近代分析实验原理：第七课-电子能量损失谱

1、七、电子能量损失谱七、电子能量损失谱（Electron energy-loss spectrometry）近代分析实验原理（Introduction of modern analytical methods）121. Why do Electron energy-loss spectrometry （EELS）Appl. Phys. Lett., Vol. 77, No. 5, 31 July 2000Appl. Phys. Lett., Vol. 78, No. 23, 4 June 20013Phys. Rev. B 71, 092401 (2005)Ag M5 and M4 edges

2、are located at 367 eV and 373 eV4The advantages of EELS:1. EELS can detect and quantify all the elements in the periodic table and is especially good for analyzing the light elements.2. EELS offers even better spatial resolution and analytical sensitivity (both at the single-atom level) in addition

3、to providing much more than just elemental identification.52. The Energy-Loss Spectruminformation from the more weakly boundconduction and valence-band electronselemental information from the more tightly bound, core-shell electrons and also details about bonding and atomic distribution。6l the zero-

4、loss peak is very intense, which can be both an advantage and a hindrance.l the intensity range is enormous; this graph uses a logarithmic scale as the only way to display the whole spectrum.l the low-loss regime containing the plasmon peak is relatively intense.l the element-characteristic features

5、 called ionization edges, are relatively low in intensity compared to the background.l the overall signal intensity drops rapidly with increasing energy loss, reaching negligible levels above 2 keV, which really defines the energy limits of the technique (and this is about the energy when XEDS reall

6、y comes into its own, emphasizing again their complementarity).73. EELS instrumentationSpectrometersEnergy filtersmanufactured by Gatan, Inc. termed a parallel-collection EELS or PEELSa magnetic-prism (sometimes called a magnetic-sector) systemThe post-column Gatan Image Filter (GIF)the Omega (O) fi

7、lter (the in-column filter), pioneered by Zeiss and also now used by JEOL8parallel-collection EELS or PEELS9We define the dispersion as the distance in the spectrum (dx) between electrons differing in energy by dE.For the Gatan magnet, the radius of curvature (R) of electrons traveling on axis is ab

8、out 200 mm, and for 100-keV electrons dx/dE is 2 m/eV.We define the energy resolution of the spectrometer as the FWHM of the focused zero loss peak.The type of electron source determines the resolution.At 100 keV, a W source has the worst energy resolution (3 eV), a LaB6 is slightly better at 1.5 eV

9、, a Schottky field emitter can give 0.7 eV, and a cold FEG gives the best value of 0.3 eV.10Schematic diagram of parallel collection of the energy loss spectrum onto a YAG scintillator fiber-optically coupled to a semiconductor photo-diode array (PDA) or CCD in the dispersion plane of the spectromet

10、er.CCDs show lower gain variation, 30 better sensitivity, higher dynamic range, and improved energy resolution compared with PDAs.11You basically select (or filter out) electrons of a specific energy coming through the spectrometer and form either an image or a DP.the in-column () filterthe post-col

11、umn Gatan Image Filter (GIF)In-column filters are placed in the heart of the TEM imaging system, between the intermediate and projector lenses such that the recording CCD detector only receives electrons that have come through the filter. So all images/DPs consist of electrons of a specific selected

12、 energy.The post-column GIF is added below the TEM viewing screen, just like a PEELS and therefore you can choose to use it or not. The GIF can be seen as either a more flexible instrument or one that limits you to having to decide whether or not to filter your images.1213(Gatan Image Filter)144. Lo

13、w-Loss and No-Loss Spectra and Imagesl The zero-loss peak, which primarily contains elastic, forward-scattered electrons, but also includes electrons that have suffered small energy losses. Forming images and DPs with the zero-loss electrons offers tremendous advantages over unfiltered images, parti

14、cularly from thicker specimens.l The low-loss region up to an (arbitrary) energy loss of 50 eV contains electrons which have interacted with the weakly bound, outer-shell electrons of the atoms. Thus, this part of the spectrum reflects the dielectric response of the specimen to high-energy electrons

15、. We can also form images from these low-loss electrons that reveal information about the electronic structure and other characteristics of the specimen.15Inelastic scattering is primarily an electron-electron interaction and entails both a loss of energy and a change of momentum.l Single scattering

16、 occurs when each electron undergoes at most one scattering event as it traverses the specimen.l Plural (1 and 20) scattering only occurs in very thick specimens or with very low energy electrons, so is irrelevant to TEM.the energy-loss spectrum is most understandable and more easily modeledTypical

17、energy losses:single-electron scattering (inter/intraband transitions), 220 eVplasmon interactions 530 eVinner-shell ionizations, 502000 eVPhonon excitations cause losses of 0.02 eVElastic scattering, 0 eV16the spectrometer has a finite energy resolutionPhonon excitations cause losses of 0.02 eVElas

18、tic scattering, 0 eVZero-Loss Images and Diffraction Patterns(elastic image or DP)increase the image contrast and improve the quality of the DPs.unfilteredfilteredthick biological sectionunfilteredfilteredthick crystalline specimen17 50 eV1. Chemical FingerprintingAs with any fingerprinting techniqu

19、e, including the forensic variety, you must be careful to decide when a match is satisfactory. There is no black and white here, only shades of gray, so dont convict unless the statistics are with you and there is strong supporting evidence from other techniques.182. Dielectric-Constant Determinatio

20、nRe()Im()Advantage: high spatial resolution1.53 eVthe dielectric response from the infrared (800 nm) through the ultra-violet ( 400 nm) wavelength range.Higher energies correspond to various electronic transitions.the energy-loss process as the dielectricresponse of the specimen to the passage of a

21、fast electron.Single-scattering193. Plasmons 100 nmSpecimen thicknessToo thickPeak position shiftconcentrationSingle scatteringCompositionFor alloys2050Including I0Em is the average energy loss in eV which, for a material of average atomic number ZRelativistic correction factor214. Single-Electron E

22、xcitationsinter/intraband transitions for the valence electrons, ( 50 eVExtended energy-loss fine structureThe characteristic features of an inner-shell ionization edge.The characteristic features of an inner-shell ionization edge. (A) The idealized sawtooth (hydrogenic) edge. (B) The hydrogenic edg

23、e superimposed on the background arising from plural inelastic scattering. (C) The presence of ELNES. (D) The EXELFS. (E) In a thick specimen, plural scattering, such as the combination of ionization and plasmon losses, adds another peak to the post-edge structure and raises the background level.26T

24、he correspondence between the energy levels of electrons surrounding adjacent Ni and O atoms and the energy-loss spectrum.27lThe ZLP is above the potential wells since these electrons dont interact with the atom.lThe plasmon peak comes from interactions with the valence/conduction band electrons jus

25、t below the Fermi level (EF).lThe relative energy levels of the ionized shell (K, L, or M) control the position of the ionization edge in the spectrum. The closer to the nucleus, the deeper the potential well and the more the energy required to eject the electron.lThere will be a different density o

26、f states in the valence (3d) band of the Ni atom compared to the s band of the O atoms at the top of the potential wells.lThe core electrons could also be given enough energy to travel into the empty states, well above EF and, in this case, we see ELNES after the ionization edge.28EFTEM IMAGING WITH

27、 IONIZATION EDGES(A) BF image of precipitates in a stainless-steel foil. The other images were obtained from specific energy-loss electrons and illustrate both jump ratio and fully quantitative images. (B) Fe M jump ratio image; (C) Cr L jump ratio image; (D) quantitative Cr map.29ELNES and EXELFS30

28、31Relationship between the empty DOS and the ELNES intensity in the ionization edge fine structure. Note the equivalence between the Fermi energy EF and the ionization edge onset EC. Electrons ejected from the inner shells reside preferentially in regions of the DOS that have the greatest density of

29、 unfilled states. The filled states below EF are drawn as a quadratic function, but this is an approximation.32Spectra from the transition metals show a variation in the L3 and L2 white-line intensity ratios reflecting the variation in the number of core L-shell electrons ejected into unfilled d sta

30、tes. Note that Cu and Zn show no white lines because their d shells are full. The L3 and L2 white lines in the Fe L edge are the only ones that show the expected L3:L2 of 2:1.the d shell has unfilled states.3d104s23d104s133(A) Differences between the ELNES of the carbon-K edge from various forms of carbon. (B) Change in the Cu L2,3 edge ELNES as Cu metal is oxidized and the filled d states lose electrons, thus permitting the appearance of white lines.The ELNES ha