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Accueil > Équipes scientifiques > Nanosciences moléculaires > Excitonics > Excitonics

Excitonics

par Le Moal Eric, Mayne Andrew - 19 juin (modifié le 17 juillet)

Excitonics

Leader : Eric Le Moal

Participants : Elizabeth Boer-Duchemin, Gérald Dujardin, Séverine Le Moal, Andrew Mayne, Hamid Oughaddou

PhD students : Rémi Bretel, Delphine Pommier

Previous students : Ala Husseen

Motivated by the development of active optical components that can be integrated with nanoelectronics on a chip, our goal is to understand and master the emission of light from organic nanostructures and 2D materials upon excitation with electrical current (ANR project "M-Exc-ICO"). The STM is used to electrically activate this luminescence within a nanoscale tunnel junction, instead of direct connections to metallic leads, in order to preserve the electronic structure of the emitters and to spatially resolve the variations of the excitation quantum efficiency.

Techniques : STM-induced luminescence, optical spectroscopy and microscopy, STM imaging and spectroscopy, electronic spectroscopies (LEED and Auger)

News : Meet us at the NANOP2019 Conference in Munich (Sept. 4-6, 2019) and the EBSN2019 Workshop in Orsay (Sept. 16-18, 2019).

Highlights

Scanning tunneling microscope-induced excitonic luminescence of a two-dimensional semiconductor. The long sought-after goal of locally and spectroscopically probing the excitons of two-dimensional (2D) semiconductors is attained using a scanning tunneling microscope (STM). Excitonic luminescence from monolayer molybdenum diselenide (MoSe2) on a transparent conducting substrate is electrically excited in the tunnel junction of an STM under ambient conditions. By comparing the results with photoluminescence measurements, the emission mechanism is identified as the radiative recombination of bright A excitons. STM-induced luminescence is observed at bias voltages as low as those which correspond to the energy of the optical bandgap of MoSe2. The proposed excitation mechanism is resonance energy transfer from the tunneling current to the excitons in the semiconductor, i.e., through virtual photon coupling. Additional mechanisms (e.g., charge injection) may come into play at bias voltages which are higher than the electronic bandgap. Photon emission quantum efficiencies of up to 10−7 photons per electron are obtained, despite the lack of any participating plasmons. Our results demonstrate a new technique for investigating the excitonic and optoelectronic properties of 2D semiconductors and their heterostructures at the nanometer scale.

article : Delphine Pommier, Rémi Bretel, Luis E. Parra López, Florentin Fabre, Andrew Mayne, Elizabeth Boer-Duchemin, Gérald Dujardin, Guillaume Schull, Stéphane Berciaud, and Eric Le Moal, “Scanning tunneling microscope-induced excitonic luminescence of a two-dimensional semiconductor” Phys. Rev. Lett. 123, 027402 (2019), pdf, suppl. material.

Electron induced reactions of NaCl on Silver. The ionic salts, NaCl and KCl, are ideal materials with wide electronic band gaps that can be grown in thin films. These thin films can then act as spacers to electronically decouple organic molecules from a metal substrate so that the molecules retain their intrinsic electronic and optical properties. We studied the structure and stability of two different thin films of NaCl and KCl, grown on the Ag(001) substrate, using a multi-technique approach with STM and STS, LEED, and Auger electron spectroscopy (AES). We found that chemical modification of the alkali halide thin films could be induced by electron irradiation using LEED and Auger. We observed that Cl depletion follows different reaction kinetics, compared to previous studies on thick NaCl films and bulk crystals. The modification of the film proceeds through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges, and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion.

article : A. Husseen, S. Le Moal, H. Oughaddou, G. Dujardin, A. Mayne, E. Le Moal, "Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation", Phys. Rev. B 96, 235418 (2017), pdf.