Oxygen-exchange reaction between O2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N¹⁶O and ¹⁸O₂ coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N¹⁸O and ¹⁸O¹⁶O are desorbed from Pt(111) initially saturated with ¹⁸O₂ at 94 K followed by exposure of N¹⁶O. Three distinct desorption peaks are observed in N¹⁸O TPD spectra at 145, 310, and 340 K, and two peaks in ¹⁸O¹⁶O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO₂ or NO₃ is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.     

Bi-periodic behaviour in the diffusive autocatalator

We consider a reaction-diffusion system, consisting of two coupled PDEs with equal diffusion coefficients, describing autocatalysis under isothermal conditions in a slab. With enforced symmetric boundary conditions only stationary or simple periodic behaviour occurs. When the symmetry is broken by enforcing non-symmetric boundary conditions we have obtained bi-periodic solutions. We have also obtained oscillations essentially confined to one half of the spatial domain.     

Bimetal-organic frameworks derived Co/N-doped carbons for lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have received extensive attention due to their high theoretical specific energy density. However, the utilization of sulfur is seriously reduced by the shuttle effect of lithium polysulfides and the low conductivity of sulfur and lithium sulfide (Li₂S). Herein, we introduced bimetal-organic frameworks (Co/Zn-ZIF) derived cobalt and nitrogen-doped carbons (Co/N-C) into Li-S batteries through host design and separator modification. The Co/N-C in Li-S batteries effectively limits the shuttle effect through simultaneously serving as polysulfide traps and chemical catalyst. As a result, the Li-S batteries deliver a high reversible capacity of 1614.5 mAh/g and superior long-term cycling stability with a negligible capacity decay of only 0.04% per cycle after 1000 cycles. Furthermore, they have a high area capacity of 5.5 mAh/cm².     

New possibilities for materials science with STED microscopy

Up to now, STED microscopy has been mainly used to study biological systems. However with the development of the technique, many benefits are expected in materials science for imaging 1D, 2D or 3D nanomaterials. We review here the use of STED microscopy in materials science, its challenges, and opportunities.     

Radiation forces of a dielectric medium plate induced by a Gaussian beam

Based on the angular spectrum method, we investigate the radiation forces on a finite-size dielectric medium plate induced by a Gaussian beam. The formulas for the radiation force along longitudinal and transverse direction are derived, and numerically calculation is performed. It is shown that for the finite-size dielectric medium plate, the radiation forces exerted by the Gaussian beam is dependent upon the angle and position of a single ray striking on the plate and the intensity of light. Our numerical results indicate that if we choose the appropriate parameters there will be enough transverse forces to overcome the gravity force, making the plate move upwards.     

First-principles study of intrinsic defect properties in hexagonal BN bilayer and monolayer

The formation energies and transition energy levels of intrinsic defects in hexagonal BN (h-BN) bilayer and monolayer have been studied by first-principles calculations based on density functional theory. Our calculated results predict that excellent intrinsic p-type and n-type conductivities are very difficult to be realized in h-BN bilayer and monolayer. This is because of the high formation energies of acceptor-like defects (≥4.6 eV ) and the rather deep transition energy levels of donor-like defects (≥2.0 eV ). In order to obtain h-BN layers with more efficient p-type and n-type conductivity, extrinsic doping using foreign impurities is necessary.     

Non-negative infrared patch-image model: Robust target-background separation via partial sum minimization of singular values

To further enhance the small targets and suppress the heavy clutters simultaneously, a robust non-negative infrared patch-image model via partial sum minimization of singular values is proposed. First, the intrinsic reason behind the undesirable performance of the state-of-the-art infrared patch-image (IPI) model when facing extremely complex backgrounds is analyzed. We point out that it lies in the mismatching of IPI model’s implicit assumption of a large number of observations with the reality of deficient observations of strong edges. To fix this problem, instead of the nuclear norm, we adopt the partial sum of singular values to constrain the low-rank background patch-image, which could provide a more accurate background estimation and almost eliminate all the salient residuals in the decomposed target image. In addition, considering the fact that the infrared small target is always brighter than its adjacent background, we propose an additional non-negative constraint to the sparse target patch-image, which could not only wipe off more undesirable components ulteriorly but also accelerate the convergence rate. Finally, an algorithm based on inexact augmented Lagrange multiplier method is developed to solve the proposed model. A large number of experiments are conducted demonstrating that the proposed model has a significant improvement over the other nine competitive methods in terms of both clutter suppressing performance and convergence rate.     

Van der Waals Effects on semiconductor clusters

Van der Waals (vdW) interactions play an important role on semiconductors in nanoscale. Here, we utilized first‐principles calculations based on density functional theory to demonstrate the growth mode transition from prolate to multiunit configurations for Ge_n (n = 10–50) clusters. In agreement with the injected ion drift tube techniques that “clusters with n < 70 can be thought of as loosely bound assemblies of small strongly bound fragments (such as Ge₇ and Ge₁₀),” we found these stable fragments are connected by Ge₆, Ge₉, or Ge₁₀ unit (from bulk diamond), via strong covalent bonds. Our calculated cations usually fragment to Ge₇ and Ge₁₀ clusters, in accordance with the experiment results that the spectra Ge₇ and Ge₁₀ correspond to the mass abundance spectra. By controlling a germanium cluster with vdW interactions parameters in the program or not, we found that the vdW effects strengthen the covalent bond from different units more strikingly than that in a single unit. With more bonds between units than the threadlike structures, the multiunit structures have larger vdW energies, explaining why the isolated nanowires are harder to produce. © 2015 Wiley Periodicals, Inc.