Deep Infiltration of Emissive Polymers into Mesoporous Silicon Microcavities: Nanoscale Confinement and Advanced Vapor Sensing

We present the study of a nanohybrid composite with superior sensing performance consisting of an emissive sensory polymer infiltrated into a mesoporous Si one-dimensional (1D) photonic crystal with a microcavity (MC). It was found that the critical condition for deep polymer infiltration is the presence of an initial low porosity layer (porosity of 45%) in contrast to shallow infiltration governed by an initial high porosity layer (porosity of 58%). This results in a narrow fluorescence peak (due to deep infiltration) or a spectral “hole” in the fluorescence band (shallow infiltration). Such a unique effect is in agreement with the model based on capillary filling and confirmed by secondary ion mass spectrometry (SIMS) data analyzing the profile of polymer infiltration along the MC depth. In the case of deep infiltration, the characteristic filling length exceeds 2 μm, allowing the polymer to impregnate the MC layer. The infiltrated polymer is spatially confined and exists as quasi- isolated chains without pore clogging as can be concluded from the “blue” spectral shift of up to 10 nm as compared with a nonspatially confined film. Polymer isolation over a large surface area along with sufficient pore openings makes this porous Si (PSi) MC/polymer nanohybrid an ideal material for gas sensing applications. This is due to the high sensitivity in conjunction with a strong fluorescence signal which is not possible with solid polymer films or bare PSi. These results are confirmed by direct observation of higher sensitivity, enhanced specificity, and partial recovery of the optical signal for the nanohybrid composite upon exposure to trinitrotoluene vapors as compared with a conventional polymer film deposited on a flat substrate.