Angstrom-scale probing of paramagnetic centers location in nanodiamonds by He NMR at low temperatures.

In this article a method to assess the location of paramagnetic centers in nanodiamonds was proposed. The nuclear magnetic relaxation of adsorbed He used as a probe in this method was studied at temperatures of 1.5-4.2 K and magnetic fields of 100-600 mT. A strong influence of the paramagnetic centers of the sample on the He nuclear spin relaxation time T was found. Preplating the nanodiamond surface with adsorbed nitrogen layers allowed us to vary the distance from He nuclei to paramagnetic centers in a controlled way and to determine their location using a simple model. The observed T minima in temperature dependences are well described within the frame of the suggested model and consistent with the concentration of paramagnetic centers determined by electron paramagnetic resonance. The average distance found from the paramagnetic centers to the nanodiamond surface (0.5 ± 0.1 nm) confirms the well-known statement that paramagnetic centers in this type of nanodiamond are located in the carbon shell. The proposed method can be applied to detailed studies of nano-materials at low temperatures.