  Hexagonal boron nitride (h-BN) is a prominent member in the growing family of
two-dimensional materials with potential applications ranging from being an
atomically smooth support for other 2D materials to templating growth of
molecular layers. We have studied the structure of monolayer h-BN grown by
chemical vapour deposition on Ir(111) by low-temperature scanning tunneling
microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art
density functional theory (DFT) calculations. The lattice-mismatch between the
h-BN and Ir(111) surface results in the formation of a moir\'e superstructure
with a periodicity of $\sim$29 \AA\ and a corrugation of $\sim$0.4 \AA. By
measuring the field emission resonances above the h-BN layer, we find a
modulation of the work function within the moir\'e unit cell of $\sim$0.5 eV.
DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental
findings and allow us to relate the change in the work function to the subtle
changes in the interaction between boron and nitrogen atoms and the underlying
substrate atoms within the moir\'e unit cell. Hexagonal boron nitride on
Ir(111) combines weak topographic corrugation with a strong work function
modulation over the moir\'e unit cell. This makes h-BN/Ir(111) a potential
substrate for electronically modulated thin film and hetero-sandwich
structures.
