STEM aberration-corrected

The first corrected electron-optical SEM was developed by Zach [10]. Eor low-voltage SEM (LVSEM, down to 500 eV electron energy instead of the conventional energies of up to 30 keV) the spot size is extremely large without aberration correction. Combining and correction and a electrostatic objective lens, Zach showed that a substantial improvement in spot size and resolution is possible. The achievable resolution in a LVSEM is now of the order of 1-2 mn. More recently, Krivanek and colleagues succeeded in building a corrected STEM [53,M]. 

Krivanek O L, Deiiby N, Spence A J and Brown L M 1998 Sphericai aberration correction in dedicated STEM Electron Microscopy 1998 14th Int. Oonf. on Electron Microscopy (Oancun) voi 1 (Bristoi institute of Physics Pubiishing) pp 55-6... 

Aberration-corrected ETEM/STEM (130) is expected to offer superior (subatomic) resolution under catalytic reaction conditions furthermore, it will provide improved flexibility for tilting the sample to different crystallographic orientations to allow understanding of the geometry of surface structural changes, enable the use of complex sample stages, and perhaps higher gas pressures. 

Characterization techniques continue to develop and will impact their application to zeolitic systems. Aberration corrected electron microscopes are now being used to improve our understanding of catalysts and other nano-materials and will do the same for zeolites. For example, individual Pt atoms dispersed on a catalyst support are now able to be imaged in the STEM mode [252]. The application of this technique for imaging the location of rare-earth or other high atomic number cations in a zeolite would be expected to follow. Combining this with tomography... 

Blom, D.A., Bradley, S.A., Sinkler, W., and Allard, LF. (2006) Observation of Pt atoms, clusters and rafts on oxide supports, by sub-Angstrom Z-contrast imaging in an aberration-corrected STEM/TEM. Proc. Microsc. Microanal, 12, 50-51. 

Important ongoing developments in HRTEM that are expected to be valuable in catalysis research include the correction of spherical aberrations in electron microscope lenses and monochromatization of the electron beam for improvement of the spatial and spectral resolution. Recently, scanning-TEM (STEM) of atomically dispersed lanthanum atoms on alumina (63) has provided e.x situ aberration-corrected images, but it is noteworthy that there is no technical limitation in applying the correction devices to instruments used for making measurements of samples in reactive environments. 

H. Sawada, et al., STEM imaging of 47-pm-separated atomic columns by a spherical aberration-corrected electron microscope with a 300-kV cold field emission gun, J. Electron Microsc. 58 (2009) 357-361. 

F re 3.10 Aberration-corrected STEM-HAADF Z-contrast images of (a) an AuPd core shell nanoparticle, and (b) an FePt ordered alloy particle. (Micrograph A courtesy of R. Tiruvalam, Lehigh University, USA. Micrograph B courtesy of J. Wittig, Vanderbilt University, Tennessee, USA). 

Another extremely important feature of aberration-corrected STEM-HAADF imaging of supported metal catalyst systems, is that sub-nm clusters and even atomically dispersed metal atoms become visible provided there is a reasonable Z-contrast between the metal... 

Figure 3.11 Aberration-corrected STEM-HAADF Z-contrast images showing the various Au species co-existing on the ZnO support (a), (b) three-dimensional Au nanoparticles (c) sub-nm Au clusters (white arrows) (d) sub-nm Au clusters and isolated Au atoms (white circles) (e) ordered monolayer Au rafts and isolated Au atoms, and (f) linear stripes of Au. (Micrographs courtesy of Q. He, Lehigh University, USA) ...

Fig. 18.14 Aberration-corrected STEM and EDX line scan analysis of the core-shell fine structures of dealloyed Pti JMi nanoparticles (reprint with permission from ref. [56])...

Ceria-zirconia mixed oxide-supported metal nanoparticles are attractive catalysts in low-temperature WGS, CO preferential oxidation (PROX), or three-way catalysis. It was observed that after redox cycles (reduction at 1173K and then oxidation at 823 K), the reducibility of these catalysts was enhanced substantially, which greatly affected their catalytic performance. HAADF studies showed that the pyrochlore-type cation sublattice in the reduced Ce Zr O was retained in the fully oxidized mixed oxide with a Ce Zr Og stoichiometry as far as the oxidation temperature did not exceed 823 K. However, it is not clear whether compositional heterogeneity occurred at the atomic level in Ce Zr Og. Trasobares and coworkers addressed this issue by aberration-corrected STEM, atomic-resolution EELS mapping, and EELS image simulations [66], They synchronously acquired EELS and HAADE signals in... 

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