Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J?cm^?2 single-shot irradiation under UHV condition and 0.25–2.5 J?cm^?2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J?cm^?2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J?cm^?2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.     

Collective Perspective on Advances in Dyson-Schwinger Equation QCD

We survey contemporary studies of hadrons and strongly interacting quarks using QCD's Dyson-Schwinger equations, addressing the following aspects: confinement and dynamical chiral symmetry breaking; the hadron spectrum; hadron elastic and transition form factors, from small- to large-Q²; parton distribution functions; the physics of hadrons containing one or more heavy quarks; and properties of the quark gluon plasma.     

Vapor‐Phase Deposition of Monofunctional Alkoxysilanes for Sub‐Nanometer‐Level Biointerfacing on Silicon Oxide Surfaces

Improving the performance and lowering the analyte detection limits of optical and electronic biosensors is essential for advancing wide ranging applications in diagnostics and drug discovery. Most sensing methods require direct linkage of a recognition element and a sensor, which is commonly accomplished through an organic monolayer interface. Alkoxyorganosilanes are typically used to prepare sensor surfaces on dielectric oxides. However, many silanes lead to roughened or thick interfaces that degrade device sensitivity. Here, controlled vapor phase deposition of monoalkoxysilanes is found to lead to monolayers resistant to elevated temperatures and extreme pH conditions. The formation of high density, subnanometer monolayers is demonstrated by ellipsometry, XPS, and AFM. The uniform attachment of these monofunctional silanes to such biosensing platforms as microarrays, field effect devices, and the formation of surface enhanced Raman spectroscopy substrates is demonstrated. The advantages of using this silane deposition protocol for the above technologies are also discussed.     

DNA Sensing Using Nanocrystalline Surface‐Enhanced Al2O3 Nanopore Sensors

A new solid‐state, Al₂O₃ nanopore sensor with enhanced surface properties for the real‐time detection and analysis of individual DNA molecules is reported. Nanopore formation using electron‐beam‐based decomposition transforms the local nanostructure and morphology of the pore from an amorphous, stoichiometric structure (O to Al ratio of 1.5) to a heterophase crystalline network, deficient in O (O to Al ratio of ≈0.6). Direct metallization of the pore region is observed during irradiation, thereby permitting the potential fabrication of nanoscale metallic contacts in the pore region with application to nanopore‐based DNA sequencing. Dose‐dependent phase transformations to purely γ and/or α‐phase nanocrystallites are also observed during pore formation, allowing for surface‐charge engineering at the nanopore/fluid interface. DNA transport studies reveal an order‐of‐magnitude reduction in translocation velocities relative to alternate solid‐state architectures, accredited to high surface‐charge density and the nucleation of charged nanocrystalline domains. The unique surface properties of Al₂O₃ nanopore sensors make them ideal for the detection and analysis of single‐stranded DNA, double‐stranded DNA, RNA secondary structures, and small proteins. These nanoscale sensors may also serve as useful tools in studying the mechanisms driving biological processes including DNA–protein interactions and enzyme activity at the single‐molecule level.     

Tea Polyphenols Reducing Lipopolysaccharide-induced Inflammatory Responses in RAW264.7 Macrophages via NF-κB Pathway

The anti-inflammatory activity of tea polyphenols(TPs) in RAW264.7 macrophages stimulated by lipopolysaccharide(LPS) was investigated in this paper. RAW264.7 macrophages were treated with different concentrations of TP(0, 12.5, 25, 50, 100, and 200 μg/mL) and then stimulated by LPS. Another blank control group was set up. The expression of pro-inflammatory cytokines associated with the nuclear factor-kappa B(NF-κB) signaling pathway was investigated before and after TP treatment. Pretreatment of RAW264.7 cells with TP decreased the expression of tumor necrosis factor-α(TNF-α), interleukin-6(IL-6) and interleukin 1 beta(IL-1β) pro-inflammatory cytokines. In addition, TP inhibited the phosphorylation of p65 and IκB by blocking the phosphorylation and the degradation of NF-κB inhibitor protein. In conclusion, TP exerts anti-inflammatory effects by regulating the release of inflammatory mediators via the NF-κB signaling pathway.     

Tunable intramolecular multicenter H-bonding interactions in first-row metal complexes bearing bidentate redox-active ligands

Abstract

In this research article, we report the synthesis and structural characterization of a family of first-row metal complexes bearing redox-active ligands with tunable H-bonding donors. We observed that these coordination complexes can adopt three different geometries and that they are stabilized by intramolecular multicenter H-bonding interactions, which are systematically modified by changing the metal ion (Co, Ni, Cu, Zn), the ligand scaffold (variations in the diamine and ureanyl substituents used) and the solvent of crystallization.     

Series Solution for Rotating Flow of an Upper Convected MaxwellFluid over a Stretching Sheet

The equations for two-dimensional flow of an upper convected Maxwell (UCM) fluid in a rotating frame are modeled. The resulting equations are first simplified by a boundary layer approach and then solved by a homotopy analysis method (HAM). Convergence of series solution is discussed through residual error curves. The results of the influence of viscoelastic and rotation parameters are plotted and discussed.     

On MHD transient flow of a Maxwell fluid in a porous medium and rotating frame

Analytic solution for unsteady magnetohydrodynamic (MHD) flow is constructed in a rotating non-Newtonian fluid through a porous medium. Constitutive equations for a Maxwell fluid have been taken into consideration. The hydromagnetic flow in the uniformly rotating fluid is generated by a suddenly moved infinite plate in its own plane. Analytic solution of the governing flow problem is obtained by means of the Fourier sine transform. It is shown that the obtained solution satisfies both the associate partial differential equation and the initial and boundary conditions. The solution for a Navier-Stokes fluid is recovered if λ→0. The steady state solution is also obtained for t→∞.     

Heat and mass transfer analysis on the flow of a second grade fluid in the presence of chemical reaction

The laminar flow problem of convective heat transfer for a second grade fluid over a semi-infinite plate in the presence of species concentration and chemical reaction is investigated. The governing equations are transformed into a dimensionless system of three non-linear coupled partial differential equations. These equations have been solved analytically subject to the relevant boundary conditions by employing a homotopy analysis method (HAM). It is noted that for the arising system, the HAM performs extremely well in terms of efficiency and simplicity. The influence of dimensionless pertinent parameters on the velocity, temperature and concentration fields has been examined carefully.