  The spatial variation of electronic states was imaged in the lightly doped
Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2} using scanning tunneling microscopy
/ spectroscopy (STM/STS). We observed nano-scale domains with a high local
density of states within an insulating background. The observed domains have a
characteristic length scale of 2 nm (~4-5a, a:lattice constant) with preferred
orientations along the tetragonal [100] direction. We argue that such spatially
inhomogeneous electronic states are inherent to slightly doped Mott insulators
and play an important role for the insulator to metal transition.
