Phonon-assisted electron emission from individual carbon nanotubes

A question of how electrons can escape from one-atom-thick surfaces has seldom been studied and is still not properly answered. Herein, lateral electron emission from a one-atom-thick surface is thoroughly studied for the first time. We study electron emission from side surface of individual electrically biased carbon nanotubes (CNTs) both experimentally and theoretically and discover a new electron emission mechanism named phonon-assisted electron emission. A kinetic model based on coupled Boltzmann equations of electrons and optical phonons is proposed and well describes experimentally measured lateral electron emission from CNTs. It is shown that the electrons moving along a biased CNT can overflow from the one-atom-thick surface due to the absorption of hot forward-scattering optical phonons. A low working voltage, high emission density, and side emission character make phonon-assisted electron emission primarily promising in electron source applications.     

Nanomaterials: Exfoliating the inorganics

A versatile method for the production of sheets of inorganic compounds with atomic thickness has been demonstrated.     

Structural transformation, photocatalytic, and field-emission properties of ridged TiO2 nanotubes

Self-organized, freestanding TiO(2) nanotube arrays with ridged structures have been fabricated using a one-step anodic oxidation method. Their structural, photocatalytic, and field-emission (FE) properties have systematically been investigated. The as-synthesized nanostructures have been characterized using XRD, Raman spectroscopy, SEM, and HRTEM. The experimental results show that after an annealing process, the starting amorphous nanotubes have been turned into anatase phase structures, and the tube walls have been decorated with nanoparticles, different from the original ridged nanotubes. Furthermore, the anatase phase nanotubes have demonstrated better photocatalytic properties than their amorphous counterparts, which is caused by the larger surface area and improved crystallinity. With respect to FE properties, the as-grown nanotubes have the lower turn-on field E(to) and the higher field enhancement factor β compared to the annealed nanotubes. The relationship between E(to), β, and the tube arrangements and morphologies has also been discussed.     

Low-Voltage Magnetoelectric Coupling in Fe0.5Rh0.5/0.68PbMg1/3Nb2/3O3-0.32PbTiO3 Thin-Film Heterostructures

The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30 nm Fe_0.5Rh_0.5/100 nm 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe_0.5Rh_0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe_0.5Rh_0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10⁻⁸ s m⁻¹ at 325 K while applying a −0.75 V bias.     

Rounding Out the Understanding of ACD Toxicity with the Discovery of Cyclic Forms of Actin Oligomers

Actin is an essential element of both innate and adaptive immune systems and can aid in motility and translocation of bacterial pathogens, making it an attractive target for bacterial toxins. Pathogenic Vibrio and Aeromonas genera deliver actin cross-linking domain (ACD) toxin into the cytoplasm of the host cell to poison actin regulation and promptly induce cell rounding. At early stages of toxicity, ACD covalently cross-links actin monomers into oligomers (AOs) that bind through multivalent interactions and potently inhibit several families of actin assembly proteins. At advanced toxicity stages, we found that the terminal protomers of linear AOs can get linked together by ACD to produce cyclic AOs. When tested against formins and Ena/VASP, linear and cyclic AOs exhibit similar inhibitory potential, which for the cyclic AOs is reduced in the presence of profilin. In coarse-grained molecular dynamics simulations, profilin and WH2-motif binding sites on actin subunits remain exposed in modeled AOs of both geometries. We speculate, therefore, that the reduced toxicity of cyclic AOs is due to their reduced configurational entropy. A characteristic feature of cyclic AOs is that, in contrast to the linear forms, they cannot be straightened to form filaments (e.g., through stabilization by cofilin), which makes them less susceptible to neutralization by the host cell.     

Experimental performance evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Microchannel reactors appear attractive as integral parts of fuel processors to generate hydrogen (H₂) for portable and distributed fuel cell applications. The work described in this paper evaluates, characterizes, and demonstrates miniaturized H₂ production in a stand-alone ammonia-fuelled microchannel reformer. The performance of the microchannel reformer is investigated as a function of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6520–32,600 Nml g_cat⁻¹ h⁻¹). The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH₃ conversion at 32,600 Nml g_cat⁻¹ h⁻¹ and 700 °C to generate an estimated fuel cell power output of 5.7 W_e and power density of 16 kW_e L⁻¹ (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm⁻¹) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H₂ to fuel cell systems.     

Periodic TiO2 Nanorod Arrays with Hexagonal Nonclose‐Packed Arrangements: Excellent Field Emitters by Parameter Optimization

Periodic TiO₂ nanorod arrays with hexagonal nonclose‐packed (hncp) arrangements are synthesized by pulsed laser deposition (PLD) using polystyrene colloidal monolayers as templates and with subsequent annealing in air. The hncp‐array formation is governed by in situ volume shrinkage of amorphous TiO₂ nanorods in the crystallizing process during annealing. The array periodicity can easily be tuned by different sphere sizes of the colloidal template, whereas the distance between neighboring nanorods can be controlled by altering the background gas pressure during the PLD process, at a given periodicity for the nanorod array. Parameter‐controlled growth is helpful for investigating and optimizing the parameter‐dependent field‐emission properties. The hncp nanorod array exhibits an enhanced field‐emission (FE) performance compared to both particle films and nanorod arrays with top aggregation. With an increase in periodicity of a hncp nanorod array, the field‐enhancement factor decreases and the turn‐on FE field increases. FE characteristics can be further enhanced by increasing the distance between adjacent nanorods while maintaining the same periodicity. The parameter‐optimized results suggest that the arrays with a smaller periodicity and a larger distance display the best FE performance and could be highly valuable for designing field‐emission devices based on these periodic nanorod arrays.     

Water exchange-minimizing DCE-MRI protocol to detect changes in tumor vascular parameters: effect of bevacizumab/paclitaxel combination therapy

Objective

The purpose of this study was to assess changes in the tumor microvasculature induced by combination antiangiogenic therapy in MCF-7 breast tumor mouse models, using a noninvasive DCE-MRI method that minimizes the effect of water exchange.

Materials and methods

3D quantitative DCE-MRI images were acquired with a heavily T ₁-weighted saturation recovery gradient echo sequence with a recovery delay of 20 ms. Tumor vascular volume (VV) and vascular permeability-surface area product (PS) were obtained through a linear regression of the albumin-Gd-DTPA-enhanced dynamic image intensity on MCF-7 breast tumor mouse models treated with combination bevacizumab/paclitaxel therapy.

Results

Measured tumor VV values were significantly higher than the values that have been reported previously using quantitative T ₁ mapping, and are in good agreement with micro-CT (computed tomography) results reported earlier from other tumor models. A trend of decreasing tumor PS was detected in the group of MCF-7 tumor bearing mice treated with the bevacizumab/paclitaxel combination regimen.

Conclusion

VV and PS maps obtained by a heavily T ₁-weighted acquisition protocol revealed the large peripheral blood vessels as well as the permeable areas within the tumor. A 12-day/three-dose combination treatment of bevacizumab and paclitaxel resulted in delayed tumor growth and a trend of decreasing tumor vascular permeability surface area product.