Crystallization of Semiconductor Nanorods into Perfectly Faceted Hexagonal Superstructures

Super crystallization of ligand‐capped nanocrystals into defined periodic solids from solution is the definitive demonstration of their self‐organizing properties. To date, this has been mainly limited to spherical nanocrystals where organization emulates atom or molecule packing in regular crystals with the most thermodynamically stable arrangement being eventually preferred. Here, the crystallization of wurtzite CdS nanorods into micrometer‐sized CdS superstructures with regular hexagonal symmetry is demonstrated by fine‐tuning the nanorod dispersibility over time. It is shown that the supercrystals have a long nucleation stage to form monolayer hexagons followed by a relatively faster growth stage both occurring rod by rod (in‐plane) and layer by layer (out of plane). The perfectly symmetrical hexagon shape of the final structure is mapped from the wurtzite crystal structure of each individual nanorod where they pack in side by side and end to end arrangements. These well‐defined superstructures are highly attractive for applications that collectively exploit electronic or optical properties that are synthetically tunable through the size and shape of each nanorod building block.     

Oriented O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic guide

We describe our observation of strongly oriented total electronic angular momentum J in O(³P₂), Ne(³P₂), and He(³S₁) atoms emerging from a bent magnetic multipole guide, as measured by resonant multiphoton ionisation. This was contrary to our expectation because no additional (uniform) magnetic fields were applied to orient the atoms behind the exit of the guide. Two- and three-photon ionisation techniques were employed to determine the degree of J polarisation, from which we infer that atoms become oriented as a result of a combination of weak fringe fields, possible stray fields, and the fact that molecular beam packets do not oscillate around the geometric center of the bent multipole guide. We conclude that similar effects may exist in other, related experiments and that a detailed characterisation of the degree of orientation is required prior to any study of chemical dynamics or spectroscopy. This paper should serve as a warning for anybody using similar devices not to assume isotropic angular momentum distributions of atoms and molecules emerging from a magnetic guide or a decelerator, particularly when it is bent; whenever possible, the possibility for a J anisotropy should be experimentally checked.     

The impact of bubbles on measurement of drug release from echogenic liposomes

Echogenic liposomes (ELIP) encapsulate gas bubbles and drugs within lipid vesicles, but the mechanisms of ultrasound-mediated drug release from ELIP are not well understood. The effect of cavitation activity on drug release from ELIP was investigated in flowing solutions using two fluorescent molecules: a lipophilic drug (rosiglitazone) and a hydrophilic drug substitute (calcein). ELIP samples were exposed to pulsed Doppler ultrasound from a clinical diagnostic ultrasound scanner at pressures above and below the inertial and stable cavitation thresholds. Control samples were exposed to a surfactant, Triton X-100 (positive control), or to flow alone (negative control). Fluorescence techniques were used to detect release. Encapsulated microbubbles reduced the measured fluorescence intensity and this effect should be considered when assessing drug release from ELIP. The origin of this effect is not specific to ELIP. Release of rosiglitazone or calcein compared to the negative control was only observed with detergent treatment, but not with ultrasound exposure, despite the presence of stable and inertial cavitation activity. Release of rosiglitazone or calcein from ELIP exposed to diagnostic ultrasound was not observed, even in the presence of cavitation activity. Ultrasound-mediated drug delivery strategies with ELIP will thus rely on passage of the drug-loaded liposomes to target tissues.     

Thermal noise as a spectroscopic tool to determine transport properties

The utilization of thermal fluctuations or Johnson/Nyquist noise as a spectroscopic method to determine transport properties in conductors or semiconductors is developed. The autocorrelation function is obtained from power spectral density measurements thus enabling electronic transport property calculation through the Green–Kubo formalism. This experimental approach is distinct from traditional numerical methods such as molecular dynamics simulations, which have been used to extract the autocorrelation function and directly related physics only. This work reports multi-transport property measurements consisting of the electronic relaxation time, resistivity, mobility, diffusion coefficient, electronic contribution to thermal conductivity and Lorenz number from experimental data. Double validation of the experiment was accomplished through the use of a standard reference material and a standard measurement method, i.e. four-probe collinear resistivity technique. Thermal noise measurements resulted in a 1.1 and 5% agreement with the reference material and four-probe values, respectively. Additional measurements were also taken on nanoscale Au and Cu thin films. Specifically, the validated spectroscopic methodology was applied to 30 nm Au and Cu thin films to obtain transport property data that was again compared to four-probe resistivity measurements. Comparative analysis of the resistivity data showed agreement within 13.6 and 4.8% for the Au and Cu samples, respectively, thus lending further credibility to the experimental method and theory.     

乙基苯胺的转动光谱及分子结构

本文使用傅里叶变换微波谱仪研究了乙基苯胺类物质(邻乙基苯胺,间乙基苯胺,对乙基苯胺)的分子结构. 由于此类分子含氮原子(I¹⁴N=1),因此跃迁谱线中都呈现出核四级裂分. 通过比较实验测定得到的分子结构,可总结苯胺环上不同位置乙基的取代对氨基及分子整体结构的影响.     

Sample-specific diamagnetic and paramagnetic passive shimming

When homogenizing the static magnetic field over extended in vivo volumes, significant residual inhomogeneity can remain after spherical harmonic shim optimization. This is due to the low spatial orders of shims available on in vivo MR systems and the presence of higher-order inhomogeneity in the vicinity of anatomic air cavities. Mediation of this problem through the development of higher-order spherical harmonic shims is severely impeded by bore space limitations. Sample-specific passive shims are not limited to low-order spatial compensation and offer an alternative means to increased homogenization. Here, we present a novel construction protocol for sample-specific passive shims comprised of both diamagnetic (bismuth) and paramagnetic (zirconium) materials. A prototype shim is constructed and shown to significantly homogenize the mouse brain at 9.4 T. Further homogenization capabilities are simulated through alteration of the shim construction.     

Dynamic shim updating on the human brain

Dynamic alteration of shim settings during a multi-slice imaging experiment can improve static magnetic-field homogeneity over extended volumes. In this report, a pre-emphasized dynamic shim updating (DSU) system capable of rapidly updating all non-degenerate zeroth through second-order shims is presented and applied to high-field multi-slice imaging studies on the human brain. DSU is utilized in both non-oblique and oblique slicing geometries while updating in-plane and through-slice shims. Image-based magnetic-field maps are used to quantify homogeneity improvements and comparisons are made on a slice-specific basis between static global shimming and increasing orders of shim inclusion utilized DSU. The influence of oblique slicing geometry on DSU-utilized global homogeneity is also quantified computationally. Finally, the effect of DSU on susceptibility artifact reduction in single-shot axial-sliced EPI is analyzed using experimental acquisitions.     

Intensity-related dynamics of femtosecond frequency combs

We have performed systematic studies of intensity-related dynamics of the pulse repetition and carrier-envelope offset frequencies in mode-locked Ti:sapphire lasers. We compared the results far two laser systems that have different intracavity dispersion-compensation schemes. We found that the carrier-envelope phase noise and its dynamic response depend critically on the mode-locking conditions. An intensity-related shift of the laser spectrum was found to be instrumental in interpretations.     

Enhanced downconversion of UV light by resonant scattering of aluminum nanoparticles

Metallic nanoparticles are known to enhance nonlinear optical processes due to a local enhancement of the optical field. This strategy has been proposed to enhance downconversion in thin film solar cells, but has various disadvantages, among which is the fact that the enhancement occurs only in a tiny volume close to the particles. We report on a very different physical mechanism that can lead to significant downconversion enhancement, namely, that of resonant light scattering, and which is a large volume effect. We show that only a tiny amount of resonantly scattering metallic (aluminum) nanoparticles is enough to create a significant enhancement of the fluorescence of dye molecules in the visible wavelength range. The strategy can be applied in general to increase the emission of UV-absorbing constituents, and is of particular use for solar energy.