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

Abstract

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 poly͓2-methoxy-5-͑2Ј-ethylhexyloxy͒-1,4-phenylene vinylene͔ ͑MEH-PPV͒ both before and after thermal annealing. Our investigation employs a combination of third harmonic generation ͑THG͒ 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.

Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV

“…34,35,[42][43][44]64,65 Aggregate formation can occur in solutions already and in thin films of high molecular weight MEH-PPVs, which might cause chain orientation effects. 28,[36][37][38] Recently, we reported that the birefringence of thin MEH-PPV films increases strongly with M w , 34,35 ranging from nearly isotropic to almost two-dimensional chain orientation parallel to the layer plane. Consequently, the linear optical constants of the films, i.e., their absorption, refractive index and birefringence also increase significantly with M w .…”

Morphology and photophysical properties of phenyleneethynylene oligomer

“…It is believed that thermal annealing of a conjugated polymer film can provide the necessary energy to allow the individual polymer strand to explore configuration space so that they can reorganize themselves towards forming lower energy structure. The amount of intermolecular interactions of the polymers will increase as a result of ordered packing [20]. In the present case, thermal treatment of the spincoated oligomer films above its melting point and then annealing at 120 or 70 8C or quenching at room temperature leads to the formation of supramolecular packing and growth of polymeric lamellae with higher order, as indicated in Fig.…”

“…During the process of spincoating, the solvent is quickly evaporated so that the polymer chains are frozen in a random way and can form varieties of conformations. On the contrary, thermal annealing of the polymer films will provide the necessary energy to make the different conformations in the spincoated film converge to a similar morphology in which both the main chains and side chains of the polymers are packed in a more ordered fashion [20]. Among the conjugated polymers, rigid-rod poly(p-phenyleneethynylene)s (PPEs) and their corresponding oligo(pphenyleneethynylene)s (OPEs) are one of the most investigated materials because of their potential applications in polymer LEDs [21] and molecular electronics [7,22].…”

Nanoscale Inhomogeneity of Conducting‐Polymer‐Based Materials

“…The same group [116] studied the formation and properties of the interchain species in more detail using a combination of the NSOM technique with third harmonic generation (THG). The goal of their study was to clarify the origin of the domains found in these materials, as described above, and separate the effects related to the excited states of the polymer molecules/aggregates, such as photoluminescence, from the effects related to the ground states of the these species, which can be probed by THG.…”