Reduction of thermal conductivity in phononic nanomesh structures

Transient heating of Pd nanoparticles studied by x-ray diffraction with time of arrival photon detection

Pulsed laser heating of an ensemble of Pd nanoparticles, supported by a MgO substrate, is studied by x-ray diffraction. By time-resolved Bragg peak shift measurements due to thermal lattice expansion, the transient temperature of the Pd nanoparticles is determined, which quickly rises by at least 100 K upon laser excitation and then decays within 90 ns. The diffraction experiments were carried out using a Cu x-ray tube, giving continuous radiation, and the hybrid pixel detector Timepix3 operating with single photon counting in a time-of-arrival mode. This type of detection scheme does not require time-consuming scanning of the pump-probe delay. The experimental time resolution is estimated at 15 ± 5 ns, which is very close to the detector's limit and matches with the 7 ns laser pulse duration. Compared to bulk metal single crystals, it is discussed that the maximum temperature reached by the Pd nanoparticles is higher and their cooling rate is lower. These effects are explained by the oxide support having a lower heat conductivity.

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nano machining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing that they are the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depth is discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Anisotropy Reversal of Thermal Conductivity in Silicon Nanowire Networks Driven by Quasi-Ballistic Phonon Transport

Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4-300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.

MoS2 phononic crystals for advanced thermal management

Effective thermal management of electronic devices encounters substantial challenges owing to the notable power densities involved. Here, we propose layered MoS2 phononic crystals (PnCs) that can effectively reduce thermal conductivity (κ) with relatively small disruption of electrical conductivity (σ), offering a potential thermal management solution for nanoelectronics. These layered PnCs exhibit remarkable efficiency in reducing κ, surpassing that of Si and SiC PnCs with similar periodicity by ~100-fold. Specifically, in suspended MoS2 PnCs, we measure an exceptionally low κ down to 0.1 watts per meter kelvin, below the amorphous limit while preserving the crystalline structure. These findings are supported by molecular dynamics simulations that account for the film thickness, porosity, and temperature. We demonstrate the approach efficiency by fabricating suspended heat-routing structures that effectively confine and guide heat flow in prespecified directions. This study underpins the immense potential of layered materials as directional heat spreaders, thermal insulators, and active components for thermoelectric devices.

The effect of echoes interference on phonon attenuation in a nanophononic membrane

Nanophononic materials are characterized by a periodic nanostructuration, which may lead to coherent scattering of phonons, enabling interference and resulting in modified phonon dispersions. We have used the extreme ultraviolet transient grating technique to measure phonon frequencies and lifetimes in a low-roughness nanoporous phononic membrane of SiN at wavelengths between 50 and 100 nm, comparable to the nanostructure lengthscale. Surprisingly, phonon frequencies are only slightly modified upon nanostructuration, while phonon lifetime is strongly reduced. Finite element calculations indicate that this is due to coherent phonon interference, which becomes dominant for wavelengths between ~ half and twice the inter-pores distance. Despite this, vibrational energy transport is ensured through an energy flow among the coherent modes created by reflections. This interference of phonon echos from periodic interfaces is likely another aspect of the mutual coherence effects recently highlighted in amorphous and complex crystalline materials and, in this context, could be used to tailor transport properties of nanostructured materials.

Thermal Rectification in Modularly Designed Bulk Metamaterials

Thermal rectification is a phenomenon of great practical importance where heat transfer is preferential in one direction. Programmable control of heat transfer in 3D space is key to enable thermal rectification at the macroscale but is rarely realized in natural materials or in current existing devices that are constructed at the nano and microscales with high system complexity. Here, modularly designed bulk metamaterials that can break the symmetry of heat transfer from one direction to the other are created, leading to thermal rectification in convergent or divergent states by tuning the metamaterial microstructural design. These thermal metamaterials are microstructured composites made using one material composition, however, they offer sufficient microstructural design freedom to allow tunable local thermal properties for unusual macroscopic heat transfer. The strategy and performance achieved are promising for next-generation thermal management.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Enhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks

Nanomaterials are driving advances in technology due to their oftentimes superior properties over bulk materials. In particular, their thermal properties become increasingly important as efficient heat dissipation is required to realize high-performance electronic devices, reduce energy consumption, and prevent thermal damage. One application where nanomaterials can play a crucial role is extreme ultraviolet (EUV) lithography, where pellicles that protect the photomask from particle contamination have to be transparent to EUV light, mechanically strong, and thermally conductive in order to withstand the heat associated with high-power EUV radiation. Free-standing carbon nanotube (CNT) films have emerged as candidates due to their high EUV transparency and ability to withstand heat. However, the thermal transport properties of these films are not well understood beyond bulk emissivity measurements. Here, we measure the thermal conductivity of free-standing CNT films using all-optical Raman thermometry at temperatures between 300 and 700 K. We find thermal conductivities up to 50 W m-1 K-1 for films composed of double-walled CNTs, which rises to 257 W m-1 K-1 when considering the CNT network alone. These values are remarkably high for randomly oriented CNT networks, roughly seven times that of single-walled CNT films. The enhanced thermal conduction is due to the additional wall, which likely gives rise to additional heat-carrying phonon modes and provides a certain resilience to defects. Our results demonstrate that free-standing double-walled CNT films efficiently dissipate heat, enhancing our understanding of these promising films and how they are suited to applications in EUV lithography.

A physics rule to design aperiodic width-modulated waveguides for minimum phonon transmission with Bayesian optimization

Aperiodic nano-waveguides (nWVGs) and superlattices (SLs) limit phonon transmission and heat conduction much more efficiently than periodic ones. They could block parasitic heat conduction that restricts heat management and energy conversion at the nanoscale. Aperiodicity can be realized in multiple ways and with variable degrees of complexity. Machine learning optimization of hetero-SLs for minimum coherent phonon conduction showed optimal aperiodicity for moderate disorder against physics intuition. Here, we report on optimal aperiodicity in width-modulated nWVGS for maximum disorder as expected by physics. Optimizing aperiodic geometry modulation is particularly challenging due to the enormous possible configurations. We set up a feasible optimization problem removing unnecessary complexity and we demonstrate efficient Bayesian optimization. Our results confirm the predicted physics rule that minimum thermal conductance occurs for the most disordered arrays of modulation units; the degree of disorder being quantified by the number of non-identical modulation units. Our work opens a route to design geometrical aperiodicity and control transmission across metamaterials.

GaAs/GaP Superlattice Nanowires for Tailoring Phononic Properties at the Nanoscale: Implications for Thermal Engineering

The possibility to tune the functional properties of nanomaterials is key to their technological applications. Superlattices, i.e., periodic repetitions of two or more materials in one or more dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer systems at the nanoscale, as well as to combine materials that cannot be put together in conventional heterostructures due to the lattice mismatch. In this work, we investigate GaAs/GaP superlattices embedded in GaP nanowires and demonstrate the tunability of their phononic and optoelectronic properties by inelastic light scattering experiments corroborated by ab initio calculations. We observe clear modifications in the dispersion relation for both acoustic and optical phonons in the superlattices nanowires. We find that by controlling the superlattice periodicity, we can achieve tunability of the phonon frequencies. We also performed wavelength-dependent Raman microscopy on GaAs/GaP superlattice nanowires, and our results indicate a reduction in the electronic bandgap in the superlattice compared to the bulk counterpart. All of our experimental results are rationalized with the help of ab initio density functional perturbation theory (DFPT) calculations. This work sheds fresh insights into how material engineering at the nanoscale can tailor phonon dispersion and open pathways for thermal engineering.

Effect of Nanographene Coating on the Seebeck Coefficient of Mesoporous Silicon

Nanographene-mesoporous silicon (G-PSi) composites have recently emerged as a promising class of nanomaterials with tuneable physical properties. In this study, we investigated the impact of nanographene coating on the Seebeck coefficient of mesoporous silicon (PSi) obtained by varying two parameters: porosity and thickness. To achieve this, an electrochemical etching process on p + doped Si is presented for the control of the parameters (thicknesses varying from 20 to 160 µm, and a porosity close to 50%), and for nanographene incorporation through chemical vapor deposition. Raman and XPS spectroscopies confirmed the presence of nanographene on PSi. Using a homemade ZT meter, the Seebeck coefficient of the p + doped Si matrix was evaluated at close to 100 ± 15 µV/K and confirmed by UPS spectroscopy analysis. Our findings suggest that the Seebeck coefficient of the porous Si can be measured independently from that of the substrate by fitting measurements on samples with a different thickness of the porous layer. The value of the Seebeck coefficient for the porous Si is of the order of 750 ± 40 µV/K. Furthermore, the incorporation of nanographene induced a drastic decrease to approximately 120 ± 15 µV/K, a value similar to that of its silicon substrate.

Universal Behavior of Highly Confined Heat Flow in Semiconductor Nanosystems: From Nanomeshes to Metalattices

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first-principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalattice with deep nanoscale feature size and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeability component and an intrinsic viscous contribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using experiments and atomistic simulations, we show that our theory applies to a general set of highly confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires, to nanowire networks, of great interest for next-generation energy-efficient devices.

Design and fabrication of diffraction grating with optimized efficiency for transient grating spectroscopy

Transient grating spectroscopy (TGS) based on diffraction gratings is a powerful optical method for studying the transport of energy carriers such as phonons and electrons. The diffraction grating in a TGS system is a key component to form a large-area interference pattern, i.e., transient grating, and to study the mean free path distribution of energy carriers. In this work, a design method for polarization-insensitive diffraction gratings with periods in the range 2-50 µm for TGS by a combination of rigorous coupled wave analysis and genetic algorithm was discussed. The method was tested for pump/probe wavelength of 515/532 or 1030/808 nm. Each ±1st diffraction order carries 35%-40% of the incident energy and the diffraction efficiencies of the other orders are lower than 10%. The optimized diffraction gratings were fabricated by a combination of photolithography and inductively coupled plasma etching, with the processing parameters introduced in detail, and their optical characteristics were evaluated. Finally, as a demonstration, the diffraction gratings for 1030/808 nm were applied to TGS to study the thermal transport properties of Ge. This work provides a useful guide for future applications and the development of TGS.

Engineering thermal transport within Si thin films: The impact of nanoslot alignment and ion implantation

In recent years, nanoporous Si films have been intensively studied for their potential applications in thermoelectrics and the thermal management of devices. To minimize the thermal conductivity, ultrafine nanoporous patterns are required but the smallest structure size is largely limited by the spatial resolution of the employed nanofabrication techniques. Along this line, an effectively smaller characteristic length of a nanoporous film can be achieved with offset nanoslot patterns. Compared with periodic circular pores, the nanoslot pattern can achieve an even lower thermal conductivity, where a much smaller porosity is required using ultra-narrow nanoslots. The obtained low thermal conductivity can be understood from the thermally dead volume revealed by phonon Monte Carlo simulations. To further minimize the contribution from short-wavelength phonons, an additional 25% thermal conductivity reduction can be achieved with Ga ions implanted using a focused ion beam.

A CMOS compatible thermoelectric device made of crystalline silicon membranes with nanopores

Herein, we report the use of nanostructured crystalline silicon as a thermoelectric material and its integration into thermoelectric devices. The proof-of-concept relies on the partial suppression of lattice thermal conduction by introducing pores with dimensions scaling between the electron mean free path and the phonon mean free path. In other words, we artificially aimed at the well-known 'electron crystal and phonon glass' trade-off targeted in thermoelectricity. The devices were fabricated using CMOS-compatible processes and exhibited power generation up to 5.5 mW cm^-2under a temperature difference of 280 K. These numbers demonstrate the capability to power autonomous devices with environmental heat sources using silicon chips of centimeter square dimensions. We also report the possibility of using the developed devices for integrated thermoelectric cooling.

Structural and Elastic Properties of Empty-Pore Metalattices Extracted via Nondestructive Coherent Extreme UV Scatterometry and Electron Tomography

Semiconductor metalattices consisting of a linked network of three-dimensional nanostructures with periodicities on a length scale <100 nm can enable tailored functional properties due to their complex nanostructuring. For example, by controlling both the porosity and pore size, thermal transport in these phononic metalattices can be tuned, making them promising candidates for efficient thermoelectrics or thermal rectifiers. Thus, the ability to characterize the porosity, and other physical properties, of metalattices is critical but challenging, due to their nanoscale structure and thickness. To date, only metalattices with high porosities, close to the close-packing fraction of hard spheres, have been studied experimentally. Here, we characterize the porosity, thickness, and elastic properties of a low-porosity, empty-pore silicon metalattice film (∼500 nm thickness) with periodic spherical pores (∼tens of nanometers), for the first time. We use laser-driven nanoscale surface acoustic waves probed by extreme ultraviolet scatterometry to nondestructively measure the acoustic dispersion in these thin silicon metalattice layers. By comparing the data to finite element models of the metalattice sample, we can extract Young's modulus and porosity. Moreover, by controlling the acoustic wave penetration depth, we can also determine the metalattice layer thickness and verify the substrate properties. Additionally, we utilize electron tomography images of the metalattice to verify the geometry and validate the porosity extracted from scatterometry. These advanced characterization techniques are critical for informed and iterative fabrication of energy-efficient devices based on nanostructured metamaterials.

Nanoscale heat transport analysis by scanning thermal microscopy: from calibration to high-resolution measurements

Scanning thermal microscopy (SThM) is a powerful technique for thermal characterization. However, one of the most challenging aspects of thermal characterization is obtaining quantitative information on thermal conductivity with nanoscale lateral resolution. We used this technique with the cross-point calibration method to obtain the thermal contact resistance, R _c, and thermal exchange radius, b, using thermo-resistive Pd/Si₃N₄ probes. The cross-point curves correlate the dependence of R _c and b with the sample's thermal conductivity. We implemented a 3ω-SThM method in which reference samples with known thermal conductivity were used in the calibration and validation process to guarantee optimal working conditions. We achieved values of R _c = 0.94 × 10⁶ ± 0.02 K W^-1 and b = 2.41 × 10^-7 ± 0.02 m for samples with a low thermal conductivity (between 0.19 and 1.48 W m^-1 K^-1). These results show a large improvement in spatial resolution over previously reported data for the Wollaston probes (where b ∼ 2.8 μm). Furthermore, the contact resistance with the Pd/Si₃N₄ is ∼20× larger than reported for a Wollaston wire probe (with 0.45 × 10⁵ K W^-1). These thermal parameters were used to determine the unknown thermal conductivity of thermoelectric films of Ag₂Se, Ag_2-x Se, Cu₂Se (smooth vs. rough surface), and Bi₂Te₃, obtaining, in units of W m^-1 K^-1, the values of 0.63 ± 0.07, 0.69 ± 0.15, 0.79 ± 0.03, 0.82 ± 0.04, and 0.93 ± 0.12, respectively. To the best of our knowledge, this is the first time these microfabricated probes have been calibrated using the cross-point method to perform quantitative thermal analysis with nanoscale resolution. Moreover, this work shows high-resolution thermal images of the V _1ω and V _3ω signals, which can offer relevant information on the material's heat dissipation.

Observation of Collective Resonance Modes in a Chiral Spin Soliton Lattice with Tunable Magnon Dispersion

A chiral spin soliton lattice (CSL), one of the representative systems of a magnetic superstructure, exhibits reconfigurability in periodicity over a macroscopic length scale. Such coherent and tunable characteristics of the CSL lead to an emergence of elementary excitation of the CSL as phononlike modes due to translational symmetry breaking and bring a controllability of the dispersion relation of the CSL phonon. Using a broadband microwave spectroscopy technique, we directly found that higher-order magnetic resonance modes appear in the CSL phase of a chiral helimagnet CrNb_{3}S_{6}, which is ascribed to the CSL phonon response. The resonance frequency of the CSL phonon can be tuned between 16 and 40 GHz in the vicinity of the critical field, where the CSL period alters rapidly. The frequency range of the CSL phonon is expected to extend over 100 GHz as extrapolated on the basis of the theoretical model. The present results indicate that chiral helimagnets could work as materials useful for broadband signal processing in the millimeter-wave band.

Preparation of mesoporous nitrogen-doped titania comprising large crystallites with low thermal conductivity

Reducing the thermal conductivity (κ) of mesoporous N-doped titania (TiO2) is crucial for the development of TiO2-based materials that exhibit excellent electronic, photochemical, and thermoelectric properties. Mesopores can contribute to the reduction of κ via phonon scattering, and the scattering effect due to the randomness of crystal interfaces should be significantly reduced to clarify the role of mesopores in reducing thermal conductivity. Highly ordered mesoporous N-doped TiO2 comprising large crystallites was prepared with silica colloidal crystals as a template into which a Ti source was introduced, followed by calcination with urea. N-doped samples comprising large crystallites exhibiting random mesopores were also prepared and used for the investigation of the effects of the shape and arrangement of the mesopore on phonon scattering. The mesostructures of the two separately prepared N-doped TiO2 samples were retained after sintering at 873 K and 80 MPa to fabricate pellets. Furthermore, the effective suppression of the long mean-free-path phonon conduction by the thin pore walls at a nanometer scale thickness significantly reduced the thermal conductivities of both samples. The presence of ordered mesopores further contributed to the reduction of κ, which was probably due to the enhanced contribution of the backscattering of phonons caused by ordered pore wall surfaces.

Tuning Coherent-Phonon Heat Transport in LaCoO3/SrTiO3 Superlattices

Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO₃/SrTiO₃ oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices.

Structural optimization of silicon thin film for thermoelectric materials

The method to optimize nanostructures of silicon thin films as thermoelectric materials is developed. The simulated annealing method is utilized for predicting the optimized structure. The mean free path and thermal conductivity of thin films, which are the objective function of optimization, is evaluated by using phonon transport simulations and lattice dynamics calculations. In small systems composed of square lattices, the simulated annealing method successfully predicts optimized structure corroborated by an exhaustive search. This fact indicates that the simulated annealing method is an effective tool for optimizing nanostructured thin films as thermoelectric materials.

Directional thermal channeling: A phenomenon triggered by tight packing of heat sources

Understanding nanoscale thermal transport is critical for nano-engineered devices such as quantum sensors, thermoelectrics, and nanoelectronics. However, despite overwhelming experimental evidence for nondiffusive heat dissipation from nanoscale heat sources, the underlying mechanisms are still not understood. In this work, we show that for nanoscale heat source spacings that are below the mean free path of the dominant phonons in a substrate, close packing of the heat sources increases in-plane scattering and enhances cross-plane thermal conduction. This leads to directional channeling of thermal transport-a novel phenomenon. By using advanced atomic-level simulations to accurately access the lattice temperature and the phonon scattering and transport properties, we finally explain the counterintuitive experimental observations of enhanced cooling for close-packed heat sources. This represents a distinct fundamental behavior in materials science with far-reaching implications for electronics and future quantum devices.

Physics of surface vibrational resonances: pillared phononic crystals, metamaterials, and metasurfaces

The introduction of engineered resonance phenomena on surfaces has opened a new frontier in surface science and technology. Pillared phononic crystals, metamaterials, and metasurfaces are an emerging class of artificial structured media, featuring surfaces that consist of pillars-or branching substructures-standing on a plate or a substrate. A pillared phononic crystal exhibits Bragg band gaps, while a pillared metamaterial may feature both Bragg band gaps and local resonance hybridization band gaps. These two band-gap phenomena, along with other unique wave dispersion characteristics, have been exploited for a variety of applications spanning a range of length scales and covering multiple disciplines in applied physics and engineering, particularly in elastodynamics and acoustics. The intrinsic placement of pillars on a semi-infinite surface-yielding a metasurface-has similarly provided new avenues for the control and manipulation of wave propagation. Classical waves are admitted in pillared media, including Lamb waves in plates and Rayleigh and Love waves along the surfaces of substrates, ranging in frequency from hertz to several gigahertz. With the presence of the pillars, these waves couple with surface resonances richly creating new phenomena and properties in the subwavelength regime and in some applications at higher frequencies as well. At the nanoscale, it was shown that atomic-scale resonances-stemming from nanopillars-alter the fundamental nature of conductive thermal transport by reducing the group velocities and generating mode localizations across the entire spectrum of the constituent material well into the terahertz regime. In this article, we first overview the history and development of pillared materials, then provide a detailed synopsis of a selection of key research topics that involve the utilization of pillars or similar branching substructures in different contexts. Finally, we conclude by providing a short summary and some perspectives on the state of the field and its promise for further future development.

Superconducting Quantum Metamaterials from Convergence of Soft and Hard Condensed Matter Science

Superconducting quantum metamaterials are expected to exhibit a variety of novel properties, but have been a major challenge to prepare as a result of the lack of appropriate synthetic routes to high-quality materials. Here, the discovery of synthesis routes to block copolymer (BCP) self-assembly-directed niobium nitrides and carbonitrides is described. The resulting materials exhibit unusual structure retention even at temperatures as high as 1000 °C and resulting critical temperature, T_c , values comparable to their bulk analogues. Applying the concepts of soft matter self-assembly, it is demonstrated that a series of four different BCP-directed mesostructured superconductors are accessible from a single triblock terpolymer. Resulting materials display a mesostructure-dependent T_c without substantial variation of the XRD-measured lattice parameters. Finally, field-dependent magnetization measurements of a sample with double-gyroid morphology show abrupt jumps comparable in overall behavior to flux avalanches. Results suggest a fruitful convergence of soft and hard condensed matter science.

Heat Transport Control and Thermal Characterization of Low-Dimensional Materials: A Review

Heat dissipation and thermal management are central challenges in various areas of science and technology and are critical issues for the majority of nanoelectronic devices. In this review, we focus on experimental advances in thermal characterization and phonon engineering that have drastically increased the understanding of heat transport and demonstrated efficient ways to control heat propagation in nanomaterials. We summarize the latest device-relevant methodologies of phonon engineering in semiconductor nanostructures and 2D materials, including graphene and transition metal dichalcogenides. Then, we review recent advances in thermal characterization techniques, and discuss their main challenges and limitations.

Gelatin as green adhesive for the preparation of a multifunctional biobased cryogel derived from bamboo industrial waste

The effective utilization of bamboo industrial waste to produce value-added products is an important subject. In this paper, a multifunctional biobased cryogel derived from bamboo industrial waste was successfully developed. Bamboo fibres were first extracted from bamboo industrial waste and then dispersed in the gelatin solution to produce bamboo fibres/gelatin cryogels (BFs/G cryogels) by a freeze-drying process. The hydrophobicities of BFs/G cryogels were further improved by modification with methyltrichlorosilane. The prepared BFs/G cryogels possessed low density (23.9-29.5 mg/cm³), high porosity (90.41-95.85%), low thermal conductivity (0.031‒0.047 W/m·K) and excellent sound-insulating performance. The presence of rigid bamboo fibres improved the mechanical performance of the BFs/G cryogel. Furthermore, the BFs/G cryogels exhibited high oil absorption capacities of 23-66 times that of their dry weights. The successful development of this cryogel provides a path for the efficient utilization of bamboo industrial waste as a renewable biomass resource.

Heat diffusion imaging: In-plane thermal conductivity measurement of thin films in a broad temperature range

This work combines the principles of the heat spreader method and the imaging capability of the thermoreflectance measurements to measure the in-plane thermal conductivity of thin films without the requirement of film suspension or multiple thermometer deposition. We refer to this hybrid technique as heat diffusion imaging. The thermoreflectance imaging system provides a temperature distribution map across the film surface. The in-plane thermal conductivity can be extracted from the temperature decay profile. By coupling the system with a cryostat, we were able to conduct measurements from 40 K to 400 K. Silicon thin film samples with and without periodic holes were measured and compared with in-plane time-domain thermoreflectance measurements and literature data as validation for heat diffusion imaging.

Evaluating the effect of grain size distribution on thermal conductivity of thermoelectric materials

The influence of grain size (d) on the thermal conductivity (k) of thermoelectric (TE) materials has been well established through experimental studies. However, the effect of grain size distribution, described by S n , on k has not been reported before. Since thermal conductivity is a key contributor to the figure of merit (ZT) for thermoelectric materials, studying the effect of grain size distribution, an important microstructural descriptor, on k is necessary. In the current study we are evaluating the effect of S n on the k of thermoelectric materials by using data reported in literature on bismuth telluride (Bi2Te3) and lead telluride (PbTe). We first check for correlations between k and d. In literature, mathematical correlations between lattice thermal conductivity (k l ) and d have already been reported but the same is missing for electronic thermal conductivity (k e ) and d. By analysing literature data for bismuth telluride and lead telluride at 300 K, we identified a linear correlation between k e and d, wherein an increase in d leads to an increase in k e . This dependence of k e on d was combined with the dependence of k l on d to establish the overall dependence of k on d. Subsequently, the grain size distribution effect was imposed by using a log normal distribution. The analysis revealed that for a given grain size, an increase in S n leads to lowering of the thermal conductivity of the material. The analysis was also extended to bimodal grain size distributions wherein the microstructure was designed in a way to contain a mixture of both nanocrystalline and microcrystalline grains.

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.

A Comprehensive Review of Strategies and Approaches for Enhancing the Performance of Thermoelectric Module

In recent years, thermoelectric (TE) technology has been emerging as a promising alternative and environmentally friendly technology for power generators or cooling devices due to the increasingly serious energy shortage and environmental pollution problems. However, although TE technology has been found for a long time and applied in many professional fields, its low energy conversion efficiency and high cost also hinder its wide application. Thus, it is still urgent to improve the thermoelectric modules. This work comprehensively reviews the status of strategies and approaches for enhancing the performance of thermoelectrics, including material development, structure and geometry improvement, the optimization of a thermal management system, and the thermal structure design. In particular, the influence of contact thermal resistance and the improved optimization methods are discussed. This work covers many fields related to the enhancement of thermoelectrics. It is found that the main challenge of TE technology remains the improvement of materials’ properties, the decrease in costs and commercialization. Therefore, a lot of research needs to be carried out to overcome this challenge and further improve the performance of TE modules. Finally, the future research direction of TE technology is discussed. These discussions provide some practical guidance for the improvement of thermoelectric performance and the promotion of thermoelectric applications.

Anisotropic and hierarchical SiC@SiO2 nanowire aerogel with exceptional stiffness and stability for thermal superinsulation

Ceramic aerogels are promising lightweight and high-efficient thermal insulators for applications in buildings, industry, and aerospace vehicles but are usually limited by their brittleness and structural collapse at high temperatures. In recent years, fabricating nanostructure-based ultralight materials has been proved to be an effective way to realize the resilience of ceramic aerogels. However, the randomly distributed macroscale pores in these architectures usually lead to low stiffness and reduced thermal insulation performance. Here, to overcome these obstacles, a SiC@SiO₂ nanowire aerogel with a nanowire-assembled anisotropic and hierarchical microstructure was prepared by using directional freeze casting and subsequent heat treatment. The aerogel exhibits an ultralow thermal conductivity of ~14 mW/m·K, an exceptional high stiffness (a specific modulus of ~24.7 kN·m/kg), and excellent thermal and chemical stabilities even under heating at 1200°C by a butane blow torch, which makes it an ideal thermally superinsulating material for applications under extreme conditions.

Phase Change Materials for Electro-Thermal Conversion and Storage: From Fundamental Understanding to Engineering Design

Advanced functional electro-thermal conversion phase change materials (PCMs) can efficiently manage the energy conversion from electrical energy to thermal energy, thereby playing a significant role in sustainable energy utilization. Considering the inherent insulating properties of pristine PCMs, electrically conductive supporting materials are widely used to encapsulate PCMs to prepare composite PCMs for electro-thermal conversion and storage. Herein, we comprehensively review the recent advances in different electro-thermal conversion PCMs, mainly including carbon-based PCMs (carbon nanotubes [CNTs], graphene, biomass-derived carbon, graphite, highly graphitized carbon, and metal organic frameworks [MOFs]-derived carbon) and MXene-based PCMs. This review aims to provide an in-depth understanding of the electrothermal conversion mechanism and the relationships between structure design (random and array-oriented structure or single and hybrid supporting materials) and electrothermal properties, thereby contributing profound theoretical and experimental bases for the construction of high-performance electro-thermal conversion PCMs. Finally, we highlight the current challenges and future prospects.

Suppressed-to-enhanced thermal transport in a Fermi-Pasta-Ulam superlattice: Mediation roles of solitons and phonons

Managing thermal transport in nanostructured materials possesses both theoretical and application value in thermoelectric and microelectronics design. Though a suppressed thermal conductivity could be easily achieved through disorder-induced phonon scattering in a superlattice, it is challenging to enhance thermal transport in a periodically designed lattice. In this paper, we show the possibility of mediating thermal conductivity from a suppressed to an enhanced value in a Fermi-Pasta-Ulam β superlattice with periodic cells of arithmetically increased nonlinearity. When the cell length is increased, thermal conductivity in the superlattice crosses over a suppressed region into an enhanced one and it is even higher than in a homogeneous lattice with the same nonlinearity strength. The mediation originates from the long-lived nonlinear wave packets as solitons across the disorder-induced interface between cells of the superlattice, while at the same time the normal vibrational modes as phonons are suppressed. Our result shows a promising strategy to manipulate thermal transport over a wide range in a superlattice with strong nonlinearity.

Achieving Minimal Heat Conductivity by Ballistic Confinement in Phononic Metalattices

Controlling the thermal conductivity of semiconductors is of practical interest in optimizing the performance of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an effective means of achieving such control; it has been proposed that the thermal conductivity of silicon could be reduced to 1 W/m/K using this approach and that a minimum in the heat conductivity would be reached for some optimal size of the inclusions. Yet the experimental verification of this design rule has been limited. In this work, we address this question by studying the thermal properties of silicon metalattices that consist of a periodic distribution of spherical inclusions with radii from 7 to 30 nm, embedded into silicon. Experimental measurements confirm that the thermal conductivity of silicon metalattices is as low as 1 W/m/K for silica inclusions and that this value can be further reduced to 0.16 W/m/K for silicon metalattices with empty pores. A detailed model of ballistic phonon transport suggests that this thermal conductivity is close to the lowest achievable by tuning the radius and spacing of the periodic inhomogeneities. This study is a significant step in elucidating the scaling laws that dictate ballistic heat transport at the nanoscale in silicon and other semiconductors.

Realization of Nanolene: A Planar Array of Perfectly Aligned, Air-Suspended Nanowires

Air suspension and alignment are fundamental requirements to make the best use of nanowires' unique properties; however, satisfying both requirements is very challenging due to the mechanical instability of air-suspended nanowires. Here, a perfectly aligned air-suspended nanowire array called "nanolene" is demonstrated, which has a high mechanical stability owing to a C-channel-shaped cross-section of the nanowires. The excellent mechanical stability is provided through geometrical modeling and finite element method simulations. The C-channel cross-section can be realized by top-down fabrication procedures, resulting in reliable demonstrations of the nanolenes with various materials and geometric parameters. The fabrication process provides large-area uniformity; therefore, nanolene can be considered as a 2D planar platform for 1D nanowire arrays. Thanks to the high mechanical stability of the proposed nanolene, perfectly aligned air-suspended nanowire arrays with an unprecedented length of 1 mm (aspect ratio ≈5100) are demonstrated. Since the nanolene can be used in an energy-efficient nanoheater, two energy-stringent sensors, namely, an air-flow sensor and a carbon monoxide gas sensor, are demonstrated. In particular, the gas sensor achieves sub-10 mW operations, which is a requirement for application in mobile phones. The proposed nanolene will pave the way to accelerate nanowire research and industrialization by providing reliable, high-performance nanowire devices.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA-VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

A carbon nanotube approach for efficient thermally insulating material with high mechanical stability and fire-retardancy

For applications in energy-saving buildings, aerospace industry, and wearable electronic devices, thermally insulating materials (TIMs) are required to possess not only low thermal conductivity but also light weight, mechanical robustness, and environmental stability. However, conventional TIMs can rarely meet these requirements. To overcome this shortcoming, we propose a new strategy for preparing TIMs. This is based on the design of a highly porous structure from carbon nanotubes (CNTs). The CNT structure is constructed by continuous winding of a hollow cylindrical CNT assembly from a high-temperature furnace and subsequent modification by the deposition of amorphous carbon (AC). The resultant sponge-like material is shown to have a record-low density of 2–4 mg cm⁻³ and a record-low thermal conductivity of 10–14 mW m⁻¹ K⁻¹. Combined with this thermal property, the sponge material also possesses fire-retardancy during burning, mechanical robustness after repeated loading and unloading to a high strain of 90%, and environmental stability from 535 to −196 °C. Such a combination of physical and mechanical properties results from the strengthening of the porous structure by virtue of AC deposition on CNT surfaces and junctions. The high performance of the new TIM constitutes the foundation for it to be used in wide areas, especially under the harsh conditions requiring multifunctionality.

Highly mechanically stable and fire-retardant thermally insulating materials (TIMs) are prepared from a cylindrical carbon nanotube (CNT) assembly.

Lightweight, mechanically flexible and thermally superinsulating rGO/polyimide nanocomposite foam with an anisotropic microstructure

We report that lightweight, anisotropic, mechanically flexible, and high performance thermally insulating materials are fabricated by the assembly of graphene oxide (GO) and polyimide (PI). With an appropriate ratio between GO and PI building blocks, the rGO/PI thermally insulating material exhibits hierarchically aligned microstructures with high porosity. These microstructures endow the rGO/PI nanocomposite with low mass density and super-insulating property (extremely low thermal conductivity of 0.012 W m^-1 K^-1 in the radial direction). Meanwhile, the introduction of PI enhances the mechanical strength and thermal stability of rGO foam. Our rGO/PI nanocomposites as super-insulating foams with a low thermal conductivity are highly attractive for potential thermal insulation applications in aerospace, wearable devices, and energy-efficient buildings.

Strong Thermopower Enhancement and Tunable Power Factor via Semimetal to Semiconductor Transition in a Transition-Metal Dichalcogenide

Electronic band engineering is a promising approach to enhance the thermopower of thermoelectric materials. In transition-metal dichalcogenides (TMDCs), this has so far only been achieved using their inherent semiconducting nature. Here, we report the thickness-modulated band engineering of nanosheets based on semimetallic platinum diselenide (PtSe₂) resulting in a thermopower enhancement of more than 50 times than that of the bulk. We obtained this by introducing a semimetal to semiconductor (SMSC) transition resulting in the formation of a bandgap. This approach based on semimetallic TMDCs provides potential advantages such as a large variation of transport properties, a decrease of the ambipolar transport effect, and a high carrier density dependence of the transport properties. Our observations suggest that the SMSC transition in TMDCs is a promising and straightforward strategy for the development of two-dimensional nanostructured thermoelectric materials.

Cross-plane heat conduction in III-V semiconductor superlattices

Thermal management is a crucial component in analyzing the performance of III-V semiconductor superlattice-based optoelectronic devices. Here we provide a rigorous physical analysis of cross-plane thermal conduction in GaAs/AlAs and their alloy-based superlattices while rigorously accounting for phonon interlayer coupling and interfacial structural characteristics. We present a comprehensive study of superlattice thermal transport, including structure-property relations, spectral and modal descriptions, and contrast it with in-plane heat conduction thereby explaining the resultant anisotropy in III-V semiconductor superlattices. Our results provide key physical insights into rational material design for thermal modulation in optoelectronic devices.

Disorder limits the coherent phonon transport in two-dimensional phononic crystal structures

Recently, increasing efforts are being made to control thermal transport via coherent phonons in periodic phononic structures; however, the direct observation of coherent phonon transport is experimentally very difficult at ambient temperature, and the importance of coherent phonons to the total thermal conductivity has not been critically assessed to date. In this study, using the non-equilibrium molecular dynamics simulations, we studied coherent phonon transport in a C3N phononic crystal (CNPnC) structure at room temperature by changing the porosity. When the holes were randomly distributed to construct the disordered C3N (D-C3N) structure, the localization of the coherent phonons was revealed by the phonon transmission coefficient, phonon wave packet simulation, phonon participation ratio and spatial energy density, which led to a significant reduction in the thermal conductivity. Finally, the effects of the length, temperature and strain on the thermal conductivity of CNPnC and D-C3N have also been discussed. Our study provides a solid understanding of the coherent phonon transport behavior, which will be beneficial for phononic-related control based on coherent phonons.

Materials Discovery and Properties Prediction in Thermal Transport via Materials Informatics: A Mini Review

There has been increasing demand for materials with functional thermal properties, but traditional experiments and simulations are high-cost and time-consuming. The emerging discipline, materials informatics, is an effective approach that can accelerate materials development by combining material science and big data techniques. Recently, materials informatics has been successfully applied to designing thermal materials, such as thermal interface materials for heat-dissipation, thermoelectric materials for power generation, and so forth. This Mini Review summarizes the research progress associated with studies regarding the prediction and discovery of materials with desirable thermal transport properties by using materials informatics. On the basis of the review of past research, perspectives are discussed and future directions for studying functional thermal materials by materials informatics are given.

Semiconductor Nanostructure Design for Thermoelectric Property Control

We present the methodologies for developing high-performance thermoelectric materials using nanostructured interfaces by reviewing our three studies and giving the new aspect of nanostructuring results. (1) Connected Si nanocrystals exhibited ultrasmall thermal conductivity. The drastic thermal conductivity reduction was brought by phonon confinement and phonon scattering. Here, we present discussion about the new aspect for phonon transport: not only nanocrystal size but also shape can contribute to thermal conductivity reduction. (2) Si films including Ge nanocrystals demonstrated that phonon and carrier conductions were independently controlled in the films, where carriers were easily transported through the interfaces between Si and Ge, while phonons could be effectively scattered at the interfaces. (3) Embedded-ZnO nanowire structure demonstrated the simultaneous realization of power factor increase and thermal conductivity reduction. The [Formula: see text] increase was caused by the interface-dominated carrier transport. The nanowire interfaces also worked as phonon scatterers, resulting in the thermal conductivity reduction.

Enhanced Photoenergy Harvesting and Extreme Thomson Effect in Hydrodynamic Electronic Systems

The thermoelectric (TE) properties of a material are dramatically altered when electron-electron interactions become the dominant scattering mechanism. In the degenerate hydrodynamic regime, the thermal conductivity is reduced and becomes a decreasing function of the electronic temperature, due to a violation of the Wiedemann-Franz law. We here show how this peculiar temperature dependence gives rise to new striking TE phenomena. These include an 80-fold increase in TE efficiency compared to the Wiedemann-Franz regime, dramatic qualitative changes in the steady state temperature profile, and an anomalously large Thomson effect. In graphene, which we pay special attention to here, these effects are further amplified due to a doubling of the thermopower.

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.

Thermoelectrics of Nanowires

The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to enhance it, including earlier theoretical and experimental work on quantum confinement effects and semimetal-to-semiconductor transition, surface engineering and complex heterostructures to enhance the carrier mobility and power factor, and the recent emergence of topological insulator NWs. For phonon transport, we broadly categorize the work on thermal conductivity of NWs into five different effects: classic size effect, acoustic softening, surface roughness, complex NW morphology, and dimensional crossover. Finally, we discuss the integration of NWs for device applications for thermoelectric power generation and cooling. We conclude our review with some outlooks for future research.