Record Low Thermal Conductivity of Polycrystalline Si Nanowire: Breaking the Casimir Limit by Severe Suppression of Propagons

Molecular dynamics work on thermal conductivity of SiGe nanotubes

CONTEXT

SiGe nanotubes (SiGeNTs) hold significant promise for applications in nanosolar cells, optoelectronic systems, and interconnects, where thermal conductivity is critical to performance. This study investigates the effects of length, diameter, temperature, and axial strain on the thermal conductivity of armchair and zigzag SiGeNTs through molecular dynamics simulations. Results indicate that thermal conductivity increases with sample length due to ballistic heat transport and decreases with temperature as phonon scattering intensifies. Axial strain transitions from compression to tension enhance phonon propagation, improving conductivity. Chirality affects conductivity, with zigzag SiGeNTs consistently outperforming armchair structures, while diameter exhibits negligible impact.

METHODS

Non-equilibrium molecular dynamics simulations were conducted using the LAMMPS package with the Tersoff potential to model Si-Ge interactions. Thermal conductivity was computed via Fourier's law, with the system divided into regions for controlled heat input and dissipation. Lengths, diameters, temperatures (100-500 K), and axial strains (- 6% to + 9%) were varied systematically. Phonon spectrum analysis was performed using Fourier transforms of velocity autocorrelation functions to compute.

Prediction methods for phonon transport properties of inorganic crystals: from traditional approaches to artificial intelligence

In inorganic crystals, phonons are the elementary excitations describing the collective atomic motions. The study of phonons plays an important role in terms of understanding thermal transport behavior and acoustic properties, as well as exploring the interactions between phonons and other energy carriers in materials. Thus, efficient and accurate prediction of phonon transport properties such as thermal conductivity is crucial for revealing, designing, and regulating material properties to meet practical requirements. In this paper, typical strategies used to predict phonon transport properties in modern science and technologies are introduced, and relevant achievements are emphasized. Moreover, insights into the remaining challenges as well as future directions of phonon transport-related exploration are proposed. The viewpoints of this paper are expected to provide a valuable reference to the community and inspire relevant research studies on predicting phonon transport properties in the near future.

Tuning Thermal Conductivity of Hybrid Perovskites through Halide Alloying

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

The intrinsically low lattice thermal conductivity of monolayer T-Au6X2 (X = S, Se and Te)

Thermal conductivity (κ, which consists of electronic thermal conductivity κe and lattice thermal conductivity κl), as an essential parameter in thermal management applications, is a critical physical quantity to measure the heat transfer performance of materials. To seek low-κ materials for heat-related applications, such as thermoelectric materials and thermal barrier coatings. In this study, based on a complex cluster design, we report a new class of two-dimensional (2D) transition metal dichalcogenides (TMDs): T-Au6X2 (X = S, Se, and Te) with record ultralow κl values. At room temperature, the κl values of T-Au6S2, T-Au6Se2, and T-Au6Te2 are 0.25 (0.23), 0.30 (0.21), and 0.12 (0.10) W m-1 K-1 along the x-axis (y-axis) direction, respectively, exhibiting good thermal insulation. The ultralow κl originates from strong phonon softening and suppression, especially for the phonon with frequency 0-1 THz. In addition, T-Au6Te2 holds the lowest group velocity and phonon relaxation time among the three T-Au6X2 monolayers. Our study provides an alternative approach for achieving ultralow κl through complex cluster replacement. Meanwhile, this new class of TMDs is expected to shine in thermal insulation and thermoelectricity due to their ultralow κl values.

Remarkable Thermal Conductivity Reduction of Silicon Nanowires during the Bending Process

The one-dimensional geometry of silicon nanowire helps to overcome the rigid and brittle nature of bulk silicon and enables it to withstand substantial bending stresses. This provides exciting opportunities for the development of flexible electronics. The bending strain introduces atomic displacement in the lattice structure, which inherently has a significant impact on the thermal conductivity. The strain-dependent thermal conductivity of silicon nanowire is crucial to the thermal management and performance of flexible electronic devices. However, in situ thermal conductivity measurement of bending silicon nanowires remains challenging and unreported due to the varying thermal contact resistances between the sample and sensor/heat sink. In this study, the Raman spectroscopy-assisted steady state thermal conductivity measurement method is coupled with a micromanipulation system to successively monitor the thermal conductivity variation of silicon nanowires during the bending process. The result shows that the thermal conductivity of silicon nanowires steeply decreases 55-78% owing to the strain-induced structural deformation during bending. Furthermore, the proposed in situ thermal conductivity measurement method can also be extended to other nanomaterials.

Impact of Ge clustering on the thermal conductivity of SiGe nanowires: atomistic simulation study

Using non-equilibrium molecular dynamics simulations, we demonstrate that the thermal conductivity of SiGe alloy nanowires is remarkably sensitive to inhomogeneous composition distributions. Specifically, the effects of Ge clustering on the thermal conductivity of SiGe nanowires are studied. The results showed that clustering Ge atoms can improve the thermal conductivity of SiGe alloy nanowires due to the reduction of random alloy scattering centers. When the number of Ge atoms in the nanocluster increases, the thermal conductivity of such nanowires grows monotonically compared with that of random alloy nanowires. To reveal the role of inhomogeneous Ge distributions on the thermal conductivity, we performed vibrational eigenmode analyses and found the remarkable delocalization of phonon modes after Ge clustering. Through such analyses, we found that the increase in thermal conductivity was correlated with the phonon delocalization in the SiGe nanowires, where stronger delocalization indicates a better thermal performance of the nanowires. Our results are helpful not only in understanding the clustering effects on heat transport but also in modulating the thermal conductivity of SiGe nanowires.

Large Thermal Conductivity Switching in Ferroelectrics by Electric Field-Triggered Crystal Symmetry Engineering

A convenient, reversible, fast, and wide-range switching of thermal conductivity is desired for efficient heat energy management. However, traditional methods, such as temperature-induced phase transition and chemical doping, have many limitations, e.g., the lack of continuous tunability over a wide temperature range and low switching speed. In this work, a strategy of electric field-driven crystal symmetry engineering to efficiently modulate thermal conductivity is reported with first-principles calculations. By simply changing the direction of an external electric field loaded in ferroelectric PbZr_0.5Ti_0.5O₃, near the morphotropic phase boundary composition, we obtain the largest switching of thermal conductivity for ferroelectric materials at room temperature based on the dual-phonon theory, i.e., normal and diffuson-like phonons, with three different criteria. The calculation results indicate that with decreasing crystal symmetry, the degeneracy of phonon modes reduces and the avoid-crossing behavior of phonon branches enhances, leading to the increase of diffuson-like phonons and weighted phonon-phonon scattering phase space. A thermal switch prototype based on PbZr_0.5Ti_0.5O₃ is further shown that can protect the Li-ion battery by modulating its temperature up to 17.5 °C. Our studies would pave the way for designing next-generation thermal switch with high speed, a wide temperature range, and a large switching ratio.

A Review on the Processing Technologies for Corrosion Resistant Thermoelectric Oxide Coatings

Oxide coatings are corrosion resistant at elevated temperatures. They also show intensive phonon scattering and strong quantum confinement behavior. Such features allow them to be used as new materials for thermoelectric energy conversion and temperature measurement in harsh environments. This paper provides an overview on processing thermoelectric oxide coatings via various technologies. The first part deals with the thermoelectricity of materials. A comparison on the thermoelectric behavior between oxides and other materials will be made to show the advantages of oxide materials. In the second part of the paper, various processing technologies for thermoelectric metal oxide coatings in forms of thin film, superlattice, and nanograin powder will be presented. Vapor deposition, liquid phase deposition, nanocasting, solid state approach, and energy beam techniques will be described. The structure and thermoelectric property of the processed metal oxide coatings will be discussed. In addition, the device concept and applications of oxide coatings for thermoelectric energy conversion and temperature sensing will be mentioned. Perspectives for future research will be provided as well.

Significant improvement in thermoelectric performance of SnSe/SnS via nano-heterostructures

In this work, we study theoretically the electronic and phonon transport properties of heterojunction SnSe/SnS, bilayer SnSe and SnS. The energy filtering effect caused by the nano heterostructure in SnSe/SnS induces an increase in the Seebeck coefficient, causing a large power factor. We calculate the phonon relaxation time and lattice thermal conductivity κL for the three structures; the heterogeneous nanostructure could effectively reduce κL due to the enhanced phonon boundary scattering at interfaces. The average κL notably reduces from around 3.3 (3.2) W m-1 K-1 for bilayer SnSe (SnS) to nearly 2.2 W m-1 K-1 for SnSe/SnS at 300 K. As a result, the average ZT (ZTave in b and c directions) reaches 1.63 with temperature range around 300-800 K, which is improved by 63% (25%) compared with that of bilayer SnSe (SnS). Our theoretical results show that the heterogeneous nanostructure is an innovative approach for improving the Seebeck coefficient and significantly reducing κL, effectively enhancing thermoelectric properties.

Perspective on multi-scale simulation of thermal transport in solids and interfaces

Phonon-mediated thermal transport is inherently multi-scale. The wave-length of phonons (considering phonons as waves) is typically at the nanometer scale; the typical size of a phonon wave energy packet is tens of nanometers, while the phonon mean free path (MFP) can be as long as microns. At different length scales, the phonons will interact with structures of different feature sizes, which can be as small as 0D defects (point defects), short to medium range linear defects (dislocations), medium to large range 2D planar defects (stacking faults and twin boundaries), and large scale 3D defects (voids, inclusions, and various microstructures). The nature of multi-scale thermal transport is that there are different heat transfer physics across different length scales and in the meantime the physics crossing the different scales is interdependent and coupled. Since phonon behavior is usually mode dependent, thermal transport in materials with a combined micro-/nano-structure complexity becomes complicated, making modeling this kind of transport process very challenging. In this perspective, we first summarize the advantages and disadvantages of computational methods for mono-scale heat transfer and the state-of-the-art multi-scale thermal transport modeling. We then discuss a few important aspects of the future development of multi-scale modeling, in particular with the aid of modern machine learning and uncertainty quantification techniques. As more sophisticated theoretical and computational methods continue to advance thermal transport predictions, novel heat transfer physics and thermally functional materials will be discovered for the pertaining energy systems and technologies.

Ultimate suppression of thermal transport in amorphous silicon nitride by phononic nanostructure

Engineering the thermal conductivity of amorphous materials is highly essential for the thermal management of future electronic devices. Here, we demonstrate the impact of ultrafine nanostructuring on the thermal conductivity reduction of amorphous silicon nitride (a-Si₃N₄) thin films, in which the thermal transport is inherently impeded by the atomic disorders. Ultrafine nanostructuring with feature sizes below 20 nm allows us to fully suppress contribution of the propagating vibrational modes (propagons), leaving only the diffusive vibrational modes (diffusons) to contribute to thermal transport in a-Si₃N₄ A combination of the phonon-gas kinetics model and the Allen-Feldmann theory reproduced the measured results without any fitting parameters. The thermal conductivity reduction was explained as extremely strong diffusive boundary scattering of both propagons and diffusons. These findings give rise to substantial tunability of thermal conductivity of amorphous materials, which enables us to provide better thermal solutions in microelectronic devices.

Phonon-Grain-Boundary-Interaction-Mediated Thermal Transport in Two-Dimensional Polycrystalline MoS2

Although dislocations and grain boundaries (GBs) are ubiquitous in large-scale MoS₂ samples, their interaction with phonons, which plays an important role in determining the lattice thermal conductivity of polycrystalline MoS₂, remains elusive. Here, we perform a systematic study of the heat transport in two-dimensional polycrystalline MoS₂ by both molecular dynamics simulation and atomic Green's function method. Our results indicate that the thermal boundary conductance of GBs of MoS₂ is in the range from 6.4 × 10⁸ to 35.3 × 10⁸ W m^-2 K^-1, which is closely correlated with the overlap between the vibrational density of states of GBs and those of the pristine lattice, as well as the GB energy. It is found that the GBs strongly scatter the phonons with frequency larger than 2 THz, accompanied by a pronounced phonon localization effect and significantly reduced phonon group velocities. Furthermore, by comparing the results from realistic polycrystalline MoS₂ to those from different theoretical models, we observe that the Casimir model is broken down and detailed phonon dynamics at a specific GB should be taken into account to accurately describe the phonon transport in polycrystalline materials. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-GB interaction.

Thermal transport through fishbone silicon nanoribbons: unraveling the role of Sharvin resistance

Heat conduction has been shown to be greatly suppressed in Si nanomeshes, which has attracted extensive attention for potential thermoelectric applications, yet the precise suppression mechanism remains to be fully understood. Attempting to further disclose the underlying mechanisms, we report on the thermal conductivity of the building block for nanomeshes, i.e., Si nanoribbons with fins attached to the two opposite sides. By expanding only the fin width while keeping both the period length and the backbone size constant, we observed an unexpected non-monotonic trend of the effective thermal conductivity normalized with the backbone cross-section. Further analysis showed that the corrected thermal conductivity extracted with appropriate consideration of the geometrical effect on diffusion followed a monotonically decreasing trend, reaching a maximum thermal conductivity reduction of 18% at 300 K for a ribbon with the maximum explored fin width of 430 nm, as compared to that of the straight ribbon of 66 nm backbone width. We attribute the thermal conductivity reduction to the thermal constriction resistance induced by the cross-section reduction between the fin and backbone sections. For ribbons with a larger fin width, the effective phonon mean free path is longer for phonons arriving at the constriction, which boosts the ballistic constriction resistance, i.e., Sharvin resistance, and leads to a lower thermal conductivity.

Pressure-driven phase transitions and reduction of dimensionality in 2D silicon nanosheets

In-situ high-pressure synchrotron X-ray powder diffraction studies up to 21 GPa of CVD-grown silicon 2D-nanosheets establish that the structural phase transitions depend on size and shape. For sizes between 9.3(7) nm and 15.2(8) nm we observe an irreversible phase transition sequence from I (cubic) → II (tetragonal) → V (hexagonal) during pressure increase and during decompression below 8 GPa the emergence of an X-ray amorphous phase. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atomic force microscopy (AFM) images of this X-ray amorphous phase reveal the formation of significant numbers of 1D nanowires with aspect ratios > 10, which are twinned and grow along the <111> direction. We discovered a reduction of dimensionality under pressure from a 2D morphology to a 1D wire in a material with a diamond structure. MD simulations indicate the reduction of thermal conductivity in such nanowires.

Tuning phonon transport spectrum for better thermoelectric materials

The figure of merit of thermoelectric materials can be increased by suppressing the lattice thermal conductivity without degrading electrical properties. Phonons are the carriers for lattice thermal conduction, and their transport can be impeded by nanostructuring, owing to the recent progress in nanotechnology. The key question for further improvement of thermoelectric materials is how to realize ultimate structure with minimum lattice thermal conductivity. From spectral viewpoint, this means to impede transport of phonons in the entire spectral domain with noticeable contribution to lattice thermal conductivity that ranges in general from subterahertz to tens of terahertz in frequency. To this end, it is essential to know how the phonon transport varies with the length scale, morphology, and composition of nanostructures, and how effects of different nanostructures can be mutually adopted in view of the spectral domain. Here we review recent advances in analyzing such spectral impedance of phonon transport on the basis of various effects including alloy scattering, boundary scattering, and particle resonance.

Methodology Perspective of Computing Thermal Transport in Low-Dimensional Materials and Nanostructures: The Old and the New

Demands for engineering thermal transport properties are ever increasing for a wide range of modern micro- and nanodevices and materials-based energy technologies. In particular, there is a severe situation due to the rapid progress in the synthesis and processing of materials and devices with structural characteristic length on the nanometer scales, which are comparable or even smaller than the intrinsic length scales (such as mean free path and wavelength) of basic energy carriers (such as phonons, electrons, and photons). Although advanced approaches for controlling the electronic and photonic transport have been proposed in the past decades, progress on controlling lattice vibrations (i.e., the phonons) is still far behind. Gaps between the fundamental understandings of the behavior of the basic energy carriers at small scales and the technological demands still remain, particularly from a computer modeling point of view. Herewith, we give a perspective of the computational approaches for predicting the thermal transport properties of low-dimensional materials and nanostructures, which are mainly sorted into three categories: empirical molecular dynamics, anharmonic lattice dynamics based Boltzmann transport equation, and Landauer theory. The advantage and disadvantage of each method are discussed and some possible solutions are suggested. The discussion is focused on fully and accurately characterizing the mode-level phonon behavior, possible all-order phonon scattering process, and incorporation of realistic nanostructures. Moreover, emerging challenges of phonon coupling effects, such as electron-phonon, phonon-photon, and phonon-magnon coupling, are also discussed. We expect that this perspective will stimulate future research in computer modeling of micro-/nanoscale heat transfer beyond traditional phonons.

Ultimate Confinement of Phonon Propagation in Silicon Nanocrystalline Structure

Temperature-dependent thermal conductivity of epitaxial silicon nanocrystalline (SiNC) structures composed of nanometer-sized grains separated by ultrathin silicon-oxide (SiO_{2}) films (∼0.3  nm) is measured by the time domain thermoreflectance technique in the range from 50 to 300 K. The thermal conductivity of SiNC structures with a grain size of 3 and 5 nm is anomalously low at the entire temperature range, significantly below the values of bulk amorphous Si and SiO_{2}. The phonon gas kinetic model, with intrinsic transport properties obtained by first-principles-based anharmonic lattice dynamics and phonon transmittance across ultrathin SiO_{2} films obtained by atomistic Green's function, reproduces the measured thermal conductivity without any fitting parameters. The analysis reveals that mean free paths of acoustic phonons in the SiNC structures are equivalent or even below half the phonon wavelength, i.e., the minimum thermal conductivity scenario. The result demonstrates that the nanostructures with extremely small length scales and a controlled interface can give rise to ultimate classical confinement of thermal phonon propagation.

Decouple electronic and phononic transport in nanotwinned structures: a new strategy for enhancing the figure-of-merit of thermoelectrics

Thermoelectric (TE) materials manifest themselves to enable direct conversion of temperature differences to electric power and vice versa. Though remarkable advances have been achieved in the past decades for various TE systems, the energy conversion efficiency of TE devices, which is characterized by a dimensionless figure-of-merit (ZT = S²σT/(κ_el + κ_ph)), generally remains a poor factor that severely limits TE devices' competitiveness and range of employment. The bottleneck for substantially boosting the ZT coefficient lies in the strong interdependence of the physical parameters involved in electronic (S and σ, and κ_el) and phononic (κ_ph) transport. Herein, we propose a new strategy of incorporating nanotwinned structures to decouple electronic and phononic transport. Combining the new concept of nanotwinned structures with the previously widely used nanocrystalline approach, the power factor of the nanotwin-nanocrystalline Si heterostructures is enhanced by 120% compared to that of bulk crystalline Si, while the lattice thermal conductivity is reduced to a level well below the amorphous limit, yielding a theoretical limit of 0.52 and 0.9 for ZT coefficient at room temperature and 1100 K, respectively. This value is almost two orders of magnitude larger than that for bulk Si and twice that for polycrystalline Si. Even for the experimentally obtained nanotwin-nanocrystalline heterostructures (e.g. grain size of 5 nm), the ZT coefficient can be as high as 0.26 at room temperature and 0.7 at 1100 K, which is the highest ZT value among all Si-based bulk nanostructures found thus far. Such substantial improvement stems from two aspects: (1) the improvement in the power factor is caused due to an increase in the Seebeck coefficient (degeneracy of the band valley) and the enhancement of electrical conductivity (the reduction of the effective band mass) and (2) the significant reduction of the lattice thermal conductivity is mainly caused due to the extremely strong phonon-grain boundary and phonon-twin boundary scattering. Our results suggest that nanotwinned structures are excellent building blocks for enhancing TE performance in diamond-like semiconductors, and our study provides a new strategy for the innovative development of other TE materials.

Nonmonotonic Diameter Dependence of Thermal Conductivity of Extremely Thin Si Nanowires: Competition between Hydrodynamic Phonon Flow and Boundary Scattering

By carefully and systematically performing Green-Kubo equilibrium molecular dynamics simulations, we report that the thermal conductivity (κ) of Si nanowires (NWs) does not diverge but converges and increases steeply when NW diameter (D) becomes extremely small (dκ/dD < 0), a long debate of one-dimensional heat conduction in history. The κ of the thinnest possible Si NWs reaches a superhigh level that is as large as more than 1 order of magnitude higher than its bulk counterpart. The abnormality is explained in terms of the dominant normal (N) process (energy and momentum conservation) of low frequency acoustic phonons that induces hydrodynamic phonon flow in the Si NWs without being scattered. With D increasing, the downward shift of optical phonons triggers strong Umklapp (U) scattering with acoustic phonons and attenuates the N process, leading to the regime of phonon boundary scattering (dκ/dD < 0). The two competing mechanisms result in nonmonotonic diameter dependence of κ with minima at critical diameter of 2-3 nm. Our results unambiguously demonstrate the converged κ and the clear trend of κ ∼ D for extremely thin Si NWs by fully elucidating the competition between the hydrodynamic phonon flow and phonon boundary scattering.

Synthesis and thermoelectric properties of defect-containing PbSe–PbTe heterojunction nanostructures

Defect-containing and defect-free PbSe–PbTe heterogeneous (HNSs) nanostructures are synthesized. The strain distribution and defects in the interface of PbSe–PbTe HNSs affect the thermoelectric parameters of PbSe–PbTe HNSs.