arXiv subjects

Gotthard Seifert

Publications and source records attributed to Gotthard Seifert.

25 records · Page 2Linked to original sources

arXiv2012

Designing electrical contacts to MoS$_2$ monolayers: A computational study

Studying the reason, why single-layer molybdenum disulfide (MoS$_2$) appears to fall short of its promising potential in flexible nanoelectronics, we found that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS$_2$/metal contacts, we performed ab initio density functional theory calculations for the geometry, bonding and electronic structure of the contact region. We found that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS$_2$ and propose Ti as a representative example of suitable alternative electrode materials.

cond-mat.mtrl-sci↗

arXiv2011

New route for stabilization of 1T-WS2 and MoS2 phases

The phenomenon of a partial 2H\rightarrow1T phase transition within multiwalled WS2 nanotubes under substitutional Rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional calculations for the related MoS2 compound we consider a possible origin of this phase transition, which was known formerly only for WS2 and MoS2 intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

cond-mat.mtrl-sci↗

arXiv2011

Structure and electronic structure of Metal-Organic Frameworks within the Density-Functional based Tight-Binding method

Density-functional based tight-binding is a powerful method to describe large molecules and materials. Metal-Organic Frameworks (MOFs), materials with interesting catalytic properties and with very large surface areas have been developed and have become commercially available. Unit cells of MOFs typically include hundreds of atoms, which make the application of standard Density-Functional methods computationally very expensive, sometimes even unfeasible. The aim of this paper is to prepare and to validate the Self-Consistent Charge Density-Functional based Tight Binding (SCC-DFTB) method for MOFs containing Cu, Zn and Al metal centers. The method has been validated against full hybrid density-functional calculations for model clusters, against gradient corrected density-functional calculations for supercells, and against experiment. Moreover, the modular concept of MOF chemistry has been discussed on the basis of their electronic properties. We concentrate on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn4O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)) and aluminium oxide AlO4(OH)2 octahedron (MIL-53). We show that SCC-DFTB predicts structural parameters with a very good accuracy (with less than 5% deviation, even for adsorbed CO and H2O on HKUST-1), while adsorption energies differ by 12 kJ mol-1 or less for CO and water compared to DFT benchmark calculations.

cond-mat.mtrl-sci↗

arXiv2011

Hexagon Preserving Carbon Nanofoams

Carbon foams are hypothetical carbon allotropes that contain graphite-like (sp$^2$ carbon) segments, connected by sp$^3$ carbon atoms, resulting in porous structures. In this work the DFTB (Density Functional based Tight-Binding) method with periodic boundary conditions (PBC) was employed to study energetics, the stability and electronic properties of this unusual class of carbon systems. Concerning the most stable phases of carbon (graphite and diamond), foams show high structural stability at very low mass density. The electronic band structure and electronic DOS (density of states) of foams indicate a similar size dependence as carbon nanotubes. The calculated bulk moduli are in the range between that of graphite (5.5 GPa) and diamond (514 GPa). These structures may represent novel stable carbon modifications with sp$^2$+sp$^3$ hybridization.

cond-mat.mtrl-sci↗

arXiv2010

Prediction of huge magnetic anisotropies of transition-metal dimer-benzene complexes

Based on numerically accurate density functional theory (DFT) calculations, we systematically investigate the ground-state structure and the spin and orbital magnetism including the magnetic anisotropy energy (MAE) of 3d- and 4d-transition-metal dimer benzene complexes (TM2Bz, TM = Fe, Co, Ni, Ru, Rh, Pd; Bz = C6H6). These systems are chosen to model TM-dimer adsorption on graphene or on graphite. We find that Fe2, Co2, Ni2, and Ru2 prefer the upright adsorption mode above the center of the benzene molecule, while Rh2 and Pd2 are adsorbed parallel to the benzene plane. The ground state of Co2Bz (with a dimer adsorption energy of about 1 eV) is well separated from other possible structures and spin states. In conjunction with similar results obtained by ab initio quantum chemical calculations, this implies that a stable Co2Bz complex with C6v symmetry is likely to exist. Chemical bonding to the carbon ring does not destroy the magnetic state and the characteristic level scheme of the cobalt dimer. Calculations including spin- orbit coupling show that the huge MAE of the free Co dimer is preserved in the Co2Bz structure. The MAE predicted for this structure is much larger than the MAE of other magnetic molecules known hitherto, making it an interesting candidate for high-density magnetic recording. Among all the other investigated complexes, only Ru2Bz shows a potential for strong-MAE applications, but it is not as stable as Co2Bz. The electronic structure of the complexes is analyzed and the magnitude of their MAE is explained by perturbation theory.

cond-mat.mes-hall↗

arXiv2009

Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits

It is demonstrated by means of density functional and ab-initio quantum chemical calculations, that transition metal - carbon systems have the potential to enhance the presently achievable area density of magnetic recording by three orders of magnitude. As a model system, Co_2-benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order of 0.1 eV per molecule, large enough to store permanently one bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite. It is suggested that the subnanometer bits can be written by simultaneous application of a moderate magnetic and a strong electric field.

cond-mat.mtrl-sci↗

arXiv2002

Ab initio many-body calculations of static dipole polarizabilities of linear carbon chains and chain-like boron clusters

In this paper we report a theoretical study of the static dipole polarizability of two one-dimensional structures: (a) linear carbon chains C$_{n} (n=2-10)$ and (b) ladder-like planar boron chains B$_{n} (n=4-14)$. The polarizabilities of these chains are calculated both at the Hartree-Fock and the correlated level by applying accurate ab initio quantum chemical methods. Methods such as restricted Hartree-Fock, multi-configuration self-consistent field, multi-reference configuration-interaction method, Møller-Plesset second-order perturbation theory, and coupled-cluster singles, doubles and triples level of theory were employed. Results obtained from ab initio wave-function-based methods are compared with the ones obtained from the density-functional theory. For the clusters studied, directionally averaged polarizability per atom for both the systems is seen to increase with the chain size.

cond-mat↗