EELS at very high energy losses

Abstract

Electron energy-loss spectroscopy (EELS) has been investigated in the range from 2 to >10 keV using an optimized optical coupling of the microscope to the spectrometer to improve the high loss performance in EELS. It is found that excellent quality data can now be acquired up until about 5 keV, suitable for both energy loss near edge structure (ELNES) studies of oxidation and local chemistry, and potentially useful for extended energy loss fine structure (EXELFS) studies of local atomic ordering. Examples studied included oxidation in Zr, Mo and Sn, and the ELNES and EXELFS of the Ti-K edge. It is also shown that good quality electron energy-loss spectroscopy can even be performed for losses above 9.2 keV, the energy loss at which the collection angle becomes 'infinite', and this is demonstrated using the tungsten L3 edge at about 10.2 keV.

Fig. 1.

The effects of oxidation on second row transition metal L-edges: (a) Zr (t/λ = 0.33, 100 s acquisition) and ZrO₂ (t/λ = 0.51, 100 s acquisition); (b) Mo (t/λ = 0.47, 40 s acquisition) and MoO₂ (t/λ = 0.24, 60 s acquisition); (c) definitions of how the L₃ and L₂ intensities are calculated. Please note, that the vertical scale of (a) and (b) are in units of absolute differential cross section, as previously used in Craven et al. [12] and calculated in a similar way.

Fig. 2.

The effects of oxidation on Sn-L edges: (a) a raw spectrum for SnO; (b) background-subtracted and deconvolved edges for Sn, SnO and SnO₂, including an inset with a detail of the chemical shifts on the L₃ edge (t/λ = 0.53, 0.17 and 0.37; acquisition time 200, 150 and 150 s, respectively); (c) a comparison of EELS and XANES for the L₃ edge of Sn in SnO, including a slight –3 eV realignment of the energy loss scale for the EELS data to match the XANES.

Fig. 3.

A background subtracted, deconvolved Ti-K edge from amorphous TiO₂ (t/λ = 0.70, 1000 s acquisition).

Fig. 4.

The tungsten L₃ edge, both background-subtracted raw data and Fourier-ratio deconvolved using the low loss (t/λ = 0.51, 100 s acquisition).

Fig 5.

Refocus of the spectrum focus, FX, required to bring the spectrum to the sharpest possible focus as a function of energy loss.

Fig. 6.

Useable ranges for EELS at different accelerating voltages extrapolated linearly from 200 kV. The ‘Good EELS’ range was defined as the range over which the acceptance angle varies by less than 5%. The Acceptable EELS range was defined as the range over which good quality data has been taken below the crossover by the authors (0–7 keV at 200 kV). The crossover was 9.2 keV at 200 kV.