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.

Backscattering limit of nanoscale heat conduction.

Being able to achieve extremely low thermal conductivities is of fundamental importance as well as of practical interest for thermal energy conversion and storage materials. By incorporating backscattering effects on thermal phonon transport, here we introduce a new lower limit for the thermal conductivities of nanoscale engineered materials. Our formulation shows that surface backscattering allows to reach a minimum limit for thermal energy transport at the nanoscale, achieving thermal conductivities which are orders of magnitude smaller than those obtained with classical approaches.

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.

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.

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.

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.

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

Thermal transport in nanostructures has attracted considerable attention in the last decade but the precise effects of surfaces on heat conduction have remained unclear due to a limited accuracy in the treatment of phonon surface scattering phenomena. Here, we investigate the impact of phonon-surface scattering on the distribution of thermal energy across phonon wavelengths and mean free paths in Si and SiGe nanowires. We present a rigorous and accurate description of phonon scattering at surfaces and predict and analyse nanowire heat spectra for different diameters and surface conditions. We show that the decrease in the diameter and increased roughness and correlation lengths makes the heat phonon spectra significantly shift towards short wavelengths and mean free paths. We also investigate the emergence of phonon confinement effects for small diameter nanowires and different surface scattering properties. Computed results for bulk materials show excellent agreement with recent experimentally-based approaches that reconstruct the mean-free-path heat spectra. Our phonon surface scattering model allows for an accurate theoretical extraction of heat spectra in nanowires and contributes to elucidate the development of critical phonon transport modes such as phonon confinement and coherent interference effects.

Mass Separation by Metamaterials.

Being able to manipulate mass flow is critically important in a variety of physical processes in chemical and biomolecular science. For example, separation and catalytic systems, which requires precise control of mass diffusion, are crucial in the manufacturing of chemicals, crystal growth of semiconductors, waste recovery of biological solutes or chemicals, and production of artificial kidneys. Coordinate transformations and metamaterials are powerful methods to achieve precise manipulation of molecular diffusion. Here, we introduce a novel approach to obtain mass separation based on metamaterials that can sort chemical and biomolecular species by cloaking one compound while concentrating the other. A design strategy to realize such metamaterial using homogeneous isotropic materials is proposed. We present a practical case where a mixture of oxygen and nitrogen is manipulated using a metamaterial that cloaks nitrogen and concentrates oxygen. This work lays the foundation for molecular mass separation in biophysical and chemical systems through metamaterial devices.

Phonon wave interference and thermal bandgap materials.

Wave interference modifies phonon velocities and density of states, and in doing so creates forbidden energy bandgaps for thermal phonons. Materials that exhibit wave interference effects allow the flow of thermal energy to be manipulated by controlling the material's thermal conductivity or using heat mirrors to reflect thermal vibrations. The technological potential of these materials, such as enhanced thermoelectric energy conversion and improved thermal insulation, has fuelled the search for highly efficient phonon wave interference and thermal bandgap materials. In this Progress Article, we discuss recent developments in the understanding and manipulation of heat transport. We show that the rational design and fabrication of nanostructures provides unprecedented opportunities for creating wave-like behaviour of heat, leading to a fundamentally new approach for manipulating the transfer of thermal energy.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons.

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.

Sound and heat revolutions in phononics.

The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.

Narrow low-frequency spectrum and heat management by thermocrystals.

By transforming heat flux from particle to wave phonon transport, we introduce a new class of engineered material to control thermal conduction. We show that rationally designed nanostructured alloys can lead to a fundamental new approach for thermal management, guiding heat as photonic and phononic crystals guide light and sound, respectively. Novel applications for these materials include heat waveguides, thermal lattices, heat imaging, thermo-optics, thermal diodes, and thermal cloaking.

Diamond-structured photonic crystals.

Certain periodic dielectric structures can prohibit the propagation of light for all directions within a frequency range. These 'photonic crystals' allow researchers to modify the interaction between electromagnetic fields and dielectric media from radio to optical wavelengths. Their technological potential, such as the inhibition of spontaneous emission, enhancement of semiconductor lasers, and integration and miniaturization of optical components, makes the search for an easy-to-craft photonic crystal with a large bandgap a major field of study. This progress article surveys a collection of robust complete three-dimensional dielectric photonic-bandgap structures for the visible and near-infrared regimes based on the diamond morphology together with their specific fabrication techniques. The basic origin of the complete photonic bandgap for the 'champion' diamond morphology is described in terms of dielectric modulations along principal directions. Progress in three-dimensional interference lithography for fabrication of near-champion diamond-based structures is also discussed.

Exploring for 3D photonic bandgap structures in the 11 f.c.c. space groups.

The promise of photonic crystals and their potential applications has attracted considerable attention towards the establishment of periodic dielectric structures that in addition to possessing robust complete bandgaps, can be easily fabricated with current techniques. A number of theoretical structures have been proposed. To date, the best complete photonic bandgap structure is that of diamond networks having Fd3m symmetry (2-3 gap). The only other known complete bandgap in a face-centred-cubic (f.c.c.) lattice structure is that of air spheres in a dielectric matrix (8-9 gap; the so called 'inverse-opal' structure). Importantly, there is no systematic approach to discovering champion photonic crystal structures. Here we propose a level-set approach based on crystallography to systematically examine for photonic bandgap structures and illustrate this approach by applying it to the 11 f.c.c. groups. This approach gives us an insight into the effects of symmetry and connectivity. We classify the F-space groups into four fundamental geometries on the basis of the connectivity of high-symmetry Wyckoff sites. Three of the fundamental geometries studied display complete bandgaps--including two: the F-RD structure with Fm3m symmetry and a group 216 structure with F43m symmetry that have not been reported previously. By using this systematic approach we were able to open gaps between the 2-3, 5-6 and 8-9 bands in the f.c.c. systems.

Triply periodic bicontinuous structures through interference lithography: a level-set approach.

Interference lithography holds the promise of fabricating large-area, defect-free photonic structures on the sub-micrometer scale both rapidly and cheaply. There is a need for a procedure to establish a connection between the structures that are formed and the parameters of the interfering beams. There is also a need to produce self-supporting three-dimensional bicontinuous structures. A generic technique correlating parameters of the interfering beams with the symmetry elements present in the resultant structures by a level-set approach is developed. A particular space group is ensured by equating terms of the intensity equation to a representative level surface of the desired space group. Single- and multiple-exposure techniques are discussed. The beam parameters for certain cubic bicontinuous structures relevant to photonic crystals, viz.,the diamond(D), the simple cubic (P), and the chiral gyroid (G) are derived by utilizing either linear or elliptically polarized light.