A nanostructured porous silicon near insulator becomes either a p- or an n-type semiconductor upon gas adsorption

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).