High Pressure Synthesis of Ultrasmall Nanodiamonds with Nitrogen Vacancy Centers

C-H terminated nanometer scale diamonds (d = 1 to 15 nm) are synthesized from 1-fluoroadamantane at high pressure (6-8 GPa) and high temperature (500-1500 °C) in a multianvil press. High resolution transmission electron microscopy, X-ray diffraction, Raman, diffuse reflectance Fourier transform infrared, and X-ray absorption spectroscopies demonstrate the excellent crystallinity and atomically flat C-H terminated surfaces of nanodiamonds with (111) and (110) facets. The importance of hydrogen to the synthesis of nanodiamond and its faceting is discussed. Following vacancy generation, annealing and oxidation of the nanodiamonds, optically detected magnetic resonance and electron spin resonance coherence times (T₂ = 0.9 and 2.1 μs) of nitrogen vacancy (NV) centers are measured. The obtained T₂ values are equivalent to the shallow NV centers (depth <10 nm) in bulk diamond crystals and larger nanocrystals prepared by mechanical milling.