Molecular excitonics for integrated coherent optoelectronics – M-Exc-ICO

Our project goal is to understand and master the emission of coherent light from organic nanostructures upon excitation with electrical current. Our long-term motivation is the development of active optical components that can be integrated with nanoelectronics on a chip. In particular, we will establish the principle of novel electrical nanosources of coherent optical signals for tomorrow’s information and communication technologies (ICTs).

The use of light to transfer and process information in devices is a long-standing goal of the semiconductor industry. Light solves the bandwidth limitation of silicon electronics and offers additional multiplexing possibilities through a range of wavelengths and polarizations. In this context, the temporal coherence (spectral narrowness) of the sources is crucial to optimize the multiplexing and the processing of optical signals. Yet, nanofabrication technologies based on inorganic semiconductors have failed to produce such sources with sufficiently low dimensions to be integrated with nanoelectronics; as a result, the sources need to be externally bonded onto the chip.

Our approach distinguishes itself from the state of the art by the use of molecular aggregates, which have both truly nanometric dimensions and exceptionally narrow luminescence spectra. In addition, we use a nanoscale tunnel junction to electrically activate this luminescence, instead of direct connections to metallic leads, in order to preserve the electronic structure of the molecules. The unique optical features of molecular aggregates result from the delocalization of molecular excitations (excitons) over coherently coupled molecules. For specific molecular arrangements called J-aggregates, exciton delocalization yields a single narrow emission band. This effect, called super-radiant emission, has been widely studied in the case of photoluminescence, i.e. when light is used to excite molecular luminescence. Conversely, super-radiant emission upon electrical excitation in a tunnel junction remains largely unaddressed.

Our project addresses the fundamental issues that need to be solved prior to the development of technologies based on electrically driven super-radiant emission from molecular aggregates. Since these issues imply intricate physical processes at the molecular and atomic scales, we will study model systems under ultra-controlled conditions. The model systems will be composed of organic molecules forming monolayer-thick J-aggregates on thin insulating layers epitaxially grown on metallic single crystals. The experimental work will involve ultrahigh vacuum scanning tunneling microscopy (STM). Preparatory experiments will be performed at room-temperature, while the complete system will be studied at low temperature. These permit both the properties of the model systems and their electrical excitation to be controlled at the atomic scale. Optics integrated in the low temperature-STM instrument will enable the detection of the luminescence induced by the STM tip in the molecular aggregates and its spectral analysis, as well as its comparison with their photoluminescence upon laser excitation, in situ, under the same conditions.

Complementary, statistical information on the structural order, domain size and growth mode of the molecular aggregates will be obtained using spot-profile-analysis low energy electron diffraction (SPA-LEED). The electronic effects of the intermolecular and molecule-substrate interactions will be probed by X-ray spectroscopy at the Synchrotron SOLEIL. Moreover, a comprehensive model of the excitation and emission processes will be developed, which includes the electronic and electromagnetic properties of the molecular aggregates and the STM tip.