Wiedemann-Franz Law for Molecular Hopping Transport

The effect of temperature oscillations on energy storage rectification in harmonic systems

Rectification, the preferential transport of a current in one direction through a system, has garnered significant attention in molecules because of its importance for controlling thermal and electronic currents at the nanoscale. Here, we report the presence of energy storage rectification effects in a molecular chain. This phenomenon is generated by subjecting a harmonic molecular chain to an oscillating temperature gradient and showing that the energy absorption rate of the system depends on the direction of the gradient. We examine how the energy storage rectification ratios in the chain are affected by the oscillating gradient, asymmetry in the chain, and the system parameters. We find that energy storage rectification can be observed in harmonic lattice structures with time-dependent temperatures and that, correspondingly, anharmonicity is not required to generate this rectification mechanism in such systems.

Molecular heat transport across a time-periodic temperature gradient

The time-periodic modulation of a temperature gradient can alter the heat transport properties of a physical system. Oscillating thermal gradients give rise to behaviors such as modified thermal conductivity and controllable time-delayed energy storage that are not present in a system with static temperatures. Here, we examine how the heat transport properties of a molecular lattice model are affected by an oscillating temperature gradient. We use analytical analysis and molecular dynamics simulations to investigate the vibrational heat flow in a molecular lattice system consisting of a chain of particles connected to two heat baths at different temperatures, where the temperature difference between baths is oscillating in time. We derive expressions for heat currents in this system using a stochastic energetics framework and a nonequilibrium Green's function approach that is modified to treat the nonstationary average energy fluxes. We find that emergent energy storage, energy release, and thermal conductance mechanisms induced by the temperature oscillations can be controlled by varying the frequency, waveform, and amplitude of the oscillating gradient. The developed theoretical approach provides a general framework to describe how vibrational heat transmission through a molecular lattice is affected by temperature gradient oscillations.

Optical Kinetic Theory of Nonlinear Multimode Photonic Networks

Recent experimental developments in multimode nonlinear photonic circuits (MMNPCs), have motivated the development of an optical thermodynamic theory that describes the equilibrium properties of an initial beam excitation. However, a nonequilibrium transport theory for these systems, when they are in contact with thermal reservoirs, is still terra incognita. Here, by combining Landauer and kinematics formalisms we develop a universal one-parameter scaling theory that describes the whole transport behavior from the ballistic to the diffusive regime, including both positive and negative optical temperature scenarios. We also derive a photonic version of the Wiedemann-Franz law that connects the thermal and power conductivities. Our work paves the way toward a fundamental understanding of the transport properties of MMNPCs and may be useful for the design of all-optical cooling protocols.

Anderson Localization of Phonons in Thermally Superinsulating Graphene Aerogels with Metal-Like Electrical Conductivity

In the quest to improve energy efficiency and design better thermal insulators, various engineering strategies have been extensively investigated to minimize heat transfer through a material. Yet, the suppression of thermal transport in a material remains elusive because heat can be transferred by multiple energy carriers. Here, the realization of Anderson localization of phonons in a random 3D elastic network of graphene is reported. It is shown that thermal conductivity in a cellular graphene aerogel can be drastically reduced to 0.9 mW m-1 K-1 by the application of compressive strain while keeping a high metal-like electrical conductivity of 120 S m-1 and ampacity of 0.9 A. The experiments reveal that the strain can cause phonon localization over a broad compression range. The remaining heat flow in the material is dominated by charge transport. Conversely, electrical conductivity exhibits a gradual increase with increasing compressive strain, opposite to the thermal conductivity. These results imply that strain engineering provides the ability to independently tune charge and heat transport, establishing a new paradigm for controlling phonon and charge conduction in solids. This approach will enable the development of a new type of high-performance insulation solutions and thermally superinsulating materials with metal-like electrical conductivity.

Energy transport between heat baths with oscillating temperatures

Energy transport is a fundamental physical process that plays a prominent role in the function and performance of myriad systems and technologies. Recent experimental measurements have shown that subjecting a macroscale system to a time-periodic temperature gradient can increase thermal conductivity in comparison to a static temperature gradient. Here, we theoretically examine this mechanism in a nanoscale model by applying a stochastic Langevin framework to describe the energy transport properties of a particle connecting two heat baths with different temperatures, where the temperature difference between baths is oscillating in time. Analytical expressions for the energy flux of each heat bath and for the system itself are derived for the case of a free particle and a particle in a harmonic potential. We find that dynamical effects in the energy flux induced by temperature oscillations give rise to complex energy transport hysteresis effects. The presented results suggest that applying time-periodic temperature modulations is a potential route to control energy storage and release in molecular devices and nanosystems.

Electron hopping heat transport in molecules

The realization of single-molecule thermal conductance measurements has driven the need for theoretical tools to describe conduction processes that occur over atomistic length scales. In macroscale systems, the principle that is typically used to understand thermal conductivity is Fourier's law. At molecular length scales, however, deviations from Fourier's law are common in part because microscale thermal transport properties typically depend on the complex interplay between multiple heat conduction mechanisms. Here, the thermal transport properties that arise from electron transfer across a thermal gradient in a molecular conduction junction are examined theoretically. We illustrate how transport in a model junction is affected by varying the electronic structure and length of the molecular bridge in the junction as well as the strength of the coupling between the bridge and its surrounding environment. Three findings are of note: First, the transport properties can vary significantly depending on the characteristics of the molecular bridge and its environment; second, the system's thermal conductance commonly deviates from Fourier's law; and third, in properly engineered systems, the magnitude of electron hopping thermal conductance is similar to what has been measured in single-molecule devices.

Thermal Transport through Polymer-Linked Gold Nanoparticles

Polymer-nanoparticle networks have potential applications in molecular electronics and nanophononics. In this work, we use all-atom molecular dynamics to reveal the fundamental mechanisms of thermal transport in polymer-linked gold nanoparticle (AuNP) dimers at the molecular level. Attachment of the polymers to AuNPs of varying sizes allows the determination of effects from the flexibility of the chains when their ends are not held fixed. We report heat conductance (G) values for six polymers-viz. polyethylene, poly(p-phenylene), polyacene, polyacetylene, polythiophene, and poly(3,4-ethylenedioxythiophene)-that represent a broad range of stiffness. We address the multimode effects of polymer type, AuNP size, polymer chain length, polymer conformation, system temperature, and number of linking polymers on G. The combination of the mechanisms for phonon boundary scattering and intrinsic phonon scattering has a strong effect on G. We find that the values of G are larger for conjugated polymers because of the stiffness in their backbones. They are also larger in the low-temperature region for all polymers owing to the quenching of segmental rotations at low temperature. Our simulations also suggest that the total G is additive as the number of linking polymers in the AuNP dimer increases from 1 to 2 to 3.

Developing Thermal Regulating and Electromagnetic Shielding Nacre-Inspired Graphene-Conjugated Conducting Polymer Film via Apparent Wiedemann-Franz Law

In this work, we observed size-dependent behavior of filler on both the thermal and electrical conductivities of nacre-like graphene-conjugated conducting polymer films and demonstrated the display of apparent Wiedemann-Franz law and tunability of Lorenz constant in such films. The maximum thermal and electrical conductivities of as-fabricated films can reach over 73 W·m^-1·K^-1 and 1200 S·cm^-1, respectively. Furthermore, the maximum value of electromagnetic interference shielding reaches 54 dB with SSE/t over 16000 dB·cm²·g^-1. These films can not only show high-quality electromagnetic interference shielding performance with small thickness and low filler ratio but also achieve simultaneous thermal management during electromagnetic shielding. The findings in this work offer new insight into designing flexible graphene-conjugated polymers with customizable thermal and electrical properties in the broad fields of thermal management systems, electromagnetic defense systems, and flexible electronic systems.

Thermal and electrical cross-plane conductivity at the nanoscale in poly(3,4-ethylenedioxythiophene):trifluoromethanesulfonate thin films

Cross-plane electrical and thermal transport in thin films of a conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) stabilized with trifluoromethanesulfonate (OTf) is investigated in this study. We explore their electrical properties by conductive atomic force microscopy (C-AFM), which reveals the presence of highly conductive nano-domains. Thermal conductivity in the cross-plane direction is measured by null-point scanning thermal microscopy (NP-SThM). PEDOT:OTf indeed demonstrates a non-negligible electronic contribution to the thermal transport. We further investigate the correlation between electrical and thermal conductivity by applying post-treatment: chemical reduction (de-doping) to lower charge carrier concentration and hence, electrical conductivity and acid treatment (over-doping) to increase the latter. From our measurements, we find a vibrational thermal conductivity of 0.34 ± 0.04 W m^-1 K^-1. From the linear dependence or the electronic contribution of thermal conductivity vs. the electronic conductivity (Wiedemann-Franz law), we infer a Lorenz number 6 times larger than the classical Sommerfeld value as also observed in many organic materials for in-plane thermal transport. By applying the recently proposed molecular Wiedemann-Franz law, we deduced a reorganization energy of 0.53 ± 0.06 eV.

Chemical ordering and temperature effects on the thermal conductivity of Ag–Au and Ag–Pd bimetallic bulk and nanocluster systems

Understanding the heat transfer mechanisms in bimetallic nanoparticles, e.g. to promote heat transfer in a nanofluid, is a significant problem for industrial and fluid mechanics related applications.

Thermoelectric effect in a single molecular junction with a vibrational mode

We investigate thermoelectric properties of single molecular junctions with electron-phonon interaction (EPI) based on a two-level model, and explore the possibility to obtain a thermoelectric device with high efficiency by engineering the energy level splitting in the molecular junction. We derive analytical expressions for electric conductance, thermopower and electronic thermal conductance in the linear response region within the dressed tunneling approximation of EPI. The effects of EPI and the level splitting in the molecule on thermoelectric properties are discussed. We show large value of thermoelectric figure of meritZTcan be achieved for molecular junctions with strong EPI and relatively small energy level splitting between the bonding and antibonding states of the molecule.

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

We study by scanning thermal microscopy the nanoscale thermal conductance of films (40-400 nm thick) of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). We demonstrate that the out-of-plane thermal conductivity is significant along the interlayer direction, larger for BTBT (0.63 ± 0.12 W m^-1 K^-1) compared to C8-BTBT-C8 (0.25 ± 0.13 W m^-1 K^-1). These results are supported by molecular dynamics calculations (approach to equilibrium molecular dynamics method) performed on the corresponding molecular crystals. The calculations point to significant thermal conductivity (3D-like) values along the 3 crystalline directions, with anisotropy factors between the crystalline directions below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport properties featuring a two-dimensional character for these materials. In agreement with the experiments, the calculations yield larger values in BTBT compared to C8-BTBT-C8 (0.6-1.3 W m^-1 K^-1versus 0.3-0.7 W m^-1 K^-1, respectively). The weak thickness dependence of the nanoscale thermal resistance is in agreement with a simple analytical model.