  Hexagonal boron nitride (BN), one of the very few layered insulators, plays a
crucial role in 2D materials research. In particular, BN grown with a high
pressure technique has proven to be an excellent substrate material for
graphene and related 2D materials, but at the same time very hard to replace.
Here we report on a method of growth at atmospheric pressure as a true
alternative for producing BN for high quality graphene/BN heterostructures. The
process is not only more scalable, but also allows to grow isotopically
purified BN crystals. We employ Raman spectroscopy, cathodoluminescence, and
electronic transport measurements to show the high-quality of such monoisotopic
BN and its potential for graphene-based heterostructures. The excellent
electronic performance of our heterostructures is demonstrated by well
developed fractional quantum Hall states, ballistic transport over distances
around $10\,\mathrm{\mu m}$ at low temperatures and electron-phonon scattering
limited transport at room temperature.
