  Nanodiamonds have emerged as promising materials for quantum computing,
biolabeling, and sensing due to their ability to host color centers with
remarkable photostability and long spin-coherence times at room temperature.
Recently, a bottom-up, high-pressure, high-temperature (HPHT) approach was
demonstrated for growing nanodiamonds with color centers from amorphous carbon
precursors in a laser-heated diamond anvil cell (LH-DAC) that was supported by
a near-hydrostatic noble gas pressure medium. However, a detailed understanding
of the photothermal heating and its effect on diamond growth, including the
phase conversion conditions and the temperature-dependence of color center
formation, has not been reported. In this work, we measure blackbody radiation
during LH-DAC synthesis of nanodiamond from carbon aerogel to examine these
temperature-dependent effects. Blackbody temperature measurements suggest that
nanodiamond growth can occur at 16.3 GPa and 1800 K. We use Mie theory and
analytical heat transport to develop a predictive photothermal heating model.
This model demonstrates that melting the noble gas pressure medium during laser
heating decreases the local thermal conductivity to drive a high spatial
resolution of phase conversion to diamond. Finally, we observe a
temperature-dependent formation of nitrogen vacancy centers and interpret this
phenomenon in the context of HPHT carbon vacancy diffusion using CB{\Omega}
theory.
