  Honeycomb Na$_2$IrO$_3$ is a prototype spin-orbit Mott insulator and Kitaev
magnet. We report a combined structural and electrical resistivity study of
Na$_2$IrO$_3$ single crystals. Laue back-scattering diffraction indicates
twinning with $\pm 120^\circ$ rotation around the $c^*$-axis while scanning
electron microscopy displays nanothin lines parallel to all three b-axis
orientations of twin domains. Energy dispersive x-ray analysis line-scans
across such domain walls indicate no change of the Ir signal intensity, i.e.
intact honeycomb layers, while the Na intensity is reduced down to $\sim 2/3$
of its original value at the domain walls, implying significant hole doping.
Utilizing focused-ion-beam micro-sectioning, the temperature dependence of the
electrical resistance of individual domain walls is studied. It demonstrates
the tuning through the metal-insulator transition into a correlated-metal
ground state by increasing hole doping.
