[continued from page 1]The features include the most sophisticated detectors for electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS), says Watanabe. These will allow composition analysis at atomic resolution while improving stability, data acquisition speed and image quality.
The new STEM obtains an improved signal from samples with an electron “gun,” or source, that is 10 times brighter than that of any other STEM and an X-ray detector whose collection angle captures four times more signal.
Lehigh is purchasing the JEM-ARM200F from JEOL Ltd. in Japan with an NSF grant and with university matching funds. The new instrument replaces the existing JEOL 2200FS STEM, which was purchased in 2004 and was the first aberration-corrected electron microscope acquired by an American university.
The JEM-ARM200F operates at voltages as low as 60 kV, in contrast with the 200 kV minimum of the JEOL 2200FS. The lower voltages, says Watanabe, will enable the study of carbon-based and other “soft” materials that can be easily damaged by the bombardment of higher-energy electron beams.
“When we first considered purchasing the new STEM, the lowest operating voltage possible was 120 kV. JEOL said this could be reduced to 80 kV. We asked for 60 kV, which would allow us to characterize many more materials. The energy threshold of 80 kV is still too high for some organic materials that require gentle imaging conditions.”
Another piece of ancilliary equipment in the new STEM is an electron tomography stage that tilts in increments of one-half degree, making it possible to take as many as 720 2-D images of the same object. When combined, these images can provide a 3-D reconstruction of a nano-object.
“The ability to obtain 3-D reconstructions is critical for determining the location of individual nanoclusters on a support in a catalyst material,” says Watanabe.
The new STEM, which is expected to be delivered in early 2012, achieves greater stability with a larger column. Improved shielding isolates it more effectively from outside air movements, changes in temperature and acoustical waves, and electrical interference.
Similar JEM-ARM200F instruments are being installed at several other universities, says Watanabe, but they lack many of the special features possessed by Lehigh’s STEM.
“I’m sure this new instrument configuration will become standard in the next couple of years. We’re proud to be the first university to have the prototype.”
Playing pool with atoms
While electron microscopy alone can achieve angstrom-level resolution, says Israel Wachs, optical spectrometers are uniquely suited to detect the random signals given off by the amorphous surfaces where material properties are determined and where catalytic activity takes place.
Two instruments give Lehigh an unparalleled ability to study surfaces, says Wachs, professor of chemical engineering and director of the Operando Molecular Spectroscopy and Catalysis Research Lab. A third records critical events that occur in nanoseconds.
The university’s Scienta ESCA 300, one of the world’s most powerful high-resolution X-ray photoelectron spectrometers (HR-XPS), complements the high sensitivity-low energy ion-scattering spectrometer (HS-LEIS), which Lehigh recently purchased with an NSF grant.
LEIS, says Wachs, “is the only technique that can identify the atoms on the outermost layer of a solid surface. XPS provides very useful chemical information from the top 10-20 atomic layers.
“These techniques combine data from the surface and near subsurface, giving a new perspective on material surfaces while establishing the basic relationships between a material’s structure and its performance. They will assist greatly in designing advanced materials.”