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Machine learned potentials (MLPs) have been widely employed in molecular dynamics simulations to study thermal transport. However, the literature results indicate that MLPs generally underestimate the lattice thermal conductivity (LTC) of typical solids. Here, we quantitatively analyze this underestimation in the context of the neuroevolution potential (NEP), which is a representative MLP that balances efficiency and accuracy. Taking crystalline silicon, gallium arsenide, graphene, and lead telluride as examples, we reveal that the fitting errors in the machine-learned forces against the reference ones are responsible for the underestimated LTC as they constitute external perturbations to the interatomic forces. Since the force errors of a NEP model and the random forces in the Langevin thermostat both follow a Gaussian distribution, we propose an approach to correcting the LTC by intentionally introducing different levels of force noises via the Langevin thermostat and then extrapolating to the limit of zero force error. Excellent agreement with experiments is obtained by using this correction for all the prototypical materials over a wide range of temperatures. Based on spectral analyses, we find that the LTC underestimation mainly arises from increased phonon scatterings in the low-frequency region caused by the random force errors. [ABSTRACT FROM AUTHOR]

Obtaining accurate lattice thermal conductivity data of LiF under extreme conditions not only provides important reference for performance evaluation, prediction, and control of materials, but also helps to alleviate the significant challenges of precise experimental measurements. The high-temperature phonon properties and lattice thermal conductivity (LTC) of solid LiF were calculated by combining on-the-fly machine learning force fields (MLFFs) with the Green–Kubo method. The introduction of MLFF successfully combines the accuracy of ab initio molecular dynamics with the scalability advantage of classical molecular dynamics. At high temperatures, there is a significant enhancement in the vibrational coupling between the acoustic and optical branches of LiF, as well as resonant effects between Li and F atoms, resulting in strong anharmonicity. Additionally, the main peak of the phonon density of states shows a noticeable redshift compared to the harmonic case. The enhanced coupling of TO and TA modes at high temperature leads to a significant increase in phonon scattering rate. By considering higher-order phonon anharmonicity, the predicted LTC is significantly reduced compared to the results obtained from considering only three-phonon interactions. Along the Hugoniot curve up to 100 GPa (2150 K), the predicted LTC agrees well with the experimental values. These findings demonstrate the crucial role of phonon anharmonicity in promoting phonon scattering. [ABSTRACT FROM AUTHOR]

We here report the electronic structure calculation of GaInSe3 and GaInTe3 monolayers with the P3m1 (no. 156) space group. The electronic structure and thermoelectric properties of the monolayers are calculated through the Vienna Ab initio Simulation Package and BoltzTraP2 codes. The dynamic and thermodynamic stabilities were verified by calculating their phonon spectra and simulating ab initio molecular dynamics. The monolayers were found to have a direct bandgap, with both PBE + SOC and HSE06 + SOC potentials. The lattice thermal conductivity of GaInTe3 monolayer calculated using Phono3py code shows ultra-low values due to enhanced phonon–phonon scattering. Combining electrical and thermal transport, the values have been evaluated. Importantly, the p-type GaInTe3 has excellent thermoelectric properties at 700 K, with a z T value of 2, indicating that the p-type GaInTe3 has potential application in the field of thermoelectricity. [ABSTRACT FROM AUTHOR]

Thermal phonon localization, rooted in phonon wave nature, is widely observed in disordered atomic systems. Binary superlattices, with structural diversity from abundant interfaces, allow for disorder introduction by engineering interfacial structures. In this study, two different disorder entities, namely, aperiodicity (randomized layer thicknesses) and interfacial mixing, were introduced to graphene/h-BN superlattices. Molecular dynamics simulations revealed that both disordered structures can significantly reduce the thermal conductivity, with interfacial mixing more effectively impeding thermal transport. The combined effect of these disorders further decreased thermal conductivity. The underlying mechanism involves Anderson localization of phonons, demonstrated by the exponential decay of phonon transmission and suppressed phonon participation ratio. Phase-breaking interactions at higher temperatures delocalize localized modes. This study offers valuable guidance for structurally designing materials targeting low thermal conductivity through the manipulation of phonon localization. [ABSTRACT FROM AUTHOR]

The carbon nano-peapod is a representative structure with interlayer van der Waals (vdW) interactions, in which encapsulated fullerene molecules play a critical role in modulating the transport properties of the carbon nanotubes (CNTs). In particular, their influence on the thermal transport characteristics has been the focal point of considerable attention. In this study, we trained an accurate machine learning potential for fullerene-encapsulated CNTs based on the efficient NEP model to investigate their thermal properties. Using equilibrium molecular dynamics simulation along with the spectral decomposition method for thermal conductivity, we find that the thermal conductivity of fullerene-encapsulated CNTs is roughly 55 % lower than that of empty CNTs, aligning with experimental observations for CNT bundles with fullerene encapsulation [Kodama et al., Nat. Mater. 16, 892 (2017)]. The research suggests that weak vdW interactions between both the fullerene and CNTs, as well as between fullerene molecules themselves, hinder phonon propagation. The encapsulated fullerene contributes to an increase in phonon scattering within the CNTs, ultimately leading to a reduction in thermal conductivity. We utilized machine learning potential to investigate the structure of fullerene-encapsulated CNTs and their heat transport property. This approach provides valuable insights for performance research of complex systems featuring interlayer vdW interactions. [ABSTRACT FROM AUTHOR]

Nitrogen-doped graphene materials hold significant promise for diverse applications owing to their exceptional electrical properties and the tunability of thermal conductivity. Therefore, the non-equilibrium molecular dynamics simulations were used to explore the phonon transport properties of nitrogen-doped graphene nanoribbons. The findings indicate that periodic doping with a small quantity of nitrogen atoms can induce coherent phonon transport, thereby resulting in a substantial reduction in thermal conductivity. Our analysis delves into various phonon and energy transport parameters, including the phonon dispersion relation, group velocity, state density, participation rate, and spectral heat flow. Through this examination, we have elucidated the coexistence and transformation mechanisms of both coherent and incoherent phonon transport under different conditions. Furthermore, our findings revealed a notable trend: once the concentration of nitrogen atoms in the doped atomic layer reaches 37.5%, the reduction in thermal conductivity attains its maximum effectiveness. Beyond this concentration, further increases in the nitrogen atom concentration result in diminishing returns, rendering the reduction in thermal conductivity ineffective. [ABSTRACT FROM AUTHOR]

Using nonequilibrium molecular dynamics (NEMD) based direct method and spectral energy density (SED) method, we calculate the size-dependent thermal conductivities (TCs) of single layer graphene (SLG), AB-stacked bilayer graphene (AB-BLG), and 21.78 ° twisted BLG (tBLG) in a robust and consistent manner. Our NEMD analysis reveals discrepancies in high TC reported for graphene systems in some of the earlier studies. Similarly, some of the previous SED based studies were done with unreliable SED Φ ′ approach. We conduct size-dependent analysis of the graphene systems by the SED method for the first time and report that bulk TCs for SLG and tBLG systems are nearly the same when calculated by either the direct or the SED method. Contrary to studies that claim that phonon group velocities of AB-BLG and tBLG samples do not change, we find that although average group velocities in SLG and AB-BLG are almost the same, they are around 30% higher when compared to tBLG samples with different twist angles. On the other hand, average phonon lifetimes are almost similar for AB-BLG and 21.78 ° tBLG samples but around 43% lower than the average phonon lifetime of SLG. Together these trends suggest the reason behind the decreasing order of TCs across three systems. We also systematically study the basic phonon mode contributions to TCs and their properties and find that the high-symmetry modes contribute the most in all three systems. [ABSTRACT FROM AUTHOR]

We study the effects of the chemical short-range order (SRO) on the thermal conductivity of the refractory high-entropy alloy HfNbTaTiZr using atomistic simulation. Samples with different degrees of chemical SRO are prepared by a Monte Carlo scheme. With increasing SRO, a tendency of forming HfTi and TiZr clusters is found. The phonon density of states is determined from the velocity auto-correlation function and chemical SRO modifies the high-frequency part of the phonon density of states. Lattice heat conductivity is calculated by non-equilibrium molecular dynamics simulations. The heat conductivity of the random alloy is lower than that of the segregated binary alloys. Phonon scattering by SRO precipitates might be expected to reduce scattering times and, therefore, decrease thermal conductivity. We find that, in contrast, due to the increase of the conductivity alongside SRO cluster percolation pathways, SRO increases the lattice heat conductivity by around 12 %. This is expected to be a general result, extending to other HEAs. [ABSTRACT FROM AUTHOR]

Thermal management becomes increasingly important in silicon gate‐all‐around (GAA) field‐effect transistor (FETs) for 3 nm technology node and beyond. The channel thermal conductivity significantly differs from bulk silicon. Precise determination of thermal conductivity is crucial for device evaluation and optimization. This study investigates the thermal conductivity of silicon nanowires, examining the complex interplay between size and channel orientation. The conventional nonequilibrium molecular dynamics (NEMD) method is used with the standard Stillinger–Weber potential at the atomic scale. The results indicate that the thermal conductivity of silicon nanowires along the [100] direction increases monotonically with both length (L) and cross‐sectional side length (D). Conversely, the [110] direction exhibits nonmonotonic variation in thermal conductivity with D, due to increased acoustic–optic phonon scattering. For GAA FET devices with a silicon nanowire channel of L = 20 nm and D = 5 nm, the NEMD calculations yield thermal conductivities of 10.8 W m·K−1 for the [100] direction and 25.3 W m·K−1 for the [110] direction. Subsequently, the self‐heating effect (SHE) in silicon nanowire GAA FETs by technology computer‐aided design with the modified channel conductivity is analyzed. The results suggest that silicon nanowires with the [110] transport direction are more suitable for device design. [ABSTRACT FROM AUTHOR]

Strain engineering has been used as a versatile tool for regulating the thermal transport in various materials as a result of the phonon frequency shift. On the other hand, the phononic bandgap can be simultaneously tuned by the strain, which can play a critical role in wide phononic bandgap materials due to the high-order phonon anharmonicity. In this work, we investigate the complex role of uniaxial tensile strain on the lattice thermal conductivity of hydrogenated graphene-like borophene, by using molecular dynamics simulations with a machine learning potential. Our findings highlight a novel and intriguing phenomenon that the thermal conductivity in the armchair direction is non-monotonically dependent on the uniaxial armchair strain. Specifically, we uncover that the increase of phonon group velocity and the decrease of three-phonon scattering compete with the enhancement of four-phonon scattering under armchair strain, leading to the non-monotonic dependence. The enhanced four-phonon scattering originates from the unique bridged B–H bond that can sensitively control the phononic bandgap under armchair strain. This anomalous non-monotonic strain-dependence highlights the complex interplay between different mechanisms governing thermal transport in 2D materials with large phononic bandgaps. Our study offers valuable insights for designing innovative thermal management strategies based on strain. [ABSTRACT FROM AUTHOR]

Tuning the thermal transport properties of hybrid halide perovskites is critical for their applications in optoelectronics, thermoelectrics, and photovoltaics. Here, an effective strategy is demonstrated to modulate the thermal transport property of hybrid perovskites by halide alloying. A highly tunable thermal conductivity of mixed‐halide hybrid perovskites is achieved due to halide‐alloying and structural distortion. The experimental measurements show that the room temperature thermal conductivity of MAPb(BrxI1‐x)3 (x = 0─1) can be largely modulated from 0.27 ± 0.07 W m−1 K−1 (x = 0.5) to 0.47 ± 0.09 W m−1 K−1 (x = 1). Molecular dynamics simulations further demonstrate that the thermal conductivity reduction of hybrid halide perovskites results from the suppression of the mean free paths of the low‐frequency acoustic and optical phonons. It is found that halide alloying and the induced structural distortion can largely increase the scatterings of optical and acoustic phonons, respectively. The confined diffusion of MA+ cations in the octahedra cage is found to act as an additional thermal transport channel in hybrid perovskites and can contribute around 10–20% of the total thermal conductivity. The findings provide a strategy for tailoring the thermal transport in hybrid halide perovskites, which may largely benefit their related applications. [ABSTRACT FROM AUTHOR]

In this study, Non-Equilibrium Molecular Dynamics (NEMD) simulation is employed to investigate the phonon thermal conductivity (PTC) of Sn/hBN van der Waals heterostructures with different vacancy-induced defects. We deliberately introduce three types of vacancies in Sn/hBN bilayer point vacancies, bivacancies, and edge vacancies at various concentrations ranging from 0.25% to 2%, to examine their effects on PTC across temperatures from 100 K to 600 K. The key findings of our work are (i) PTC declines monotonically with increasing vacancy concentration for all types of vacancies, with a maximum reduction of ∼62% observed at room temperature compared to its pristine form. (ii) The position of defects has an impact on PTC, with a larger decrease observed when defects are present in the hBN layer and a smaller decrease when defects are in the Sn layer. (iii) The type of vacancy also influences PTC, with point vacancies causing the most substantial reduction, followed by bivacancies, and edge vacancies having the least effect. A 2% defect concentration results in a ∼62% decrease in PTC for point vacancies, ∼51% for bivacancies, and ∼32% for edge vacancies. (iv) Finally, our results indicate that for a given defect concentration, PTC decreases as temperature increases. The impact of temperature on thermal conductivity is less pronounced compared to the effect of vacancies for the defective Sn/hBN bilayer. The presence of vacancies and elevated temperatures enhance phonon-defect and phonon–phonon scattering, leading to changes in the phonon density of states (PDOS) profile and the distribution of phonons across different frequencies of Sn/hBN bilayer, thus affecting its thermal conductivity. This work offers new insights into the thermal behavior of vacancy-filled Sn/hBN heterostructures, suggesting potential pathways for modulating thermal conductivity in bilayer van der Waals heterostructures for applications in thermoelectric, optoelectronics, and nanoelectronics in future. [ABSTRACT FROM AUTHOR]

Configurational entropy manipulation strategy has been proposed for designing high-performance thermoelectric materials. Understanding the phase stability is essential to regulate the thermal conductivity for optimizing the thermoelectric performance. Herein, the lattice thermal conductivity of PbSe is found to decrease from 1.87 to 0.76 Wm−1 K−1 of PbSe 0.5 Te 0.25 S 0.25 , which mainly results from the decreased contribution from the phonon modes in the frequency range of 0.5–2 THz. Moreover, we find local chemical ordering (LCO) in PbSe 0.5 Te 0.25 S 0.25 by conducting hybrid Monte Carlo and molecular dynamics simulations based on our constructed machine-learning interatomic potential. The local chemical ordering can reduce phonon scattering with frequency in 0–2 THz, thus enhancing thermal conductivity by approximately 14%. This work unfolds the energy favorable structure with LCO in entropy-tailored thermoelectric material, which gives guidance for regulating thermal transport. [ABSTRACT FROM AUTHOR]

The effect of defects on the thermal properties of Ψ-Graphene Nanotubes (Ψ-GNTs) is investigated in this study using Molecular Dynamics simulations. The results reveal that the thermal properties of Ψ-GNTs are profoundly impacted by the presence of defects. Specifically, a significant reduction in thermal conductivity is observed with increasing defect concentrations. This reduction is attributed to the scattering of phonons by the defects, which leads to increased phonon–phonon interactions and decreased thermal transport efficiency. Furthermore, the effect of temperature on the thermal properties of defective Ψ-GNTs is investigated. The findings demonstrate a nonlinear decrease in thermal conductivity with increasing temperature. [ABSTRACT FROM AUTHOR]

Vibrational polaritons have shown potential in influencing chemical reactions, but the exact mechanism by which they impact vibrational energy redistribution, crucial for rational polariton chemistry design, remains unclear. In this work, we shed light on this aspect by revealing the role of solvent phonon modes in facilitating the energy relaxation process from the polaritons formed of a T1u mode of W(CO)6 to an IR inactive Eg mode. Ultrafast dynamic measurements indicate that along with the direct relaxation to the dark T1u modes, lower polaritons also transition to an intermediate state, which then subsequently relaxes to the T1u mode. We reason that the intermediate state could correspond to the near-in-energy Raman active Eg mode, which is populated through a phonon scattering process. This proposed mechanism finds support in the observed dependence of the IR-inactive state's population on the factors influencing phonon density of states, e.g., solvents. The significance of the Raman mode's involvement emphasizes the importance of non-IR active modes in modifying chemical reactions and ultrafast molecular dynamics. [ABSTRACT FROM AUTHOR]

There is still a gap in understanding phonon scattering by geometrical defects at the nanoscale, and it remains a significant challenge for heat transfer management in nanoscale devices and systems. In this study, we aim to explore the characteristics of phonon scattering by a single pore to gain insights into thermal transport in nanostructures. The paper outlines a methodology for assessing the spatial distribution of the magnitude of the radial, azimuthal, and polar components of the velocity of scattered phonons by a spherical pore. We demonstrated that the size parameter, commonly employed in electromagnetic wave scattering theory, is vital in determining the scattering regime. Specifically, we show that the calculated scattering efficiency has the same pattern as that commonly obtained in classical wave scattering theory. However, we found that crystallographic directions are pivotal in shaping the scattering patterns, especially in the regions where scattering patterns are defined by the Mie resonances. This observation holds significance in understanding the influence of phonon coherence on thermal transport in nanostructured materials. [ABSTRACT FROM AUTHOR]

In this study, we have investigated numerous influential factors such as length, diameter, impurity introduction, and vacancy defects on the thermal conductivity of carbon nanotubes (CNTs). These investigations were conducted through molecular dynamics simulations using the large-scale atomic/molecular massively parallel simulator (LAMMPS). It is observed that longer CNTs tend to exhibit heightened thermal conductivity, a consequence of the increased support for phonon vibration modes that facilitate efficient thermal transport. Furthermore, CNTs with larger diameters display superior thermal characteristics owing to reduced phonon scattering effects. The introduction of boron doping reduces CNTs thermal conductivity by approximately 3% with the inclusion of 6% boron atoms, whereas nitrogen doping increases it by a similar margin. These doping effects hold great potential for optimizing the performance of MEMS and NEMS devices. This duality in doping offers a versatile means to fine-tune the thermal conductivity of CNTs, enabling effective heat management in micro/nanodevices. By strategically modulating thermal conductivity, we can optimize the heat transfer properties of CNT-based materials and devices. This optimization is of utmost importance in ensuring efficient heat dissipation and averting thermal-induced issues, such as overheating, performance degradation, or failure. Additionally, this paper explores how vacancy defects impact the thermal conductivity of CNTs. By varying the vacancy concentration from 1 to 6%, a decrease in thermal conductivity of approximately 2% to 4% was observed in both SWCNTs and DWCNTs. These results emphasize the pivotal role of defects in perturbing the efficient phonon transport mechanisms in CNTs and suggest the potential for customizing CNTs with specific defect concentrations to enhance their suitability for thermoelectric devices and thermal insulation materials. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials have attracted more and more attention due to their excellent properties. In this work, we systematically explore the heat transport properties of Graphene-C3B (GRA-C3B) superlattices and van der Waals (vdW) heterostructures using molecular dynamics method. The effects of interface types and heat flow directions on the in-plane interfacial thermal resistance (ITRip) are analyzed. Obvious thermal rectification is detected in the more energy stable interface, GRA zigzag-C3B zigzag (ZZ) interface, which also has the minimum value of ITRip. The dependence of the superlattices thermal conductivity (k) of the ZZ interface on the period length (l p ) is investigated. The results show that when the l p is 3.5 nm, the k reaches a minimum value of 35.52 W m−1 K−1, indicating a transition stage from coherent phonon transport to incoherent phonon transport. Afterwards, the effects of system size, temperature, coupling strength and vacancy defect on the out-of-plane interfacial thermal resistance (ITRop) are evaluated. With the increase of temperature, coupling strength and vacancy defect, ITRop are found to reduce effectively due to the enhanced Umklapp phonon scattering and increased probability of energy transfer. Phonon density of states and phonon participation ratio is evaluated to reveal phonon behavior during heat transport. This work is expected to provide essential guidance for the thermal management of nanoelectronics based on 2D monolayer GRA and C3B. [ABSTRACT FROM AUTHOR]

Nanoparticles embedded within a crystalline solid serve as impurity phonon scattering centers that reduce lattice thermal conductivity, a desirable result for thermoelectric applications. Most studies of thermal transport in nanoparticle-laden composite materials have assumed the nanoparticles to possess a single size. If there is a distribution of nanoparticle sizes, how is thermal conductivity affected? Moreover, is there a best nanoparticle size distribution to minimize thermal conductivity? In this work, we study the thermal conductivity of nanoparticle-laden composites through a molecular dynamics approach which naturally captures phonon scattering processes more rigorously than previously used analytical theories. From thermal transport simulations of a systematic variety of nanoparticle configurations, we empirically formulate how nanoparticle size distribution, particle number density, and volume fraction affect the lattice thermal conductivity. We find at volume fractions below 10%, the particle number density is by far the most impactful factor on thermal conductivity and at fractions above 10%, the effect of the size distribution and number density is minimal compared to the volume fraction. In fact, upon comparisons of configurations with the same particle number density and volume fractions, the lattice thermal conductivity of a single nanoparticle size can be lower than that of a size distribution which contradicts intuitions that a single size would attenuate phonon transport less than a spectrum of sizes. The random alloy, which can be considered as a single size configuration of maximum particle number density where the nanoparticle size is equal to the lattice constant, is the most performant in thermal conductivity reduction at volume fractions below 10%. We conclude that nanoparticle size distribution only plays a minor role in affecting lattice thermal conductivity with the particle number density and volume fraction being the more significant factors that should be considered in fabrication of nanoparticle-laden composites for potential improved thermoelectric performance. [ABSTRACT FROM AUTHOR]

The miniaturization and higher power density of modern electronics pose a significant challenge in thermal management. A key focus in addressing this challenge revolves around the advancement of thermal interfaces within microchip packaging, aiming to enhance thermal energy dissipation and optimization of performance. Copper nanowires are extensively employed in the chip industry as interconnects for signal transmission and thermal management purposes. Investigating the impact of reduced cross-section on the thermal transport properties of nanowires is crucial. In this study, the thermal conductivity of copper and silicon nanowires is studied with variations in the length of the nanowires. The simulation is conducted with the Equilibrium Molecular Dynamics (EMD) process. The cross-section of the nanowire is kept fixed (10 × 10 nm) and with the increase in length, its thermal conductivity is studied. At room temperature for a 50 nm length, the lattice thermal conductivity value is 1.68 and 0.037 W m − 1 K − 1 for silicon and copper nanowires, respectively. We further studied the phonon scattering, mean free path, and group velocity of silicon and copper lattices. Our study may help to design more thermally efficient microchips and innovate new cooling methods of microelectronics. [ABSTRACT FROM AUTHOR]

Multi-layered stacking structures and atomic mixing interfaces were constructed. The effects of various factors on the thermal conductivity of different lattice structures were studied by non-equilibrium molecular dynamics simulations, including the number of atomic mixing layers, temperature, total length of the system, and period length. The results showed that the mixing of two and four layers of atoms can improve the thermal conductivities of the multi-layer structure with a small total length due to a phonon "bridge" mechanism. When the total length of the system is large, the thermal conductivity of the multi-layer structure with atomic mixing interfaces decreases significantly compared with that of the perfect interfaces. The interfacial atom mixing destroys the phonon coherent transport in the multi-layer structure and decreases the thermal conductivity to some extent. The thermal conductivity of the multi-layer structure with perfect interfaces is significantly affected by temperature, whereas the thermal conductivity of the multi-layer structures with atomic mixing is less sensitive to temperature. [ABSTRACT FROM AUTHOR]

High-temperature driven solid–solid reaction at coke (C)/calcium oxide (CaO) interfaces is attractive for industrial-scale production of calcium carbide (CaC2). However, the reaction rate limited by the low thermal conductivity (k) results from a high Kapitza thermal resistance (Rk) at C/CaO interfaces. Identifying various factors including temperature and absorbed moisture on the k of C/CaO pellets is significant for heat transfer enhancement. Here, we developed a modified effective medium assumption model considering the particle-packed configuration to predict the influence of temperature and moisture on the k of C/CaO pellets, in which the R k is evaluated by the non-equilibrium molecular dynamics. The results show that the k of C/CaO pellets increases from 0.48 to 0.55 W/(m K) when the temperature increases from 300 to 900 K, which is attributed to a 19.7% decrease in the R k of C/CaO interfaces caused by a rising temperature activated inelastic interfacial phonon scattering. Moreover, it is found that the k of C/CaO pellets decreases from 0.48 to 0.44 W/(m K) after inserting absorption water layers with a thickness of 0.5 nm at C/CaO interfaces. A further 38.4% degeneration in k is harvested when increasing the thickness of the absorption water layers from 0.5 to 1.3 nm. This work provides an overall insight into the interfacial effect on the k of C/CaO porous pellets and guides the heat transfer optimization for particle-packed systems. [ABSTRACT FROM AUTHOR]

We derive a theoretical model for phonon thermal boundary conductance across solid interfaces in the high temperature classical limit using quasi-harmonic thermodynamics, an approach that accounts for phonon anharmonicity effects on energy density changes via thermal expansion. Commonly used predictive models based on harmonic theory predict a thermal boundary conductance in the classical limit that is that constant and independent of temperature. Thus, these theories do not capture the increase in thermal boundary conductance with increasing temperature that has been reported in numerous molecular dynamics and anharmonic non-equilibrium Green's function simulations. Our model accounts for anharmonic effects on the thermal boundary conductance via an increased internal energy of the material through an additional quasi-harmonic term that includes the material's Grüneisen parameter. We show good agreement between our model calculations and the predicted thermal boundary conductance across a heavy argon/argon interface determined via molecular dynamics simulations. Further, our results also capture the contribution of inelastic scattering to thermal boundary conductance across a silicon/germanium interface predicted from anharmonic nonequilibrium Green's functions simulations. Our quasi-harmonic thermodynamic-based theory suggests that an increase in thermal boundary conductance with an increase in temperature above the Debye temperature is due to anharmonicity in the materials adjacent to the interface, which is captured by the thermal expansion-driven phonon energy density changes in the materials. This theory is also consistent with prior molecular dynamics and anharmonic non-equilibrium Green's function simulations that suggest that inelastic scattering effects on thermal boundary conductance are driven by phononic processes in materials near the interface and not at the interface. This model can help in screening materials for high interface density composites to increase thermal conductance and mitigate temperature in a range of applications. [ABSTRACT FROM AUTHOR]

This work combines first-principles calculations, spin–lattice dynamics, and the non-equilibrium Green's function (NEGF) method to compute thermal boundary conductance at a three-dimensional Co–Cu interface, considering spin–lattice interactions. Spin–lattice interactions are quantified through exchange interactions between spins, and the exchange constants are obtained from first-principles calculations. Equilibrium molecular dynamics is used to calculate the heat flux across the interface after the spin and lattice subsystems are in equilibrium. Because of the weak interaction between Co and Cu layers adjacent to the interface, spin-wave transmission is low. Spins are scattered by phonons inside the Co contact, and interfacial thermal conductance is reduced. We also compare the results to the NEGF method. Phonon and magnon scattering rates are incorporated into Büttiker probes attached to the device. The NEGF method shows a similar trend in thermal boundary conductance with spins included. Green's function is solved recursively; therefore, it can be applied to large devices. [ABSTRACT FROM AUTHOR]

Atomistic simulation methods, including anharmonic lattice dynamics combined with the Boltzmann transport equation, equilibrium and non-equilibrium molecular dynamics simulations, and Landauer formalism, are vital for the prediction of thermal conductivity and the understanding of nanoscale thermal transport mechanisms. However, for years, the simulation results using different methods, or even the same method with different simulation setups, lack consistency, leading to many arguments about the underlying physics and proper numerical treatments on these atomistic simulation methods. In this perspective, we review and discuss the recent advances in atomistic simulation methods to predict the thermal conductivity of solid materials. The underlying assumptions of these methods and their consequences on phonon transport properties are comprehensively examined. Using silicon and graphene as examples, we analyze the influence of higher-order phonon scatterings, finite-size effects, quantum effects, and numerical details on the thermal conductivity prediction and clarify how to fairly compare the results from different methods. This perspective concludes with suggestions on obtaining consistent thermal conductivity prediction of different material systems and also provides perspective on efficient and accurate simulations of thermal transport in more complex and realistic conditions. [ABSTRACT FROM AUTHOR]

Utilizing molecular dynamics (MD) simulations based on the highly precise force-fields, we find that phonon scattering strengths induced by the cubic anharmonicity can be significantly underestimated by the perturbation theory (PT) approach in materials with sizable frequency gaps or branch bunching. We trace this result to the additional three-phonon scatterings in MD enabled by the fluctuating phonon energy and the continuous energy exchange between modes. These channels are essential to accurately evaluate the zone-center phonon linewidth in boron arsenide as compared to the experiment and could noticeably lower the lattice thermal conductivity of beryllium telluride and tungsten carbide. Accordingly, due to the stronger three-phonon scatterings, four-phonon scatterings would become less important than previously believed in this type of material. Moreover, our work emphasizes the different phonon scattering processes in MD and PT simulations, offering new insights for an improved description of anharmonic properties. [ABSTRACT FROM AUTHOR]

Nanostructured semiconductors are promising candidates for thermoelectric materials owing to their superior thermal insulating properties over their bulk counterparts. In this study, a one-dimensional, crystalline nanostructure synthesized by sintering Si nanoparticles, called Nano Particle Chain (NPC) structures, is proposed. The structure is systematically analyzed for its thermal transport properties and compared with the nanowire counterparts. Both classical molecular dynamics and lattice dynamics tools were employed to evaluate lattice thermal conductivity (k) and to perform phonon mode level decomposition. A marked reduction in the phonon relaxation time of the NPC structure was observed indicating possible effects of phonon-boundary/constriction scatterings. This has resulted in a two-order reduction in k in NPC structures over bulk Si. Further, one order reduction of k of NPC structures was attained with respect to a nanowire of the same constriction size, indicating the effectiveness of the mismatch of particle and constriction diameters as an efficient thermal suppression mechanism. With the addition of a second material of different mass, the NPC structures can be further diversified to core/shell configurations. It was also identified that a non-monotonic variation of k exists, with a minimum in core/shell NPC structures. This effect is materialized by using a Ge-like fictitious material to coat the original Si nanoparticles, owing to competing effects of two phonon suppression mechanisms. Moreover, these core/shell NPC structures are compared with previously reported diameter modulated core/shell nanowire structures [E. Blandre et al., Phys. Rev. B, 91, 115404 (2015)] to highlight their capability to enhance the thermoelectric performance over conventional one-dimensional nanostructure configurations. [ABSTRACT FROM AUTHOR]

The effects of length, diameter, temperature, and axial strain on the thermal conductivity of armchair and zigzag WS 2 nanotubes are systematically investigated by nonequilibrium molecular dynamics simulations. It is found that the thermal conductivity gradually increases with the increase in the length, while it is insensitive to nanotube diameter variation. The thermal conductivity of armchair and zigzag WS 2 nanotubes is remarkably reduced as temperature increases due to the increment of phonon–phonon scattering and reduction of the phonon mean free path. In addition, compressive strain can increase thermal conductivity due to increased contributions from low-frequency phonons, while the opposite is true in the case of tensile strain. The chirality has a slight influence on the thermal transport properties of the WS 2 nanotube. [ABSTRACT FROM AUTHOR]

Effective heat dissipation is the bottleneck problem for the development and commercialization of GaN-based high-power electronic and photonic devices. To address this challenge and explore the underlying mechanism of phonon transportation across the GaN/AlN heterointerface, in this work, we formed three types of GaN/AlN heterostructures with distinctively different interfacial morphologies by annealing recrystallization approach. It is found that the interfacial thermal conductance (ITC) of GaN/AlN heterostructures can be remarkably improved by tailoring the interfacial crystal structure and phase morphology. Besides the commonly amorphous phase and ideal ordered wurtzite phase, we further found that AlN may present an additional stable rock salt phase at the interfacial region, and its significant effect on interfacial thermal transport has been observed. Using molecular dynamics simulation, we systematically investigated the effects of different GaN/AlN heterojunctions on the ITCs. Our results suggest that heat dissipation at the GaN/AlN interface is dominated by phonons scattered diffusely by the amorphous region at interfaces and the ITC can be significantly enhanced by recrystallizing the amorphous AlN to rock salt one. Furthermore, through phonon vibrational spectrum, we revealed that phonon modes dominate the energy transport across the interfaces of wurtzite AlN/GaN, amorphous AlN/GaN, and rock salt AlN/GaN are significantly different. Finally, we found the ITC increased with the temperature due to the enhanced inelastic phonon scattering and the presence of additional excited phonon modes at higher temperatures. The findings elucidated here provide a clearer insight into the effect of interfacial microstructures on the interfacial thermal resistance of GaN–substrate interface, which also provide a viable heat management strategy for the high-power GaN-based devices. [ABSTRACT FROM AUTHOR]

In this work, the effects of in-plane shear stress on the thermal conductivities of two important kinds of graphene configurations (rectangular graphene and annular graphene) are investigated using molecular dynamics simulations. The various strain angles are set as θ = 0°, 3°, 5°, 10°, and 15° for rectangular graphene and θ = 0°, 3°, 5°, and 8° for annular graphene. A spectral energy density approach is applied to carry out phononic analyses of the two configurations under in-plane shear stress for the first time. It is found that for rectangular graphene, the thermal conductivity shows a significant decreasing trend from 239.6 to 79.1 W m−1 K−1 with the θ increasing from 0° to 15°. Besides, the application of shear stress can significantly widen the energy bands of branches, which indicates shorter phonon lifetimes. Further phononic analysis shows that most of the phonon scattering caused by in-plane shear stress emanates from the in-plane mode, which is an updated conclusion in this field. On the other hand, the thermal conductivity of annular graphene also shows a decreasing evolution with the increasing strain angles, but phonon transport in rectangular graphene is more susceptible to shear stress than in annular graphene due to a stronger phonon-boundary scattering in the latter configuration. In this work, the monotonically decreasing trends of the thermal conductivities of both graphene configurations with the increasing strain angle are different from the V-shape trend reported in the literature. Based on phononic analyses, it can be concluded that an abnormal increase of the thermal conductivities at a low shear stress can be attributed to the application of the AIREBO potential model in the literature. [ABSTRACT FROM AUTHOR]

The graphite intercalation compounds have attracted wide interest due to the superconductivity. In this work, the thermal transport in bilayer graphene intercalated with Ca atoms (C6CaC6) at room temperature is studied by using non-equilibrium molecular dynamics simulations. Our simulation results show that the in-plane lattice thermal conductivity (κ L) of C6CaC6 is significantly lower than that of the bilayer graphene. The detailed phonon mode analysis reveals that the reduction of κ L is because of the mode hybridization and flatbands induced by the intercalated Ca atoms, leading to the decrease in phonon group velocity and the enhancement of phonon scattering. Unlike the role of van der Waals interactions in multilayer graphene and supported graphene, increasing coupling strength between intercalated Ca atoms and graphene brings an enhanced κ L in C6CaC6. The spectral phonon analysis uncovers that such anomalous phenomenon is caused by the redistribution of phonon scattering phase space originated from the shift of the flatbands. This study indicates that atom intercalation is an effective way to regulate the heat transport in two-dimensional materials. [ABSTRACT FROM AUTHOR]

Exploring the thermal transport properties of the interface structure of low-dimensional nanomaterials contributes to a deeper understanding of the interface phonon modes and may provide theoretical support for efficient chip cooling devices. In this manuscript, we have simulated in detail the effects of system temperature, substrate location, and heat flow density on the thermal transport at the SWCNT/Si interface using the Non-equilibrium Molecular Dynamics approach and predicted the interfacial thermal conductance of SWCNT/Si for second, third and fourth order phonon using the Anharmonic Inelastic model. The results show that the low-frequency acoustic branch of SWCNT is suppressed by phonon scattering from the substrate, and the low-frequency phonon branch of SWCNT is boosted by about 1 THz. The anharmonic channels and inelastic phonon scattering significantly affect the interface phonon modes at higher temperatures, and the anharmonic interactions could increase additional thermal transport channels, which result in an increased number of additional phonon peaks. The increase in temperature gradually consumes the phonons incident at the interface in SWCNT, which enhances the anharmonic scattering and weakens the nonlinear characteristics of the material heterostructure, while the weakening of the acoustic branch accompanied by the temperature increase makes the LA phonon branch thermal conduction rate slower and the interfacial thermal conductance gradually stabilizes, which leads to the weakening of the thermal rectification effect. [ABSTRACT FROM AUTHOR]

Thermal transport in amorphous lithium‐sulfur (a‐LixS) is systematically investigated using molecular dynamics and the contributions from different types of heat carriers are quantitatively evaluated. In general, the thermal conductivity (TC) of a‐LixS changes largely by varying the concentration (x) of Li ions in a‐LixS. Interestingly, the TC of a‐LixS shows three distinct regimes of dependence on Li concentration. For low Li concentration (x = 0.4–1.2), the TC grows slowly, followed by a rapid increase in TC for medium Li concentration (x = 1.2–1.6), where the growth rate is three times that of the first regime, and finally, the TC is independent of Li concentration (x = 1.6–2.0). The TC enhancement in the first and second regimes is mainly attributed to propagating and non‐propagating vibrational modes in a‐LixS, respectively. In contrast, the stable thermal transport regime is governed by the competition between propagating and non‐propagating phonons. These investigations provide quantitative TC data of various polysulfides for shuttling analysis, and a fundamental understanding of the thermal transport mechanism of complex a‐LixS structures, which is beneficial for the rational design of thermal management of Li‐S batteries. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) is a promising material for thermoelectric power generation. The characterization of thermal transport properties is essential to understanding their applications in thermoelectric devices. The existence of stacking faults, which originate from the "wrong" stacking sequences of Si–C bilayers, is a general feature of SiC. However, the effects of stacking faults on the thermal properties of SiC are not well understood. In this study, we evaluated the accuracy of Tersoff, MEAM, and GW potentials in describing the thermal transport of SiC. Additionally, the thermal conductivity of 3C/4H-SiC nanowires was investigated using non-equilibrium molecular dynamics simulations (NEMD). Our results show that thermal conductivity exhibits an increase and then saturation as the total lengths of the 3C/4H-SiC nanowires vary from 23.9 nm to 95.6 nm, showing the size effect of molecular dynamics simulations of the thermal conductivity. There is a minimum thermal conductivity, as a function of uniform period length, of the 3C/4H-SiC nanowires. However, the thermal conductivities of nanowires weakly depend on the gradient period lengths and the ratio of 3C/4H. Additionally, the thermal conductivity of 3C/4H-SiC nanowires decreases continuously from compressive strain to tensile strain. The reduction in thermal conductivity suggests that 3C/4H-SiC nanowires have potential applications in advanced thermoelectric devices. Our study provides insights into the thermal transport properties of SiC nanowires and can guide the development of SiC-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

Thermal conductivity of hydrogenated hexagonal boron nitride (h-BN) nanosheets was investigated using molecular dynamics simulation method. A newly parameterized reactive force field (ReaxFF) for hydrogen and h-BN interactions was used. ReaxFF was used due to its higher accuracy compared to other simpler interatomic potentials. Accurate thickness selection of a monolayer h-BN nanosheet has been shown to produce high thermal conductivity values for pristine armchair and zigzag nanosheets. It was further found that hydrogenation diminishes thermal conductivity of hydrogenated h-BN nanosheets. This reduction in thermal conductivity was due to the occurrence of sp2 to sp3 bonding transition when hydrogen atoms were placed on top of B and N atoms. The increase in temperature was also found to diminish thermal conductivity due to the occurrence of phonon–phonon scattering at higher temperatures. N-vacancy defect has then been shown to exhibit lower thermal conductivity compared to B-vacancy defect. Furthermore, the removal of more atoms contributes to higher decline in thermal conductivity. However, vacancy defect constructed along vertical direction provides the highest reduction in thermal conductivity. It is expected that this work provides useful insights for the design of an effective hydrogen storage system using these novel h-BN nanosheets. [ABSTRACT FROM AUTHOR]

Thermal management is extremely important for designing high-performance devices. The lattice thermal conductivity of materials is strongly dependent on detailed structural defects at different length scales, particularly point defects like vacancies, line defects like dislocations, and planar defects such as grain boundaries. Traditionally, the McKelvey–Shockley phonon Boltzmann's transport equation (BTE) method, combined with molecular dynamics simulations, has been widely used to evaluate the phonon mean free paths (MFPs) in defective systems. However, this method can only provide the aggregate MFPs of the whole sample, as it is challenging to extract the MFPs in different regions with varying thermal conductivities. In this study, the 1D McKelvey–Shockley phonon BTE method was extended to model inhomogeneous materials, where the contributions of defects to the phonon MFPs are explicitly obtained. Then, the method was used to study the phonon scattering with the core structure of an edge dislocation. The phonon MFPs in the dislocation core were obtained and were found to be consistent with the analytical model in a way that high frequency phonons are likely to be scattered in this area. This method not only advances the knowledge of phonon–dislocation scattering but also shows the potential to investigate phonon transport behaviors in more complicated materials. [ABSTRACT FROM AUTHOR]

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This paper studies thermal transport in nanoporous silicon with a significant specific surface area. First, the equilibrium molecular dynamics approach was used to obtain the dependence of thermal conductivity on a specific surface area. Then, a modified phonon transport kinetic theory-based approach was developed to analyze thermal conductivity. Two models were used to evaluate the phonon mean free path in the porous materials. The first model assumes that the dependence of the mean free path only relies on the specific surface area, and the second one also considers the mean free path variation with the porosity. Both approaches approximate molecular dynamics data well for the smaller porosity values. However, the first model fails for highly porous matrixes, while the second one matches well with molecular dynamics simulations across all considered ranges of the porosities. This work illustrates that the phonon mean free path dependence with the porosity/volume fraction of composite materials is essential for describing thermal transport in systems with significant surface-to-volume ratio. [ABSTRACT FROM AUTHOR]

The ultralow thermal conductivity (TC) (0.1–0.3 W m−1 K−1) of amorphous epoxy resins (EPs) limits their applications to high heat flux; thus, designing EPs with high TC is of great significance. In this work, the individual and synergistic effects of three different factors, chain length, tensile strain, and curing agent structures, on the intrinsic TC of single crosslinked EP chains were investigated mainly by equilibrium molecular dynamics simulations. The results show that, compared with the linear polyethylene chain, EP chains in the unstretched state retain a much lower TC below 3 W m−1 K−1 due to the more significant phonon scattering in the complex crosslinked structures. The TC can be significantly improved over a critical tensile strain due to the significant reduction in the folding and kink structures. The critical tensile strain varies with the types of crosslinked EP chains. A high TC of EP chains can be easily achieved due to the synergistic effect of chain length and stretching. By constructing two types of structural evolutions, the molecular structure of the curing agents was found to have a significant effect on the TC of crosslinked EP chains. The additional functional groups on the benzene ring cause a significant reduction in the TC of single EP chains. In addition, the decrease rate generally slows down as the benzene ring number increases. [ABSTRACT FROM AUTHOR]

Improving the performance of molecular qubits is a fundamental milestone towards unleashing the power of molecular magnetism in the second quantum revolution. Taming spin relaxation and decoherence due to vibrations is crucial to reach this milestone, but this is hindered by our lack of understanding on the nature of vibrations and their coupling to spins. Here we propose a synergistic approach to study a prototypical molecular qubit. It combines inelastic X-ray scattering to measure phonon dispersions along the main symmetry directions of the crystal and spin dynamics simulations based on DFT. We show that the canonical Debye picture of lattice dynamics breaks down and that intra-molecular vibrations with very-low energies of 1-2 meV are largely responsible for spin relaxation up to ambient temperature. We identify the origin of these modes, thus providing a rationale for improving spin coherence. The power and flexibility of our approach open new avenues for the investigation of magnetic molecules with the potential of removing roadblocks toward their use in quantum devices. Understanding phonon-induced relaxation in molecular qubits is a crucial step in realizing their application potential. Garlatti at al. use a combination of inelastic X-ray scattering and density functional theory to investigate the role of low-energy phonons on spin relaxation of a prototypical molecular qubit. [ABSTRACT FROM AUTHOR]

Ferroelectric domain walls (DWs) of perovskite oxide materials, which can be written and erased by an external electric field, offer the possibility to dynamically manipulate phonon scattering and thermal flux behavior. Different from previous ferroelectric materials, such as BaTiO3, PbTiO3, etc., with an immutable and low Curie temperature. The Curie temperature of perovskite oxide KTa1−xNbxO3 (KTN) crystal can be tuned by altering the Ta/Nb ratio. In this work, the ferroelectric KTa0.6Nb0.4O3 (KTN) single crystal is obtained by the Czochralski method. To understand the role of ferroelectric domains in thermal transport behavior, we perform a nonequilibrium molecular dynamics (NEMD) calculation on monodomain and 90° DWs of KTN at room temperature. The calculated thermal conductivity of monodomain KTN is 9.84 W/(m·k), consistent with experimental results of 8.96 W/(m·k), and distinctly decreased with the number of DWs indicating the outstanding performance of the thermal switch. We further evaluate the thermal boundary resistance (TBR) of KTN DWs. An interfacial thermal resistance value of 2.29 × 10−9 K·m2/W and a large thermal switch ratio of 4.76 was obtained for a single DW of KTN. Our study shows that the ferroelectric KTN can provide great potential for the application of thermal switch at room temperature and over a broad temperature range. [ABSTRACT FROM AUTHOR]

Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu2SnS3 (CTS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass–electron-crystal behavior—a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs. [ABSTRACT FROM AUTHOR]

The Nowotny–Juza α-MgAgSb has been demonstrated to be a promising candidate for room-temperature thermoelectric material, owing to its ultralow lattice thermal conductivity. The challenge of enhancing its figure of merit (ZT) for commercial applications is how to effectively decouple the electrical and thermal transport with available experimental strategies. With a synergic pressure and doping strategy, we demonstrate from first principles that the bandgap of α-MgAgSb enlarges and its electrical and thermal transport can be decoupled. From the perspective of lattice dynamics, the locally vibrating three-centered Mg-Ag-Sb bonds generate multiple low-lying optical phonons which contribute large scattering channels among heat-carrying phonons and thus result in a strong anharmonicity. Under hydrostatic pressure from ambient to 50 GPa, the chemical bonds are strengthened and low-lying optical phonons move upward, which reduces the anharmonic three-phonon scattering events and thus increases lattice thermal conductivity. Under hydrostatic pressure, α-MgAgSb maintains high mechanical stability even at 550 K and 50 GPa, as verified by first-principles molecular dynamics simulations. By combining the pressure and the doping strategy to engineer density of states near the Fermi level, the thermoelectric power factor can be tuned to be significantly high while the thermal conductivity remains reasonably low. The physical insights gained from this work pave the way for decoupling electrical and thermal transport of α-MgAgSb via the synergic pressure and doping strategy toward improving its thermoelectric performance. [ABSTRACT FROM AUTHOR]

It is shown using classical molecular dynamics simulations that phonon scattering from disorder in the interatomic forces introduced by charge transfer and not from mass disorder is needed to explain the thermal conductivity reduction experimentally measured that accompanies the addition of a sixth cation to the entropy stabilized oxide J14 [(Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5]. The simulations were performed on five entropy-stabilized oxides, J14, and J14 plus Sc, Sn, Cr, or Ge in equi-molar cation proportions. Comparing the simulation results to predictions from the Bridgman equation using properties from the simulations suggests that despite phonon scattering from disorder in both atomic forces and mass, the thermal conductivity for these systems is still above an analytical limit for an amorphous structure. [ABSTRACT FROM AUTHOR]

Phonon energies at finite temperatures shift away from their harmonic values due to anharmonicity. In this paper, we have realized the rigorous calculation of phonon energy shifts of silicon by three- and four-phonon scattering from first principles. The anharmonic fourth-order force constants are calculated by considering up to the fifth nearest neighbors. The results agree reasonably well with available data from inelastic neutron scattering throughout the Brillouin zone. Surprisingly, the frequency shifts of optical phonon modes near the Γ point are sensitive to the cutoff radius of the fourth-order force constants, in contrast to the four-phonon scattering rates, which nearly saturate when considering the second nearest neighbors. We have also compared the results with ab initio molecular dynamics simulations and found that the higher order of anharmonicity is important for optical phonons. Our work provides critical insight into the anharmonic phonon frequency shift and will have a significant impact on the thermal and optical applications. [ABSTRACT FROM AUTHOR]

The manipulation of phonon transport with coherent waves in solids is of fundamental interest and useful for thermal conductivity design. Based on equilibrium molecular dynamics simulations and lattice dynamics calculations, the thermal transport in SiGe superlattice nanowires with a tuned Si/Ge interface density was investigated by using the core-shell and phononic structures as the primary stacking layers. It was found that the thermal conductivity decreased with the increase of superlattice period lengths ( L p ) when L p was larger than 4 nm. This is because introducing additional Si/Ge interfaces can enhance phonon scattering. However, when L p < 4 nm, the increased interface density could promote heat transfer. Phonon density-of-state analysis demonstrates that new modes between 10 and 14 THz are formed in structures with dense Si/Ge interfaces, which is a signature of coherent phonon transport as those modes do not belong to bulk Si or Ge. The density of the newly generated modes increases with the increase of interface density, leading to an enhanced coherent transport. Besides, with the increase of interface density, the energy distribution of the newly generated modes becomes more balanced on Si and Ge atoms, which also facilitates heat transfer. Our current work is not only helpful for understanding coherent phonon transport but also beneficial for the design of new materials with tunable thermal conductivity. [ABSTRACT FROM AUTHOR]

Polymers are a unique class of materials from the perspective of normal mode analysis. Polymers consist of individual chains with repeating units and strong intra-chain covalent bonds, and amorphous arrangements among chains with weak inter-chain van der Waals and for some polymers also electrostatic interactions. Intuitively, this strong heterogeneity in bond strength can give rise to special features in the constituent phonons, but such effects have not been studied deeply before. Here, we use lattice dynamics and molecular dynamics to perform modal analysis of the thermal conductivity in amorphous polymers. We find an abnormally large population of localized modes in amorphous polymers, which is fundamentally different from amorphous inorganic materials. Contrary to the common picture of thermal transport, localized modes in amorphous polymers are found to be the dominant contributors to thermal conductivity. We find that a significant portion of the localization happens within individual chains, but heat is dominantly conducted when localized modes involve two chains. These results suggest localized modes generally play a key role in thermal transport for different polymers. The results provide an alternative perspective on why polymer thermal conductivity is generally quite low and gives insight into how to potentially change it. Lattice dynamics and molecular dynamics simulations are used to analyze normal modes and thermal conductivity of amorphous polymers. Here, the authors show that the localized modes are dominant, both in mode population and in their contributions to thermal conductivity for all three examples of amorphous polymers studied in this work. [ABSTRACT FROM AUTHOR]

Thermal conductivity of porous graphene can be affected by defect concentration, nanopore shape and distribution, and it is hard to clarify the effects due to the correlation of those factors. In this work, molecular dynamics simulation is used to compare the thermal conductivity of graphene with three shapes of regularly arranged nanopores. The results prove the dominant role of defect concentration under certain circumstances in reducing thermal conductivity, while the coupling effect of nanopore shape should be noticed. When the atoms at the local phonon scattering area around each nanopore are properly removed, the abnormal increment of thermal conductivity can be detected with the increase of defect concentration. Heat flux vector angles can effectively characterize the local phonon scattering area, which can be used to describe the effect of nanopore shape. The coupling effect of defect concentration and pore shape with similar heat flux path is clarified according to this process. By adjusting vertex angle of triangle defect, there is a balanced state of the effect factors between the variation of defect concentration and the same phonon scattering area. It provides a possible way to describe the weighing factors of the coupling effect. The results suggest a feasible approach to optimize and regulate thermal properties of porous graphene in nanodevice. [ABSTRACT FROM AUTHOR]

In this study, the non-equilibrium molecular dynamics simulation (NEMD) has been used to evaluate the thermal properties, especially the rectification of ultra-narrow edge-functionalized graphene with hydrogen atoms. The system's small width equals 4.91 Å (equivalent to two hexagonal rings). The dependence of the thermal rectification on the mean temperature, hydrogen concentration, and temperature difference between the two baths was investigated. Results reveal that the thermal rectification increases to 100% at 550 K by increasing the mean temperature. Also, it is disclosed that hydrogen concentration plays a vibrant role in thermal rectification. As a result of maximum phonon scattering at the interface, a thorough rectification is obtained in a half-fully hydrogenated system. As well, the effects of temperature difference of baths ΔT on thermal rectification has been calculated. As a result, the thermal rectification decreases even though the current heat increases with ΔT. Finally, the thermal resistance at the interface using a mismatching factor between the two-phonon density of states (DOS) on both sides of the interface has been explained. [ABSTRACT FROM AUTHOR]

• Swift heavy ion (SHI) irradiation induces straining and point defect formation in ZnO, while retaining its crystallinity even at high ion doses. • Nanoscale thermal transport is assessed by femtosecond laser-based time-domain thermoreflectance. • Fluence-dependent reduction and saturation of thermal conductivity were captured by the evolution of radiation damage derived from the regimes of single ion path impacts and nanoscale overlapping of ions paths, respectively. • Novel computational approach to directly model the impact of SHI irradiation on nano-scale heat propagation without using free parameters. • Presented results promote a novel way of making a new generation of radiation-controlled thermally functional materials. Near-surface nanoscale thermal conductivity (k) variation of ion-irradiated single-crystalline ZnO was studied by time-domain thermoreflectance. ZnO was irradiated by 710 MeV Bi swift heavy ions (SHI) in the 1010–1013 ion/cm2 fluence range to investigate the progression of radiation damage both from single ion impacts and ion path overlapping regimes. Structural characterization using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy indicated the absence of amorphization. The degradation in k was attributed primarily due to phonon scattering on point defects. The results of measured k were used to validate several models including the semi-analytical Klemens-Callaway model, and a novel hybrid modeling approach based on the Monte-Carlo code TREKIS coupled with molecular dynamics simulations which captures the effects of single ion and ion path overlapping regimes , respectively. The findings promote a novel approach to developing radiation-controlled thermally functional materials. [Display omitted] [ABSTRACT FROM AUTHOR]