Mesoscale optical properties of conjugated polymers probed by near-field scanning optical microscopy

Dearo, J. A.; Weston, Kenneth D.; Buratto, Steven K.; Lemmer, Uli

doi:10.1016/s0009-2614(97)00882-8

Cited by 58 publications

(60 citation statements)

References 26 publications

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“…As shown on the topographic profile, position 1 is on the top of one of the bumps and position 2 is centered in one of the flat regions of the film. Even though the two SRPL spectra are collected only ∼1 µm apart, they are clearly different, showing that the emission from MEH-PPV films, like that of other conjugated polymers, 21,29,35,39 is spatially inhomogeneous. The SRPL spectrum collected from the flat region (position 2, solid curve) strongly resembles that of MEH-PPV in solution, while the spectrum collected on top of the bump (position 1, dashed curve) shows a red tail and a relative enhancement of the vibronic feature near 620 nm.…”

Section: Resultsmentioning

confidence: 97%

“…A similar set of experiments performed by Buratto and co-workers on films of (unsubstituted) PPV found aggregated domains ∼100 nm in diameter in which there was a correlation between the film topography, the PL intensity, and the emission polarization. 35 While none of this previous work investigated the spectrum of the spatially resolved photoluminescence (SRPL), the measurements from both groups do show that aggregated species may exhibit spatial localization, indicating that the optical properties of a conjugated polymer sample can vary from location to location within the film. NSOM has also been used to study films of liquid crystalline poly(9,9-dialkylfluorenes), 29 which reveal two distinct types of morphology: 50-150 nm polymer clusters that have a lower PL quantum yield than the rest of the film, and 50-500 nm polymer domains that show highly polarized luminescence.…”

Section: Introductionmentioning

confidence: 98%

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Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films

Nguyen¹,

Schwartz²,

Schaller³

et al. 2001

J. Phys. Chem. B

125

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It is becoming increasingly clear that the way in which a conjugated polymer film is cast affects the interactions between polymer chains and thus the optical and electrical properties of the film. Given that conjugated polymer films cast in different ways also show different nanometer-scale surface topographies, the question that arises is: What is the correlation between surface topography, local chain packing, and the local electronic properties of a conjugated polymer film? In this paper, we address this question using fluorescence near-field scanning optical microscopy (NSOM) to examine films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) that were prepared in different ways. The spatially resolved photoluminescence (SRPL) spectra collected on top of the nanometer-scale topographic features ("bumps") exhibited by spin-cast MEH-PPV films show an enhancement of the red portion of the emission relative to spectra collected from flat regions of the film. Moreover, photooxidative damage (signified by a red-shift and drop in quantum yield of the SRPL) occurs much more quickly in the flat regions of the MEH-PPV films than on the topographic bumps. Taken together, these observations suggest that the bumps on the films correspond to regions in which the chains are packed more tightly: the red-shifted emission results from increased interchain interactions, while the decreased photooxidation rate results from the fact that oxygen cannot easily diffuse between the tightly packed polymer chains. We also find that the spatial homogeneity of MEH-PPV films can be greatly improved by annealing: heating the films above the glass transition temperature removes the topographic features and produces a uniform but weak and red-shifted SRPL due to increased interchain interactions. In contrast to spin-cast films, the SRPL of annealed films undergoes a blue-shift upon photooxidation. This result can be explained by considering the differences between the local chain packing in annealed and nonannealed films, combined with the fact that excitations in the film tend to migrate to low-energy aggregatedchain "traps". All of these results provide insight into how polymer film morphology can be controlled through film processing conditions to improve the optical properties and the performance of electroluminescent devices based on this class of materials.

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Section: Resultsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 98%

Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films

Nguyen¹,

Schwartz²,

Schaller³

et al. 2001

J. Phys. Chem. B

125

View full text Add to dashboard Cite

show abstract

“…0.1 "m [559,560]. On the other hand, Cho et al [561] have discovered that the methoxy-substituted PPV precursor polymer (i.e.…”

Section: Advanced Microfabrications Of Oriented and Patterned Conjugamentioning

confidence: 99%

Advanced syntheses and microfabrications of conjugated polymers, C60-containing polymers and carbon nanotubes for optoelectronic applications

Dai

1999

Polym. Adv. Technol.

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“…Near-field scanning optical microscopy (NSOM) shows that the optical properties of these films vary from location to location, consistent with the idea of spatially localized aggregate species. 24,26,27 All of these results suggest that conjugated polymer chains can aggregate to form electronic species with ground-and excited-state properties distinct from that of an isolated polymer chain.…”

Section: Introductionmentioning

confidence: 99%

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

et al. 1999

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The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated polymer films, and that the degree of interchain interactions can be controlled by varying the solvent and polymer concentration of the solution from which the films are cast. Thus, much of the controversy in the literature can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of interchain interaction. The photoluminescence (PL) of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes both its spectral shape and quantum yield when the films are prepared from different solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film's PL, a result assigned to a combination of changes in the vibronic structure of the PL of the exciton and increased numbers of weakly emissive interchain species. Photoluminescence excitation spectroscopy shows that excitation to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a distinct ground state absorption. Scanning force microscopy shows topographic features that correlate with the degree of interchain interactions, verifying that the morphology of conjugated polymer films changes with polymer concentration, choice of solvent, and spin-casting speed. Even at low excitation intensities, photooxidative damage occurs quickly in MEH-PPV films excited in air, and the rate at which damage occurs is sensitive to the packing of the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excitedstate absorption dynamics that do not match those of the emissive species, provide direct evidence for the production of interchain aggregates. Annealing an MEH-PPV film produces a photophysical signature similar to photooxidation, implying that defects in conjugated polymer films are intrinsic and depend on the details of how the chains are packed. At higher excitation intensities, we find that exciton-exciton annihilation occurs, and that the probability for annihilation can vary by an order of magnitude depending on the degree of interchain contact in the film. Finally, we show that changing the film morphology has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of carriers at the expense of lower quantum efficiencies for electroluminescence. Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer films for particular device applications.

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Mesoscale optical properties of conjugated polymers probed by near-field scanning optical microscopy

Cited by 58 publications

References 26 publications

Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films

Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films

Advanced syntheses and microfabrications of conjugated polymers, C60-containing polymers and carbon nanotubes for optoelectronic applications

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

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