2020 Volume 61 Issue 9 Pages 1707-1710
We disperse InP nanocrystals into nano-polycrystalline diamond during the direct conversion from graphite as a possible technique to control its solid-state properties. We synthesize diamond, using the high-pressure, high-temperature technique, which encapsulates an InP alloy in close contact with the graphite starting material. X-ray diffraction of the synthesized sample suggests the formation of polycrystalline diamond where the mixed crystals contain InP. Cross-sectional transmission electron microscopy shows the existence of InP nanocrystals with sizes up to approximately 100 nm. The existence of the InP elements can promote the formation of larger crystalline diamond grains arising from liquid sintering, which show a larger grain size over 400 nm compared with the regions without InP, where a grain size of approximately 50 nm is observed.
Nanoparticle dispersion is a method that enables the strategic tuning of the solid-state properties, such as mechanical, thermal, optical, or dielectric, of the host matrix.1–4) Using high pressure high temperature (HPHT) techniques, diamond can be synthesized by a direct conversion from graphite. Diamond is a material that possesses unique properties.5,6) In addition to its attractive appearance, hardness, and thermal conductivity, diamond exhibits characteristic electrical, optical, and spintronic properties under appropriate conditions and/or treatments,7–22) which show great potential for the realization of future materials. Nano-polycrystalline diamond (NPD) has been synthesized using HPHT conditions of over 10 GPa and 2000°C by the Ehime University group.23) The characterizations have mainly been focused on the structural and mechanical properties, as well as on its unique ultra-hard nature.24–30) Recently, we observed that the as-synthesized NPD exhibits semiconducting properties, especially at high temperatures.31) The discovery of a pathway to control the solid-state properties of HPHT diamond is expected to extend the potential for this material. InP is a III–V compound that may control the electric and dielectric properties of diamond that consists of group-IV carbon.3,22) In this report, we attempt to synthesize nano-polycrystalline diamond from graphite encapsulated with InP as a nanoscale particle.
A Kawai-type multi-anvil device operated on a 60 MN press was employed for the synthesis of NPD by the direct conversion of graphite under high pressure and high temperature.25) The synthesis was carried out by encapsulating the InP alloy that was in close contact with the graphite starting material, with the expectation that the constituent P, a possible n-type dopant with a relatively small atomic number, would diffuse into the graphite/diamond during the synthesis. The sample structure introduced into the HPHT cell is shown in Fig. 1(a). The graphite was cylindrical with a diameter of 3 mm and a height of 3.2 mm. A slice of InP was inserted into the graphite, as shown in Fig. 1(a). Since InP becomes a liquid under high temperature, it was enclosed in the graphite cell so that the liquid InP could not leak out. The HPHT synthesis was carried out at 15 GPa and 2300°C for 20 minutes. The synthesized sample fragments were characterized using micro-beam X-ray diffraction (XRD) with a spot size of approximately 10 µm and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS). Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with EDS were carried out for the thin film sample processed by a focused ion beam (FIB). Atomic force microscopy (AFM) was carried out for the interior of the sample which was exposed by cross-section polishing. The microscopic structural characteristics were also investigated by Raman spectroscopy with a spot size of approximately 1 mm.
(a) Schematic illustration of the sample structure of the starting materials introduced in the HPHT cell. (b) Microscopy observation for a fragment of the sample. The numbered dots indicate the measurement points for XRD. (c) Wide-range X-ray θ-2θ scan on a logarithmic scale at the points indicated in (b). The red-colored solid lines show diffraction peaks related to diamond, and the blue dashed lines show diffraction peaks related to the InP crystals. The curves are obtained at the numbered spots indicated in (b).
Figure 1(b) shows the microscopy observation for a fragment of the sample. The spot size was 100 µm. We were able to observe the formation of transparent diamond, which is indicated by the dashed line in the figure. The numbered dots indicate the measurement points for XRD that was carried out as follows. Figure 1(c) shows a wide-range X-ray θ-2θ scan for the points indicated in Fig. 1(b) on a logarithmic scale. The red-colored dash-dotted lines show the diffraction peaks related to diamond, and the blue dashed lines show the diffraction peaks related to the InP crystals. We observed peaks that originated from diamond at certain points, which indicated the existence of diamond at those points. At point 1, many peaks were observed and several of these were broad but identified to be close to those of InP. The results suggest that these areas were crystallized under the influence of the InP elements. We observed intensity drops at the higher angle side of the (111) and (022) peaks at point 1. These drops seemed to arise from the interference induced by the surrounding materials. Notably, at point 2, the observed peaks originated from diamond and the other peaks were marginal. Consequently, the area was proposed to consist of high purity diamond. At point 2, the diffraction peaks were in good agreement with the known peak positions of the diamond. However, especially at point 1, the peak position shifted to the lower angle side. This may imply a lattice expansion of the crystal induced by the effect of In and P on the region.
The surface of the synthesized diamond sample fragments may become contaminated by the introduced InP species, as well as the elements constituting the HPHT cell.25) Hence, we exposed the interior of the fragments by applying cross-section polishing and investigated the exposed polished internal surface. Figure 2(a) shows the SEM image of the sample fragments. The EDS elemental maps for the sample are shown in Fig. 2(b) for C, (c) for In, and (d) for P. A carbon intensity was recognized throughout the sample, which originated from the existence of diamond. We also observed an In and P intensity over a large part of the sample that corresponded to the polished internal surface in the image. Hence, we confirmed the co-existence of a pure-diamond region and diamond containing In and P elements at the polished part in the image.
(a) Cross-sectional SEM image of the sample fragments. The interior of the sample was exposed by cross-section polishing and the exposed area is indicated with red-colored dashed lines. The measurement area for the following EDS measurements are indicated by the rectangle. EDS elemental maps for (b) C, (c) In, and (d) P.
Figure 3(a) shows the results of cross-sectional TEM observation for the sample in the area containing In and P elements. The diffraction pattern for the indicated area is shown in Fig. 3(b). In the diffraction pattern, Debye-Scherrer rings that originated from the polycrystalline diamond were observed. In addition, we observed diffraction spots that were identified as InP (111), as shown in the figure. These results suggested the co-existence of diamond and InP, which supported the XRD data shown in Fig. 1(c). As shown in the figure, we observed P and In intensity within the diamond, as shown in Fig. 3(e) and (f). The P and In intensity was observed as grains having diameters between tens to several hundreds of nanometers, whose locations corresponded to each other. Further, the locations of the P and In grains also agreed with the dark spots in the bright-field TEM image shown in Fig. 3(c) and the elemental map of C shown in Fig. 3(d). These results suggested that the InP nanocrystals were uniformly dispersed in diamond.
TEM, ED, STEM, and EDS elemental maps for the sample having diamond and InP elements. (a) Cross-sectional TEM image, the green circle indicates the area where the electron diffraction pattern shown in (b) was taken. (c) Cross-sectional STEM observation and its EDS elemental mapping for (d) C, (e) P, and (f) In.
The fragment after cross-sectional polishing was also investigated by AFM. Figure 4(a) shows the AFM observation for the sample at the regions without In and P. Specifically, we observed a grained structure in the image. The grain size was approximately 50 nm in diameter, which was close to that of NPD reported previously.32) Figure 4(b) shows the AFM observation in the region containing In and P. In contrast to the image shown in Fig. 4(a), a larger grain size of 400 nm, with a morphology that seemed to originate from the coalesced grains of smaller sizes, was observed. This feature was proposed to be induced by the effect of the existence of InP during the synthesis. These elements would promote the formation of diamond with the larger grain size owing to the liquid-assisted sintering.
AFM observation for the sample after cross-section polishing at the regions (a) without and (b) with In and P elements.
As in the Raman spectra shown in Fig. 5, we observed many broad peaks, which included a sharp peak related to the D band at 1333 cm−1 and a graphite-related G band at 1550 cm−1, from the area without InP.33,34) However, the area with InP showed simple spectra where a sole D-band peak without any observable surrounding peaks, similar to the spectra from the single crystalline diamond, was observed.34) There was no deviation for the peak positions and the widths between the spectra. Hence, the area without InP contained the residual precursor graphite. The area with InP had a similar feature to pure crystalline diamond, which suggested that the existence of InP promoted the synthesis of diamond. These findings were in agreement with the result from AFM, where the area with InP showed a larger grain size than the area without InP.
Raman spectra of the sample taken at the area without and near InP. The inset shows enlarged spectra around the D-band peak.
We show the dispersive addition of InP nanocrystals into nano-polycrystalline diamond during the direct conversion from graphite. We synthesized diamond using the high pressure and high temperature technique, and encapsulated InP alloy in close contact with the graphite starting material. X-ray diffraction of the synthesized sample suggested the formation of a polycrystalline diamond with the mixed crystals containing InP. From the observation of cross-sectional transmission electron microscopy, we observed the existence of nanometer-scale In and P within the sample, in correspondence with their locations. The existence of these elements promoted the formation of larger crystalline diamond grains owing to liquid sintering, which showed a larger grain size of over 400 nm compared with the regions without In and P having a grain size of approximately 50 nm.
This study was partly supported by the Research Unit project and the Joint Usage/Research Center PRIUS by Ehime University, and KAKENHI (Nos. 17K18883 and 15K13957) from the JSPS.
