A force-constant model of graphene for conductivity calculations

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 ▲]

Raman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron–phonon coupling in graphene layers

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 ▲]

Excitons in a mirror: Formation of “optical bilayers” using MoS2 monolayers on gold substrates

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 ▲]

Semi-empirical phonon calculations for graphene on different substrates

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 ▲]

Phonon dispersion and optical properties of layered materials: influence of the environment

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 ▲]

Phonon dispersion and optical properties of layered materials: influence of the environment

Presentation (2014, January 16)

F center in lithium fluoride revisited: Comparison of solid-state physics and quantum-chemistry approaches

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 ▲]

Screening of electron-phonon coupling in graphene on Ir(111)

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 ▲]

Efficient Gate-tunable light-emitting device made of defective boron nitride nanotubes: from ultraviolet to the visible

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 ▲]

Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2

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 ▲]