July 7, 2004

NSLS 2004 Annual Users’ Meeting Workshop

Nanoprobes for Nanosciences

A workshop on synchrotron-based Nanoprobes was held on May 19th, 2004 as part of the National Synchrotron Light Source (NSLS) and Center for Functional Nanomaterials (CFN) annual users’ meeting. The goal of the workshop was to explore the scientific opportunities offered by synchrotron-based Nanoprobes, with emphasis on combined Low-Energy/Photo-Electron Microscopy (LEEM/PEEM), Soft X-ray Microscopy and X-ray Micro-Diffraction.

Bob Hwang, director of CFN at Brookhaven National Laboratory (BNL), opened the workshop by giving an overview talk about the status of the CFN, current CFN capabilities that are open for users, including scanning probes, Transmission Electron Microsocopy (TEM) and NSLS beamlines, and the plan to establish close collaborations with other BNL research departments. He also outlined the vision to provide the Nanoscience user community 24/7 access to a broad range of “off the shelf” instruments, as well as develop new and novel characterization methods in conjunction with partner users to achieve a more efficient transfer of information and technology.

After the introduction, the workshop was divided into three dedicated sessions. In the soft x-ray microscopy session, Harald Ade from North Carolina State University presented an extensive review of his work on characterization of polymers using soft x-ray transmission microscopy (XTM). The chemical sensitivity in Near Edge X-ray Absorption Fine Structure (NEXAFS) and linear dichroism are exploited to provide a unique contrast mechanism to discriminates different components in the sample as well as characterize the crystallinity and morphology of the sample. The second speaker of the session was Chris Jacobsen from SUNY Stony Brook. Chris first gave an excellent review of x-ray microscopy using zone-plates as well as the advances made in recent years in the fabrication of high-resolution x-ray zone-plates. In the second half of his talk, Chris talked about diffraction imaging, a new technique that has attracted a great deal of attention recently because the possibility of achieving higher spatial resolution then that of the zone-plate based x-ray microscopy. The key in diffraction imaging is of course recovering the phase information that is lost in the measurement. Chris gave a detailed account of a very efficient iterative algorithm recently developed.

After a short break, Cev Noyan from IBM and Columbia University started the session on hard x-ray micro-spectroscopy and micro-diffraction. Cev presented a study of strain in SiGe crystals grown on Si crystals. It was a very nice demonstration of the power of micro-diffraction. In this case, he was able to map out the strain in both the SiGe layer as well as that of the substrate. He also discussed in some length the challenges in finding and focusing on single grain using x-ray microbeams. The second talk in the session was given by Barry Lai from Advance Photon Source (APS) at Argonne National Laboratory. Barry talked about the applications of micro-fluoresence and X-ray absorption near edge spectroscopy (XANES) spectroscopy to life science problems, in particular the role of trace amounts of metal and metal oxide in immunology and intracellular processes. At the 2ID of APS, x-rays beamsize down to 200 nm x 200 nm, with intensities up to 2 x 10⁹ ph/sec, is achieved using a hard x-ray zone-plate. These hard x-ray zone-plates are extremely difficult to produce because of the narrow width and the height/width aspect ratio required. Barry also shared his experience in how to accurately align optics and position samples, a very challenging problem as the spatial resolution of the experiment improves to sub-micron level. The third speaker of the session is Ken Evans-Lutterodt of NSLS/BNL. Ken gave an overview of the microdiffraction project at the X13B beamline at the NSLS. The project is funded by the Department of Energy to serve the increasing needs of nanoscience users. The instrument, taking advantage of the small source size at the X13 straight section of the ring, is designed to perform sub-micron x-ray micro-diffraction and imaging. It has the unique capability of allowing experimenters to choose the focusing optics to optimize their measurement. The beamline will also serve at test bench for new emerging focusing optics and x-ray imaging techniques.

The third session, focused on low-energy electron / photoelectron microscopy (LEEM/PEEM), began after the lunch. Rudolph Tromp from IBM gave a broad overview of the history, current state-of-the-art, and the future possibilities of this powerful technique. For example, structural information with 4 nm spatial resolution can be achieved with LEEM today. There are also a variety of contrast mechanisms that can be exploited in LEEM, for example using bright and dark field imaging, as well as work function and electron energy dependence. A LEEM instrument can also be combined with a photon source to perform photoelectron microscopy (PEEM) to obtain chemical contrast. The potential of combining the structural information from LEEM and chemical information from PEEM is extremely exciting. Finally, by using a spin-polarized electron source, LEEM can be used to study magnetic surfaces and interfaces (SPLEEM). Time-resolved study of magnetization dynamics has also been demonstrated. Stefan Heun from ELETTRA Trieste/Italy presented the capability and results from the X-PEEM instrument located at Synchrotrone Trieste. The unique feature of the instrument is the energy filtering of photoelectrons. It enhances the chemical sensitivity significantly. The session ended with a talk on the next generation of PEEM developed at the Advanced Light Source (ALS) by Jun Feng from LBNL. Jun talked about the design of the PEEM3 project at LBNL. The goal of PEEM3 is to have spatial resolution down to 5 nm, almost an order of magnitude better than the PEEMs currently operating at various synchrotron facilities. Jun discussed in detail the critically important aberration correction system, including an electron mirror aberration corrector and an aberration-free magnetic beam separator.

Following the three focused sessions, there was a lively discussion period, chaired by Peter Sutter of CFN/BNL, Ken Evans-Lutterodt and Chris Jacobsen, for the workshop attendees to give their input. These valuable ideas will be taken into consideration in the design of the new LEEM/PEEM end station and for the future upgrade of the x-ray microscopy programs at the NSLS.

ACKNOWLEDGEMENTS
The National Synchrotron Light Source at Brookhaven National Laboratory is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.

FOR MORE INFORMATION
Cecilia Sanchez-Hanke
Building 725D
National Synchrotron Light Source
Brookhaven National Laboratory
Upton, NY
11973-5000
Tel.: (621) 344-5699
Fax: (631) 344-3238
Email: hanke@bnl.gov