2024 |
Woo, S. Y., Shao, F., Arora, A., Schneider, R., Wu, N., Mayne, A. J., Ho, C. - H., Och, M., Mattevi, C., Reserbat-Plantey, A., Moreno, A., Sheinfux, H. H., Watanabe, K., Taniguchi, T., Michaelis de Vasconcellos, S., Koppens, F. H. L., Niu, Z., Stephan, O., Kociak, M., Garcia de Abajo, F. J., Bratschitsch, R., Konecna, A., & Tizei, L. H. G. (2024). Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation. NANO LETTERS, 24(12), 3678–3685.
Résumé: Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS(2), MoSe(2), and WSe(2) van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe(2)/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.
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2022 |
Shao, F., Woo, S. Y., Wu, N., Schneider, R., Mayne, A. J., de Vasconcellos, S. M., Arora, A., Carey, B. J., Preuß, J. A., Bonnet, N., Och, M., Mattevi, C., Watanabe, K., Taniguchi, T., Niu, Z., Bratschitsch, R., & Tizei, L. H. G. (2022). Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening. Physical Review Materials, 6, 074005.
Résumé: The excitonic states of transition metal dichalcogenide (TMD) monolayers are heavily influenced by their external dielectric environment and depend on the substrate used. In this work, various wide band gap dielectric materials, namely hexagonal boron nitride (h−BN) and amorphous silicon nitride (Si3N4), under different configurations as support or encapsulation material for WS2 monolayers, are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy in a scanning transmission electron microscope. In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified, in increasing order of importance, by monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron-beam-induced damage on bare TMD monolayers is also revealed to be responsible for irreversible linewidth increases. h−BN not only provides clean surfaces of TMD monolayers and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which h−BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.
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Shao, F., Woo, S. Y., Wu, N., Schneider, R., Mayne, A. J., Michaelis, S., Arora, A., Carey, B. J., Preuß, J. A., Bonnet, N., Mattevi, C., Watanabe, K., Taniguchi, T., Bratschitsch, R., & Tizei, L. H. G. (2022). Disentangling Exciton Linewidth Broadening Factors in Transition Metal Dichalcogenide Monolayer with Electron Energy Loss Spectroscopy. In MICROSCOPY & MICROANALYSIS (Vol. 28, pp. 1778–1779).
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Woo, S. Y., Shao, F., Wu, N., Schneider, R., Arora, A., Preuß, J. A., Carey, B. J., de Vasconcellos, S. M., Mayne, A. J., Bratschitsch, R., & Tizei, L. H. G. (2022). Strain Relaxation and Excitonic Absorption of Atomically-Reconstructed WSe2 Moiré Superlattices. In MICROSCOPY & MICROANALYSIS (Vol. 28, pp. 2462–2463).
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2021 |
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Shao, F., Woo, S. Y., Wu, N., Mayne, A. J., Schneider, R., Michaelis, S., Arora, A., Carey, B., Preuß, J., Bratschitsch, R., & Tizei, L. H. G. (2021). Understanding transition metal dichalcogenide absorption line widths in electron energy loss spectroscopy. In MICROSCOPY & MICROANALYSIS (Vol. 27, pp. 1170–1172).
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Vallejo Bustamante, J., Wu, N. J., Fermon, C., Pannetier-Lecoeur, M., Wakamura, T., Watanabe, K., Taniguchi, T., Pellegrin, T., Bernard, A., Daddinounou, S., Bouchiat, V., Gueron, S., Ferrier, M., Montambaux, G., & Bouchiat, H. (2021). Detection of graphene's divergent orbital diamagnetism at the Dirac point. SCIENCE, 374(6573), 1399–1402.
Résumé: The electronic properties of graphene have been intensively investigated over the past decade. However, the singular orbital magnetism of undoped graphene, a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions, has been challenging to measure in a single flake. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage–dependent magnetization of a single graphene monolayer encapsulated between boron nitride crystals. The signal exhibits a diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. Our measurements offer a means to monitor Berry phase singularities and explore correlated states generated by the combined effects of Coulomb interactions, strain, or moiré potentials.
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2020 |
Wakamura, T., Wu, N. J., Chepelianskii, A. D., Gueron, S., Och, M., Ferrier, M., Taniguchi, T., Watanabe, K., Mattevi, C., & Bouchiat, H. (2020). Spin-Orbit-Enhanced Robustness of Supercurrent in Graphene/WS2 Josephson Junctions. PHYSICAL REVIEW LETTERS, 125(26), 266801.
Résumé: We demonstrate the enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high out-of-plane magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on WS{2}, where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions, however, only graphene on WS{2} exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasiballistic edge states stabilized by the strong SOIs induced in graphene by WS_{2}.
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2018 |
Zhao, M., Almarzouqi, F., Duverger, E., Sonnet, P., Dujardin, G., & Mayne, A. J. (2018). Sub-molecular spectroscopy and temporary molecular charging of Ni-phthalocyanine on graphene with STM. Phys. Chem. Chem. Phys., 20(29), 19507–19514.
Résumé: In this study, the self-assembled molecular network and electronic properties of Ni-phthalocyanine (NiPc) molecules on monolayer graphene (MLG)/6H-SiC(0001) were studied by room temperature Scanning Tunnelling Microscopy (STM) and Density Functional Theory (DFT) calculations. In this study, a very weak electronic coupling between the graphene and the NiPc molecules is found. This is due to the very small charge transfer of only 0.035e- per molecule. The weak molecule-graphene interaction has two observable consequences: sub-molecular resolution was obtained in the STM spectroscopy at room-temperature with the molecules adsorbed directly on the graphene, and the occupied and unoccupied molecular resonance peaks were observed to shift their position in energy as a function of the tip-surface distance. This is due to the temporary local charging (either positive or negative) that is achieved by decreasing the surface voltage under the STM tip. This may have important consequences for future studies of the opto-electronic properties of such hybrid graphene-molecule systems.
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2016 |
Debiossac, M., Zugarramurdi, A., Mu, Z., Lunca-Popa, P., Mayne, A. J., & Roncin, P. (2016). Helium diffraction on SiC grown graphene: Qualitative and quantitative descriptions with the hard-corrugated-wall model. PHYSICAL REVIEW B, 94(20), 205403.
Résumé: Monolayer epitaxial graphene grown on 6H-SiC(0001) was recently investigated by grazing-incidence fastatom diffraction and analyzed with an ab initio electronic density calculation and with exact atomic diffraction methods. With these results as a reference, the hard-corrugated-wall model (HCW) is used as a complementary analytic approach to link binary potentials to the observed atomic corrugation. The main result is that the HCW model reproduces the macroscopic corrugation of the moir´e pattern on a quantitative level, suggesting that soft-wall corrections may be neglected for macroscopic superstructures, allowing straightforward analysis in terms of a one-dimensional corrugation function.
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2015 |
Zugarramurdi, A., Momeni, A., Debiossac, M., Lunca-Popa, P., Mayne, A.J., Borisov, A.G., Mu, Z., Roncin, P. & Khemliche, H. (2015). Determination of the geometric corrugation of graphene on SiC(0001) by grazing incidence fast atom diffraction. Appl. Phys. Lett., 106(10), 101902.
Résumé: We present a grazing incidence fast atom diffraction (GIFAD) study of monolayer graphene on 6H-SiC(0001). This system shows a Moiré-like 13x13 superlattice above the reconstructed carbon buffer layer. The averaging property of GIFAD results in electronic and geometric corrugations that are well decoupled; the graphene honeycomb corrugation is only observed with the incident beam parallel to the zigzag direction while the geometric corrugation arising from the superlattice is revealed along the armchair direction. Full-quantum calculations of the diffraction patterns show the very high GIFAD sensitivity to the amplitude of the surface corrugation. The best agreement between the calculated and measured diffraction intensities yields a corrugation height of 0.27 +- 0.3A° .
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2014 |
Rogez, B., Yang, H., Le Moal, E., Lévêque-Fort, S., Boer-Duchemin, E., Yao, F., Lee, Y. - H., Zhang, Y., Wegner, D., Hildebrandt, N., Mayne, A. J., & Dujardin G. (2014). Fluorescence lifetime and blinking of individual semiconductor nanocrystals on graphene. J. Phys. Chem. C, 118, 18445–18452.
Résumé: A new class of optoelectronic nanodevices consisting of 0D semiconductor nanocrystals and 2D single graphene layers is attracting much attention. In particular, such a system may be used to investigate and control the transfer of energy and charge in low-dimensional systems. To this end, the fluorescence dynamics of individual colloidal quantum dots(QDs) on graphene are investigated on both the 10−9 to 10−8 s time scale (fluorescence lifetime) and the 1−100 s time scale(blinking statistics) in this paper. We find that (i) a nonradiative energy transfer rate of ≈5 × 10+8 s−1 is obtained from the reduced lifetimes of QDs on graphene as opposed to those on insulating substrates such as glass; (ii) QDs still exhibit fluorescence intermittency (“blinking”) on graphene; (iii) the cumulative distribution functions of the “off” times may be described by power-law statistics; (iv) QD coupling to graphene increases the time spent in the “on” state while the time spent in the “off” state remains relatively unchanged; and (v) the fluorescence emission spectrum of the QDs is practically unaltered by the QD−graphene coupling.
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2013 |
Yang, H., Mayne, A. J., Cejas, C., Dujardin, G., & Kuk, Y. (2013). Manipulation at a distance: Atomic-scale observation of ballistic electron transport in single layer graphene. Appl. Phys. Lett., 102(22), 223104.
Résumé: We present scanning tunneling microscopy manipulation experiments on epitaxial graphene and the carbon buffer layer grown on hexagonal silicon carbide. Low voltage pulses applied to the graphene layer with the microscope tip induce nonlocal modifications of a bare carbon buffer region 10 nm away. The graphene itself is not affected. This is direct evidence for ballistic hot electrons propagating along the graphene layer to the graphene edge. High energy states in the graphene band structure (Van Hove Singularities) may explain both the electron transport and the coupling of the graphene edge to the adjacent bare carbon buffer region.
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Yang, H., Mayne, A. J., Comtet, G., Dujardin, G., Kuk, Y., Sonnet, P., Stauffer, L., Nagarajan, S., & Gourdon, A. (2013). STM imaging, spectroscopy and manipulation of a self-assembled PTCDI monolayer on epitaxial graphene. Phys. Chem. Chem. Phys., 15(14), 4939–4946.
Résumé: Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and manipulation studies were performed on an ordered self-assembled monolayer (SAM) of N,N'-bis(1-hexylheptyl)perylene-3,4: 9,10-bis(dicarboximide) molecules on epitaxial graphene on hexagonal silicon carbide – SiC(0001). Four novel aspects of the molecular SAM on graphene are presented. Molecules adsorb in both armchair and zig-zag configurations, giving rise to six orientations of the molecular layer with respect to the underlying substrate. The interaction between the molecules and the graphene surface shifts the LUMO towards the Fermi level, inducing a charge transfer and the opening of a band gap in the graphene, with the LUMO inside. This decouples the LUMO from the surface rendering it invisible in the dI/dV spectroscopy. The HOMO only becomes visible at short tip-surface distances, as its energy lies within the band gap of the SiC substrate. Finally, the observed molecular defects are very particular, being composed exclusively of molecular dimers. These molecular dimers have a stronger interaction with the graphene than other molecules.
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2011 |
Park, C., Yang, H., Mayne, A. J., Dujardin, G., Seo, S., Kuk, Y., Ihm, J., & Kim, G. (2011). Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation. PNAS, 108(46), 18622–18625.
Résumé: We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.
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Vizzini, S., Enriquez, H., Chiang, S., Oughaddou, H., & Soukiassian, P. (2011). Nano-structures developing at the graphene/silicon carbide interface. SURFACE SCIENCE, 605(5-6), L6–L11.
Résumé: We use scanning tunneling microscopy and spectroscopy to study defects on epitaxial graphene grown on a 4H-SiC(000-1) C-face substrate. At the graphene/SiC interface, we discover a few isolated small areas covered by nano-objects confined vertically and forming mesas, suggestive of packed carbon nanotubes and leading to electronic interface states. Nano-crack defects are also found at the SiC surface. They are covered with an unbroken graphene layer going deep into the crack showing no electronic interface state, and thus would probably not affect the carrier mobility. (C) 2011 Elsevier B.V. All rights reserved.
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2010 |
Yang, H., Mayne, A. J., Boucherit, M., Comtet, G., Dujardin, G., & Kuk, Y. (2010). Quantum Interference Channeling at Graphene Edges. Nano Lett., 10(3), 943–947.
Résumé: Electron scattering at graphene edges is expected to make a crucial contribution to the electron transport in graphene nanodevices by producing quantum interferences. Atomic-scale scanning tunneling microscopy (STM) topographies of different edge structures of monolayer graphene show that the localization of the electronic density of states along the C-C bonds, a property unique to monolayer graphene, results in quantum interference patterns along the graphene carbon bond network, whose shapes depend only on the edge structure and not on the electron energy.
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2008 |
Yang, H., Baffou, G., Mayne, A. J., Comtet, G., Dujardin, G., & Kuk, Y. (2008). Topology and electron scattering properties of the electronic interfaces in epitaxial graphene probed by resonant tunneling spectroscopy. Phys. Rev. B, 78(4), 041408.
Résumé: Z-V scanning tunneling spectroscopy is used to probe the topology and electron scattering properties of the electronic interfaces of monolayer and bilayer graphenes, expitaxially grown on SiC(0001). The dZ/dV spectra validate existing calculations of the interface topology and provide evidence for new electron scattering properties due to changes in the electronic character of the bonding. Two sharp boundaries are observed: between the vacuum and the graphene pi state lying above the graphene atom plane and a subsurface barrier between the carbon-rich layer and the bulk SiC.
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