Study of Chromium-Doped Diamond-Like Carbon by Z-Contrast Imaging and Electron Energy Loss Spectroscopy
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atomic number, Z. The Z-contrast image can directly reveal atomic positions as the brighter spots correspond to the positions of heavier atoms. The EELS data can be collected simultaneously through the hole of the dark-field detector allowing direct correlation between the images and spectra. EXPERIMENTAL Diamond-like carbon films of 0.1-5.0 microns thickness were grown on alumina substrates using a plasma chemical deposition technique described elsewhere [6]. The doping of chromium was performed by co-evaporation of chromium into the reactor volume while growing the carbon film[7,8]. The carbon film was prepared for transmission electron microscopy by mechanical dimpling from the substrate side and ion beam milling to electron transparency. Two samples doped at low (-6at%Cr) and high levels (- 12at%Cr) were examined by Z-contrast imaging in a VG HB603 STEM and a VG HB501. The EELS data were collected on VG HB501 STEM and Philips CM200 transmission electron microscopes. Fig.l(a) is a phase contrast image of highly doped sample which is similar to a typical amorphous structure. The Cr is not readily distinguishable. Fig.l(b) is a dark-field Z-contrast image of the same area. Since Cr atoms are heavier than C, the brighter areas directly correspond to Cr-rich regions. The average Cr-rich cluster size is estimated to be 2nm with a separation of 3.5nm between clusters. The low-doped sample does not show any Cr-rich regions, indicating uniform Cr distribution throughout the DLC matrix (picture not shown).
329 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society Downloaded from https://www.cambridge.org/core. Columbia University - Law Library, on 04 Aug 2019 at 12:24:02, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-593-329
Fig. I Chromium doped diamond-like carbon. (a)phase contrast image showing amorphous carbon matrix. (b)Z-contrast image showing Cr clusters. The EEL spectra of carbon the K-edge and Cr L-edge were collected from both samples, as well as from reference samples of chromium carbide Cr3C2, metallic Cr and synthetic graphite. Spectra, shown in Fig.2(a-e), were taken in image mode from a scanned area of approximately 100 nm 2 . Since the highly doped film has uneven Cr distribution, spatially resolved EELS were also collected on the VG HB 501 with a probe size less than 3A from both Cr-rich regions and Crdeficient regions, as shown in Fig.3. The probe position was determined from the Z-contrast image taken simultaneously with the EEL spectra. Using the intensity ratio of Ic and 'Cr and calibrating with the reference sample Cr3C 2, the concentrations of Cr are found to be an average of 6at% for the low doped sample. For the highly doped sample, bright regions show 7-17at% Cr whereas dark regions show 1-3at% Cr. C K-edge
Cr L-edge
low doped DLC
(1a) CU
highly doped DLC
(b)
2 '(d) graphite 2 300b 30 '
Cchromium (c)
carbide (e)
metallic chromium
560 570 080 590 600 610 620 630
Energy Loss (eV)
Fig.2 EEL
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