Nanostructured metal oxide modification of a porous silicon interface for sensor applications: the question of water interaction, stability, platform diversity and sensitivity, and selectivity

Abstract

Key parameters of a nanostructure modified porous silicon (PSi) template that can affect the development and performance of PSi-based sensors are considered. The importance of pore selection and direct in-situ nitrogen functionalization are emphasized. Metal oxide nanostructured island sites deposited to select, well defined, and reproducible nano-pore walled, micron sized p and n-type silicon pores (0.7–1.5 μ diameter) provide enhanced selectivity. The micron-sized pores facilitate rapid Fickian analyte diffusion to these highly active sites. The metal oxide nanoparticles are trapped by a thin nanopored wall covering preventing their sintering at elevated temperatures. The varying sensitivities of the highly active metal oxide nanostructured sites are well predicted within the recently developed Inverse Hard/Soft Acid/Base (IHSAB) model. Nitrogen functionalization of the nanostructure decorated surfaces provides the conversion of these PSi interfaces from hydrophilic to hydrophobic character. The decrease of water interaction provides greatly enhanced stability. Selectivity can be extended to the measurement of multiple gases using a combination of nanostructured island site determined detection matrices, p and n-type charge carrier variation, time dependent diffusion response, and pore structure influenced sensitivity. The range of variable responses is dominated by the molecular electronic structure of the nanostructured island sites as evaluated using the IHSAB concept. Pulsed mode operation can facilitate low analyte consumption and high analyte selectivity and further provide the ability to assess false positive signals using Fast Fourier Transfer techniques. The modeling of the PSi sensor response with a new Fermi energy distribution –based response isotherm is found to be superior to other isotherms. The rate of sensor response, linearity of measurement, and hysteresis of response are also considered within the framework of the decorated micron sized pore structure.