A few-layer covalent network of fullerenes.

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces-a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)∞ by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Pushing the Limits of Heat Conduction in Covalent Organic Frameworks Through High-Throughput Screening of Their Thermal Conductivity.

Tailor-made materials featuring large tunability in their thermal transport properties are highly sought-after for diverse applications. However, achieving `user-defined' thermal transport in a single class of material system with tunability across a wide range of thermal conductivity values requires a thorough understanding of the structure-property relationships, which has proven to be challenging. Herein, large-scale computational screening of covalent organic frameworks (COFs) for thermal conductivity is performed, providing a comprehensive understanding of their structure-property relationships by leveraging systematic atomistic simulations of 10,750 COFs with 651 distinct organic linkers. Through the data-driven approach, it is shown that by strategic modulation of their chemical and structural features, the thermal conductivity can be tuned from ultralow (≈0.02 W m-1 K-1) to exceptionally high (≈50 W m-1 K-1) values. It is revealed that achieving high thermal conductivity in COFs requires their assembly through carbon-carbon linkages with densities greater than 500 kg m-3, nominal void fractions (in the range of ≈0.6-0.9) and highly aligned polymeric chains along the heat flow direction. Following these criteria, it is shown that these flexible polymeric materials can possess exceptionally high thermal conductivities, on par with several fully dense inorganic materials. As such, the work reveals that COFs mark a new regime of materials design that combines high thermal conductivities with low densities.

Validation of the Wiedemann-Franz Law in Solid and Molten Tungsten above 2000 K through Thermal Conductivity Measurements via Steady-State Temperature Differential Radiometry.

We measure the thermal conductivity of solid and molten tungsten using steady state temperature differential radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz law to capture the electronic thermal conductivity of metals in their molten phase. We further support this conclusion using ab initio molecular dynamics simulations with a machine-learned potential. Our results show that at these high temperatures, the vibrational contribution to thermal conductivity is negligible compared to the electronic component.

Wavelet Coherence Analysis of Plasma Beta, Alfven Mach Number, and Magnetosonic Mach Number during Different Geomagnetic Storms.

We study the variation in plasma beta, Alfven Mach number, and magnetosonic Mach number during different geomagnetic storms of solar cycles 23, 24, and 25. In addition, we employ measurements of the solar wind's flow pressure, proton density, interplanetary magnetic field (IMF) along the z-direction (Bz), temperature, velocity, and geomagnetic index SYM-H. Here, the wavelet coherence (WTC) approach of plasma beta, the Alfven Mach number, and the magnetosonic Mach number have been used with the symmetrical H component (SYM-H) index, which are critical indicators of the plasma behavior and magnetic field interactions. A solar CME or, much less severely, a corotating interaction region (CIR), which is formed at the leading edge of a high-speed stream, is the source of the magnetic storm. The key objective of this study is to reveal the possible dependencies of the geomagnetic indices on whether a storm is driven by a CME or CIR. For CIR-associated storms, large amplitude waves occur preferentially with the rising Alfven Mach number and plasma beta. At the same time, the magnetosonic Mach number lacks variability during the storms caused by shock on the arrival of Earth's environment. This is different for CME-driven storms, where the variations of the magnetosonic Mach number do not show much fluctuation compared to the Alfven Mach number and plasma beta. WTC between SYM-H and our derived parameters indicates periodicities between 64 and 512 minutes and noticeable regions of significantly enhanced power on November 07-09, 2004, and June 21-23, 2015. However, the magnetosonic Mach number showed a noticeable coherence with SYM-H between 64 and 250 minutes on September 06-08, 2017. Although, during March 19-21, 2021, both the Alfven Mach number and magnetosonic Mach number showed a noticeable coherence with SYM-H, plasma beta showed none. These parameters can be used in the prediction of geomagnetic storms of the category above G3.

Direct Measurement of Ballistic and Diffusive Electron Transport in Gold.

We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Thermally Conductive Self-Healing Nanoporous Materials Based on Hydrogen-Bonded Organic Frameworks.

Hydrogen-bonded organic frameworks (HOFs) are a class of nanoporous crystalline materials formed by the assembly of organic building blocks that are held together by a network of hydrogen-bonding interactions. Herein, we show that the dynamic and responsive nature of these hydrogen-bonding interactions endows HOFs with a host of unique physical properties that combine ultraflexibility, high thermal conductivities, and the ability to "self-heal". Our systematic atomistic simulations reveal that their unique mechanical properties arise from the ability of the hydrogen-bond arrays to absorb and dissipate energy during deformation. Moreover, we also show that these materials demonstrate relatively high thermal conductivities for porous crystals with low mass densities due to their extended periodic framework structure that is comprised of light atoms. Our results reveal that HOFs mark a new regime of material design combining multifunctional properties that make them ideal candidates for gas storage and separation, flexible electronics, and thermal switching applications.

Supramolecular Interactions Lead to Remarkably High Thermal Conductivities in Interpenetrated Two-Dimensional Porous Crystals.

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon-phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

Hybridization from Guest-Host Interactions Reduces the Thermal Conductivity of Metal-Organic Frameworks.

We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metal-organic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H4-TCNQ). This allows for modification of the interaction strength between the guest and host, presenting an opportunity to study the fundamental atomic scale mechanisms of how guest molecules impact the thermal conductivity of large unit cell porous crystals. The thermal conductivities of the guest@MOF systems decrease significantly, by on average a factor of 4, for all infiltrated samples as compared to the uninfiltrated, pristine HKUST-1. This reduction in thermal conductivity goes in tandem with an increase in density of 38% and corresponding increase in heat capacity of ∼48%, defying conventional effective medium scaling of thermal properties of porous materials. We explore the origin of this reduction by experimentally investigating the guest molecules' effects on the mechanical properties of the MOF and performing atomistic simulations to elucidate the roles of the mass and bonding environments on thermal conductivity. The reduction in thermal conductivity can be ascribed to an increase in vibrational scattering introduced by extrinsic guest-MOF collisions as well as guest molecule-induced modifications to the intrinsic vibrational structure of the MOF in the form of hybridization of low frequency modes that is concomitant with an enhanced population of localized modes. The concentration of localized modes and resulting reduction in thermal conductivity do not seem to be significantly affected by the mass or bonding strength of the guest species.

Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks.

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m-1 K-1) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties.

Heat Transfer Mechanisms and Tunable Thermal Conductivity Anisotropy in Two-Dimensional Covalent Organic Frameworks with Adsorbed Gases.

Two-dimensional covalent organic frameworks (2D COFs) are a novel class of materials that are ideal for gas storage and separation technologies due to their high porosities and large surface areas. In this work we study the heat transfer mechanisms in 2D COFs with the addition of gas adsorbates, demonstrating the remarkably tunable anisotropic response of the phonon thermal conductivity in 2D COFs during gas adsorption. More specifically, our results from atomistic simulations on COF-5/methane systems show that, as the gas density increases, the cross-plane thermal conductivity along the direction of the laminar pores increases, whereas the in-plane thermal conductivity along the 2D sheets is monotonically decreased. We show that a large portion of heat is conducted along the laminar pore channels by the gas molecules colliding with the solid framework and is directly related to the gas diffusivities.

Uniquely anisotropic mechanical and thermal responses of hybrid organic-inorganic perovskites under uniaxial strain.

The complete understanding of the mechanical and thermal responses to strain in hybrid organic-inorganic perovskites holds great potential for their proper functionalities in a range of applications, such as in photovoltaics, thermoelectrics, and flexible electronics. In this work, we conduct systematic atomistic simulations on methyl ammonium lead iodide, which is the prototypical hybrid inorganic-organic perovskite, to investigate the changes in their mechanical and thermal transport responses under uniaxial strain. We find that the mechanical response and the deformation mechanisms are highly dependent on the direction of the applied uniaxial strain with a characteristic ductile- or brittle-like failure accompanying uniaxial tension. Moreover, while most materials shrink in the two lateral directions when stretched, we find that the ductile behavior in hybrid perovskites can lead to a very unique mechanical response where negligible strain occurs along one lateral direction while the length contraction occurs in the other direction due to uniaxial tension. This anisotropy in the mechanical response is also shown to manifest in an anisotropic thermal response of the hybrid perovskite where the anisotropy in thermal conductivity increases by up to 30% compared to the unstrained case before plastic deformation occurs at higher strain levels. Along with the anisotropic responses of these physical properties, we find that uniaxial tension leads to ultralow thermal conductivities that are well below the value predicted with a minimum thermal conductivity model, which highlights the potential of strain engineering to tune the physical properties of hybrid organic-inorganic perovskites.

Ultralow Thermal Conductivity of Two-Dimensional Metal Halide Perovskites.

We report on the thermal conductivities of two-dimensional metal halide perovskite films measured by time domain thermoreflectance. Depending on the molecular substructure of ammonium cations and owing to the weaker interactions in the layered structures, the thermal conductivities of our two-dimensional hybrid perovskites range from 0.10 to 0.19 W m-1 K-1, which is drastically lower than that of their three-dimensional counterparts. We use molecular dynamics simulations to show that the organic component induces a reduction of the stiffness and sound velocities along with giving rise to vibrational modes in the 5-15 THz range that are absent in the three-dimensional counterparts. By systematically studying eight different two-dimensional hybrid perovskites, we show that the thermal conductivities of our hybrid films do not depend on the thicknesses of the organic layers and instead are highly dependent on the relative orientation of the organic chains sandwiched between the inorganic constituents.

Nanoscale Wetting and Energy Transmission at Solid/Liquid Interfaces.

Understanding the effects and limitations of solid/liquid interfaces on energy transport is crucial to applications ranging from nanoscale thermal engineering to chemical synthesis. Until now, the majority of experimental evidence regarding solid/liquid interactions has been limited to macroscale observations and experiments. The lack of experimental works exploring nanoscale solid/liquid interactions has been accentuated as the body of knowledge from theory and simulations at these scales has exploded in recent years. In this study, we expand on current nanoscale thermal measurement techniques in order to more fully understand solid/liquid interfacial energy transport. We use thermal ablation threshold measurements on thick Au films in various liquids as a metric to describe thermal transport at the Au/liquid interface. Furthermore, using ultrafast pump-probe experiments, we gain insight into this transport through picosecond ultrasonic coupling at solid/liquid interfaces with known macroscopic observations. We find significant variations in both the ablation threshold and the damping of the acoustic modes within the Au films depending on nanoscopic interactions at the solid/liquid interface rather than typical macroscale metrics such as acoustic mismatch, measured contact angle, and work of adhesion.

Thermal Boundary Conductance Across Heteroepitaxial ZnO/GaN Interfaces: Assessment of the Phonon Gas Model.

We present experimental measurements of the thermal boundary conductance (TBC) from 78-500 K across isolated heteroepitaxially grown ZnO films on GaN substrates. This data provides an assessment of the underlying assumptions driving phonon gas-based models, such as the diffuse mismatch model (DMM), and atomistic Green's function (AGF) formalisms used to predict TBC. Our measurements, when compared to previous experimental data, suggest that TBC can be influenced by long wavelength, zone center modes in a material on one side of the interface as opposed to the '"vibrational mismatch"' concept assumed in the DMM; this disagreement is pronounced at high temperatures. At room temperature, we measure the ZnO/GaN TBC as 490[+150,-110] MW m-2 K-1. The disagreement among the DMM and AGF, and the experimental data at elevated temperatures, suggests a non-negligible contribution from other types of modes that are not accounted for in the fundamental assumptions of these harmonic based formalisms, which may rely on anharmonicity. Given the high quality of these ZnO/GaN interfaces, these results provide an invaluable, critical, and quantitative assessment of the accuracy of assumptions in the current state of the art computational approaches used to predict phonon TBC across interfaces.

Titanium contacts to graphene: process-induced variability in electronic and thermal transport.

Contact resistance (R C) is a major limiting factor in the performance of graphene devices. R C is sensitive to the quality of the interface and the composition of the contact, which are affected by the graphene transfer process and contact deposition conditions. In this work, a linear correlation is observed between the composition of Ti contacts, characterized by x-ray photoelectron spectroscopy, and the Ti/graphene contact resistance measured by the transfer length method. We find that contact composition is tunable via deposition rate and base pressure. Reactor base pressure is found to effect the resultant contact resistance. The effect of contact deposition conditions on thermal transport measured by time-domain thermoreflectance is also reported. Interfaces with higher oxide composition appear to result in a lower thermal boundary conductance. Possible origins of this thermal boundary conductance change with oxide composition are discussed.

Analytical model for thermal boundary conductance and equilibrium thermal accommodation coefficient at solid/gas interfaces.

We develop an analytical model for the thermal boundary conductance between a solid and a gas. By considering the thermal fluxes in the solid and the gas, we describe the transmission of energy across the solid/gas interface with diffuse mismatch theory. From the predicted thermal boundary conductances across solid/gas interfaces, the equilibrium thermal accommodation coefficient is determined and compared to predictions from molecular dynamics simulations on the model solid-gas systems. We show that our model is applicable for modeling the thermal accommodation of gases on solid surfaces at non-cryogenic temperatures and relatively strong solid-gas interactions (ε(sf) ≳ k(B)T).

Electron-electron scattering limits thermal conductivity of metals under extremely high electron temperatures.

We report on the thermal transport properties of noble metals (gold, silver and copper) under conditions of extremely high electron temperatures (that are on the order of the Fermi energy). We perform parameter-free density functional theory calculations of the electron temperature-dependent electron-phonon coupling, electronic heat capacities, and thermal conductivities to elucidate the strong role played by the excitation of the low lyingd-bands on the transport properties of the noble metals. Our calculations show that, although the three metals have similar electronic band structures, the changes in their electron-phonon coupling at elevated electron temperatures are drastically different; while electron-phonon coupling decreases in gold, it increases in copper and, it remains relatively unperturbed for silver with increasing electron temperatures of up to ∼60 000 K (or 5 eV). We attribute this to the varying contributions from acoustic and longitudinal phonon modes to the electron-phonon coupling in the three metals. Although their electron-phonon coupling changes with electron temperature, the thermal conductivity trends with electron temperature are similar for all three metals. For instance, the thermal conductivities for all three metals reach their maximum values (on par with the room-temperature values of some of the most thermally conductive semiconductors) at electron temperatures of ∼6000 K, and thereafter monotonically decrease due to the enhanced effect of electron-electron scattering for electronic states that are further away from the Fermi energy. As such, only accounting for electron-phonon coupling and neglecting electron-electron scattering can lead to large over-predictions of the thermal conductivities at extremely high electron temperatures. Our results shed light on the microscopic understanding of the electronic scattering mechanisms and thermal transport in noble metals under conditions of extremely high electron temperatures and, as such, are significant for a plethora of applications such as in plasmonic devices that routinely leverage hot electron transport.

Reversible and high-contrast thermal conductivity switching in a flexible covalent organic framework possessing negative Poisson's ratio.

The ability to dynamically and reversibly control thermal transport in solid-state systems can redefine and propel a plethora of technologies including thermal switches, diodes, and rectifiers. Current material systems, however, do not possess the swift and large changes in thermal conductivity required for such practical applications. For instance, stimuli responsive materials, that can reversibly switch between a high thermal conductivity state and a low thermal conductivity state, are mostly limited to thermal switching ratios in the range of 1.5 to 4. Here, we demonstrate reversible thermal conductivity switching with an unprecedented 18× change in thermal transport in a highly flexible covalent organic framework with revolving imine bonds. The pedal motion of the imine bonds is capable of reversible transformations of the framework from an expanded (low thermal conductivity) to a contracted (high thermal conductivity) phase, which can be triggered through external stimuli such as exposure to guest adsorption and desorption or mechanical strain. We also show that the dynamic imine linkages endow the material with a negative Poisson's ratio, thus marking a regime of materials design that combines low densities with exceptional thermal and mechanical properties.