Electrical friction modulation on MoS2 using electron beam radiation without electrostatic interactions.

Nanoscale friction under different electronic states and the corresponding friction controlling methods are both scientifically interesting and technologically important. However, friction measurements under electrical modulation are severely hampered by electrostatic forces induced by the charge-trapping effect. Therefore, in this study, we developed a new modulation method free from the charge-trapping effect through electron beam radiation; this method successfully modulated the friction between few-layer MoS2 and the silicon tip on atomic force microscopy. Friction on monolayer MoS2 increased under electron beam radiation. Strong correlations between the accelerating voltage, beam current, and friction force were found, and constant adhesion force demonstrate that the influence of static electricity was eliminated in this method. Excited electron states caused by electron injection could be possible mechanisms for friction modulation. However, the electron beam radiation had a negligible influence on the friction of bilayer MoS2. This study is the first of its kind, revealing the effect of electron beam radiation and electronic states on friction, which is important for the development of tribological theories and nanoelectromechanical systems, and offers a new electrical modulation method for friction tuning.

Superlubricity between a silicon tip and graphite enabled by the nanolithography-assisted nanoflakes tribo-transfer.

Superlubricity between a cone-shaped (sharp) silicon tip and graphite remains a challenge in the nanotribology field. In this paper, an efficient method of achieving superlubricity between a cone-shaped silicon tip and graphite was proposed. Graphite nanoflakes were transferred onto the silicon tip by repeatedly rubbing against the scratches produced by nanolithography on a graphite surface. The superlubricity between the graphite nanoflakes-wrapped tip and highly oriented pyrolytic graphite (HOPG) was attained, and the friction coefficient was reduced to 0.0007. At low normal loads, the frictional force was small and showed a strong correlation with the sliding angle, but as the normal load increased, this dependence gradually decreased. It was firstly found that the transferred graphite nanoflakes on the contact zone of the silicon tip could be transformed into amorphous carbon layers induced by the shear force and high pressure during the superlubricity test process.

Thickness dependent friction on few-layer MoS2, WS2, and WSe2.

Nanoscale friction on two-dimensional (2D) materials is closely associated with their mechanical, electronic and photonic properties, which can be modulated through changing thickness. Here, we investigated the thickness dependent friction on few-layer MoS2, WS2, and WSe2 using atomic force microscope at ambient condition and found two different behavior. When a sharp tip was used, the regular behavior of decreasing friction with increasing thickness was reproduced. However, when a pre-worn and flat-ended tip was used, we observed an abnormal trend: on WS2 and WSe2, friction increased monotonically with thickness, while for MoS2, friction decreased from monolayer to bilayer and then subsequently increased with thickness. As suggested by the density functional theory calculation, we hypothesize that the overall frictional behavior is a competition between the puckering effect and the intrinsic energy corrugation within the compressive region. By varying the relative strength of the puckering effect via changing the tip shape, the dependence of friction on sample thickness can be tuned. Our results also suggest a potential means to measure intrinsic frictional properties of 2D materials with minimum impact from puckering.

Strain-Gradient Modulated Exciton Emission in Bent ZnO Wires Probed by Cathodoluminescence.

Photoelectrical properties of semiconductor nanostructures are expected to be improved significantly by strain engineering. Besides the local strain, the strain gradient is promising to tune the luminescence properties by modifying the crystal symmetry. Here, we report the investigation of strain-gradient induced symmetry-breaking effect on excitonic states in pure bending ZnO microwires by high spatial-resolved cathodoluminescence at low temperature of 80 K. In addition to the local-strain induced light emission peak shift, the bound exciton emission photon energy shows an extraordinary jump of ∼16.6 meV at a high strain-gradient of 1.22% µm-1, which is ascribed to the strain gradient induced symmetry-breaking. Such a symmetry-breaking lifts the energy degeneracy of the electronic band structures, which significantly modifies the electron-hole interactions and the fine structures of the bound exciton states. These results provide a further understanding of the strain gradient effect on the excitonic states and possess a potential for the applications in optoelectronic devices.

A multiphase mixture model for substrate concentration distribution characteristics and photo-hydrogen production performance of the entrapped-cell photobioreactor.

A multiphase mixture model was developed for revealing the interaction mechanism between biochemical reactions and transfer processes in the entrapped-cell photobioreactor packed with gel granules containing Rhodopseudomonas palustris CQK 01. The effects of difference operation parameters, including operation temperature, influent medium pH value and porosity of packed bed, on substrate concentration distribution characteristics and photo-hydrogen production performance were investigated. The results showed that the model predictions were in good agreement with the experimental data reported. Moreover, the operation temperature of 30 °C and the influent medium pH value of 7 were the most suitable conditions for photo-hydrogen production by biodegrading substrate. In addition, the lower porosity of packed bed was beneficial to enhance photo-hydrogen production performance owing to the improvement on the amount of substrate transferred into gel granules caused by the increased specific area for substrate transfer in the elemental volume.

[Spatial-temporal variations of extractable organic matter in atmospheric PM10 and PM2.5 in Beijing].

PM10 and PM2.5 samples were collected in parallel in different function zones of Beijing during four seasons of 2005. The pollution level, distribution characteristics of the extractable organic matter (EOM) and relationship between EOM (PM10) and EOM (PM2.5) were illustrated. The results show that: the annual mean concentrations of organic compound in PM10 and PM2.5 are 41.39 microg/m3 and 34.84 microg/m3, being 1.44 times and 1.26 times higher than Ming Tombs site. The concentrations of EOM in winter are 67.04 microg/m3 (PM10) and 64.64 microg/m3 (PM2.5), which are 1.15 and 1.82 times, 2.06 and 2.26 times, 4.53 and 6.26 times higher than that in spring, autumn, summer, respectively. Ratios of EOM in PM2.5 to that in PM10 in different seasons exceed 0.60. In different function zones the concentrations of EOM present industrial and commercial zones > living, traffic and contrast zones. The influence of EOM (PM2.5) on EOM (PM10) in different districts are various. The order of annual concentrations of organic compositions is non-hydrocarbons > asphaltenes > aromatics > saturated hydrocarbon. The seasonal emissions of pollution sources play an important role in seasonality of compositions of EOM.

[Determination of 16 PAHs from atmospheric particulate matter PM2.5 in certain ironworks district by HPLC].

The paper describes the determination of the 16 polycyclic aromatic hydrocarbons (PAHs) prescribed by Environmental Protection Agency in PM2.5 in certain ironworks district of Beijing by HPLC. A HPLC method of VWD-FLD connected in series was founded. The 16 PAHs were completely separated by the method. The earlier eluted ten PAHs were detected by VWD and the rest were detected by FLD. The minimum detection limits of the 16 PAHs vary from 0.29 microg x L(-1) (for Bkf) to 50.6microg x L(-1) (for Nap), the average recoveries range between 85.1% (for Nap) and 103.2% (for Fla) and the relative standard deviations are between 4.31% (for Flu) and 9.93% (for Ind). The detection results indicate that the pollution situation is still very serious in this district.