Comparison between SAD and CBEDYOUNES SINA AND UK HUH

MATERIAL SCIENCE AND ENGINEERING DEPARTMENT, THE UNIVERSIT Y OF TENNESSEE, KNOXVILLE, TN

In TEM, there are many methods such as Kikuchi, Fresnel, CTEM,

HRTEM, Lorentz, SE, BE, BF, DF, HAADF, CBED, SAD, EELS, etc.

Among them, SAD & CBED are two major methods to obtain crystal

structure.

CBED is easier and more useful method than SAD to obtain crystal structure information in CBED is easier and more useful method than SAD to obtain crystal structure information in

TEM. If you want to:TEM. If you want to:

Check for the presence of weak reflections; look for structure in the diffuse scattering; get Check for the presence of weak reflections; look for structure in the diffuse scattering; get

diffraction from a polycrystalline region, amorphous samples, or a region with more than onediffraction from a polycrystalline region, amorphous samples, or a region with more than one

grain use SAD. In the other cases, use CBED. grain use SAD. In the other cases, use CBED.

Illumination of the sample in SAD is parallel beam and in CBED is focused beam. If weIllumination of the sample in SAD is parallel beam and in CBED is focused beam. If we

remove our specimen in SAD, we see a single bright spot and in CBED, a single disc.remove our specimen in SAD, we see a single bright spot and in CBED, a single disc.   In CBED, we can focus the beam on a small area (30 - 100 nm). The smallest area in caseIn CBED, we can focus the beam on a small area (30 - 100 nm). The smallest area in case

of SAD is about 0.5 of SAD is about 0.5 μμm. Therefore SAD is more desirable for a region contains more than m. Therefore SAD is more desirable for a region contains more than

one grain.one grain.   Although position of the spots in SAD has a clear meaning but intensities of the spot has aAlthough position of the spots in SAD has a clear meaning but intensities of the spot has a

Little information. Intensities of discs in CBED contain about the sample such as: 3DLittle information. Intensities of discs in CBED contain about the sample such as: 3D

information about the lattice, symmetry details, lattice parameter & thickness changes.information about the lattice, symmetry details, lattice parameter & thickness changes.   If the specimen is polycrystalline, SAD is to be preferred because SAD will produce aIf the specimen is polycrystalline, SAD is to be preferred because SAD will produce a

sharp ring pattern. If the specimen is an amorphous material, SAD pattern has preference sharp ring pattern. If the specimen is an amorphous material, SAD pattern has preference thanthan

CBED.CBED.   CBED has preference for tilting and aligning the sample in order to navigation inCBED has preference for tilting and aligning the sample in order to navigation in

reciprocal space, because Kikuchi lines in CBED are more clearly visible.reciprocal space, because Kikuchi lines in CBED are more clearly visible.

1.1. D. B. Williams and C. B. Carter (1996), D. B. Williams and C. B. Carter (1996), Transmission Electron Microscopy, Transmission Electron Microscopy, Plemun, New Plemun, New York, U.S.A.York, U.S.A.

2.2. J.A.Eades, educational article: J.A.Eades, educational article: When to use selected-area diffraction and when to use When to use selected-area diffraction and when to use convergent-beam diffractionconvergent-beam diffraction,, Acta Microscopica Acta Microscopica Vol. 17, No. 1, 2008, pp.101-105Vol. 17, No. 1, 2008, pp.101-105

3.3. Reimer. Transmission Electron Microscopy. Springer, Berlin, 1993 (3rd edition).Reimer. Transmission Electron Microscopy. Springer, Berlin, 1993 (3rd edition).

Left) Kikuchi lines can be observed in CBED. Right) Kikuchi lines are hardly visible in SAD.

* Kikuchi line have very valuable information about determining the orientation of the sample.

Diffraction pattern types:

• Spot (Selected area diffraction)• Ring (Polycrystals)• Amorphous (Diffuse rings)• CBED (Convergent beam)• Kikuchi (Inelastic scattering)• HOLZ (Higher Order Laue Zones)

From diffraction patterns we can:

measure the average spacing between layers or rows of atoms

determine the orientation of a single crystal or grain

find the crystal structure of an unknown material

measure the size, shape and internal stress of small crystalline regions

This poster presents the strengths and weaknesses of SAD and CBED.

CBED is the most useful method and should be generally used. SAD

should be used only in certain specific cases which are discussed.

INTRODUCTION

GOAL

COMPARISON RESULTS AND SUMMARY

DIFFRACTION PATTERN TYPES AND USES

REFERENCES

LOGO

• • Parallel beams are focused at the back focal planeParallel beams are focused at the back focal plane• • Parallel illumination results sharp spots at the planeParallel illumination results sharp spots at the plane• • Conical illumination results discs at the planeConical illumination results discs at the plane

CBED patterns of five fold, three fold, and two fold respectivelyCBED patterns of five fold, three fold, and two fold respectively

SAD patterns of polycrystalline, texture, amorphous SAD patterns of polycrystalline, texture, amorphous

SAD and CBED; Two major techniques in TEMSAD and CBED; Two major techniques in TEM