Chemical Analysis in TEM: XEDS, EELS and EFTEM HRTEM PhD course Lecture 5 1
Part IV Subject Chapter Prio x-ray spectrometry 32 1 Spectra and mapping 33 2 Qualitative XEDS 34 1 Quantitative XEDS 35.1-35.4 1 35.5 2 35.6-35.8 2 35.9-35.10 3 x-ray resolut., MDM, MMF 36 2 EELS monochromator 37.5 1 37 1 Low loss region 38 2 High/Core loss region 39 1 ELNES and EXELFS 40 2 40.8 3 2
Inelastic scattering Incoming electrons ionize the atom (core-shell excitation), loses energy collect emitted x-rays (XEDS) Or - the energy loss can be measured (EELS), typically up to 3kV loss. 3
Inelastic scattering Energy-loss electrons (characteristic) Beam damage: heating, knock-on, sputtering Plasmons and phonons Bremsstrahlung X-rays Characteristic X-rays (XEDS) Auger electrons Secondary electrons Cathodoluminescence (semiconductors) 4
Plasmons and phonons Plasmons: collective oscillations of free electron gas most common inelastic interaction! Typical energy 15-25 ev forward scattering Phonons: collective oscillation of atoms in a lattice Results in specimen heating Very small energy loss (< 0.1 ev) but high scattering angle 5
Plasmons and Phonons 6
Bremsstrahlung X-rays Electrons may lose momentum when passing close to the nucleus (Coulomb interaction) X-ray is emitted to compensate for energy loss Any energy up to the beam energy is possible, though high energies exceedingly rare Background in XEDS spectrum 7
Characteristic X-ray: Generation High-energy incoming electron Emitted x-ray, E = hν = E Electron from higher energy level relaxes to fill hole E Energy loss electron 8
Characteristic X-ray: XEDS X-ray Energy Dispersive Spectroscopy (XEDS, EDS, EDX, EDXS) Analysis of characteristic X-rays generated when the electron beam hits the specimen Provides elemental information about the sample Quick and easy, extremely common 9
High resolution analytical electron microscopy (AEM) XEDS, energy dispersive x-ray spectrometer HAADF, High angle annular dark field detector EELS, electron energy loss spectrometer (EFTEM) 10
XEDS: X-ray Energy Dispersive Spectroscopy Analysis of Characteristic X-rays 11
Characteristic X-rays electronic states 12
Possible transitions See Figure 4.3! 13
Relative intensity of transitions 14
Energy dispersive spectrometer How it works: An X-ray is detected by a semiconductor detector The detector generates a charge pulse proportional to the X-ray energy The pulse is converted to a voltage The signal is amplified, isolated and identified The signal is stored in a channel assigned to that specific energy in the computer Note this is a serial process only one X-ray can be analysed at a time! (but appears parallel) 15
Detectors: semiconductor detector Most detectors are made from Si (p-i-n diode) doped with Li to compensate impurity doping When the semiconductor absorbs X-rays, electrons are transferred from valence to conduction band (creating electron-hole pairs) The energy per pair is known (3.8 ev for Si) so the number of pairs is linked to the X-ray energy Electrons and holes separated in p-i-n diode by a reverse bias and the resulting electron pulse measured 16
Detector Design 17
Semiconductor detector Cooling to very low temperature necessary Thermal energy creates e-h pairs (noise) FET noise (signal processing) Prevent Li atoms from diffusing Results in delicate detector, sensitive to vacuum quality Detector windows (to column) to prevent ice buildup None: appropriate only in UHV (but best signal) Conventional Be: Robust, some X-ray absorption Thin windows: Less robust, but less absorption
Si Drift Detectors (SDD) Much higher count rates Better signal in SEM Minimal cooling More robust Less contamination Higher collection angle Better counts in TEM! 19
Other detectors (read 32.4-32.6) Intrinsic Ge (IG): no compensation doping needed, much more robust than Si(Li), minimal cooling required Better X-ray absorption: better signal-to-noise, can detect much higher energies Disadvantage: much more difficult and more expensive to produce, only small areas are feasible Wavelength dispersive spectrometer (WDS): MUCH better energy resolution, current versions very slow and very low efficiency Bolometer : very good energy resolution and fast, but small area (low counts) and require very high cooling (liquid He) 20
Comparison of detectors 21
The X-ray spectrum Element-specific characteristic peaks Natural linewidth 1-5 ev XEDS output linewidth 100-150 ev (electronics) Continuous non-characteristic background Artifact peaks originating from: the detector the specimen outside the region of interest elsewhere in the system 22
The X-ray Spectrum 23
Other X-rays Generated Bremsstrahlung X-rays occur when electrons are decelerated by interaction with atomic nuclei any energy up to beam energy Spurious X-rays come from the specimen but not from the chosen analysis region (from Bremsstrahlung X-rays or uncollimated electrons) System X-rays can come from elsewhere in the microscope than the specimen when electrons scattered by the sample strike other parts of the system 24
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XEDS resolution Bulk sample Thin sample (transmission) Figure 36.1 26
Qualitative XEDS analysis Critical to understand what sample consists of Without proper qualitative XEDS, quantitative analysis is meaningless! ALL peaks must be identified, such that artifacts are understood and all present elements are accounted for Otherwise quantification cannot be accurate The more you know about the sample BEFORE analysis, the more reliable your analysis will be 27
Peak Identification Procedure Check most intense first Determine if it fits K, L, M Look for family peaks Look for artifacts Go to next intense peak and repeat procedure 28
Family peaks 1. Kα and Kβ 2. Kαβ 3. L family peaks 4. L family (diminished) 5. Kα and Kβ and Lαβ 6. L and M peaks 29
Sample spectrum 1 30
Sample Spectrum 1 31
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Qualitative X-ray mapping Normally we select a single position in our sample, and take a spectrum of counts versus energy It is also possible to select a single energy, and plot counts versus position This gives an x-ray image for a single energy, corresponding to a specific element It is possible to obtain compositional maps! 33
X-ray line mapping 34
X-ray area mapping Instead of plotting position versus counts in 1D, we can produce a 2D plot of position - Note the number of counts will be low! 35
Quantitative XEDS Once we know what our sample consists of, we want to know how much of each element it contains We can investigate the relative intensity of characteristic X-ray peaks to get this information Current quantification techniques give reasonably accurate results, with room for improvement 36
Quantitative XEDS The concentration of a present element is proportional to the intensity of the observed characteristic X-ray signal Since it is difficult to measure an absolute intensity compare measured values for two elements to each other 37
Cliff-Lorimer technique k AB is a sensitivity factor, called the Cliff-Lorimer factor 38
Ternary systems 39
Exercise: AuIn nanoparticle 40
Exercise: AuIn nanoparticle 41
Energy Loss Electrons EELS: Electron energy loss spectroscopy EFTEM: Energy filtered TEM 42
Magnetic prism Electrons curve in the magnetic field according to energy (speed) The dispersion is the spreading of the electrons after the prism (vertical plane) spectroscopy! The design also makes it into a focussing lens for e - entering with different angles (but same energy). 43
(P)EELS Parallel Electron Energy Loss spectrometer 44
EELS Spectrum - Intensity vs Energy Loss 45
EELS spectrum: log intensity scale Question if we have 14,000 counts in the zero loss peak, how many counts in the first edge? Answer about 14, and that includes the background! 46
The Zero Loss Peak (ZLP) Unscattered, elastically scattered, electrons generating phonons when scattering 47
The low-loss region Contains electrons generating plasmons Fingerprint region, characteristic of sample chemistry Compare to ZLP to determine thickness 48
Fingerprint region for Al. Different types of crystal structure affects the plasmon peak shape 49
Thickness determination by EELS I l = the intensity in the low-loss region, 0-50 ev including the ZLP I 0 = The intensity in the zero-loss peak. λ = mean free path for inelastic scattering (usually approximated by plasmon crossection) 50
High-loss region Contains element-specific characteristic edges Compare XEDS spectra and EELS - Energy resolution - energy position - fine structure EELS uses the primary event (XEDS is a secondary event) 51
Notation of core loss edges < Electron configuration 52
Fine structure of EELS edges 53
Quantification of EELS spectra 1. Background subtraction 2. Deconvolution (too thick?) 3. Edge integration 4. Know partial ionization crossections 54
EELS + Imaging: Gatan Image Filter 5 cm 5 cm Magnetic sector disperses electrons according to energy. Can re-create image, using selected energy electrons only 55
When/how do we use EFTEM? Cleaning images from diffuse scattering - Zero-loss imaging Mapping of elements - Jump ratio - Three-window technique 56
Unstained/osmicated cebellar cortex (thickness=0.4 μm, 100 kev) ZERO-LOSS IMAGING Allowing only the elastically scattered electrons (zero-loss filtering) to contribute to the image (or diffraction pattern) removes the inelastic fog for Z <12 the inelastic cross-section is larger than the elastic cross-section limited use for very thick specimens where zero-loss intensity < 1% of spectrum intensity 57
Mapping of elements A PE1 A PE A E A PE2 A E Jump ratio Record images with one window before edge, and the same window after the edge. Divide each pixel A E /A PE. Reduces diffraction contrast Three-window technique Record three images; one after the edge, two before to calculate the background contribution to each pixel in A E. Subtract background 58
EFTEM images of λ phage (virus) a) C map, b) P map, c) Mg map d) combination of the carbon and phosphor maps, carbon appearing yellow, phosphor details blue., C P 1) Is there any DNA in the tail? 2) Role of Mg for packing DNA Mg C+P 59
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Quick comparison XEDS Quick, easy, routine Low energy resolution (100 ev) EELS/EFTEM More difficult to do/ complicated to interpret High energy resolution (1 ev) Fine structure: chemistry/binding/structure... Suitable for energies >~1 kev Not good for light elements Suitable for energies <~3 kev Good for light elements Heavier elements possible using L,M not all elements reasonable/easy 61