  The role of image charges in nanoporous semiconductor materials is
investigated within the framework of the effective mass and envelope function
approximations. We show that nanometric air bubbles in these materials can act
as electron-trapping centers. This trapping capability originates from a deep
stabilizing self-polarization potential well induced by the air - semiconductor
dielectric mismatch which can surpass the electroaffinity barrier. The trapping
strength is a function of the pore size and the bulk parameters of the matrix
material. A {\it trapping parameter} characteristic for each semiconductor
material is defined. This parameter provides a simple way to ascertain the
maximum pore size in a given material which is able to induce self-trapping of
excess electrons.
