Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires

Localized Phonon Transport Study of GaN/SiO2 Core/shell Nanowires under Thermal-Stress Coupling

In order to comprehensively explore the intricate mechanisms of thermo-mechanical interactions, this study employs the molecular dynamics method to investigate the influence of heat flux density, shell thickness and length, as well as stress on the radial interface phonon transport in GaN/SiO2 core/shell nanowire. Additionally, the surface eigenmode decomposition method is employed to analyze the interface phonon dispersion curves. The investigation reveals that with increasing heat flux density, internal thermal stresses intensify, leading to a complex distribution of thermal stresses within the system. Under the influence of thermal stress, the nonlinear acoustic properties interact with phonon scattering, resulting in the pronounced localization of interface phonons. Compressive stress causes an upshift in low-frequency phonons, while tensile stress induces a downward shift in the high-frequency optical branches at the interface. The localized phonon vibrations at the SiO2/GaN interface under nonuniform stress are identified as the primary cause for the abundant presence of nondispersive phonon modes at the radial interface. By elucidating the subtle interplay between lattice vibrations and stress fields, this study offers a novel and profound understanding of thermo-mechanical coupling effects, thereby providing innovative theoretical foundations for the design and performance management of thermoelectric devices.

Super-Suppression of Long-Wavelength Phonons in Constricted Nanoporous Geometries

In a typical semiconductor material, the majority of the heat is carried by long-wavelength, long-mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean free paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessen's rule. In general, however, long-wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale molecular-dynamics simulations, non-equilibrium Green's function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries that create narrow constrictions in the passage of phonons lead to anticorrelated heat currents in the phonon spectrum. This effect results in super-suppression of long-wavelength phonons due to heat trapping and reductions in the thermal conductivity to values well below those predicted by Matthiessen's rule.

Ab initio study of mechanical and thermal properties of GeTe-based and PbSe-based high-entropy chalcogenides

GeTe-based and PbSe-based high-entropy compounds have outstanding thermoelectric (TE) performance and crucial applications in mid and high temperatures. Recently, the optimization of TE performance of high-entropy compounds has been focused on reducing thermal conductivity by strengthening the phonon scattering process to improve TE performance. We report a first-principles investigation on nine GeTe-based high-entropy chalcogenide solid solutions constituted of eight metallic elements (Ag, Pb, Sb, Bi, Cu, Cd, Mn, and Sn) and 13 PbSe-based high-entropy chalcogenide solid solutions: Pb0.99-ySb0.012SnySe1-2xTexSx (x = 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and y = 0) and Pb0.99-ySb0.012SnySe1-2xTexSx (y = 0.05, 0.1, 0.15, 0.2, 0.25 and x = 0.25). We have investigated the mechanical properties focusing on Debye temperature (ΘD), thermal conductivity (κ), Grüneisen parameter (γα), dominant phonon wavelength (λdom), and melting temperature (Tm). We find that the lattice thermal conductivity is significantly reduced when GeTe is alloyed into the following compositions: Ge0.75Sb0.13Pb0.12Te, Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te, and Ge0.61Ag0.11Sb0.13Pb0.12Mn0.05Bi0.01Te. This reduction is due to the mass increase and strain fluctuations. The results also show that Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te solid solution has the lowest Young's modulus (30.362 GPa), bulk and shear moduli (18.626 and 12.359 GPa), average sound velocity (1653.128 m/sec), Debye temperature (151.689 K), lattice thermal conductivity (0.574 W.m-1.K-1), dominant phonon wavelength (0.692 Å), and melting temperature (535.91 K). Moreover, Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te has the highest Grüneisen parameter with a reduced and temperature-independent lattice thermal conductivity. The positive correlation between ΘD and κ is revealed. Alloying of PbSe-based high-entropy by Sb, Sn, Te, and S atoms at the Se and Pb sites resulted in much higher shear strains resulted in the reduction of phonon velocity, a reduced ΘD, and a lower lattice thermal conductivity.

Experimental Evidence of Superdiffusive Thermal Transport in Si0.4Ge0.6 Thin Films

Superdiffusive thermal transport represents a unique phenomenon in heat conduction, which is characterized by a size (L) dependence of thermal conductivity (κ) in the form of κ ∝ L^β with a constant β between 0 and 1. Although superdiffusive thermal transport has been theoretically predicted for SiGe alloys, direct experimental evidence is still lacking. Here, we report on a systematic experimental study of the thickness-dependent thermal conductivity of Si_0.4Ge_0.6 thin films grown by molecular beam epitaxy. The cross-plane thermal conductivity of Si_0.4Ge_0.6 thin films spanning a thickness range from 20 to 1120 nm was measured in the temperature range 120-320 K via a differential three-omega method. Results show that the thermal conductivity follows a consistent κ ∝ t^0.26 power law with the film thickness (t) at different temperatures, providing direct experimental evidence that alloy-scattering dominated thermal transport in SiGe is superdiffusive.

Phonon localization and resonance in thermal transport of pillar-based GaAs nanowires

Exploring the possibility of nanostructures to modulate thermal conductivity (TC) contributes to promote a deeper comprehension of phonon diffusion and transport processes with the design of thermally insulated devices with high ZT values, and the GaAs nanowires (NWs) widely used in optoelectronic and microelectronic devices exhibit nondiffusive phonon thermal transport phenomena attributed to size effects, while ignoring the wave effects of phonons. Here, we simulate the TC of pillar-based GaAs NWs using non-equilibrium molecular dynamics and Monte Carlo simulations. The spatial distribution of density of states, temperature and heat flow distribution clouds, phonon participation rate, dispersion curves and phonon transmittance of atoms were calculated to investigate the phonon thermal transport processes in pillar-based NWs. The calculation results show that the pillar-based surface reduce the TC by 16%, the TC of pristine NW increases with axial and equivalent diameter, and the TC of pillar-based NW increases nonlinearly with axial length and increases with radial length. The phonon-surface scattering intensity is enhanced by the perturbation introduced by the pillared surface with a substantial decrease in phonon transmission capacity and a break in long-wavelength phonon transport even annihilated, which leads to surface phonon localization. Nanopillars not only enhance the phonon-surface scattering intensity at low frequencies, but also reconfigure the dispersion curve to reduce the group velocity. A series of flat resonance phonon modes are generated throughout the whole spectrum due to the hybridization between the local resonance phonon modes of the nanopillar and the phonon modes of the substrate NWs, resulting in the phonon modes shifting to lower frequencies. The pillar-based surface induced surface phonon localization and local resonance phenomenon contributes to the modulation of phonon thermal transport in GaAs-based field-effect transistors.

Lévy walk of quasiballistic phonons in nanowires

Phonon transport in square-cross-section nanowires is studied using spectral Monte Carlo simulations. Our results show the evolution of the different transport regimes described by Lévy statistics as a function of the surface roughness-to-thermal wavelength ratio σ/λ. More precisely, the relationship between the Lévy index γ describing the mean free path distribution Ψ(Λ) and σ/λ is established for the classical diffusive regime, the superdiffusive regime, and the ballistic regime in the nanowire. Besides the conventional superdiffusive regime that is marked by Ψ(Λ) with a single heavy-tailed peak, we reveal an unconventional superdiffusive subregime featuring Ψ(Λ) with sawtooth oscillations when σ/λ∼0.01. Investigation of the direction of propagation of phonons shows a significant narrowing of the angular distribution around the long axis of the nanowire due to the diffuse scattering at rough boundaries when σ/λ>0.1. These results shed light on the transport mechanisms of quasiballistic phonons and will help in nanowire design for specific applications.

Impact of Porosity and Boundary Scattering on Thermal Transport in Diameter-Modulated Nanowires

We study the thermal conductivity of diameter-modulated Si nanowires to understand the impact of different nanoscale transport mechanisms as a function of nanowire morphology. Our investigation couples transient suspended microbridge measurements of diameter-modulated Si nanowires synthesized via vapor-liquid-solid growth and dopant-selective etching with predictive Boltzmann transport modeling. We show that the presence of a low thermal conductivity phase (i.e., porosity) dominates the reduction in effective thermal conductivity and is supplemented by increased phonon-boundary scattering. The relative contributions of both mechanisms depend on the details of the nanoscale morphology. Our findings provide valuable insights into the factors that govern thermal conduction in complex nanoscale materials.

Mean Free Path Suppression of Low-Frequency Phonons in SiGe Nanowires

Accurate measurements of the size-dependent lattice thermal conductivity (κ_l) of alloy nanostructures are challenging but help to address outstanding questions on the effects of atomic disorder and surface roughness on low-frequency vibrational modes in functional materials. Here, we report sensitive κ_l measurements of multiple segments of the same individual SiGe nanowires. In contrast to a previous report of ballistic thermal transport over several microns in SiGe nanowires, the obtained κ_l are nearly independent of the segment length from 2 to 10 μm and the temperature between 150 and 300 K. The results are in agreement with a theoretical calculation based on the virtual crystal approximation of the vibrational modes as phonons with mean free paths suppressed by purely diffuse surface scattering. The findings inform continuing theoretical efforts for understanding the roles of different types of vibrational modes in thermal transport in disordered thermoelectric and electronic materials.

Coherent and Incoherent Impacts of Nanopillars on the Thermal Conductivity in Silicon Nanomembranes

Nanostructuring is the dominant approach for effective thermal conduction control in nanomaterials. In the past decade, researchers have been interested in thermal conduction control by the coherent effects in phononic crystal (PnC) systems. Recent theoretical works predicted that nanopillars on the surface of silicon membranes could cause a dramatic thermal conductivity reduction due to the phonon local resonances. However, this remarkable prediction has not been experimentally verified yet with the deep-nanoscale pillar-based PnCs. Here, we fabricate nanopillars on suspended silicon membranes using damageless neutral-beam etching and investigate the impact of nanopillars on the thermal conductivity of the membranes in the 4-300 K range. We found that thermal conductivity reduction caused by the nanopillars does not exceed 16%, which is much weaker than that predicted by the theoretical works. Moreover, this reduction remains temperature independent. These facts make the coherence an unlikely reason for the observed reduction. Indeed, our Monte Carlo simulations can reproduce the experimental results under a purely incoherent approximation. Our study shows that the coherent control of heat conduction by PnC nanostructures is more challenging to observe experimentally in reality than predicted in near-ideal modeling.

Research Progress on Thermal Conductivity of Graphdiyne Nanoribbons and its Defects: A Review

BACKGROUND

Graphdiyne has a unique pi-conjugated structure, perfect pore distribution and adjustable electronic properties of sp2, sp hybrid planar framework. Due to the presence of acetylenic bonds, it has more excellent properties compared to grapheme, such as a unique structure-dependent Dirac cone, abundant carbon bonds and a large bandgap. As one of the important raw materials for nanodevices, it is extremely important to study the thermal properties of graphdiyne nanoribbon.

OBJECTIVE

This paper mainly introduces and discusses recent academic research and patents on the preparation methods and thermal conductivity of graphdiyne nanoribbons. Besides, the applications in engineering and vacancy defects in the preparation process of graphdiyne are described.

METHODS

Firstly, taking thermal conductivity as an index, the thermal conductivity of graphdiyne with various vacancy defects is discussed from the aspects of length, defect location and defect type. In addition, the graphdiyne nanoribbons were laterally compared with the thermal conductivity of the graphene nanoribbons.

RESULTS

The thermal conductivity of graphdiyne with defects increases with the length and width, which is lower than the intrinsic graphdiyne. The thermal conductivity of the acetylene chain lacking one carbon atom is higher than the one lacking the benzene ring. Typically, the thermal conductivity is larger in armchair than that of zigzag in the same size. Moreover the thermal conductivity of nanoribbons with double vacancy defects is lower than those nanoribbons with single vacancy defects, which can also decrease with the increase of temperature and the number of acetylene chains. The thermal conductivity is not sensitive to shear strain.

CONCLUSION

Due to the unique structure and electronic characteristics, graphdiyne has provoked an extensive research interest in the field of nanoscience. Graphdiyne is considered as one of the most promising materials of next-generation electronic devices.

Phononic pathways towards rational design of nanowire heat conduction

Thermal conduction in semiconductor nanowires is controlled by the transport of atomic vibrations also known as thermal phonons. The ability of nanowires to tailor the transport of thermal phonons stems from their precise atomic scale growth coupled with high structural surface to volume ratios. Understanding and manipulating thermal transport properties at the nanoscale is central for progress in the fields of microelectronics, optoelectronics, and thermoelectrics. Here, we review state-of-the-art advances in the understanding of nanowire thermal phonon transport and the design and fabrication of nanowires with tailored thermal conduction properties. We first introduce the basic physical mechanisms of thermal conduction at the nanoscale and detail recent developments in employing nanowires as thermal materials. We discuss and provide insight on different strategies to modulate nanowire thermal properties leveraging the underlying phonon transport processes occurring in nanowires. We also highlight challenges and key areas of interest to motivate future research and create exceptional capabilities to control heat flow in nanowires.

Thermoelectric Photosensor Based on Ultrathin Single-Crystalline Si Films †

Ultrathin Si films have a reduced thermal conductivity in comparison to Si bulk due to phonon scattering at the surfaces. Furthermore, the small thickness guarantees a reduced thermal mass (in the µJ/K range), which opens up the possibility of developing thermal sensors with a high sensitivity. Based on these premises, a thermoelectric (TE) microsensor based on ultrathin suspended Si films was developed and used as a thermal photosensor. The photoresponse of the device was evaluated with an argon laser (λ = 457 nm) with a variable power ranging from 0 to 10 mW in air at atmospheric pressure, with laser diodes at 406 nm, 520 nm and 638 nm wavelengths, and fixed powers in high vacuum conditions. The responsivity per unit area, response time (τ) and detectivity (D*) of the device were determined in air at ambient pressure, being 2.6 × 10⁷ V/Wm², ~4.3 ms and 2.86 × 10 7   c m H z ( 1 / 2 ) W - 1 , respectively. Temperature differences up to 30 K between the central hot region and the Si frame were achieved during open-circuit voltage measurements, with and without laser diodes. During illumination, the photogeneration of carriers caused a slight reduction of the Seebeck coefficient, which did not significantly change the sensitivity of the device. Moreover, the measurements performed with light beam chopped at different frequencies evidenced the quick response of the device. The temperature gradients applied to the thermoelectric Si legs were corrected using finite element modeling (FEM) due to the non-flat temperature profile generated during the experiments.

Vertical Silicon Nanowire Thermoelectric Modules with Enhanced Thermoelectric Properties

Thermoelectric modules based on silicon nanowires (Si-NWs) have recently attracted significant attention as they show an improved thermoelectric efficiency due to a decrease in thermal conductivity. Here, we adopt a top-down fabrication method to dramatically reduce the thermal conductivity of vertical Si-NWs. The thermal conductivity of a vertical Si-NW is significantly suppressed with an increasing surface roughness, decreasing diameter, and increasing doping concentration. This large suppression is caused by enhanced phonon scattering, which depends on the phonon wavelength. The boron- and phosphorus-doped rough Si-NWs with a diameter of 200 nm and surface roughness of 6.88 nm show the lowest thermal conductivity of 10.1 and 14.8 W·m^-1·K^-1, respectively, which are 5.1- and 3.6-fold lower than that of a smooth intrinsic nanowire and 14.8- and 10.1-fold lower than that of bulk silicon. A thermoelectric module was fabricated using this doped rough Si-NW array, and its thermoelectric performance is compared with previously reported Si-NW modules. The fabricated module exhibits an excellent performance with an open circuit voltage of 216.8 mV·cm^-2 and a maximum power of 3.74 μW·cm^-2 under a temperature difference of 180 K, the highest reported for Si-NW thermoelectric modules.

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m⁻¹·K⁻¹ for 90 nm diameter wires with 35–40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Thermal conductivity reduction in silicon fishbone nanowires

Semiconductor nanowires are potential building blocks for future thermoelectrics because of their low thermal conductivity. Recent theoretical works suggest that thermal conductivity of nanowires can be further reduced by additional constrictions, pillars or wings. Here, we experimentally study heat conduction in silicon nanowires with periodic wings, called fishbone nanowires. We find that like in pristine nanowires, the nanowire cross-section controls thermal conductivity of fishbone nanowires. However, the periodic wings further reduce the thermal conductivity. Whereas an increase in the wing width only slightly affects the thermal conductivity, an increase in the wing depth clearly reduces thermal conductivity, and this reduction is stronger in the structures with narrower nanowires. Our experimental data is supported by the Callaway-Holland model, finite element modelling and phonon transport simulations.

Enhancing Thermal Transport in Layered Nanomaterials

A comprehensive rational thermal material design paradigm requires the ability to reduce and enhance the thermal conductivities of nanomaterials. In contrast to the existing ability to reduce the thermal conductivity, methods that allow to enhance heat conduction are currently limited. Enhancing the nanoscale thermal conductivity could bring radical improvements in the performance of electronics, optoelectronics, and photovoltaic systems. Here, we show that enhanced thermal conductivities can be achieved in semiconductor nanostructures by rationally engineering phonon spectral coupling between materials. By embedding a germanium film between silicon layers, we show that its thermal conductivity can be increased by more than 100% at room temperature in contrast to a free standing thin-film. The injection of phonons from the cladding silicon layers creates the observed enhancement in thermal conductivity. We study the key factors underlying the phonon injection mechanism and find that the surface conditions and layer thicknesses play a determining role. The findings presented here will allow for the creation of nanomaterials with an increased thermal conductivity.

Heat conduction tuning by wave nature of phonons

The world communicates to our senses of vision, hearing, and touch in the language of waves, because light, sound, and even heat essentially consist of microscopic vibrations of different media. The wave nature of light and sound has been extensively investigated over the past century and is now widely used in modern technology. However, the wave nature of heat has been the subject of mostly theoretical studies because its experimental demonstration, let alone practical use, remains challenging due to its extremely short wavelengths. We show a possibility to use the wave nature of heat for thermal conductivity tuning via spatial short-range order in phononic crystal nanostructures. Our experimental and theoretical results suggest that interference of thermal phonons occurs in strictly periodic nanostructures and slows the propagation of heat. This finding expands the methodology of heat transfer engineering to the wave nature of heat.

Phonon Surface Scattering and Thermal Energy Distribution in Superlattices

Thermal transport at small length scales has attracted significant attention in recent years and various experimental and theoretical methods have been developed to establish the reduced thermal conductivity. The fundamental understanding of how phonons move and the physical mechanisms behind nanoscale thermal transport, however, remains poorly understood. Here we move beyond thermal conductivity calculations and provide a rigorous and comprehensive physical description of thermal phonon transport in superlattices by solving the Boltzmann transport equation and using the Beckman-Kirchhoff surface scattering theory with shadowing to precisely describe phonon-surface interactions. We show that thermal transport in superlattices can be divided in two different heat transport modes having different physical properties at small length scales: layer-restricted and extended heat modes. We study how interface conditions, periodicity, and composition can be used to manipulate the distribution of thermal energy flow among such layer-restricted and extended heat modes. From predicted frequency and mean free path spectra of superlattices, we also investigate the existence of wave effects. The results and insights in this paper advance the fundamental understanding of heat transport in superlattices and the prospects of rationally designing thermal systems with tailored phonon transport properties.

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures

Unlike classical heat diffusion at macroscale, nanoscale heat conduction can occur without energy dissipation because phonons can ballistically travel in straight lines for hundreds of nanometres. Nevertheless, despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a method to control the directionality of ballistic phonon transport using silicon membranes with arrays of holes. First, we demonstrate that the arrays of holes form fluxes of phonons oriented in the same direction. Next, we use these nanostructures as directional sources of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce thermal lens nanostructures, in which the emitted phonons converge at the focal point, thus focusing heat into a spot of a few hundred nanometres. These results motivate the concept of ray-like heat manipulations at the nanoscale.

Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon Nanostructures

Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and optoelectronic devices. Amorphous materials are considered to possess the lower limit to the thermal conductivity (κ), which is ∼1 W·m^-1 K^-1 for a-Si. However, recent work suggested that κ of micrometer-thick a-Si films can be greater than 3 W·m^-1 K^-1, which is contributed to by propagating vibrational modes, referred to as "propagons". However, precise determination of κ in a-Si has been elusive. Here, we used structures of a-Si nanotubes and suspended a-Si films that enabled precise in-plane thermal conductivity (κ_∥) measurement within a wide thickness range of 5 nm to 1.7 μm. We showed unexpectedly high κ_∥ in a-Si nanostructures, reaching ∼3.0 and 5.3 W·m^-1 K^-1 at ∼100 nm and 1.7 μm, respectively. Furthermore, the measured κ_∥ is significantly higher than the cross-plane κ on the same films. This unusually high and anisotropic thermal conductivity in the amorphous Si nanostructure manifests the surprisingly broad propagon mean free path distribution, which is found to range from 10 nm to 10 μm, in the disordered and atomically isotropic structure. This result provides an unambiguous answer to the century-old problem regarding mean free path distribution of propagons and also sheds light on the design and performance of numerous a-Si based electronic and optoelectronic devices.

Ballistic thermal transport in silicon nanowires

We have experimentally investigated the impact of dimensions and temperature on the thermal conductivity of silicon nanowires fabricated using a top-down approach. Both the width and temperature dependences of thermal conductivity agree with those in the existing literature. The length dependence of thermal conductivity exhibits a transition from semi-ballistic thermal phonon transport at 4 K to fully diffusive transport at room temperature. We additionally calculated the phonon dispersion in these structures in the framework of the theory of elasticity and showed that the thermal conductance increases with width. This agrees with our experimental observations and supports the pertinence of using the modified phonon dispersion at low temperatures.

Length Scale of Diffusive Phonon Transport in Suspended Thin Silicon Nanowires

Recent experimental advances have revealed that the mean free path (mfp) of phonons contributing significantly to thermal transport in crystalline semiconductors can be several microns long. Almost all of these experiments are based on bulk and thin film materials and use techniques that are not directly applicable to nanowires. By developing a process with which we could fabricate multiple electrically contacted and suspended segments on individual heavily doped smooth Silicon nanowires, we measured phonon transport across varying length scales using a DC self-heating technique. Our measurements show that diffusive thermal transport is still valid across O(100) nm length scales, supporting the diffuse nature of phonon-boundary scattering even on smooth nanowire surfaces. Our work also showcases the self-heating technique as an important alternative to the thermal bridge technique to measure phonon transport across short length scales relevant to mapping the phonon mfp spectrum in nanowires.