Zhiting Tian

High Resolution Image Download MS PowerPoint Slide Conspectus The relentless increase in power density and advancements in nanoengineering in modern electronic and energy conversion technologies have pushed thermal management to its physical limits, where ballistic transport phenomena and the wave nature of phonons become important. Our research tackles this frontier using a dual approach: advanced experimental tools to accurately probe thermal transport across ballistic and diffusive regimes, and predictive theoretical methods to resolve wave-based phonon transport with atomic precision. On the experimental front, transient thermal grating (TTG) spectroscopy has emerged as a crucial noncontact optical technique for probing complex material systems that are inaccessible to other methods, including new classes of materials such as two-dimensional (2D) covalent organic frameworks (COFs) and hybrid perovskites. Studying in-plane thermal transport in these materials is particularly challenging due to their small dimensions or fragility. On the theoretical front, the atomistic Green’s function (AGF) is a quantum-mechanical framework that describes phonon transport as wave propagation and accounts for interactions with the atomic structure, defects, and interfaces from first-principles, providing fundamental physical insights. This Account summarizes how we leveraged TTG and AGF for scientific discoveries. For instance, we used TTG to directly measure the in-plane thermal conductivity of novel 2D COFs, revealing a high value of ∼ 1.18 W/(m·K). We also applied TTG to unveil remarkably weak anisotropy of 1.5 in thermal conductivity of 2D hybrid perovskites. Furthermore, we developed TTG to uniquely characterize micrometer-thick metallic interfaces, smaller than the laser beam size. In the theoretical domain, we developed anharmonic AGF method for three-dimensional (3D) interfaces and discovered enhanced thermal interface conductance due to anharmonicity at the interface. We also found enhanced conductance due to a bridging effect caused by atomic mixing. We applied AGF to directly capture phonon Anderson localization in aperiodic superlattices and coherent phonon transport in periodic superlattices. The collective insights gained from these distinct experimental and theoretical advances are building a framework for the rational design of materials with tailored thermal transport properties, paving the way for next-generation solutions in electronics packaging, thermoelectrics, thermal insulation, and beyond.

ABSTRACT Although significant efforts have focused on hot electron transport in two‐dimensional hybrid organic–inorganic perovskites (2D HOIPs), exploring the mobility of carriers when in thermal equilibrium with the lattice may widen their applicability as semiconductors in energy conversion and electronics. While Dion–Jacobson (DJ) perovskites with conjugated ligands are expected to enhance charge transport through dielectric screening and interlayer delocalization, we report a surprising finding that conjugated DJ perovskite films exhibit significantly lower conductivity than Ruddlesden–Popper (RP) perovskite films with insulating ligands, both in intrinsic and doped states. In addition to weaker orbital overlap within the inorganic layer due to increased octahedral tilting in the conjugated DJ phase, we experimentally demonstrate that the highest occupied molecular orbital (HOMO) of the conjugated organic layer lies ∼1.5 eV above the valence band maximum (VBM) of the inorganic layer. The HOMO offset leads to hole trapping, thereby reducing free carrier concentration in the inorganic layer and suppressing the bulk conductivity of the conjugated DJ perovskite. These findings reveal that carrier delocalization through the conjugated ligands can only be advantageous when the HOMO‐VBM offset is sufficiently small and comparable to carrier thermal energy.

ABSTRACT As nanoscale devices continue to shrink in size and increase in complexity, their thermal performance becomes increasingly governed by interfacial phenomena. In particular, thermal boundary resistance (TBR) plays a critical role in controlling heat dissipation. In the conventional phonon Landauer formalism, TBR depends on the interfacial reflection coefficient for each phonon mode, a quantity for which direct experimental probes are limited. Here, we introduce a novel analysis, acoustic phonon reflectometry, based on picosecond acoustics that enables direct experimental determination of the mode‐resolved phonon reflection coefficient at semiconductor interfaces at a single frequency. Applying this approach to longitudinal acoustic phonons in aluminum nitride, we observe excellent agreement with predictions from the acoustic mismatch model across three distinct solid‐solid interfaces. This work establishes a new method for probing phonon transport at interfaces and provides critical insights into the microscopic mechanisms governing interfacial heat transfer at the nanoscale.

Conjugated polymers are promising materials for flexible electronics, with poly(3-hexylthiophene) (P3HT) standing out for its ease of solution-based processing. However, the influence of the molecular arrangement on thermal transport remains less understood, particularly with respect to thin-film thickness. Here, we investigate the in-plane thermal conductivity of P3HT thin films with thicknesses ranging from 12 to 136 nm using transient thermal grating (TTG) spectroscopy, a precise noncontact optical technique. A pronounced thickness dependence is observed, with the highest thermal conductivity of 0.88 W/m·K in the thinnest film and the lowest value of 0.25 W/m·K in the thickest film. The enhancement in thermal conductivity in thinner films is attributed to denser lamellar packing in face-on configurations, an increased contribution of face-on domains, and spatial confinement-induced improvements in short-range ordering within the amorphous regions of thinner films. These findings demonstrate that the film thickness is an effective parameter for tuning in-plane thermal conductivity in P3HT thin films.

How ordered phases melt in low-dimensional quantum materials remain difficult to resolve because the relevant fluctuations are dynamic and charge neutral. In this work, we show that thermal transport provides a sensitive probe of these hidden fluctuations in the layered transition metal dichalcogenide 2H-TaSe2. We observe a striking V-shaped temperature dependence of the thermal conductivity that cannot be explained by conventional phonon-phonon scattering. Instead, it originates from scattering by persistent local charge-density-wave (CDW) correlations, consistent with our phenomenological model linking thermal transport to spatial CDW fluctuation. Electron diffraction reveals short-range periodic lattice distortions persisting to at least 300 K, while X-ray diffraction shows thermal hysteresis of the CDW wavevector. Together, these results reveal a dislocation- and fluctuation-driven weak first-order melting of the CDW state.

How ordered phases melt in low-dimensional quantum materials remain difficult to resolve because the relevant fluctuations are dynamic and charge neutral. In this work, we show that thermal transport provides a sensitive probe of these hidden fluctuations in the layered transition metal dichalcogenide 2H-TaSe2. We observe a striking V-shaped temperature dependence of the thermal conductivity that cannot be explained by conventional phonon-phonon scattering. Instead, it originates from scattering by persistent local charge-density-wave (CDW) correlations, consistent with our phenomenological model linking thermal transport to spatial CDW fluctuation. Electron diffraction reveals short-range periodic lattice distortions persisting to at least 300 K, while X-ray diffraction shows thermal hysteresis of the CDW wavevector. Together, these results reveal a dislocation- and fluctuation-driven weak first-order melting of the CDW state.

Abstract In this commentary, we clarify that the Landauer formula is not limited to the phonon gas model. It is fundamentally more general and applies to both particle- and wave-based descriptions of phonons, provided the transmission function is well defined. In the harmonic regime, the phonon transmission function and the resulting Landauer expression for heat current are exact. They can be rigorously derived using the atomistic Green’s function method, which treats phonons as waves and does not require phonon dispersion in the interface region. In short, the Landauer framework remains valid for ideal, disordered, and defective interfaces, as long as an appropriate transmission function is used. Graphical abstract

We report plasma-assisted molecular beam epitaxial growth of AlBN thin films on a nitrided c-plane Al2O3 substrate. The AlBN film epitaxially grows in rotational alignment with an out-of-plane/in-plane directions of AlBN [0001¯]/[101¯0]‖AlN nucleation layer [0001¯]/[101¯0]‖Al2O3[0001]/[112¯0]. The B composition of the AlBN layer is varied from 0% to 15% by varying the growth temperature, exploiting the reaction rate-controlled growth mechanism. X-ray diffraction and high-resolution transmission electron microscopy are used to determine the structural properties as a function of boron composition. A monotonic decrease in the c-lattice constant and a non-monotonic change in the a-lattice constant are observed with an increase in the B content in AlBN films grown on nitrided sapphire. While the control AlN film showed a bandgap of 6.1 eV, the AlBN films with ∼15% boron showed a bandgap of 5.9 eV. The AlBN films with 15% B exhibit a fivefold increase in the nonlinear second-harmonic generation intensity compared to AlN. AlBN films exhibit higher thermal conductivity than AlScN films with comparable alloy compositions, and at equal or smaller thicknesses. The findings indicate several opportunities for AlBN films in applications of deep UV optoelectronics, nonlinear photonics, and high-power electronics devices, especially in high-voltage and high-temperature environments.

In this commentary, we clarify that the Landauer formula is not limited to the phonon gas model. It is fundamentally more general and applies to both particle- and wave-based descriptions of phonons, provided the transmission function is well defined. In the harmonic regime, the phonon transmission function and the resulting Landauer expression for heat current are exact. They can be rigorously derived using the atomistic Green's function method, which treats phonons as waves and does not require phonon dispersion in the interface region. In short, the Landauer framework remains valid for ideal, disordered, and defective interfaces, as long as an appropriate transmission function is used.

In this commentary, we clarify that the Landauer formula is not limited to the phonon gas model. It is fundamentally more general and applies to both particle- and wave-based descriptions of phonons, provided the transmission function is well defined. In the harmonic regime, the phonon transmission function and the resulting Landauer expression for heat current are exact. They can be rigorously derived using the atomistic Green's function method, which treats phonons as waves and does not require phonon dispersion in the interface region. In short, the Landauer framework remains valid for ideal, disordered, and defective interfaces, as long as an appropriate transmission function is used.

Two-dimensional (2D) materials hold significant promise for next-generation nanoelectronics while introducing critical thermal management challenges. The extreme thinness, anisotropic heat transport, and unique interfacial coupling make the thermal design principles of 2D materials-based electronics fundamentally different from that of bulk systems. In this Perspective, we discuss exciting opportunities that leverage recent advances in thermal science to unlock unprecedented thermal management capabilities, thereby providing new insights into the design of 2D materials-based electronics. We first provide an overview of key thermophysical properties of 2D materials that govern thermal management performance, including in-plane thermal conductivity, interfacial thermal conductance, and thermal expansion coefficient. Then, we not only highlight important physical phenomena distinct from bulk materials but more notably illustrate how the interplay among these thermophysical properties ultimately dictates the unique characteristics of heat dissipation and thermomechanical stress in 2D materials-based electronic devices. With both material- and device-level insights, we identify key thermal bottlenecks in existing 2D materials-based electronic devices and present a fully quantitative roadmap toward an electrical and thermal co-design strategy for substantially improved thermal management. Bridging thermal innovations to the device design, we envision this Perspective can foster next-generation thermal management technologies for reliable 2D materials-based electronics.

Recent years have seen major developments in thermal management approaches for semiconductors and thermoelectric materials, which serve as critical technologies for achieving carbon neutrality. Modern electronic and optoelectronic devices require effective heat dissipation and thermal energy conversion to achieve better performance and maintain reliability and efficiency. In particular, as device dimensions continue to shrink to the nanoscale, conventional bulk thermal transport theories become inadequate, necessitating a deeper understanding of phonon transport mechanisms at interfaces, in nanostructures, and across heterogeneous systems. The field of phonon engineering has emerged through the convergence of several scientific disciplines: Theoretical modeling of phonon heat transport together with nanoscale thermal measurement methods, advanced materials development, and materials informatics approaches have driven the development of phonon engineering. The combination of multiple scientific disciplines has sped up advancements in our knowledge and ability to control thermal transport at micro- and nanoscale levels.

Two-dimensional covalent organic frameworks are promising low-density porous materials for lightweight thermal management, yet comprehensive thermal conductivity measurements remain scarce. Particularly, direct in-plane thermal conductivity data for large-area, fully suspended covalent organic framework thin films has not been reported previously. This study addresses this gap by measuring in-plane and cross-plane thermal conductivities of two-dimensional covalent organic frameworks with varying pore sizes using laser-based pump-probe techniques. Transient thermal grating spectroscopy revealed a high in-plane thermal conductivity of 1.18 ± 0.21 W/(m⋅K) for a sample with a 1.4 nm pore size, highlighting a notable pore size effect. Cross-plane thermal conductivity measured via frequency-domain thermoreflectance indicated weak thermal anisotropy for samples with larger pores. Grazing-incident wide-angle X-ray scattering provided structural insights and clarified heat conduction mechanisms. These direct in-plane thermal conductivity measurements enhance understanding of thermal transport behaviors in covalent organic frameworks, supporting their development as advanced thermal management materials. The few options available for the measurement of in-plane and cross-plane thermal conductivity of covalent organic frameworks films limit their application for lightweight thermal management. Here, the authors measure both, the in-plane and cross-plane thermal conductivity of two dimensional covalent organic frameworks with different pore sizes using laser-based pump-probe techniques.

Abstract Energy‐efficient, durable low‐emissivity (low‐E) glass windows are in demand for reducing energy consumption and comfortable living environments. However, commercial low‐E coating materials are expensive, prone to abrasion, and hence coated only on the interior side of windows, limiting their energy efficiency. Here, a new material is introduced, namely chemically inert and transparent carbon (C) doped boron nitride (BN) nano‐coatings on glass surfaces at room temperature using pulsed laser deposition, that shows promising long‐wave infrared emissivity (ε LWIR ≈0.42). The hydrophilic C‐BN coatings on glass show excellent environmental stability including high temperature‐high humidity degradation resistance, UV‐light, thermal cycling, freezing condition, and saltwater resilience. Furthermore, the coating shows promising adhesion on the glass surface with full scratch protection. In an actual‐sized building energy simulation for cold‐climates, the intended exterior‐side C‐BN coated low‐E glass shows 2.9% energy savings compared to the interior‐side coated commercial low‐E glass. C‐BN would be a useful coating material for durable and energy‐efficient low‐E glass window technology.

Photothermal biomodulation is an emerging technique that leverages the deep optical penetration of near-infrared light in biological tissues, enabling a range of diagnostic and therapeutic applications. Given that photothermal agents are used within the body, ensuring long-term safety is essential, necessitating the development of safer, biodegradable agents. In this work, we developed biodegradable photothermal particles based on the FDA-approved polylactic- co -glycolic acid (PLGA) polymer and confined water. We hypothesize that confined water acts as a photothermal transducer due to its lower heat capacity compared to surrounding bulk water, while the polymer layer provides thermal insulation, effectively retaining the generated heat within the particles and creating a thermal gradient in their immediate vicinity. Fluorescent thermometry and IR camera results demonstrate the strong photothermal performance of the developed particles, enabling localized heating instead of global heating in surrounding environments. Additionally, we confirm the presence of confined water within the particles through Fourier transform infrared (FTIR) and X-ray diffraction (XRD) results. Further in vitro validation using lysozyme enzyme activity tests and cell viability experiments with EO771 cancer cells expressing LanYFP fluorescent protein confirmed the biocompatibility and efficacy of the developed particles. These particles successfully induced localized heating in the cellular environment without compromising cell viability, making them highly promising for safe biomedical applications in photothermal therapy and biomodulation.

High thermal conductivity is essential for polymer applications such as electronic chip encapsulation, where efficient heat dissipation ensures system functionality and reliability. Here, we introduce a novel strategy to enhance through-plane thermal conductivity in 2D covalent organic frameworks (COFs). A highly crystalline edge-on 2D polyamide (v2DPA) film achieves a thermal conductivity of 1.16 ± 0.05 W/(mK) at 310 K, surpassing the previous record (1.03 W/(mK) in COF-5 [Evans et al. Nat. Mater. 2021, 20, 1142]) and aligning with molecular dynamics predictions (1.11 ± 0.07 W/(mK)). This value is nearly three times higher than that of bulk PA (0.34 ± 0.03 W/(mK)). Phonon dispersion calculations attribute this enhancement to strong covalent bonding, increasing phonon lifetimes, and group velocities. Our findings highlight the effectiveness of orienting 2D polymer and layer-stacked 2D COF films in an edge-on configuration to improve through-thickness thermal conductivity, offering a promising pathway for their integration into electronic thermal management applications.

Thermal transport in amorphous carbon has attracted immense attention due to its extreme thermal properties: It has been reported to have among the highest thermal conductivity for bulk amorphous solids up to ~37 W m −1 K −1 , comparable to crystalline sapphire ( α -Al 2 O 3 ). However, mechanism behind the high thermal conductivity remains elusive due to many variables at play. In this work, we perform large-scale (~10 5 atoms) molecular dynamics simulations utilizing a machine learning potential based on neural networks with first-principles accuracy. Through spectral decomposition of thermal conductivity which enables a quantum correction to classical heat capacity, we find that propagating vibrational excitations govern thermal transport in amorphous carbon (~100 % of thermal conductivity) in sharp contrast to the convention that diffusive vibrational excitations dominate thermal transport in amorphous solids. This remarkable behavior resembles thermal transport in simple crystals. Our work, therefore, provides a perspective that deepens our understanding of intermediate thermal transport mechanisms between the two ends of spectrum of solids: crystalline and amorphous solids.

<title>Abstract</title> Understanding the anharmonic phonon properties of crystal compounds—such as phonon lifetimes and thermal conductivities—is essential for investigating and optimizing their thermal transport behaviors. These properties also impact optical, electronic, and magnetic characteristics through interactions between phonons and other quasiparticles and fields. In this study, we develop an automated first-principles workflow to calculate anharmonic phonon properties and build a comprehensive database encompassing more than 6,000 inorganic compounds. Utilizing this dataset, we train a graph neural network model to predict thermal conductivity values and spectra from structural parameters, demonstrating a scaling law in which prediction accuracy improves with increasing training data size. High-throughput screening with the model enable the identification of materials exhibiting extreme thermal conductivities—both high and low. The resulting database offers valuable insights into the anharmonic behavior of phonons, thereby accelerating the design and development of advanced functional materials.

The increasing power density and continued miniaturization of microelectronics impose significant challenges on thermal management. Interfaces between two mating surfaces can represent a substantial fraction of the total thermal resistance to heat flow to the surroundings. A deep understanding of the interface structure and its thermal transport properties is imperative for better thermal interface design strategies. Soldering, sintering, and direct bonding are widely used in electronics packaging to attach the die to the heat spreader or heat sink. Yet, a direct probing method for the thermal interface properties is lacking. For the first time, a laser-induced transient thermal gratings (TTG) spectroscopy method is presented to investigate such interfaces. Using soldered interfaces as an example, it is demonstrated that the thermal conductivity for bond line thicknesses of 55 and 11 µm can be resolved. The technique enables the identification of structure-property relationships for interfaces from various die-attach methods. It offers a powerful and convenient probe of the interface quality and benefits the future design of high-performance microelectronic devices with low contact resistance.

Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of mechanical strength, ductility, dielectric constant, thermal conductivity, and exceptional neutron radiation shielding capability. Through exhaustive characterizations we reveal that SPS induces non-basal plane crystallinity, twisting of layers, and facilitates inter-grain fusion with a high degree of in-plane alignment across macroscale dimensions, resulting in near-theoretical density and improved properties. Our findings highlight the importance of material design, via new approaches such as layer twisting and interlayer interconnections, to create novel ceramics with properties that could go beyond their intrinsic limits.

A systematic investigation was performed on the impact of the β-gallium oxide (Ga2O3) epitaxial buffer layer thickness grown by close-injection showerhead metalorganic chemical vapor deposition (CIS-MOCVD) on the film’s structural, electrical, and thermal characteristics. Varying thicknesses of unintentionally doped β-Ga2O3 epitaxial layers were grown by CIS-MOCVD on Fe-doped (010) β-Ga2O3 substrates, followed by a 10 nm β-Ga2O3 Si-doped layer with a Si concentration of 1019 cm−3. Gate-recessed lateral metal–oxide–semiconductor field-effect transistors were fabricated with these epilayer films. The device characteristics and secondary ion mass spectroscopy results highlighted the need for precise Si doping within the channel, as well as minimizing the Si accumulation at the epilayer-substrate interface for proper device operation. The results from positron annihilation spectroscopy did not indicate a strong correlation between the epilayer thickness and Ga-related vacancies, and the thermal conductivities of the epilayers were consistent with increasing thickness as shown in the device-level frequency-domain thermoreflectance analysis.

Abstract The interaction of very low frequency (VLF) and extremely low frequency (ELF) electromagnetic waves with nanocomposites is rarely explored. It is demonstrated that low‐dimensional electrically conducting fillers are able to shield extremely long wavelengths, provided they form extended conduction paths through percolation. Other mechanisms that synergistically augment the shielding of the high frequencies, such as skin effect, interfacial polarization, and multiple internal scattering, have insignificant effects in the low‐frequency range. In this regard, high aspect ratio 1D conductors having the lowest percolation thresholds provide the best shielding performance, both gravimetrically and volumetrically. Shielding in these materials are observed majorly occur mostly through reflection, and hence, these materials can be employed for both shielding and guiding low frequencies. The correlation proposed to estimate shielding effectiveness based on conductivity and frequency enables convenient material design for low‐frequency modulation.

We synthesized and characterized the advanced multifunctional features of mycelium–coir-based composites as a replacement for fossil-based foams used in building insulation.

Donor-acceptor conjugated polymers are emerging as a new class of organic semiconductors, where the donor and acceptor moieties function as hole and electron transporters, respectively. The potential of being doped as both p-type and n-type makes them attractive for scalable manufacturing, and they have been widely explored for organic photovoltaics. They can be particularly appealing for organic thermoelectrics, primarily due to their high interchain mobility alongside intrachain mobility. The high intrinsic mobility, resulting from the push-pull effect of the donor-acceptor moieties, ensures high electrical conductivity with minimal doping, which is crucial for maintaining a high Seebeck coefficient in thermoelectric materials. In this review, we explain the molecular structure and energetics, as well as their relationship to the electronic structure of donor-acceptor polymers. We also review the existing literature on how structural and energetic modifications can be implemented to modulate interchain transport, intrachain transport, and doping efficiencies. Based on these, we propose that improvements in molecular design, characterization methods, and the integration of data science and machine learning can accelerate research on donor-acceptor polymers for thermoelectrics and beyond.

. We utilized wide-angle X-ray scattering to elucidate the molecular structural changes that led to this thermal conductivity enhancement. Furthermore, we conducted a device-cooling experiment and showed a 39% hot spot temperature reduction compared to a commercial ceramic-filled silicone thermal pad under a heating power of 3.6 W. Thus, this bulk-scale thermally conductive PE bar with nanoscale structural refinement demonstrated superior cooling performance, offering potential as an advanced thermal interface material for thermal management in microelectronics.

Abstract As the feature size of microelectronic circuits is scaling down to nanometer order, the increasing interconnect crosstalk, resistance-capacitance (RC) delay and power consumption can limit the chip performance and reliability. To address these challenges, new low- k dielectric ( k &lt; 2) materials need to be developed to replace current silicon dioxide ( k = 3.9) or SiCOH, etc. However, existing low- k dielectric materials, such as organosilicate glass or polymeric dielectrics, suffer from poor thermal and mechanical properties. Two-dimensional polymers (2DPs) are considered promising low- k dielectric materials because of their good thermal and mechanical properties, high porosity and designability. Here, we report a chemical-vapor-deposition (CVD) method for growing fluoride rich 2DP-F films on arbitrary substrates. We show that the grown 2DP-F thin films exhibit ultra-low dielectric constant (in plane k = 1.85 and out-of-plane k = 1.82) and remarkable mechanical properties (Young’s modulus &gt; 15 GPa). We also demonstrated the improved performance of monolayer MoS 2 field-effect-transistors when utilizing 2DP-F thin films as dielectric substrates.

High-efficiency thermoelectric materials are vital for sustainable energy. This work presents a reliable method to fabricate freestanding, flexible micrometer-thick films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), achieving a power factor of 204 μW m –1 K –2, 4 orders of magnitude higher than pristine films. This improvement stems from polymer chain realignment and enhanced charge transport via solvent doping, coupled with PSS reduction during post-treatment, which also strengthens film mechanics. Crucially, the freestanding nature ensures transferability, making them ideal for versatile applications. Besides thermoelectrics, these highly conductive, robust, and flexible PEDOT:PSS films provide attractive solutions for organic solar cells, battery electrodes, and flexible electronics.

Abstract As device miniaturization advances, managing heat at the nanoscale becomes increasingly critical. Nanoscale heat transfer presents unique challenges, including size effect, ballistic transport, and complex phonon interactions, which conventional macroscopic theories cannot fully address. Molecular dynamics (MD) simulations have been a powerful tool for directly modeling atomistic motion and interactions, offering valuable insights into thermal phenomena. This article provides an overview of MD methods and their contributions to understanding thermal transport in inorganic crystals, amorphous solids, polymers, and interfaces. Additionally, we offer our perspective on the emerging trends and future research directions in MD simulations, emphasizing their potential to unravel complex thermal phenomena and guide the design of next-generation thermal materials and devices.

The control of heat flow in disordered materials presents a significant challenge due to the limitations of conventional phonon transport models in systems lacking periodic long-range crystal order. This study investigates energy dissipation mechanisms induced by structural irregularities, utilizing folded polymers, particularly proteins, as model systems. Proteins, macromolecules characterized by coexisting periodic amino acid chains folded into irregular three-dimensional structures, serve as useful platforms for examining the impact of irregular topologies on vibrational properties. Our research reveals an important enhancement of the phonon density of states at mid-band frequencies, diverging from the van Hove singularities typically expected at Brillouin-zone edges in perfect crystals. This state redistribution exhibits similarities to observations in some disordered electronic and optical systems, generally known as Lifshitz tails. By interpreting this effect as a resonance between multiple degrees of freedom tuned by gradients of an effective phonon confinement potential, we provide a rational for interpreting the ubiquitous ``boson peak'' reported in disordered materials. Furthermore, this study elucidates how disorder allows heat to be channeled in narrow frequency bands. To this purposes, we present mathematical tools that enable rapid and sharp estimation of the phonon density of states and thermal currents, circumventing the need for solving expensive eigenvalue problems. Our methodology may facilitate the characterization and control of heat transport in specific amorphous and disordered solids, with implications for tailoring thermal materials through strategic manipulation of structural disorder.

Understanding thermal transport mechanisms in phase change materials is critical to elucidating the microscopic picture of phase transitions and advancing thermal energy conversion and storage. Experiments consistently show that cubic phase germanium telluride (GeTe) has an unexpected increase in lattice thermal conductivity with rising temperature. Despite its ubiquity, resolving its origin has remained elusive. In this work, we carry out temperature-dependent lattice thermal conductivity calculations for cubic GeTe through efficient, high-order machine-learned models and additional corrections for coherence effects. We corroborate the calculated phonon properties with our inelastic X-ray scattering measurements. Our calculated lattice thermal conductivity values agree well with experiments and show a similar increasing trend. Through additional bonding strength calculations, we propose that a major contributor to the increasing lattice thermal conductivity is the strengthening of second-nearest neighbor interactions. The findings herein serve to deepen our understanding of thermal transport in phase-change materials. Anomalous lattice thermal conductivity increase with temperature in cubic GeTe is correlated with strengthening of second-nearest neighbor bonds at temperatures near that of the phase transition, enhancing our understanding of thermal transport in phase-change materials.

Organic molecule-doped n-type single-walled carbon nanotube (SWCNT) networks are promising candidates for advanced energy applications, such as flexible thermoelectrics and photovoltaics. Yet charge transport in n-type SWCNTs is limited by two factors: i) charge localization impeding inter-tube transport caused by disordered mesostructure of randomly oriented SWCNTs and ii) reduction of charge carrier concentration driven by oxidation. Herein, studied the relationship between the mesostructure and thermoelectric properties of n-type SWCNTs obtained by surfactant-functionalization and polymer-dopant grafting. Surprisingly, the electrical conductivity of the polymer-doped SWCNTs keeps increasing with increasing polymer content, even after the saturation of carrier concentration, resulting in 12x higher conductivity on polymer-doping compared to surfactant-functionalization. While hopping transport typically dominates in disordered systems, it is shown that a bridging effect from the polymer causes unusual band-like conduction in polymer-doped SWCNTs. Additionally, since surfactants are essential to prevent oxidation and retain n-type over a long duration, shows that SWCNTs obtained through a dual-functionalization strategy using both polymer-dopant and surfactant, demonstrates a long-term stable high n-type thermoelectric power factor, when the surfactant amount is carefully controlled. Besides thermoelectrics, the findings are of general interest to developing stable and conductive n-type SWCNTs for various energy and electronic applications.

Vitrimers, an emerging class of polymer materials, are thermosets with dynamic covalent cross-linkers, allowing for topology rearrangement at elevated temperatures. However, vitrimers have several drawbacks, such as slow response times and often lack photothermal catalytic activity. Herein, we demonstrate that embedding functional nanofillers, i.e., hierarchically assembled plasmonic gold nanoparticles (AuNPs) on graphene nanoplatelets (GNPIs) into a vitrimer matrix, induces an ultrafast photothermal healing response. Unlike previous research that mainly focused on bulk materials, our exploration of vitrimer nanocomposite films uncovers unique advantages, such as optical transparency in the visible wavelength, flexibility, and ultrafast localized healing upon exposure to a 532 nm wavelength laser. These remarkable properties of vitrimer nanocomposite films were demonstrated with three various filler compositions and concentrations, where AuNPs/GNPls serve as a powerful filler. Photothermally activated self-healing of these hybrid materials is demonstrated by taking advantage of the localized surface plasmon resonance (LSPR) of AuNPs and the broad absorbance wavelength and high thermal conductivity of GNPls. Furthermore, profilometry is utilized to quantify the volume percent recovery of healing, providing quantitative evidence of increased healing with a higher filler concentration and laser dosage. This localized, ultrafast healing is pivotal for future coating applications, where bulk heating could lead to undesirable deformations. Our comprehensive understanding of the role of filler composition, filler concentration, and laser dosage in the self-healing properties of films opens up a wide array of potential applications for these light-responsive functional materials. The potential applications of these materials span from self-healing coatings to flexible electronics, inspiring a new era of innovative solutions.

High-temperature polymers are attractive for applications in extreme temperatures, where they maintain their mechanical flexibility and electrical insulating properties. However, their heat dissipation capability is limited due to their intrinsically low thermal conductivities. Hexagonal boron nitride (hBN) is a chemically inert, thermally stable, and electrically insulative compound with a high thermal conductivity, making it an ideal candidate as a filler within a high-temperature polymer matrix to increase the thermal conductivity. This study evaluates the effect of filler size and dispersion on thermal conductivity by producing homogeneous composite samples using a combination of solvent mixing and resonant acoustic mixing (RAM). We carefully characterized our samples, including the spread of the size distribution, and observed that the smaller sized hBN centered around 5 μm was able to integrate more seamlessly into the polytetrafluoroethylene (PTFE) matrix with particle size in the 15 μm range and hence outperformed 30 μm, in contrast to the conventional wisdom, which asserts that larger fillers universally perform better than smaller ones. Our thermal conductivity of hBN/PTFE composites at 30 wt % is 2× higher than the literature values. Notably, we reached the record-high value of 3.5 W/m K at 40 wt % with an onset of percolation at 20 wt %, attributed to optimized hBN dispersion that facilitates the formation of thermal percolation. Our findings provide general guidelines to enhance the thermal conductivity of polymer composites for thermal management, ranging from power transmission to microelectronics cooling.

Thermal transport properties of amorphous carbon has attracted increasing attention due to its extreme thermal properties: It has been reported to have among the highest thermal conductivity for bulk amorphous solids up to $\sim$ 37 Wm\textsuperscript{-1}K\textsuperscript{-1}, comparable to crystalline sapphire ($α$-Al\textsubscript{2}O\textsubscript{3}). Further, large density dependence in thermal conductivity demonstrates a potential for largely tunable thermal conductivity. However, mechanism behind the high thermal conductivity and its large density dependence remains elusive due to many variables at play. In this work, we perform large-scale ($\sim$ 10\textsuperscript{5} atoms) molecular dynamics simulations utilizing a machine learning potential based on neural networks. Through spectral decomposition of thermal conductivity which enables a quantum correction to classical heat capacity, we find that propagating vibrational excitations govern thermal transport in amorphous carbon ($\sim$ 100 \% of thermal conductivity) in sharp contrast to the conventional wisdom that diffusive vibrational excitations dominate thermal transport in amorphous solids. Instead, this remarkable behavior resembles thermal transport in simple crystals. Moreover, our temperature dependent spectral diffusivity and velocity current correlation analyses reveal that the density dependent thermal conductivity originates from anharmonicity sensitive propagating excitations. Our work suggests a novel insight and design principle into developing mechanically hard, thermally conductive amorphous solids.

Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of mechanical strength, ductility, dielectric constant, thermal conductivity, and exceptional neutron radiation shielding capability. Through exhaustive characterizations we reveal that SPS induces non-basal plane crystallinity, twisting of layers, and facilitates inter-grain fusion with a high degree of in-plane alignment across macroscale dimensions, resulting in near-theoretical density and improved properties. Our findings highlight the importance of material design, via new approaches such as layer twisting and interlayer interconnections, to create novel ceramics with properties that could go beyond their intrinsic limits.

Phonon-induced Cooper pair breaking, inciting nonequilibrium quasiparticle (QP) bursts, is known to deteriorate the performance of superconducting devices. However, a detailed understanding of QP-phonon dynamics is lacking due to the absence of a well-established theoretical framework. This paper presents a fully ab initio scheme of calculating nonequilibrium, polarization-dependent QP-phonon dynamics in superconductors. The authors find that with only an 8% deviation from the equilibrium phonon Bose–Einstein distribution, the resulting nonequilibrium QP population is 83 times larger than the equilibrium Fermi–Dirac distribution, and the longitudinal acoustic (LA) phonon polarization is most responsible for QP generation. The authors demonstrated that the qubit transition rate in Josephson junction-based transmon qubits is increased by orders of magnitude when taking these nonequilibrium distributions into account, compared to equilibrium distributions. This framework allows an in-depth exploration of nonequilibrium QP-phonon dynamics in various Josephson-junction-based superconducting devices. It paves the way for formulating advanced phonon shielding strategies to target the LA polarization, potentially leading to enhanced device performance, such as increased coherence time of transmon qubits or reduced thermal noise in cryogenics.

With the continuous miniaturization of microelectronics, the need to effectively prevent thermal crosstalk between adjacent functional blocks is increasing. This study focuses on mitigating thermal crosstalk between high- and low-power components within the die-embedded glass interposer by incorporating a thermal isolation material─polyimide (PI) aerogels. Polyimide aerogels possess ultralow thermal conductivity of 0.029 W/(m K), low dielectric constant in the range of 1–3, and high-temperature stability up to 400 °C, which makes them an ideal candidate for a thermal isolation material. Given the complexity of the actual configurations, we leverage simulations to evaluate the isolation performance in this initial study. We first conducted infrared (IR) camera measurements on a simplified test setup to validate the thermal model. The results confirm the superior thermal isolation performance of PI aerogels, thereby demonstrating their potential as highly effective thermal isolation materials for microelectronic applications.

We used temperature-dependent spark plasma sintering to induce phase transformations of metastable 3D c-BN to mixed-phase 3D/2D c-BN/h-BN and ultimately to the stable 2D h-BN phase at high temperature, useful for extreme-temperature technology.