Measurement of the dielectric function of α-Al2O3 by transmission electron microscopy – Electron energy-loss spectroscopy without Cerenkov radiation effects

•The dielectric function of α-Al2O3 was deduced even with Cerenkov radiation present.•An accurate dielectric function at a probe size less than 5 nm was obtained.•An effective method for dielectric function measurement avoids Cerenkov radiation.

The dielectric function of α-Al2O3 was measured by electron energy-loss spectroscopy (EELS) coupled with the difference method. The influence of Cerenkov radiation was significant in measurements using a 200 kV transmission electron microscope (TEM) and the correct dielectric function could not be obtained using the conventional EELS procedure. However, a good agreement between the optical data and EELS for the dielectric functions was obtained via a 60 kV TEM. Combining EELS and the difference method, however, provided an accurate measurement of the dielectric function for α-Al2O3 even at an accelerating voltage of 200 kV. The combination of EELS and the difference method in the nano-beam diffraction mode could derive an accurate dielectric function with superior spatial resolution regardless of the occurrence of Cerenkov radiation.