Fig. 1. Schematic of the synthesis of nanodiamonds via the two-step HPHT method. 1. cylindrical sample of the precursor naphthalene; 2. graphite container; 3. ZrO₂ sleeve; 4. octahedron pressure medium of MgO.

Fig. 1. (a) XRD patterns and (b) Raman spectra of the naphthalene precursor and samples NX (X = 1–4) prepared using different carbonization time at 11.5 GPa and 1700 °C.

Fig. 2. (a)–(d) Grain size distributions of samples NX (X = 1–4) prepared using different carbonization time. The red curves show fitting of the lognormal distribution function. Inset shows the corresponding SEM images.

Fig. 3. Average grain size of samples NX (X = 1–4) as a function of carbonization time.

Fig. 4. (a) TEM image of sample N3. Inset shows HRTEM image of a single diamond particle from the white rectangle area marked in (a). (b) Selected area electron diffraction pattern of sample N3.

Fig. 5. (a) XRD patterns, (b) Raman spectra, and (c)–(f) SEM images of samples NX (X = 5–8) obtained using different carbonization time at 11.5 GPa and 700 °C.

Fig. 6. Raman spectra of diamond obtained employing (a) anthracene and (b) acridine as precursors in the two-step HPHT method.