Thomas Garm Pedersen

Bandgap opening in graphene induced by patterned hydrogen adsorption

Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many emerging areas such as graphene-based electronic devices. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice, but experimental evidence for this is so far lacking. Here, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the Moiré superlattice positions of graphene grown on an Ir(111) substrate.

Electronic properties of graphene antidot lattices

Graphene antidot lattices constitute a novel class of nano-engineered graphene devices with controllable electronic and optical properties. An antidot lattice consists of a periodic array of holes that causes a band gap to open up around the Fermi level, turning graphene from a semimetal into a semiconductor. We calculate the electronic band structure of graphene antidot lattices using three numerical approaches with different levels of computational complexity, efficiency and accuracy. Fast finite-element solutions of the Dirac equation capture qualitative features of the band structure, while full tight-binding calculations and density functional theory (DFT) are necessary for more reliable predictions of the band structure. We compare the three computational approaches and investigate the role of hydrogen passivation within our DFT scheme.

Plasmon−Phonon Coupling in Large-Area Graphene Dot and Antidot Arrays Fabricated by Nanosphere Lithography

Nanostructured graphene on SiO2 substrates paves the way for enhanced light-matter interactions and explorations of strong plasmon-phonon hybridization in the mid-infrared regime. Unprecedented large-area graphene nanodot and antidot optical arrays are fabricated by nanosphere lithography, with structural control down to the sub-100 nm regime. The interaction between graphene plasmon modes and the substrate phonons is experimentally demonstrated, and structural control is used to map out the hybridization of plasmons and phonons, showing coupling energies of the order 20 meV. Our findings are further supported by theoretical calculations and numerical simulations.

Optical properties of graphene antidot lattices

Undoped graphene is semimetallic and thus not suitable for many electronic and optoelectronic applications requiring gapped semiconductor materials. However, a periodic array of holes (antidot lattice) renders graphene semiconducting with a controllable band gap. Using atomistic modeling, we demonstrate that this artificial nanomaterial is a dipole-allowed direct-gap semiconductor with a very pronounced optical-absorption edge. Hence, optical infrared spectroscopy should be an ideal probe of the electronic structure. To address realistic experimental situations, we include effects due to disorder and the presence of a substrate in the analysis.

Clar sextet analysis of triangular, rectangular, and honeycomb graphene antidot lattices.

Pristine graphene is a semimetal and thus does not have a band gap. By making a nanometer scale periodic array of holes in the graphene sheet a band gap may form; the size of the gap is controllable by adjusting the parameters of the lattice. The hole diameter, hole geometry, lattice geometry, and the separation of the holes are parameters that all play an important role in determining the size of the band gap, which, for technological applications, should be at least of the order of tenths of an eV. We investigate four different hole configurations: the rectangular, the triangular, the rotated triangular, and the honeycomb lattice. It is found that the lattice geometry plays a crucial role for size of the band gap: the triangular arrangement displays always a sizable gap, while for the other types only particular hole separations lead to a large gap. This observation is explained using Clar sextet theory, and we find that a sufficient condition for a large gap is that the number of sextets exceeds one-third of the total number of hexagons in the unit cell. Furthermore, we investigate nonisosceles triangular structures to probe the sensitivity of the gap in triangular lattices to small changes in geometry.

Theory of excitonic second-harmonic generation in monolayer MoS2

Recent experimental results have demonstrated the ability of monolayer

Density functional study of graphene antidot lattices: Roles of geometrical relaxation and spin

{\mathrm{MoS}}_{2}

Characterization of azobenzene chromophores for reversible optical data storage: molecular quantum calculations

to efficiently generate second harmonic fields with susceptibilities between 0.1 and 100 nm/V. However, few theoretical calculations exist with which to interpret these findings. In particular, it is of interest to theoretically estimate the modulus of the second harmonic response since experimental reports on this differ by almost three orders of magnitude. Here, we present calculations of the second harmonic response based on a tight-binding band structure and implementation of excitons in a Bethe-Salpeter framework. We compare directly with recent experimental findings demonstrating a good agreement with the excitonic theory regarding, e.g., peak position. Furthermore, we predict an off-resonance susceptibility on the order of 0.1 nm/V, while on-resonance values rise to 4 nm/V.

Reliability of point source approximations in compact LED lens designs

Received 8 April 2009; revised manuscript received 29 June 2009; published 18 September 2009Graphene sheets with regular perforations, dubbed as antidot lattices, have theoretically been predicted tohave a number of interesting properties. Their recent experimental realization with lattice constants below 100nanometers stresses the urgency of a thorough understanding of their electronic properties. In this work, weperform calculations of the band structure for various hydrogen-passivated hole geometries using both spin-polarized density functional theory DFT and DFT based tight-binding DFTB and address the importance ofrelaxation of the structures using either method or a combination thereof. We find from DFT that all structuresinvestigated have band gaps ranging from 0.2 to 1.5 eV. Band gap sizes and general trends are well capturedby DFTB with band gaps agreeing within about 0.2 eV even for very small structures. A combination of thetwo methods is found to offer a good trade-off between computational cost and accuracy. Both methods predictnondegenerate midgap states for certain antidot hole symmetries. The inclusion of spin results in a spin-splitting of these states as well as magnetic moments obeying the Lieb theorem. The local-spin texture of bothmagnetic and nonmagnetic symmetries is addressed.DOI: 10.1103/PhysRevB.80.115117 PACS number s : 73.63.Fg

Correlation and dimensional effects of trions in carbon nanotubes

Azobenzene chromophores are promising as molecularly engineered materials for reversible optical data storage based on molecular reorientation. In this paper, the optical properties of several different azobenzene chromophores are studied using molecular quantum calculations. Special emphasis is put on molecular anisotropy since a high degree of anisotropy is essential for the storage performance. The trans isomers are all found to be practically one-dimensional whereas the anisotropy of the cis isomers is highly dependent on substituents. Molecular reorientation of chromophores in liquid-crystalline polymers is simulated in order to study the influence of lacking cis anisotropy. It is demonstrated that photoinduced birefringence is significantly reduced in materials characterized by a low degree of cis anisotropy.