2, Adamas Nanotechnologies, Raleigh, North Carolina, United States
Nitrogen-vacancy centers (N-V) in nanocrystalline diamonds have been studied extensively for their interesting photoluminescence properties. Negatively-charged N-V centers (N-V-) can emit visible light that is readily detectable, even at room temperature. Observation of N-V centers, particularly the optically active N-V-, has becoming routine using optical fluorescence microscopy. Combined with AFM, the locations of the N-V- as well as the particle size can be measured. However, the questions remain as to what the effect of the host structure and the surface environment to the activation of the N-V centers. To answer this question, a technique that is capable of detecting both active and non-active N-V centers as well as the surrounding host structure is needed.
Here we demonstrate that we can measure the signals arising from the N-V centers in individual nanoparticles. The measurement is based on high-energy resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Using EELS, we can measure the electronic transitions within the nanodiamond bandgap to detect N-V- and N-V0 (and others). Such STEM can operate at low voltages (40-100kV) with both high spatial (~0.1 nm) and high-energy (16 meV) resolutions. Such capability allows us to observe the inter-band transition as well as the atomic structure of the particles at the same time.
Our initial result on a 70 nm nanodiamond confirms that we can indeed detect signals at 1.95 and 2.16 eV transitions in the particle. Interestingly, we found that not all particles exhibit these transition states. The reason, we then confirmed is due to the lack of nitrogen in the particle. These observations highlight the importance of dopant control for nanodiamond particles.