4In normal imaging mode, the illumination is approximately parallel and the contrast in the image comes from the fact that the electrons are scattered..
5Electrons which leave the specimen in the same direction come to the same point in the diffraction pattern.Conversely, electrons which travel in the same direction at the diffraction pattern come from the same place on the sample and go to the same place in the image.
10An electron after scattering is going in a direction which is 2 away from the direction it had before the scattering.2 in a direction perpendicular to the planes which diffract.
11In convergent-beam diffraction, we do not use parallel illumination. We focus the electrons so that they form a focussed probe at the specimen.At the sample, the electrons are travelling in a range of directions inside a cone.
12Convergent-beam with no sample The electrons in each different direction, in the illumination cone, come to a different place in the diffraction pattern.Since the directions in the cone of illumination fill the cone, the electrons in the diffraction pattern fill a circle.In the diffraction pattern there is a bright disc.
13With a specimen The electrons are scattered though 2. Electrons are scattered from all the directions in the convergent conical illumination.Each point in the direct beam disc is one direction of illumination so each point in the disc can be scattered by the same 2.
15Therefore the diffracted electrons also form a disc. A convergent-beam pattern has an array of discs - one for each Bragg reflection.For every spot in a diffraction pattern with parallel illumination, there will be a disc in the convergent-beam pattern
32Disadvantages 1 Weak reflections harder to see 2 Does not show diffuse scatter For example, from disordered materials3 Not good for powder patterns – ring patterns.
33Golden Rules Golden rule I: Start with something easy Golden rule II: Take lots of pictures
34Practical details1 Use a large spot size for tilting and set up. Go to a small spot size only just before taking the picture.2 Choose a condenser aperture size to give the convergence angle that you want.3 In many cases, the ideal convergence is that which makes the discs just touch.
35ConclusionThere is every reason to use convergent-beam diffraction as the standard form of diffraction.Only use selected-area diffraction for:checking for weak reflectionslooking for structure in the diffuse scatterfor ring patterns
36Zone AxesA zone axis is a direction in a crystal that is parallel to more than one set of planesAt a zone-axis orientation, the electron beam travels down rows of atomsAt a zone-axis orientation, the diffraction pattern consists of a regular net of spots or discs
40Laue ZonesAt a zone-axis orientation, the reflections in the diffraction pattern break up into zones called Laue zonesThe central zone is called the zero-order Laue zoneThe first ring is called the first-order Laue zone - and so onThe first-order, second-order, third order (and so on) are known collectively as the higher-order Laue zones
42HOLZ HOLZ is the acronym for higher-order Laue zone The rings of reflections outside the central, zero-order Laue zone are the HOLZBecause the narrow, dark, straight lines in the bright field disc are associated with diffraction into a HOLZ reflection, they are known as HOLZ linesDo not confuse HOLZ with HOLZ lines
47The Tanaka MethodsTraditional microscopy taught that the microscope should be focussed on the specimen or on the diffraction pattern in the back focal plane.Tanaka liberated us and gave rise to a family of new techniques by telling us to look in other places.
53References on convergent-beam diffraction General bookThere is good basic information inTransmission Electron MicroscopyD. B. Williams and C. B. CarterPlenum New York, 1996Specific topicsMore detailed information on specific topics is to be found in:Electron MicrodiffractionJ. C, H. Spence and J. M. ZuoPlenum, New York, 1992Large-Angle Convergent-Beam Electron Diffraction (LACBED)J-P MorniroliSociete Francaise des Microscopies, Paris, 2002The atlas of convergent beam patterns from Bristol is:Convergent Beam Electron Diffraction of Alloy PhasesThe Bristol Group (Compiled by J. Mansfield)Adam Hilger, Bristol, 1984and the supplement (which includes an erratum list for the book) isThe Library of Convergent Beam Electron Diffraction Update: No 1.J. F. Mansfield, Y. P. Lin and R. J. GrahamNorelco Reporter: Electron OpticsThe other major group on convergent-beam diffraction is the group of Michiyoshi Tanaka at Sendai, Japan. They have produced a series of excellent books:Convergent-Beam Electron DiffractionM. Tanaka and M. TerauchiJEOL, Tokyo, 1985Convergent-Beam Electron Diffraction IIM. Tanaka, M. Terauchi and T. KaneyamaJEOL, Tokyo, 1988Convergent-Beam Electron Diffraction IIIM. Tanaka, M. Terauchi and K. TsudaJEOL, Tokyo, 1994Convergent-Beam Electron Diffraction IVM. Tanaka, M. Terauchi, K. Tsuda and K. SaitohJEOL, Tokyo, 2002Review article on the basics:Convergent-Beam DiffractionJ. A. EadesA chapter in "Electron Diffraction Techniques Volume 1", ed. J. M. Cowley,International Union of Crystallography and Oxford University Press(Oxford) 1992 ppThis two-volume set make a very good place to start on all electron diffraction topics.ReferencesThe books listed above contain many detailed references to research papers. I would draw attention to three tutorial papers of my own, which try to provide a clear introduction to some topics:Symmetry determination:'Symmetry Determination by Convergent-beam Diffraction'EUREM 88; IOP Conf. Series 93 (1988) Vol. 1, 3-12'Glide Planes and Screw Axes in Convergent-beam Diffraction: The Standard Procedure'Microbeam Analysis 1988 (D. E. Newbury Ed.) (1988) 75-80The Tanaka method (though, of course, there is a lot on this topic in his books):'Zone-Axis Patterns by the 'Tanaka' Method'J. Electron Microscopy Technique (1984) 1,
54AcknowledgmentThe convergent beam patterns used for this talk have been stolen from many different people especially the Bristol Group.