Type of presentation: Oral

MS-7-O-2931 Spatial distribution of different diamond phases in compressed graphite studied by STEM-EELS

Sato Y.¹, Bugnet M.², Terauchi M.¹, Botton G.², Yoshiasa A.³

¹Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, ²Department of Materials Science and Engineering, McMaster University, ³Graduate School of Science and Technology, Kumamoto University

y-sato@tagen.tohoku.ac.jp

Electronic structure of hexagonal diamond (Lonsdaleite, h-DIA) is different from that of cubic diamond (c-DIA) [1]. h-DIA specimen was synthesized by direct transfer from graphite under high pressure and high temperature, where c-DIA and graphite phases always coexist. It has been an interesting subject to clarify how graphite phase (sp²) transfers to h-DIA or c-DIA phases (sp³) in the compression process. X-ray diffraction analyses have reported the h-DIA, c-DIA and graphite phases in the specimen exist as particle with a few nanometer in diameter [2]. However, direct observation of spatial distribution of the h-DIA and other allotropes has not been reported.
As the electronic structure of those carbon materials are different, K-shell excitation spectra, which reflect the density of states of conduction bands, should be different and effective to distinguish each crystal phases. In this study, spectroscopic imaging method by using STEM-EELS technique was applied. By fitting the spectra with linear combination of the three kinds of reference spectra of h-DIA, c-DIA, and graphite crystal phases (MLLS fitting) [3], spatial distribution of three kinds of the carbon crystal phases in the compressed graphite specimen was investigated.
Carbon K-shell excitation spectra was obtained by using FEI Titan 80-300, equipped with monochromator and a high-resolution spectrometer operated at 80 kV. K-shell excitation spectrum of h-DIA shows a different intensity distribution from that of c-DIA (Fig.1) [1]. There is no π* peak intensity (indicated by an arrow) means that present spectral profile of h-DIA specimen produced consist of only σ* component formed by sp³ orbital. Intensity distribution of K-shell excitation spectrum at each measured points was fitted with a linear combination of the three reference spectra (Figure 2b). Figure 2c shows a spectroscopic imaging of the compressed graphite specimen, where blue, green, and red colors indicate h-DIA, c-DIA, and graphite, respectively. The image shows the individual crystal phases are a few tens nanometer in diameter, which is about ten times larger than the estimated value by X-ray diffraction. This indicates that actual crystal phases exist as larger sized masses, which should include many crystal defects. The spectroscopic imaging is effective to distinguish different carbon crystal phases in real space and it makes easy to understand how three kinds of carbon phases exist in this compressed graphite.

References
[1] Y. Sato et al., Diamond & Related Materials 25, 40-44 (2012).
[2] A. Yoshiasa et al., Jpn. J. Appl. Phys., 42, 1694-1704 (2003).
[3] R. D. Leapman and C.R. Swyt, Ultramicroscopy, 26, 393-404 (1988).

The experimental work was carried out at the Canadian Centre for Electron Microscopy (CCEM), a national facility supported by NSERC and McMaster University.

Fig. 1: Carbon K-shell excitation spectra of h-DIA and c-DIA.

Fig. 2: (a) ADF image of compressed graphite specimen. (b) Spectrum fitting of carbon K-shell excitation spectra with reference spectra. Dots and black line are the experimental and the fitting spectra. (c) Spectroscopic image of the specimen. Blue, green, and red colors indicate h-DIA, c-DIA, and graphite phases, respectively.