charges, triggered by ultrafast optical, thermal and electronic initiations. The ongoing efforts include studying phase transitions, collective phenomena and correlation effects in complex solids, hot electron dynamics at interfaces, and processes that are extremely far-from-equilibrium. To the extent necessitated by the sciences, we continue to develop techniques that enhance resolutions and enable new sciences. These efforts include producing brighter, faster electron pulses, combining spectroscopy, local probe and diffraction to correlate structure, dynamics and property. At the bottom of the length scale for material investigations everything looks like a big molecule, and can be viewed as complex entities with unusual capabilities. In the laboratory as well as from modern sophisticated molecular dynamics simulations, we now begin to have access to the multi-scaled world of matters with atoms and molecules gradually zoomed in for our perception.
Z. Tao, T.-R. T. Han, S.D. Mahanti, P.M. Duxbury, F. Yuan, C.-Y. Ruan, K. Wang, J. Wu, "Decoupling of Structural and Electronic Phase Transitions in VO2". Phys. Rev. Lett. 109, 166406 (2012).
T.-R. T. Han, Z. Tao, S.D. Mahanti, K. Chang, C.-Y. Ruan, C.D. Malliakas M. G. Kanatzidis, "Structural dynamics of two-dimensional charge-density waves in CeTe3 investigated by ultrafast electron crystallography, Phys. Rev. B 86, 075145 (2012).
K. Chang, R.A. Murdick, Z. Tao, T.-R. T. Han, C.-Y. Ruan, “Ultrafast electron diffractive voltammetry: General formalism and applications”. (Review) Mod. Phys. Lett. B 25, 2099 (2011).
C.-Y. Ruan, Y. Murooka, R.K. Raman, R.A. Murdick, R. J. Worhatch, A. Pell, "The development and applications of ultrafast electron nanocrystallography". (Review) Micros. Microanal. 15, 323 (2009).
R.A. Murdick, R.K. Raman, Y. Murooka, C.-Y. Ruan, “Photovoltage dynamics of the hydroxylated Si(111) surface investigated by ultrafast electron diffraction”. Phys. Rev. B 77, 245329 (2008).
R.K. Raman, Y. Murooka, C-Y. Ruan, T. Yang, S. Berber, D. Tomanek, “Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography”. Phys. Rev. Lett. 101, 077401 (2008).
C.-Y. Ruan, V. Franco, V.A. Lobastov, S. Chen, A.H. Zewail, “Ultrafast electron crystallography : transient structures of molecules, surfaces and phase transitions”. Proc. Natl. Acad. Sci. U.S.A. (101), 1123 (2004).
Physical Review Focus, "Diamonds
© 2006 | All Rights Reserved |