Thermoelectric energy harvesting-the transformation of waste heat into useful electricity-is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500-900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In(4)Se(3-delta), which achieves the ZT value of 1.48 at 705 K-very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.
First-principles calculations have been used to investigate the effects of Al and Mg doping on the prevention of degradation phenomena in Li(NiCoMn)O cathode materials. Specifically, we have examined the effects of dopants on the suppression of oxygen evolution and cation disordering, as well as their correlation. It is found that Al doping can suppress the formation of oxygen vacancies effectively, while Mg doping prevents the cation disordering behaviors, i.e., excess Ni and Li/Ni exchange, and Ni migration. This study also demonstrates that formation of oxygen vacancies can facilitate the construction of the cation disordering, and vice versa. Delithiation can increase the probabilities of formation of all defect types, especially oxygen vacancies. When oxygen vacancies are present, Ni can migrate to the Li site during delithiation. However, Al and Mg doping can inhibit Ni migration, even in structures with preformed oxygen defects. The analysis of atomic charge variations during delithiation demonstrates that the degree of oxidation behavior in oxygen atoms is alleviated in the case of Al doping, indicating the enhanced oxygen stability in this structure. In addition, changes in the lattice parameters during delithiation are suppressed in the Mg-doped structure, which suggests that Mg doping may improve the lattice stability.
The colloidal quantum dots (QDs) have inherent multiple dangling bonds (DBs) on the surface atoms due to the intrinsic weak bonding nature and steric hindrance of organic ligands. Such DBs can be the trap sites for charge carriers, leading to the reduction of luminescence efficiency, but their detailed characteristics are still unclear. In this study, we disclose the electronic and optical features of the surface DBs of InP QDs via density functional calculations combined with experimental evidence. For InP core, both In-DB and P-DB create invariant DB energy levels with respect to the core size, and their optical transition intensities exhibit an order of magnitude smaller than the band-edge transition. The In-DB and P-DB generate a deep trap level at −3.947 eV and a shallow trap level at −5.717 eV, and the deep trap level corresponds to the origin to induce the trap emissions. The passivation with ZnS shell on InP core significantly modifies the optical properties of both DBs to the radiative transition even when the passivating shell partially covers the InP surface. The ZnS shell growth pushes the energy levels of the In-DB and P-DB to near the band edges and makes the orbitals more delocalized. Such modified roles of the DBs significantly improve the optical intensities comparable to those of the band-edge transition, which is validated by the absorption calculations and luminescence measurements.
Theoretical calculations based on density functional theory were performed to provide better understanding of the size dependent electronic properties of InP quantum dots (QDs). Using a hybrid functional approach, we suggest a reliable analytical equation to describe the change of energy band gap as a function of size. Synthesizing colloidal InP QDs with 2-4 nm diameter and measuring their optical properties was also carried out. It was found that the theoretical band gaps showed a linear dependence on the inverse size of QDs and gave energy band gaps almost identical to the experimental values.
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