EELS Measurements with Low-Energy Incident Electrons
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EELS measurements in EMs have mostly been taken with high energy incident electron beams. However, EELS signal generated by low energy incident electron beams is also useful in some cases, e.g. for biological materials which is radiation-sensitive. For instance, an acceleration voltage of 100 kV was used in the EELS analysis of carbon nanotubes to avoid irradiation damage [7]. Ruthemann first presented transmission EELS spectra with a 2–10 keV incident electron energy. [1, 2] For instance, he reported an EELS spectrum of a thin collodion film, consisted of nitrogen, oxygen, and carbon elements, at an incident beam energy of 7.5 keV. [2] Hillier and Baker performed elemental microanalysis with an incident beam energy of 25–75 keV. [3] Recently, Luo and Khursheed studied amorphous carbon using second-order geometric aberration corrected EELS spectrometer at an incident beam energy of 30 keV in their SEM system.

Sasaki et al. [5] used Cs correctors to correct their lower-voltage STEM/TEM system. For the Cs corrected microscope, Figure 4294a shows the energy spreads, determined from the FWHM (full width at half maximum) of the zero-loss peaks are 0.30 and 0.27 eV at 30 and 60 kV, respectively, with an exposure time of 0.05 s.

Zero-loss peaks of EELS spectra measured at (a) 30 kV and (b) 60 kV in a Cs corrected microscope

Figure 4294a. Zero-loss peaks of EELS spectra measured at (a) 30 kV and (b) 60 kV in the Cs corrected microscope. [1]

The energy changes of incident electrons associated with thermal diffuse scattering (TDS) are of the order of kT ≈ 0.05 eV and are too small to be measured by EELS tools because most EELS instruments have typical energy resolution of >0.1 eV.

Furthermore, Brown and Ho [6] used an EELS spectrometer with an incident electron energy of 10 eV, consisting of a double-pass cylindrical monochromator and single-pass cylindrical analyzer, to analyze the interaction between methyl chloride and Si( 100) 2 x 1. Figure 4294b shows the EEL spectra obtained after 1 L exposures of methyl chloride (CH3CI) onto the Si(100) surface held at 85 and 300 K. The origin of the peaks, related to the existence of molecularly adsorbed species, is listed in Table 4294.

EEL spectra obtained after 1 L exposures of methyl chloride onto the Si(100) surface held at 85 and 300 K

Figure 4294b. The EEL spectra obtained after 1 L exposures of methyl chloride onto the Si(100) surface held at 85 and 300 K. [6]

Table 4294. The origin of the peaks in Figure 4294b.
Energy loss Origin
67 meV The Si-Cl stretch: an adsorbed chlorine atom
89 meV The C-Cl stretch of a weakly bound methyl chloride molecule
85 meV The Si-(CH3) stretch
157 meV The CH3 symmetric deformation mode
178 meV The CH3 asymmetric deformation mode
368 meV The C-H stretch modes

Electron energy-loss spectroscopy (EELS) has also been applied to study the excitation of the inner-shell electrons of gaseous atoms [8] and molecules [9-12] with low-energy incident electrons, e.g. at 2.5 keV. At these low energies, the spectra are dipole dominated and are analogous to photoabsorption spectra obtainable with bremsstrahlung or synchrotron radiation. The continuously variable energy loss is similar to a tuneable photon energy. For instance, the highest electron energy losses studied in gases have been about 700 eV in the region of F 1s excitation [9,10,13] and about 880 eV in the region of Ne 1s (K-shell) excitation as the extension of inner-shell EELS [14].

 

 

 

 

 

 

 

 

 

 

 

[1] G. Ruthemann, Naturwiss. 29, 648 (1941).
[2] G. Ruthemann, Naturwiss. 30, 145 (1942).
[3] J. Hillier and R. F. Baker, J. Appl. Phys. 15, 663 (1944).
[4] T. Luo and A. Khursheed, Second-order aberration corrected electron energy loss spectroscopy attachment for scanning electron microscopes, Review of Scientific Instruments, 77, 043103 (2006).
[5] Takeo Sasaki, Hidetaka Sawada, Fumio Hosokawa, Yuji Kohno, Takeshi Tomita, Toshikatsu Kaneyama, Yukihito Kondo, Koji Kimoto, Yuta Sato, and Kazu Suenaga, Performance of low-voltage STEM/TEM with delta corrector and cold field emission gun, Journal of Electron Microscopy 59(Supplement): S7–S13 (2010).
[6] Kyle A. Brown and W. Ho, The interaction of methyl chloride and Si( 100) 2 x 1, Surface Science 338 (1995) 111-116.
[7] S. Suzuki, C. Bower, O. Zhou, In-situ TEM and EELS studies of alkali–metal intercalation with single-walled carbon nanotubes, Chemical Physics Letters 285 1998. 230–234.
[8] King G C, Tronc M, Read F H and Bradford R C 1977 J. Phys. B: Atom. Molec. Phys. 10 2479.
[9] Hitchcock A P and Brion C E 1978 J. Electron Spectrosc. 13 193.
[10] Hitchcock A P and Brion C E 1978 Chem. Phys. 33 55.
[11] Hitchcock A P and Brion C E 1980 J. Electron Spectrosc. 18 1.
[12] Tronc M, King G C and Read F H 1979 J. Phys. B: Atom. Molec. Phys. 12 137.
[13] Wight G R and Brion C E, 1974 J. Electron Spectrosc. 4 327.
[14] A P Hitchcock, and C E Brion, Neon K-shell excitation studied by electron energy-loss spectroscopy, 1980 J. Phys. B: At. Mol. Phys. 13 3269.

 

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