Nanofibers Paired with Gold Nanohole Substrate Boost Exciton Transport in Organic Solids

Researchers at the Institute of Science Tokyo found that self-assembling, anthracene-based supramolecular nanofibers can carry excitons, energy-carrying particle pairs created when light is absorbed, over hundreds of nanometers. Pairing the nanofibers with a plasmonic gold nanohole substrate, which uses metal nanostructures to concentrate light's electric field, doubled the exciton diffusivity. The approach addresses the limited diffusivity of singlet excitons in organic semiconductors, carbon-based materials that conduct electricity, and offers a new strategy for optoelectronic devices.

For decades, excitons in organic semiconductors have typically diffused only 5 to 20 nanometers before recombining, limiting device performance. The study, conducted by Professor Martin Vacha and Associate Professor Yoshimitsu Sagara of the Institute of Science Tokyo with Dr. Takatoshi Fujita of the National Institute for Quantum Science and Technology, is published in the journal Nano Letters.

"Developing a universal design principle for long-range exciton transport has remained an elusive goal, necessitating a deeper comprehension of intrinsic molecular factors to strategically enhance migration within organic systems," Vacha said. The team built molecules from 9,10-bis(phenylethynyl)anthracene (BPEA), modifying the BPEA chromophore with amide groups so hydrogen bonding drove self-assembly into ordered one-dimensional nanofibers, while hydrophilic dendritic side chains ensured solubility. This arrangement formed "J-aggregates," ordered assemblies that support efficient electronic coupling and energy transport.

Confocal fluorescence microscopy with position-dependent fluorescence lifetime measurements showed exciton transport lengths up to 350 nanometers and diffusion coefficients reaching 0.7 square centimeters per second, among the highest reported for organic solids. Quantum-chemical calculations showed excitons delocalized across two to three neighboring molecular units, with mixing between locally excited and charge-transfer states strengthening intermolecular electronic coupling.

When deposited onto substrates patterned with regularly spaced gold nanohole arrays, the nanofibers reached diffusion coefficients 1.3 square centimeters per second and transport lengths beyond 550 nanometers under optimal alignment, more than double the values on glass substrates. Simulations showed the enhancement was strongest when nanofibers aligned parallel to the periodic nanohole array, while misaligned nanofibers showed weaker enhancement.