Enhanced NV Fluorescence in Flake Nanodiamond Revealed by Correlative Photoluminescence and Transmission Electron Microscopy

Nitrogen-vacancy (NV) centers in fluorescent nanodiamonds (FND) have attracted immense interest due to their room-temperature emission, exceptionally high photo- and chemical stability, and excellent biocompatibility. Such outstanding properties have gifted NV centers broad application prospects in the fields like quantum sensing and biomedical diagnostic applications. However, the lack of precise understanding of the factors affecting NV optical properties (e.g., NV distribution, particle morphology, and surface properties of particles) limits its performance control and optimization. Therefore, an in-depth exploration of these factors is essential to further explore the potential of NV centers in FND.<br/>Unlike most other fluorescent nanoparticles, FND samples typically contain a wide range of particle size and shape distributions. And, it has been reported that the same size (and same fabrication process) individual FND particles with NV centers can exhibit a 4-5-fold variation in photoluminescent (PL) intensity. The above characteristics bring a challenge to our study, namely, it is not feasible to only focus on particle structure information or to only measure the particle shape distribution.<br/>To overcome this, a new correlative photoluminescence-transmission electron microscopy (PL-TEM) method has been developed. The method combines confocal PL microscopy with TEM imaging of the same sample region, offering direct and accurate optical properties and nanostructure correlation at the individual particle level while measuring statistically significant particle numbers.<br/>Here, we perform a direct correlation between PL images and TEM montage maps of the same region of the sample. Morphological (size and shape) data and PL intensity/ lifetime of individual particles in the region were measured by bright-field TEM and confocal PL microscopy, respectively. High-resolution TEM, and high-energy resolution electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) were used to measure the surface properties of the FND particles. The FND sample used here is nominally 100 nm size FND (with NV) particles produced by ionization irradiation and high-temperature annealing, with two different surface treatments (acid treatment, molten salt treatment) for comparison.<br/>PL-TEM correlation results also demonstrate the fluctuation of NV brightness in particles of the same size (volume). However, the particle morphological dependency of NV fluorescence is revealed after adding the three-dimensional shape of the particles into consideration. We found that flake-shape particles produced enhanced fluorescence: after normalizing the volume of FND particles and performing statistical analysis, the results showed that the relative brightness of thin, flake-shaped FND particles is several times higher than that of the thicker, three-dimensional-shaped particles. Furthermore, by comparing high-resolution measurements on particle surface with optical measurements, we found that this trend is robust to different surface oxidation treatments. Our theoretical calculations also support this finding. Such results offer new possibilities for fluorescence-optimized sensing applications of FND by controlling the particle thickness and shape.