  Graphene nanoribbons (GNRs) with widths of a few nanometres are promising
candidates for future nano-electronic applications due to their structurally
tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability.
However, the direct growth of micrometre-long GNRs on insulating substrates,
which is essential for the fabrication of nano-electronic devices, remains an
immense challenge. Here, we report the epitaxial growth of GNRs on an
insulating hexagonal boron nitride (h-BN) substrate through
nanoparticle-catalysed chemical vapor deposition (CVD). Ultra-narrow GNRs with
lengths of up to 10 {\mu}m are synthesized. Remarkably, the as-grown GNRs are
crystallographically aligned with the h-BN substrate, forming one-dimensional
(1D) moir\'e superlattices. Scanning tunnelling microscopy reveals an average
width of 2 nm and a typical bandgap of ~1 eV for similar GNRs grown on
conducting graphite substrates. Fully atomistic computational simulations
support the experimental results and reveal a competition between the formation
of GNRs and carbon nanotubes (CNTs) during the nucleation stage, and van der
Waals sliding of the GNRs on the h-BN substrate throughout the growth stage.
Our study provides a scalable, single-step method for growing micrometre-long
narrow GNRs on insulating substrates, thus opening a route to explore the
performance of high-quality GNR devices and the fundamental physics of 1D
moir\'e superlattices.
