arXiv subjects

Thomas Heine

Publications and source records attributed to Thomas Heine.

At least 55 records · Page 3Linked to original sources

arXiv2017

The effects of halogen elements on the opening of an icosahedral B12 framework

The fully halogenated or hydrogenated B12X12 (X = H, F, Cl, Br and I) clusters are confirmed to be icosahedral. On the other hand, the bare B12 cluster is shown to have a planar structure. A previous study showed that a transformation from an icosahedron to a plane happens when 5 to 7 iodine atoms are substituted. Later, the transition was confirmed to be seven iodine substitutions based on an infrared spectroscopy study. In this study, we investigated the effects of different halogen atoms on the opening of the B12 icosahedral cage by means of density functional theory calculations. We found that the halogen elements do not have significant effects on the geometries of the clusters. The computed IR spectra show similar representative peaks for all halogen substituted clusters. Interestingly, we found a blue-shift in the IR spectra with the increase in the mass of the halogen atoms. Further, we compared the Gibbs free energies at different temperatures for different halogen atoms. The results show that the Gibbs free energy differences between open and close structures of B12X7 become larger when heavier halogen atoms are present. This interesting finding was subsequently investigated by energy decomposition analysis.

physics.chem-ph↗

arXiv2017

Defect healing and charge transfer mediated valley polarization in MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructures

Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe$_2$ is completely recovered if the material is sandwiched in MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructures. We show by means of density-functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS$_2$ layers are donated to heal Se vacancy defects in the middle MoSe$_2$ layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe$_2$ without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality.

cond-mat.mes-hall↗

arXiv2017

Explicit treatment of hydrogen bonds in the Universal Force Field: validation and application for Metal-Organic Frameworks, hydrates and host-guest complexes

A straightforward means to include explicit hydrogen bonds within the Universal Force Field is presented. Instead of treating hydrogen bonds as non-bonded interaction subjected to electrostatic and Lennard-Jones potentials, we introduce an explicit bond with negligible bond order, thus maintaining the structural integrity of the H-bonded complexes and avoiding the necessity to assign arbitrary charges to the system. The explicit hydrogen bond changes the coordination number of the acceptor site and the approach is thus most suitable for systems with under-coordinated atoms, such as many metal-organic frameworks, however, it also shows excellent performance for other systems involving a hydrogen-bonded framework. In particular, it is an excellent means for creating starting structures for molecular dynamics and for investigations employing more sophisticated methods. The approach is validated for the hydrogen bonded complexes in the S22 dataset and then employed for a set of metal-organic frameworks from the Computation-Ready Experimental (CoRE) database and several hydrogen bonded crystals including water ice and clathrates. We show that direct inclusion of hydrogen bonds reduces the maximum error in predicted cell parameters from 66% to only 14% and the mean unsigned error is similarly reduced from 14% to only 4%. We posit that with the inclusion of hydrogen bonding, the solvent-mediated breathing of frameworks such as MIL-53 is now accessible to rapid UFF calculations, which will further the aim of rapid computational scanning of metal-organic frameworks while providing better starting points for electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2017

Double Dirac Point Semimetal in Two-Dimensional Material: Ta2Se3

Here, we report by first-principles calculations one new stable 2D Dirac material, Ta2Se3 monolayer. For this system, stable layered bulk phase exists, and exfoliation should be possible. Ta2Se3 monolayer is demonstrated to support two Dirac points close to the Fermi level, achieving the exotic 2D double Dirac semimetal. And like 2D single Dirac and 2D node-line semimetals, spin-orbit coupling could introduce an insulating state in this new class of 2D Dirac semimetals. Moreover, the Dirac feature in this system is layer-dependent and a metal-to-insulator transition is identified in Ta2Se3 when reducing the layer-thickness from bilayer to monolayer. These findings are of fundamental interests and of great importance for nanoscale device applications.

cond-mat.mtrl-sci↗

arXiv2017

GeP3: A small indirect band gap 2D crystal with high carrier mobility and strong interlayer quantum confinement

We propose a two-dimensional crystal which possesses low indirect band gaps of 0.55 eV (monolayer) and 0.43 eV (bilayer) and high carrier mobilities similar to those of phosphorene: GeP3. GeP3 has a stable three-dimensional layered bulk counterpart which is metallic and is known from experiment since 1970. It has a small cleavage energy, which suggests exfoliation of bulk material as viable means for the preparation of mono- and few-layer materials. The material shows strong interlayer quantum confinement effects, resulting in a band gap reduction from mono- to bilayer, and then to a semiconductor-metal transition between bi- and triple layer. Under biaxial strain, the indirect band gap can be turned into a direct one. Pronounced light absorption in the spectral range from ~600 to 1400 nm is predicted for monolayer and bilayer and promises applications in photovoltaics.

cond-mat.mtrl-sci↗

arXiv2017

Electronic structure and oxygen reduction on tunable [Ti(IV)Pc]2+and Ti(II)Pc titanyl-phthalocyanines: A quantum chemical prediction

The high cost of platinum-based catalysts has hampered the commercialization of polymer electrolyte membrane fuel cells (PEMFCs). Hence, the electronic structure and oxygen reduction ability of [Ti(IV)Pc]2+, Ti(II)Pc titanyl-phthalocyanines, and their tailored peripheral and axial ligand complexes were theoretically investigated to determine non-precious cathode catalysts. Our results revealed that the peripherally substituted and unsubstituted Ti(II)Pc triplet complexes can spontaneously reduce peroxide. The singlet [Ti(IV)Pc]2+ parent complex has a 6.45 eV barrier. However, fluorine substitution at peripheral positions reduced the energy barrier up to 0.45 eV. In addition, chlorine substitution has shown spontaneous peroxide reduction as in the case of triplets. The high catalytic activity of Ti(II)Pc complexes and singlet chlorine substituted complex is attributed to the optimal charge transfer between dioxygen molecule and the novel catalyst complexes. As a result, Ti(II)Pcs and chlorine substituted singlet complexes are considered as potential substitutions for the noble Pt-based catalyst for the PEMFCs.

cond-mat.mtrl-sci↗

arXiv2016

Vibrationally resolved UV/Vis spectroscopy with time-dependent density functional based tight binding

We report a time-dependent density functional based tight-binding (TD-DFTB) scheme for the calculation of UV/Vis spectra, explicitly taking into account the excitation of nuclear vibrations via the adiabatic Hessian Franck-Condon (AH|FC) method with a harmonic approximation for the nuclear wavefunction. The theory of vibrationally resolved UV/Vis spectroscopy is first summarized from the viewpoint of TD-DFTB. The method is benchmarked against time-dependent density functional theory (TD-DFT) calculations for strongly dipole allowed excitations in various aromatic and polar molecules. Using the recent 3ob:freq parameter set of Elstner's group, very good agreement with TD-DFT calculations using local functionals was achieved.

physics.chem-ph↗

arXiv2016

Spin polarization in SCC-DFTB

We evaluate the performance of spin-polarized DFTB within the SCC-DFTB (also known as DFTB2) model. The method has been implemented in the ADF modeling suite. We briefly review how spin polarization is incorporated into the DFTB2 method and validate the method in terms of structural parameters and energy using the GMTKN30 test set, from which we used 288 spin-polarized systems.

physics.chem-ph↗

arXiv2016

Discovery of a two-dimensional topological insulator in SiTe

Two-dimensional (2D) topological insulators (TIs), a new state of quantum matter, are promising for achieving the low-power-consuming electronic devices owning to the remarkable robustness of their conducting edge states against backscattering. Currently, the major challenge to further studies and possible applications is the lack of suitable materials, which should be with high feasibility of fabrication and sizeable nontrivial gaps. Here, we demonstrate through first-principles calculations that SiTe 2D crystal is a promising 2D TI with a sizeable nontrivial gap of 0.220 eV. This material is dynamically and thermally stable. Most importantly, it could be easily exfoliated from its three-dimensional superlattice due to the weakly bonded layered structure. Moreover, strain engineering can effectively control its nontrivial gap and even induce a topological phase transition. Our results provide a realistic candidate for experimental explorations and potential applications of 2D TIs.

cond-mat.mtrl-sci↗

arXiv2016

Tight-Binding Approximations to Time-Dependent Density Functional Theory - a fast approach for the calculation of electronically excited states

We propose a new method of calculating electronically excited states that combines a density functional theory (DFT) based ground state calculation with a linear response treatment that employs approximations used in the time-dependent density functional based tight binding (TD-DFTB) approach. The new method termed TD-DFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive time-dependent density functional theory (TD-DFT) calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TD-DFTB.

physics.chem-ph↗

arXiv2015

Two-Dimensional Transition Metal Dichalcogenides with a Hexagonal Lattice: Room Temperature Quantum Spin Hall Insulators

So far, several transition metal dichalcogenides (TMDCs) based two-dimensional (2D) topological insulators (TIs) have been discovered, all of them based on a tetragonal lattice. However, in 2D crystals, the hexagonal rather than the tetragonal symmetry is the most common motif. Here, based on first-principles calculations, we propose a new class of stable 2D TMDCs of composition MX2 (M=Mo, W, X=S, Se, Te) with a hexagonal lattice. They are all in the same stability range as other 2D TMDC allotropes that have been demonstrated experimentally, and they are identified to be practical 2D TIs with large band gaps ranging from 41 to 198 meV, making them suitable for applications at room-temperature. Besides, in contrast to tetragonal 2D TMDs, their hexagonal lattice will greatly facilitate the integration of theses novel TI states van-der-Waals crystals with other hexagonal or honeycomb materials, and thus provide a route for 2D-material-based devices for wider nanoelectronic and spintronic applications. The nontrivial band gaps of both WSe2 and WTe2 2D crystals are 198 meV, which are larger than that in any previously reported TMDC-based TIs. These large band gaps entirely stem from the strong spin-orbit coupling strength within the d orbitals of Mo/W atoms near the Fermi level. Our findings will significantly broaden the scientific and technological impact of both 2D TIs and TMDCs.

cond-mat.mtrl-sci↗

arXiv2015

Precise and reversible band gap tuning in single-layer MoSe2 by uniaxial strain

We present photoluminescence (PL) spectroscopy measurements of single-layer MoSe2 as a function of uniform uniaxial strain. A simple clamping and bending method is described that allows for application of uniaxial strain to layered, 2D materials with strains up to 1.1% without slippage. Using this technique, we find that the electronic band gap of single layer MoSe2 can be reversibly tuned by -27 +- 2 meV per percent of strain. This is in agreement with our density-functional theory calculations, which estimate a modulation of -32 meV per percent of strain, taking into account the role of deformation of the underlying substrate upon bending. Finally, due to its narrow PL spectra as compared with that of MoS2, we show that MoSe2 provides a more precise determination of small changes in strain making it the ideal 2D material for strain applications.

cond-mat.mtrl-sci↗

arXiv2015

Quantum Spin Hall Effect and Topological Phase Transition in Two-Dimensional Square Transition Metal Dichalcogenides

Two-dimensional (2D) topological insulators (TIs) hold promise for applications in spintronics based on the fact that the propagation direction of edge electrons of a 2D TI is robustly linked to their spin origination. Here, with the use of first-principles calculations, we predict a family of robust 2D TIs in monolayer square transition metal dichalcogenides (MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2). Sizeable intrinsic nontrivial bulk band gaps ranging from 24 to 187 meV are obtained, guarantying the quantum spin Hall (QSH) effect observable at room temperature in these new 2D TIs. Significantly different from most known 2D TIs with comparable band gaps, these sizeable energy gaps originate from the strong spin-orbit interaction related to the pure d electrons of the Mo/W atoms around the Fermi level. A single pair of topologically protected helical edge states is established for the edge of these systems with the Dirac point locating in the middle of the bulk band gap, and their topologically nontrivial states are also confirmed with nontrivial topological invariant Z2 = 1. More interestingly, by controlling the applied strain, a topological quantum phase transition between a QSH phase and a metallic phase or a trivial insulating phase can be realized in these 2D materials, and the detailed topological phase diagram is established.

cond-mat.mtrl-sci↗

arXiv2015

Two-Dimensional Inversion Asymmetric Topological Insulators in Functionalized III-Bi Bilayers

The search for inversion asymmetric topological insulators (IATIs) persists as an effect for realizing new topological phenomena. However, so for only a few IATIs have been discovered and there is no IATI exhibiting a large band gap exceeding 0.6 eV. Using first-principles calculations, we predict a series of new IATIs in saturated Group III-Bi bilayers. We show that all these IATIs preserve extraordinary large bulk band gaps which are well above room-temperature, allowing for viable applications in room-temperature spintronic devices. More importantly, most of these systems display large bulk band gaps that far exceed 0.6 eV and, part of them even are up to ~1 eV, which are larger than any IATIs ever reported. The nontrivial topological situation in these systems is confirmed by the identified band inversion of the band structures and an explicit demonstration of the topological edge states. Interestingly, the nontrivial band order characteristics are intrinsic to most of these materials and are not subject to spin-orbit coupling. Owning to their asymmetric structures, remarkable Rashba spin splitting is produced in both the valence and conduction bands of these systems. These predictions strongly revive these new systems as excellent candidates for IATI-based novel applications.

cond-mat.mtrl-sci↗

arXiv2014

Efficient Calculation of Electronic Absorption Spectra by Means of Intensity-Selected TD-DFTB

During the last two decades density functional based linear response approaches have become the de facto standard for the calculation of optical properties of small and medium-sized molecules. At the heart of these methods is the solution of an eigenvalue equation in the space of single-orbital transitions, whose quickly increasing number makes such calculations costly if not infeasible for larger molecules. This is especially true for time-dependent density functional tight binding (TD-DFTB), where the evaluation of the matrix elements is inexpensive. For the relatively large systems that can be studied the solution of the eigenvalue equation therefore determines the cost of the calculation. We propose to do an oscillator strength based truncation of the single-orbital transition space to reduce the computational effort of TD-DFTB based absorption spectra calculations. We show that even a sizeable truncation does not destroy the principal features of the absorption spectrum, while naturally avoiding the unnecessary calculation of excitations with small oscillator strengths. We argue that the reduced computational cost of intensity-selected TD-DFTB together with its ease of use compared to other methods lowers the barrier of performing optical properties calculations of large molecules, and can serve to make such calculations possible in a wider array of applications.

physics.chem-ph↗

arXiv2014

Hydrogen adsorption in metal-organic frameworks: the role of nuclear quantum effects

The role of nuclear quantum effects on the adsorption of molecular hydrogen in metal-organic frameworks (MOFs) has been investigated on grounds of Grand-Canonical Quantized Liquid Density-Functional Theory (GC-QLDFT) calculations. For this purpose, we have carefully validated classical H2 -host interaction potentials that are obtained by fitting Born-Oppenheimer ab initio reference data. The hydrogen adsorption has first been assessed classically using Liquid Density-Functional Theory (LDFT) and the Grand-Canonical Monte Carlo (GCMC) methods. The results have been compared against the semi-classical treatment of quantum effects by applying the Feynman-Hibbs correction to the Born-Oppenheimer-derived potentials, and by explicit treatment within the Grand-Canonical Quantized Liquid Density-Functional Theory (GC-QLDFT). The results are compared with experimental data and indicate pronounced quantum and possibly many-particle effects. After validation calculations have been carried out for IRMOF-1 (MOF-5), GC-QLDFT is applied to study the adsorption of H2 in a series of MOFs, including IRMOF-4, -6, -8, -9, -10, -12, -14, -16, -18 and MOF-177. Finally, we discuss the evolution of the H2 quantum fluid with increasing pressure and lowering temperature.

physics.chem-ph↗

arXiv2014

Transition-metal dichalcogenide bilayers: switching materials for spin- and valleytronic applications

We report that an external electric field applied normal to bilayers of transition-metal dichalcogenides TX2, M = Mo, W, X = S, Se, creates significant spin-orbit splittings and reduces the electronic band gap linearly with the field strength. Contrary to the TX2 monolayers, spin-orbit splittings and valley polarization are absent in bilayers due to the presence of inversion symmetry. This symmetry can be broken by an electric field, and the spin-orbit splittings in the valence band quickly reach similar values as in the monolayers (145 meV for MoS2... 418 meV for WSe2) at saturation fields less than 500 mV A-1. The band gap closure results in a semiconductor-metal transition at field strength between 1.25 (WX2) and 1.50 (MoX2) V A-1. Thus, by using a gate voltage, the spin polarization can be switched on and off in TX2 bilayers, thus activating them for spintronic and valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Electron Transport in MoWSeS Monolayers in Presence of an External Electric Field

The influence of an external electric field on single-layer transition-metal dichalcogenides TX2 with T = Mo, W and X = S, Se (MoWSeS) have been investigated by means of density-functional theory within two-dimensional periodic boundary conditions under consideration of relativistic effects including the spin-orbit interactions. Our results show that the external field modifies the band structure of the monolayers, in particular the conduction band. This modification has, however, very little influence on the band gap and effective masses of holes and electrons at the K point, and also the spin-orbit splitting of these monolayers is almost unaffected. Our results indicate a remarkable stability of the electronic properties of TX2 monolayers with respect to gate voltages. A reduction of the electronic band gap is observed only starting from field strengths of 2.0 V per Å (3.5 V per Å) for selenides (sulphides), and the transition to a metallic phase would occur at fields of 4.5 V per Å (6.5 V per Å).

cond-mat.mtrl-sci↗