Lin Ju

Magnetic properties of transition metal doped AlN nanosheet: First-principle studies

We carry out our first-principles calculations within density functional theory to study the 3d transition metal (TM) doped AlN nanosheets. The calculated results indicate that a stoichiometric AlN nanosheet is graphene-like structure and nonmagnetic. The TM impurities can induce magnetic moments, localized mainly on the 3d TM atoms and neighboring N atoms. Our calculated results of TM-doped nanosheet systems indicate a strong interaction between 3d orbit of TM atom and the 2p orbit of N atoms. In addition, the Mn- and Ni-doped AlN nanosheet with half-metal characters seems to be good candidates for spintronic applications. When substituting two Al atoms, the relative energies of the states between ferromagnetic and antiferromagnetic coupling are investigated sufficiently. The exchange coupling of Co- and Ni-doped AlN nanosheets exhibits a transformation with different distances of two TM atoms and that of Cr-, Mn-, and Fe-doped AlN nanosheets is not changed.

Microstructure, dielectric properties and impedance spectroscopy of Ni doped CaCu3Ti4O12 ceramics

Ni doped CaCu3Ti4O12 ceramics (CaCu3−xNixTi4O12, x = 0, 0.05, 0.1, 0.2) were prepared by a sol–gel method and the influence on the microstructure, dielectric properties and impedance spectroscopy has been investigated. CaCu3−xNixTi4O12 ceramics can cause a remarkable increase in the grain size and exhibit a very high dielectric constant (e′) over a wide frequency range at room temperature. The e′ value of the CaCu2.9Ni0.1Ti4O12 ceramic sintered at 1060 °C for 8 h was between 7.1 × 104 and 9.6 × 104 over the frequency range of 20 Hz to 100 kHz. Besides, a very low dielectric loss (tan δ) of 0.025 was observed for the CaCu2.95Ni0.05Ti4O12 ceramic sintered at 1060 °C for 8 h with e′ ∼ 4.2 × 104 at about 1 kHz. These results indicate that Ni doping at the Cu site may improve the dielectric properties of CaCu3Ti4O12. The giant dielectric response behavior can be attributed to the internal barrier layer capacitance (IBLC) effect.

Room-temperature magnetoelectric coupling in nanocrystalline Na0.5Bi0.5TiO3

Nanocrystalline Na0.5Bi0.5TiO3 was prepared by sol-gel method. The nanocrystalline Na0.5Bi0.5TiO3 plates present room temperature ferromagnetism (FM). The reduction of FM for the plate with subsequent long time and high temperature air-annealing indicating that the observed ferromagnetism is connected with the vacancies at/near the surface of nanograins. For Na0.5Bi0.5TiO3 powders after annealing at 900 °C in air, a subsequent annealing in vacuum at 900 °C for 20 min weakens the room-temperature FM, but a subsequent annealing in oxygen atmosphere at 900 °C for 20 min enhances the room-temperature FM, which means that the observed FM in nanocrystalline Na0.5Bi0.5TiO3 may originate from cation vacancy at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect (LDA + U) method on the magnetism of Na0.5Bi0.5TiO3 (100) surface show that Na vacancies can introduce a nonzero magnetic moment. The Na0.5Bi0.5TiO3 plates annealed a...

Magnetoelectric coupling in nanocrystalline Pb0.82La0.18TiO3

The combined effect of Pb vacancies and La dopant can induce ferromagnetism below 75 K for Pb0.82La0.18TiO3 plates annealed at 700,1000, and 1200 °C for 1 h. Pb0.82La0.18TiO3 plates annealed at 1000 °C for 1 h show the coexistence of ferromagnetism resulting from oxygen vacancies at/near surfaces of nanograins and ferroelectricity near room temperature. Under application of magnetic field, the dielectric constant decreases and the ferroelectric transition temperature shifts to high temperature for Pb0.82La0.18TiO3 multiferroic plates. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for Pb0.82La0.18TiO3 multiferroic plates, showing a strong magnetoelectric coupling.

Adsorption and gas-sensing characteristics of a stoichiometric α-Fe2O3 (0 0 1) nano thin film for carbon dioxide and carbon monoxide with and without pre-adsorbed O2

Herein, for the first time, the adsorption and gas-sensing characteristics of the CO2 and CO molecules on a stoichiometric α-Fe2O3 (0 0 1) nano thin film with and without pre-adsorbed O2 molecules have been studied using the density functional theory (DFT) method. Without pre-adsorbed O2 molecules, the CO2 molecule plays the role of an acceptor and obtains electrons from the stoichiometric α-Fe2O3 (0 0 1) nano thin film. For the O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film system, the CO2 molecule also plays the role of an acceptor. However, less number of electrons are transferred to the CO2 molecule as compared to the pre-adsorbed O2 molecules. Different from the CO2 molecule, the CO molecule always plays the role of a donor for the α-Fe2O3 (0 0 1) nano thin film system with and without pre-adsorbed O2. The theoretical results verify that the CO molecule can react with the lattice oxygen or adsorbed oxygen of the α-Fe2O3 (0 0 1) nano thin film. The electrons transferred to the stoichiometric α-Fe2O3 (0 0 1) nano thin film from the CO molecule or newly formed CO2 molecule are more than that transferred to the O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film. For the stoichiometric or O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film, the CO2 and CO molecules exhibited opposite behaviors of charge transformation. In addition, pre-adsorbed O2 molecules displayed competitive adsorption with the CO2 or CO molecule. The pre-adsorbed O2 molecules hinder electron transfer to the CO2 molecules from the α-Fe2O3 (0 0 1) nano thin film or hinder electron transfer to the α-Fe2O3 (0 0 1) nano thin film from the CO molecule. Theoretical results demonstrate that the surface of α-Fe2O3 materials (0 0 1) could be prepared for use as adsorbents or gas sensors for CO2 and CO molecules. Their structures are stable after CO2 molecules are adsorbed or after the reaction of CO molecules with the lattice oxygen or adsorbed oxygen of the α-Fe2O3 (0 0 1) nano thin film.

Design of a multiband terahertz perfect absorber

A thin-flexible multiband terahertz metamaterial absorber (MA) has been investigated. Each unit cell of the MA consists of a simple metal structure, which includes the top metal resonator ring and the bottom metallic ground plane, separated by a thin-flexible dielectric spacer. Finite-difference time domain simulation indicates that this MA can achieve over 99% absorption at frequencies of 1.50 THz, 3.33 THz, and 5.40 THz by properly assembling the sandwiched structure. However, because of its asymmetric structure, the MA is polarization-sensitive and can tune the absorptivity of the second absorption peak by changing the incident polarization angle. The effect of the error of the structural parameters on the absorption efficiency is also carefully analyzed in detail to guide the fabrication. Moreover, the proposed MA exhibits high refractive-index sensing sensitivity, which has potential applications in multi-wavelength sensing in the terahertz region.

Room-temperature ferromagnetism in nanocrystalline (Na0.7K0.3)0.5Bi0.5TiO3

The nanocrystalline (Na0.7K0.3)0.5Bi0.5TiO3 plates present room temperature ferromagnetism. The reduction of ferromagnetism for the (Na0.7K0.3)0.5Bi0.5TiO3 plate with subsequent long time and high temperature air-annealing indicates that the observed ferromagnetism is connected with the cation vacancies at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect method on the magnetism of (Na2/3K1/3)0.5Bi0.5TiO3 supercell show that Na vacancies can introduce a nonzero magnetic moment. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for (Na0.7K0.3)0.5Bi0.5TiO3 plates, showing a strong magnetoelectric coupling.

Room-temperature magnetoelectric coupling in nanocrystalline (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25)

Nanocrystalline (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25) plates exhibit ferromagnetism at room temperature. The reduction of ferromagnetism for the (Na1−xKx)0.5Bi0.5TiO3 plates over time and the high temperature air-annealing indicates that the observed ferromagnetism is connected with the cation vacancies at/near the surface of nanograins. The density functional theory calculation with the local density approximation plus on-site effect method on the magnetism of the (Na5/6K1/6)0.5Bi0.5TiO3 supercell shows that Na vacancies can introduce a nonzero magnetic moment. The (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25) plates when annealed at 900 °C for 1 hour present d0 multiferroicity with the coexistence of ferromagnetism and ferroelectricity at room-temperature. A room-temperature magnetodielectric effect is observed in (Na1−xKx)0.5Bi0.5TiO3 plates and the appropriate substitution of potassium can increase the magnetodielectric effect. In addition, electric field treatment leads to an enormous enhancement of saturation magnetization for (Na1−xKx)0.5Bi0.5TiO3 multiferroic plates, resulting in a strong magnetoelectric coupling.

Construction of coaxial ZnSe/ZnO p–n junctions and their photovoltaic applications

Coaxial ZnSe/ZnO nanostructures were fabricated by coating a ZnO thin film on the surface of presynthesized p-type ZnSe 1D nanostructures by a sputtering method. Owing to the n-type behavior of ZnO resulting from intrinsic defects, coaxial ZnSe/ZnO p–n junctions were realized and showed a pronounced rectifying behavior. Photovoltaic devices based on the coaxial ZnSe/ZnO p–n junction showed a power conversion efficiency of 1.24% and a large open-circuit voltage of 0.87 V under UV light. The large bandgaps of ZnSe and ZnO and the high quality of the ZnSe/ZnO interface were considered to be related to the high performance of the devices.

Disorder-enhanced spin polarization of the Zn1−xCoxO1−v concentrated magnetic semiconductor

Amorphous concentrated magnetic semiconductor Zn0.32Co0.68O1−v (v refers to oxygen vacancies) thin film was investigated by magnetic and electrical transport measurements as well as Andreev reflection spectroscopy. At a low temperature range, the electrons in the Zn0.32Co0.68O1−v are strongly localized, and electrical transport obeys the Efros variable range hopping law. Spin polarization was measured by Andreev reflection spectroscopy. As high as 64 ± 5% of spin polarization was attained through fitting of the modified Blonder–Tinkham–Klapwijk (BTK) theory. This enhanced spin polarization of Zn0.32Co0.68O1−v likely relates with the structure disorder and high concentration of magnetic cobalt ions, which lead to a spin imbalance impurity band in the tail of the conduction band. Considering room temperature ferromagnetism and high spin polarization, this material appears to be promising for spintronics device applications.