Upconverted Nanoparticle for 2D Nano-mapping of Excitation Energy Transfer in Conjugated Polymer

Real-space evaluation of exciton migration in nano-domain potentially provides significant information in advanced material design; that is however still challenging as optical excitation and detection are diffraction limited to submicrometer resolutions. To achieve the real-space evaluation, we utilized rare-earth upconverting nanoparticles with thulium ions as emitters (TmUCNP) for nanometric photoexcitation of conducting polymer, MEH-PPV. The TmUCNPs in MEH-PPV solid were photoexcited with a near-infrared laser. The visible emission of the Tm ions allowed for the production of excited states of the MEH-PPV through excitation energy transfer. The emission of MEH-PPV spectrally separatable from Tm ions permitted individual imaging of their emission spots and single-particle emission dynamics measurement. The luminescence images of the TmUCNPs and MEH-PPV film, analyzed using the localization method, provided 2D nanoscale distributions of the emissive trap sites in the MEH-PPV surrounding the TmUCNPs. From the 2D map of the emissive sites, we determined the length of EET in the MEH-PPV solid to be < 40 nm.