Dependence of Hydrocarbon Sensitivity on the Distance of Linked Phenyl Group to CdSe Quantum Dots Surfaces

Abstract

The hydrocarbon sensing properties of CdSe semiconductor quantum dots (QDs) tailored with benzoic, phenylacetic, 4-phenylbutanoic, or 6-phenylhexanoic acids as a surface enhancement agent have been studied to determine their dependence on the phenyl group distance from the QD surface. QDs of roughly the same size were used to attach each of the mentioned surface agents with similar surface coverages, as determined by nuclear magnetic resonance technique, followed by drop-coating and drying of the QD solutions onto porous anodic aluminum oxide substrates for sensing tests. Results indicated that normalized photoluminescence (PL) enhancement did not vary noticeably for QDs with longer chained phenyl groups compared to those with shorter length groups upon film exposure to xylenes vapors over the range of 15 to 9400 ppm in a balance of air. The quantum yield of CdSe QDs with the 6-phenylhexanoic acid surface ligand has a higher value, 29.4%, compared to those with benzoic acid, 4.5%. Moreover, the PL sensitivity toward xylenes exposure for QDs with 6-phenylhexanoic acid versus those with the surface-bound benzoic acid was about 8 times and 16 times greater at xylenes concentrations below 2500 ppm and above 4500 ppm, respectively. These results suggest that the QDs with longer length surface-enhancing ligands are more promising for this type of hydrocarbon sensing applications.