  We have performed a detailed high-resolution electron spectroscopic
investigation of NiS$_2$ and related Se-substituted compounds
NiS$_{2-x}$Se$_x$, which are known to be gapped insulators in the bulk at all
temperatures. A large spectral weight at the Fermi energy of the room
temperature spectrum, in conjunction with the extreme surface sensitivity of
the experimental probe, however, suggests that the surface layer is metallic at
300 K. Interestingly, the evolution of the spectral function with decreasing
temperature is characterized by a continuous depletion of the single-particle
spectral weight at the Fermi energy and the development of a gap-like structure
below a characteristic temperature, providing evidence for a metal-insulator
crossover behavior at the surfaces of NiS$_2$ and of related compounds. These
results provide a consistent description of the unusual transport properties
observed in these systems.
