Why does polycrystalline natural diamond turn black after annealing?

Transcription

1 phys. stat. sol. (a) 201, No.11, (2004) / DOI /pssa Why does polycrystalline natural diamond turn black after annealing? B. Willems *, 1, K. De Corte 2, and G. Van Tendeloo 1 1 EMAT, Department of Physics, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium 2 Hoge Raad voor Diamant HRD, GDI Department, Plaslaar 50, 2500 Lier, Belgium Received 10 March 2004, revised 9 July 2004, accepted 31 July 2004 Published online 7 September 2004 PACS Bi, y, g, Uw Low-quality polycrystalline natural diamonds become black when annealing them in an inert atmosphere at sufficiently high temperatures. Different carbon phases (the coexistence of diamond, graphite and amorphous carbon) are identified after this treatment using Raman spectroscopy. Internal structural changes in the microstructure upon annealing are reported and defects are characterised by transmission electron microscopy (TEM) analysis. 1 Introduction Natural black diamonds are rare in nature. Different methods such as electron and neutron irradiation, ion-implantation and high-temperature (HT) treatments are available to produce a black diamond. In this paper we focus on the high-temperature annealing in inert atmosphere of low-quality polycrystalline natural diamonds, also known as boart or coated diamond. They consist of a monocrystalline core surrounded by a polycrystalline coat. It is known that polycrystalline diamond contains a variety of structural defects such as dislocations, twins and grain boundaries [1 4]. In chemical vapour deposited (CVD) polycrystalline diamond films these defects play an important role during annealing. Nistor et al. demonstrated that graphite-like material forms at grain boundaries and nearby defected regions within the grains [5]. It can be expected that the annealing of polycrystalline natural diamond results in similar internal structural transformations. 2 Experimental procedure Translucent natural coated polycrystalline diamonds of various colour (yellowish, greenish, reddish, greyish, brownish) were subjected to temperatures above 1000 C for several hours in vacuum. A COMDIAM D-scope was used to study the optical characteristics. The samples were examined by Raman spectroscopy before and after annealing. Raman spectra covering the range cm 1 were collected at room temperature using two Raman spectroscopy set-ups, a Jobin Yvon LabRAM HR800 High Resolution analyzer system and a Renishaw Raman 2000 system. Here Raman spectroscopy was carried out at 514,5 and 632,817 nm. In order to pinpoint the Raman measurements an optical microscope was used (magnification 10, 50 and 100). * Corresponding author: Bert.Willems@ua.ac.be, Phone: , Fax:

2 phys. stat. sol. (a) 201, No.11 (2004) / Fig. 1 A low-quality natural diamond (3,83 3,15 1,87 mm in size) after HT annealing: the monocrystalline core remains unchanged and hence transparent in zones free of cracks and inclusions. The polycrystalline coat becomes completely black. For the transmission electron microscopy (TEM) analysis, plan-view samples were produced by laser sawing thin slices (ca. 2 mm 2 mm 150 µm) of treated and untreated polycrystalline diamond. The diamond slabs were subsequently thinned to electron transparency by Ar + -ion milling. In total 10 samples, before and after annealing, were investigated. TEM was carried out on a Philips CM20. 3 Results and discussion 3.1 Optical microscopy After annealing, the complete polycrystalline zone (known as the coat ) becomes black (Fig. 1). The monocrystalline core remains essentially unchanged in zones free from cracks, cleavages and inclusions. The actual annealing parameters given, graphite formation in perfect single crystals is inhibited by the stress due to the difference in specific volume of graphite and diamond [6]. The darkening of the polycrystalline zone is independent of its initial colour and/or aggregation state. A detailed view learns that the blackening is caused by small inclusions and at cracks. 3.2 Raman spectroscopy Before annealing, all samples show an intense peak at 1330 to 1333 wavenumbers (cm 1 ), the 1 st order diamond Raman line. After heating, the diamond Raman line is still detected, indicating that the sp 3 fraction is larger than 80% [7]. A significant shift of the spectral position of the diamond Raman line before and after annealing is not observed. A broadening of the diamond Raman line, measured by the full width at half maximum (FWHM), is observed for all heated samples. In a sample, the FWHM of the diamond Raman line changes from 2,8 cm 1 before annealing to 4,7 cm 1 after annealing. By focussing on cracks or black surface features, all heated samples show in addition a broad Gaussian-shaped band at 1600 cm 1 (Fig. 2). Based on its features, the 1600 cm 1 band is identified as the Fig. 2 Black cracks and inclusions on the surface of an annealed sample (a). Raman spectrum showing the G-band around 1600 cm 1 and the broadened diamond line at 1332 cm 1 in an annealed sample (632,817 nm excitation, recorded at room temperature) (b).

3 2488 B. Willems et al.: Why does polycrystalline natural diamond turn black after annealing? A rb i t r a r y Fig. 3 Variations in Raman scattering within a single sample after annealing illustrated by two spectra. U ni t s Wavenumbers (cm -1 ) well-documented G-band, attributed to the presence of defective or microcrystalline graphite structures or nanocrystalline (glassy) carbon [7], implying a non-diamond carbon content. After an annealing of 2 hours at 1300 C, the intensity of the 1600 cm 1 band is always low compared to the intensity of the diamond Raman line. However after annealing at higher temperatures during a longer period (e.g. 6 hours at 1500 C) it becomes clear that the intensity of the diamond Raman line is in close relationship with the intensity of the 1600 cm 1 band, showing an apparent anti-correlation. This is illustrated in Fig. 3. In general, the intensity of the G-band relative to the one of the diamond Raman line varies remarkably with the intensity of the 1600 cm 1 band. Moreover as the latter becomes stronger, it may be accompanied by a band at 1350 cm 1 that is identified as the D-band. Previous studies have attributed the latter to various forms of non-diamond carbon phases: disordered or nanocrystalline defective graphite, disordered glassy carbon, sp 2 -hybridized carbon phases, microcrystalline defective graphite, inclusions of amorphous diamond-like carbon, or a diamond precursor phase [7]. In rather rare cases, a peak at 1585 cm 1 is observed by studying the black material at cracks or inclusions. The presence of well-crystallized graphite gives rise to this peak. Although optical microscopy revealed that the black inclusions are small but abundant in the coat, there is limited evidence of non-diamond phases as detected by Raman spectroscopy in reflection. Therefore the suspected internal structural transformation in the microstructure is studied by TEM as in analogy with the HT annealing of polycrystalline CVD films. Fig. 4 Electron diffraction (ED) pattern (a) and TEM image (b) of a polycrystalline non-diamond inclusion surrounded by dislocations. ED pattern (c) and TEM image (d) of a dislocation pile up near non-diamond inclusions. Images are recorded in a sample after annealing.

4 phys. stat. sol. (a) 201, No.11 (2004) / Fig. 5 Non-diamond phases (>5 µm in size) appear after annealing. Optical micrographs between crossed polarizers (a) and corresponding low magnification BF TEM image (b). 3.3 TEM Defects such as dislocations, inclusions and grain boundaries are common defects, which are observed before and after HT annealing. The plan-view bright-field (BF) TEM observations of polycrystalline diamond TEM samples after HT annealing confirm the presence of these common defects. Figure 4 illustrates this after annealing. Inclusions distort the diamond lattice. This produces lattice strain which is released by inducing dislocations around the inclusions, as depicted in Fig. 4(b). Optical micrographs of TEM samples, recorded between crossed polarizers, provide a detailed view of the non-diamond phases (>5 µm in size), present at connected cracks and less frequently at isolated inclusions. A typical area after annealing is shown in Fig. 5(a) as observed by optical microscopy and in Fig. 5(b) by TEM. After heating the electron diffraction (ED) pattern in Fig. 6(a) provides a direct evidence of the presence of highly disordered polycrystalline graphite within a µm-sized inclusion. The ED pattern is taken from both the diamond matrix and the inclusion. Note the high density of stress-induced dislocations in Fig. 6(d) on both sides of this inclusion. Due to the difference in specific volume between diamond and non-diamond carbon phases, the lattice is strained. In order to lower the strain dislocations are induced, and hence the dislocation density increases significantly. It is however remarkable that isolated µm-sized graphite-like inclusions can exist in the diamond matrix. From the ED pattern in Fig. 7(a) it becomes clear that the material within the junction of two transformed cracks in Fig. 7(b) contains graphite-like material as well. Figure 8 and Fig. 9 show examples of transformed grain boundaries which contain well-organised graphite layers in HT-annealed black diamond. The corresponding ED patterns in Fig. 8(b) and Fig. 9(b) are taken from the diamond grain and the graphitized region. The ED pattern depicts a highly ordered graphite phase revealing the epitaxial orientation relationship between the two carbon phases; i.e. the {0002} plane of graphite is parallel to Fig. 6 Observation of a µm-sized graphite-like inclusion (> 5 µm). ED pattern showing (0002) reflections of polycrystalline graphite (a) and corresponding low magnification BF TEM image (b). Enlargement of the inset in (b) onto dislocations emerging from the inclusion (c), (d).

5 2490 B. Willems et al.: Why does polycrystalline natural diamond turn black after annealing? Fig. 7 ED pattern (a) and TEM image (b) of a transformed crack after annealing at 1300 C for 2 hours. Fig. 8 ED pattern (a) showing (0002) graphite reflections and TEM image (b) of a transformed grain boundary after annealing at 1300 C for 2 hours. one of the {111} planes of diamond. The high degree of ordering of the graphitic planes is illustrated in the high magnification, multi-beam TEM image in Fig. 9(a) using the (0002) graphite reflections. The high degree of ordering of the graphite planes in the grain boundary can be explained by the preferential epitaxial relationship with the diamond phase. The observation of graphite along grain boundaries is in accordance with the findings of Nistor et al. who reported the formation of amorphous carbon and/or graphite layers along grain boundaries in CVD diamond upon annealing [5]. In general the significant increase of dislocation density in annealed samples can be explained by dilatations caused by the non-diamond inclusions (amorphous carbon, graphite) at µm-sized inclusions, cracks and grain boundaries. The lattice mismatch due to the difference in specific volume between diamond and non-diamond phases creates lattice strain which is accommodated by inducing extra dislocations. 4 Conclusion Annealing at temperatures above 1000 C in vacuum renders all polycrystalline natural diamond black. Raman spectroscopy reveals the presence of graphite and amorphous carbon by highlighting a band at 1600 cm 1 in all heated samples. In some cases a peak at 1585 cm 1 indicative of well-crystallized graphite is observed. TEM analysis confirms the internal structural transformation of the microstructure upon HT annealing of polycrystalline natural diamond. TEM observations also complement the results obtained by Raman spectroscopy regarding the coexistence of different carbon phases, diamond and non-diamond phases such as amorphous carbon and graphite. During the transformation non-diamond phases are introduced in cracks, but also at µm-sized isolated inclusions and grain boundaries. On average a significant increase in dislocation density near the defects mentioned above is noted. Acknowledgements The authors wish to thank Samantha Quinn and Bert Nouwen for helpful discussions. The GDI team is thanked for the given support. Furthermore a special acknowledgement goes to Ludo Rossou for preparing the TEM samples. IAP V-1 is acknowledged for financial support. Fig. 9 ED pattern (a) and high magnification, multibeam TEM image (b) with (0002) graphite reflections after annealing.

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