Single Molecule Architectures as Lowest-scale Light Emitting Devices – SMAL'LED

The first objective of the project consisted in probing the transport and optical properties of single-molecular quantum dot (QD) suspended in a STM junction by insulating organic chains. To obtain such structures on a metallic surface we explored two strategies: (i) we directly evaporated the molecular QD already functionalized with its insulating chains, (ii) we evaporated the molecular QDs and the chains separately and we attached them following a thermally-induced polymerisation reaction, directly on the surface.

(i) Here, we choose a perylene derivative (PTCDI) as a molecular QD, functionalized with insulating alkane chains. This molecule was directly sublimed under UHV on a silver sample and imaged with STM. Then, we lifted the molecule in the STM junction and acquired conductance (dI/dV vs V) spectra. The sharp peaks observed in these spectra strongly hint that the molecular QD is decoupled from the electrodes. These data could be fitted using a model describing a sequential tunnelling process. In the scope of publication this model needs now to be refined. Moreover, the present molecular junctions were unstable, probably because the part remaining on the surface was not sufficiently anchored. As a consequence, the molecular wires were often jumping on the tip, which constitutes an important drawback for the luminescence measurements. This is why we rapidly explored another strategy (ii).

(ii) We evaporated the molecular QDs (here a porphyrine) and the insulating chains (polythiophene) separately on a gold surface. Then, we bind them to each other directly on the surface using an on-surface polymerization method. At first we studied the properties of the insulating chains and of the molecular QDs alone. This permitted us to identify the impact of an elevated temperature – required for the polymerization reaction – on the chemical structure of the two components. It also led to a first publication in the frame of the project Next, we co-evaporated the chains and the QDs and co-polymerized them on the surface. The STM images reveal long molecular wires containing several molecular QDs that provided highly stable junctions once lifted between tip and sample. Electrically-excited optical spectra of a single QD lifted in the junction have been recorded. They reveal extremely sharp emission lines (the sharpest ever measured with STM) that are related the fluorescence of the molecular QD. These spectra also reveal vibronic peaks that ressemble Raman emission.

Our publication in Physical Review Letters is, to my opinion, a very important step for the understanding of the complex interaction between molecular exitons and plasmons and for the realization of nanoplasmonic devices integrating electrically gated molecular junctions as controllable excitation sources. This work will be highlighted (within the next few days) as an “actualité scientifique” of the Institute of Physics (INP) of the CNRS.

1. J. Phys. Chem. Lett. 6, 2987 (2015)
2. J. Phys. Chem. Cond. Mat. 28, 165001 (2016)
3. Phy. Rev. Lett. 116, 036802 (2016)

Our project aims at combining molecular electronics and nanophotonics, two extremely active research areas in nanoscience. We will explore the interactions between charge transport and light emission at the level of a single-molecule junction by designing and realizing model electroluminescent light sources made of individual molecules either as isolated entities or as highly integrated assemblies.

The important progresses made over the past 4 decades in the understanding of single-molecule electronic junctions led researchers and technologists to realize that the quantum nature of such objects opens up new functionalities going much beyond today's paradigms of electronics. This recently led to renewed interest for the promising concept of single-molecule optoelectronics; in particular probing and controlling the interactions between local near-field light (e.g. plasmon wave), and a molecular junction. Here, the light can be either excited by electrons passing through the molecular junction (i.e. electroluminescent processes), or used to alter the transport properties of the molecular junction through photo-mechanical or photo-electronic processes.

In this project we propose to exploit an unconventional method based on cryogenic scanning tunneling microscopy (STM) to control the opto-electronic properties of individual and interacting molecular junctions strongly coupled to plasmonic fields, with the final aim of building light emitting diodes (LED) of molecular scale. To this end we will study the light emission properties of single, dimers and short chains of organic quantum emitters having individual molecules as optically active components. The interactions of the emitters with the plasmonic STM resonator as well as between the different emitters will be addressed. Therefore we have designed a new type of optically-active molecular structure specifically adapted to the STM environment. It is based on a succession of luminophores separated by insulating organic sub-units. Suspended between the STM-tip and the sample, these tuneable structures will reveal new phenomena related to the emission of coupled molecular quantum dots. In a second part of the project, mesoscopic samples based on the self-assembly of these structures will be designed and probed in “real life conditions” with an STM-AFM working at air. These studies will allow probing energy transfer (FRET) between the electronically excited molecular strands. Finally, we will actively pursue the realization of a colour controlled LED-device prototype based on the concept developed in the previous steps. This prototype will make use of the quantum nature of the individual molecular quantum dots through the exploitation of their specific electronic transport properties.

This project relies on the complementarity of four different teams (three partners) having competences in organic synthesis, STM manipulation and STM induced light emission measurements, molecular photonics in interaction with nanostructures, optoelectonic organic devices and first principles calculations.