Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO : From basics towards applications

ZnO is presently experiencing a research boom with more than 2000 ZnO-related publications in 2005. This phenomenon is triggered, for example, by hope to use ZnO as a material for blue/UV optoelectronics as an alternative to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits that are transparent in the visible or for semiconductor spintronics. Currently, however, the main problem is to achieve high, reproducible and stable p-doping. Herein, we critically review aspects of the material growth, fundamental properties of ZnO and ZnO-based nanostructures and doping as well as present and future applications with emphasis on the electronic and optical properties including stimulated emission.

ZnO: Material, Physics and Applications

This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

Ferrite nanoparticles (FNPs) have attracted a great interest due to their wide applications in several areas such as biomedical, wastewater treatment, catalyst and electronic device. This review focuses on the synthesis, characterisation and application of FNPs in electronic device with more emphasis on the recently published works. The most commonly used synthesis techniques along with their advantages and limitations are discussed. The available characterisation techniques and their application in electronic materials such as sensors and biosensors, energy storage, microwave device, electromagnetic interference shielding and high-density recording media are briefly reviewed.

Ferrite nanoparticles: Synthesis, characterisation and applications in electronic device

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

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Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

High-quality Co-doped ZnO single crystalline films with a wide range of carrier concentration and good reproducibility have been grown by molecular beam epitaxy. After the systematic studies of the magnetic and transport properties of the films, we suggest that there are two distinct ferromagnetic mechanisms in different conductivity regimes. In the insulating regime, carriers tend to be localized, favoring the formation of bound magnetic polarons, which leads to ferromagnetism. In the metallic regime, however, most carriers are weakly localized and the free carrier-mediated exchange is dominant. Our experimental observations are well consistent with the recent theoretical description of magnetism in Co-doped ZnO and helpful for understanding the ferromagnetic mechanism in oxide-based diluted magnetic semiconductors.

The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to free carrier-mediated exchange interaction

The structure, electrical property, and magnetism of 7.5% V-doped ZnO powders that were hydrogenated annealed at 300, 500, and 700°C have been systematically investigated. The saturation magnetization and conductivity of the V:ZnO powders increase with hydrogenated annealing temperature (Tha), and the magnetization-temperature curves transform from paramagnetism to weak ferromagnetism (∼10−4μB∕V atom) as Tha increases. The results suggest that ferromagnetism in V:ZnO powders is highly correlated to the structural defects.

Effects of hydrogenated annealing on structural defects, conductivity, and magnetic properties of V-doped ZnO powders

Rare earth elements (REEs) or "technology metals" were coined by the U.S. Department of Energy, a group of seventeen elements found in the Earth's crust. These chemical elements are vital and irreplaceable to the world of technology owing to their unique physical, chemical, and light-emitting properties, all of which are beneficial in modern healthcare, telecommunication, and defense. Rare earth elements are relatively abundant in Earth's crust, with critical qualities to the device performance. The reuse and recycling of rare earth elements through different technologies can minimize impacts on the environment; however, there is insufficient data about their biological, bioaccumulation, and health effects. The increasing usage of rare earth elements has raised concern about environmental toxicity, which may further cause harmful effects on human health. The study aims to review the toxicity analysis of these rare earth elements concerning aquatic biota, considering it to be the sensitive indicator of the environment. Based on the limited reports of REE effects, the review highlights the need for more detailed studies on the hormetic effects of REEs. Aquatic biota is a cheap, robust, and efficient platform to study REEs' toxicity, mobility of REEs, and biomagnification in water bodies. REEs' diverse effects on aquatic life forms have been observed due to the lack of safety limits and extensive use in the various sectors. In accordance with the available data, we have put in efforts to compile all the relevant research results in this paper related to the topic "toxicity effect of REEs on aquatic life".

An Updated Review of Toxicity Effect of the Rare Earth Elements (REEs) on Aquatic Organisms

Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films.

Hua-Shu Hsu

Papers

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to free carrier-mediated exchange interaction

Effects of hydrogenated annealing on structural defects, conductivity, and magnetic properties of V-doped ZnO powders

An Updated Review of Toxicity Effect of the Rare Earth Elements (REEs) on Aquatic Organisms

Carrier-mediated ferromagnetism in single crystalline (Co, Ga)-codoped ZnO films