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After the non-destructive characterisation techniques used to analyse the recovered sample were complete (as shown in Sections 7.4.2 and 7.4.3), the sample could be prepared for electron microscopy measurements. A FIB was used to prepare several TEM lamellae from both the inner and annular regions of the recovered sample. An SEM image of the sample after several lamellae had been created is shown in Fig. 7.11.

Shown for reference in Figs. 7.12(a-b) is a HRTEM image and SADP of the pre- cursor GC. The HRTEM image reveals the entangled layered graphitic nanostructure of the material at ambient, and the SADP (which has been indexed to graphite) shows that the material has an isotropic structure. Figure 7.12(c) shows a magni- fied image of a single layered graphitic nanostructure. An intensity profile measured across this image [shown in Figure 7.12(d)] indicates that the graphitic layers in this nanostructure are separated by a distance of 0.36 nm.

Figure 7.11: An SEM image of the sample captured in the FIB of the sample after several lamellae have been removed for imaging and analysis in the TEM.

Figure 7.12: (a) A TEM image and (b) SADP of the precursor GC. The SADP shows that the structure is isotropic and has been indexed to graphite. (c) A magnified image of a layered graphitic nanostructure and (d) an intensity profile across this nanostructure showing that the interlayer spacing is 0.36 nm.

shown in Figs. 7.13(a-b). At the bottom of the HRTEM image is a schematic of a DAC which shows the orientation of the sample with respect to the compression axis. Similar to the results of the room temperature compression experiments presented in Chapter 5, the HRTEM image shows the extended layered graphitic nanostructures is reduced and those that remain appear to be oriented perpendicular to the compression axis of the DAC. The strong asymmetric arcs in the {002} reflection in the SADP shown as part of the same figure confirm this result. A close-up image of a single

layered graphitic nanostructure is shown in Fig. 7.13(c), and an intensity profile across this image [shown in Fig. 7.13(c)] indicates that the separation between layers in the nanostructures in the inner region of the sample is 0.36 nm, which is the same as the precursor.

Figure 7.13: (a) A TEM image and (b) SADP of the inner region of the recovered sample shown in Fig. 7.5(b). A schematic drawing of a DAC has been included with blue arrows to show the compression axis. The SADP has been indexed to graphite and shows that the structure is no longer isotropic, but has become aligned relative to the compression axis in the DAC. (c) A magnified image of a layered graphitic nanostructure and (d) an intensity profile which shows that the interlayer spacing is 0.36 nm.

A HRTEM image and a SADP of the annular region of the recovered sample are shown in Figs. 7.14(a-b). At the bottom of the HRTEM image is a schematic of a DAC which shows the orientation of the sample with respect to the compression axis. The HRTEM image shows that the nanostructure of the material is very different in the annular region compared to the inner region or the precursor. This is expected, as the measurements presented in Sections 7.4.2 and 7.4.3 propose that this region is comprised of nanocrystalline diamond with a majority of hexagonal stacking. The nanocrystals are so small (∼1-2 nm) that the structure observed in this image appears to be almost amorphous. Some small layered graphitic nanostructures do remain scattered throughout the annular region of the sample. However, as can be seen in a magnified image of one of these remaining graphitic regions [shown in Fig. 7.13(c)] the layers are permanently compressed with a layer separation of only 0.29 nm, as

shown in Fig. 7.14(d).

Figure 7.14: (a) A TEM image and (b) SADP of the annular region of the recovered sample shown in Fig. 7.5(b). A schematic drawing of a DAC has been included with blue arrows to show the compression axis. The SADP has been indexed to hexagonal diamond, and shows that the material has become aligned relative to the compression axis in the DAC. (c) A magnified image of a small graphitic inclusion with (d) an intensity profile showing that the layer separation in this inclusion is 0.29 nm.

The SADP shown in Fig. 7.14 is also quite different to that of the inner region and precursor. There is no longer any observable contribution from graphitic material, as is expected. What is interesting about this SADP is that it shows that the structure of the sp3 bonded nanocrystalline annular region of the recovered sample is not isotropic, as some preferred orientation can be seen in the main inner ring of the diffraction pattern.

To investigate the preferred orientation of the nanocrystals, the SADP from Fig. 7.14 (which is viewed perpendicular to the DAC compression axis) was further anal- ysed by assuming that the sample is comprised of either cubic or hexagonal diamond. The models generated, which are shown in Fig. 7.15, assume a random crystal orien- tation around the DAC compression axis. For hexagonal diamond this axis is aligned along the <100>, and for cubic diamond it is aligned along the <111>. For each model the reciprocal lattice is rotated around the compression axis and the inter- section points with the Ewald sphere are indicated overlaying the SADP for cubic diamond in Fig. 7.15(a), and for hexagonal diamond in Fig. 7.15(b). The recip- rocal lattice points can be thought of as narrow arcs because the alignment of the

nanocrystals around the compression axis is not perfect. The hexagonal diamond model provides a better fit to the key features of the SADP. This includes a better fit in the vertical and horizontal directions of the inner main diffraction ring, which is a result of the unequal diameters of the {100} and {002} reflections of hexagonal diamond.

Figure 7.15: (a) The SADP of the annular region (aligned so that the compression axis of the DAC is vertical). Overlaying this are the predicted reflections for cubic diamond with theh111i direction vertical. The spots indicate the intersection of the Ewald sphere with reciprocal lattice rotating around the h111i. (b) The same SADP overlayed with spots generated by the same method for hexagonal diamond with the

h100i direction vertical.

integrated in vertical and horizontal directions (with respect to the page) to show the unequal diameters of the main inner diffraction ring. This SADP analysis generated a c/a ratio of 1.71 that is within error of the XRD refinement, which is shown alongside other published results in Table 7.2.

Figure 7.16: A plot showing the unequal diameters of the main inner ring in the SADP show in Fig. 7.15 produced by the different{100} and {002}d-spacings.

To probe the microscopic density of the inner and annular regions of the recovered sample low loss EELS measurements were performed. A low loss EELS spectrum of the precursor GC is shown in Fig. 7.17, along with spectra from the inner and annular regions, and two spectra of crystalline cubic and hexagonal diamond which are included for comparison [235]. It is possible to determine the microscopic density of each region of the sample using the method described by Titantah et al. [176], which relates density to the position of the bulk plasmon peak in the spectrum. In the spectrum of the precursor the bulk plasmon peak is centered at 22.7 eV, which corresponds to a density of 1.63 g/cm3_{. This is similar to the value determined from}

the low loss EELS spectra presented in Chapter 5. In the spectrum of the inner region the bulk plasmon peak has shifted substantially to the right, and can be seen centred at 27 eV, which corresponds to a density of 2.29 g/cm3. This is very similar to the density of graphite (within error) which is 2.27 g/cm3 _{at ambient. In the spectrum}

of the annular region the bulk plasmon peak has shifted to 32 eV, corresponding to a density of 3.22 g/cm3_{. This is lower than the density expected for pure diamond which}

is∼3.5 g/cm3_{, but it is consistent with a material that is comprised of nanocrystalline}

diamond with a trace amount of graphitic material.

To probe the sp2 _{and sp}3 _{bonding fractions of the inner and annular regions of}

K-edge EELS spectrum of the precursor GC is shown in Fig. 7.18, along with spectra from the inner and annular regions, and two spectra of crystalline cubic and hexagonal diamond which are included for comparison [235].

Figure 7.17: Low loss EELS spectra taken of the precursor (Sigradur-G), the inner region, and the annular region from the sample shown in Fig. 7.5(b). The shift to the right of the bulk plasmon peak indicates an increase in material density. For comparison spectra from Sato et al. of cubic and hexagonal diamond are included [235].

The strong 1s-π* peak at 285 eV in the carbon K-edge EELS spectrum of the precursor indicates that it is predominantly sp2 bonded. The spectrum of the inner region also has a strong 1s-π* peak suggesting that it also in comprised mostly of sp2 bonds. It is not possible to make an accurate determination of the exact sp2 bonding fraction from these spectra as the measurements were not performed under the “magic EELS” conditions [177, 209, 210]. For a detailed explanation of this see Section 5.3. The intensity of the 1s-π* peak in the spectrum of the annular region is greatly diminished but still exists, as opposed to pure diamond (either cubic or hexagonal) where the 1s-π* peak would not be present. This supports previous results which suggest that the material in the annular region is mostly sp3 _bonded,

Figure 7.18: Carbon K-edge EELS spectra taken of the precursor (Sigradur-G), the inner region, and the annular region from the sample shown in Fig. 7.5(b). The loss of the 1s-π* peak (located at 285 eV) in the annular region spectrum indicates a reduction in sp2 _{bonding. For comparison spectra from Sato et al. of cubic and}

hexagonal diamond are included [235].

has been used by Huet al. to determine the sp3 _{bonding fraction of recovered glassy}

carbon samples that have been compressed to∼15 GPa and up to 1200◦C [206]. Their results show similar broad features and a reduction in the 1s-π* peak intensity.

In summary, the results presented in this section support the results from the pre- vious section in showing that both the inner and annular regions of the recovered sam- ple are distinctly different. The inner region is best described as oriented nanocrys- talline graphite which has a density (of∼2.29 g/cm3_{) similar to graphite. The annular}

region is comprised of nanocrystalline diamond with predominantly hexagonal stack- ing that contains a trace number of graphitic inclusions and has a density of ∼3.22 g/cm3_.