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Nanostructuring technology has been widely employed to reduce the thermal conductivity of thermoelectric materials because of the strong phonon-boundary scattering. Optimizing the carrier concentration can not only improve the electrical properties, but also affect the lattice thermal conductivity significantly due to the electron-phonon scattering. The lattice thermal conductivity of silicon nanostructures considering electron-phonon scattering is investigated for comparing the lattice thermal conductivity reductions resulting from nanostructuring technology and the carrier concentration optimization. We performed frequency-dependent simulations of thermal transport systematically in nanowires, solid thin films and nanoporous thin films by solving the phonon Boltzmann transport equation using the discrete ordinate method. All the phonon properties are based on the first-principles calculations. The results show that the lattice thermal conductivity reduction due to the electron-phonon scattering decreases as the feature size of nanostructures goes down and could be ignored at low feature sizes (50 nm for n-type nanowires and 20 nm for p-type nanowires and n-type solid thin films) or a high porosity (0.6 for n-type 500 nm-thick nanoporous thin films) even when the carrier concentration is as high as 10 cm. Similarly, the size effect due to the phonon-boundary scattering also becomes less significant with the increase of carrier concentration. The findings provide a fundamental understanding of electron and phonon transports in nanostructures, which is important for the optimization of nanostructured thermoelectric materials.

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The strain-sensitive heterostructure, as a type of lowdimensional technique, has attracted extensive attention, but the influence mechanism of the biaxial strain on its thermoelectric properties is still unclear. In this paper, the first principles based on density functional theory and the BoltzTrap transport equation with relaxation time calculated by deformation potential theory are employed to figure out the biaxial strain effect on the band structure and transport performance of the MoS 2 /WS 2 heterostructure. The lattice thermal conductivity under different strains is also investigated through nonequilibrium molecular dynamics. The results indicate that the strain-induced convergence of the valence and conduction bands can significantly improve the Seebeck coefficient of p-and ntype doping systems, respectively. The effective mass also changes with a tunable band structure, which increases the electrical conductivity under the tensile strain. Additionally, the biaxial strain is beneficial to reduce the lattice thermal conductivity. The final figure of merit significantly increases at large strains or at strains where band convergence can be achieved. This work shows that the biaxial strain is a highly efficient strategy to increase the thermoelectric properties of heterostructures.

Efficient production of metastable quantum states of nuclei (isomers) is critical for exotic applications, like nuclear clocks, nuclear batteries, clean nuclear energy, and nuclear gamma-ray lasers [1][2][3][4][5][6]. However, due to low reaction cross sections and quick decay, it is extremely difficult to acquire significant amount of isomers with short lifetimes via traditional accelerators or reactors. Here, we present femtosecond pumping of nuclear isomeric states by the Coulomb excitation of ions with the quivering electrons induced by laser fields for the first time. Nuclear isomers populated on the second excited state of 83 Kr, are generated with a rate of 3.84 × 10 17 per second from a table-top hundreds-TW laser system. This high efficiency of isomer production can be explained by Coulomb collision[7] of ions with the quivering electrons during the laser-cluster interactions at nearly solid densities.