Effect of Surface Modification on the Optical Properties of Nanocrystalline Zinc Oxide Materials

The wide band gap and unique photoluminescence (PL) spectrum of nanocrystalline zinc oxide (nano-ZnO) make it useful for a variety of photonics and sensor applications. Toward the goal of modifying the electronic structure and optical properties of nano-ZnO, nanorods were functionalized with electron withdrawing organosilanes, 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDS) and pentafluorophenyltriethoxysilane (PFS), and a partially conjugated heterobifunctional molecule, p-maleimidophenyl isothiocyanate (PMPI). Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) confirmed the presence of the modifiers on the nano-ZnO surface and verified covalent attachment. PL spectroscopy was performed to evaluate the influence of the modifiers on the nano-ZnO inherent optical behavior. An increase in the nano-ZnO near-band edge emission (UV) was evident for the organosilane modifiers, despite their differing electronic structures, while the defect emission (visible) remained unchanged. However, surface modification with the non-silane modifier PMPI resulted in unaltered UV and visible emission intensity. The varying influence of the modifiers may be due to the absence of a silane group in the PMPI, allowing for more efficient electron transport to the modifier. The influence of size/shape of the nanocrystalline ZnO was also examined by reacting spherical nanoparticles with PFDS. Preliminary results indicate that PFDS modification of the nanospheres resulted in similar PL behavior as the nanorods; although, the inherent PL of the spheres differs from the nanorods. These studies will elucidate the role of modifier structure on surface-modified nano-ZnO optical behavior, so that optical tailoring of the nano-ZnO inherent PL can be realized.