Mechanical Manipulation of Photophysical Properties of Single Conjugated Polymer Nanoparticles

Results on simultaneous atomic force microscopy (AFM) and fluorescence microspectroscopy on single nanopartides of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) are presented. Single particle fluorescence spectra and defocused images confirm that the particles are composed of large numbers of randomly oriented conjugated segments (chromophores) with a wide distribution of the segment lengths. Mechanical contact of the particle surface with a sharp tip of the AFM probe causes reversible modification of the particle fluorescence intensity and spectra. The majority of the partides show an increase of fluorescence intensity and blue spectral shift upon contact with the tip. The fluorescence increase is correlated with the particle size and is most pronounced for the smallest particles. The phenomena are interpreted in terms of local disturbance of the MEH-PPV chain conformation by the probe tip.