The previous model on surface free energy has been extended to calculate size dependent thermodynamic properties (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature and specific heat capacity) of nanoparticles. According to the quantitative calculation of size effects on the calculated thermodynamic properties, it is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation. In other words, the size dependent thermodynamic properties P(n) have the form of P(n) = P(b)(1 -K/D), in which P(b) is the corresponding bulk value and K is the material constant. This may be regarded as a scaling law for most of the size dependent thermodynamic properties for different materials. The present predictions are consistent literature values.

Based on the rigorous consideration of the bond broken rule and surface relaxation, a model for the size-dependent surface free energy of face-centered-cubic nanoparticles and nanocavities is presented, where the surface relaxation is calculated by the BOLS relationship. It is found that the surface free energy of nanoparticles and nanocavities represents a reverse size effect-the surface free energy of nanoparticles decreases with the decrease of particle size while it rises with the shrinkage of cavities. The size effect on the surface free energy of nanoparticles and nanocavities is not evident in large size ranges, while it becomes more and more distinct with decreasing size, especially for sizes smaller than 10 nm. The present predictions are in good agreement with the available literature data.

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This work provides a detailed mechanism for exciton dynamics in D–π–A–π–D TADF molecules and strategies for further efficiency improvement.

Redox-active organics based on a multi-electron mechanism are of great interest in battery electrode materials as they are capable of delivering high capacity per molecular weight. However, most of such organics shows huge voltage gap that is inherited from their stepwise redox reactions occurring in the same conjugated redox moiety. This study focuses on the voltage tailoring of polymeric dihydrophenazine derivative, which shows high specific capacity as a cathode electrode material and decent cycling stability, but suffers huge voltage gap of ca. 0.8 V. We demonstrate a strategy to modify the voltage gap of dihydrophenazine derivatives through the incorporation of functional groups with different electron affinity near the redox moiety. The as-designed dihydrophenazine derivatives are further copolymerized to yield a polymeric material with significantly smoothened charge-discharge profiles and good capacity retention. We further demonstrate through theoretical calculation based on density-functional theory that the substitute site and types of functional groups are of great importance in voltage tailoring as well as structural stability of the dihydrophenazine derivatives.

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Improving the control of heat flow at the nanoscale is essential for promoting its applications in many fields, such as energy conversion, thermal informatics, and communication technologies. Here we perform a systematic study on the synergistic effect of screw dislocations and surface resonators on thermal transport of Si nanowires and the corresponding mechanisms based on molecular dynamics simulations. We uncover that screw dislocations reduce the thermal conductivity by enhancing the anharmonicity of nanowires due to the non-homogeneous stress field. For resonant structures, we demonstrate that the suppression of relaxation time is the main mechanism for thermal conductivity reduction. The suppression of relaxation time by more than two orders of magnitude below 4 THz dramatically reduces the resonant structure thermal conductivity, while the previously proposed group velocity reduction mechanism can only impede phonon transport beyond 4 THz slightly. By comparing the mechanisms produced by dislocations and resonators, we find that the resonators have a stronger effect over screw dislocations in impeding the phonon transport at low-frequencies while it becomes opposite at high-frequencies. As a result, they can be combined together to manipulate phonon transport synergistically at all frequencies. Our findings not only provide new insights on the mechanisms of thermal conductivity engineering by screw dislocations and surface resonators, but also illustrate a new paradigm for ultralow thermal conductivity design through the tailoring of the entire frequency range of phonon transport.

Nanophononic metamaterials have broad applications in fields such as heat management, thermoelectric energy conversion, and nanoelectronics. Phonon resonance in pillared low-dimensional structures has been suggested to be a feasible approach...