arXiv subjects

Matthias Kuehne

Publications and source records attributed to Matthias Kuehne.

5 recordsLinked to original sources

arXiv2026

Unified 1D Theory and Design Principles for Harmonic Electrothermal Characterization of Nanoscale Conductors

Electrothermal characterization based on the third or other harmonics of an ac Joule heating current is widely deployed for the thermal analysis of solid conductors and their environment, including solid substrates and fluids. However, a unified theory that bridges heat transfer in two archetypal experimental geometries - suspended vs. substrate-supported conductor - has been missing. Here, we present and validate such a theory that explicitly accounts for finite conductor length, thermal mass, and environmental coupling through a unified thermal transfer function. This framework enables the prediction of voltage responses at all harmonics of the driving current (dc, 1$\omega$, 2$\omega$, 3$\omega$) and the formulation of design principles for the characterization of nanoscale conductors. The conductor length $l$ is the primary parameter controlling the frequency regime at which the conductor's thermal mass dominates the thermal response, with the characteristic frequency $\omega_\mathrm{c}=\alpha/l^2$, where $\alpha$ is the conductor's thermal diffusivity - closely related to a criterion previously reported for suspended wires free from environmental coupling. Our unified framework generalizes this result, revealing that sufficiently weak environmental coupling is a necessary condition for $\omega_\mathrm{c}$ to govern the onset of thermal-mass-dominated response. Optimization of interfacial thermal resistance and environmental thermal impedance may further improve temperature resolution and facilitate on-substrate implementations.

physics.app-ph↗

arXiv2025

Hydrogen bonding in water under extreme confinement

Fluids under extreme confinement exhibit unique structures and intermolecular bonding, distinct from their bulk analogs, driving innovative applications at the water-energy nexus. Probing confined water experimentally at the length scale of intermolecular and surface forces has, however, remained a challenge. Here, we report direct molecular-level observations of hydrogen bonding in water confined inside individual carbon nanotubes, enabled by in-situ vibrational electron energy-loss spectroscopy with nanoscale resolution. Hydrogen bonding is probed via the intramolecular O-H stretching frequency, which serves as a sensitive spectral signature of the local intermolecular bonding environment. Water in larger carbon nanotubes exhibit the bonded O-H vibrations of bulk water, but at smaller diameters, the frequency blueshifts to near the free O-H stretch found in water vapor and hydrophobic surfaces, indicating a highly dispersed, non-H-bonded environment. Theoretical analysis based on quantum vibrational oscillators indicates that enhanced damping rates, corresponding to rapid hydrogen-bond fluctuations, leads the bimodal spectral peaks to merge into a single broad feature, matching the experimental observation. Furthermore, cryogenic experiments provide insights into complex structural phase transitions of confined water. This research reveals the quantum and dynamic nature of hydrogen bonds under confinement and the potential impact of unveiling molecular-level structure and bonding in confined fluids.

physics.chem-ph↗

arXiv2025

Thermal characterization of suspended fine wires across continuum to free-molecular gas regimes using the 3$ω$ method

The 3$ω$ method is widely used to measure the thermal conductivity and the specific heat of wires and thin films. These measurements are typically performed under high vacuum conditions, which justify the use of heat transfer models that exclude thermal losses to a surrounding fluid. Here, we study the effect of thermal conduction from a joule-heated wire to a surrounding gas on pressure-dependent 3$ω$ measurements, and show how a one-dimensional (1D) heat-transfer model may be used to reliably determine the wire's thermal properties. We derive a full analytical solution of the 1D heat-transfer equation with finite heat-transfer coefficient $h$ and validate it experimentally using a 16-$μ$m diameter platinum wire in air across pressures from $10^{-5}$ to $10^3$ mbar. We introduce a model for heat transfer between the wire and the surrounding gas based on kinetic gas theory that accurately describes the data across continuum to free-molecular gas regimes, with $h$ varying from near-zero in high vacuum to approximately 700 W/(m$^2\cdot$K) at atmospheric pressure. We show that use of a validated $h(p)$ model allows extracting both thermal conductivity $κ$ and volumetric heat capacity $ρc_p$, whereas volumetric heat capacity can be extracted even without invoking a specific $h(p)$ model. Our approach facilitates the characterization of fine wires with moderate to low thermal conductivities and may enable accurate thermal measurements of suspended wires with diameters on the nanometer scale.

physics.app-ph↗

arXiv2021

An Irreversible Synthetic Route to an Ultra-Strong Two-Dimensional Polymer

Polymers that extend covalently in two dimensions have attracted recent attention as a means of combining the mechanical strength and in-plane energy conduction of conventional two-dimensional (2D) materials with the low densities, synthetic processability, and organic composition of their one-dimensional counterparts. Efforts to date have proven successful in forms that do not allow full realization of these properties, such as polymerization at flat interfaces or fixation of monomers in immobilized lattices. A frequently employed synthetic approach is to introduce microscopic reversibility, at the cost of bond stability, to achieve 2D crystals after extensive error correction. Herein we demonstrate a synthetic route to 2D irreversible polycondensation directly in the solution phase, resulting in covalently bonded 2D polymer platelets that are chemically stable and highly processable. Further fabrication offers highly oriented, free-standing films which exhibit exceptional 2D elastic modulus and yield strength at 50.9 +- 15.0 GPa and 0.976 +- 0.113 GPa, respectively. Platelet alignment is evidenced by polarized photoluminescence centered at 580 and 680 nm from different dipole transitions. This new synthetic route provides opportunities for 2D polymers in applications ranging from composite structures to molecular sieving membranes.

cond-mat.soft↗

arXiv2019

Observation and Spectral Assignment of a Family of Hexagonal Boron Nitride Lattice Defects

Atomic vacancy defects in single unit cell thick hexagonal boron nitride are of significant interest because of their photophysical properties, including single-photon emission, and promising applications in quantum communication and computation. The spectroscopic assignment of emission energies to specific atomic vacancies within the triangular lattice is confounded by the exponential scaling of defect candidates with the number of removed atoms. Herein, we collect more than 1000 spectra consisting of single, isolated zero-phonon lines between 1.69 and 2.25 eV, observing 6 quantized zero-phonon lines arising from hexagonal boron nitride vacancies. A newly developed computational framework for isomer cataloguing significantly narrows the number of candidate vacancies. Direct lattice imaging of hexagonal boron nitride, electronic structure calculations, and subsequent boric acid etching are used to definitively assign the 6 features. Systematic chemical etching supports the assignment by demonstrating the sequence of growth of successively larger vacancy centres from smaller ones, with the defects including a single B vacancy and a 16-atom triangular defect. These features exhibit a range of emission lifetimes from 1 to 6 ns, and phonon sidebands offset by the dominant lattice phonon in hexagonal boron nitride near 1370 cm-1. This assignment should significantly advance the solid-state chemistry and photophysics of such vacancy emitters.

cond-mat.mtrl-sci↗