Nanodiamonds (NDs) represent a novel nanomaterial applicable from biomedicine to
spintronics. Here we study ability of air annealing to further decrease the
typical 5 nm NDs produced by detonation synthesis. We use atomic force
microscopy (AFM) with sub-nm resolution to directly measure individual
detonation nanodiamonds (DNDs) on a flat Si substrate. By means of particle
analysis we obtain their accurate and statistically relevant size distributions.
Using this approach, we characterize evolution of the size distribution as a
function of time and annealing temperature: i) at constant time (25 min) with
changing temperature (480, 490, 500°C) and ii) at constant temperature
(490°C) with changing time (10, 25, 50 min). We show that the mean size
of DNDs can be controllably reduced from 4.5 nm to 1.8 nm without noticeable
particle loss and down to 1.3 nm with 36% yield. By air annealing the size
distribution changes from Gaussian to lognormal with a steep edge around 1 nm,
indicating instability of DNDs below 1 nm.