Nanoparticles synthesis by electron beam radiolysis

Electron beam (EB) irradiation is a useful method to generate stable silver nanoparticles without the interference of inherent impurities generated from chemical reactions. Our experiments were carried out using linear electron beam accelerators with two different EB absorbed dose rates: 2 kGy min^?1 and 7–8 kGy s^?1, and with different absorbed dose levels. The optimum conditions for silver nanoparticles (AgNPs) generation by radiolysis, or by radiolysis combined with chemical reduction, were established. In order to obtain a good yield for AgNPs synthesized by radiolysis, a high dose rate is required, resulting in a rapid production process. At low absorbed dose rates, the utilization of a stabilization agent is advisable. By modifying the experimental conditions, the ratio between the chemical and radiolytic reduction process can be adjusted, thus it is possible to obtain nanoparticles with tailored characteristics, depending on the desired application.

Open image in new window

     

Dynamic Cost-Benefit Analysis of Drug Substitution Programs

This paper studies optimal spending for drug substitution programs in the context of a dynamic epidemic model of both drug use and drug use-related infections. Two types of costs are considered in addition to control costs: social costs resulting from individuals being dependent on drugs; additional costs due to drug users being vulnerable to infections like hepatitis C or HIV. Analysis of the model demonstrates that the long-run equilibrium is not necessarily unique. Instead, there may be multiple equilibria. Which of these equilibria is optimal depends on the initial conditions for the number of drug addicts and the number of those who are infected. So, for a given set of epidemic parameters, it may be optimal to spend a lot on substitution programs that reduce the number of drug addicts or to spend little and to accept a high level of drug use.     

Explicit min–max polynomials on the disc

Denote by ${\Pi }_{n+m?1}^2?\{{\sum }_{0\le i+j\le n+m?1}{c}_{i,j}{x}^i{y}^j:c_{i,j}\in \mathbb{R}\}$ the space of polynomials of two variables with real coefficients of total degree less than or equal to $n+m?1$. Let $b_0,b_1,\dots ,b_l\in \mathbb{R}$ be given. For $n,m\in \mathbb{N},n\ge l+1$ we look for the polynomial $b_0{x}^n{y}^m+b_1{x}^{n?1}{y}^{m+1}+?+b_l{x}^{n?l}{y}^{m+l}+q(x,y),q(x,y)\in {\Pi }_{n+m?1}^2$, which has least maximum norm on the disc and call such a polynomial a min–max polynomial. First we introduce the polynomial $2P_{n,m}(x,y)=x{G}_{n?1,m}(x,y)+y{G}_{n,m?1}(x,y)=2x^n{y}^m+q(x,y)$ and $q(x,y)\in {\Pi }_{n+m?1}^2$, where $G_{n,m}(x,y)?1/2^{n+m}(U_n\left(x\right)U_m\left(y\right)+U_{n?2}\left(x\right)U_{m?2}\left(y\right))$, and show that it is a min–max polynomial on the disc. Then we give a sufficient condition on the coefficients $b_j,j=0,\dots ,l,l$ fixed, such that for every $n,m\in \mathbb{N},n\ge l+1$, the linear combination ${\sum }_{\nu =0}^l{b}_{\nu }{P}_{n?\nu ,m+\nu }(x,y)$ is a min–max polynomial. In fact the more general case, when the coefficients $b_j$ and $l$ are allowed to depend on $n$ and $m$, is considered. So far, up to very special cases, min–max polynomials are known only for $x^n{y}^m$,$n,m\in {\mathbb{N}}_0$.     

Growth and structural characterization of YMnO3 thin films grown by pulsed liquid injection MOCVD on Si and SrTiO3 substrates

Optimum parameters for the growing of YMnO₃ films by pulsed liquid injection metalorganic chemical vapor deposition have been studied. Si substrates were used for the optimization of the deposition process. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that polycrystalline single phase YMnO₃ films can be obtained for an optimal ratio of Y and Mn on the injected solution and either amorphous, metastable orthorhombic, and/or hexagonal YMnO₃ phases can be obtained depending on the deposition temperature and precursors ratio. In a second stage, YMnO₃ films were grown on SrTiO₃ substrates. Pure epitaxial orthorhombic YMnO₃ phase was confirmed by XRD. The films microstructure, characterized by scanning electron microscopy and TEM, shows a columnar growth. Each columnar grain grows epitaxially with three possible orientations.     

Unoccupied electronic structure of TiOCl studied using x-ray absorption near-edge spectroscopy

We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl using x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. We acquire data both in total electron and fluorescence yield modes (TEY and FY, respectively). While only the latter allows us to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl, the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent in the incommensurate intermediate phase within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by means of cluster calculations in D(4h) and D(2h) symmetries using two different codes. We extract values of the crystal-field splitting and parameterize our results using the commonly seen notation of Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.     

Structure and orbital ordering of ultrathin LaVO3 probed by atomic resolution electron microscopy and Raman spectroscopy

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO₃ films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature‐dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm^–1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO₃ films at about 130 K.     

Vibrational mode selectivity in hyperfine interactions: polarization quantum beat spectroscopy of HCF(A1A")

We report on the vibrational mode dependence of the 19F and 1H hyperfine interaction constants in the A1A" state of HCF, determined using polarization quantum beat spectroscopy. The nuclear spin/overall rotation coupling constants display a pronounced energy dependence and mode selectivity which can be traced to variations in both the A rotational constant and nuclear spin/electron orbital coupling constant a. In particular, modes containing C-F stretching excitation display significantly larger 19F spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2p(pi) orbitals.     

Development of the IR laser pyrolysis for the synthesis of iron–doped TiO2 nanoparticles: Structural properties and photoactivity

The preparation of TiO₂-based nanoparticles of closely controlled sizes and purity gives rise to considerable interest in the frame of environmental applications, e.g. in photocatalysis. When nanoparticles instead of their bulk counterpart are used the synthesis method plays a fundamental role in defining specific structural properties. Between the different gas-phase synthesis techniques, the CO₂ laser pyrolysis is a versatile method allowing for the preparation of nanostructures of various chemical compositions. Here we demonstrate that pure and Fe–doped TiO₂ nanoparticles with rather low Fe concentration may be prepared by applying the sensitized IR laser pyrolysis to a gas mixtures containing titanium tetrachloride, air and iron pentacarbonyl (vapors). The structures of TiO₂-based particles were systematically investigated by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy, selected area electron diffraction and X-ray Photoelectron Spectroscopy. Depending on the synthesis parameters, the nanoparticle system contains mixtures of anatase and rutile, with a preponderance of the anatase phase. Higher rutile proportion was found in the iron-doped samples. Mean particle diameters of around 14 nm and 12 nm were estimated for undoped and doped anatase titania, respectively. From UV–Vis diffuse reflectance spectra, higher absorbance and red shifted absorption were evidenced at higher amount of doped iron. Preliminary evaluation tests of the UV photoactivity of samples were performed by using the scanning electrochemical microscopy for determining the evolution of the oxygen consumption in the presence of IV-chlorophenol. They show that the undoped nano titania samples perform better than the reference P25 Degussa sample. A drop of the nano-titania photoactivity as a consequence of Fe doping was observed. Possible reasons of this effect are tentatively discussed.