高分辨电子显微学

The physical baபைடு நூலகம்is of imaging and diffraction
The objective lens
Newton´s Lens Equation q 1 1 1 —— + —— = —— u v f Magnification
Astigmatic Image
Astigmatism Corrected
HRTEM of Au Catalyst
200
A particle i l on [001] zone axis i
VI. Forming image and contrast mechansim
Back focal plane and image plane of objective lens
E. Ruska: Über Ü Fortschritte im Bau und in der Leistung des magnetischen Elektronenmikroskops, Z. Physik 87, (1934) 580‐602 (eingeg.: 12.12.1933)
Nobel Prize for his contribution in development of TEM
2700 1700 300 109 3 10-2 100 1010 1.5 1 5 10-4 500 1013 0.3 0 3 10-8 >1000
II Lenses II. L and d Aberration Ab ti
Lorentz Force and Electromagnetic lenses
u
f v
v M = — u
Magnification, Demagnification
In an electron microscope, changing magnification is obtained by changing the strength of the lens. High magnification is achieved by repeat this for several se eral lenses in tandem one after the other.
高分辨电子显微学方法
苏党生
Jin Hua, Sept. 27, 2010
dangsheng@fhi‐berlin.mpg.de
Die Erfindung des Elektronenmikroskops
Erstes höher als das Lichtmikroskop p vergrößerndes (zweistufiges) Elektronenmikroskop
Lens rotation centers
The objective lens field should be centered around the optic axis, so that the direct electrons emerging from the specimen p see a symmetric y field as they pass through the lens.
Electron lenses are the magnetic equivalent of the glass lenses in an optical microscope, microscope and, and to a large extent, we can draw comparisons between the two. two
When an electron with i charge q (= ( -e) ) enters a magnetic i field of strength B, it experiences a force F (Lorentz force) F = q (E + v x B) = -e (v x B)
Effect of defocus and aberration
High resolution images of an Au nanoparticle
D l li i effect Delocalization ff i in high hi h‐resolution l i image i
1
Human eyes can´t detect intensity changes < 5%, even < 10% is difficult.
What is image contrast
Do not confuse intensity with contrast when you describe your images. You can have strong or weak contrast but not bright or dark contrast. contrast The term bright and dark refer to density (number/unit area) of electrons hitting the sceree/detector (and the subsequent light emission that you can see). Lower the overall intensity y Condense the beam onto a small area stronger g contrast lower the image contrast
Eucentric Plane
Eucentric plane is defined as a standard object plane: If the specimen is in the eucentric plane, l the image of the object will not move as you tilt the specimen; objective lens strength is always the same when the image on the screen is in focus; the objective lens current is an optimum value If the image is in focus (working at a fixed objective lens magnification).
Electron Sources
Field Emission Gun
Cathode V1 ext volt V0 acc volt
Probe diameter 1‐2 nm Brightness 1013 A/m2ster
Focusing Anodes
Vacuum required 10‐8 Pa
Diffraction pattern
Selected‐Area Diffraction (SAD)
Selected‐Area Diffraction (SAD)
Use select area aperture to include many crystallites and single crystallite
MoO2-polycrystalline
MoO3-[010]
What is image contrast
Contrast C t t (C) as th the difference diff in i intensity (ΔI) between two adjacent areas
(I2 - I1) C= I1 ΔI = —— I
Nobel Prize 1986
Nobel Prize 2007
Part I: Basics of (S)TEM
Transmission electron microscope
Construction of a TEM
* Illumination (electron source) * Lenses and apertures * Specimen holder * Recording system
Chromatic Aberration
The objective lens bends electrons of lower energy more strongly and thus electrons from a point in the object form a disk image In general, the chromatic aberration become smaller for thinner specimen.
v M=— u To resolve a lattice distance of 0.2 nm a magnification of 5•106 is needed, assuming human eyes can resolve two points of 0.1 mm
Exciting the lens strength – focus
I Illumination (electron source) I.
Electron Sources
An LaB6 crystal
An FEG tip, showing the extraordinarily fine W needle
Electron Sources
LaB6 Gun
Probe diameter > 5 nm Brightness 109 A/m2ster Vacuum required 10‐4 Pa
The procedue to centering lens rotation is called current centering.
Correction of astigmatism in the imaging lenses