Undoped Organic Semiconductors as a New SERS Platform

Nanostructured molecular organic semiconductor films are promising Surface-Enhanced Raman Spectroscopy (SERS) platforms for both fundamental and technological research. In this presentation we report that nanostructured films of oligothiophenes functionalized with different fluorinated alkyl or aryl groups demonstrate unprecedent Raman enhancement factor (>10⁵) and a low limit of detection (10^-9 M) for the methylene blue probe molecule. This data is comparable to those reported for the best inorganic semiconductors and even intrinsic plasmonic metal-based SERS platforms. Photoluminescence spectroscopy and computational analysis suggest that both charge-transfer energy and effective molecular interactions, leading to a small but non-zero oscillator strength in the charge-transfer state between the organic semiconductor film and the analyte molecule, are required to achieve large SERS enhancement factors and low molecular sensitivities in these systems. Our results provide not only a considerable experimental advance in organic SERS figure-of-merits but also guidance for the molecular design of more sensitive SERS platforms based on organic (macro)molecules.