Michael Rohlfing

Michael Rohlfing
University of Münster | WWU · Institute of Solid State Theory

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159
Publications
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Publications

Publications (159)
Article
We report on the direct observation of electron transfer between the surface and the second layer of the prototypical transition-metal dichalcogenide 2H−MoS2. We are able to disentangle the excitation and the transfer process in our measurement. Thereby, we determine both the momentum location and the duration of the electron transfer between the f...
Article
We employ density functional theory (DFT) to analyze the dispersion of the electronic state that exists at the commensurate interface between a monolayer of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) and the Ag(111) surface. First, we present and verify a hydrogen-termination approach which allows a meaningful DFT description of t...
Article
Weakly bound systems, like noble-gas dimers or two-dimensional layered materials (graphite, hexagonal boron nitride, or transition-metal dichalcogenides), exhibit excited electronic states of a particular nature. These so-called exciplex states combine on-site (or intralayer) and charge-transfer (or interlayer) configurations in a well-balanced way...
Article
Defects play crucial roles in the photonic and chemical activities of TiO2. The origin of the deep band-gap defect state Sbg in the rutile TiO2(110) surface has remained controversial for quite a long time. Using many-body Green's function theory, we believe that Sbg can be attributed only to σ bonds formed between 3d orbitals at the Ti interstitia...
Article
Monolayers of transition metal dichalcogenides (TMDCs) have unique optoelectronic properties. Density functional theory allows only for a limited description of the electronic excitation energies in these systems, while a more advanced treatment within many-body perturbation theory employing the GW/BSE approximation is often rather time consuming....
Article
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The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of t...
Preprint
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The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques also opened the door to study the optical properties of these nanomaterials. We present a comprehensive study o...
Article
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Recently, spatially indirect (“interlayer”) excitons have been discovered in bulk 2H-MoTe2. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (XIL) in bulk 2H-MoSe2 and 2H-WSe2 using valley-resolved magneto-...
Article
We present a physically intuitive model of molecular quantum dots beyond the constant interaction approximation. It accurately describes their charging behavior and allows the extraction of important molecular properties that are otherwise experimentally inaccessible. The model is applied to data recorded with a noncontact atomic force microscope o...
Article
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Atomic force microscopy is an impressive tool with which to directly resolve the bonding structure of organic compounds1–5. The methodology usually involves chemical passivation of the probe-tip termination by attaching single molecules or atoms such as CO or Xe (refs 1,6–9). However, these probe particles are only weakly connected to the metallic...
Article
Transition-metal dichalcogenides (TMDCs) are a focus of current research due to their fascinating optical and electronic properties with possible technical applications. ReSe2 is an interesting material of the TMDC family, with unique anisotropic properties originating from its distorted 1T structure (1T '). To develop a fundamental understanding o...
Article
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Transition-metal dichalcogenides (TMDCs) are a focus of current research due to their fascinating optical and electronic properties with possible technical applications. ReSe 2 is an interesting material of the TMDC family, with unique anisotropic properties originating from its distorted 1T structure (1T '). To develop a fundamental understanding...
Article
Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the...
Article
The power of the GW formalism is, to a large extent, based on the explicit treatment of dynamical correlations in the self-energy. This dynamics is taken into account by calculating the energy dependence of the screened Coulomb interaction W, followed by a convolution with the Green's function G. In order to obtain the energy dependence of W the pr...
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It is unclear whether there is an intermediate dark state between the S2 and S1 states of carotenoids. Previous two-dimensional electronic spectroscopy measurements support its existence and its involvement in the energy transfer from carotenoids to chlorophylls, but there is still considerable debate on the origin of this dark state and how it reg...
Article
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Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin...
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Article
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Almost all experiments and future applications of transition metal dichalcogenide monolayers rely on a substrate for mechanical stability, which can significantly modify the optical spectra of the monolayer. Doping from the substrate might lead to the domination of the spectra by trions. Here we show by ab initio many-body theory that the negative...
Article
One- and two-dimensional materials are presently being intensively investigated due to their interesting properties for next-generation opto-electronic devices. Among these, armchair-edged graphene nanoribbons are very promising candidates with optical properties which are dominated by excitons. In the presence of additional charges, trions (i.e. c...
Article
Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel opto-electronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2....
Article
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Since their discovery single-layer semiconducting transition metal dichalcogenides have attracted much attention thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. I...
Article
The optical control of spin currents in topological surface states opens new perspectives in (opto-) spintronics. To understand these processes, a profound knowledge about the dispersion and the spin polarization of both the occupied and the unoccupied electronic states is required. We present a joint experimental and theoretical study on the unocc...
Article
In this paper we review a recently introduced microscopy technique, scanning quantum dot microscopy (SQDM), which delivers quantitative maps of local electrostatic potential near surfaces in three dimensions. The key to achieving SQDM imaging is the functionalization of a scanning probe microscope tip with a π-conjugated molecule that acts as a gat...
Article
Due to their unique band structure, single layers of transition metal dichalcogenides are promising for new atomic-scale physics and devices. It has been shown that the band structure and the excitonic transitions can be tuned by straining the material. Recently, the discovery of single-photon emission from localized excitons has put monolayer WSe2...
Article
Employing the GW method, we discuss the electronic and topological properties of Bi2Te3 and Sb2Te3 thin films consisting of one to six quintuple layers (QLs). Although both bulk materials are three-dimensional topological insulators, the two-dimensional topological phases of their thin films differ. We find the nontrivial quantum spin Hall phase, t...
Article
Trion states of three correlated particles (e.g., two electrons and one hole) are essential to understand the optical spectra of doped or gated nanostructures, like carbon nanotubes or transition-metal dichalcogenides. We develop a theoretical many-body description for such correlated states using an ab initio approach. It can be regarded as an ext...
Article
The magnetic properties of nanostructures that consist of a small number of atoms or molecules are typically determined by magnetic exchange interactions. Here, we show that non-magnetic, chemical interactions can have a similarly decisive effect if spin-moment-carrying orbitals extend in space and therefore allow the direct coupling of magnetic pr...
Article
We discuss topological and electronic properties of thin films of the topological insulator Bi2Se3 employing the GW method. We obtain a topologically trivial phase for films of one to six quintuple layers thickness, whereas a quantum spin Hall phase is observed in neither of these films. This challenges recent contradictory reports on quantum spin...
Article
This paper reports the quasiparticle band structure and the optical absorption spectrum of SrO, using many-body perturbation theory. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, electron-hole pair states and optical excitations are obtained by solving the Bethe-...
Article
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We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole fiel...
Article
By means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), we have investigated the adsorption of single Au atoms on a PTCDA monolayer physisorbed on the Au(111) surface. A chemical reaction between the Au atom and the PTCDA molecule leads to the formation of a radical that has an unpaired electron in its highest occupi...
Article
We investigate the influence of potassium adsorption and selenium vacancies in the surface layer on the electronic properties of the prototypical topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. These modifications of the surface give rise to oscillations in the charge density that extend deep into the crystal. They result in a long-rang...
Article
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Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory base...
Article
The topological crystalline insulator SnTe belongs to the recently discovered class of materials in which a crystalline symmetry ensures the existence of topologically protected surface states. The bulk band structure of SnTe is characterized by a band inversion at the four equivalent $L$ points, giving rise to a mirror Chern number of ${n}_{\mathc...
Article
We present a comparative study of excited states in push-pull oligomers of PCPDTBT and PSBTBT and prototypical complexes with a Coo acceptor using many-body Green's functions theory within the GW approximation and the Bethe-Salpeter equation. We analyze excitations in oligomers up to a length of 5 nm and find that for both materials the absorption...
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Charge-transfer (CT) excited states play an important role in the excited-state dynamics of DNA in aqueous solution. However, there is still much controversy on their energies. By ab initio many-body Green's function theory, together with classical molecular dynamics simulations, we confirm the existence of CT states at the lower energy side of the...
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We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule on the silver (001) surface. Adsorption structures, electronic properties, and scanning tunneling microscopy (STM) images are calculated within density-functional theory. Adsorption occurs most favorably in on-top configuration, with the C=C double bond directly above a si...
Article
We present the quasiparticle band structure and the optical excitation spectrum of bulk LiCl, using many-body perturbation theory. Density-functional theory is used to calculate the ground-state geometry of the system. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration...
Article
Before transport data can be understood quantitatively, a few prerequisites have to be fulfilled: the geometric and the electronic structures of the metal/molecule contacts have to be known, and electron correlation effects have to be taken into account. Here we discuss experimental and theoretical approaches to tackle these challenges. On the theo...
Article
We report the quasiparticle band structure and optical absorption spectrum of bulk LiBr calculated from first-principles approaches. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, the optical excitation is investigated by solving the Bethe-Salpeter equation for th...
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Recent experiments have indicated that dopants and defects can trigger new redshifted photoluminescence (PL) peaks below the E_{11} peak in single-walled carbon nanotubes (SWCNTs). To understand the origin of the new PL peaks, we study theoretically the excited-state properties of SWCNTs with some typical dopants and defects by ab initio many-body...
Article
The influence of the buckling type of the Si(111)-2×1 surface on the electronic structure is studied with high-resolution scanning tunneling microscopy and scanning tunneling spectroscopy and compared to ab initio calculations. We utilize the multitude of domain boundaries to identify differently buckled domains. I(V) measurements with high spatial...
Article
Excited states of donor–acceptor dimers are studied using many-body Green’s functions theory within the GW approximation and the Bethe–Salpeter equation. For a series of prototypical small-molecule based pairs, this method predicts energies of local Frenkel and intermolecular charge-transfer excitations with the accuracy of tens of meV. Application...
Article
Excited states of dicyanovinyl-substituted oligothiophenes are studied using many-body Green’s functions theory within the GW approximation and the Bethe-Salpeter equation. By varying the number of oligomer repeat units, we investigate the effects of resonant–antiresonant transition coupling, dynamical screening, and molecular conformations on calc...
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We present the results of calculations of SrTiO 3 and CaTiO 3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO 3 or CaO 3) of...
Article
Full-text available
We present the results of calculations of SrTiO3 and CaTiO3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO3 or CaO3) of the...
Article
Using the first principles ground state method, we systematically studied the DX center in bulk GaAs and GaAs(110). The DX center is found to be a metastable state in bulk GaAs and completely unstable in the top few layers of GaAs(110). We find that the charge states of SiGa defects on the GaAs(110) surface are localized, and the extra charge is ma...
Article
We report upon controlled switching of a single 3,4,9,10-perylene tetracarboxylic diimide derivative molecule on a rutile TiO2(110) surface using a non-contact atomic force microscope at room temperature. After submonolayer deposition, the molecules adsorb tilted on the bridging oxygen row. Individual molecules can be manipulated by the atomic for...
Article
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A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map...
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Optical experiments on the Si(111)-2×1 surface show that the absorption peak due to dangling-bond transitions exhibits a blueshift upon oxygen exposure. The effect is interpreted as due to quantum confinement of surface electrons in π -bonded chains of atoms, whose length decreases with oxygen uptake (a particle-in-a-box effect, the box being one d...
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We present the results of calculations of surface relaxations, rumplings, energetics, optical band gaps, and charge distribution for the SrZrO(3) and PbZrO(3) (001) and (011) surfaces using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We consider both SrO(PbO) and ZrO(2) terminations of the (001) surface and Sr(P...
Article
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The electronic spectrum of a chemically contacted molecule in the junction of a scanning tunneling microscope can be modified by tip retraction. We analyze this effect by a combination of density functional, many-body perturbation and numerical renormalization group theory, taking into account both the non-locality and the dynamics of electronic co...
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The dynamics of a molecular junction consisting of a PTCDA molecule between the tip of a scanning tunneling microscope and a Ag(111) surface have been investigated experimentally and theoretically. Repeated switching of a PTCDA molecule between two conductance states is studied by low-temperature scanning tunneling microscopy for the first time, an...
Article
The adsorption behavior of trimesic acid (TMA) on rutile TiO(2)(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functional theory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imaging reveals individual molecules, centered above the surface titanium rows. Based on the NC-AFM results alone it is...
Article
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A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single D2 molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultra-high geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-di...
Article
We present the calculations of quasiparticle bandstructures and lifetimes for noble metals Cu and Ag within the GW approximation based on localized Gaussian orbital basis sets. For Cu, both the calculated positions of the d bands and the width of the d bands are within 0.1 eV compared to the experimental results. For Ag, partial core correction sho...
Article
We apply the so-called diabatization method by Baer (1975 Chem. Phys. Lett. 35 112) to describe photo-excited electronic states. The diabatization is a unitary transformation which, applied to the adiabatic eigenstates of the quantum Hamiltonian, allows for treating the non-adiabatic effects correctly. These effects are all those which appear in th...
Article
Excited states of the negatively charged nitrogen-vacancy color center in diamond are studied using ab initio many-body perturbation theory (within GW approximation and Bethe-Salpeter equation), including electronic exchange, correlation, and electron-hole interaction effects. We find that three singlets, one of which exhibits an intersystem crossi...