Structural, Optical, Charge-Transport, and Dielectric Properties of Double-Perovskite La2Co1-zFezMnO6 (z = 0, 0.2-1.0).

A series of double-perovskite La2Co1−zFezMnO6 (z = 0, 0.2−1.0) ceramics were synthesized using a well-established sol−gel method. The series of samples with a monoclinic phase and a P21/n symmetry were characterized by XRD, FTIR, conductivity, and capacitance measurement to extract charge-transport and dielectric characteristics at room temperature. The obtained IR spectra fitted well with the Lorentz oscillator model to calculate the damping factor, optical frequency, and oscillator strength and compared with the theory, which gave better agreement. The calculated activation energies from the Arrhenius plot supported the semiconducting nature of all samples. The temperature and frequency-dependent dielectric parameters, such as the real part (εr'), imaginary part (ε″) of the dielectric constant, dielectric loss (tanδ), and ac-conductivity (σac) were extracted. The dielectric constant (εr', ε″) and dielectric loss (tanδ) were enhanced at a low frequency, while the ac-conductivity (σac) displayed higher values at higher frequencies. The enhancement in the dielectric parameters with increasing iron concentrations arose due to the higher surface volume fraction of iron (Fe3+) ions than the cobalt (Co3+) ions. The radius of the Fe3+ (0.645 Å) was relatively higher than the Co3+ ions (0.61 Å), significantly influenced by the grains and grain boundaries, and enhanced the barrier for charge mobility at the grain boundaries that play a vital role in space charge polarization.

Effects of Rhenium Substitution of Co and Fe in Spinel CoFe2O4 Ferrite Nanomaterials.

In this work, nanoparticles of Co1-xRexFe2O4 and CoFe2-xRexO4 (0 ≤ x ≤ 0.05) were synthesized by the sol-gel method. The Rietveld refinement analysis of XRD and Raman data revealed that all of the prepared samples were single phase with a cubic spinel-type structure. With the substitution of Re, the lattice parameters were slightly increased, and Raman spectra peak positions corresponding to the movement of the tetrahedral sublattice shifted to a higher energy position. Furthermore, Raman spectra showed the splitting of T2g mode into branches, indicating the presence of different cations at crystallographic A- and B-sites. The SEM micrograph confirms that surface Re exchange changes the coordination environment of metals and induces Fe-site structure distortion, thereby revealing more active sites for reactions and indicating the bulk sample's porous and agglomerated morphology. The vibrating sample magnetometer (VSM) results demonstrated that the synthesized nanoparticles of all samples were ferromagnetic across the entire temperature range of 300-4 K. The estimated magnetic parameters, such as the saturation magnetization, remanent magnetization, coercivity, blocking temperature (TB), and magnetic anisotropy, were found to reduce for the Co-site doping with the increasing doping ratio of Re, while in the Fe site, they enhanced with the increasing doping ratio. The ZFC-FC magnetization curve revealed the presence of spin-glass-like behavior due to the strong dipole-dipole interactions in these ferrite nanoparticles over the whole temperature range. Finally, the dielectric constant (εr') and dielectric loss (tanδ) were sharply enhanced at low frequencies, while the AC conductivity increased at high frequencies. The sharp increases at high temperatures are explained by enhancing the barrier for charge mobility at grain boundaries, suggesting that samples were highly resistive. Interestingly, these parameters (εr', tanδ) were found to be higher for the Fe-site doping with the increasing Re doping ratio compared with the Co site.

Operation of silicon-germanium heterojunction bipolar transistors with different structures at deep cryogenic temperature.

In this work, the device performances of discrete and integrated SiGe heterojunction bipolar transistors (HBTs) with different device structures from 300 to 4.8 K were investigated. The turn-on voltages of base-emitter and base-collector junctions increased non-linearly with temperature cooled to 4.8 K. Energy bandgap engineering was taken into account for the analytical model of the turn-on voltage versus temperature. Incomplete ionization occurred in the base-collector junction because of the low doping concentration. The trap-assisted tunneling current in the forward base current was clearly observed below 20 K. The ideality factor and saturation current were shown to be temperature dependence. The ideality factor was much larger than 2 below 40 K, indicating that the current is not only contributed by drift, diffusion and recombination, but also by tunneling. The peak current gain of the discrete SiGe HBTs achieved the maximum value of 3,388 at 80 K, while that of the integrated was 546 at 140 K. The transconductance in logarithm was linearly dependent on reciprocal temperature above 50 K, but flattened below 50 K. Early effect was evidently observed below 77 K in the fixed base current output characteristics of the discrete SiGe HBTs, and it was not obvious for the integrated SiGe HBTs.

Ultra-sensitive and plasmon-tunable graphene photodetectors for micro-spectrometry.

We demonstrate an ultra-sensitive photodetector based on a graphene/monolayer MoS2 vertical heterostructure working at room temperature. Highly confined plasmon waves are efficiently excited through a periodic array of monolayer graphene ribbons in which plasmon resonance has remarkably large oscillator strength, resulting in a sharp optical absorption peak in the normal-incidence transmission spectrum. A significant amount of electron-hole pairs are produced in graphene ribbons by optical absorption, separated by the built-in electric field across the graphene/MoS2 heterojunction. The responsivity reaches up to 1 × 107 A W-1 at room temperature due to very strong resonance in the heterostructure, yielding a highly sensitive graphene-based photodetector. Additionally, the absorption can be tuned over a wide spectral range (6-16 µm) by varying gate biasing. The ultra-sensitive, spectrally tunable photodetector could be potentially used as a promising candidate for mid-infrared micro-spectrometers.