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Oliver Kühn

Publications and source records attributed to Oliver Kühn.

At least 19 recordsLinked to original sources
arXiv2026

On the importance of multi-configurational and exchange effects in molecular aggregates

We present an extension of the Frenkel exciton model to incorporate exchange interactions between monomers in molecular aggregates in conjunction with {a multi-reference electronic structure approach}. Our derivation, {which combines the Frenkel exciton Hamiltonian and the single-electron pair exchange approximation}, yields a non-perturbative, variational expression for the exchange coupling that naturally excludes any basis set superposition error. The method has been implemented in OpenMolcas and enables combination with multi-reference electronic structure techniques. The main objective of the present study is to assess the role of exchange in systems with strong multi-configurational character. Illustrative examples demonstrate how the inclusion of exchange at different levels of approximation can substantially alter the magnitude and sign of intermonomer couplings and thus, for instance, potentially converting the predicted classification of the aggregate from H-type to J-type. Comparison with TDDFT-based couplings highlights significant discrepancies arising from multi-reference effects, double excitations, and Rydberg transitions. Overall, this approach advances the predictive modeling of photophysical and photochemical processes in aggregates of polyacenes, carotenoids, and other systems where multi-configurational and Rydberg states are essential.

physics.chem-ph↗
arXiv2026

Davydov Splitting Without a Davydov Pair and Highly Mobile Singlet Excitons in Perylene Red Microcrystals

We investigate the excitonic properties of perylene red microcrystals, whose unit cell contain eight molecules, using both experimental and theoretical methods. Only two of the nominal eight Davydov transitions are experimentally observed, with an apparent splitting of 610 cm$^{-1}$ that is reasonably reproduced by a Frenkel-Holstein exciton model parametrized from density functional theory. A symmetry-based analysis of the eigenstates reveals that the two observed transitions do not belong to the same Davydov pair, so the splitting does not directly report on the Coulomb coupling within the unit cell. The complex mixture of local excitations produces a J-like band for the dominant transitions, favouring exciton transfer through increased spectral overlap. Consistent with this picture, time-correlated single-photon counting of the emission and ultrafast transient absorption spectroscopy show that the dynamics is dominated by highly mobile singlet Frenkel excitons. The exciton mobility extracted from experiment agrees very well with that obtained from Kinetic Monte Carlo simulations, supporting a picture of incoherent hopping transport.

cond-mat.mtrl-sci↗
arXiv2026

Mismatch between Raman shear modes and ferroelectric polarization in 3R-MoS$_{2}$

Sliding ferroelectricity in parallel-stacked two-dimensional van der Waals materials enables a broad range of novel device concepts, but exploiting it requires reliable, non-destructive assignment of the underlying stacking order and polarization state. Here, we combine Kelvin-probe force microscopy (KPFM) with low-frequency Raman spectroscopy to probe the polarization domains and stacking configurations of a exfoliated trilayer 3R-MoS$_{2}$ flake on a hBN substrate. We find that ABA and BAB - both stackings with zero net polarization - are indistinguishable in KPFM, yet show drastically different low-frequency shear modes. This observation is reproduced across multiple flakes and is corroborated by low-temperature photoluminescence. Notably, the standard bond-polarizability model does not account for the difference in shear-mode activity between the ABA and BAB configurations, indicating that the interlayer Raman response of these stackings is governed by physics beyond a simple polarizability picture. Our results show that none of the here-used individual techniques alone is sufficient to assign sliding-ferroelectric stacking order and motivate a combined spectroscopic-scanning-probe approach.

cond-mat.mes-hall↗
arXiv2026

Stacking-Directed Polarization and Excitonic Engineering in MoS$_2$/MoSe$_2$ van der Waals Heterostructures

The stacking-dependent polarization and excitonic response of MoS$_2$/MoSe$_2$ heterostructures were investigated using GW+BSE many-body perturbation theory. While homobilayer MoS$_2$ exhibited a switchable interlayer dipole driven by registry-induced symmetry breaking, the MoS$_2$/MoSe$_2$ hetero-interface remained pinned by the intrinsic chemical potential mismatch between sulfur and selenium. In 2L-MoS$_2$/MoSe$_2$ trilayers, the stacking sequence enabled a deterministic control of photogenerated electrons between the central and bottom MoS$_2$ layers, governed by internal electric fields and quasiparticle band-edge shifts of 60--70~meV. Our calculations predicted a 36~meV interlayer excitonic shift, in remarkable agreement with recent experiments. These results elucidate the microscopic link between atomic registry and many-body interactions, establishing transition metal dichalcogenide trilayers as a potential platform for sliding ferroelectricity and programmable optoelectronic functionalities.

cond-mat.mtrl-sci↗
arXiv2025

Ferroelectric Control of Interlayer Excitons in 3R-MoS$_{2}$ / MoSe$_{2}$ Heterostructures

We investigate the interaction between interlayer excitons and ferroelectric domains in hBN-encapsulated 3R-MoS$_2$/MoSe$_2$ heterostructures, combining photoluminescence experiments with density functional theory and many-body Green's function calculations. Low-temperature photoluminescence spectroscopy reveals a strong redshift of the interlayer exciton energy with increasing MoS$_2$ layer thickness, attributed to band renormalization and dielectric effects. We observe local variations in exciton energy that correlate with local ferroelectric domain polarization of the 3R-MoS$_2$ layer, showcasing distinct domain-dependent interlayer exciton transition energies. Gate voltage experiments demonstrate that the interlayer exciton energy can be tuned by electrically induced domain switching. These results highlight the potential for interlayer exciton control by local ferroelectric order and establish a foundation for future ferroelectric optoelectronic devices based on van der Waals heterostructures.

cond-mat.mtrl-sci↗
arXiv2025

Chemical Control of Mechanical Anisotropy and Band Alignment in Perylene-based Two-dimensional MoS$_2$-Organic Hybrids

This study presents a comprehensive investigation of hybrid interfaces formed by monolayer MoS$_2$ coupled with the organic molecules perylene (P), perylene diimide (PDI), and perylene orange (PO). Using density functional theory, we demonstrate the extent to which the mechanical and electronic properties of a hybrid system can be altered by the chemical modification of a given chromophore. The three systems exhibit distinct differences due to their chemical composition and van der Waals contact enabled by their geometry. All systems are structurally stable. The binding energies follow the order PD$>$P$>$PO due to the large $π$-system (PD) and strong structural distortion (PO). Young's modulus and Poisson's ratio exhibit pronounced anisotropy in all cases. PO exhibits the greatest anisotropy due to steric effects and a permanent dipole, which introduce directionality to the molecule-surface interaction. Physisorption is accompanied by net charge transfer in the same order as the binding energies. The associated interfacial polarization results in a change in the work function compared to pristine MoS$_2$ in the order P$>$PO$>$PD. Finally, the presence of organic molecules introduces states into the MoS$_2$ energy gap, with the band alignment being either type II (P, PO) or type I (PD).

cond-mat.mtrl-sci↗
arXiv2025

Growth of few-layer molecular crystals of PTCDI on hexagonal boron nitride by microspacing air-gap sublimation

Extended two-dimensional (2D) crystals of dye molecules adsorbed on 2D material substrates like boron nitride have recently become a subject of intense study, with potential applications ranging from quantum technology to optoelectronics. The most established technique for the production of these films is physical vapor transport in vacuum. We demonstrate that few-layer crystalline films of the organic dye molecule PTCDI on boron nitride can be produced by microspacing in-air sublimation, a radically simplified technique, not requiring complicated vacuum systems. The resulting layers display clearly resolved atomic step terraces in atomic force microscopy, and a clear polarization anisotropy in their fluorescence, confirming molecular alignment and long-range order. Using density functional theory and classical molecular dynamics simulations, the canted motive is identified as the most likely building block for the morphology of a PTDCI monolayer on the hBN substrate.

cond-mat.mtrl-sci↗
arXiv2025

Chasing Charge Carriers: Diffusion Dynamics in Mixed-n Quasi-Two-Dimensional Colloidal MAPbBr3 Perovskites

In optoelectronic applications, metal halide perovskites (MHPs) are compelling materials because of their highly tuneable and intensely competitive optical properties. Colloidal synthesis enables the controlled formation of various morphologies of MHP nanocrystals, all with different carrier properties and, hence, different optical and carrier transport behaviours. We characterized three different methylammonium lead tribromide perovskite (MAPbBr3) morphologies: nanoplatelets (NPLs), nanosheets (NSs), and nanostripes (NSTs) synthesized by hot-injection synthesis protocols with slightly different parameters. A fluorescence imaging microscope (FLIM) for time- and space-resolved measurements of the carrier migration was employed to quantify the charge carriers' migration process upon photoexcitation. The results are rationalized in the two-dimensional diffusion model framework, considering funnelling and trapping processes in mixed-n colloidal MHPs. Subdiffusion mode was found to prevail in the nanocrystals, whereby the highest carrier diffusivity was found for bulk-like NSTs, followed by layered NSs and a film of NPLs. These findings provide a better understanding of optoelectronic processes in perovskites relevant to photovoltaic and light-emitting devices.

cond-mat.mtrl-sci↗
arXiv2024

From Structural Stability to Electronic Flexibility: Unveiling Strain-induced Effects in a MoS$_2$/Perylene Orange Hybrid System

This study delves into the interaction between a monolayer of molybdenum disulfide (MoS$_2$) and a single Perylene Orange (PO) molecule, representative of inorganic and organic semiconductor materials, respectively. Investigation of the amalgamation of these materials under mechanical strains reveals significant alterations in the electronic properties of the MoS$_2$/PO interface. Tensile strain induces a reduction in the bandgap, while compressive strain initially engenders an increase, followed by a subsequent decrease. Notably, MoS$_2$ undergoes a transition from a direct to an indirect bandgap under both stretching and compression conditions. These alterations stem from shifts in the density of states and band structure adjustments resulting from lattice deformations induced by applied strain. Remarkably, under specific compression conditions, the MoS$_2$/PO system manifests a transition between type II and type I band alignments. The detailed analysis of a range of strain magnitudes yields profound insights into the behavior of MoS$_2$ and MoS$_2$/PO systems under mechanical strain, with potential implications for nano- and optoelectronics applications.

cond-mat.mtrl-sci↗
arXiv2023

Quantum Dynamics of Photoactive Transition Metal Complexes. A Case Study of Model Reduction

Transition metal complexes for photochemical applications often feature a high density of electron-vibrational states characterized by nonadiabatic and spin-orbit couplings. Overall, the dynamics after photoexcitation is shaped by rapid transitions between states of different character and multiplicity. Even though transient absorption experiments enable characterization in terms of kinetic rates, the complexity of the systems usually prevents a more detailed analysis. Quantum dynamics simulations using quantum chemically determined model Hamiltonians may provide such details. In particular, one is tempted to pursue a model reduction, such as to identify couplings or vibrational modes most relevant for the dynamics. Here, we address how such an endeavor is challenged by the particular nature of transition metal complexes. For that purpose, we performed quantum dynamics simulations for a recently studied iron(II) homoleptic complex.

physics.chem-ph↗
arXiv2023

Formally Exact Simulations of Mesoscale Exciton Diffusion in a Photosynthetic Aggregate

The photosynthetic apparatus of plants and bacteria combine atomically precise pigment-protein complexes with dynamic membrane architectures to control energy transfer on the 10-100 nm length scales. Recently, synthetic materials have integrated photosynthetic antennae proteins to enhance exciton transport, though the influence of artificial packing on the excited-state dynamics in these biohybrid materials remains unclear. Here, we use the adaptive Hierarchy of Pure States (adHOPS) to perform a formally exact simulation of excitation energy transfer within artificial aggregates of light harvesting complex 2 (LH2) with a range of packing densities. We find that LH2 aggregates support a remarkable exciton diffusion length ranging from 100 nm at a biological packing density to 300 nm at the densest packing previously suggested in an artificial aggregate. The unprecedented scale of these calculations also underscores the efficiency with which adHOPS simulates excited-state processes in realistic molecular materials.

physics.chem-ph↗
arXiv2021

Strong Exciton-Vibrational Coupling in Molecular Assemblies. Dynamics using the Polaron Transformation in HEOM Space

In the context of Frenkel exciton dynamics in aggregated molecules the polaron transformation technique facilitates a treatment where diagonal elements attributed to electronic excited-state populations are decoupled from fluctuations associated with vibrational degrees-of-freedom. In this article we describe for the first time how the polaron transformation can be applied in the context of the "Hierarchical Equations of Motion" (HEOM) technique for treatment of open quantum systems with all vibrational components attributed to an environment. By using a generating function approach to introduce a shift in the excited state potential energy surface, we derive hierarchical equations for polaron transformation in analogy to those for time propagation. We demonstrate the applicability of the developed approach by calculating the dynamics of underdamped and overdamped oscillators coupled to electronic excitation of a monomer without and with previous polaron transformation and study the dynamics of the expectation value of the respective vibrational coordinates. Furthermore, we investigate the dynamics of a dimer with a barrier comparable to the thermal energy between the minima of the lower excitonic potential energy surface. It turns out that the assumption of localization at the monomer unit with energetically higher potential minimum, introduced via polaron transformation, has a substantial influence on the transfer dynamics. Here, it makes a clear difference whether the polaron transformation is performed in the local or exciton basis. This reflects the fact that the polaron transformation only accounts for equilibration of the vibrational, but not of the excitonic dynamics. We sketch an approach to compensate this shortcoming in view of obtaining an initial state for the calculation of emission spectra of molecular aggregates.

physics.chem-ph↗
arXiv2020

Site-selective and real-time observation of bimolecular electron transfer during photocatalytic water splitting

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system for the first time. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [Ir$^{\rm III}$(ppy)$_2$(bpy)]$^+$ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe$_3$(CO)$_{12}$ as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale. The present findings enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

physics.chem-ph↗
arXiv2020

Direct Optimal Control Approach to Laser-Driven Quantum Particle Dynamics

Optimal control theory is usually formulated as an indirect method requiring the solution of a two-point boundary value problem. Practically, the solution is obtained by iterative forward and backward propagation of quantum wavepackets. Here, we propose direct optimal control as a robust and flexible alternative. It is based on a discretization of the dynamical equations resulting in a nonlinear optimization problem. The method is illustrated for the case of laser-driven wavepacket dynamics in a bistable potential. The wavepacket is parameterized in terms of a single Gaussian function and field optimization is performed for a wide range of particle masses and lengths of the control interval. Using the optimized field in a full quantum propagation still yields reasonable control yields for most of the considered cases. Analysis of the deviations leads to conditions which have to be fulfilled to make the semiclassical single Gaussian approximation meaningful for field optimization.

quant-ph↗
arXiv2020

Exciton Transfer Using Rates Extracted From the "Hierarchical Equations of Motion''

Frenkel exciton population dynamics of an excitonic dimer is studied by comparing results from a quantum master equation (QME) involving rates from second-order perturbative treatment with respect to the excitonic coupling with non-perturbative results from ``Hierarchical Equations of Motion'' (HEOM). By formulating generic Liouville-space expressions for the rates, we can choose to evaluate them either via HEOM propagations or by applying cumulant expansion. The coupling of electronic transitions to bath modes is modeled either as overdamped oscillators for description of thermal bath components or as underdamped oscillators to account for intramolecular vibrations. Cases of initial nonequilibrium and equilibrium vibrations are discussed. In case of HEOM initial equilibration enters via a polaron transformation. Pointing out the differences between the nonequilibrium and equilibrium approach in the context of the projection operator formalism, we identify a further description, where the transfer dynamics is driven only by fluctuations without involvement of dissipation. Despite this approximation, also this approach can yield meaningful results in certain parameter regimes. While for the chosen model HEOM has no technical advantage for evaluation of the rate expressions compared to cumulant expansion, there are situations where only evaluation with HEOM is applicable. For instance, a separation of reference and interaction Hamiltonian via a polaron transformation to account for the interplay between Coulomb coupling and vibrational oscillations of the bath at the level of a second-order treatment can be adjusted for a treatment with HEOM.

physics.chem-ph↗
arXiv2019

Multi-reference quantum chemistry protocol for simulating autoionization spectra: Test of ionization continuum models for the neon atom

In this contribution we present a protocol to evaluate partial and total Auger decay rates combining the restricted active space self-consistent field electronic structure method for the bound part of the spectrum and numerically obtained continuum orbitals in the single-channel scattering theory framework. On top of that, the two-step picture is employed to evaluate the partial rates. The performance of the method is exemplified for the prototypical Auger decay of the neon $1s^{-1}3p$ resonance. Different approximations to obtain the continuum orbitals, the partial rate matrix elements, and the electronic structure of the bound part are tested against theoretical and experimental reference data. It is demonstrated that the partial and total rates are most sensitive to the accuracy of the continuum orbitals. For instance, it is necessary to account for the direct Coulomb potential of the ion for the determination of the continuum wave functions. The Auger energies can be reproduced quite well already with a rather small active space. Finally, perspectives of the application of the proposed protocol to molecular systems are discussed.

physics.chem-ph↗
arXiv2019

The light-harvesting complex 2 of allochromatium vinosum: B800 absorption band splitting and exciton relaxation

Allochromatium (Alc.) vinosum has a double-peak structure of its absorption band around 800~nm. Previously, the excitonic origin of this feature has been demonstrated experimentally, but a detailed understanding still lacks a model Hamiltonian being able to reproduce absorption as well as exciton relaxation time scales. Here, we propose a system-bath model which accommodates these observables. It combines Frenkel exciton theory for a dimerized and energetically heterogeneous B800 pigment pool with a quantum master equation approach describing phase and energy relaxation according to an experimental spectral density. The analysis of this model shows that the LH2 of Alc. vinosum features an interesting interplay of two excitonic bands, which are originating from the different pigment pools.

physics.chem-ph↗