arXiv subjects

Mischa Bonn

Publications and source records attributed to Mischa Bonn.

At least 19 recordsLinked to original sources

arXiv2026

Relaxing Coherence Requirements on Laser Sources for Nanoscopy through Optical Fiber Technique

High numerical-aperture (NA) focusing of cylindrical vector beams (CVBs) typically requires costly, high-quality lasers to supply a stable vector pupil field. Here, we introduce fiber-conditioned vectorial nanofocusing, in which an optical-fiber-based mode-selective coupler projects a strongly distorted beam of a diode that costs two orders of magnitude less than the reference laser into radially or azimuthally polarized CVBs. A three-tolerance analysis clarifies its operating principle by establishing separate requirements for spatial state, temporal coherence and residual wavefront error. The resulting fields reproduce expected high-NA focal signatures and generate orientation-sensitive single-molecule excitation patterns consistent with reference-laser measurements, lowering the barrier to structured-light experiments.

physics.optics↗

arXiv2026

A hidden bulk polymorph governs charge transport dimensionality in an organic semiconductor

Organic semiconductors (OSCs) are widely explored for flexible optoelectronic technologies, with performance governed not only by molecular design, but also by solid-state packing, which can give rise to polymorphism. Dinaphthothienothiophene (DNTT) is a benchmark OSC that has long been considered monomorphic. Here, we discover, isolate, and resolve the crystal structure of a previously unrecognised bulk polymorph of DNTT, termed blue DNTT owing to its characteristic blue emission. Coexisting with the well-known (green) DNTT in commercial powders, yet previously overlooked, blue DNTT represents the thermodynamically stable form. By combining X-ray diffraction, Raman, and THz spectroscopy with simulations, we demonstrate that polymorphism in DNTT reshapes the low-frequency phonon landscape and transfer-integral network, impacting charge transport. While green DNTT exhibits two-dimensional charge transport with holes more mobile than electrons, blue DNTT shows charge transport along all crystallographic directions enabled by a distinct herringbone packing. Electron mobility along the crystallographic a and b-axes in blue DNTT exceeds twice the hole mobility in the green phase. To our knowledge, this is the first reported acene-based semiconductor exhibiting three-dimensional charge transport. Polymorphism emerges as a key lever to tune charge transport dimensionality and carrier efficiency in organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Electrical tunability of terahertz nonlinearity in graphene

Graphene is conceivably the most nonlinear optoelectronic material. Its nonlinear optical coefficients in the terahertz (THz) frequency range surpass those of other materials by many orders of magnitude. This, in particular, allows one to use graphene for extremely efficient up-conversion of sub-THz electronic input signals into the THz frequency range at room temperature and under ambient conditions, thus paving the way for practical graphene-based ultrahigh-frequency electronic technology. Here, we show that the THz nonlinearity of graphene can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in THz third-harmonic generation in graphene by about two orders of magnitude. This essentially converts graphene from an almost perfectly linear, inert electronic material to a material with the highest possible THz nonlinearity. We demonstrate gating control of THz nonlinearity of graphene for both ultrashort single-cycle and quasi-monochromatic multi-cycle input signals. Our experimental results are in quantitative agreement with a physical model of graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultra-high frequencies.

cond-mat.mes-hall↗

arXiv2026

Dielectric control of ultrafast carrier dynamics and transport in graphene

Understanding the ultrafast dynamics of photoexcited charges in graphene is essential, as the microscopic mechanisms underlying these dynamics determine many of graphene's optical, optothermal, and optoelectronic properties. These are crucial properties for many functionalities and devices enabled by graphene, such as high-speed photodectors. Therefore, beyond scientific understanding, it is highly desirable to control ultrafast carrier dynamics for practical applications. Here, we establish this control by engineering the dielectric environment of graphene, thereby regulating both heating and cooling dynamics without altering the Fermi energy, optical power, or ambient temperature. By combining optical pump-terahertz probe experiments with theoretical calculations, we show that dielectric screening suppresses carrier-carrier interactions and slows the dynamics. In particular, reduced carrier-carrier scattering delays the formation of a quasi-equilibrium hot electron distribution, thus slowing carrier heating. It also slows carrier cooling because re-thermalization after optical-phonon emission depends on the same interactions. The enhanced screening further reduces the energy of electron-hole puddles, thereby increasing charge mobility and the Seebeck coefficient. This ability to externally control internal graphene dynamics and transport properties enables the optimization of device performance, such as the sensitivity of photodetectors for data communication and wireless communication applications.

cond-mat.mes-hall↗

arXiv2026

Suppressed excitonic effects enable high mobility, high-yield photoconductivity in a two-dimensional polymer crystal with axial pyridine coordination

Two-dimensional polymers (2DPs) and their layer-stacked covalent organic frameworks (2D COFs) offer modular, atomically precise platforms for organic optoelectronics, yet their photoconductive responses remain fundamentally constrained by strong excitonic effects and localized charge transport. Here, we demonstrate that a diyne-linked 2DP crystal with axial pyridine coordination overcomes this limitation, enabling simultaneous efficient free-carrier generation and band-like transport. Introducing pyridine ligands that axially coordinate to Cu-porphyrin nodes transforms weak van der Waals stacking into a pyridine-bridged architecture with pronounced interlayer band dispersion and substantially reduced carrier effective masses. The resulting strong interlayer electronic coupling suppresses the exciton binding energy to well below thermal energy, such that optical excitation directly populates delocalized electronic states. Time-resolved terahertz spectroscopy reveals Drude-type photoconductivity with room-temperature mobilities approaching 500 cm^2 V^-1 s^-1 and a photon-to-free-carrier conversion ratio of ~0.4, yielding a photoconductive response that exceeds that of state-of-the-art organic and many inorganic photoactive materials. These results establish interlayer coordination as a powerful strategy for mitigating excitonic effects and accessing inorganic-like charge transport in organic 2D crystals, opening a pathway toward highly efficient photo-to-electricity conversion in organic-based systems.

cond-mat.mtrl-sci↗

arXiv2026

Generalized thermodynamic closure in ultrafast phonon dynamics

Driven-dissipative dynamics underlie a wide range of nonequilibrium phenomena in quantum materials, yet reduced descriptions beyond the quasi-equilibrium picture remain difficult to establish. Here, we experimentally demonstrate that a resonantly driven phonon mode admits a generalized thermodynamic description in which coherence and energy jointly organize the nonequilibrium evolution. Beyond a threshold driving field strength, we observe a delayed ultrafast response of a coherently driven phonon mode. Combined with experimentally constrained Lindblad dynamics, we show that this delay reflects the finite-time spreading of excitations across many phonon levels. At the same time, the full density-matrix trajectories for three driving conditions collapse onto a common surface defined by energy and coherence. Our results establish a coherence-extended thermodynamic regime for driven phonons and provide a framework for broader state engineering in driven-dissipative bosonic excitations.

cond-mat.mtrl-sci↗

arXiv2025

Observation of a phonon bottleneck effect on the thermal depopulation from a photoexcited shallow defect in silicon

We report the observation of a phonon bottleneck effect impacting the thermal depopulation of photoexcited shallow defects in high-resistivity silicon. Using time-resolved terahertz (THz) spectroscopy, near-band-gap excitation produces a pronounced temporal delay in photoconductivity, indicating that a fraction of photogenerated charge carriers is temporarily trapped immediately after excitation. By analyzing the frequency-resolved complex photoconductivity as a function of pump-probe delay and photon energy, we attribute this delay to the presence of a localized shallow state situated approximately 40 meV from the band edge, which competes with silicon's indirect band-to-band absorption. The zero-order kinetic profile of the temporal delay, its invariance with respect to photon flux, and its temperature dependence collectively support the existence of a phonon bottleneck that hinders the thermal release of electrons from this shallow trap. this represents experimental evidence of a phonon bottleneck effect associated with the thermal activation of shallow traps in photoexcited silicon. These findings provide microscopic insight into carrier relaxation dynamics in silicon and highlight the significance of electron-phonon interactions in the ultrafast processes governing materials used in optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Breaking the Air-Water Paradigm: Ion Behavior at Hydrophobic Solid-Water Interfaces

Hydrophobic solid-water interfaces underpin processes in nanofluidics, electrochemistry, and energy technologies. Microscopic insights into these systems are often inferred from our understanding of the air-water interface, which is assumed to exhibit similar behavior. Here, we challenge this paradigm by combining heterodyne-detected vibrational sum-frequency generation spectroscopy with machine-learning molecular dynamics simulations at first-principles accuracy to investigate the graphene-NaCl(aq) interface as a prototypical hydrophobic solid-water system. Spectroscopic results suggest that ions have a minimal effect on the structure of the interfacial water, while simulations reveal that Na$^{+}$ and Cl$^{-}$ accumulate densely at the surface. Together, these findings reveal a new adsorption mechanism that departs from the established air-water interface paradigm, where interfacial ion adsorption is typically associated with, and often detected through, pronounced alteration of the interfacial water alignment and orientation. This difference arises because ions cannot penetrate the solid boundary and reside at a similar depth as the interfacial water molecules. As a consequence, large ion populations can be accommodated within the extended two-dimensional hydrogen-bond network at the interface, causing only minor local distortions but significant changes to its longer-range connectivity. These results reveal a distinct mechanism of electrolyte organization at aqueous-carbon interfaces, relevant to energy applications, where performance is highly sensitive to the local organization of interfacial water.

physics.chem-ph↗

arXiv2025

Wetting Transparency of Graphene: A macroscopic Window but Nanoscopic Mirror

Graphene supported on a substrate in contact with water underpins a wide range of processes and technologies, yet its wettability remains controversial. Understanding how substrate charges and graphene's properties influence water organization is crucial. Here, we combine heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy with molecular dynamics simulations to investigate CaF$_2$-supported graphene interfaces in contact with water. We find that interfacial water orientation is primarily governed by the CaF$_2$ substrate's pH-dependent local electrostatics, confirming graphene's macroscopic wetting transparency. However, at the nanoscale, graphene's polarizability induces a local inversion of water's molecular orientation above substrate charges, revealing subtle structural ordering that is masked in spatially averaged measurements. These insights elucidate the molecular origins of graphene's wetting behavior and suggest new avenues to tailor interfacial phenomena in graphene-based nanofluidic, sensing, and energy applications.

physics.chem-ph↗

arXiv2025

On the Slipperiness of Surfactants: Charge-Mediated Friction Control at the Molecular Scale

From soap-covered dishes to freshly cleaned floors, surfactants can make surfaces slippery; yet, the underlying mechanism remains poorly understood. Here, we identify the molecular origin behind this ubiquitous phenomenon using macroscopic tribology and surface molecular spectroscopy. We demonstrate that reducing friction through surfactants hinges on charge complementarity: surfactants of opposite charge to the solid surface reduce friction even at extreme contact pressures, whereas like-charged or neutral surfactants are ineffective. Oppositely charged surfactants self-assemble into dense and robust molecular brushes, creating a persistent lubrication beyond the limits of conventional mechanisms. This charge-mediated approach offers a universal and scalable framework for friction control across length scales without significant surface modification.

physics.chem-ph↗

arXiv2025

Reevaluating Anomalous Electric Fields at the Air-Water Interface: A Surface-Specific Spectroscopic Survey

The notion that large electric fields at the air-water interface catalyze spontaneous chemical reactions has sparked significant debate, with far reaching implications for atmospheric chemistry and interfacial reactivity. Using vibrational sum frequency generation spectroscopy, we test this hypothesis by directly comparing local electric field strengths at the air-water interface and in bulk water. By applying established vibrational frequency-to-field mappings to the OH stretch of interfacial and bulk water, we extract effective electric field distributions under ambient conditions. Contrary to prevailing claims, our results reveal no spectroscopic evidence for exceptionally strong or long-lived interfacial electric fields. Instead, bulk water consistently exhibits broader distributions and statistically larger field magnitudes. The absence of key spectral signatures, such as redshifted continua, or slowed spectral diffusion, further undermines the idea of anomalous surface fields. Our findings call into question the growing narrative that electrostatic forces at pristine and charge neutral water surfaces can drive chemical reactions and instead highlight the importance of rigorous spectroscopic benchmarks when evaluating interfacial phenomena.

physics.chem-ph↗

arXiv2025

Temperature- and charge carrier density-dependent electronic response in methylammonium lead iodide

Understanding carrier dynamics in photoexcited metal-halide perovskites is key for optoelectronic devices such as solar cells (low carrier densities) and lasers (high carrier densities). Trapping processes at low carrier densities and many-body recombination at high densities can significantly alter the dynamics of photoexcited carriers. Combining optical-pump/THz probe and transient absorption spectroscopy we examine carrier responses over a wide density range (10^14-10^19 cm-3) and temperatures (78-315K) in the prototypical methylammonium lead iodide perovskite. At densities below ~10^15 cm-3 (room temperature, sunlight conditions), fast carrier trapping at shallow trap states occurs within a few picoseconds. As excited carrier densities increase, trapping saturates, and the carrier response stabilizes, lasting up to hundreds of picoseconds at densities around ~10^17 cm-3. Above 10^18 cm-3 a Mott transition sets in: overlapping polaron wavefunctions lead to ultrafast annihilation through an Auger recombination process occurring over a few picoseconds. We map out trap-dominated, direct recombination-dominated, and Mott-dominated density regimes from 78-315 K, ultimately enabling the construction of an electronic phase diagram. These findings clarify carrier behavior across operational conditions, aiding material optimization for optoelectronics operating in the low (e.g. photovoltaics) and high (e.g. laser) carrier density regimes.

cond-mat.mtrl-sci↗

arXiv2025

Spontaneous Surface Charging and Janus Nature of the Hexagonal Boron Nitride-Water Interface

Boron, nitrogen and carbon are neighbors in the periodic table and can form strikingly similar twin structures-hexagonal boron nitride (hBN) and graphene-yet nanofluidic experiments demonstrate drastically different water friction on them. We investigate this discrepancy by probing the interfacial water and atomic-scale properties of hBN using surface-specific vibrational spectroscopy, atomic-resolution atomic force microscopy (AFM), and machine learning-based molecular dynamics. Spectroscopy reveals that pristine hBN acquires significant negative charges upon contacting water at neutral pH, unlike hydrophobic graphene, leading to interfacial water alignment and stronger hydrogen bonding. AFM supports that this charging is not defect-induced. pH-dependent measurements suggest OH- chemisorption and physisorption, which simulations validate as two nearly equally stable states undergoing dynamic exchange. These findings challenge the notion of hBN as chemically inert and hydrophobic, revealing its spontaneous surface charging and Janus nature, and providing molecular insights into its higher water friction compared to carbon surfaces.

physics.chem-ph↗

arXiv2025

Beyond the band edge: Unveiling high-mobility hot carriers in a two-dimensional conjugated coordination polymer

Hot carriers, inheriting excess kinetic energy from high-energy photons, underpin numerous optoelectronic applications involving non-equilibrium transport processes. Current research on hot carriers has predominantly focused on inorganic materials, with little attention paid to organic-based systems due to their ultrafast energy relaxation and inefficient charge transport. Here, we overturn this paradigm by demonstrating highly mobile hot carriers in solution-processable, highly crystalline two-dimensional conjugated coordination polymer (2D c-CP) Cu3BHT (BHT = benzenehexathiol) films. Leveraging a suite of ultrafast spectroscopic and imaging techniques, we unravel the microscopic charge transport landscape in Cu3BHT films following non-equilibrium photoexcitation across temporal, spatial, and frequency domains, revealing two distinct high-mobility transport regimes. In the non-equilibrium transport regime, hot carriers achieve ultrahigh mobility of ~2,000 cm2 V-1 s-1, traversing grain boundaries up to 300 nm within a picosecond. In the quasi-equilibrium transport regime, free carriers exhibit Drude-type band-like transport with a remarkable mobility of ~400 cm2 V-1 s-1 and an intrinsic diffusion length exceeding 1 micrometer. These findings establish 2D c-CPs as versatile platforms for exploring high-mobility non-equilibrium transport, unlocking new opportunities for organic-based hot carrier applications.

cond-mat.mtrl-sci↗

arXiv2024

Curved graphene nanoribbons derived from tetrahydropyrene-based polyphenylenes via one-pot K-region oxidation and Scholl cyclization

Precise synthesis of graphene nanoribbons (GNRs) is of great interest to chemists and materials scientists because of their unique opto-electronic properties and potential applications in carbon-based nanoelectronics and spintronics. In addition to the tunable edge structure and width, introducing curvature in GNRs is a powerful structural feature for their chemi-physical property modification. Here, we report an efficient solution synthesis of the first pyrene-based GNR (PyGNR) with curved geometry via one-pot K-region oxidation and Scholl cyclization of its corresponding well-soluble tetrahydropyrene-based polyphenylene precursor. The efficient A2B2-type Suzuki polymerization and subsequent Scholl reaction furnishes up to 35 nm long curved GNRs bearing cove- and armchair-edges. The construction of model compound, as a cutout of PyGNR, from a tetrahydropyrene-based oligophenylene precursor proves the concept and efficiency of the one-pot K-region oxidation and Scholl cyclization, which is clearly revealed by single crystal X-ray diffraction analysis. The structure and optical properties of PyGNR are investigated by Raman, FT-IR, solid-state NMR and UV-Vis analysis with the support of DFT calculations. PyGNR shows the absorption maximum at 680 nm, exhibiting a narrow optical bandgap of 1.4 eV, qualifying as a low-bandgap GNR. Moreover, THz spectroscopy on PyGNR estimates its macroscopic charge mobility of 3.6 cm2/Vs, outperforming other curved GNRs reported via conventional Scholl reaction.

physics.app-ph↗

arXiv2024

Anisotropic electron-phonon interactions in 2D lead-halide perovskites

Two-dimensional hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interactions between charge carriers and ionic rearrangements. Here, we investigate the interaction of photo-generated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide, varying the inorganic lammelae thickness in the 2D single crystals. We determined the directionality of the transition dipole moments of the relevant phonon modes (in the 0.3-3 THz range) by angle-and-polarization dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a 10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

cond-mat.mtrl-sci↗

arXiv2023

Interfaces Govern Structure of Angstrom-scale Confined Water

Water plays a crucial role in geological, biological, and technological processes. Nanoscale water confinement occurs in many of these settings, including sedimentary rocks, water channel proteins, and applications like desalination and water purification membranes. The structure and properties of water in nanoconfinement can differ significantly from bulk water, exhibiting, for instance, modified hydrogen bonds, dielectric constant, and phase transitions. Despite the importance of strongly nanoconfined water, experimentally elucidating the nanoconfinement effect on water, such as its orientation and hydrogen bond (H-bond) network, has remained challenging. Here, we study two-dimensionally nanoconfined aqueous electrolyte solutions with tunable confinement from nanoscale to angstrom-scale sandwiched between a graphene sheet and CaF2. We employ heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy, a surface-specific vibrational spectroscopy capable of directly and selective probing water orientation and H-bond environment at interfaces and under confinement. Remarkably, the vibrational spectra of the nanoscale confined water can be described quantitatively by the sum of the individual water surface signals from the CaF2/water and water/graphene interfaces until the confinement reduces to angstrom-scale (< ~8 {\AA}). Ab initio molecular dynamics simulations confirm our experimental observation. These results manifest that interfacial, rather than nanoconfinement effects, dominate the water structure until angstrom-level confinement.

physics.chem-ph↗

arXiv2023

Signatures of Mode-Resolved, Nonlocal Electron-Phonon Coupling in Two-Dimensional Spectroscopy

Electron-phonon coupling (EPC) is foundational in condensed matter physics, determining intriguing phenomena and properties in both conventional and quantum materials. In this manuscript, we propose and demonstrate a novel two dimensional (2D) EPC spectroscopy which allows for direct extraction of EPC matrix elements for specific phonon modes and different electron energies, simultaneously. Using this technique, we are able to measure the electron-energy dependence of the EPC strength for individual phonon modes. This capability allows us to identify unique signatures distinguishing nonlocal Su-SchriefferHeeger (SSH)-type couplings from local Holstein-type couplings. In application to a methylammonium lead iodide (MAPI) perovskite, we find that two pronounced phonon modes at room temperature exhibit highly distinctive EPC behaviors, concerning strength, anisotropy, and temperature-dependence across the structural phase transition. Our approach paves the way for unraveling the microscopic origin of EPC, the change of the phonon-mode-specific EPC with external conditions, and phonon-mediated ultrafast control of condensed materials.

cond-mat.mtrl-sci↗