Study of mean absorptive potential using Lenz model: toward quantification of phase contrast from an electrostatic phase plate.

An electrostatic phase plate can provide better phase contrast, a fact that plays a promising role for the high-resolution observation of specimens containing light elements. However, in order to quantify the "phase" contrast from images recorded using the phase plate, the "absorption" (or scattering) contrast arising from electrons scattered elastically and inelastically outside of the phase-plate ring must be analyzed. Angular distributions of the elastic and inelastic scattering are predicted using the Lenz model. The mean absorptive potential, V(o) serving as an index for the contribution of "absorption" contrast, is calculated from the reciprocal mean free path of elastic and inelastic scattering, and is verified experimentally. The mean absorptive potential of a particular phase plate with inner and outer radii of 0.25microm (theta(1)=0.09mrad) and 1mum (theta(2)=0.4mrad), respectively, is approximately 0.11eV for carbon and is equivalent to that of an objective aperture of semiangle 17mrad (cutoff frequency 6.7nm(-1)).

Valence state map of iron oxide thin film obtained from electron spectroscopy imaging series.

This paper demonstrates the applicability of electron-spectroscopic imaging (ESI) for valence-state mapping of the iron oxide system. We have previously developed a set of signal-processing methods for an ESI series, to allow mapping of sp(2)/sp(3) ratio, dielectric function and energy bandgap. In this study, these methods are applied to generate a valence-state map of an iron oxide thin film (Fe/alpha-Fe(2)O(3)). Two problems, data undersampling and a convolution effect associated with extraction of the image-spectrum from the core loss image series, were overcome by using cubic-polynomial interpolation and maximum-entropy deconvolution. As a result, the reconstructed image-spectrum obtained from the ESI series images has a quality as good as that of conventional electron energy-loss spectra. The L(3)/L(2) ratio of the reconstructed ESI spectrum is determined to be 3.30+/-0.30 and 5.0+/-0.30 for Fe and alpha-Fe(2)O(3), respectively. Our L(3)/L(2) ratio mapping shows an accurate correspondence across the Cu/Fe/alpha-Fe(2)O(3) region. The effect of delocalization and chromatic aberration on the ESI resolution is discussed and estimated to be about 2 nm for the case of L(3)/L(2) ratio mapping.