2009: Strategic Partnership Group (SPG) Funded for Material Imaging at the Space-Time Limit

  • Jan 3, 2009

The MSU Foundation, through the Strategic Partnership Fund initiative, has funded a group of scientists based in Physics (Chong-Yu Ruan, Martin Berz, and Phil Duxbury), NSCL (Marc Doleans), Chemistry (Marcos Dantus), and Material Sciences (Martin Crimp) for a joint effort to push the current limit of material imaging capabilities to the most fundamental level.

The composition and atomic arrangement in nanostructured materials, with length scales of 1-100nm, play significant roles in defining their properties and behavior, including phase transitions, chemical reactions, electronic processes, and macromolecular functions. These non-equilibrium processes, which involve the exchange of charge and energy, control the material properties by adjusting the electronic and consequently the atomic configurations on the time scale of femtoseconds to picoseconds. Studying the underlying physical and chemical properties at the most elementary level requires the development of an instrument capable of 3D imaging of a nanomaterial with Angstrom spatial resolution and femtosecond temporal resolution. This is a grand challenge problem in studying complex materials that the research groups of Ruan, Duxbury, Dantus, and Crimp face. An instrument capable of recording real-time movies of isolated nanostructures performing functions with atomic scale resolution is a holy grail in material research. Professor Duxbury, who is a world expert in solving nanostructure problems with limited experimental information, understands the intricate needs of reaching this goal. There is only so much one can do if the images are blurred due to averaging over random motions and inhomogeneous distributions as occur in experiments with limited space-time resolution. To solve the nanostructure problem, one needs to be able to reach space-time resolution on a single particle level, a research problem graduate student Zhensheng Tao is working on using the ultrafast electron diffraction (UED) method.

Currently UED represents a promising alternative approach to X-ray diffraction for high resolution space-time imaging, in particular for examining structural changes in nanostructured materials and macromolecules, due to its five orders of magnitude stronger scattering cross-section.  Ultrafast nanoscale electron crystallography using femtosecond electron pulses has recently been demonstrated in Ruan’s group in studying melting phenomena in isolated nanoparticles of gold supported on a substrate.  Similar to time-resolved X-ray crystallography, this technique uses laser pulses to trigger the structural changes, but use electron pulses to image the atomic rearrangement of nanoparticles at specific times in the rapid transformation induced by fs laser pulses – a technique pioneered initially at Caltech in Ahmed Zewail’s group where both Dantus and Ruan worked prior their coming to MSU.  The amazing feature of using electrons rather than X-rays for studying nanomaterials is the high-sensitivity that allows only a handful of particles to be investigated at a time.  This is in contrast to X-ray and neutron diffraction approaches in which samples usually have high volume density in the micrometer to millimeter size scale.  Materials studies with X-rays also face significant damage if a high flux source is used to boost the signal strength.  Nevertheless, the jury is still out on the viability of electron pulse sources because it is fundamentally difficult to pack a large number of electrons into a narrowly confined space-time volume, because of the strong Coulomb forces between them. This makes achieving femtosecond-nanometer imaging fundamentally difficult when employing pulsed electron beams.  Professor Crimp is an expert of implementing electron diffraction in a high-resolution transmission electron microscope.  He understands the great advantage of using an electron microscope to investigate individual nanoparticles and functioning domain sites for material research, and worries about having strong Coulomb forces in the electron beam column that will effect the spatial coherence of the electrons needed for resolving atomic details within the nanostructure. These fundamental issues pertaining to the so-called ‘space charge’ effects in a densely packed electron bunch will be solved using established accelerator physics concepts.

Professor Berz is a world leading expert in beam physics, who has been involved in the design of major accelerators, including the Large Hadron Collider.  His expertise in solving the beam dynamics problem is key to the development of a new type of electron optical system, which is specially tailored to the need of the dynamical electron microscope in solving the space-charge problem. Working with Berz, Professor Makino and graduate student He Zhang are working to extend their differential map approach to treat the beam dynamics of the femtosecond electron bunches.  A mini-accelerator will be built based on their calculations, to compress and shape the electron pulses into image forming rays.  But this is not possible without the insight of an accelerator physicist with expertise in bringing a highly technical design into reality.  Professor Doleans from the National Superconducting Cyclotron Laboratory will help design and construct the low energy accelerator. Doleans has significant experience in working with time-dependent electron optics used to shape and control electron bunches.  His insight into the key issues related to space charge problems has led to the materialization of the SPG project.

In terms of generating the most intense femtosecond laser electron bunches, the laser-material interactions at the photocathode surface need to be optimized to yield a well-defined initial condition for the downstream electron acceleration and compression. Professor Dantus’s pioneering femtosecond pulse-shaping technique will be incorporated to control photoemission and to achieve this goal.

This collaboration between six world class research groups funded by the MSU Foundation will allow them to work toward achieving the fastest, most precise and efficient imaging instrument and to focus it on solving complex material problems of the 21st century.