Electron energy loss spectroscopy

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Experimental electron energy loss spectrum, showing the major features: zero-loss peak, plasmon peaks and core loss edge. Electron energy loss spectrum feature overview.svg
Experimental electron energy loss spectrum, showing the major features: zero-loss peak, plasmon peaks and core loss edge.

Electron energy loss spectroscopy (EELS) is a form of electron microscopy in which a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected. The amount of energy loss can be measured via an electron spectrometer and interpreted in terms of what caused the energy loss. Inelastic interactions include phonon excitations, inter- and intra-band transitions, plasmon excitations, inner shell ionizations, and Cherenkov radiation. The inner-shell ionizations are particularly useful for detecting the elemental components of a material. For example, one might find that a larger-than-expected number of electrons comes through the material with 285  eV less energy than they had when they entered the material. This is approximately the amount of energy needed to remove an inner-shell electron from a carbon atom, which can be taken as evidence that there is a significant amount of carbon present in the sample. With some care, and looking at a wide range of energy losses, one can determine the types of atoms, and the numbers of atoms of each type, being struck by the beam. The scattering angle (that is, the amount that the electron's path is deflected) can also be measured, giving information about the dispersion relation of whatever material excitation caused the inelastic scattering. [1]

Contents

History

The technique was developed by James Hillier and RF Baker in the mid-1940s [2] but was not widely used over the next 50 years, only becoming more widespread in research in the 1990s due to advances in microscope instrumentation and vacuum technology. With modern instrumentation becoming widely available in laboratories worldwide, the technical and scientific developments from the mid-1990s have been rapid. The technique is able to take advantage of modern aberration-corrected probe forming systems to attain spatial resolutions down to ~0.1 nm, while with a monochromated electron source and/or careful deconvolution the energy resolution can be 0.1 eV or better. [3] This has enabled detailed measurements of the atomic and electronic properties of single columns of atoms, and in a few cases, of single atoms. [4] [5]

Comparison with EDX

EELS is spoken of as being complementary to energy-dispersive x-ray spectroscopy (variously called EDX, EDS, XEDS, etc.), which is another common spectroscopy technique available on many electron microscopes. EDX excels at identifying the atomic composition of a material, is quite easy to use, and is particularly sensitive to heavier elements. EELS has historically been a more difficult technique but is in principle capable of measuring atomic composition, chemical bonding, valence and conduction band electronic properties, surface properties, and element-specific pair distance distribution functions. [6] EELS tends to work best at relatively low atomic numbers, where the excitation edges tend to be sharp, well-defined, and at experimentally accessible energy losses (the signal being very weak beyond about 3 keV energy loss). EELS is perhaps best developed for the elements ranging from carbon through the 3d transition metals (from scandium to zinc). [7] For carbon, an experienced spectroscopist can tell at a glance the differences between diamond, graphite, amorphous carbon, and "mineral" carbon (such as the carbon appearing in carbonates). The spectra of 3d transition metals can be analyzed to identify the oxidation states of the atoms. [8] Cu(I), for instance, has a different so-called "white-line" intensity ratio than Cu(II) does. This ability to "fingerprint" different forms of the same element is a strong advantage of EELS over EDX. The difference is mainly due to the difference in energy resolution between the two techniques (~1 eV or better for EELS, perhaps a few tens of eV for EDX).

Variants

Example of inner shell ionization edge (core loss) EELS data from La0.7Sr0.3MnO3, acquired on a scanning transmission electron microscope. Electron energy loss spectroscopy coreloss lsmo.svg
Example of inner shell ionization edge (core loss) EELS data from La0.7Sr0.3MnO3, acquired on a scanning transmission electron microscope.

There are several basic flavors of EELS, primarily classified by the geometry and by the kinetic energy of the incident electrons (typically measured in kiloelectron-volts, or keV). Probably the most common today is transmission EELS, in which the kinetic energies are typically 100 to 300 keV and the incident electrons pass entirely through the material sample. Usually this occurs in a transmission electron microscope (TEM), although some dedicated systems exist which enable extreme resolution in terms of energy and momentum transfer at the expense of spatial resolution.[ citation needed ]

Other flavors include reflection EELS (including reflection high-energy electron energy-loss spectroscopy (RHEELS)), typically at 10 to 30 keV, and aloof EELS (sometimes called near-field EELS), in which the electron beam does not in fact strike the sample but instead interacts with it via the long-ranged Coulomb interaction. Aloof EELS is particularly sensitive to surface properties but is limited to very small energy losses such as those associated with surface plasmons or direct interband transitions.[ citation needed ]

Within transmission EELS, the technique is further subdivided into valence EELS (which measures plasmons and interband transitions) and inner-shell ionization EELS (which provides much the same information as x-ray absorption spectroscopy, but from much smaller volumes of material). The dividing line between the two, while somewhat ill-defined, is in the vicinity of 50 eV energy loss.

Instrumental developments have opened up the ultra-low energy loss part of the EELS spectrum, enabling vibrational spectroscopy in the TEM. [9] Both IR-active and non-IR-active vibrational modes are present in EELS. [10]

EEL spectrum

The electron energy loss (EEL) spectrum can be roughly split into two different regions: the low-loss spectrum (up until about 50eV in energy loss) and the high-loss spectrum. The low-loss spectrum contains the zero-loss peak as well as the plasmon peaks, and contains information about the band structure and dielectric properties of the sample. The high-loss spectrum contains the ionisation edges that arise due to inner shell ionisations in the sample. These are characteristic to the species present in the sample, and as such can be used to obtain accurate information about the chemistry of a sample. [11]

Thickness measurements

EELS allows quick and reliable measurement of local thickness in transmission electron microscopy. [6] The most efficient procedure is the following: [12]

The spatial resolution of this procedure is limited by the plasmon localization and is about 1 nm, [6] meaning that spatial thickness maps can be measured in scanning transmission electron microscopy with ~1 nm resolution.

Pressure measurements

The intensity and position of low-energy EELS peaks are affected by pressure. This fact allows mapping local pressure with ~1 nm spatial resolution.

Use in confocal geometry

Scanning confocal electron energy loss microscopy (SCEELM) is a new analytical microscopy tool that enables a double corrected transmission electron microscope to achieve sub-10 nm depth resolution in depth sectioning imaging of nanomaterials. [16] It was previously termed as energy filtered scanning confocal electron microscopy due to the lack to full spectrum acquisition capability (only a small energy window on the order of 5 eV can be used at a time). SCEELM takes advantages of the newly developed chromatic aberration corrector which allows electrons of more than 100 eV of energy spread to be focused to roughly the same focal plane. It has been demonstrated that a simultaneous acquisition of the zero loss, low-loss, and core loss signals up to 400 eV in the confocal geometry with depth discrimination capability.[ citation needed ]

See also

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References

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  2. ↑ Baker, J.; Hillier, R. F. (September 1944). "Microanalysis by means of electrons". J. Appl. Phys. 15 (9): 663–675. Bibcode:1944JAP....15..663H. doi:10.1063/1.1707491.
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  4. ↑ Ramasse, Quentin M.; Seabourne, Che R.; Kepaptsoglou, Despoina-Maria; Zan, Recep; Bangert, Ursel; Scott, Andrew J. (October 2013). "Probing the Bonding and Electronic Structure of Single Atom Dopants in Graphene with Electron Energy Loss Spectroscopy". Nano Letters. 13 (10): 4989–4995. Bibcode:2013NanoL..13.4989R. doi:10.1021/nl304187e. ISSN   1530-6984. PMID   23259533. S2CID   68082.
  5. ↑ Tan, H.; Turner, S.; Yücelen, E.; Verbeeck, J.; Van Tendeloo, G. (September 2011). "2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy". Phys. Rev. Lett. 107 (10): 107602. Bibcode:2011PhRvL.107j7602T. doi:10.1103/PhysRevLett.107.107602. hdl: 10067/912650151162165141 . PMID   21981530.
  6. 1 2 3 Egerton 1996.
  7. ↑ Ahn C C (ed.) (2004) Transmission electron energy loss spectrometry in material science and the EELS Atlas, Wiley, Weinheim, Germany, doi : 10.1002/3527605495, ISBN   3527405658
  8. ↑ Riedl, T.; T. Gemming; W. Gruner; J. Acker; K. Wetzig (April 2007). "Determination of manganese valency in La1−xSrxMnO3 using ELNES in the (S)TEM". Micron. 38 (3): 224–230. doi:10.1016/j.micron.2006.06.017. PMID   16962785.
  9. ↑ Krivanek, Ondrej L.; Lovejoy, Tracy C.; Dellby, Niklas; Aoki, Toshihiro; Carpenter, R. W.; Rez, Peter; Soignard, Emmanuel; Zhu, Jiangtao; Batson, Philip E.; Lagos, Maureen J.; Egerton, Ray F. (2014). "Vibrational spectroscopy in the electron microscope". Nature. 514 (7521): 209–212. Bibcode:2014Natur.514..209K. doi:10.1038/nature13870. ISSN   0028-0836. PMID   25297434. S2CID   4467249.
  10. ↑ Venkatraman, Kartik; Levin, Barnaby D.A.; March, Katia; Rez, Peter; Crozier, Peter A. (2019). "Vibrational spectroscopy at atomic resolution with electron impact scattering". Nature Physics. 15 (12): 1237–1241. arXiv: 1812.08895 . doi:10.1038/s41567-019-0675-5. S2CID   119452520.
  11. ↑ Hofer, F.; et al. (2016). "Fundamentals of electron energy loss spectroscopy". IOP Conference Series: Materials Science and Engineering. 109: 012007. doi: 10.1088/1757-899X/109/1/012007 .
  12. ↑ Iakoubovskii, K.; Mitsuishi, K.; Nakayama, Y.; Furuya, K. (2008). "Thickness measurements with electron energy loss spectroscopy" (PDF). Microscopy Research and Technique. 71 (8): 626–31. CiteSeerX   10.1.1.471.3663 . doi:10.1002/jemt.20597. PMID   18454473. S2CID   24604858.
  13. ↑ Iakoubovskii, Konstantin; Mitsuishi, Kazutaka; Nakayama, Yoshiko; Furuya, Kazuo (2008). "Mean free path of inelastic electron scattering in elemental solids and oxides using transmission electron microscopy: Atomic number dependent oscillatory behavior" (PDF). Physical Review B. 77 (10): 104102. Bibcode:2008PhRvB..77j4102I. doi:10.1103/PhysRevB.77.104102.
  14. ↑ Taverna, D.; Kociak, M.; Stéphan, O.; Fabre, A.; Finot, E.; Décamps, B.; Colliex, C. (2008). "Probing Physical Properties of Confined Fluids within Individual Nanobubbles". Physical Review Letters. 100 (3): 035301. arXiv: 0704.2306 . Bibcode:2008PhRvL.100c5301T. doi:10.1103/PhysRevLett.100.035301. PMID   18232994. S2CID   4028240.
  15. ↑ Iakoubovskii, Konstantin; Mitsuishi, Kazutaka; Furuya, Kazuo (2008). "Structure and pressure inside Xe nanoparticles embedded in Al" (PDF). Physical Review B. 78 (6): 064105. Bibcode:2008PhRvB..78f4105I. doi:10.1103/PhysRevB.78.064105.
  16. ↑ Xin, Huolin L.; et al. (2013). "Scanning Confocal Electron Energy-Loss Microscopy Using Valence-Loss Signals". Microscopy and Microanalysis. 19 (4): 1036–1049. Bibcode:2013MiMic..19.1036X. doi:10.1017/S1431927613001438. PMID   23692691. S2CID   25818886.

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