1 Å Resolution in 200 kV STEM/SEM
imaging using both surface-generated secondary electrons and transmitted electrons provides correlative information for understanding what is happening on the surface and inside the sample. Te advantage of acquiring SE and STEM images
Figure 2: SE image of a Si (110) surface. The Si dumbbells, composed of two projected Si columns separated by a distance of 1.4 Å, can be clearly resolved.
shows simultaneously collected BF-STEM, HAADF-STEM, and SE images of a gold nanoparticle. Te capability of simultaneously collecting images in multiple imaging modes is an important advantage of STEM/SEM systems. Te BF-STEM image is similar to the TEM phase contrast image, which gives the morphology and atomic lattice information of materials. Te HAADF-STEM image is a dark-field image that carries chemical information because the image contrast is proportional to Z1.7 (Z denotes atomic number) [4]. Tis is oſten called the Z-contrast image. Te heavier the atom, the brighter the contrast is in HAADF-STEM images. It should be emphasized that both BF- and HAADF-STEM images are formed by electrons transmitted through the specimen so the images reflect a projection of the interior structure of specimen. Te SE image is formed by secondary electrons emitted from the specimen surface, revealing surface structural characteristics. Te simultaneous atomic resolution
simultaneously is also shown in Figure 4, which contains an SE and an HAADF-STEM image for polymer-coated Au nanoparticles. Te images were obtained using a Hitachi HD-2300A filed emission STEM/SEM operated at 200 kV [5]. Te sub-80 nm diameter Au nanoparticles were coated with cross-linked polymer shells. Under the SE imaging mode, a group of the Au/polymer core-shell nanoparticles appeared as close-packed spheres. However, the simultaneously acquired HAADF-STEM image shows up to 20 nm gaps among the Au nanoparticles. Obviously, the gaps should not be empty as they appear to be in the STEM image; they were actually filled with polymer shells coating the Au particles. Because of the weak scattering power of the polymer, these polymer shells did not have sufficient contrast in the transmitted Z-contrast images. On the other hand, the SE imaging is more sensitive to low-Z elements than HAADF imaging and good topographical view [6], therefore it could image the polymer shells. Te 3-D visualization effect of SE imaging may be combined
with in-situ heating electron microscopy to study the migration of nanocatalysts on a support surface. Using the SE detector on a Hitachi 300 kV field emission HF-3300 TEM/STEM/SEM, 1 nm diameter gold nanocatalysts supported on an iron oxide substrate were observed at elevated temperatures. When heated in-situ in the microscope, Au nanoparticles became mobile. Figure 5 shows a sequence of SE images taken at 700oC, the two gold nanoparticles indicated by arrows moved toward each other and finally merged to form a single particle. Terraces on the support surface are also visible [7]. Combined information about support surface and the behavior of nanocatalysts, as well as structural evolution in catalysts upon heating as imaged using the transmitted electrons (not shown), can be valuable for the study of catalysts.
Discussion Shortly aſter its commercial introduction, the designers
of SEMs settled on a maximum electron accelerating voltage of
30 kV to balance the
Figure 3: BF-, HAADF-STEM, and SE images of a gold nanoparticle. The three images were collected simultaneously on a Cs-corrected HD-2700 STEM/SEM. Accelerating voltage of 200 kV and a 0.1 nm diameter electron probe were used. Au (111) lattice fringes with a 0.24 nm lattice spacing are marked in the SE image.
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demands of resolution, X-ray excitation, manufacturing cost, and engineering aspects. However, it was also noticed from theory that higher electron beam energy would improve the spatial resolution and reduce the effect of lens aberrations. Figures 2–5 show that 200–300 kV TEM/STEM/ SEM can provide atomic- resolution characterization of both interior and surface structures of materials.
www.microscopy-today.com • 2011 September
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