  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.
