  We have studied the chemical potential shift in La$_{2-x}$Sr$_x$NiO$_4$ and
the charge ordering transition in La$_{1.67}$Sr$_{0.33}$NiO$_4$ by
photoemission spectroscopy. The result shows a large ($\sim$ 1 eV/hole)
downward shift of the chemical potential with hole doping in the high-doping
regime ($\delta \gtrsim$ 0.33) while the shift is suppressed in the low-doping
regime ($\delta \lesssim$ 0.33). This suppression is attributed to a
segregation of doped holes on a microscopic scale when the hole concentration
is lower than $\delta \simeq 1/3$. In the $\delta = 1/3$ sample, the
photoemission intensity at the chemical potential vanishes below the charge
ordering transition temperature $T_{\rm CO}=$ 240 K.

Strongly Correlated Electrons;Materials Science