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.     

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.     

Spin-Flip Raman Scattering on Electrons and Holes in Two-Dimensional (PEA)2PbI4 Perovskites

The class of Ruddlesden–Popper type (PEA)₂PbI₄ perovskites comprises 2D structures whose optical properties are determined by excitons with a large binding energy of about 260 meV. It complements the family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III–V and II–VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin-flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field up to 10 T. From the recorded data, the electron and hole Landé factors (g-factors) are evaluated, their signs are determined, and their anisotropies are measured. The electron g-factor value changes from +2.11 out-of-plane to +2.50 in-plane, while the hole g-factor ranges between -0.13 and -0.51. The spin flips of the resident carriers are arranged via their interaction with photogenerated excitons. Also the double spin-flip process, where a resident electron and a resident hole interact with the same exciton, is observed showing a cumulative Raman shift. Dynamic nuclear spin polarization induced by spin-polarized holes is detected in corresponding changes of the hole Zeeman splitting. An Overhauser field of the polarized nuclei acting on the holes as large as 0.6 T can be achieved.