N-/Si-co-doped nanocrystalline diamonds as highly sensitive luminescent thermometers

Diamonds can host a variety of luminescent point defects, such as nitrogen vacancies and silicon vacancies, making them remarkably promising materials for luminescence nanothermometry. They also exhibit desirable characteristics, including low cytotoxicity, good biocompatibility and the benefits of carbon chemistry for tuneable surface functionalization with target molecules. In this work, N-/Si co-doped nanocrystalline diamonds were synthesized via microwave plasma-enhanced chemical vapor deposition under varying methane flow and temperature conditions, aiming for maximum photoluminescence intensity. In-depth analysis of the luminescence characteristics of NV^- and SiV^- emitting point defects with temperature, based on both experiments and ab initio calculations, suggested the existence of thermal coupling between their states. This coupling enabled ratiometric approaches to be exploited for more sensitive luminescent thermometry. A maximum thermal relative sensitivity of 5.5% K^-1 was achieved. This is remarkable for in vitro temperature measurements of biological tissues.