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.

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.

Drive-in diffusion of Mg implanted into GaN during ultra-high pressure annealing leads to low surface acceptor concentrations. This favors p-type Schottky contact formation, which severely increases the on-state resistance of Mg-implanted GaN pn diodes (PNDs). This work aims to reduce the resistance of contacts to Mg-implanted p-GaN by incorporating Mg deposition and annealing into the contact stack, achieving a rectification ratio (RR) over 10 ^12 , a current density above 1 kA cm ^−2 and a record-low differential specific on-resistance ( R _ON ) of 0.65 mΩ.cm ^2 in Mg-implanted PNDs, offering a potential solution for improving the performance and manufacturability of vertical GaN devices that require contacts to Mg-implanted p-GaN.

Professor Gang Chen turns 60 this year. He has made seminal contributions to nanoscale heat transfer and energy sciences in his career spanning over three decades. He has achieved both fundamental progresses and technological advancements with a profound impact on the scientific community and society at large. He is also a prolific educator, having mentored over 100 graduate students and postdocs, many of whom now occupy research and teaching positions in top institutions around the world. With this Special Issue in the Journal of Heat and Mass Transfer, we celebrate Professor Gang Chen's multi-faceted contribution to science and the scientific community by compiling research papers reporting recent progress by former members of his group, including one by Professor Chen himself. This Editorial briefly reviews Professor Gang Chen's scientific contributions in five main areas: (1) Nondiffusive phonon transport; (2) Nanostructured thermoelectrics; (3) Nanoscale thermal radiation; (4) Thermal transport in polymers; (5) Solar-photovoltaic-thermal energy engineering. By no means do we attempt to cover the entire scope of research documented in Gang's more than 400 journal publications. Instead, we discuss his representative works and summarize their scientific significance within a broader context, including a survey of the impact of these original results on defining new frontiers in each area.

Electron and hole impact ionization coefficients are obtained from measurements of high‐Al content Al x Ga 1‐ x N p–n diodes grown on AlN substrates. The photomultiplication method using a 193 nm pulsed laser is applied to measure the multiplication factor. The impact ionization coefficients are modeled using Chynoweth's formulation, based on electric field profiles determined from the solution of Poisson's equation. A least‐squares fit of the theoretical multiplication factor to the measured multiplication factor yields the impact ionization coefficients and the extracted electron impact ionization coefficients are consistent with previous numerical predictions. Furthermore, the theoretical breakdown voltage and critical electric field are computed based on the extracted impact ionization coefficients. These results provide much‐needed data to further optimize the design of optoelectronic, power switching, and high‐power RF devices based on ultrawide bandgap AlGaN.

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 similar to 3.8 V, on/off current ratio (I-on/I-off) > 10(8), and sub-Boltzmann subthreshold swing (SS) similar to 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.

The neutral (VN-3MgGa)0 complex was found to be the primary compensator in Mg-doped N-polar GaN. The experimental data showed a sharp drop in hole concentration once [Mg] exceeded ∼1019 cm−3. Temperature-dependent Hall measurements, in conjunction with a charge balance model, revealed that the carrier loss was due to a drastic reduction in acceptor concentration (NA), suggesting that a significant fraction of Mg atoms was incorporated in an electrically neutral configuration. A quantitative semi-empirical model based on the grand canonical formalism pointed to the formation of (VN-3MgGa)0 complexes as the primary cause for the observed carrier loss.

We report on vertical GaN junction barrier Schottky (JBS) diodes formed by Mg ion implantation and ultrahigh -pressure annealing (UHPA). The static ON-state characteristics of the diodes show an ideality factor of 1.05, a turn-on voltage of $\sim$ 0.7 V, a current rectification ratio of $\sim$ 10 $^{11}$ , and a low differential specific ON-resistance that scales with Schottky stripe width in fair agreement with the analytical model. The reverse leakage dependence on Schottky stripe width also agrees well with the analytical model. Implanted p-n junction diodes fabricated on the same wafer exhibit avalanche breakdown in reverse bias with a positive temperature coefficient, but the forward current is limited by a series barrier. Temperature-dependent current–voltage measurements of th p-n diodes verify the presence of the implanted p-n junction and reveal an additional 0.43-eV barrier, which we hypothesize arises from a p-Schottky contact and forms a second diode back-to-back with the p-n junction. This interpretation is supported by analysis of the capacitance–voltage characteristics of the implanted p-n diodes, epitaxial p-n diodes fabricated with intentional p-Schottky contacts, and comparison to TCAD simulations. Ultimately, the presence of the p-Schottky contact does not hinder JBS diode operation. The use of diffusion-aware designs and/or diffusion reduction represents future directions for Mg implantation technology in GaN power devices.

High-resolution thermometry is critical for probing nanoscale energy transport. Here, we demonstrate how high-resolution thermometry can be accomplished using vanadium oxide (VOx ), 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 similar to 337 K and systematically quantify their temperature-dependent resistance, noise characteristics, and temperature resolution. We show that VOx sensors can achieve, in a bandwidth of similar to 16 mHz, a temperature resolution of similar to 5 mu K at room temperature (similar to 300 K) and a temperature resolution of similar to 1 mu K at the MIT (similar to 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 VOx-based thermometers offer a similar to 10-50-fold improvement in resolution over widely used Pt-based thermometers.

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.

AlN Schottky barrier diodes with low ideality factor (<1.2), low differential ON-resistance (<0.6 mΩ cm ^2 ), high current density (>5 kA cm ^−2 ), and high breakdown voltage (680 V) are reported. The device structure consisted of a two-layer, quasi-vertical design with a lightly doped AlN drift layer and a highly doped Al _0.75 Ga _0.25 N ohmic contact layer grown on AlN substrates. A combination of simulation, current–voltage measurements, and impedance spectroscopy analysis revealed that the AlN/AlGaN interface introduces a parasitic electron barrier due to the conduction band offset between the two materials. This barrier was found to limit the forward current in fabricated diodes. Further, we show that introducing a compositionally-graded layer between the AlN and the AlGaN reduces the interfacial barrier and increases the forward current density of fabricated diodes by a factor of 10 ^4 .

Quantum interference (QI) can strongly affect electric and thermoelectric properties of molecular junctions (MJs). So far, however, a limited number of experimental studies have explored the influence of QI on thermoelectric transport in MJs. To address this open point, we synthesized derivatives of meta-OPE3 with an electron-withdrawing nitro (-NO2) substituent or an electron-donating N,N-dimethyl amine (-NMe2) substituent, attached at two different positions of the central phenylene ring, and systematically studied the electrical conductance and thermopower of the corresponding gold-molecule-gold junctions. We show that (i) the electrical conductance of MJs depends weakly on the polarity of the substituents but strongly on the substitution position and (ii) MJs with the N,N-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.

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing.

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.

AbstractControl 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 (VOx)-based planar gate electrode, whose dielectric properties can be adjusted by changing its temperature. We demonstrate that when the gate is located close ( < ~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.

Recent successful integration of semiconductors into spintronic THz emitters has demonstrated a new pathway of control over terahertz (THz) radiation through ultrafast demagnetization dynamics. Here, the spintronic THz emission from different ultrawide bandgap (UWBG) semiconductors interfaced with ferromagnets is studied. The authors show that the Schottky barrier in the UWBG semiconductor AlN acts as a spin filter that increases the polarization of the spin current injected from the ferromagnet. Furthermore, the authors show that the two‐dimensional electron gas at the interface between Al 0.25 Ga 0.75 N and GaN enhances the magnitude of the emitted radiation due to the high spin‐to‐charge conversion efficiency induced by the Rashba effect that results in a hallmark asymmetry in emission amplitude. The results provide a framework for future engineering of semiconducting/ferromagnet heterostructures for ultrafast communications technologies beyond 5G.

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.

We investigate the electrical characteristics of Ni Schottky contacts on n-type GaN films that have undergone ultra-high-pressure annealing (UHPA), a key processing step for activating implanted Mg. Contacts deposited on these films exhibit low rectification and high leakage current compared to contacts on as-grown films. By employing an optimized surface treatment to restore the GaN surface following UHPA, we obtain Schottky contacts with a high rectification ratio of ∼10 9 , a near-unity ideality factor of 1.03, and a barrier height of ∼0.9 eV. These characteristics enable the development of GaN junction barrier Schottky diodes employing Mg implantation and UHPA.