How to Identify, Attribute, and Quantify Triplet Defects in Ensembles of Small Nanoparticles

Nanodiamonds containing negatively charged triplet (having an electron spin S = 1) nitrogen-vacancy (NV^-) centers are an extraordinary room-temperature quantum system, whose electron spins may be polarized and read out optically even in a single nanocrystal. In this Viewpoint we promote a simple but reliable method to identify, attribute, and quantify these triplet defects in a polycrystalline sample using electron paramagnetic resonance (EPR) spectroscopy. The characterization relies on a specific "forbidden" transition ("ΔM = 2"), which appears at about half the central magnetic field and shows a remarkably small anisotropy. In particular, we emphasize that this method is by far not limited to NV^- centers in diamond but could become an important characterization tool for novel triplet defects in various types of nanoparticles.