Mesoporous silicon exchanges electrons extensively and reversibly with adsorbed species, e.g., NO2 and NH3. Such an exchange has been reported for systems that maintain their original p or n character. In the present case, a solid, which is a p-type semiconductor in bulk, converts to a "near insulator" when nanostructured. The near insulator may adopt either a p or an n nature with electron exchange, depending on the type of adsorbed molecule (see Figure).