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Nanoscopic interchain aggregate domain formation in conjugated polymer films studied by third harmonic generation near-field scanning optical microscopy

2002; American Institute of Physics; Volume: 117; Issue: 14 Linguagem: Inglês

10.1063/1.1499479

1520-9032

Richard D. Schaller, Preston T. Snee, Justin C. Johnson, Lynn F. Lee, Kevin R. Wilson, Louis H. Haber, Richard J. Saykally, Thuc‐Quyen Nguyen, Benjamin J. Schwartz,

Force Microscopy Techniques and Applications

The electronic structure of conjugated polymer films is of current interest due to the wide range of potential applications for such materials in optoelectronic devices. A central outstanding issue is the significance of interchain electronic species in films of these materials. In this paper, we investigate the nature of interchain species in films of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) both before and after thermal annealing. Our investigation employs a combination of third harmonic generation (THG) and near-field scanning optical microscopy to measure the wavelength and spatial dependence of the THG efficiency. These chemically selective imaging measurements reveal new, low-energy absorption features in nanometer-scale spatially distinct regions of annealed films that are only infrequently observed prior to annealing. This suggests that the polymer strands in annealed MEH-PPV films pack together closely enough that significant ground-state wave function overlap can occur: thermal annealing creates nanoscopic aggregation domains. THG polarization studies indicate that polymer chain segments in these domains have a preferred orientational alignment. The spatial correlation of these aligned nanoscopic regions within the annealed films suggests that they form via a nucleation and growth type mechanism. In combination with previous work, these data support the idea that the nature and spatial distribution of interchain interactions in conjugated polymer films are complex; conjugated polymer films likely contain an inhomogeneous spatial distribution of both ground- and excited-state interchain species.