II. Experimental techniques
3.2 Results and discussion
3.2.3 XRD characterization
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500-nm thick Pt films sputtered onto the D(100) surface were studied and the results suggest a critical role of an oxide layer on such a sub-surface of the metal film in the process of diamond dissolution, because we observed that the dissolution rates were significantly smaller for the Pt/D(100) samples than for the Ni/D(100) and Co/D(100) samples under the same conditions (a temperature of 1000±1 ℃, and similar partial pressures of water vapor in the RSR-M system in different experimental runs), even though the solubility and diffusion rates of C in Pt31 are both close to those in Ni32 and Co25. The main difference between Pt and Co/Ni is that a “thick” oxide layer was not formed on or near the Pt surface33 but was formed for Co and Ni (Figure 3.19). The SEM images show that without water vapor a graphitic film formed on the top surface of Pt film which we found could also be removed by water vapor. In a separate 3-hour experiment, a 500-nm thick Pt film was deposited by sputtering on a D(100) sample and the resulting sample was heated at 1000±1 ℃ in the presence of water vapor (bubbler temperature of 25 ℃). No obvious dissolution of the diamond was found. This might be due to the absence of an oxide layer. We note that whether in the quartz tube furnace or the cold wall system, the graphite film could be etched away by water vapor at temperatures in the range studied, as reported by others34, while bare D(100) and D(110) did not etch when exposed to water vapor at around 1000 ℃ in any of our experiments.
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Figure 3.20 Synchrotron-based GIXD patterns of (a) an as-deposited (pristine) Ni film on the D(100) surface, (b) Ni film after heating at 1009±1 ℃ for 1 h without water vapor and (c) Ni oxide/Ni film on the D(100) surface obtained after heating at 1009±1 ℃ for 10 min with water vapor (bubbler temperature 25 ℃). (d) In-situ XRD study of a 500-nm thick Ni film on the D(100) surface. (e) Powder XRD (P-XRD) patterns of bare single crystal diamond with a (100) surface (upper curve) and nickel coated diamond heated at 1009±1 ℃ for 10 min with water vapor present (bubbler temperature 25 ℃) contributing to the dissolution of diamond into the Ni film (lower curve).
Ni/Co-D(100) variable temperature in-situ XRD. The results of variable temperature in-situ XRD measurements conducted under a helium atmosphere at about 75 Torr with no water vapor present (see details in the Experimental section) are shown in Figure 3.20d for the Ni/D(100) sample. It can be seen that a nickel carbide36 (Ni3C) phase was apparently not present in the Ni/D system during the entire heating and cooling process. Ni3C is reported to decompose at about 350 ℃37 into Ni and graphite. The XRD patterns show that a graphitic film with the (002) orientation was formed at 1000 ℃, and that the Ni(111) peak disappeared at 900 ℃ during the heating stage, but the Ni(200) peak was still observed during the whole measurement process due to its epitaxial relation with the diamond substrate. This
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result shows that the preferred orientation of Ni crystals is evolved over 900 ℃, as the conventional reflection-mode XRD measurement only provides out-of-plane information of the sample. Figure 3.20e shows a P-XRD pattern of the Ni film for Ni/D(100) after a diamond dissolution experiment with water vapor present in the quartz tube furnace, and also a pattern of the bare single crystal diamond 100 substrate used in our study. We found that after heating at 1009±1 ℃, the deposited polycrystalline Ni film on D(100) substrate converted primarily to a Ni(100) film (inset of Figure 3.20e). Besides, the variable temperature in-situ XRD measurement for a 500-nm thick Co film on the D(100) surface are shown in Figure 3.21a. This film was converted to FCC-Co(100) during the heat treatment. No peaks corresponding to the cobalt carbide (Co3C) phase were found in the XRD pattern of the Co-coated D(100) sample heat treated without water vapor. Heat treatment of a polycrystalline Co film on the D(100) surface at ~1000 ℃ in the absence of water vapor resulted in a Co(100) film with single crystalline features after returning the sample to room temperature.
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Figure 3.21 Variable temperature in-situ XRD analyses of (a) 500-nm thick Co film on the D(100) surface, (b) 500-nm thick Ni film and (c) Co film on the D(110) surface. (d) P-XRD measurements of 500-nm thick Ni film on the D(111) surface. Note that the peak intensity is weak since the 1 mm x 1 mm area of the Ni film region on the D(111) surface is small.
Ni/Co-D(110) variable temperature in-situ XRD. The same characterization on D(110) substrates are shown in Figures 3.21b-c, and the XRD patterns suggest that the Ni and Co films were converted to fcc films with single crystalline features and a (110) surface after heating and cooling to room temperature. For a 500-nm thick Ni film coated on D(111) substrate heated at 1009 ℃ for 1 hour in the absence of water vapor in the quartz tube furnace the film was converted to a Ni(111) film with single crystalline features according to the XRD data shown in Figure 3.21d.
It is not possible to introduce water vapor into our variable temperature in-situ XRD instrument, and so we did P-XRD measurements on the samples (Ni/Co on D(100), D(110)) after the dissolution experiments. Figure 3.22 shows the XRD patterns for these samples, in which we found that the fcc- M(200) peak is the main peak for the D(100) samples and fcc-M(220) is the main peak for the D(110) samples, for both Ni and Co.
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Figure 3.22 P-XRD analysis of 500-nm thick Ni/Co film on (a-b) D(100) substrates and (c-d) D(110) substrates after dissolution experiments. We suggest that only fcc-Co exists (not hcp) in the Co/D(100) and Co/D(110) samples because the XRD pattern for Co/D(100) (b) is comparable to the fcc-Ni/D(100) XRD pattern (a), and the XRD pattern for Co/D(110) (d) is comparable to the fcc-Ni/D(110) XRD pattern (c), and also that the CoO peaks shown in (b), (d) are in agreement with the fcc-CoO phases but not the hcp-CoO phases. In other words, and possibly due to the influence of the diamond substrates, the fcc-Co film does not convert to hcp after cooling to room temperature.
The results described above mean that the Ni and Co films likely have an epitaxial interface with the single crystal diamond substrates with 100 and 110 orientations and that the as deposited films are converted to films with single crystalline features with the same surface orientation as the diamond. For Ni films deposited on 111 single crystal diamond plates that we have briefly studied, the Ni film is
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apparently converted to a film with 111 orientation and with single crystalline features, perhaps epitaxial to the diamond. We have not yet tested Co.