References of "Wirtz, Ludger 50003339"
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See detailQuantum Interference Effects in Resonant Raman Spectroscopy of Single- and Triple-Layer MoTe2 from First-Principles
Pereira Coutada Miranda, Henrique UL; Reichardt, Sven UL; Froehlicher, Guillaume et al

in Nano Letters (2017), 17(4), 2381--2388

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See detailRaman Spectroscopy of Graphene
Reichardt, Sven UL; Wirtz, Ludger UL

in Binder, Rolf (Ed.) Optical Properties of Graphene (2017)

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See detailExcitons in boron nitride single layer
Galvani, Thomas; Paleari, Fulvio UL; Pereira Coutada Miranda, Henrique UL et al

in Physical Review. B : Condensed Matter (2016), 94(125303),

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed ... [more ▼]

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π) and conduction (π∗) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme. [less ▲]

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See detailTheoretical Study of the Raman G Peak Intensity of Graphene
Reichardt, Sven UL; Wirtz, Ludger UL

Poster (2016, February 18)

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See detailTemperature-dependent excitonic effects in the optical properties of single-layer MoS2
Molina-Sanchez, Alejandro UL; Palummo, Maurizia; Marini, Andrea et al

in Physical Review. B : Condensed Matter (2016), 93

The electron-phonon interaction alters substantially the conventional picture of the band structure. It also changes the properties of excitonic states, which are very pronounced in many 2D materials ... [more ▼]

The electron-phonon interaction alters substantially the conventional picture of the band structure. It also changes the properties of excitonic states, which are very pronounced in many 2D materials. Using many-body perturbation theory, the authors describe how the inclusion of temperature modifies the electronic bands of single-layer MoS2. Different bands and different regions in the Brillouin zone are affected in different ways by electron-phonon coupling. Using the temperature-broadened bands as input for the Bethe-Salpeter equation, the authors explain why, for the bound A and B excitons, the electron-phonon coupling changes mainly the position, and for the C exciton, only the width is affected by temperature, while the energy is rather constant. [less ▲]

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See detailVibrational and optical properties of MoS2: From monolayer to bulk
Molina-Sanchez, Alejandro UL; Hummer, Kerstin; Wirtz, Ludger UL

in Surface Science Reports (2015), 70(4), 554-586

Molybdenum disulfide, MoS2, has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore ... [more ▼]

Molybdenum disulfide, MoS2, has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of ~1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS2 exhibits remarkably different physical properties compared to bulk MoS2 due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS2 is indirect, it becomes direct with decreasing number of layers. In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS2. In particular, we focus on the effects of spin–orbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS2 and layered materials. The effect of external strain on the band gap of single-layer MoS2 and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the the phonon dispersion relations of single-layer, few-layer and bulk MoS2. Based on the latter, we explain the behavior of the Raman-active A1gA1g and View the MathML sourceE2g1 modes as a function of the number of layers. Finally, we compare theoretical and experimental results of Raman, photoluminescence, and optical-absorption spectroscopy. [less ▲]

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See detailUnified Description of the Optical Phonon Modes in N-Layer MoTe2
Froehlicher, Guillaume; Lorchat, Etienne; Fernique, François et al

in Nano Letters (2015), 15

N-layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three-dimensional) and monolayer (quasi-two-dimensional) limits ... [more ▼]

N-layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three-dimensional) and monolayer (quasi-two-dimensional) limits. Here, using high-resolution micro-Raman spectroscopy, we report a unified experimental description of the Γ-point optical phonons in N-layer 2H-molybdenum ditelluride (MoTe2). We observe series of N-dependent low-frequency interlayer shear and breathing modes (below 40 cm–1, denoted LSM and LBM) and well-defined Davydov splittings of the mid-frequency modes (in the range 100–200 cm–1, denoted iX and oX), which solely involve displacements of the chalcogen atoms. In contrast, the high-frequency modes (in the range 200–300 cm–1, denoted iMX and oMX), arising from displacements of both the metal and chalcogen atoms, exhibit considerably reduced splittings. The manifold of phonon modes associated with the in-plane and out-of-plane displacements are quantitatively described by a force constant model, including interactions up to the second nearest neighbor and surface effects as fitting parameters. The splittings for the iX and oX modes observed in N-layer crystals are directly correlated to the corresponding bulk Davydov splittings between the E2u/E1g and B1u/A1g modes, respectively, and provide a measurement of the frequencies of the bulk silent E2u and B1u optical phonon modes. Our analysis could readily be generalized to other layered crystals [less ▲]

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See detailElectronic and Vibrational proprieties of graphene on Ir(111) and SiC(100)
Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Poster (2015, September)

In the last years, graphene has become one of the most studied materials due to its peculiar electronic, optical, thermal, and mechanical properties. It is thus of major importance, for practical ... [more ▼]

In the last years, graphene has become one of the most studied materials due to its peculiar electronic, optical, thermal, and mechanical properties. It is thus of major importance, for practical applications, to study how the electronic and vibrational proprieties of graphene change when deposited on a substrate. The non-commensurability of the unit cell of graphene with the substrate leads to the formation of Moiré patterns with accordingly large supercell sizes. Ab-initio calculations using standard plane-wave based codes on these large systems are of high computational cost even for the ground-state calculations. We show the effect that such Moiré patterns have on the band structure by projecting the resulting electronic structure and phonon dispersion onto the unit cell of free-standing graphene with an unfolding scheme. We compare our results with HREELS measurements of the phonon dispersion of graphene on Ir(111). The accurate knowledge of the interaction graphene-substrate will provide important information for future applications of graphene on electronic devices. Work performed in collaboration with the experimental groups of J. Kroeger (TU Ilmenau, Germany) and T. Seyller (TU Chemnitz, Germany). [less ▲]

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See detailPhonon-limited carrier mobility and resistivity from carbon nanotubes to graphene
Li, Jing; Pereira Coutada Miranda, Henrique UL; Niquet, Yann-Michel et al

in Physical Review. B: Condensed Matter and Materials Physics (2015)

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon ... [more ▼]

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon-limited electrical mobility in graphene and in a wide range of CNTs of different types to address this issue. The theoretical study is based on a tight-binding method and a force-constant model from which all possible electron-phonon couplings are computed. The electrical resistivity of graphene is found in very good agreement with experiments performed at high carrier density. A common methodology is applied to study the transition from one to two dimensions by considering CNTs with diameter up to 16 nm. It is found that the mobility in CNTs of increasing diameter converges to the same value, i.e., the mobility in graphene. This convergence is much faster at high temperature and high carrier density. For small-diameter CNTs, the mobility depends strongly on chirality, diameter, and the existence of a band gap. [less ▲]

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See detailAb-initio study of the temperature effects on the optical properties of transition metal dichalcogenides
Molina-Sanchez, Alejandro UL; Palummo, Maurizia; Marini, Andrea et al

Scientific Conference (2015, March 05)

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See detailA force-constant model of graphene for conductivity calculations
Pereira Coutada Miranda, Henrique UL; Wirtz, Ludger UL

Poster (2015, January)

Transport in graphene is strongly limited by the electron-phonon interaction. Accurate description of the phonon dispersion relations is essential for the study of this interaction. Using current state-of ... [more ▼]

Transport in graphene is strongly limited by the electron-phonon interaction. Accurate description of the phonon dispersion relations is essential for the study of this interaction. Using current state-of-the-art ab initio density-functional theory plane-wave codes, we are limited to systems with few atoms. For larger systems (e.g., nanotubes, nanoribbons), accurate semi-empircal models are needed. We have developed a force constant model for the phonon dispersion of graphene. Our implementation can include a large number of neighbours, which allows us to simulate accurately long-range interaction effects. As shown in previous publications it is possible to reproduce the phonon dispersion frequencies of graphene with a 4th nearest neighbours force constant model. However, some features can only be captured using long-range interactions (Kohn-anomalies, certain phonon eigenvectors). Using an ab initio phonon dispersion calculated with DFPT as reference, we show the nature of the long-range interactions and explore different ways to include them in our semi-empirical model. We also study the dependence of the force constants on charge and strain. Work in collaboration with Jing Li, Yann-Michel Niquet, Luigi Genovese, and Ivan Duchemin from L_Sim, SP2M, UMR-E CEA/UJF-Grenoble 1, INAC, Grenoble, France and Christophe Delerue from IEMN - Dept. ISEN, UMR CNRS 8520, Lille, France [less ▲]

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See detailAb initio perspective on the Mollwo-Ivey relation for F centers in alkali halides
Tiwald, Paul; Karsai, Ferenc; Laskowski, Robert et al

in Physical Review. B: Condensed Matter and Materials Physics (2015), 92

We revisit the well-known Mollwo-Ivey relation that describes the ``universal'' dependence of the absorption energies of F-type color centers on the lattice constant a of alkali-halide crystals, E-abs ... [more ▼]

We revisit the well-known Mollwo-Ivey relation that describes the ``universal'' dependence of the absorption energies of F-type color centers on the lattice constant a of alkali-halide crystals, E-abs proportional to a(-n). We perform both state-of-the-art ab initio quantum chemistry and post-DFT calculations of F-center absorption spectra. By ``tuning'' independently the lattice constant and the atomic species we show that the scaling with the lattice constant alone 2 in agreement with the ``particle-in-the-box'' model. Keeping the lattice constant fixed and changing the atomic species enables us to quantify the ion-size effects which are shown to be responsible for the exponent n approximate to 1.8. [less ▲]

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See detailRaman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron–phonon coupling in graphene layers
Chacón-Torres, Julio; Wirtz, Ludger UL; Pichler, Thomas

in Physica Status Solidi B. Basic Research (2014), 251(12), 23372355

Graphite intercalation compounds (GICs) are an interesting and highly studied field since 1970’s. It has gained renewed interest since the discovery of superconductivity at high temperature for CaC6 and ... [more ▼]

Graphite intercalation compounds (GICs) are an interesting and highly studied field since 1970’s. It has gained renewed interest since the discovery of superconductivity at high temperature for CaC6 and the rise of graphene. Intercalation is a technique used to introduce atoms or molecules into the structure of a host material. Intercalation of alkali metals in graphite has shown to be a controllable procedure recently used as a scalable technique for bulk production of graphene, and nano-ribbons by induced exfoliation of graphite. It also creates supra-molecular interactions between the host and the intercalant, inducing changes in the electronic, mechanical, and physical properties of the host. GICs are the mother system of intercalation also employed in fullerenes, carbon nanotubes, graphene, and carbon-composites. We will show how a combination of Raman and ab-initio calculations of the density and the electronic band structure in GICs can serve as a tool to elucidate the electronic structure, electron–phonon coupling, charge transfer, and lattice parameters of GICs and the graphene layers within. This knowledge of GICs is of high importance to understand superconductivity and to set the basis for applications with GICs, graphene and other nano-carbon based materials like nanocomposites in batteries and nanoelectronic devices. [less ▲]

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See detailMoiré-induced replica of graphene phonons on Ir(111)
Endlich, Michael; Pereira Coutada Miranda, Henrique UL; Molina-Sanchez, Alejandro UL et al

in Annalen der Physik (2014), 526(9-10), 372-380

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure ... [more ▼]

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure. Calculations for a linear chain of C atoms attached to an infinitely heavy substrate reveal that imposing a superstructure by periodically varying the C-C interaction and the C-substrate coupling induces replicated phonons at wave vectors reflecting the supercell periodicity. Deviations between the phonon dispersion of graphene on Ir(111) and of pristine graphene are analyzed and rationalized in terms of the weak graphene-Ir(111) interaction. [less ▲]

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See detailExcitons in a mirror: Formation of “optical bilayers” using MoS2 monolayers on gold substrates
Mertens, Jan; Shi, Yumeng; Molina-Sanchez, Alejandro UL et al

in Applied Physics Letters (2014), 104

We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas ... [more ▼]

We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a “mirror biexciton” with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS2 on gold thus resembles a bilayer of MoS2 which has been created by optical coupling. Additional top-mirrors produce an “optical bulk.” [less ▲]

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See detailPhonons of graphene on metallic and semiconductor surfaces, an ab-inito approach
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

Scientific Conference (2014, April 04)

The interaction of graphene with substrates can alter its electronic and vibrational properties and is relevant for the practical use of graphene. In this work, we describe the graphene-substrate ... [more ▼]

The interaction of graphene with substrates can alter its electronic and vibrational properties and is relevant for the practical use of graphene. In this work, we describe the graphene-substrate interaction through the theoretical study of the vibrational properties. We focus on three paradigmatic cases where the interaction strength changes gradually: graphene@BN, graphene@Ir(111), and graphene@SiC (i.e., the buffer layer). We use ab-initio methods to obtain the phonon modes, the density of states, and the strength of the electron-phonon coupling. When we deal with large supercells, we use an unfolding scheme to visualize the phonon bands in the primitive unit cell. Thus, we can distinguish clearly the changes in the phonon dispersion of perturbed-graphene with respect to the one of pristine graphene. Graphene on boron nitride exhibits a weak interaction but a non-negligible shift of the 2D Raman band. We explain this observation as due to a weakening of the electron-phonon interaction via screening of electron-electron correlation by the dielectric substrate. Graphene on iridium, also displays weak interaction but the underlying material is a metal. This leads to an even more pronounced screening of the electron-electron interaction in graphene. In the last case, we study the buffer layer of graphene on silicon carbide. The hybridization of graphene with silicon carbide changes the electronic structure of graphene and the phonon bands. [less ▲]

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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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