Edgar Meyhöfer

ABSTRACT Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants with poorly understood sublethal effects on insects. Perfluorooctanoic acid (PFOA), one of the most widely distributed legacy PFAS is increasingly recognized for altering organismal physiology beyond traditional toxicity endpoints. Here, we use the fruit fly Drosophila melanogaster as a model to examine how PFOA exposure during larval (juvenile) development reshapes insect life-history progression and metabolic homeostasis. Our studies reveal that at environmentally relevant concentrations (nM to low µM), PFOA induces precocious expression of developmentally-regulated genes and leads to metabolic changes that persist into adulthood. At higher concentrations used to probe mechanism, PFOA accelerates larval development, disrupts mitochondrial membrane potential, and increases whole-organism metabolic heat production – results that suggest altered mitochondrial energetic efficiency. Consistent with this tradeoff, PFOA-exposed larvae that develop faster under permissive conditions exhibit heightened sensitivity to environmental stressors, including elevated temperature and reduced food hydration. Together, these findings demonstrate that PFOA disrupts metabolic and developmental processes in a dose- and context-dependent manner, highlighting sublethal effects that may influence insect resilience under environmental stress. SYNOPSIS STATEMENT Here we describe how PFOA alters the growth, development, and metabolism of the fruit fly Drosophila melanogaster . Specifically, we find that PFOA accelerates Drosophila juvenile growth while also rendering exposed larvae sensitive to environmental stress. These observations suggest that widespread PFOA contamination may impair the developmental fitness of insect populations.

Nanoscale heat transport plays an important role in energy conversion and thermal management. Therefore, understanding how nanoscale heat transport can be tuned is critical for developing novel technologies, including cooling strategies for microelectronics and nanostructured materials and devices for high-efficiency energy conversion. To probe nanoscale thermal transport phenomena, many calorimetric tools and approaches have been developed. Specifically, suspended microcalorimeters featuring picowatt resolution have been extensively employed for measuring thermal transport in low-dimensional materials, radiative heat transfer in nanoscale gaps, and between subwavelength structures. Further, scanning calorimetric probes, combined with atomic force microscopy and scanning tunneling microscopy, have been utilized for probing atomic-scale thermal transport and near-field thermal radiation. Here, we discuss these advances in calorimetric tools and their use for studying nanoscale thermal transport. We conclude by discussing open experimental challenges and highlighting the importance of future developments in subpicowatt resolution calorimetric tools for accessing unexplored nanoscale thermal transport phenomena.

Quantitative insights into brain metabolism are essential for advancing our understanding of the energy dynamics in the brain. Here, we present a nanowatt-resolution biocalorimeter capable of real-time metabolic heat output measurements of individual, live Drosophila melanogaster brains. Using this platform, we show that female brains, across multiple genotypes, exhibit a significantly higher metabolic rate (∼10%-15%) than male brains at a young age (<10 days old) and follow distinct metabolic trajectories across the lifespan. We also find that parkin mutants, a genetic model for Parkinson's disease, exhibit a ∼15% reduction in brain metabolic output relative to controls, revealing that defective mitophagy due to parkin deficiency affects brain metabolism. Further, we demonstrate that the metabolic output of a Drosophila brain is ∼2.5-fold higher than reproductive tissues like ovary and testis. Together, these advances open new avenues for investigating the impact of aging, neurodegeneration, and disease states on brain metabolism.

Source data for figures

Source data for figures

Thermal currents arise in the presence of temperature gradients, analogous to electrical currents driven by voltage differentials. Beyond such normal currents, persistent electrical currents flowing without a voltage differential arise in superconductors. However, persistent thermal currents in the absence of temperature differentials have never been experimentally detected. Here, we report a direct experimental detection of a persistent thermal current via calorimetric measurements performed using a custom-fabricated platform consisting of three magneto-optical Indium Gallium Arsenide discs positioned symmetrically in the near-field of each other. When reciprocity is broken by applying an orthogonal magnetic field, an asymmetric thermal conductance appears between the discs, providing an unambiguous signature of a persistent heat current. The direction of this current is reversed with the magnetic field orientation and the magnitude scales with the magnetic field, confirming its origin in broken reciprocity. This work uncovers a completely novel transport phenomenon that can enable the control of energy flow in non-reciprocal devices created from magneto-optical and topological materials and unlocks transformative energy storage and thermal management applications.

Thermal currents arise in the presence of temperature gradients, analogous to electrical currents driven by voltage differentials. Beyond such normal currents, persistent electrical currents flowing without a voltage differential arise in superconductors. However, persistent thermal currents in the absence of temperature differentials have never been experimentally detected. Here, we report a direct experimental detection of a persistent thermal current via calorimetric measurements performed using a custom-fabricated platform consisting of three magneto-optical Indium Gallium Arsenide discs positioned symmetrically in the near-field of each other. When reciprocity is broken by applying an orthogonal magnetic field, an asymmetric thermal conductance appears between the discs, providing an unambiguous signature of a persistent heat current. The direction of this current is reversed with the magnetic field orientation and the magnitude scales with the magnetic field, confirming its origin in broken reciprocity. This work uncovers a completely novel transport phenomenon that can enable the control of energy flow in non-reciprocal devices created from magneto-optical and topological materials and unlocks transformative energy storage and thermal management applications.

Radiative heat transfer between nanoscale (i.e., subwavelength) structures, with dimensions smaller than the thermal wavelength, can significantly surpass the far-field blackbody limit (Thompson, D.; et al. Nature 2018 ). This enhanced thermal coupling, called super-Planckian radiative heat transfer, limits the performance of high-resolution calorimeters [often made of silicon nitride (SiN)] used in nanoscale thermal sensing. Here, via computational and experimental work, we show that super-Planckian coupling can be significantly attenuated by employing polymers. Our calculations show that Parylene-C (a polymer) exhibits a lower density of guided-modes and reduced absorption across the thermal spectrum, suppressing this coupling by up to 10-fold compared to SiN. Experiments performed with custom-fabricated Parylene-C and SiN devices confirm that the radiative coupling is indeed attenuated in Parylene-C. Our findings highlight how the super-Planckian coupling can be attenuated for improved performance in high-resolution calorimetry.

We investigate self-heating effects (SHE) and thermal mitigation strategies in ferroelectric ScAlN/GaN high-electron-mobility transistors (HEMTs). Molecular beam epitaxy (MBE)-grown devices demonstrate a large memory window (MW) of ~3.8 V, on/off current ratio (I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub>/I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub>) >10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup>, and sub-Boltzmann subthreshold swing (SS) ~20 mV/dec, enabled by ScAlN polarization control of the two-dimensional electron gas (2DEG). Under high drain bias, SHE degrades memory and subthreshold characteristics. The thermal origin of degradation is confirmed by external heating and transconductance analysis. Multi-frequency conductance measurements reveal charge trapping and detrapping, which may be accelerated by SHE. Heat sink integration effectively reduces temperature rise, as verified by scanning thermal microscopy. These results underscore the importance of thermal management for reliable ferroelectric HEMT operation in high-power and extreme-environment applications.

Abstract Quantitative insights into brain metabolism are essential for advancing our understanding of energy dynamics in the brain. However, current approaches for tracking brain metabolism, metabolic profiling and respirometry, provide only static snapshots of metabolite levels or lack the required resolution. Here, we develop a novel nanowatt-resolution biocalorimeter capable of real-time continuous measurements of heat output to quantitatively measure the metabolism of individual live Drosophila melanogaster brains and investigate how sex, genotype, age, and disease affect brain metabolism. We show for the first time that female brains, across multiple wild-type genotypes, exhibit a significantly higher metabolic rate (∼10%) than male brains at a young age (&lt;10 days old) and follow distinct metabolic trajectories across the lifespan. We also find that parkin mutants, a genetic model for Parkinson’s disease, exhibit a ∼15% reduction in brain metabolic output relative to controls, revealing that defective mitophagy due to parkin deficiency affects brain metabolism. Furthermore, we measure the metabolic rate of reproductive tissues of Drosophila , highlighting the broad applicability of our biocalorimeter. Together, these advances open new avenues for investigating how tissue-specific metabolism is impacted by aging, neurodegeneration, and disease states. Teaser Direct measurement of metabolic rate of individual Drosophila brains to investigate how sex, genotype, age, and disease affect brain metabolism.

High Resolution Image Download MS PowerPoint Slide Anisotropic thermal transport was measured in imine-linked two-dimensional polymer (2DP) films that were prepared by interfacial polymerization. Measurements of both in-plane ( k ∥ ) and cross-plane ( k ⊥ ) thermal conductivities relied on preparing free-standing 2DP films that were readily transferred for different measurement configurations. We polymerized two 2DP (Per-PDA and TAPPy-PDA) films at a liquid–liquid interface. These polycrystalline, imine-linked 2DP films are 100–200 nm in thickness and were measured by frequency domain thermoreflectance to extract k ⊥ and a suspended calorimetric platform technique to evaluate k ∥ . We find that k ∥ is larger than k ⊥ in both materials at room temperature, leading to anisotropy ratios ( k ∥ / k ⊥ ) as high as 2.3. We attribute this behavior to the fact that the stiff, in-plane covalent bonds of 2DPs transport heat more effectively than the flexible, supramolecular cross-plane interactions. Variable–temperature measurements revealed a positive correlation between temperature and thermal conductivity, which we attribute to phonon scattering from grain boundaries and defects in the polycrystalline 2DP films. Molecular dynamics simulations of pristine crystals predict larger thermal conductivities and anisotropy ratios exceeding 7. The simulations suggest that as higher quality 2DP films become available, higher thermal conductivities and anisotropy ratios will also manifest.

Classical transport of electrons and holes in nanoscale devices leads to heating that severely limits performance, reliability, and efficiency. In contrast, recent theory suggests that interband quantum tunneling and subsequent thermalization of carriers with the lattice results in local cooling of devices. However, internal cooling in nanoscale devices is largely unexplored. Here, using a novel scanning thermal microscopy technique with millikelvin temperature resolution and nanometer spatial resolution, we directly record the cross-sectional temperature in functional InGaAs tunnel diodes. Our measurements reveal large, localized cooling of 2-3 W/cm^{2} at the tunnel junction, which is in quantitative agreement with the bipolar Peltier process associated with interband tunneling. These advances hold significant potential for integration into electronic and energy conversion devices and improving their performance.

Abstract Control of heat flow is critical for thermal logic devices and thermal management and has been explored theoretically. However, experimental progress on active control of heat flow has been limited. Here, we describe a nanoscale radiative thermal transistor that comprises of a hot source and a cold drain (both are ~250 nm-thick silicon nitride membranes), which are analogous to the source and drain electrodes of a transistor. The source and drain are in close proximity to a vanadium oxide (VO x )-based planar gate electrode, whose dielectric properties can be adjusted by changing its temperature. We demonstrate that when the gate is located close ( &lt; ~1 µm) to the source-drain device and undergoes a metal-insulator transition, the radiative heat transfer between the source and drain can be changed by a factor of three. More importantly, our nanomembrane-based thermal transistor features fast switching times ( ~ 500 ms as opposed to minutes for past three-terminal thermal transistors) due to its small thermal mass. Our experiments are supported by detailed calculations that highlight the mechanism of thermal modulation. We anticipate that the advances reported here will open new opportunities for designing thermal circuits or thermal logic devices for advanced thermal management.

High-resolution thermometry is critical for probing nanoscale energy transport. Here, we demonstrate how high-resolution thermometry can be accomplished using vanadium oxide (VO x ), which features a sizable temperature-dependence of its resistance at room temperature and an even stronger dependence at its metal–insulator-transition (MIT) temperature. We microfabricate VO x nanofilm-based electrical resistance thermometers that undergo a metal–insulator-transition at ∼337 K and systematically quantify their temperature-dependent resistance, noise characteristics, and temperature resolution. We show that VO x sensors can achieve, in a bandwidth of ∼16 mHz, a temperature resolution of ∼5 μK at room temperature (∼300 K) and a temperature resolution of ∼1 μK at the MIT (∼337 K) when the amplitude of temperature perturbations is in the microkelvin range, which, in contrast to larger perturbations, is found to avoid hysteric resistance responses. These results demonstrate that VO x -based thermometers offer a ∼10–50-fold improvement in resolution over widely used Pt-based thermometers.

Achieving micro-kelvin (µK) temperature stability is critical for many calorimetric applications. For example, sub-nanowatt resolution biocalorimetry requires stabilization of the temperature of the calorimeter to µK levels. Here, we describe how µK temperature stability can be accomplished in a prototypical calorimetric system consisting of two nested shields and a suspended capillary tube, which is well suited for biocalorimetry applications. Specifically, we show that by employing nested shields with µTorr-levels of vacuum in the space between them as well as precise feedback control of the temperature of the shields (performed using high-resolution temperature sensors), the effect of ambient temperature fluctuations on the inner shield and the capillary tube can be attenuated by ∼100 dB. We also show that this attenuation is key to achieving temperature stabilities within ±1 and ±3 µK (amplitude of oscillations) for the inner shield and the capillary tube sensor, respectively, measured in a bandwidth of 1 mHz over a period of 10 h at room temperature (∼20.9 ± 0.2 °C). We expect that the methods described here will play a key role in advancing biocalorimetry.

-dimethyl amine group feature a higher thermopower than MJs with the nitro group. We also present calculations based on first principles, which explain these trends and show that the transport properties are highly sensitive to microscopic details in junctions, exhibiting destructive QI features.

Recent experiments, at room temperature, have shown that near-field radiative heat transfer (NFRHT) via surface phonon polaritons (SPhPs) exceeds the blackbody limit by several orders of magnitude. Yet, SPhP-mediated NFRHT at cryogenic temperatures remains experimentally unexplored. Here, we probe thermal transport in nanoscale gaps between a silica sphere and a planar silica surface from 77-300 K. These experiments reveal that cryogenic NFRHT has strong contributions from SPhPs and does not follow the T^{3} temperature (T) dependence of far-field thermal radiation. Our modeling based on fluctuational electrodynamics shows that the temperature dependence of NFRHT can be related to the confinement of heat transfer to two narrow frequency ranges and is well accounted for by a simple analytical model. These advances enable detailed NFRHT studies at cryogenic temperatures that are relevant to thermal management and solid-state cooling applications.

High Resolution Image Download MS PowerPoint Slide Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn–P1 and Zn–P2 . Past theory suggests that P1 and Zn–P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn–P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn–P1 junctions is relatively close, and the same holds for P2 and Zn–P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn–P1 junctions show the largest thermopower and Zn–P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn–P1 junctions is robustly predicted to be larger than those of P2 and Zn–P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn–P1 and Zn–P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Recent measurements of near-field radiative heat transfer (NFRHT) between objects separated by nanometer-sized vacuum gaps have revealed that thermal radiation at the nanoscale is remarkably distinct from far-field thermal radiation and can exceed the blackbody radiation limit by orders of magnitude. Given the technological relevance of thin films, there remains a significant need to experimentally explore how such films influence NFRHT. Here, we report direct measurements of the thickness-dependence of NFRHT between planar nanofilms of magnesium fluoride (thickness ranging from 20 to 500 nm) performed using microfabricated devices and a custom-developed nanopositioner. These results directly demonstrate for the first time that nanofilms can enhance thermal radiation up to 800-fold above the blackbody limit and are as effective as bulk materials when nanoscale gaps have dimensions smaller than the film thickness. Finally, calculations based on fluctuational electrodynamics show good agreement with the measured gap-size dependence of the heat transfer coefficient for films of all thicknesses and provide physical insight into the observed dependence. The experimental techniques and insights reported here pave the way for systematically exploring novel thin films for near-field thermal and energy systems.

Control of nanoscale thermal transport is essential for novel energy conversion, thermal management, and thermal logic devices. Recent experimental and theoretical studies have shown that far-field radiative heat transfer in planar membranes with nanoscale thickness can exceed the blackbody limit. Past computations suggest that the observed enhancements are due to highly directional and spatially confined in-plane heat transfer between these membranes. However, experimental evidence for this confinement is lacking. Here, we perform experiments on submicron-thick, planar silicon nitride membranes to directly quantify the spatial extent over which heat transfer occurs and show that, at room temperature, heat transfer is confined to ∼10 μm above and below the plane containing the membranes, a distance comparable to Wien’s wavelength. Furthermore, we provide calculations of Poynting fluxes that enable detailed explanations of our experimental observations. The resulting insights could lead to novel approaches and devices for active control of heat flow at the nanoscale.

Thermophotovoltaic (TPV) energy conversion is a promising power-generation technology for converting heat to electricity. Recent studies have explored TPV devices featuring nanoscale gaps, which take advantage of near-field effects that enable much larger radiative fluxes and power density. The authors review the physics of near-field thermal radiation, and assess theoretical and experimental advances in predicting and validating near-field enhancements of power output and efficiency in TPV devices. Their discussion of the near-field photonic heat engines presented here will help to guide future engineering solutions in developing practical near-field energy-conversion devices.

Near-field radiative heat transfer (NFRHT) arises between objects separated by nanoscale gaps and leads to dramatic enhancements in heat transfer rates compared to the far-field. Recent experiments have provided first insights into these enhancements, especially using silicon dioxide (SiO 2 ) surfaces, which support surface phonon polaritons (SPhP). Yet, theoretical analysis suggests that SPhPs in SiO 2 occur at frequencies far higher than optimal. Here, we first show theoretically that SPhP-mediated NFRHT, at room temperature, can be 5-fold larger than that of SiO 2, for materials that support SPhPs closer to an optimal frequency of 67 meV. Next, we experimentally demonstrate that MgF 2 and Al 2 O 3 closely approach this limit. Specifically, we demonstrate that near-field thermal conductance between MgF 2 plates separated by 50 nm approaches within nearly 50% of the global SPhP bound. These findings lay the foundation for exploring the limits to radiative heat transfer rates at the nanoscale.