Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2.

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

Intrinsically Low Thermal Conductivity in the Most Lithium-Rich Binary Stannide Crystalline Li5Sn.

Using ab initio lattice dynamics and a unified heat transport theory, we compute the lattice thermal conductivity (κL) of Li5Sn, a newly synthesized crystalline material for Li-ion batteries. The weak bonding in the Li-rich environment leads to significant softening of the optical phonon modes, temperature-induced hardening, and strong anharmonicity. This complexity is captured in the particle-like and glass-like components of κL by accounting for the temperature-dependent interatomic force constants acting on the renormalized phonon frequencies and three- and four-phonon scatterings contributing to the phonon lifetime. We predict very low room-temperature κL values of 0.857, 0.599, and 0.961 W/mK for the experimental Cmcm phase and 0.996, 0.908, and 1.385 W/mK for the theoretically predicted Immm phase along the main crystallographic directions. Both phases display complex crystal behavior with glass-like transport exceeding 20% above room-temperature and an unusual κL temperature dependence. Our results can be used to inform system-level thermal models of Li-ion batteries.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 from ab initio Lattice Dynamics.

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κL ) and a non-standard κL ∝T-0.491 dependence. At high T, the departure from the standard κL ∝T-1 law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κL =2.98 W mK-1 arises mainly from the conventional particle-like propagation of phonons, there is also a significant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κL with a weak T-dependence points to a new utility for LaWN3 in energy technology applications, and motivates synthesis and exploration of nitride perovskites.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2.

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS2) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity (κph). With the further inclusion of phonon-electron scattering, κph reduces to 1.78 W mK-1. Nevertheless, the total thermal conductivity (κtotal) of 7.82 W mK-1 is dominated by the electronic thermal conductivity (κe) of 6.04 W mK-1. Due to the electron-phonon coupling, κe differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κe are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κe and κph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

Anisotropic Phononic and Electronic Thermal Transport in BeN4.

Beryllium polynitride (BeN4) has been recently synthesized under high-pressure conditions [Bykov et al. Phys. Rev. Lett. 2021, 126, 175501]. Its anisotropic lattice structure dependent on the applied pressure motivates exploration of its thermal transport properties with a theoretical framework that combines the Boltzmann transport equation with ab initio calculations. The bonding anisotropy (impacting the phonon and electron group velocities) and bonding anharmonicity (captured through three- and four-phonon scatterings) are reflected in the strong anisotropy of both phononic and electronic components of the thermal conductivity. Moreover, the pressure-driven evolution of the interlayer Be-N bonding, from partially covalent (under high-pressure synthesis conditions) to van der Waals (under ambient pressure), drives a largely interlayer thermal conductivity. These findings highlight an alternative strategy for achieving directional control of the thermal transport in synthetic materials.

First-principles prediction of infrared phonon and dielectric function in biaxial hyperbolic van der Waals crystal α-MoO3.

Layered biaxial hyperbolic molybdenum trioxide (α-MoO3) with weak van der Waals (vdW) interlayer bonding recently received extensive attention due to its anisotropic dielectric response to infrared (IR) radiation, which couples to the lattice vibrations and allows for manipulating the radiative energy transport. However, the understanding of IR-active phonon modes and dielectric function of it has not yet been fully achieved. Here, by utilizing mode-level first-principles analysis based on density functional theory (DFT), the phonon modes contributing to the IR dielectric response of α-MoO3 are fully determined. The anisotropic IR-active modes are identified from lattice vibration analysis, allowing for a clear evaluation of the IR absorption contribution from the weak or strong IR phonon modes. By further employing anharmonic-lattice dynamics calculations, the damping of the corresponding IR modes is directly obtained. This approach enables predictions of IR optical properties without any fitting or assumed parameters. Our predictions bridge the scientific gap of comprehensively understanding the unreported IR-active phonon modes of α-MoO3 and overall agree well with available experimental data, placing our DFT-based method at a privileged stage for accurately predicting the IR optical properties of α-MoO3. These comprehensive understandings of the IR phonons and dielectric properties of α-MoO3 pave the way for nanophotonic devices with tunable functionalities and enable design of α-MoO3 for advanced optical devices.

Phononic Thermal Transport along Graphene Grain Boundaries: A Hidden Vulnerability.

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra-narrow and aligned along the temperature gradient direction. Non-equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κp ) along linear GBs comprising periodically repeating pentagon-heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κp reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon-phonon scattering. The non-monotonic dependence with dislocation density of κp uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport.

Ultralow Thermal Conductivity in Two-Dimensional MoO3.

Monolayer molybdenum trioxide (MoO3) is an emerging two-dimensional (2D) material with high electrical conductivity but unexplored thermal conductivity. Using first-principles calculations and a Boltzmann transport theoretical framework, we predict a record low room-temperature phonon thermal conductivity (κp) of 1.57 and 1.26 W/mK along the principal in-plane directions of the MoO3 monolayer. The behavior is attributed to the combination of soft flexural and in-plane acoustic modes, which are coupled through the finite layer thickness, and to the strong bonding anharmonicity, which gives rise to significant 3- and 4-phonon scattering. These insights suggest new indicators for guiding the search of 2D materials with low κp and motivates κp measurements in MoO3 and its applications as a thermoelectric and thermally protective material.

Ab initio predictions of graphite-like phase with anomalous grain boundaries and flexoelectricity from collapsed carbon nanotubes.

Although large-radius carbon nanotubes (CNTs) are now available in macroscopic quantities, little is known about their condensed phase. Large-scale density functional theory calculations predict a low energy phase in which the same-diameter "dog-bone" collapsed CNTs form a graphite-like phase with complex, anomalous grain boundaries (GBs). The excess GB volume does not prevent the strong van der Waals coupling of the flattened CNT sides into AB stacking. The associated GB energetics is dominated by the van der Waals energy penalty and high curvature bending of the loop CNT edges, which exhibit reactivity and flexoelectricity. The large density and superior mechanical rigidity of the proposed microstructural organization as well as the GB flexoelectricity are desirable properties for developing ultra-strong composites based on large-radius CNTs.

One-dimensional intergrowths in two-dimensional zeolite nanosheets and their effect on ultra-selective transport.

Zeolite MFI is a widely used catalyst and adsorbent that also holds promise as a thin-film membrane. The discovery of nanometre-thick two-dimensional (2D) MFI nanosheets has facilitated methods for thin-film zeolite fabrication that open new horizons for membrane science and engineering. However, the crystal structure of 2D-MFI nanosheets and their relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cell-wide intergrowths of zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1D or near-1D-MEL domains, and show that a fraction of nanosheets have high (~25% by volume) MEL content while the majority of nanosheets are MEL-free. Atomistic simulations show that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores compared to pristine MFI nanosheets, and permeation experiments show a separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed. Confined growth in graphite is shown to increase the MEL content in MFI nanosheets. Our observation of these intergrowths suggests strategies for the development of ultra-selective zeolite membranes.

Intrinsic twist in Iß cellulose microfibrils by tight-binding objective boundary calculations.

Objective boundary conditions are used to simulate at the atomistic scale cellulose Iß microfibrils. The method enables for the first time a direct calculation of the structural twist from a self-consistent charge density-functional-based tight-binding description of interatomic interactions. Calculations reveal that microfibrils are stabilized under a uniform right-handed twist whose magnitude depends on the area and the shape of the microfibril cross-section. The latter behavior highlights the distinct structural effects imprinted by the complex hydrogen bonding network and the differences in the relative shear strength between the hydrogen and van der Waals interactions: While the intrachain bonding gives a disposition for severe twisting in the glycosidic linkages, the interchain hydrogen and van der Waals bonding contribute to the development of twist at the microfibril level. The interchain hydrogen bonding is much more effective than the van der Waals one in counterbalancing the intrinsic tendency for twist of the microfibril.

Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals.

Thin bilayers of contrasting nanomaterials are ubiquitous in solution-processed electronic devices and have potential relevance to a number of applications in flexible electronics. Motivated by recent mesoscopic simulations demonstrating synergistic mechanical interactions between thin films of single-wall carbon nanotubes (SWCNTs) and spherical nanocrystal (NC) inclusions, we use a thin-film wrinkling approach to query the compressive mechanics of hybrid nanotube/nanocrystal coatings adhered to soft polymer substrates. Our results show an almost 2-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of a thin interpenetrating overlayer of semiconductor NCs. Mesoscopic distinct-element method simulations further support the experimental findings by showing that the additional noncovalent interfaces introduced by nanocrystals enhance the modulus of the SWCNT network and hinder network wrinkling.

Excluded Volume Approach for Ultrathin Carbon Nanotube Network Stabilization: A Mesoscopic Distinct Element Method Study.

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.

Zig-zag twins and helical phase transformations.

We demonstrate the large bending deformation induced by an array of permanent magnets (applied field ∼0.02 T) designed to minimize poles in the bent state of the crystal. Planar cantilevers of NiMnGa (5M modulated martensite) ferromagnetic shape memory alloy deform into an arched shape according to theory, with a zig-zag microstructure that complies with the kinematic and magnetic compatibility between adjacent twin variants. A general theory of bent and twisted states is given, applicable to both twinning and austenite/martensite transformations. Some of these configurations achieve order-of-magnitude amplification of rotation and axial strain. We investigate also atomistic analogues of these bent and twisted configurations with perfect interfaces between phases. These mechanisms of large deformation, induced by small magnetic fields or temperature changes, have potential application to the development of new actuation technologies for micro-robotic systems.

Thermal Transport across Surfactant Layers on Gold Nanorods in Aqueous Solution.

Ultrafast transient absorption experiments and molecular dynamics simulations are utilized to investigate the thermal transport between aqueous solutions and cetyltrimethylammonium bromide (CTAB)- or polyethylene glycol (PEG)-functionalized gold nanorods (GNRs). The transient absorption measurement data are interpreted with a multiscale heat diffusion model, which incorporates the interfacial thermal conductances predicted by molecular dynamics. According to our observations, the effective thermal conductance of the GNR/PEG/water system is higher than that of the GNR/CTAB/water system with a surfactant layer of the same length. We attribute the enhancement of thermal transport to the larger thermal conductance at the GNR/PEG interface as compared with that at the GNR/CTAB interface, in addition to the water penetration into the hydrophilic PEG layer. Our results highlight the role of the GNR/polymer thermal interfaces in designing biological and composite-based heat transfer applications of GNRs, and the importance of multiscale analysis in interpreting transient absorption data in systems consisting of low interfacial thermal conductances.

Nanowires with dislocations for ultralow lattice thermal conductivity.

Nanostructures grown by screw dislocations have been successfully synthesized in a range of materials, including thermoelectric materials, but the impact of these extended crystallographic defects on thermal properties of these nanostructures is not known. We investigate thermal transport in PbSe and SiGe nanowires storing screw dislocations via equilibrium molecular dynamics simulations. The inherent one dimensionality and the combined presence of a reconstructed surface and dislocation yield ultralow thermal conductivity values. Our simulations suggest that the large dislocation strain field in nanowires may play a key role in suppressing the thermal conductivity of thermoelectric nanomaterials to increase their thermoelectric figure of merit.

Rings and rackets from single-wall carbon nanotubes: manifestations of mesoscale mechanics.

We combine experiments and distinct element method simulations to understand the stability of rings and rackets formed by single-walled carbon nanotubes assembled into ropes. Bending remains a soft deformation mode in ropes because intra-rope sliding of the constituent nanotubes occurs with ease. Our simulations indicate that the formation of these aggregates can be attributed to the mesoscopic mechanics of entangled nanotubes and to the sliding at the contacts. Starting from the single-walled carbon nanotubes, the sizes of the rings and rackets' heads increase with the rope diameter, indicating that the stability of the experimental aggregates can be largely explained by the competition between bending and van der Waals adhesion energies. Our results and simulation method should be useful for understanding nanoscale fibers in general.

Nanomechanics of Twisted Mono- and Few-Layer Graphene Nanoribbons.

Enabled by the technique of objective molecular dynamics, we reveal the unusual mechanics exhibited by nanoscale twisted graphene nanoribbons containing up to seven layers. Unlike in a linear-elastic plate, we find that the deformation practically does not contain contributions associated with in-plane shearing but largely with inhomogeneous stretching and compression of the constituent layers. The whole twisted structure undergoes shortening when no axial force is applied, while the constituent layers store various strain energies, depending on their location. We capture this behavior with a simple model and show that the deviations from the plate model are increasing with the number of layers and width of the ribbon. Our results are especially relevant for the experimental efforts of measuring graphene's shear modulus.

Electrically active screw dislocations in helical ZnO and Si nanowires and nanotubes.

While the presence of axial screw dislocations in helical nanowires and nanotubes is known to be due to the growth process, their effect on the electronic properties remains unexplored. Relying on objective molecular dynamics simulations coupled to density functional tight-binding models for ZnO and Si, and supporting density functional theory calculations, we predict significant screw-dislocation-induced band gap modifications in both materials. The effect originates in the highly distorted cores and should be present at radii larger than those considered in our simulations (maximum ∼2 nm) as well as in other materials. The observed band gap dependences on the size of the Burgers vector and wall thickness could motivate new strategies for growing, via the screw dislocation mechanism, stable nanostructures with desired band gaps.

Nanoscale bending of multilayered boron nitride and graphene ribbons: experiment and objective molecular dynamics calculations.

By combining experiments performed on nanoribbons in situ within a high-resolution TEM with objective molecular dynamics simulations, we reveal common mechanisms in the bending response of few-layer-thick hexagonal boron nitride and graphene nanoribbons. Both materials are observed forming localized kinks in the fully reversible bending experiments. Microscopic simulations and theoretical analysis indicate platelike bending behavior prior to kinking, in spite of the possibility of interlayer sliding, and give the critical curvature for the kinking onset. This behavior is distinct from the rippling and kinking of multi- and single-wall nanotubes under bending. Our findings have implications for future study of nanoscale layered materials, including nanomechanical device design.