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Electron Energy Loss Spectrometry (EELS)
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Electron Energy Loss Spectrometry (EELS)
Inelastic scattering causes loss of the energy of electrons Electron-electron interactions Loss in Energy + Change in Momentum Energy loss electrons leads to higher chromatic aberration Thin specimen required EELS spectrometer has a very high energy resolution [(FEG ~ 0.3 eV), (XEDS → resolution ~ 100eV)] Note: beam energy can be 400 kV Can be used in forming energy filtered images + diffraction patterns Can be used in forming energy filtered images + diffraction patterns Not just elemental information (as in XEDS) but Chemical information like bonding Material parameters like dielectric constant
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Beam Interaction with the specimen
Changes Spatial and Angular distribution of electrons Energy distribution Coherency
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Spectrometers Magnetic Prism (Gatan)
Omega Filter (LEO- formerly Zeiss)
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Inter of intra band transitions
Part of the zero loss peak. Not resolved in an EELS spectrum Causes specimen to heat up. Phonon excitation (~ 0.2 eV, 5-15 mrad) Low loss region (< 50 eV) Inter of intra band transitions Signature of the structure (5-25 eV, 5-10 mrad) EELS Transverse waves Half the energy of bulk plasmons Surface plasmon Plasmon excitation (~5-25 eV, < 0.1 mrad) Bulk plasmon Longitudinal waves High loss region (> 50 eV) λp ~ 100nm Inner shell ionization Elemental information (~ eV, 1-5 mrad)
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COLLECTIVE OSCILLATIONS
PLASMONS PHONONS Collective oscillations of free electrons Most common inelastic interaction Damped out in < 1015 s Localized to < 10 nm Predominant in metals (high free electron density) Angles → < 0.1 mrad Collective oscillations of atoms Can be generated by other inelastic processes. (Auger / X-ray energy) Will heat up the specimen Small energy loss < 0.1 eV Phonon scattered electrons to large angle (5 – 15 mrads) Diffuse background
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Plasmon excitation Longitudinal wave like oscillations of weakly bound electrons Rapidly damped (lifetime ~1015 sec, localized to < 10 nm) Dominate in materials with free electrons (n) (Li, Na, Mg, Al ) But occur in all materials to some extent or other EP = f(n) Microanalytical information Carry contrast formation, limit image resolution through chromatic aberration Can be removed by energy filtering EPlasmon → Energy lost by the electron beam when it generates a plasmon h → Planck’s constant m → Mass of electron n → Free electron density (sometimes plasmon peaks are observed from materials without free electrons) P → Frequency of the plasmon generated
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Al specimen Plasmon Peaks Thin sample Thick sample
Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Inter- and Intra Band Transitions
Excitations in outermost orbital → delocalized, interatomic bonding → reflects the solid state character of the sample Change in orbital state of the core electron Interactions with molecular orbital → can be used for finger printing (by storing low loss spectra of known specimens in a database) Secondary electron emission (< 20eV) (hence in same energy-loss regime as band transitions) Weakly bound outer-shell electrons control the reaction of a material to an external field → controls the dielectric response (can measure with the signal from the < ~ 10eV region)
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Direct Beam (Energy loss electrons) SPECIMEN
Incident High-kV Beam Direct Beam (Energy loss electrons) SPECIMEN Secondary Electrons (E < 20eV) The electrons causing SE emission appear in the loss regime
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Inter- and Intra Band Transitions
Energy-loss (eV) → Spectra vertically displaced for easy visibility → Low loss spectrum from Al and Al compounds → The differences in the spectra are due to differences in bonding Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Inner shell ionization
The other side of the coin of XEDS. Small cross section (ΔE ↑ cross section ↓) (Ionization edge intensity much smaller than the plasmon peak) K , L (L1 , L2 , L3) , M (M1 , M2 , M3 , M4;5). Combination of ionization loss with plasmon loss can occur. There are intensity variations superimposed on the ionization edge ELNES (Energy Loss Near Edge Structure) (starting from about 30 eV of the edge and extending ~100s of eV) EXELFS (Extended Energy Loss Fine Structure) (~50 eV after ELNES) ELNES and EXELFS arise due to the ionization process imparting more than critical energy (Ec) for ionization Li → 50 eV to ionize K shell electron (Z ↑ Eionization ↑) U → 99 KeV to ionize K shell electron → Use L, M edges for heavy elements
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Decreasing probability of ionization ↓ as E↑
Intensity Ec → minimum energy required to ionize a atom Ec Energy Loss Idealized Ionization edge Only found for isolated Hydrogen atoms
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Bonding Effects Ionization loss Plasmon loss
Edge superimposed on plural scattering Idealized edge Bonding Effects EXELFS Diffraction Effects from atoms surrounding the ionized atom ELNES Thick Specimen → Combination of ionization and plasmon losses Ionization loss Plasmon loss Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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EELS Ionization Edge vs Characteristic X-Rays in EDX
Can lead to Detection of the beam electron that ionized the atom is independent of the atom emitting Auger electron or a X-ray (hence EELS is not affected by the fluorescence yield limitation that limits X-ray analysis by EDX)
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Inner shell ionization edges
The EELS spectrum Gain Change 100 Zero loss peak ELNES EXELFS Inner shell ionization edges Intensity Energy Loss (eV) 500
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The EELS spectrum Io 40 80 Ionization Edge Zero loss peak Plasmon peak
Forward scattered (cone of few mrad) 000 spot of DP Bragg diffracted peak (~20mrad) rarely enters the spectrometer) Includes energy loss of ~0.3 eV Includes phonon loses EELS does not resolve phonon loses Bonding effects ~50 eV after ELNES Diffraction effects from the atoms surrounding the ionized atom Ionization Edge Zero loss peak Io ELNES FWHM defines the energy resolution E resolution kV resolution EXELFS Plasmon peak v 40 80 250 300 350 400 450 Low loss region ~ 50 eV Energy-loss (eV) Coordination, Bonding effects, Density of states, Radial distribution function ELNES- Energy Loss Near Edge Structure EXELFS- EXtended Energy Loss Fine Structure
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Parts to the low loss region
1 Information available in the low loss region of the spectrum 1a Valley before the plasmon peak Parts to the low loss region 1b The plasmon peak Free electron density (plasmon peak) Composition of the specimen (In some binary free electron systems the plasmon peak shift reflects the composition of the specimen) Dielectric constant of the specimen Io Plasmon peak v 40 80 Low loss region ~ 50 eV
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Intraband transition characteristic of Polystyrene
Band gap differences manifesting itself in the low loss region of the EELS spectrum 1a Low loss region before Plasmon peak Intraband transition characteristic of Polystyrene Transmission Electron Microscopy, David B. Williams & C. Barry Carter, Plenum Press, New York, 1996.
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Al → free electron metal
Fe → Transition metal 1b Plasmon peak NiO NiO – ZrO2 interface 6 nm interface ZrO2 Transmission Electron Microscopy, David B. Williams & C. Barry Carter, Plenum Press, New York, 1996.
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Information available in ELNES and EXELFS
2 Information available in ELNES and EXELFS Arise as more than critical energy for ionization (Ec) imparted to the core electron The excess energy can be thought of as a wave emanating from the ionized atom If the excess energy is ~ few eV → the electron undergoes plural elastic scattering from the surrounding atoms → ELNES → Bonding between atoms Excess energy is greater → interaction can be approximated to a single scattering event→ EXELFS → Local atomic arragement 250 300 350 400 450 Energy-loss (eV) ELNES EXELFS Bonding of the ionized atom Coordination of the ionized atom The density of states of the solid The radial distribution function
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For convenience 250 300 350 400 450 Energy-loss (eV) ELNES EXELFS
Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Characteristics of the three principal sources operating at 100kV
Units Tungsten LaB6 Field emission Current density A/m2 5 104 106 1010 Brightness A/m2/Sr 109 5 1010 1013 Energy spread eV 3 1.5 0.3 Vacuum Pa 102 104 108 Lifetime hr 100 500 >1000
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Resolution Chemical analysis (structural and elemental) using EELS
Thin specimen is better (plasmon peak intensity < 1/10 th zero loss peak) Use high E0 (scattering cross-section , but benefit is in of plural scattering edge signal to noise ratio ) Energy resolution limited by electron source Resolution STEM Limited by size of probe (~1nm) Spatial Resolution Limited by selecting aperture at spectrometer entrance (its effective size at the plane of the specimen) TEM Somewhat better than XEDS FEG < 1nm resolution Energy Resolution 1 eV (incident energy eV!) Using FEG DSTEM Krivanek et al. [Microsc. Microanal. Microst. 2, 257 (1991).] detected single atom of Th on C film
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Equal amounts of B and N but intensities very different
Difficult to do quick semi-quantitative analysis (as possible with XEDS) Equal amounts of B and N but intensities very different Variation of ionization cross section with E Varying nature of the plural scattering background C and N edges sit on tails of preceding edges EELS spectrum (high loss region) from a particle of BN over a ‘holey’ C film Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Ti L2,3 TiC TiN Stainless steel specimen EELS spectra
Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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~8º TILT BOUNDARY IN THE SrTiO3 POLYCRYSTAL
No visible Grain Boundary 2.761 Å Fourier filtered image Dislocation structures at the Grain boundary Si peak at 1839 eV Sr L2,3 peaks Grain Boundary Grain eV 1900 2000 EELS 2100 2200 ~8º TILT BOUNDARY IN THE SrTiO3 POLYCRYSTAL VG microscope
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EELS microanalysis EELS microanalysis detection of a single atom of Th on C film! Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Differences from C K edge from graphite and diamond
ELNES Cu L edge from Cu and CuO Differences from C K edge from graphite and diamond Si L edge ELNES changes across a Si-SiO2 interface due to change in Si bonding atomic level images with spectra localized to individual atomic columns Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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Carbon K shell Excitation of: Carbon K shell e (1s) → antibonding orbital Diamond, graphite and fullerene are the matters which consists of only carbon, so that, all of these specimens have absorption peaks around 284 eV in EELS corresponding to the existence of carbon atom. From the fine structure of the absorption peak, the difference in bonding state and local electronic state can be detected. The sharp peak at absorption edge corresponds to the excitation of carbon K-shell electron (1s electron) to empty anti-bonding pi-orbital. It is not observed for diamond, because of no pi-electron in it.
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Energy Filtering Imaging Diffraction
Filter out the inelastically scattered electrons Imaging Diffraction
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Differences between C edge between graphite and diamond
ELNES [1] L3 [1] Graphite L2 CuO Diamond Cu 280 v 290 300 310 320 920 v 940 960 980 Energy-loss (eV) → Energy-loss (eV) → Differences between C edge between graphite and diamond Change in Cu L edge as Cu metal is oxidized [1] Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.
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