Femtosecond Time-Resolved Transient Absorption Spectroscopy with Sub-Diffraction-Limited Spatial Resolution Reveals Accelerated Exciton Loss at Gold-Poly(3-Hexylthiophene) Interface

Molecules are known to change properties when in contact with metal surfaces. Therefore, dynamics of photoinduced molecular excitons in a semiconductor also are expected to be influenced by a metal contact. This effect, which is of considerable interest also for applications, is limited to interface excitons generated within just a few nanometer proximity to a metal layer. Up to now, however, a highly localized access to such excitonic events has not been presented, and diffraction-limited microspectroscopy did not yield any pattern in exciton dynamics other than that of bulk excitons, irrespective of an existing metal interface. In our work, we have combined femtosecond time-resolved spectroscopy with scanning near-field optical microscopy (SNOM) to study the interfacial dynamics of a gold-poly­(3-hexylthiophene) system (Au–P3HT) making use of tip-enhancement of the light fields of the ultrashort laser pulses by a gold rim surrounding the SNOM fiber tip, which collects the signal light. Next to annihilation of free excitons in P3HT, which is an efficient loss mechanism at the laser powers employed, a direct exciton decay highly confined within the near-field range right at the Au–P3HT interface has been observed. We show that the occurrence of the gold-coated SNOM-tip-induced near-field enhancement of the optical fields permits selective access to the highly confined interfacial exciton decay. The experiments reveal that the early ultrafast loss of charge pairs in P3HT becomes significantly faster at the Au–P3HT interface because of an additional pathway.