arXiv subjects

Stefan Lochbrunner

Publications and source records attributed to Stefan Lochbrunner.

7 recordsLinked to original sources

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↗

arXiv2025

Unlocking klockmannite: formation of colloidal quasi-2D CuSe nanocrystals and photo-physical properties arising from crystal anisotropy

Copper selenide is an exceptional quasi-layered monolithic material that exhibits both semiconducting and metallic properties in adjacent visible and near-infrared (NIR) spectral ranges. Here we introduce a thiol-free colloidal synthesis for generating quasi-2D klockmannite copper selenide nanocrystals via hot injection method, achieving shape control by tuning the injection temperature and precursor concentrations without any additional ligands. This approach produces large klockmannite nanosheets with lateral sizes from 200 nm to several micrometres, as well as uniform triangular nanoplatelets with sizes of 12-25 nm that are monocrystalline and display strong NIR plasmonic absorption. The spectral features of the anisotropic klockmannite phase in the NIR have been analysed using complex-scaled discrete dipole approximation (CSDDA) calculations, which reveal pronounced optical anisotropy and the emergence of hyperbolic regime. The combined effect of propagating and evanescent fields is regarded as the underlying reason of such modes in the hyperbolic domain. Finally, the ultrafast photophysical behaviour of the material in klockmannite phase is examined, including hot-hole cooling, trapping, and coherent phonons generation. Our findings emphasize the important role of the intrinsic crystal anisotropy in governing the physical properties of nanoscale klockmannite.

cond-mat.mtrl-sci↗

arXiv2025

Zone-sectored organic crystals with spatially resolved exciton dynamics

Among the organic semiconductors, rubrene stands out in terms of hole mobility, luminescence yield and exciton migration distance. A novel type of rubrene microcrystal is prepared in the orthorhombic phase, exhibiting zone-sectored tabular domains with distinct photoluminescence (PL) characteristics. These sectors exhibit distinct PL spectra and time-evolution, arising from differences in the in-plane orientation of the orthorhombic unit cell relative to the crystal surface. A combination of polarised optical microscopy, fluorescence lifetime imaging microscopy (FLIM), and atomic force microscopy (AFM) is used to characterise the samples in terms of crystal orientation, fluorescence lifetime, and photoluminescence spectra. Spatially resolved PL spectroscopy reveals that the redshifted 650 nm emission band has polarisation along the transition dipole moment and is associated with high photon absorption due to the alignment of excitation polarisation and transition dipole moment and selectively localized within specific sectors of the crystal. The detected photon originates from direct emission of a geminate coherent triplet pair, or from its fusion. This band exhibits pure mono-exponential dynamics with 3.7 ns lifetime. The triplet fusion behaviour in the succeeding time regimes can be treated in the framework of power law scaling and random walk. The emission kinetics are modelled using rate equations describing geminate and non-geminate exciton fusion processes, enabling a quantitative interpretation of the spatially resolved PL kinetics. These findings introduce a material-based strategy, opening novel routes for photonic applications and light harvesting.

physics.optics↗

arXiv2023

Synthesis, Optoelectronic Properties, and Charge Carrier Dynamics of Colloidal Quasi-two-dimensional Cs3Bi2I9 Perovskite Nanosheets

Non-toxicity and stability make two-dimensional (2D) bismuth halide perovskites better alternatives to lead-based ones for optoelectronic applications and catalysis. In this work, we synthesize sub-micron size colloidal quasi-2D Cs3Bi2I9 perovskite nanosheets and study their generation and relaxation of charge carriers. Steady-state absorption spectroscopy reveals an indirect bandgap of 2.07 eV, which is supported by the density functional theory calculated band structure. The nanosheets do not show detectable photoluminescence at room temperature at band-edge excitation which is attributed to the indirect bandgap. However, cathodoluminescence spanning a broad range from 500 nm to 750 nm with an asymmetric and Stokes-shifted spectrum is observed, indicating the phonon- and trap-assisted recombination. We study the ultrafast charge carrier dynamics in Cs3Bi2I9 nanosheets using a femtosecond transient absorption spectroscopy. The samples are excited with pump energies higher than their bandgap, and the results are interpreted in terms of hot carrier generation (<1 ps), thermalization with local phonons (~1 ps), and cooling (>30 ps). Further, a relatively slow relaxation of excitons (>3 ns) at the band edge suggests the formation of stable polarons which decay nonradiatively by emitting phonons.

cond-mat.mtrl-sci↗

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↗

arXiv2012

Size dependent exciton dynamics in one-dimensional perylene bisimide aggregates

The size dependent exciton dynamics of one-dimensional aggregates of substituted perylene bisimides are studied by ultrafast transient absorption spectroscopy and kinetic Monte-Carlo simulations in dependence on the temperature and the excitation density. For low temperatures the aggregates can be treated as infinite chains and the dynamics is dominated by diffusion driven exciton-exciton annihilation. With increasing temperature the aggregates decompose into small fragments consisting of very few monomers. This scenario is also supported by the time dependent anisotropy deduced from polarization dependent experiments.

physics.chem-ph↗

arXiv2012

Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

The theoretical description and the properties of Frenkel excitons in non-covalently bonded molecular aggregates are reviewed from a multi-exciton perspective of dissipative quantum dynamics. First, the photophysical and quantum chemical characterization of the monomeric dye building blocks is discussed, including the important aspect of electron-vibrational coupling within the Huang-Rhys model. Supplementing the model by the Coulombic interactions between monomers, the description of aggregates in terms of excitonic or vibrational-excitonic bands follows. Besides of giving rise to complex absorption and emission line shapes, exciton-vibrational interaction is responsible for energy and phase relaxation and thereby limits the size of coherent excitations in larger aggregates. Throughout, emphasis is put on the electronic three-level model as a minimum requirement to describe nonlinear spectroscopies including effects of two-exciton states such as excited state absorption and exciton-exciton annihilation. The experimentally observed characteristics of stationary absorption and fluorescence spectra of aggregates as well as their temperature dependence are discussed. Examples for ultrafast spectroscopic experiments including pump-probe studies, photon echo and two-dimensional spectroscopy are presented and results on the size of coherence domains and on intra- and interband relaxation are given. Finally, experimental signatures for exciton-exciton annihilation and their analysis with respect to the mobility of excitons are described.

physics.chem-ph↗