Lars Matthes

Infrared absorbance of silicene and germanene

Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K′ contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology.

Optical properties of two-dimensional honeycomb crystals graphene, silicene, germanene, and tinene from first principles

We compute the optical conductivity of 2D honeycomb crystals beyond the usual Dirac-cone approximation. The calculations are mainly based on the independent-quasiparticle approximation of the complex dielectric function for optical interband transitions. The full band structures are taken into account. In the case of silicene, the influence of excitonic effects is also studied. Special care is taken to derive converged spectra with respect to the number of k points in the Brillouin zone and the number of bands. In this way both the real and imaginary parts of the optical conductivity are correctly described for small and large frequencies. The results are applied to predict the optical properties reflection, transmission and absorption in a wide range of photon energies. They are discussed in the light of the available experimental data.

Silicene on metal and metallized surfaces: ab initio studies

The deposition of silicene on several metals is investigated. For fcc crystals the (111) surfaces while for hexagonal ones the (0001) surfaces are used. The Ca(111)1 ? 1 substrate is found to be the most promising candidate. The silicene adsorption on Ca-functionalized Si(111)1 ? 1 and 2 ? 1 surfaces is also studied. The 1 ? 1 substrates lead to overlayer silicene with hexagonal symmetry and Dirac cones. However, the Dirac points are below the Fermi level, and small energy gaps are opened. In the case of 2 ? 1 surfaces, strong lattice relaxation occurs. Only rudiments of conical linear bands remain visible.

Optical absorption and emission of α-Sn nanocrystals from first principles

We investigate the optical properties of hydrogenated α-Sn nanocrystals up to diameters of 3.6 nm in the framework of an ab initio pseudopotential method including spin-orbit interaction (SOI) and the repeated supercell approximation. The fundamental absorption and emission edges are determined including quasiparticle effects and electron-hole interaction. The atomic geometries in the ground and excited electronic states follow from total energy optimization. We discuss the oscillator strengths of the optical absorption near the fundamental gaps for the most important transitions. We demonstrate that the spectra can only be correctly described including SOI. The strongly size-dependent Stokes shifts between optical absorption and emission are shown to be mainly a consequence of the different atomic geometries.

Flexible 2D Crystals of Polycyclic Aromatics Stabilized by Static Distortion Waves

The epitaxy of many organic films on inorganic substrates can be classified within the framework of rigid lattices which helps to understand the origin of energy gain driving the epitaxy of the films. Yet, there are adsorbate–substrate combinations with distinct mutual orientations for which this classification fails and epitaxy cannot be explained within a rigid lattice concept. It has been proposed that tiny shifts in atomic positions away from ideal lattice points, so-called static distortion waves (SDWs), are responsible for the observed orientational epitaxy in such cases. Using low-energy electron diffraction and scanning tunneling microscopy, we provide direct experimental evidence for SDWs in organic adsorbate films, namely hexa-peri-hexabenzocoronene on graphite. They manifest as wave-like sub-Ångström molecular displacements away from an ideal adsorbate lattice which is incommensurate with graphite. By means of a density-functional-theory based model, we show that, due to the flexibility in the adsorbate layer, molecule–substrate energy is gained by straining the intermolecular bonds and that the resulting total energy is minimal for the observed domain orientation, constituting the orientational epitaxy. While structural relaxation at an interface is a common assumption, the combination of the precise determination of the incommensurate epitaxial relation, the direct observation of SDWs in real space, and their identification as the sole source of epitaxial energy gain constitutes a comprehensive proof of this effect.

Modeling of structure and properties of silicene and related novel 2D crystals

Silicene and related 2D crystals with honeycomb symmetry are of increasing interest because of their compatibility with the Si-device technology. By means of density functional and many-body perturbation theory we model their principal properties and growth on substrates. In the case of hydrogenated crystals also excitons in two dimensions are studied.

Charge-Carrier Transport Through Guanine Crystals and Stacks

We study small-polaron motion through guanine-based materials. The temperature dependence and anisotropy of charge carrier (hole) transport in crystalline guanine is investigated by employing the Kubo formalism and calculating the hole mobilities with ab initio DFT material parameters. We discuss our findings in relation to transport pathways in DNA-based structures like guanine quadruplexes and ribbons which are considered to play a major role in DNA-based nanoelectronics. The mobility results are interpreted by help of a novel visualization method for transport channels which we derive from overlapping wavefunctions. An analysis of coherent and incoherent contributions to the mobility shows that even in materials with high purity and long-range order like crystals only incoherent phonon-assisted hopping occurs at room temperature.