Surface Characterization Techniques: Diffraction and Scattering

Diffraction and Scattering

Wave Mechanics

Diffraction and scattering are vital in wave mechanics for understanding wave behaviors. These principles are applied in spectrometry, optics, acoustics, high-energy research, and building designs.

Diffraction

Diffraction refers to the behavior of waves when they encounter an obstacle, involving the bending of waves around small obstacles and the spreading of waves through small openings. This is observable using a ripple tank. The extent of diffraction depends on the size of the opening and the wavelength of the wave. Observable diffraction occurs when the width of the hole and the wavelength are of the same or nearly equal order. If the wavelength is much larger or smaller than the hole's width, significant diffraction is not produced.

Diffraction of light through a small slit demonstrates the wave nature of light. Famous experiments include Young's single and double slit experiments. The diffraction grating is a useful product based on diffraction theory, used to obtain high-resolution spectra.

Scattering

Scattering is the deviation of waves due to irregularities in space. Radiations like light and sound, and even small particles, can be scattered by particles, density anomalies, or surface anomalies. Scattering is an interaction between two particles, important for proving the wave-particle duality of light, illustrated by the Compton Effect. Rayleigh scattering explains why the sky is blue; blue light from the sun is scattered more than other wavelengths.

Other forms of scattering include Brillouin scattering and inelastic X-ray scattering.

Difference between Scattering and Diffraction

  1. Diffraction is exclusive to waves, while scattering occurs in both waves and particles.

  2. Diffraction is a wave propagation property, whereas scattering is a wave interaction property.

  3. Diffraction proves the wave nature of light, while Compton scattering proves the particle nature of light.

  4. Diffraction is coherent, requiring a regular surface/medium relative to the wavelength. Scattering is incoherent and occurs when the interface/surface is rough on a scale comparable to the wavelength.

Surface Characterization Techniques

These techniques analyze patterns produced when a sample is illuminated by X-rays. Diffraction patterns reveal the atomic structure of molecules like powders, small molecules, or large molecules like protein crystals. Diffraction measures strains in materials under load by monitoring changes in atomic plane spacing. Scattering experiments determine the molecular structure of non-crystalline materials, including complex biological samples and polymers.

The interface between materials in composites (e.g., glass fiber and resin) is critical to performance. Chemical analysis identifies what is present, its location, and quantity, for all elements and molecular fragments up to 10,000 mass units.

Synchrotron Techniques

The main diffraction and scattering techniques include:

  1. Macromolecular Crystallography (MX)

  2. Grazing Incidence X-ray Diffraction (GIXD)

  3. Powder Diffraction

  4. X-Ray Reflectivity (XRR)

  5. Scattering-SAXS and WAXS

  6. X-ray Photoelectron Spectroscopy (XPS) in Surface Chemical Analysis

  7. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) in Surface Characterization

  8. Adhesion Failure Analysis of Composite Materials

  9. Dynamic Secondary Ion Mass Spectrometry (DSIMS)

  10. 3D Profilometry Using White Light Interferometry

These techniques involve measurements using suitable beam lines and are employed in four main categories:

  1. Diffraction/scattering for crystallography.

  2. Spectroscopy for chemical composition analysis down to the nanometer level.

  3. Polarimetry for measuring the shape of complex molecules and the properties of magnetic materials.

  4. Imaging for detailed imaging of small animals and biological/physical materials using light from infrared to hard X-rays.

1. Macromolecular Crystallography (MX)

Macromolecular Crystallography, also known as Protein Crystallography, determines the 3D structures of large biological molecules. It is essential for linking structure with function, rational drug design, investigating protein folding, and relating structural information.

MX beam lines enable structure determination by molecular replacement, standard replacement with heavy atoms, Single wavelength Anomalous Dispersion (SAD), or Multi-wavelength Anomalous Dispersion (MAD) measurements, where the X-ray wavelength is tuned with the natural absorption wavelength of selected atoms in the macromolecule. This detects small differences in the diffraction pattern, allowing phase determination to determine the structure through crystallographic methods.

Protein Crystallography

This single-crystal X-ray diffraction technique measures diffraction patterns of protein crystals to determine their structure. Protein crystallography is used in drug design, allowing researchers to tailor pharmaceuticals to interact with proteins and other large biological molecules.

Applications

  1. Rational drug design

  2. Enzyme mechanisms

  3. Supramolecular structure

  4. Molecular recognition of compounds like nucleic acids

2. Grazing Incidence X-ray Diffraction (GIXD)

Many material properties are determined by their surfaces and interfaces, including electronic, magnetic, and chemical behavior. GIXD is ideal for determining the morphology of novel materials in realistic operating conditions, leading to technological development.

Applications

  1. Examining the atomic structure of materials, surfaces, and interfaces.

  2. Measuring strain at interfaces to predict component function in realistic conditions.

  3. Investigating complex alloy semiconductors, oxide surfaces, polymers, and biological films using interchangeable environmental stages.

3. Powder Diffraction

Powder diffraction examines small, weakly interacting crystals in random orientations. It studies polycrystalline materials like metals and alloys, ceramics, superconductors, pharmaceuticals, geochemicals, zeolites, and related porous solids.

Applications

  1. Refinement of structures and P-T mapping of organic molecular pharmaceutical solids.

  2. Determination of competing effects in highly correlated oxides and chalcogens.

  3. Study of structural changes in polymer lithium-ion conductors.

  4. Structure of porous materials and nanoscale systems within pores, and the time dependence of intersite cation orders in mineral phases.

  5. Technological areas including catalysis, energy storage, radioactive waste treatment, magnetic recording, structural biology, and pharmaceuticals.

4. X-Ray Reflectivity (XRR)

X-ray reflectivity (XRR) studies the surface properties of materials by probing electron density perpendicular to the surface, to obtain information about surface roughness, thin film thickness and density. The technique measures the reflected X-ray intensity as a function of incidence angle over a range of angles close to the critical angle for total reflection.

Applications

XRR studies thin films or multilayers of metals and semiconductors or soft matter (polymers, surfactants, biological systems, etc.) at both solid and liquid interfaces.

5. Scattering-SAXS and WAXS

Small Angle X-ray Scattering (SAXS) provides essential information on the structure and dynamics of large molecular assemblies, also called Non-crystalline diffraction (NCD), characteristic of living organisms and complex materials like polymers and colloids. SAXS covers the angular range up to 1^\circ while WAXS typically covers 5-60^\circ.

Anomalous SAXS (ASAXS) uses tuneable synchrotron X-rays with energies close to the absorption edges of the element under study, providing information on the specific composition and density fluctuation of the sample.

Applications

  1. Studies of supramolecular organization in biological systems.

  2. Corneal transparency.

  3. Structure and function of muscle filaments.

  4. Biological membranes

  5. Polymer processing and design.

  6. Self-assembly of mesoscopic metal particles, colloids, and liquid crystals devices.

Wide-angle-X-ray scattering (WAXS) is similar to SAXS, but diffracts at larger angles. Conducting both WAXS and SAXS in a single run is possible, depending on the instrument. The diffraction pattern allows determination of chemical composition, film texture, crystallite size, and film stress. Crystalline solids contain uniformly spaced atoms (electrons) described by imaginary planes, where the distance between these planes is known as d-spacing. Each crystalline solid has a unique d-spacing pattern, and the intensity of these patterns is directly proportional to the number of electrons (atoms) present in imaginary planes.

6. X-ray Photoelectron Spectroscopy (XPS) in Surface Chemical Analysis

XPS analyzes the surface by using an X-ray beam for its elemental chemical composition. The top 10 nm of the sample surface is quantitatively analyzed at an accuracy of 0.1 atomic percent. In high resolution mode, it enables the nature of elemental bonding (e.g., C-O and C-C) and the oxidation state of metallic elements to be identified. The technique is also used to construct two-dimensional chemical maps for particular elements and for quantitative elemental depth profiling of the top 10 nm via argon ion beam sputtering.

Applications

  1. Determine elements and their quantities within the top 1-12 nm of the sample surface.

  2. Determine contamination in the sample bulk.

  3. Determine empirical formula of material.

  4. Determine the chemical state, and local bonding of atoms.

  5. Determine binding energy of electronic states.

  6. Determine density of electronic states.

  7. Determine thickness of different materials within the top 12 mm of the surface.

7. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) in Surface Characterization

Static secondary ion mass spectrometry uses a primary ion beam to gather information about the material from the sample surface. The instrument detects both neutral and ionic species, focusing the secondary ions towards a charge detector using ion optics. Mass detection is achieved by using an extended flight path such that lighter ions arrive at the detector before heavier ions, giving 'time-of-flight' spectral separation. The outcome extends to molecular fragments as large as 10000 mass units. ToF-SIMS samples the top 3 nm of the surface and is sensitive to ppm levels. ToF-SIMS is used to investigate organic material at surfaces and interfaces.

8. Adhesion Failure Analysis of Composite Materials

Adhesion failure analysis is a common application where contaminants can be confirmed. Specific images of species are obtained by scanning the sample surface with the primary ion beam and generating spectra for every image. This data is then used to select a characteristic mass fragment and plot its distribution within the sampled area with micron-scale resolution. When this is done on a cross-section, bulk distribution in complex systems can be investigated.

9. Dynamic Secondary Ion Mass Spectrometry (DSIMS)

For many applications, the region of interest is the immediate sub-surface. Dynamic Secondary Ion Mass Spectrometry (DSIMS) is the technique of choice in this application. In this case, a continuous primary ion beam is used with sufficient energy to generate a crater in the material under investigation. The secondary ions generated are continuously detected and plotted against the rate, which is subsequently calibrated against the crater depth.

10. 3D Profilometry Using White Light Interferometry (3DP)

3D profilometry is a technique for measuring surface topography, that generates quantitative information on the physical nature of surfaces and sub-surfaces by using white light interferometry. The 3D image is half micron pixellated in the x-y plane but nanometre resolved in the vertical (z) axis. The technique can generate line scans, 2D color height maps, and 3D video output. This method gives statistical surface roughness averaging, which is useful in situations where adhesion between different materials is critical. For transparent coatings such as lacquers, coating thickness can be determined by profiling both the lacquer and substrate surfaces and subtracting for the difference.

For example, surface deterioration measurement (e.g. on a turbine blade leading edge) can be accurately quantified by the 3P technique as an inservice monitoring method by using the replication procedure.