Synchrotron-Assisted HPHT Annealing Of Quantum Nanodiamonds: In Situ Control Of Phase Stability

Group-IV color centers in nanodiamond, such as silicon-vacancy (SiV) defects, are promising quantum emitters for sensing and photonics due to their bright and narrow zero-phonon line (ZPL) emission, representing up to 80% of their total luminescence [1–3]. However, in as-grown CVD nanodiamonds, residual lattice strain induces significant spectral broadening, preventing access to the fine electronic structure of SiV centers even at cryogenic temperatures. To mitigate this effect, we carried out a controlled high-pressure and high-temperature (HPHT) annealing process using the Paris–Edinburgh press, performed in situ at the PSICHE beamline of Synchrotron SOLEIL [4]. The synchrotron X-ray diffraction and tomography measurements were crucial for the precise calibration of pressure and temperature conditions within the assembly, enabling us to finely tune the HPHT parameters to promote strain relaxation while avoiding the onset of graphitization. These in situ synchrotron investigations provided real-time insight into nanodiamond phase stability and strain-release mechanisms, which were confirmed ex situ by optical measurements, leading to the first observation of resolved fine-structure transitions in SiV centers after HPHT treatment. Importantly, the calibrated HPHT setup can now be reliably operated off-beam, enabling systematic post-annealing studies on different types of nanodiamonds and color centers. This methodology establishes a robust experimental reference for future off-beam HPHT annealing treatments and serves as a benchmark for the broader quantum diamond research community. REFERENCES 1. T. Muller et al., Nat. Commun. 5, 3328 (2014). 2. M. De Feudis et al., Adv. Mater. Interfaces 7, 1901408 (2019). 3. B. Vindolet et al., Phys. Rev. B 106, 214109 (2022). 4. L. Henry et al., J. Synchrotron Rad. 29, 853-861 (2022).