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11/20/11

X-ray techniques for material analysis

X-rays
X-rays were discovered in 1895 by German physicist Roentgen. X-rays are a form of electromagnetic radiation of very short wavelengths in the angstrom and nanometre region. To make images of the internal structure of bodies and objects X-rays are scattered by the internal lattices of solid objects and are used.
Crystal lattice
A crystal lattice of a material is a regular three-dimensional distribution (cubic, rhombic, etc.) of atoms in space arranged in such a way that they form a series of parallel planes separated from one another by a distance which varies according to the nature of the material. For any crystal, planes exist in a number of different orientations, each with its own specific spacing.
Constructive interference
When a monochromatic X-ray beam is projected onto a crystalline material at an angle, diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by a complete number of wavelengths.
Bragg's Law
By varying the angle, the Bragg's Law conditions are satisfied by different spacing in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffracted peaks of radiation produces a pattern, which is characteristic of the sample. Where a mixture of different phases is present, the resultant diffractogram is formed by addition of the individual patterns.
X-ray diffraction
X-ray diffraction is a tool for the investigation of the structure of matter and is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials.
Using X-ray diffraction, a wealth of structural, physical and chemical information about a material investigated can be obtained. A host of application techniques for various material classes is available, each revealing its own specific details of the sample studied besides applying in chemical analysis, stress and strain measurement, the study of phase equilibrium, measurement of particle size, as well as crystal structure.
X-ray fluorescence (XRF)
The wavelength-dispersive x-ray fluorescence unit (XRF) is used chiefly for the determination of major element (Si, Al, Fe, Na, K, Mg, Ca, Mn, Ti, P) in rocks, minerals, ceramics, nanocompounds, cements, clays, alloys, etc. and major trace elements such as Rb, Sr, Y, Zr, Nb, Zn, Co, Cu, Ni, Ba, and Cr. Qualitative or semi-quantitative scans can be run on elements such as carbon through uranium.
Soft X-ray Appearance Potential Spectroscopy (SXAPS)
Soft X-ray Appearance Potential Spectroscopy SXAPS is a member of the Appearance Potential Spectroscopies. The experimental apparatus has a filament mounted near the sample which emits electrons which are accelerated towards the sample.
X-rays generated within the sample are detected via photoelectrons generated by the X-ray within the detector. SXAPS is not particularly surface sensitive, but as it is a threshold technique, the incident electrons will only travel a short distance before they are unable to excite the level of interest and suffers from poor signal.
Extended X-ray Absorption Fine Structure (EXAFS)
In EXAFS, operates with a monochromatic X-ray beam whose energy is gradually increased such that it traverses one of the absorption edges of the elements contained within the sample. Below the absorption edge, the photons cannot excite the electrons of the relevant atomic level and thus absorption is low.
The probability of X-ray absorption will depend on the photon energy (as the photoelectron energy will depend on the photon energy). The net result is a series of oscillations on the high photon energy side of the absorption edge. These oscillations can be used to determine the atomic number, distance and coordination number of the atoms surrounding the element whose absorption edge is being examined. The necessity to sweep the photon energy implies the use of synchrotron radiation in EXAFS experiments.
Extended X ray Absorption Fine Structure spectroscopy (REFLEXAFS)
By reflecting the X-rays from a surface at grazing incidence and detecting the resultant X-ray fluorescence with a Si(Li) detector, a more surface sensitive signal can be obtained. This technique is known as REFLEXAFS.
EDX - Energy Dispersive X-ray Analysis or EPMA - Electron Probe Micro Analysis
This technique is used in conjunction with SEM and is not a surface science technique. An electron beam strikes the surface of a conducting sample. This causes X-rays to be emitted from the point the material. The energy of the X-rays emitted depends on the material under examination. The X-rays are generated in a region about 2 microns in depth, and thus EDX is not a surface science technique. By moving the electron beam across the material an image of each element in the sample can be acquired in a manner similar to SAM. Due to the low X-ray intensity, images usually take a number of hours to acquire. Elements of low atomic number are difficult to detect by EDX.
NEXAFS - Near Edge X-ray Absorption Fine Structure and XANES - X-ray Absorption Near Edge Structure
If an X-ray has just sufficient energy to excite a core level, then the resultant photoelectron will leap into unoccupied states. This is the region that is explored by NEXAFS and XANES and is generally regarded as being the energy region between the absorption edge and where the EXAFS oscillations begin.
NEXAFS has particular application to chemisorbed molecules on surfaces. Information concerning the orientation of the molecule can be inferred from the polarization dependence. NEXAFS is sensitive to bond angles and frequently dominated by intra-molecular resonances of pi or sigma symmetry. The energy, intensity and polarization dependence of these resonances can be used to determine the orientation and intramolecular bond lengths of the molecule on the surface.
X-ray Absorption Near Edge Structure (XANES)
XANES can provide information about vacant orbitals, electronic configuration and site symmetry of the absorbing atom. The absolute position of the edge contains information about the oxidation state of the absorbing atom. In the near edge region, multiple scattering events dominate. Theoretical multiple scattering calculations are compared with experimental XANES spectra in order to determine the geometrical arrangement of the atoms surrounding the absorbing atom. Hence the technique provides complementary information to EXAFS.
Angle-resolved X-ray Photoelectron Spectroscopy
The effective sampling depth in XPS can be varied (for flat samples) by changing the angle of the sample with respect to the detector. The actual depth sampled, d, is given by the equation: d = 3λ sin θ, where λ is the inelastic mean free path of the photoelectron and θ is the angle between the sample surface and the analyzer acceptance plane.
By comparing the relative intensities of peaks at the same kinetic energy over a number of different takeoff angles it is possible to calculate layer thickness. Alternately, comparing relative intensities at low and high take off angles indicates whether a species is enriched or depleted in the surface region. This is useful for the analysis of thin films on surfaces where it is possible to determine the molecular orientation to the surface.
Angle Resolved X-Ray Photoelectron Spectroscopy (AR-XPS)
It is a non-destructive depth profile analysis of surfaces by measuring XPS data at surfaces of solid materials when changing the angle between analyzer acceptance axis and sample surface. Angle resolved XPS is an important tool for non-destructive near surface and ultra thin film analysis. The Ultra HSA provides the highest energy resolution at all take off angles as well as detecting very low concentrations of near surface species. Automated eucentric positioning of the sample ensures that the analysis position remains constant as the sample is tilted. Automatic charge neutralization allows all types of sample to be analysed by ARXPS automatically.

1 Responses to “X-ray techniques for material analysis”

bibhuti das said...
December 17, 2013 at 6:31 AM

A thin-film sample support window is a substance used for retaining liquid, powdered, slurry or solid specimens in XRF Sample Cups. Of the many different types of materials available, few possess the necessary combination of consistency and chemical and physical properties to serve x-ray spectrochemical needs.

XRF Thin Film


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