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See detailFermi energy dependence of the optical emission in core/shell InAs nanowire homostructures
Möller, Michael; Oliveira, DS; Sahoo, PK et al

in Nanotechnology (2017), 28

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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 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 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 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 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 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 detailTight-Binding Simulations of Nanowires
García-Cristóbal, Alberto; Molina-Sanchez, Alejandro UL

in Bhushan, Bharat (Ed.) Encyclopedia of Nanotechnology (2015)

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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 detailOptical Properties of MoS2: Excitons Beyond the Bandgap
Molina-Sanchez, Alejandro UL

Scientific Conference (2014, August 26)

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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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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 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 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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