Nitrogen is the most common impurity in diamond, incorporated into the crystal lattice as atomically dispersed entities or aggregates to form C centers (isolated substitutional nitrogen atoms), A centers (two nearest-neighbor substitutional nitrogen atoms), or B centers (four substitutional nitrogen atoms surrounding a vacancy). When diamond is exposed to high-energy particles such as electrons, neutrons, protons, helium ions, or gamma rays, vacancies can be created by the radiation damage. Upon thermal annealing at 600 °C or above, the vacancies are mobile and subsequently trapped by nitrogen atoms to form different color centers, depending on the type of the starting material used in the production.

The negatively charged nitrogen-vacancy center, NV−, is a point defect consisting of a substitutional nitrogen atom adjacent to a carbon atom vacancy with C3v symmetry. It exhibits a zero-phonon line (ZPL) at 638 nm, accompanied with a broad phonon sideband at 560 nm for the 3A → 3E transition. When excited by green yellow light, the center emits far-red fluorescence at 690 nm with a near-unity quantum yield. Moreover, the fluorescence is perfectly stable, showing no sign of photoblinking or photobleaching even under continuous high-power laser excitation at room temperature. Due to this outstanding feature, the NV− center has been employed as a single-photon source for quantum information application.

Apart from the negatively charged state, the NV center can also exist in the neutral form, i.e. NV0. Both centers are found in type Ib diamond, typically synthesized by high-pressure-high-temperature (HPHT) methods and containing about 100 ppm of atomically isolated nitrogen as impurity. The NV0 center is characterized by its distinct ZPL at 575 nm, which can be readily distinguished from its negative counterpart.

The third type of color center that has been produced in NDs at some higher nitrogen concentrations is the H3 center. The center consists of a nitrogen-vacancy-nitrogen complex, N-V-N, originating from the A aggregate in type Ia diamond, which typically contains 1000 ppm of nitrogen as impurity. When excited by blue light at its maximum absorption (470 nm), the H3 center emits green fluorescence at 531 nm with a fluorescence quantum yield close to 1. Similar to NV−, the center emits exceptionally stable fluorescence without photobleaching or blinking in the study of single H3 centers.

Lastly, ND can also emit blue fluorescence from the N3 center. This structural defect is composed of three nitrogen atoms (known as the B aggregate) surrounding a vacancy in type Ia diamond. These centers (with a ZPL at 415 nm) are often produced concurrently with the H3 centers in natural diamond. The NDs containing high-density ensembles of these fluorescent color centers (either NV−, NV0, H3, or N3) are called fluorescent nanodiamonds (FNDs).

Applications of fluorescent nanodiamonds include:
  • Single particle tracking
  • Biomolecular labeling
  • Cellular imaging
  • Superresolution imaging
  • In vivo imaging
  • Time-gated fluorescence imaging
  • Stem cell tracking
  • Tumor targeting
  • Cathodoluminescence imaging
  • Nanoscale thermometry
  • Nanoscale magnetometry 

Further reading:
Y. Y. Hui and H.-C. Chang, “Recent developments and applications of nanodiamonds as versatile bioimaging agents,” J. Chin. Chem. Soc. 61, 67–76 (2014).