Pavel N. Brunkov

Picosecond-Range Avalanche Switching of High-Voltage Diodes: Si Versus GaAs Structures

We present a comparative study of Si and GaAs high-voltage diodes operated in the delayed impact ionization breakdown mode. We use an experimental setup that allows measuring current and voltage on the diode simultaneously and independently during a 100-ps high-voltage switching transient. Si and GaAs structures with identical geometries and a stationary breakdown voltage of ~1 kV were investigated. All devices trigger at close to 2 kV and are capable of forming a voltage ramp with a kilovolt amplitude and a 100-ps rise time. We found that Si p+nn+ and p+pnn+ structures differ in residual voltage: a relatively low residual voltage Vres of 100-200 V was observed only for p±nn± structures, whereas for p+pnn+ structures Vres is about 1 kV. We report observing the lock-on effect in GaAs structures: after 100-ps avalanche switching GaAs diodes remain in a high conducting state as long as the applied voltage pulse lasts, whereas within the same time of 2 ns reference Si diodes fully recover the blocking capability.

New silver nanoparticles induce apoptosis-like process in E. coli and interfere with mammalian copper metabolism

Silver nanoparticles (SNPs) are new functional materials that are widely used in biomedical and industrial technologies. Two main features that make SNPs valuable are their strong antibacterial effects and low toxicity to eukaryotes. In this study, SNPs were synthesized using a modified method of reducing the metal ions to their atomic state followed by crystallization. SNPs were characterized by UV/vis spectroscopy, X-ray diffractometry, atomic force microscopy, and transmission electron microscopy (TEM). The SNPs were spherically shaped with an average linear dimension of 20 nm. In aqueous solution, the SNPs were beige-yellow in color, and they formed a black color in bacteria-rich growth media. The toxicity and bioavailability of the SNPs were tested using Escherichia coli cells and C57Bl/6 mice. Although the SNPs displayed bactericidal activity, an E. coli cell strain transformed with an expression plasmid carrying a human CTR1 ectodomain with three motives that bind Cu(II), Cu(I), and Ag(I) demonstrated increased resistance to treatment with SNPs. TEM showed that the SNPs were absorbed by the E. coli cell, and flow cytometry showed that the SNPs induced apoptosis-like death. In mice treated with SNPs (daily intraperitoneal injection of 10 μg SNPs/g body weight over 4 days), the ceruloplasmin (Cp) oxidase activity in the blood serum decreased. However, level of Cp gene expression, the relative contents of the Cp protein in the Golgi complex and in the serum did not change. Treatment with SNPs did not influence the activity of superoxide dismutase 1 in the liver and had no apparent toxic effects in mice. These findings expand the scope of application for the use of new SNPs. The data are discussed in a paradigm, in which the effects of SNPs are caused by the interference of silver ions with copper metabolism.

Facile reduction of graphene oxide suspensions and films using glass wafers

This paper reports a facile and green method for conversion of graphene oxide (GO) into graphene by low-temperature heating (80 °C) in the presence of a glass wafer. Compared to conventional GO chemical reduction methods, the presented approach is easy-scalable, operationally simple, and based on the use of a non-toxic recyclable deoxygenation agent. The efficiency of the proposed method is further expanded by the fact that it can be applied for reducing both GO suspensions and large-scale thin films formed on various substrates prior to the reduction process. The quality of the obtained reduced graphene oxide (rGO) strongly depends on the type of the used glass wafer, and, particularly, magnesium silicate glass can provide rGO with the C/O ratio of 7.4 and conductivity of up to 33000 S*cm−1. Based on the data obtained, we have suggested a mechanism of the observed reduction process in terms of the hydrolysis of the glass wafer with subsequent interaction of the leached alkali and alkali earth cations and silicate anions with graphene oxide, resulting in elimination of the oxygen-containing groups from the latter one. The proposed approach can be efficiently used for low-cost bulk-quantity production of graphene and graphene-based materials for a wide field of applications.

Delayed avalanche breakdown of high-voltage silicon diodes: Various structures exhibit different picosecond-range switching behavior

We present a comparative study of silicon high-voltage diodes exhibiting the effect of delayed superfast impact-ionization breakdown. The effect manifests itself in a sustainable picosecond-range transient from the blocking to the conducting state and occurs when a steep voltage ramp is applied to the p+-n-n+ diode in the reverse direction. Nine groups of diodes with graded and abrupt pn-junctions have been specially fabricated for this study by different techniques from different Si substrates. Additionally, in two groups of these structures, the lifetime of nonequilibrium carriers was intentionally reduced by electron irradiation. All diodes have identical geometrical parameters and similar stationary breakdown voltages. Our experimental setup allows measuring both device voltage and current during the kilovolt switching with time resolution better than 50 ps. Although all devices are capable of forming a front with kilovolt amplitude and 100 ps risetime in the in-series load, the structures with graded pn-junctions have anomalously large residual voltage. The Deep Level Transient Spectroscopy study of all diode structures has been performed in order to evaluate the effect of deep centers on device performance. It was found that the presence of deep-level electron traps negatively correlates with parameters of superfast switching, whereas a large concentration of recombination centers created by electron irradiation has virtually no influence on switching characteristics.We present a comparative study of silicon high-voltage diodes exhibiting the effect of delayed superfast impact-ionization breakdown. The effect manifests itself in a sustainable picosecond-range transient from the blocking to the conducting state and occurs when a steep voltage ramp is applied to the p+-n-n+ diode in the reverse direction. Nine groups of diodes with graded and abrupt pn-junctions have been specially fabricated for this study by different techniques from different Si substrates. Additionally, in two groups of these structures, the lifetime of nonequilibrium carriers was intentionally reduced by electron irradiation. All diodes have identical geometrical parameters and similar stationary breakdown voltages. Our experimental setup allows measuring both device voltage and current during the kilovolt switching with time resolution better than 50 ps. Although all devices are capable of forming a front with kilovolt amplitude and 100 ps risetime in the in-series load, the structures with graded...

The Extracellular Domain of Human High Affinity Copper Transporter (hNdCTR1), Synthesized by E. coli Cells, Chelates Silver and Copper Ions In Vivo

There is much interest in effective copper chelators to correct copper dyshomeostasis in neurodegenerative and oncological diseases. In this study, a recombinant fusion protein for expression in Escherichia coli cells was constructed from glutathione-S-transferase (GST) and the N-terminal domain (ectodomain) of human high affinity copper transporter CTR1 (hNdCTR1), which has three metal-bound motifs. Several biological properties of the GST-hNdCTR1 fusion protein were assessed. It was demonstrated that in cells, the protein was prone to oligomerization, formed inclusion bodies and displayed no toxicity. Treatment of E. coli cells with copper and silver ions reduced cell viability in a dose- and time-dependent manner. Cells expressing GST-hNdCTR1 protein demonstrated resistance to the metal treatments. These cells accumulated silver ions and formed nanoparticles that contained AgCl and metallic silver. In this bacterial population, filamentous bacteria with a length of about 10 µm were often observed. The possibility for the fusion protein carrying extracellular metal binding motifs to integrate into the cell’s copper metabolism and its chelating properties are discussed.

Reduction of the graphene oxide films by soft UV irradiation

We have studied the UV reduction process of thin graphene oxide films, deposited on silicon substrate from ethanol suspension. Chemical structure of obtained material was analyzed by XPS method. TEM images showed holes formation during reduction process, that are connected into network. Films with observed structure have great variety of possible future applications, such as gas-sensors and different organic/nonorganic nanocomposites.

Increasing the quantum efficiency of GaAs solar cells by embedding InAs quantum dots

Development of Metalorganic Vapor Phase Epitaxy (MOVPE) technology of InAs quantum dots (QDs) in GaAs for photovoltaic applications is presented. The growth peculiarities in InAs-GaAs lattice-mismatched system were considered. The photoluminescence (PL) intensity dependences on different growth parameters were obtained. The multimodal distribution of QDs by sizes was found using AFM and PL methods. GaAs solar cell nanoheterostructures with imbedded QD arrays were designed and obtained. Ones have been demonstrated a significant increase of quantum efficiency and photogenerated current of QD solar cells due to photo effect in InAs QD array (0.59 mA/cm2 for AM1.5D and 82 mA/cm2 for AM0).

High-voltage subnanosecond avalanche sharpening diodes: A comparative study of silicon and gallium arsenide structures

Subnanosecond avalanche switching of dynamically overvoltaged diodes is known to occur for both Si and GaAs diodes. Here we present a novel comparative study of Si and GaAs structures. Original experimental setup that allows measuring current and voltage on the diode simultaneously and independently has been used for measurements. Si p⁺nn⁺ and p⁺ pnn⁺ and GaAs p⁺nn⁺ structures with similar geometrical parameters and stationary breakdown voltage ~1 kV have been investigated. All devices under study trigger at ~2 kV and are capable of forming a voltage ramp with kilovolt amplitude and 100-ps risetime. We found a drastic difference between Si p⁺nn⁺ and p⁺pnn⁺ relatively low residual voltage V_res~150 V has been observed only for p⁺nn⁺ structures whereas for p⁺pnn⁺ structures V_res~1 kV. The difference between Si and GaAs structures is much more dramatic: after 100-ps avalanche switching GaAs diode remains in high conducting state as long as the applied voltage pulse lasts. Within the same time of 2 ns the reference Si diode fully recovers the blocking capability. Next, the residual voltage on GaAs diode does not exceed several dozen volts. The discovered effect resembles the well-know lock-on effect in optically activated switches.