Electron Microscopy Electron Microscopy and Diffractionand Diffraction

7. X-Ray Elemental Microanalysis 7. X-Ray Elemental Microanalysis

Do Minh NghiepMaterials Science Center

Part of the figures, texts are quoted from internet resources. All the copyrights belong to the original authors.

All the references made here are for educational purpose only.

ContentContent

• Origin of X-rays Origin of X-rays

• Characteristic radiation-discontinuum Characteristic radiation-discontinuum spectrumspectrum

• Bremstrahlung-continuum spectrumBremstrahlung-continuum spectrum

• Energy Dispersive Spectroscopy (EDS)Energy Dispersive Spectroscopy (EDS)

• Wavelength Dispersive Spectroscopy (WDS)Wavelength Dispersive Spectroscopy (WDS)

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X-raysX-rays

Origin of X-raysOrigin of X-rays An inelastic collision

between a primary beam electron and an inner orbital electron results in the emission of that electron from the atom.

The energy released from an electron replacement event produces a photon with an energy exactly equal to the drop in energy.

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Primary X-raysPrimary X-rays

X-rays can have an energy nearly

equal to that of the primary beam

electron and thus can escape from very deep within the specimen.

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Secondary X-rays: fluoresenceSecondary X-rays: fluoresence

Xray-atom interaction

E-atom interaction

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Fe-K familyFe-K family

When an electron from a K-shell is replaced by one from the next closest shell (L), it is designated as a Kα event.

When an electron from a K-shell is replaced by one from the second closest shell (M), it is designated as a Kβ event.

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Fe-L familyFe-L family

When an electron from a L-shell is replaced by one from the next closest shell (M), it is designated as a Lα event.

The K shell will never donate its electron as this would require an increase in energy, not a drop.

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Certain events such as Mα, Lβ, and Kγ are only possible in atoms of sufficient atomic weight (N- shell).

System of spectrum lines System of spectrum lines

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There are actually a wide variety of subsets of these X-rays, since each electron shell has multiple orbitals. Example: K-shell (1), L-shell (3), M-shell (5) ...

System of spectrum lines System of spectrum lines

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Bremstrahlung-continuum spectrumBremstrahlung-continuum spectrum

Bremsstrahlung X-rays are the major part of the continuum X-ray

signal that can escape from the deepest

portion of the interaction region.

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Continuum spectrumContinuum spectrum

"Bremsstrahlung" means "braking radiation" and comes from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they interact with the specimen. Although they contribute to the total X-ray signal, they contain no useful information, because their energies are nonspecific and therefore are considered as part of the background: Eo = E + Ei (mv2/2 = E + hC/.

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Origin of Origin of continuum spectrumcontinuum spectrum

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Because E = Eo-Ei different,

energy of X-ray photons Ei

changes continuously from E0 > E1 > E2 > E3 >…> E n (o<1<2<3<...<n)

X-ray X-ray spectroscopyspectroscopy

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Two X-ray spectrometers for Two X-ray spectrometers for microanalysis by EDS and WDSmicroanalysis by EDS and WDS

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ED

S:

En

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Sp

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How X-rays are collectedHow X-rays are collectedin EDS and WDS in EDS and WDS

EDS: by Si detector WDS: by monocrystal

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EDS histogram

WDS histogram

X-ray X-ray HistogramsHistograms

Energy Energy Dispersive Dispersive

Spectroscopy Spectroscopy (EDS)(EDS)

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Principal scheme of EDSPrincipal scheme of EDS

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Semi-Semi-conductor conductor

detector and detector and spectrum spectrum

measurement measurement

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Construction of detector systemConstruction of detector system

Collimator to limit BSE and stray X-rays

Window usually made of beryllium (limited to sodium, atomic number 11) or thin plastic to detect down to boron (atomic number 5) protects cooled crystal from air.

Detector crystal silicon wafer with lithium added in. For each 3.8 eV from an X-ray, produce an electron and hole. This produces a pulse of current, the voltage of which is proportional to the X-ray energy. Must keep the crystal at Liquid Nitrogen temperature to keep noise to a minimum.

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Expanded view of an EDS detectorExpanded view of an EDS detector

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Elements of semiconductor detectorElements of semiconductor detector

Lithium doped silicon (SiLi) crystal detector acts as a semiconductor that carries current in a rate proportional to the number of ionization events and acts as an indirect measurement of the energy contained in the X-ray.

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Function of semiconductor detectorFunction of semiconductor detector

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Each ionized atom of silicon absorbs 3.8 eV of energy, so an X-ray of 3.8

keV will ionize approximately

1000 silicon atoms.

Factors affecting signal collectionFactors affecting signal collection

Distance between detector and X-ray source, Angle at which detector is struck,

Volume of signal collected.08.04.23 25

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For a given angle of electron incidence, the length of the absorption path is directly proportional to the cosecant of the take-off angle, φ

take-off angle (góc thoát tia)

Factors affecting signal collection: Factors affecting signal collection: take-off angle take-off angle

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Factors affecting signal collection: Factors affecting signal collection: solid anglesolid angle

The solid angle Ω of a detector is defined as angle of the cone of signal entering the detector. The greater the size of the detector surface area (or the closer to the source) the

greater will be the solid angle.

Ω: solid angle (góc chiếu)

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One can also increase the solid angle by placing the detector closer to the source.

One then tries to maximize both the solid angle and the take-off angle.

How to increase solid and take-off anglesHow to increase solid and take-off angles

Semiconductor detectorSemiconductor detector

One reason that the final lens of an SEM is conical in shape is so that the EDS detector can be positioned at a high take-off angle and inserted

close to the specimen for a high solid angle.08.04.23 29

Characteristics of detectorCharacteristics of detector

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Larger SiLi crystals will be able to sample a larger volume of signal (better Ω), but because of imperfections in the crystal they have

slightly greater noise and thus slightly lower resolution.

((ΩΩ))

MCA consists of an analog to digital converter which “scores” the analog signal coming from the field effect transistor (FET). Newer systems employ a digital pulse processor which converts the signal on the fly.

Then Now

Multi-channel analyzer: MCAMulti-channel analyzer: MCA

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An EDS histogram An EDS histogram

The changes in conductivity of the SiLi crystal can be counted for a given time and displayed as a histogram using a multichannel analyzer.

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Qualitative analysis by EDS/EDX Qualitative analysis by EDS/EDX

Positive identification of an element is best done by collection of the entire family of peaks for a given element.08.04.23 33

X-ray spectrumX-ray spectrum

An X-ray spectrum for a sample is composed of all the possible signals for that given set of elements.

These will differ in terms of energies (keV) and probabilities (likelihood) scored as number of such signals collected over a given period of time.

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X-ray energy in keV

# C

ou

nts

X-ray emission and absorption energy (keV)X-ray emission and absorption energy (keV)

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Each element has a family of characteristic X-rays associated with it

SE image of Chrysotile Asbestos Fibers(sợi amiăng)

EDS histogram of Chrysotile Asbestos Fibers

Chemical composition of the fibers: Si, Mg and Fe

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Example 1: Asbestos materialsExample 1: Asbestos materials

Bullet fragments (blue, vết đạn) can be identified on cloth fibers and distinguished from other metal pieces by their elemental composition.

Example 2: Example 2: Gunshot Residue (GSR) analysisGunshot Residue (GSR) analysis

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BSE image of cloth fibers

EDS histogramof cloth fibers

GSRGSR(Gunshot (Gunshot

Residue-GSR) Residue-GSR)

Gunshot Residue (GSR) analysis: Gunshot Residue (GSR) analysis: criminal investigation by elemental analysis of GSR, of GSR, identifying a gun firingidentifying a gun firing

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Gunshot Residue (GSR) Gunshot Residue (GSR) analysis:analysis:

Particles are very characteristic, therefore presence of these particles forms evidence of firing a gun.

Particles normally consist of Pb (lead), Sb (antimony) and Ba (barium).

New ammunition: environmentally

friendly (no Sb).

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GSRGSR(Gunshot Residue)(Gunshot Residue)

The proportion of elements present in GSR differ slightly and databases of GSR from different manufacturers can be used to identify what ammunition was used in a crime.

GSR is often found on criminals and also on victims if shot at close range.

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GSRGSR(Gunshot Residue)(Gunshot Residue)

X-Ray elemental mappingX-Ray elemental mapping

Ca Al Si

S K Mg

Fe Na BEI08.04.23 41

Elemental mapsElemental maps

SEI

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Combined with backscatter

imaging (BSEI) and X-ray maps of Au (a), Ba (b) and

Ca (c) different layers of paint can

be identified.

Analysis of painting materialsAnalysis of painting materials

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Au

Ba Ca

Wavelength Wavelength Dispersive Dispersive

Spectroscopy Spectroscopy (WDS)(WDS)

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William Henry BraggWilliam Henry Bragg 1862 – 19421862 – 1942

Nobel Prize in Physics Nobel Prize in Physics 19151915

Like an electron beam an X-ray has its own wavelength which is proportional to its energy.

Diffraction in crystals Diffraction in crystals

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Crystal: A solid formed by the solidification of a chemical and having a highly regular atomic

structure. May be composed of a single

element (C / diamond) or multiple elements

(BaTiO3).

Crystal detector Crystal detector

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Close packingClose packingof crystalsof crystals

Cubic Hexagonal

HÖ s¸u ph ¬ng

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If a wavelength enters a crystal at the appropriate angle it will be diffracted rather than being absorbed or scattered by the crystal.

Vulf-Bragg’s law of diffraction Vulf-Bragg’s law of diffraction

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Diffraction lawDiffraction law

For a given wavelength λ there is a specific angle θ (Bragg’s angle) at which diffraction will occur. Bragg’s angle is determined by the d-spacing (interplanar spacing) of the crystal and the order of diffraction (n = 1, 2, 3….). 08.04.23 49

Crystal detectorCrystal detector

A WDS detector takes advantage of the fact that an X-ray of a given wavelength can be focused by a crystal if it encounters the crystal at the proper Bragg’s angle.

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Crystal-monochromatorCrystal-monochromator

To better accomplish this crystals are bent and ground to form a curved surface which will bring all the diffracted X-ray wavelengths to a single focal point, thus the crystal acts as a focusing lens.

Focused Non-focused

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Crystal-monochromatorCrystal-monochromator

To change the Bragg’s angle the diffracting crystal and detector can be moved together relative to the stationary specimen along a circle known as the Rowland Circle.

Rawland circle

Rawland circle

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Wavelength Dispesive SpectrometerWavelength Dispesive Spectrometer

Rawland

circle

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WDWDspectrometerspectrometer

WDS detectors are quite large and must be positioned around the specimen chamber at an angle to take advantage of maximum take-off angle and maximum solid angle.

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An electron probe microanalyzer EPMA An electron probe microanalyzer EPMA (Multi-channel EDS and WDS) (Multi-channel EDS and WDS)

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A Microprobe is a specialized SEM that is outfitted with an EDS detector and array of several WDS detectors.

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An electron probe microanalyzer EPMAAn electron probe microanalyzer EPMA

Different diffracting crystals can only diffract certain wavelengths (even with the changes in Bragg’s angle).

So an array of detectors must be used if one is to be able to detect K, L, and M events for many different elements.

Since WDS detectors do not need to be cooled they are windowless and can detect down to Berylium

LiF - Lithium fluoride, PET - Pentaerythritol, and TAP - Thallium acid phthalateLiF - Lithium fluoride, PET - Pentaerythritol, and TAP - Thallium acid phthalate

Crystals for detectorCrystals for detector

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How to chose a crystal-monochromatorHow to chose a crystal-monochromator

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Samples must be conductive since high keV is used (carbon coating if not naturally conductive).

Samples must be flat (polished) as geometry of sample to detector is crucial and also minimizes artifacts when doing quantitative measurements.

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Specimen preparation for WDSSpecimen preparation for WDS

Characteristics WDS EDS

Spectral resolution, eV high: 2-6 low: 130-155

Collection efficiency low (slower) high (faster)

Ration P/B (peak/background) higher lower

Effect of sample geometry highly sensitive to geometric effects

low sensivity

Spectral artifacts few several

Liquid nitrogen no required required / LN-free

Moving mechanical parts Yes no / yes

Beam current relatively high

typicallower feasible

Purchase very expensive less expensive

WDS vs EDSWDS vs EDS

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WDS vs WDS vs EDS EDS

in in SEMSEM

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Comparison of EDS and WDSComparison of EDS and WDS

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WDS vs EDSWDS vs EDS

A comparison of two spectra collected with EDS and WDS shows how peak overlap and energy spread can serve to obscure the information in an EDS spectrum.

BaTiO3

BaTiO3

(Ba-L + Ti-K)

Ba-L1,2Ti-K1,2

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EDS: overlaping, Ba and Ti can’t resolved

WDS: no overlaping, better resolution

Elemental Elemental quantitative analysisquantitative analysis

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Calculation formularCalculation formular

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stdstdstd

specspec CkC

N

NC ..

Cspec, Cstd - concentration of specimen and standardNspec, Nstd - peak intensity of specimen and standard

k - correction factor

Quantitative X-ray analysisQuantitative X-ray analysis

If one wants to quantify the relative amounts of different elements present in a complex sample one has to account for a number of

factors and carry out a correction of the data08.04.23 66

Correction factor ZAFCorrection factor ZAF

Several methods to correct the spectra. ZAF takes into account the Atomic Weight (Z), effects of Absorbance (A) and effects of Fluorescence (F) in adjusting the data to give the correct values.

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Correction of overlapingCorrection of overlaping

One must account for other elements present in the sample and whether their individual peaks overlap with each other creating a “shoulder” that

can mask the presence of one element or distort the midpoint of another.

As

Cd

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EDS can be added as a component of a TEM. It requires an angled detector (for take-off angle) and scan coils in the column to function as a Scanning Transmission Electron Microscope or STEM.

EDS microanalysis in STEM modeEDS microanalysis in STEM mode

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EDS can be used to identify elements present vacuoles or inclusions. Must take into account elements present in the embedding medium.

Application of Application of EDS in STEMEDS in STEM

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X-ray microanalysis enables the SEM and TEM to be used as analytical instruments.

Limitations include the quantities of given element and an inability to distinguish isotopes.

By placing a focussed beam over an object allows one to carry out microchemical analysis on very small or very discrete objects or to map the presence of various elements within a specimen.

Application of EDS and WDSApplication of EDS and WDS

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Element Element linescanlinescan

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ApplicationApplication- diffusion - diffusion

zonezone

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Some elemental analysis techniquesSome elemental analysis techniques

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Some elemental analysis techniquesSome elemental analysis techniques

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