II. Experimental techniques
3.2 Results and discussion
3.2.2 Cold wall system (RSR-M) experiments
To further study the mechanism of diamond dissolution at the M-D interfaces and removal of carbon at the open metal surfaces, we used a home-built cold wall system because it can be very rapidly heated to 1000 ℃ and from that temperature we can quench the samples to room temperature in 8 seconds. A spectropyrometer was used to measure the sample temperature and its emissivity as a function of time (See details in the Experimental section). As shown in Figure 3.14a, the changes in the emissivity (blue data points) from about 0.4 to 0.75 and in the free Ni surface temperature (red data points) from about 1010 ℃ to 950 ℃ are caused by the formation of graphite on the open Ni surface30. Then as we repeated the same process for the same sample (see Figure 3.14b), the emissivity and the temperature remained almost unchanged at about 950 ℃. This is because a stable graphite/nickel/diamond structure was formed within 10 secs in the 1st round of heat treatment at about 1000 ℃.
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Figure 3.14 (a-b) Spectropyrometer measurements of the emissivity of 500-nm thick Ni coated D(100) sample in the cold wall system and (c-d) time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiles measured for the Ni /D(100) sample with water vapor present for the conditions used in the continuous diamond dissolution experiments (c) and in the experiments with a fully oxidized metal layer (d).
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Figure 3.15 An optical microscope (OM) image of a pierced D(100) plate obtained after the dissolution experiment (3 hours at 1050±1 ℃, 770±5 Torr pressure, under continuous flow (1000 sccm Ar(g)) of water vapor (bubbler temperature, 25 ℃).
Heat treatment at 1050±1 ℃ in the presence of water vapor (25 ℃ bubbler temperature) for 3 hours resulted in the complete dissolution of the diamond covered with the Ni film (the Ni film pierced through the 0.3 mm thick diamond). An optical microscope image of the pierced diamond plate is shown in Figure 3.15.
Figures 3.14a-b show the results of the spectropyrometer measurements of the emissivity of the 500- nm thick Ni coated D(100) sample. The time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiles shown in Figures 3.14c-d were obtained for the samples that had been heated from room temperature to 1000±1 ℃ in 5 mins under continuous flow of 1000 sccm Ar at 770±5 Torr; Figure 3.14c shows the depth profiles of the sample heat treated for 10 mins at 1000±1 ℃ with a continuous flow of water vapor (25 ℃ bubbler temperature) using Ar (1000 sccm) as a carrier gas; the depth profile measurements of the sample heat treated for 3 hours at 1000±1 ℃ and 63 ℃ water bubbler temperature are shown in Figure 3.14d.
Depth distribution of C in the metal (Ni, Co) film with and without water vapor. A series of fast heating and fast cooling (quenching) experiments on the 500-nm thick Ni coated D(100) samples with and without water vapor were performed. Figure 3.14c shows the ToF-SIMS depth profiles of C, Ni, and NiO for the sample exposed to water vapor. This nickel oxide layer (region) was present at the (sub)surface. It took ~3 seconds to cool from 1000±1 ℃ to below 500 ℃. We chose the fastest possible cooling rate to try to quench the sample and to “freeze” the C distribution inside the metal film. Under these conditions and based on the depth profiles shown in Figure 3.14c, the surface region (to the depth of ~50 nm) is oxygen-deficient possibly due to the formation of CO(g) at the metal surface. The ToF- SIMS data also suggest that O is not present at the metal-diamond interface region, indicating that there is pure Ni at the interface. Considering the C- and C3- plots, we suggest that a concentration gradient of dissolved C atoms was formed in the Ni film. Figure 3.16 shows a ToF-SIMS depth profile of the C distribution for a Co film coated sample exposed to water vapor at the same conditions as the Ni sample.
In that sample a concentration gradient of dissolved carbon was also found. Figure 3.17a shows the ToF-SIMS data of a Ni-coated diamond sample that was heat treated at 1000±1 ℃ for 10 mins without water vapor in which the minimum intensity of the C- signal was about 6E+4 counts (for a sputter time of ~2000 s). Note that no obvious C- signal was detected in the pristine Ni-coated diamond sample (Figure 3.17b). Neither was there any transition from NiO to metallic Ni observed for the sample which had been heat treated with no water present in the chamber.
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Figure 3.16 Time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiles of a 500-nm thick Co/D(100) sample after heat treatment at 1000±1 ℃ for 10 mins with water vapor present in the reaction chamber.
Figure 3.17 Time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiles of (a) a 500- nm thick Ni/D(100) sample after heat treatment at 1000±1 ℃ for 10mins without water vapor in the reaction chamber; (b) a pristine 500-nm thick Ni/D(100) sample.
The ToF-SIMS data for a fully oxidized sample are shown in Figure 3.14d. O- and NiO- signals remained high throughout the Ni film and the C- and C3- signals were essentially undetectable, indicating that diamond does not dissolve in the NiO film. Figure 3.18 shows the SEM images of the
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bare diamond surface after removing the NiO film. These images show that on the diamond surface a small amount of carbon was dissolved in the not fully oxidized Ni film at the beginning of the exposure to the water vapor. Yet, even after a 3-hour-long experiment (500 sccm Ar(g); bubbler temperature of 63 ℃ and a sample temperature of 1000±1 ℃ ), no considerable amount of diamond was dissolved.
There was almost no difference in diamond mass before and after this experiment (42.02 mg before and 42.01 mg after) because the Ni film oxidized fairly quickly all the way to the Ni/D interface. In short, too high a concentration of water vapor quickly converts the Ni film to an oxidized one which completely inhibits dissolution of the diamond.
Figure 3.18 SEM images of the diamond surface taken (a) in the bare region (uncoated by Ni), (b) at the edge of the Ni-coated region, and (c) within the Ni-coated region after removing the NiO layer.
Figure 3.19 SEM images of (a) graphite film formed on the Pt surface after 1000±1 ℃ heat treatment for 3 hours (770±5 Torr) without water vapor present in the cold wall system reaction chamber; (b) Pt surface after exposure to a continuous flow of Ar(g) and 1000 sccm of water vapor (bubbler temperature, 25 ℃) at 1000±1 ℃. No graphite was observed.
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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.