Aberration Corrected Electron Microscopy

After decades of work and anticipation, systems that correct for aberrations of the electromagnetic lenses used in electron microscopy have now been developed [44-46]. The resolution of the standard transmission electron microscope (TEM) was described in Section 3.1.4.3, and the spherical aberration coefficient (Cs) appears in 

There is still a noise limit to resolution, and this requires that a number of electrons pass through each resolved feature. Following the calculations in Section 3.4.4, if a 50pm feature has 50% contrast and needs a 5 1 signal to noise ratio, it needs 50 electrons to pass through it, and this corresponds to 0.3 Ccm . From Fig. 

38 this is just possible for some organic crystals, but not for most polymers. Even so, it would not be wise to rule out some applications of this new technique. Negative 

In the scanning electron microscope (SEM), particularly at low voltages, the chromatic aberration is as important as the spherical aberration and an aberration corrected SEM (ACSEM) has both Cs and Q reduced to very small values 

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Figure 7.18 Direct observation of motions of the Er3N cluster and Er3N 4-C8o in CNTs (a) HRTEM images (upper, taken at 37 s lower, taken at 2 s) (b) suggested orientations (c) simulated images [185]. (Reprinted with permission from Y. Sato, et al., Structures of Dsd-Cso and 7h-Er3N C8o fullerenes and their rotation inside carbon nanotubes demonstrated by aberration-corrected electron microscopy, Nano Letters, 7, 3704-3708, 2007. 2007 American Chemical Society).

Sato, Y, Suenaga, K., Okubo, S. et al. (2007) Structures of Da-C%Q and /h-ErsNOCgo fullerenes and their rotation inside carbon nanotubes demonstrated by aberration-corrected electron microscopy. Nano Letters, 7, 3704-3708. 

Third, the notion (and reality) of such structural infractions as twins and coherent intergrowths - as is seen by Yacaman et al. in a 923-atom nanoalloy of AuPd [35] - is meaningless in our molecular bimetallic nanoparticles. In the nanoaUoys of Yacaman et al. [35, 42] and others [43], one may discern directly, by aberration-corrected electronic microscopy, thin bands of hexagonal close-packed and face-centered cubic packed sheets. In a typical molecular nanoparticle of the kind that we have studied (also by aberration-corrected electron microscopy [39]), it is directly established (in line with theoretical predictions [44]) that a single bimetallic cluster of RUj Pt does indeed possess molecular character. Furthermore, when six or more such clusters coalesce into larger entities containing ca 200 atoms they adopt the regular crystalline, and faceted state of a bulk metal. 

Ward EPW, Arslan 1, Midgley PA, Bleloch A, Thomas JM (2005) Direct visuahsation, by aberration-corrected electron microscopy, of the crystallisation of bimetallic nanoparticle catalysts. Chem Commun 5805... 

Cullen DA, Mctc KL, Reeves KS, Vemstrom GD, Atanasoska LL, Haugen GM, Atanasoski RT (2011) CharacterizatiCTi of durable nanostmctured thin film catalysts tested imder transient conditiCTts using analytical aberration-corrected electron microscopy. ECS Trans 41(1) 1099-1103... 

We have also not mentioned the last breakthrough in electron microscopy, where the spherical aberration problem has been finally solved by means of multipole lenses [117,118], and new aberration corrected electron microscopes are entering the market. 

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... 

A limiting factor in electron microscopy is the quality of the electron beam. Aberrations introduced by the optics limit both spatial resolution and analytical capabilities. There is a need to correct for the spherical and chromatic aberrations introduced by the electron optics. This will result in improved coherence of the beam and improved imaging and diffraction. In particular, these advances will permit the analysis of amorphous samples. Smaller beam sizes can also be achieved, allowing for sub-Angstrom resolution chemical analysis of samples. Development of higher-quality electron beams and short pulses of electron beams would broaden and deepen the application of electron microscopy. 

A very important reeent advanee in eleetron microscope development is in the correction of lens aberration. From the very start of electron microscopy development, the importance of lens aberration as a limiting factor of the spatial resolution of the electron microscope has been recognised [3.41, 3.42]. Ways to correct these aberrations were proposed a long time ago and put into actual microscopes with partial success at least about 30 years ago [3.43]. The main idea is to compensate the positive spherieal aberration of the round lens with the negative aberration of multipole lenses. 

R. C. Tiruvalam et al. Aberration corrected anal)tical electron microscopy studies of sol-immobilized Au -I- Pd, Au Pd and Pd Au catalysts used for benzyl alcohol oxidation and hydrogen peroxide production, Faraday Discuss., 2011, 152(0), 63-86. 

Fig. 1.2 Aberration-corrected high-resolution transmission electron microscopy (HR-TEM) images of HIF-Pt/C catalysts, showing the high density of atomic steps [33]...

Fig. 9.4 Aberration-corrected transmission electron microscopy image of partially unzipped outer walls of CNTs forming graphene-CNT complex catalyst with a high degree of edge exposure (reprinted with permission from Macmillan Publishers Ltd [32], copyright 2012)...

Urban KW (2008) Studying atomic structures by aberration-corrected transmission electron microscopy. Science 321(5888) 506-510... 

FIGURE 19 (a) The corrector of Fig. 18 incorporated in a transmission electron microscope, (b) The phase contrast transfer function of the corrected microscope. Dashed line no correction. Full line corrector switched on, energy width (a measure of the temporal coherence) 0.7 eV. Dotted line energy width 0.2 eV. Chromatic aberration remains a problem, and the full benefit of the corrector is obtained only if the energy width is very narrow. [From Haider, M., et al. (1998). J. Electron Microsc. 47,395. Copyright Japanese Society of Electron Microscopy.]... 

Urban KW et al (2009) Negative spherical aberration ultrahigh-resolution imaging in corrected transmission electron microscopy. Philos Trans R Soc A 367 3735-3753. doi 10.1098/rsta. 2009.0134... 

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