arXiv subjects

Gotthard Seifert

Publications and source records attributed to Gotthard Seifert.

At least 19 recordsLinked to original sources

arXiv2026

Graphene-based technologies for energy applications, challenges and perspectives

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

cond-mat.mtrl-sci↗

arXiv2024

Adsorption of molecular hydrogen on honeycomb ZnO monolayers: A quantum density-functional theory perspective

We investigate the adsorption of molecular hydrogen on pristine zinc oxide (ZnO) platelets. The volumetric and gravimetric hydrogen storage capacities of the ZnO monolayers are evaluated in a broad range of thermodynamic conditions (i.e., for temperatures in the range 77 K < T < 450 K, and for external gas pressures up to 200 bar). The thermodynamic properties and the microscopic spatial distribution of the adsorbed hydrogen fluid are assessed within the density functional theory of liquids for quantum fluids at finite temperature (QLDFT), and the adsorption enthalphies are obtained by fitting the computed adsorption densities to the Toth model isotherm. Compared to graphene platelets, the ZnO sheets impose a rather tighter confinement to the motion of the hydrogen molecules parallel to the surface. The isosteric heat of adsorption approaches 3.2 kJ/mol in the low density regime. This quantity shows a fairly smooth dependence on the hydrogen uptake for temperatures below 100 K, while it is shown to depend quite sensitively on the adsorbate density above this temperature.

cond-mat.mtrl-sci↗

arXiv2023

Giant Apparent Flexoelectricity in Semiconductors Driven by Insulator-to-metal Transition

We elucidate the flexoelectricity of materials in the high strain gradient regime, of which the underlying mechanism is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high strain gradient-induced insulator-to-metal transition. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film, resulting in a spatial separation of electron and hole. Therefore, upon the insulator-to-metal transition, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to pronounced polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high, with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

cond-mat.mtrl-sci↗

arXiv2020

Liquid Flow through Defective Layered Membranes: A Phenomenological Description

We present a realistic phenomenological description of liquid transport through defective, layered membranes. We derive general expressions based on conventional models of laminar flow and extend the formalism to accommodate slip flow. We consider different types of defects including in-layer vacancies that provide an activation-free tortuous path through the membrane. Of the many factors that affect flow, the most important is the radius of in-layer vacancy defects, which enters in the fourth power in expressions for the flux density. We apply our formalism to water transport through defective multilayer graphene oxide membranes and find that the flow remains in the laminar regime. Our results show that observed high water permeability in this system can be explained quantitatively by a sufficient density of in-layer pores that shorten the effective diffusion path.

cond-mat.mtrl-sci↗

arXiv2020

Insights into regularity of 2D 3d transition metal monocarbides formation

Recently several theoretical predictions were made about 2D planar FeC, CoC, NiC, and CuC while their bulk phases still remain unknown. Here, we present generalization of 2D family of 3d transition metal monocarbides (TMC) by searching their stable configurations with DFT methods and evolutionary algorithm. It is found that in the TMC row (TM = Sc-Cu) a tendency of 3D rocksalt phase formation is monotonously interchanging by 2D phase appearance, namely planar orthorhombic TMC characterized by carbon dimers inside metal hexagons. Among them, orthorhombic CoC and FeC monocarbides would be likely formed rather than any other 2D metal carbide phase or metal/graphene interface.

cond-mat.mtrl-sci↗

arXiv2020

Exciton g-factors of van der Waals heterostructures from first principles calculations

External fields are a powerful tool to probe optical excitations in a material. The linear energy shift of an excitation in a magnetic field is quantified by its effective g-factor. Here we show how exciton g-factors and their sign can be determined by converged first principles calculations. We apply the method to monolayer excitons in semiconducting transition metal dichalcogenides and to interlayer excitons in MoSe$_2$/WSe$_2$ heterobilayers and obtain good agreement with recent experimental data. The precision of our method allows to assign measured g-factors of optical peaks to specific transitions in the band structure and also to specific regions of the samples. This revealed the nature of various, previously measured interlayer exciton peaks. We further show that, due to specific optical selection rules, g-factors in van der Waals heterostructures are strongly spin- and stacking-dependent. The calculation of orbital angular momenta requires the summation over hundreds of bands, indicating that for the considered two-dimensional materials the basis set size is a critical numerical issue. The presented approach can potentially be applied to a wide variety of semiconductors.

cond-mat.mtrl-sci↗

arXiv2019

Thermodynamic stability of Borophene, $\mathrm{B_2O_3}$ and other $\mathrm{B_{1-x}O_x}$ sheets

The recent discovery of borophene, a two-dimensional allotrope of boron, raises many questions about its structure and its chemical and physical properties. Boron has a high chemical affinity to oxygen but little is known about the oxidation behavior of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional $\mathrm{B_{1-x}O_x}$ for compositions ranging from $x=0$ to $x=0.6$, which correspond to borophene and $\mathrm{B_2O_3}$ sheets, respectively. Our results indicate that no stable compounds except borophene and $\mathrm{B_2O_3}$ sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and $\mathrm{B_2O_3}$. Other hypothetical crystals such as $\mathrm{B_2O}$ are unstable and some of them were found to undergo spontaneous disproportionation into borophene and $\mathrm{B_2O_3}$. It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming $\mathrm{B_2O_3}$ at the edges. All findings can be rationalized by oxygen's preference of two-fold coordination which is incompatible with higher in-plane coordination numbers preferred by boron. These results agree well with recent experiments and pave the way to understand the process of oxidation of borophene and other two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2019

Luminescent emission of excited Rydberg excitons from monolayer WSe2

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of g-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

cond-mat.mes-hall↗

arXiv2019

An efficient way to model complex magnetite: assessment of SCC-DFTB against DFT

Magnetite has attracted increasing attention in recent years due to its promising and diverse applications in biomedicine. Theoretical modelling can play an important role in understanding magnetite-based nanomaterials at the atomic scale for a deeper insight into the experimental observations. However, calculations based on density functional theory (DFT) are too costly for realistically large models of magnetite nanoparticles. Classical force field methods are very fast but lack of precision and of the description of electronic effects. Therefore, a cheap and efficient quantum mechanical simulation method with comparable accuracy than DFT is highly desired. Here, a less computational demanding DFT-based method, i.e. self-consistent charge density functional tight-binding (SCC-DFTB), is adopted to investigate magnetite bulk and low-index (001) surface with newly proposed parameters for Fe-O interactions. We report that SCC-DFTB with on-site Coulomb correction provides results in quantitatively comparable agreement with those obtained by DFT+U and hybrid functional methods. Therefore, SCC-DFTB is valued as an efficient and reliable method for the description magnetite. This assessment will promote SCC-DFTB computational studies on magnetite-based nanostructures that attract increasing attention for medical applications.

cond-mat.mtrl-sci↗

arXiv2018

Momentum-space indirect interlayer excitons in transition metal dichalcogenide van der Waals heterostructures

Monolayers of transition metal dichalcogenides (TMDCs) feature exceptional optical properties that are dominated by excitons, tightly bound electron-hole pairs. Forming van der Waals heterostructures by deterministically stacking individual monolayers allows to tune various properties via choice of materials and relative orientation of the layers. In these structures, a new type of exciton emerges, where electron and hole are spatially separated. These interlayer excitons allow exploration of many-body quantum phenomena and are ideally suited for valleytronic applications. Mostly, a basic model of fully spatially-separated electron and hole stemming from the $K$ valleys of the monolayer Brillouin zones is applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first principle calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS$_2$/WSe$_2$ heterostructures residing at the $Γ$ and $K$ valleys. We control the emission energy of this new type of momentum-space indirect, yet strongly-bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in TMDC heterostructures and devices.

cond-mat.mes-hall↗

arXiv2017

Chemical and Electronic Repair Mechanism of Sulfur Defects in MoS$_2$ Monolayers

Using {\em ab initio} density functional theory calculations, we characterize changes in the electronic structure of MoS$_{2}$ monolayers introduced by missing or additional adsorbed sulfur atoms. We furthermore identify the chemical and electronic function of substances that have been reported to reduce the adverse effect of sulfur vacancies in quenching photoluminescence and reducing electronic conductance. We find that thiol-group containing molecules adsorbed at vacancy sites may re-insert missing sulfur atoms. In presence of additional adsorbed sulfur atoms, thiols may form disulfides on the MoS$_{2}$ surface to mitigate the adverse effect of defects.

physics.chem-ph↗

arXiv2016

Localized defect states in MoS$_2$ monolayers: electronic and optical properties

Defects usually play an important role in tuning and modifying various properties of semiconducting or insulating materials. Therefore we study the impact of point and line defects on the electronic structure and optical properties of MoS2 monolayers using density-functional methods. The different types of defects form electronic states that are spatially localized on the defect. The strongly localized nature is reflected in weak electronic interactions between individual point or line defect and a weak dependence of the defect formation energy on the defect concentration or line defect separation. In the electronic energy spectrum the defect states occur as deep levels in the band gap, as shallow levels very close to the band edges, as well as levels in-between the bulk states. Due to their strongly localized nature, all states of point defects are sharply peaked in energy. Periodic line defects form nearly dispersionless one-dimensional band structures and the related spectral features are also strongly peaked. The electronic structure of the monolayer system is quite robust and it is well preserved for point defect concentrations of up to 6%. The impact of point defects on the optical absorption for concentrations of 1% and below is found to be very small. For higher defect concentrations molybdenum vacancies were found to quench the overall absorption and sulfur defects lead to sharp absorption peaks below the absorption edge of the ideal monolayer. For line defects, we did not find a considerable impact on the absorption spectrum. These results support recent experiments on defective transition metal chalcogenides.

cond-mat.mes-hall↗

arXiv2013

Defect-induced conductivity anisotropy in MoS2 monolayers

Various types of defects in MoS2 monolayers and their influence on the electronic structure and transport properties have been studied using the Density-Functional based Tight-Binding method in conjunction with the Green's Function approach. Intrinsic defects in MoS2 monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS2, strong anisotropy is observed in the presence of defects. Localized mid-gap states are observed in semiconducting MoS2 that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models.

cond-mat.mtrl-sci↗

arXiv2013

Theory of second harmonic generation in few-layered MoS2

Recent experimental results have demonstrated the ability of monolayer MoS$_2$ to efficiently generate second harmonic fields with susceptibilities between 0.1 and 100 nm/V. However, no 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 single-particle calculations of the second harmonic response based on a tight-binding band structure. We compare directly with recent experimental findings and include in the discussion also spectral features and the effects of multiple layers.

cond-mat.mes-hall↗

arXiv2013

Optimizing electronic structure and quantum transport at the graphene-Si(111) interface: An ab-initio density-functional study

We use ab initio density functional calculations to determine the interaction of a graphene monolayer with the Si(111) surface. We found that graphene forms strong bonds to the bare substrate and accommodates the 12% lattice mismatch by forming a wavy structure consisting of free-standing conductive ridges that are connected by ribbon-shaped regions of graphene, which bond covalently to the substrate. We perform quantum transport calculations for different geometries to study changes in the transport properties of graphene introduced by the wavy structure and bonding to the Si substrate. Our results suggest that wavy graphene combines high mobility along the ridges with efficient carrier injection into Si in the contact regions.

cond-mat.mtrl-sci↗

arXiv2013

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS2 are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS2 is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS2 layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS2. The most stable non-stoichiometric defect structures are observed experimentally, one of which contains metallic Mo-Mo bonds and another one bridging S atoms.

cond-mat.mtrl-sci↗

arXiv2013

Graphene nanoflakes - structural and electronic properties

The structures, cohesive energies and HOMO-LUMO gaps of graphene nanoflakes and corresponding polycyclic aromatic hydrocarbons for a large variety of size and topology are investigated at the density functional based tight-binding level. Polyacene-like and honeycomb-like graphene nanoflakes were chosen as the topological limit structures. The influence of unsaturated edge atoms and dangling bonds on the stability is discussed. Our survey shows a linear trend for the cohesive energy as function of Ns/N (N - total number of atoms and Ns is number of edge atoms). For the HOMO-LUMO gap the trends are more complex and include also the topology of the edges.

cond-mat.mtrl-sci↗

arXiv2012

Density-functional study of LixMoS2 intercalates (0<=x<=1)

The stability of Lithium intercalated 2H- and 1T allotropes of Molybdenum disulfide (LixMoS2) is studied within a density-functional theory framework as function of the Li content (x) and the intercalation sites. Octahedral coordination of Li interstitials in the van der Waals gap is found as the most favorite for both allotropes. The critical content of Lithium, required for the initialization of a 2H->1T phase transition is estimated to x ~ 0.4. For smaller Li contents the hexagonal 2H crystal structure is not changed, while 1T-LixMoS2 compounds adopt a monoclinic lattice. All allotropic forms of LixMoS2 - excluding the monoclinic Li1.0MoS2 structure - show metallic-like character. The monoclinic Li1.0MoS2 is a semiconductor with a band gap of 1.1 eV. Finally, the influence of Li intercalation on the stability of multiwalled MoS2 nanotubes is discussed within a phenomenological model.

cond-mat.mtrl-sci↗