Structural, Optical, and Magnetic Properties of Gd Doped CdTe Quantum Dots for Magnetic Imaging Applications

The effect of pressure on the electronic and optical properties of SrAl2O4 up to 25 GPa was studied by means of the pseudo-potential plane waves method within the generalized gradient approximation for exchange and correlation. The calculated lattice parameters are consistent with available experimental and theoretical data. By analyzing the electronic and optical properties, the pressure dependences of the electronic structures and optical constants were investigated. The band structures show an indirect band gap for this compound and the calculated band gaps expend with increasing pressure. Meanwhile, the optical properties including the dielectric spectra, absorption coefficient spectra, reflectivity, and the real part of the refractive index spectra in the low energy range have a blue shift. Given this, the optical properties of SrAl2O4 could be tuned by changing pressure to some degree, which is beneficial to the optical applications.

Communication—Electron-Beam Stimulated Release of Dislocations from Pinning Sites in GaN

To achieve low leakage in GaN-based power devices and improve reliability in optoelectronic devices such as laser diodes, it is necessary to reduce dislocation density in epitaxial layers and control their introduction during processing. We have previously shown that dislocations can be introduced at room temperature in GaN. The effect of electron-beam irradiation at fixed points on the shift of such freshly introduced dislocations in GaN is reported. Dislocations can be displaced up to 10-15 µm from the beam position. We conclude the main reason limiting the dislocation travelling distance is the existence of a high number of pinning sites.

Communication—Function-Oriented Design of 3D Carbon Networks Toward Negative Permittivity at kHz Frequencies

Three-dimensional (3D) carbon networks composed of graphene (GR) and carbon nanotube (CNT) were constructed in copper calcium titanate (CCTO) in order to realize negative permittivity behavior. The results show that negative permittivity can be obtained at kHz frequencies above percolation threshold when 3D carbon networks are successfully constructed. Negative permittivity originates from the low-frequency plasmonic state which is explained by the Drude model. The magnitude of negative permittivity was tuned between 105 and 106 which significantly correlates with concentration of free carriers. Moreover, the reactance spectra clarify the inductive character of negative permittivity materials.

Synthesis, Structure and Magnetic Properties of NiFe3O5

A new CaFe3O5-type phase NiFe3O5 (orthorhombic Cmcm symmetry, cell parameters a = 2.89126(7), b = 9.71988(21) and c = 12.52694(27) Å) has been synthesised under pressures of 12-13 GPa at 1200 °C. NiFe3O5 has an inverse cation site distribution and reveals an interesting evolution from M2+(Fe3+ )2Fe2+O5 to Fe2+(M2+ 0.5Fe3+ 0.5)2Fe3+O5 distributions over three distinct cation sites as M2+ cation size decreases from Ca to Ni. Magnetic susceptibility measurements show successive transitions at 275, ~150, and ~20 K and neutron diffraction data reveal a series of at least three spin-ordered phases with evolving propagation vectors k = [0 0 0] [0 ky 0]  [½ ½ 0] on cooling. The rich variety of magnetically ordered phases in NiFe3O5 likely results from frustration of Goodenough-Kanamori exchange interactions between the three spin sublattices, and further interesting magnetic materials are expected to be accessible within the CaFe3O5-type family.

Hybrid La2O3-cPVP Dielectric for Organic Thin Film Transistor Applications

In this work, we have studied the electrical performance of cross-linked polyvinyl phenol (cPVP) modified lanthanum oxide (La2O3) bilayer dielectric film in pentacene thin film transistors (TFT). A simple spin-coating and room temperature operated cross-linking reaction of the hydroxyl moieties of PVP and the nitrogen groups of PMF were carried out to form the cross-linked PVP. The deposition of a thin 30 nm cPVP layer over the La2O3 layer provided a low leakage current (<10−7A/cm2), causing a reduction in the interface trap density. Besides, the modified surface properties of the La2O3 layer were favorable for the growth of pentacene organic semiconductors. As a result, the current on-off ratio and the sub-threshold slope was improved from 104 and 1.0 V/decade to 105 and 0.67 V/decade. The La2O3∕cPVP pentacene TFT operated at −10 V also exhibited improvement in the field-effect mobility to 0.71 cm2/Vs from 0.48 cm2/Vs for the single-layer La2O3 (130 nm) device. Thus, our work demonstrates that the rare earth oxide La2O3 with cPVP is an excellent dielectric system in the context of emerging transistors with hybrid polymer gate dielectrics.

EPR Study of the Mn-Doped Magnesium Titanate Ceramics

Manganese-doped magnesium titanate ceramic samples obtained by a solid-state reaction via sintering in the air from a mixture of MgO and TiO2 powders of different molar ratios were analyzed by electron paramagnetic resonance (EPR) technique. The EPR signals of Mn2+ ions (S = 5/2, І = 5/2) in crystal phases of MgO, Mg2TiO4, and MgTiO3 were detected. We have obtained the following spin Hamiltonian parameters for Mn2+ ions: g = 2.0015, A ~ 81 ∗ 10-4 cm-1 (in MgO phase); g = 2.0029, A ~ 73.8 ∗ 10-4 cm-1, b2 0 = 35 ∗ 10-4 cm-1 (in Mg2TiO4 phase); g = 2.004, A ~ 79 ∗ 10-4 cm-1, b2 0 = 165 ∗ 10-4 cm-1 (in MgTiO3 phase). Despite the presence of Mn4+ centers in both Mg2TiO4:Mn and MgTiO3:Mn ceramics confirmed by previous optical studies, no EPR signals related to Mn4+ ions (S = 3/2, І = 5/2) were found. The Mn2+ EPR signals are proposed as structural probes in manganese-doped magnesium titanate ceramics.

Growth of Interconnected ZnO Nanostructures-Its Photocatalytic and Electrochemical Properties

We report the synthesis of interconnected ZnO nano structures through the addition of polyvinyl pyrrolidone (PVP) in a growth medium consisting of ZnCl2 and NaOH at a temperature of 70 0C with a reaction time of 24 hrs. The formation of interconnected ZnO is evaluated in accordance with the reaction time and reaction temperature used for the synthesis, and samples were characterized by powder X-ray diffraction, Fourier transform infra- red (FTIR) spectroscopy, Brunauer-Emmett-Teller(BET) analysis, Field emission scanning electron microscopy (FESEM), Photoluminescence (PL) and Electrochemical methods. BET studies show the mesoporous nature of ZnO grown with the addition of PVP in the growth medium. Interconnected ZnO nanostructures exhibit efficient visible light driven photo catalytic degradation of methylene blue (MB) attributed to interconnected morphology of ZnO. Electro chemical studies have shown that the interconnected ZnO nanostructures give higher order specific capacitance.

Effect of Pressure on Electronic and Optical Properties of SrAl2O4

The effect of pressure on the electronic and optical properties of SrAl2O4 up to 25 GPa was studied by means of the pseudo-potential plane waves method within the generalized gradient approximation for exchange and correlation. The calculated lattice parameters are consistent with available experimental and theoretical data. By analyzing the electronic and optical properties, the pressure dependences of the electronic structures and optical constants were investigated. The band structures show an indirect band gap for this compound and the calculated band gaps expend with increasing pressure. Meanwhile, the optical properties including the dielectric spectra, absorption coefficient spectra, reflectivity, and the real part of the refractive index spectra in the low energy range have a blue shift. Given this, the optical properties of SrAl2O4 could be tuned by changing pressure to some degree, which is beneficial to the optical applications.

Plug and Play Electrodeposition Cell: A Case Study of Bismuth Ferrite Thin Films for Photoelectrochemical Water Splitting

In the present study, we designed and fabricated cost-effective miniaturized versatile electrochemical deposition cell, which is found to be at par performance as compared with conventional electrodeposition techniques. A case study is being undertaken for the electrodeposition of varied thickness of bismuth ferrite (BiFeO3) films on FTO glass substrates. X-ray diffraction (XRD) patterns confirms the structural perovskite phase of BiFeO3 (BFO). UV-Visible absorption spectra and Tauc plot of BFO estimates the direct band gap which lies between 1.9 to 2.1 eV. The properties of bismuth ferrite crystal system such as electronic band structure and density of states (DOS) are investigated theoretically. Photoelectrochemical (PEC) water splitting application is carried out to investigate the best performance of BFO films of varied thickness. The best performer (BFO15) working electrode yields a photocurrent density of ~ 35 µA/cm2 at 0.2 V vs RHE under visible LED (light intensity of 100mW/cm2) in neutral 0.5 M Na2SO4 electrolyte. Incident photon to current conversion (IPCE) measurements, electrochemical impedance spectroscopy (EIS) and Mott-Schottky characteristics confirms the best performance of BFO15 photocathode film.

Enhanced Reverse Recovery Performance in Superjunction MOSFET with Reduced Hole-Barrier

High reverse recovery charge (QRR) and resultant high switching losses have become the main factors that constrain the performance and application area of superjunction MOSFET (SJ-MOSFET). To reduce QRR, an SJ-MOSFET with reduced hole-barrier is proposed and demonstrated. By introducing a Schottky contact on the bottom of the n-pillar at the drain side, the barrier for the hole carrier is dramatically reduced in the reverse conduction state. As a result, the hole carrier in the drift region is significantly reduced, which results in a low QRR and enhanced reverse recovery performance. Compared with the conventional SJ-MOSFET (Conv-SJ-MOSFET), the proposed device achieves 64.6% lower QRR with almost no sacrifice in other characteristics. The attenuated QRR accounts for a 19.6% ~ 46.8% reduction in total power losses with operation frequency at 5 ~ 200 kHz, demonstrating the great potential of the proposed SJ-MOSFET used in power conversion systems.