Optical properties of the Folic Acid/APTMS/TiO$_{2}$ nanosystems*

Our photoluminescence experiments on folic acid (FA) conjugated nanoparticles of ${\rm TiO_{2}}$, ${\rm CeO_{2}}$ and ${\rm SiO_{2}}$ show great promise for a variety of optoelectronic applications for these materials, in particular in the field of modern molecular photoelectronic devices, since they demonstrate a dramatic increase of the photoemission intensity at wavelengths between 500 to 700 nm when the nanoparticles are coated with the 3-aminopropyltrimethoxylane (APTMS) linker/spacer molecule. We report here results of accompanying time-dependent density- functional theory studies of the FA/APTMS/${\rm TiO_{2}}$ nanosystems by using the B3LYP exchange-correlation potential. We demonstrate that the large increase of the photoemission is due to enhanced optical transitions which involve the intermediate energy levels related to the APTMS states. We present details of the geometric and electronic structure and excited states of our nanosystems and their dependence on the characteristics of the nanoparticle. We discuss possible optoelectronic applications for this effect.