Size-dependent photoluminescence of zinc oxide quantum dots through organosilane functionalization

Organosilane grafting on the surface of ZnO nanoparticles is a possible means of fabricating hybrid nanomaterials with tuned optical and physico-chemical properties. We report here the synthesis, morpho-structural characterization and optical properties of hybrid ZnO quantum dots (QDs) grafted with variable amounts of the surfactant 3-(trimethoxysilyl) propylmethacrylate (MPS) (2, 5 and 10% molar ratios of Si/Zn). This organosilane surfactant was chosen to evaluate its ability to both prevent the agglomeration of the nanoparticles and to tune the optical properties of the resulting hybrid nanomaterials. The grafted ZnO QDs were prepared using a modified precipitation method and were characterized by X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared spectrometry and thermogravimetric analysis; their optical properties were studied by UV-visible spectrometry. Unmodified ZnO and MPS-grafted ZnO QDs showed optical transmittance between 85 and 90% and low reflectance in the visible domain, whereas a significant blue shift of the photoemission bands was observed from 578 nm in unmodified ZnO to 546 nm in ZnO grafted with 10% MPS. This shift is associated with the reduction in size of the ZnO QDs with increasing amounts of MPS. A decrease in the band gap energies from 3.494 eV for unmodified ZnO to 3.377 eV for ZnO-MPS 10% was detected and gives a new insight into the relationship between the reduction in nanoparticle size promoted by an organic surfactant with electron injection into the ZnO bands and the value of band gap energy.