News

Research Assistant (RA) Openings

There will be up to four new RA positions in the coming years in the laboratory of ultrafast electron crystallography. We seek first and second year graduate students who are potentially interested in Condensed Matter Physics, Atomic-Molecular-Optical Physics, and Nanosciences. The student(s) will be expected to join our continued exploration in areas of nanoscience and energy sciences by implementing and developing novel ultrafast techniques using electrons and lasers. New students will initially complement existing research projects (see group projects) as a way to get familiar with the lab, the instruments and the experimental techniques. We are glad to arrange a lab tour and presentation for interested students and encourage conversations with our group members. 

Postdoctoral Research Associate Opening

We are seeking postdoctoral researcher to perform researches on ultrafast nanoscale imaging. Candidates who receive or will receive Ph.D. in Physics, Chemistry, or Material Sciences in the areas of ultrafast lasers, accelerators, and microscopy are welcome to apply.

Interested candidates may contact Prof. Chong-Yu Ruan at ruan[AT]pa[DOT]msu[DOT]edu.

Oct 2009: Imaging ultrafast photoelectron dynamics

Understanding the mechanism of vacuum space charge emission and surface charge accumulation is crucial to the development of pulsed laser driven electron technologies, such as time-resolved photoemission, ultrafast electron diffraction and microscopy. Recently, a novel method to directly image the spatiotemporal evolution of the photoemitted electron packet generated over a femtosecond laser excited surface has been developed in UEC group . This method possesses sufficient sensitivity to image electron density as small as 10^10 e/cm^3 and permits quantitative measurement of the instantaneous electron density distribution and its collected translational and expansion speed near the surface. These initial kinetic energy distributions are representative of the hot electron profile within the materials responsible for the photoemission. This technique thus can provide information equivalent to those obtained from time-resolved photoemission spectroscopy, but can be operated at a strongly driven condition with a direct resolution to distinguish the presence of space charge effects. In the study of photoemission from a graphite surface, we found that the dynamical profile of photoelectrons reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and reduction of electron expansion (cooling). The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud.  (Applied Physics Letter 95, 181108(2009))

July 2009: Quantitative modeling of excited state structural dynamics to visualize surface melting of Au nanoparticles

Our group developed a progressive refinement method to analyze excited state data by adopting a reverse Monte Carlo approach for modeling the excited state structures . The tracking of particle dynamics is constrained by using the inherent dynamical correlations between structural functions obtained from two neighboring time frames. The ability of 'making molecular movies' is tantalizingly close from analyzing the photoinduced structural dynamics for the strong scattering Au and Ag nanoparticles, based on this method. (Microscopy and Microanalysis Vol. 15, 323-337; Special issue on ultrafast electron microscopy and ultrafast sciences)

July 2008: Shaping up graphite with light

Can we make diamond from graphite without resorting to extreme heat and pressure as diamond was made in the Earth in geological time? The UEC team raise such a possibility by shining a short burst of intense femtosecond laser pulse on graphite and watching using ultrafast diffraction the movements of some loosely separated carbon atoms in graphene layers undergoing rehybridization, forming more tightly bound diamond-like bonds. Whereas this intriguing intermediate structure is short-lived, approximately for 30 picoseconds, this experiment shows the possibility of a structural transformation purely induced by light in this very versatile class of material. Assisted by density functional theory calculation performed by Tomanek's group at MSU, the cause for such a transformation is attributed to the electronic structure changes and the Coulomb field buildup following the photoexcitation.  We are currently exploring different strategies in optically converting graphite into more permanent form of diamonds on graphite surface and ultrathin slab. 

(Phys. Rev. Lett. 101, 077401 (2008))

June 31, 2008 : Measuring photovoltage at ultrafast speed

We detailed a method to directly measure the photovoltage at nanointerfaces using femtosecond electron pulses.  The transient surface voltage is determined by observing Coulomb refraction changes induced by interfacial charging.  Using coherent diffraction signal, we can trace the origin of the refractive beam trajectory in material, thus obtain the nanoscale scale sensitivity of the charge transfer process.   We are currently extending this approach to investigate molecular electronic processes. 

(Phys. Rev. B 77, 245329 (2008)).

31 March 2007:  Ultrafast Meets Ultrasmall

Our first paper outlining ultrafast electron nanocrystallography and the studies of photomelting of size-selected Au nanoparticles is now published online (Nano Letters: Author ASAP). Download PDF: with subscription, without subscription.

We report in the Nano Letters the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with full-profile atomic density maps determined with sub-picosecond and picometer accuracies. In the ultrasmall environment, the melting process induced by photons is ultrafast and far-from-equilibrium, transforming nanocrystals into shelled nanoliquids.  The energy of the photons is transferred to the atoms within just a few picoseonds. However, it takes tens of picoseond for the atoms to be released from the crystalline motif into a liquid shelled structure. This dynamical transformation is strongly influenced by a coherent shearing motion within fcc structure that is determined as the pathway leading to the photomelting. The nanoliquids is more icosahedral-like, and it reverts to the fcc crystalline state in 100 ps. The structural excitation is displacive which results in a coherent transformation with crystal/liquid coexistence and that differs from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle. The reversible and coherent transformation on the ultrafast time scale demonstrates the directed dynamics on the energy landscape of finite systems. This methodology is general and could be implemented to study a wide class of phenomena pertaining to nanoscaled materials.

Time-resolved melting dynamics of 2nm Au nanoparticles. (Left) Radial distribution function  (RDF) map constructed by stacking RDF of UEC patterns at a sequence of delays between -5-2300 ps at irradiation fluence F=31 mJ/cm². Surface melting (enclosed by the dashed white line) is visible. (Right) RDF map for F=75 mJ/cm². Full scale melting is observed. The liquid state (enclosed by dashed white line) is characterized by the drop of 2^nd nearest density (at ~ 5Å) to (1-1/e) of the static value (at negative time).