Low-Loss Region in EELS
- Practical Electron Microscopy and Database -
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The EELS recorded up to a few tens of electron volts is sometimes called the low-loss region. In general, the term “low-loss’’ is used to describe the energy-loss of electrons in the range up to ~50 eV. The low-loss region is dominated by collective excitations (plasmons) [1] or by inter- or intra-band transitions. Typical values of the plasmon energies of materials are between 5 eV and 30 eV. The plasmon peaks are thus in the low energy loss region. Even though this signal is intensive, the interpretation is difficult because the excitation probability depends on both valence and conduction band states.

Actually, the EELS in the low-energy loss region less than 50 eV is particularly called valence electron energy loss spectroscopy (VEELS). This low loss region mainly reflects:
        i) Excitation of valence band electrons (outer shell electrons), such as interband transition (single excitation) and plasmon excitation (collective excitation).
        ii) Electronic structure.

Both i and ii) above determine the optical properties of materials. VEELS can be applied to analyze, for instance, local electronic and optical properties of materials [2] in nanoscale with STEM [3–8]. The STEM-VEELS method provides various advantages over conventional optical spectroscopy, for instance, it can measure a wider energy (wavelength) range and local electronic and optical properties from a small volume in a nano-region. The difficulty of STEM-VEELS application is that it is hard to fit a zero-loss tail, to extract ELF (energy loss function) attributable to the large zero-loss tail and to interpret the collective excitation such as plasmon, etc.  

On the other hand, zero-loss can be originally from interaction between incident electron beam and atomic nuclei and also arise from inelastic scattering by conduction or valence electrons.

The EEL spectrum can be described in a dielectric formulation [2] by,

               Cross-section for Inelastic Scattering in EELS Measurements -------- [4776]

where,
          v -- The speed of the incident electron;
          na -- The number of atoms per unit volume;
          θE -- The characteristic scattering angle (θE = E/γm0v2);
          Im(−1/ε) -- The energy loss function

Based on Equation 4776 and a Kramers–Kronig analysis, the complex dielectric function ε = ε1 + iε2 can be obtained from the low-loss EEL spectrum.

In general, the requirements of TEM specimen thickness for EELS and EFTEM measurements are:
         i) The specimens should be sufficiently thin to prevent any multiple inelastic scattering, but the degree of single inelastic scattering should be relatively high. For most materials, the optimized specimen thickness is in the range of 25-100 nm, depending on the average atomic number and the beam energy.
         ii) To avoid surface effects, the specimen thickness cannot be less than 25 nm if low-loss EELS spectra are measured.


 

 

 

 

 


 

 

 

 

 

[1] Egerton R. F., Electron Energy Loss Spectroscopy in the Electron Microscope. New York: Plenum, 1986.
[2] Egerton, R. F., Electron Energy Loss Spectroscopy in the Electron Microscope, Plenum Press, New York, 1996.
[3] N. Ikarashi, K. Manabe, J. Appl. Phys. 94 (2003) 480–486.
[4] N. Ikarashi, K. Manabe, Appl. Phys. Lett. 80 (2002) 4127–4129.
[5] N. Ikarashi, M. Murata, K. Masuzaki, T. Tastumi, Appl. Phys. Lett. 84 (2002) 3672–3674.
[6] D.W. McComb, Phys. Rev. B 54 (1996) 7094–7102.
[7] L. Ryen, X.Wang, U. Helmersson, E. Olsson, J. Appl. Phys. 85 (1999) 2828–2834.
[8] P.E. Batson, K.L. Kavanagh, J.M. Woodall, J.W. Mayer, Phys. Rev. Lett. 57 (1986) 2729–2732.

 

 

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