Electron-Beam Synthesis of Dielectric Coatings Using Forevacuum Plasma Electron Sources (Review)

This is a review of current developments in the field of ion-plasma and beam methods of synthesis of protective and functional dielectric coatings. We give rationales for attractiveness and prospects of creating such coatings by electron-beam heating and following evaporation of dielectric targets. Forevacuum plasma electron sources, operating at elevated pressure values from units to hundreds of pascals, make it possible to exert the direct action of an electron beam on low-conductive materials. Electron-beam evaporation of aluminum oxide, boron, and silicon carbide targets is used to exemplify the particular features of electron-beam synthesis of such coatings and their parameters and characteristics.

Oligo-heterotrophic activity of Marinobacter subterrani creates an indirect Fe(II)-oxidation phenotype in gradient tubes

Autotrophic bacteria utilizing Fe(II) as their energy and electron sources for growth affect multiple biogeochemical cycles. Some chemoheterotrophic bacteria have also been considered to exhibit an Fe(II) oxidation phenotype. For example, several Marinobacter strains have been reported to oxidize Fe(II) based on formation of oxidized iron bands in semi-solid gradient tubes that produce opposing concentration gradients of Fe(II) and oxygen. While gradient tubes are a simple and visually compelling method to test for Fe(II) oxidation, this method alone cannot confirm if, and to what extent, Fe(II) oxidation is linked to metabolism in chemoheterotrophic bacteria. Here we probe the possibility of protein-mediated and metabolic byproduct-mediated Fe(II) oxidation in Marinobacter subterrani JG233, a chemoheterotroph previously proposed to oxidize Fe(II). Results from conditional and mutant studies, along with measurements of Fe(II) oxidation rates suggest M. subterrani is unlikely to facilitate Fe(II) oxidation under microaerobic conditions. We conclude that the Fe(II) oxidation phenotype observed in gradient tubes inoculated with M. subterrani JG233 is a result of oligo-heterotrophic activity, shifting the location where oxygen dependent chemical Fe(II) oxidation occurs, rather than a biologically-mediated process. Importance Gradient tubes are the most commonly used method to isolate and identify neutrophilic Fe(II)-oxidizing bacteria. The formation of oxidized iron bands in gradient tubes provides a compelling assay to ascribe the ability to oxidize Fe(II) to autotrophic bacteria whose growth is dependent on Fe(II) oxidation. However, the physiological significance of Fe(II) oxidation in chemoheterotrophic bacteria is less well understood. Our work suggests that oligo-heterotrophic activity of certain bacteria may create a false-positive phenotype in gradient tubes by altering the location of the abiotic, oxygen-mediated oxidized iron band. Based on the results and analysis presented here, we caution against utilizing gradient tubes as the sole evidence for the capability of a strain to oxidize Fe(II) and that additional experiments are necessary to ascribe this phenotype to new isolates.

Ab Initio Quantum–Mechanical Predictions of Semiconducting Photocathode Materials

Ab initio quantum–mechanical methods are well-established tools for material characterization and discovery in many technological areas. Recently, state-of-the-art approaches based on density-functional theory and many-body perturbation theory were successfully applied to semiconducting alkali antimonides and tellurides, which are currently employed as photocathodes in particle accelerator facilities. The results of these studies have unveiled the potential of ab initio methods to complement experimental and technical efforts for the development of new, more efficient materials for vacuum electron sources. Concomitantly, these findings have revealed the need for theory to go beyond the status quo in order to face the challenges of modeling such complex systems and their properties in operando conditions. In this review, we summarize recent progress in the application of ab initio many-body methods to investigate photocathode materials, analyzing the merits and the limitations of the standard approaches with respect to the confronted scientific questions. In particular, we emphasize the necessary trade-off between computational accuracy and feasibility that is intrinsic to these studies, and propose possible routes to optimize it. We finally discuss novel schemes for computationally-aided material discovery that are suitable for the development of ultra-bright electron sources toward the incoming era of artificial intelligence.

THz-Enhanced DC Ultrafast Electron Diffractometer

Terahertz- (THz-) based electron manipulation has recently been shown to hold tremendous promise as a technology for manipulating and driving the next generation of compact ultrafast electron sources. Here, we demonstrate an ultrafast electron diffractometer with THz-driven pulse compression. The electron bunches from a conventional DC gun are compressed by a factor of 10 and reach a duration of ~180 fs (FWHM) with 10,000 electrons/pulse at a 1 kHz repetition rate. The resulting ultrafast electron source is used in a proof-of-principle experiment to probe the photoinduced dynamics of single-crystal silicon. The THz-compressed electron beams produce high-quality diffraction patterns and enable the observation of the ultrafast structural dynamics with improved time resolution. These results validate the maturity of THz-driven ultrafast electron sources for use in precision applications.

C-type cytochrome-initiated reduction of bacterial lytic polysaccharide monooxygenases.

The release of glucose from lignocellulosic waste for subsequent fermentation into biofuels holds promise for securing humankind's future energy needs. The discovery of a set of copper dependent enzymes known as lytic polysaccharide monooxygenases (LPMOs) has galvanized new research in this area. LPMOs act by oxidatively introducing chain breaks into cellulose and other polysaccharides, boosting the ability of cellulases to act on the substrate. Although several proteins have been implicated as electron sources in fungal LPMO biochemistry, no equivalent bacterial LPMO electron donors have been previously identified, although the proteins Cbp2D and E from Cellvibrio japonicus have been implicated as potential candidates. Here we analyze a small c-type cytochrome (CjX183) present in Cellvibrio japonicus Cbp2D, and show that it can initiate bacterial CuII/I LPMO reduction and also activate LPMO-catalyzed cellulose-degradation. In the absence of cellulose, CjX183-driven reduction of the LPMO results in less H2O2 production from O2, and correspondingly less oxidative damage to the enzyme than when ascorbate is used as the reducing agent. Significantly, using CjX183 as the activator maintained similar cellulase boosting levels relative to the use of an equivalent amount of ascorbate. Our results therefore add further evidence to the impact that the choice of electron source can have on LPMO action. Furthermore, the study of Cbp2D and other similar proteins may yet reveal new insight into the redox processes governing polysaccharide degradation in bacteria.