The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

Antibacterial activity of zinc oxide nanoparticles (ZnO-NPs) has received significant interest worldwide particularly by the implementation of nanotechnology to synthesize particles in the nanometer region. Many microorganisms exist in the range from hundreds of nanometers to tens of micrometers. ZnO-NPs exhibit attractive antibacterial properties due to increased specific surface area as the reduced particle size leading to enhanced particle surface reactivity. ZnO is a bio-safe material that possesses photo-oxidizing and photocatalysis impacts on chemical and biological species. This review covered ZnO-NPs antibacterial activity including testing methods, impact of UV illumination, ZnO particle properties (size, concentration, morphology, and defects), particle surface modification, and minimum inhibitory concentration. Particular emphasize was given to bactericidal and bacteriostatic mechanisms with focus on generation of reactive oxygen species (ROS) including hydrogen peroxide (H2O2), OH− (hydroxyl radicals), and O2 −2 (peroxide). ROS has been a major factor for several mechanisms including cell wall damage due to ZnO-localized interaction, enhanced membrane permeability, internalization of NPs due to loss of proton motive force and uptake of toxic dissolved zinc ions. These have led to mitochondria weakness, intracellular outflow, and release in gene expression of oxidative stress which caused eventual cell growth inhibition and cell death. In some cases, enhanced antibacterial activity can be attributed to surface defects on ZnO abrasive surface texture. One functional application of the ZnO antibacterial bioactivity was discussed in food packaging industry where ZnO-NPs are used as an antibacterial agent toward foodborne diseases. Proper incorporation of ZnO-NPs into packaging materials can cause interaction with foodborne pathogens, thereby releasing NPs onto food surface where they come in contact with bad bacteria and cause the bacterial death and/or inhibition.

/pdf/review-on-zinc-oxide-nanoparticles-antibacterial-activity-2smaigi8sc.pdf

Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism.

ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G ∼ 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (τ ∼ 20 ns) and slow (τ ∼ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than ∼10 GHz. The high gain and low power consumption of NW photodetec...

http://www1.spms.ntu.edu.sg/~oson/publications/papers/NL%207,%201003%20(2007).pdf

ZnO Nanowire UV Photodetectors with High Internal Gain

Green synthesis of metal nanoparticles using plants

In this study, EOG signals recorded by an developed electronic hardware are processed by digital signal processing techniques and classified with Nearets Neighbor method. The DC component of the signal is suppressed with the digital signal processing techniques, and filtered with different algorithms, a classification is performed. The difference between the filtering processes can be analyzed with different algorithms by using the software that is being developed. This study can be used effectively on the human computer interface application.

Digital signal processing and classification study for electrooculogram signals

A multi-channel measurement system used to measure electrocardiogram (ECG), electroencephalogram (EEG), electromyogram (EMG) and electrooculogram (EOG) biosignals has been designed and prototyped. The designed system has 16 configurable measurement channels. Of the 16 channels the developed system has, 8 have been designed for EEG, 2 for EMG, 2 for EOG, 1 for ECG measurements, the remaining 3 have been reserved as backup channels. In circuit design, biosignal amplifier design principles have been applied by taking into account the characteristics of the biosignal to be measured for each channel, such as bandwidth, frequency, amplitude, noise level. Modules such as instrumentation amplifier, filter, DC suppression unit, amplifier, DC level determination unit, analogdigital converter, optical isolation unit, power supply have been designed to perform biosignal measurements through these channels. Biosignals measured by the developed system can be shifted to the desired threshold level with the help of the analog output reference voltage, converted to digital data 10-bit resolution and transferred to the computer environment in real time. The data transferred to the computer can be used in C#, Excel, MATLAB, and LabVIEW platforms. The novelty of the developed system is that any of the four desired biosignal types can be measured from any channel. In addition, another feature of the system is that it can work with real-time data without being dependent on the databases serving for human-computer interface applications. In experimental studies with some researchers for the performance tests of the system, ECG, EEG, EMG and EOG signals have been recorded with different module configurations, and signal processing stages were carried out to be used for human-computer applications.

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Development of a Multichannel Bioinstrumentation System for Human-Computer Interface Applications

Transient photocapacitance (TPC) measurements were performed to investigate deep centers in GaN∕AlGaN multiple quantum wells The influence of the persistent photovoltaic effect was successfully separated during the TPC experiments The resolution obtained by the TPC measurements is much better than that of steady-state photocapacitance The spectral dependence of photoionization cross section of deep centers in GaN is quantitatively determined in the energy range from 168to330eV The absolute values of photoionization cross sections of these centers are found to be of the order of 10−15–10−14cm2

Photoionization study of deep centers in GaN∕AlGaN multiple quantum wells

The current conduction in GaN is very topical and is the topic of a vast amount of research. By simultaneously mapping the topography and the current distribution, conductive atomic force microscopy (C-AFM) has the potential to establish a correlation between topological features and localized current paths. In this study, this technique was applied to image the conduction properties of as-grown and post-growth chemically etched samples GaN epitaxial layers on a microscopic scale. Our results show that prismatic planes have a significantly higher conductivity than the surrounding areas of the sample surface. A large and stable local current was mainly observed from the walls of the etched pits, under forward and reverse bias of the metallized AFM tip/semiconductor junction.

Investigation of Epitaxial GaN Films by Conductive Atomic Force Microscopy

With the rising demand for energy, concepts such as energy efficiency, energy-efficient devices, and efficiency have grown in importance in recent years. It's well known that a substantial amount of energy produced around the world is consumed in industry, with electric motors accounting for the majority of this consumption. As a result, it is critical to conduct research to improve the efficiency of newly manufactured electric motors in the high efficiency class, as well as electric motors that are still in use because they have not reached the end of their economic lives. The advantages of the proposed system over the conventional fan-cooled system were investigated in this study, which proposed a vortex tube-supported cooling system for cooling the squirrel-caged asynchronous electric motor of the IE3 efficiency class. In this context, an Ansys Motor-CAD model of the test engine was created, and a vortex tube-supported experimental setup was built based on the data. As a result of the simulation and experimental studies, the experimental motor's stator winding temperature improved by 68.64%, and its efficiency improved by 1.79% according to the motor nameplate value. The proposed method increased efficiency without requiring any structural changes to the existing electric motor. 

Seydi Doğan

Papers

Digital signal processing and classification study for electrooculogram signals

Development of a Multichannel Bioinstrumentation System for Human-Computer Interface Applications

Photoionization study of deep centers in GaN∕AlGaN multiple quantum wells

Investigation of Epitaxial GaN Films by Conductive Atomic Force Microscopy

A New Cooling Approach to Increase Efficiency for the Induction Motor : Vortex Tube