BME 210 Lecture 8 Electron Microscopy

BME 210 Lecture 8 Electron Microscopy - 8. Electron...

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8. Electron Microscopy
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The principle of Electron Microscopy 1. Electrons can be deflected and focused exhibiting the “lensing” effect – they can be used for imaging 2. Electron’s wavelength can be very short providing high imaging resolution
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Electron wavelength Electrons have wave properties – just like photons Electron wavelength – De Broglie λ : λ DB = h/mu m – electron mass , u – velocity, h – Planck constant The existence of electron wavelength was postulated by Louis De Broglie in 1924 (Nobel Prize in 1929) Larger electron velocity shorter λ DB For electron accelerated by voltage V : mu 2 /2 = eV u = √(2eV/m) λ DB = h/√(2meV) For a typical acceleration V of 100 kV: λ DB ≈ 0.004 nm Like photons, electrons can diffract, interfere, etc. (“wave nature” of matter)
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Electron focusing – electron lens Electron lens – typically, magnetic - a wire coil with electrical current creating non- uniform magnetic field (electrical lens is also possible) Principle of operation: • Because of the electron charge, magnetic field exerts Lorentz force on a moving electron deflecting it • Electrons near the edge are deflected stronger than those close to the center (central electrons are not deflected at all) Parallel electron beam can be focused into a point – this is a lens • The electron lens can be considered in the same way as an optical lens Lorentz force magnetic field coil e -
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Electron lens resolution The theoretical resolution limit of EM is the same as for optical lens (the diffraction limit) : λ = λ DB 0.004 nm at 100 kV n 1 a
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This note was uploaded on 04/28/2010 for the course BME eng. biolo taught by Professor Fast during the Spring '10 term at University of Alabama at Birmingham.

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BME 210 Lecture 8 Electron Microscopy - 8. Electron...

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