Modified Metal Oxide Nanostructure Deposited Semiconductor Interfaces: Tuning the Lewis Acidity of Reversibly Interacting Sites

We study the reversible response matrices formed at room temperature as metal oxide nanostructure decorated interfaces are modified through in-situ nitridation. Nanostructured  TiO₂, SnO_x, NiO, and Cu_xO (x= 1,2), in order of decreasing Lewis acidity are deposited to an n-type nanopore coated microporous porous silicon (PS) interface to direct a dominant electron transduction process for reversible chemical sensing in the absence of significant chemical bond formation. The metal oxide sensing sites can be modified to decrease their Lewis acidity in a process of amination (triethylamine) which substitutes nitrogen, providing a weak interaction to form sites of the metal oxynitrides. This trend is characterized by XPS measurements of the shift in the binding energy from that of the oxides and by infrared spectroscopy. The in-situ modification of the metal oxides deposited to the interface changes the reversible interaction with the analytes NH₃ and NO.  Response measurements as a function of NH₃ and NO concentration demonstrate the significant effect which the nitridation and apparent oxynitride formation has on the coupling to the decorated semiconductor majority charge carriers. The relative responses of the nitridated interfaces are not simply the result of creating more basic surface sites. As the substitution of nitrogen for oxygen lowers the Lewis acidity of the metal sites the nitridated metal oxides shift on the acidity scale of effectively greater basic character. However, the analyte basicity or acidity does not change. The responses to the analyte change in distinctly different ways which are predictable within the framework of the IHSAB model. Thus, nitridation significantly changes the response matrix for a given gas and, creating a distinct new family of response matrices which are determined by the acidity/basisity of a given analyte relative to the nanostructures which decorate the PS interface. The response matrix for a given analyte due to the metal oxide decorated PS interface is therefore changed in a substantial and predictable way The results of this modification are well explained by the recently developed IHSAB model of reversible electron transduction. This work is now being extended to the study of interaction with the sulfur based analytes of the sulfides (basic) and thiols (acidic)