Max-Planck-Institut für Festkörperforschung , Heisenbergstraße 1, 70569 Stuttgart, Germany.
Institut de Physique, École Polytechnique Fédérale de Lausanne , 1015 Lausanne, Switzerland.
ACS Nano. 2017 Feb 28;11(2):1230-1237. doi: 10.1021/acsnano.6b08471. Epub 2017 Jan 20.
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.
有机薄膜的电致发光是有机发光二极管 (OLED) 的核心方面，广泛应用于当前的显示技术中。然而，其在分子尺度上的空间变化基本上还没有被探索过。在这里，我们通过使用扫描隧道显微镜 (STM) 来解决这个问题，并对在 Ag(111) 和 Au(111) 表面上的薄 C 膜 (<10 单层) 的电致发光进行了深入研究。与 OLED 类似，金属基底和 STM 针尖注入互补的载流子，这些载流子可能在分子薄膜内复合；然而，针尖的原子级定义的电荷注入使得可以在亚分子尺度上对局部电致发光进行映射。我们表明，固态 C 中的辐射复合被限制在各种结构缺陷中，其发射特性可以单独处理。发射精细结构揭示了与 C 的 Jahn-Teller 活性振动模式的耦合，这意味着其奇偶禁戒的最低单重态跃迁在发射中心变得局部允许。在几纳米的横向距离内，只能检测到针尖诱导等离子体的微弱发射。它们的激发证明了两种载流子类型的注入，并证实它们在远离结构缺陷的位置无法进行辐射复合。最后，我们证明在电致发光图谱中可见的分子轨道模式能够明确区分有机薄膜中电子-空穴对的固有辐射复合与针尖诱导等离子体的技术相关发射。这种能力对于巩固 STM 作为探索有机薄膜纳米尺度发光的工具至关重要。
