In-Plane Anisotropic Rectification and Photovoltaic Effects on ZnO Nonpolar (101̅0) Crystal Plane and the Physical Mechanism

The concept of spontaneous electric field (Es) in polar structures is crucial for understanding the physical and chemical properties of compound semiconductors and improving their performanes. However, this concept has not been widely accepted so far. Here, we show the first observation of rectification and photovoltaic effects in the polar [0001] direction on the nonpolar ZnO (1010) crystal plane. However, no rectification and photovoltaic effects are observed in the nonpolar [1210] direction perpendicular to the [0001]. When a stress was applied in the [0001] direction of the ZnO single crystal, the rectification and photovoltaic effects are abated and disappeared. The disappearance of the two effects results from the pressure-induced disappearance of the polar structure. The results fully demonstrated that the rectification and photovoltaic effects arise from the existence of Es in the polar [0001] direction. The Es motivates the directional transfer of the electrons and photocreated charges along the [0001] direction, and the rectification and photovoltaic effects are thus observed. These results provide direct evidence for the polar structure theory and suggest that the polar structures can be employed to develop new types of photovoltaic and other electronic and photoelectronic devices.

Hardening Effect in Lead-Free KNN-Based Piezoelectric Ceramics with CuO Doping

As the most promising lead-free piezoelectric ceramics to replace lead zirconate titanate (PZT) ceramics, potassium sodium niobate (KNN) ceramics have been widely studied for their application prospects in various electronic devices. Increasing Q_m while maintaining a high piezoelectric activity is quite important for piezoelectric ceramics applied in ultrasonic devices. A KNN-based ceramic with high d₃₃ and Q_m is prepared by a conventional solid-state technique to construct polycrystalline phase boundaries and induce defect dipoles. The best overall performance can reach d₃₃ = 260 pC/N, Q_m = 210, and T_C = 293 °C. The temperature dependence of the relevant parameters is tested, where Q_m increases but d₃₃ decreases with the rise of temperature accompanied by escaping ferroelectric boundary, which shows that the polarization rotation plays an important role in the two parameters. The hardening effect of KNN-based ceramics with CuO doping is further studied by first-principles calculations, demonstrating that the Cu doping strongly disturbs the ferroelectric order, but the formation of defect dipoles could stabilize the ferroelectric order. It is illustrated that defect dipoles always find their ground state at the site near the domain walls and the oriented defect dipoles hinder the polarization rotation severely, confirming the role of the defect dipoles in KNN-based materials.

Development of a Model Based on Physical Mechanisms for the Explanation of Drug Release: Application to Diclofenac Release from Polyurethane Films

In this study, we present a method for prediction of the drug-release profile based on the physical mechanisms that can intervene in drug release from a drug-carrier. The application presented here incorporates the effects of drug concentration and Reynolds number defining the circulating flow in the testing vein. The experimental data used relate to the release of diclofenac from samples of non-degradable polyurethane subjected to static and continuous flow. This case includes simultaneously three mechanisms: burst-release, diffusion and osmotic pressure, identified beforehand here as being able to contribute to the drug liberation. For this purpose, authors coded the Sequential Quadratic Programming Algorithm to solve the problem of non-linear optimization. The experimental data used to develop the mathematical model obtained from release studies carried out in water solution at 37 °C, for three concentrations of diclofenac and two water flow rates. We discuss the contribution of mechanisms and kinetics by considering two aforementioned parameters and, following that, we obtain the specific-model and compare the calculated results with the experimental results for the reserved cases. The results showed that drug percentage mostly affect the burst release, however flow rate has affected the osmotic release. In addition, release kinetics of all the mechanisms have increased by increasing the values of two considered parameters.

One- and Two-Photon Absorption: Physical Principle and Applications

We introduce the principle and applications of one-photon absorption (OPA) and two-photon absorption (TPA) controlled by external electric fields. The physical mechanism of OPA and TPA are firstly introduced, which can visually promote thoroughly understanding of principle and physical analysis. Secondly, the applications of different molecules in OPA and TPA with and without external electric field are introduced in detail. The effect of the external electric field on the charge transfer during the absorption process is also exemplified. Furthermore, the external electric field on the molecular orbital wave function is visualized through the charge transfer process in the excited state transitions. The purpose of this review is to deepen the understanding of the types of charge transfer under linear and non-linear absorption in different systems.

Surface/Interface Engineering for Constructing Advanced Nanostructured Light-Emitting Diodes with Improved Performance: A Brief Review

With the rise of nanoscience and nanotechnologies, especially the continuous deepening of research on low-dimensional materials and structures, various kinds of light-emitting devices based on nanometer-structured materials are gradually becoming the natural candidates for the next generation of advanced optoelectronic devices with improved performance through engineering their interface/surface properties. As dimensions of light-emitting devices are scaled down to the nanoscale, the plentitude of their surface/interface properties is one of the key factors for their dominating device performance. In this paper, firstly, the generation, classification, and influence of surface/interface states on nanometer optical devices will be given theoretically. Secondly, the relationship between the surface/interface properties and light-emitting diode device performance will be investigated, and the related physical mechanisms will be revealed by introducing classic examples. Especially, how to improve the performance of light-emitting diodes by using factors such as the surface/interface purification, quantum dots (QDs)-emitting layer, surface ligands, optimization of device architecture, and so on will be summarized. Finally, we explore the main influencing actors of research breakthroughs related to the surface/interface properties on the current and future applications for nanostructured light-emitting devices.

High-Performance 0-3 Type Niobate-Based Lead-Free Piezoelectric Composite Ceramics with ZnO Inclusions

Because of their high toxicity, lead-based materials in electronic devices must be replaced by lead-free piezoelectric materials. However, some issues remain that hinder the industrial applications of these alternative ceramics. Here, we report the construction of a 0-3-type ceramic composite (KNNS-BNKZ: xZnO), where the Sb-doped ZnO submicronic particles were randomly distributed throughout the potassium-sodium niobate-based ceramic matrix. In this (K,Na)NbO₃ (KNN)-based ceramic composite, superior temperature stability, excellent piezoelectric properties, and a high Curie temperature were simultaneously achieved. The unipolar strain varied from +20 to -16% when the temperature was increased from 23 to 200 °C in KNNS-BNKZ: xZnO with x = 0.75. By increasing the ZnO content from x = 0 to x = 5.0, the Curie temperature was increased from 227 to 294 °C. More importantly, the piezoelectric coefficient remained high ( d₃₃ = 480-510 pC/N) for a wide range of compositions, x = 0.25-1.0. Transmission electron microscopy (TEM) experiments showed that the compensatory electric fields generated by the Sb-doped ZnO submicronic particles were responsible for the improved temperature stability. The high piezoelectricity was due to the existence of nanodomains, which were clearly observed in the TEM experiments. The results presented in this work clarify some of the physical mechanisms in this KNN-based ceramic composite, thus advancing the development of lead-free ceramics.

Antimicrobial properties of nanorods: killing bacteria via impalement

Silver is known to possess anti-microbial properties that are of chemical origin. It is believed that either Ag atoms bind to thiol groups in bacterial enzymes or Ag+ ions enter bacterial cells and denature the DNA molecule to kill bacteria. Silver nanorods, however, may kill bacteria by another mechanism: it is possible that the sharp tips of the nanorods puncture bacterial cells and kill bacteria via impalement-a physical mechanism. To test if this can indeed happen, we have compared the anti-microbial properties of silver and CdS nanorods. No significant difference is found between the two even though CdS does not possess the chemical properties of silver. This indicates that the physical kill mechanism is indeed likely and therefore nanorods of any material may possess anti-microbial properties. In that case, it is possible to overcome serious short- and long-term health hazard issues which have been posed by silver nanoparticles by replacing them with nanorods of innocuous elements or compounds. A surface containing nanorods of varying heights presents an undulating bed of spikes to microbes and is most inhospitable to bacteria.

Relationship between Poling Characteristics and Phase Boundaries of Potassium-Sodium Niobate Ceramics

The controversy about the optimum poling conditions of (K,Na)NbO3 (KNN)-based lead-free ceramics was still unresolved and the relationships between poling characteristics and phase boundary types were rarely mentioned. Here, we tried to unveil the relationships between poling characteristics and phase boundary types of these ceramics. The optimum poling temperatures should be chosen near their corresponding phase transition temperatures. In addition, a large piezoelectricity can be attained in the ceramics with a multiphase coexistence under a lower poling electric field (<E(C)), while a higher poling electric field (≥E(C)) is required to achieve the sufficient polarization in the ones with single O or T phase. More interestingly, it is the first time to report that the ceramics with different phase boundaries can be fully poled after the measurement of P-E loops, where the d33 values match those of the corresponding ones poled by the DC electric field. We believe that this modified poling process can benefit the improved piezoelectricity of KNN-based ceramics.

Strong piezoelectricity in (1 - x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free binary system: identification and role of multiphase coexistence

Here we report a strong piezoelectric activity in (1 - x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free ceramics by designing different phase boundaries. The phase boundaries concerning rhombohedral-orthorhombic-tetragonal (R-O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were attained by changing BKZS and Sn contents and then were identified by the X-ray diffraction patterns as well as temperature-dependent permittivity and ν1 Raman modes associated with BO6 perovskite octahedron. A high strain (strain = 0.21-0.28% and d33* = 707-880 pm/V) and a strong piezoelectric coefficient (d33 = 415-460 pC/N) were shown in the ceramics located at the multiphase coexistence region. The reported results of this work are superior to that (d33* ∼ 570 pm/V and d33 ∼ 416 pC/N) of the textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics [Nature 2004, 432, 84]. We believe that the material system of this work will become one of the most promising candidates for piezoelectric actuators.