arXiv subjects

Tobias Hertel

Publications and source records attributed to Tobias Hertel.

18 recordsLinked to original sources

arXiv2026

A Visible-Frequency Excitonic Reststrahlen Band in (PEA)$_2$PbI$_4$ Slabs

Layered halide perovskites host exceptionally strong excitons, whose optical signatures are usually interpreted as absorptive resonances on a smooth dielectric background. Strong excitons, however, can also reshape the dielectric response itself and drive the real permittivity negative, opening a reflective band: the visible, excitonic analogue of an infrared Reststrahlen band. Whether bare (PEA)$_2$PbI$_4$ slabs reach this regime has remained unclear. Here we show that low-temperature transmission of (PEA)$_2$PbI$_4$ slabs, driven by the intralayer-exciton manifold, evolves with increasing thickness from an excitonic dip into a broad near-zero-transmission interval with compressed Fabry-Pérot-like fringes. Transfer-matrix analysis with an effective Lorentz-oscillator dielectric response reproduces this crossover, reconstructs a finite negative-Re($\varepsilon$) window, and implies near-ultrastrong exciton-photon coupling. Calculated field maps show suppressed in-plane field penetration within this interval and a driven longitudinal response near the high-energy ($\varepsilon=0$) edge. These results identify (PEA)$_2$PbI$_4$ slabs as a cavity-free visible-frequency excitonic Reststrahlen material.

cond-mat.mtrl-sci↗

arXiv2026

Liquid-phase encapsulation of $π$-conjugated dyes in boron nitride nanotubes: Ensemble and single-nanotube optical characterization

Boron nitride nanotubes (BNNTs) provide wide-bandgap, optically transparent one-dimensional hosts for molecular dyes, limiting direct electronic participation of the host. Whether dye@BNNT systems produce bright, well-defined J- or H-aggregates or instead heterogeneous emissive ensembles whose character depends on chain length and local packing remains only partly resolved. We address this question using ensemble extinction, photoluminescence, quantum-yield measurements, and TCSPC-derived radiative and non-radiative rates, together with polarization-resolved single-nanotube microscopy on encapsulated quaterthiophene, sexithiophene, octithiophene, and Nile Red, selected from a ten-dye screening. In the oligothiophene series, confinement modifies spectra and excited-state dynamics in a length-dependent manner, with all three oligothiophenes forming weakly emissive ensembles with suppressed effective radiative rates and 6T showing the strongest redistribution between effective radiative and non-radiative decay. The absence of radiative-rate enhancement or fluorescence-lifetime shortening across the series disfavors bright J-aggregate assignments. Polarization-resolved single-nanotube microscopy reveals strongly polarized emission, but with tube-to-tube and intratube variations, identifying oligothiophene@BNNTs as ordered yet structurally heterogeneous confined ensembles. Nile Red provides a complementary case in which the dominant response is dielectric tuning of a solvatochromic charge-transfer state rather than oligothiophene-like aggregate formation. These findings establish dye-filled BNNTs as optically quiet nanoconfined systems in which molecular ordering, dielectric confinement, and guest-guest coupling can be distinguished through combined ensemble and single-nanotube spectroscopy.

cond-mat.mtrl-sci↗

arXiv2026

Intra- and Interlayer Excitonic Fine Structure of the Two-Dimensional Perovskite (PEA)$_2$PbI$_4$

Two-dimensional halide perovskites host strongly bound excitons whose fine structure controls polarization selection rules and radiative recombination, yet several spectral features in (PEA)$_2$PbI$_4$ remain controversially assigned. Here, polarization-resolved low-temperature photoluminescence combined with first-principles G$_0$W$_0$+BSE calculations resolves both the intralayer and interlayer excitonic fine structure of this prototypical n=1 Ruddlesden-Popper perovskite. The low-energy multiplet is consistently described as a purely excitonic intralayer fine structure governed by crystal symmetry, octahedral distortions, and the two-layer unit cell, without invoking Rashba or exciton-polaron mechanisms as the primary origin. A weaker doublet ~45 meV above the bright intralayer states is identified as interlayer excitons from its agreement with the calculated interlayer manifold in energy and splitting. Although the static calculations underestimate their oscillator strength and do not reproduce the observed orthogonal polarizations, distortion-induced mixing with bright intralayer excitons strongly enhances interlayer optical activity and provides a plausible explanation for their visibility. Our results establish interlayer excitons in (PEA)$_2$PbI$_4$ and refine the excitonic description of fine structure in two-dimensional perovskites.

cond-mat.mes-hall↗

arXiv2025

From Canonical to Complex: Benchmarking LLM Capabilities in Undergraduate Thermodynamics

Large language models (LLMs) are increasingly considered as tutoring aids in science education. Yet their readiness for unsupervised use in undergraduate instruction remains uncertain, as reliable teaching requires more than fluent recall: it demands consistent, principle-grounded reasoning. Thermodynamics, with its compact laws and subtle distinctions between state and path functions, reversibility, and entropy, provides an ideal testbed for evaluating such capabilities. Here we present UTQA, a 50-item undergraduate thermodynamics question answering benchmark, covering ideal-gas processes, reversibility, and diagram interpretation. No leading 2025-era model exceeded our 95\% competence threshold: the best LLMs achieved 82\% accuracy, with text-only items performing better than image reasoning tasks, which often fell to chance levels. Prompt phrasing and syntactic complexity showed modest to little correlation with performance. The gap concentrates in finite-rate/irreversible scenarios and in binding visual features to thermodynamic meaning, indicating that current LLMs are not yet suitable for unsupervised tutoring in this domain.

physics.ed-ph↗

arXiv2024

Onset of Spin Entanglement in Doped Carbon Nanotubes Studied by EPR

Nanoscale semiconductors with isolated spin impurities have been touted as promising materials for their potential use at the intersection of quantum, spin, and information technologies. Electron paramagnetic resonance (EPR) studies of spins in semiconducting carbon nanotubes have overwhelmingly focused on spins more strongly localized by $\rm sp^3$-type lattice defects. However, the creation of such impurities is irreversible and requires specific reactions to generate them. Shallow charge impurities, on the other hand, are more readily and widely produced by simple redox chemistry, but have not yet been investigated for their spin properties. Here we use EPR to study p-doped (6,5) semiconducting single-wall carbon nanotubes (s-SWNTs) and elucidate the role of impurity-impurity interactions in conjunction with exchange and correlation effects for the spin behavior of this material. A quantitative comparison of the EPR signals with phenomenological modeling combined with configuration interaction electronic structure calculations of impurity pairs shows that orbital overlap, combined with exchange and correlation effects, causes the EPR signal to disappear due to spin entanglement for doping levels corresponding to impurity spacings of $14\,\rm nm$ (at 30 K). This transition is predicted to shift to higher doping levels with increasing temperature and to lower levels with increasing screening, providing an opportunity for improved spin control in doped s-SWNTs.

cond-mat.mtrl-sci↗

arXiv2023

Electronic Structure and Scaling of Coulomb Defects in Carbon Nanotubes from Modified Hückel Calculations

Controlled doping and understanding its underlying microscopic mechanisms is crucial for advancement of nanoscale electronic technologies, especially in semiconducting single-wall carbon nanotubes (s-SWNTs), where adsorbed counterions are known to govern redox-doping levels. However, modeling the associated 'Coulomb defects' is challenging due to the need for large-scale simulations at low doping levels. Using modified Hückel calculations on 120 nm long s-SWNTs with adsorbed $\rm Cl^-$ ions, we study the scaling properties of shallow Coulomb defect states at the valence band edge and quantum well (QW) states in the conduction band. Interestingly, the QW states may underlie observed exciton band shifts of inhomogeneously doped semiconductors. Binding energies of Coulomb defects are found to scale with counterion distance, effective band mass, relative permittivity and counterion charge according to $d^{α-2}m^{α-1}ε_r^{-α}|z_j|^α$, with $α$ as an empirical parameter, deepening our understanding of s-SWNT doping.

cond-mat.mtrl-sci↗

arXiv2023

Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes

Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of $2.0\,\rm ps^{-1}$. Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.

cond-mat.mes-hall↗

arXiv2023

Trions, Exciton Dynamics and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism

Doping substantially influences the electronic and photophysical properties of semiconducting single-wall carbon nanotubes (s-SWNTs). Although prior studies have noted that surplus charge carriers modify optical spectra and accelerate non-radiative exciton decay in doped s-SWNTs, a direct mechanistic correlation of trion formation, exciton dynamics and energetics remains elusive. This work examines the influence of doping-induced non-radiative decay and exciton confinement on s-SWNT photophysics. Using photoluminescence, continuous-wave absorption, and pump-probe spectroscopy, we show that localization of and barrier formation by trapped charges can be jointly quantified using diffusive exciton transport- and particle-in-the-box models, yielding a one-to-one correlation between charge carrier concentrations derived from these models. The study highlights the multifaceted role of exohedral counterions, which trap charges to create quenching sites, form barriers to exciton movement, and host trion states. This contributes significantly to understanding and optimizing the photophysical properties of doped SWNTs.

cond-mat.mtrl-sci↗

arXiv2021

Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes

Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.

physics.chem-ph↗

arXiv2019

Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy

Controlling doping is essential for a successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being 'high' or 'low' or it is entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting carbon nanotubes (s-SWNTs) using VIS-NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 $\rm nm^{-1}$ can be quantified with a sensitivity of roughly one charge per $10^4$ carbon atoms assuming in-homogeneous or homogeneous carrier distributions.

cond-mat.mtrl-sci↗

arXiv2014

High energetic excitons in carbon nanotubes directly probe charge-carriers

Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously with sizable quantum yield even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D, suggesting an initial charge carrier separation of the same order of the exciton correlation length.

cond-mat.mes-hall↗

arXiv2007

Intersubband decay of 1-D exciton resonances in carbon nanotubes

We have studied intersubband decay of E22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n, m) can be mapped onto a connectivity grid with curves of constant (n-m) and (2n+m). Moreover, the global behavior of E22 linewidths is best characterized by a strong increase with energy irrespective of their (n-m) mod(3)= \pm 1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E22 linewidths are dominated by phonon assisted coupling to higher momentum states of the E11 and E12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.

cond-mat.mtrl-sci↗

arXiv2003

The interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene and ovalene at sub-monolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of 2-D adsorbate islands up to the desorption temperature. Pre-exponential frequency factors are found to be in the range $10^{14}$-$10^{21} s^{-1}$ as obtained from both Falconer--Madix (isothermal desorption) analysis and Antoine's fit to vapour pressure data. The resulting binding energy per carbon atom of the PAH is $52\pm$5 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is $61\pm5$~meV/atom which is considerably larger than previously reported experimental values.

cond-mat.mtrl-sci↗

arXiv2002

Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite

We present a study on the kinetics of oxygen adsorption and desorption from single-wall carbon nanotube (SWNT) and highly oriented pyrolytic graphite (HOPG) samples. Thermal desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K which is shifted to significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O2 binding energy on SWNT bundles, 18.5 kJ/mol, is 55% higher than that for adsorption on HOPG, 12.0 kJ/mol. In combination with molecular mechanics calculations we show that the observed binding energies for both systems can be attributed to van der Waals interactions, i.e. physisorption. The experiments provide no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption.

cond-mat.mtrl-sci↗

arXiv2002

Charge-carrier dynamics in single-wall carbon nanotube bundles: A time-domain study

We present a real-time investigation of ultrafast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow to study the processes governing the subpicosecond and the picosecond dynamics of non-equilibrium charge-carriers. On the subpicoseond timescale the dynamics are dominated by ultrafast electron-electron scattering processes which lead to internal thermalization of the laser excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi-level - the highest energy studied experimentally. The subsequent cooling of the laser heated electron gas down to the lattice temperature by electron-phonon interaction occurs on the picosecond time-scale and allows to determine the electron-phonon mass enhancement parameter lambda. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper.

cond-mat.mtrl-sci↗

arXiv2002

Desorption kinetics and interaction of Xe with single-wall carbon nanotube bundles

We present a study on the kinetics of xenon desorption from single-wall carbon nanotube (SWNT) bundles using thermal desorption spectroscopy (TDS). TD-spectra from SWNT samples show a broad desorption feature peaked at significantly higher temperature than the corresponding low-coverage desorption feature on graphite. The observations are explained using a coupled desorption-diffusion (CDD) model, which allows the determination of the low-coverage Xe binding energy for adsorption on SWNT bundles, 27 kJ/mol. This energy is about 25% higher than the monolayer binding energy on graphite, 21.9 kJ/mol. By comparison with molecular mechanics calculations we find that this increase of the binding energy is consistent with adsorption in highly coordinated groove-sites on the external bundle surface.

cond-mat↗

arXiv2001

Anisotropy of Quasiparticle Lifetimes and the Role of Disorder for Ultrafast Electron Dynamics in Graphite

Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 eV and 1.5 eV - the vicinity of a saddle point in the graphite bandstructure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g. defects or phonons, increases electron energy relaxation rates.

cond-mat↗

arXiv1999

Influence of excited electron lifetimes on the electronic structure of carbon nanotubes

We have studied the dynamics of electrons in single wall carbon nanotubes using femtosecond time-resolved photoemission. The lifetime of electrons excited to the pi* bands is found to decrease continuously from 130 fs at 0.2 eV down to less than 20 fs at energies above 1.5 eV with respect to the Fermi level. This should lead to a significant lifetime--induced broadening of the characteristic van Hove singularities in the nanotube DOS.

cond-mat.mtrl-sci↗