Objective lens high-resolution electron microscope

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). 

Aberration correction using exit wave restoration or commercial aberration correcting lenses generally only compensate the coherent objective lens aberrations up to either third or fifth order spherical aberration. For these microscopes the information limit resolution is no longer determined by the coherent objective lens aberrations but by chromatic aberrations coupled to the stability of the electron source, lens current and power supply. Correction of the remaining chromatic aberration is seen by many as the next step in the future of high-resolution electron microscopy. ... 

Alignment of the electron microscope. It is absolutely essential that the microscope be accurately aligned and that the astigmatism of the objective lens be fully corrected with the same objective aperture as used for the high-resolution imaging. 

High-resolution transmission electron microscopy can be understood as a general information-transfer process. The incident electron wave, which for HRTEM is ideally a plane wave with its wave vector parallel to a zone axis of the crystal, is diffracted by the crystal and transferred to the exit plane of the specimen. The electron wave at the exit plane contains the structure information of the illuminated specimen area in both the phase and the amplitude.. This exit-plane wave is transferred, however affected by the objective lens, to the recording device. To describe this information transfer in the microscope, it is advantageous to work in Fourier space with the spatial frequency of the electron wave as the relevant variable. For a crystal, the frequency spectrum of the exit-plane wave is dominated by a few discrete values, which are given by the most strongly excited Bloch states, respectively, by the Bragg-diffracted beams. 

The analogous light microscope method uses a phase plate to produce a phaseshift. Defocusing the objective lens is the method used to produce this phaseshift in the electron microscope. The situation is quite complex. The nature of the defocus-phaseshift relation must be well known in order to interpret the resulting images accurately. Phase contrast is always present, but can often be ignored except at high resolution or deliberate defocus [40,49, 63] (Section 3.1.4). 

Objective lens and specimen stage is the heart of a transmission electron microscope. Modem objective lenses are a twin lens instead of a single lens. It has an upper and a lower pole-piece with a gap in between. The resolution of a microscope is directly determined by the pole-piece gap. Small pole-piece gaps allow high resolution, but limit tilt angles. A larger pole piece gap reduces resolution but allows other TEM applications such as tomography, cryo TEM, environmental TEM, and dynamic experiments. 

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