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carbon at room temperature:

Ex

situ

observations

5.1

Introduction

Several recent studies have aimed to characterise the high pressure behaviour of GC [62,63,65,97]. These studies have left some questions unanswered. Such as, does the bonding and structure of GC change permanently after it is compressed beyond some threshold pressure? Or put more accurately, is there a threshold pressure at which the inherent properties of GC, including its superelasticity and non-graphitisability, are lost? There is a distinct lack of characterisation or analysis of the samples recovered following compression in the studies mentioned [62, 63,65,97] that could potentially shed some light on this, which is where the work presented in this chapter becomes relevant.

From early experiments it was observed that the Raman spectrum was different between recovered samples that had been compressed to relatively low pressures (∼10 GPa) compared to those which had been compressed to relatively high pressures (∼50 GPa). These Raman spectroscopy results indicated that a pressure threshold exists somewhere between 10-50 GPa. This finding incited many separate high pressure experiments in which GC was compressed in a Boehler plate-DAC [31] to different maximum pressures ranging from 4-54 GPa. After decompression, the samples were then recovered before their bonding and structure were thoroughly characterised.

The motivation for the experiments presented in this chapter became the search for this exact pressure threshold beyond which the structure of GC was permanently altered, at which the non-graphitising and superelastic properties may also be lost.

With the addition of more experiments and the use of several electron microscopy techniques (including HRTEM imaging, SADPs, and EELS) to analyse the recovered samples, the pressure range that incorporates this threshold has been narrowed to 35-45 GPa.

5.2

Experimental details

The characterisation of several recovered carbon samples from individual high pres- sure experiments are presented in this chapter. In all experiments a Sigradur-G GC precursor was loaded with a ruby but without a pressure medium into a Boehler plate- DAC [31], as detailed in Section 3.1. Figure 5.1(a) shows an image of the Boehler plate-DAC used in this work. The diamond anvils used in these experiments had culet diameters of 400µm. Stainless steel gaskets were used, which were pre-indented to a thickness of ∼50 µm. After pre-indentation a hole with a diameter of 200 µm was drilled into the centre using an Almax-Easylab electrode micro-driller with a tungsten-carbide drill bit.

Figure 5.1: (a) A photo of the Boehler plate-DAC used for these experiments. (b) An SEM image of a carbon sample lodged in a stainless steel gasket following recovery after decompression. This image shows examples of the different stages of the TEM sample preparation process outlined in Section 3.2.2. The red dashed circle indicates the boundary between the sample and gasket. 1 - A platinum deposition. 2,3 - Successful lamellae extractions. 4,5 - Failed lamellae extractions. This particular sample had been compressed to a maximum pressure 45 GPa.

In each of these separate experiments the samples were raised to different max- imum pressures (of 4, 10, 25, 35, 45, and 54 GPa), held at the maximum pressure for at least an hour, before being decompressed back to ambient. The main diamond

Raman line and the R1-ruby fluorescence line were used to measure the pressure, using the method described in Section 3.1.

The samples that were recovered from the DAC after decompression remained embedded in the stainless steel gaskets. The samples were first analysed with Raman spectroscopy, using a 532 nm excitation wavelength on a Renishaw micro-Raman spectrometer, with a 50× objective lens (providing a spot size ∼1 µm2_{) and a laser}

power of∼0.7 mW. After this a FEI dualbeam Scios FIB was used to prepare several lamellae from each recovered sample for analysis using TEM imaging, SADPs, and EELS measurements to be conducted on a JEOL 2100-F FEG-TEM/STEM equipped with a Gatan Imaging Filter. The system operated at 200 kV while taking the TEM images and SADPs, and at 80 kV for the EELS measurements. An SEM image following the lamella preparation process (outlined in Section 3.2.2) is shown in Fig. 5.1(b).

5.3

Results

The results presented here are split into three sections. Firstly, Raman spectroscopy is used to examine the bonding and level of structural order in the recovered samples. Then TEM imaging and SADPs are used to observe changes to the layered graphitic nanostructures and the isotropic nature of the material. Finally, low loss and carbon K-edge EELS measurements are performed to quantify any permanent changes to the density and the sp2 bonding fraction of the samples, respectively.

Raman spectroscopy

Once the samples had been decompressed and recovered from the DAC, Raman spec- troscopy measurements were performed. This technique was performed first, as it is non-destructive when a low laser power is used. Several dozen Raman scans were taken on the top and bottom surfaces of each of the recovered samples. These scans were very consistent across each individual sample. The Raman spectrum of the pre- cursor (Sigradur-G) is shown in Fig. 5.2, along with a Raman spectrum from each of the recovered samples.

Seven Gaussian peaks are required to fit to each spectrum, as shown in Section 4.4. These peaks include the intense D and G-peaks, which indicate that these samples are mostly comprised of sp2 _{bonded graphitic material. The presence of such a large}

D-peak indicates that the recovered material remains highly disordered. It should be noted that there is also no sharp diamond peak at ∼1332 cm−1_{. However, there still}

Figure 5.2: The Raman spectra of uncompressed Sigradur-G and the recovered sam- ples following compression in a plate-DAC to different maximum pressures. All scans have been scaled to the height of the G-peak, and a line has been added to act as the baseline for each scan. The D, G, and D’-peaks broaden above the 45 GPa threshold, and can be seen by the lack of definition of the D’-peak and an increase in intensity in the region between the D and G-peaks.

could be small percentages of sp3 _{bonds distributed throughout the samples, as will}

be discussed later in this chapter.

It can be seen that the D and G-peaks broaden slightly in the scans of recovered samples that have been compressed to 45 GPa and beyond. To show this change more clearly, a ratio of the integrated intensity of the D and G-peaks is used to estimate the average in-plane crystallite size (of graphene sheets), L[a], using the method described in Section 4.4 [163]. The calculated L[a] are plotted in Fig. 5.3 with respect to pressure. This figure shows a sudden drop in L[a] for the samples that have been compressed to 45 GPa and above. This is an indication that there is a permanent change in the structure of GC when compressed beyond this pressure threshold at 35-45 GPa, which is highlighted by the pink shaded region in Fig. 5.3.

Figure 5.3: The average in-plane graphitic size, L[a], determined from Raman spectra of the uncompressed precursor and other samples after recovery following compression with respect to pressure. The permanent transition between 35-45 GPa is highlighted by the pink shaded region.

To ensure the repeatability of this result, several more Sigradur-G samples were compressed to maximum pressure both above and below the pressure threshold at 35-45 GPa, and the results are consistent.

Electron microscopy

After the Raman analysis was complete, a FIB was used to prepare lamellae from each of the recovered samples, and for the precursor Sigradur-G, as outlined in Section 3.2.2.

A HRTEM image of the precursor GC is shown in Fig. 5.4(a). It shows that the Sigradur-G sample is comprised of an isotropic entanglement of layered graphitic nanostructures. The isotropic orientation of the nanostructures is confirmed by the broad uniform diffraction rings in the SADP shown in Fig. 5.4(b), which has been indexed to graphite. In this SADP rings with spacings consistent with the {002},

{100}, and {110} reflections of graphite can be seen.

Similar HRTEM images and SADPs were also taken from all of the samples re- covered following compression. All of the samples that were compressed to maximum pressures up to 35 GPa and below show similar features. Because of this, only results from the sample compressed to 35 GPa are shown (in Fig. 5.5) as this is just below the pressure threshold identified from Raman analysis.

Figure 5.4: (a) A high resolution TEM image of the uncompressed Sigradur-G showing the characteristic layered graphitic nanostructures and (b) an associated SADP which is indexed to graphite.

Figure 5.5: (a) A high resolution TEM image of the sample recovered following compression to 35 GPa showing that the layered nanostructures remain and (b) an associated SADP which shows that the bulk of the structure has remained isotropic. The SADP has been indexed to graphite. (inset) The compression axis of the DAC is indicated by the blue arrows.

Figure 5.5(a) shows a HRTEM image of a region of the lamella produced from the sample recovered after compression to 35 GPa. This image, similar to the image of the precursor, shows that the recovered sample is comprised of a twisted entanglement of layered graphitic nanostructures. The SADP shown in Fig. 5.5(b) taken on the same sample shows that the material has remained structurally isotropic and is graphitic.

This result was the same for all samples compressed to maximum pressures below 35 GPa. The SADP from the 35 GPa sample exhibits the same graphitic reflections as the precursor, albeit they are slightly sharper. This line sharpening is consistent across several lamellae produced from these samples, and the origin of this is still somewhat unknown. It is possible that the high pressure applied has allowed the graphene sheets within the layered nanostructures to relax slightly more to perhaps lessen the effect of sheet curvature.

The HRTEM image shown in Fig. 5.6(a) is from a lamella extracted from the sample shown in Fig. 5.1(b) which was compressed to 45 GPa. This image shows that a large portion of the layered graphitic nanostructures have vanished, and have been replaced with smaller oriented graphitic crystallites. A SADP taken on this lamella is shown in Fig. 5.6(b), and has been indexed to graphite. It shows that the remaining nanostructures are no longer oriented isotropically. The transformation of the {002} reflection into two “arcs” indicates that the remaining graphitic layers are aligned perpendicular to the compression axis in the DAC, which is indicated by the inset in Fig. 5.6(b).

Figure 5.6: (a) A high resolution TEM image of the sample recovered following compression to 45 GPa and (b) its associated SADP. The SADP has been indexed to graphite. (inset) The compression axis of the DAC is indicated by the blue arrows.

All lamella generated from samples that have been compressed to 45 GPa and above display very similar features. They all show that the majority of large layered graphitic nanostructures are gone, and those that remain are now aligned perpendic- ular to the compression axis of the DAC. The HRTEM images and SADPs shown

here strongly support the Raman analysis, which indicated that a permanent change occurs to the material when compressed beyond 35-45 GPa.