The transition between high and low wear regimes under multidirectional reciprocating sliding

The wear phenomena and wear characteristics of reciprocating sliding wear with superimposed lateral vibrations were investigated using a ball-on-disc tribometer. The tribometer enabled two orthogonal oscillations, whereas one oscillation had a constant amplitude of 1 mm (primary oscillation) and the other one had a variable amplitude from 0 to 20.2 μm (secondary oscillation). Ball and disc were made of AISI 52100 steel. The ball surface was polished and the disc surface was unidirectionally grinded parallel to the direction of primary oscillation. Two regimes with different wear rates were found, being separated by a characteristic transition amplitude of 2.7 ± 0.4 μm in the secondary oscillation. This transition correlated with a change of wear mechanisms from tribochemically to mechanically dominated wear. A wear model based on surface topography and particle motion was developed. The wear model is able to predict the value of the transition amplitude by means of characteristic topographical data and the size of wear particles.     

Engineering Potato Virus X Particles for a Covalent Protein Based Attachment of Enzymes

Plant virus nanoparticles are often used to display functional amino acids or small peptides, thus serving as building blocks in application areas as diverse as nanoelectronics, bioimaging, vaccination, drug delivery, and bone differentiation. This is most easily achieved by expressing coat protein fusions, but the assembly of the corresponding virus particles can be hampered by factors such as the fusion protein size, amino acid composition, and post‐translational modifications. Size constraints can be overcome by using the Foot and mouth disease virus 2A sequence, but the compositional limitations cannot be avoided without the introduction of time‐consuming chemical modifications. SpyTag/SpyCatcher technology is used in the present study to covalently attach the Trichoderma reesei endoglucanase Cel12A to Potato virus X (PVX) nanoparticles. The formation of PVX particles is confirmed by western blot, and the ability of the particles to display Cel12A is demonstrated by enzyme‐linked immunosorbent assays and transmission electron microscopy. Enzymatic assays show optimal reaction conditions of 50 °C and pH 6.5, and an increased substrate conversion rate compared to free enzymes. It is concluded that PVX displaying the SpyTag can serve as new scaffold for protein display, most notably for proteins with post‐translational modifications.     

Modelling for hydrogen diffusion in metals with traps revisited

A rigorous concept for diffusion of hydrogen is presented that allows for the role of trapping and/or hydrostatic stress. Nonlinear partial differential equations for the concentration of freely diffusing hydrogen in the lattice and for the total concentration of hydrogen both in the lattice and traps are derived. A generalized chemical diffusion coefficient can be deduced and is compared with reported relations from the literature. Simulation results are presented for different amounts of traps and their different energetic levels (depths), both for hydrogen concentration profiles and the total amount of hydrogen in the case of charging or discharging a cylindrical specimen. A significant charging/discharging asymmetry occurs for high hydrogen concentrations and high amounts of traps of sufficient depth. Comparisons of the chemical diffusion coefficient with experimental results from the literature are also presented.     

Laser heating of uncoated optics in a convective medium

Powerful, long-pulse lasers have a variety of applications. In many applications, optical elements are employed to direct, focus, or collimate the beam. Typically the optic is suspended in a gaseous environment (e.g., air) and can cool by convection. The variation of the optic temperature with time is obtained by combining the effects of laser heating, thermal conduction, and convective loss. Characteristics of the solutions in terms of the properties of the optic material, laser beam parameters, and the environment are discussed and compared with measurements at the Naval Research Laboratory, employing kW-class, 1 μm wavelength, continuous wave lasers and optical elements made of fused silica or BK7 glass. The calculated results are in good agreement with the measurements, given the approximations in the analysis and the expected variation in the absorption coefficients of the glasses used in the experiments.     

Connecting dopant bond type with electronic structure in N-doped graphene

Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.     

Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

Hybrid passivated colloidal quantum dot solids

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.     

In vivo Quantum‐Dot Toxicity Assessment

Quantum dots have potential in biomedical applications, but concerns persist about their safety. Most toxicology data is derived from in vitro studies and may not reflect in vivo responses. Here, an initial systematic animal toxicity study of CdSe–ZnS core–shell quantum dots in healthy Sprague–Dawley rats is presented. Biodistribution, animal survival, animal mass, hematology, clinical biochemistry, and organ histology are characterized at different concentrations (2.5–15.0 nmol) over short‐term (<7 days) and long‐term (>80 days) periods. The results show that the quantum dot formulations do not cause appreciable toxicity even after their breakdown in vivo over time. To generalize the toxicity of quantum dots in vivo, further investigations are still required. Some of these investigations include the evaluation of quantum dot composition (e.g., PbS versus CdS), surface chemistry (e.g., functionalization with amines versus carboxylic acids), size (e.g., 2 versus 6 nm), and shape (e.g., spheres versus rods), as well as the effect of contaminants and their byproducts on biodistribution behavior and toxicity. Combining the results from all of these studies will eventually lead to a conclusion regarding the issue of quantum dot toxicity.     

Emotion regulation and vulnerability to depression: Spontaneous versus instructed use of emotion suppression and reappraisal.

Emotion dysregulation has long been thought to be a vulnerability factor for mood disorders. However, there have been few empirical tests of this idea. In this study, we tested the hypothesis that depression vulnerability is related to difficulties with emotion regulation by comparing recovered-depressed and never-depressed participants (N = 73). In the first phase, participants completed questionnaires assessing their typical use of emotion regulation strategies. In the second phase, sad mood was induced using a film clip, and the degree to which participants reported to have spontaneously used suppression versus reappraisal to regulate their emotions was assessed. In the third phase, participants received either suppression or reappraisal instructions prior to watching a second sadness-inducing film. As predicted, suppression was found to be ineffective for down-regulating negative emotions, and recovered-depressed participants reported to have spontaneously used this strategy during the first sadness-inducing film more often than controls. However, the groups did not differ regarding the effects of induced suppression versus reappraisal on negative mood. These results provide evidence for a role for spontaneous but not instructed emotion regulation in depression vulnerability. (PsycINFO Database Record (c) 2010 APA, all rights reserved)