Controlled HPHT annealing of SiV-doped nanodiamonds at SOLEIL synchrotron: structural and optical investigations

Diamonds and nanodiamonds (NDs) containing quantum color centers, such as nitrogen-vacancy (NV) centers, have been widely studied in recent years for their promising applications across various fields, including cryptography and telecommunications, medical sciences, and sensing. In the context of sensing, recent studies on color centers such as silicon-vacancy (SiV) and germanium-vacancy (GeV) centers have shown great potential for measuring physical quantities like magnetic fields, temperature, and strain. This is due to their intense zero-phonon line (ZPL) optical signal, about 80% of their total luminescence, which is characteristic of group-IV color centers (G4V) [1]. In our recent papers, we demonstrated that as-grown CVD NDs containing SiV and GeV centers, featuring luminescent ZPLs and excellent photostability at room temperature, can function effectively as nanosensors of high stress and pressure conditions (up to 180 GPa) [2, 3]. To advance our research, in this work we investigate the optical properties of SiV centers in as-grown NDs at cryogenic temperatures. For SiV centers, which have spin 1/2, the fine structure should become accessible through the Jahn-Teller effect, which lifts the degeneracy at cryogenic temperatures (< 20 K). However, experiments on our as-grown NDs reveal significant spectral broadening even at around 9 K, caused by lattice strain, which prevents access to the optically addressable electronic spin states of the SiV center. To overcome this limitation, we propose an original post-treatment involving high-pressure and high-temperature (HPHT) annealing using the Paris–Edinburgh press. In fact, the gradual increase of temperature and pressure allows for control over the undesired diamond-to-graphite phase transition. HPHT annealing experiments were conducted at the PSICHE beamline of Synchrotron SOLEIL [4], where X-ray diffraction and tomography analyses enabled precise monitoring of P,T parameters and made it possible for the first time to access single fine-structure transitions of the in such SiV-NDs. References [1] C. Bradac, et al., Nat. Commun. 10, 1 (2019). [2] B. Vindolet, et al., Phys. Rev. B 106, 214109 (2022). [3] M. De Feudis, et al., Adv. Mater. Interfaces 7, 1901408 (2019). [4] L. Henry et al., J. Synchrotron Rad. 29 (2022).