Photoluminescence Plasmonic Quenching Of Conjugated Polymers By Graphene Nanodots Supported Silver Nanoparticles

Graphene nanodots(GNDs)are a class of fluorescent carbon nanoparticles bearing both x,y-confined graphene lattice and rich functional groups.In this thesis,the viewpoint of quasi-molecules is introduced to describe the physical image of GNDs and thus to clarify the relationship between its structure and photoluminescence(PL)properties.GNDs-supported silver nanoparticles(GNDs/Ag)have been prepared by using the GNDs as reducing agent and template.The PL quenching of conjugated polymer by GNDs/Ag is investigated systematically,which provide necessary guidance for plasmonic enhancement of optoelectronic devices.The main contributions are summarized as follows.Using two dimensional photoluminescence excitation(2D-PLE)map,molecular orbital calculation,reduction level dependent PL analysis,absorption spectroscopy and time-resolved PL spectroscopy,three quasi-molecular PL are determined in ethylenediamine(EDA)reduced GNDs,including C=O related electronic state,graphenol related electronic state and large π-conjugated domains.The graphenol structure is expected to be created via nucleophilic addition-elimination reactions between epoxide groups and EDA,contributing most to the blue-shifted and enhanced PL of GNDs.The multiple quasi-molecular PL provides deeper insights into the commonly called “excitation wavelength dependent PL”.An effort is made to utilize the heterogeneous photoluminescence through the phosphor-based light-emitting diodes employing reduced GNDs as a phosphor,which is capable of converting blue light into white light.GNDs-supported silver nanoparticles(GNDs/Ag)have prepared by using GNDs as template and reducing agent via photo-reduction in situ.The average size of silver nanoparticles is 4 nm,and feature with uniform particle size and narrow distribution.GNDs/Ag-polymer composite films are prepared by assembling the light-emitting conjugated polymer with blue(F8BT),green(PFO)and red(MEH-PPV)emissions on the surface of GNDs/Ag layers.The UV-vis spectra,steady and transient state PL spectra confirm that GNDs have dramatic fluorescence quenching effects on the three polymers.As the surface density of GNDs/Ag increases,the optical absorption of the polymer composite films increase,and the PL intensity and quantum yield decrease.Due to the Purcell effect,the fluorescence lifetimes shorten,and thenon-radiation decay rate increase.Time resolved PL spectral integrals show that the efficiency and rate constant of the energy transfer of PFO to GNDs/Ag are much larger than that of the F8 BT and MEH-PPV.Nanoscale thickness controlled film has been accurately assembled on the GNDs/Ag surface by using the layer-by-layer(LBL)of the poly(diallyl dimethylammonium)chloride : poly(styrenesulfonic acid)(PDDA:PSS).The numbers of layers,thickness and roughness of the self-assembled film are characterized by atomic force microscope,UV-vis spectra and contact angle test.The distance between GNDs/Ag surface and blue light-emitting polyfluorene derivatives(PCFOz)is regulated by the numbers of self-assembled PDDA:PSS layers(0≤d≤7).The UV-vis spectra,steady and transient state PL spectra confirm that the GNDs/Ag quenched polymer PL can be controlled by regulating the thickness of self-assembly layers.With the d decreases,the photo-absorption of the GNDs/Ag-PCFOz composite film increases,and the PL intensity and quantum yield decrease exponentially,but the energy transfer efficiency and rate constant increase.We find the effective distance of PL quenching of PCFOz by GNDs/Ag is about 6 nm,which provide evidence for near field properties of plasmon-exciton coupling.