High-performance 2D electronic devices enabled by strong and tough two-dimensional polymer with ultra-low dielectric constant.

As the feature size of microelectronic circuits is scaling down to nanometer order, the increasing interconnect crosstalk, resistance-capacitance (RC) delay and power consumption can limit the chip performance and reliability. To address these challenges, new low-k dielectric (k < 2) materials need to be developed to replace current silicon dioxide (k = 3.9) or SiCOH, etc.

Anomalous lattice thermal conductivity increase with temperature in cubic GeTe correlated with strengthening of second-nearest neighbor bonds.

Understanding thermal transport mechanisms in phase change materials is critical to elucidating the microscopic picture of phase transitions and advancing thermal energy conversion and storage. Experiments consistently show that cubic phase germanium telluride (GeTe) has an unexpected increase in lattice thermal conductivity with rising temperature. Despite its ubiquity, resolving its origin has remained elusive.

Unusual Electrical Conductivity Enhancement in Stable n-Type Carbon Nanotube Networks.

Organic molecule-doped n-type single-walled carbon nanotube (SWCNT) networks are promising candidates for advanced energy applications, such as flexible thermoelectrics and photovoltaics. Yet charge transport in n-type SWCNTs is limited by two factors: i) charge localization impeding inter-tube transport caused by disordered mesostructure of randomly oriented SWCNTs and ii) reduction of charge carrier concentration driven by oxidation. Herein, studied the relationship between the mesostructure and thermoelectric properties of n-type SWCNTs obtained by surfactant-functionalization and polymer-dopant grafting.

Significantly Enhanced Thermal Conductivity of hBN/PTFE Composites: A Comprehensive Study of Filler Size and Dispersion.

High-temperature polymers are attractive for applications in extreme temperatures, where they maintain their mechanical flexibility and electrical insulating properties. However, their heat dissipation capability is limited due to their intrinsically low thermal conductivities. Hexagonal boron nitride (hBN) is a chemically inert, thermally stable, and electrically insulative compound with a high thermal conductivity, making it an ideal candidate as a filler within a high-temperature polymer matrix to increase the thermal conductivity.

Thermal Conductivity of Covalent-Organic Frameworks.

Covalent-organic frameworks (COFs) are a highly promising class of materials that can provide an excellent platform for thermal management applications. In this Perspective, we first review previous works on the thermal conductivities of COFs. Then we share our insights on achieving high, low, and switchable thermal conductivities of future COFs.

Phase Stability of Hexagonal/Cubic Boron Nitride Nanocomposites.

Boron nitride (BN) is an exceptional material, and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate, and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the coexistence of two phases can lead to strong nonlinear optical properties and low thermal conductivity at room temperature.

Pore-Confined Polymers Enhance the Thermal Conductivity of Polymer Nanocomposites.

Molecularly confined polymer fillers in nanopores were found to give superior mechanical properties of polymer nanocomposites. In this work, we study the thermal conductivity of such nanocomposites and unveil the effect of polymer confinement on thermal conductivity. Using the time-domain thermoreflectance method, we measure the cross-plane thermal conductivity of polymer nanocomposites that consist of polystyrene fillers confined within a nanoporous organosilicate matrix.

Thermal Percolation in Well-Defined Nanocomposite Thin Films.

Thermal percolation in polymer nanocomposites─the rapid increase in thermal transport due to the formation of networks among fillers─is the subject of great interest in thermal management ranging from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold.

Doping-Enabled Reconfigurable Strongly Correlated Phase in a Quasi-2D Perovskite.

Highlighted by the discovery of high-temperature superconductivity, strongly correlated oxides with highly distorted perovskite structures serve as intriguing model systems for pursuing emerging materials physics and testing technological concepts. While 3d correlated oxides with a distorted perovskite structure are not uncommon, their 4d counterparts are unfortunately rare. In this work, we report the tuning of the electrical and optical properties of a quasi-2D perovskite niobate CsBiNbO via hydrogenation.

Remarkably Weak Anisotropy in Thermal Conductivity of Two-Dimensional Hybrid Perovskite Butylammonium Lead Iodide Crystals.

Two-dimensional (2D) hybrid organic-inorganic perovskites consisting of alternating organic and inorganic layers are a new class of layered structures. They have attracted increasing interest for photovoltaic, optoelectronic, and thermoelectric applications, where knowing their thermal transport properties is critical. We carry out both experimental and computational studies on thermal transport properties of 2D butylammonium lead iodide crystals and find their thermal conductivity is ultralow (below 0.

Single-Crystal SnSe Thermoelectric Fibers via Laser-Induced Directional Crystallization: From 1D Fibers to Multidimensional Fabrics.

Single-crystal tin selenide (SnSe), a record holder of high-performance thermoelectric materials, enables high-efficient interconversion between heat and electricity for power generation or refrigeration. However, the rigid bulky SnSe cannot satisfy the applications for flexible and wearable devices. Here, a method is demonstrated to achieve ultralong single-crystal SnSe wire with rock-salt structure and high thermoelectric performance with diameters from micro- to nanoscale.

Supercompliant and Soft (CH_{3}NH_{3})_{3}Bi_{2}I_{9} Crystal with Ultralow Thermal Conductivity.

In this Letter, we show the phonon dispersion of (CH_{3}NH_{3})_{3}Bi_{2}I_{9} single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft.

Anderson Localization for Better Thermoelectrics?

Solid-state thermoelectrics are of great interest because they directly convert between thermal and electrical energy. However, wider application of thermoelectrics is dependent upon improving their performance. Anderson localization of charge carriers has been generally perceived as detrimental to thermoelectrics, but in their paper in this issue of ACS Nano, Lee et al.

Experimental Phonon Dispersion and Lifetimes of Tetragonal CHNHPbI Perovskite Crystals.

Hybrid organic-inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetragonal CHNHPbI single crystals at both 200 and 300 K by high-energy resolution inelastic X-ray scattering, which enables a thorough understanding of the underlying mechanisms for the ultralow thermal conductivity. Notably, we observed unusual and significant phonon dips along the [100] and [110] directions at both temperatures.

Tunable thermal conductivity of π-conjugated two-dimensional polymers.

Two-dimensional (2D) polymers are organic analogues of graphene. Compared to graphene, 2D polymers offer a higher degree of tunability in regards to structure, topology, and physical properties. The thermal transport properties of 2D polymers play a crucial role in their applications, yet remain largely unexplored.

Modeling assisted evaluation of direct electricity generation from waste heat of wastewater via a thermoelectric generator.

The thermal energy represents a significant portion of energy potential in municipal wastewater and may be recovered as electricity by a thermoelectric generator (TEG). Converting heat to all-purpose electricity by TEG has been demonstrated with large heat gradients, but its application in waste heat recovery from wastewater has not been well evaluated. Herein, a bench-scale BiTe-based waste heat recovery system was employed to generate electricity from a low temperature gradient through a combination of experiments and mathematical modeling.

Thermal Switching of Thermoresponsive Polymer Aqueous Solutions.

Thermal switches are of great importance to thermal management in a wide variety of applications. However, traditional thermal switches suffer from being large and having slow transition rates. To overcome these limitations, we took advantage of abrupt second-order phase transitions in thermoresponsive polymer aqueous solutions to enable fast thermal switching.

Significantly High Thermal Rectification in an Asymmetric Polymer Molecule Driven by Diffusive versus Ballistic Transport.

Tapered bottlebrush polymers have novel nanoscale polymer architecture. Using nonequilibrium molecular dynamics simulations, we showed that these polymers have the unique ability to generate thermal rectification in a single polymer molecule and offer an exceptional platform for unveiling different heat conduction regimes. In sharp contrast to all other reported asymmetric nanostructures, we observed that the heat current from the wide end to the narrow end (the forward direction) in tapered bottlebrush polymers is smaller than that in the opposite direction (the backward direction).

Unusually low and density-insensitive thermal conductivity of three-dimensional gyroid graphene.

Graphene has excellent mechanical, thermal and electrical properties. However, there are limitations in utilizing monolayers of graphene for mechanical engineering applications due to its atomic thickness and lack of bending rigidity. Synthesizing graphene aerogels or foams is one approach to utilize graphene in three-dimensional bulk forms.

Importance of the Hubbard correction on the thermal conductivity calculation of strongly correlated materials: a case study of ZnO.

The wide bandgap semiconductor, ZnO, has gained interest recently as a promising option for use in power electronics such as thermoelectric and piezoelectric generators, as well as optoelectronic devices. Though much work has been done to improve its electronic properties, relatively little is known of its thermal transport properties with large variations in measured thermal conductivity. In this study, we examine the effects of a Hubbard corrected energy functional on the lattice thermal conductivity of wurtzite ZnO calculated using density functional theory and an iterative solution to the Boltzmann transport equation.

Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys.

PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated.

Resonant bonding leads to low lattice thermal conductivity.

Understanding the lattice dynamics and low thermal conductivities of IV-VI, V2-VI3 and V materials is critical to the development of better thermoelectric and phase-change materials. Here we provide a link between chemical bonding and low thermal conductivity. Our first-principles calculations reveal that long-ranged interaction along the 〈100〉 direction of the rocksalt structure exist in lead chalcogenides, SnTe, Bi2Te3, Bi and Sb due to the resonant bonding that is common to all of them.

Effect of aluminum on the thermoelectric properties of nanostructured PbTe.

In the present work, the effect of aluminum (Al) on the thermoelectric properties of PbTe is studied. Aluminum doped PbTe samples, fabricated by a ball milling and hot pressing, have Seebeck coefficients between -100 and -200 μV K-1 and electrical conductivities of (3.6-18) × 104 S m-1 at room temperature, which means that Al is an effective donor in PbTe.