High-pressure high-temperature annealing of quantum nanodiamonds: in situ synchrotron investigation of nanoscale graphitization and optical quality

Color centers in nanodiamonds are promising solid-state quantum emitters for sensing and quantum technologies. However, nanodiamonds often exhibit significant lattice strain and structural disorder, which degrade their optical properties and limit the resolution of their fine spectral structure at cryogenic temperatures. High-pressure high-temperature (HPHT) annealing has been proposed as an effective route to relax internal strain and improve the crystalline quality of quantum nanodiamonds, although the phase stability of diamond at the nanoscale under these conditions remains poorly constrained. Here we investigate the structural evolution of nanodiamonds during HPHT annealing using in situ synchrotron energy-dispersive X-ray diffraction (EDXRD) in a Paris–Edinburgh press. A dedicated sample assembly combining nanodiamonds, NaCl and Pt was developed to allow accurate pressure–temperature calibration together with simultaneous XRD, radiography and tomography measurements. The diffraction data reveal the onset of the diamond-to-graphite transformation at temperatures higher than typically reported for bulk diamond. At 2 GPa, the appearance of graphite reflections is observed at approximately 1800 K (Fig. 1a), while experiments performed at 4 GPa show graphitization only at higher temperature, around 2120 K (Fig. 1b). These observations highlight the distinct phase stability of diamond at the nanoscale under HPHT conditions. Recovered samples, which had been annealed at temperatures below the graphitization threshold, were further investigated by photoluminescence spectroscopy. Low-temperature measurements reveal a clear improvement in the optical response of the treated nanodiamonds, with resolved components of the silicon-vacancy color-center fine structure becoming observable at 12 K after HPHT annealing. These results demonstrate that controlled HPHT treatments provide a powerful approach to probe nanoscale diamond stability while improving the optical quality of quantum nanodiamonds.