Nanostructure‐Directed Physisorption vs Chemisorption at Semiconductor Interfaces: The Inverse of the HSAB Concept

A concept, complementary to that of hard and soft acid–base interactions (HSAB‐dominant chemisorption) and consistent with dominant physisorption to a semiconductor interface, is presented. We create a matrix of sensitivities and interactions with several basic gases. The concept, based on the reversible interaction of hard‐acid surfaces with soft bases, hard‐base surfaces with soft acids, or vice versa, corresponds 1) to the inverse of the HSAB concept and 2) to the selection of a combination of semiconductor interface and analyte materials, which can be used to direct a physisorbed vs chemisorbed interaction. The technology, implemented on nanopore coated porous silicon micropores, results in the coupling of acid–base chemistry with the depletion or enhancement of majority carriers in an extrinsic semiconductor. Using the inverse‐HSAB (IHSAB) concept, significant and predictable changes in interface sensitivity for a variety of gases can be implemented. Nanostructured metal oxide particle depositions provide selectivity and complement a highly efficient electrical contact to a porous silicon nanopore covered microporous interface. The application of small quantities (much less than a monolayer) of nanostructured metals, metal oxides, and catalysts which focus the physisorbtive and chemisorbtive interactions of the interface, can be made to create a range of notably higher sensitivities for reversible physisorption. This is exemplified by an approach to reversible, sensitive, and selective interface responses. Nanostructured metal oxides developed from electroless gold (Au_xO), tin (SnO₂), copper (Cu_xO), and nickel (NiO) depositions, nanoalumina, and nanotitania are used to demonstrate the IHSAB concept and provide for the detection of gases, including NH₃, PH₃, CO, NO, and H₂S, in an array‐based format to the sub‐ppm level.