Peer-reviewed Publications |
Aguillon, F., & Borisov, A. G. (2023). Atomic-Scale Defects Might Determine the Second Harmonic Generation from Plasmonic Graphene Nanostructures. J. Phys. Chem. Lett., 14, 238–244.
Résumé: In this work, we theoretically investigate the impact of the atomic scale lattice imperfections of graphene nanoflakes on their nonlinear response enhanced by the resonance between an incident electromagnetic field and localized plasmon. As a case study, we address the second harmonic generation from graphene plasmonic nanoantennas of different symmetries with missing carbon atom vacancy defects in the honeycomb lattice. Using the many-body time-dependent density matrix approach, we find that one defect in the nanoflake comprising over five thousand carbon atoms can strongly impact the nonlinear hyperpolarizability and override the symmetry constraints. The effect reported here cannot be captured using the relaxation time approximation within the quantum or classical framework. Results obtained in this work have thus important implications for the design of nonlinear graphene devices.
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Babaze, A., Neuman, T., Esteban, R., Aizpurua, J., & Borisov, A. G. (2023). Dispersive surface-response formalism to address nonlocality in extreme plasmonic field confinement. Nanophotonics, 12(16), 3277–3289.
Résumé: The surface-response formalism (SRF), where quantum surface-response corrections are incorporated into the classical electromagnetic theory via the Feibelman parameters, serves to address quantum effects in the optical response of metallic nanostructures. So far, the Feibelman parameters have been typically obtained from many-body calculations performed in the long-wavelength approximation, which neglects the nonlocality of the optical response in the direction parallel to the metal–dielectric interface, thus preventing to address the optical response of systems with extreme field confinement. To improve this approach, we introduce a dispersive SRF based on a general Feibelman parameter d⊥(ω, k‖), which is a function of both the excitation frequency, ω, and the wavenumber parallel to the planar metal surface, k‖. An explicit comparison with time-dependent density functional theory (TDDFT) results shows that the dispersive SRF correctly describes the plasmonic response of planar and nonplanar systems featuring extreme field confinement. This work thus significantly extends the applicability range of the SRF, contributing to the development of computationally efficient semiclassical descriptions of light–matter interaction that capture quantum effects.
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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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Mukherjee, A., Momeni, A., Allouche, A. R., Staicu Casagrande, E. M., Minea, T., & Khemliche, H. (2023). Grazing incidence fast atom diffraction in high-pressure conditions. Surf. Interfaces, 37, 102754.
Résumé: Grazing Incidence Fast Atom Diffraction (GIFAD) is a recent technique for characterizing surface structures and real-time monitoring of thin film growth. Up to now, GIFAD has only been used in Ultra-High-Vacuum conditions, typically in the range of 10−10 to 10−8 mbar, and has therefore only been considered for high vacuum deposition methods like Molecular Beam Epitaxy or very low-pressure Chemical Vapor Deposition (CVD). At pressures exceeding 10−6 mbar, gas phase collisions along the atom beam trajectory not only reduce the mean free path but also degrade the beam coherence length and thus potentially suppress the diffraction signal. In addition, pressures lower than 10−5 mbar are required to maintain a low noise level on the scattered particle detector. In a new configuration, we demonstrate that GIFAD can operate at pressure as high as 10−2 mbar of argon with well-contrasted diffraction patterns. This opens wide avenues for the study of surface reactivity, thin film growth in Magnetron Sputtering Deposition, where electron diffraction is inevitably perturbed by the electromagnetic fields. This High-Pressure version of GIFAD could also be extended to Reactive Pulsed Laser Deposition and many CVD variants.
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Pommier, D., Hufschmitt, Z., Zhang, C., Lai, Y., Dujardin, G., Le Moal, E., Sauvan, C., Greffet, J. - J., Wang, J., & Boer-Duchemin, E. (2023). Nanoscale Electrical Excitation of Surface Plasmon Polaritons with a Nanoantenna Tunneling Junction. ACS Photon., 10(8), 2641–2649.
Résumé: Quantum tunneling-driven optical nanoantennas are key components for the development of integrated plasmonic nanodevices. In this work, we use the tunneling junction between a nanoantenna and a thin gold film to electrically excite propagating surface plasmons on the nanoscale. The nanoantenna is a chemically synthesized gold nanocube (∼50 nm side length) that is separated from a thin (50 nm) gold film by an insulating molecular layer (1,8-octanedithiols, ∼1 nm thick). A novel method for completing the electrical circuit between the nanoantenna and the gold film using an atomic force microscope (AFM) is developed. Based on the results of numerical modeling, the nanoantenna modes exciting the propagating surface plasmon polaritons are identified as hybridized gap and antenna modes. Our results demonstrate the ability to interrogate individual tunneling-driven nanoantennas, a crucial step toward the development of electrical nanosources of surface plasmon polaritons and light.
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Quijada, M., Babaze, A., Aizpurua, J., & Borisov, A. G. (2023). Nonlinear Optical Response of a Plasmonic Nanoantenna to Circularly Polarized Light: Rotation of Multipolar Charge Density and Near-Field Spin Angular Momentum Inversion. ACS Photon., 10(11), 3963–3975.
Résumé: The spin and orbital angular momentum carried by electromagnetic pulses open new perspectives to control nonlinear processes in light–matter interactions, with a wealth of potential applications. In this work, we use time-dependent density functional theory (TDDFT) to study the nonlinear optical response of a free-electron plasmonic nanowire to an intense, circularly polarized electromagnetic pulse. In contrast to the well-studied case of the linear polarization, we find that the nth harmonic optical response to circularly polarized light is determined by the multipole moment of order n of the induced nonlinear charge density that rotates around the nanowire axis at the fundamental frequency. As a consequence, the frequency conversion in the far field is suppressed, whereas electric near fields at all harmonic frequencies are induced in the proximity of the nanowire surface. These near fields are circularly polarized with handedness opposite to that of the incident pulse, thus producing an inversion of the spin angular momentum. An analytical approach based on general symmetry constraints nicely explains our numerical findings and allows for generalization of the TDDFT results. This work thus offers new insights into nonlinear optical processes in nanoscale plasmonic nanostructures that allow for the manipulation of the angular momentum of light at harmonic frequencies.
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Wang, J., Ouvrard, A., Zheng, W., Carrez, S., Ghalgaoui, A., & Bourguignon, B. (2023). In situ study of catalytic CO oxidation on ultrathin MgO film supported Pd nanoparticles by sum frequency generation: size and site effects. Phys. Chem. Chem. Phys., 25(15), 10845–10852.
Résumé: Controlling the reactive sites of nanoparticles (NPs) is crucial to improve catalyst efficiency. In this work, sum-frequency generation is used to probe CO vibrational spectra on MgO(100) ultrathin film/Ag(100) supported Pd nanoparticles ranging from 3 to 6 nm in diameter and compared to those of coalesced Pd NPs and Pd(100) single crystals. We aim to demonstrate in situ the role played by active adsorption sites in the catalytic CO oxidation reactivity trends varying with the NP size. From ultrahigh vacuum to the mbar range and temperatures from 293 K to 340 K, our observations suggest that bridge sites are the main active sites for CO adsorption and catalytic oxidation. On Pd(100) single crystals at 293 K, CO oxidation predominates over CO poisoning at a pressure ratio of O2/CO greater than 300; on Pd NPs, both the site coordination due to NP geometry and MgO-induced Pd–Pd interatomic distance change impact the reactivity trend varying with size in different ways. Edge sites with low coordination are more reactive than facet sites, while facet sites with a smaller Pd–Pd atomic length are more reactive than that with a larger length. The interplay of both site and size effects gives rise to a non-monotonic reactivity trend of CO on the MgO(100) ultrathin film supported Pd NPs: the reactivity of Pd NPs increases for the smaller NP size side due to a higher edge/facet ratio and meanwhile increases for the larger NP size side due to the terrace facet with a smaller Pd–Pd atomic length at the NP surface and a lower diffusion barrier.
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