Stable resistive switching characteristics of Ce:HfOx film induced by annealing process

In this work, Ce:HfO_x films were fabricated and the resistive switching characteristics were investigated. The chemical bonding states of the films were explored by X-ray photoelectron spectroscopy. The annealing process was carried out to modulate the concentration of oxygen vacancies in the film to confirm the dominant role of oxygen vacancies on resistive switching behaviors, which resulted in the elimination of unstable oxygen vacancies and the introduction of oxygen vacancy near Ce dopants due to the reduction of Ce⁴⁺. Benefiting from the oxygen vacancies near Ce dopants, stable resistive switching performance can be achieved for the annealed Ce:HfO_x sample. A schematic diagram based on the formation and rupture of oxygen vacancy filaments was proposed to illustrate the switching behaviors of annealed Ce:HfO_x sample.     

Effects of annealing on the polymer solar cells based on CdSe-PVK electron acceptor

CdSe-poly(N-vinylcarbazole) (CdSe-PVK) nanocomposite was synthesized and utilized as the electron acceptor in the active layer of polymer solar cells. The photovoltaic properties of the polymer solar cells based on poly(3-hexylthiophene) (P3HT):CdSe-PVK as the active layer were investigated in detail. The effects of annealing temperature (100-200 °C) and time (5-60 min) on the device performance were studied. At annealing temperature of 150 °C for 30 min, the device demonstrated an optimal efficiency of 0.235% under AM 1.5 (100 mW cm⁻²) solar simulated light irradiation. The improved efficiency under the optimal conditions was confirmed by the highest light harvest in UV-vis spectra due to the increased crystallinity of P3HT after thermal annealing. Photoluminescence of these devices also exhibited that the quench effect increases with the increasing of annealing temperature, indicating that the charge separation between electron-donating (P3HT) and electron-accepting (CdSe-PVK) molecules was increased after heat treatment. Atomic force microscopy (AFM) images showed that the phase segregation and 3D interpenetrating networks of P3HT:CdSe-PVK were responsible for the enhancement of the device efficiency.     

Spray-coating deposition techniques for polymeric semiconductor blends

We develop a universal method for spray-deposition of polymeric semiconductor blends, based on blends of polyfluorenes (F8TBT), polythiophenes (P3HT) and fullerenes (PCBM), as suitable for large areas. A multi-faceted characterisation approach, studying photoluminescence quenching, together with atomic force and optical microscopy, illustrates favourable results in terms of layer thickness, uniformity, and mesoscale morphology. With key engineering tolerances in mind, thermal (melt) and solvent-vapour annealing are investigated as post-processing methods, for improving the planarity of craterform layers and blend photophysical characteristics.     

Electrical characterization of defects introduced during electron beam deposition of W Schottky contacts on n-type 4H-SiC

We have studied the defects introduced in n-type 4H-SiC during electron beam deposition (EBD) of tungsten by deep-level transient spectroscopy (DLTS). The results from current-voltage and capacitance-voltage measurements showed deviations from ideality due to damage, but were still well suited to a DLTS study. We compared the electrical properties of six electrically active defects observed in EBD Schottky barrier diodes with those introduced in resistively evaporated material on the same material, as-grown, as well as after high energy electron irradiation (HEEI). We observed that EBD introduced two electrically active defects with energies E_C – 0.42 and E_C – 0.70 eV in the 4H-SiC at and near the interface with the tungsten. The defects introduced by EBD had properties similar to defect attributed to the silicon or carbon vacancy, introduced during HEEI of 4H-SiC. EBD was also responsible for the increase in concentration of a defect attributed to nitrogen impurities (E_C – 0.10) as well as a defect linked to the carbon vacancy (E_C – 0.67). Annealing at 400 °C in Ar ambient removed these two defects introduced during the EBD.     

Synthesis,structural and optical properties of Er doped,Li doped and Er + Li co-doped ZnO nanocrystallites by solution-combustion method

Pure zinc oxide (ZnO) and 2 mol% of Er, 1 mol% of Li individually doped and Er + Li co-doped ZnO nanopowders were synthesized by auto-combustion method. Crystal structure and grain size were characterized by X-ray diffractometer and found that all synthesized samples have hexagonal wurtzite crystal structure. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) studies were used to determine the morphology and size of the nanocrystallites. A UV–VIS measurement shows four absorbance peaks in the visible regions and the band gap is slowly blue shifted after annealing at 800 °C. The presence of Er, Li and the hexagonal wurtzite structure was confirmed by FT-IR studies. FT-Raman spectroscopy has been employed to study the crystalline quality and structural disorders.     

Chemically grown Bi2S3 nanorod films for hydrogen evolution

Bi₂S₃ nanorod films were grown on ITO-coated glass substrates through chemical bath deposition (CBD) and annealing in a sulfur atmosphere. The as-deposited films were amorphous/nanocrystalline, with a particle size of 20 nm and a direct optical band gap of 1.87 eV. Upon annealing at 350 °C, the films exhibited a nanorod morphology with a length of 300 nm. Further increasing the temperature from 400 to 450 °C resulted in an increased diameter of nanorods. The direct optical band gap decreased from 1.68 to 1.47 eV upon increasing the annealing temperature from 350 to 400 °C. Photoelectrochemical (PEC) measurements showed that the nanorod films grown on ITO-coated glass substrates exhibited significantly increased PEC activity owing to their nanorod structures. The Bi₂S₃ nanorod films formed at 400 °C exhibited a maximum photocurrent density of 6.1 mA/cm² at 1 V, which was 2.5 times higher than that of the as-deposited films. The enhancement in the photocurrent density could be due to the effective visible-light absorption of Bi₂S₃ nanorods as a result of the increased crystallinity and decreased band gap. This study demonstrates the synthesis route involving a simple and inexpensive CBD method of Bi₂S₃ nanorod films for the optimized PEC water-splitting applications.     

Effect of annealing temperature on structural and magnetic properties of Co2TiO4 ceramics prepared by sol-gel method

In this paper, polycrystalline Co₂TiO₄ ceramics have been synthesized using a sol-gel process followed by annealing at different temperatures. The lattice size and the average grain size of the samples increases with rise in annealing temperature. The temperature-dependent inverse paramagnetic susceptibilities recorded under zero-field-cooling condition have been fitted according to the Néel's expression for ferrimagnets. Subsequently, the molecular field constants and the corresponding exchange constants have been calculated. The fitting result shows that the magnetic interaction in the system becomes weaker as the annealing temperature rises. In addition, negative magnetization is observed during field-cooling process. The higher annealing temperature is beneficial to the growth of tetrahedral sublattice, leading to a decrease on compensation temperature. Furthermore, magnetization hysteresis loops for all the samples demonstrate the crucial role of grain size on the magnetic properties.     

Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature

Vertically aligned ZnO nanorods (NRs) on aluminum-doped zinc oxide (AZO) substrates were fabricated by a single-step aqueous solution method at low temperature. In order to optimize optical quality, the effects of annealing on optical and structural properties were investigated by scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. We found that the annealing temperature strongly affects both the near-band-edge (NBE) and visible (defect-related) emissions. The best characteristics have been obtained by employing annealing at 400°C in air for 2 h, bringing about a sharp and intense NBE emission. The defect-related recombinations were also suppressed effectively. However, the enhancement decreases with higher annealing temperature and prolonged annealing. PL study indicates that the NBE emission is dominated by radiative recombination associated with hydrogen donors. Thus, the enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors. On the other hand, the reduction of visible emission is mainly attributed to the annihilation of OH groups. Our results provide insight to comprehend annealing effects and an effective way to improve optical properties of low-temperature-grown ZnO NRs for future facile device applications.     

Optimization by record dynamics

Large dynamical changes in thermalizing glassy systems are triggered by trajectories crossing record sized barriers, a behavior revealing the presence of a hierarchical structure in configuration space. The observation is here turned into a novel local search optimization algorithm dubbed record dynamics optimization, or RDO. RDO uses the Metropolis rule to accept or reject candidate solutions depending on the value of a parameter akin to the temperature and minimizes the cost function of the problem at hand through cycles where its ‘temperature’ is raised and subsequently decreased in order to expediently generate record high (and low) values of the cost function. Below, RDO is introduced and then tested by searching for the ground state of the Edwards–Anderson spin-glass model, in two and three spatial dimensions. A popular and highly efficient optimization algorithm, parallel tempering (PT), is applied to the same problem as a benchmark. RDO and PT turn out to produce solutions of similar quality for similar numerical effort, but RDO is simpler to program and additionally yields geometrical information on the system’s configuration space which is of interest in many applications. In particular, the effectiveness of RDO strongly indicates the presence of the above mentioned hierarchically organized configuration space, with metastable regions indexed by the cost (or energy) of the transition states connecting them.     

Controlled suppression of the photoluminescence superlinear dependence on excitation density in quantum dots

We have shown that it is possible to tune, up to complete suppression, the photoluminescence superlinear dependence on the excitation density in quantum dot samples at high temperatures by annealing treatments. The effect has been attributed to the reduction of the defectivity of the material induced by annealing.