苏党生电镜课件EM Part I

I. Illumination (electron source)
Electron Sources
An LaB6 crystal
An FEG tip, showing the extraordinarily fine W needle
Field Emission Gun
Cathode V1 ext volt V0 acc volt
2700 1700 300 1010 1.5 10-4 1013 0.3 10-8
A/m2/ 109 sr eV 3 Pa 10-2
Life time
hr
100
500
>1000
II. Lenses and Aberration
Lorentz Force and Electromagnetic lenses
Ray paths in transmission electron microscope
Selected-Area Diffraction (SAD)
Interaction of electrons with specimen
Diffraction equations
Bragg equation The electron waves behave as if they were reflected off atomic planes
When an electron with charge q (= -e) enters a magnetic field of strength B, it experiences a force F (Lorentz force) F = q (E + v x B) = -e (v x B)
Electron lenses are the magnetic equivalent of the glass lenses in an optical microscope, and, to a large extent, we can draw comparisons between the two.
The physical basis of imaging and diffraction
Newton´ Lens Equation s
1 1 1 —— + —— = —— u v f Magnification
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
Effect of defocus and aberration
High resolution images of an Au nanoparticle
Delocalization effect in high-resolution image
III. Alignment and stigmation
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.
Transmitted beam gives (000) spot, and diffracted beam by (200) plane gives (200) diffraction spot.
Diffraction pattern
Lens Back focal plane
(200) (000)
Real Lenses and Aberration
Spherical Aberration
The spherical aberration is caused by the lenes field acting inhomogeneously on the off-axis rays. The further off axis the electron is, the more strongly it is bent back toward the axis. As a result, a point object is imaged as a disk of finite size, which limits our ability to magnify detail because the detail is degraded by the imaging process.
* Non-perfectly cylindrically symmertrical magnetic field down the bore of soft iron polepieces * Mictrostructural inhomogeneities of the soft iron * Not precisely centered apertures around the axis. * Contamination of aperture (charging up)
High magnification is achieved by repeat this for several lenses in tandem one after the other.
v M=— u
Exciting the lens strength – focus
Chromatic Aberration
misalignement of objecive lens aperture
residual contamination of the aperture
Astigmatic Image
Astigmatism Corrected
VI. Forming image and diffraction pattern
Magnification, Demagnification
In an electron microscope, changing magnification is obtained by changing the strength of the lens. In addition, the object in a TEM is close to the lens, making u small and M large.
Using lens to view diffraction pattern
Incident beam
(200)
Lens = Fourier transform (Real Reciprocal)
Parallel beam will be focused on one point at back focal plane by the lens.
Electron Diffraction Pattern from a Polycrystalline Polycrystalline
The reflected wave remains in phase If the path difference AB + BC (2 d sin ) follows the Bragg equation


d

A B C
n λ = 2 d sin θ
Thinking in reciprocal space
Lens rotation centers
The objective lens field should be centered around the optic axis, so that the direct electrons emerging from the specimen see a symmetric field as they pass through the lens.
The procedue to centering lens rotation is called current centering.
Correction of astigmatism in the imaging lenses
Objective lens astigmatism is mainly due to
Eucentric Plane
Eucentric plane is defined as a standard object plane: If the specimen is in the eucentric plane,
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).
DICP, Nov 14, 2006
Electron Microscopy and Its Application in Heterogeneous Catalysis
A basic lecture to understanding electron microscopy
Electron Microscopes at Fritz Haber Institute