  Fluorescent nanoparticles are widely utilized in a large range of nanoscale
imaging and sensing applications. While ultra-small nanoparticles (size <10 nm)
are highly desirable, at this size range their photostability can be
compromised due to effects such as intensity fluctuation and spectral diffusion
caused by interaction with surface states. In this letter, we demonstrate a
facile, bottom-up technique for the fabrication of sub-10-nm hBN nanoparticles
hosting photostable bright emitters via a catalyst-free hydrothermal reaction
between boric acid and melamine. We also implement a simple stabilization
protocol that significantly reduces intensity fluctuation by ~85% and narrows
the emission linewidth by ~14% by employing a common sol-gel silica coating
process. Our study advances a promising strategy for the scalable, bottom-up
synthesis of high-quality quantum emitters in hBN nanoparticles.
