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The type-II Dirac fermions are characterized by a tilted Dirac cone and sustain exotic quantum transport phenomena. Here, we show the emergence of a type-II Dirac nodal loop in the SrVO 3 / SrTiO 3 quantum well structure by density functional theory calculations and symmetry arguments. When the spin–orbit coupling (SOC) is neglected, the unavoidable crossing between two t 2 g bands gives rise to a nodal loop in the Brillouin zone, which is protected by the mirror symmetry. Such a nodal loop is fully gapped by including SOC, which results in a large spin Hall effect due to large spin Berry curvatures. Our findings add the unexplored functionality to oxide quantum wells and offer a practical platform to explore the interplay between topological fermions and spin transport phenomena. [ABSTRACT FROM AUTHOR]

Motivated by the booming development of spintronics based on quantum dot systems, we employed the standard nonequilibrium Green's function theory to derive the transport formula and the heat generation formula of a quantum dot coupled to a substrate and study the relation between spin current and its accompanying heating or cooling. Our results demonstrate that (i) a thermal bias combined with Zeeman splitting can generate steady spin current in a limited dot level range, while a rotating magnetic field can generate time-average spin current in a global range and pure spin current can induce more heat generation than non-pure spin current; (ii) magnetic resonance of spin current can also effectively enhance heat generation; (iii) appropriate environmental temperature in conjunction with a thermal bias makes cooling, while increasing the frequency of the rotating magnetic can easily give rise to the transition from cooling to heating; and (iv) enhancing the coupling between quantum dots and substrates can effectively reduce heat generation while maintaining the fundamental properties of pure spin current. [ABSTRACT FROM AUTHOR]

The enhancement of thermoelectric properties in CoS b 3 through atom substitution and hydrostatic pressure application is a promising avenue. Herein, we conducted a comprehensive theoretical investigation into the structural, electronic, and thermoelectric characteristics of CoS b 3 − x A x (A = Ge, Se, Te; x = 0.125, 0.250) using density functional theory coupled with Boltzmann transport theory. By subjecting the system to pressures ranging from 0 to 20 GPa and substituting Sb atoms, we evaluated the enthalpy of formation to predict stability, with CoS b 2.875 T e 0.125 exhibiting superior stability under 20 GPa. The bandgap of doped compounds is direct, ranging from 0.33 to 0.56 eV along the Γ point, and was calculated to elucidate electronic properties. Additionally, employing the Slack model, we computed lattice thermal conductivity based on elastic constants to provide a comprehensive analysis of thermoelectric efficiency. Remarkably, our study not only highlights the effect of hydrostatic pressure on structural and electronic properties but also reveals a beneficial impact on increasing Z T values to 2.77 for CoS b 2.750 G e 0.250 at 800 K and 20 GPa, indicating predominantly p -type behavior. [ABSTRACT FROM AUTHOR]

2D Janus Rashba semiconductors, which break both the mirror symmetry in the crystal structure and the spin degeneracy in the energy band, provide a promising platform to optimize thermoelectric performance. Herein, we use first-principles and Boltzmann transport theory to investigate the electron and phonon transport properties for Janus semiconductors of monolayer Si2AsSb and Si2SbBi. The strong Rashba spin-splitting is found in both Janus monolayers especially for Si2SbBi, which decreases the bandgaps and makes the valence bands more dispersive, resulting in decreased p-type Seebeck coefficient and increased p-type electrical conductivity. The lattice thermal conductivities of both monolayers are not low due to the weak phonon anharmonicity, strong chemical bonding, and long phonon relaxation time. The low lattice thermal conductivity of Si2SbBi than Si2AsSb mainly originates from the low phonon group velocity. Both monolayers exhibit better thermoelectric performance in n-type than in p-type. The competition among Seebeck coefficient, electrical conductivity, and electronic thermal conductivity makes the difference of optimal thermoelectric figure of merits in n-type without and with Rashba spin–orbit coupling slight for Si2AsSb, but it is significant for Si2SbBi. Within Rashba spin–orbit coupling, the optimal figure of merits at 700 K reach 0.65 and 0.59 for Si2AsSb and Si2SbBi, respectively, which indicate the potential thermoelectric applications, and will stimulate the broad study on thermoelectric properties of 2D Janus Rashba semiconductors. [ABSTRACT FROM AUTHOR]

Among the numerous advantages of the high-power impulse magnetron sputtering (HiPIMS) technique, the most important is the enhanced ionization degree of sputtered species contributing to the film growth. Consequently, the quality of deposited thin films is highly improved. Still, the optimization process is challenging due to the complexity associated with the intricate transport of the sputtered species, ionized or neutrals. The scarce knowledge available on the spatial distribution of these species when operating a HiPIMS discharge makes the quantitative prediction of any deposition feature particularly difficult. In this paper, we discuss the influence of experimentally controllable quantities, such as gas pressure and target current density, on the transport of sputtered titanium in non-reactive (argon) HiPIMS, namely, on the behavior of metal atoms and metal ion fluxes intercepting the substrate. Systematic quantitative measurements were performed in a diameter normal plane on a circular planar target. Hence, the 2D spatial distribution of the ionized flux fraction (IFF) and the total flux of titanium sputtered particles (deposition rate) are evaluated by biasing a quartz crystal microbalance equipped with an electron magnetic filter. The wide range of parameters we examined allows us to predict and optimize the flux of sputtered species based on complete mapping of the IFF of sputtered particles. [ABSTRACT FROM AUTHOR]

The deviations of cryogenic collector current–voltage characteristics of SiGe heterojunction bipolar transistors (HBTs) from ideal drift-diffusion theory have been a topic of investigation for many years. Recent work indicates that direct tunneling across the base contributes to the non-ideal current in highly scaled devices. However, cryogenic discrepancies have been observed even in older-generation devices for which direct tunneling is negligible, suggesting that another mechanism may also contribute. Although similar non-ideal current–voltage characteristics have been observed in Schottky junctions and were attributed to a spatially inhomogeneous junction potential, this explanation has not been considered for SiGe HBTs. Here, we experimentally investigate this hypothesis by characterizing the collector current ideality factor and built-in potential of a SiGe HBT vs temperature using a cryogenic probe station. The temperature dependence of the ideality factor and the relation between the built-in potential as measured by capacitance–voltage and current–voltage characteristics are in good qualitative agreement with the predictions of a theory of electrical transport across a spatially inhomogeneous junction. These observations suggest that inhomogeneities in the base–emitter junction potential may contribute to the cryogenic non-idealities. This work helps to identify the physical mechanisms limiting the cryogenic microwave noise performance of SiGe HBTs. [ABSTRACT FROM AUTHOR]

The mesmerizing physical properties of two-dimensional (2D) nanomaterials have resulted in their enormous potential for high-power solar energy conversion and long-term stability devices. The present work systematically investigated the fundamental properties of monolayered 2D group-IV–V materials using a combined approach of first-principles calculations and Boltzmann transport theory, specifically the thermoelectric and optical properties, for the first time. The structural and lattice dynamics analysis disclosed the energetic, dynamical, and mechanical stabilities of 17 out of 25 considered materials. The electronic properties' calculation shows that all the stable materials exhibit a semiconducting nature. Additionally, the energy–momentum relation in a few systems reveals the quartic Mexican-hat-like dispersion in their valence band edges. Owing to the larger depth of Mexican-hat dispersion and the larger height of density step function modes, the hole carrier mobilities of SnN (761.43 m2/Vs), GeN (422.80 m2/Vs), and SiN (108.90 m2/Vs) materials were found to be significantly higher than their electron mobilities at room temperature. The achieved high Seebeck coefficient and electrical conductivity at room temperature result in excellent thermoelectric power factors for GeN (3190 mW/mK2), SiN (1473 mW/mK2), and CAs (774 mW/mK2) materials, manifesting their potential for thermoelectric devices. Further, the calculated optical and solar parameters demonstrate an exceptionally high value (27.25%) of theoretical limits of power conversion efficiency for the SnBi material, making it a suitable candidate as a light-absorbing material in solar cell devices. The present theoretical work filters out the potential 2D group-IV–V materials for solar and heat energy-harvesting devices. [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]

The topological antiferromagnet γ -FeMn is a fascinating material for studying unconventional magnetic properties and topological transport phenomena though high-quality γ -FeMn has been obtained on the Cu substrate for magnetic research. However, the difficulty of growing single-crystalline γ -FeMn films on insulating substrates has prevented experiments from exactly verifying many theoretical predictions on their transport properties. Ultrathin Cu buffer films have been considered for solving this problem but failed because the crystal quality and thickness of Cu films achieved in previous works do not satisfy the growth buffer criteria for γ -FeMn. In this work, the growth of 5-nm-thick Cu films on SrTiO 3 (001) substrates was studied. It was found that single-crystalline ultrathin Cu films with roughness as low as 1 nm are obtained on the insulating substrate. We subsequently obtained high crystalline quality γ -FeMn films with smooth surface and verified their antiferromagnetism. Finally, two aspects of the influence of Cu films on adjacent films have been analyzed. Our results facilitate the experimental exploration of the fascinating properties of γ -FeMn. [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]

Here, we report the synthesis of single crystalline magnetic Weyl semimetal Co3Sn2S2. The synthesized crystal is characterized through various tools, viz. x-ray diffraction, field emission electron microscopy, and x-ray photoelectron spectroscopy. A clear ferromagnetic transition is observed in magnetization and heat capacity at around 175 K, which is further verified through electrical transport measurements. Hysteresis is observed in ρ–T measurements in a cooling and a warming cycle, showing the presence of the first order phase transition and charge ordering in the synthesized sample. Synthesized Co3Sn2S2 exhibits a high magnetoresistance of around 230% at 2 K. The transport phenomenon in synthesized Co3Sn2S2 appears to have contributions from topological surface states at low temperature below, say, 70 K, and above that, the same is found to be strongly dependent on its bulk magnetic state. Magnetoconductivity data at low fields of up to ±1 T (Tesla) are fitted with the Hikami Larkin Nagaoka model, which shows the presence of a weak antilocalization (WAL) effect in the synthesized Co3Sn2S2 crystal at low temperatures below 30 K. Angle dependent magnetotransport measurements confirm that the observed WAL is the topological surface state dominated phenomenon. [ABSTRACT FROM AUTHOR]

Here, we report the synthesis of single crystalline magnetic Weyl semimetal Co3Sn2S2. The synthesized crystal is characterized through various tools, viz. x-ray diffraction, field emission electron microscopy, and x-ray photoelectron spectroscopy. A clear ferromagnetic transition is observed in magnetization and heat capacity at around 175 K, which is further verified through electrical transport measurements. Hysteresis is observed in ρ–T measurements in a cooling and a warming cycle, showing the presence of the first order phase transition and charge ordering in the synthesized sample. Synthesized Co3Sn2S2 exhibits a high magnetoresistance of around 230% at 2 K. The transport phenomenon in synthesized Co3Sn2S2 appears to have contributions from topological surface states at low temperature below, say, 70 K, and above that, the same is found to be strongly dependent on its bulk magnetic state. Magnetoconductivity data at low fields of up to ±1 T (Tesla) are fitted with the Hikami Larkin Nagaoka model, which shows the presence of a weak antilocalization (WAL) effect in the synthesized Co3Sn2S2 crystal at low temperatures below 30 K. Angle dependent magnetotransport measurements confirm that the observed WAL is the topological surface state dominated phenomenon. [ABSTRACT FROM AUTHOR]

We have theoretically explored the electronic and thermoelectric properties of the C2P4 monolayer with the interface of density functional theory and semi-classical transport theory. Our calculation shows a high Seebeck coefficient and low electronic thermal conductivity in the vicinity of zero chemical potential (μ = 0), resulting in a good power factor (PF) and a high figure of merit (ZT). More particularly, the electronic figure of merit (ZTe) exhibits two high peak values around μ = 0 due to the significant contribution of thermoelectric parameters. Furthermore, ZTe decreases by increasing the temperature, giving a peak value of 0.98 in the negative chemical potential (μ), whereas, for μ > 0, the peak value increases slightly with temperature. Additionally, the ZTe peak value is robust against ±10% of uni- and biaxial strains at room temperature. To make our calculation more realistic, we add phonon contributions to the thermal conductivity in pristine C2P4 and calculate the total ZT. We have found that phonon contribution dominates at low temperatures, and the ZT peak is reduced to 0.78. These optimal thermoelectric parameters of the C2P4 monolayer may be suitable for demonstrating the feasibility of a good thermoelectric material. [ABSTRACT FROM AUTHOR]

The development of glassy nanocomposites, xFe-(1−x) (0.5 V 2 O 5 –0.4 CdO–0.1 ZnO) is particularly important not only for exploring their microstructures using x-ray diffraction, FT-IR, and UV–Vis techniques but also for exploring their electrical conduction mechanism in terms of hopping of small polarons. The presence of various nanophases, such as ZnO, CdO, Cd9.5Zn0.5, ZnV, and Zn3V2O8, have been identified and the size of estimated nanocrystallites is found to decrease with more incorporation of the Fe content in the compositions. As the value of lattice strain increases with the increase of the Fe content in the compositions, the present system becomes more and more unstable, which may be favorable for better electrical transport phenomena via the polaron hopping process. Electrical conductivity of the system has been analyzed using modified correlated barrier hopping model, Almond–West formalism, and the alternating-current conductivity scaling. Experimental data reveal that both optical photon and acoustical phonon transitions are responsible for the entire electrical conduction process. Polaron hopping is expected to be of percolation type, which has been validated from an estimated range of frequency exponents. All experimental data have been used to frame a schematic model to explore the conduction mechanism inside the present glassy system. [ABSTRACT FROM AUTHOR]

For more than 30 years, scanning thermal microscopy (SThM) has been used for thermal imaging and quantitative thermal measurements. It has proven its usefulness for investigations of the thermal transport in nanoscale devices and structures. However, because of the complexity of the heat transport phenomena, a quantitative analysis of the experimental results remains a non-trivial task. This paper shows the SThM state-of-art, beginning with the equipment and methodology of the measurements, through its theoretical background and ending with selected examples of its applications. Every section concludes with considerations on the future development of the experimental technique. Nowadays, SThM has passed from its childhood into maturity from the development stage to its effective practical use in materials research. [ABSTRACT FROM AUTHOR]

Here, we have presented the results of the detailed theoretical study of thermoelectric properties of two Rashba compounds KSnSb and KSnBi using first principles calculations based on density functional theory and Boltzmann transport theory taking spin–orbit coupling (SOC) into account. As these compounds have layered-type crystal structures, their transport parameters are found to be highly anisotropic. For KSnBi (KSnSb), the calculated lattice thermal conductivity κ l along its crystallographic c axis is found to have ultralow value of 0.49 W m−1 K−1 (0.78 W m−1 K−1) even at room temperature, whereas almost twofold larger value of κ l is estimated along its crystallographic a axis. However, large values of other transport parameters like electrical conductivity σ and thermopower S desirable for a high power factor (S 2 σ) are found along the a axis of these compounds. For KSnSb, the optimum a axis Z T = 2.6 can be reachable for an electron concentration of 3.3 × 1019 cm−3 and at a temperature of 800 K. Comparable value of optimum a axis Z T = 2.5 is also noted for KSnBi despite its strong susceptibility to bipolar conduction. Both these non-centrosymmetric compounds exhibit SOC-driven Rashba spin splitting of electronic bands, which affects both thermopower and electrical conductivity of these compounds. However, such Rashba spin splitting induced change in thermopower is almost negated by the concomitant change in electrical conductivity, resulting in no appreciable impact on power factor and hence Z T of the studied compounds. [ABSTRACT FROM AUTHOR]

We systematically studied the intrinsic electron transport of the recently fabricated two-dimensional (2D) monolayer MoSi2N4 and WSi2N4 (belong to the MA2Z4 family, where M is a transition metal; A is Si or Ge; and Z is N, P, or As) by using the Boltzmann transport theory with the scattering rates calculated from first-principles calculations. It is found that both materials have moderate room temperature electron mobility of 87 cm2/V s for MoSi2N4 and 119 cm2/V s for WSi2N4. Our detailed analysis shows that their electron mobility difference is attributed to the different average effective mass. Moreover, by comparing studies with monolayer MoS2 and WS2, we reveal that the polar optical phonon pattern of MoSi2N4 and WSi2N4 is governed by the antiparallel silicon–nitrogen bond oscillation, different from MoS2 and WS2, in which is governed by the antiparallel transition metal–sulfur bond oscillation. This feature is arising from the large electronegativity difference between N and Si. Nitrogen has the fourth largest electronegativity, so it always tends to attract plenty of electrons from Si and thus Si has a large Born effective charge, thereby resulting in strong Fröhlich interaction. We further found that all the 2D semiconducting MA2Z4 have large Born effective charges, thereby indicating a generally strong electron–phonon coupling strength. [ABSTRACT FROM AUTHOR]

The McKelvey–Shockley (McK–S) flux method is a semi-classical transport theory that captures ballistic and non-equilibrium effects and can treat carrier flow from the nano-scale to the macro-scale. This work introduces a revised formulation of the McK–S flux equations for electron transport, in order to resolve the energy dependence of the fluxes, capture the effect of electric field, and include acoustic/optical phonon scattering. This updated McK–S formalism is validated by simulating electron transport across a finite-length semiconductor under the influence of a constant electric field under varying conditions, from ballistic to diffusive and from near-equilibrium to non-equilibrium, and benchmarked against solutions of the Boltzmann transport equation (BTE). The McK–S results display good agreement with those of the BTE, including the directed fluxes and heating profiles, with the electron density showing larger differences when far from equilibrium. Compared to other more rigorous techniques, the McK–S flux method is physically intuitive and computationally efficient and, thus, well suited to treat systems that are complex and/or span multiple length scales. [ABSTRACT FROM AUTHOR]

We systematically studied the intrinsic electron transport of the recently fabricated two-dimensional (2D) monolayer MoSi2N4 and WSi2N4 (belong to the MA2Z4 family, where M is a transition metal; A is Si or Ge; and Z is N, P, or As) by using the Boltzmann transport theory with the scattering rates calculated from first-principles calculations. It is found that both materials have moderate room temperature electron mobility of 87 cm2/V s for MoSi2N4 and 119 cm2/V s for WSi2N4. Our detailed analysis shows that their electron mobility difference is attributed to the different average effective mass. Moreover, by comparing studies with monolayer MoS2 and WS2, we reveal that the polar optical phonon pattern of MoSi2N4 and WSi2N4 is governed by the antiparallel silicon–nitrogen bond oscillation, different from MoS2 and WS2, in which is governed by the antiparallel transition metal–sulfur bond oscillation. This feature is arising from the large electronegativity difference between N and Si. Nitrogen has the fourth largest electronegativity, so it always tends to attract plenty of electrons from Si and thus Si has a large Born effective charge, thereby resulting in strong Fröhlich interaction. We further found that all the 2D semiconducting MA2Z4 have large Born effective charges, thereby indicating a generally strong electron–phonon coupling strength. [ABSTRACT FROM AUTHOR]

We devise a simple heuristic method for obtaining the relaxation time and electrical conductivity dependence on the temperature of carriers in 2D semiconductors. The approach is computationally straightforward. It relies on the BoltzTraP algorithm (from the Boltzmann transport equation), on a direct fitting procedure, and on a proper scaling at a reference temperature. The approach provides a good estimate for the figure of merit ZT, an important characterization of thermoelectricity in materials. We employ our approach to analyze promising 2D systems for thermoelectric applications, namely, nitrogenated holey graphene (NHG), boron-doped NHG, and tungsten disulfide 2D- WS 2. In all these cases, our results agree with computationally expensive calculations available in the literature at a fraction of the computing time. [ABSTRACT FROM AUTHOR]

The recent hydrodynamic phonon transport theory for graphitic materials has been supported by the measurements of the second sound at temperatures up to about 100 K. When boundary scattering becomes comparable to momentum-conserving normal phonon scattering processes that are responsible for phonon hydrodynamics, Poiseuille phonon flow phenomena can emerge to give rise to unique size-dependent thermal conductivity in thin graphite. Here, we examine the thickness range for the Poiseuille phonon flow to become observable in thin graphite with the use of both deviational Monte Carlo simulation of the Peierls-Boltzmann transport equation and four-probe thermal transport measurements. As the basal-plane thermal conductivity calculated by prior first-principles theories saturates to the graphite value when the thickness is increased to five graphene layers, the phonon dispersion of graphite is used in the current calculations of thin graphite of micrometer thickness and a 23-layer thick ultrathin graphite (UTG) sample. The calculations show that diffuse surface scattering by surface defects can lead to Poiseuille phonon flow at 50 K in thin graphite with the thickness close to several micrometers but not in the 65 μm thin graphite and 23-layer UTG, where phonon scattering by the top and bottom surfaces become, respectively, much less and more frequent than the normal processes. In addition, the calculation results with the bulk graphite dispersion and diffuse surface scattering show decreased basal-plane thermal conductivity with decreasing thickness, opposite to recent thermocouple measurements of thin graphite samples. In comparison, the calculation results reveal that partially diffuse surface defect scattering can yield the four-probe measurement results of UTG samples, which are prepared here with an improved process to minimize surface contamination. [ABSTRACT FROM AUTHOR]

Phonons and magnons, which are respectively quanta of lattice vibrations and spin dynamics, are both bosonic quasi-particles and constitute two fundamental collective excitations in condensed-matter physics. The fundamental physics of spin-heat coupling via the interactions between magnons and phonons have attracted much attention in recent years among both experimental and theoretical physicists, given its promising applications in the fields of energy, data storage, and spintronics. In this perspective, we highlight the impacts of magnon–phonon interactions on the thermal and magnetic transport properties of various magnetic materials. Several representative applications will also be discussed as the enabling techniques resulting from such interwoven transport phenomena, including metrology development, magnon contributions to thermal transport and storage, and temperature-dependent magnetic dynamics for recording and spintronic applications. [ABSTRACT FROM AUTHOR]

Thermal transport through various nanowires has attracted extensive attention in the past two decades. Nanowires provide an excellent platform to dissect phonon transport physics because one can change the wire size to impose systematically varying boundary conditions that can help to distinguish the contributions of various scattering mechanisms. Moreover, novel confinement phenomena beyond the classical size effect promise opportunities to achieve highly desirable properties. Based on a summary of research progresses in nanowire thermal properties, we discuss more intriguing observations due to the classical size effect, coupling between mechanical and thermal properties, and divergent thermal conductivity as a result of conversion from three-dimensional to one-dimensional phonon transport, showcasing the superdiffusive thermal transport phenomenon. We hope that these discussions could provide a new perspective on further exploring thermal transport in nanowires, which may eventually lead to breakthroughs such as achieving thermal conductivity values higher than that of any known materials. [ABSTRACT FROM AUTHOR]

In this study, by utilizing the first-principles calculation coupled with the Boltzmann transport theory, we comprehensively study the thermoelectric (TE) properties of the Sb2C monolayer. The calculated results show that the Sb2C monolayer owns an inherent ultra-low lattice thermal conductivity of 0.88 W m−1 K−1 at 300 K, which originates from small phonon group velocities, large Grüneisen parameters, and short phonon lifetimes. The Sb2C monolayer also exhibits excellent electrical transport properties mainly due to the degeneration of the bottom conduction bands, which increases the Seebeck coefficient of the n-type doped samples and thus yields a larger power factor. Based on the extremely low lattice thermal conductivity and superior electrical transport performance, a large ZT value of 2.71 for the n-type doped Sb2C monolayer at 700 K is obtained. Our results quantify Sb2C monolayers as promising candidates for building outstanding thermoelectric devices. [ABSTRACT FROM AUTHOR]

Template-assisted selective area growth techniques have gained popularity for their ability to grow epitaxial materials in prefabricated dielectric templates. Confined epitaxial lateral overgrowth (CELO) is one such technique that uses dielectric templates to define the geometry of the grown nanostructures. Two terminal low-temperature magneto-transport measurements were used to determine electronic properties. For doped In0.53Ga0.47As CELO nanostructures, we observe Shubnikov–De Hass oscillations in the longitudinal magnetoresistance and utilize these to estimate effective mass, carrier density, and mobilities. This analysis both reveals the presence of defects in these nanostructures and material variabilities between growth runs. Electron beam lithography and contact deposition for transport measurements were enabled by parasitic growth removal. In the future, this approach can enable other material systems to be explored for confined lateral epitaxy, improve material quality, and investigate a variety of quantum transport phenomenon in such nanoscale devices. [ABSTRACT FROM AUTHOR]

Random alloy fluctuations significantly affect the electronic, optical, and transport properties of (In,Ga)N-based optoelectronic devices. Transport calculations accounting for alloy fluctuations currently use a combination of modified continuum-based models, which neglect to a large extent atomistic effects. In this work, we present a model that bridges the gap between atomistic theory and macroscopic transport models. To do so, we combine atomistic tight-binding theory and continuum-based drift–diffusion solvers, where quantum corrections are included via the localization landscape method. We outline the ingredients of this framework in detail and present first results for uni-polar electron transport in single and multi- (In,Ga)N quantum well systems. Overall, our results reveal that both random alloy fluctuations and quantum corrections significantly affect the current–voltage characteristics of uni-polar electron transport in such devices. However, our investigations indicate that the importance of quantum corrections and random alloy fluctuations can be different for single and multi-quantum well systems. [ABSTRACT FROM AUTHOR]

Transparent conducting Al -doped ZnO films were grown by atomic layer deposition (ALD). Al -doping was introduced by inserting 1 A l 2 O 3 cycle per 28 ZnO cycles. The x-ray photoelectron spectroscopy showed that the density of the Al donors is 2 × 10 21 – 3 × 10 21 cm − 3 , while the Hall-effect measurements showed a ten times lower electron density. This low doping efficiency is a well-known inherent problem of the ALD method, and we wanted to explain its origin. We have found that the electron density is reduced by electron traps at the grain surface; however, the effect was too weak to explain the low doping efficiency. Therefore, the mechanism of the A l 2 O 3 doping was analyzed. We have proposed that each A l 2 O 3 molecule ideally provides two single-electron Al donors accompanied by one Zn vacancy, which acts as a two-electron acceptor. This would cause a perfect compensation; however, the compensation is in reality not perfect, which results in weakly efficient doping. Calculations also showed that each Zn vacancy creates a bound pair with an Al donor. To verify our doping model experimentally, it was inserted into the metallic transport theory and compared with the electron transport measurements. A good agreement was found for a broad range of experimental conditions. In the regime of weak localization, the conductivity showed the temperature dependence σ (T) = a + b T 3 / 4 , which is a signature of weak localization and electron–electron scattering in a 3D dirty metal. [ABSTRACT FROM AUTHOR]

Interfaces are ubiquitous in electronics, photonics, and advanced materials. Interface engineering has become an essential strategy for developing functional materials with low thermal conductivities such as thermoelectric materials and thermal barrier coatings. On the other hand, interfaces are becoming a bottleneck for thermal management in electronic devices. Recent experiments have shown that a fin-like nanostructured interface with a size of 30–100 nm could enhance thermal transport across interfaces. Since phonon mean free paths span from several nanometers to dozens of micrometers, depending on the material, the size of the interface features may significantly affect the phonon transport regime and interface conductance. Here, the Monte Carlo simulation, with ab initio-based phonon properties as input parameters, was developed to study thermal conductance of a fin-like nanostructured interface. Simulated results indicate that the nanofin size (i.e., width, spacing, and height) significantly affects interface thermal conductance. Interface conductance is found to first increase and then decrease with increasing width of the nanofin when its height is 100 nm. This phenomenon is attributed to competition between the enlarged interface area and increased backscattering of transmitted phonons. This study demonstrates the existence of an optimal nanofin size for maximizing interface conductance, which could be important for thermal management of high-power electronics using nanostructured interfaces. [ABSTRACT FROM AUTHOR]

Theoretical modeling of phonon transport process in strongly anharmonic materials at a finite temperature needs to accurately capture the effects of lattice anharmonicity. The anharmonicity of potential energy surface would result in not only strong phonon scatterings but also shifts of phonon frequencies and eigenvectors. In this work, we evaluated the roles of anharmonicity-renormalized phonon eigenvectors in predicting phonon transport properties of anharmonic crystals at high temperatures using molecular dynamics-based normal mode analysis (NMA) methods in both time domain and frequency domain. Using PbTe as a model of strongly anharmonic crystal, we analyzed the numerical challenges to extract phonon lifetimes using NMA methods when phonon eigenvectors deviate from their harmonic values at high temperatures. To solve these issues, we proposed and verified a better fitting strategy, Sum-up Spectrum Fitting Method (SSFM) than the original frequency-domain NMA method. SSFM is to project the total spectrum energy density data of all phonon modes onto an inaccurate (harmonic or quasi-harmonic) eigenvector base and then manually sum up the peaks that belong to the same phonon mode (at the same frequency). The SSFM relaxes the requirement for accurate temperature-dependent eigenvectors, making it robust for analyzing strongly anharmonic crystals at high temperatures. [ABSTRACT FROM AUTHOR]

Several proposals are available to get selective spin transmission through different nano-junctions and in all the cases the regulation is done either by applying a magnetic field or by tuning spin–orbit (SO) coupling. In the present work, we explore a separate scheme where the spin-dependent transport is regulated externally by irradiating a quantum ring that bridges the contact electrodes. This is a new proposal of generating spin selective transmission through a nano-junction, to the best of our knowledge. A high degree of spin polarization along with its phase alteration can be achieved by suitably adjusting the irradiation, circumventing the regulation of magnetic field and/or SO coupling. The effect of irradiation is included through the well-known Floquet-Bloch ansatz, where all the spin-dependent transport phenomena are worked out using Green's function formalism following the Landauer–Büttiker prescription within a tight-binding framework. Precise dependencies of light irradiation, SO coupling, magnetic flux threaded by the ring, interface sensitivity, system temperature, and impurities on spin polarization are critically investigated. Our analysis may give a new platform for spin selective electron transmission and make it applicable to other complex nano-structured materials also. We strongly believe that the present proposal can be examined in a suitable laboratory. [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]

Thermoelectric materials that can work at operating temperatures of T ≥ 900 K are highly desirable since the key thermoelectric factors of most thermoelectric materials degrade at high temperatures. In this work, we investigate the high temperature thermoelectric performance of EuFeO3 using a combination of first-principles methods and semi-classical Boltzmann transport theory. High temperature thermoelectric performance is achieved owing to the presence of corrugated flatbands in the valence band region and extremely flatbands in the conduction band region. The lowest energetic structure of EuFeO3 lies within a G-type antiferromagnetic configuration, and the effect of compressive and tensile strains (−7% to +7%) along the (a, b) axes on thermoelectric performance is systematically analyzed. An extremely high value of the Seebeck coefficient (more than 1000 μV/K) is consistently recorded in the high temperature region between 900 K and 1400 K in this material. Furthermore, electrical conductivities and power factors are high and electronic thermal conductivities are low in the considered range of temperatures. The calculated theoretical minimum lattice conductivity is small, estimated at around 1.47–1.54 W m−1 K−1. A compressive strain of −3% is revealed to be the optimum level of strain for enhancing the key thermoelectric factors. Overall, p-type doping shows better thermoelectric performance than n-type doping in EuFeO3. [ABSTRACT FROM AUTHOR]

We extend the application of the nonlocal theory of Mahan and Claro [Phys. Rev. B 38, 1963 (1988)] to solve the steady-state Boltzmann–Peierls transport equation within the framework of the single mode relaxation time approximation using the modified Debye–Callaway model. We consider the case of a semi-infinite semiconductor (SC) crystal with a boundary condition at its top surface that can be considered reasonably representative of time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) techniques. The approach allows us to obtain three different contributions to the heat flux density current that shed further light on the fundamental role of nonlocality and nonlinearity in heat transport by phonons in SC crystals. Through their intrinsic and implicit shuffling effect of the crystal momentum, phonon–phonon Normal scattering processes play a key role in the onset of thermal conduction as they introduce the temperature Laplacian as a second driving potential force for the heat flux density current in addition to the conventional Fourier's temperature gradient. The developed model suits quite fairly to interpret the frequency behavior of the reduced effective thermal conductivity of SC crystals that is observed in TDTR and FDTR experiments. We obtain an expression of the effective thermal conductivity of the SC crystal that is characterized with a universal spectral suppression function that captures and describes the role, the weight, and the contribution of quasi-ballistic and non-diffusive phonons. The spectral suppression function only depends on the ratio between the phonon mean free path and the thermal penetration depth as defined based on the diffusive Fourier's law. [ABSTRACT FROM AUTHOR]

In this study, the phonon thermal transport in monolayer C 3 N under biaxial strains ranging from 0% to 10% has been investigated using first-principles calculations based on the Boltzmann transport equation. It is found that the thermal conductivity κ of C 3 N shows a nonmonotonic up-and-down behavior in response to tensile strain, and the maximum κ occurs at a strain of 6%. Interestingly, the thermal conductivity of monolayer C 3 N shows a remarkable high strain tunability, as its value at 6% strain is about 13.2 times higher than the value of κ in an unstrained monolayer. A mode-by-mode phonon level analysis shows that a competition between different phonon properties is responsible for such variations in the thermal conductivity. We found that the decrease in group velocity of the transverse acoustic, longitudinal acoustic, and optical modes as well as the increase in the three-phonon phase space of all the acoustic modes tend to reduce the thermal conductivity with strain. However, the group velocity of the z -direction acoustic mode and the Grüneisen parameter of all acoustic modes change in the direction of increasing the phonon lifetime and the thermal conductivity with increasing strain. Upon stretching, the change in the Grüneisen parameter and the phonon lifetime of the acoustic modes is found to be drastically higher than the change in other properties. The competition between these opposite effects leads to the up-and-down behavior of the thermal conductivity in C 3 N. [ABSTRACT FROM AUTHOR]

Recent studies indicated the interesting metal-to-semiconductor transition when layered bulk GeP3 and SnP3 are restricted to the monolayer or bilayer, and the SnP3 monolayer has been predicted to possess high carrier mobility and promising thermoelectric performance. Here, we investigate the biaxial strain effect on the electronic and thermoelectric properties of the SnP3 monolayer. Our first-principles calculations combined with Boltzmann transport theory indicate that the SnP3 monolayer has the "pudding-mold-type" valence band structure, giving rise to a large p-type Seebeck coefficient and a high p-type power factor. The compressive biaxial strain can decrease the energy gap and result in metallicity. In contrast, the tensile biaxial strain increases the energy gap, increases the n-type Seebeck coefficient, and decreases the n-type electrical conductivity. Although the lattice thermal conductivity becomes larger at a tensile biaxial strain due to the increased maximum frequency of the acoustic phonon modes and the increased phonon group velocity, it is still low, e.g., only 4.1 W m−1 K−1, at room temperature with 6% tensile strain. The tensile strain decreases the figure of merit, but the value is still considerable, and it can reach 2.01 for p-type doping at 700 K with 6% tensile strain. Therefore, the SnP3 monolayer is a good thermoelectric material with low lattice thermal conductivity and promising figure of merit even at 6% tensile strain. [ABSTRACT FROM AUTHOR]

Transparent oxide materials, such as CuAlO2, a p-type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. TCO materials are generally low cost and non-toxic. The potential to engineer them through strain and nano-structuring are two promising avenues toward continuously tuning the electronic and thermal properties to achieve high zT values and low $cost/kW h devices. In this work, the strain-dependent lattice thermal conductivity of 2H CuAlO2 is computed by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. While the average bulk thermal conductivity is around 32 W/(m K) at room temperature, it drops to between 5 and 15 W/(m K) for typical experimental grain sizes from 3 nm to 30 nm. We find that strain can offer both an increase as well as a decrease in the thermal conductivity as expected; however, the overall inclusion of small grain sizes dictates the potential for low thermal conductivity in this material. [ABSTRACT FROM AUTHOR]

Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under different strain values, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from the first principles method, and it is found to decrease the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on the strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates. Under a compressive strain state, the decreases of lattice thermal conductivity are 19% and 23% for in-plane and cross-plane conditions, respectively, comparable with those under a free state. However, the decreases are small under the tensile strain state, because of the decreased electron-phonon scattering rates and increased phonon anharmonicity. [ABSTRACT FROM AUTHOR]

In this paper, the thermoelectric (TE) properties of Janus MXY monolayers (M = Pd, Pt; X, Y = S, Se, Te) are systematically studied using first principles and the Boltzmann transport theory. The thermal conductivity (k), Seebeck coefficient (S), power factor (PF), and TE figure of merit (ZT) are calculated accurately for various carrier concentrations. The lattice thermal conductivities of these six materials sequentially decrease in the order PtSSe, PtSTe, PtSeTe, PdSSe, PdSTe, and PdSeTe. PdSeTe and PtSeTe monolayers have a high ZT close to one at 300 K. In addition, we predicted the TE properties at high temperatures and found that the maximum ZT (2.54) is achieved for a monolayer of PtSeTe at 900 K. The structural and electronic properties of these six Janus transition-metal dichalcogenide (TMD) monolayers were systematically studied from first principles. Our results show that all six materials are semiconductors with bandgaps between 0.77 eV and 2.26 eV at the Heyd-Scuseria-Ernzerhof (HSE06) level. The present work indicates that the Janus MXY TMD monolayers (M = Pd, Pt; X, Y = S, Se, Te) are potentially TE materials. [ABSTRACT FROM AUTHOR]

The thermal conductivities along the basal and hexagonal directions of α -quartz silica, the low-temperature form of crystalline SiO2, are predicted from the solution of the Boltzmann transport equation combined with the van Beest, Kramer, and van Santen potential for the temperature up to 900 K and the pressure as high as 4 GPa. The thermal conductivities at atmospheric pressure, which show a negative and nonlinear dependence on temperature, are in reasonable agreement with the experimental data. The influence of pressure on thermal conductivity is positive and linear. The pressure (P) and temperature (T) dependences of the thermal conductivity (λ) in basal and hexagonal directions are fitted to a function of the form λ = (b + c P) T a. The thermal conductivity, influenced by temperature and pressure, is analyzed based on phonon properties, including spectral thermal conductivity, dispersion relation, phonon density of states, phonon lifetime, and phonon probability density distribution function. [ABSTRACT FROM AUTHOR]

Monolayer silicon phosphide (SiP) and germanium phosphide (GeP) are predicted to exhibit fascinating electronic characters with highly stable structures, which indicate their potential applications in future electronic technologies. By using first-principles calculations combined with the semiclassical Boltzmann transport theory, we systematically investigate the thermoelectric properties of monolayer SiP and GeP. High anisotropy is observed in both phonon and electron transport of monolayer SiP and GeP where the thermal and electrical conductivity along the xx crystal direction are smaller than those along the yy crystal direction. The lattice thermal conductivity (room temperature) along the xx crystal direction is about 11.05 W/mK for monolayer SiP and 9.48 W/mK for monolayer GeP. However, monolayer SiP and GeP possess almost isotropic Seebeck coefficient, and the room temperature values with both n- and p-type doping approach 2.9 mV/K and 2.5 mV/K, respectively. Based on the electron relaxation time estimated from the deformation potential theory, the maximum thermoelectric figure of merit of monolayer SiP and GeP with n-type doping approach 0.76 and 0.78 at 700 K, respectively. The results presented in this work shed light upon the thermoelectric performance of monolayer SiP and GeP and foreshow their potential applications in thermoelectric devices. [ABSTRACT FROM AUTHOR]

In this work, the electronic structure and carrier mobility of single-walled boron arsenide nanotubes (BAsNTs) have been systematically studied by using Boltzmann transport equation with the relaxation time approximation. We found that the ionic characteristic of B–As bond results in the dipole shells in the optimized BAsNTs. It is predicted that both zigzag BAs nanotubes (ZNTs) and armchair BAs nanotubes are semiconductors, and the strong σ*–π* hybridization in small ZNTs leads to a rapid drop of bandgap with a decrease of radius. Interestingly, as the size (n) of the NTs decreases, the hole mobility (μh) of ZNTs has an evident 3p (p is an integer) oscillation but electron mobility (μe) basically falls down, which falls even faster when the radius gets smaller. Comparing the carrier mobility between BAsNTs and its unzipping nanoribbons, we found that rolling BAs nanoribbons (BAsNRs) into BAsNTs would increase the μe but decrease the μh. The different behavior of the carrier mobility in BAsNRs and BAsNTs results from their distinct bond features of edge states, which vary with different widths (for BAsNRs) or radii (for BAsNTs). [ABSTRACT FROM AUTHOR]

Recent electron microscopy experiments have used focused electron beams as nanoscale heat sources or thermometers to enable high spatial resolution studies of heat transfer in nanostructures. When the electron beam radius is smaller than the heat carrier mean free path, Fourier's law will underpredict the temperature rise due to electron beam-induced heating, motivating the development of subcontinuum models to interpret thermal electron microscopy measurements. Here, electron beam-induced heating of nonmetallic samples is modeled by applying a recently developed general solution of the governing Boltzmann transport equation (BTE) under the relaxation time approximation. The analytical BTE solution describes thermal phonon transport from a time-periodically heated cylindrical region in a homogeneous infinite medium. The BTE results show that ballistic phonon effects in this radial heat spreading scenario are more conveniently represented using a ballistic thermal resistance rather than an effective thermal conductivity. Calculations of this ballistic resistance for three semiconductors (Si, GaAs, and 3C-SiC) show that ballistic effects dominate the total thermal resistance to radial heat flow for typical STEM or SEM beam radii (<10 nm), indicating that the ballistic resistance could potentially be measured using thin-film electron beam heating experiments. However, combining the BTE solution with recent calorimetric measurements shows that the magnitude of the temperature rise remains negligibly small (<1 K) under typical electron microscopy conditions, even when considering these ballistic effects. These BTE modeling results can be used to quantify electron beam-induced heating or to design experiments probing ballistic phonon transport using electron beam heat sources. [ABSTRACT FROM AUTHOR]

Due to the compatibility with the well-developed Si-based semiconductor technology, the properties of silicene and silicene-based materials have attracted tremendous attention. Among them, the thermal conductivity (TC) is of special importance for electronic devices. However, unlike graphene, the poor quality of empirical potentials hinders the reliable evaluation of TC for silicene using molecular dynamics (MD). Here, we present a Gaussian approximation potential (GAP) for silicene based on ab initio derived training data. The potential can precisely describe the geometries, mechanical properties, as well as phonon dispersion of free-standing sheet, outperforming any other empirical ones. Using sinusoidal approach-to-equilibrium MD simulations based on the GAP potential, the TC of silicene is found to be 32.4 ± 2.9 W / m K at room temperature. Importantly, our result achieves a good agreement with Boltzmann transport equation (BTE) based first-principles predictions (∼ 30 W / m K), such that the TC value of silicene is confirmed via both MD and BTE; thus, we prove that the accuracy of machine learning potentials, like GAP, can enable a faithful prediction of TC at a density functional theory (DFT) level. [ABSTRACT FROM AUTHOR]

In this work, we present a concurrent atomistic-continuum (CAC) method for modeling and simulation of crystalline materials. The CAC formulation extends the Irving-Kirkwood procedure for deriving transport equations and fluxes for homogenized molecular systems to that for polyatomic crystalline materials by employing a concurrent two-level description of the structure and dynamics of crystals. A multiscale representation of conservation laws is formulated, as a direct consequence of Newton's second law, in terms of instantaneous expressions of unit cell-averaged quantities using the mathematical theory of distributions. Finite element (FE) solutions to the conservation equations, as well as fluxes and temperature in the FE representation, are introduced, followed by numerical examples of the atomic-scale structure of interfaces, dynamics of fracture and dislocations, and phonon thermal transport across grain boundaries. In addition to providing a methodology for concurrent multiscale simulation of transport processes under a single theoretical framework, the CAC formulation can also be used to compute fluxes (stress and heat flux) in atomistic and coarse-grained atomistic simulations. [ABSTRACT FROM AUTHOR]

Recently, two-dimensional transition metal carbides and nitrides (MXenes) have gained significant attention in electronics and electrochemical energy conversion and storage devices where the heat production significantly affects the safety and performance of these devices. In this paper, we have studied the thermal transport in monolayer T i 3 C 2 T z , the first and most studied MXene, using density functional theory and the phonon Boltzmann transport equation and quantified the effect of surface termination (bare, fluorine, and oxygen) on its lattice thermal conductivity. We found that the thermal conductivity of fluorine-terminated T i 3 C 2 T z (108 W/m K) is approximately one order of magnitude higher than its oxygen-terminated counterpart (11 W/m K). Our calculations reveal that the increased thermal conductivity for the fluorine-terminated structure is due to its enhanced specific heat and group velocity and diminished scattering rate of phonons. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of two-dimensional (2D) Dirac materials are calculated within linearized Boltzmann transport theory and relaxation time approximation. We find that the gapless 2D Dirac material exhibits poorer thermoelectric performance than the gapped one. This fact arises due to the cancelation effect from electron-hole contributions to the transport quantities. Opening the bandgap lifts this cancelation effect. Furthermore, there exists an optimal bandgap for maximizing figure of merit (Z T) in the gapped 2D Dirac material. The optimal bandgap ranges from 6 k B T to 18 k B T , where k B is the Boltzmann constant and T is the operating temperature in kelvin. This result indicates the importance of having narrow gaps to achieve the best thermoelectrics in 2D systems. Larger maximum Z T s can also be obtained by suppressing the lattice thermal conductivity. In the most ideal case where the lattice thermal conductivity is very small, the maximum Z T in the gapped 2D Dirac material can be many times the Z T of commercial thermoelectric materials. [ABSTRACT FROM AUTHOR]

Efficiently decreasing the lattice thermal conductivity, κ L , is one of the main concerns in the field of thermoelectrics (TE). Herein, we theoretically investigate κ L for single-crystal and polycrystalline CrSi 2 using first-principles and the phonon Boltzmann transport equation. Though CrSi 2 is known as a potential TE material because of its reasonable power factor, controlling its κ L remains as a challenge to be solved. In this study, we discuss how to decrease κ L efficiently on the basis of the calculation. The phonon band structure and density of states are computed via harmonic calculation. In addition, the achievable lowest lattice thermal conductivity, κ L 0 , and cumulative lattice thermal conductivity, κ cum , are estimated using the Cahill model and anharmonic calculation, respectively. We predict κ L 0 for CrSi 2 to be around 2.2 W m − 1 K − 1 at 650 K, which suggests that CrSi 2 is a potential TE material with high z T over 0.39 at 650 K. The phonon mean-free path dependence of κ cum indicates that the critical crystallite size for decreasing κ L for polycrystalline CrSi 2 is 70 nm at 600 K. In addition, it is revealed that the crystallite size should be as small as 7 nm to decrease κ L to half. These calculational findings offer useful insights into how to control κ L for CrSi 2. [ABSTRACT FROM AUTHOR]

Thermal transport properties of graphene nanoribbons (GNRs) are investigated using phonon transport studies. Ribbons of varying widths are considered to investigate the transition of key thermal properties with width. The lattice structure of the ribbons is fully resolved, and phonon transport is modeled by accounting for all three-phonon scattering processes using a solution of the linearized Boltzmann transport equation. A 3× reduction in intrinsic thermal conductivity is observed compared to bulk graphene arising from increased strength of three-phonon scattering due to the additional nondegenerate phonon modes that appear due to the finite edges of confined nanoribbons. Strong dependence of thermal conductivity on ribbon width is also observed. The underlying mechanisms for thermal conductivity reduction and width dependence are presented by analyzing frequency- and polarization-resolved phonon transport. The additional scattering pathways present in 1D GNRs lead to a significant reduction in the thermal conductivity of otherwise highly conducting flexural phonons in bulk graphene. In contrast, confinement-induced changes to the density of states, specific heat or group velocity, and the subsequent impact on lattice thermal conductivity are found to be relatively small. [ABSTRACT FROM AUTHOR]

Bismuth compounds have been playing an important role in thermoelectric (TE) applications. To match the high figure of merit (ZT) of n-type Bi-based TE materials for constructing effective TE devices, searching for p-type Bi-based TE materials with high ZT is highly desirable. Inspired by the successful exfoliation of BiOBr atomic layers [Wu et al., Angew. Chem. 130(28), 8855 (2018)], we systematically study the TE properties of single-layer BiOBr by using density functional theory and semiclassical Boltzmann transport theory, and we find that the p-type doped single-layer BiOBr exhibits a peak ZT value of 1.84 at 800 K, exceeding the highest value ever achieved among bulk p-type Bi-based TE materials (ZT = 1.4) [Poudel et al., Science 320(5876), 634 (2008)]. [ABSTRACT FROM AUTHOR]

Thermal conductivity integral models including a suppression function to account for boundary scattering have had considerable success in explaining and predicting the thermal conductivity of nanostructures. However, the suppression function is analytically defined only for some simple structures, e.g., thin films and nanowires. For arbitrary nanostructures, Monte Carlo (MC)-based methods have been developed to calculate the suppression function. Here, we focus on two main types of MC-based methods: path sampling methods and ray tracing simulations. For the path sampling method, a more computationally efficient sampling algorithm is proposed based on the analytical solution of the average distance phonons can travel before a collision. The physical meaning of the path sampling method is rigorously given for the first time by comparing to the analytical solution of the Boltzmann Transport Equation for symmetric structures. Several limitations of the path sampling method are discussed based on assumptions in the derivation. Ray tracing simulations are well defined when a converged boundary mean free path (MFP) can be found. However, convergence is not guaranteed for arbitrary structures. More generally, we propose a modified formula to approximate the full-range suppression function with a characteristic length, which is determined by fitting to the calculated suppression function at selected MFPs. Ultimately, the accuracy of each calculated suppression function is evaluated by comparing the calculated thermal conductivity accumulation function for nanostructures including thin films, nanowires, and anisotropic modulated nanostructures. Our results provide guidance for selecting the appropriate techniques for calculating the suppression function and predicting the thermal conductivity of nanostructures. [ABSTRACT FROM AUTHOR]