Low-frequency conductivity of low wear high-entropy alloys.

High-entropy alloys (HEAs) provide new research avenues for alloy combinations in the periodic table, opening numerous possibilities in novel-alloy applications. However, their electrical characteristics have been relatively underexplored. The challenge in establishing an HEA electrical conductivity model lies in the changes in electronic characteristics caused by lattice distortion and complexity of nanostructures. Here we show a low-frequency electrical conductivity model for the Nb-Mo-Ta-W HEA system. The cocktail effect is found to explain trends in electrical-conductivity changes in HEAs, while the magnitude of the reduction is understood by the calculated plasma frequency, free electron density, and measured relaxation time by terahertz spectroscopy. As a result, the refractory HEA Nb15Mo35Ta15W35 thin film exhibits both high hardness and excellent conductivity. This combination of Nb15Mo35Ta15W35 makes it suitable for applications in atomic force microscopy probe coating, significantly improving their wear resistance and atomic-scale image resolution.

Comparative Study of the Orientation and Order Effects on the Thermoelectric Performance of 2D and 3D Perovskites.

Calcium titanium oxide has emerged as a highly promising material for optoelectronic devices, with recent studies suggesting its potential for favorable thermoelectric properties. However, current experimental observations indicate a low thermoelectric performance, with a significant gap between these observations and theoretical predictions. Therefore, this study employs a combined approach of experiments and simulations to thoroughly investigate the impact of structural and directional differences on the thermoelectric properties of two-dimensional (2D) and three-dimensional (3D) metal halide perovskites. Two-dimensional (2D) and three-dimensional (3D) metal halide perovskites constitute the focus of examination in this study, where an in-depth exploration of their thermoelectric properties is conducted via a comprehensive methodology incorporating simulations and experimental analyses. The non-equilibrium molecular dynamics simulation (NEMD) was utilized to calculate the thermal conductivity of the perovskite material. Thermal conductivities along both in-plane and out-plane directions of 2D perovskite were computed. The NEMD simulation results show that the thermal conductivity of the 3D perovskite is approximately 0.443 W/mK, while the thermal conductivities of the parallel and vertical oriented 2D perovskites increase with n and range from 0.158 W/mK to 0.215 W/mK and 0.289 W/mK to 0.309 W/mK, respectively. Hence, the thermal conductivity of the 2D perovskites is noticeably lower than the 3D ones. Furthermore, the parallel oriented 2D perovskites exhibit more effective blocking of heat transfer behavior than the perpendicular oriented ones. The experimental results reveal that the Seebeck coefficient of the 2D perovskites reaches 3.79 × 102 µV/K. However, the electrical conductivity of the 2D perovskites is only 4.55 × 10-5 S/cm, which is one order of magnitude lower than that of the 3D perovskites. Consequently, the calculated thermoelectric figure of merit for the 2D perovskites is approximately 1.41 × 10-7, slightly lower than that of the 3D perovskites.

Properties of Halide Perovskite Photodetectors with Little Rubidium Incorporation.

This study investigates the effects of Rb doping on the Rb-formamidinium-methylammonium-PbI3 based perovskite photodetectors. Rb was incorporated in the perovskite films with different contents, and the corresponding photo-response properties were studied. Doping of few Rb (~2.5%) was found to greatly increase the grain size and the absorbance of the perovskite. However, when the Rb content was greater than 2.5%, clustering of the Rb-rich phases emerged, the band gap decreased, and additional absorption band edge was found. The excess Rb-rich phases were the main cause that degraded the performance of the photodetectors. By space charge limit current analyses, the Rb was found to passivate the defects in the perovskite, lowering the leakage current and reducing the trap densities of carriers. This fact was used to explain the increase in the detectivity. To clarify the effect of Rb, the photovoltaic properties were measured. Similarly, h perovskite with 2.5% Rb doping increased the short-circuit current, revealing the decline of the internal defects. The 2.5% Rb doped photodetector showed the best performance with responsivity of 0.28 AW-1 and ~50% quantum efficiency. Detectivity as high as 4.6 × 1011 Jones was obtained, owing to the improved crystallinity and reduced defects.

Resistive Switching Property of Organic-Inorganic Tri-Cation Lead Iodide Perovskite Memory Device.

: In this study, a glass/indium tin oxide (ITO)/formamidinium-methylammonium-cesium (FA-MA-Cs) tri-cation lead iodide perovskite/poly(methyl methacrylate (PMMA)/Al memory device with a controlled composition of (FA0.75MA0.25)1-xCsxPbI3 (x = 0-0.1) is demonstrated to exhibit bipolar resistive switching behavior. The tri-cation organic-inorganic metal halide perovskite film was prepared by a one-step solution process in which the amount of Cs was varied to modify the property of FA0.75MA0.25PbI3. It was found that the microstructure and defect properties of films are highly dependent on the contents of FA, MA, and Cs in the perovskite. The results found that 5% CsI doping is the optimized condition for improving the quality of FA0.75MA0.25PbI3, forming a high quality tri-cation perovskite film with a smooth, uniform, stable and robust crystalline grain structure. The resistive switching on/off ratio of the (FA0.75MA0.25)0.95Cs0.05PbI3 device is greater than 103 owing to the improved thin-film quality. Moreover, for the 5% CsI doped FA0.75MA0.25PbI3 films, the endurance and the stability of retention are better than the non-doped film. The improved microstructure and memory properties are attributed to the balance stress of FA/MA/Cs with different ionic size. It suggests the potential to achieve a desired resistive memory property of tri-cationic perovskite by carefully adjusting the cation ratios.

Improving Two-Step Prepared CH3NH3PbI3 Perovskite Solar Cells by Co-Doping Potassium Halide and Water in PbI2 Layer.

Incorporating additives into organic halide perovskite solar cells is the typical approach to improve power conversion efficiency. In this paper, a methyl-ammonium lead iodide (CH3NH3PbI3, MAPbI3) organic perovskite film was fabricated using a two-step sequential process on top of the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole-transporting layer. Experimentally, water and potassium halides (KCl, KBr, and KI) were incorporated into the PbI2 precursor solution. With only 2 vol% water, the cell efficiency was effectively improved. Without water, the addition of all of the three potassium halides unanimously degraded the performance of the solar cells, although the crystallinity was improved. Co-doping with KI and water showed a pronounced improvement in crystallinity and the elimination of carrier traps, yielding a power conversion efficiency (PCE) of 13.9%, which was approximately 60% higher than the pristine reference cell. The effect of metal halide and water co-doping in the PbI2 layer on the performance of organic perovskite solar cells was studied. Raman and Fourier transform infrared spectroscopies indicated that a PbI2-dimethylformamide-water related adduct was formed upon co-doping. Photoluminescence enhancement was observed due to the co-doping of KI and water, indicating the defect density was reduced. Finally, the co-doping process was recommended for developing high-performance organic halide perovskite solar cells.

ZnO/Silicon-Rich Oxide Superlattices with High Thermoelectric Figure of Merit: A Comprehensive Study by Experiment and Molecular Dynamic Simulation.

ZnO is a direct band gap material that has numerous optoelectronic applications. Recently, the thermoelectric behavior of ZnO has drawn much attention because it is expected to enrich the multifunctional application of ZnO. However, the high thermal conductivity nature of ZnO (∼50 W/(m·K)) is a challenge to further increase its thermoelectrtic figure of merit ( ZT). In this paper, a way to increase the ZT of ZnO thin films by insertion of silicon-rich oxide (SRO) interlayers is reported. All of the constituents are earth-abundant and environmental friendly. The effects of the number of SRO layers, thickness, grain size, heat treatment, and crystallinity of ZnO of the superlattices on the thermoelectric behaviors of ZnO were investigated. The thermoelectric ZT was determined by the transient Harman method by measuring the Seebeck voltage. The thermal conductivity of the ZnO/SRO superlattices that is crucial to elucidate the ZT behaviors is calculated using molecular dynamic simulation, in which the Zn-O and Zn-Zn interactions were described by the Born-Mayer potential and the short-range non-Coulombic O-O interaction was described by the Morse potential. For a given total ZnO/SRO thickness, the grain size of the ZnO decreases monotonically with the increasing number of SRO layers, thus leading to a decrease of the thermal conductivity and an increase of the ZT of the superlattices. As the best result, the annealed 45 nm thick ZnO thin film with three SRO interlayers presents a high ZT of ∼0.16 at room temperature. A comprehensive study on the ZnO/SRO superlattice-based thermoelectrtic devices was carried out by the experiment and theoretical simulation. The results imply potential thermoelectric application of the ZnO/SRO superlattices.

Enhancing the electroluminescence efficiency of Si NC/SiO2 superlattice-based light-emitting diodes through hydrogen ion beam treatment.

This paper presents a novel method for enhancing the electroluminescence (EL) efficiency of ten-period silicon-rich oxide (SRO)/SiO2 superlattice-based light-emitting diodes (LEDs). A hydrogen ion beam (HIB) was used to irradiate each SRO layer of the superlattices to increase the interfacial roughness on the nanoscale and the density of the Si nanocrystals (Si NCs). Fowler-Nordheim (F-N) tunneling was the major mechanism for injecting the carriers into the Si NCs. The barrier height of the F-N tunneling was lowered by forming a nano-roughened interface and the nonradiative Pb centers were passivated through the HIB treatment. Additionally, the reflectance of the LEDs was lowered because of the nano-roughened interface. These factors considerably increased the slope efficiency of EL and the maximum output power of the LEDs. The lighting efficiency increased by an order of magnitude, and the turn-on voltage decreased considerably. This study established an efficient approach for obtaining bright Si NC/SiO2 superlattice-based LEDs.

Enhanced white photoluminescence in silicon-rich oxide/SiO2 superlattices by low-energy ion-beam treatment.

This study presents the crystalline and luminescence properties of silicon-rich oxide (SRO)/SiO2 superlattices in which the SRO layers were prepared with a low-energy (<60 eV) argon ion-beam treatment. Experimental results evidenced that density of the Si nanocrystals (NCs) in the SRO layer was increased by ion-beam treatment after annealing, increasing the surface roughness. The stoichiometry of the as-prepared SRO layer was unchanged but the phase separation of the annealed SRO layer was enhanced by the ion-beam treatment, yielding visible white photoluminescence from the E' centers and Si NCs.

GaN-based stacked micro-optics system.

A prototype of a GaN-based stacked micro-optics system is demonstrated. The system consists of a GaN microlens, GaN membrane gratings, six spacers, a spatial filter, and a 980 nm VCSEL. The laser beam is collimated by the GaN microlens and diffracted by the GaN membrane grating. The systems can be used in blue-violet-UV micro-optics systems.