Submolecular Electroluminescence Mapping of Organic Semiconductors.

The electroluminescence of organic films is the central aspect in organic light emitting diodes (OLEDs) and widely used in current display technology. However, its spatial variation on the molecular scale is essentially unexplored. Here, we address this issue by using scanning tunneling microscopy (STM) and present an in-depth study of the electroluminescence from thin C films (<10 monolayers) on Ag(111) and Au(111) surfaces. Similar to an OLED, the metal substrate and STM tip inject complementary charge carriers that may recombine within the molecular film; however, the atomically defined charge injection by the tip enables mapping of the local electroluminescence down to the submolecular scale. We show that the radiative recombination in solid C is restricted to various structural defects, whose emission characteristics can be addressed individually. The emission fine structure reveals a coupling to Jahn-Teller active vibrational modes of C, which implies that its parity-forbidden lowest singlet transition becomes locally allowed at the emission centers. At lateral distances of a few nanometers, only a weak emission from tip-induced plasmons is detectable. Their excitation evidences the injection of both charge carrier types and confirms that they are unable to recombine radiatively at positions far from structural defects. Finally, we demonstrate that the molecular orbital pattern visible in electroluminescence maps enables an unambiguous discrimination between the intrinsic radiative recombination of electron-hole pairs in the organic film and the technique-related emission of tip-induced plasmons. This capability is essential to consolidate STM as a tool to explore the light generation from organic films on the nanoscale.