X-ray_Diffraction.ppt
- 1. X-ray Diffraction The Basics Followed by a few examples of Data Analysis by Wesley Tennyson NanoLab/NSF NUE/Bumm
- 2. NanoLab/NSF NUE/Bumm X-ray Diffraction Bragg’s Law Lattice Constants Laue Conditions θ - 2θ Scan Scherrer’s Formula Data Analysis Examples
- 3. Bragg’s Law nλ = 2 d sin θ • Constructive interference only occurs for certain θ’s correlating to a (hkl) plane, specifically when the path difference is equal to n wavelengths.
- 4. NanoLab/NSF NUE/Bumm Bragg condition’s The diffraction condition can be written in vector form 2k∙G + G2 = 0 k - is the incident wave vector k’ - is the reflected wave vector G - is a reciprocal lattice vector such that where G = ∆k = k - k’ the diffraction condition is met
- 5. NanoLab/NSF NUE/Bumm Lattice Constants The distance between planes of atoms is d(hkl) = 2π / |G| Since G can be written as G = 2π/a (h*b1+ k*b2 +l*b3) Substitute in G d(hkl) = a / (h2 + k2 + l2)(1/2) Or a = d * (h2 + k2 + l2)(1/2) a is the spacing between nearest neighbors
- 6. NanoLab/NSF NUE/Bumm Laue Conditions a1∙∆k = 2πυ1 a2∙∆k = 2πυ2 a3∙∆k = 2πυ3 Each of the above describes a cone in reciprocal space about the lattice vectors a1, a2, and a3. the υi are integers When a reciprocal lattice point intersects this cone the diffraction condition is met, this is generally called the Ewald sphere.
- 7. NanoLab/NSF NUE/Bumm Summary of Bragg & Laue When a diffraction condition is met there can be a reflected X-ray Extra atoms in the basis can suppress reflections Three variables λ, θ, and d λ is known θ is measured in the experiment (2θ) d is calculated From the planes (hkl) a is calculated sin 2 n d 2 2 2 l k h d a
- 8. NanoLab/NSF NUE/Bumm θ - 2θ Scan The θ - 2θ scan maintains these angles with the sample, detector and X-ray source Normal to surface Only planes of atoms that share this normal will be seen in the θ - 2θ Scan
- 9. NanoLab/NSF NUE/Bumm θ - 2θ Scan The incident X-rays may reflect in many directions but will only be measured at one location so we will require that: Angle of incidence (θi) = Angle of reflection (θr) This is done by moving the detector twice as fast in θ as the source. So, only where θi = θr is the intensity of the reflect wave (counts of photons) measured.
- 10. NanoLab/NSF NUE/Bumm θ - 2θ Scan
- 11. Smaller Crystals Produce Broader XRD Peaks
- 12. t = thickness of crystallite K = constant dependent on crystallite shape (0.89) = x-ray wavelength B = FWHM (full width at half max) or integral breadth B = Bragg Angle Scherrer’s Formula B cos B K t
- 13. Scherrer’s Formula What is B? B = (2θ High) – (2θ Low) B is the difference in angles at half max 2θ high Noise 2θ low Peak
- 14. When to Use Scherrer’s Formula Crystallite size <1000 Å Peak broadening by other factors Causes of broadening • Size • Strain • Instrument If breadth consistent for each peak then assured broadening due to crystallite size K depends on definition of t and B Within 20%-30% accuracy at best Sherrer’s Formula References Corman, D. Scherrer’s Formula: Using XRD to Determine Average Diameter of Nanocrystals.
- 15. Data Analysis Plot the data (2θ vs. Counts) Determine the Bragg Angles for the peaks Calculate d and a for each peak Apply Scherrer’s Formula to the peaks
- 16. Bragg Example
- 17. Bragg Example d = λ / (2 Sin θB) λ = 1.54 Ǻ = 1.54 Ǻ / ( 2 * Sin ( 38.3 / 2 ) ) = 2.35 Ǻ Simple Right!
- 18. Scherrer’s Example Au Foil 98.25 (400) 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 95 95.5 96 96.5 97 97.5 98 98.5 99 99.5 100 100.5 101 101.5 102 2 Theta Counts
- 19. Scherrer’s Example B B t cos 89 . 0 t = 0.89*λ / (B Cos θB) λ = 1.54 Ǻ = 0.89*1.54 Ǻ / ( 0.00174 * Cos (98.25/ 2 ) ) = 1200 Ǻ B = (98.3 - 98.2)*π/180 = 0.00174 Simple Right!