Linker-Induced Anomalous Emission of Organic-Molecule Conjugated Metal-Oxide Nanoparticles
journal contributionposted on 26.06.2012, 00:00 by Volodymyr Turkowski, Suresh Babu, Duy Le, Amit Kumar, Manas K. Haldar, Anil V. Wagh, Zhongjian Hu, Ajay S. Karakoti, Andre J. Gesquiere, Benedict Law, Sanku Mallik, Talat S. Rahman, Michael N. Leuenberger, Sudipta Seal
Semiconductor nanoparticles conjugated with organic- and dye-molecules to yield high efficiency visible photoluminescence (PL) hold great potential for many future technological applications. We show that folic acid (FA)-conjugated to nanosize TiO2 and CeO2 particles demonstrates a dramatic increase of photoemission intensity at wavelengths between 500 and 700 nm when derivatized using aminopropyl trimethoxysilane (APTMS) as spacer-linker molecules between the metal oxide and FA. Using density-functional theory (DFT) and time-dependent DFT calculations we demonstrate that the strong increase of the PL can be explained by electronic transitions between the titania surface oxygen vacancy (OV) states and the low-energy excited states of the FA/APTMS molecule anchored onto the surface oxygen bridge sites in close proximity to the OVs. We suggest this scenario to be a universal feature for a wide class of metal oxide nanoparticles, including nanoceria, possessing a similar band gap (∼3 eV) and with a large surface-vacancy-related density of electronic states. We demonstrate that the molecule–nanoparticle linker can play a crucial role in tuning the electronic and optical properties of nanosystems by bringing optically active parts of the molecule and of the surface close to each other.