X-ray Diffraction- Applications

Question 1

Advantages of x-ray scattering for materials characterisation

Answer 1

Easy phase identification and structure determination. Non-destructive.
Versatile: single crystal, fibre, powder
Length scale from atomic to colloidal (100nm)
Inorganic, organic, biological
Bulk structure (in comparison to SEM, AFM)

Question 2

Disadvantages of x-ray scattering for materials characterisation

Answer 2

An indirect method (not microscopy). Diffraction is always an average so difficult for analysis of local defects. Interpretation of data can be ambiguous

Question 3

X-ray diffraction for protein crystallography and difficulties

Answer 3

Can find the shape of the molecule (protein folding) and how it is related to its functions.
However they are large molecules (over 10^3 amino acid groups) so dimensions of crystal lattice also large. Means many diffraction peaks. They are non-centrosymmetric and are chiral

Question 4

How protein crystallography works

Answer 4

Crucially need to grow a good single crystal of the protein. Use rotating crystal x-ray diffraction. Structure determination procedures well developed including softwares, isomorphous replacement, anomalous scattering (use of absorption edge)

Question 5

SAXS

Answer 5

Small angle x-ray scattering. For larger scale periodicities (fluctuations). Ideal for probing nanostructures: biological systems, surfactant systems, liquid crystals, microelectronics, nanoparticles, colloidal systems.
Requires high intensity source and good optics (synchrotron radiation source)

Question 6

Microbeam SAXS

Answer 6

E.g microfluid device of a surfactant system. Can find the internal orientation by looking at different depths

Question 7

X-ray study of thin films types

Answer 7

Reflectivity. Sample is flat and the incident angle is the same as scattered angle.
Off-specular scan. Sample at some angle and the difference between incident and scattered angle is twice this tilt Δθ.
GISAXS. Sample flat but scattering can move over two dimensions, incident angle is scattered angle with respect to x-axis

Question 8

Optical index of x-rays in a medium

Answer 8

Is less than 1. Is equal to 1-δ where δ is extremely small (about 10^-5)

Question 9

Total reflection of x-rays

Answer 9

Occurs when θ less than θc. Where Cosθc=n=1-δ
θc is very small as well (under 0.5°). X-rays will then be reflected by the outer surface with incident and reflected angles equal

Question 10

What is normally used for x-ray reflectivity instead of θ?

Answer 10

q=4πsin(θ)/λ
And qc=3.75x10^-2 rt(ρe)
Where ρe is electron density in units e-/Å^3

Question 11

Reflectivity

Answer 11

Intensity of reflected radiation over intensity of incident radiation.
R(θ)=(θ-x/θ+x)^2
Where x=rt(θ^2-θc^2)
Same form for R(q) but swap q for θ

Question 12

Reflectivity with multiple layer materials

Answer 12

Get oscillations in absolute reflectivity. These are due to constructive interference between waves reflected at the different interface between layers. Using a Fourier transform you can find the number of and depth of different layers

Question 13

GISAXS

Answer 13

Grazing incidence small angle x-ray scattering. The incident beam is reflected by the sample but can also be rotated. The detection plane shows the distribution of reflected beam and transmitted beam