  The unique physical, mechanical, chemical, optical, and electronic properties
of hexagonal boron nitride (hBN) make it a promising two-dimensional material
for electronic, optoelectronic, nanophotonic, and quantum devices. Here we
report on the changes in hBN's properties induced by isotopic purification in
both boron and nitrogen. Previous studies on isotopically pure hBN have focused
on purifying the boron isotope concentration in hBN from its natural
concentration (approximately 20 at$\%$ $^{10}$B, 80 at$\%$ $^{11}$B) while
using naturally abundant nitrogen (99.6 at$\%$ $^{14}$N, 0.4 at$\%$ $^{15}$N),
i.e. almost pure $^{14}$N. In this study, we extend the class of
isotopically-purified hBN crystals to $^{15}$N. Crystals in the four
configurations, namely h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N,
and h$^{11}$B$^{15}$N, were grown by the metal flux method using boron and
nitrogen single isotope ($>99\%$) enriched sources, with nickel plus chromium
as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate
the high quality of the monoisotopic hBN crystals with vibrational and optical
properties of the $^{15}$N-purified crystals at the state of the art of
currently available $^{14}$N-purified hBN. The growth of high-quality
h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N, and h$^{11}$B$^{15}$N
opens exciting perspectives for thermal conductivity control in heat
management, as well as for advanced functionalities in quantum technologies.
