Influence of thickness on the optical properties of amorphous GeSe2 thin films: analysis using Raman spectra,Urbach energy and Tauc parameter

An analysis is reported of thickness-induced defects in amorphous GeSe₂ thin films deposited by the vacuum evaporation technique. X-ray diffraction studies confirmed the amorphous nature of the thin films. Optical absorption measurements revealed an indirect transition with an energy gap that increases with film thickness. A blue shift in optical transmittance edges was observed in annealed GeSe₂ thin films. The obtained lower values of Urbach energy (E _U) indicate that as thickness increases more ordered films can be produced. Raman spectra suggest that annealing promotes corner-sharing GeSe_4/2 tetrahedra and edge-sharing Ge₂Se_8/2 bi-tetrahedra bonding and leads to the reduction in disorder in bonding network, which is amply supported by the way of increase in band gap, increase in Tauc parameter (B ^1/2) and reduction in E _U from the analysis of transmittance spectra. Increasing the thickness promotes tetrahedral and bi-tetrahedral bonding through the reduction in bonding defects.     

Synthesis of novel twisted carbazole-quinoxaline derivatives with 1,3,5-benzene core: bipolar molecules as hosts for phosphorescent OLEDs

A series of carbazole/quinoxaline hybrids have been synthesized by classic Ullmann and Pd/Cu-catalyzed Sonogashira coupling reaction. Their photophysical, thermal, and electrochemical properties were investigated. The introduction of electron rich carbazole and electron deficient quinoxaline on to the 1,3,5-benzene center leads to a twisted structure with good glass forming property and imparts a bipolar character. The triplet energies in the range of 2.34-2.53 eV indicate them as potential host materials in phosphorescent OLEDs.     

PVA and BSA stabilized silver nanoparticles based surface-enhanced plasmon resonance probes for protein detection

To perform biosensing using nanoparticles in solution, silver particles were coated with bovine serum albumin (BSA) and polyvinyl alcohol (PVA) as control stabilizer. The plasmon resonance (420 nm) of the silver nanoparticles in solution was shifted slightly to longer wavelength (443 nm) when they were coated with BSA. The biointeractions of these engineered nanoparticles were studied using a mouse model. No significant changes in behavior or toxicity were observed. The nanoparticles were detected in all tissues including the brain. Antibody recognition was monitored via the change in light absorption which accompanied binding, indicating that the particles can be used as a biosensor to gain more insight into cellular mechanisms governing the function of organs in general, and the blood brain barrier (BBB) and brain in particular.     

Surface transport and stable trapping of particles and cells by an optical waveguide loop

Waveguide trapping has emerged as a useful technique for parallel and planar transport of particles and biological cells and can be integrated with lab-on-a-chip applications. However, particles trapped on waveguides are continuously propelled forward along the surface of the waveguide. This limits the practical usability of the waveguide trapping technique with other functions (e.g. analysis, imaging) that require particles to be stationary during diagnosis. In this paper, an optical waveguide loop with an intentional gap at the centre is proposed to hold propelled particles and cells. The waveguide acts as a conveyor belt to transport and deliver the particles/cells towards the gap. At the gap, the diverging light fields hold the particles at a fixed position. The proposed waveguide design is numerically studied and experimentally implemented. The optical forces on the particle at the gap are calculated using the finite element method. Experimentally, the method is used to transport and trap micro-particles and red blood cells at the gap with varying separations. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip, e.g. microfluidics or optical detection, to make an on-chip system for single cell analysis and to study the interaction between cells.     

Explicit all-atom modeling of realistically sized ligand-capped nanocrystals

We present a study of an explicit all-atom representation of nanocrystals of experimentally relevant sizes (up to 6 nm), "capped" with alkyl chain ligands, in vacuum. We employ all-atom molecular dynamics simulation methods in concert with a well-tested intermolecular potential model, MM3 (molecular mechanics 3), for the studies presented here. These studies include determining the preferred conformation of an isolated single nanocrystal (NC), pairs of isolated NCs, and (presaging studies of superlattice arrays) unit cells of NC superlattices. We observe that very small NCs (3 nm) behave differently in a superlattice as compared to larger NCs (6 nm and above) due to the conformations adopted by the capping ligands on the NC surface. Short ligands adopt a uniform distribution of orientational preferences, including some that lie against the face of the nanocrystal. In contrast, longer ligands prefer to interdigitate. We also study the effect of changing ligand length and ligand coverage on the NCs on the preferred ligand configurations. Since explicit all-atom modeling constrains the maximum system size that can be studied, we discuss issues related to coarse-graining the representation of the ligands, including a comparison of two commonly used coarse-grained models. We find that care has to be exercised in the choice of coarse-grained model. The data provided by these realistically sized ligand-capped NCs, determined using explicit all-atom models, should serve as a reference standard for future models of coarse-graining ligands using united atom models, especially for self-assembly processes.     

Nucleophilic ring-opening of activated aziridines: A one-step method for labeling biomolecules with fluorine-18

The direct labeling of biomolecules with fluorine-18 is highly desirable. An option is the ring-opening of an activated aziridine moiety in a biomolecule using ¹⁸F-fluoride. Therefore, a series of aziridine-based model compounds and three aziridine-based biomolecules four aziridine-based model compounds were synthesized and evaluated as potential precursors for a direct one-step radiolabeling with fluorine-18. High to moderate yields of ¹⁸F-incorporation were achieved under mild labeling conditions. The influence of different activating groups, reaction temperature, solvent and base was investigated. The applicability of this method for the direct ¹⁸F-radiolabeling of biomolecules for positron emission tomography (PET) studies is illustrated with examples.     

Low‐Temperature Growth of Carbon Nanotubes Catalyzed by Sodium‐Based Ingredients

Synthesis of low‐dimensional carbon nanomaterials such as carbon nanotubes (CNTs) is a key driver for achieving advances in energy storage, computing, and multifunctional composites, among other applications. Here, we report high‐yield thermal chemical vapor deposition (CVD) synthesis of CNTs catalyzed by reagent‐grade common sodium‐containing compounds, including NaCl, NaHCO₃, Na₂CO₃, and NaOH, found in table salt, baking soda, and detergents, respectively. Coupled with an oxidative dehydrogenation reaction to crack acetylene at reduced temperatures, Na‐based nanoparticles have been observed to catalyze CNT growth at temperatures below 400 °C. Ex situ and in situ transmission electron microscopy (TEM) reveal unique CNT morphologies and growth characteristics, including a vaporizing Na catalyst phenomenon that we leverage to create CNTs without residual catalyst particles for applications that require metal‐free CNTs. Na is shown to synthesize CNTs on numerous substrates, and as the first alkali group metal catalyst demonstrated for CNT growth, holds great promise for expanding the understanding of nanocarbon synthesis.     

Cholesterol packing around lipids with saturated and unsaturated chains: a simulation study

The fundamental role of cholesterol in the regulation of eukaryotic membrane structure is well-established. However the manner in which atomic level interactions between cholesterol and lipids, with varying degrees of chain unsaturation and polar groups, affect the overall structure and organization of the bilayer is only beginning to be understood. In this paper we describe a series of Molecular Dynamics simulations designed to provide new insights into lipid-cholesterol interactions as a function of chain unsaturation. We have run simulations of varying concentrations of cholesterol in dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleyol phosphatidylcholine (POPC), and dioleyol phosphatidylcholine (DOPC) bilayers. Structural analysis of the simulations reveals both atomistic and systemic details of the interactions and are presented here. In particular, we find that the minimum partial molecular area of cholesterol occurs in POPC-Chol mixtures implying the most favorable packing. Physically, this appears to be related to the fact that the two faces of the cholesterol molecule are different from each other and that the steric cross section of cholesterol molecules drops sharply near the small chain tails.     

Effects of contact resistance on the evaluation of charge carrier mobilities and transport parameters in amorphous zinc tin oxide thin-film transistors

Accurate determination of the charge transport characteristics of amorphous metal-oxide transistors requires the mitigation of the effects of contact resistance. The use of additional electrodes as voltage probes can overcome contact resistance-related limitations and yields accurate charge carrier mobility values, trap depths and temperature and carrier density dependencies of mobility as well as trap depths. We show that large differences in measured charge carrier mobility values are obtained when such contact resistances are not factored out. Upon exclusion of the contact resistance, the true temperature dependence of charge carrier mobility appears in the form of two clearly distinct mobility regimes. Analyzing these revealed mobility regions leads to a more accurate determination of the underlying transport physics, which shows that contact resistance-related artefacts yield incorrect trends of trap depth with gate voltage, potentially leading to a misconstruction of the charge transport picture. Furthermore, a comparison of low- and high-mobility samples indicates that the observed effects are more general.