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See detailSemi-empirical phonon calculations for graphene on different substrates
Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Poster (2014, April 02)

We investigate the graphene-substrate interaction via changes in the phonon dispersion of graphene. Ab-initio calculations on these systems are of high computational cost due to the non-commensurability ... [more ▼]

We investigate the graphene-substrate interaction via changes in the phonon dispersion of graphene. Ab-initio calculations on these systems are of high computational cost due to the non-commensurability of the unit cells of graphene and the substrate. This leads to the formation of Moiré patterns with accordingly large supercell sizes. We use a semi-empirical force constant model for the calculation of phonons of graphene on different metallic and insulating substrates. The interaction of graphene with the substrate is described via suitably chosen spring constants. The phonon dispersion in the primitive unit cell of graphene is obtained via an “unfolding procedure” similar to the ones used for the discussion of ARPES (angular resolved photo-emission spectroscopy) of graphene on incommensurate substrates. [less ▲]

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See detailMaterials science in Luxembourg
Kreisel, Jens UL; Wirtz, Ludger UL; Schiltz, Marc

in Nature Materials (2014), 13

With its strategic location and firm commitment to investing in research, Luxembourg has ambitious plans to become a significant player in the international research arena.

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See detailPhonon dispersion and optical properties of layered materials: influence of the environment
Wirtz, Ludger UL

Scientific Conference (2014, February 11)

Since graphene and related hexagonal monolayers (BN, MoS2) are atomically thin 2D materials, their properties can be strongly influenced by the environment. In particular, the substrate on which they are ... [more ▼]

Since graphene and related hexagonal monolayers (BN, MoS2) are atomically thin 2D materials, their properties can be strongly influenced by the environment. In particular, the substrate on which they are deposited can modify the phonon dispersion and the optical properties. Besides the direct "mechanical" interaction via chemisorption or physisorption, the dielectric (or metallic) screening by the substrate plays an important role. It can reduce the Kohn-Anomaly at the high-symmetry point K of the highest optical phonon branch of graphene by partially screening the electron-phonon coupling. We give several examples how this effect can be observed in Raman and HREEL spectra: graphene on boron nitride [1], graphene on Ir(111) [2], graphene on Ni(111) [3]. Furthermore, we present ab-initio calculations of the electronic band-structure and optical absorption spectra of mono and multi-layers of molybdenum disulfide: we discuss the role of spin-orbit coupling and inter-layer coupling on the splitting of the valence band maximum. The absorption spectra display prominent excitonic effects [4]. The excitonic binding energy strongly depends on the number of layers and the dielectric environment. [1] F. Forster, A. Molina-Sánchez, S. Engels, A. Epping, K. Watanabe, T. Taniguchi, L. Wirtz, and C. Stampfer, Phys. Rev. B 88, 085419 (2013). [2] M. Endlich, A. Molina-Sánchez, L. Wirtz, and J. Krüger, Phys. Rev. B 88, 205403 (2013). [3] A. Allard and L. Wirtz, Nano Letters 10, 4335 (2010). [4] A. Molina-Sánchez, D. Sangalli, K. Hummer, A. Marini, and L. Wirtz, Phys. Rev. B 88, 045412 (2013). [less ▲]

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See detailOptical and vibrational properties of layered MoS2
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Scientific Conference (2014, January 24)

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See detailF center in lithium fluoride revisited: Comparison of solid-state physics and quantum-chemistry approaches
Karsai, Ferenc; Tiwald, Paul; Laskowski, Robert et al

in Physical Review. B, Condensed Matter (2014), 89

We revisit the theoretical description of the F color center in lithium fluoride employing advanced complementary ab initio techniques.We compare the results from periodic supercell calculations involving ... [more ▼]

We revisit the theoretical description of the F color center in lithium fluoride employing advanced complementary ab initio techniques.We compare the results from periodic supercell calculations involving density-functional theory (DFT) and post-DFT techniques with those from the embedded-cluster approach involving quantumchemical many-electron wave-function techniques. These alternative approaches yield results in good agreement with each other and with the experimental data provided that correlation effects are properly taken into account. [less ▲]

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See detailElectron-Phonon Interaction in Single-Layer MoS2. Influence on the Electronic Properties and Bandgap Renormalization
Molina-Sanchez, Alejandro UL; Palummo, Maurizia; Marini, Andrea et al

Scientific Conference (2014)

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See detailScreening of electron-phonon coupling in graphene on Ir(111)
Endlich, M.; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(205403),

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon ... [more ▼]

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon branches are observed at the ¯ and ¯K point of the surface Brillouin zone. At ¯K the Kohn anomaly is weaker than observed for pristine graphene and graphite. This observation is rationalized in terms of a decrease of the electron-phonon coupling due to screening of graphene electron correlations by the metal substrate. [less ▲]

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See detailThe phonon dispersion of graphene: influence of the environment
Wirtz, Ludger UL

Presentation (2013, October 23)

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See detailAnomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots
Habinshuti, J.; Kilian, O.; Cristini-Robbe, O. et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(11), 115313

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up ... [more ▼]

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement. [less ▲]

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See detailEfficient Gate-tunable light-emitting device made of defective boron nitride nanotubes: from ultraviolet to the visible
Attaccalite, Claudio; Wirtz, Ludger UL; Marini, Andrea et al

in Scientific Reports (2013), 3

Boron nitride is a promising material for nanotechnology applications due to its two-dimensional graphene-like, insulating, and highly-resistant structure. Recently it has received a lot of attention as a ... [more ▼]

Boron nitride is a promising material for nanotechnology applications due to its two-dimensional graphene-like, insulating, and highly-resistant structure. Recently it has received a lot of attention as a substrate to grow and isolate graphene as well as for its intrinsic UV lasing response. Similar to carbon, one-dimensional boron nitride nanotubes (BNNTs) have been theoretically predicted and later synthesised. Here we use first principles simulations to unambiguously demonstrate that i) BN nanotubes inherit the highly efficient UV luminescence of hexagonal BN; ii) the application of an external perpendicular field closes the electronic gap keeping the UV lasing with lower yield; iii) defects in BNNTS are responsible for tunable light emission from the UV to the visible controlled by a transverse electric field (TEF). Our present findings pave the road towards optoelectronic applications of BN-nanotube-based devices that are simple to implement because they do not require any special doping or complex growth. [less ▲]

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See detailEffect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2
Molina-Sanchez, Alejandro UL; Sangalli, Davide; Hummer, Kerstin et al

in Physical Review. B, Condensed Matter and Materials Physics (2013)

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ... [more ▼]

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ) and the calculation of the absorption spectra (on the level of the Bethe-Salpeter equation) explicitly include spin-orbit coupling, using the full spinorial Kohn-Sham wave functions as input. Without excitonic effects, the absorption spectra would have the form of a step function, corresponding to the joint density of states of a parabolic band dispersion in two dimensions. This profile is deformed by a pronounced bound excitonic peak below the continuum onset. The peak is split by spin-orbit interaction in the case of single-layer and (mostly) by interlayer interaction in the case of double-layer and bulk MoS2. The resulting absorption spectra are thus very similar in the three cases, but the interpretation of the spectra is different. Differences in the spectra can be seen in the shape of the absorption spectra at 3 eV where the spectra of the single and double layers are dominated by a strongly bound exciton. [less ▲]

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See detailOptical and vibrational properties of 2D materials with ab-initio techniques
Wirtz, Ludger UL

Presentation (2013, April 13)

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See detailManifestation of charged and strained graphene layers in the Raman response of graphite intercalation compounds
Wirtz, Ludger UL; Chacon, Julio; Pichler, Thomas

Scientific Conference (2013)

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See detailContribution of the buffer layer to the Raman spectrum of epitaxial graphene on SiC(0001)
Fromm, F.; Jr, M. H. Oliveira; Molina-Sanchez, Alejandro UL et al

in New Journal of Physics (2013), 15(4), 043031

We report a Raman study of the so-called buffer layer with $(6 3 6 3)R30^ $ periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface ... [more ▼]

We report a Raman study of the so-called buffer layer with $(6 3 6 3)R30^ $ periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a non-vanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer layer. [less ▲]

Detailed reference viewed: 170 (5 UL)