Semiconductor Quantum Dot/Polymer Thin Film Based Hydrocarbon Sensor: Characterization of Sensing Properties

Semiconductor CdSe quantum dots (QDs) have been surface functionalized with aromatic ligands for the sensitive and selective detection of hydrocarbons (HCs). By fabricating rough nanostructured films on silicon substrates using the QDs embedded in poly(methyl methacrylate) (PMMA), we have achieved so-far the lowest detection limit for HCs (e.g. ~15 ppm xylenes) within QD based sensing techniques. The introduced rough nanostructure of these films is thought to enhance the local electromagnetic field (EF), aiding in the observed high hydrocarbon (HC) sensitivity. Further, a detailed study of the sensing properties has revealed both QD photoluminescence (PL) enhancement and quenching processes, with the inversion point in the signal change occurring at increased HC levels, and being dependent on the QD surface ligand, QD loading, and polymer matrices. The following events appear to compete with each other, and determine the characteristic HC sensing behavior: 1) PL enhancement due to HCs adsorption on the QD surfaces or via pi-pi stacking interaction between the HCs and the QD surface groups, which dominates at low level HC exposures 2) Rough nanostructure registered local EF enhancement for enhanced photon absorption and emission from QDs, which diminishes with increased HC absorption in the film due to wetting effects at high level HC exposures, leading to significant PL quenching, and 3) Increased collection of background signal reflected from Si substrate under film wetting conditions, which may or may not be visible depending on QD loading and film thickness.