Figure 1 – High-resolution transmission electron microscopy (TEM) images of fullerenes (C60 molecules) embedded into single-walled carbon nanotubes at 80 kV (left, conventional TEM) and 30 kV (right, SALVE). Conventional imaging at 80 kV using a spherical aberration (Cs)-corrected Titan TEM led to polymerization of the electron-beam-sensitive molecules at doses of 108 e-/nm2 (this dose corresponds to ca. 20 single images or ca. 20 seconds of exposure time). The electron irradiation at 30 kV using the SALVE TEM does not damage the fullerenes. The spherical and chromatic aberration (Cs/Cc)-corrected SALVE TEM provides higher resolution at much lower electron voltage, and allows imaging with white-atom contrast (atoms appear white on black). (Images provided by Johannes Biskupek, Ulm University; samples provided by Andrei Khlobystov, The University of Nottingham.)For most of my scientific life, I’ve been intrigued with the possibility of directly observing molecules. Atomic force microscopes (AFMs) in a myriad of forms produce some exciting images, but I was looking for something faster and more direct. Thus, when FEI (Hillsboro, Ore.) and CEOS (Corrected Electron Optical Systems, Heidelburg, Germany) announced delivery of the first sub-Angstrom, low-voltage electron (SALVE) microscope to Ulm University in Germany, I had to know more. The SALVE microscope will facilitate direct observation of atomic structures with sub-Angstrom spatial resolution. SALVE also often improves contrast, as shown in Figure 1. The clear 30-kV images are carbon fullerenes in a carbon nanotube (horizontal lines). Carbon-containing organic materials have been difficult to image in electron microscopes because they interact weakly with low-power X-rays. In addition, carbon structures are more easily damaged by high-energy electrons. Lower-energy electrons (30 kV) interact more strongly and cause less damage. However, the performance of conventional electron microscopes (optimized for 80 kV) deteriorates at lower energies because small variations in energy among the individual electrons of the beam become larger relative to the average beam energy.
Figure 2 – The SALVE instrument is large and aptly named Titan.In addition to correcting both spherical and chromatic aberrations, SALVE reduces the energy spread of the beam with a monochromator to provide high-energy resolution. This can facilitate spectroscopic analysis of composition, electronic structures and bonding states.
Where is this all going? Bert Freitag, director of product marketing, materials science, at FEI, forecasts: “In addition to imaging performance, [SALVE] supports cryo-EM techniques at liquid nitrogen temperatures and provides high-resolution electron energy-loss spectroscopy (EELS) information at low voltages. These capabilities will extend the range of our … Titan Themis TEM platform (Figure 2) into entirely new application spaces, such as research on molecules and new 2-D materials.”
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected].
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