Electron microscope, theoretical resolution

High-resolution electron microscopic studies employed a modified JEOL-JEM200CX (8) operated at 200 kV with objective lens characteristics Cs = 0.52 mm, Cc = 1.05 mm leading to a theoretical point resolution as defined by the first zero in the phase contrast transfer function of 1.95 A at the optimum or Scherzer (9) defocus position (400 A underfocus). 

Scherzer, O. The theoretical resolution limit of the electron microscope. J. Appl. Phys. 1948, 20, 20-29. 

Q.7.17 An electron microscope is designed with an accelerating potential of 200 kV. What is the wavelength of the electron beam in this device What is the theoretical resolution of this microscope Use an angle of illumination of 4 X 10 radians. 

Electron microscopic immunocytochemistry takes advantage of the increased resolution of the electron microscope to localize label to specific cellular organelles. Optical microscopes limit resolution to the wavelength of light, about 0.2 p,m or 200 nm. The electron microscope has theoretical resolution of less than 0.2 nm. Practically for biological samples, resolution is limited to about 2 nm because of the... 

The electron microscopes can be divided into two types (166) scanning electron microscopes (SEM), which use a 10-nm electron beam at the specimen surface, and transmission electron microscopes (TEM). With current TEMs, resolution of about 0.2 nm can be achieved, provided very thin (<20 nm) samples are available. With conventional inorganic oxide-supported metal catalysts, particles of approximately 1 nm can be detected. Scanning transmission electron microscopes (STEM) use a high brightness dark-field emission gun to produce a probe about 0.3 nm in diameter and combine the techniques of SEM and TEM. Further experimental and theoretical aspects of electron microscopy applied to catalysis have been reviewed recently (113, 167-169). 

Electron microscopy is a powerful direct experimental technique. Using electron microscopy information can be obtained on the presence of inhomogeneities, and on their shapes, sizes, size dispersion and number density. The experimental and theoretical aspects of this technique have been reviewed by Hirsh a/. (1965). There are two methods of observation. In the first, the topography of the sample surface is replicated and it is this replica, and not the sample which is then examined in the electron microscope. Generally carbon is used as the replicating material and shadowing at an angle with heavy elements (Pt) is used to accentuate the surface reUef. The resolution limit is about 50 A due to a microstructure in the rephca. As the sample itself is not examined, a diffraction pattern is not obtained. The sample surface can be either etched or unetched. An unetched surface wiU reveal cracks, voids, and polyphase microstructures if the various phases... 

When the electron microscope was first conceptualized, the goal was the ability to visualize individual atoms. Although this was not achieved for a few decades, the early electron microscopes were able to use electron beams to view objects. By the late 1930 s, electron microscopes, which had theoretical resolutions of 10 nanometers (nm), were being produced. By 1944, the resolution was decreased to 2 nm—avast improvement on the light microscope, which had a theoretical resolution of 200 nm. Other parts of the instrument were also improved for example, the voltage accelerations were increased, which resulted in better resolution. Also, better technology in the electron lens decreased the amount of optical aberrations, and the vacuum systems and the electron guns were refined. 

FIGURE 1.10 Transfer function calculated for optimum contrast (a) Reproduction of the phase shift sin % from Scherzer (1949) in case of optimum contrast, usually improperly called the extended Scherzer plateau (at 870 A defocus position for the high-resolution transmission electron microscopy used), (b) Effect of defocus on cos %. (Adapted from N. Uyeda et al. Molecular image resolution in electron microscopy. J. Appl. Phys. 43, 5181-5189 (1972). With permission.) (c) Scherzer plateau calculated for Philips CM 20 (200 kV, Cj = 1.2 mm). (From O. Scherzer. The theoretical resolution limit of the electron microscope. J. Appl. Phys. 20, 20-29 (1949). With permission.)... 

---