Efficient radiative cooling of low-cost BaSO paint-paper dual-layer thin films.

Many materials have been explored for the purpose of creating structures with high radiative cooling potential, such as nanocellulose-based structures and nanoparticle-based coatings, which have been reported with environmentally friendly attributes and high solar reflectance in current literature. They each have their own advantages and disadvantages in practice. It is worth noting that nanocellulose-based structures have an absorption peak in the UV wavelengths, which results in a lower total solar reflectance and, consequently, reduce radiative cooling capabilities.

Defect scattering can lead to enhanced phonon transport at nanoscale.

Defect scattering is well known to suppress thermal transport. In this study, however, we perform both molecular dynamics and Boltzmann transport equation calculations, to demonstrate that introducing defect scattering in nanoscale heating zone could surprisingly enhance thermal conductance of the system by up to 75%. We further reveal that the heating zone without defects yields directional nonequilibrium with overpopulated oblique-propagating phonons which suppress thermal transport, while introducing defects redirect phonons randomly to restore directional equilibrium, thereby enhancing thermal conductance.

Highly Tunable Cellulosic Hydrogels with Dynamic Solar Modulation for Energy-Efficient Windows.

Smart windows that can passively regulate incident solar radiation by dynamically modulating optical transmittance have attracted increasing scientific interest due to their potential economic and environmental savings. However, challenges remain in the global adoption of such systems, given the extreme variability in climatic and economic conditions across different geographical locations. Aiming these issues, a methylcellulose (MC) salt system is synthesized with high tunability for intrinsic optical transmittance (89.

Phonon Scattering Engineered Unconventional Thermal Radiation at the Nanoscale.

We show that engineering phonon scattering, such as through isotope enrichment and temperature modulation, offers the potential to achieve unconventional radiative heat transfer between two boron arsenide bulks at the nanoscale, which holds promise in applications for nonlinear thermal circuit components. A heat flux regulator is proposed, where the temperature window for stabilized heat flux exhibits a wide tunability through phonon scattering engineering. Additionally, we propose several other nonlinear thermal radiative devices, including a negative differential thermal conductance device, a temperature regulator, and a thermal diode, all benefiting from the design space enabled by isotope and temperature engineering of the phonon linewidth.

Unconventional and Dynamically Anisotropic Thermal Conductivity in Compressed Flexible Graphene Foams.

Although a variety of methods to predict the effective thermal conductivity of porous foams have been proposed, the response of such materials under dynamic compressive loading has generally not been considered. Understanding the dynamic thermal behavior will widen the potential applications of porous foams and provide insights into methods of modifying material properties to achieve desired performance. Previous experimental work on the thermal conductivity of a flexible graphene composite under compression showed intriguing behavior: the cross-plane thermal conductivity remained approximately constant with increasing compression, despite the increasing mass density.

Atmospheric Water Harvesting by Large-Scale Radiative Cooling Cellulose-Based Fabric.

Atmospheric water harvesting (AWH) has received tremendous interest because of population growth, limited freshwater resources, and water pollution. However, key challenges remain in developing efficient, flexible, and lightweight AWH materials with scalability. Here, we demonstrated a radiative cooling fabric for AWH via its hierarchically structured cellulose network and hybrid sorption-dewing mechanisms.

Raman Linewidth Contributions from Four-Phonon and Electron-Phonon Interactions in Graphene.

The Raman peak position and linewidth provide insight into phonon anharmonicity and electron-phonon interactions in materials. For monolayer graphene, prior first-principles calculations have yielded decreasing linewidth with increasing temperature, which is opposite to measurement results. Here, we explicitly consider four-phonon anharmonicity, phonon renormalization, and electron-phonon coupling, and find all to be important to successfully explain both the G peak frequency shift and linewidths in our suspended graphene sample over a wide temperature range.

Wide range continuously tunable and fast thermal switching based on compressible graphene composite foams.

Thermal switches have gained intense interest recently for enabling dynamic thermal management of electronic devices and batteries that need to function at dramatically varied ambient or operating conditions. However, current approaches have limitations such as the lack of continuous tunability, low switching ratio, low speed, and not being scalable. Here, a continuously tunable, wide-range, and fast thermal switching approach is proposed and demonstrated using compressible graphene composite foams.

Ultrawhite BaSO Paints and Films for Remarkable Daytime Subambient Radiative Cooling.

Radiative cooling is a passive cooling technology that offers great promises to reduce space cooling cost, combat the urban island effect, and alleviate the global warming. To achieve passive daytime radiative cooling, current state-of-the-art solutions often utilize complicated multilayer structures or a reflective metal layer, limiting their applications in many fields. Attempts have been made to achieve passive daytime radiative cooling with single-layer paints, but they often require a thick coating or show partial daytime cooling.

Ultrahigh Thermal Conductivity of θ-Phase Tantalum Nitride.

Extracting long-lasting performance from electronic devices and improving their reliability through effective heat management requires good thermal conductors. Taking both three- and four-phonon scattering as well as electron-phonon and isotope scattering into account, we predict that semimetallic θ-phase tantalum nitride (θ-TaN) has an ultrahigh thermal conductivity (κ), of 995 and 820  W m^{-1} K^{-1} at room temperature along the a and c axes, respectively. Phonons are found to be the main heat carriers, and the high κ hinges on a particular combination of factors: weak electron-phonon scattering, low isotopic mass disorder, and a large frequency gap between acoustic and optical phonon modes that, together with acoustic bunching, impedes three-phonon processes.

Prediction of BiTe-SbTe Interfacial Conductance and Superlattice Thermal Conductivity Using Molecular Dynamics Simulations.

Bismuth telluride (BiTe) and its alloys with antimony telluride (SbTe) have long been considered to be the best room-temperature bulk thermoelectric (TE) materials. In recent decades, proof-of-concept demonstrations on BiTe-SbTe nanostructures have shown high TE performance due to reduction in lattice thermal conductivities. Particularly, ultra-low thermal conductivities have been observed in BiTe-SbTe 1D superlattices, leading to thermoelectric figures of merit () as high as 2.

Vibrational hierarchy leads to dual-phonon transport in low thermal conductivity crystals.

Many low-thermal-conductivity (κ) crystals show intriguing temperature (T) dependence of κ: κ ∝ T (crystal-like) at intermediate temperatures whereas weak T-dependence (glass-like) at high temperatures. It has been in debate whether thermal transport can still be described by phonons at the Ioffe-Regel limit. In this work, we propose that most phonons are still well defined for thermal transport, whereas they carry heat via dual channels: normal phonons described by the Boltzmann transport equation theory, and diffuson-like phonons described by the diffusion theory.

Enhanced Thermoelectric Performance of As-Grown Suspended Graphene Nanoribbons.

Conventionally, graphene is a poor thermoelectric material with a low figure of merit () of 10-10. Although nanostructuring was proposed to improve the thermoelectric performance of graphene, little experimental progress has been accomplished. Here, we carefully fabricated as-grown suspended graphene nanoribbons with quarter-micron length and ∼40 nm width.

Decomposition of the Thermal Boundary Resistance across Carbon Nanotube-Graphene Junctions to Different Mechanisms.

Three different mechanisms are identified to contribute to thermal resistances across a carbon nanotube-graphene junction: material mismatch, nonplanar junction, and defects. To isolate the contributions of each mechanism, we have designed five types of junctions and performed nonequilibrium molecular dynamics simulations. The results show that the contributions from the three mechanisms are similar, each at around 2.

Manipulating Band Structure through Reconstruction of Binary Metal Sulfide for High-Performance Thermoelectrics in Solution-Synthesized Nanostructured Bi S I.

Reconstructing canonical binary compounds by inserting a third agent can significantly modify their electronic and phonon structures. Therefore, it has inspired the semiconductor communities in various fields. Introducing this paradigm will potentially revolutionize thermoelectrics as well.

Optical Generation and Detection of Local Nonequilibrium Phonons in Suspended Graphene.

The measured frequencies and intensities of different first- and second-order Raman peaks of suspended graphene are used to show that optical phonons and different acoustic phonon polarizations are driven out of local equilibrium inside a submicron laser spot. The experimental results are correlated with a first-principles-based multiple temperature model to suggest a considerably lower equivalent local temperature of the flexural phonons than those of other phonon polarizations. The finding reveals weak coupling between the flexural modes with hot electrons and optical phonons.

Nanocomposites from Solution-Synthesized PbTe-BiSbTe Nanoheterostructure with Unity Figure of Merit at Low-Medium Temperatures (500-600 K).

A scalable, low-temperature solution process is used to synthesize precursor material for Pb-doped Bi Sb Te thermoelectric nanocomposites. The controllable Pb-doping leads to the increase in the optical bandgap, thus delaying the onset of bipolar conduction. Furthermore, the solution synthesis enables nanostructuring, which greatly reduces thermal conductivity.

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large-Scale Solution-Synthesized Bi Te Se Hollow Nanostructures.

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ) and excellent figure of merit (z T). Herein we report a scalable (ca. 11.

Welding of Semiconductor Nanowires by Coupling Laser-Induced Peening and Localized Heating.

We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enhances film quality and charge transfer while largely maintaining basic particle morphology. During the laser peening phase, a shockwave is used to compress the film. Laser sintering comprises the second step, where a nanosecond pulse laser beam welds the nanowires.

Enhancing photo-induced ultrafast charge transfer across heterojunctions of CdS and laser-sintered TiO2 nanocrystals.

Enhancing the charge transfer process in nanocrystal sensitized solar cells is vital for the improvement of their performance. In this work we show a means of increasing photo-induced ultrafast charge transfer in successive ionic layer adsorption and reaction (SILAR) CdS-TiO2 nanocrystal heterojunctions using pulsed laser sintering of TiO2 nanocrystals. The enhanced charge transfer was attributed to both morphological and phase transformations.

Phonon lateral confinement enables thermal rectification in asymmetric single-material nanostructures.

We show that thermal rectification (TR) in asymmetric graphene nanoribbons (GNRs) is originated from phonon confinement in the lateral dimension, which is a fundamentally new mechanism different from that in macroscopic heterojunctions. Our molecular dynamics simulations reveal that, though TR is significant in nanosized asymmetric GNRs, it diminishes at larger width. By solving the heat diffusion equation, we prove that TR is indeed absent in both the total heat transfer rate and local heat flux for bulk-size asymmetric single materials, regardless of the device geometry or the anisotropy of the thermal conductivity.

Measurement of thermal conductivity of PbTe nanocrystal coated glass fibers by the 3ω method.

Fiber-based thermoelectric materials can conform to curved surfaces to form energy harvesting devices for waste heat recovery. Here we investigate the thermal conductivity in the axial direction of glass fibers coated with lead telluride (PbTe) nanocrystals using the self-heated 3ω method particularly at low frequency. While prior 3ω measurements on wire-like structures have only been demonstrated for high thermal conductivity materials, the present work demonstrates the suitability of the 3ω method for PbTe nanocrystal coated glass fibers where the low thermal conductivity and high aspect ratio result in a significant thermal radiation effect.

Optical properties of ordered carbon nanotube arrays grown in porous anodic alumina templates.

We have synthesized ordered carbon nanotube (CNT) arrays in porous anodic alumina (PAA) matrix, and have characterized their total optical reflectance and bi-directional reflectance distribution function after each processing step of the microwave plasma chemical vapor deposition process (MPCVD). For a PAA sample without CNT growth, the reflectance shows an oscillating pattern with wavelength that agrees reasonably with a multilayer model. During the MPCVD process, heating the sample significantly reduces the reflectance by 30-40%, the plasma treatment reduces the reflectance by another 5-10%, and the CNT growth further reduces the reflectance by 2-3%.

Synthesis and thermoelectric properties of compositional-modulated lead telluride-bismuth telluride nanowire heterostructures.

We demonstrate the rational solution-phase synthesis of compositional modulated telluride nanowire heterostructures containing lead telluride (PbTe) and bismuth telluride (Bi2Te3). By tuning the ratio between PbTe and Bi2Te3 through adjusting the amount of critical reactants and precursors during the synthesis, the influence of composition on the thermoelectric properties of the nanowire heterostructures has been investigated in hot pressed nanocomposite pellets. Measurements of the thermoelectric properties show strongly reduced thermal conductivity that leads to an enhanced thermoelectric figure of merit (ZT) of 1.

Energy relaxation in CdSe nanocrystals: the effects of morphology and film preparation.

Ultrafast time-resolved absorption spectroscopy is used to investigate exciton dynamics in CdSe nanocrystal films. The effects of morphology, quantum-dot versus quantum-rod, and preparation of nanocrystals in a thin film form are investigated. The measurements revealed longer intraband exciton relaxation in quantum-rods than in quantum-dots.