Donor-acceptor recombination emission in hydrogen-terminated nanodiamond: Novel single-photon source for room-temperature quantum photonics

In fluorescence spectra of nanodiamonds (NDs) synthesized at high pressure from adamantane and other organic compounds, very narrow (~1 nm) lines of unknown origin are observed in a wide spectroscopic range from ~500 to 800 nm. Here, we propose and experimentally substantiate the hypothesis that these mysterious lines arise from radiative recombination of donor-acceptor pairs (DAPs). To confirm our hypothesis, we study the fluorescence spectra of undoped and nitrogen-doped NDs of different sizes, before and after thermal oxidation of their surface. The results obtained with a high degree of confidence allowed us to conclude that the DAPs are formed through the interaction of donor-like substitutional nitrogen present in the diamond lattice, and a 2D layer of acceptors resulting from the transfer doping effect on the surface of hydrogen-terminated NDs. A specific behavior of the DAP-induced lines was discovered in the temperature range of 100-10 K: their energy increases and most lines are split into 2 or more components with decreasing temperature. It is shown that the majority of the studied DAP emitters are sources of single photons, with an emission rate of up to>1 million counts/s at room temperature, which significantly surpasses that of nitrogen-vacancy and silicon-vacancy centers under the same detection conditions. Despite an observed temporal instability in the emission, the DAP emitters of H-terminated NDs represent a powerful room-temperature single-photon source for quantum optical technologies.