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Electronic devices get smaller and smaller in every generation. In micro-/nano-electronic devices such as high electron mobility transistors, heat dissipation has become a crucial design consideration due to the ultrahigh heat flux that has a negative effect on devices' performance and their lifetime. Therefore, thermal transport performance enhancement is required to adapt to the device size reduction. β-Ga2O3 has recently gained significant scientific interest for future power devices because of its inherent material properties such as extremely wide bandgap, outstanding Baliga's figure of merit, large critical electric field, etc. This work aims to use a machine learning approach to search promising substrates or heat sinks for cooling β-Ga2O3, in terms of high interfacial thermal conductance (ITC), from large-scale potential structures taken from existing material databases. With the ITC dataset of 1633 various substrates for β-Ga2O3 calculated by full density functional theory, we trained our recently developed convolutional neural network (CNN) model that utilizes the fused orbital field matrix (OFM) and composition descriptors. Our model proved to be superior in performance to traditional machine learning algorithms such as random forest and gradient boosting. We then deployed the CNN model to predict the ITC of 32 716 structures in contact with β-Ga2O3. The CNN model predicted the top 20 cubic and noncubic substrates with ITC on the same level as density functional theory (DFT) results on β-Ga2O3/YN and β-Ga2O3/MgO interfaces, which has the highest ITC of 1224 and 1211 MW/m2K, respectively, among the DFT-ITC datasets. Phonon density of states, group velocity, and scattering effect on high heat flux transport and consequently increased ITC are also investigated. Moderate to high phonon density of states overlap, high group velocity, and low phonon scattering are required to achieve high ITC. We also found three Magpie descriptors with strong Pearson correlation with ITC, namely, mean atomic number, mean atomic weight, and mean ground state volume per atom. Calculations of such descriptors are computationally efficient, and therefore, these descriptors provide a new route for quickly screening potential substrates from large-scale material pools for high-performance interfacial thermal management of high-electron mobility transistor devices. [ABSTRACT FROM AUTHOR]

Considering the strong built-in electric field caused by polarization in wurtzite nitride heterostructures, the energy levels and wave functions of the two-dimensional electrons in AlyGa1−yN back-barrier Al2O3/InxAl1−xN/GaN heterostructures are calculated using the finite element difference method. The dispersion relations and electrostatic potentials of optical phonons are obtained by the transfer matrix method. The electron mobility under optical phonon scattering is studied based on the theory of Lei–Ting force balance equation. The influences of AlyGa1−yN back barrier, ternary mixed crystal effect, and size effect are also analyzed by comparing with the Al2O3/InxAl1−xN/GaN heterostructure without back barrier. It is found that the introduction of a back barrier can attenuate the effect of gate-insulating layer, but enhance the effect of barrier and channel layers on electronic states. Then, the interaction between electrons and optical phonons is weakened, so the electron mobility in AlyGa1−yN back-barrier Al2O3/InxAl1−xN/GaN heterostructure is higher than that in Al2O3/InxAl1−xN/GaN structure under the same conditions. It is also found that using a thinner AlyGa1−yN film with y = 0.25 as the back-barrier layer of Al2O3/InxAl1−xN/GaN heterostructure is more conducive to improving 2DEG mobility. These conclusions can provide references for the preparation of InxAl1−xN/GaN heterojunction devices. [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]

The electron–phonon scattering plays a crucial role in determining the electronic, transport, optical, and thermal properties of materials. Here, we employ a non-Markovian stochastic Schrödinger equation (NMSSE) in momentum space, together with ab initio calculations for energy bands and electron–phonon interactions, to reveal the phonon-mediated ultrafast hole relaxation dynamics in the valence bands of monolayer black phosphorus. Our numerical simulations show that the hole can initially remain in the high-energy valence bands for more than 100 fs due to the weak interband scatterings, and its energy relaxation follows single-exponential decay toward the valence band maximum after scattering into low-energy valence bands. The total relaxation time of holes is much longer than that of electrons in the conduction band. This suggests that harnessing the excess energy of holes may be more effective than that of electrons. Compared to the semiclassical Boltzmann equation based on a hopping model, the NMSSE highlights the persistence of quantum coherence for a long time, which significantly impacts the relaxation dynamics. These findings complement the understanding of hot carrier relaxation dynamics in two-dimensional materials and may offer novel insights into harnessing hole energy in photocatalysis. [ABSTRACT FROM AUTHOR]

Hafnia (HfO2) is a potential candidate for the high-k gate dielectrics in next-generation high-power electronics. Its thermal transport properties, which determine the performance of these related high-power electronics, are critical while rarely investigated. Here, the thermal transport properties of HfO2 in a wide temperature range of 300–2000 K with a phase transition between monoclinic and tetragonal phases at ∼1765 K, are systematically studied based on the temperature-dependent effective potential landscapes with both propagating and coherence thermal transport considered. It is found that the cage-like structure of monoclinic HfO2 results in the avoid crossing in the phonon band structures, which increases the three-phonon scattering largely. Some phonon modes with significant scattering matrix can have relatively larger 3ph and 4ph scattering rates in tetragonal HfO2. Consequently, the thermal conductivity of HfO2 is only 11.95–1.72 W/mK at 300–2000 K. Our results further show that propagating phonon channels dominate the thermal transport in HfO2 and contribute at least 70% to the total thermal conductivity. The rest of the thermal conductivity of HfO2 results from the coherence thermal transport channels, which is caused by the overlap of phonons. Four-phonon scatterings are found to be significant for the thermal transport in tetragonal HfO2, which can result in a thermal conductivity reduction of ∼50%. Our results here advance the understanding of the thermal transport in HfO2, which may benefit the performance optimization of HfO2-related electronics. [ABSTRACT FROM AUTHOR]

The surface optical (SO) phonon replica in photoluminescence (PL) spectroscopy of nitride nanowires (NWs) was theoretically investigated in this study. The dispersive relationships of SO phonon mode in anisotropic wurtzite (WZ) and isotropic zinc-blende (ZB) crystal structure NWs with circular and square cross sections (CSs) were derived within the framework of the dielectric continuum model. Based on the energy and momentum conservation laws, a constraint relationship between the frequency and wave-number was constructed for SO phonon-assisted excitonic PL spectra in the NW structure. By combining the dispersive and constraint relationships, the frequency and wave-number of the SO phonon replica in the PL spectra could be determined. The WZ and ZB crystal structures of nitride semiconductor were considered. The influences of surface factors including the CS shape, dielectric medium, and environment temperature on the frequency and photon wavelength of the band-edge emission of the SO phonon replica were studied in detail. Numerical results reveal that the crystal structure, surface factors, and environment temperature greatly affect the frequency and photon wavelength of the band-edge emission of the SO phonon replica. The calculated results for the photon wavelength agree well with the experimental values of the SO phonon replica in AlN NWs. The results of the dielectric effect obtained here are also supported by previous experimental and theoretical results for nitrides and other semiconductor NWs. The present theoretical scheme and numerical results can be used to analyze and design the SO phonon replica in PL spectra of nanostructures. [ABSTRACT FROM AUTHOR]

The rising temperature due to a self-heating or thermal environment not only degrades the subthreshold performance but also intensifies thermal stress, posing a severe challenge to device performance and reliability design. The thermal stress effects on the ON-state performance of the p-type fin field-effect transistor were previously studied. However, as far as we know, how thermal stress affects its subthreshold conduction remains unclear, which is studied in this manuscript. The impact of thermal stress due to the self-heating of adjacent devices on subthreshold conduction is investigated by solving the quantum transport, thermal conduction, and force balance equations for ballistic transport and dissipative transport with phonon scattering. Then, the thermal stress effects at different ambient temperatures are further discussed and analyzed. The simulation results show that the OFF-state leakage current can be reduced by thermal stress, even up to 9.28% for the (110)/[001] device operating at an ambient temperature of 550 K, and its reduction is the comprehensive result of the thermal stress effects on the band structure, potential profile, carrier distribution, and source-to-drain tunneling. In addition, the thermal stress has no significant effects on subthreshold swing although it can change the magnitude of the subthreshold current. Moreover, the effect of thermal stress on subthreshold conduction is highly dependent on the thermal environment of the device and the crystal orientation of the channel semiconductor material. [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]

For multi-layered 2D materials, although its c-axis has a much lower thermal conductivity than the a-axis, its phonon mean free path has been confirmed to be very long, e.g., in the order of 100s nm at room temperature for multi-layered graphene. An anisotropic specific heat concept has been proposed in the past to explain this very long mean free path. This work carries out detailed atomistic modeling to quantify the anisotropic specific heat concept and reports the discovery of anisotropic temperatures in multi-layered 2D materials under ultrafast surface heating. Extremely fast c-phonon energy transport is discovered, and the non-Fourier effect is observed for both a-phonons and c-phonons. The energy coupling factor between these two modes of phonons is determined to be in the order of 1016 W K−1 m−3, with the specific number depending on the structure location. The anisotropic temperature concept is also quantitatively confirmed based on the lattice Boltzmann method simulation. The anisotropic temperature concept does not violate the physics that temperature is a scalar; rather, it is developed to distinguish the temperatures of phonons that travel in different directions. This concept is universally applicable to other 2D materials to describe the heat conduction in the in-plane and out-of-plane directions that feature different interatomic bonds. [ABSTRACT FROM AUTHOR]

This paper investigates a systematic theoretical study of the lattice dynamical, electronic, and thermoelectric transport properties of PbTe, which is subjected to hydrostatic pressure and uniaxial stress along the [100], [110], and [111] directions, by using first-principles calculations. Our study demonstrates that stress is an effective tool for regulating thermoelectric properties in materials, and different types of stress affect these properties through distinct mechanisms. Specifically, under hydrostatic pressure and uniaxial stress along the [100] direction, PbTe exhibits similar electronic behavior but different lattice dynamical properties. Bandgap closure and reopening are observed under hydrostatic pressure and uniaxial stress along the [100] direction, whereas only the gap opposite trends are seen for uniaxial stress along the [110] and [111] directions. Under uniaxial stress along the [100] direction, phonon lifetimes decrease, leading to low thermal conductivity. In contrast, under hydrostatic pressure and uniaxial stress in the [110] and [111] directions, phonon lifetimes are enhanced due to a weakening of the anharmonic effect. Our findings provide a comprehensive understanding of the implication of different stress types on the thermoelectric properties of PbTe. [ABSTRACT FROM AUTHOR]

Recently, Chiloyan et al. [Appl. Phys. Lett. 116, 163102 (2020)] have reported that phonon transport could exceed bulk heat conduction if low-frequency phonons with long mean free path (MFP) remain in the nonthermal regime in silicon. To gain a better understanding of their findings, we investigated the effects of temperature-induced anharmonicity on both Landau–Rumer damping and Akhiezer damping, including polarization. To do this, we follow a rigorous procedure for calculating the Akhiezer model and use phonon kinetic theory based on the Boltzmann transport equation. Consequently, we find that in the Akhiezer regime, the longitudinal acoustic phonon modes (LA) are strongly suppressed by phonon anharmonicity compared to the transverse acoustic phonon modes. Therefore, the low-frequency phonons with a long MFP of LA can help to exceed bulk heat conduction if they remain in the regime of nonthermal phonon transport where there are no appreciable scatterings with other phonons. It is also shown that Akhiezer damping eliminates thermal conductivity by 16.8% at 500 K, which is higher than the observed reduction (12.6%) at 300 K in silicon, uncovering a novel regime where the Akhiezer damping, previously deemed insignificant in the thermal conduction of bulk silicon, becomes crucial. [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]

The delafossite semiconductor CuInO 2 has shown great potential in transparent electronics for its bipolar dopability. However, little is known about the limiting factors about its carrier mobility, which impedes its further development. Applying a b i n i t i o Boltzmann transport formalism, here we calculate the intrinsic, phonon-limited carrier mobility of CuInO 2 and study its carrier–phonon coupling mechanisms. The calculated room-temperature electron and hole mobilities along the in-plane direction are μ e = 97.6 cm 2 V − 1 s − 1 and μ h = 1.4 cm 2 V − 1 s − 1 , respectively. We find that the electron mobility is limited by the combination of acoustic phonons and polar longitudinal optical (LO) phonons, while the hole mobility is mainly limited by carrier–acoustic phonon scattering. We further show that the electron effective mass and bandgap of CuInO 2 can be tuned through strain engineering for improved carrier transport properties. Our work uncovers the underlying factors that govern the intrinsic carrier mobility of the transparent bipolar semiconductor CuInO 2 and sheds light on the design and exploration of bipolar conducting transparent conductive oxides (TCOs) based on delafossite semiconductors. [ABSTRACT FROM AUTHOR]

Tetrahedrite, Cu12Sb4S13, an eco-friendly thermoelectric material with earth-abundant and low-cost constituents, has garnered global interest. This study investigated the thermoelectric characteristics of Ag-added tetrahedrites, which were prepared through solid-state synthesis and subsequently hot-pressed. The minimal temperature dependence of the transport coefficient indicated the presence of strong electron–phonon coupling in the samples. It was observed that Ag addition in tetrahedrite could successfully scatter acoustic phonons, reducing the lattice thermal conductivity while minimally affecting the power factor. The enhanced anharmonicity induced by Ag addition is the primary cause of reduced lattice thermal conductivity. Raman spectroscopy data showed that Ag addition could weaken the Sb–S bond, further supporting the previous argument. Consequently, the lattice thermal conductivity was lowered to ∼0.27 W m−1 K−1 and obtained for the composition Ag0.025 added Cu11.975Sb4S13. A relatively high power factor of ∼1.3 mW m−1 K−2 was obtained for the same composition. Owing to the lowest total thermal conductivity ∼1.09 W m−1 K−1, the sample with composition Ag0.025 added Cu11.975Sb4S13 showed the highest thermoelectric figure of merit of 0.87 at 738 K. [ABSTRACT FROM AUTHOR]

In this work, we calculated the lattice thermal conductivity of monolayer InTe by means of phonon Boltzmann transport theory with first-principles calculated inter-atomic force constants. The higher-order phonon anharmonicity was found to play a strong impact on thermal transport in InTe. With the involvement of the phonon–phonon scattering process up to the fourth-order, the in-plane lattice thermal conductivity of monolayer InTe is 5.1 W m−1 K−1 at room temperature, which is 35% of that considering only third-order force constants. Furthermore, strain was found to be an effective way to manipulate the thermal transport in InTe, which reduces to one half when applying 5% in-plane tensile strain. The strain adjustment is due to the decreases in the phonon group velocity as well as the increase in the phonon scattering rates. These findings can enrich thermal transport properties of group-III monochalcogenides and benefit the material design of thermoelectrics and thermal management electronic devices. [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]

Precise positioning of single site-controlled inverted pyramidal InGaAs quantum dots (QDs) at the antinode of a GaAs photonic crystal cavity with nanometer-scale accuracy holds unique advantages compared to self-assembled QDs and offers great promise for practical on-chip photonic quantum information processing. However, the strong coupling regime in this geometry has not yet been achieved due to the low cavity Q-factor based on the (111)B-oriented membrane structures. Here, we reveal the onset of phonon-mediated coherent exciton–photon interaction on our tailored single site-controlled InGaAs QD–photonic crystal cavity. Our results present the Rabi-like oscillation of luminescence intensity between excitonic and photonic components correlated with their energy splitting pronounced at small detuning. Such Rabi-like oscillation is well reproduced by modeling the coherent exchange of the exciton-photon population. The modeling further reveals an oscillatory two-time covariance at QD-cavity resonance, which indicates that the system operates at the onset of the strong coupling regime. Moreover, by using the cavity mode as a probe of the virtual state of the QD induced by phonon scattering, it reveals an increase in phonon scattering rates near the QD–cavity resonance and asymmetric phonon emission and absorption rate of even around 50 K. [ABSTRACT FROM AUTHOR]

The thermal conductivity of group IV elements—germanium, silicon, and diamond—is described in order to demonstrate various important and interesting aspects of the mechanism of phonon heat transfer in single-crystalline semiconductors and dielectrics. The measured temperature dependence of thermal conductivity κ (T) for these materials reveals different phonon scattering processes that determine thermal conductivity. In addition to the intrinsic processes of phonon–phonon scattering, scattering by isotopes, dopants, free electrons, sample surfaces, the effects of phonon focusing, irradiation with high-energy particles, and phonon hydrodynamics are discussed. [ABSTRACT FROM AUTHOR]

In this study, the ungated AlGaN/GaN devices with special mesa structures were designed. The hypothesis of effective width expansion was tested experimentally by using ungated samples with different sizes, and an empirical expression for calculating the conduction channel effective width was given according to the experimental results. Finally, split-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with different gate lengths were prepared, and the channel electron mobility of split-gate samples was calculated by using the empirical expression and polarization Coulomb field (PCF) scattering theoretical model. The results showed that, for split-gate AlGaN/GaN HFETs, with the increase in the gate length, the total amount of additional polarization charges underneath the gate increases, resulting in the enhancement of the PCF scattering intensity. [ABSTRACT FROM AUTHOR]

The material dependence of phonon-polariton-based in-plane thermal conductance is investigated by examining systems composed of air and several wurtzite and zinc-blende crystals. Phonon-polariton-based thermal conductance varies by over an order of magnitude (∼ 0.5 – 60 nW/K), which is similar to the variation observed in the materials corresponding to bulk thermal conductivity. Regardless of the material, phonon-polaritons exhibit similar thermal conductance to that of phonons when layers become ultrathin (∼ 10 nm), suggesting the generality of the effect at these length-scales. A figure of merit is proposed to explain the large variation of in-plane polariton thermal conductance that is composed entirely of easily predicted and measured optical phonon energies and lifetimes. Using this figure of merit, in-plane phonon-polariton thermal conductance enlarges with increases in (1) optical phonon energies, (2) splitting between transverse and longitudinal mode pairs, and (3) phonon lifetimes. [ABSTRACT FROM AUTHOR]

Monolayer HfSe2, in the family of transition metal dichalcogenides (TMDCs), is a potential thermoelectric candidate due to its low thermal conductivity. While its mobility remains low as in other 2D TMDCs is inconceivable for electronic and thermoelectric applications. Earlier theoretical attempts have failed to give justification for the orders of low experimental mobility obtained for monolayer HfSe2. We calculate the carrier mobility in the framework of the density functional perturbation theory in conjunction with the Boltzmann transport equation and correctly ascertain the experimental value. We also calculate the carrier mobility with the previously employed method, the deformation potential (DP) model, to figure out the reason for its failure. We found that it is the strong electron-optical phonon interaction that is causing the low mobility. As the DP model does not account for the optical phonons, it overestimates the relaxation time by an order of two and also underestimates the temperature dependence of mobility. A strong polar type interaction is evidenced as a manifestation of a discontinuity in the first derivative of the optical-phonons at the K and Γ points as well as a dispersive optical phonon at the K point. We also included the spin–orbit coupling which leads to an energy splitting of ∼330 meV and significantly affects mobility and scattering rates. [ABSTRACT FROM AUTHOR]

The lattice mismatch, defect, and weak interlayer coupling severely constrain the practical application of van der Waals heterojunctions (vdWHs) in the field of optoelectronic devices. Here, we introduced the 2D ferroelectric (FE) material AgBiP2Se6 to construct defect-free, low lattice-mismatched AgBiP2Se6/MoSe2 heterojunctions with different polariton directions (I, II, III). The AgBiP2Se6 layer can provide an excellent FE electric field to enhance the interlayer coupling and stiffness. The larger interlay stiffness reduces the probability of electron–phonon scattering and then results in significant carrier mobility (∼0.5 × 104 cm2 V−1 s−1) for configurations I and II. Phase transition of FE to paraelectric AgBiP2Se6 in the AgBiP2Se6/MoSe2 heterojunctions can be achieved under specific biaxial strain, which can effectively regulate the electronic structure. Applying the strain and electric field can regulate the bandgap and band alignment of configurations I and II. The photoelectric conversion efficiency of configuration I can reach as high as 20.54% under 2% biaxial strain. Furthermore, configuration II holds a nearly free electron state near the Fermi level under an electric field, which can act as a favorable electron transport channel. A design to strengthen interlayer coupling in the FE-based AgBiP2Se6/MoSe2 heterojunction has been proposed, and it can provide a new way to break through the traditional bottleneck in the development of optoelectronic devices. [ABSTRACT FROM AUTHOR]

This study provides experimental evidence of charged interface states limited scattering in III-nitride heterostructures. Temperature-dependent Hall measurements and temperature-dependent ID–VG measurements indicate a significant influence of the charged interface states on the electron mobility in different AlGaN/GaN heterostructures where the characteristic of the interface is controlled by modulating the growth conditions. Charged interface states at the AlGaN/GaN heterointerface lead to electron scattering as the distance between the centroid of the two-dimensional electron gas and the interface decreases with increasing electron density. It is observed that a component of experimental Hall mobility, which ranges between 9.2 × 103 and 3.4 × 104 cm2/V s among the three samples, obtained after adding all the scattering events using Matthiessen's rule cannot be explained completely by considering all the conventional scattering mechanisms such as phonon–phonon scattering, interface roughness scattering, and dislocation density scattering. An in-depth analysis reveals a significant scattering of channel electrons by the charged states at the GaN/AlN/AlGaN interface. Furthermore, the estimated interface states from the temperature-dependent subthreshold slopes conducted on the fabricated high electron mobility transistors are in good agreement with the charged interface states extracted from the temperature-dependent Hall measurements. A good understanding on this new scattering mechanism in the GaN heterostructure may help in designing high-performance III-nitride devices in the future. [ABSTRACT FROM AUTHOR]

Surface Phonon Polaritons (SPhPs) as an evanescent electromagnetic surface wave supports long propagation which readily surpasses the mean free path of classical heat carriers, e.g., phonons and electrons in solids. SPhPs have emerged as a promising candidate to dominate heat transfer in thin films. Polaritonic heat transfer has two distinct advantages: superior thermal conduction and a wide range of manipulation. Here, we study the upper limit of the thermal conductivity mediated by long-range polaritons in asymmetric surrounding media, where its surface effect overwhelms the volumetric one. The thin film structure strengthens the interactions of two surface waves at the top and bottom surfaces, and the asymmetric surrounding media makes an evanescent surface wave to further penetrate the free space with a higher refractive index, but it requires a fine tuning of asymmetric permittivity of the surrounding media to reach the upper limit of energy transmission efficiency near to the modal cut-off, where the transverse wavevector becomes zero. Both analytical and numerical simulations were introduced to investigate dispersion in asymmetric surrounding media and to model the thermal conductivity of glass thin films. Anomalously high thermal conductivity of 248 W/m-K was achieved with a 50 nm thick SiO2 film in asymmetric surrounding media, yet subtly dissimilar. [ABSTRACT FROM AUTHOR]

Tunable lattice thermal conductivity is a critical issue promoting thermoelectric performance of potential thermoelectric materials. Herein, the effects of biaxial strain and an external electric field on the lattice thermal conductivity of monolayer AlP3 have been investigated systematically by solving the phonon Boltzmann transport equation based on first-principles calculations. Our results imply that the lattice thermal conductivity of monolayer AlP3 can be effectively modulated in a wide range depending on the applied in-plane biaxial tensile strains or out-of-plane external electric fields. Once the biaxial tensile strain is applied to the monolayer, the lattice thermal conductivity exhibits an up-and-down behavior with an increase in the tensile strain. The maximum thermal conductivity is obtained at a tensile strain of 6%, and a surprising peak value of 13.8 times higher than that of the pristine monolayer can be achieved, whereas the lattice thermal conductivity of monolayer AlP3 is able to be further suppressed by applying an external electric field, and a minimum value of about 47.5% of the pristine one can be observed by utilizing an electric field of 0.06 eV/Å. Thus, this work highlights that biaxial strain combined with an external electric field can provide effective ways to realize robust thermal management for 2D triphosphides. [ABSTRACT FROM AUTHOR]

Thermoelectric transport properties of Janus monolayers M2P2S3Se3 (M = Zn and Cd) are investigated by the first-principles based transport theory. The Zn2P2S3Se3 and Cd2P2S3Se3 monolayers are indirect-gap semiconductors. The high polarizability of M-Se/S bonds in the MS3Se3 distorted octahedrons leads to anharmonic phonon behavior, which produces an intrinsic lattice thermal conductivity (κl) as low as 1.06 and 1.99 W m−1 K−1 at 300 K for Zn2P2S3Se3 and Cd2P2S3Se3 monolayers, respectively. The lower κl of the Zn2P2S3Se3 monolayer is mainly attributed to more pronounced flat modes of the phonon dispersion in a frequency range of 1–1.7 THz caused by the softer Zn–Se/S bonds. The polar optical phonon scattering of carriers surprisingly plays a dominant role in carrier transport of both the monolayers, which greatly suppresses the electrical conductivity and thereby the power factor by about an order of magnitude. The predicted figure of merit (zT) increases monotonically with the temperature at the optimal carrier density, and at the operating temperature of 1200 K, it reaches an optimal value of 0.86 at an optimal electron density of ∼ 1.5 × 1 0 13 cm−2 for the n-type Zn2P2S3Se3 monolayer and 0.30 at an optimal electron density of ∼ 7 × 1 0 12 cm−2 for the n-type Cd2P2S3Se3 monolayer. [ABSTRACT FROM AUTHOR]

Polycrystalline thin films are widely used for devices and energy-related applications, such as power electronics, solar cells, and thermal management of devices. In many cases, large-scale crystallization during thin-film growth is challenging, so columnar grains are often found in metal and semiconductor thin films. These rough columnar grain boundaries may also have different phonon specularities from that for typically smoother top/bottom film surfaces. A simple analytical model to separately treat these boundaries and interfaces for phonon scattering is currently unavailable, although the in-plane thermal transport is critical to heat spreading within thin-film devices. In this paper, we extend the effective medium formulation from three-dimensional polycrystalline bulk materials to columnar-grained thin films. The model predictions agree well with those given by frequency-dependent phonon Monte Carlo simulations, considering varied phonon specularity at top/bottom film surfaces and grain-boundary phonon transmissivity. The analytical model is further used to analyze the existing data on polycrystalline ZnO thin films with columnar grains. [ABSTRACT FROM AUTHOR]

Twisted bilayer graphene (tBLG) is an intriguing low-dimensional material due to the possible modulation of electronic and thermal properties and a subject of intense research, both for its fundamental physics as well as for its potential in technological applications. Here, the thermoelectric properties of bilayer graphene are investigated for twist angles of 0 ° and 20 °. The thermoelectric properties are calculated using density functional theory, molecular dynamics, and Boltzmann transport theory. An increase in the power factor is observed for 20 ° tBLG due to an increase in the Seebeck coefficient by 2.2 times at 700 K. The thermal conductivity for 20 ° tBLG is reduced by 20% and 22% for 325 and 700 K, respectively, as compared to BLG. Consequently, an overall ∼ 3 times enhancement of a thermoelectric figure of merit (ZT) for 20 ° tBLG compared to BLG at 700 K is obtained. A strong effect of boundary scattering on thermal transport is observed. However, for electron transport, it is negligible for 20 ° tBLG. Due to this combined effect, an increase of 194 times in ZT is obtained at a ribbon width of L = 10 nm and T = 700 K for 20 ° tBLG. This indicates that 20 ° twisted bilayer graphene could be an efficient thermoelectric power generator and can be a suitable material for carbon-based technology and devices. [ABSTRACT FROM AUTHOR]

We address the electron-spin-phonon coupling in an effective model Hamiltonian for DNA to assess its role in spin transfer involved in the Chiral-Induced Spin Selectivity (CISS) effect. The envelope function approach is used to describe semiclassical electron transfer in a tight-binding model of DNA at half filling in the presence of intrinsic spin–orbit coupling. Spin-phonon coupling arises from the orbital-configuration dependence of the spin–orbit interaction. We find spin-phonon coupling only for the acoustic modes, while the optical modes exhibit electron–phonon interaction without coupling to spin. We derive an effective Hamiltonian whose eigenstates carry spin currents that are protected by spin-inactive stretching optical modes. As optical phonons interact more strongly than acoustic phonons, side buckling and tilting optical base modes will be more strongly associated with decoherence, which allows for the two terminal spin filtering effects found in CISS. [ABSTRACT FROM AUTHOR]

In the present work, we demonstrate the ability of a technique based on Monte Carlo resolution of the Boltzmann transport equation associated to the Green–Kubo autocorrelation of the phonon heat flux to predict, at thermal equilibrium, the thermal conductivity tensor of nanoporous structures. This methodology, which is derived from a former work [D. Lacroix, M. Isaiev, G. Pernot, Phys. Rev. B 104, 165 202 (2021)] developed in the case of bulk systems, is used to predict thermal transport properties of Si porous matrices and Si phononic membranes at room temperature. A broad range of porosities and different pore network organizations are considered. Our results are compared to available experimental data and former modeling techniques. In addition, analytical models based on the phonon mean free path are detailed and compared to numerical simulations. [ABSTRACT FROM AUTHOR]

Thermal conductivity and power factor are key factors in evaluating heat transfer performance and designing thermoelectric conversion devices. To search for materials with ultralow thermal conductivity and a high power factor, we proposed a set of universal statistical interaction descriptors (SIDs) and developed accurate machine learning models for the prediction of thermoelectric properties. For lattice thermal conductivity prediction, the SID-based model achieved the state-of-the-art results with an average absolute error of 1.76 W m−1 K−1. The well-performing models predicted that hypervalent triiodides XI3 (X = Rb, Cs) have ultralow thermal conductivities and high power factors. Combining first-principles calculations, the self-consistent phonon theory, and the Boltzmann transport equation, we obtained the anharmonic lattice thermal conductivities of 0.10 and 0.13 W m−1 K−1 for CsI3 and RbI3 in the c-axis direction at 300 K, respectively. Further studies show that the ultralow thermal conductivity of XI3 arises from the competition of vibrations between alkali metal atoms and halogen atoms. In addition, at 700 K, the thermoelectric figure of merit ZT values of CsI3 and RbI3 are 4.10 and 1.52, respectively, at the optimal hole doping level, which indicates hypervalent triiodides are potential high performance thermoelectric materials. [ABSTRACT FROM AUTHOR]

Tailoring the specific properties to practical applications by structural modification is of vital importance for the envisioned development of two-dimensional ferroelectric materials. Herein, a comprehensive investigation on the effects of single doping on the ferroelectric properties and electronic transport in a monolayer of α-In2Se3 was carried out via the combination of first-principles density functional theory calculations and electron–phonon coupling simulations. Our results show that single-doping in In2Se3 can reduce effective mass of carriers and thereby enhance the high carrier mobility potential of the material. Moreover, the ferroelectric phonon mode in single-doped In2Se2X features a lower scattering rate, associating with the single-doping atom, and indicates reduced hindrance to carrier transport during ferroelectric switching. Compared to pristine In2Se3, the obtained smaller ferroelectric barriers (<1 eV) of single-doped ones promote more promising ferroelectricity from the analysis of the ferroelectric soft mode. Interestingly, the observed variations in ferroelectric behaviors resulting from doping of different elements highlight the significance of single-doping in modifying the ferroelectric properties of monolayers. Furthermore, strain engineering results reveal that single doping obviously affects the dependence of gap on strains: linear relationship for doping ones and nonlinearity for pristine one. Our study provides valuable insights into achieving higher carrier mobility in these critical materials. [ABSTRACT FROM AUTHOR]

Following recent molecular dynamic simulations [M. Dinpajooh and A. Nitzan, J. Chem. Phys. 153, 164903 (2020)], we theoretically analyze how the phonon heat transport along a single polymer chain may be affected by varying the chain configuration. We suggest that phonon scattering controls the phonon heat conduction in strongly compressed (and tangled) chain when multiple random bends act as scattering centers for vibrational phonon modes, which results in the diffusive character of heat transport. As the chain is straightening up, the number of scatterers decreases, and the heat transport acquires nearly ballistic character. To analyze these effects, we introduce a model of a long atomic chain made out of identical atoms where some atoms are put in contact with scatterers and treat the phonon heat transfer through such a system as a multichannel scattering problem. We simulate the changes in the chain configurations by varying the number of the scatterers and mimic a gradual straightening of the chain by a gradual reducing of the number of scatterers attached to the chain atoms. It is demonstrated, in agreement with recently published simulation results, that the phonon thermal conductance shows a threshold-like transition from the limit where nearly all atoms are attached to the scatterers to the opposite limit where the scatterers vanish, which corresponds to a transition from the diffusive to the ballistic phonon transport. [ABSTRACT FROM AUTHOR]

The energy band structure is crucial for solid insulation research. The traditional thermally stimulated current is used mostly for direct trap estimation but has some limitations, which can be overcome by the space charge decay method. With the help of a modified pulsed-electro acoustic system, we perform the test under different low temperatures and obtain a more comprehensive band spectrum including shallow traps and different polarities. This study chooses isotactic polypropylene (PP) to investigate in a temperature range of −120 to 20 °C. Signal processing and recovery are considered. Two basic trap estimation methods are compared and the one based on the modified isothermal relaxation current theory is used here. The electron and hole trap densities near different electrode interfaces are considered in the experiment. In the preliminary results, it is found that the traps tend to move toward a shallow direction with a decrease in temperature. A model of the retrapping process caused by phonon scattering is put forward to explain the results. The retrapping possibility is, therefore, assumed to follow a power relation with temperature. An attempt to rectify the trap depth is performed on such a model by numerical approximation and reaches the result that the retrapping possibility may be dependent upon the second power of temperature. Finally, the broadband energy spectrum is calculated after correction. In the normalized coordinate system, the traps in PP are mainly distributed in a deep range; few shallow traps are found maybe because the space charge density is low. [ABSTRACT FROM AUTHOR]

The article presents a new concept of A 2 B 2 O 7 pyrochlore materials design dedicated to thermal barrier coating based on the simultaneous use of several structural mechanisms responsible for phonon wave scattering. The basis for these analyses is the La 2 O 3 -Gd 2 O 3 -ZrO 2 ternary system, in which a smaller Gd3+ gadolinium atom was introduced into the La 2 Zr 2 O 7 compound in the substitution position A, with the simultaneous introduction of an excess B atom (Zr4+) and its partial localisation in position A. This resulted in the cumulative effect of the difference in mass and radius of rare earth elements cations, with the simultaneous formation of a non-stoichiometric type of lattice and the rattling effect. Initially, this effect was achieved using substitution atoms with a much smaller radius, such as Tm3+ or Yb3+, but without creating non-stoichiometric structures. Such systems were also characterised by a strong tendency to decompose into pyrochlore and fluorite phases. Using a new approach, a phase-stable material with ultra-low thermal conductivity was obtained, with a two-phase structure, which, however, was not formed as a result of the compound's decomposition but is the result of the presence of excess Zr4+ and a shift in the ternary system to the two-phase region. As a result, a material with extremely low thermal conductivity and a course independent of temperature increase was obtained, which suggests the maximum level of crystal lattice defect. In addition, thermal conductivity was effectively reduced by using relatively cheap rare earth oxides, reducing their amount compared to pyrochlore compounds based on lanthanum with ytterbium or thulium. [ABSTRACT FROM AUTHOR]

Nanostructured Ni-substituted Co1−xNixSb3 (0 ≤ x ≤ 0.08) samples were synthesized using a hydrothermal method. X-ray diffraction analysis confirmed the phase purity of the samples, and a secondary phase was observed in samples with higher Ni substitution ratios (x ≥ 0.06). SEM and TEM images reveal the spherical morphology of the samples with monodispersed particles. Different modes of optic phonon vibrations were observed in Raman spectra, and the intensity of the peaks increased with Ni content and slightly decreased in samples with x values ≥0.06. The electrical resistivity of the samples decreased with Ni content up to an x value of 0.06 and slightly increased in Co0.92Ni0.08Sb3 due to secondary phase formation. The positive Seebeck coefficient values of pure CoSb3 confirm that holes were the majority carriers, whereas the negative Seebeck coefficients of Co1−xNixSb3 samples reveal that Ni substitution enhanced electron concentration in the sample. Co0.96Ni0.04Sb3 exhibited a higher power factor compared with other samples due to its low resistivity. Ni substitution in CoSb3 effectively reduced lattice thermal conductivity (κL) as Co0.96Ni0.04Sb3 showed a lower κL of 2.0 W m−1 K−1 than that of the pure sample due to nanostructuring and the rattling effect of Ni. As a result, a figure of merit of 1.72 was achieved at 553 K for the Co0.96Ni0.04Sb3 sample. [ABSTRACT FROM AUTHOR]

The pyrochlore oxide La 2 Hf 2 O 7 (LHO) has attracted extensive attention in thermal management applications. Accurate determination and comprehensive understanding of thermal conductivity (κ) stand as imperative prerequisites for the effective utilization and manipulation of LHO in various applications. In this study, with the assistance of machine learning interatomic potential, we investigate the microscopic mechanisms of phonon thermal transport in LHO crystals. Our findings show that the thermal transport in LHO defies a precise explication through the conventional framework of the Boltzmann transport equation (BTE) due to the emergence of diffusive phonon modes, engendered by strong anharmonicity. Using the state-of-the-art dual-phonon model incorporating both propagating and diffusive modes, we successfully reproduce the experimentally measured results in both magnitude and temperature dependence. It is also shown that the thermal conduction in LHO crystals is dominated by the propagating modes at low temperatures while by the diffusive modes at high temperatures, which synergistically lead to a low κ for LHO crystals. The novel insights into phonon thermal transport provide a stepping stone for regulating the thermal transport properties of LHO crystals. [ABSTRACT FROM AUTHOR]

Porous SiC ceramics with three different pore sizes (∼7, ∼45, and ∼98 μm) and two different porosities (∼54% and ∼63%) were prepared to investigate the effects of pore size on electrical and thermal properties of the porous ceramics. With an increase in pore size from ∼7 to ∼98 μm at the same porosity (∼54%), the electrical and thermal conductivities of the porous ceramics increased from ∼1.01 to ∼2.30 Ω−1·cm−1 and ∼14.3 to ∼26.2 W·m−1K−1, respectively. Similar trends have been obtained in the ceramics with ∼63% porosity. The porous SiC ceramic with larger pore size has smaller pore/strut interface area at the same porosity, which leads to less carrier scattering and phonon‐pore scattering at the pore/strut interfaces, resulting in higher electrical and thermal conductivities, respectively. These results suggest that the electrical and thermal properties of porous SiC ceramics can be successfully tuned by adjusting the pore size and porosity depending on the application. [ABSTRACT FROM AUTHOR]

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are highly promising for waste heat harvesting. Although thermoelectric materials research has expanded over the years, bismuth telluride‐based alloys are still the best for near‐room‐temperature applications. In this work, a ≈38% enhancement of the average ZT (300−473 K) to 1.21 is achieved by mixing Bi0.4Sb1.6Te3 with an emerging thermoelectric material Sb2Si2Te6, which is significantly higher than that of most BiySb2−yTe3‐based composites. This enhancement is facilitated by the unique interface region between the Bi0.4Sb1.6Te3 matrix and Sb2Si2Te6‐based precipitates with an orderly atomic arrangement, which promotes the transport of charge carriers with minimal scattering, overcoming a common factor that is limiting ZT enhancement in such composites. At the same time, high‐density dislocations in the same region can effectively scatter the phonons, decoupling the electron‐phonon transport. This results in a ≈56% enhancement of the thermoelectric quality factor at 373 K, from 0.41 for the pristine sample to 0.64 for the composite sample. A single‐leg device is fabricated with a high efficiency of 5.4% at ΔT = 164 K further demonstrating the efficacy of the Sb2Si2Te6 compositing strategy and the importance of the precipitate‐matrix interface microstructure in improving the performance of materials for relatively low‐temperature applications. [ABSTRACT FROM AUTHOR]

Clean energy advances can drive solid heat storage technology. Because of their high-temperature, corrosion, and excellent thermal shock resistance properties, refractory oxides are widely used as solid heat storage materials. However, their applications are limited owing to their poor heat-transfer performance resulting from their multicomponent, multiphase, and complex nature and thus understanding their heat-transfer mechanism will be crucial for their performance optimization. In this study, the thermal properties and mechanisms of corundum, high-alumina, and magnesia bricks were characterized using heat-transfer theories and models. Macroscopic analysis of the bricks revealed that they are all "internal porosity" materials. By comparing the weighting parameter j , the multiphase high-alumina brick shows poor heat-transfer path patency and the measured thermal conductivity is the lowest at 3.55 W m−1 K−1. Microscopic analysis found that in contrast to the thermal conductivity of other materials, the thermal conductivity of high-alumina bricks was significantly affected by phonon intrinsic scattering. Moreover, corundum and magnesia bricks exhibited a wider range of ultimate heat-transfer performance than the high-alumina brick. Hence, the heat-transfer performance of refractory oxide can be enhanced by increasing their particle contact, reducing the negative effects of microcracks, phases, and impurities, and by reducing the number of defects on phonon heat transfer, thereby providing an optimization scheme for improving the heat-transfer performance of the materials. [ABSTRACT FROM AUTHOR]

1% vanadium doped ZnO powders have been successfully synthesized via two different preparation techniques viz., glycine assisted combustion method and mechanical attrition method to understand their impact on the structure-property relation. X-ray Diffraction study validates the Wurtzite phase formation of ZnO with hexagonal structure. X-ray diffraction peak discloses the marked role of size and strain component. Evidently, the shift of lattice strain with the preparation techniques from compressive to tensile nature show the importance of structure property relations in the ZnO compounds. SEM micrographs depict the formation of densely populated nanograins with networking like structures which is characteristics of combustion synthesized product whereas as top down approach exhibits the presence of coarse grains amidst the broken sea of fine particles. Selected area electron diffraction (SAED) images display ring and bright circular spot pattern with changes in method of preparation, which further clarifies the distinct nature of investigated ZnO:V particles. Vibrational spectral study gives clear evidence on the structural disorder as deduced from the changes associated with longitudinal optical mode phonon scattering. Interestingly, the UV spectral study divulges the increment in optical band gap as well as a strong visible light absorption (about 30 %) for milled ZnO:V over their chemically prepared counterparts. Photoluminescence spectra of milled V doped ZnO upheld the structural disorder taken place with the suppression of Near Band Edge (NBE) transition and the shifting of broad asymmetric defect emission band towards the lower wavelength regime. Impedance study enlightens the crucial role of synthesis method with 5 times enhanced grain boundary resistance along with suppression of grain contribution in the milled samples. Magnetic study highlights the remarkable distinction of preparation route which alters the ferromagnetic ordering in the combustion synthesized samples to conventional diamagnetic signature of the host ZnO in the milled samples. [ABSTRACT FROM AUTHOR]

In this study, we conducted calculations to determine the thermal conductivity of silicon nanowires with various shapes and cross-sectional designs using the Monte Carlo method combined with Green–Kubo heat flow autocorrelation. This computational approach is known for its reliability in predicting thermal properties of complex nanostructured devices. We specifically examined nanowires with circular or rectangular sections combined to complex serpentine and fishbone modulations. Our findings indicate that the nanowire's geometry significantly impacts the phonon mean free path and thermal transport. More specifically, patterns with serpentine modulations exhibit a pronounced reduction in thermal conductivity attributable to enhanced phonon boundary scatterings. Our simulation results are compared with the available experimental data, highlighting the potential of our method for structural optimization in thermal management applications, particularly in devices like thermoelectric converters. [ABSTRACT FROM AUTHOR]

Due to their amorphous‐like ultralow lattice thermal conductivity both below and above the superionic phase transition, crystalline Cu‐ and Ag‐based superionic argyrodites have garnered widespread attention as promising thermoelectric materials. However, despite their intriguing properties, quantifying their lattice thermal conductivities and a comprehensive understanding of the microscopic dynamics that drive these extraordinary properties are still lacking. Here, an integrated experimental and theoretical approach is adopted to reveal the presence of Cu‐dominated low‐energy optical phonons in the Cu‐based argyrodite Cu7PS6. These phonons yield strong acoustic‐optical phonon scattering through avoided crossing, enabling ultralow lattice thermal conductivity. The Unified Theory of thermal transport is employed to analyze heat conduction and successfully reproduce the experimental amorphous‐like ultralow lattice thermal conductivities, ranging from 0.43 to 0.58 W m−1 K−1, in the temperature range of 100–400 K. The study reveals that the amorphous‐like ultralow thermal conductivity of Cu7PS6 stems from a significantly dominant wave‐like conduction mechanism. Moreover, the simulations elucidate the wave‐like thermal transport mainly results from the contribution of Cu‐associated low‐energy overlapping optical phonons. This study highlights the crucial role of low‐energy and overlapping optical modes in facilitating amorphous‐like ultralow thermal transport, providing a thorough understanding of the underlying complex dynamics of argyrodites. [ABSTRACT FROM AUTHOR]

Strain provides an additional mechanism in tuning the thermal/electrical properties of nanostructures and therefore has triggered lots of interest in recent years. However, experimental research about the strain effect on the thermal conductivity of nanowires is still limited, especially in the low-temperature range, which is important in understanding the physics of strain-induced regulation in thermal conductivity. Here, we present thermal transport measurements of bent silicon carbide nanowires at temperatures ranging from 20 to 300 K. Reduction in thermal conductivity compared to their straight counterparts is observed. More specifically, the relative change is up to 55% at 20 K and descends with temperature, which is due to the inhomogeneous strain-induced phonon scattering. This study will deepen the understanding of thermal properties in nanostructures with strain. [ABSTRACT FROM AUTHOR]

SnSe materials have garnered significant interest due to the intrinsic low thermal conductivity. However, its limited electrical conductivity hinders their practical implementation. This study achieved a high ZT value of 1.40 at 773 K in the samples of n-type polycrystalline SnSe 0.95 + x wt% SmCl 3 + y wt% ErCl 3 (x = 0, 0.75, 1.0, 1.25, and 1.5; y = 0, 0.0675, 0.125, and 0.25) since (SmCl 3 , ErCl 3) co-doping decoupled the electrical-thermal transport properties. First, the released electron carrier concentration due to SmCl 3 doping enhanced the electrical transport properties. The SnSe 0.95 + 1.25 wt% SmCl 3 sample exhibited a high power factor of 563 μWm−1K−2 at 773 K. Then, the lattice thermal conductivity markedly decreased with further ErCl 3 doping due to the high-density dislocations and coherent boundary, resulting in effective the phonon scattering. Additionally, the special microtopography was conducive to the decreased lattice thermal conductivity. Therefore, the SnSe 0.95 + 1.25 wt% SmCl 3 + 0.125 wt% ErCl 3 sample achieved a low lattice thermal conductivity value of 0.28 Wm−1K−1 at 773 K. Simultaneously, the (SmCl 3 , ErCl 3) co-doped samples maintained the electrical transport properties than the SmCl 3 -doped samples. Consequently, the SnSe 0.95 + 1.25 wt% SmCl 3 + 0.125 wt% ErCl 3 sample obtained a peak ZT value of 1.40 at 773 K and a competitive average ZT value of 0.37 from 323 to 773 K, and a theoretically calculated conversion efficiency reached 7.2%, which can be applied in the deep space for power generation. This strategy can be utilized to optimize the thermoelectric performance of other thermoelectrical material systems. [ABSTRACT FROM AUTHOR]

High-entropy rare-earth niobates have garnered significant attention in the field of thermal barrier coatings (TBCs) due to their ultra-low thermal conductivity. However, the exceptional thermal properties of fluorite-phase high-entropy rare-earth niobates and the outstanding mechanical properties of monoclinic-phase high-entropy rare-earth niobates cannot be simultaneously harnessed. To address this limitation, this paper investigates and synthesizes five single-phase high-entropy rare-earth niobates: (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5) 3 NbO 7 (RE = Ce, Sm, Eu, Dy, Lu), and five composite-phase high-entropy rare-earth niobates with a molar ratio of (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5)NbO 7 : (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5)NbO 4 = 1:2 (RE = Ce, Sm, Eu, Dy, Lu). The objective is to fabricate niobate coatings that exhibit both outstanding thermal and mechanical properties. The test results reveal that the intrinsic thermal conductivity of the composite-phase samples stands at a mere 1.46 W m−1·K−1 at 1000 °C. Comparatively, the experimental thermal conductivity of the single-phase samples slightly surpasses that of the composite-phase at 1000 °C, registering a minimum of 1.57 W m−1·K−1. Importantly, both values adhere to the TBC standard. The mechanisms behind the low thermal conductivity of the composite-phase and single-phase samples are elucidated by the phonon scattering mechanism and the Cahill model, respectively. Furthermore, the thermal expansion coefficient of the composite-phase samples exceeds that of the single-phase samples overall, and they demonstrate exceptional mechanical properties, offering nearly twice the fracture toughness of the single-phase samples while maintaining high hardness. Experimental evidence shows that composite-phase niobates exhibit enhanced thermodynamic properties compared to single-phase niobates. This work serves as a valuable reference for the utilization of high-entropy rare-earth niobates in the realm of thermal barrier coatings. [ABSTRACT FROM AUTHOR]

This study investigates spatially resolved phonon-mediated thermal transport across nano-sized grains and grain boundaries (GBs) in 3C-SiC using molecular dynamics (MD) simulations. This investigation involves controlling the complete range of inter-grain misorientation tilt angles (θ = 0°–90°) and nanoscale grain sizes (d = 2.18–130.77 nm). The grain boundary energy and interfacial thermal transport are found to be highly θ -sensitive and asymmetric with respect to θ = 45° due to the low symmetry associated with two interpenetrating diatomic SiC fcc lattices. When adjacent grains are tilted at θ = 14.25°, the interfacial heat conduction is highly suppressed compared to other θ values, especially for larger grains. The most stable atomic configuration of the GB region associated with the minimal GB energy results in the highest suppression of heat conduction across the GB interface. Spatially resolved thermal anisotropy reveals a strong GB-mediated nanoscale hydrodynamic phonon Poiseuille effect when heat flows parallel to the GB planes, as shown by our perturbed MD study. With the reduction of d , the intra-grain and inter-GB thermal conductivities decrease due to the enhanced phonon scattering from interfaces, but the difference between these conductivities becomes negligible for the heat flow normal to the GB planes. It is envisioned that nanoscale spatially resolved control of thermal energy will provide useful guidance to engineer nanocrystalline ceramics with tunable interfacial thermal properties. [ABSTRACT FROM AUTHOR]