Observation of chemical state changes during sample cooling using a soft X-ray emission spectrometer

Abstract text (incl. figure legends and references)

Introduction / Objective:

In recent years, the needs for chemical state analysis of materials in bulk samples using a scanning electron microscope (SEM) has increased in the fields such as batteries, steel and polymer materials, and more and more research results have been reported.

Terauchi et al. reported the development of a wavelength-dispersive soft X-ray emission spectrometer (WD-SXES) combined with a transmission electron microscope, and demonstrated high energy-resolution [1,2]. We have developed and commercialized a general-purpose soft X-ray emission spectrometer (SXES) that can be mounted on a kind of scanning electron microscope (SEM/EPMA*) based on the Terauchi"s spectrometer [3]. This contributes to the generalization of chemical state analysis in bulk samples.

In addition, the technique of observing a sample while it is cooled on a SEM system is becoming more common. It is known that cooling a sample changes its chemical state, but only a limited number have been reported. In this study, the correlation between sample temperature and chemical state was investigated using SXES.

In a pioneering work, Skinner experimentally and theoretically reported that the intensity distribution near the Fermi level (Fermi edge) in the soft X-ray emission spectrum becomes sharp as the sample cooled [4]. The observed edge width is 0.22 eV at 300 deg C and 0.14 eV at 110 deg C for metal Al.

In this study, similar experiments were carried out using a SEM equipped with a liquid nitrogen cooling stage and a high energy resolution SXES , and the comparison and interpretation of the obtained spectra are reported.

Materials & Methods:

A JSM-IT800HL SEM and an SS-94000SXES spectrometer from JEOL, a C1003 Liquid nitrogen cooling stage from GATAN, Inc. were used. Metal aluminum (Al) was used for the sample.

Results:

The Al L_2,3-M-emission SXES spectra obtained at different sample temperatures from room temperature to -185 deg C are shown in Figure 1. The intensity rise from around 73.0 eV to the lower energy side is the L₃-edge corresponding to the Fermi edge. As shown in the figure, the width of the intensity rise changes with sample cooling and shows a correlation with the sample temperature (from 0.110 eV to 0.088 eV).

This indicates that the Fermi level of the valence electron is broadened by the temperature factor. In other words, we have observed a phenomenon corresponding to the temperature dependence of the Fermi-Dirac distribution function.

Conclusions:

We have shown that by combining a general-purpose SXES commercialized by JEOL and a sample cooling mechanism, it becomes possible to acquire higher energy-resolution SXES spectra with reduced blurring due to the temperature factor. We think that this resolution improvement is of great benefit for more detailed chemical state analysis of materials.

[1] Terauchi, M., & Kawana, M. (2006). Ultramicroscopy, 106, 1069-1075. doi: 10.1016/j.ultramic.2006.04.021

[2] Terauchi, M et al. (2012). J. Electron Microsc., 61, 1-8. doi: 10.1093/jmicro/dfr076

[3] Takahashi, H et al. (2016). IOP Conf. Ser.: Mater. Sci. Eng., 109, 012017. doi: 10.1088/1757-899X/109/1/012017

[4] Skinner, H. W. B. (1940). Philos. Trans. R. Soc. A, 239, 95-134. doi: 10.1098/rsta.1940.0009

*　Electron Probe Microanalyzer