LabAdviser/314/Microscopy 314-307/SEM/Nova/Transmission Kikuchi diffraction: Difference between revisions
| Line 2: | Line 2: | ||
The formation of Kikuchi patterns can be described as a two-step process, as shown schematically in Fig. 1. The first step consists of an incoherent inelastic, large angle scattering process of the primary beam electrons, producing a point-like electron source inside of the crystal. The second step is the elastic and inelastic coherent | The formation of Kikuchi patterns can be described as a two-step process, as shown schematically in Fig. 1. The first step consists of an incoherent inelastic, large angle scattering process of the primary beam electrons, producing a point-like electron source inside of the crystal. The second step is the elastic and inelastic coherent | ||
scattering of these electrons by the crystal planes of the material: the electrons can be Bragg reflected at lattice planes with reciprocal lattice vector -g if the Bragg angle is ±''θ''B or if the direction of incidence k''θ'' lies on one of the Kossel cones, which have an aperture 90°- ''θ''B and the g direction (normal to the lattice planes) as axis. There is a pair of Kossel cones for g, another pair for 2g and so on. The intersection between the Kossel cones and the plane of observation results in the formation of Kikuchi lines. The intersection is an hyperbola, but the lines are approximately straight owing to the low value of ''θ''B. The formed Kikuchi lines do not have the same intensity: one line, called the ''excess'' line, is brighter than the other, called the ''defect'' line. An example of a formed Kikuchi pattern is visible in Fig. 2, showing the excess (bright) and defect (black) Kikuchi lines and a zone axis, indicating an intersection of planes | scattering of these electrons by the crystal planes of the material: the electrons can be Bragg reflected at lattice planes with reciprocal lattice vector -g if the Bragg angle is ±''θ''B or if the direction of incidence k''θ'' lies on one of the Kossel cones, which have an aperture 90°- ''θ''B and the g direction (normal to the lattice planes) as axis. There is a pair of Kossel cones for g, another pair for 2g and so on. The intersection between the Kossel cones and the plane of observation results in the formation of Kikuchi lines. The intersection is an hyperbola, but the lines are approximately straight owing to the low value of ''θ''B. The formed Kikuchi lines do not have the same intensity: one line, called the ''excess'' line, is brighter than the other, called the ''defect'' line. An example of a formed Kikuchi pattern is visible in Fig. 2, showing the excess (bright) and defect (black) Kikuchi lines and a zone axis, indicating an intersection of planes. | ||
<gallery widths="350px" heights="350px" perrow="2" halign="center"> image:Picture11.png|Fig. 1: Schematic representation of Kikuchi lines formation. | <gallery widths="350px" heights="350px" perrow="2" halign="center"> image:Picture11.png|Fig. 1: Schematic representation of Kikuchi lines formation. | ||
image:Picture4.png|Fig. 2: Image of a Kikuchi pattern from an Au thin-film, recorded on a FEI Nova 600 NanoSEM at 30 kV acceleration voltage. </gallery> | image:Picture4.png|Fig. 2: Image of a Kikuchi pattern from an Au thin-film, recorded on a FEI Nova 600 NanoSEM at 30 kV acceleration voltage. </gallery> | ||
The simple geometry of Kikuchi diffraction makes indexing and orientation determination from Kikuchi patterns straightforward: the position of the diffracting lattice plane in the crystal can be easily determined from the center line of a pair of Kikuchi lines, together with the lattice plane spacing from the distance between the lines themselves. | |||
= Indexing of Kikuchi patterns = | = Indexing of Kikuchi patterns = | ||