| A challenge of future technology is the realization of devices that can not only control the electron charge, but also control the electron spin, electronic quantum state, as well as photon behavior. The spintronics born from the need of lower power consumption, higher speed and larger integration density of devices, shows a concept of synergetic and multifunctional use of charge and spin dynamics of electrons. The magnetic semiconductors, which have a Curie temperature above room temperature, are promising candidates to realize spintronic devices. Dilute magnetic semiconductors(DMSs) have recently attracted considerable interest due to their promising applications in the next-generation multifunctional “spintronic” devices. Although doping semiconductors with transition metal(TM) is a route to obtain DMSs, this method has some intrinsic weaknesses, including cluster or secondary phases. However, due to the poor solubility of the magnetic elements, the precipitation of second phases or clusters may limit the application of DMS and also complicate the explanation of magnetic signals. Using defects as the dopant other than TM element to develop magnetism can preclude the undesirable second phases or clusters. Based on nano oxides ABOx, we choose Ba Nb O3、Ba Mo O4、Cd WO4 to study magnetism.Owing to environmental deterioration caused by the demand and combustion of fossil fuels, the increasing demand for energy leads to searching for new energy sources, especially clean and renewable energy. Thermoelectric materials have attracted widespread research interest in potential applications for clean and renewable energy sources, due to their capability of reversibly converting between heat and electricity with the advantages of being reliable and environmentally friendly. However, due to limit experiment condition, the optimal temperature and doping concentration have not been given yet in experiment. Therefore, in order to improve the ZT value and give a theoretical guidance to experiment, it is necessary to have a better understanding of the dependence of the thermoelectric properties on doping concentration and working temperature. Based on Bismuth sulfur compounds, we choose Bi2S3、Bi2O2Se、Bi2Se3 to study thermoelectric property.The main contents can be summarized as follows:(1) Optical and magnetic properties of the perovskite barium niobate(Ba Nb O3) nanocubes are investigated before and after UV irradiation. The energy gap changes from 3.94 e V to 3.88 e V and light absorption ability increases after UV irradiation. The XPS spectra reveal that the Ba Nb O3 nanocubes contain oxygen vacancy. The Ba Nb O3 nanocubes show weak ferromagnetism, but the saturation magnetization is enhanced from 4.34×10-3 to 6.60×10-3 emu/g after UV irradiation. The calculations indicate that the ferromagnetism is due to two +2 charge oxygen vacancies(2 2O1 O5 V V+ +-) which cause unpaired electrons around oxygen vacancy occupying the d state of the nearest-neighbor Nb atom. The total magnetic moment increases from the 0.942 μB to 1.76 μB owing to an increase of the concentration of oxygen vacancy in Ba Nb O3. The theoretical consequence well matches the laboratorial result.(2) Diluted magnetic semiconductors(DMSs), with Curie temperature at room temperature, are of technological and fundamental importance. Defect engineering has been an effective way to introduce magnetic moment in various non-magnetic systems. Here, we report for the first time that Ba Mo O4 with oxygen vacancy shows ferromagnetic behaviors. The first-principles calculations suggest that the oxygen vacancy is responsible for the ferromagnetism. When one oxygen vacancy is introduced, the related occupation state of Mo 4d is1 02 g gt e-, and a local magnetic moment of 1.0 μB is found. When two oxygen vacancies are introducd the related occupation state of Mo 4d is 2 02 g gt e-, and a local magnetic moment is up to 2.0 μB. Therefore, the magnetism results from the unpaired electrons on d orbital, which show high spin states. Our findings demonstrate that room-temperature ferromagnetism can also be induced through defect engineering.(3) The monoclinic Cd WO4 nanodods with diameter of 80 nm and length of 300 nm are obtained by a hydrothermal method at 200 ℃ for 24 h. The temperature-dependent photoluminescence(PL) is studied from 20 K to 300 K, showing two peaks centered at 438 nm(2.83 e V) and 490 nm(2.53 e V). We find that the emission intensity of peaks at 438 nm and 490 nm decreases with temperature due to the thermal quenching by nonradiative recombination. The strong temperature dependency of emission peaks could be explained by electron-phonon coupling. The theoretical calculations demonstrate that the oxygen vacancy 0OV and 1OV+ lead to the peaks at 438 nm and 490 nm in PL spectrum respectively. In addition, we find that the partly filled W t2 g orbit around 1OV+with one electron shows high spin state, which has been demonstrated in magnetic measurement. The Cd WO4 nanodods without any exotic magnetic dopant presents room temperature ferromagnetism(RTFM).(4) The electronic structure and thermoelectric property of Bi2S3 are investigated. The electron and hole effective mass of Bi2S3 is analyzed in detail, from which we find that the thermoelectric transportation varies in different directions in Bi2S3 crystal. Along ac plane the higher figure of merit(ZT) could be achieved. For n-type doped Bi2S3, the optimal doping concentration is found in the range of 1.0×1019-5.0 × 1019 cm-3, in which the maximal ZT reaches 0.21 at 900 K, but along ZZ direction, the maximal ZT reaches 0.36. These findings provide a new understanding of thermoelectricity-dependent structure factors and improving ZT ways. The donor concentration N increases as T increases at one bar of pressure under a suitable chemical potential μ, but above this chemical potential μ, the donor concentration N keeps a constant.(5) We report an investigation of the temperature and carrier concentration dependent thermoelectric behavior of Bi2Se3 single layers. The results are based on a combination of experimental data and calculated transport functions and electronic structures obtained from Boltzmann transport theory and from the first-principles. When the carrier concentration is fixed at 3.24 × 1018 cm-3, the maximal value of ZT is 19 at 900 K, while considering the anisotropy of ZT, the higher ZT = 25 can be achieved along xx direction. When the carrier concentration is increased to 4 × 1019 cm-3, the maximal value of ZT is 38 at 900 K, while considering the anisotropy of ZT, the higher ZT = 72 can be achieved along yy direction at carrier concentration of 5.2 × 1019 cm-3. Compared with the experimental data, we find that our calculated results coincide roughly with experimental data, indicating that our calculations are reliable and reasonable. As more and more 2D materials have been realized besides graphene, our calculation results would be important to provide new strategy to explore high efficiency thermoelectric materials.(6) Electronic and transport properties of Bi2O2 Se under strain are calculated using Tran-Blaha modified Becke-Johnson(TB-m BJGGA) potential and semi-classical Boltzmann transport theories. The electronic band gap decreases with tensile and compressive in-plane strain. We predict that the n-type Seebeck coefficient can be increased under compressive in-plane strain while the p-type Seebeck coefficient can be increased under tensile in-plane strain. Further, the power factor of n-type doping Bi2O2 Se can be increased under compressive in-plane strain while that of p-type doping Bi2O2 Se can be increased under tensile in-plane strain. For p-type doping Bi2O2 Se, large thermoelectric figure of merit(ZT ~1.42) could be obtained under tensile strain(2.3%) at 800 K. Moreover, a higher ZT~1.76 could be achieved along ZZ direction. This study demonstrates that the electronic and thermoelectric properties can be controlled by strain engineering in thermoelectric material. |
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