2023 |
Bretel, R., Le Moal, S., Oughaddou, H., & Le Moal, E. (2023). Hydrogen-bonded one-dimensional molecular chains on ultrathin insulating films: Quinacridone on KCl/Cu(111). Phys. Rev. B, 108, 125423.
Résumé: We report on the growth of one-dimensional (1D) chains of the prochiral quinacridone (QA) molecule on ultrathin KCl films on Cu(111) in ultrahigh vacuum. Using low-temperature scanning tunneling microscopy (STM), we observe straight homochiral 1D chains of QA molecules on one (1L), two (2L), and three (3L) atomic layer thick (100)-terminated KCl islands. The KCl films mostly consist of 2L-thick KCl islands delineated by long polar and short nonpolar edges. These 2L-thick KCl islands are topped by smaller one-atom-thick KCl islands or pits, which are delineated by nonpolar step edges. We find that QA chains can nucleate at these nonpolar step edges or on top of KCl terraces without assistance of step edges. In both cases, the longest straight QA chains observed grow along the KCl ⟨100⟩ directions or slightly rotated (typically less than 10∘) from them. Intermolecular distances ranging from 6.4 Å to 6.8 Å are measured for QA chains on KCl/Cu(111), which is compatible with hydrogen bonds between neighboring flat-lying QA molecules. These intermolecular distances being larger than the measured KCl lattice parameter (i.e., 6.21 Å at 78 K), QA chain growth on KCl/Cu(111) is incommensurate. Molecular arrangement models for the QA chains on KCl are proposed, based on the analysis of the STM images.
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Jiang, S., Neuman, T., Bretel, R., Boeglin, A., Scheurer, F., Le Moal, E., & Schull, G. (2023). Many-Body Description of STM-Induced Fluorescence of Charged Molecules. Phys. Rev. Lett., 130, 126202.
Résumé: A scanning tunneling microscope is used to study the fluorescence of a model charged molecule (quinacridone) adsorbed on a sodium chloride (NaCl)-covered metallic sample. Fluorescence from the neutral and positively charged species is reported and imaged using hyperresolved fluorescence microscopy. A many-body model is established based on a detailed analysis of voltage, current, and spatial dependences of the fluorescence and electron transport features. This model reveals that quinacridone adopts a palette of charge states, transient or not, depending on the voltage used and the nature of the underlying substrate. This model has a universal character and clarifies the transport and fluorescence mechanisms of molecules adsorbed on thin insulators.
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Le Moal, S., Krieger, I., Kremring, R.: W., S. Yang, X.: Soubatch, S, Tautz, F. S., Silly, M., Borisov, A. G., Sokolowski, M., & Le Moal, E. (2023). Core-Level Binding Energy Shifts in Ultrathin Alkali-Halide Films on Metals: KCl on Ag(100). J. Phys. Chem. C, 127(50), 24253–24265.
Résumé: We present an experimental and theoretical analysis of the core-level binding energy shifts in metal-supported ultrathin KCl films, i.e., a case from a broader class of fewatom-thick, wide-bandgap insulating layers that is increasingly used in nanosciences and nanotechnologies. Using synchrotron-based high-resolution photoemission spectroscopy (HRPES) measurements, we identify the different contributions to the core-level binding energy shifts for the Cl– anions and K+ cations of two to three atomic layer-thick KCl films grown on Ag(100). The distances of the Cl– and K+ ions of the first two atomic layers of the KCl film from the metal substrate are determined from normal incidence X-ray standing wave measurements. We also calculate the core-level binding energy shifts using an analytical electrostatic model and find that the theoretical results are in agreement with the experimental HRPES results only when polarization and substrateinduced image charge effects are taken into account. Finally, our results evidence the effect of the third atomic layer of the KCl film, which partially covers and screens the first two atomic layers of KCl wetting the metal substrate.
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2022 |
Peña Román, R. J., Bretel, R., Pommier, D., Parra López, L. E., Lorchat, E., Boer-Duchemin, E., Dujardin, G., Borisov, A. G., Zagonel, L. F., Schull, G., Berciaud, S., & Le Moal, E. (2022). Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS2. Nano Lett., 22(23), 9244–9251.
Résumé: The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip–sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors.
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Peña Román, R. J., Pommier, D., Bretel, R., Parra López, L. E., Lorchat, E., Chaste, J., Ouerghi, A., Le Moal, S., Boer-Duchemin, E., Dujardin, G., Borisov, A. G., Zagonel, L. F., Schull, G., Berciaud, S., & Le Moal, E. (2022). Electroluminescence of monolayer WS2 in a scanning tunneling microscope: Effect of bias polarity on spectral and angular distribution of emitted light. Phys. Rev. B, 106, 085419.
Résumé: Inelastic electron tunneling in a scanning tunneling microscope is used to generate excitons in monolayer tungsten disulfide (WS2). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments.
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2020 |
Humberg, N., Bretel, R., Eslam, A., Le Moal, E., & Sokolowski, M. (2020). Hydrogen-Bonded One-Dimensional Chains of Quinacridone on Ag(100) and Cu(111): The Role of Chirality and Surface Bonding. J. Phys. Chem. C, 124(45), 24861–24873.
Résumé: The adsorption and ordering of the prochiral molecule quinacridone (QA) on the Ag(100) and Cu(111) surfaces were studied by low-energy electron diffraction and scanning tunneling microscopy. Upon adsorption, the molecules form parallel homochiral chains of flat-lying molecules linked together via hydrogen bonds on both surfaces, but these chains show significant surface-dependent differences concerning their lateral order. On both substrates, the chains are not thermodynamically stable but only metastable and stabilized by kinetic barriers. On the Ag(100) surface, annealing induces a phase transition to a highly ordered and heterochiral structure with a reduced density of hydrogen bonds. The related loss of bonding energy is overcompensated by a stronger bonding to the substrate, yielding a commensurate structure. For QA on Ag(100), we propose that during the initial chain formation and the phase transition upon annealing, the molecules can change their handedness by rotating around their long axes. In contrast, the initial chain formation and the phase transitions of QA on the Cu(111) surface appear to be subject to stronger kinetic limitations. These are explained by stronger substrate molecule interactions on Cu(111), which reduce the diffusion and the possibility for a change of handedness in comparison to QA on Ag(100). We discuss how the intermolecular hydrogen bonds, the 2D chirality, and the different chemical reactivities of the two surfaces [Ag(100) and Cu(111)] influence the structural formation of QA aggregates. We compare our results to the results for QA on Ag(111) reported previously by Wagner et al. [JPCC2014, 118, 10911-10920].
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Le Moal, E., Boer-Duchemin, E. (2020). La nano-optique sous la pointe d’un microscope à effet tunnel. Photoniques, 102, 31–34.
Résumé: Le microscope à effet tunnel (STM) n’est pas seulement un outil de sciences des surfaces qui produit de magnifiques images résolues à l’échelle atomique. Le courant tunnel sous la pointe du STM est également une source d’excitation optique extrêmement locale, ce qui en fait un extraordinaire outil de la nano-optique. Ici, nous donnons un aperçu des possibilités de cet outil en plasmonique et en électroluminescence, lorsque STM et microscopie optique sont associés dans un même instrument.
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2019 |
Pommier, D., Bretel, R., López, L. E. P., Fabre, F., Mayne, A., Boer-Duchemin, E., Dujardin, G., Schull, G., Berciaud, S., & Le Moal, E. (2019). Scanning Tunneling Microscope-Induced Excitonic Luminescence of a Two-Dimensional Semiconductor. Phys. Rev. Lett., 123(2), 027402.
Résumé: 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 that correspond to the energy of the optical band gap 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 that are higher than the electronic band gap. 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.
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2017 |
Husseen, A., Le Moal, S., Oughaddou, H., Dujardin, G., Mayne, A., & Le Moal, E. (2017). Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation. Phys. Rev. B, 96(23), 235418.
Résumé: We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiationwith electrons of energy 52–60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM).We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed 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. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.
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